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Absolute emission intensity studies on the halogen afterglows and excited molecular oxygen 1969

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ABSOLUTE EMISSION INTENSITY STUDIES ON THE HALOGEN AFTERGLOWS AND EXCITED MOLECULAR OXYGEN by ROBERT JAMES BROWNE B . S c , U n i v e r s i t y Of Western Ontario, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF June BRITISH COLUMBIA 1969 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l - ment o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y w i l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n w i l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f The U n i v e r s i t y Of B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada TABLE OF CONTENTS A b s t r a c t L i s t of Tables L i s t of I l l u s t r a t i o n s Acknowledgements v i PART I : A STUDY OF THE HALOGEN AFTERGLOWS INTRODUCTION 1 E l e c t r o n i c States of the Halogens 3 Halogen Atom Recombination Studies 7 Rate Studies on Halogen Atom Recombination 9 Studies on the Luminescence from Halogen Atom Recombination 12 Purpose of t h i s I n v e s t i g a t i o n 18 EXPERIMENTAL The Flow System 19 M a t e r i a l s 23 Produ c t i o n of the E x c i t e d Species Ca) C h l o r i n e and Bromine Atoms 24 (b) Production of Iodine Atoms i n a Flow System 26 Atom D e t e c t i o n and Measurement 27 Spectroscopic Measurements (al Equipment 34 (b) Measurement of Spectra 35 Cc) S i g n a l D e t e c t i o n 36 (d) S i g n a l A m p l i f i c a t i o n 38 Ce) C a l i b r a t i o n of Detectors f o r Absolute Emission I n t e n s i t i e s 39 Cf) Experimental Determination of the Transmission of the O p t i c a l System and Detector S e n s i t i v i t y 44 RESULTS The Bromine Aft e r g l o w Spectrum 4 6 K i n e t i c s of the Bromine Aft e r g l o w 49 Ca) Dependence of Emission I n t e n s i t y on [Br] 50 (bl Pressure Dependence of the Emission I n t e n s i t y 51 Ccl Absolute Rate Constant Measurements 51 Emission from Iodine Atom Recombination 53 The C h l o r i n e A f t e r g l o w Spectrum 54 K i n e t i c s of the C h l o r i n e Afterglow Emission 56 (a) Dependence of the Emission I n t e n s i t y on [Cl] 57 (b) Dependence of the Emission I n t e n s i t y on [Cl-?] 58 Cc) Absolute Rate Constant Measurements 59 Es t i m a t i o n of E r r o r i n the Rate Constants 60 page i i i i i v TABLE OF CONTENTS ( C o n t i n u e d ) page DISCUSSION 61 C o n t r i b u t i o n of Two Body R a d i a t i v e Recombination to the Halogen Afterglows 62 (a) C h l o r i n e 63 (b) Bromine 66 The Iodine Afterglow 67 The O r i g i n of the Bromine Afte r g l o w 67 K i n e t i c s of the Bromine Afterglow 73 Mechanism of the Emission Reaction (a). Formation of E x c i t e d States i n the B r 2 Afterglow 75 (b). R e l a x a t i o n Processes i n the E x c i t e d States 78 O r i g i n of the C h l o r i n e Emission 81 K i n e t i c s of the C h l o r i n e Afterglow (a) Order of Emission I n t e n s i t y w i t h Respect to [Cl] 82 (b) Pressure Dependence of the Emission I n t e n s i t y 84 Mechanism of the Emission Reaction i n C h l o r i n e Ca). Formation of the E m i t t i n g State 85 (b) R e l a x a t i o n of the 3 n Q + u State 88 (A) V i b r a t i o n a l R e l a x a t i o n 89 ( i i ) Quenching by I C I 2 J 89 ( i i i ) . Quenching by Atoms 90 Cc) P a r t i c i p a t i o n of Other E l e c t r o n i c States i n the C l 2 Afterglow 93 Suggestions f o r Further Study of Halogen Afterglows 94 PART I I : STUDIES ON EXCITED MOLECULAR OXYGEN INTRODUCTION E l e c t r o n i c States of Molecular Oxygen 96 Studies on E x c i t e d Molecular Oxygen 99 Purpose of t h i s I n v e s t i g a t i o n 102 EXPERIMENTAL Production of 0 2 ( A g ) and 0 2 ( Eg) Molecules 103 Measurement of Emission from the C l 2 - H 20 2 System 104 RESULTS Es t i m a t i o n of [ 0 2 C 1 ^ g ) J from Absolute Emission 106 Absolute Emission I n t e n s i t y of the 6340A Band 108 The Chlorine-Hydrogen Peroxide System 109 DISCUSSION R a d i a t i v e L i f e t i m e of the ( 0 2 ) 2 Complex 113 Chemiluminescence from the C1 2-H 20 2 System 117 APPENDIX 122 BIBLIOGRAPHY i ABSTRACT The bromine afterglow emission was studied using the discharge-flow technique. The spectrum of discharged bromine O 0 was observed to extend from 6000A to 22000A and has been a t t r i b u t e d to the Br„ l 3 I I + — a n d B r 0 ( 3 n , —» - 2 o u g 2 l u g' t r a n s i t i o n s . The dependence of the emission i n t e n s i t y on atom concentration was observed to vary between I « [Br]^"^ ± at short wavelengths, and I « iBr]"*"^ ± i n the long wave- length region of the spectrum. In the pressure range studied CO.5 to 2 t o r r l , the .intensity was found to be independent of the molecular bromine concentration. By measuring the o o absolute emission i n t e n s i t y between 6000A and 12000A, values of the apparent rate constant, defined as k app " I / I B r l 2 [ B r 2 ] , were measured. These rate constants were found to depend on atom concentration and pressure and varied between k = app 13 15 2 -2 -1 5.4 x 10 and 1.3 x 10 cc .mole sec . A mechanism of the emission reaction i s discussed and i t i s suggested that as much as 15% of the t o t a l recombination of bromine atoms may be proceeding v i a excited states. In a s i m i l a r study of the chlorine afterglow, a l l of the emission was a t t r i b u t e d to the C l - (3IT + —>- 1 Z + ) tran- 2 o u g s i t i o n . A study of the emission i n t e n s i t y i n narrow bands revealed that I « ICIJ^*^ ~ at short wavelengths, 3 corresponding to low v i b r a t i o n a l l e v e l s of the n 0 + u state, while I = [ C l ] 1 * ^ ± at long wavelengths and higher v i - brational l e v e l s . S i m i l a r l y , the pressure dependence of the in t e n s i t y changed from I « I C ^ J ^ * ^ ± at long wavelengths to I oc [ C ^ ] ^ * ^ * at short wavelengths. Absolute emission o i n t e n s i t y measurements were made i n the region from 5000A to o 12000A and the rate constants, defined by k = I/IC1] 2[C1 0] , app 2 were found. Extrapolation of these values of k to zero app 14 2 -2 -1 atom concentration y i e l d s k = 1.8 x 10 cc mole sec J • app A mechanism for the formation and relaxation of the excited state i s discussed. The study of absolute emission i n t e n s i t i e s was extended to measurements on excited oxygen molecules i n a flow system. o i + 3 _ Observations on the 7619A band of the O J S —»• Z ) tran- 2 g g 1 + -9 -1 s i t i o n yielded [0» (. E )] = 1.77 x 10 moles 1. , thereby ^ 9J confirming that t h i s species i s a minor constituent i n the products of discharged oxygen. Absolute i n t e n s i t y studies on the 6340A and 7030A bands of the (0 oC 1A ) ) 0 — ( 0 o ( 3 E ~ ) ) o 2 g z z g z t r a n s i t i o n indicate that the half l i f e of the (°2 ) ) 2 c o l ~ l i s i o n complex i s around 0.1 seconds. The emission spectrum produced when CI2 i s reacted with H2O2 i n solution was observed to originate i n various tran- L g s i t i o n s involving excited molecular oxygen. A y i e l d of 0-(^"A ) of 10% from t h i s reaction was estimated. i i i LIST OF TABLES Table 1 - Table 4 Table 5 Table 6 Table 8 Table 9 Table 10 Table 11 Rate Of Recombination Of Iodine And Bromine Atoms At Room Temperature Table 2 - Measurement Of The Band Heads Of The Br. Table 3 - A f t e r g l o w In The Region 7000A - 8300A Bands In The B r 2 A f t e r g l o w Spectrum Recorded By The H i l g e r Monochromator The Dependence Of I, Upon [Br] Values Of n In The Expression 1^ °c [Br] n Table 7 - Values Of k For Emission Between 6000A o app And 12000A In The Bromine Afte r g l o w Band Heads Recorded From The C h l o r i n e A f t e r - glow Spectrum - Dependence Of I. Upon [Cl] - Values Of n In The Expression I, « [Cl] n Values Of m In The Expression 1^ <* IC1 2J m Values Of k For The C h l o r i n e A f t e r g l o w app Emission Table 12 - Bands Observed In The Spectrum Of The C l 2 - H 20 2 System i v LIST OF ILLUSTRATIONS Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Chlorine p o t e n t i a l energy diagram. Bromine p o t e n t i a l energy diagram. Iodine p o t e n t i a l energy diagram. Schematic diagram of experimental apparatus, Reaction tube used i n bromine afterglow studies. C i r c u i t diagram of the detector bridge. Relative response of the RCA 7265 photo- m u l t i p l i e r . Relative response of the RCA 7102 photo- m u l t i p l i e r . Relative response of PbS detector. Recorder trace of the bromine afterglow spectrum. Assignments to the ^ 0 + u l v + I t r a n s i t i o n g i n the spectrum of discharged bromine. Spectral d i s t r i b u t i o n of the bromine a f t e r - glow spectrum obtained by PbS detector. True spectral d i s t r i b u t i o n of the bromine afterglow spectrum. Plots of (a) log 1^ vs. log [Br] and (b) log C^I^dA). vs. log [Br]. Plots of (a) log 1^ vs. log [Br] and (b) log (^I^dA) vs. log [Br] for a pressure of 1.82 t o r r . The spectrum of discharged 1^. Iodine afterglow spectrum produced by IC1 + C l reaction. The change i n the spectral d i s t r i b u t i o n of the C l 2 afterglow with atom concentration. The change i n spectral d i s t r i b u t i o n of the C l 2 afterglow spectrum with pressure. LIST OF ILLUSTRATIONS (Continued) Figure 20 - True spectral d i s t r i b u t i o n of the chlorine afterglow spectrum. Figure 21 - Plots of log 1^ vs. log [Cl] for a pressure C a ) of 1.70 t o r r . (b) - Plot of log ( I^dX) vs. log [Cl] for a pressure of 1.70 t o r r . Figure 22 - Plots of log I. vs. log IC1] for a pressure Ca) of 3.08 t o r r . (b) - Plot of log C v s * lo<3 £ c ll f o r a Pressure of 3.08 t o r r . Figure 23 - p l o t of log P vs. log 1^ for bands centred at f i v e wavelengths. Figure 24 - Plot of k vs. 1/[C1J for a pressure of app 1.70 t o r r . Figure 25 - Plot of k vs. 1/[C1] for a pressure of app 2.33 t o r r . TOTAL Figure 26 - Plot of k vs. 1/[C1] for f i v e pressures. app _ Figure 27 - Plot of AG' .. vs. v' for the II, state V T I / 2. X U of Br-. 2 Figure 28 - Plot of k vs. iBr ] for a pressure of 0.92 app t o r r . Figure 29 - plots of 1/k vs. [BrJ. app 2 Figure 30 - Plot of k vs. IC1] for a pressure of 1.70 t o r r . TOTAL Figure 31 - Plots of k/k vs. [C1J i n the pressure app range 0.83 to 3.08 t o r r . Figure 32 - pressure dependence of the chlorine afterglow emission i n t e n s i t y . Figure 33 - Schematic diagram for proposed mechanism of Cl atom recombination. Figure 34 - Potential energy curves for the oxygen molecule, Figure 35 - Emission spectrum from.the reaction of chlorine with hydrogen peroxide. Figure 36 - E f f e c t i v e p o t e n t i a l energy curves for the Lennard-Jones C6-12) p o t e n t i a l . V i ACKNOWLEDGEMENTS I would l i k e t o e x p r e s s my s i n c e r e a p p r e c i a t i o n t o : Dr. E. A. O g r y z l o a t whose s u g g e s t i o n t h i s i n v e s t i g a t i o n was u n d e r t a k e n and under whose encouragement and g u i d a n c e t h i s work was c a r r i e d o u t , Dr. A. V. Bree and Dr. N. Basco f o r h e l p f u l d i s c u s s i o n s and s u g g e s t i o n s . The N a t i o n a l R e s e a r c h C o u n c i l o f Canada f o r a B u r s a r y and S c h o l a r s h i p s d u r i n g t h e c o u r s e o f t h i s r e s e a r c h , And f i n a l l y , Mr. Norman F i n l a y s o n f o r h i s e n t h u s i a s t i c h e l p i n w r i t i n g computer programs, and M i s s Thea Alma f o r her p a t i e n c e i n t y p i n g and p r o o f - r e a d i n g t h e m a n u s c r i p t . To My Aunt Jessie P A R T O N E STUDY OF THE HALOGEN AFTERGLOWS 1. INTRODUCTION I t i s a s u r p r i s i n g f a c t t h a t one of the s i m p l e s t conceivable r e a c t i o n s , the recombination of atoms to form diatomic molecules, i s s t i l l p o o r l y understood. Not only i s the o v e r a l l mechanism of recombination open to q u e s t i o n , but the r o l e played by e l e c t r o n i c a l l y ex-, c i t e d s t a t e s i s u n c e r t a i n . I t was i n an attempt to shed some l i g h t on t h i s l a t t e r problem t h a t t h i s research was undertaken. The question which we sought t o answer was what p a r t i n the a s s o c i a t i o n process i s played by c o l l i s i o n s i n t o e l e c t r o n i c a l l y e x c i t e d s t a t e s . The technique which shows the g r e a t e s t p o t e n t i a l f o r t h i s type of study i s the "discharge—flow" method by which atoms and e x c i t e d molecules can be produced at pressures at which the elementary processes can be observed. P r i o r to the development of the discharge-flow technique, a number of i n v e s t i g a t o r s undertook a study of these pro- cesses by d i s c h a r g i n g gases i n s t a t i c systems. The f i r s t r e p o r t of t h i s type of study was given by Bequerel (1) i n 1859 who passed an e l e c t r i c c u r r e n t through oxygen. The development of the e l e c t r i c a l discharge method by Wood C2) i n 1923 and l a t e r by Bonhoeffer (3) made 2. p o s s i b l e a more s a t i s f a c t o r y method of studying atom r e - a c t i o n s , and a great d e a l of work on n i t r o g e n atoms was done. F i n a l l y the advent of the e l e c t r o d e l e s s d i s c h a r g e , made p o s s i b l e by the development of high i n t e n s i t y micro- wave sources, provided the best source of e x c i t e d species and t h i s i s the method most widely used today. U n t i l r e c e n t l y , the m a j o r i t y of experiments were performed on " a c t i v e " n i t r o g e n and oxygen and the r e s u l t s were h i g h l y q u a l i t a t i v e i n nature. However, the a p p l i - c a t i o n of discharge-flow experiments to the study of the upper atmosphere ( 4 ) and to c e r t a i n b i o p h y s i c a l phenomena ( 5 ) , has s t i m u l a t e d a renewed i n t e r e s t i n the technique. Recent developments have made p o s s i b l e the study of halogen atoms (6) and a l s o of e x c i t e d oxygen molecules ( 7 ) i n flow systems. Many atom r e a c t i o n s have accompanying chemilumines- cent emissions, as i n the case of the r e a c t i o n of oxygen atoms w i t h n i t r i c oxide (8), but u n t i l r e c e n t l y , most i n v e s t i g a t i o n s of these emissions i n v o l v e d only a spec- t r o s c o p i c i d e n t i f i c a t i o n of the e x c i t e d s p e c i e s . Today, however, the development of s e n s i t i v e d e t e c t o r s i n the v i s i b l e and near i n f r a r e d r e g i o n s , together w i t h the a v a i l a b i l i t y of a convenient standard f o r absolute emis- s i o n i n t e n s i t y C9) has made p o s s i b l e a more d e t a i l e d study of these emissions,and i t i s now p o s s i b l e to measure the absolute emission i n t e n s i t i e s from a s s o c i a t i o n r e a c t i o n s . In the present work we have attempted such measurements on the discharge a f t e r g l o w of oxygen and the halogens. Before d e s c r i b i n g some of the previous work on r e - combination luminescence, however, we w i l l review b r i e f l y our knowledge about the e x c i t e d s t a t e s of Cl^, B r 2 and I 2 . E l e c t r o n i c States Of The Halogens Applying the Wigner-Witmer c o r r e l a t i o n r u l e s (10), the f o l l o w i n g s t a t e s can be shown t o c o r r e l a t e w i t h two 2 P atoms: X A 3 A i-TT X T T 3 T T 3 T T 1 V + 1 V + A , A , n , n , I I , n , z , z . g' u' u' g' u' g' g' u' 1 - 3 + ' 3 + 3 V- ^u' Lu' LMf g * M u l l i k e n (11, 12) suggested t h a t the lowest energy e l e c t r o n c o n f i g u r a t i o n s of the halogens which l e a d t o a "̂Zg ground s t a t e are the f o l l o w i n g : C l 2 [ K K L L ( z a ) 2 ( y a ) 2 ( x a ) 2 ( w T T ) 4 ( v T r ) 4 ] B r 2 [KKLLMM(za) 2 (ya) 2 (xa) 2 (WTT) 4 (VTT) 4 ] • I 2 iKKLLMMNN(za) 2 (ya) 2 (xa) 2 (WIT)4 (VTT) 4 ] . In order to understand the v a r i o u s e l e c t r o n i c t r a n - s i t i o n s which the halogens undergo, the c o u p l i n g of the v a r i o u s angular momenta w i t h i n the molecule must be understood. The coupling i n the halogens i s not s t r i c t l y Hund's case (a) or (c) but f a l l s i n between these two c l a s s i f i c a t i o n s . In the excited states,, the tendency i s more toward case (c) coupling, e s p e c i a l l y i n the heavier atoms. Moreover, i n the excited states of the halogen molecules the energies of d i s s o c i a t i o n are very small, (D = .55 eV. for I 2 ; .39 eV. for Br 2; .23 eV. for C l 2 ) (13) and are roughly about equal to the doublet separ- 2 2 ations between the ^2/2 a n < ^ P l / 2 s u b l e v e l s * F r o m these figures we conclude that, e s p e c i a l l y i n the heavier halogens, the i n t e r a c t i o n between L and S i s stronger than the i n t e r a c t i o n with the internuclear axis. Thus the quantum numbers of the i n d i v i d u a l atoms L^,S^,J^ and L 2 , S 2 , J 2 r e t a i n t h e i r s i g n i f i c a n c e to a considerable ex- tent i n the molecule, and the quantum numbers A (the component of the e l e c t r o n i c o r b i t a l angular momentum along the internuclear axis) and Z for the molecule as a whole are no longer good quantum numbers. The only e l e c t r o n i c quantum number which i s s i g n i f i c a n t i s fl (ft = |M̂  + M2| where M^ and M 2 are projections of and J2 on the internuclear a x i s ) . Nonetheless, these quantum numbers can be assigned to a state by imagining that the atoms are forced closer together creating a stronger f i e l d , breaking down the L,S coupling and thereby ap- proaching Hund's case Ca), The convention which has been adopted i n naming the e l e c t r o n i c states of the halogens u t i l i z e s a mixture of case (a) and (c) nomenclature. 3 + For example the II + state would be named 0 i f case o u u (c) naming were s t r i c t l y adhered to. In p r e d i c t i n g the possible excited states of the halogens, Mulliken (14) v i s u a l i z e d a d i f f e r e n t type of coupling. Using his scheme, the excited states are ob- tained by adding one electron i n an "excited o r b i t a l " to any of the various states of the ions X 2 +. Each ex- c i t e d o r b i t a l has an i o n i z a t i o n p o t e n t i a l which i s smaller than that of any o r b i t a l present i n the un- excited molecule. In the case of I 2 , such an o r b i t a l * * i s the anti-bonding 5 a o r b i t a l (referred to as a ) p u which has an i o n i z a t i o n p o t e n t i a l of the same order of magnitude as the ordinary i o n i z a t i o n p o t e n t i a l of the molecule. Thus the lowest excited state i s obtained by * + adding a o"u electron to the normal state of ^ : I-: a 2 TT4 TT3 a* , lr\ (2) 2 g u g u u 3 Transitions from the ground state of X„ to the II + and 2 o 3 11̂  l e v e l s of (2) give the well known v i s i b l e and i n f r a - red absorption bonds of the halogens. If we write (2) i n a s l i g h t l y d i f f e r e n t form where the X 2 + core i s separated, we get V H 2 " V V \ l / 2 3 ' 0 * ! o f l u ( 3 ) ( r 2 3 4 2 n ' , *, ! [ a g ^u^.g' n e 3/2 ] a i 2 , l u In (3), we have a core with good quantum numbers Sc> A , E , ft and an electron i n the excited o r b i t a l which c' c c i s weakly coupled to the magnetic axis giving a resultant ft = ftc ± 1/2 . This type of coupling i s c a l l e d ft-s or ft,w coupling and i s s i m i l a r to J - j coupling i n atoms. The molecule as a whole has A and ft as good quantum numbers while S has l o s t i t s s i g n i f i c a n c e . The s e l e c t i o n rules which govern t r a n s i t i o n s between states with ft,co coupling are as follows: (1) t r a n s i t i o n s within the core are the same as i f the excited electron were absent (AA .Aft = 0, + 1 with c Aft = AA ) and c c (2> Aft = 0, + 1 . Because ft,w coupling depends on strong s p i n - o r b i t coupling i n the core, one might expect i t to be modified by a tendency toward atomic spin-orbit (case (c)) coup- l i n g , i n which case A c = A i s not well defined and the rules AA = 0, ± 1 and AA = Aft c are not s t r i c t . Tran- s i t i o n s from ordinary case' (a) or (b) to ft^oo states are governed by Aft = 0, ± 1, AA = 0, ± 1 with no Aft = AA r e s t r i c t i o n . From a study of experimental and t h e o r e t i c a l values 3 of Av, the i n t e r v a l s between the (0,0) bands of two II systems of each molecule, Mulliken (14,15) concluded that i n the e l e c t r o n i c l e v e l s of the halogen molecules, the coupling i s , for the most part, between the A,s and ft,co types, but has strong tendencies, e s p e c i a l l y i n 7. the heavier atoms, toward separate atom case (c) coup- l i n g . On t h i s b a s i s he p r e d i c t e d a number of s t a t e s i n the halogen molecules some of which are shown as dotted curves i n the p o t e n t i a l energy diagrams (Figures 1,2 and 3). Halogen Atom Recombination Studies The experimental techniques used to produce the high c o n c e n t r a t i o n s of atomic species necessary f o r r e - combination s t u d i e s f a l l i n t o three c a t e g o r i e s : thermal methods, photochemical d i s s o c i a t i o n , and e l e c t r i c a l d i s charge. The f i r s t s t u d i e s were conducted by heating the halogens i n sealed tubes (16, 17) to approximately 1300°K a t which temperature the gases were found to emit v i s i b l e r a d i a t i o n . The same phenomenon i s observed i n shock tubes and extensive study of shock heated c h l o r i n e and bromine has been made by Palmer (18, 19) f Palmer and Hornig (20) , B r i t t o n (21) and Burns and Hornig (22). D i r e c t p h o t o l y s i s of the halogens was used by Rabinowitch and Wood (23) to determine the r a t e s of atom recombination i n the presence of many f o r e i g n gases. However, the g r e a t e s t advance i n t h i s technique came w i t h the development of f l a s h p h o t o l y s i s . This method i s p a r t i c u l a r l y s u i t e d to the study of i o d i n e because i t s l a r g e e x t i n c t i o n c o e f f i c i e n t allows a high Figure 1. Chlorine p o t e n t i a l energy diagram. Figure 2. Bromine po t e n t i a l energy diagram. Figure 3. Iodine p o t e n t i a l energy diagram. r(A) r (A)  8. i n i t i a l concentration of atoms to be formed, and the decay can be e a s i l y followed photometrically. The numerous publications by Porter (see, for example, reference 24) are evidence of the stimulus which t h i s technique has given to the study of atom recombination. Although e l e c t r i c a l discharge techniques were very early shown to be useful for N 2 and 0 2 studies, the fact that metal electrodes were used limited the types of gases which could be studied. This problem was circumvented by the advent of the radio frequency and microwave d i s - charges which u t i l i z e d external antennae and resonance c a v i t i e s to sustain the discharge and thus avoided con- tamination of the gas under study. Investigations of the halogens, however, seem to have been discouraged by the work of Schwab (25) who reported that chlorine and bromine atoms recombined so r a p i d l y on glass and quartz that t h e i r study i n a d i s - charge-flow system was impossible. In 1961 Ogryzlo (6) reported that c e r t a i n oxy-acids (called "poisons") when coated on the walls of a flow system reduced wall re- combination to such an extent that large atom concentra- tions could be e a s i l y maintained. Although any non-metal oxy-acid was found to be e f f e c t i v e , phosphoric acid seemed to be the best poison for halogen atoms. 9 . Rate Studies On Halogen Atom Recombination Since c h l o r i n e has a very low e x t i n c t i o n c o e f f i c i e n t , the technique of f l a s h p h o t o l y s i s i s not s u i t a b l e f o r studying the k i n e t i c s of recombination, and most q u a n t i - t a t i v e work has been c a r r i e d out using flow systems. The e a r l y work i n t h i s f i e l d has been reviewed (26) and only the recent work w i l l be considered here. For the termolecular recombination r e a c t i o n C l + C l + M — C l 2 + M (4) 16 6 — 2 — 1 Bader (26) found k £ ± 2 = 2.45 x 10 cm moles sec and 16 6 — 2 — 1 k T T = 0.3 x 10 cm moles sec where k„„ i s d e f i n e d by He M J ~ d [ c l J = 2 k M l C l ] 2 [ M ] . • (5) dt For the concurrent w a l l recombination r e a c t i o n C l + w a l l —»- 1/2 C l 2 + w a l l (6) he found k = 3 . 9 sec"''" from which the surface recom- -5 b m a t i o n c o e f f i c i e n t was found to be a = 6.81 x 10 These values were l a t e r confirmed by Hutton and Wright 16 6 — 2 — 1 (27) whose value of k_,, = 2.0 x 10 cm moles sec CI2 was i n good agreement w i t h Bader, but both were almost two orders of magnitude gr e a t e r than t h a t reported by L i n n e t t and Booth (28). In c o n t r a s t to the dearth of m a t e r i a l a v a i l a b l e on C l 2 recombination, a great d e a l of data have been pub- l i s h e d on B r 2 and 1^- The f i r s t e xtensive work on i o d i n e and bromine atom recombination was done by Rabinowitch (29) who derived values of k M for a v a r i e t y of gases from photostationary~measurements. The development of f l a s h photolysis has made possible the d i r e c t measure- ment of the recombination of bromine and iodine atoms and k^ values for a large number of gases have been obtained. Some of these values are given i n Table 1. The method of f l a s h photolysis uses the absorption of B r 2 or I 2 i n the ground state to measure the concen- t r a t i o n of atoms at any time a f t e r the f l a s h . I m p l i c i t i n t h i s measurement i s the assumption that most of the atoms undergo a d i r e c t recombination into the ground state. I f , however, a large percentage of the atoms combine through a bound excited state before being re- laxed to the ground state, the measurement of these rate constants could be i n error. Discharge-flow experiments, on the.other hand, do not suffer from t h i s problem because the atom concentration i s measured d i r e c t l y . Two pr i n c i p a l experimental facts have evolved from studies of the rate of halogen atom recombination: (1) the e f f i c i e n c y of various t h i r d bodies varies over a fa c t o r of at le a s t 10^ and (2) the rate constants e x h i b i t a negative temperature dependence. Two theo- r e t i c a l approaches have been u t i l i z e d to explain these experimental f i n d i n g s . The f i r s t i s the energy transfer mechanism which TABLE 1 RATE OF RECOMBINATION OF IODINE ATOMS AT ROOM TEMPERATURE D i l u e n t Gas k x 10" 1 6 6 -2 cm mole sec -1 Reference He 0.15 24 Ar 0.30 24 ° 2 0.67 24 c o 2 1.34 24 Benzene 7.95 24 Toluene 19.3 24 CH 3CII 2I 26.0 24 Mesitylene 40.2 24 J2 138.0 24 RATE OF RECOMBINATION OF BROMINE ATOMS AT ROOM TEMPERATURE D i l u e n t Gas k x 10" 1 6 6 , -2 cm mole sec -1 Reference He 0.13 22 Ar 0.3 22 N 2 0.17 36 co 2 0.39 37 °2 0*40 36 Br 13.0 37 11. may be represented by the f o l l o w i n g equations * X + X ^=i: X 2 (7a) X* + M > X 2 + M (7b) * X 2 i s a c o l l i s i o n complex w i t h a d e f i n i t e l i f e t i m e , and i t i s supposed t h a t a net recombination occurs i f t h i s complex c o l l i d e s during i t s l i f e t i m e w i t h a t h i r d body M, t r a n s f e r r i n g to i t (with a c e r t a i n p r o b a b i l i t y P) enough energy so t h a t the molecule X 2 cannot subsequently d i s s o c i a t e . The t h i r d - o r d e r r a t e constant k f o r r e - r combination i s given by k r = PgZK (8) where Z i s the c o l l i s i o n number f o r r e a c t i o n (7b) and K i s the e q u i l i b r i u m constant f o r r e a c t i o n (7a). g i s the e l e c t r o n i c degeneracy f a c t o r which a r i s e s because each 2 of the two ground s t a t e {_ ̂ 2/2^ a t o m s ^ a s ^ t s lowest energy l e v e l s p l i t i n t o four components by i n t e r a c t i o n w i t h the other. Of the r e s u l t i n g 16 p o s s i b l e combinations 1 + only one leads t o the Eg ground s t a t e , so that i f no e l e c t r o n i c t r a n s i t i o n s occur, g = 1/16. This theory pre- d i c t s the c o r r e c t magnitude of k^, but cannot account f o r the negative temperature dependence unless P i s assumed to vary w i t h temperature (30). The a l t e r n a t e mechanism, X + M 5 ? = = XM (9a) XM + X > X 2 + M (9b) 12. has a l s o been shown to be i n s u b s t a n t i a l accord w i t h the observations 131,32). For t h i s scheme to p r e d i c t the c o r r e c t temperature dependence, the intermediate species XM must be bound, the w e l l depth of the i n t e r m o l e c u l a r p o t e n t i a l being of the order of a few k i l o c a l o r i e s (33). At present, an unequivocal choice between these two t h e o r e t i c a l approaches cannot be made. Studies on the Luminescence from Halogen Atom Recombination The f i r s t s t u d i e s of emission i n the v i s i b l e from the halogens were reported by Kondratjew and Leipunsky C16). By heating the halogens i n quartz tubes to 1300°K they produced an emission which at low pressures con- s i s t e d of bands i n the long wavelength r e g i o n and a continuum i n the v i o l e t . An inc r e a s e i n the pressure was found to e l i m i n a t e the band s t r u c t u r e u n t i l , at a few hundred mm. Hg, only the continuum remained. Since emission was found at wavelengths s h o r t e r than the con- vergence l i m i t , these i n v e s t i g a t o r s suggested t h a t two body recombination f o l l o w e d by the immediate emission of a quantum of r a d i a t i o n was the predominant process : C K 2P 3 / 2) + C K 2P 1 / 2) * C l * . (10) Photometric measurements of the change of i n t e n s i t y of the continuum w i t h temperature, compared to th a t pre^ d i e t e d from simple k i n e t i c theory, was considered 13. reasonable confirmation that the two body process was responsible for the continuum. Uchida (17) l a t e r studied the halogens by heating the gas i n a c a p i l l a r y of 0.5 mm diameter and observing the emission spectra. He found that at long wavelengths the emission was almost exclusively banded and r a i s i n g the pressure did not eliminate the bands, but caused them to be overwhelmed by the i n t e n s i t y of the continuum. By taking spectra of the emission near the walls and i n the middle of the c a p i l l a r y , he discovered that the bands originated near the walls and the continuum pre- dominated i n the middle of the tube. He suggested that the wall acted as an energy sink, s t a b i l i z i n g the re- 2 2 3 combination of a P-. and a P, ,„ atom into the IT + 3/2 1/2 o u state, which then radiated. Uchida found evidence that the i n t e n s i t y of the bands and also of the continuum was proportional to the fourth power of the degree of d i s s o c i a t i o n . This he took to be proof that atoms i n 2 the p x/2 state were produced by the process 3ciC 2P 3 / 2) * C l 2 ( 1 Z g ) + CK2P1/2) (11) 3 and then the formation of the n + proceeded v i a o u c C1 C 2P 3^ 21 + C1C2P1^21 + wall — - C l 2 ( 3 n o + u ) + w a l l . (12) Recent investigations of halogen emissions have been conducted i n flow systems at low pressures where banded 14. emission predominates. Using a photographic method to determine i n t e n s i t i e s , Bader and Ogryzlo (34) concluded that the emission i n t e n s i t y varied according to i = k [ c i ] 2 r c i 2 ] , d 3 ) which i s i n con t r a d i c t i o n to the findings of Uchida. 3 The bands were i d e n t i f i e d as belonging to the H 0 + u — ^ " E + t r a n s i t i o n i n accord with e a r l i e r work, but they noted a change i n spe c t r a l d i s t r i b u t i o n with pres- sure which had not previously been seen, higher pressures favouring the longer wavelength emission. The f a c t that no t r a n s i t i o n s above v' = 13 were observed, they took 3 as evidence of the crossing of the R Q + U state by the ^"II^u, leading to the mechanism C l ( 2 P 3 / 2 ) + C l ( 2 P 3 / 2 ) — C l 2 ( \ u ) (14a) C 1 2 c l l I l u ) + C l ( 2 p 3 / 2 ) — C 1 2 { \ + J + C 1 < 2 p 3 / 2 ) (14b) C 12 (Vu j t + C l 2 — C 1 2 { \ \ ) + C 1 2 ( 1 4 c ) C l 2 ( 3 n Q + u ) — * C l 2 + hv (14d) 2 2 and p r e d i c t i n g I = k{Cl ( ̂ 2/2^ '•C12-'* H e r e ' t n e notation Cl,. (3JI + f represents a v i b r a t i o n a l l y excited molecule. 2 o u c J Bader and Ogryzlo were, however, unable to decide between t h i s mechanism and the following, which predicts the same k i n e t i c order but does not require the intermediate 15. formation of the ̂ H^u state: C1(2P3^2) + C l 2 + C l 2 ̂ —- C l 3 + C l 2 (15a) C l , + C l C 2 P . / o ) C l , ( 3 n + ) + Cl„ (15b) 3 6 / 2 2 O U 2 C l „ ( 3 n + ) »• C l _ + hv . (15c) 2 o u 2 In a si m i l a r study of chlorine atom recombination, Hutton and Wright (27) also found the emission i n t e n s i t y to be proportional to the square of the atom concentration and to the f i r s t power of the C l 2 pressure. They reported the absolute rate constant for the chlorine emission to 13 6 "̂2 1 be 1.5 x 10 cm mole'" sec" . This determination, however, o o was made over the region 5200A to 8000A, since they could not detect longer wavelength emission, so that t h i s con- stant may not represent the absolute rate i f the emission extends beyond t h e i r measureable range. These authors 3 favoured the d i r e c t formation of the IT + state from o u 2 2 one ground state ^^/2 a n ( ^ o n e e x c i t e d P i / 2 a t o m ' since 2 at 20°C the equilibrium concentration of C l ( P-j./2̂  ^ s 2 1% that of C l C P3/2^* T h e v therefore proposed the f o l - lowing mechanism to explain the emission k i n e t i c s : CIC 2P 3 / 2) + CIC 2P 1 / 21 + c i 2 — c i 2 ( 3 n o + u ) + c i 2 (16a) C l 2 ( 3 n o + u ) — * C l 2 + hv (16b) C l 2 ( 3 n Q + u ) + C l 2 — * C l 2 + C l 2 . (16c) Equation C16c) represents a deactivation of the excited state by C l 2 , and was invoked by Hutton and Wright be- cause they found that the emission i n t e n s i t y was i n - dependent of [C1 2J f o r pressures above 2 t o r r . Bader and Ogryzlo, on the other hand, attributed t h i s obser- vation to a s h i f t i n the emission maximum to longer wavelengths. Much less work has been done on r a d i a t i v e recom- bination from bromine l a r g e l y owing to the greater d i f f i c u l t i e s involved i n atom detection. Using an i s o - thermal c a l o r i m e t r i c detector and a photographic method of determining i n t e n s i t i e s s i m i l a r to that employed by Bader and Ogryzlo, Gibbs C35) investigated the bromine afterglow emission. He found that, as i n the case of chlorine, the emission i n t e n s i t y was proportional to the square of the atom concentration and the f i r s t power of the B r 2 concentration: I = k [ B r J 2 ! B r 2 ] . (17) The spectrum of the afterglow was found to consist of a large number of bands degraded towards the red which 3 1 + Gibbs assigned to the B r 2 ( KQ+U —•» £ g) t r a n s i t i o n . At the pressures and temperatures encountered i n discharge-flow experiments, the banded emission a r i s i n g from three-body processes predominates over any con- t r i b u t i o n from two-body atom recombinations, which 17. would g i v e r i s e to continuous emission. At the high pressures and temperatures used i n shock tube e x p e r i - ments, however, the emission spectrum i s continuous. A study of the emission from shock heated bromine l e d Palmer (18) to conclude t h a t two body recombination can 3 3 1 take place i n t o the B r 0 ( II, ), Br~ ( n + ) and Br« ( II, ) c 2 l u ' 2 o u 2 l u s t a t e s v i a the processes B r ( 2 P 3 / 2 ) + B r ( 2 P 1 / 2 ) - Br* (18) 2 B r ( 2 P 3 / 2 ) —*. Br* . (19) In a s i m i l a r , but more q u a n t i t a t i v e i n v e s t i g a t i o n of the c h l o r i n e emission, Palmer (19, 20) suggested a means of c a l c u l a t i n g the concentrations of e x c i t e d C l 2 molecules i n a t t r a c t i v e or r e p u l s i v e s t a t e s based on e q u i l i b r i u m s t a t i s t i c a l mechanics. 18. Purpose Of This Investigation The purpose of t h i s research was to study the luminescence from halogen atom recombination i n order to gain a better understanding of the ro l e played by e l e c t r o n i c a l l y excited states i n the t o t a l recombination process. To do t h i s , we hoped to be able to: Ca) devise a method of producing iodine atoms i n a discharge flow-system and of observing the re- combination emission, (b) study the entire afterglow spectra of B r 2 and C l 2 and i d e n t i f y the e l e c t r o n i c state (s) giving r i s e to the luminescence. We also hoped to determine the k i n e t i c order of the emission i n t e n s i t y with respect to atom concentration and pressure. Using the 0 + NO glow as a standard of emission i n t e n s i t y , we undertook to determine the absolute rate constants for emission i n the halogen afterglows and from these measurements, to estimate the f r a c t i o n of the t o t a l recombination proceeding through e l e c t r o n i c a l l y excited states. F i n a l l y , from an analysis of these data, and those of other workers, we hoped to be able to give a general mechanism for recombination into excited states i n the halogens. 19. EXPERIMENTAL The F l o w S y s t e m A l l d i s c h a r g e - f l o w e x p e r i m e n t s were p e r f o r m e d u s i n g a f l o w s y s t e m , p a r t o f w h i c h i s i l l u s t r a t e d i n f i g u r e 4. The a p p a r a t u s c o n s i s t e d o f a p u r i f i c a t i o n l i n e , a m a i n f l o w s y s t e m , i n w h i c h t h e r e a c t i o n t u b e was l o c a t e d , and f o u r a u x i l i a r y g a s . s t o r a g e b u l b s and c a p i l l a r y f l o w m e t e r s c o n n e c t e d t o t h e vacuum s y s t e m a t p o i n t s A, B and C ( f i g u r e 4 ) . S i n c e i t was p e r i o d i c a l l y n e c e s s a r y t o remove t h e r e a c t i o n t u b e f o r c l e a n i n g and p o i s o n i n g , t h i s s e c t i o n o f t h e vacuum s y s t e m was c o n s t r u c t e d t o a l l o w f o r i t s c o m p l e t e r e m o v a l . A number o f d i f f e r e n t t y p e s o f r e a c t i o n t u b e were u s e d i n t h i s work; d e t a i l s o f t h e one u s e d f o r b r o m i n e a r e shown i n f i g u r e 5. To p r e v e n t l i g h t f r o m t h e d i s - c h a r g e r e g i o n b e i n g r e f l e c t e d (or c o n d u c t e d ) by t h e g l a s s i n t o t h e o b s e r v a t i o n s e c t i o n , two r i g h t - a n g l e d l i g h t t r a p s were p l a c e d a t t h e end o f t h e d i s c h a r g e t u b e . The o u t s i d e o f t h e s e l i g h t t r a p s was p a i n t e d w i t h f l a t b l a c k e n a m e l . M u l t i p l e i n l e t j e t s A and B were u s e d t o i n t r o d u c e NO, N0~ and N0C1 i n t o t h e m a i n Figure 4. Schematic diagram of experimental apparatus. DISCHARGE REGION A B rlLJL, J L I I •I- MONOCHROMATOR- DETECTOR AMPLIFIER — FLOWMETER RECORDER - ® * V TO PUMP F i g u r e 5. R e a c t i o n t u b e . u s e d i n b r o m i n e a f t e r - g l o w s t u d i e s . Light Trap Optical Window Gas Flow Titration Jets Water Jacket Detector Coil 20. gas stream and were found to giv e the best mixing c h a r a c t e r i s t i c s of any type t e s t e d . The s e c t i o n of the r e a c t i o n tube viewed by the spectrophotometer was covered by an o p t i c a l l y f l a t quartz window. The w a l l s of the v e s s e l surrounding the d e t e c t o r c o i l were maintained at a constant temperature of 25°C by c i r c u l a t i n g water from a temperature bath through the water j a c k e t . Although the dete c t o r operates i s o - t h e r m a l l y , and t h e r e f o r e w i l l not cause a temperature v a r i a t i o n of the w a l l s a f t e r e q u i l i b r i u m has been ob- t a i n e d , i t was found t h a t r e l a t i v e l y s m a ll changes i n room temperature could cause a p p r e c i a b l e d e v i a t i o n s i n the d e t e c t o r c u r r e n t . This d i f f i c u l t y was overcome by c i r c u l a t i n g water from a constant temperature bath through a water j a c k e t surrounding the dete c t o r c o i l . • In s p e c t r o s c o p i c experiments which r e q u i r e d maximum i n t e n s i t y from the afterglows so t h a t s m a l l s l i t w i d t h s could be used, another type of r e a c t i o n tube was em- ployed. This v e s s e l was one i n c h i n diameter, four inches long and was viewed "end on" by the spectro- photometer. The w a l l s of t h i s tube were coated w i t h MgO which has 98% r e f l e c t i v i t y (38) and which helped to- i n c r e a s e the i n t e n s i t y . This, type of tube was im- p r a c t i c a l f o r k i n e t i c measurements, however, because of the decrease i n the i n t e n s i t y of the emissions down the l e n g t h of the tube, p a r t i c u l a r l y at higher pressures. 21. Gas flow rates were measured by observing the pressure difference established across a c a p i l l a r y as the gas flowed through i t . This pressure d i f f e r e n c e was indicated on a U-tube manometer f i l l e d with either s u l f u r i c acid or dibutylphthalate, since most of the gases used attacked mercury. C a l i b r a t i o n of these flowmeters, i n the case of non-condensable gases, consisted of c o l l e c t i n g a measured volume of the pump exhaust, noting the time required, and applying the i d e a l gas laws to ca l c u l a t e the flow rate. The flow rate of n i t r i c oxide was assessed each time i t was used by observing the pressure drop i n the storage bulb i n a fi x e d i n t e r v a l of time. To measure the flow rates of N0C1 and other condensible gases with vapour pressures greater than one atmosphere, a sp e c i a l c a l i b r a t i o n bulb was constructed. The volume of t h i s bulb was determined accurately and the pressure of any gas contained i n i t could be read d i r e c t l y from a mercury manometer. A U-tube f i l l e d with dibutylphtha- l a t e and an air-bleeding system separated the mercury manometer from the bulb, and permitted pressure measure- ments without having the gas come into contact with the mercury. Flow rates of bromine and nitrogen dioxide were calc u l a t e d by trapping out the gas and weighing i t i n a s p e c i a l l y constructed thin-walled weighing vessel. Since no attempt was made to construct the c a p i l l a r i e s of the flowmeters i n such a way that gas flow through them conformed to the Foiseuille equation (51) , each value of the t o t a l system pressure required a separate c a l i - bration of the flowmeters. Gas flows were co n t r o l l e d by eit h e r of two types of needle valve. In most cases, Edwards high vacuum needle valves proved to be s a t i s f a c t o r y , but when ex- posed to the halogens they were found to become cor- roded. For t h i s reason, a Fischer-Porter glass and t e f l o n valve was used for bromine, while the tank regulator was found to give s u f f i c i e n t l y f i n e c o n t r o l i n the case of c h l o r i n e . T o t a l system pressure was measured by either of two t i l t i n g McLeod gauges, one containing mercury, the other s u l f u r i c a c i d . The l a t t e r was designed to have a pressure measuring range of 0.1 to 5 t o r r and was c a l i b r a t e d against the mercury gauges. The t o t a l pressure at any flow rate of the gas could be v a r i e d by p a r t i a l l y c l o s i n g a 20 mm. stopcock j u s t upstream from the l i q u i d nitrogen cold trap. O r i g i n a l l y , a c o n s t r i c t i o n was placed at t h i s point so that the pressure could be varied by changing the flow r a t e . This procedure was l a t e r abandoned because i t permitted the use of only one flow rate at a par- t i c u l a r pressure. 23. Although pumping speed i s i r r e l e v a n t i n the case of condensible gases, s i n c e the e f f e c t i v e pressure at the l i q u i d n i t r o g e n t r a p i s zero, i t i s important when working w i t h oxygen. I n i t i a l l y a Welsh Duo-Seal model 1397 vacuum pump was used which had a pumping speed of 425 l./min. This was l a t e r r eplaced by two pumps, a Welsh model 1403 and a 1405H, operated i n p a r a l l e l . Both pumps were used only f o r oxygen s t u d i e s and during the O + NO c a l i b r a t i o n . M a t e r i a l s C h l o r i n e from Matheson of Canada L i m i t e d (99.5% minimum pur i t y ) , was used without f u r t h e r p u r i f i c a t i o n . N i t r o s y l c h l o r i d e was obtained from Matheson (93% minimum p u r i t y ) and was f u r t h e r p u r i f i e d by tr a p to tr a p d i s t i l l a t i o n using dry ice-acetone and a l c o h o l s l u s h baths and stored i n a 22 1. bulb. The i n f r a - r e d a b s o r p t i o n s p e c t r a of samples withdrawn from the s t o r - age bulb and from the pump t r a p showed t h a t l i t t l e or no decomposition had r e s u l t e d from passing the gas through the metal needle v a l v e s . Nitrogen d i o x i d e was obtained from Matheson Company and t r a c e s of the c h i e f i m p u r i t y , NO, were removed by s t o r i n g the gas w i t h oxygen u n t i l the s o l i d was pure white i n co l o u r . The gas was kept trapped down wit h dry i c e and before being used, was expanded i n t o a 2 1. bulb. N i t r i c oxide was obtained from Matheson and i t s c h i e f impurity was found to be NO^. Trap to trap d i s - t i l l a t i o n s using l i q u i d nitrogen and an ethanol slush bath were performed u n t i l the gas was colourless and the s o l i d was pure white. The n i t r i c oxide was stored, ready f o r use, i n a 2 1. bulb with a mercury manometer attached, the l a t t e r serving.to measure the bulb pres- sure and scavenge any NC^ formed a f t e r p u r i f i c a t i o n . Reagent grade bromine from the Baker Chemical Company was f i r s t d r i ed over s i l i c a g e l , thoroughly de-gased by freezing and pumping, and then the middle f r a c t i o n was slowly d i s t i l l e d into the cold trap of a 22 1. storage bulb. When not i n use, the bromine was kept trapped down i n a dry ice-acetone bath to avoid con- tamination. Matheson extra dry grade oxygen was found to contain l i t t l e N 2, the most objectionable impurity i n gas-discharge experiments, and was used d i r e c t l y from the c y l i n d e r without further p u r i f i c a t i o n . Production of the Excited Species Ca) Chlorine and Bromine Atoms Atoms were formed i n an electrodeless discharge produced by applying 2450 mc./sec. microwave power from a Raytheon generator to the discharge region of the reaction tube. Since the f l u c t u a t i o n of l i n e v o l t a g e a f f e c t e d the atom concentrations q u i t e marked- l y , the microwave generator was run o f f a 220 V Sola transformer to s t a b i l i z e the v o l t a g e . A h wave d i s - charge c a v i t y constructed from the s p e c i f i c a t i o n s of Broida et a l C39) was found to be the most s a t i s f a c t o r y type of c a v i t y f o r halogen s t u d i e s . Adjustments could be made f o r tuning t h i s c a v i t y to resonance and a l s o f o r o p t i m i z i n g the r e f l e c t e d power. The increased e f f i c i e n c y thus obtained extended the pressure range over which the discharge could be maintained and, i n f a c t , i t was found t h a t oxygen discharges could be s u s t a i n e d at pressures c l o s e to one atmosphere. I t appears t h a t high concentrations of halogen atoms are obtained when the discharge region i s q u i t e hot, so t h a t the h i g h l y l o c a l i z e d discharge produced by t h i s c a v i t y proved to be advantageous. The discharge tube was constructed of 12 mm. diameter pyrex and c o o l i n g was achieved by blowing a stream of a i r through the i n l e t provided on the c a v i t y . In order to decrease w a l l recombination t o an acceptable l e v e l , the technique f i r s t used by Ogryzlo (6) of c o a t i n g the w a l l s of the r e a c t i o n v e s s e l w i t h phosphoric a c i d was employed. In p r a c t i c e only the w a l l s of the discharge region were "poisoned" s i n c e t h i s seemed to be s u f f i c i e n t . Coating the w a l l s of the whole system was undesirable s i n c e t h i s caused fogging 26. of the o p t i c a l windows. The surface of the g l a s s was prepared by c l e a n i n g i t thoroughly w i t h soap and water and then w i t h hot, concentrated NaOH s o l u t i o n before the poison was a p p l i e d . The tube was then f i l l e d w i t h a 20% s o l u t i o n of H^PO^, drained , and then evacuated to remove the excess water. F i n a l l y the tube was thoroughly d r i e d by d i s c h a r g i n g pure argon through i t f o r an hour before any experiments were performed. Cb) Production Of Iodine Atoms In A Flow System In the hope th a t t h i s study could be extended to an i n v e s t i g a t i o n of 1^, some p r e l i m i n a r y experiments on the production of i o d i n e atoms i n a flow system were undertaken. The main problem encountered was t h a t of o b t a i n i n g a high enough flow r a t e of i o d i n e through the system to produce measurable q u a n t i t i e s of i o d i n e atoms. I n i t i a l l y , t h i s problem was overcome by heating the i o d i n e c r y s t a l s i n a small g l a s s furnace. This furnace was connected to the discharge tube through a c a p i l l a r y and both the furnace and c a p i l l a r y were wrapped w i t h heating wire and i n s u l a t i n g m a t e r i a l . The discharge tube was maintained at a high tempera- tur e by surrounding i t w i t h a j a c k e t f i l l e d w i t h CCl^ which was heated by the discharge i t s e l f . Carbon t e t r a c h l o r i d e was used f o r t h i s purpose since i t does not absorb the microwave r a d i a t i o n . A second method used to provide high concentra- t i o n s of i o d i n e , and one which was found u s e f u l when using 1^ as a t i t r a n t i n gas r e a c t i o n s , was to pass an i n e r t gas through a v e s s e l of heated i o d i n e c r y s t a l s . Because the high r a t e of recombination of i o d i n e atoms caused a r a p i d decay of i n t e n s i t y down the reac- t i o n tube, observations of the emission were done very c l o s e to the discharge r e g i o n . Two r i g h t - a n g l e d l i g h t t r a p s ensured t h a t l i g h t from the discharge d i d not enter the monochromator s l i t . The w a l l s of the r e a c t i o n tube were jacketed so tha t the e n t i r e v e s s e l could be heated and a removable c o l d t r a p was placed immediately a f t e r the tube. This t r a p permitted easy removal of the i o d i n e a f t e r i t had passed through the system. Because of the obvious d i f f i c u l t i e s e n t a i l e d i n the measurement of i o d i n e f l o w r a t e s , no attempt was made to perform any q u a n t i t a t i v e experiments. Since the emission was expected to be i n the i n f r a r e d , the PbS d e t e c t o r was employed f o r a l l observations. Atom Detec t i o n and Measurement The two most widely used methods f o r measuring atom c o n c e n t r a t i o n i n flow systems are chemical t i t r a - t i o n and i s o t h e r m a l - c a l o r i m e t r i c d e t e c t i o n . In order to u t i l i z e a t i t r a t i o n procedure a substance must be a v a i l a b l e which r e a c t s r a p i d l y and s t o i c h i o m e t r i c a l l y 28. w i t h the atomic species but does not generate a product which re a c t s w i t h the atoms at a comparable r a t e . Spealman and Rodebush (40) f i r s t suggested the use of NC>2 f o r the t i t r a t i o n of oxygen atoms: 0 + N0 2 — * NO + 0 2 0 + NO • N0 2 + hv . When N0 2 i s added to a stream of oxygen atoms, the c h a r a c t e r i s t i c green a i r after g l o w i s observed and as more N0 2 i s added, the i n t e n s i t y of t h i s emis- s i o n reaches a maximum (when the flow of N0 2 i s about h of the oxygen atom f l o w ) . A d d i t i o n of more N0 2 causes the i n t e n s i t y of the glow t o d i m i n i s h and f i n a l l y ex- t i n g u i s h e s i t sharply to w i t h i n 3 cm. of the gas i n l e t , the l a t t e r d i s t a n c e being governed by the mixing c h a r a c t e r i s t i c s of the i n l e t j e t . At t h i s p o i n t the flow of N0 2 i s equal to the flow of oxygen atoms. A pr e c a u t i o n necessary i n t h i s procedure i s to keep the [ 0 ] / [ 0 2 ] r a t i o s m all so t h a t no s i g n i f i c a n t pressure change occurs during the t i t r a t i o n f o r oxygen atoms. Kaufman (8) found th a t e f f i c i e n c y of the discharge changed w i t h t o t a l pressure thus causing a change i n the oxygen atom flow r a t e . The use of NOCl as a t i t r a n t f o r c h l o r i n e atoms was f i r s t suggested by Ogryzlo (6) and was used by Hutton and Wright (271 f o r atom c o n c e n t r a t i o n measure- ments . The r e a c t i o n i s N0C1 + C l *• NO + C l 2 . (21) Although t h i s r e a c t i o n i s reasonably f a s t (41), our p r e l i m i n a r y experiments showed t h a t the e x t i n c t i o n of the c h l o r i n e a f t e r g l o w d i d not provide a s a t i s f a c t o r y endpoint f o r the t i t r a t i o n . When a p h o t o m u l t i p l i e r was placed 10 cm. downstream from the i n l e t j e t , as suggested by Hutton and Wright, and NOC1 was added u n t i l e x t i n c t i o n of the c h l o r i n e a f t e r g l o w occurred at t h i s p o i n t , a n a l y s i s of the r e a c t i o n products showed the presence of con s i d e r a b l e NOC1. This f a c t , and the o b s e r v a t i o n t h a t t h i s procedure gave con- s i s t e n t l y higher atom flow r a t e s than i n d i c a t e d by the i s o t h e r m a l d e t e c t o r , leads us to b e l i e v e t h a t t h i s i s not a very s a t i s f a c t o r y procedure. For t h i s reason, and a l s o because no s a t i s f a c t o r y chemical t i t r a t i o n procedure has been found f o r bromine, the iso t h e r m a l c a l o r i m e t r i c detector was used f o r a l l halogen atom measurements. The isothermal c a l o r i m e t r i c d e t e c t o r was f i r s t d e s c r i b e d by Ogryzlo (42) who used i t i n the measure- ment of oxygen atoms and l a t e r found i t to be u s e f u l f o r the halogen atoms (6). The d e t e c t o r , i n the form i n which i t was used f o r c h l o r i n e and bromine atoms, 30. c o n s i s t e d of a h e l i c a l l y wound s p i r a l of platinum wire e l e c t r o p l a t e d w i t h n i c k e l . The n i c k e l was p l a t e d from a s o l u t i o n of n i c k e l c h l o r i d e and ammonium c h l o r i d e using a p l a t i n g c u r r e n t of about 10 m i l l i a m p s s u p p l i e d from a 6 v o l t storage b a t t e r y . Halogen atoms recombine on the n i c k e l s u r f a c e w i t h almost 100% e f f i c i e n c y r e - l e a s i n g an amount of heat equal to the d i s s o c i a t i o n energy per p a i r of combining atoms. By c o n t r o l l i n g the c u r r e n t to the d e t e c t o r one i s able to hold the temperature constant so t h a t the d e t e c t o r operates i s o t h e r m a l l y . The decrease i n c u r r e n t necessary to maintain the.detector temperature from the value w i t h no atoms flowing, to t h a t when atoms are recombining on the surface, permits c a l c u l a t i o n of the atomic flow r a t e . The galvanometer-potentiometer and a s s o c i a t e d w i r i n g used f o r the c o n t r o l and measurement of the d e t e c t o r c u r r e n t i s shown i n f i g u r e 6. The d e t e c t o r forms one arm of a Wheatstone brid g e . In o p e r a t i o n , the bridge i s f i r s t balanced w i t h the discharge o f f , by adjustment of the c u r r e n t and/or the decade r e s i s - tance box. The c u r r e n t passing through the d e t e c t o r i s found by measuring the p o t e n t i a l drop across a 1 ohm p r e c i s i o n r e s i s t o r . The discharge i s then i n i - t i a t e d and the c u r r e n t through the d e t e c t o r i s reduced to maintain the d e t e c t o r at i t s i n i t i a l temperature. The c u r r e n t i s read again and the atom flow r a t e Figure 6. C i r c u i t diagram of the detector bridge  31. calculated from the formula 2 atom flow = A ( i )R gm. atom/sec. (22) 4.18 D/2 2 2 2 where A ( i ) i s the current squared decrease = i ^ - ±^ R = resistance of the detector i n ohms D = d i s s o c i a t i o n energy of the bond being formed (cal./mole) 4.18= the e l e c t r i c a l power to heat conversion factor (4.18 joules/cal.) The concentration of gas i n the system can be c a l - culated using the i d e a l gas laws which are assumed to be v a l i d for t h i s work. A number of problems were encountered i n using the detector for halogen atom measurements. The most serious problem which arose i n the work with chlorine was that when the detector c o i l was exposed to high atom concentrations, a contamination of the n i c k e l surface occurred a f t e r a period of time. This caused a reduction i n the recombining e f f i c i e n c y of the c o i l and was r e a d i l y recognized by the appearance of the afterglow downstream from the detector. At moderate atom flow rates, t h i s contamination usually required up to two hours before atoms began to sweep past the detector. When th i s occurred, the detector current reading began r i s i n g slowly so that the bridge could no longer be properly balanced and the orange chlorine a f t e r g l o w was v i s i b l e downstream from the n i c k e l c o i l . Since t h i s phenomenon had not been reported i n pre- vious work using t h i s technique, a systematic search f o r the source of the t r o u b l e was undertaken. F i r s t , f u r t h e r p u r i f i c a t i o n and d r y i n g of the c h l o r i n e was c a r r i e d out w i t h no n o t i c e a b l e e f f e c t on the r a p i d i t y of contamination. Further evidence t h a t i m p u r i t i e s were not causing l o s s of e f f i c i e n c y was provided by the f a c t t h a t undischarged c h l o r i n e d i d not cause contamination over a p e r i o d of s i x hours. To ensure t h a t the phos- p h o r i c a c i d poisoning was not being t r a n s f e r r e d to the d e t e c t o r , only the discharge region was coated. The water was then thoroughly removed by baking the tube a t 200°C under vacuum f o r two hours before p l a c i n g the d e t e c t o r back i n t o the system. This procedure d i d not produce any change. F i n a l l y , a number of d i f f e r e n t techniques of p l a t i n g the n i c k e l onto the c o i l were t r i e d , from using d i f f e r e n t e l e c t r o p l a t i n g c u r r e n t s to d e p o s i t the n i c k e l , to heat t r e a t i n g the c o i l . Again t h i s d i d not produce any n o t i c e a b l e improvement. Con- sequently, measurements had to be taken r a p i d l y , w i t h i n the l i f e t i m e of a given c o i l . This was f e a s i b l e because i t was observed t h a t the l o s s of d e t e c t o r e f f i c i e n c y occurred r a t h e r suddenly and was e a s i l y recognized by a sudden in c r e a s e i n d e t e c t o r c u r r e n t and appearance of the a f t e r g l o w behind the c o i l . Although undoubtedly 33. the contamination proceeded at a constant r a t e , the l a r g e area provided by the long c o i l of n i c k e l wire meant t h a t contamination occurred f i r s t a t the top of the c o i l and then proceeded to the bottom. Evidence f o r t h i s was t h a t i n i t i a l l y only the f i r s t c o i l of wire was needed to k i l l a l l the atoms, even at high flow r a t e s . The second e f f e c t which made the use of t h i s technique p o s s i b l e was tha t the c o i l could be r e - a c t i v a t e d i n s i t u merely by applying 1.5-2 amps to the c o i l f o r about one minute. At f i r s t , the number of atoms being swept by the c o i l increased sharply and then slowly decreased u n t i l no atoms were g e t t i n g by. The detec t o r c u r r e n t was then reduced to i t s o r i g i n a l value and a l - lowed to r e - e q u i l i b r a t e . I n i t i a l l y a second photomulti- p l i e r was placed downstream of the detec t o r so t h a t any emission i n t h i s r e g i o n could be detected, but s i n c e a l l experiments were performed i n the dark, i t became obvious t h a t the eye could detect t h i s as q u i c k l y as the p h o t o m u l t i p l i e r . In using t h i s technique to f i n d bromine atom con- c e n t r a t i o n s , q u i t e a d i f f e r e n t problem arose. Because of i t s higher molecular weight, bromine d i d not t r a n s - f e r heat from the c o i l to the w a l l s of the r e a c t i o n tube as q u i c k l y as d i d c h l o r i n e . As a r e s u l t the det e c t o r operated a t a higher temperature, and i f high 34. atom flows were used, n i c k e l was found to d i s t i l l o f f the c o i l and onto the w a l l s . This r e s u l t e d i n a con- tinuous increase i n the d e t e c t o r c u r r e n t but at no time was there any evidence t h a t atoms were being swept by the c o i l . F u rther v e r i f i c a t i o n of t h i s phenomenon came w i t h the di s c o v e r y t h a t upon removing the d e t e c t o r and a l l o w i n g discharged oxygen t o flow through the system, a band of v i o l e t chemiluminescence on the w a l l s of the r e a c t i o n tube marked the former p o s i t i o n of the de t e c t o r c o i l . This glow has r e c e n t l y been i d e n t i f i e d (43) as the Herzberg bands of 0^ which are produced i n the c a t a l y z e d recombination of oxygen atoms on metal s u r f a c e s . This problem was circumvented by using low Br flow r a t e s and operating the de t e c t o r at low cu r r e n t v a l u e s . Spectroscopic Measurements (a) Equipment A H i l g e r and Watts l a r g e aperture prism mono- chromator No. D285 equipped w i t h interchangeable g l a s s and quartz prisms was used f o r most s p e c t r o s c o p i c measurements. This instrument was w e l l s u i t e d f o r the low l i g h t i n t e n s i t i e s encountered i n t h i s work because of i t s f/4.5 aperture, i t s high t r a n s m i s s i o n , r e s u l t i n g from the use of f r o n t surfaced m i r r o r s f o r i n t e r n a l f o c u s i n g , and the wide s l i t widths o b t a i n a b l e (to 1.25 mm.). Although the glass prism and associated c a l i b r a t e d wavelength drum were used f o r most ex- periments, the quartz prism was used i n investigations of emissions beyond 2\i because of the higher trans- mission of quartz i n the in f r a r e d . When working i n the i n f r a r e d region, the monochromator was sealed and flooded with dry nitrogen to reduce absorption due to atmospheric CC^ and water. A wavelength drive mechanism u t i l i z i n g a 1 rpm synchronous motor was added and by means of interchanging gears, any of four drive speeds could be selected. The wavelength c a l i b r a t i o n of the instrument was checked p e r i o d i c a l l y using the mercury emission' l i n e s from a low pressure mercury lamp or using neon or argon lamps i n the infr a r e d . (b) Measurement of Spectra The procedure used to measure accurately band head positions i n Cl^ and Br^ emission spectra was as follows. F i r s t , a neon spectrum was recorded and measured. The po s i t i o n and wavelength of a number of the bands were then used to compute the l i n e a r d i s - persion curve of the monochromator by f i t t i n g a cubic equation to the data. When the emission spectra were o recorded, the 7503A Ne l i n e was superimposed on the recorder trace. The band heads were then measured r e l a t i v e to t h i s l i n e and these measurements were used to f i n d the correct wavelength from the dispersion curve. These c a l c u l a t i o n s were c a r r i e d out on the IBM 7044 computer and the program also calculated vacuum wave numbers. (c) Signal Detection Since the systems studied emitted r a d i a t i o n over a wide wavelength range, i t was necessary to use three d i f f e r e n t detectors. An RCA 7265 photomultiplier, operated at a voltage of 1750 v o l t s , detected r a d i a t i o n i n the v i s i b l e region. In the near infrared region o o C6800A - 12000A) an RCA 7102 photomultiplier was used, operated at 1250 v o l t s and cooled with l i q u i d nitrogen. Both photomultipliers were adapted to f i t into a metal casing on which was mounted a metal dewar. The glass envelope of each phototube came into contact with a brass r i n g attached to the bottom of t h i s dewar so that e f f i c i e n t cooling could be achieved. The metal casing could be evacuated and the o p t i c a l window was f i t t e d with a heater to prevent fogging. Because con- densation on the r e s i s t o r s and associated wiring was found to cause a sharp increase i n the noise l e v e l , a heater was placed around the r e s i s t o r chain and pro- v i s i o n was made for blowing dry nitrogen through the wiring associated with the phototube. S t a b i l i z e d D.C. voltages were supplied from an Interstate power supply model 304. The determination of absolute emission i n t e n s i t i e s r e q u i r e s a high degree of p h o t o m u l t i p l i e r s t a b i l i t y and f o r t h i s reason, c a r e f u l warmup procedures were f o l l o w e d before each experiment was performed. Both tubes were run at operat i n g v o l t a g e s f o r one hour be- f o r e being used. In a d d i t i o n , the RCA 7102 was cooled f o r one hour before the vo l t a g e was a p p l i e d . N e i t h e r tube was exposed to l i g h t of high i n t e n s i t y w h i l e being used. Using a c a l i b r a t e d lamp and a s t a b l e A.C. power supply, the output c u r r e n t of the p h o t o m u l t i p l i e r s was found to be a l i n e a r f u n c t i o n of the e x c i t i n g i l l u m i - n a t i o n at i n t e n s i t i e s s i m i l a r to those encountered i n an experiment. At high i n t e n s i t y l e v e l s , the phototubes were observed to Undergo some f a t i g u e a f t e r prolonged exposure, but t h i s was not expected to be a problem s i n c e the emission i n t e n s i t i e s observed i n an experiment were very low. To check f o r s l i g h t s e n s i t i v i t y changes from day to day, the s i g n a l from a tungsten s t r i p lamp operated at set voltage and cu r r e n t was measured. In the medium i n f r a r e d r e g i o n (1.2y - 2.6y) emis- s i o n was detected by a lead s u l f i d e photoconductive c e l l obtained from I n f r a r e d I n d u s t r i e s Incorporated. The s e n s i t i v e area of t h i s d e t e c t o r was 2.54 mm. by 5.08 mm. and i t was mounted i n s i d e a dewar equipped w i t h a sapphire o p t i c a l window. The de t e c t o r was 38. operated i n series with a 500 KQ. load r e s i s t o r and voltage was supplied from a 300 v. battery. The c e l l and associated wiring were mounted inside a grounded brass case. Since optimum values of sign a l to noise r a t i o and s e n s i t i v i t y were obtained when the c e l l was operated at -78°C, the dewar was f i l l e d with dry i c e and acetone before the voltage was applied. A small, front-surfaced spherical mirror was used to focus the l i g h t coming from the e x i t s l i t of the monochromator onto the se n s i t i v e area of the photocell. The apparatus was c a r e f u l l y shielded to prevent extraneous l i g h t from entering the photocell. (dl Signal Amplification Light s t r i k i n g the entrance s l i t of the mono- chromator was chopped at 165 c.p.s. by a rotating toothed wheel so that the output of a l l the detectors could be amplified as an A.C. s i g n a l . A l o c k - i n am- p l i f i e r constructed by E l e c t r o n i c s , M i s s i l e s and Communications Incorporated amplified the output from the photomultipliers and the signal was displayed on a Leeds and Northrup Speedomax G recorder. One of the problems associated with absolute emission studies i s that r a d i a t i o n of widely varying i n t e n s i t i e s must be compared. For example, the 0 + NO afterglow, by which the detectors were calibrated,was much more intense than the emission from the recombination of halogen 39. atoms. However, the use of the p r e c i s i o n step a t - tenuators on the a m p l i f i e r g r e a t l y s i m p l i f i e d t h i s procedure and made the use of " n e u t r a l d e n s i t y " f i l t e r s unnecessary. 4 Although a g a i n of 1.2 x 10 was o b t a i n a b l e from the l o c k - i n a m p l i f i e r , the weak output from the lead s u l f i d e d e t e c t o r n e c e s s i t a t e d the use of a pre- a m p l i f i e r . A low noise P r i n c e t o n A p p l i e d Research (model CR-4) a m p l i f i e r having an i n t e r n a l power supply, (a rechargeable nickel-cadmium b a t t e r y pack) was used i n t h i s a p p l i c a t i o n . The small p h y s i c a l s i z e of t h i s u n i t permitted i t s placement c l o s e to the p h o t o c e l l so tha t short leads could be used. The output of t h i s p r e - a m p l i f i e r was fed i n t o the l o c k - i n a m p l i f i e r . In some experiments, the simultaneous use of two d e t e c t o r systems was necessary. For t h i s purpose a second phototube, chopper, and frequency s e n s i t i v e a m p l i f i e r were used. This a m p l i f i e r was s e n s i t i v e to 4 a frequency of 27.5 Hz and had an o v e r a l l gam of 10 . (e) C a l i b r a t i o n of Detectors f o r Absolute Emission I n t e n s i t i e s Because gas phase chemiluminescent r e a c t i o n s are d i f f u s e sources of l i g h t , t h e i r c h a r a c t e r i z a t i o n w i t h respect to absolute quantum y i e l d s i s d i f f i c u l t . In these systems, the normal c a l i b r a t i o n procedures 40. a p p l i c a b l e to p o i n t sources of l i g h t cannot be used. Recently, however, F o n t i j n , Meyer and S c h i f f (9) have st u d i e d the r a t e of l i g h t emission from the r e a c t i o n of oxygen atoms w i t h NO and have suggested i t s use as a standard. The k i n e t i c s of the o v e r a l l r e a c t i o n have been i n v e s t i g a t e d by Kaufman (8) and can be represented by 0 + NO »- N0 2 + hv (23a) 0 + NO + M • N0 2 + M (23b) O + N0 2 ». NO + 0 2 . (23c) Since r e a c t i o n (23c) i s very much f a s t e r than (23a) or (23b) the NO c o n c e n t r a t i o n remains e s s e n t i a l l y constant during the course of the r e a c t i o n . The pro- cess may then be considered simply as a n i t r i c oxide c a t y l i z e d recombination of oxygen atoms. The l i g h t i n t e n s i t y i s p r o p o r t i o n a l to the product [O][NO] and i s independent of the t o t a l pressure at pressures above 0.1 t o r r . The procedure used i n f i n d i n g the r a t e constant of emission of a r e a c t i o n i s simply to observe the i n t e n s i t y of the 0 + NO glow i n the same r e a c t i o n tube and under the same c o n d i t i o n s ( i e . spec- trometer s l i t w i d t h , etc.) as the unknown glow. The use of t h i s procedure makes i t unnecessary to know the t r u e s p e c t r a l d i s t r i b u t i o n of the unknown emission and s i n c e both the 0 + NO glow and the unknown glow 41. emit i n the same volume, a l l geometric factors cancel out. Let us suppose that the afterglow under study radiates according to the rate equation I = k Q [ B ] n . The i n t e n s i t y of t h i s emission i n a wavelength i n t e r - v a l AX centred at a wavelength X, i s I (X n) = Fo^lUhko  [B]IV =K (V AoW (24) o 1 z o In t h i s equation F Q(X^) i s the spectral f l u x density o (photons/cc/sec/Al and L Q i s the t o t a l i n t e n s i t y i n comparable units, i e . Xmax L Q = \ F Q(X)dX (25) V i s the volume of the rad i a t i n g gas, k Q i s the t o t a l l i g h t production rate constant for the unknown glow and [B] i s the measured concentration of the emitting species. The constant K(X) contains a l l the geometrical fa c t o r s , the transmission of the observation tube window and the monochromator and the phototube sensi- t i v i t y . Since the monochromator i s a prism instrument, the constant KCX) would also account for the v a r i a t i o n i n s p e c tral s l i t width,AX, with wavelength. F i n a l l y , i Q ( X ^ ) i s the photomultiplier current produced by I (A,), and A i s the amplifier attenuation, the l a t t e r o 1 o *• 42, being the only parameter varied i n the detector system between observation of the 0 + NO "continuum" and the unknown afterglow. A si m i l a r equation to (24) can be written for the in t e n s i t y of the N0 2 "continuum" i n the same wavelength i n t e r v a l AX centred at X-̂ : I s U l ) = F:st*1->AA1ks.[Oj INOJV = K ( X 1 ) A g i s ( X 1 ) (26) _ s where the symbols a l l have t h e i r former meaning, the subscript s denoting the parameters of the "standard" reaction. Dividing equation (1) by (3) we get W k o = Vo^ 1)F s(X 1)k slO] [NO] (27) L o [ B ] n A s i s (X,) L s Integrating over a l l wavelengths between Xmin and Xmax, between which a l l of the i n t e n s i t y of the unknown emis- sion l i e s , we obtain the following equation for the t o t a l l i g h t producing rate constant Xmax /Xmax k = k [0] INOJA^ ( F (X)i (X) d x / f F (X) dX (28) Xmin Xmin In equation (28) k g and F g can be found using the data of Fontijn, Meyer and Sc h i f f (9) and a l l the other values can be found experimentally. 43. I f , for example, a monochromator trace i s obtained for an unknown emission, the curve i s f i r s t corrected by taking i n t e n s i t y readings at a number of wavelengths and multiplying each of these by F ( X ) / i ( X ) . The curve i s then re-plotted and the i n t e g r a l i n the numerator of equation (281 i s evaluated by finding the area under t h i s curve. This was done o r i g i n a l l y by a procedure of cutting and weighing, or by the use of a planimeter. However the whole c a l c u l a t i o n was eventually c a r r i e d out numerically on the IBM 7044 computer. In order to ensure that the reaction tube did not change p o s i t i o n r e l a t i v e to the spectrometer s l i t between c a l i b r a t i o n s , i t was clamped so that i f i t s removal was necessary, i t could be relocated i n i t s o r i g i n a l p o s i t i o n . Before using the 0 + NO c a l i b r a t i o n procedure, the only change made to the system was to remove the de- tector c o i l since the products of t h i s reaction quickly contaminated the metal surface of the detector. Also, a small c o i l of s i l v e r wire was placed downstream from the reaction tube to destroy the oxygen atoms i n the gas stream and thereby prevent ozone from c o l l e c t i n g i n the l i q u i d nitrogen cold traps. The 0 + NO glow was observed at a t o t a l system pressure of 1 t o r r and at an oxygen flowrate of between 50 and lOOy moles/sec, The gas flows were allowed to e q u i l i b r a t e f o r an hour before the spectrum was scanned A f t e r scanning the spectrum the NO flow was cut o f f and the N 0 2 t i t r a t i o n f o r oxygen atoms was q u i c k l y per- formed. This procedure was repeated three or four times and an average of the r e s u l t s was taken. Although the use of t h i s c a l i b r a t i o n procedure made the determination of the s p e c t r a l s e n s i t i v i t y of the monochromator-photomultiplier combination un- necessary over the r e g i o n covered by the N 0 2 "con- tinuum", t h i s f u n c t i o n had to be known f o r emissions beyond 1.4y. The use of t h i s f u n c t i o n then permitted the c a l c u l a t i o n of the true s p e c t r a l d i s t r i b u t i o n of such emissions. A lamp of standard s p e c t r a l i r r a d i a n c e operated from a constant c u r r e n t power supply (the u n i t was obtained from E l e c t r o O p t i c s A s s o c i a t e s ) was used f o r t h i s purpose. The lamp was c a l i b r a t e d against a black-body r a d i a t i o n source and covered the r e g i o n from 0.25 to 2.5y . Cf) Experimental Determination of the Transmission of the O p t i c a l System and Detector S e n s i t i v i t y In order t o compare the s p e c t r a l response of the d e t e c t o r s used i n t h i s i n v e s t i g a t i o n , r e l a t i v e s p e c t r a l s e n s i t i v i t y curves were found f o r each d e t e c t o r . These curves represent not only the response of each d e t e c t o r but a l s o the t r a n s m i s s i o n of the e n t i r e o p t i c a l sys- tem i n c l u d i n g the monochromator and a l l o p t i c a l windows used i n the course of a normal experiment. They were obtained using the lamp of standard spectral irradiance, described i n the previous section, i n conjunction with a number of neutral density f i l t e r s which were needed to reduce the high i n t e n s i t y of the 100 watt quartz- iodine lamp. Figure (7) was obtained using the RCA 7265 photomultiplier at room temperature operated at 1750 v o l t s and using a monochromator s l i t w i d t h of 200u. Figure (8) shows the r e l a t i v e spectral s e n s i t i v i t y of the RCA 7102 phototube cooled to l i q u i d nitrogen tem- perature and operated at 1250 v o l t s . The spectral response of the pbS c e l l (cooled to -78°C) i s shown in figure 9. Figure 7. Relative response of the RCA 7265 photomultiplier and monochromator (sl i t w i d t h 0.2 mm).  Figure 8. Relative response of the RCA 7102 photomultiplier and monochromator (slit w i d t h 0.1 mm).  Figure 9. Relative response of PbS detector and monochromator (sli t w i d t h 0.15 mm).  RESULTS The Bromine Afte r g l o w Spectrum When a discharge i s i n i t i a t e d i n gaseous bromine, a f a i n t red glow v i s i b l e to the eye i s produced. The spectrum of t h i s emission was recorded using the Hilger-Watts monochromator and the RCA 7102 photo- m u l t i p l i e r cooled to l i q u i d n i t r o g e n temperature. The high r a t e of bromine atom recombination, r e s u l t i n g i n a r a p i d decay of the a f t e r g l o w down the length of the r e a c t i o n tube, l i m i t e d the pressure range over which the emission could be s t u d i e d . At pressures above 2 t o r r , the i n t e n s i t y of the emission was i n s u f f i c i e n t t o a l l o w d e t e c t i o n at reasonable s l i t widths. A t y p i - c a l recorder t r a c e of the bromine spectrum measured at a pressure of 0.8 t o r r i s shown i n f i g u r e 10. V a r i a t i o n of the c o n d i t i o n s of pressure and atom c o n c e n t r a t i o n under which the spectrum was recorded had a marked e f f e c t on the appearance of the bands. Not only was the r e l a t i v e i n t e n s i t y of each band found to be a f u n c t i o n of pressure, but i n some cases the p o s i t i o n of the band appeared to s h i f t . This l a t t e r e f f e c t was most n o t i c e a b l e i n the i n f r a r e d r e g i o n of Figure 10. Recorder trace of the bromine a f t e r - glow spectrum. INTENSITY o § 9 < co m r~ m o H x 3 o o CO O CD the spectrum where the lower d i s p e r s i o n of the mono- chromator could cause extensive overlapping of the bands. Thus, before any assignment of these bands was attempted, a high r e s o l u t i o n spectrum of the bromine a f t e r g l o w was examined. This bromine a f t e r g l o w spectrum was photographed w i t h a Czerny-Turner f/6.3 p l a n e - g r a t i n g spectrograph o blazed at 7500A on Kodak 1-N p l a t e s so t h a t the region o o between 7200A and 8400A was recorded. The spectrum was observed to be q u i t e complex, c o n s i s t i n g of a l a r g e number of red degraded bands and e x h i b i t i n g e x t ensive overlap i n some re g i o n s . The recorded band heads are l i s t e d i n Table 2 together w i t h the proposed assignments. The values of v , were c a l c u l a t e d using 3 the spectroscopic constants of the II^ s t a t e given 3 by Darbyshire (4 4) and Horsley (45) and f o r the RQ+U s t a t e , c a l c u l a t i o n s were based on the constants given by Horsley and Barrow (46). The bands have been assigned to the 3 n , —*• 1Z* and 3 n + — » 1 E + t r a n s i t i o n s of l u g o u g 3 molecular bromine. The emission from the IT, s t a t e l u was observed to o r i g i n a t e i n v i b r a t i o n a l l e v e l s higher than v' = 5 while assignments corresponding to v 1 = 1 3 and 2 have been made f o r the II + ( f i q u r e 11) . o u ^ The band p o s i t i o n s measured on the spectrophotometer tr a c e s are given i n Table 3 together with t h e i r t e n t a t i v e TABLE 2 MEASUREMENT OF THE BAND HEADS OF THE B r 2 AFTERGLOW IN THE REGION 7000 - 8300A V vac. (cm - ) R e l a t i v e I n t e n s i t y v» l u - V g V 3 n + o u - V g i v" v c a l c . (cm - 1) 1 v l l i V , c a l c . (cm - ) 11689.5 S 5 9 11671.2 11702.1 M 1 14 11704.9 11716.6 M 11 11 11715.9 11734.0 M 8 10 11727.5 11798.8 M 6 9 11798.7 11835.0 W 9 10 11831.0 4 8 11839.2 11984.4 S 5 8 11975.1 11991.3 S 1 13 11997.9 12004.3 M 11 10 12015 (3 7) (12000.8) 12032.1 W 8 9 12029.2 12046.0 W 12104.6 S 6 8 12102.5 12143.2 w (9 9) 12132.7 4 7 12145.2 [12172.6) w (13 10) 12171.2 12227.8 w 7 8 12221.8 10 9 12228.6 14 10 12238.8 TABLE 2 Continued 3 I K V 3n + — V l u g o u g 1 r 1 Vvac. Relative v' v" v v' v" v i T ^ 4 - ^ ^ „ ,• 4 . . . c a l c . c a l c . (cm - 1) Intensity (cm ) (cm ' 12277.9 S 12297.9 S 12336.0 M 12417.6 S 12437.7 S 12488.6 M 12533.7 S 12543.9 W 12575.7 S 12595.6 S 12638.8 M 12717.7 S 12736.3 S 12774.6 W 12841.0 S 12848.2 W 12895.0 M 12926.5 W 12948.6 M (5 7) (12289.3) 8 8 12333.1 6 7 12408.6 9 8 12436.6 (13 9) 12472.9 7 7 12527.8 10 8 12532.5 14 9 12540.5 5 6 12589.3 8 7 12639.1 6 6 12716.8 9 7 12742.6 13 8 12776.8 7 6 12836.0 10 7 12838.5 14 8 12844.4 5 5 12899.7 11 7 12927.0 8 6 12947.3 1 12 12293.2 1 11 12590.5 1 10 12890.1 TABLE 2 Continued V vac. (cm - 1) R e l a t i v e I n t e n s i t y l u V g 3n + - o u g v ' v" v c a l c . (cm - 1) t V 1 V" V c a l c . (cm - 1) 13019.9 W (12 7) (13008.4) 13035.6 S 6 5 13027.1 13051.7 S 9 6 13050.8 2 10 13051.0 13149.0 S 7 5 13146.4 10 6 13146.7 14 7 13150.4 13192.1 M 1 9 13191.8 13240.3 W 11 6 13235.2 5 4 13212.2 13259.7 W 8 5 13257.7 13318.2 M 12 6 13316.6 13337.9 M 6 4 13339.6 13359.5 M 9 5 13361.2 2 9 13352.7 13392.2 W 13 6 13391.0 13461.1 S 7 4 13458.9 10 5 13457.1 14 6 13458.6 13502.4 M 1 8 13495.7 13544.0 W 11 5 13545.6 13572.6 M 8 4 13570.2 13626.0 S 12 5 13627.0 TABLE 2 Continued V vac. (cm"1) Relative Intensity -» V g 3 J I + — V o u g v" V c a l c . (cm" ) v 1 v" v , c a l c . (cm - 1) 13700.7 W 13 5 13701.3 13768.9 S 7 3 13773.6 10 4 13769.6 14 5 13769.0 3 1 + Figure 11. Assignments to the n + —»• E 3 ^ o u g t r a n s i t i o n i n the spectrum of d i s - charged bromine. F u l l l i n e 0.7 t o r r , broken l i n e 1.5 t o r r . t. I V'- l 12 13 14 ' 15 i \ . f t V '' 3 T T • — ' y + •V"0/ 14 15 16 17 18 19 7500 8000 9000 10,000 Wavelength ( A ) TABLE 3 BANDS IN THE B r 2 AFTERGLOW SPECTRUM RECORDED BY HILGER MONOCHROMATOR V vac. (cm" ) V' Xu- l y + g 3 n +. o U g V" V c a l c . (cm - 1) v" V c a l c . (cm - 1) 14604 (18 3) (14603) 14417 1 5 14420 14286 (18 5) (14288) 2 6 14271 14120 1 6 14110 13985 9 3 13988 13962 2 7 13962 13876 8 3 13885 13807 1 7 13802 13648 2 8 13657 13629 12 5 13627 7 4 13459 13447 10 5 13457 14 6 13459 13350 2 9 13352 13492 1 8 13496 13186 1 9 13192 7 5 13146 13134 10 6 13146 14 7 13150 TABLE 3 Continued V vac. (cm - 1) v» l u g v' 3 n + o u — V g • V " c a l c . Icm"1) V " V c a l c (cm - 1 13038 6 5 13027 2 10 13051 12948 8 6 12947 12881 05 51 U2899) 1 10 12890 12839 7 6 12836 10 7 12839 12742 9 7 12743 12709 6 6 12717 12596 5 6 C12589)* 1 11 12591 (.12607) f 12526 7 7 12528 12418. 6 7 12409 12304 1 12 12293 12279 5 7 (12281)* (12298)t 12229 7 8 12222 12131 9 9 12133 12079 6 8 (12103)* (12071) t 11971 5 8 11975 11839 9 10 11831 4 8 (11839) * TABLE 3 Continued V vac. (cm - 1) v« l u g v' • 3 n + o u g v" v c a l c . Ccro"1) V " V c a l c . (cm" ) 11756 1 7 (11762)t 11710 1 14 11705 11673 5 9 (11671)* 11599 2 8 (11598)t 11540 0 14 11541 11416 1 15 11414 11370 5 10 C11370) * C11387) t 11248 C4 10) (11261)t 0 15 11250 11153 1 9 (11152)t 11098 5 11 (11088)t 10950 4 11 (10962)t 0 16 10961 10859 1 10 (10851)t 10673 0 17 10674 10648 2 11 (10636) * 4 12 (10664) t 10528 3 12 (10532)t 10389 (4 13) C103691t 0 18 10389 10230 3 13 ( 1 0 2 3 8 ) t 1 12 (10254)t TABLE 3 Continued V v a c . ( c m " 1 ) v ' l u — V g V ' 3 n + o u g i V " V c a l c . C c n T 1 ) v " v c a l c . (cm ) 10080 C4 14) U0076) t 0 19 10107 9827 0 13 C 9811)t 0 20 9827 9782 9678 9531 0 14 9518 0 21 9549 9483 9231 0 15 ( 9227)+ 9065 1 16 ( 9035)t 8940 0 16 ( 8938)+ * v calculated from references 44 and 45. + v calculated from reference 48. 48. assignments. In the region between 11,500 and 14,000 cm the measured bands show good agreement with the strong (s) bands which appear on the photographic plate and band assignments i n t h i s region were made by comparing the two spectra. In the more in t e r e s t i n g spectral region around 10,000 cm \ a number of intense and well separated peaks are observed but the o r i g i n of these bands was more d i f f i c u l t to determine for two reasons. F i r s t of a l l , the l i n e a r dispersion of the monochromator i s quite low i n t h i s region so that measurement of band positions i s less accurate than i n the higher energy region. Secondly, since absorp- t i o n has never been observed into l e v e l s below v 1 =6 3 i n the J I ^ U state (44, 47), the spectroscopic constants given by Darbyshire (44) may not be v a l i d for the lowest v i b r a t i o n a l l e v e l s . For those assignments which are i n question, the band positions calculated from the revised constants given by Clyne and Coxon (48) are also shown i n Table 3. To f i n d the wavelength range over which the emis- sion extended, the RCA 7265 photomultiplier and the Infratron PbS detector were used to f i n d the high and low energy l i m i t s r espectively. The bromine afterglow i n t e n s i t y was below the l i m i t s of the detectors at o o wavelengths less than 6000A and greater than 19000A. The spectral d i s t r i b u t i o n , obtained using the lead s u l f i d e c e l l , i s shown i n figure 12. Figure 12. Spectral d i s t r i b u t i o n of the bromine afterglow spectrum obtained by PbS detector.  The s p e c t r a l d i s t r i b u t i o n of the bromine a f t e r - glow was found to be dependent upon atom c o n c e n t r a t i o n and t o t a l pressure. Under c o n d i t i o n s of low pressure and high atom c o n c e n t r a t i o n the maximum i n t e n s i t y ap- o peared at 7700A as shown i n f i g u r e 13(a). At pressures o above 0.5 t o r r the maximum i n t e n s i t y appeared a t 94 0OA ( f i g u r e 13(b)). This spectrum was recorded a t a t o t a l pressure of 1 t o r r u sing argon as a d i l u e n t and was o unusual i n t h a t very l i t t l e emission below 8 000A was observed. These l a s t two spectra represent the t r u e s p e c t r a l d i s t r i b u t i o n of the bromine a f t e r g l o w and were obtained by c o r r e c t i n g the o r i g i n a l spectra f o r t r a n s m i s s i o n of the o p t i c a l system and s e n s i t i v i t y of the p h o t o m u l t i p l i e r . The spectra were then r e c o n s t r u c t e d p o i n t by p o i n t on a s c a l e l i n e a r i n wavelength. K i n e t i c s Of The Bromine Afterglow Before measuring the r a t e constants f o r the t o t a l emission, a more q u a n t i t a t i v e study of the v a r i a t i o n of s p e c t r a l d i s t r i b u t i o n w i t h atom c o n c e n t r a t i o n and pres- sure was undertaken. To do t h i s , the i n t e n s i t i e s i n narrow bands centred at a number of wavelengths across the a f t e r g l o w spectrum were observed as a func- t i o n of I Br] and { B r 0 ] . At the same time, the dependence Figure 13. True spectral d i s t r i b u t i o n of the bromine afterglow spectrum. Curve (a): pressure = 0.25 torr Curve (b): pressure = 1 to r r W A V E L E N G T H (A) of the i n t e g r a t e d emission i n t e n s i t y on atom concen- t r a t i o n and pressure was i n v e s t i g a t e d . (a) Dependence Of Emission I n t e n s i t y On [Br] At a f i x e d t o t a l pressure, the i n t e n s i t y i n a small wavelength i n t e r v a l , 1^, was measured a t v a r i o u s atom co n c e n t r a t i o n s . These measurements were repeated at a number of pressures between 0.5 and 2 t o r r . The r e s u l t s o o o obtained f o r bands centred at 7200A, 8400A and 10400A are l i s t e d i n Table 4. The reg i o n of the spectrum covered o o by each of these bands i s : 7165A < X < 7235A f o r the 7200A band, 8335A < X < 8465A f o r the band centred at 8400A, and 10395A < X < 10505A f o r the 10400A band. I f the dependence of 1^ on [Br] i s assumed to be of the form 1^ a l B r ] n , then a p l o t of l o g 1^ agai n s t l o g [Br] ( f i g u r e s 14(a) and 15(a)) w i l l y i e l d a value of the parameter n. When t h i s was done, i t was found t h a t 1.2 € n 4 2, and only at the high energy end of the spectrum d i d n approach a value of 2 (Table 5). The i n t e g r a t e d emission i n t e n s i t i e s (^I^dX) between o o 6800A and 12000A were obtained by scanning the emission spectrum using l a r g e monochromator s l i t w i d t h s to o b t a i n s t r u c t u r e l e s s s p e c t r a . These spectra were then c o r r e c t e d to o b t a i n the tru e s p e c t r a l d i s t r i b u t i o n s and the areas under these curves were measured. The dependence of the i n t e g r a t e d i n t e n s i t y on atom c o n c e n t r a t i o n i s shown i n Table 4. P l o t s of l o g ((i.dX) against [Br] ( f i g u r e s 14(b), TABLE 4 THE DEPENDENCE OF 1^ UPON [Br]. INTENSITY IN ARBITRARY UNITS Pressure (torr) 0.53 iBrJ x 10' (m/cc) 0.079 0.32 0.55 7200 1.0 7.0 16.0 8400 9.0 45.0 114 10400 18.0 96. a 210 12000A I xdX 6000A 70. 7 312 709 0.92 0.45 0.68 0.99 1.44 0.15 0.56 2.36 4.80 13.7 22.0 50.0 0,65 7.50 70.0 43.2 93.8 176 310 7.15 70.0 700 91.2 186 346 540 17.6 139 1200 288 608 1120 1900 57.0 450 4800 1.18 0.67 1.00 0.58 0. 80 1.43 0.150 12.8 28.4 8.8 16. 0 50.0 0.0 104 211 61.3 128 325 9.6 208 384 119 238 550 20.0 679 1300 •413 790 1920 69.8 1.50 0.41 0.34 0.60 0.82 8.0 4.0 10.0 15.7 34.4 29.6 70. 0 126 72 .0 56.0 152 246 234 188 492 830 TABLE 4 Continued Pressure iBrj x 10 9 I 7 2 0 Q I g 4 0 0 I 1 Q 4 0 0 \ I AdX (torr) (m/cc) J 0 6000A 12000A 1.82 •i 0.064 0.12 0.18 0.23 0.31 0.37 0.47 1.2 2.4 0.8 2.4 4.0 5.6 12.5 9.6 23.2 13.6 29.6 43.2 54.5 99.0 19. 2 48.0 28.8 57.0 88.0 117 182 64.2 157 93.0 189 286 372 612 TABLE 5 VALUES OF n IN THE EXPRESSION I x - [ B r ] n n Pressure ( t o r r ) 12000A I7200 I8400 I10400 6000A 0.52 1.5 ± 0.2 1.2 ± 0.2 1.2 ± 0. 2 1. 2 ± 0.2 0.92 2.0 ± 0.1 1.6 ± 0.1 1.5 ± 0. 1 1. 6 ± 0.1 1.18 2.0 + 0.1 1.6 + 0.1 1.5 ± 0. 1 1. 7 ± 0.1 1.50 - 1.8 + 0.1 1.7 ± 0. 1 1. 7 ± 0.1 1.82 2.3 + 0.2 1.8 ± 0.1 1.6 ± 0. 1 1. 8 ± 0.1 Figure 14: Plots of Ca).. log 1^ vs. log [Br] and Cbl log C^I^dA vs. log [Br] for a pressure of 0.92 t o r r . Intensity i n a r b i t r a r y u n i t s . Points i n Ca) obtained for bands centred o at the following wavelengths: A 7200A, • 8400A, • 10400A.  Figure 15. Plots of Ca) log 1̂  vs. log [Br] and (b) log ($1 dX) vs. log [Br] for a pressure of 1.82 t o r r . Intensity i n ar b i t r a r y units. Points i n Ca) obtained for bands centred o at the following wavelengths: A 7200A, • 8400A, and • 10400A. _ i _ l _ 1 100 9-5 9 0 Log [Br] 51. 15(b) and Table 5) indicate that the dependence of the integrated i n t e n s i t y upon iBr] increases with increasing pressure, but does not show a squared dependence even at the highest pressure studied. (b) Pressure Dependence Of The Emission Intensity Since i t was not possible to devise an experiment i n which the t o t a l pressure was varied while the atom concen- t r a t i o n remained constant, an attempt was made to obtain the v a r i a t i o n of 1^ with I B ^ J by i n t e r p o l a t i o n of the 1^ vs. IBr] data. However, owing to the large amount of scatter i n the r e s u l t s , no p o s i t i v e c o r r e l a t i o n could be made. The v a r i a t i o n of the integrated i n t e n s i t y with pres- sure was, however, obtained by i n t e r p o l a t i o n , although the data show considerable scatter. At IBr] = 5.0 x 10 "^moles/ c c , these r e s u l t s indicate that I^dX <* [Br,J 0.0 ± 0.2 over the pressure range 0.5 to 1.8 t o r r . This dependence could not be determined at higher [Br] values because of the l i m i t e d range of atom concentration over which the data overlapped. (c) Absolute Rate Constant Measurements If the apparent rate constant for the emission i s defined as kapp " \otal ' t B r ] 2 [ B r 2 ] , (29) 52. t h e n e q u a t i o n 28 can be r e w r i t t e n , f o r t h e case o f t h e bromine a f t e r g l o w , t o g i v e : 12000A / l 2 0 0 0 A k a p p ° k s I O : i l N O ] A B r 2 f F s ( - A ) i B r 2 ( X ) d V f F s ( X ) d X ( 3 0 ) ! B r j 2 l B r 2 3 A s ) i (X) J o / o 6000A ' 6000A where a l l symbols have t h e i r f o r m e r meaning. F o r each v a l u e o f i B r ] and l B r 2 J a s t r u c t u r e l e s s s p e c t r o - photometer t r a c e was o b t a i n e d , u s i n g a s l i t w i d t h o f o o 500 m i c r o n s , f o r e m i s s i o n from 6000A t o 12000A. V a l u e s o o f i D (X) were r e a d from t h e s e t r a c e s a t 200A i n t e r v a l s B r 2 and t h e i n t e g r a l i n t h e numerator o f e q u a t i o n (30) was e v a l u a t e d n u m e r i c a l l y . The v a l u e s o f k found i n t h i s 1 app way a r e l i s t e d i n T a b l e 6. S i n c e a s i g n i f i c a n t c o n t r i b u t i o n t o t h e t o t a l e m i s- o s i o n i n t e n s i t y i s made by r a d i a t i o n beyond 12000A, t h e r a t e c o n s t a n t s c a l c u l a t e d above w i l l be lo w e r t h a n those c a l c u l a t e d f o r t h e t o t a l e m i s s i o n . To e s t i m a t e the f r a c t i o n o f t h e t o t a l e m i s s i o n w h i c h appears beyond o 12000A, s p e c t r a were r e c o r d e d u s i n g t h e l e a d s u l f i d e d e t e c t o r , and t h e n c o r r e c t e d t o y i e l d t h e t r u e s p e c t r a l d i s t r i b u t i o n . However, because o f t h e low i n t e n s i t y l e v e l s i n v o l v e d , and s i n c e the s i g n a l / n o i s e r a t i o o f t h i s d e t e c t o r i s f a r below t h a t o f the p h o t o m u l t i p l i e r s , s p e c t r a a t TABLE 6 - VALUES OF k FOR EMISSION BETWEEN 6000A AND 12000A IN THE BROMINE AFTERGLOW app Pressure Flow of B r 2 Flow of Br i B r 2 ^ x 1 q 8 I B r ^ x 1 q 9 k x I O - 1 4 t o r r n / y moles/sec , , moles/cc y moles/sec ' moles/cc ' 6 , - 2 - 1 ' • ' cm moles sec 0.53 17.6 0.048 17.6 0.193 17.6 0.335 0.92 18.6 0.168 18.6 0.253 18.6 0.370 18.6 0.534 18.6 0.0564 18.6 0.208 18.6 0.879 1.18 18.6 0.193 18.6 0.292 18.6 0.168 18.6 0.231 2.88 0.0785 12.7 2.86 0.316 3.88 2.85 0.548 2.94 4.98 0.451 1.01 4.96 0.680 0.939 4.95 0.994 0.813 4.93 1.44 0.667 4.99 0.152 1.76 4.97 0.559 1.03 4.88 2.36 0.625 6.38 0.665 0.852 6.36 1.01 0.715 6.38 0.579 0.684 6.37 0.796 0.695 TABLE 6 Continued 8 9 -14 Pressure Flow of B r 2 Flow of Br * B r2^ x 1 0 ^ B r^ x 1 0 k x 1 0 M » W . « > - » > W « c m o l e s / c c moles/cc ^ 2 x 1.18 18.6 0.416 " 18.6 0.0433 1.50 18.7 0.0937 18.7 0.0769 18.7 0.138 18.7 0.188 1.82 21.5 0.0140 21.5 0.0271 21.5 0.0510 21.5 0.0692 21.5 0.0805 21.5 0.1030 6.34 1.43 0.524 6.40 0.149 1.74 8.13 0.408 0.612 8.13 0.335 0.730 8.12 0.601 0.596 8.11 0.819 0.542 9.88 0.0644 5.56 9.88 0.125 3.64 9.88 0.235 1.24 9.87 0.318 1.02 9.87 0.370 0.976 9.86 0.474 0.985 53. low atom concentrations could not be measured. Using high atom c o n c e n t r a t i o n , 13% of the emission was observed o t o l i e beyond 12000A at 0.28 t o r r and t h i s value increased to 22% f o r pressures above 0.5 t o r r . In view of the f a c t t h a t emission i n the i n f r a r e d i s favoured by low [ B r ] , the r a t e constants f o r the t o t a l emission are probably i n the range of 20 to 30% higher than those l i s t e d i n Table 6. Emission From Iodine Atom Recombination Two methods of producing i o d i n e atoms i n a flow system were used: d i r e c t discharge of i o d i n e and the chemical t i t r a t i o n of c h l o r i n e atoms w i t h IC1. The emission produced i n each case was of low i n t e n s i t y and appeared i n the r e g i o n 0.8u to 2.4y. The spect r a obtained using the PbS d e t e c t o r are shown i n f i g u r e s 16 and 17. The i n t e n s i t y of the emission was lower than t h a t of the other halogen afterglows because high concentrations of i o d i n e were d i f f i c u l t to o b t a i n i n the flow system. The production of e x c i t e d i o d i n e atoms by the chemical t i t r a t i o n procedure occurs v i a the f o l l o w i n g steps: i c i + C U 2 P 3 / 2 ) - * c i 2 + K 2 P 3 / 2 ) (31) I ( 2 P 3 / 2 ) + K 2 P 3 / 2 ) + M — I * + M . However, i n using t h i s method, care had to be taken not Figure 16. The spectrum of discharged DISCHARGED I2 0 6 0 - 8 1 - 4 1 - 8 2 - 2 WAVELENGTH (microns) Figure 17. Iodine afterglow spectrum produced by IC1 + Cl reaction. Broken curve gives the true spectral d i s t r i b u t i o n . ICI + CI WAVELENGTH (microns) 54. to t i t r a t e more than a stoichiometric amount of IC1 into the stream of chlorine atoms. This was necessary to en- sure that emission a r i s i n g from the formation of excited I C l did not contribute to the afterglow: I ( 2 P 3 / 2 ) + C l C 2 P 3 / 2 ) + M — • I C l ( 3 n i ) + M . (32) Clyne and Coxon (30) studied the emission spectrum of ICl produced i n a discharge-flow system, and found a o number of strong t r a n s i t i o n s below 8000A. The f a c t that o no emission below 8000A was detected i n our experiments was taken to indicate that reaction (32) did not c o n t r i - bute s i g n i f i c a n t l y to the afterglow. Because of the d i f f i c u l t i e s involved i n measuring iodine flow rates and atom concentrations, k i n e t i c measurements on the iodine afterglow were not attempted. The Chlorine Afterglow Spectrum The emission from the recombination of chlorine atoms o o was observed to extend from 5000A to 15000A. The region o o from 6Q00A to 11000A i n the emission spectrum was charac- t e r i z e d by a large number of red-degraded bands, the p o s i t i o n of which was determined by measuring the mono- chromator traces. T h i r t y band heads i n the afterglow were measured, and these have a l l been i d e n t i f i e d as a r i s i n g from the 3JJ + —•> t r a n s i t i o n of molecular chlorine, o u g The calculated band head positions shown i n Table 7 are TABLE 7 ND HEADS RECORDED FROM THE CHLORINE AFTERGLOW SPECTRUM w a v e l e n g t h 0 (A) V v a c . (cm" 1 ) V . v ' u g v " V c a l c . 10551 9474 2 17 9455 10071 9926 0 15 9909 9618 10394 0 14 10388 2 15 10396 9195 10872 0 13 10872 8979 11134 1 13 11121 8814 11342 0 12 11362 2 13 11359 8611 11610 1 12 11610 8441 11886 0 11 11857 2 12 11849 8265 12096 1 11 12105 8108 12330 0 10 12357 2 11 12344 7944 12585 3 11 12571 1 10 12606 7780 12850 2 10 12844 7635 13094 3 10 13072 1 9 13112 7480 13365 2 9 13350 7360 13583 3 9 13578 7226 13835 2 8 13856 7100 14081 3 8 14086 6992 14302 4 8 14305 6868 14572 3 7 14605 6750 14811 4 7 14821 6655 15032 5 7 15029 TABLE 7 Continued wavelength v 3 n 1 + v+ (A) , -1. 1 ^ 5 c a l c . (cm ) 6580 15193 6 7 15226 6522 15333 4 6 15343 6438 15528 5 6 15551 6364 15718 6 6 15748 6296 15879 4 5 15871 6225 16071 5 5 16078 6154 16255 6 5 16276 6083 16442 7 5 16461 6020 16607 5 4 16611 t calculated from reference (50) 55. based on the revised spectroscopic constants of Clyne and Coxon (50) . Spectral d i s t r i b u t i o n changes s i m i l a r to those exhibited by the bromine afterglow were observed i n the chlorine emission. Maintaining the t o t a l pressure at a constant value while varying the atom concentration had the e f f e c t of s h i f t i n g the i n t e n s i t y maximum from "shorter wavelengths, at high IC1J, to longer wavelengths at low {C1J (figure 18). On the other hand, high pressures were found to favour emission i n the red region while lowering the pressure s h i f t e d the i n t e n s i t y maximum towards the blue (figure 19). The true spectral d i s t r i b u t i o n of the chlorine a f t e r - glow spectrum i s shown i n figure 20. These curves were constructed i n a manner s i m i l a r to those of figure 13 for the bromine emission. In order to rule out the p o s s i b i l i t y that these i n - te n s i t y s h i f t s were due to changing discharge temperatures, experiments were performed to investigate the e f f e c t of varying the reaction tube temperature on the spectrum. However, no measurable change i n in t e n s i t y d i s t r i b u t i o n was observed when the temperature of the walls of the reaction tube was varied over the range of -20°C to 150°C. \ Figure 18. The change i n the spectral d i s t r i b u t i o n of the C l 2 afterglow with atom concen- t r a t i o n . Pressure fixed at 1.0 t o r r . 6 0 0 0 8 0 0 0 1 0 , 0 0 0 Wavelength (A) Figure 19. The change i n spectral d i s t r i b u t i o n of the C l ~ afterglow spectrum with -9 pressure. [C1J = 1.2 x 10 mole/cc, , monochromator s l i t w i d t h = 0.5 mm, pressures i n t o r r .  F i g u r e 20. T r u e s p e c t r a l d i s t r i b u t i o n o f t h e c h l o r i n e a f t e r g l o w s p e c t r u m . T 56. K i n e t i c s Of The C h l o r i n e A f t e r g l o w Emission Since the c h l o r i n e a f t e r g l o w emission was found to o o extend over such a wide wavelength range (5000A to 15000A) and because no s i n g l e d e t e c t o r was s u i t a b l e f o r studying the e n t i r e spectrum, the emission was s t u d i e d i n three r e g i o n s . The RCA 7265 p h o t o m u l t i p l i e r was used to study o o the r e g i o n between 5000A and 6800A (hereafter c a l l e d the O .0 v i s i b l e r e g i o n ) , w h i l e the p o r t i o n from 6800A to 12000A ( i n f r a r e d region) was covered by the RCA 7102 ptioto- o m u l t i p l i e r . Beyond 12000A, the emission i n t e n s i t y was low, and measurements w i t h the PbS d e t e c t o r showed t h a t t h i s r e g i o n c o n t r i b u t e d not more than 5% to the t o t a l i n t e n s i t y . In view of the small c o n t r i b u t i o n from t h i s region and because of the low s e n s i t i v i t y of the PbS d e t e c t o r , de- o t a i l e d k i n e t i c s t u d i e s were not attempted beyond 12000A. The v i s i b l e and i n f r a r e d regions were s t u d i e d at a number of d i f f e r e n t pressures ranging from 0.83 to 3.08 t o r r . The l i m i t s on the pressure range were imposed by the requirement of only a small pressure g r a d i e n t down the l e n g t h of the r e a c t i o n tube and by the need to operate under c o n d i t i o n s where emission i n t e n s i t i e s were e a s i l y measurable. At pressures above 3.5 t o r r , the i n t e n s i t y became too low f o r d e t e c t i o n , and below 0.83 t o r r the pressure g r a d i e n t , as c a l c u l a t e d from the P o i s e u i l l e equation (51), became s i z a b l e . At each value of the t o t a l system pressure, the atom c o n c e n t r a t i o n was v a r i e d by a d j u s t i n g the microwave power or the c a v i t y resonance. (a) Dependence Of The Emission Intensity On [Cl] The emission i n t e n s i t y i n various regions of the chlorine afterglow spectrum was studied as a function of atom concentration at constant t o t a l pressure. This was done by i s o l a t i n g f i v e regions of the spectrum, the positions and bandwidths of which are l i s t e d below. Band Centre Bandwidth o o A A 5500 5468 < X < 5532 6200 6152 < X < 6248 7000 6930 < X < 7070 8600 8475 < X < 8725 10600 10390 < X < 10810 The i n t e n s i t y measured i n each of these bands at various atom concentrations i s l i s t e d i n Table 8 for f i v e values of the pressure. P l o t t i n g the logarithm of 1^ against log [Cl] y i e l d s a value of n i n the expression 1^ <* [ C l ] n , and the values of n obtained i n t h i s way are l i s t e d i n Table 9. Figures 21 and 22 show log 1^ vs. log [Cl] plot s for two values of the t o t a l pressure. From an examination of the values of n l i s t e d i n Table 9, i t appears that the i n t e n s i t y at longer wave- lengths depends on the f i r s t power of the atom concen- t r a t i o n while that at short wavelengths i s proportional to the square of the atom concentration. The integrated emission i n t e n s i t y i n the v i s i b l e and infr a r e d regions was also observed as a function of [Cl] TABLE 8 Ca} DEPENDENCE OF I UPON I C l ] . INTENSITY IN ARBITRARY UNITS Pressure I C l ] I 5 5 0 Q I \l,dX t o r r moles/cc x 10 "5000A J; 6800A 0.83 1.29 1.80 2.05 2.99 1.42 1.50 1.83 2.37 2.84 4.0 8.0 11.2 30.4 4.80 4.80 8.80 13.6 19.2 24.8 46.5 62.5 114 28.0 30.4 45.6 68.9 103 61.4 111 146 275 68.5 75.5 113 169 246 1.32 0.943 1.16 1.52 1.96 2 .54 2.54 3.28 3.09 1.50 3.00 4.50 8.00 14.4 14.4 25.0 22.5 15.2 20. 0 28.5 49. 5 81.6 80.0 130 118 35.2 50.9 71.1 122 199 198 316 286 1.70 0.721 1.02 1.52 1.82 2.49 2.32 1.0 2.0 6.0 8.0 14.3 13.0 8.0 16. 0 34.0 46.0 85.9 74.1 21.9 40. 9 85.7 116 21.2 186 TABLE 8 Ca) Continued 6800 Pressure t o r r I C I J moles/cc x 10 ^ s o o I6200 l l A d X J5000 2.33 0.484 0.567 0.616 0.815 1.24 1.90 0.7 0.8 1.0 2.0 4.5 10.5 6.0 7.0 9.0 13.5 28.0 66.0 16.1 18.1 23.5 35.3 72.1 165 3.03 0.501 0.575 0.531 1.10 1.38 0.8 1.2 1.0 3.0 4.5 4.0 5.0 4.5 11.0 18.0 11.2 13.8 11.9 30. 7 48.6 TABLE 8(b) DEPENDENCE OF I, ON ICl]. INTENSITY IN ARBITRARY UNITS 12000A Pressure ICl] I I I U^dX t o r r moles/cc x 10 J o 68 00A 0.83 1.20 82.5 111 47.5 448 II 1.24 80.0 106 47.5 440 ti 1.15 65.0 92 .5 40. 0 373 ii 0.716 27.5 50.0 25.0 198 II 0.153 6.25 17.5 8.00 62.8 1.32 4.62 442 488 195 2030 II 5.24 507 542 208 2280 II 2.95 236 • 307 132 1240 II 3.21 264 334 144 1370 II 3.96 360 416 176 1730 II 5.83 572 601 247 2560 1.70 0.746 40. 0 89.5 41.7 343 II 1.32 89.5 165 72 .0 618 II 2.35 196 300 12 0 1180 II 5.53 485 600 250 2500 n 4.95 430 540 230 2240 II 7.09 816 881 368 3710 II 3.57 296 406 176 1650 II 4.48 400 504 216 2090 II 5.86 540 655 280 2760 n 6.54 663 767 32 5 3240 2.33 0.434 12.5 40.0 21.2 155 II 1.12 50.0 115 55.0 426 II 0.712 25.0 70.0 35.0 258 II 0. 863 30.0 83.9 42.5 310 II 1.28 55.0 125 65.0 473 TABLE 8 (b) Continued 12000A Pressure t o r r I C U moles/cc x 10 Z7000 I8600 •"•10600 J 6800A 3.08 0.216 8.76 32.5 18.8 124 0.299 11.2 45.0 25.0 165 0.518 21.2 70.0 40.0 262 0. 668 32. 5 98.9 55.0 372 0.953 47.5 135 75. 0 505 1.26 70. 0 180 92 .5 664 TABLE 9 VALUES OF n IN THE EXPRESSION I, « [Cl] n Pressure torr 5500 n '6200 "7000 '8600 "10600 6800 I xdX 5000 . 12000 6800 0.83 2.1 + 0.1 1.9 ± 0.1 1.5 ± 0.2 1.1 ± 0.2 1.0 ± 0.2 1.8 ± 0.1 1.2 ± 0.2 1.32 2.2 ± 0.1 1.8 ± 0.1 1.3 ± 0.1 1.0 ± 0.1 0.910.1 1.7+0.1 . 1.0 ± 0.1 1.70 2.0 ± 0.1 1.9 ± 0.1 1.3 ± 0.1 1.0 ± 0.1 0.9 ± 0.1 1.8 ± 0.1 1.0 ± 0.1 2.33 2.0 ± 0.1 1.8 ± 0.1 1.6 ± 0.1 1.1 + 0.1 1.0 ± 0.1 1.7 ± 0.1 1.0 ± 0.1 3.03 1.6 ± 0.1 1.4 ± 0.1 1.2 + 0.1 1.0 ± 0.1 0.9 ± 0.1 1.4 ± 0.1 1.0 ± 0.1 Figure 21(a). Plots of log 1^ vs. log [Cl] for a pressure of 1.70 t o r r . Intensity i n a r b i t r a r y u n i t s . Points obtained for bands centred at the following wavelengths: A 5500A, A6200A, • 7000A, B 8600A, O10600A. Figure 21(b). Plot of log (Jl^dX) vs. log [Cl] for a pressure of 1.70 t o r r . Closed c i r c l e s for infrared region o o (6800A - 12000A), open c i r c l e s for v i s i b l e region (5000A - 6800A).   F i g u r e 22Cal. P l o t s of l o g 1^ vs. l o g [Cl] f o r a pressure of 3.08 t o r r . I n t e n s i t y i n a r b i t r a r y u n i t s . P o i n t s obtained f o r bands centred at the f o l l o w i n g wavelengths: A5500A, A 6200A, • 7000A, B 8 600A, O10600A. Figur e 22Cb). P l o t of l o g (Jl^dX) vs. l o g [Cl] f o r a pressure of 3.08 t o r r . Closed c i r c l e s f o r i n f r a r e d r e g i o n C6800A - 12000A), open c i r c l e s f o r v i s i b l e r e g i o n (5000A - 6800A).   58. (Table 8 and f i g u r e s 21 and 22). The v i s i b l e r e g i o n was found to e x h i b i t a higher dependence on [Cl] than the i n f r a r e d r e g i o n (Table 9). The i n t e n s i t y i n t h i s l a t t e r p o r t i o n of the spectrum was found to depend on the f i r s t power of the atom c o n c e n t r a t i o n at every pressure s t u d i e d . (b) Dependence Of The Emission I n t e n s i t y On [ C ^ ] Since i t was not p o s s i b l e t o devise an experiment i n which the pressure was v a r i e d w h i l e the atom concen- t r a t i o n was held constant, the dependence of the i n t e n s i t y on pressure had to be found by i n t e r p o l a t i o n of the 1^ vs. [C1J data. The data obtained i n t h i s way e x h i b i t c o n s i d e r a b l e s c a t t e r s i n c e they are a f f e c t e d by day to day changes i n p h o t o m u l t i p l i e r and d e t e c t o r s e n s i t i v i t y . However, values of m i n the expression 1^ « [C^]™ have been obtained by p l o t t i n g l o g 1^ a g a i n s t l o g P at con- s t a n t [C1J ( f i g u r e 23), and these are shown i n Table 10. These data show t h a t the i n t e n s i t y i n the short wave- leng t h r e g i o n of the spectrum i s independent of pressure, but the dependence increases w i t h wavelength u n t i l the i n t e n s i t y i s found to be p r o p o r t i o n a l to [ C ^ ] ^ " ^ ~ ^'^ at 10600A. The i n t e g r a t e d i n t e n s i t y i n the v i s i b l e r e g i o n was found to be independent of pressure w h i l e that i n the i n f r a r e d region was observed to have small pressure de- pendence ( f i g u r e 23 and Table 10). Figure 23. Plot of log P vs. log 1^ for bands centred at f i v e wavelengths. Atom -9 concentration fixed at 1.3 x 10 mole/cc.  TABLE 10 VALUES OF m IN THE EXPRESSION I. <* [Cl J A 2 m [Cl] x 10 (moles/cc) -9 m 5500 7000 8600 10600 S 6800A I xdX 5000A 5 12000A 6800A 1.3 0.0 ± 0.2 0.0 + 0.2 0.3 + 0.2 0.5 -± 0.2 0.0 + 0.2 0.3 ± 0.2 2.0 0.0 ± 0.2 0.0 ± 0.2 0.3 + 0.2 0.4 ± 0.2 0.0 ± 0.2 0.1 ± 0.2 (c) Absolute Rate Constant Measurements If we define the apparent rate constant for the chlorine emission as k a P P = W / ™ 2 ' ^ 1 ' then the rate constants can be evaluated using equation 28. However, because separate measurements have been c a r r i e d out i n the v i s i b l e and i n f r a r e d regions of the chlorine afterglow, rate constants corresponding to the VIS emission i n these two regions were calculated, k for aPP ° ° IR the region 5000A to 6800A, and k for the emission y app o o between 6800A and 12000A. The r e s u l t s of these measure- ments are l i s t e d i n Table 11. The most important data to be extracted from these r e s u l t s are the rate constants for the t o t a l emission. A simple addition of the rate constants i n the two regions VIS IR was not possible since both k and k varied with atom c app app concentration. However, i t was found that straight l i n e s were obtained by p l o t t i n g k against 1/[C1] (figures app 24 and 25), and t h i s fact made i t possible to obtain rate constants f o r any value of ICl] by i n t e r p o l a t i o n . Thus, the rate constant for the t o t a l emission at any atom con- centration could be determined by adding the values of k V I S and k I R found from these pl o t s . The resultant k T 0 T A L app app ^ app vs. 1/IC1J p l o t s , constructed i n t h i s way, are shown i n f i g u r e 26, The s o l i d l i n e s in t h i s figure represent the VIS IR values of 1/IC1J over which the k a p p a n d k a p p d a t a overlap VALUES OF k app TABLE 11 FOR THE CHLORINE AFTERGLOW EMISSION PRESSURE = 0.83 t o r r Spectral Region Flow of C l 2 u moles/sec Flow of C l u moles/sec IC1 2J x 10' moles/cc IC1] x 10 gm atom/cc -12 k x 10 U app 6 , - 2 cm moles sec 5000 - 6800A 32.1 32.1 32, 32, 32, 32, 32, 32, 0.918 1.28 1.46 2,13 1,01 32.1 1, 1, 1, 2, 07 30 69 02 4.45 4.42 4.41 4.36 4, 4, 4, 4, 4, 44 43 42 39 37 1.29 1.80 2.05 2.99 1.42 1.50 1.83 2 .37 2.84 3.42 3.20 3.26 2.90 3.16 3.11 3.16 2.81 2.88 6800 - 12000A 32.1 32.1 32.1 32.1 32.1 0. 855 0.879 0.815 0.510 0.109 4,45 4.45 4.45 4.47 4.50 1.20 1.24 1.15 0.716 0.153 14.5 13.5 13.3 17.9 12.4 TABLE 11 Continued PRESSURE = 1.32 t o r r Spectral Region Flow of C l 2 y moles/sec Flow O f C l y moles/sec IC1 2J x 10 moles/cc ICl] x 10* gm atom/cc k x 10 1 2 app 6 .. -2 -1 cm moles sec 5000 - 6800A 33.0 33.0 33.0 33.0 33.0 33.0 33.0 33.0 0.434 0.534 0.697 0.901 1.17 1.17 1.51 1.42 7.12 7.11 7. 7, 7, 7, 7, 7, 10 07 04 04 00 02 0, 1, 1, 2, 2, 943 16 1.52 96 54 54 3.28 3.09 2, 2, 1, 1, 1, 1, 1, 29 19 80 86 80 79 73 1.76 6800 - 12000A 32.6 32.6 32.6 32.6 32.6 32.6 2 .10 2.38 1.34 1.46 1.80 2.65 6.94 6.91 7.02 7.01 6.97 6.88 4.62 5.24 2.95 3.21 3.96 5.83 2.85 2.50 4.24 3.96 3.30 2.28 TABLE 11 Continued PRESSURE =1.70 t o r r Spectral Region Flow of C l 2 u moles/sec Flow of C l u moles/sec IC1 2J x 10' moles/cc [Cl] x 10 gm atom/cc k x 10 1 2 app 6 n -2 cm moles sec 5000 - 6 80 OA 31.0 31.0 31.0 31.0 31.0 0.242 0.341 0.511 0.610 0.835 9.20 9.19 9.16 9.15 9.11 0.721 1.02 1.52 1.82 2.49 1.89 1. 78 1.66 1,59 1.55 6800 - 12000A 32.8 32.8 32.8 35 37 37 36 36 36 36 1 5 5 7 7 7 7 0.265 0.469 0.833 2.10 2.01 2.88 1.42 1.78 2.33 2.60 9.28 9.17 9.12 8.96 8.99 8.88 9.06 9.01 8.94 8.91 0.746 1.32 2.35 5.53 4.95 7.09 3.57 4.48 5.86 6.54 1.40 8.05 4.92 1.90 2.12 1. 73 2.98 2.41 1. 87 1.77 TABLE 11 Continued PRESSURE = 2.33 t o r r Spectral Flow of C l 2 Flow of C l - t c l 2 J x 1 q B t c l J x 1 q 9 k x 10 1 2 R e 9 i o n y moles/sec y * o W s e c m o l e s / c c gm atom/cc aPP _ 2 _ M ' . cm moles sec 38.2 0.146 38.2 0.171 5000 - 38.2 0.186 6800A 38.2 0.246 38.2 0.373 38.2 0.574 31.2 0.107 31.2 0.275 ' 6800 - 31.1 0.175 12000A 31.1 0.212 31.1 0.314 12.6 0.484 2.24 12.6 0.567 1.84 12.6 0.616 2.02 12.6 0.815 1.74 12.6 1.24 1.55 12.6 1.90 1.50 12.6 0.434 13,6 12.6 1.12 5.66 12.6 7.12 8.40 12.6 8.63 6.88 12.6 1.28 4.79 TABLE 11 Continued PRESSURE =3.08 t o r r Spectral Region Flow of C l 2 u moles/sec Flow of C l u moles/sec IC1 2J x 10( moles/cc [Cl] x 10* gm atom/cc -12 k x 10 i Z app 6 , - 2 cm moles sec 5000 - 6800A 38.1 38.1 38.1 38.1 38.1 0.116 0.133 0.123 0.255 0.320 16.4 16.4 16.4 16.4 16.4 0.501 0.575 0.531 1.10 1.38 1.12 1.05 1. 05 0.634 0.639 6800 - 12000A 38.1 38.1 38.1 38.1 38.1 38.1 0.0491 0.0681 0.118 0.152 0.217 0.287 16.7 16.7 16.7 16.7 16.7 16.7 0.216 0.299 0.518 0.668 0.953 1.26 33.1 23.0 12.2 10.4 6.94 5.20 Figure 24. Plot of k vs. 1/[C1] for a pressure app o of 1.70 t o r r . o v i s i b l e region (5000A o o - 6800A), ©infrared region (6800A - o 12000A), addition of rate con- stants for the two regions.  Figure 25. Plot of k vs. 1/IC1] for a pressure app 0 of 2.33 t o r r . O v i s i b l e region (5000A - 6800A), • in f r a r e d region (6800A - o 12000A) , addition of rate con- stants for the two regions.  T O T A L Figure 26. Plot of k u x ^ J j vs. 1/fCl] for f i v e pressures. Pressures are i n t o r r .  60. w h i l e the broken l i n e s represent regions obtained by an e x t r a p o l a t i o n of e i t h e r set of data. The e r r o r i n the r a t e o constants caused by n e g l e c t i n g the emission beyond 12 000A i s l e s s than 5%. E s t i m a t i o n Of E r r o r In The Rate Constants In e v a l u a t i n g the emission r a t e constants f o r the c h l o r i n e and bromine a f t e r g l o w s , the l a r g e s t source of e r r o r i s i n the value of k , the r a t e constant f o r the s standard emission. S c h i f f et a l (9) r e p o r t t h i s value w i t h i n an accuracy of 30%. The e r r o r i n the measurement of atom concentrations i s 2% f o r c h l o r i n e atoms and around 5% f o r bromine atoms. Other e r r o r s such as those caused by changes i n p h o t o m u l t i p l i e r s e n s i t i v i t y are d i f f i c u l t to evaluate but probably do not c o n t r i b u t e more than 5%. Therefore, we estimate t h a t the values of k app f o r the c h l o r i n e a f t e r g l o w are accurate to w i t h i n 40% w h i l e those f o r the bromine a f t e r g l o w are w i t h i n 50%. DISCUSSION There were two major questions towards which t h i s i n - v e s t i g a t i o n of the emission from halogen atom recombination was d i r e c t e d : (1) what f r a c t i o n of the t o t a l recombination occurs v i a e l e c t r o n i c a l l y e x c i t e d s t a t e s , and (2) what p a r t do elementary processes such as v i b r a t i o n a l r e l a x a t i o n and e l e c t r o n i c quenching play i n the recombination luminescence. The former question can be answered from a knowledge of the absolute r a t e constants f o r the emission without making any assumptions about the mechanism of the r e a c t i o n . The l a t t e r q u e s t ion demands a more d e t a i l e d knowledge of the k i n e t i c s of the emission and of the e n e r g e t i c a l l y favourable s t a t e s which are a v a i l a b l e f o r p o p u l a t i o n . The molecule f o r which the most d e t a i l e d i n f o r m a t i o n about e l e c t r o n i c s t a t e s i s a v a i l a b l e i s i o d i n e , but we were unable to make k i n e t i c measurements on the i o d i n e a f t e r g l o w . On the other hand, our most accurate k i n e t i c r e s u l t s were obtained f o r c h l o r i n e , but r e l a t i v e l y l i t t l e i s known about i t s e l e c t r o n i c s t a t e s . Intermediate between these two cases i s bromine, f o r which two e l e c t r o n i c a l l y e x c i t e d s t a t e s are w e l l known, and f o r which f a i r l y complete k i n e t i c data has been obtained. 62. C o n t r i b u t i o n Of Two Body R a d i a t i v e Recombination To The Halogen Afterglows Two body recombination i n t o a r e p u l s i v e s t a t e , or the r e p u l s i v e p o r t i o n of a bound s t a t e , may give r i s e to con- tinuous emission. This process might be expected to be important i f the r a d i a t i v e l i f e t i m e of such a s t a t e i s short and emission from bound l e v e l s i s prevented. The l a t t e r s i t u a t i o n could a r i s e because of slow termolecular recombination, r a p i d r e d i s s o c i a t i o n , quenching of the bound s t a t e s or some combination of these c o n d i t i o n s . In atom recombination s t u d i e s at high pressure and temperature such as are produced i n shock tube.experiments, continuous emission has been found to predominate over banded emission, the l a t t e r a r i s i n g from three-body r e - combination. Although i n the present work the predominant emission was banded, i t was necessary to i n v e s t i g a t e the p o s s i b i l i t y t h a t continuous emission could be making a s i z a b l e c o n t r i b u t i o n to the t o t a l i n t e n s i t y . Since no experimental technique of separating a con- tinuum from the banded spectrum could be devised, i t was necessary to c a l c u l a t e t h e o r e t i c a l l y the c o n t r i b u t i o n which might be expected. To do t h i s , we have used the expressions d e r i v e d by Palmer (52) f o r the p o p u l a t i o n of the r e p u l s i v e r e g i o n of an e x c i t e d s t a t e and a p p l i e d these to the cases of two body r a d i a t i v e recombination i n c h l o r i n e and bromine. 63. (a) C h l o r i n e The two body process i n c h l o r i n e may be represented i n the f o l l o w i n g equations: k C l + C l -7-^ C l * (34a) k 2 C l * — C l 2 + hv (34b) I = k, [ C l ! ] = k 3 k l I C 1 ] 2 = k 3 K * ( r ) [ C 1 ] 2 ' ( 3 5 ) * The e q u i l i b r i u m constant K (r) r e l a t e s the c o n c e n t r a t i o n * of atoms to the c o n c e n t r a t i o n of C l 2 molecules having an i n t e r n u c l e a r s e p a r a t i o n from r to r + dr. For each acces- s i b l e e x c i t e d s t a t e there w i l l be an equation 35, and the t o t a l i n t e n s i t y would then be the sum of the c o n t r i b u t i o n s from a l l of these s t a t e s . In the case of c h l o r i n e , p o p u l a t i o n of the r e p u l s i v e 3 p o r t i o n of the ^ 0 + u s t a t e and subsequent r a d i a t i o n would o give r i s e to emission at wavelengths below 4500A. Since the spectrum of discharged c h l o r i n e was not observed to o extend below 5000A, we conclude t h a t t h i s s t a t e cannot be a source of continuous emission. The only remaining s t a t e w i t h a s u f f i c i e n t l y short r a d i a t i v e l i f e t i m e i s the ^11, , l u ' f o r which the lower energy p o r t i o n of the p o t e n t i a l curve i s not a c c u r a t e l y known. However, based on the p o t e n t i a l curve as drawn i n f i g u r e 1, a rough c a l c u l a t i o n can be * made. The e q u i l i b r i u m constant K ( r ) , f o r a homonuclear diatomic molecule, has been given by Palmer: * , , O T r 2 , * . -U*(r)/kT , K (r) = 2JIr tg /g vg„) e " dr . x. y * In t h i s equation, g i s the s t a t i s t i c a l weight of the ex- c i t e d s t a t e , g x and g are the s t a t i s t i c a l weights of the * atomic s t a t e s and U (r) i s the energy of the s t a t e , a t i n t e r n u c l e a r s e p a r a t i o n r , above the energy of the f r e e atoms, dr i s f i x e d by the s p e c t r a l s l i t width of the mono- chromator and i s determined from the p o t e n t i a l energy * diagram as i s U ( r ) . To determine, f o r example, the r a t e o of two body emission i n a band at 6500A and f o r a s p e c t r a l o s l i t w i dth of 120A, the f o l l o w i n g values are obtained: dr = 0.02A, r = 2.41A, U* (r) = 2253 cm - 1, and K*(r) = — 6 3 —1 6.3 x 10 cm mole . Taking Palmer's (19) value of ^2 microseconds f o r the r a d i a t i v e l i f e t i m e of the ^11, s t a t e l u * - 1 - 1 of C±2, we c a l c u l a t e k^K (r) = 3.2 cc mole sec . This i s e q u i v a l e n t to a. termolecular r e a c t i o n w i t h k = 2.0 x 10 2 -2 -1 cc mole sec a t a pressure of 3.03 t o r r , and t h i s i s s e v e r a l orders of magnitude smaller than the r a t e constant f o r the e x p e r i m e n t a l l y observed process ( t y p i c a l l y around 10 2 -2 -1 ° ° 10 cc mole sec i n a 120A band centred at 6500A). o * S i m i l a r l y , f o r emission at 7500A, K ( r ) k 3 / [ C l 2 ] = 7.4 x 10 2 -2 -1 cc moles sec . From these c a l c u l a t i o n s , we conclude t h a t the c o n t r i b u t i o n to the t o t a l emission i n t e n s i t y from two^body r a d i a t i v e processes i s n e g l i g i b l e . I t should be pointed out, however, that the values obtained for K (r) are very s e n s i t i v e to changes i n the shape and p o s i t i o n of the C^H^ ) state. Since the low energy portion of t h i s state has never been d i r e c t l y observed, i t s p o s i t i o n 3 and point of crossing of the n o + u state are the subject of some conjecture. We have drawn the ^"n^ to cross the 3 n + between the thirteenth and fourteenth v i b r a t i o n a l o u l e v e l s , following Bader and Ogryzlo (34) who observed no emission o r i g i n a t i n g i n l e v e l s greater than v 1 = 13 i n the spectrum of discharged C ^ . It was therefore postulated that the l e v e l s above the thirteenth were being predis- sociated by a crossing state, assumed to be the " ^ n ^ u . Bader and Ogryzlo's argument i n favour of locating the 1 I I ^ u , based on a perturbation of the r o t a t i o n a l l e v e l s at v' => 14, has lar g e l y been discounted (50) by the f a c t that such d i s c o n t i n u i t i e s i n the Br values are probably ex- perimental i n o r i g i n . If the ^H^u were relocated to cross 3 the II + at around v' =8, the calculated contribution to o u ' the t o t a l i n t e n s i t y from the continuum becomes si z a b l e . o At 7500A for example, I c o n t / I b a n d s = 0.72. Although measuring an underlying continuum presents many experimental d i f f i c u l t i e s , i t would be expected that a continuum of t h i s i n t e n s i t y could be observed, e s p e c i a l l y at longer wave- lengths where band overlap i s minimal. For t h i s reason we conclude that the ^ n ^ u state must cross above the eighth 3 v i b r a t i o n a l l e v e l of the n + state. o u (b) Bromine Similar considerations apply i n the case of bromine. If continuous emission arises from molecules on the re- 3 3 pulsive portions of the n + or the II. states, such r r o u l u emission would be detected at wavelengths shorter than o o 5100A and 6350A respe c t i v e l y . Since the observed i n t e n s i t y o i s e f f e c t i v e l y zero at 6000A, contributions from these sources must be n e g l i g i b l e . Therefore, continuous emission i s possible only from the repulsive "'"n^ state. The radia- t i v e l i f e t i m e of t h i s state has not been measured, but since we are concerned here with estimating the maximum possible r a d i a t i o n from two-body recombination, we w i l l — 6 place a lower l i m i t of 10 sec. on i t s value. For a * pressure of 0.92 t o r r , we c a l c u l a t e K ( r ) k 3 / [ B r 2 J = 11 2 - 2 - 1 ° 2 x 10 cc mole sec for emission at 8500A. This l a t t e r rate constant i s less by a factor of 100 than the experi- mentally determined rate constant for a band centred at ° 13 2 -2 -1 8500A (k = 1.2 x 10 cc mole sec ). Since the exp p o s i t i o n of the "^n^u state i s more firmly established i n the case of bromine (53) than chlorine, and i n view of the f a c t that we have taken a lower l i m i t for the l i f e t i m e of the ^H^ u state, we can be more confident here that any underlying continuum i s contributing a n e g l i g i b l e amount to the t o t a l i n t e n s i t y . The Iodine Afterglow In view of the known and calculated p o t e n t i a l energy curves shown i n fi g u r e 3, the iodine afterglow most l i k e l y 3 o r i g i n a t e s from the n ^ u state of I 2 . The energy diff e r e n c e 2 2 -1 between the ?2./2 a n c ^ P3/2 s t a t e s ^ n iodine i s 7616 cm . This s p l i t t i n g i s far too large to allow for any population 3 of the II + state, e i t h e r by thermal e x c i t a t i o n , or i n -o u •* verse p r e d i s s o c i a t i o n v i a one of the repulsive states 2 c o r r e l a t i n g with ground state ^3/2 a t o i n s « Also, the short 3 -7 r a d i a t i v e l i f e t i m e of the II + state (7 x 10 sec) o u (54) precludes i t s o r i g i n i n the discharge. Thus we would expect to see only the 3 n ^ state, and the f a c t that the afterglow extends from 0.8y to beyond 2.3y (figures 16 and 17) substantiates t h i s assignment. The maximum of the emission appears to be at 1.25y which corresponds to a v e r t i c a l t r a n s i t i o n from v 1 = 3 to v." =19. The O r i g i n Of The Bromine Afterglow Gibbs and Ogryzlo (49) have measured some of the band heads i n the spectrum of discharged bromine between o o 6200A and 8300A in the pressure range of 0.5 to 2.5 torr, They i d e n t i f i e d four series of bands o r i g i n a t i n g i n the 3 0, 1, 2, 3 l e v e l s of the II + state and i n addition, o u mentioned several bands appearing further into the red which they were unable to i d e n t i f y . 68. o I n t h e p r e s e n t s t u d y o f t h e e m i s s i o n b e t w e e n 6850A o and 1 2 0 0 0 A we have i d e n t i f i e d b a n d s o r i g i n a t i n g i n t h e 3 0, 1, 2 l e v e l s o f t h e II + s t a t e s , b u t n o t f r o m v ' = 3. ' ' o u ' A l a r g e number o f bands i n t h i s r e g i o n , however, have 3 1 + b e e n a s s i g n e d t o t h e II, — * • £ t r a n s i t i o n w i t h most o f . l u g t h e e m i s s i o n a r i s i n g f r o m v ' = 5 t o v ' = 14 o f t h e ex- c i t e d s t a t e . I d e n t i f i c a t i o n o f t h e t r a n s i t i o n s g i v i n g r i s e t o t h e o bands b e y o n d 9000A p o s e s a s p e c i a l p r o b l e m . I t h a s a l r e a d y b e e n m e n t i o n e d t h a t t h e s e bands a r e p r o b a b l y t h e r e s u l t o f t h e o v e r l a p p i n g o f a number o f t r a n s i t i o n s . S i n c e t h e r e s o l u t i o n o f t h e monochromator i s q u i t e low i n t h i s r e - g i o n , t h e c o n t r i b u t i n g b a n d s c a n n o t be s e p a r a t e d . I n a r e c e n t s t u d y o f t h e r o t a t i o n a l f i n e s t r u c t u r e o f n i n e 1 + 3 bands i n t h e E — n ^ u s y s t e m o f b r o m i n e ( i n a b s o r p t i o n ) , H o r s l e y (45) c a l c u l a t e d t h e e q u i l i b r i u m i n t e r n u c l e a r d i s - 3 ° t a n c e f o r t h e n ^ u s t a t e t o be 2.55A. T h i s v a l u e i s some- what l o w e r t h a n h as been p r e d i c t e d (44) a n d l e a d s t o a f u r t h e r c o m p l i c a t i o n i n t h e i d e n t i f i c a t i o n o f t h e i n f r a r e d b a n d s i n t h e e m i s s i o n s p e c t r u m . The p r e d i c t e d F r a n c k - Condon maximum f o r e m i s s i o n f r o m t h e v ' =0 l e v e l o f t h e 3 H^ u s t a t e , b a s e d on t h i s new v a l u e o f t h e i n t e r n u c l e a r d i s t a n c e , i s a t 10700 cm" 1 (0-12), w h i l e e m i s s i o n f r o m t h e 3 v 1 - 0 l e v e l o f t h e II + i s e x p e c t e d t o be a maximum a t o u 10400 cm" 1 (0-17). S p e c t r a r e c o r d e d a t h i g h p r e s s u r e have o a maximum i n t e n s i t y a t a r o u n d 10,500A as w o u l d be e x p e c t e d i f v i b r a t i o n a l relaxation were populating the zero v i b r a - t i o n a l l e v e l of the excited states. However, because of the s i m i l a r i t y of the Franck-Condon factors i n t h i s region, i t i s d i f f i c u l t to determine whether or not emission from one state i s predominant. The fac t that the v i b r a t i o n a l 3 l e v e l spacing of the n ^ u state below v' = 8 i s uncertain makes assignments i n t h i s region even more d i f f i c u l t . Decreasing the t o t a l pressure and increasing the atom concentration s h i f t s the maximum emission i n t e n s i t y to higher energy. This i s consistent with a decrease i n v i b r a t i o n a l relaxation causing emission predominantly from higher v i b r a t i o n a l l e v e l s . At the lowest pressure at which the afterglow spectrum was recorded (0.3 t o r r ) , the true spectral d i s t r i b u t i o n (figure 13) shows the maximum i n - te n s i t y occurs at around 13050 cm"''". In t h i s region, 3 emission from the n + state originates i n the f i r s t and o u second v i b r a t i o n a l l e v e l s . The most intense band from v' = 2 i s predicted to be the (2, 10) t r a n s i t i o n which occurs at 13051 cm"1. It thus appears that at low pressures 3 the population of the ^ Q + u maY be approaching an i n i t i a l d i s t r i b u t i o n , i f as Gibbs and Ogryzlo (49) suggest, the recombination into t h i s state occurs v i a the " ^n^ u state. 3 This state i s assumed to in t e r s e c t the II + pot e n t i a l o u c curve i n the location of the t h i r d v i b r a t i o n a l l e v e l (53), so that recombination into the repulsive " ^ n^ u state, followed by a c o l l i s i o n induced crossing to the bound 70. 3 II + state, would favour formation into the second and o u ' t h i r d v i b r a t i o n a l l e v e l s . However, just how large a con- t r i b u t i o n to the t o t a l r a d i a t i v e recombination scheme i s 3 made by formation into the II + state i s uncertain, since J o u ' we cannot estimate the f r a c t i o n of the t o t a l emission o r i g i n a t i n g i n t h i s state. In a study of the spectrum of discharged bromine c a r r i e d out concurrently with the present work, Clyne and Coxon (4 8) i d e n t i f i e d a large number of bands, most of which they assigned to the 3H^ u »• ^E* t r a n s i t i o n . We have observed many of the same bands reported by these authors. However, our r e s u l t s d i f f e r i n the assignment of 3 bands to the n + state. Clyne and Coxon have i d e n t i f i e d o u J the following s e r i e s : 3 n + v' = 0 1 2 o u V v" = 1 to 4 v" = 2 to 8 v" = 2 to 8 g Reference to figure 2 shows that these bands should be r e l a t i v e l y weak since they occur at s i g n i f i c a n t l y higher energies than the predicted Franck-Condon maxima for each l e v e l . However, no assignments to these l e v e l s have been made i n the spectral regions where the maximum i n t e n s i t y would be expected. o Clyne and Coxon found that many bands beyond 8 0 00A could not be reconciled with Darbyshire 1s (44) v i b r a t i o n a l analysis of the "extreme red" (3JT —*. 1 E + ) system and i t s i n f r a r e d extension. Beyond 8000A marked d e v i a t i o n s between the observed and p r e d i c t e d wave numbers began to appear, and t h i s f o r c e d them to conclude t h a t e i t h e r Darbyshire's v i b r a t i o n a l assignments were i n e r r o r , or a new band system was beginning to appear i n t h i s r e g i o n . In e a r l i e r s t u d i e s of the a b s o r p t i o n spectrum of bromine, Darbyshire (44) and Brown (47) d i d not observe t r a n s i t i o n s below v' = 6 and, thus, a r a t h e r long e x t r a p o l a t i o n was necessary to o b t a i n the value of u' . Because reasonable doubt as to the p o s i t i o n and spacing of the low v i b r a t i o n a l l e v e l s of t h i s s t a t e does e x i s t , Clyne and Coxon (48) chose to i n t e r p r e t system. This r e q u i r e d a m o d i f i c a t i o n of Darbyshire's o r i g i n a l a n a l y s i s which i n v o l v e d r e a s s i g n i n g three bands and dropping f i v e o t h e r s , a l l o w i n g a smooth c o n t i n u a t i o n from the bands observed i n absorption to Clyne's new bands. The new s p e c t r o s c o p i c constants were then c a l c u l a t e d f o r 3 -1 the II ̂  s t a t e y i e l d i n g u)^ = 153 ± 2 cm which i s much lower than the o r i g i n a l 170.7 cm 1 c a l c u l a t e d by Darbyshire. Our observations on the i n f r a r e d bands suggest t h a t t h i s procedure i s j u s t i f i e d , but there are a number of i n - c o n s i s t e n c i e s i n the work of Clyne and Coxon. C a l c u l a t i o n s of the v i b r a t i o n a l energy l e v e l s using these new constants are i n f a i r agreement wi t h the observed t r a n s i t i o n s up to around the tenth v i b r a t i o n a l l e v e l , but f o r higher l e v e l s d e v i a t i o n s become q u i t e l a r g e . The (17-3) t r a n s i t i o n , f o r e ° 3 the bands beyond 8 000A as an extension of the II l u 72. example, i s calculated to appear at 14635 cm 1 while i t a c t u a l l y i s located (45) at 14559 cm-''". The cause of t h i s discrepancy can be understood by p l o t t i n g the values of AG*(v + 1/2) against (v1 + 1/2) i e . the v i b r a t i o n a l energy spacing vs. v i b r a t i o n a l number (figure 27). Using Clyne's r e s u l t s together with those of Darbyshire (44) and Brown (47), t h i s curve exhibits a region of p o s i t i v e curvature at high v i b r a t i o n a l numbers, a sharp point of i n f l e c t i o n at v 1 = 9, followed by a region of s l i g h t negative cur- vature to v 1 = 0. To obtain constants which would be consistent with t h i s type.of curve, the.data would have to be f i t t e d to an equation containing higher powers of (v1 + 1/2) , i e . a) x and u) y should have been calculated. ' ' e e eJ e For t h i s reason Clyne's spectroscopic constants are v a l i d over the range v' = 0 to v' = 8 only. In the spectral region observed photographically, the 3 lowest l e v e l of the Jl, observed was the f i f t h l e v e l , so l u ' we were unable to confirm Clyne's assignments for the low v i b r a t i o n a l l e v e l s of t h i s state. The bands which were assigned to the f i f t h and sixth l e v e l s however, did not agree well with either^ Darbyshire's or Clyne's scheme. In summary, the emission spectrum of discharged bromine i s very complex consisting of a large number of d i f f u s e red-degraded bands. The spectrum does show the following features. Figure 27. Plot of ^ G \ + 1 / f 2 v s * v ' f o r t h e 3 j I l u state of B r 2 . Open c i r c l e s are values of Darbyshire (44) and Brown (47) and closed c i r c l e s are data of Clyne and Coxon (48) .  73. 3 (a) Most of the bands i d e n t i f i e d a r i s e from the — 1 I I + t r a n s i t i o n . g 3 (b) Emission from the 0, 1, 2 l e v e l s of the n o + u n a s been observed. High pressures favour emission from the zeroth l e v e l , low pressures the second v i b r a - t i o n a l l e v e l . K i n e t i c s Of The Bromine Afte r g l o w Using a photographic technique to measure emission i n t e n s i t y , Gibbs and Ogryzlo (49) s t u d i e d the dependence of the bromine a f t e r g l o w i n t e n s i t y on atom c o n c e n t r a t i o n and pressure. F i t t i n g t h e i r data to the equation I = k [ B r J n [ B r 2 J m , they obtained n = 1.9 ± 0.2 and m = 0.8 ± 0.2 and concluded t h a t w i t h i n experimental e r r o r , I = k [ B r J 2 ! B r 2 J . We have determined the value of the l i g h t emission 2 r a t e constant d e f i n e d by k = I/[Br] [ B r 0 ] , by measuring app 2 the absolute emission i n t e n s i t y of the bromine aft e r g l o w . I f the k i n e t i c order i s t h a t p r e d i c t e d by Gibbs and Ogryzlo, then k should be constant at a l l values of atom concen-app t r a t i o n and pressure. This has not been observed to be the case, as i s i l l u s t r a t e d i n f i g u r e 28. This v a r i a t i o n of k can be understood by examining app J ^ o o the dependence of (.Î dA (6000A to 12000A) on atom con- 2 Figure 28. Plot of k a p p v s . IBr] for a pressure of 0.92 t o r r . »  centration, as shown i n Table 8. The integrated emission 1 2 + 0 2 i n t e n s i t y was found to vary as [Br] " " ' at 0.52 t o r r , and as [ B r ] 1 * 8 ~ at 1.82 t o r r , but was not found to depend on the square of the atom concentration under any of the experimental conditions used. Assuming that the rate of formation of the excited states i s second order i n atom concentration, the f a c t that less than a second order dependence was observed for the o v e r a l l emission process suggests that"bromine atoms are involved i n quenching the luminescence. Therefore, i n order to obtain the rate of emission i n the absence of atom quenching, we must evaluate k at very low [Br]. app We have plotted 1/k against [Br] i n figure 29 and ex- "̂PP trapolated to zero atom concentration. The most r e l i a b l e intercepts w i l l be obtained from figure 29 (b) since the atom concentration range of these data i s less by an order of magnitude than that of figure 29(a). The values of rate constants at zero IBr], obtained at pressures of 0.53 and 16 2 2 1 1.50 t o r r , are k = 0.4 x 10 cc mole" sec - and k = app app 16 2 2 1 2.0 x 10 cc mole sec" respectively. Comparing these 16 2 — 2 — 1 rates with the value of 13 x 10 cc mole sec" obtained for the o v e r a l l rate of recombination using Br 2 as a t h i r d body (371, we f i n d that between 3 and 15% of the t o t a l recombination takes place into e l e c t r o n i c a l l y excited states. In view of our study of the emission i n t e n s i t y at various wayelengths as a function of atom concentration Figure 29. Plots of 1/k vs. [Br] for four ^ ' app values of pressure: Ca) O 0.92 torr and • 1.18 t o r r Cb) A 0.53 t o r r and • 1.50 torr,   (Table 5 ), we can now suggest a p o s s i b l e e x p l a n a t i o n f o r the r e s u l t s obtained by Gibbs and Ogryzlo. The technique used by these authors to measure emission i n t e n s i t y i n - volved photographing the afte r g l o w , and then r e l a t i n g the o p t i c a l d e n s i t y of the f i l m to i n t e n s i t y . The photographic o f i l m used was s e n s i t i v e only to 8800A, so t h a t the long wavelength r a d i a t i o n was not recorded. Reference t o Table 5 shows t h a t i t was a t these shorter wavelengths t h a t c l o s e to a second order dependence on [Br] was observed, and t h i s 2 l e d the authors to suggest t h a t I a [Br] . Our r e s u l t s on the pressure dependence of the i n t e - grated i n t e n s i t y i n d i c a t e t h a t some process i n v o l v i n g bromine molecules i s a l s o quenching the luminescence. Although these data are subject to con s i d e r a b l e e r r o r , they show a l e s s than f i r s t order dependence of the r a t e of emission on the pressure. Mechanism Of The Emission Reaction (a) Formation Of E x c i t e d States In The B r 2 Afterglow U n l i k e the i o d i n e a f t e r g l o w , i n which a l l the emission 3 o r i g i n a t e s i n the I I ^ u s t a t e , two e l e c t r o n i c a l l y e x c i t e d s t a t e s are i n v o l v e d i n the bromine emission. These s t a t e s 3 2 are the n l u , which c o r r e l a t e s w i t h two ground state. 3 atoms, and the II + , which c o r r e l a t e s w i t h one ground o u 3 2 2 s t a t e P3/2 a n d o n e e x c i t e < ^ P i / 2 a t o m ( f i g u r e 2). 3 The d i r e c t formation of the II + s t a t e from one ex- o u c i t e d and one ground s t a t e atom does not appear to be a favourable process because of the low concentration of 2 P. / 9 atoms i n the gas stream. The equilibrium concen-1/2 t r a t i o n of the excited atom i s expected to be very small C48) ^ 2 p i / 2 J / l 2 p 3 / 2 J ^ 1 x 1 0 ~ 8 a t 3 0 0 ° K ) d u e t o t h e 2 2 large energy difference between the ^2/2 a n d P l / 2 a t o m i c states. Recent f l a s h photolysis studies (55) have shown that molecular bromine i s very e f f i c i e n t at causing spin- 2 o r b i t relaxation of p^/2 atoms. For t h i s reason, excited atoms formed i n the discharge would be extremely short l i v e d and could not be considered as a possible source 3 of the II + state, o u Gibbs and Ogryzlo (49) suggested an a l t e r n a t i v e 3 mechanism by which the ^- 0 + u state could be populated. This involved the formation of an intermediate state, cor- r e l a t i n g with ground state atoms, which could then undergo a c o l l i s i o n - i n d u c e d t r a n s i t i o n into the emitting state. The two possible e l e c t r o n i c states which could f u l f i l l 3 t h i s r o l e are the n - which i s predicted to i n t e r s e c t o u 3 the II + at small values of internuclear distance (34), o u ' and the , which i s assumed to cross at about the t h i r d 3 v i b r a t i o n a l l e v e l of the n 0 + u ( 5 3 ) • ° u r observations on the afterglow spectrum indicate that the population of the 3 . II o+ u state takes place into the second or t h i r d v i b r a t i o n a l l e v e l s , since at low pressures the maximum emission o r i g i - nates from v' =2, This evidence, together with the fact that emission from l e v e l s higher than v' = 3 has never been observed, suggests that the ^n^u i s the intermediate 3 3 state i n the formation of the n + . The II - state, on o u o u ' the other hand, would be expected to populate a much wider range of v i b r a t i o n a l l e v e l s at, or below, the d i s s o c i a t i o n l i m i t . Therefore, we propose the following mechanism for 3 the formation of the n + state: o u B r C 2 P 3 / 2 ) + B r t 2 P 3 / 2 ) — B r 2 ( \ u ) (36) B r 2 C l n i u > + B r 2 — B ^ 2 ( 3 V u ) v ' = 3 + B r 2 • { 3 7 ) Any a l t e r n a t i v e mechanism, such as one involving Br^ or a simple termolecular c o l l i s i o n , would be expected to 3 populate c h i e f l y the zeroth v i b r a t i o n a l l e v e l of the II + c J on state, since t h i s i s the only l e v e l which l i e s below the d i s s o c i a t i o n l i m i t of ground state atoms. 3 The formation of the II + state i n the t h i r d v i b r a - o u t i o n a l l e v e l (equation 37) would involve a f a i r l y large a c t i v a t i o n energy since t h i s l e v e l l i e s 430 cm - 1 above the energy of two ground state atoms. The formation of the 3 H l u state, on the other hand, should involve no a c t i v a t i o n energy. B r ( 2 P 3 / 2 ) + B r ( 2 P 3 ^ 2 ) + B r 2 —>• B r 2 ( 3 n i u ) + B r 2 (38) Since i t has not been possible to separate the con- t r i b u t i o n s to the t o t a l emission from each of these states, i t i s not possible to state unequivocally which i s predominant, However, considering the a c t i v a t i o n energy required to form 78, 3 the IT + s t a t e , and the l a r g e number of bands which we have o u . 3 3 assigned to the 1T Û s t a t e , i t would appear t h a t the l a t t e r i s more important i n the recombination process. (b) R e l a x a t i o n Processes In The E x c i t e d States 3 F o l l o w i n g the formation of the IT + s t a t e , the 3 o u molecule may r a d i a t e , r e d i s s o c i a t e , or be v i b r a t i o n a l l y r e l a x e d to the zeroth and f i r s t v i b r a t i o n a l l e v e l s : Br-C 3 n + ) , - , + B r 0 Br „ ( 3 n + ) , n + Br„ (39) 2 o u v*=3 2 2 o u v'=0 2 B r 2 ( 3 n Q + u ) + B r 2 ». 2Br + B r 2 (40) B r 0 ( 3 n + ) • B r 0 ( 1 Z + ) + hv (41) 2 o u 2 g v ' V i b r a t i o n a l r e l a x a t i o n would most probably occur one quantum at a time and thus equation (39) represents the r e s u l t s of a number of c o l l i s i o n s . Since we observe a l e s s than f i r s t order pressure dependence f o r the t o t a l emission, some process i n v o l v i n g B r 2 must be removing e x c i t e d molecules. 3 In the ^ 0+ u, t h i s would most probably occur through the d i s s o c i a t i o n shown i n equation (40), t h i s being the reverse of the formation process. 3 There may be another process by which the ^ 0 + u s t a t e i s r e l a x e d and t h a t i s by a c o l l i s i o n induced c r o s s i n g to 3 the. n^^. I f quenching by B r 2 i s e f f e c t i v e i n the upper s t a t e , there i s no reason f o r assuming t h a t d e a c t i v a t i o n d i r e c t l y to the ground s t a t e occurs. In f a c t , Franck- Condon f a c t o r s and symmetry c o n s i d e r a t i o n s (56) would probably favour a conversion to another component of the 3 II s t a t e . Thus, we i n c l u d e the f o l l o w i n g r e a c t i o n : B r2 ( 3 noV + B r 2 — B r 2 ( 3 l I l u ) + B r 2 • ( 4 2 ) The r a d i a t i v e l i f e t i m e s of these two e x c i t e d s t a t e s of bromine have not been measured. We assume, however, t h a t 3 the r a d i a t i v e l i f e t i m e of the n + s t a t e of bromine l i e s o u between the l i f e t i m e s of the e q u i v a l e n t s t a t e s i n i o d i n e and c h l o r i n e , or around 10"^sec. On the other hand, c a l - c u l a t i o n s based on the t h e o r e t i c a l work of M u l l i k e n (15) 3 -4 i n d i c a t e a r a d i a t i v e l i f e t i m e of the 11̂  s t a t e of 10 sec. I t would be expected t h a t processes l e a d i n g to the de- a c t i v a t i o n of the e x c i t e d s t a t e s would be more e f f e c t i v e 3 f o r the I I ^ u s t a t e , s i n c e these processes would be com- p e t i n g w i t h spontaneous emission. We propose t h a t the 3 r e l a x a t i o n of the n ^ u s t a t e i s the r e s u l t of the f o l l o w i n g r e a c t i o n s : B r 2 C 3 n l u l + Br —•* B r 2 ( 1 E g ) + Br (43) B r 2 c 3 j I l u > + B r 2 + B r 2 ( 4 4 ) B r 2 C 3 n i u ) . — * B r 2 ( 1 E g ) + hv . (45) 3 Throughout the n ^ u e l e c t r o n i c s t a t e , the energy i n t e r v a l between adjacent v i b r a t i o n a l l e v e l s i s l e s s than kT at room temperature (44). I t i s known t h e o r e t i c a l l y t h a t when energy l e v e l s are t h i s c l o s e l y spaced, the p r o b a b i l i t y of c o l l i s i o n - i n d u c e d t r a n s i t i o n s between them becomes very 80. high (57). E x c i t e d i o d i n e molecules, which have v i b r a t i o n a l spacings very s i m i l a r to these e x c i t e d bromine molecules, were found to undergo v i b r a t i o n a l t r a n s i t i o n s at p r a c t i c a l l y every c o l l i s i o n (58). Thus, r a p i d v i b r a t i o n a l r e l a x a t i o n of t h i s s t a t e of bromine would be expected, and T i f f a n y (59) has suggested t h a t no more than 100 c o l l i s i o n s would be necessary to reach v i b r a t i o n a l and r o t a t i o n e q u i l i b r i u m . This i s c o n s i s t e n t w i t h our observation t h a t a t high pres- o sures, the maximum emission occurs a t 9400A. Although we have assigned some of the bands i n t h i s r e g i o n to the (0,v") t r a n s i t i o n s of the 3 n + —*• system, t h i s ' o u g * ' wavelength a l s o corresponds to the p r e d i c t e d Franck-Condon 3 maximum from the zeroth l e v e l of the IK s t a t e , which i s l u ' undoubtedly c o n t r i b u t i n g to the t o t a l i n t e n s i t y . The measurement of the v a r i a t i o n of 1/k w i t h atom app c o n c e n t r a t i o n and pressure i s c o n s i s t e n t w i t h the above mechanism. Although the data c o n t a i n a l a r g e amount of s c a t t e r , they do show three trends. F i r s t of a l l , the value of k , the r a t e constant f o r the t o t a l emission, app decreases w i t h i n c r e a s i n g atom co n c e n t r a t i o n i n d i c a t i n g t h a t quenching by atoms does occur. Secondly, the slope of the 1/k v s . i B r J p l o t s ( f i g u r e 29) increases w i t h i n - c r e a s i n g pressure. This i s explained q u a l i t a t i v e l y by a 3 3 c o l l i s i o n induced c r o s s i n g from the n + to the Jl, v o u l u (equation 42)., followed by a quenching by atoms which i s expected to be more e f f i c i e n t i n the l a t t e r s t a t e . The 81. t h i r d observation which we can make about these 1/k app vs. [Br] plots i s that they are curved and that they ap- pear to f l a t t e n out at high values of [Br]. This f l a t t e n i n g out would correspond to a "steady state" being reached 3 between the c o l l i s i o n induced crossing to the II^ state and the removal from t h i s state by atomic quenching. Origin Of The Chlorine Emission A l l of the emission a r i s i n g from the recombination 3 1 + of chlorine atoms has been assigned to the II + —*• Z 3 o u g t r a n s i t i o n . The fac t that no bands i n the emission spectrum could be attributed to the —*- transition, i s con- lu g v s i s t e n t with Mulliken's (14) t h e o r e t i c a l prediction, and also with recent absorption studies (60). The observed change i n spectral d i s t r i b u t i o n with pressure can be attributed to v i b r a t i o n a l relaxation i n the emitting state caused by c o l l i s i o n s with chlorine molecules. Thus, at low pressures, the emission maximum o was observed at 67 00A, corresponding to rad i a t i o n from 3 the fourth and f i f t h v i b r a t i o n a l l e v e l s of the II + . At o u higher pressures, t h i s maximum was observed to s h i f t u n t i l at 3 t o r r , emission from the zeroth and f i r s t v i b r a t i o n a l l e v e l s predominated. 82. K i n e t i c s Of The C h l o r i n e A f t e r g l o w (a) Order Of Emission I n t e n s i t y With Respect To [Cl] In previous i n v e s t i g a t i o n s of c h l o r i n e a f t e r g l o w (27, 34), the emission i n t e n s i t y was found to vary as the square of the atom c o n c e n t r a t i o n and the f i r s t power of the t h i r d body c o n c e n t r a t i o n . For C l 2 as a t h i r d body, the r a t e equation then becomes I = k l C l J 2 ! C l 2 J . (46) We have determined the value of the l i g h t emission r a t e constant, as defined i n equation (46) , f o r a number o of values of [C1 2J and [Cl] over the range 5000A to ° 2 12000A. When t h i s r a t e constant was p l o t t e d a g a i n s t [Cl] , as i n f i g u r e 30, i t s value was observed to increase 2 sha r p l y at low values of [C1J . Since the formation of the e x c i t e d s t a t e must proceed v i a a termolecular r e a c t i o n , t h i s apparent change i n order w i t h respect to c h l o r i n e atom c o n c e n t r a t i o n i n d i c a t e s a quenching of the lumines- cence by atoms. Therefore, to f i n d the r a t e of recombination 3 i n t o the II + s t a t e , we must determine the value of the o u ' emission r a t e constant at very low atom c o n c e n t r a t i o n . In TOTAL f i g u r e 31 we have constructed p l o t s of 1/k agai n s t ^PP IC1J, us i n g the data of f i g u r e 26. By e x t r a p o l a t i n g these curves to zero atom c o n c e n t r a t i o n , the average value of (at [C1J = 0) was found to be 1.8 x 1 0 1 4 c m 6 m o l e - 2 aPP r-l sec . Compared to the t o t a l r a t e of recombination, 2 Figure 30. Plot of k vs. [C1J for a pressure app v of 1.70 t o r r .  TOTAT . Figure 31. plots of 1/k u vs. [Cl] i n the app pressure range 0.83 to 3.08 t o r r . Broken curves obtained by extrapol- ation.  83. k = 2.0 x lO^cm^mole^sec 1 (27), t h i s value indicates that 1% of the t o t a l recombination takes place into the 3 II + state, o u The error made by previous investigators i n deter- mining the order of the emission i n t e n s i t y with respect to [Cl] can be attributed to the fa c t that only the short wavelength region of the afterglow spectrum was studied. We have found a large v a r i a t i o n i n the dependence of emission i n t e n s i t y on atom concentration over the entire emitting region (see Table 9). At a pressure of 1.7 0 t o r r , for example, t h i s dependence decreased from I a [Cl] " u _ ' at 5500A to I « [ C l ] u , y " u * 1 at 10600A. In t h e i r study of the chlorine afterglow, Bader and Ogryzlo (34) measured i n t e n s i t y using a photographic technique by r e l a t i n g the o p t i c a l density of the f i l m to emission i n t e n s i t y . Although i n f r a r e d - s e n s i t i v e f i l m was. o used, and a f i l t e r eliminated radiat i o n below 6250A, i t i s possible that much of the emission recorded lay below o 7000A. In that case, the in f r a r e d portion would not have contributed to the i n t e n s i t y and a second order dependence would have been observed. Our c a l c u l a t i o n of the. integrated o o emission i n t e n s i t y i n the v i s i b l e region (5000A - 6800A) yielded I <= [ C l ] 1 , 8 * 0 , 1 while Bader's (26) r e s u l t s indicated I <* [ C l ] 1 " 9 ± <^'1. The same explanation could apply to the Work.of Hutton and Wright (27) who used a photomultiplier having i t s maximum s e n s i t i v i t y i n the v i s i b l e region, 84. (b) Pressure Dependence Of The Emission Intensity On the question of the dependence of I upon [ C l 2 ] , there appears to be a discrepancy between the r e s u l t s of Bader and Ogryzlo and Hutton and Wright. The former workers claim a f i r s t power dependence, but t h e i r determination was based on experiments performed at only two pressures and no r e s u l t s were obtained at pressures greater than 1.6 t o r r . Hutton and Wright concluded that the emission i n t e n s i t y was dependent on the molecular chlorine concen- t r a t i o n below 2 t o r r , but above t h i s pressure was i n - dependent of [ C l 2 3 • ,If we assume that i n the wavelength range over which Hutton and Wright's i n t e n s i t y measurements were made 2 I « ICl] , we can compare our values of i n t e n s i t y at 2 fix e d atom concentration with t h e i r values of I/[C1] . This has been done i n the logarithmic p l o t of figure 32, where the slope of these curves corresponds to m i n the equation I = k l C l ] n l C l 2 J m . Our r e s u l t s are i n e s s e n t i a l agreement with those of Hutton and Wright, since they indicate a change i n k i n e t i c order around 2 torr and show a less than f i r s t order dependence below t h i s pressure. The slope of our plots at IC1 2J = 1 t o r r gives I <* I C ^ J ^ * ^ ~ while Hutton and Wright's r e s u l t s y i e l d 0 4 I a ICl,,] * . This discrepancy can be attributed to the fa c t that these workers measured the emission i n t e n s i t y o below 8000A, where we observed a lower order dependence on pressure (Table 10 and figure 23). Our measured Figure 32. Pressure dependence of the chlorine afterglow emission i n t e n s i t y . Results of Hutton and Wright (A) plotted as I r e l / { C 1 J 2 vs. P; t h i s work (9 and O ) plotted as I r e j _ vs. P. Pressure (torr) 85. o i n t e n s i t y values i n c l u d e a l l the emission from 5000A to o 12000A and are thus expected to i n d i c a t e a higher o v e r a l l order i n molecular c h l o r i n e c o n c e n t r a t i o n . In a p r e l i m i n a r y study of the c h l o r i n e a f t e r g l o w emission c a r r i e d out at the same time as t h i s work, Clyne and Steadman (61) observed a s i m i l a r behaviour of the i n - t e n s i t y w i t h pressure. The i n t e n s i t y i n a narrow wavelength o band at 5800A was found to be pressure independent above o 1 t o r r w h i l e the i n t e n s i t y i n a band at 7800A was found to be dependend on [ C ^ J • These workers were not able to measure t o t a l i n t e n s i t i e s owing to the low s e n s i t i v i t y of t h e i r p h o t o m u l t i p l i e r at long wavelengths, but they sug- gested t h a t the t o t a l i n t e n s i t y c ould be p r o p o r t i o n a l to IMJ i n the pressure range s t u d i e d . This we have not found to be the case, Clyne and Steadman a l s o examined the e f f e c t of using argon as a t h i r d body and found t h a t w h i l e an o v e r a l l decrease i n i n t e n s i t y was observed, there was no s i g n i f i c a n t d i f f e r e n c e i n i n t e n s i t y d i s t r i b u t i o n between argon and c h l o r i n e systems. Mechanism Of The Emission Reaction In C h l o r i n e (a) Formation Of The E m i t t i n g State The c h l o r i n e a f t e r g l o w emission has been shown to 3 o r i g i n a t e i n the II + e l e c t r o n i c s t a t e of molecular o u c h l o r i n e . Since t h i s s t a t e c o r r e l a t e s w i t h one ground 2 2 s t a t e P-J/T atom and one e x c i t e d P, ,„ atom, we are faced 86. with the problem of how t h i s state i s formed i n the re- combination process. 3 Di r e c t recombination into the n + state by one o u normal and one excited atom has been suggested by Hutton and Wright (27) on the basis that the small doublet separ- ation of the atoms (881 cm ^) allows a s u f f i c i e n t thermal 2 population of the excited ^\/2 a t o m i c state. At room temperature, however, only about 0.7% of the atoms are i n the excited state. In recent E.P.R. studies on the discharge products of CF^Cl and mixtures of CF^Cl and C l 2 , Carrington et a l (62) detected signals corresponding to the excited 3 C l ( ^2./2^ a t o m s . By comparing these signals to those from the more abundant ground state atoms, they were able to estimate the r e l a t i v e populations of the two states, ° ° —2 and calculated N]_/2 / / N3/2 = x 10 . Since t h i s represents a population d i s t r i b u t i o n f i v e times larger than that ex- pected for a Boltzmann d i s t r i b u t i o n , i t may in d i c a t e that excited atoms are produced by some reaction subsequent to 2 the discharge. However, the population of the P ] y 2 s t a t e i s s t i l l too low to explain the large concentration of 3 H o+ u observed i n the chlorine afterglow, i f d i r e c t f o r - mation of t h i s state from one normal and one excited atom i s assumed, 3 The II + state must therefore be formed i n some o u process involving ground state atoms. One mechanism by . 87. which t h i s might occur could involve two ground state atoms combining into an intermediate state, which can 3 then undergo a r a d i a t i o n l e s s t r a n s i t i o n into the II + 3 o u emitting state. Such an intermediate i s probably another e l e c t r o n i c a l l y excited state of C l 2 such as the ̂ n ^ u 3 repulsive state, or the n o + u - The process may be represented by the equations: C l t 2 P 3 / 2 ) + C U 2 P 3 / 2 ) — C l 2 ( \ u ) (47) C 1 2 ( l n i u ) + C 1 2 C 1 2 ( 3 V u ) • ( 4 8 ) Since there i s add i t i o n a l evidence (page 76) that 1 3 the Ik i s the precursor of the II + state i n bromine, l u r o u ' we have assumed that t h i s i s also the case for chlorine. However, unlike bromine, where a considerable a c t i v a t i o n energy was required to populate the v' = 3 l e v e l of the 3 n Q + u , none i s required to populate the highest observed 3 v i b r a t i o n a l l e v e l of the C l 2 ( n o + u ) state. The energy difference between two ground state atoms and the v 1 = 13 3 _1 l e v e l of the II + state i s 162 cm , and t h i s can be made o u up by the mean k i n e t i c energy of approach of the reactant p a r t i c l e s (kT ̂  200 cm"1 at 300°K). The Lewis-Rayleigh nitrogen afterglow exhibits many c h a r a c t e r i s t i c s s i m i l a r to the halogen afterglows. The recombination of nitrogen atoms leads to emission pre-3 3 + dominantly i n the f i r s t p o s i t i v e (B II —>• A 88. system of molecular nitrogen. The emitting B state, however, does not correlate with ground state atoms and hence must be populated v i a some intermediate state. Bayes and Kistiakowsky (63) suggested that a steady state popu- 5 + l a t i o n of the E state followed by a c o l l i s i o n induced 3 r a d i a t i o n l e s s t r a n s i t i o n to the B II state was responsible g * for the emission. However, i n a recent study of the absolute emission from the nitrogen afterglow, Campbell and Thrush (64) determined that half of the t o t a l recombination was 3 occurring v i a the B IT state. This high rate of formation of nitrogen molecules i n the B state could not be accounted 5 + for by a steady state population of the E^ state since i t s binding energy i s only of the order of 2 Kcal/mole. They thus concluded that atoms recombine into the A 3 E * state and population of the B state proceeds v i a a c o l - l i s i o n induced crossing from the A state. Although such r a d i a t i o n l e s s t r a n s i t i o n s as the 5 E + — - 3 E + of N 0 and the X n , —*• 3II + of C l _ are f o r -g u 2 l u o u 2 bidden because of spin and symmetry requirements, Zener (65) has pointed out that the e l e c t r i c f i e l d of a c o l l i d i n g molecule can relax the " o r b i t a l " s e l ection rules, and the spin change i s probably unimportant for these states of chlorine (page 6). (b) Relaxation Of The 3II + State o u The experimental observations on the chlorine a f t e r - glow provide evidence for the following relaxation processes. 89. (i) V i b r a t i o n a l Relaxation The observation that the.spectral d i s t r i b u t i o n of the emission s h i f t s to the red with increasing pressure, suggests that v i b r a t i o n a l relaxation by c o l l i s i o n with C l 2 i s taking place. I f , as was suggested i n the previous 3 1 section, formation into the II + takes place v i a the II, ' o u ^ l u state, v i b r a t i o n a l relaxation would depopulate the l e v e l s close to the point of crossing of t h i s intermediate state. Presumably, at very low pressures a spectral d i s t r i b u t i o n corresponding to emission from these high v i b r a t i o n a l l e v e l s would be observed. For emission i n narrow band widths, we f i n d that I .« ICI,,]^ for emission from high v i b r a t i o n a l l e v e l s while the pressure dependence approaches [Cl,,] 1 for the v 1 = 0,1 l e v e l s . I f formation into the high v i b r a t i o n a l l e v e l s i s * 2 a t h i r d order p r o c e s s , ( d l C l 2 J / d t = k f [ C l 2 ] [ C l ] ) and re- moval i s f i r s t order i n IC1 2J, the o v e r a l l process w i l l be pressure independent. If the lower lev e l s are formed ex- c l u s i v e l y by rela x a t i o n from higher v i b r a t i o n a l l e v e l s , a f i r s t order dependence on [C1 2J should be observed. Cii) Quenching By IC1 2J If v i b r a t i o n a l relaxation were the only process involving chlorine molecules, the pressure dependence of the integrated emission i n t e n s i t y would be Jl^dA <* [ C l 2 ] . Our data indicate that j l dA « {Cl ] 0 * 6 1 0 , 1 at 1 t o r r , 90. and t h i s implies that quenching by [ C l 2 ] i s taking place. ( i i i ) Quenching By Atoms Although previous workers (2 7, 34) have overlooked the p o s s i b i l i t y of atom quenching of the emitting state i n chlorine, our data provide conclusive evidence that t h i s i s an extremely important process. Furthermore, 3 atoms are most e f f i c i e n t at quenching the II + state ^ 3 o u in low v i b r a t i o n a l l e v e l s as i s indicated by the fac t that 1^ * [ C l ] 1 , 0 ± 0 , 1 for emission at 10600A. For high v i b r a t i o n a l l e v e l s 1^ « I C I ] 2 * ^ ± implying that atom quenching does not play an important part. This l a t t e r observation i s d i f f i c u l t to inte r p r e t unless the 3 l i f e t i m e of the II + state i s shorter at high v i b r a t i o n a l o u 3 l e v e l s than at low. Owing to a misinterpretation of the 3 v term in the Ei n s t e i n r a d i a t i o n law, Bayes and K i s t i a - 3 kowsky (.63) stated that t h i s was the case for the B II state of nitrogen. However, Douglas (66) pointed out that the correct use of t h i s term implies that high v i b r a t i o n a l l e v e l s should a c t u a l l y have longer l i f e t i m e s . The v a r i a t i o n of the e l e c t r i c t r a n s i t i o n moment with internuclear distance i s another e f f e c t which w i l l cause a v a r i a t i o n in the l i f e t i m e of an elec t r o n i c state. During a v i b r a t i o n of large amplitude, in which the molecule approaches d i s s o c i a t i o n , the t r a n s i t i o n moment of chlorine must vary from that for the normal 3JT + t r a n s i t i o n , J o u g 3 3 to a value approaching the p j / 2 *" P3/2 a t o m i c t r a n ~ s i t i o n . Since t h i s l a t t e r t r a n s i t i o n i s forbidden, the high v i b r a t i o n a l l e v e l s would be expected to have a longer r a d i a t i v e l i f e t i m e than the lower l y i n g l e v e l s . This has 3 been found to be the case for the n + state of iodine o u (54) where a sevenfold increase i n l i f e t i m e was observed between the v 1 =10 and v 1 =50 v i b r a t i o n a l l e v e l s . Thus, the p r e f e r e n t i a l quenching by atoms cannot be attributed to l i f e t i m e v a r i a t i o n . 3 3 The II - state i s predicted (15) to cross the II + o u . ^ o u near the bottom of the p o t e n t i a l w e l l . Bader and Ogryzlo 3 (34) have drawn i t to approach the n 0 + u state at the inside c l a s s i c a l turning point at low v i b r a t i o n a l l e v e l s . If the process by which the atoms quench the excited state 3 involves an induced crossing to the II - state, the • o u ' quenching would be most e f f e c t i v e near the point of crossing, and therefore, at low v i b r a t i o n a l l e v e l s . This mechanism can also explain another experimentally observed phenomenon which i s that the atomic quenching e f f i c i e n c y increases with increasing pressure. At high pressure, v i b r a t i o n a l relaxation forces the excited molecules into the low v i b r a t i o n a l l e v e l s where, because of the proximity 3 of the ^ 0 ~ u state, they are more e a s i l y quenched by atoms Without a c t u a l l y specifying how t h i s low-level quenching occurs, l e t us see i f the mechanism thus far described q u a n t i t a t i v e l y explains the experimental r e s u l t s 92. 3 To do t h i s , l e t us assume that the excited II + state ' o u 3 * consists of two l e v e l s , such that ^ Q + u represents the 3 high v i b r a t i o n a l l e v e l s and ^ 0 + u represents the lowest v i b r a t i o n a l l e v e l s (figure 33). The following equations may be written: C l + C l + C l 2 — C l 2 ( 3 n Q + u ) * + C l 2 (49) C 12 ( 3Vu )* + C l 2 ^ C 12 lVu' + C 1 2 (5°) C 12 ( 3Vu )* + C 1 2 C 1 2 ( 3 ' E g ) + C 1 2 ( 5 1 ) C l 2 ( 3 n Q + u ) * — C l 2 ( 1 E g ) + hv (52) C 12 ( 3Vu ) + C 1 2 — L * C 1 2 ( l z g } + C 1 2 ( 5 3 ) C 1 2 ( 3 l I o + u ) + C 1 — ^ C 1 2 ( l E g ) + C 1 ( 5 4 ) C l 2 ( 3 n Q + u ) —Z— C l 2 ( 1 E g ) + hv (55) Making the usual steady state assumptions we calculate I = k C L ( 3 n + )* + k C l „ ( 3 n + ) r 2 o u r 2 o u k r k f r c i J 2 ! C l 2 J k r k v k f [ C 1 ] 2 [ C 1 2 ] 2 ( k r + k v [ c i 2 J + k 1 [ c i 2 ] ) ( k r + k v [ c i 2 ] + k 1 [ c i 2 ] ) ( k r + k q [ c i ] + k 2 [ c i 2 ] (56) and since k a p p = I/IC1J 2JC1 2 J , Figure 33. Schematic diagram for proposed mechanism of C l atom recombination. Non-radiative processes shown as straight arrows.  93. 1 1 k" = FT 1 app f 1 + v 2 2 1 r 1 2 2 \ [ C l 2 ] k (k +k [ClJ+k [Cl0]+k„[Cl„]) r r q v 2 2 2 k (k +k ) [Cl ] + I 1 r 1 1 I [Cl] (57) \ k k (k +k ICIJ +k IC1 ] +k [Cl J ) J These equations provide a q u a l i t a t i v e d e s c r i p t i o n of the experimental r e s u l t s . The intercepts of the 1/k e f / app vs. [C1J plots are predicted to be a function of pressure and although t h e i r determination involved an extrapolation of the data, these are found to increase with pressure (figure 31). The second term predicts that the slopes of the 1/k vs. [C1J plots increase with pressure and that app at large values of ICIJ, these should " f l a t t e n out" as the term k2lClJ i n the denominator becomes larger. (c> P a r t i c i p a t i o n Of Other E l e c t r o n i c States In The C l 2 Afterglow 3 The n Q ~ u has already been mentioned as a possible intermediate i n the quenching of emission by atoms. There 3 i s , however, a p o s s i b i l i t y that the TI^u may play an important part i n atom recombination, since much of the recombination occurs into t h i s state i n the case of 3 bromine. The r a d i a t i v e l i f e t i m e of the IT. state of l u chlorine i s undoubtedly long, since absorption into this state has never been observed. Quenching by atoms would be expected to be more e f f i c i e n t i n t h i s s t a t e than i n 3 the IT + because of t h i s longer l i f e t i m e . A c o l l i s i o n -o u 3 induced c r o s s i n g between the two s t a t e s , and a more r a p i d 3 quenching of the IÎ  s t a t e by atoms would be e n t i r e l y c o n s i s t e n t w i t h our observation t h a t the quenching e f - f i c i e n c y i ncreases w i t h i n c r e a s i n g pressure. Suggestions For Further Study Of Halogen Afterglows (a) Bromine The accurate measurement of bromine atom concentra- t i o n s remains one of the l a r g e s t b a r r i e r s i n o b t a i n i n g q u a n t i t a t i v e k i n e t i c r e s u l t s . A q u a n t i t a t i v e chemical t i t r a t i o n f o r Br atoms e q u i v a l e n t to the N0C1 + C l r e - a c t i o n has not been found, and the isothermal d e t e c t o r has not proven to be r e l i a b l e . Bromine atoms, however, do g i v e a good ESR s i g n a l , and t h i s method of d e t e c t i o n may warrant f u r t h e r study. The spectrum of the bromine a f t e r g l o w i n the r e g i o n o o 9000A to 12000A should be a c c u r a t e l y measured on a high r e s o l u t i o n spectrophotometer. The use of a time averaging computer could be used, i n t h i s a p p l i c a t i o n , to accumulate s u f f i c i e n t s i g n a l to be measured. A study of the bromine a f t e r g l o w at a number of temperatures might provide the 3 f i n a l answer to the question of the o r i q i n of the IT + ^ 3 o u s t a t e . At very low temperatures, very l i t t l e emission would be seen from t h i s s t a t e i f formation proceeds v i a the """II. . (b) Chlorine To understand the quenching processes which are operative i n the C l 2 afterglow, studies of i n d i v i d u a l tran- s i t i o n s could be undertaken. I t might be possible to d i s - cover whether atom quenching occurs i n c e r t a i n s p e c i f i c 3 l e v e l s near the bottom of the ^- 0 + n state providing evidence for a crossing by another state i n that region. P A R T T W O STUDIES ON EXCITED MOLECULAR OXYGEN INTRODUCTION During the course of t h i s research, the very i n - t e r e s t i n g problem of simultaneous e l e c t r o n i c t r a n s i t i o n s i n two oxygen molecules i n the gas phase arose. Two fea t u r e s of our apparatus (as described i n Se c t i o n 1) made i t i d e a l l y s u i t e d to a study of these t r a n s i t i o n s . F i r s t l y , our equipment was designed f o r d e t e c t i n g emission i n the near i n f r a r e d r e g i o n of the spectrum and many of o these "double molecule" t r a n s i t i o n s appear between 6000A o and 13000A. Secondly, the c a l i b r a t i o n of the de t e c t o r s f o r absolute emission i n t e n s i t y made p o s s i b l e the study of t r a n s i t i o n p r o b a b i l i t i e s . Considering these f a c t s , we hoped to make absolute i n t e n s i t y measurements on the (0 2 ) 2 bands and extend t h i s study to e x c i t e d oxygen produced i n s o l u t i o n . E l e c t r o n i c States Of Molecular Oxygen The l i t e r a t u r e on the subject of oxygen spectroscopy i s e xtensive and no attempt w i l l be made here to give a d e t a i l e d review. However, a b r i e f d i s c u s s i o n of the lower e l e c t r o n i c s t a t e s and some of the t r a n s i t i o n s i n which these are i n v o l v e d w i l l be presented. 97. There are, as yet, only comparatively few diatomic molecules for which a large number of e l e c t r o n i c states have been established on the basis of t h e i r observed band spectra. However, i n the case of oxygen, seven bound states have been i d e n t i f i e d . Six of these states correlate with 3 ground state P atoms, and one correlates with one normal 3 1 P atom and one excited D atom. These states are shown i n the p o t e n t i a l diagram of figure 34 as drawn by Gilmore (67) . The lowest e l e c t r o n i c configuration of , as pre- dicted by molecular o r b i t a l theory (68), i s 0„lKK(a2s) 2 (a*2s) 2 (a2P) 2 (TT 2P) 2(TT 2P) 2 (TT 2P) Z(TT 2 P r j y ^ 3 - 1 1 + and t h i s gives r i s e to the states Z , A and Z . The g g g 3 - ground state i s the Z^ state, and the f a c t that i t i s a t r i p l e t accounts for the observed paramagnetic properties of oxygen. The "*"A state of 0~ i s the lowest excited state, g 2 l y i n g 0.98 eV above the ground 3Z~ state (69). The half l i f e for spontaneous r a d i a t i v e emission has been found to be of the order of 45 minutes (70). In the i s o l a t e d molecule, t r a n s i t i o n s between the ground l e v e l and the two upper l e v e l s (''"A and Ẑ"1") are of the magnetic dipole g g type since the s e l e c t i o n rules for t h i s r a d i a t i o n include the r o t a t i o n a l l e v e l combination + ••->» + which i s s t r i c t l y Figure 34. Potential energy curves for the oxygen molecule. E (electron volts) 98. forbidden for e l e c t r i c dipole r a d i a t i o n . These t r a n s i t i o n s are weak, even for magnetic dipole transitons, because they involve s i n g l e t - t r i p l e t intercombinations. Further- 3 - 1 more, the E^ •*—*• A t r a n s i t i o n i s doubly forbidden because i t also v i o l a t e s the AA = 0 ± 1 r u l e . This would explain the exceptionally long r a d i a t i v e l i f e t i m e of the A species. 1 3 - The t r a n s i t i o n s between A and E form the i n f r a - g g red atmospheric absorption system which l i e s i n the range of 13000A to 730OA. The (0-0) t r a n s i t i o n l i e s at 1.27y and as predicted, has an i n t e n s i t y of about 1/4 00 that of 1 + 3 -the E ••—*• E system. g g * The (0-0) and (0-1) emission bands have also been recorded i n the t w i l i g h t and airglow spectrum of the at- mosphere (71) . The "'"Eg state of oxygen l i e s 1.626 eV above the ground state and has an estimated r a d i a t i v e l i f e t i m e of 7 seconds (72) . The 3 E ~ t r a n s i t i o n i s known as the red atmos- g g o pheric system and has i t s (0-0) band at 7619A. The i n t e n s i t y of t h i s system i s low since t h i s t r a n s i t i o n involves a s i n g l e t - t r i p l e t intercombination and i s also symmetry f o r - bidden. Bands of the red atmospheric system are found i n absorption i n l i q u i d (72) and gaseous (73) oxygen, and i n emission i n the airglow (74) and aurora (75). The other bound states of oxygen which have been 3 1 - 3 observed spectroscopically are the A u, E u and A E u 3 -which c o r r e l a t e w i t h ground s t a t e atoms, and the B Z^ 1 3 which c o r r e l a t e s w i t h one D and one P atom. The strongest band system i n oxygen i s the Schuman- 3 + 3 - Runge system (B Z u X Z ) , which i s a f u l l y allowed e l e c t r i c d i p o l e t r a n s i t i o n w i t h an o s c i l l a t o r s t r e n g t h of f = 0.16 (76). These bands are found i n the u l t r a - o v i o l e t and converge to a very c l e a r l i m i t (at 1759A) 3 _ which corresponds to the d i s s o c i a t i o n l i m i t of ( Z u ) . Studies On E x c i t e d Molecular Oxygen For many years d i s c h a r g e - f l o w techniques have been used to produce high concentrations of oxygen atoms i n a flow system. Ogryzlo (7) found t h a t i f the atoms are removed from a stream of discharged oxygen by d i s t i l l i n g mercury i n t o the discharge r e g i o n , the gas stream s t i l l contained an e x c i t e d s p e c i e s . Mass spectrometric (77) and c a l o r i m e t r i c (78) measurements have shown t h a t the predominant e x c i t e d s t a t e of O,, i n such a system i s "*"A , and i t has been found th a t up to 10% of the t o t a l stream can be produced i n the discharge products. In a s p e c t r o s c o p i c study of a stream of e x c i t e d oxygen molecules, Bader and Ogryzlo (7) found two unique emission o o bands, one a t 6340A and the other at 7030A. The i n t e n s i t y o of the 634OA band was found to be p r o p o r t i o n a l to the square of the 0 2 (-̂A ) c o n c e n t r a t i o n as measured by a c a l o r i m e t r i c d e t e c t o r , and both bands were observed to 100. be broad and s t r u c t u r e l e s s . Noting t h a t the energy of the o 634 0A band i s e q u i v a l e n t to twice the e x c i t a t i o n energy of Oj ("'"A ) , Bader and Ogryzlo proposed t h a t the bands arose z g as a r e s u l t of the f o l l o w i n g processes: 20- I A ) 0. >- 20„t £ ) n + hv(6340A) 2 g ^ 4 2 g v=0 20 0 ( A ) 0. » 0 o ( JZ ) n + 0 o CZ ) , + hv (7030A). 2 g ^ 4 2 g v=0 2 g v=l Although Bader and Ogryzlo s t a t e d t h a t the bin d i n g energy of the 0^ double molecule was 600 c a l o r i e s , a remeasurement o of the temperature dependence of the 634OA band by Arnold 079) l e d to the c o n c l u s i o n t h a t the 0^ double molecule was a c t u a l l y a c o l l i s i o n complex, and hence could more c o r r e c t l y be represented as ~ 0 2. Simultaneous e l e c t r o n i c t r a n s i t i o n s i n two molecules were f i r s t suggested by E l l i s and Kneser (80) i n 1933 to e x p l a i n bands appearing i n the absorption spectrum of l i q u i d oxygen. They suggested a d i s s o c i a t i o n l i m i t of 142 cal/mole from a study of the l i n e shapes of the absorp- t i o n bands. In 1936, Salow and St e i n e r (81) found these bands i n the abs o r p t i o n spectrum of the compressed gas and determined t h a t the i n t e n s i t y was dependent on the square of the oxygen pressure but independent of the pressure of added gases. More r e c e n t l y Dianov-Klokov (82) studied the temperature dependence of the band i n t e n s i t i e s and concluded th a t the absorption was due to an 0„ - 0 ? c o l l i s i o n complex. 101. E x c i t e d oxygen molecules have a l s o been observed i n the products of some chemiluminescent r e a c t i o n s i n s o l u t i o n . The extremely i n t e r e s t i n g o b s e r v a t i o n t h a t a red chemiluminescence i s produced during the r e a c t i o n of hydrogen peroxide and sodium h y p o c h l o r i t e i n aqueous s o l u t i o n was reported by S e l i g e r (83). Using a low r e s o l u - t i o n apparatus, he recorded one emission band and gave i t s o peak wavelength as 6348A. The work was extended by Khan o and Kasha (84) who found a second band at 7032A having the same h a l f width as the f i r s t . They a t t r i b u t e d these bands to the (0-0) and (0-1) bands of the 1 E + —*- 3 E ~ t r a n s i t i o n g g of molecular oxygen. This assignment was based on the f a c t t h a t the band se p a r a t i o n of 1567 cm 1 c o i n c i d e s , w i t h i n experimental e r r o r , to the ground s t a t e v i b r a t i o n a l f r e - quency (1580 cm ^) of molecular oxygen. The authors con- cluded t h a t the discrepancy between the p o s i t i o n of these bands i n s o l u t i o n and i n the gas phase was caused by a sol v e n t s h i f t of the bands of 2593 cm 1 . Subsequent c r y s t a l f i e l d c a l c u l a t i o n s on hydrated oxygen (85) suggested t h a t such a high frequency s h i f t was t h e o r e t i c a l l y p o s s i b l e . However, i n a study of discharged oxygen i n the gas phase, Ogryzlo e t a l (86, 7) found these same bands, thereby r u l i n g out t h i s i n t e r p r e t a t i o n . These authors suggested t h a t the emission arose from simultaneous e l e c t r o n i c t r a n - s i t i o n s i n two oxygen molecules. 102. Purpose Of This Investigation The determination of the absolute rate of emission i n three bands of the spectrum of discharged molecular oxygen was undertaken. From the rate constant of the emis- ° 1 + 3 - sion from the 7619A band a r i s i n g from the 0»( I — • £ ) 2 g g t r a n s i t i o n , we hoped to be able to estimate the concen- t r a t i o n of 0 oC 1£ +) i n the gas stream. We also undertook a 2 g 3 o measurement of the absolute i n t e n s i t i e s of the 6340A (1A ) - — » ( 3 E _ ) 0 and 7030A (1A ) o(0,0) — - ( 3E _)„(0,1) g 2 g 2 g 2 ' g 2 ' bands i n order to determine the r a d i a t i v e l i f e t i m e of the ( 0 2 ( 1 A g ) ) 2 complex. We hoped to extend the foregoing i n v e s t i g a t i o n to a study of the chemiluminescence from the reaction of ^2^2 and C l 2 i n solution and to i d e n t i f y the excited species giving r i s e to th i s emission. 103. EXPERIMENTAL Production Of 0 o C1 A ) And 0„ i1!.*) Molecules 2 g 2 g When commercially avai l a b l e tank oxygen i s discharged, the products are oxygen atoms, excited oxygen molecules Ĉ A and 1 E + ) , and small amounts of nitrogen atoms which g g y subsequently react to produce NO and N0 2. By using Matheson medical grade oxygen, the amount of N0 2 i n the discharge products was minimized, as indicated by the low i n t e n s i t y of the 0 + NO afterglow. To obtain a pure stream of excited oxygen molecules, removal of the oxygen atoms produced i n the discharge was necessary. This was accomplished by d i s t i l l i n g a small amount of mercury into the discharge region. The mercuric oxide ring which forms immediately aft e r the discharge i s very e f f e c t i v e at removing atoms while i t does not de- activate the excited molecules. A drop of mercury was placed 3 cm. upstream from the microwave cavity and the temperature of the mercury was controlled by means of a heating tape wrapped around the reservoir. After the d i s - charge was i n i t i a t e d and allowed to warm up, the temperature of the mercury was raised to a point at which just enough mercury was d i s t i l l e d into the discharge to cause an 104. e x t i n c t i o n of the residual 0 + NO glow. The control and c a l i b r a t i o n of gas flowrates has been described i n Part 1 and w i l l not be discussed here. The reaction tube used i n the oxygen work, however, d i f - fered from those described i n connection with the halogens i n that the monochromator viewed the emission down the length of a 10 cm. section of the tube. The detector and water jacket were placed at the end of t h i s section around a 90° bend. The discharge region was jacketed, allowing cooling a i r to be blown through i t , and a standard C type microwave antenna was used. The flow of 0~ Ĉ A ) was measured with the isothermal 2 g calo r i m e t r i c detector described previously. However, since cobalt has been found to be more e f f i c i e n t than n i c k e l i n deactivating the 0^ ̂ A^) molecules, the platinum c o i l was electro-plated with cobalt. This e l e c t r o - p l a t i n g was found to give best r e s u l t s when done at a current of 10 ma. and a voltage of 6 v o l t s for about one hour. The e l e c t r o - p l a t i n g solution was a d i l u t e solution of cobalt chloride and ammonium chloride. Measurement Of Emission From The Cl^ - H2°2 S v s t e m A very simple procedure was employed i n making measurements on the C l ^ - ^2^2 s Y s ^ e m * Approximately 10 ml. of a d i l u t e solution of ammonium hydroxide was cooled i n an ice bath and then 1 ml. of 90% H o0„ was added. Chlorine was 105. bubbled into the solution through a small glass j e t placed i n such a way that a f i n e stream of bubbles was directed against the wall of the glass vessel. The small reaction zone so produced was found to be an advantage i n increasing the length of time the reaction could be viewed, since re- actants were used up more slowly and the temperature of the solution did not r i s e too r a p i d l y . The red emission from the bubbles was focused on the s l i t of the mono- chromator by means of a lens system, and the wall of the glass vessel viewed by the monochromator was kept free of condensation by blowing dry nitrogen over i t . The reaction normally proceeded for four to f i v e minutes aft e r which time the solution heated up quite r a p i d l y and the emission terminated. RESULTS E s t i m a t i o n Of I.0_ ( £ )]. From Absolute Emission 2 g In using the isothermal c a l o r i m e t r i c d e t e c t o r to estimate 0„ ("*"A ) c o n c e n t r a t i o n i n a stream of e x c i t e d 2 g oxygen molecules, the assumption i s made t h a t there are no other e x c i t e d species i n the gas stream capable of r e l e a s i n g heat to the d e t e c t o r . The only e x c i t e d s t a t e of 0 2 w i t h low enough energy to be formed i n the discharge, or i n subsequent r e a c t i o n s , i s the •""£* s t a t e . Most of the emission from t h i s s t a t e appears i n the (0-0) band of 1 + 3 - 0 the 0~ ( £ — * £ ) t r a n s i t i o n a t 7619A, so t h a t by f i n d i n g 2 g g the absolute emission from t h i s band, an e s t i m a t i o n of the c o n c e n t r a t i o n of 0 2 ("*"£*) should be p o s s i b l e . The peak was scanned using the cooled RCA 7102 p h o t o m u l t i p l i e r and the c a l c u l a t i o n s were performed as described i n the experimenta s e c t i o n of P a r t 1. In t h i s case, however, the r a t e of emis- s i o n f o l l o w s from the equation I = k { 0 9 (V)] 2 g and the r a t e constant f o r spontaneous emission k i s known to be 0.14 sec 1 (72). Rearranging equation (28) we have 107. 7700 / . 7700 [ 0 2 ( 1 Z g ) j = k s{OJlNOjA o JF SU);L OU)CU //(0.14)A g ^ F G(A)dA 7500 i s ( X ) / 7500 Under the following experimental conditions P = 2 t o r r [0 2J = 1.08 x 10~ 4 moles/1. [OJ = 3.81 x I O - 6 gm.atoms/1. [NO] = 5.43 x 10~ 6 moles/1. the i n t e g r a l s were calculated to be 7700 A \F (A)i CA)dA = 0.101 I s o 7500 i s ( X ) 7700 A s \ F s ^ ) d X = H.24 7500 from which a value of the concentration of i s calculated g to be [0„( 1E +)J = 1.77 x 10~ 9moles/l. / g Under s i m i l a r experimental conditions, the concentration 1 -6 of 0 2 ( A ) i s usually of the order of 10 moles/1. This measurement, therefore, confirms the previous suggestions (7, 79) that 0~ ( 1Z +) i s a minor constituent i n the products l. g of discharged oxygen i n flow systems. Consequently, the 108. error introduced by the assumption that the excited stream i s e n t i r e l y "'"A J i s n e g l i g i b l e . o Absolute Emission Intensity Of The 6340A Band o The 634 0A peak of molecular oxygen was observed using the monochromator and the cooled RCA 7102 photomultiplier and the concentration of the excited 0 2 ( 1 A g ) species was measured using the cobalt-plated isothermal c a l o r i m e t r i c detector. A number of measurements were taken under con- d i t i o n s of varied t o t a l pressure and excited molecule con- centration and an average value of the rate constant for emission i n t h i s band was found. Bader and Ogryzlo (7) o showed that the i n t e n s i t y of the 6340A band i s proportional to the square of the "'"A concentration; thus we define the g rate constant for the emission to be I = k' l O ^ A ) ] 2 . (58) g Using equation 28 as outlined i n the experimental section and the 0 + NO c a l i b r a t i o n procedure, an average -1 -1 value of k 1 was calculated to be k' = 0.090 l.mole sec A t y p i c a l set of values used i n equation (28) to obtain t h i s value of k' was IOJ = 3.20 x 10~ 6moles/l. INOJ = 2.96 x 10~ 6moles/l. [ 0 2 ( 1 A )] = 8.14 x 10~ 6moles/l. A /A = 3.2 x 10~ 4 rv o 109. 7000 / 7000 F (A) i (A)dA / I F (A)dA = 0.242 6000 / 6000 The Chlorine-Hydrogen Peroxide System When chlorine i s bubbled into an a l k a l i n e solution of hydrogen peroxide i n water, the chlorine i s consumed and molecular oxygen i s formed. The equation for the o v e r a l l reaction can be written as C l 2 + H0~ + OH" —»• 2C1~ + H 20 + 0 2 (59) The bubbles formed i n the reaction emit a red l i g h t of quite high i n t e n s i t y . Making the solution more basic had the e f f e c t of reducing the size of the bubbles formed and also of ex- tending the length of time the reaction lasted. Typical proportions of reactants which were found to produce the best emission were 5 ml. of NH^OH, 5 ml. of water and 10 drops of 90% H 20 2. The spectrum of the emission o r i g i n a t i n g i n the bubbles was recorded using the f/4.5 Hilger and Watts monochromator and a cooled RCA 7102 photomultiplier and i s shown i n f i g u r e 35, A l l the bands i n the spectrum can be assigned : + ) , 0 0 ( 1 A ] g ' 2 g to known t r a n s i t i o n s involving 0 2 (̂Z"*" , 0 2 (''"A ) and the c o l l i s i o n a l pair (0 0 ("*"A ) ) 0 . These are l i s t e d i n Table 12, Z a Z F i g u r e 3 5 . E m i s s i o n s p e c t r u m f r o m t h e r e a c t i o n o f c h l o r i n e w i t h h y d r o g e n p e r o x i d e . S o l i d l i n e i s f r o m t h e aqueous-ammonia s y s t e m . B r o k e n l i n e i s e m i s s i o n f r o m t h e c h l o r o f o r m - p y r i d i n e s y s t e m . 5 5 0 0 6 0 0 0 7 0 0 0 8 0 0 0 1 0 , 0 0 0 Wavelength (A) TABLE 12 BANDS OBSERVED IN THE SPECTRUM OF THE C1 2"H 20 2 SYSTEM Peak Number Wavelength E l e c t r o n i c States V i b r a t i o n a l Levels o (A) 1 5800 C 1 A g ) 2 — ^ ( 3Zg) 2 (0,1 — K 0,0) 2 6340 c l V 2 • ' • ^ Z g V (0,0 0,0) 3 7030 (̂ A ) - — * ( 3 Z ~ ) 9 (0,0 — * 0,1) 4 7619 1 Z + 3 E " (0,0) 5 7700 1 Z + — * 3 E " (1,1) g g 8645 1 + 3 - ^g — * % 10,1) X A g — V (2,0) 10,700 1L v 3 Z " (1,0) 12,700 X A g >- 3Z~ (0,0) 110. Because water i s known to be an extremely e f f i c i e n t d e a c t i v a t o r f o r both 0 ~ ( 1 Z + ) and f o r v i b r a t i o n a l l y ex- 2 g •* c i t e d oxygen (87), experiments were performed to see i f reducing the water vapour content of the bubbles had any e f f e c t on the spectrum. F i r s t , a l l s o l u t i o n s were cooled to 5°C before being used and during the experiment, a c o l d f i n g e r helped to maintain the low temperature. This r e - s u l t e d i n higher i n t e n s i t i e s i n peaks (5) and (6) as might be expected. A second set of experiments were conducted using a non-aqueous system to produce the chemiluminescence. A 50% s o l u t i o n of chloroform and p y r i d i n e was cooled and then a l a y e r of 90% H2C>2 was added. The mixture was shaken to e x t r a c t some hydrogen peroxide i n t o the c h l o r o - form l a y e r . The c h l o r i n e was then bubbled i n t o t h i s l a y e r . F i g u r e 35 shows the r e s u l t i n g spectrum i n which bands due to v i b r a t i o n a l l y e x c i t e d oxygen are n o t i c e a b l y absent. The small i n t e n s i t y of the 7619A ( 1 Z + >• 3 Z ~ ) band i n - d i c a t e s the lower c o n c e n t r a t i o n of 0 „ ( 1 Z + ) i n t h i s system. 2 g An estimate of the y i e l d of 0~ ("'"A ) can be obtained J 2 g by comparing the i n t e n s i t i e s of bands found i n the s o l u t i o n work w i t h those found i n the study of discharged gaseous oxygen. As was mentioned p r e v i o u s l y , the emission i n t e n s i t y o of the 6340A band has been found to be p r o p o r t i o n a l to the square of the ^A c o n c e n t r a t i o n (7, 79): ^340 = k . l O ^ A g ) ] 2 • < 5 8) o 2_ Since the band at 12700A arises from one 0-( A ) molecul 2 g we can write ^2700 = k ' l ° 2 ( 1 V ] • ( 6 0 ) The r a t i o of the emission i n t e n s i t i e s i n these two bands i s given by R = I6340 = k' 10, (XA )1 . (61) •"•12700 When 0 2 (''"A ) i s obtained from an e l e c t r i c a l discharge, the concentration of the excited species can be found using the cobalt detector previously described. A value for k'/k" ^ n equation 61 can then be found by measuring the r a t i o of the i n t e n s i t i e s of the two emission l i n e s . With t h i s value of k'/k" i t ^ s possible to estimate the 0 o C"*"A ) concentration i n any system simply by measuring z g R for that system. In contrast to other methods of deter mining chemiluminescence y i e l d s , geometric factors and c o l l i s i o n a l quenching rates need not be accounted for when equation 61 i s used. Using the f/4.5 monochromator and the RCA 7102 photomultiplier and with a s l i t width of 500u, the following r e s u l t s were obtained when 0^^^g) w a s measured i n the products of an e l e c t r i c a l discharge: 112 IO„(1A U = 1.5 x 10~ 5mole l . " 1 2 g R = 36 k'/k" = 2 ' 4 x 1 0 6 l . m o l e ~ 1 Using the same o p t i c a l system to measure the emission 3 from the C l 2 - H 20 2 system, R was found t o be 1.8 x 10 . However, Badger, Wright and Whitlock (70) have shown t h a t o 1/5 of the i n t e n s i t y i n the 12700A band i s due to spon- taneous emission, the remaining i n t e n s i t y a r i s i n g from c o l l i s i o n induced t r a n s i t i o n s . Thus, i n c a l c u l a t i n g the c o n c e n t r a t i o n of 0~ ("*"A ), a value of R f i v e times 2 g ' gre a t e r than the measured value should be used, i e . 3 . . R = 9.0 x 10 . Using t h i s c o r r e c t e d i n t e n s i t y r a t i o i n 1 - 3 - 1 equation 61, we c a l c u l a t e { 0 2 ( A )] = 3.75 x 10 mole 1. ^ g I f we assume t h a t the bubble temperature i s 300°K, t h i s corresponds t o a p a r t i a l pressure of 7 0 t o r r . Since the t o t a l oxygen pressur i n the bubbles i s about 750 t o r r , \ g t h i s corre ponds to a y i e l d of 10% 0^ ("*"A ) i n the r e a c t i o n , 113. DISCUSSION Radiative Lifetime Of The CO,,) 2 Complex Bader and Ogryzlo (7) have proposed the following o o mechanism to explain the emission at 6340A and 7030A i n the spectrum of discharged oxygen: k K2 (62) k C 0 2 C 1 A g ) ) 2 >̂ C0 2( 32:g)) 2 + hv(6340,7030A) (63) I = O ^ / k ^ k - j l O ^ A ) ] 2 . (64) The value of the rate constant for emission i n the o 6340A band can be used to estimate the r a d i a t i v e l i f e t i m e of the ( 0 2 ) 2 c o l l i s i o n a l complex. If we assume that the o p r o b a b i l i t i e s of the t r a n s i t i o n s giving r i s e to the 6340A o and 7030A bands are equal, since the i n t e n s i t i e s of these bands are almost equal, we can c a l c u l a t e the value of the t o t a l l i g h t emission rate i n equation 64: ( k 1 / k 2 ) k 3 = 2(k') = 2(0.090) = 0.18 1. mole" 1sec~ 1 . 0 The rate constants k^ and k 2 can be estimated on a basis of simple c o l l i s i o n theory, assuming that d i s s o c i a t i o n of the complex occurs w i t h i n one v i b r a t i o n . Thus = I O 1 1 1. mole "''sec "*", k 2 = 1 0 1 3 s e c 1 and k^ i s then c a l - c u l a t e d to be 18. Expressed as the r a d i a t i v e h a l f l i f e of ( 0 2 ( 1 A g ) ) 2 : t 1 / 2 = 0.693/k 3 = 37.5 msec. Obviously t h i s estimate could be c o n s i d e r a b l y low depending upon how strong are the a t t r a c t i v e f o r c e s between two 0„ ("*"A ) molecules. 2 g The problem, then, i s one of f i n d i n g an accurate e q u i l i b r i u m constant f o r equation 62 (K = k ^ / k 2 ) . In an attempt to estimate the c o n t r i b u t i o n of bound species to the second v i r i a l c o e f f i c i e n t , Stogryn and H i r s c h f e l d e r (88) d e r i v e d an equation enabling t h i s e q u i l i b r i u m constant to be c a l c u l a t e d . The authors d i v i d e d the second v i r i a l c o e f f i c i e n t i n t o three p a r t s : B(T) = B f (T) + B b(T) + B m(T) . (65) In t h i s equation, B^ (T) a r i s e s from c o l l i s i o n s between f r e e molecules, B^tT) i s r e l a t e d to the e q u i l i b r i u m con- s t a n t f o r the formation of bound double molecules i n the gas and B (T) i s r e l a t e d to the e q u i l i b r i u m constant f o r m the formation of "metastable" double molecules. The d i f f e r e n c e between these species can be understood i f an e f f e c t i v e p o t e n t i a l energy of i n t e r a c t i o n between molecules i s d e f i n e d as 4> e f f(r,L) = 4>tr) + L / r 2 (66) 2 where L/r i s c a l l e d the c e n t r i f u g a l p o t e n t i a l and 115. represents the energy of r o t a t i o n a l motion. <J>(r) can be approximated by a Lennard-Jones (6-12) p o t e n t i a l <f>(r) = 4 e l ( a / r ) 1 2 - ( a / r ) 6 ] . (67) where C-e) i s the maximum energy of a t t r a c t i o n between two molecules and a i s the l o w - v e l o c i t y c o l l i s i o n diameter. When the c e n t r i f u g a l p o t e n t i a l term has a f i n i t e v alue, the e f f e c t i v e p o t e n t i a l energy curve contains a "hump" or r o t a t i o n a l b a r r i e r . In f i g u r e 36 one such p o t e n t i a l energy curve i s shown f o r a small value of L. Region B i n t h i s f i g u r e corresponds to the reg i o n of phase space occupied by those two-molecule systems where the t o t a l energy i s l e s s than the energy of the separated molecules. These are termed bound dimers and can only be f r e e d by a c o l l i s i o n w i t h another molecule. Region M contains the metastable species which have greater energy than the separated molecules. Although c l a s s i c a l l y , these can only be fre e d by a c o l l i s i o n w i t h another molecule, quantum mechanically they can d i s s o c i a t e by leakage through the energy b a r r i e r . Depending on whether the h a l f - l i f e of d i s s o c i a t i o n i s greater or l e s s than the average time between c o l l i s i o n s , t h i s species can behave l i k e a bound or f r e e molecule. Using a s t a t i s t i c a l mechanical argument, Stogryn and H i r s c h f e l d e r d e r i v e d an expression f o r the e q u i l i - brium constant f o r the formation of both the bound and Figure 36. E f f e c t i v e p o t e n t i a l energy curves for the Lennard-Jones (6-12) p o t e n t i a l . Energy o 116. metastable s p e c i e s . For 0^ gas at 294°C, t h e i r expression y i e l d s K = 0.0254 1. mole" 1. Since Kk 3 = 0.18 1. mole" 1 sec 1 , s u b s t i t u t i o n of t h i s value of K y i e l d s = 7.1 -1 . 1/2 sec . Expressing t h i s as a r a d i a t i v e h a l f l i f e g i v e s t ' = 0.69/7.1 % 0.1 seconds. In a t h e o r e t i c a l treatment of gas v i s c o s i t y , Kim and Ross (89) extended the work of Stogryn and H i r s c h - f e l d e r to i n c l u d e the e f f e c t of "quasidimers" i n a d d i t i o n to the bound and metastable dimers. Kim and Ross define d a dimer complex by a s s i g n i n g i t a phase space bounded by L 4 L F $ F F 4 0.8, R 4 R (q) where L = l a r g e s t value of angular momentum f o r which there appears an i n f l e c t i o n p o i n t i n the <j> curve, and R (q) i s the locus of the * r e f f ' m ^ maximum i n t h i s curve. Thus, i n a d d i t i o n t o the dimers considered by Stogryn,and H i r s c h f e l d e r which a r i s e as a r e s u l t of three-or-more-body c o l l i s i o n s , Kim and Ross consider the quasidimer which a r i s e s from two-body c o l l i - s i o n s . This takes i n t o account molecules which approach each other w i t h small r e l a t i v e k i n e t i c energy so t h a t o r b i t i n g occurs i n the v i c i n i t y of the maximum of the e f f e c t i v e p o t e n t i a l . In the case of atomic recombination, Bunker (30) considers t h i s the most important type of c o l l i s i o n l e a d i n g to r e a c t i o n . The e q u i l i b r i u m constant c a l c u l a t e d f o r a l l three regions of phase space (bound, metastable and quasidimers) f o r the case of O^i i s K = 0.079 1. mole '''.Comparing t h i s value w i t h the K 117. obtained from the theory of Stogryn and Hirschfelder, we see that the addition of quasidimers increases the e q u i l i - brium constant considerably, k^ i s now calculated to be 2.3 s e c - 1 and t 1 / 2 = 0.3 seconds for the r a d i a t i v e h a l f - l i f e of the C0 2( 1A g) ) 2 species. Since r e l a t i v e l y l i t t l e i s known about the mechanism of double t r a n s i t i o n s i n molecules, no decision can be made as to which regions of phase space should be con- sidered i n c a l c u l a t i n g the equilibrium constant for the ( 0 2 ) 2 complex. Chemiluminescence From The .C.l.2-H202 System We have assumed that the chemiluminescence observed i n the reaction of chlorine and hydrogen peroxide o r i g i n - ates from excited gaseous oxygen i n the bubbles. This assumption i s reinforced by the observation that the i n - t e n s i t i e s of the (0-0) and (1-0) bands of the —*- 3 Z ~ g g t r a n s i t i o n are i n the same r a t i o as those observed i n atmospheric absorption studies (73). The i n t e n s i t i e s of the (1-0) and (0-0) t r a n s i t i o n s of the main chemilumines- cence bands, however, are roughly equal, as i s the case in compressed (90). This fa c t supports the interpre- t a t i o n of these bands as c o l l i s i o n induced 0^ - 0^ pair t r a n s i t i o n s . Water i s known to have a high e f f i c i e n c y i n deac- t i v a t i n g v i b r a t i o n a l l y excited oxygen (87), and for t h i s 118. 1 3 - reason the high i n t e n s i t y of the (1,0) ( A -—>• E ) band i s surp r i s i n g . I t has not, however, been c l e a r l y established whether water maintains t h i s high e f f i c i e n c y of v i b r a t i o n a l quenching i n excited oxygen molecules. It i s quite l i k e l y that the e f f i c i e n c y of water i n t h i s respect may be due to the s i m i l a r i t y of the v i b r a t i o n a l 3 - -1 frequency of 0_ ( E ) (1580 cm ) and the frequency of the "wagging mode" i n R̂ O (1595 cm "*"). In that case, since the v i b r a t i o n a l frequency of ("'"Ag) i s 1509 cm-"'" and of ("*"£*) i s 1433 cm 1 , the e f f i c i e n c y with which v i b r a t i o n a l energy transfer occurs may be considerably decreased for these excited molecules. There are a number of reasons for believing that the y i e l d of O20~A ) *~s a c t u a l l Y larger than the 10% found experimentally. Since the reaction i s 62 kcal exothermic, the temperature of the gas i n the bubbles i s probably somewhat above room temperature. Direct measurement of the bubble temperature was found to be impossible. How- ever, the "vi b r a t i o n a l temperature" (69, page 203) of the emitting gas can be estimated from the r e l a t i v e i n - t e n s i t y of the emission from v i b r a t i o n a l l y excited states. 1 3 -From the i n t e n s i t y of the ( A ) , — * - ( E ) „ t r a n s i t i o n g v=l g v=0. o at 10670A, an estimated v i b r a t i o n a l temperature of about 600°K can be calculated. Assuming that the true tempera- ture of the gas i s somewhat less than t h i s value, the reported y i e l d can, at most, be low by a factor of 2. 119. A second e f f e c t c o n t r i b u t i n g to the low value f o r the y i e l d could be the d i l u t i o n of the 0„ ("""A ) by Cl„ u 2 g 2 and H 20 vapor, s i n c e the r e a c t i o n i s not i n f i n i t e l y f a s t . Both these e f f e c t s , however, have been minimized by working w i t h cooled s o l u t i o n s where the vapor pressure of water i s low and the bubble temperature i s nearer to 25°C. A l s o , by using a very b a s i c s o l u t i o n the r e a c t i o n r a t e was maximized. Badger, Wright and Whitlock (70) have measured the absolute i n t e n s i t i e s of the d i s c r e t e - l i n e a bsorption o band and the u n d e r l y i n g continuous absorption at 12600A i n oxygen gas at pressures up to 4.3 atmospheres. They concluded t h a t the d i s c r e t e absorption i s a measure of the i n t r i n s i c t r a n s i t i o n p r o b a b i l i t y i n i s o l a t e d molecules, and the continuum a r i s e s from an enhancement of the t r a n - s i t i o n p r o b a b i l i t y i n c o l l i s i o n complexes. By studying the pressure dependence of the i n t e g r a t e d absorption c o e f f i c i e n t s of each of these absorptions, they d e r i v e d an equation f o r the r e c i p r o c a l mean l i f e t i m e of "̂A oxygen molecules subject to decay only by r a d i a t i v e pro- cesses: ( l / x m ) = A = (2.6 x 10" 4) (1 + 3.8P Q + 3.0P C Q + 0.7P N ) (68) where P i s p a r t i a l pressures i n atmospheres. The h a l f l i f e of an i s o l a t e d """A molecule i s then about 45 minutes, which g i n pure oxygen i s reduced to 9.2 minutes at one atmosphere. 120. o This means th a t only 1/5 of the 12600A band i n t e n s i t y i s due to the spontaneous emission process, the remaining i n t e n s i t y a r i s i n g from c o l l i s i o n induced t r a n s i t i o n s . For t h i s reason, i n c a l c u l a t i n g the y i e l d of 0 2 (̂ "A ) t R i - n equation (61) has been increased by a f a c t o r of f i v e . From the observed i n t e n s i t y sequence i n a b s o r p t i o n , Badger et a l p r e d i c t e d t h a t the (0-1) t r a n s i t i o n proba- b i l i t y i n the ( ^ A ^ ) 2 c o l l i s i o n complex would be r e l a t i v e l y l a r g e w h i l e the i n t e n s i t y of the (0-2) and higher l e v e l s would be n e g l i g i b l e . The f a c t t h a t we have not observed the (0-2) t r a n s i t i o n confirms t h i s p r e d i c t i o n . The importance of t h i s r e a c t i o n i s i n i t s a p p l i c a b i l i t y to the i n t e r p r e t a t i o n of many o x i d a t i o n r e a c t i o n s where there i s organic molecule chemiluminescence. The extreme p r o h i b i t i o n of the s i n g l e t •->• t r i p l e t (g*-+-g) t r a n s i t i o n r e s u l t s i n a r e l a t i v e l y high s t a b i l i t y of s i n g l e t e x c i t e d oxygen. For t h i s reason, energy t r a n s f e r processes between e x c i t e d oxygen and acceptor molecules having s u i t a b l e energy l e v e l s occur q u i t e r e a d i l y . I f , f o r example, a small amount of dibenzanthrone i s added to the s o l u t i o n i n which the c h l o r i n e - p e r o x i d e react i o n , i s proceeding, an extremely b r i g h t red chemiluminescence i s seen. Examination of the spectrum shows t h a t i t i s i d e n t i c a l to the fluorescence band of dibenzanthrone. Many other substances are capable of being e x c i t e d i n t h i s r e a c t i o n as was shown by M a l l e t (91), and t h i s i s a m a n i f e s t a t i o n 1 2 1 . o f t h e many e n e r g y l e v e l s i n t h e o x y g e n s y s t e m a v a i l a b l e f o r t r a n s f e r r i n g . e n e r g y . Khan a nd K a s h a ( 9 2 ) h a v e r e c e n t l y o f o u n d a b a n d a t 4780A i n t h i s s y s t e m , w h i c h i s u n d o u b t e d l y due t o t h e (̂ "A , ^E ) c o l l i s i o n c o m p l e x , a nd t h i s c o n f i r m s 9 9 t h e p r e s e n c e o f l e v e l s o f s u f f i c i e n t l y h i g h e n e r g y t o e x - c i t e many l a r g e a r o m a t i c compounds. I t i s p r o b a b l e , t h e r e - f o r e , a s Khan a n d K a s h a h a v e s u g g e s t e d , t h a t i n a n y r e a c t i o n p r o d u c i n g s i n g l e t o x y g e n , c h e m i l u m i n e s c e n c e may o c c u r i f t h e r e e x i s t s a s p e c i e s c a p a b l e o f a c c e p t i n g t h e e n e r g y f r o m t h e o x y g e n . R e c e n t l y , O g r y z l o a n d P e a r s o n ( 9 3 ) h a v e s t u d i e d t h e e x c i t a t i o n o f v i o l a n t h r o n e b y s i n g l e t o x y g e n c o n f i r m i n g t h i s h y p o t h e s i s . T h e s e a u t h o r s h a v e a t t r i b u t e d t h e l u m i n e s c e n c e o f t h e v i o l a n t h r o n e t o t h e f o l l o w i n g r e - a c t i o n s : ° 2 c l A g ) + L Y 0 — ° 2 ( 3 V + \ ( 6 9 ) 0 2 ( 1 A g ) + 3 V ] _ — • 0 2 ( 3 E ~ ) + 1 V 1 ( 7 0 ) 1 V 1 * 1 V ( ) + hv ( 7 1 ) w h e r e ^"VQ i s t h e s i n g l e t g r o u n d s t a t e o f v i o l a n t h r o n e , 3 1 i s t h e l o w e s t t r i p l e t e x c i t e d s t a t e a n d i s t h e l o w e s t s i n g l e t e x c i t e d s t a t e . A P P E N D I X 122. C a l c u l a t i o n Of P o t e n t i a l Energy Curves Accurate p o t e n t i a l energy diagrams were drawn f o r each of the halogens to a s s i s t i n the i n t e r p r e t a t i o n of the emission s p e c t r a . When r o t a t i o n a l data were a v a i l a b l e f o r an e l e c t r o n i c s t a t e , the p o t e n t i a l f u n c t i o n used to c a l c u l a t e the shape of the curve was the H u l b u r t - H i r s c h - f e l d e r p o t e n t i a l (94): V = D [ ( l - e " X ) 2 + c x 3 e " 2 x (1 + bx) ] (72) x = io e 2(B D ) 1 / 2 e r - r e r e where b and c are constants c a l c u l a t e d from spectroscopic data, and the remaining symbols have t h e i r usual spectro- scopic meaning. When the r o t a t i o n a l constants of a s t a t e were not known, a Morse (95) f u n c t i o n was used i n the c a l c u l a t i o n . Computations were c a r r i e d out on the IBM 7044 computer and the output was p l o t t e d from each molecule. Large s c a l e drawings were a l s o made and these were used i n the a c t u a l i n t e r p r e t a t i o n a l work. (a) C h l o r i n e 1 + The ground s t a t e E g curve f o r c h l o r i n e was c a l c u - l a t e d using the H u l b u r t - H i r s c h f e l d e r p o t e n t i a l and the 3 data of Douglas, M i l l e r and S t o i c h e f f (60). The n o + u curve was taken d i r e c t l y from the paper of Todd, Richards and Byrne (96) who used the more accurate R.K.R. method 123. of f i n d i n g the c l a s s i c a l t u r n i n g p o i n t s . Very l i t t l e i s known about the low energy p o r t i o n of the ^CT Û s t a t e and 3 i t s p o s i t i o n and p o i n t of c r o s s i n g of the ^ Q + u curve i s i n d e f i n i t e . F o l l o w i n g Bader and Ogryzlo (34) we have drawn 3 i t to cross the n + s t a t e between the t h i r t e e n t h and o u f o u r t e e n t h v i b r a t i o n a l l e v e l s w The high energy p o r t i o n of t h i s s t a t e has been drawn from the data of Palmer (19). Recently Clyne and Coxon (50) have studied the spectrum of discharged c h l o r i n e and have observed t r a n - s i t i o n s to a l l the low l y i n g v i b r a t i o n a l l e v e l s of the 3 1 II + s t a t e . Since t h e i r c a l c u l a t i o n of co avoids the o u e long e x t r a p o l a t i o n from v' = 6 made by Douglas et a l (60), t h e i r value f o r t h i s constant has been used i n c a l c u l a t i n g the p o s i t i o n of the v i b r a t i o n a l l e v e l s . (b) Bromine Horsley and Barrow (4 6) have r e c e n t l y done a c a r e f u l study of the a b s o r p t i o n spectrum of bromine and have r e - c a l c u l a t e d the s p e c t r o s c o p i c constants f o r the ground "*"E+ 3 and n + s t a t e s . The ground s t a t e p o t e n t i a l curve was o u c drawn using t h e i r data and the H u l b e r t - H i r s c h f e l d e r poten- t i a l . The ^ n o + u curve was taken d i r e c t l y from the paper by Todd, Richards and Byrne (96) who used the more accurate Rydberg-Klein-Rees method (97) t o l o c a t e the t u r n i n g p o i n t s , The lower p o r t i o n of the "*"II, curve has been drawn to ^ l u 3 cross the II + s t a t e between the t h i r d and f o u r t h v i b r a -o u t i o n a l l e v e l s as suggested by B a y l i s s and Rees (53). The 3 shape of the IT^u s t a t e i s l e s s a c c u r a t e l y known because of the d i f f i c u l t i e s i n v o l v e d i n a n a l y z i n g the spectrum. The v i b r a t i o n a l assignment of t h i s s t a t e was f i r s t made by Brown (47) and l a t e r r e v i s e d by Darbyshire (44). The numbering of t h i s s t a t e suggested by Darbyshire has been used, but could be i n e r r o r by ±2 u n i t s . The p o t e n t i a l energy curve of t h i s s t a t e was c a l c u l a t e d using the spec- t r o s c o p i c constants given by Horsley (45) who has s t u d i e d 3 the r o t a t i o n a l f i n e s t r u c t u r e of a number of the II, l u •«—• bands. The upper p o r t i o n s of the curves were e x t r a p o l a t e d from the diagram i n the paper of K i s t i a - kowsky and Sternberg (98). (c) Iodine Much more accurate s p e c t r o s c o p i c data are a v a i l a b l e f o r i o d i n e than the other halogens, so t h a t the p o t e n t i a l energy diagram i s much more p r e c i s e l y known. The ground s t a t e f u n c t i o n f o r was c a l c u l a t e d using the data of 3 Rank and Rao (99) w h i l e the II + curve was taken from o u the paper of S t e i n f e l d , Zare, Jones, Lesk and Klemperer 3 (100) . The II ̂  s t a t e was c a l c u l a t e d using a Morse poten- t i a l , and the data of Mathieson and Rees (101). 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