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Absolute emission intensity studies on the halogen afterglows and excited molecular oxygen Browne, Robert James 1969-12-31

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ABSOLUTE EMISSION INTENSITY STUDIES ON THE HALOGEN AFTERGLOWS AND EXCITED MOLECULAR OXYGEN by ROBERT JAMES BROWNE B.Sc, University Of Western Ontario, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF June BRITISH COLUMBIA 1969 In presenting this thesis in partial fulfil ment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library will make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain will not be allowed without my written permission. Department of The University Of British Columbia Vancouver 8, Canada TABLE OF CONTENTS Abstract List of Tables List of Illustrations Acknowledgements vi PART I: A STUDY OF THE HALOGEN AFTERGLOWS INTRODUCTION 1 Electronic 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 this Investigation 18 EXPERIMENTAL The Flow System 9 Materials 23 Production of the Excited Species Ca) Chlorine and Bromine Atoms 24 (b) Production of Iodine Atoms in a Flow System 26 Atom Detection and Measurement 7 Spectroscopic Measurements (al Equipment 3(b) Measurement of Spectra 35 Cc) Signal Detection 36 (d) Signal Amplification 8 Ce) Calibration of Detectors for Absolute Emission Intensities 9 Cf) Experimental Determination of the Transmission of the Optical System and Detector Sensitivity 44 RESULTS The Bromine Afterglow Spectrum 4 6 Kinetics of the Bromine Afterglow 9 Ca) Dependence of Emission Intensity on [Br] 50 (bl Pressure Dependence of the Emission Intensity 51 Ccl Absolute Rate Constant Measurements 5Emission from Iodine Atom Recombination 3 The Chlorine Afterglow Spectrum 54 Kinetics of the Chlorine Afterglow Emission 56 (a) Dependence of the Emission Intensity on [Cl] 57 (b) Dependence of the Emission Intensity on [Cl-?] 58 Cc) Absolute Rate Constant Measurements 59 Estimation of Error in the Rate Constants 60 page i iii iv TABLE OF CONTENTS (Continued) page DISCUSSION 61 Contribution of Two Body Radiative Recombination to the Halogen Afterglows 62 (a) Chlorine 3 (b) Bromine 6 The Iodine Afterglow 67 The Origin of the Bromine Afterglow 6Kinetics of the Bromine Afterglow 73 Mechanism of the Emission Reaction (a). Formation of Excited States in the Br2 Afterglow 75 (b). Relaxation Processes in the Excited States 78 Origin of the Chlorine Emission 81 Kinetics of the Chlorine Afterglow (a) Order of Emission Intensity with Respect to [Cl] 82 (b) Pressure Dependence of the Emission Intensity 84 Mechanism of the Emission Reaction in Chlorine Ca). Formation of the Emitting State 85 (b) Relaxation of the 3nQ+u State 8 (A) Vibrational Relaxation 9 (ii) Quenching by ICI2J 8(iii). Quenching by Atoms 90 Cc) Participation of Other Electronic States in the Cl2 Afterglow 93 Suggestions for Further Study of Halogen Afterglows 94 PART II: STUDIES ON EXCITED MOLECULAR OXYGEN INTRODUCTION Electronic States of Molecular Oxygen 96 Studies on Excited Molecular Oxygen 9 Purpose of this Investigation 102 EXPERIMENTAL Production of 02( Ag) and 02( Eg) Molecules 103 Measurement of Emission from the Cl2 - H202 System 104 RESULTS Estimation of [02C1^g)J from Absolute Emission 106 Absolute Emission Intensity of the 6340A Band 108 The Chlorine-Hydrogen Peroxide System 109 DISCUSSION Radiative Lifetime of the (02)2 Complex 113 Chemiluminescence from the C12-H202 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 attributed to the Br„ l3II + —and Br0 (3n, —»-2 o u g 2 lu g' transitions. The dependence of the emission intensity on atom concentration was observed to vary between I « [Br]^"^ ± at short wavelengths, and I « iBr]"*"^ ± in the long wave length region of the spectrum. In the pressure range studied CO.5 to 2 torrl, the .intensity was found to be independent of the molecular bromine concentration. By measuring the o o absolute emission intensity between 6000A and 12000A, values of the apparent rate constant, defined as kapp " I/IBrl2[Br2], 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 is discussed and it is suggested that as much as 15% of the total recombination of bromine atoms may be proceeding via excited states. In a similar study of the chlorine afterglow, all of the emission was attributed to the Cl- (3IT + —>- 1Z+) tran-2 o u g sition. A study of the emission intensity in narrow bands revealed that I « ICIJ^*^ ~ at short wavelengths, 3 corresponding to low vibrational levels of the n0+u state, while I = [Cl]1*^ ± at long wavelengths and higher vi brational levels. Similarly, the pressure dependence of the intensity changed from I « IC^J^*^ ± at long wavelengths to I oc [C^]^*^ * at short wavelengths. Absolute emission o intensity measurements were made in the region from 5000A to o 12000A and the rate constants, defined by k = I/IC1] 2[C10] , app 2 were found. Extrapolation of these values of k to zero app 14 2 -2 -1 atom concentration yields k = 1.8 x 10 cc mole sec J • app A mechanism for the formation and relaxation of the excited state is discussed. The study of absolute emission intensities was extended to measurements on excited oxygen molecules in a flow system. o i + 3 _ Observations on the 7619A band of the OJ S —»• Z ) tran-2 g g 1 + -9 -1 sition yielded [0» (. E )] = 1.77 x 10 moles 1. , thereby ^ 9J confirming that this species is a minor constituent in the products of discharged oxygen. Absolute intensity studies on the 6340A and 7030A bands of the (0oC1A ))0 — (0o(3E~))o 2 g z z g z transition indicate that the half life of the (°2 ) ) 2 col~ lision complex is around 0.1 seconds. The emission spectrum produced when CI2 is reacted with H2O2 in solution was observed to originate in various tran-L g sitions involving excited molecular oxygen. A yield of 0-(^"A ) of 10% from this reaction was estimated. iii 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 -Afterglow In The Region 7000A - 8300A Bands In The Br2 Afterglow Spectrum Recorded By The Hilger 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 Afterglow Band Heads Recorded From The Chlorine After glow Spectrum - Dependence Of I. Upon [Cl] - Values Of n In The Expression I, « [Cl] n Values Of m In The Expression 1^ <* IC12J m Values Of k For The Chlorine Afterglow app Emission Table 12 - Bands Observed In The Spectrum Of The Cl2 - H202 System iv 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 potential energy diagram. Bromine potential energy diagram. Iodine potential energy diagram. Schematic diagram of experimental apparatus, Reaction tube used in bromine afterglow studies. Circuit diagram of the detector bridge. Relative response of the RCA 7265 photo-multiplier . Relative response of the RCA 7102 photo-multiplier . Relative response of PbS detector. Recorder trace of the bromine afterglow spectrum. Assignments to the ^0+u lv+ I transition g in the spectrum of discharged bromine. Spectral distribution of the bromine after glow spectrum obtained by PbS detector. True spectral distribution 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 torr. The spectrum of discharged 1^. Iodine afterglow spectrum produced by IC1 + Cl reaction. The change in the spectral distribution of the Cl2 afterglow with atom concentration. The change in spectral distribution of the Cl2 afterglow spectrum with pressure. LIST OF ILLUSTRATIONS (Continued) Figure 20 - True spectral distribution of the chlorine afterglow spectrum. Figure 21 - Plots of log 1^ vs. log [Cl] for a pressure Ca) of 1.70 torr. (b) - Plot of log ( I^dX) vs. log [Cl] for a pressure of 1.70 torr. Figure 22 - Plots of log I. vs. log IC1] for a pressure Ca) of 3.08 torr. (b) - Plot of log C vs* lo<3 £cll for a Pressure of 3.08 torr. Figure 23 - plot of log P vs. log 1^ for bands centred at five wavelengths. Figure 24 - Plot of k vs. 1/[C1J for a pressure of app 1.70 torr. Figure 25 - Plot of k vs. 1/[C1] for a pressure of app 2.33 torr. TOTAL Figure 26 - Plot of k vs. 1/[C1] for five pressures. app _ Figure 27 - Plot of AG' .. vs. v' for the II, state VTI/ 2. XU of Br-. 2 Figure 28 - Plot of k vs. iBr ] for a pressure of 0.92 app torr. Figure 29 - plots of 1/k vs. [BrJ. app 2 Figure 30 - Plot of k vs. IC1] for a pressure of 1.70 torr. TOTAL Figure 31 - Plots of k/k vs. [C1J in the pressure app range 0.83 to 3.08 torr. Figure 32 - pressure dependence of the chlorine afterglow emission intensity. 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 - Effective potential energy curves for the Lennard-Jones C6-12) potential. Vi ACKNOWLEDGEMENTS I would like to express my sincere appreciation to: Dr. E. A. Ogryzlo at whose suggestion this investigation was undertaken and under whose encouragement and guidance this work was carried out, Dr. A. V. Bree and Dr. N. Basco for helpful discussions and suggestions. The National Research Council of Canada for a Bursary and Scholarships during the course of this research, And finally, Mr. Norman Finlayson for his enthusiastic help in writing computer programs, and Miss Thea Alma for her patience in typing and proof-reading the manuscript. To My Aunt Jessie PART ONE STUDY OF THE HALOGEN AFTERGLOWS 1. INTRODUCTION It is a surprising fact that one of the simplest conceivable reactions, the recombination of atoms to form diatomic molecules, is still poorly understood. Not only is the overall mechanism of recombination open to question, but the role played by electronically ex-, cited states is uncertain. It was in an attempt to shed some light on this latter problem that this research was undertaken. The question which we sought to answer was what part in the association process is played by collisions into electronically excited states. The technique which shows the greatest potential for this type of study is the "discharge—flow" method by which atoms and excited molecules can be produced at pressures at which the elementary processes can be observed. Prior to the development of the discharge-flow technique, a number of investigators undertook a study of these pro cesses by discharging gases in static systems. The first report of this type of study was given by Bequerel (1) in 1859 who passed an electric current through oxygen. The development of the electrical discharge method by Wood C2) in 1923 and later by Bonhoeffer (3) made 2. possible a more satisfactory method of studying atom re actions, and a great deal of work on nitrogen atoms was done. Finally the advent of the electrodeless discharge, made possible by the development of high intensity micro wave sources, provided the best source of excited species and this is the method most widely used today. Until recently, the majority of experiments were performed on "active" nitrogen and oxygen and the results were highly qualitative in nature. However, the appli cation of discharge-flow experiments to the study of the upper atmosphere (4) and to certain biophysical phenomena (5), has stimulated a renewed interest in the technique. Recent developments have made possible the study of halogen atoms (6) and also of excited oxygen molecules (7) in flow systems. Many atom reactions have accompanying chemilumines cent emissions, as in the case of the reaction of oxygen atoms with nitric oxide (8), but until recently, most investigations of these emissions involved only a spec troscopic identification of the excited species. Today, however, the development of sensitive detectors in the visible and near infrared regions, together with the availability of a convenient standard for absolute emis sion intensity C9) has made possible a more detailed study of these emissions,and it is now possible to measure the absolute emission intensities from association reactions. In the present work we have attempted such measurements on the discharge afterglow of oxygen and the halogens. Before describing some of the previous work on re combination luminescence, however, we will review briefly our knowledge about the excited states of Cl^, Br2 and I2. Electronic States Of The Halogens Applying the Wigner-Witmer correlation rules (10), the following states can be shown to correlate with two 2 P atoms: XA 3A i-TT XTT 3TT 3TT 1V+ 1V + A, A, n, n, II, n, z , z . g' u' u' g' u' g' g' u' 1 - 3 + ' 3 + 3V-^u' Lu' LMf g * Mulliken (11, 12) suggested that the lowest energy electron configurations of the halogens which lead to a ^"Zg ground state are the following: Cl2 [KKLL(za)2(ya)2(xa)2(wTT)4(vTr)4] Br2 [KKLLMM(za) 2 (ya) 2 (xa) 2 (WTT) 4 (VTT) 4] • I2 iKKLLMMNN(za)2 (ya)2 (xa)2 (WIT)4 (VTT)4] . In order to understand the various electronic tran sitions which the halogens undergo, the coupling of the various angular momenta within the molecule must be understood. The coupling in the halogens is not strictly Hund's case (a) or (c) but falls in between these two classifications. In the excited states,, the tendency is more toward case (c) coupling, especially in the heavier atoms. Moreover, in the excited states of the halogen molecules the energies of dissociation are very small, (D = .55 eV. for I2; .39 eV. for Br2; .23 eV. for Cl2) (13) and are roughly about equal to the doublet separ-2 2 ations between the ^2/2 an<^ Pl/2 sublevels* From these figures we conclude that, especially in the heavier halogens, the interaction between L and S is stronger than the interaction with the internuclear axis. Thus the quantum numbers of the individual atoms L^,S^,J^ and L2,S2,J2 retain their significance to a considerable ex tent in the molecule, and the quantum numbers A (the component of the electronic orbital angular momentum along the internuclear axis) and Z for the molecule as a whole are no longer good quantum numbers. The only electronic quantum number which is significant is fl (ft = |M^ + M2| where M^ and M2 are projections of and J2 on the internuclear axis). Nonetheless, these quantum numbers can be assigned to a state by imagining that the atoms are forced closer together creating a stronger field, breaking down the L,S coupling and thereby ap proaching Hund's case Ca), The convention which has been adopted in naming the electronic states of the halogens utilizes a mixture of case (a) and (c) nomenclature. 3 + For example the II + state would be named 0 if case o u u (c) naming were strictly adhered to. In predicting the possible excited states of the halogens, Mulliken (14) visualized a different type of coupling. Using his scheme, the excited states are ob tained by adding one electron in an "excited orbital" to any of the various states of the ions X2+. Each ex cited orbital has an ionization potential which is smaller than that of any orbital present in the un-excited molecule. In the case of I2, such an orbital * * is the anti-bonding 5 a orbital (referred to as a ) p u which has an ionization potential of the same order of magnitude as the ordinary ionization potential of the molecule. Thus the lowest excited state is obtained by * + adding a o"u electron to the normal state of ^ : I-: a 2 TT4 TT3 a* , lr\ (2) 2 g ugu u 3 Transitions from the ground state of X„ to the II + and 2 o 3 11^ levels of (2) give the well known visible and infra red absorption bonds of the halogens. If we write (2) in a slightly different form where the X2 + core is separated, we get V H2 "VV \ l/23'0*!oflu (3) ( r 2 3 4 2n ' , *, ![ag ^u^.g' ne 3/2] a i2,lu In (3), we have a core with good quantum numbers Sc> A , E , ft and an electron in the excited orbital which c' c c is weakly coupled to the magnetic axis giving a resultant ft = ftc ± 1/2 . This type of coupling is called ft-s or ft,w coupling and is similar to J-j coupling in atoms. The molecule as a whole has A and ft as good quantum numbers while S has lost its significance. The selection rules which govern transitions between states with ft,co coupling are as follows: (1) transitions within the core are the same as if 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 spin-orbit coupling in the core, one might expect it to be modified by a tendency toward atomic spin-orbit (case (c)) coup ling, in which case Ac = A is not well defined and the rules AA = 0, ±1 and AA = Aftc are not strict. Tran sitions from ordinary case' (a) or (b) to ft^oo states are governed by Aft = 0, ± 1, AA = 0, ± 1 with no Aft = AA restriction. From a study of experimental and theoretical values 3 of Av, the intervals between the (0,0) bands of two II systems of each molecule, Mulliken (14,15) concluded that in the electronic levels of the halogen molecules, the coupling is, for the most part, between the A,s and ft,co types, but has strong tendencies, especially in 7. the heavier atoms, toward separate atom case (c) coup ling. On this basis he predicted a number of states in the halogen molecules some of which are shown as dotted curves in the potential energy diagrams (Figures 1,2 and 3). Halogen Atom Recombination Studies The experimental techniques used to produce the high concentrations of atomic species necessary for re combination studies fall into three categories: thermal methods, photochemical dissociation, and electrical discharge. The first studies were conducted by heating the halogens in sealed tubes (16, 17) to approximately 1300°K at which temperature the gases were found to emit visible radiation. The same phenomenon is observed in shock tubes and extensive study of shock heated chlorine and bromine has been made by Palmer (18, 19)f Palmer and Hornig (20) , Britton (21) and Burns and Hornig (22). Direct photolysis of the halogens was used by Rabinowitch and Wood (23) to determine the rates of atom recombination in the presence of many foreign gases. However, the greatest advance in this technique came with the development of flash photolysis. This method is particularly suited to the study of iodine because its large extinction coefficient allows a high Figure 1. Chlorine potential energy diagram. Figure 2. Bromine potential energy diagram. Figure 3. Iodine potential energy diagram. r(A) r (A) 8. initial concentration of atoms to be formed, and the decay can be easily followed photometrically. The numerous publications by Porter (see, for example, reference 24) are evidence of the stimulus which this technique has given to the study of atom recombination. Although electrical discharge techniques were very early shown to be useful for N2 and 02 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 dis charges which utilized external antennae and resonance cavities 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 rapidly on glass and quartz that their study in a dis charge-flow system was impossible. In 1961 Ogryzlo (6) reported that certain 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 easily maintained. Although any non-metal oxy-acid was found to be effective, phosphoric acid seemed to be the best poison for halogen atoms. 9. Rate Studies On Halogen Atom Recombination Since chlorine has a very low extinction coefficient, the technique of flash photolysis is not suitable for studying the kinetics of recombination, and most quanti tative work has been carried out using flow systems. The early work in this field has been reviewed (26) and only the recent work will be considered here. For the termolecular recombination reaction Cl + Cl + M —Cl2 + M (4) 16 6 — 2 — 1 Bader (26) found k£±2 = 2.45 x 10 cm moles sec and 16 6 — 2 — 1 kTT = 0.3 x 10 cm moles sec where k„„ is defined by He M J ~ d[clJ = 2kMlCl]2[M] . • (5) dt For the concurrent wall recombination reaction Cl + wall —»- 1/2 Cl2 + wall (6) he found k =3.9 sec"''" from which the surface recom--5 bmation coefficient was found to be a = 6.81 x 10 These values were later confirmed by Hutton and Wright 16 6 — 2 — 1 (27) whose value of k_,, = 2.0 x 10 cm moles sec CI2 was in good agreement with Bader, but both were almost two orders of magnitude greater than that reported by Linnett and Booth (28). In contrast to the dearth of material available on Cl2 recombination, a great deal of data have been pub lished on Br2 and 1^- The first extensive work on iodine and bromine atom recombination was done by Rabinowitch (29) who derived values of kM for a variety of gases from photostationary~measurements. The development of flash photolysis has made possible the direct 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 in Table 1. The method of flash photolysis uses the absorption of Br2 or I2 in the ground state to measure the concen tration of atoms at any time after the flash. Implicit in this measurement is the assumption that most of the atoms undergo a direct recombination into the ground state. If, 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 in error. Discharge-flow experiments, on the.other hand, do not suffer from this problem because the atom concentration is measured directly. Two principal experimental facts have evolved from studies of the rate of halogen atom recombination: (1) the efficiency of various third bodies varies over a factor of at least 10^ and (2) the rate constants exhibit a negative temperature dependence. Two theo retical approaches have been utilized to explain these experimental findings. The first is the energy transfer mechanism which TABLE 1 RATE OF RECOMBINATION OF IODINE ATOMS AT ROOM TEMPERATURE Diluent Gas k x 10"16 6 -2 cm mole sec -1 Reference He 0.15 24 Ar 0.30 24 °2 0.67 24 co2 1.34 24 Benzene 7.95 24 Toluene 19.3 24 CH3CII2I 26.0 24 Mesitylene 40.2 24 J2 138.0 24 RATE OF RECOMBINATION OF BROMINE ATOMS AT ROOM TEMPERATURE Diluent Gas k x 10"16 6 , -2 cm mole sec -1 Reference He 0.13 22 Ar 0.3 22 N2 0.17 36 co2 0.39 37 °2 0*40 36 Br 13.0 37 11. may be represented by the following equations * X + X ^=i: X2 (7a) X* + M > X2 + M (7b) * X2 is a collision complex with a definite lifetime, and it is supposed that a net recombination occurs if this complex collides during its lifetime with a third body M, transferring to it (with a certain probability P) enough energy so that the molecule X2 cannot subsequently dissociate. The third-order rate constant k for re-r combination is given by kr = PgZK (8) where Z is the collision number for reaction (7b) and K is the equilibrium constant for reaction (7a). g is the electronic degeneracy factor which arises because each 2 of the two ground state {_ ^2/2^ atoms ^as ^ts lowest energy level split into four components by interaction with the other. Of the resulting 16 possible combinations 1 + only one leads to the Eg ground state, so that if no electronic transitions occur, g = 1/16. This theory pre dicts the correct magnitude of k^, but cannot account for the negative temperature dependence unless P is assumed to vary with temperature (30). The alternate mechanism, X + M 5?== XM (9a) XM + X > X2 + M (9b) 12. has also been shown to be in substantial accord with the observations 131,32). For this scheme to predict the correct temperature dependence, the intermediate species XM must be bound, the well depth of the intermolecular potential being of the order of a few kilocalories (33). At present, an unequivocal choice between these two theoretical approaches cannot be made. Studies on the Luminescence from Halogen Atom  Recombination The first studies of emission in the visible from the halogens were reported by Kondratjew and Leipunsky C16). By heating the halogens in quartz tubes to 1300°K they produced an emission which at low pressures con sisted of bands in the long wavelength region and a continuum in the violet. An increase in the pressure was found to eliminate the band structure until, at a few hundred mm. Hg, only the continuum remained. Since emission was found at wavelengths shorter than the con vergence limit, these investigators suggested that two body recombination followed by the immediate emission of a quantum of radiation was the predominant process : CK2P3/2) + CK2P1/2) * Cl* . (10) Photometric measurements of the change of intensity of the continuum with temperature, compared to that pre^ dieted from simple kinetic theory, was considered 13. reasonable confirmation that the two body process was responsible for the continuum. Uchida (17) later studied the halogens by heating the gas in a capillary of 0.5 mm diameter and observing the emission spectra. He found that at long wavelengths the emission was almost exclusively banded and raising the pressure did not eliminate the bands, but caused them to be overwhelmed by the intensity of the continuum. By taking spectra of the emission near the walls and in the middle of the capillary, he discovered that the bands originated near the walls and the continuum pre dominated in the middle of the tube. He suggested that the wall acted as an energy sink, stabilizing 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 intensity of the bands and also of the continuum was proportional to the fourth power of the degree of dissociation. This he took to be proof that atoms in 2 the px/2 state were produced by the process 3ciC2P3/2) * Cl2(1Zg) + CK2P1/2) (11) 3 and then the formation of the n + proceeded via o u c C1C2P3^21 + C1C2P1^21 + wall —- Cl2(3no+u) + wall. (12) Recent investigations of halogen emissions have been conducted in flow systems at low pressures where banded 14. emission predominates. Using a photographic method to determine intensities, Bader and Ogryzlo (34) concluded that the emission intensity varied according to i = k[ci]2rci2] , d3) which is in contradiction to the findings of Uchida. 3 The bands were identified as belonging to the H0+u —^"E+ transition in accord with earlier work, but they noted a change in spectral distribution with pres sure which had not previously been seen, higher pressures favouring the longer wavelength emission. The fact that no transitions above v' = 13 were observed, they took 3 as evidence of the crossing of the RQ+U state by the ^"II^u, leading to the mechanism Cl(2P3/2) + Cl(2P3/2) — Cl2(\u) (14a) C12cllIlu) + Cl(2p3/2) — C12{\+J + C1<2p3/2) (14b) C12(Vujt + Cl2 — C12{\\) + C12 (14c) Cl2(3nQ+u) —* Cl2 + hv (14d) 2 2 and predicting I = k{Cl ( ^2/2^ '•C12-'* Here' tne notation Cl,. (3JI + f represents a vibrationally excited molecule. 2 o u c J Bader and Ogryzlo were, however, unable to decide between this mechanism and the following, which predicts the same kinetic order but does not require the intermediate 15. formation of the ^H^u state: C1(2P3^2) + Cl2 + Cl2 ^—- Cl3 + Cl2 (15a) Cl, + ClC2P./o) Cl,(3n + ) + Cl„ (15b) 3 6/2 2 O U 2 Cl„(3n + ) »• Cl_ + hv . (15c) 2 o u 2 In a similar study of chlorine atom recombination, Hutton and Wright (27) also found the emission intensity to be proportional to the square of the atom concentration and to the first power of the Cl2 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 this con stant may not represent the absolute rate if the emission extends beyond their measureable range. These authors 3 favoured the direct formation of the IT + state from o u 2 2 one ground state ^^/2 an(^ one excited Pi/2 atom' since 2 at 20°C the equilibrium concentration of Cl ( P-j./2^ ^s 2 1% that of ClC P3/2^* Thev therefore proposed the fol lowing mechanism to explain the emission kinetics: CIC2P3/2) + CIC2P1/21 + ci2 — ci2(3no+u) + ci2 (16a) Cl2(3no+u) —* Cl2 + hv (16b) Cl2(3nQ+u) + Cl2 —* Cl2 + Cl2 . (16c) Equation C16c) represents a deactivation of the excited state by Cl2, and was invoked by Hutton and Wright be cause they found that the emission intensity was in dependent of [C12J for pressures above 2 torr. Bader and Ogryzlo, on the other hand, attributed this obser vation to a shift in the emission maximum to longer wavelengths. Much less work has been done on radiative recom bination from bromine largely owing to the greater difficulties involved in atom detection. Using an iso thermal calorimetric detector and a photographic method of determining intensities similar to that employed by Bader and Ogryzlo, Gibbs C35) investigated the bromine afterglow emission. He found that, as in the case of chlorine, the emission intensity was proportional to the square of the atom concentration and the first power of the Br2 concentration: I = k[BrJ2!Br2] . (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 Br2 ( KQ+U —•» £g) transition. At the pressures and temperatures encountered in discharge-flow experiments, the banded emission arising from three-body processes predominates over any con tribution from two-body atom recombinations, which 17. would give rise to continuous emission. At the high pressures and temperatures used in shock tube experi ments, however, the emission spectrum is continuous. A study of the emission from shock heated bromine led Palmer (18) to conclude that two body recombination can 3 3 1 take place into the Br0 ( II, ), Br~ ( n + ) and Br« ( II, ) c 2 lu ' 2 o u 2 lu states via the processes Br(2P3/2) + Br(2P1/2) - Br* (18) 2Br(2P3/2) —*. Br* . (19) In a similar, but more quantitative investigation of the chlorine emission, Palmer (19, 20) suggested a means of calculating the concentrations of excited Cl2 molecules in attractive or repulsive states based on equilibrium statistical mechanics. 18. Purpose Of This Investigation The purpose of this research was to study the luminescence from halogen atom recombination in order to gain a better understanding of the role played by electronically excited states in the total recombination process. To do this, we hoped to be able to: Ca) devise a method of producing iodine atoms in a discharge flow-system and of observing the re combination emission, (b) study the entire afterglow spectra of Br2 and Cl2 and identify the electronic state (s) giving rise to the luminescence. We also hoped to determine the kinetic order of the emission intensity with respect to atom concentration and pressure. Using the 0 + NO glow as a standard of emission intensity, we undertook to determine the absolute rate constants for emission in the halogen afterglows and from these measurements, to estimate the fraction of the total recombination proceeding through electronically excited states. Finally, 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 in the halogens. 19. EXPERIMENTAL The Flow System All discharge-flow experiments were performed using a flow system, part of which is illustrated in figure 4. The apparatus consisted of a purification line, a main flow system, in which the reaction tube was located, and four auxiliary gas. storage bulbs and capillary flow meters connected to the vacuum system at points A, B and C (figure 4). Since it was periodically necessary to remove the reaction tube for cleaning and poisoning, this section of the vacuum system was constructed to allow for its complete removal. A number of different types of reaction tube were used in this work; details of the one used for bromine are shown in figure 5. To prevent light from the dis charge region being reflected (or conducted) by the glass into the observation section, two right-angled light traps were placed at the end of the discharge tube. The outside of these light traps was painted with flat black enamel. Multiple inlet jets A and B were used to introduce NO, N0~ and N0C1 into the main 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 Figure 5. Reaction tube.used in bromine after glow studies. Light Trap Optical Window Gas Flow Titration Jets Water Jacket Detector Coil 20. gas stream and were found to give the best mixing characteristics of any type tested. The section of the reaction tube viewed by the spectrophotometer was covered by an optically flat quartz window. The walls of the vessel surrounding the detector coil were maintained at a constant temperature of 25°C by circulating water from a temperature bath through the water jacket. Although the detector operates iso-thermally, and therefore will not cause a temperature variation of the walls after equilibrium has been ob tained, it was found that relatively small changes in room temperature could cause appreciable deviations in the detector current. This difficulty was overcome by circulating water from a constant temperature bath through a water jacket surrounding the detector coil. • In spectroscopic experiments which required maximum intensity from the afterglows so that small slitwidths could be used, another type of reaction tube was em ployed. This vessel was one inch in diameter, four inches long and was viewed "end on" by the spectro photometer. The walls of this tube were coated with MgO which has 98% reflectivity (38) and which helped to- increase the intensity. This, type of tube was im practical for kinetic measurements, however, because of the decrease in the intensity of the emissions down the length of the tube, particularly at higher pressures. 21. Gas flow rates were measured by observing the pressure difference established across a capillary as the gas flowed through it. This pressure difference was indicated on a U-tube manometer filled with either sulfuric acid or dibutylphthalate, since most of the gases used attacked mercury. Calibration of these flowmeters, in the case of non-condensable gases, consisted of collecting a measured volume of the pump exhaust, noting the time required, and applying the ideal gas laws to calculate the flow rate. The flow rate of nitric oxide was assessed each time it was used by observing the pressure drop in the storage bulb in a fixed interval of time. To measure the flow rates of N0C1 and other condensible gases with vapour pressures greater than one atmosphere, a special calibration bulb was constructed. The volume of this bulb was determined accurately and the pressure of any gas contained in it could be read directly from a mercury manometer. A U-tube filled with dibutylphtha late 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 calculated by trapping out the gas and weighing it in a specially constructed thin-walled weighing vessel. Since no attempt was made to construct the capillaries of the flowmeters in such a way that gas flow through them conformed to the Foiseuille equation (51) , each value of the total system pressure required a separate cali bration of the flowmeters. Gas flows were controlled by either of two types of needle valve. In most cases, Edwards high vacuum needle valves proved to be satisfactory, but when ex posed to the halogens they were found to become cor roded. For this reason, a Fischer-Porter glass and teflon valve was used for bromine, while the tank regulator was found to give sufficiently fine control in the case of chlorine. Total system pressure was measured by either of two tilting McLeod gauges, one containing mercury, the other sulfuric acid. The latter was designed to have a pressure measuring range of 0.1 to 5 torr and was calibrated against the mercury gauges. The total pressure at any flow rate of the gas could be varied by partially closing a 20 mm. stopcock just upstream from the liquid nitrogen cold trap. Originally, a constriction was placed at this point so that the pressure could be varied by changing the flow rate. This procedure was later abandoned because it permitted the use of only one flow rate at a par ticular pressure. 23. Although pumping speed is irrelevant in the case of condensible gases, since the effective pressure at the liquid nitrogen trap is zero, it is important when working with oxygen. Initially a Welsh Duo-Seal model 1397 vacuum pump was used which had a pumping speed of 425 l./min. This was later replaced by two pumps, a Welsh model 1403 and a 1405H, operated in parallel. Both pumps were used only for oxygen studies and during the O + NO calibration. Materials Chlorine from Matheson of Canada Limited (99.5% minimum purity), was used without further purification. Nitrosyl chloride was obtained from Matheson (93% minimum purity) and was further purified by trap to trap distillation using dry ice-acetone and alcohol slush baths and stored in a 22 1. bulb. The infra-red absorption spectra of samples withdrawn from the stor age bulb and from the pump trap showed that little or no decomposition had resulted from passing the gas through the metal needle valves. Nitrogen dioxide was obtained from Matheson Company and traces of the chief impurity, NO, were removed by storing the gas with oxygen until the solid was pure white in colour. The gas was kept trapped down with dry ice and before being used, was expanded into a 2 1. bulb. Nitric oxide was obtained from Matheson and its chief impurity was found to be NO^. Trap to trap dis tillations using liquid nitrogen and an ethanol slush bath were performed until the gas was colourless and the solid was pure white. The nitric oxide was stored, ready for use, in a 2 1. bulb with a mercury manometer attached, the latter serving.to measure the bulb pres sure and scavenge any NC^ formed after purification. Reagent grade bromine from the Baker Chemical Company was first dried over silica gel, thoroughly de-gased by freezing and pumping, and then the middle fraction was slowly distilled into the cold trap of a 22 1. storage bulb. When not in use, the bromine was kept trapped down in a dry ice-acetone bath to avoid con tamination. Matheson extra dry grade oxygen was found to contain little N2, the most objectionable impurity in gas-discharge experiments, and was used directly from the cylinder without further purification. Production of the Excited Species Ca) Chlorine and Bromine Atoms Atoms were formed in 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 fluctuation of line voltage affected the atom concentrations quite marked ly, the microwave generator was run off a 220 V Sola transformer to stabilize the voltage. A h wave dis charge cavity constructed from the specifications of Broida et al C39) was found to be the most satisfactory type of cavity for halogen studies. Adjustments could be made for tuning this cavity to resonance and also for optimizing the reflected power. The increased efficiency thus obtained extended the pressure range over which the discharge could be maintained and, in fact, it was found that oxygen discharges could be sustained at pressures close to one atmosphere. It appears that high concentrations of halogen atoms are obtained when the discharge region is quite hot, so that the highly localized discharge produced by this cavity proved to be advantageous. The discharge tube was constructed of 12 mm. diameter pyrex and cooling was achieved by blowing a stream of air through the inlet provided on the cavity. In order to decrease wall recombination to an acceptable level, the technique first used by Ogryzlo (6) of coating the walls of the reaction vessel with phosphoric acid was employed. In practice only the walls of the discharge region were "poisoned" since this seemed to be sufficient. Coating the walls of the whole system was undesirable since this caused fogging 26. of the optical windows. The surface of the glass was prepared by cleaning it thoroughly with soap and water and then with hot, concentrated NaOH solution before the poison was applied. The tube was then filled with a 20% solution of H^PO^, drained, and then evacuated to remove the excess water. Finally the tube was thoroughly dried by discharging pure argon through it for an hour before any experiments were performed. Cb) Production Of Iodine Atoms In A Flow System In the hope that this study could be extended to an investigation of 1^, some preliminary experiments on the production of iodine atoms in a flow system were undertaken. The main problem encountered was that of obtaining a high enough flow rate of iodine through the system to produce measurable quantities of iodine atoms. Initially, this problem was overcome by heating the iodine crystals in a small glass furnace. This furnace was connected to the discharge tube through a capillary and both the furnace and capillary were wrapped with heating wire and insulating material. The discharge tube was maintained at a high tempera ture by surrounding it with a jacket filled with CCl^ which was heated by the discharge itself. Carbon tetrachloride was used for this purpose since it does not absorb the microwave radiation. A second method used to provide high concentra tions of iodine, and one which was found useful when using 1^ as a titrant in gas reactions, was to pass an inert gas through a vessel of heated iodine crystals. Because the high rate of recombination of iodine atoms caused a rapid decay of intensity down the reac tion tube, observations of the emission were done very close to the discharge region. Two right-angled light traps ensured that light from the discharge did not enter the monochromator slit. The walls of the reaction tube were jacketed so that the entire vessel could be heated and a removable cold trap was placed immediately after the tube. This trap permitted easy removal of the iodine after it had passed through the system. Because of the obvious difficulties entailed in the measurement of iodine flowrates, no attempt was made to perform any quantitative experiments. Since the emission was expected to be in the infrared, the PbS detector was employed for all observations. Atom Detection and Measurement The two most widely used methods for measuring atom concentration in flow systems are chemical titra tion and isothermal-calorimetric detection. In order to utilize a titration procedure a substance must be available which reacts rapidly and stoichiometrically 28. with the atomic species but does not generate a product which reacts with the atoms at a comparable rate. Spealman and Rodebush (40) first suggested the use of NC>2 for the titration of oxygen atoms: 0 + N02 —* NO + 02 0 + NO • N02 + hv . When N02 is added to a stream of oxygen atoms, the characteristic green air afterglow is observed and as more N02 is added, the intensity of this emis sion reaches a maximum (when the flow of N02 is about h of the oxygen atom flow). Addition of more N02 causes the intensity of the glow to diminish and finally ex tinguishes it sharply to within 3 cm. of the gas inlet, the latter distance being governed by the mixing characteristics of the inlet jet. At this point the flow of N02 is equal to the flow of oxygen atoms. A precaution necessary in this procedure is to keep the [0]/[02] ratio small so that no significant pressure change occurs during the titration for oxygen atoms. Kaufman (8) found that efficiency of the discharge changed with total pressure thus causing a change in the oxygen atom flow rate. The use of NOCl as a titrant for chlorine atoms was first suggested by Ogryzlo (6) and was used by Hutton and Wright (271 for atom concentration measure ments . The reaction is N0C1 + Cl *• NO + Cl2. (21) Although this reaction is reasonably fast (41), our preliminary experiments showed that the extinction of the chlorine afterglow did not provide a satisfactory endpoint for the titration. When a photomultiplier was placed 10 cm. downstream from the inlet jet, as suggested by Hutton and Wright, and NOC1 was added until extinction of the chlorine afterglow occurred at this point, analysis of the reaction products showed the presence of considerable NOC1. This fact, and the observation that this procedure gave con sistently higher atom flow rates than indicated by the isothermal detector, leads us to believe that this is not a very satisfactory procedure. For this reason, and also because no satisfactory chemical titration procedure has been found for bromine, the isothermal calorimetric detector was used for all halogen atom measurements. The isothermal calorimetric detector was first described by Ogryzlo (42) who used it in the measure ment of oxygen atoms and later found it to be useful for the halogen atoms (6). The detector, in the form in which it was used for chlorine and bromine atoms, 30. consisted of a helically wound spiral of platinum wire electroplated with nickel. The nickel was plated from a solution of nickel chloride and ammonium chloride using a plating current of about 10 milliamps supplied from a 6 volt storage battery. Halogen atoms recombine on the nickel surface with almost 100% efficiency re leasing an amount of heat equal to the dissociation energy per pair of combining atoms. By controlling the current to the detector one is able to hold the temperature constant so that the detector operates isothermally. The decrease in current necessary to maintain the.detector temperature from the value with no atoms flowing, to that when atoms are recombining on the surface, permits calculation of the atomic flow rate. The galvanometer-potentiometer and associated wiring used for the control and measurement of the detector current is shown in figure 6. The detector forms one arm of a Wheatstone bridge. In operation, the bridge is first balanced with the discharge off, by adjustment of the current and/or the decade resis tance box. The current passing through the detector is found by measuring the potential drop across a 1 ohm precision resistor. The discharge is then ini tiated and the current through the detector is reduced to maintain the detector at its initial temperature. The current is read again and the atom flow rate Figure 6. Circuit 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 ) is the current squared decrease = i^ - ±^ R = resistance of the detector in ohms D = dissociation energy of the bond being formed (cal./mole) 4.18= the electrical power to heat conversion factor (4.18 joules/cal.) The concentration of gas in the system can be cal culated using the ideal gas laws which are assumed to be valid for this work. A number of problems were encountered in using the detector for halogen atom measurements. The most serious problem which arose in the work with chlorine was that when the detector coil was exposed to high atom concentrations, a contamination of the nickel surface occurred after a period of time. This caused a reduction in the recombining efficiency of the coil and was readily recognized by the appearance of the afterglow downstream from the detector. At moderate atom flow rates, this contamination usually required up to two hours before atoms began to sweep past the detector. When this occurred, the detector current reading began rising slowly so that the bridge could no longer be properly balanced and the orange chlorine afterglow was visible downstream from the nickel coil. Since this phenomenon had not been reported in pre vious work using this technique, a systematic search for the source of the trouble was undertaken. First, further purification and drying of the chlorine was carried out with no noticeable effect on the rapidity of contamination. Further evidence that impurities were not causing loss of efficiency was provided by the fact that undischarged chlorine did not cause contamination over a period of six hours. To ensure that the phos phoric acid poisoning was not being transferred to the detector, only the discharge region was coated. The water was then thoroughly removed by baking the tube at 200°C under vacuum for two hours before placing the detector back into the system. This procedure did not produce any change. Finally, a number of different techniques of plating the nickel onto the coil were tried, from using different electroplating currents to deposit the nickel, to heat treating the coil. Again this did not produce any noticeable improvement. Con sequently, measurements had to be taken rapidly, within the lifetime of a given coil. This was feasible because it was observed that the loss of detector efficiency occurred rather suddenly and was easily recognized by a sudden increase in detector current and appearance of the afterglow behind the coil. Although undoubtedly 33. the contamination proceeded at a constant rate, the large area provided by the long coil of nickel wire meant that contamination occurred first at the top of the coil and then proceeded to the bottom. Evidence for this was that initially only the first coil of wire was needed to kill all the atoms, even at high flow rates. The second effect which made the use of this technique possible was that the coil could be re activated in situ merely by applying 1.5-2 amps to the coil for about one minute. At first, the number of atoms being swept by the coil increased sharply and then slowly decreased until no atoms were getting by. The detector current was then reduced to its original value and al lowed to re-equilibrate. Initially a second photomulti-plier was placed downstream of the detector so that any emission in this region could be detected, but since all experiments were performed in the dark, it became obvious that the eye could detect this as quickly as the photomultiplier. In using this technique to find bromine atom con centrations, quite a different problem arose. Because of its higher molecular weight, bromine did not trans fer heat from the coil to the walls of the reaction tube as quickly as did chlorine. As a result the detector operated at a higher temperature, and if high 34. atom flows were used, nickel was found to distill off the coil and onto the walls. This resulted in a con tinuous increase in the detector current but at no time was there any evidence that atoms were being swept by the coil. Further verification of this phenomenon came with the discovery that upon removing the detector and allowing discharged oxygen to flow through the system, a band of violet chemiluminescence on the walls of the reaction tube marked the former position of the detector coil. This glow has recently been identified (43) as the Herzberg bands of 0^ which are produced in the catalyzed recombination of oxygen atoms on metal surfaces. This problem was circumvented by using low Br flow rates and operating the detector at low current values. Spectroscopic Measurements (a) Equipment A Hilger and Watts large aperture prism mono-chromator No. D285 equipped with interchangeable glass and quartz prisms was used for most spectroscopic measurements. This instrument was well suited for the low light intensities encountered in this work because of its f/4.5 aperture, its high transmission, resulting from the use of front surfaced mirrors for internal focusing, and the wide slit widths obtainable (to 1.25 mm.). Although the glass prism and associated calibrated wavelength drum were used for most ex periments, the quartz prism was used in investigations of emissions beyond 2\i because of the higher trans mission of quartz in the infrared. When working in the infrared region, the monochromator was sealed and flooded with dry nitrogen to reduce absorption due to atmospheric CC^ and water. A wavelength drive mechanism utilizing a 1 rpm synchronous motor was added and by means of interchanging gears, any of four drive speeds could be selected. The wavelength calibration of the instrument was checked periodically using the mercury emission' lines from a low pressure mercury lamp or using neon or argon lamps in the infrared. (b) Measurement of Spectra The procedure used to measure accurately band head positions in Cl^ and Br^ emission spectra was as follows. First, a neon spectrum was recorded and measured. The position and wavelength of a number of the bands were then used to compute the linear dis persion curve of the monochromator by fitting a cubic equation to the data. When the emission spectra were o recorded, the 7503A Ne line was superimposed on the recorder trace. The band heads were then measured relative to this line and these measurements were used to find the correct wavelength from the dispersion curve. These calculations were carried out on the IBM 7044 computer and the program also calculated vacuum wave numbers. (c) Signal Detection Since the systems studied emitted radiation over a wide wavelength range, it was necessary to use three different detectors. An RCA 7265 photomultiplier, operated at a voltage of 1750 volts, detected radiation in the visible region. In the near infrared region o o C6800A - 12000A) an RCA 7102 photomultiplier was used, operated at 1250 volts and cooled with liquid nitrogen. Both photomultipliers were adapted to fit into a metal casing on which was mounted a metal dewar. The glass envelope of each phototube came into contact with a brass ring attached to the bottom of this dewar so that efficient cooling could be achieved. The metal casing could be evacuated and the optical window was fitted with a heater to prevent fogging. Because con densation on the resistors and associated wiring was found to cause a sharp increase in the noise level, a heater was placed around the resistor chain and pro vision was made for blowing dry nitrogen through the wiring associated with the phototube. Stabilized D.C. voltages were supplied from an Interstate power supply model 304. The determination of absolute emission intensities requires a high degree of photomultiplier stability and for this reason, careful warmup procedures were followed before each experiment was performed. Both tubes were run at operating voltages for one hour be fore being used. In addition, the RCA 7102 was cooled for one hour before the voltage was applied. Neither tube was exposed to light of high intensity while being used. Using a calibrated lamp and a stable A.C. power supply, the output current of the photomultipliers was found to be a linear function of the exciting illumi nation at intensities similar to those encountered in an experiment. At high intensity levels, the phototubes were observed to Undergo some fatigue after prolonged exposure, but this was not expected to be a problem since the emission intensities observed in an experiment were very low. To check for slight sensitivity changes from day to day, the signal from a tungsten strip lamp operated at set voltage and current was measured. In the medium infrared region (1.2y - 2.6y) emis sion was detected by a lead sulfide photoconductive cell obtained from Infrared Industries Incorporated. The sensitive area of this detector was 2.54 mm. by 5.08 mm. and it was mounted inside a dewar equipped with a sapphire optical window. The detector was 38. operated in series with a 500 KQ. load resistor and voltage was supplied from a 300 v. battery. The cell and associated wiring were mounted inside a grounded brass case. Since optimum values of signal to noise ratio and sensitivity were obtained when the cell was operated at -78°C, the dewar was filled with dry ice and acetone before the voltage was applied. A small, front-surfaced spherical mirror was used to focus the light coming from the exit slit of the monochromator onto the sensitive area of the photocell. The apparatus was carefully shielded to prevent extraneous light from entering the photocell. (dl Signal Amplification Light striking the entrance slit of the mono chromator was chopped at 165 c.p.s. by a rotating toothed wheel so that the output of all the detectors could be amplified as an A.C. signal. A lock-in am plifier constructed by Electronics, Missiles 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 is that radiation of widely varying intensities 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 precision step at tenuators on the amplifier greatly simplified this procedure and made the use of "neutral density" filters unnecessary. 4 Although a gain of 1.2 x 10 was obtainable from the lock-in amplifier, the weak output from the lead sulfide detector necessitated the use of a pre amplifier. A low noise Princeton Applied Research (model CR-4) amplifier having an internal power supply, (a rechargeable nickel-cadmium battery pack) was used in this application. The small physical size of this unit permitted its placement close to the photocell so that short leads could be used. The output of this pre-amplifier was fed into the lock-in amplifier. In some experiments, the simultaneous use of two detector systems was necessary. For this purpose a second phototube, chopper, and frequency sensitive amplifier were used. This amplifier was sensitive to 4 a frequency of 27.5 Hz and had an overall gam of 10 . (e) Calibration of Detectors for Absolute Emission Intensities Because gas phase chemiluminescent reactions are diffuse sources of light, their characterization with respect to absolute quantum yields is difficult. In these systems, the normal calibration procedures 40. applicable to point sources of light cannot be used. Recently, however, Fontijn, Meyer and Schiff (9) have studied the rate of light emission from the reaction of oxygen atoms with NO and have suggested its use as a standard. The kinetics of the overall reaction have been investigated by Kaufman (8) and can be represented by 0 + NO »- N02 + hv (23a) 0 + NO + M • N02 + M (23b) O + N02 ». NO + 02 . (23c) Since reaction (23c) is very much faster than (23a) or (23b) the NO concentration remains essentially constant during the course of the reaction. The pro cess may then be considered simply as a nitric oxide catylized recombination of oxygen atoms. The light intensity is proportional to the product [O][NO] and is independent of the total pressure at pressures above 0.1 torr. The procedure used in finding the rate constant of emission of a reaction is simply to observe the intensity of the 0 + NO glow in the same reaction tube and under the same conditions (ie. spec trometer slitwidth, etc.) as the unknown glow. The use of this procedure makes it unnecessary to know the true spectral distribution of the unknown emission and since both the 0 + NO glow and the unknown glow 41. emit in the same volume, all geometric factors cancel out. Let us suppose that the afterglow under study radiates according to the rate equation I = kQ[B]n. The intensity of this emission in a wavelength inter val AX centred at a wavelength X, is I (Xn) = Fo^lUhko [B]IV = K(VAoW (24) o 1z  o In this equation FQ(X^) is the spectral flux density o (photons/cc/sec/Al and LQ is the total intensity in comparable units, ie. Xmax LQ = \ FQ(X)dX (25) V is the volume of the radiating gas, kQ is the total light production rate constant for the unknown glow and [B] is the measured concentration of the emitting species. The constant K(X) contains all the geometrical factors, the transmission of the observation tube window and the monochromator and the phototube sensi tivity. Since the monochromator is a prism instrument, the constant KCX) would also account for the variation in spectral slit width,AX, with wavelength. Finally, iQ(X^) is the photomultiplier current produced by I (A,), and A is the amplifier attenuation, the latter o 1 o *• 42, being the only parameter varied in the detector system between observation of the 0 + NO "continuum" and the unknown afterglow. A similar equation to (24) can be written for the intensity of the N02 "continuum" in the same wavelength interval AX centred at X-^: IsUl) = F:st*1->AA1ks.[Oj INOJV = K(X1)Agis(X1) (26) _ s where the symbols all have their former meaning, the subscript s denoting the parameters of the "standard" reaction. Dividing equation (1) by (3) we get Wko = Vo^1)Fs(X1)kslO] [NO] (27) Lo [B]n As is (X,) Ls Integrating over all wavelengths between Xmin and Xmax, between which all of the intensity of the unknown emis sion lies, we obtain the following equation for the total light producing rate constant Xmax /Xmax k = k [0] INOJA^ (F (X)i (X) dx/fF (X) dX (28) Xmin Xmin In equation (28) kg and Fg can be found using the data of Fontijn, Meyer and Schiff (9) and all the other values can be found experimentally. 43. If, for example, a monochromator trace is obtained for an unknown emission, the curve is first corrected by taking intensity readings at a number of wavelengths and multiplying each of these by F (X)/i (X). The curve is then re-plotted and the integral in the numerator of equation (281 is evaluated by finding the area under this curve. This was done originally by a procedure of cutting and weighing, or by the use of a planimeter. However the whole calculation was eventually carried out numerically on the IBM 7044 computer. In order to ensure that the reaction tube did not change position relative to the spectrometer slit between calibrations, it was clamped so that if its removal was necessary, it could be relocated in its original position. Before using the 0 + NO calibration procedure, the only change made to the system was to remove the de tector coil since the products of this reaction quickly contaminated the metal surface of the detector. Also, a small coil of silver wire was placed downstream from the reaction tube to destroy the oxygen atoms in the gas stream and thereby prevent ozone from collecting in the liquid nitrogen cold traps. The 0 + NO glow was observed at a total system pressure of 1 torr and at an oxygen flowrate of between 50 and lOOy moles/sec, The gas flows were allowed to equilibrate for an hour before the spectrum was scanned After scanning the spectrum the NO flow was cut off and the N02 titration for oxygen atoms was quickly per formed. This procedure was repeated three or four times and an average of the results was taken. Although the use of this calibration procedure made the determination of the spectral sensitivity of the monochromator-photomultiplier combination un necessary over the region covered by the N02 "con tinuum", this function had to be known for emissions beyond 1.4y. The use of this function then permitted the calculation of the true spectral distribution of such emissions. A lamp of standard spectral irradiance operated from a constant current power supply (the unit was obtained from Electro Optics Associates) was used for this purpose. The lamp was calibrated against a black-body radiation source and covered the region from 0.25 to 2.5y . Cf) Experimental Determination of the Transmission of the Optical System and Detector Sensitivity In order to compare the spectral response of the detectors used in this investigation, relative spectral sensitivity curves were found for each detector. These curves represent not only the response of each detector but also the transmission of the entire optical sys tem including the monochromator and all optical windows used in the course of a normal experiment. They were obtained using the lamp of standard spectral irradiance, described in the previous section, in conjunction with a number of neutral density filters which were needed to reduce the high intensity of the 100 watt quartz-iodine lamp. Figure (7) was obtained using the RCA 7265 photomultiplier at room temperature operated at 1750 volts and using a monochromator slitwidth of 200u. Figure (8) shows the relative spectral sensitivity of the RCA 7102 phototube cooled to liquid nitrogen tem perature and operated at 1250 volts. The spectral response of the pbS cell (cooled to -78°C) is shown in figure 9. Figure 7. Relative response of the RCA 7265 photomultiplier and monochromator (slitwidth 0.2 mm). Figure 8. Relative response of the RCA 7102 photomultiplier and monochromator (slitwidth 0.1 mm). Figure 9. Relative response of PbS detector and monochromator (slitwidth 0.15 mm). RESULTS The Bromine Afterglow Spectrum When a discharge is initiated in gaseous bromine, a faint red glow visible to the eye is produced. The spectrum of this emission was recorded using the Hilger-Watts monochromator and the RCA 7102 photo multiplier cooled to liquid nitrogen temperature. The high rate of bromine atom recombination, resulting in a rapid decay of the afterglow down the length of the reaction tube, limited the pressure range over which the emission could be studied. At pressures above 2 torr, the intensity of the emission was insufficient to allow detection at reasonable slit widths. A typi cal recorder trace of the bromine spectrum measured at a pressure of 0.8 torr is shown in figure 10. Variation of the conditions of pressure and atom concentration under which the spectrum was recorded had a marked effect on the appearance of the bands. Not only was the relative intensity of each band found to be a function of pressure, but in some cases the position of the band appeared to shift. This latter effect was most noticeable in the infrared region of Figure 10. Recorder trace of the bromine after glow spectrum. INTENSITY o § 9 < co m r~ m o H x 3 o o CO O CD the spectrum where the lower dispersion of the mono chromator could cause extensive overlapping of the bands. Thus, before any assignment of these bands was attempted, a high resolution spectrum of the bromine afterglow was examined. This bromine afterglow spectrum was photographed with a Czerny-Turner f/6.3 plane-grating spectrograph o blazed at 7500A on Kodak 1-N plates so that the region o o between 7200A and 8400A was recorded. The spectrum was observed to be quite complex, consisting of a large number of red degraded bands and exhibiting extensive overlap in some regions. The recorded band heads are listed in Table 2 together with the proposed assignments. The values of v , were calculated using 3 the spectroscopic constants of the II^ state given 3 by Darbyshire (4 4) and Horsley (45) and for the RQ+U state, calculations were based on the constants given by Horsley and Barrow (46). The bands have been assigned to the 3n, —*• 1Z* and 3n + —» 1E+ transitions of lu g o u g 3 molecular bromine. The emission from the IT, state lu was observed to originate in vibrational levels higher than v' = 5 while assignments corresponding to v1 = 1 3 and 2 have been made for the II + (fiqure 11) . o u ^ The band positions measured on the spectrophotometer traces are given in Table 3 together with their tentative TABLE 2 MEASUREMENT OF THE BAND HEADS OF THE Br2 AFTERGLOW IN THE REGION 7000 - 8300A V vac. (cm- ) Relative Intensity v» lu - V g V 3n + o u - V g i v" vcalc. (cm-1) 1 vll i V , calc. (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 3IK V 3n + — V lu g o u g 1 r 1 Vvac. Relative v' v" v v' v" v i T^4-^^„ ,• 4... calc. calc. (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) Relative Intensity lu V g 3n + -o u g v' v" vcalc. (cm-1) t V1 V" Vcalc. (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 3JI+ — V o u g v" Vcalc. (cm" ) v1 v" v , calc. (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 transition in the spectrum of dis charged bromine. Full line 0.7 torr, broken line 1.5 torr. t. I V'-l 12 13 14 ' 15 i \ .ft V '' 3TT • —'y+ •V"0/ 14 15 16 17 18 19 7500 8000 9000 10,000 Wavelength (A) TABLE 3 BANDS IN THE Br2 AFTERGLOW SPECTRUM RECORDED BY HILGER MONOCHROMATOR V vac. (cm" ) V' Xu- ly+ g 3n +. o U g V" Vcalc. (cm-1) v" Vcalc. (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» lu g v' 3n + o u — V g • V" calc. Icm"1) V" Vcalc (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« lu g v' • 3n + o u g v" vcalc. Ccro"1) V" Vcalc. (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 (10238)t 1 12 (10254)t TABLE 3 Continued V vac. (cm"1) v' lu — V g V' 3n + o u g i V" Vcalc. CcnT1) v" vcalc. (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 in this region were made by comparing the two spectra. In the more interesting spectral region around 10,000 cm \ a number of intense and well separated peaks are observed but the origin of these bands was more difficult to determine for two reasons. First of all, the linear dispersion of the monochromator is quite low in this region so that measurement of band positions is less accurate than in the higher energy region. Secondly, since absorp tion has never been observed into levels below v1 =6 3 in the JI^U state (44, 47), the spectroscopic constants given by Darbyshire (44) may not be valid for the lowest vibrational levels. For those assignments which are in question, the band positions calculated from the revised constants given by Clyne and Coxon (48) are also shown in Table 3. To find the wavelength range over which the emis sion extended, the RCA 7265 photomultiplier and the Infratron PbS detector were used to find the high and low energy limits respectively. The bromine afterglow intensity was below the limits of the detectors at o o wavelengths less than 6000A and greater than 19000A. The spectral distribution, obtained using the lead sulfide cell, is shown in figure 12. Figure 12. Spectral distribution of the bromine afterglow spectrum obtained by PbS detector. The spectral distribution of the bromine after glow was found to be dependent upon atom concentration and total pressure. Under conditions of low pressure and high atom concentration the maximum intensity ap-o peared at 7700A as shown in figure 13(a). At pressures o above 0.5 torr the maximum intensity appeared at 94 0OA (figure 13(b)). This spectrum was recorded at a total pressure of 1 torr using argon as a diluent and was o unusual in that very little emission below 8 000A was observed. These last two spectra represent the true spectral distribution of the bromine afterglow and were obtained by correcting the original spectra for transmission of the optical system and sensitivity of the photomultiplier. The spectra were then reconstructed point by point on a scale linear in wavelength. Kinetics Of The Bromine Afterglow Before measuring the rate constants for the total emission, a more quantitative study of the variation of spectral distribution with atom concentration and pres sure was undertaken. To do this, the intensities in narrow bands centred at a number of wavelengths across the afterglow spectrum were observed as a func tion of I Br] and {Br0]. At the same time, the dependence Figure 13. True spectral distribution of the bromine afterglow spectrum. Curve (a): pressure = 0.25 torr Curve (b): pressure = 1 torr WAVELENGTH (A) of the integrated emission intensity on atom concen tration and pressure was investigated. (a) Dependence Of Emission Intensity On [Br] At a fixed total pressure, the intensity in a small wavelength interval, 1^, was measured at various atom concentrations. These measurements were repeated at a number of pressures between 0.5 and 2 torr. The results o o o obtained for bands centred at 7200A, 8400A and 10400A are listed in Table 4. The region of the spectrum covered o o by each of these bands is: 7165A < X < 7235A for the 7200A band, 8335A < X < 8465A for the band centred at 8400A, and 10395A < X < 10505A for the 10400A band. If the dependence of 1^ on [Br] is assumed to be of the form 1^ a lBr]n, then a plot of log 1^ against log [Br] (figures 14(a) and 15(a)) will yield a value of the parameter n. When this was done, it was found that 1.2 € n 4 2, and only at the high energy end of the spectrum did n approach a value of 2 (Table 5). The integrated emission intensities (^I^dX) between o o 6800A and 12000A were obtained by scanning the emission spectrum using large monochromator slitwidths to obtain structureless spectra. These spectra were then corrected to obtain the true spectral distributions and the areas under these curves were measured. The dependence of the integrated intensity on atom concentration is shown in Table 4. Plots of log ((i.dX) against [Br] (figures 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 IxdX 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 109 I720Q Ig400 I1Q400 \ IAdX (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 Ix - [Br]n n Pressure (torr) 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 torr. Intensity in arbitrary units. Points in Ca) obtained for bands centred o at the following wavelengths: A 7200A, • 8400A, • 10400A. Figure 15. Plots of Ca) log 1^ vs. log [Br] and (b) log ($1 dX) vs. log [Br] for a pressure of 1.82 torr. Intensity in arbitrary units. Points in Ca) obtained for bands centred o at the following wavelengths: A 7200A, • 8400A, and • 10400A. _i _l _1 100 9-5 90 Log [Br] 51. 15(b) and Table 5) indicate that the dependence of the integrated intensity 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 it was not possible to devise an experiment in which the total pressure was varied while the atom concen tration remained constant, an attempt was made to obtain the variation of 1^ with IB^J by interpolation of the 1^ vs. IBr] data. However, owing to the large amount of scatter in the results, no positive correlation could be made. The variation of the integrated intensity with pres sure was, however, obtained by interpolation, although the data show considerable scatter. At IBr] = 5.0 x 10 "^moles/ cc, these results indicate that I^dX <* [Br,J 0.0 ± 0.2 over the pressure range 0.5 to 1.8 torr. This dependence could not be determined at higher [Br] values because of the limited range of atom concentration over which the data overlapped. (c) Absolute Rate Constant Measurements If the apparent rate constant for the emission is defined as kapp " \otal ' tBr]2[Br2], (29) 52. then equation 28 can be rewritten, for the case of the bromine afterglow, to give: 12000A /l2000A kapp° ksIO:ilNO]ABr2 fFs(-A)iBr2(X)dV fFs(X)dX (30) !Brj2lBr23As ) i (X) J o / o 6000A ' 6000A where all symbols have their former meaning. For each value of iBr] and lBr2J a structureless spectro photometer trace was obtained, using a slitwidth of o o 500 microns, for emission from 6000A to 12000A. Values o of iD (X) were read from these traces at 200A intervals Br2 and the integral in the numerator of equation (30) was evaluated numerically. The values of k found in this 1 app way are listed in Table 6. Since a significant contribution to the total emis-o sion intensity is made by radiation beyond 12000A, the rate constants calculated above will be lower than those calculated for the total emission. To estimate the fraction of the total emission which appears beyond o 12000A, spectra were recorded using the lead sulfide detector, and then corrected to yield the true spectral distribution. However, because of the low intensity levels involved, and since the signal/noise ratio of this detector is far below that of the photomultipliers, spectra at TABLE 6 - VALUES OF k FOR EMISSION BETWEEN 6000A AND 12000A IN THE BROMINE AFTERGLOW app Pressure Flow of Br2 Flow of Br iBr2^ x 1q8 IBr^ x 1q9 k x IO-14 torr 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 Br2 Flow of Br *Br2^ x 10 ^Br^ x 10 k x 10 M»W.«> -»>W«c moles/cc 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 concentration, 13% of the emission was observed o to lie beyond 12000A at 0.28 torr and this value increased to 22% for pressures above 0.5 torr. In view of the fact that emission in the infrared is favoured by low [Br], the rate constants for the total emission are probably in the range of 20 to 30% higher than those listed in Table 6. Emission From Iodine Atom Recombination Two methods of producing iodine atoms in a flow system were used: direct discharge of iodine and the chemical titration of chlorine atoms with IC1. The emission produced in each case was of low intensity and appeared in the region 0.8u to 2.4y. The spectra obtained using the PbS detector are shown in figures 16 and 17. The intensity of the emission was lower than that of the other halogen afterglows because high concentrations of iodine were difficult to obtain in the flow system. The production of excited iodine atoms by the chemical titration procedure occurs via the following steps: ici + CU2P3/2) -* ci2 + K2P3/2) (31) I(2P3/2) + K2P3/2) + M — I* + M . However, in using this method, care had to be taken not Figure 16. The spectrum of discharged DISCHARGED I2 06 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 distribution. ICI + CI WAVELENGTH (microns) 54. to titrate more than a stoichiometric amount of IC1 into the stream of chlorine atoms. This was necessary to en sure that emission arising from the formation of excited ICl did not contribute to the afterglow: I(2P3/2) + ClC2P3/2) + M —• ICl(3ni) + M . (32) Clyne and Coxon (30) studied the emission spectrum of ICl produced in a discharge-flow system, and found a o number of strong transitions below 8000A. The fact that o no emission below 8000A was detected in our experiments was taken to indicate that reaction (32) did not contri bute significantly to the afterglow. Because of the difficulties involved in measuring iodine flow rates and atom concentrations, kinetic 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 in the emission spectrum was charac terized by a large number of red-degraded bands, the position of which was determined by measuring the mono chromator traces. Thirty band heads in the afterglow were measured, and these have all been identified as arising from the 3JJ + —•> transition of molecular chlorine, o u g The calculated band head positions shown in Table 7 are TABLE 7 ND HEADS RECORDED FROM THE CHLORINE AFTERGLOW SPECTRUM wavelength 0 (A) V vac. (cm"1) V. v' u g v" Vcalc. 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 3n 1 + v+ (A) , -1. 1^ 5 calc. (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 distribution changes similar to those exhibited by the bromine afterglow were observed in the chlorine emission. Maintaining the total pressure at a constant value while varying the atom concentration had the effect of shifting the intensity 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 in the red region while lowering the pressure shifted the intensity maximum towards the blue (figure 19). The true spectral distribution of the chlorine after glow spectrum is shown in figure 20. These curves were constructed in a manner similar to those of figure 13 for the bromine emission. In order to rule out the possibility that these in tensity shifts were due to changing discharge temperatures, experiments were performed to investigate the effect of varying the reaction tube temperature on the spectrum. However, no measurable change in intensity distribution 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 in the spectral distribution of the Cl2 afterglow with atom concen tration. Pressure fixed at 1.0 torr. 6000 8000 10,000 Wavelength (A) Figure 19. The change in spectral distribution of the Cl~ afterglow spectrum with -9 pressure. [C1J = 1.2 x 10 mole/cc, , monochromator slitwidth = 0.5 mm, pressures in torr. Figure 20. True spectral distribution of the chlorine afterglow spectrum. T 56. Kinetics Of The Chlorine Afterglow Emission Since the chlorine afterglow emission was found to o o extend over such a wide wavelength range (5000A to 15000A) and because no single detector was suitable for studying the entire spectrum, the emission was studied in three regions. The RCA 7265 photomultiplier was used to study o o the region between 5000A and 6800A (hereafter called the O .0 visible region), while the portion from 6800A to 12000A (infrared region) was covered by the RCA 7102 ptioto-o multiplier. Beyond 12000A, the emission intensity was low, and measurements with the PbS detector showed that this region contributed not more than 5% to the total intensity. In view of the small contribution from this region and because of the low sensitivity of the PbS detector, de-o tailed kinetic studies were not attempted beyond 12000A. The visible and infrared regions were studied at a number of different pressures ranging from 0.83 to 3.08 torr. The limits on the pressure range were imposed by the requirement of only a small pressure gradient down the length of the reaction tube and by the need to operate under conditions where emission intensities were easily measurable. At pressures above 3.5 torr, the intensity became too low for detection, and below 0.83 torr the pressure gradient, as calculated from the Poiseuille equation (51), became sizable. At each value of the total system pressure, the atom concentration was varied by adjusting the microwave power or the cavity resonance. (a) Dependence Of The Emission Intensity On [Cl] The emission intensity in various regions of the chlorine afterglow spectrum was studied as a function of atom concentration at constant total pressure. This was done by isolating five regions of the spectrum, the positions and bandwidths of which are listed 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 intensity measured in each of these bands at various atom concentrations is listed in Table 8 for five values of the pressure. Plotting the logarithm of 1^ against log [Cl] yields a value of n in the expression 1^ <* [Cl]n, and the values of n obtained in this way are listed in Table 9. Figures 21 and 22 show log 1^ vs. log [Cl] plots for two values of the total pressure. From an examination of the values of n listed in Table 9, it appears that the intensity at longer wave lengths depends on the first power of the atom concen tration while that at short wavelengths is proportional to the square of the atom concentration. The integrated emission intensity in the visible and infrared regions was also observed as a function of [Cl] TABLE 8 Ca} DEPENDENCE OF I UPON ICl] . INTENSITY IN ARBITRARY UNITS Pressure ICl] I550Q I \l,dX torr 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 torr ICIJ moles/cc x 10 ^soo I6200 llAdX 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 torr moles/cc x 10 Jo 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 torr ICU 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 IxdX 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 torr. Intensity in arbitrary units. 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 torr. Closed circles for infrared region o o (6800A - 12000A), open circles for visible region (5000A - 6800A). Figure 22Cal. Plots of log 1^ vs. log [Cl] for a pressure of 3.08 torr. Intensity in arbitrary units. Points obtained for bands centred at the following wavelengths: A5500A, A 6200A, • 7000A, B 8 600A, O10600A. Figure 22Cb). Plot of log (Jl^dX) vs. log [Cl] for a pressure of 3.08 torr. Closed circles for infrared region C6800A - 12000A), open circles for visible region (5000A - 6800A). 58. (Table 8 and figures 21 and 22). The visible region was found to exhibit a higher dependence on [Cl] than the infrared region (Table 9). The intensity in this latter portion of the spectrum was found to depend on the first power of the atom concentration at every pressure studied. (b) Dependence Of The Emission Intensity On [C^] Since it was not possible to devise an experiment in which the pressure was varied while the atom concen tration was held constant, the dependence of the intensity on pressure had to be found by interpolation of the 1^ vs. [C1J data. The data obtained in this way exhibit considerable scatter since they are affected by day to day changes in photomultiplier and detector sensitivity. However, values of m in the expression 1^ « [C^]™ have been obtained by plotting log 1^ against log P at con stant [C1J (figure 23), and these are shown in Table 10. These data show that the intensity in the short wave length region of the spectrum is independent of pressure, but the dependence increases with wavelength until the intensity is found to be proportional to [C^]^"^ ~ ^'^ at 10600A. The integrated intensity in the visible region was found to be independent of pressure while that in the infrared region was observed to have small pressure de pendence (figure 23 and Table 10). Figure 23. Plot of log P vs. log 1^ for bands centred at five 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 IxdX 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 kaPP = W/™2'^1' then the rate constants can be evaluated using equation 28. However, because separate measurements have been carried out in the visible and infrared regions of the chlorine afterglow, rate constants corresponding to the VIS emission in 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 results of these measure ments are listed in Table 11. The most important data to be extracted from these results are the rate constants for the total emission. A simple addition of the rate constants in the two regions VIS IR was not possible since both k and k varied with atom c app app concentration. However, it was found that straight lines were obtained by plotting k against 1/[C1] (figures app 24 and 25), and this fact made it possible to obtain rate constants for any value of ICl] by interpolation. Thus, the rate constant for the total emission at any atom con centration could be determined by adding the values of kVIS and kIR found from these plots. The resultant kT0TAL app app ^ app vs. 1/IC1J plots, constructed in this way, are shown in figure 26, The solid lines in this figure represent the VIS IR values of 1/IC1J over which the kapp and kapp data overlap VALUES OF k app TABLE 11 FOR THE CHLORINE AFTERGLOW EMISSION PRESSURE = 0.83 torr Spectral Region Flow of Cl2 u moles/sec Flow of Cl u moles/sec IC12J 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 torr Spectral Region Flow of Cl2 y moles/sec Flow Of Cl y moles/sec IC12J x 10 moles/cc ICl] x 10* gm atom/cc k x 10 12 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 torr Spectral Region Flow of Cl2 u moles/sec Flow of Cl u moles/sec IC12J x 10' moles/cc [Cl] x 10 gm atom/cc k x 10 12 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 torr Spectral Flow of Cl2 Flow of Cl -tcl2J x 1qB tclJ x 1q9 k x 10 12  Re9ion y moles/sec y *oWsec moles/cc 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 torr Spectral Region Flow of Cl2 u moles/sec Flow of Cl u moles/sec IC12J x 10( moles/cc [Cl] x 10* gm atom/cc -12 k x 10 iZ 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 torr. o visible region (5000A 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 torr. O visible region (5000A - 6800A), • infrared region (6800A -o 12000A) , addition of rate con stants for the two regions. TOTAL Figure 26. Plot of k ux^Jj vs. 1/fCl] for five pressures. Pressures are in torr. 60. while the broken lines represent regions obtained by an extrapolation of either set of data. The error in the rate o constants caused by neglecting the emission beyond 12 000A is less than 5%. Estimation Of Error In The Rate Constants In evaluating the emission rate constants for the chlorine and bromine afterglows, the largest source of error is in the value of k , the rate constant for the s standard emission. Schiff et al (9) report this value within an accuracy of 30%. The error in the measurement of atom concentrations is 2% for chlorine atoms and around 5% for bromine atoms. Other errors such as those caused by changes in photomultiplier sensitivity are difficult to evaluate but probably do not contribute more than 5%. Therefore, we estimate that the values of k app for the chlorine afterglow are accurate to within 40% while those for the bromine afterglow are within 50%. DISCUSSION There were two major questions towards which this in vestigation of the emission from halogen atom recombination was directed: (1) what fraction of the total recombination occurs via electronically excited states, and (2) what part do elementary processes such as vibrational relaxation and electronic quenching play in the recombination luminescence. The former question can be answered from a knowledge of the absolute rate constants for the emission without making any assumptions about the mechanism of the reaction. The latter question demands a more detailed knowledge of the kinetics of the emission and of the energetically favourable states which are available for population. The molecule for which the most detailed information about electronic states is available is iodine, but we were unable to make kinetic measurements on the iodine afterglow. On the other hand, our most accurate kinetic results were obtained for chlorine, but relatively little is known about its electronic states. Intermediate between these two cases is bromine, for which two electronically excited states are well known, and for which fairly complete kinetic data has been obtained. 62. Contribution Of Two Body Radiative Recombination  To The Halogen Afterglows Two body recombination into a repulsive state, or the repulsive portion of a bound state, may give rise to con tinuous emission. This process might be expected to be important if the radiative lifetime of such a state is short and emission from bound levels is prevented. The latter situation could arise because of slow termolecular recombination, rapid redissociation, quenching of the bound states or some combination of these conditions. In atom recombination studies at high pressure and temperature such as are produced in shock tube.experiments, continuous emission has been found to predominate over banded emission, the latter arising from three-body re combination. Although in the present work the predominant emission was banded, it was necessary to investigate the possibility that continuous emission could be making a sizable contribution to the total intensity. Since no experimental technique of separating a con tinuum from the banded spectrum could be devised, it was necessary to calculate theoretically the contribution which might be expected. To do this, we have used the expressions derived by Palmer (52) for the population of the repulsive region of an excited state and applied these to the cases of two body radiative recombination in chlorine and bromine. 63. (a) Chlorine The two body process in chlorine may be represented in the following equations: k Cl + Cl -7-^ Cl* (34a) k2 Cl* —Cl2 + hv (34b) I = k,[Cl!] = k3kl IC1]2 = k3K*(r)[C1]2' (35) * The equilibrium constant K (r) relates the concentration * of atoms to the concentration of Cl2 molecules having an internuclear separation from r to r + dr. For each acces sible excited state there will be an equation 35, and the total intensity would then be the sum of the contributions from all of these states. In the case of chlorine, population of the repulsive 3 portion of the ^0+u state and subsequent radiation would o give rise to emission at wavelengths below 4500A. Since the spectrum of discharged chlorine was not observed to o extend below 5000A, we conclude that this state cannot be a source of continuous emission. The only remaining state with a sufficiently short radiative lifetime is the ^11, , lu' for which the lower energy portion of the potential curve is not accurately known. However, based on the potential curve as drawn in figure 1, a rough calculation can be * made. The equilibrium constant K (r), for a homonuclear diatomic molecule, has been given by Palmer: * , , OTr 2 , * . -U*(r)/kT , K (r) = 2JIr tg /gvg„) e " dr . x. y * In this equation, g is the statistical weight of the ex cited state, gx and g are the statistical weights of the * atomic states and U (r) is the energy of the state, at internuclear separation r, above the energy of the free atoms, dr is fixed by the spectral slit width of the mono-chromator and is determined from the potential energy * diagram as is U (r). To determine, for example, the rate o of two body emission in a band at 6500A and for a spectral o slit width of 120A, the following 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 for the radiative lifetime of the ^11, state lu * -1-1 of C±2, we calculate k^K (r) = 3.2 cc mole sec . This is equivalent to a. termolecular reaction with k = 2.0 x 10 2 -2 -1 cc mole sec at a pressure of 3.03 torr, and this is several orders of magnitude smaller than the rate constant for the experimentally observed process (typically around 10 2 -2 -1 ° ° 10 cc mole sec in a 120A band centred at 6500A). o * Similarly, for emission at 7500A, K (r)k3/[Cl2] = 7.4 x 10 2 -2 -1 cc moles sec . From these calculations, we conclude that the contribution to the total emission intensity from two^body radiative processes is negligible. It should be pointed out, however, that the values obtained for K (r) are very sensitive to changes in the shape and position of the C^H^ ) state. Since the low energy portion of this state has never been directly observed, its position 3 and point of crossing of the no+u state are the subject of some conjecture. We have drawn the ^"n^ to cross the 3 n + between the thirteenth and fourteenth vibrational o u levels, following Bader and Ogryzlo (34) who observed no emission originating in levels greater than v1 = 13 in the spectrum of discharged C^. It was therefore postulated that the levels above the thirteenth were being predis-sociated by a crossing state, assumed to be the "^n^u. Bader and Ogryzlo's argument in favour of locating the 1II^u, based on a perturbation of the rotational levels at v' => 14, has largely been discounted (50) by the fact that such discontinuities in the Br values are probably ex perimental in origin. If the ^H^u were relocated to cross 3 the II + at around v' =8, the calculated contribution to o u ' the total intensity from the continuum becomes sizable. o At 7500A for example, Icont/Ibands = 0.72. Although measuring an underlying continuum presents many experimental difficulties, it would be expected that a continuum of this intensity could be observed, especially at longer wave lengths where band overlap is minimal. For this reason we conclude that the ^n^u state must cross above the eighth 3 vibrational level of the n + state. o u (b) Bromine Similar considerations apply in 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 lu emission would be detected at wavelengths shorter than o o 5100A and 6350A respectively. Since the observed intensity o is effectively zero at 6000A, contributions from these sources must be negligible. Therefore, continuous emission is possible only from the repulsive "'"n^ state. The radia tive lifetime of this state has not been measured, but since we are concerned here with estimating the maximum possible radiation from two-body recombination, we will — 6 place a lower limit of 10 sec. on its value. For a * pressure of 0.92 torr, we calculate K (r)k3/[Br2J = 11 2-2-1 ° 2 x 10 cc mole sec for emission at 8500A. This latter rate constant is 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 position of the "^n^u state is more firmly established in the case of bromine (53) than chlorine, and in view of the fact that we have taken a lower limit for the lifetime of the ^H^u state, we can be more confident here that any underlying continuum is contributing a negligible amount to the total intensity. The Iodine Afterglow In view of the known and calculated potential energy curves shown in figure 3, the iodine afterglow most likely 3 originates from the n^u state of I2. The energy difference 2 2 -1 between the ?2./2 anc^ P3/2 states ^n iodine is 7616 cm . This splitting is far too large to allow for any population 3 of the II + state, either by thermal excitation, or in-o u •* verse predissociation via one of the repulsive states 2 correlating with ground state ^3/2 atoins« Also, the short 3 -7 radiative lifetime of the II + state (7 x 10 sec) o u (54) precludes its origin in the discharge. Thus we would expect to see only the 3n ^ state, and the fact that the afterglow extends from 0.8y to beyond 2.3y (figures 16 and 17) substantiates this assignment. The maximum of the emission appears to be at 1.25y which corresponds to a vertical transition from v1 =3 to v." =19. The Origin Of The Bromine Afterglow Gibbs and Ogryzlo (49) have measured some of the band heads in the spectrum of discharged bromine between o o 6200A and 8300A in the pressure range of 0.5 to 2.5 torr, They identified four series of bands originating in the 3 0, 1, 2, 3 levels of the II + state and in addition, o u mentioned several bands appearing further into the red which they were unable to identify. 68. o In the present study of the emission between 6850A o and 12000A we have identified bands originating in the 3 0, 1, 2 levels of the II + states, but not from v' = 3. ' ' o u ' A large number of bands in this region, however, have 3 1 + been assigned to the II, —*• £ transition with most of . lu g the emission arising from v' = 5 to v' = 14 of the ex cited state. Identification of the transitions giving rise to the o bands beyond 9000A poses a special problem. It has already been mentioned that these bands are probably the result of the overlapping of a number of transitions. Since the resolution of the monochromator is quite low in this re gion, the contributing bands cannot be separated. In a recent study of the rotational fine structure of nine 1 + 3 bands in the E —n^u system of bromine (in absorption) , Horsley (45) calculated the equilibrium internuclear dis-3 ° tance for the n^u state to be 2.55A. This value is some what lower than has been predicted (44) and leads to a further complication in the identification of the infrared bands in the emission spectrum. The predicted Franck-Condon maximum for emission from the v' =0 level of the 3 H^u state, based on this new value of the internuclear distance, is at 10700 cm"1 (0-12), while emission from the 3 v1 - 0 level of the II + is expected to be a maximum at o u 10400 cm"1 (0-17). Spectra recorded at high pressure have o a maximum intensity at around 10,500A as would be expected if vibrational relaxation were populating the zero vibra tional level of the excited states. However, because of the similarity of the Franck-Condon factors in this region, it is difficult to determine whether or not emission from one state is predominant. The fact that the vibrational 3 level spacing of the n^u state below v' = 8 is uncertain makes assignments in this region even more difficult. Decreasing the total pressure and increasing the atom concentration shifts the maximum emission intensity to higher energy. This is consistent with a decrease in vibrational relaxation causing emission predominantly from higher vibrational levels. At the lowest pressure at which the afterglow spectrum was recorded (0.3 torr), the true spectral distribution (figure 13) shows the maximum in tensity occurs at around 13050 cm"''". In this region, 3 emission from the n + state originates in the first and o u second vibrational levels. The most intense band from v' = 2 is predicted to be the (2, 10) transition which occurs at 13051 cm"1. It thus appears that at low pressures 3 the population of the ^Q+u maY be approaching an initial distribution, if as Gibbs and Ogryzlo (49) suggest, the recombination into this state occurs via the "^n^u state. 3 This state is assumed to intersect the II + potential o u c curve in the location of the third vibrational level (53), so that recombination into the repulsive "^n^u state, followed by a collision induced crossing to the bound 70. 3 II + state, would favour formation into the second and o u ' third vibrational levels. However, just how large a con tribution to the total radiative recombination scheme is 3 made by formation into the II + state is uncertain, since J o u ' we cannot estimate the fraction of the total emission originating in this state. In a study of the spectrum of discharged bromine carried out concurrently with the present work, Clyne and Coxon (4 8) identified a large number of bands, most of which they assigned to the 3H^u »• ^E* transition. We have observed many of the same bands reported by these authors. However, our results differ in the assignment of 3 bands to the n + state. Clyne and Coxon have identified o u J the following series: 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 relatively weak since they occur at significantly higher energies than the predicted Franck-Condon maxima for each level. However, no assignments to these levels have been made in the spectral regions where the maximum intensity would be expected. o Clyne and Coxon found that many bands beyond 8 0 00A could not be reconciled with Darbyshire1s (44) vibrational analysis of the "extreme red" (3JT —*. 1E+) system and its infrared extension. Beyond 8000A marked deviations between the observed and predicted wave numbers began to appear, and this forced them to conclude that either Darbyshire's vibrational assignments were in error, or a new band system was beginning to appear in this region. In earlier studies of the absorption spectrum of bromine, Darbyshire (44) and Brown (47) did not observe transitions below v' = 6 and, thus, a rather long extrapolation was necessary to obtain the value of u' . Because reasonable doubt as to the position and spacing of the low vibrational levels of this state does exist, Clyne and Coxon (48) chose to interpret system. This required a modification of Darbyshire's original analysis which involved reassigning three bands and dropping five others, allowing a smooth continuation from the bands observed in absorption to Clyne's new bands. The new spectroscopic constants were then calculated for 3 -1 the II ^ state yielding u)^ = 153 ± 2 cm which is much lower than the original 170.7 cm 1 calculated by Darbyshire. Our observations on the infrared bands suggest that this procedure is justified, but there are a number of in consistencies in the work of Clyne and Coxon. Calculations of the vibrational energy levels using these new constants are in fair agreement with the observed transitions up to around the tenth vibrational level, but for higher levels deviations become quite large. The (17-3) transition, for e ° 3 the bands beyond 8 000A as an extension of the II lu 72. example, is calculated to appear at 14635 cm 1 while it actually is located (45) at 14559 cm-''". The cause of this discrepancy can be understood by plotting the values of AG*(v + 1/2) against (v1 + 1/2) ie. the vibrational energy spacing vs. vibrational number (figure 27). Using Clyne's results together with those of Darbyshire (44) and Brown (47), this curve exhibits a region of positive curvature at high vibrational numbers, a sharp point of inflection at v1 = 9, followed by a region of slight negative cur vature to v1 = 0. To obtain constants which would be consistent with this type.of curve, the.data would have to be fitted to an equation containing higher powers of (v1 + 1/2) , ie. a) x and u) y should have been calculated. ' ' e e eJ e For this reason Clyne's spectroscopic constants are valid over the range v' = 0 to v' = 8 only. In the spectral region observed photographically, the 3 lowest level of the Jl, observed was the fifth level, so lu ' we were unable to confirm Clyne's assignments for the low vibrational levels of this state. The bands which were assigned to the fifth and sixth levels however, did not agree well with either^ Darbyshire's or Clyne's scheme. In summary, the emission spectrum of discharged bromine is very complex consisting of a large number of diffuse red-degraded bands. The spectrum does show the following features. Figure 27. Plot of ^G\+1/f2 vs* v' for the 3jIlu state of Br2. Open circles are values of Darbyshire (44) and Brown (47) and closed circles are data of Clyne and Coxon (48) . 73. 3 (a) Most of the bands identified arise from the — 1II+ transition. g 3 (b) Emission from the 0, 1, 2 levels of the no+u nas been observed. High pressures favour emission from the zeroth level, low pressures the second vibra tional level. Kinetics Of The Bromine Afterglow Using a photographic technique to measure emission intensity, Gibbs and Ogryzlo (49) studied the dependence of the bromine afterglow intensity on atom concentration and pressure. Fitting their data to the equation I = k[BrJn[Br2Jm , they obtained n = 1.9 ± 0.2 and m = 0.8 ± 0.2 and concluded that within experimental error, I = k[BrJ2!Br2J . We have determined the value of the light emission 2 rate constant defined by k = I/[Br] [Br0], by measuring app 2 the absolute emission intensity of the bromine afterglow. If the kinetic order is that predicted by Gibbs and Ogryzlo, then k should be constant at all values of atom concen-app tration and pressure. This has not been observed to be the case, as is illustrated in figure 28. This variation of k can be understood by examining app J ^ o o the dependence of (.I^dA (6000A to 12000A) on atom con-2 Figure 28. Plot of kapp vs. IBr] for a pressure of 0.92 torr. » centration, as shown in Table 8. The integrated emission 12 + 02 intensity was found to vary as [Br] " " ' at 0.52 torr, and as [Br]1*8 ~ at 1.82 torr, 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 is second order in atom concentration, the fact that less than a second order dependence was observed for the overall emission process suggests that"bromine atoms are involved in quenching the luminescence. Therefore, in order to obtain the rate of emission in the absence of atom quenching, we must evaluate k at very low [Br]. app We have plotted 1/k against [Br] in figure 29 and ex-^"PP trapolated to zero atom concentration. The most reliable intercepts will be obtained from figure 29 (b) since the atom concentration range of these data is 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 torr, 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 overall rate of recombination using Br2 as a third body (371, we find that between 3 and 15% of the total recombination takes place into electronically excited states. In view of our study of the emission intensity 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 torr Cb) A 0.53 torr and • 1.50 torr, (Table 5), we can now suggest a possible explanation for the results obtained by Gibbs and Ogryzlo. The technique used by these authors to measure emission intensity in volved photographing the afterglow, and then relating the optical density of the film to intensity. The photographic o film used was sensitive only to 8800A, so that the long wavelength radiation was not recorded. Reference to Table 5 shows that it was at these shorter wavelengths that close to a second order dependence on [Br] was observed, and this 2 led the authors to suggest that I a [Br] . Our results on the pressure dependence of the inte grated intensity indicate that some process involving bromine molecules is also quenching the luminescence. Although these data are subject to considerable error, they show a less than first order dependence of the rate of emission on the pressure. Mechanism Of The Emission Reaction (a) Formation Of Excited States In The Br2 Afterglow Unlike the iodine afterglow, in which all the emission 3 originates in the II^u state, two electronically excited states are involved in the bromine emission. These states 3 2 are the nlu, which correlates with two ground state. 3 atoms, and the II + , which correlates with one ground o u 3 2 2 state P3/2 and one excite<^ Pi/2 atom (figure 2). 3 The direct formation of the II + state from one ex-o u cited and one ground state atom does not appear to be a favourable process because of the low concentration of 2 P./9 atoms in the gas stream. The equilibrium concen-1/2 tration of the excited atom is expected to be very small C48) ^2pi/2J/l2p3/2J ^ 1 x 10~8 at 300°K) due to the 2 2 large energy difference between the ^2/2 and Pl/2 atomic states. Recent flash photolysis studies (55) have shown that molecular bromine is very efficient at causing spin-2 orbit relaxation of p^/2 atoms. For this reason, excited atoms formed in the discharge would be extremely short lived and could not be considered as a possible source 3 of the II + state, o u Gibbs and Ogryzlo (49) suggested an alternative 3 mechanism by which the ^-0+u state could be populated. This involved the formation of an intermediate state, cor relating with ground state atoms, which could then undergo a collision-induced transition into the emitting state. The two possible electronic states which could fulfill 3 this role are the n - which is predicted to intersect o u 3 the II + at small values of internuclear distance (34), o u ' and the , which is assumed to cross at about the third 3 vibrational level of the n0+u (53)• °ur observations on the afterglow spectrum indicate that the population of the 3 . IIo+u state takes place into the second or third vibrational levels, since at low pressures the maximum emission origi nates from v' =2, This evidence, together with the fact that emission from levels higher than v' = 3 has never been observed, suggests that the ^n^u is the intermediate 3 3 state in 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 vibrational levels at, or below, the dissociation limit. Therefore, we propose the following mechanism for 3 the formation of the n + state: o u BrC2P3/2) + Brt2P3/2) — Br2 (\u) (36) Br2Clniu> + Br2 — B^2(3Vu)v'=3 + Br2 • {37) Any alternative mechanism, such as one involving Br^ or a simple termolecular collision, would be expected to 3 populate chiefly the zeroth vibrational level of the II + c J on state, since this is the only level which lies below the dissociation limit of ground state atoms. 3 The formation of the II + state in the third vibra-o u tional level (equation 37) would involve a fairly large activation energy since this level lies 430 cm-1 above the energy of two ground state atoms. The formation of the 3 Hlu state, on the other hand, should involve no activation energy. Br(2P3/2) + Br(2P3^2) + Br2 —>• Br2 (3niu) + Br2 (38) Since it has not been possible to separate the con tributions to the total emission from each of these states, it is not possible to state unequivocally which is predominant, However, considering the activation energy required to form 78, 3 the IT + state, and the large number of bands which we have o u . 3 3 assigned to the 1T^U state, it would appear that the latter is more important in the recombination process. (b) Relaxation Processes In The Excited States 3 Following the formation of the IT + state, the 3 o u molecule may radiate, redissociate, or be vibrationally relaxed to the zeroth and first vibrational levels: Br-C3n + ),-,+ Br0 Br„(3n + ) , n + Br„ (39) 2 o u v*=3 2 2 o u v'=0 2 Br2(3nQ+u) + Br2 ». 2Br + Br2 (40) Br0(3n + ) • Br0(1Z+) + hv (41) 2 o u 2 g v ' Vibrational relaxation would most probably occur one quantum at a time and thus equation (39) represents the results of a number of collisions. Since we observe a less than first order pressure dependence for the total emission, some process involving Br2 must be removing excited molecules. 3 In the ^0+u, this would most probably occur through the dissociation shown in equation (40), this being the reverse of the formation process. 3 There may be another process by which the ^0+u state is relaxed and that is by a collision induced crossing to 3 the. n^^. If quenching by Br2 is effective in the upper state, there is no reason for assuming that deactivation directly to the ground state occurs. In fact, Franck-Condon factors and symmetry considerations (56) would probably favour a conversion to another component of the 3 II state. Thus, we include the following reaction: Br2(3noV + Br2 — Br2(3lIlu) + Br2 • (42) The radiative lifetimes of these two excited states of bromine have not been measured. We assume, however, that 3 the radiative lifetime of the n + state of bromine lies o u between the lifetimes of the equivalent states in iodine and chlorine, or around 10"^sec. On the other hand, cal culations based on the theoretical work of Mulliken (15) 3 -4 indicate a radiative lifetime of the 11^ state of 10 sec. It would be expected that processes leading to the de activation of the excited states would be more effective 3 for the II^u state, since these processes would be com peting with spontaneous emission. We propose that the 3 relaxation of the n^u state is the result of the following reactions: Br2C3nlul + Br —•* Br2(1Eg) + Br (43) Br2c3jIlu> + Br2 + Br2 (44) Br2C3niu). —* Br2(1Eg) + hv . (45) 3 Throughout the n^u electronic state, the energy interval between adjacent vibrational levels is less than kT at room temperature (44). It is known theoretically that when energy levels are this closely spaced, the probability of collision-induced transitions between them becomes very 80. high (57). Excited iodine molecules, which have vibrational spacings very similar to these excited bromine molecules, were found to undergo vibrational transitions at practically every collision (58). Thus, rapid vibrational relaxation of this state of bromine would be expected, and Tiffany (59) has suggested that no more than 100 collisions would be necessary to reach vibrational and rotation equilibrium. This is consistent with our observation that at high pres-o sures, the maximum emission occurs at 9400A. Although we have assigned some of the bands in this region to the (0,v") transitions of the 3n + —*• system, this ' o u g * ' wavelength also corresponds to the predicted Franck-Condon 3 maximum from the zeroth level of the IK state, which is lu ' undoubtedly contributing to the total intensity. The measurement of the variation of 1/k with atom app concentration and pressure is consistent with the above mechanism. Although the data contain a large amount of scatter, they do show three trends. First of all, the value of k , the rate constant for the total emission, app decreases with increasing atom concentration indicating that quenching by atoms does occur. Secondly, the slope of the 1/k vs. iBrJ plots (figure 29) increases with in-creasing pressure. This is explained qualitatively by a 3 3 collision induced crossing from the n + to the Jl, v o u lu (equation 42)., followed by a quenching by atoms which is expected to be more efficient in the latter state. The 81. third observation which we can make about these 1/k app vs. [Br] plots is that they are curved and that they ap pear to flatten out at high values of [Br]. This flattening out would correspond to a "steady state" being reached 3 between the collision induced crossing to the II^ state and the removal from this state by atomic quenching. Origin Of The Chlorine Emission All of the emission arising from the recombination 3 1 + of chlorine atoms has been assigned to the II + —*• Z 3 o u g transition. The fact that no bands in the emission spectrum could be attributed to the —*- transition, is con-lu g v sistent with Mulliken's (14) theoretical prediction, and also with recent absorption studies (60). The observed change in spectral distribution with pressure can be attributed to vibrational relaxation in the emitting state caused by collisions with chlorine molecules. Thus, at low pressures, the emission maximum o was observed at 67 00A, corresponding to radiation from 3 the fourth and fifth vibrational levels of the II + . At o u higher pressures, this maximum was observed to shift until at 3 torr, emission from the zeroth and first vibrational levels predominated. 82. Kinetics Of The Chlorine Afterglow (a) Order Of Emission Intensity With Respect To [Cl] In previous investigations of chlorine afterglow (27, 34), the emission intensity was found to vary as the square of the atom concentration and the first power of the third body concentration. For Cl2 as a third body, the rate equation then becomes I = klClJ2!Cl2J . (46) We have determined the value of the light emission rate constant, as defined in equation (46) , for a number o of values of [C12J and [Cl] over the range 5000A to ° 2 12000A. When this rate constant was plotted against [Cl] , as in figure 30, its value was observed to increase 2 sharply at low values of [C1J . Since the formation of the excited state must proceed via a termolecular reaction, this apparent change in order with respect to chlorine atom concentration indicates a quenching of the lumines cence by atoms. Therefore, to find the rate of recombination 3 into the II + state, we must determine the value of the o u ' emission rate constant at very low atom concentration. In TOTAL figure 31 we have constructed plots of 1/k against ^PP IC1J, using the data of figure 26. By extrapolating these curves to zero atom concentration, the average value of (at [C1J = 0) was found to be 1.8 x 1014cm6mole-2 aPP r-l sec . Compared to the total rate of recombination, 2 Figure 30. Plot of k vs. [C1J for a pressure app v of 1.70 torr. TOTAT. Figure 31. plots of 1/k u vs. [Cl] in the app pressure range 0.83 to 3.08 torr. Broken curves obtained by extrapol ation. 83. k = 2.0 x lO^cm^mole^sec 1 (27), this value indicates that 1% of the total recombination takes place into the 3 II + state, o u The error made by previous investigators in deter mining the order of the emission intensity with respect to [Cl] can be attributed to the fact that only the short wavelength region of the afterglow spectrum was studied. We have found a large variation in the dependence of emission intensity on atom concentration over the entire emitting region (see Table 9). At a pressure of 1.7 0 torr, for example, this dependence decreased from I a [Cl] "u _ ' at 5500A to I « [Cl]u,y " u*1 at 10600A. In their study of the chlorine afterglow, Bader and Ogryzlo (34) measured intensity using a photographic technique by relating the optical density of the film to emission intensity. Although infrared-sensitive film was. o used, and a filter eliminated radiation below 6250A, it is possible that much of the emission recorded lay below o 7000A. In that case, the infrared portion would not have contributed to the intensity and a second order dependence would have been observed. Our calculation of the. integrated o o emission intensity in the visible region (5000A - 6800A) yielded I <= [Cl]1,8 * 0,1 while Bader's (26) results indicated I <* [Cl]1"9 ± <^'1. The same explanation could apply to the Work.of Hutton and Wright (27) who used a photomultiplier having its maximum sensitivity in the visible region, 84. (b) Pressure Dependence Of The Emission Intensity On the question of the dependence of I upon [Cl2] , there appears to be a discrepancy between the results of Bader and Ogryzlo and Hutton and Wright. The former workers claim a first power dependence, but their determination was based on experiments performed at only two pressures and no results were obtained at pressures greater than 1.6 torr. Hutton and Wright concluded that the emission intensity was dependent on the molecular chlorine concen tration below 2 torr, but above this pressure was in dependent of [Cl23• ,If we assume that in the wavelength range over which Hutton and Wright's intensity measurements were made 2 I « ICl] , we can compare our values of intensity at 2 fixed atom concentration with their values of I/[C1] . This has been done in the logarithmic plot of figure 32, where the slope of these curves corresponds to m in the equation I = klCl]nlCl2Jm. Our results are in essential agreement with those of Hutton and Wright, since they indicate a change in kinetic order around 2 torr and show a less than first order dependence below this pressure. The slope of our plots at IC12J = 1 torr gives I <* IC^J^*^ ~ while Hutton and Wright's results yield 0 4 I a ICl,,] * . This discrepancy can be attributed to the fact that these workers measured the emission intensity 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 intensity. Results of Hutton and Wright (A) plotted as Irel/{C1J2 vs. P; this work (9 and O ) plotted as Irej_ vs. P. Pressure (torr) 85. o intensity values include all the emission from 5000A to o 12000A and are thus expected to indicate a higher overall order in molecular chlorine concentration. In a preliminary study of the chlorine afterglow emission carried out at the same time as this work, Clyne and Steadman (61) observed a similar behaviour of the in tensity with pressure. The intensity in a narrow wavelength o band at 5800A was found to be pressure independent above o 1 torr while the intensity in a band at 7800A was found to be dependend on [C^J • These workers were not able to measure total intensities owing to the low sensitivity of their photomultiplier at long wavelengths, but they sug gested that the total intensity could be proportional to IMJ in the pressure range studied. This we have not found to be the case, Clyne and Steadman also examined the effect of using argon as a third body and found that while an overall decrease in intensity was observed, there was no significant difference in intensity distribution between argon and chlorine systems. Mechanism Of The Emission Reaction In Chlorine (a) Formation Of The Emitting State The chlorine afterglow emission has been shown to 3 originate in the II + electronic state of molecular o u chlorine. Since this state correlates with one ground 2 2 state P-J/T atom and one excited P, ,„ atom, we are faced 86. with the problem of how this state is formed in the re combination process. 3 Direct 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 sufficient thermal 2 population of the excited ^\/2 atomic state. At room temperature, however, only about 0.7% of the atoms are in the excited state. In recent E.P.R. studies on the discharge products of CF^Cl and mixtures of CF^Cl and Cl2, Carrington et al (62) detected signals corresponding to the excited 3 Cl ( ^2./2^ atoms. By comparing these signals to those from the more abundant ground state atoms, they were able to estimate the relative populations of the two states, ° ° —2 and calculated N]_/2//N3/2 = x 10 . Since this represents a population distribution five times larger than that ex pected for a Boltzmann distribution, it may indicate that excited atoms are produced by some reaction subsequent to 2 the discharge. However, the population of the P]y2 state is still too low to explain the large concentration of 3 Ho+u observed in the chlorine afterglow, if direct for mation of this state from one normal and one excited atom is assumed, 3 The II + state must therefore be formed in some o u process involving ground state atoms. One mechanism by . 87. which this might occur could involve two ground state atoms combining into an intermediate state, which can 3 then undergo a radiationless transition into the II + 3 o u emitting state. Such an intermediate is probably another electronically excited state of Cl2 such as the ^n^u 3 repulsive state, or the no+u- The process may be represented by the equations: Clt2P3/2) + CU2P3/2) — Cl2(\u) (47) C12(lniu) + C12 C12(3Vu) • (48) Since there is additional evidence (page 76) that 1 3 the Ik is the precursor of the II + state in bromine, lu r o u ' we have assumed that this is also the case for chlorine. However, unlike bromine, where a considerable activation energy was required to populate the v' = 3 level of the 3 nQ+u, none is required to populate the highest observed 3 vibrational level of the Cl2 ( no+u) state. The energy difference between two ground state atoms and the v1 = 13 3 _1 level of the II + state is 162 cm , and this can be made o u up by the mean kinetic energy of approach of the reactant particles (kT ^ 200 cm"1 at 300°K). The Lewis-Rayleigh nitrogen afterglow exhibits many characteristics similar to the halogen afterglows. The recombination of nitrogen atoms leads to emission pre-3 3 + dominantly in the first positive (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 via some intermediate state. Bayes and Kistiakowsky (63) suggested that a steady state popu-5 + lation of the E state followed by a collision induced 3 radiationless transition to the B II state was responsible g * for the emission. However, in a recent study of the absolute emission from the nitrogen afterglow, Campbell and Thrush (64) determined that half of the total recombination was 3 occurring via the B IT state. This high rate of formation of nitrogen molecules in the B state could not be accounted 5 + for by a steady state population of the E^ state since its binding energy is only of the order of 2 Kcal/mole. They thus concluded that atoms recombine into the A 3E* state and population of the B state proceeds via a col lision induced crossing from the A state. Although such radiationless transitions as the 5E+ —- 3E+ of N0 and the Xn, —*• 3II + of Cl_ are for-g u 2 lu o u 2 bidden because of spin and symmetry requirements, Zener (65) has pointed out that the electric field of a colliding molecule can relax the "orbital" selection rules, and the spin change is probably unimportant for these states of chlorine (page 6). (b) Relaxation Of The 3II + State o u The experimental observations on the chlorine after glow provide evidence for the following relaxation processes. 89. (i) Vibrational Relaxation The observation that the.spectral distribution of the emission shifts to the red with increasing pressure, suggests that vibrational relaxation by collision with Cl2 is taking place. If, as was suggested in the previous 3 1 section, formation into the II + takes place via the II, ' o u ^ lu state, vibrational relaxation would depopulate the levels close to the point of crossing of this intermediate state. Presumably, at very low pressures a spectral distribution corresponding to emission from these high vibrational levels would be observed. For emission in narrow band widths, we find that I .« ICI,,]^ for emission from high vibrational levels while the pressure dependence approaches [Cl,,]1 for the v1 = 0,1 levels. If formation into the high vibrational levels is * 2 a third order process,(dlCl2J/dt = kf[Cl2][Cl] ) and re moval is first order in IC12J, the overall process will be pressure independent. If the lower levels are formed ex clusively by relaxation from higher vibrational levels, a first order dependence on [C12J should be observed. Cii) Quenching By IC12J If vibrational relaxation were the only process involving chlorine molecules, the pressure dependence of the integrated emission intensity would be Jl^dA <* [Cl2] . Our data indicate that jl dA « {Cl ]0 *6 1 0,1 at 1 torr, 90. and this implies that quenching by [Cl2] is taking place. (iii) Quenching By Atoms Although previous workers (2 7, 34) have overlooked the possibility of atom quenching of the emitting state in chlorine, our data provide conclusive evidence that this is an extremely important process. Furthermore, 3 atoms are most efficient at quenching the II + state ^ 3 o u in low vibrational levels as is indicated by the fact that 1^ * [Cl]1,0 ± 0,1 for emission at 10600A. For high vibrational levels 1^ « ICI]2*^ ± implying that atom quenching does not play an important part. This latter observation is difficult to interpret unless the 3 lifetime of the II + state is shorter at high vibrational o u 3 levels than at low. Owing to a misinterpretation of the 3 v term in the Einstein radiation law, Bayes and Kistia-3 kowsky (.63) stated that this was the case for the B II state of nitrogen. However, Douglas (66) pointed out that the correct use of this term implies that high vibrational levels should actually have longer lifetimes. The variation of the electric transition moment with internuclear distance is another effect which will cause a variation in the lifetime of an electronic state. During a vibration of large amplitude, in which the molecule approaches dissociation, the transition moment of chlorine must vary from that for the normal 3JT + transition ,J o u g 3 3 to a value approaching the pj/2 *" P3/2 atomic tran~ sition. Since this latter transition is forbidden, the high vibrational levels would be expected to have a longer radiative lifetime than the lower lying levels. This has 3 been found to be the case for the n + state of iodine o u (54) where a sevenfold increase in lifetime was observed between the v1 =10 and v1 =50 vibrational levels. Thus, the preferential quenching by atoms cannot be attributed to lifetime variation. 3 3 The II - state is predicted (15) to cross the II + o u . ^ o u near the bottom of the potential well. Bader and Ogryzlo 3 (34) have drawn it to approach the n0+u state at the inside classical turning point at low vibrational levels. 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 effective near the point of crossing, and therefore, at low vibrational levels. This mechanism can also explain another experimentally observed phenomenon which is that the atomic quenching efficiency increases with increasing pressure. At high pressure, vibrational relaxation forces the excited molecules into the low vibrational levels where, because of the proximity 3 of the ^0~u state, they are more easily quenched by atoms Without actually specifying how this low-level quenching occurs, let us see if the mechanism thus far described quantitatively explains the experimental results 92. 3 To do this, let us assume that the excited II + state ' o u 3 * consists of two levels, such that ^Q+u represents the 3 high vibrational levels and ^0+u represents the lowest vibrational levels (figure 33). The following equations may be written: Cl + Cl + Cl2 —Cl2(3nQ+u)* + Cl2 (49) C12(3Vu)* + Cl2 ^ C12lVu' + C12 (5°) C12(3Vu)* + C12 C12(3'Eg) + C12 (51) Cl2(3nQ+u)* —Cl2(1Eg) + hv (52) C12(3Vu) + C12 —L* C12(lzg} + C12 (53) C12(3lIo+u) + C1 —^ C12(lEg) + C1 (54) Cl2(3nQ+u) —Z— Cl2(1Eg) + hv (55) Making the usual steady state assumptions we calculate I = k CL (3n + )* + k Cl„(3n + ) r 2 o u r 2 o u krkf rciJ2!Cl2J krkvkf [C1]2[C12]2 (kr+kv[ci2J+k1[ci2]) (kr+kv[ci2]+k1[ci2]) (kr+kq[ci]+k2[ci2] (56) and since kapp = I/IC1J2JC12J, Figure 33. Schematic diagram for proposed mechanism of Cl 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 \ [Cl2] k (k +k [ClJ+k [Cl0]+k„[Cl„]) rr 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 qualitative description of the experimental results. The intercepts of the 1/k e f / app vs. [C1J plots are predicted to be a function of pressure and although their 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 "flatten out" as the term k2lClJ in the denominator becomes larger. (c> Participation Of Other Electronic States In The Cl2 Afterglow 3 The nQ~u has already been mentioned as a possible intermediate in the quenching of emission by atoms. There 3 is, however, a possibility that the TI^u may play an important part in atom recombination, since much of the recombination occurs into this state in the case of 3 bromine. The radiative lifetime of the IT. state of lu chlorine is undoubtedly long, since absorption into this state has never been observed. Quenching by atoms would be expected to be more efficient in this state than in 3 the IT + because of this longer lifetime. A collision-o u 3 induced crossing between the two states, and a more rapid 3 quenching of the II^ state by atoms would be entirely consistent with our observation that the quenching ef ficiency increases with increasing pressure. Suggestions For Further Study Of Halogen Afterglows (a) Bromine The accurate measurement of bromine atom concentra tions remains one of the largest barriers in obtaining quantitative kinetic results. A quantitative chemical titration for Br atoms equivalent to the N0C1 + Cl re action has not been found, and the isothermal detector has not proven to be reliable. Bromine atoms, however, do give a good ESR signal, and this method of detection may warrant further study. The spectrum of the bromine afterglow in the region o o 9000A to 12000A should be accurately measured on a high resolution spectrophotometer. The use of a time averaging computer could be used, in this application, to accumulate sufficient signal to be measured. A study of the bromine afterglow at a number of temperatures might provide the 3 final answer to the question of the oriqin of the IT + ^ 3 o u state. At very low temperatures, very little emission would be seen from this state if formation proceeds via the """II. . (b) Chlorine To understand the quenching processes which are operative in the Cl2 afterglow, studies of individual tran sitions could be undertaken. It might be possible to dis cover whether atom quenching occurs in certain specific 3 levels near the bottom of the ^-0+n state providing evidence for a crossing by another state in that region. PART TWO STUDIES ON EXCITED MOLECULAR OXYGEN INTRODUCTION During the course of this research, the very in teresting problem of simultaneous electronic transitions in two oxygen molecules in the gas phase arose. Two features of our apparatus (as described in Section 1) made it ideally suited to a study of these transitions. Firstly, our equipment was designed for detecting emission in the near infrared region of the spectrum and many of o these "double molecule" transitions appear between 6000A o and 13000A. Secondly, the calibration of the detectors for absolute emission intensity made possible the study of transition probabilities. Considering these facts, we hoped to make absolute intensity measurements on the (02 )2 bands and extend this study to excited oxygen produced in solution. Electronic States Of Molecular Oxygen The literature on the subject of oxygen spectroscopy is extensive and no attempt will be made here to give a detailed review. However, a brief discussion of the lower electronic states and some of the transitions in which these are involved will be presented. 97. There are, as yet, only comparatively few diatomic molecules for which a large number of electronic states have been established on the basis of their observed band spectra. However, in the case of oxygen, seven bound states have been identified. 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 in the potential diagram of figure 34 as drawn by Gilmore (67) . The lowest electronic configuration of , as pre dicted by molecular orbital theory (68), is 0„lKK(a2s)2 (a*2s)2 (a2P)2 (TT 2P)2(TT 2P)2 (TT 2P)Z(TT 2Prj y ^ 3-1 1 + and this gives rise to the states Z , A and Z . The g g g 3 -ground state is the Z^ state, and the fact that it is a triplet accounts for the observed paramagnetic properties of oxygen. The "*"A state of 0~ is the lowest excited state, g 2 lying 0.98 eV above the ground 3Z~ state (69). The half life for spontaneous radiative emission has been found to be of the order of 45 minutes (70). In the isolated molecule, transitions between the ground level and the two upper levels (''"A and ^Z"1") are of the magnetic dipole g g type since the selection rules for this radiation include the rotational level combination + ••->» + which is strictly Figure 34. Potential energy curves for the oxygen molecule. E (electron volts) 98. forbidden for electric dipole radiation. These transitions are weak, even for magnetic dipole transitons, because they involve singlet-triplet intercombinations. Further-3-1 more, the E^ •*—*• A transition is doubly forbidden because it also violates the AA = 0 ± 1 rule. This would explain the exceptionally long radiative lifetime of the A species. 1 3 -The transitions between A and E form the infra-g g red atmospheric absorption system which lies in the range of 13000A to 730OA. The (0-0) transition lies at 1.27y and as predicted, has an intensity 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 in the twilight and airglow spectrum of the at mosphere (71) . The "'"Eg state of oxygen lies 1.626 eV above the ground state and has an estimated radiative lifetime of 7 seconds (72) . The 3E~ transition is known as the red atmos-g g o pheric system and has its (0-0) band at 7619A. The intensity of this system is low since this transition involves a singlet-triplet intercombination and is also symmetry for bidden. Bands of the red atmospheric system are found in absorption in liquid (72) and gaseous (73) oxygen, and in emission in the airglow (74) and aurora (75). The other bound states of oxygen which have been 3 1 - 3 observed spectroscopically are the Au, Eu and A Eu 3 -which correlate with ground state atoms, and the B Z^ 1 3 which correlates with one D and one P atom. The strongest band system in oxygen is the Schuman-3 + 3 -Runge system (B Zu X Z ) , which is a fully allowed electric dipole transition with an oscillator strength of f = 0.16 (76). These bands are found in the ultra-o violet and converge to a very clear limit (at 1759A) 3 _ which corresponds to the dissociation limit of ( Zu). Studies On Excited Molecular Oxygen For many years discharge-flow techniques have been used to produce high concentrations of oxygen atoms in a flow system. Ogryzlo (7) found that if the atoms are removed from a stream of discharged oxygen by distilling mercury into the discharge region, the gas stream still contained an excited species. Mass spectrometric (77) and calorimetric (78) measurements have shown that the predominant excited state of O,, in such a system is "*"A , and it has been found that up to 10% of the total stream can be produced in the discharge products. In a spectroscopic study of a stream of excited oxygen molecules, Bader and Ogryzlo (7) found two unique emission o o bands, one at 6340A and the other at 7030A. The intensity o of the 634OA band was found to be proportional to the square of the 02 (-^A ) concentration as measured by a calorimetric detector, and both bands were observed to 100. be broad and structureless. Noting that the energy of the o 634 0A band is equivalent to twice the excitation energy of Oj ("'"A ) , Bader and Ogryzlo proposed that the bands arose z g as a result of the following processes: 20- IA ) 0. >- 20„t £ ) n + hv(6340A) 2 g ^ 4 2 g v=0 200 (A ) 0. » 0o (JZ ) n + 0o CZ ) , + hv (7030A). 2 g ^ 4 2 g v=0 2 g v=l Although Bader and Ogryzlo stated that the binding energy of the 0^ double molecule was 600 calories, a remeasurement o of the temperature dependence of the 634OA band by Arnold 079) led to the conclusion that the 0^ double molecule was actually a collision complex, and hence could more correctly be represented as ~ 02. Simultaneous electronic transitions in two molecules were first suggested by Ellis and Kneser (80) in 1933 to explain bands appearing in the absorption spectrum of liquid oxygen. They suggested a dissociation limit of 142 cal/mole from a study of the line shapes of the absorp tion bands. In 1936, Salow and Steiner (81) found these bands in the absorption spectrum of the compressed gas and determined that the intensity was dependent on the square of the oxygen pressure but independent of the pressure of added gases. More recently Dianov-Klokov (82) studied the temperature dependence of the band intensities and concluded that the absorption was due to an 0„ - 0? collision complex. 101. Excited oxygen molecules have also been observed in the products of some chemiluminescent reactions in solution. The extremely interesting observation that a red chemiluminescence is produced during the reaction of hydrogen peroxide and sodium hypochlorite in aqueous solution was reported by Seliger (83). Using a low resolu tion apparatus, he recorded one emission band and gave its 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 half width as the first. They attributed these bands to the (0-0) and (0-1) bands of the 1E+ —*- 3E~ transition g g of molecular oxygen. This assignment was based on the fact that the band separation of 1567 cm 1 coincides, within experimental error, to the ground state vibrational fre quency (1580 cm ^) of molecular oxygen. The authors con cluded that the discrepancy between the position of these bands in solution and in the gas phase was caused by a solvent shift of the bands of 2593 cm 1. Subsequent crystal field calculations on hydrated oxygen (85) suggested that such a high frequency shift was theoretically possible. However, in a study of discharged oxygen in the gas phase, Ogryzlo et al (86, 7) found these same bands, thereby ruling out this interpretation. These authors suggested that the emission arose from simultaneous electronic tran sitions in two oxygen molecules. 102. Purpose Of This Investigation The determination of the absolute rate of emission in 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 arising from the 0»( I —• £ ) 2 g g transition, we hoped to be able to estimate the concen tration of 0oC1£+) in the gas stream. We also undertook a 2 g 3 o measurement of the absolute intensities of the 6340A (1A )-—» (3E_)0 and 7030A (1A )o(0,0) —- (3E_)„(0,1) g 2 g 2 g 2 ' g 2 ' bands in order to determine the radiative lifetime of the (02(1Ag))2 complex. We hoped to extend the foregoing investigation to a study of the chemiluminescence from the reaction of ^2^2 and Cl2 in solution and to identify the excited species giving rise to this emission. 103. EXPERIMENTAL Production Of 0o C1 A ) And 0„ i1!.*) Molecules 2 g 2 g  When commercially available tank oxygen is discharged, the products are oxygen atoms, excited oxygen molecules C^A and 1E+), and small amounts of nitrogen atoms which g g y subsequently react to produce NO and N02. By using Matheson medical grade oxygen, the amount of N02 in the discharge products was minimized, as indicated by the low intensity of the 0 + NO afterglow. To obtain a pure stream of excited oxygen molecules, removal of the oxygen atoms produced in the discharge was necessary. This was accomplished by distilling a small amount of mercury into the discharge region. The mercuric oxide ring which forms immediately after the discharge is very effective at removing atoms while it 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 dis charge was initiated and allowed to warm up, the temperature of the mercury was raised to a point at which just enough mercury was distilled into the discharge to cause an 104. extinction of the residual 0 + NO glow. The control and calibration of gas flowrates has been described in Part 1 and will not be discussed here. The reaction tube used in the oxygen work, however, dif fered from those described in connection with the halogens in 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 this section around a 90° bend. The discharge region was jacketed, allowing cooling air to be blown through it, and a standard C type microwave antenna was used. The flow of 0~ C^A ) was measured with the isothermal 2 g calorimetric detector described previously. However, since cobalt has been found to be more efficient than nickel in deactivating the 0^ ^A^) molecules, the platinum coil was electro-plated with cobalt. This electro-plating was found to give best results when done at a current of 10 ma. and a voltage of 6 volts for about one hour. The electro plating solution was a dilute solution of cobalt chloride and ammonium chloride. Measurement Of Emission From The Cl^ - H2°2 Svstem A very simple procedure was employed in making measurements on the Cl^ - ^2^2 sYs^em* Approximately 10 ml. of a dilute solution of ammonium hydroxide was cooled in an ice bath and then 1 ml. of 90% Ho0„ was added. Chlorine was 105. bubbled into the solution through a small glass jet placed in such a way that a fine stream of bubbles was directed against the wall of the glass vessel. The small reaction zone so produced was found to be an advantage in 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 rise too rapidly. The red emission from the bubbles was focused on the slit 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 it. The reaction normally proceeded for four to five minutes after which time the solution heated up quite rapidly and the emission terminated. RESULTS Estimation Of I.0_ ( £ )]. From Absolute Emission 2 g  In using the isothermal calorimetric detector to estimate 0„ ("*"A ) concentration in a stream of excited 2 g oxygen molecules, the assumption is made that there are no other excited species in the gas stream capable of releasing heat to the detector. The only excited state of 02 with low enough energy to be formed in the discharge, or in subsequent reactions, is the •""£* state. Most of the emission from this state appears in the (0-0) band of 1 + 3 - 0 the 0~ ( £ —* £ ) transition at 7619A, so that by finding 2 g g the absolute emission from this band, an estimation of the concentration of 02 ("*"£*) should be possible. The peak was scanned using the cooled RCA 7102 photomultiplier and the calculations were performed as described in the experimenta section of Part 1. In this case, however, the rate of emis sion follows from the equation I = k{09 (V)] 2 g and the rate constant for spontaneous emission k is known to be 0.14 sec 1 (72). Rearranging equation (28) we have 107. 7700 / . 7700 [02(1Zg)j = ks{OJlNOjAo JFSU);LOU)CU //(0.14)Ag ^ FG(A)dA 7500 is(X)/ 7500 Under the following experimental conditions P = 2 torr [02J = 1.08 x 10~4 moles/1. [OJ = 3.81 x IO-6 gm.atoms/1. [NO] = 5.43 x 10~6 moles/1. the integrals were calculated to be 7700 A \F (A)i CA)dA = 0.101 Is o 7500 is(X) 7700 As \Fs^)dX = H.24 7500 from which a value of the concentration of is calculated g to be [0„(1E+)J = 1.77 x 10~9moles/l. / g Under similar experimental conditions, the concentration 1 -6 of 02( A ) is usually of the order of 10 moles/1. This measurement, therefore, confirms the previous suggestions (7, 79) that 0~ (1Z+) is a minor constituent in the products l. g of discharged oxygen in flow systems. Consequently, the 108. error introduced by the assumption that the excited stream is entirely "'"A J is negligible. 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 02(1Ag) species was measured using the cobalt-plated isothermal calorimetric detector. A number of measurements were taken under con ditions of varied total pressure and excited molecule con centration and an average value of the rate constant for emission in this band was found. Bader and Ogryzlo (7) o showed that the intensity of the 6340A band is proportional to the square of the "'"A concentration; thus we define the g rate constant for the emission to be I = k' lO^A )]2. (58) g Using equation 28 as outlined in the experimental section and the 0 + NO calibration procedure, an average -1 -1 value of k1 was calculated to be k' = 0.090 l.mole sec A typical set of values used in equation (28) to obtain this value of k' was IOJ = 3.20 x 10~6moles/l. INOJ = 2.96 x 10~6moles/l. [02(1A )] = 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 is bubbled into an alkaline solution of hydrogen peroxide in water, the chlorine is consumed and molecular oxygen is formed. The equation for the overall reaction can be written as Cl2 + H0~ + OH" —»• 2C1~ + H20 + 02 (59) The bubbles formed in the reaction emit a red light of quite high intensity. Making the solution more basic had the effect 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% H202. The spectrum of the emission originating in the bubbles was recorded using the f/4.5 Hilger and Watts monochromator and a cooled RCA 7102 photomultiplier and is shown in figure 35, All the bands in the spectrum can be assigned :+), 00(1A ] g ' 2 g to known transitions involving 02 (^Z"*"  , 02 (''"A ) and the collisional pair (00 ("*"A ))0. These are listed in Table 12, Z a Z Figure 35. Emission spectrum from the reaction of chlorine with hydrogen peroxide. Solid line is from the aqueous-ammonia system. Broken line is emission from the chloroform-pyridine system. 5500 6000 7000 8000 10,000 Wavelength (A) TABLE 12 BANDS OBSERVED IN THE SPECTRUM OF THE C12"H202 SYSTEM Peak Number Wavelength Electronic States Vibrational Levels o (A) 1 5800 C1Ag)2 —^ (3Zg)2 (0,1 —K 0,0) 2 6340 clV2 •'•^ZgV (0,0 0,0) 3 7030 (^A ) - —*(3Z~)9 (0,0 —* 0,1) 4 7619 1Z+ 3E" (0,0) 5 7700 1Z+ —* 3E" (1,1) g g 8645 1 + 3 -^g —* % 10,1) XAg — V (2,0) 10,700 1L v 3Z" (1,0) 12,700 XAg >- 3Z~ (0,0) 110. Because water is known to be an extremely efficient deactivator for both 0~(1Z+) and for vibrationally ex-2 g •* cited oxygen (87), experiments were performed to see if reducing the water vapour content of the bubbles had any effect on the spectrum. First, all solutions were cooled to 5°C before being used and during the experiment, a cold finger helped to maintain the low temperature. This re sulted in higher intensities in 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% solution of chloroform and pyridine was cooled and then a layer of 90% H2C>2 was added. The mixture was shaken to extract some hydrogen peroxide into the chloro form layer. The chlorine was then bubbled into this layer. Figure 35 shows the resulting spectrum in which bands due to vibrationally excited oxygen are noticeably absent. The small intensity of the 7619A (1Z+ >• 3Z~) band in-dicates the lower concentration of 0„(1Z+) in this system. 2 g An estimate of the yield of 0~ ("'"A ) can be obtained J 2 g by comparing the intensities of bands found in the solution work with those found in the study of discharged gaseous oxygen. As was mentioned previously, the emission intensity o of the 6340A band has been found to be proportional to the square of the ^A concentration (7, 79): ^340 = k.lO^Ag)]2 • <58) o 2_ Since the band at 12700A arises from one 0-( A ) molecul 2 g we can write ^2700 = k'l°2(1V] • (60) The ratio of the emission intensities in these two bands is given by R = I6340 = k' 10, (XA )1 . (61) •"•12700 When 02 (''"A ) is obtained from an electrical 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 ratio of the intensities of the two emission lines. With this value of k'/k" it ^s possible to estimate the 0o C"*"A ) concentration in any system simply by measuring z g R for that system. In contrast to other methods of deter mining chemiluminescence yields, geometric factors and collisional quenching rates need not be accounted for when equation 61 is used. Using the f/4.5 monochromator and the RCA 7102 photomultiplier and with a slit width of 500u, the following results were obtained when 0^^^g) was measured in the products of an electrical discharge: 112 IO„(1A U = 1.5 x 10~5mole l."1 2 g R = 36 k'/k" = 2'4 x 106l.mole~1 Using the same optical system to measure the emission 3 from the Cl2 - H202 system, R was found to be 1.8 x 10 . However, Badger, Wright and Whitlock (70) have shown that o 1/5 of the intensity in the 12700A band is due to spon taneous emission, the remaining intensity arising from collision induced transitions. Thus, in calculating the concentration of 0~ ("*"A ), a value of R five times 2 g ' greater than the measured value should be used, ie. 3 . . R = 9.0 x 10 . Using this corrected intensity ratio in 1 -3-1 equation 61, we calculate {02( A )] = 3.75 x 10 mole 1. ^ g If we assume that the bubble temperature is 300°K, this corresponds to a partial pressure of 7 0 torr. Since the total oxygen pressur  in the bubbles is about 750 torr, \ g this corresponds to a yield of 10% 0^ ("*"A ) in the reaction, 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 in the spectrum of discharged oxygen: k K2 (62) k C02C1Ag))2 ^> C02(32:g))2 + hv(6340,7030A) (63) I = O^/k^k-jlO^A )]2 . (64) The value of the rate constant for emission in the o 6340A band can be used to estimate the radiative lifetime of the (02)2 collisional complex. If we assume that the o probabilities of the transitions giving rise to the 6340A o and 7030A bands are equal, since the intensities of these bands are almost equal, we can calculate the value of the total light emission rate in equation 64: (k1/k2)k3 = 2(k') = 2(0.090) = 0.18 1. mole"1sec~1 . 0 The rate constants k^ and k2 can be estimated on a basis of simple collision theory, assuming that dissociation of the complex occurs within one vibration. Thus = IO11 1. mole "''sec "*", k2 = 1013sec 1 and k^ is then cal culated to be 18. Expressed as the radiative halflife of (02(1Ag))2: t1/2 = 0.693/k3 = 37.5 msec. Obviously this estimate could be considerably low depending upon how strong are the attractive forces between two 0„ ("*"A ) molecules. 2 g The problem, then, is one of finding an accurate equilibrium constant for equation 62 (K = k^/k2). In an attempt to estimate the contribution of bound species to the second virial coefficient, Stogryn and Hirschfelder (88) derived an equation enabling this equilibrium constant to be calculated. The authors divided the second virial coefficient into three parts: B(T) = Bf (T) + Bb(T) + Bm(T) . (65) In this equation, B^ (T) arises from collisions between free molecules, B^tT) is related to the equilibrium con stant for the formation of bound double molecules in the gas and B (T) is related to the equilibrium constant for m the formation of "metastable" double molecules. The difference between these species can be understood if an effective potential energy of interaction between molecules is defined as 4>eff(r,L) = 4>tr) + L/r2 (66) 2 where L/r is called the centrifugal potential and 115. represents the energy of rotational motion. <J>(r) can be approximated by a Lennard-Jones (6-12) potential <f>(r) = 4el(a/r)12 - (a/r)6] . (67) where C-e) is the maximum energy of attraction between two molecules and a is the low-velocity collision diameter. When the centrifugal potential term has a finite value, the effective potential energy curve contains a "hump" or rotational barrier. In figure 36 one such potential energy curve is shown for a small value of L. Region B in this figure corresponds to the region of phase space occupied by those two-molecule systems where the total energy is less than the energy of the separated molecules. These are termed bound dimers and can only be freed by a collision with another molecule. Region M contains the metastable species which have greater energy than the separated molecules. Although classically, these can only be freed by a collision with another molecule, quantum mechanically they can dissociate by leakage through the energy barrier. Depending on whether the half-life of dissociation is greater or less than the average time between collisions, this species can behave like a bound or free molecule. Using a statistical mechanical argument, Stogryn and Hirschfelder derived an expression for the equili brium constant for the formation of both the bound and Figure 36. Effective potential energy curves for the Lennard-Jones (6-12) potential. Energy o 116. metastable species. For 0^ gas at 294°C, their expression yields K = 0.0254 1. mole"1. Since Kk3 = 0.18 1. mole"1 sec 1, substitution of this value of K yields = 7.1 -1 . 1/2 sec . Expressing this as a radiative halflife gives t ' = 0.69/7.1 % 0.1 seconds. In a theoretical treatment of gas viscosity, Kim and Ross (89) extended the work of Stogryn and Hirsch-felder to include the effect of "quasidimers" in addition to the bound and metastable dimers. Kim and Ross defined a dimer complex by assigning it a phase space bounded by L 4 L F$ FF 4 0.8, R 4 R (q) where L = largest value of angular momentum for which there appears an inflection point in the <j> curve, and R (q) is the locus of the * reff ' m ^ maximum in this curve. Thus, in addition to the dimers considered by Stogryn,and Hirschfelder which arise as a result of three-or-more-body collisions, Kim and Ross consider the quasidimer which arises from two-body colli sions. This takes into account molecules which approach each other with small relative kinetic energy so that orbiting occurs in the vicinity of the maximum of the effective potential. In the case of atomic recombination, Bunker (30) considers this the most important type of collision leading to reaction. The equilibrium constant calculated for all three regions of phase space (bound, metastable and quasidimers) for the case of O^i is K = 0.079 1. mole '''.Comparing this value with the K 117. obtained from the theory of Stogryn and Hirschfelder, we see that the addition of quasidimers increases the equili brium constant considerably, k^ is now calculated to be 2.3 sec-1 and t1/2 = 0.3 seconds for the radiative half-life of the C02(1Ag) )2 species. Since relatively little is known about the mechanism of double transitions in molecules, no decision can be made as to which regions of phase space should be con sidered in calculating the equilibrium constant for the (02)2 complex. Chemiluminescence From The .C.l.2-H202 System We have assumed that the chemiluminescence observed in the reaction of chlorine and hydrogen peroxide origin ates from excited gaseous oxygen in the bubbles. This assumption is reinforced by the observation that the in tensities of the (0-0) and (1-0) bands of the —*- 3Z~ g g transition are in the same ratio as those observed in atmospheric absorption studies (73). The intensities of the (1-0) and (0-0) transitions of the main chemilumines cence bands, however, are roughly equal, as is the case in compressed (90). This fact supports the interpre tation of these bands as collision induced 0^ - 0^ pair transitions. Water is known to have a high efficiency in deac tivating vibrationally excited oxygen (87), and for this 118. 1 3 -reason the high intensity of the (1,0) ( A -—>• E ) band is surprising. It has not, however, been clearly established whether water maintains this high efficiency of vibrational quenching in excited oxygen molecules. It is quite likely that the efficiency of water in this respect may be due to the similarity of the vibrational 3 - -1 frequency of 0_ ( E ) (1580 cm ) and the frequency of the "wagging mode" in R^O (1595 cm "*"). In that case, since the vibrational frequency of ("'"Ag) is 1509 cm-"'" and of ("*"£*) is 1433 cm 1, the efficiency with which vibrational energy transfer occurs may be considerably decreased for these excited molecules. There are a number of reasons for believing that the yield of O20~A ) *~s actuallY larger than the 10% found experimentally. Since the reaction is 62 kcal exothermic, the temperature of the gas in the bubbles is probably somewhat above room temperature. Direct measurement of the bubble temperature was found to be impossible. How ever, the "vibrational temperature" (69, page 203) of the emitting gas can be estimated from the relative in tensity of the emission from vibrationally excited states. 1 3 -From the intensity of the ( A ) , —*- ( E ) „ transition g v=l g v=0. o at 10670A, an estimated vibrational temperature of about 600°K can be calculated. Assuming that the true tempera ture of the gas is somewhat less than this value, the reported yield can, at most, be low by a factor of 2. 119. A second effect contributing to the low value for the yield could be the dilution of the 0„ ("""A ) by Cl„ u 2 g 2 and H20 vapor, since the reaction is not infinitely fast. Both these effects, however, have been minimized by working with cooled solutions where the vapor pressure of water is low and the bubble temperature is nearer to 25°C. Also, by using a very basic solution the reaction rate was maximized. Badger, Wright and Whitlock (70) have measured the absolute intensities of the discrete-line absorption o band and the underlying continuous absorption at 12600A in oxygen gas at pressures up to 4.3 atmospheres. They concluded that the discrete absorption is a measure of the intrinsic transition probability in isolated molecules, and the continuum arises from an enhancement of the tran sition probability in collision complexes. By studying the pressure dependence of the integrated absorption coefficients of each of these absorptions, they derived an equation for the reciprocal mean lifetime of ^"A oxygen molecules subject to decay only by radiative pro cesses: (l/xm) = A = (2.6 x 10"4) (1 + 3.8PQ + 3.0PCQ + 0.7PN ) (68) where P is partial pressures in atmospheres. The halflife of an isolated """A molecule is then about 45 minutes, which g in pure oxygen is reduced to 9.2 minutes at one atmosphere. 120. o This means that only 1/5 of the 12600A band intensity is due to the spontaneous emission process, the remaining intensity arising from collision induced transitions. For this reason, in calculating the yield of 02 (^"A ) t R i-n equation (61) has been increased by a factor of five. From the observed intensity sequence in absorption, Badger et al predicted that the (0-1) transition proba bility in the (^A^)2 collision complex would be relatively large while the intensity of the (0-2) and higher levels would be negligible. The fact that we have not observed the (0-2) transition confirms this prediction. The importance of this reaction is in its applicability to the interpretation of many oxidation reactions where there is organic molecule chemiluminescence. The extreme prohibition of the singlet •->• triplet (g*-+-g) transition results in a relatively high stability of singlet excited oxygen. For this reason, energy transfer processes between excited oxygen and acceptor molecules having suitable energy levels occur quite readily. If, for example, a small amount of dibenzanthrone is added to the solution in which the chlorine-peroxide reaction, is proceeding, an extremely bright red chemiluminescence is seen. Examination of the spectrum shows that it is identical to the fluorescence band of dibenzanthrone. Many other substances are capable of being excited in this reaction as was shown by Mallet (91), and this is a manifestation 121. of the many energy levels in the oxygen system available for transferring.energy. Khan and Kasha (92) have recently o found a band at 4780A in this system, which is undoubtedly due to the (^"A , ^E ) collision complex, and this confirms 9 9 the presence of levels of sufficiently high energy to ex cite many large aromatic compounds. It is probable, there fore, as Khan and Kasha have suggested, that in any reaction producing singlet oxygen,chemiluminescence may occur if there exists a species capable of accepting the energy from the oxygen. Recently, Ogryzlo and Pearson (93) have studied the excitation of violanthrone by singlet oxygen confirming this hypothesis. These authors have attributed the luminescence of the violanthrone to the following re actions: °2clAg) + LY0 — °2(3V + \ (69) 02(1Ag) + 3V]_ —• 02(3E~) + 1V1 (70) 1V1 * 1V() + hv (71) where ^"VQ is the singlet ground state of violanthrone, 3 1 is the lowest triplet excited state and is the lowest singlet excited state. APPENDIX 122. Calculation Of Potential Energy Curves Accurate potential energy diagrams were drawn for each of the halogens to assist in the interpretation of the emission spectra. When rotational data were available for an electronic state, the potential function used to calculate the shape of the curve was the Hulburt-Hirsch-felder potential (94): V = D[(l - e"X)2 + cx3 e"2x (1 + bx) ] (72) x = io e 2(B D)1/2 e r - r e r e where b and c are constants calculated from spectroscopic data, and the remaining symbols have their usual spectro scopic meaning. When the rotational constants of a state were not known, a Morse (95) function was used in the calculation. Computations were carried out on the IBM 7044 computer and the output was plotted from each molecule. Large scale drawings were also made and these were used in the actual interpretational work. (a) Chlorine 1 + The ground state Eg curve for chlorine was calcu lated using the Hulburt-Hirschfelder potential and the 3 data of Douglas, Miller and Stoicheff (60). The no+u curve was taken directly from the paper of Todd, Richards and Byrne (96) who used the more accurate R.K.R. method 123. of finding the classical turning points. Very little is known about the low energy portion of the ^CT^U state and 3 its position and point of crossing of the ^Q+u curve is indefinite. Following Bader and Ogryzlo (34) we have drawn 3 it to cross the n + state between the thirteenth and o u fourteenth vibrational levelsw The high energy portion of this state has been drawn from the data of Palmer (19). Recently Clyne and Coxon (50) have studied the spectrum of discharged chlorine and have observed tran sitions to all the low lying vibrational levels of the 3 1 II + state. Since their calculation of co avoids the o u e long extrapolation from v' = 6 made by Douglas et al (60), their value for this constant has been used in calculating the position of the vibrational levels. (b) Bromine Horsley and Barrow (4 6) have recently done a careful study of the absorption spectrum of bromine and have re calculated the spectroscopic constants for the ground "*"E+ 3 and n + states. The ground state potential curve was o u c drawn using their data and the Hulbert-Hirschfelder poten tial. The ^no+u curve was taken directly from the paper by Todd, Richards and Byrne (96) who used the more accurate Rydberg-Klein-Rees method (97) to locate the turning points, The lower portion of the "*"II, curve has been drawn to ^ lu 3 cross the II + state between the third and fourth vibra-o u tional levels as suggested by Bayliss and Rees (53). The 3 shape of the IT^u state is less accurately known because of the difficulties involved in analyzing the spectrum. The vibrational assignment of this state was first made by Brown (47) and later revised by Darbyshire (44). The numbering of this state suggested by Darbyshire has been used, but could be in error by ±2 units. The potential energy curve of this state was calculated using the spec troscopic constants given by Horsley (45) who has studied 3 the rotational fine structure of a number of the II, lu •«—• bands. The upper portions of the curves were extrapolated from the diagram in the paper of Kistia-kowsky and Sternberg (98). (c) Iodine Much more accurate spectroscopic data are available for iodine than the other halogens, so that the potential energy diagram is much more precisely known. The ground state function for was calculated using the data of 3 Rank and Rao (99) while the II + curve was taken from o u the paper of Steinfeld, Zare, Jones, Lesk and Klemperer 3 (100) . The II ^ state was calculated using a Morse poten tial, and the data of Mathieson and Rees (101). 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