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A spectroscopic study of hexafluoromanganates (IV) Pfeil, Achim 1971

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A SPECTROSCOPIC STUDY OF HEXAFLUOROMANGANATES(I V) by ACHIM PFEIL D i p l . C h e m . , Johann -Wo l fgang -Goethe -Un i ve rs i ta t zu F r a n k f u r t am Main , 1966 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the requ i red s t a n d a r d . THE UNIVERSITY OF BRITISH COLUMBIA October , 1971 In presenting th i s thesis i n p a r t i a l fulf i lment of the requirements for an advanced, degree at the Univers i ty of B r i t i s h Columbia, I agree that the Library s h a l l make i t f reely avai lable for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholar ly purposes may he granted by the Head of my Department or by h i s representatives. I t i s understood that copying or publ ica t ion of th i s thesis for f i n a n c i a l gain s h a l l not be allowed without my wri t ten permission. Department of Cfr£"M ( STfty The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date i i ABSTRACT The pr imary processes of photoexc i ted hexaf1uoromanganate (l V ) have been i n v e s t i g a t e d through s p e c t r o s c o p i c s t u d i e s . The emiss ion and a b s o r p t i o n s p e c t r a , emiss ion quantum y i e l d s , and l i f e t i m e s of I^MnF^, Rb^MnF^, Cs^MnF^ and Mn in the m a t r i x of K^GeF^ and Cs^GeF^ have been measured as s i n g l e c r y s t a l s and in s o l u t i o n at temperatures between 10 and 600°K. The v i b r a t i o n a l s t r u c t u r e in the s p e c t r a has been c o r r e l a t e d in many cases w i t h the f r e q u e n c i e s determined from IR and Raman s p e c t r a and o r i g i n s of e l e c t r o n i c t r a n s i t i o n s have been a s s i g n e d . 2 Phosphorescence from the E s t a t e has been observed . Thermal ly 2 k a c t i v a t e d reverse i n t e r s y s t e m c r o s s i n g from the E^ to the T^^ s t a t e has been e s t a b l i s h e d f o r Cs^MnF^ from the occur rence of delayed f l u o r e s -cence at h igh temperatures . The a c t i v a t i o n energy of t h i s process is c o n s i s t e n t w i t h the s e p a r a t i o n of the o r i g i n s of these s t a t e s obta ined from low- temperature h i g h - r e s o l u t i o n s p e c t r a . k+ The l i f e t i m e of Mn :Cs2GeF^ i s r a d i a t i v e up to kS0°K and on ly weakly dependent on temperature . The temperature dependence f o l l o w s that p r e d i c t e d by a v i b r o n i c mechanism. Above k50°K the complex i s s t r o n g l y d e a c t i v a t e d by a r a d i a t i o n 1 ess process which dominates the thermal a c t i v a t e d reverse i n t e r s y s t e m c r o s s i n g . In s o l u t i o n a s t rong temperature dependent r a d i a t i o n 1 ess process occurs above 130°K. The nature of these r a d i a t i o n 1 ess processes is d i s c u s s e d . TABLE OF CONTENTS Page T i t l e Page i A b s t r a c t .' i i Table of Contents i i i L i s t of F igures v i L i s t of Tables i x Acknowledgements x I I n t roduct ion 1 1. E l e c t r o n i c S ta tes 2 2 . Emiss ion and Absorp t ion Spect ra . . . 2 3 . Pr imary Processes 5 4. Temperature Dependence of Phosphorescence L i f e t i m e s . 9 5. Purpose of Th is Work 12 I 1 Exper imental 15 1. P r e p a r a t i o n 15 2. IR Spect ra 16 3- Raman Spect ra 16 k. The Dewar 17 5. Absorp t ion Spect ra 19 6. Emiss ion Spect ra 20 7. L i f e t i m e Measurements 2h 8. The F lash Lamp 26 i V Page III Fundamental Frequencies of the Groundstate 31 1. Resu l t s 33 2 . D i s c u s s i o n 33 i ) The Raman Spectrum 33 i i ) The IR Spectrum 41 i i i ) Summary 44 IV E l e c t r o n i c and V i b r o n i c S ta tes 47 1. Resu l t s 47 i ) Absorp t ion Spect ra 47 i i ) Emiss ion Spect ra 59 2. D i s c u s s i o n , 68 2 2 4 i) T, , E -<->- A„ t r a n s i t i o n s 72 lg 9 2g i i ) \ . , N"- -«- A 0 t r a n s i t i o n s 83 l g 2g 2g i i i ) Summary 90 V Temperature Dependence of the Phosphorescence L i f e t i m e . . 97 1 . R e s u l t s 97 2. D i s c u s s i o n 102 i) The Temperature Dependence of Phosphorescence L i f e t i m e 102 2 -i i ) The Pr imary Photophysica1 Processes of [MnF^] 111 VI Conc lus ions 114 1. Spect ra 114 V Page 2. Temperature Dependence of the Phosphorescence L i f e t i m e 115 i ) The Rate Constant J J I J i|) The Rate Constant k_^ J J I J 3 . Suggest ions f o r Fur ther Work 116 B i b l i o g r a p h y . . . . . 118 v i LIST OF FIGURES Page 3 1. Tanabe-Sugano diagram f o r oc tahedra l d systems 3 3 2. J a b l o n s k i diagram f o r a d complex 6 3. Schematic of the setup f o r a b s o r p t i o n , emiss ion and l i f e t i m e measurements 18 4. P r o f i l e of the standard lamp, response of the d e t e c t i n g system and curve of c o r r e c t i o n f a c t o r s 2 3 5- Cross s e c t i o n a l view of the f l a s h lamp 2 7 6. Decay curves of the f l a s h lamp 30 7 . Raman spectrum of p o l y c r y s t a l1 i n e Cs„MnF, . . ' 34 L b 8. Raman spectrum of p o l y c r y s t a l1 i n e K^MnF^ 35 9. I n f ra red spectrum of s o l i d K.MnF, . . . . 36 Z o 10. Comparison of Raman, emiss ion and a b s o r p t i o n spectrum of Cs 2 MnF 6 38 11. Comparison of Raman, emiss ion and a b s o r p t i o n spectrum of K 2 MnF 6 39 12. Abso rp t ion and r e f l e c t a n c e spectrum of K^MnF^ at 300°K. . 49 13- Abso rp t ion spectrum of K^MnF^ at 85°K, s i n g l e c r y s t a l . . 51 14. Abso rp t ion spectrum of Cs^MnF^ at 10°K, s i n g l e c r y s t a l . . 52 15- Abso rp t ion spectrum of M n ^ t c s . G e F , a t 80°K, s i n g l e 2 D c r y s t a l 53 2 4 16. O s c i l l a t o r s t r e n g t h f of E •<- A 2 ^ of Cs 2MnFg as a f u n c t i o n of temperature 54 vi i Page 17. Absorption spectrum of Cs^MnF^ at 80°K, single crystal . . 55 18. Absorption spectrum of Cs2MnF^ at 10°K, single crystal . . 56 19- Details of the absorption spectrum of Cs^MnF^ at 85°K, single crystal 57 20. Details of the absorption of I^MnF^ at 85°K, single crystal 58 21 . Emission spectrum of Cs2MnF^ at 10°K, single crystal . . . 60 22. Emission spectrum of Mn^:Cs^eF^ (k%) at 80°K, single crystal 62 23. Emission spectrum at 590°K of Mn^+:Cs2GeF^ (k%) compared to the absorption spectrum at 300°K. . 64 2k. Quantum yields of phosphorescence Q^  and delayed fluor-escence Qj. as a function of temperature 65 25. Details of the emission spectrum of K^MnF^ at 85°K . . . . 69 26. Emission spectrum of Mn^r^GeF^ ( 0 . U ) at 80°K, single crystal 70 k+ 27. Absorption and emission spectra of Mn :Cs2GeF^ (k%) at 80°K, single crystal . (Assignments) 73 28. Absorption spectrum of K^MnF^ and emission spectrum of Mn:K^GeF^ (0.1%), single crystals. (Assignments) 80 I, 29. Vibrational structure of T, band of Cs„MnF, at 80°K, 1 g 2 b single crystal . (Assignments) 85 30. Absorption spectrum of Cs_MnF-., single crysta l . (Assignments) 93 v i i I Page 31. Abso rp t ion spectrum of K^MnF^ at 85°K, s i n g l e c r y s t a l . (Ass ignments) 96 k+ 32. Phosphorescence l i f e t i m e of s i n g l e c r y s t a l s of Mn :Cs„MnF, 2 o (0.1% mole r a t i o ) and Cs^MnF^ as a f u n c t i o n of temperature 98 33. Phosphorescence l i f e t i m e of C s ^ n F ^ in s o l u t i o n (20% HF) as a f u n c t i o n of temperature (c = 2 x 10 M ) 100 3^. The n a t u r a l l i f e t i m e of phosphorescence of Cs^MnF, as 2 b c a l c u l a t e d from the o s c i l l a t o r s t reng th 101 4+ 35. Quantum y i e l d s and l i f e t i m e s of emiss ion of Mn :Cs„GeF , . 103 2 0 36. Temperature dependence of the l i f e t i m e of Mn -.Cs^MnF, 2 b (0.1%) and Cs MnFg in deoxygenated 20% HF 109 2-37- The pr imary photophys ica l processes of [MnF^] in C s 2 G e F 6 . . 112 i x LIST OF TABLES Page 1. Observed f requenc ies in the Raman spectrum of C s ^ n F ^ compared to f requenc ies observed in emiss ion and absorp t ion s p e c t r a 42 2. Observed f requenc ies in the Raman spectrum of K^MnF^ compared to f requenc ies observed in emiss ion and absorp -t i o n s p e c t r a 43 3. Fundamental f requenc ies of some hexaf1uor ides w i th t e t r a -va lent cat ions . 46 4. D e t a i l s of the v i b r o n i c s t r u c t u r e in the emiss ion spectrum 4+ of Mn :Cs 2GeF 6(4%) at 80°K, s i n g l e c r y s t a l 63 5. D e t a i l s of the v i b r o n i c s t r u c t u r e in the emiss ion spectrum 4+ of Mn : K 2 G e F 6 (0.1%) at 80°K, s i n g l e c r y s t a l 71 6. Comparison and assignment of f requenc ies as observed in 2 4 emiss ion and a b s o r p t i o n of E •*->• A_ 75 9 2g 4 7. Assignment of s t r u c t u r e of the Tj a b s o r p t i o n band at 85°K of Cs 2 MnF 6 84 8 . D e t a i l s and assignment of s t r u c t u r e of T 2 a b s o r p t i o n spectrum of C s ^ n F ^ . . 91 4 9- D e t a i l s and assignments of the s t r u c t u r e of T 2 absorp -t i o n spectrum of I^MnF^ Sk 2-10. The fundamental f requenc ies of [MnF^] 95ACKNOWLEDGEMENTS I wish to express my s i n c e r e thanks to Dr. G.B. P o r t e r fo r h i s constant h e l p , c r i t i c i s m and encouragement throughout the course of t h i s work. Thanks are due to my co l leagues in the l a b o r a t o r y f o r many e n l i g h t e n i n g d i s c u s s i o n s and to Mrs. E l a i n e Hunt f o r her p a t i e n t typ ing of the manuscr ip t . G r a t e f u l acknowledgement is a l s o made to the U n i v e r s i t y of B r i t i s h Columbia f o r the award of a Graduate F e l l o w s h i p , 1968-1970. Gerda, Joachim und E r i ka gewi dmet 1 I I N T R O D U C T I O N Especially as transition metal ions have found application in 1 2 phosphors and in lasers much attention has been focussed on optical studies of the fate of electronic excitation energy and the factors affecting i t . The understanding of the fate of energy absorbed by a molecule is of general importance. The present knowledge has been obtained from studies of the excited states by absorption and emission spectros-3 4 5 copy, photochemistry and lifetime measurements. ' ' However, i t has become obvious that such a task is of considerable complexity. Among the transition metal coordination compounds by far the most extensive studies in this regard have been carried out on C r ( l l l ) complexes and on C r ( l l l ) ions incorporated in various crystall ine hosts.^ These compounds are the most suitable for such an investigation: a large number with a widely varying ligand f ie ld strength are known and have been prepared in a high degree of purity; the compounds are chem-ical ly well characterized and stable; the optical spectra are generally well understood. Above a l l , however, almost a l l C r ( l l l ) compounds show luminescence which is essential for such an investigation. Before the results of previous work in this f ie ld are summarized (see sections 3 and 4 of this chapter) a brief outline of the theoretical and empirical facts is given which characterize the C r ( l l l ) coordination compounds and furthermore, are important for the understanding of this work. 2 1 . ELECTRONIC S T A T E S 6 , 7 3 Ignor ing s p i n - o r b i t c o u p l i n g , an oc tahedra l 3d complex has k 3 a A^g ground s t a t e a r i s i n g from the o r b i t a l c o n f i g u r a t i o n t 2 . The e x c i t e d s t a t e s which are of i n t e r e s t in the present s t u d y , are the 2 2 2 three doublet s t a t e s E , T, and T. ( a r i s i n g from the same con -9 l g 2g 3 k h f i g u r a t i o n t ^ ) and the three quar te t s t a t e s ^ , ^ l g ^ o t n ^ r o m 2 4 2 t „ e ) and T, ( t „ e ) . The energ ies of these l e v e l s as a f u n c t i o n 2g g lg 2g g ' of l i gand f i e l d s t reng th were c a l c u l a t e d by Tanabe and Sugano and g t a b u l a t e d in form of diagrams such as is shown in F i g . 1. From these diagrams the r e l a t i v e p o s i t i o n s and approximate energ ies of the l e v e l s of a g iven complex can be o b t a i n e d , once the parameters B and Dq are known, which are unique fo r each complex. B and 10 Dq are the intere1ectronic r e p u l s i o n parameter and the l i gand f i e l d s t r e n g t h , r e s p e c t i v e l y , and can be determined from the a b s o r p t i o n spectrum of a complex. 2. EMISSION AND ABSORPTION SPECTRA^ Luminescence is observed in the c r y s t a l l i n e m a t e r i a l and in r i g i d g l a s s y s o l u t i o n s at low temperatures . Two types of luminescence, phosphorescence and f l u o r e s c e n c e , have been d i s t i n g u i s h e d . Phosphores-cence i s emiss ion from the lowest l y i n g doublet s t a t e and is a s p i n -fo rb idden p r o c e s s . F luorescence i s s p i n - a l l o w e d and occurs from the lowest l y i n g e x c i t e d quar te t s t a t e . The type of luminescence which 3 4 occurs f o r a p a r t i c u l a r compound has been c o r r e l a t e d w i t h the r e l a -2 4 9 t i v e energ ies of the E and T „ s t a t e s . 9 2g In most c a s e s , however, phosphorescence on ly is observed . The phosphorescence s p e c t r a are c h a r a c t e r i z e d by a set of sharp l i n e s (10 cm ' or less ) which are u s u a l l y the m i r r o r images of the c o r r e s -ponding l i n e s of the s p i n - f o r b i d d e n absorp t ion s p e c t r a . The f l u o r e s -cence s p e c t r a , on the other hand, are broad and s t r u c t u r e l e s s w i t h a h a l f - h e i g h t w idth of approx imate ly 3000 cm ' . Besides by t h e i r w i d t h , the bands in the a b s o r p t i o n s p e c t r a are c h a r a c t e r i z e d by t h e i r i n t e n s i t i e s . The i n t e n s i t y is expressed by the o s c i l l a t o r s t reng th f and is c a l c u l a t e d from the area under the absorp -t i o n band in a p l o t of the e x t i n c t i o n c o e f f i c i e n t , e , versus wavenumber, 3 v. The a b s o r p t i o n bands of 3d complexes are approx imate ly of Gaussian shape, and the o s c i l l a t o r s t reng th can be est imated from the formula f = 4 .6 x 1 0 " 9 e v 1 / 0 (1) max 1/2 where e is the va lue of the e x t i n c t i o n c o e f f i c i e n t at the band max-max imum and v j / 2 ' s t n e width of the band in cm ' at h a l f - h e i g h t . This express ion is w ide l y used to c a l c u l a t e the o s c i l l a t o r s t r e n g t h of absorp-3 t i o n bands of d complexes. The i n t e n s i t y of a band is an important parameter in the a n a l y s i s of a spectrum because i t helps to c h a r a c t e r i z e the e l e c t r o n i c t r a n s i t i o n . The o s c i l l a t o r s t reng th of the va r ious types 3 of t r a n s i t i o n s in d complexes spans severa l orders of magnitude, and r e p r e s e n t a t i v e i n t e n s i t y values a r e , f o r example'^*: 5 Type of T r a n s i t i o n S p i n - f o r b i d d e n , Laporte fo rb idden S p i n - a l l o w e d , Laporte fo rb idden S p i n - a l l o w e d , Laporte a l lowed ( e . g . charge t r a n s f e r ) Laporte fo rb idden t r a n s i t i o n s are metal l o c a l i z e d t r a n s i t i o n s of the d e l e c t r o n s and may be s p i n - f o r b i d d e n or s p i n - a l l o w e d . f e max i o " 7 i o " 1 - 5 - 2 3 10 3 - 10 10 - 1 0 J io"1 io1* 3 . PRIMARY PROCESSES The f a t e of the energy absorbed by a molecule is best desc r ibed in terms of the pr imary p rocess . The pr imary process comprises the i n i t i a l act of a b s o r p t i o n of a photon to produce an e x c i t e d e l e c t r o n i c s t a t e , and a l l subsequent events which lead e i t h e r to decomposit ion of the molecule (primary photochemical p r o c e s s ) , or to the re tu rn of the molecule to i t s the rmal l y e q u i l i b r a t e d ground e l e c t r o n i c s t a t e (primary photophysica1 p r o c e s s e s ) . As in o r g a n i c systems, the e x c i t a t i o n energy may be d i s s i p a t e d in severa l ways: p h o t o c h e m i c a l l y , r a d i a t i v e l y as luminescence, or non-r a d i a t i v e l y , a l l of which are desc r ibed by a J a b l o n s k i d i a g r a m . ' ' Such 3 k 2 2 k a diagram f o r a d complex i n v o l v i n g the T „ , T „ , E and A„ s t a t e s 2g 2g g 2g is shown in F i g . 2. E x c i t a t i o n i n t o the high energy s t a t e s (P ) , 4 T J ^ ( F ) i s fo l lowed by very rap id d e a c t i v a t i o n to the lowest e x c i t e d 6 F i g u r e . 2 J a b l o n s k i diagram fo r a d complex. Legend: , i n t r i n s i c f l u o r e s c e n c e e m i s s i o n ; k 0 , i n t e r n a l c o n v e r s i o n ; k^, photochemical r e a c t i o n from the T^^ s t a t e ; k^, in te rsys tem c r o s s i n g ; k_^, thermal l y a c t i v a t e d reverse in te rsys tem c r o s s i n g ; k^, i n t r i n s i c phosphorescence e m i s s i o n ; k^, in te rsys tem c r o s s i n g to ground s t a t e ; k^, photochemical r e a c t i o n from the "E s t a t e . 9 7 4 state ^ 2 g " ~*"his process, called internal conversion, has been estimated from the absence of fluorescence from upper states to be - 1 1 4 complete in about 10 sec. After excitation to the 1^ state, the complex may return to the ground state by radiative (fluores-cence, kj) or nonradiative (k^) processes, or may undergo a photo-chemical decomposition (k^). In most cases, however, intersystem 2 2 crossing (k^) to or states competes favorably with the processes just described as judged by the appearance of phosphores-cence (kj.) . Relaxation within the doublet manifold is also con-sidered to occur rapidly so that a l l processes following intersystem 2 2 crossing take place from the equilibrated E^ state. Once the E^ state is reached, much the same alternatives are again presented: besides phosphorescence, there may be intersystem crossing to the ground state (k^), a photochemical process (k^) and, in addition, a thermally activated reverse intersystem crossing (k_^) which may be 1 2 important at higher temperatures. The symbols ky k^, k^ denote f i r s t order rate constants and represent the probabilities of these intramolecular processes. Certain of these processes cannot be directly observed. But the three quantities, luminescence quantum yield 0, experimental lifetime x and integrated absorption coefficient /edv can be related to the rate con-stants kj , k^. The relationships in the absence of photochemical reactions and thermally activated back intersystem crossing (k_ = k , = k_= 0) for the V_ -> ^ A„ transition are"* 3 - 4 7 2g 2g 8 0 f= k 1/(k 1 + k 2 + kk) (2) x f = l/(k, + k 2 + kh) (3) 2 L and f o r the E -> A„ t r a n s i t i o n are 9 2g V ki,V ( k5 + k 6 ) ( k l + k 2 + k4 } ( A ) x p = l/(k5 + k 6) (5) The assumption k^  = k_^ = k^ = 0 is v a l i d fo r some C r ( l l l ) compounds at low temperatures , but was int roduced on ly to s i m p l i f y the r e l a t i o n s above. In the general c a s e , these processes may be important and must be c o n s i d e r e d . The f o l l o w i n g express ion is w ide l y used to r e l a t e the i n t e g r a t e d a b s o r p t i o n c o e f f i c i e n t /e(v)dv and the l i f e t i m e : f . j = 4.3 x 10~ 9 /e(v)dv (6) T Q = 0 .8 (g ./g. ) l / ( f . . v 2 i j ) (7) f . . i s the o s c i l l a t o r s t reng th of the t r a n s i t i o n j i and can be c a l -c u l a t e d from Equat ion 1, page 4. g . and g .^ are the degeneracies of the ground and e x c i t e d s t a t e s , r e s p e c t i v e l y , and v . . the frequency of the no-phonon t r a n s t i o n of j i . T q represents the natu ra l l i f e t i m e , i . e . the r a d i a t i v e l i f e t i m e in the absence of competing n o n - r a d i a t i v e p r o c e s s -e s . The r e c i p r o c a l of the f l u o r e s c e n c e n a t u r a l l i f e t i m e is k^  and that of phosphorescence is k^: k, = 1/T ( f luorescence) (8) 1 o k^- = l/x o (phosphorescence) (9) 4 4 For T ^ <- ^2g' ^ e r a t ' ° 9 j / 9 j = 3 i s g e n e r a l l y assumed and f o r 2 4 E *- A„ the r a t i o i s 2 . g 2g 9 In p r i n c i p l e , a l l these rate constants can be c a l c u l a t e d from these data at low temperature , i f = k ^ = = 0. But complete data are not a v a i l a b l e f o r any complex, not even f o r ruby, which is the most e x t e n s i v e l y s t u d i e d t r a n s i t i o n metal compound.'' O f t e n , on ly one type of emiss ion is d i r e c t l y observed in most c a s e s . Fur ther c o m p l i c a t i o n s a r i s e from inaccura te de te rminat ion of the a b s o l u t e emiss ion y i e l d (the exper imental e r r o r can be as large as 100%) and from the l i m i t e d a p p l i c a t i o n s of Equat ion J. S t r i c t l y s p e a k i n g , Equat ion 7 i s a p p l i c a b l e only to atomic systems where t r a n s i t i o n s are sharp . In somewhat mod i f ied form i t has been shown to hold ra ther w e l l f o r o r g a n i c molecules w i t h s t r o n g l y a l lowed t r a n s i t i o n s which show m i r r o r symmetry between - a b s o r p t i o n and emiss ion s p e c t r a . However, the d-d t r a n s i t i o n s of t r a n s i t i o n metal compounds are f o r -bidden and al though a mi r ro r - image r e l a t i o n s h i p between the a b s o r p t i o n and emiss ion s p e c t r a has been observed f o r some C r ( l l l ) compounds,'' Equation 7 must be used in a q u a l i t a t i v e way. 4. TEMPERATURE DEPENDENCE OF PHOSPHORESCENCE LIFETIMES As po inted out b e f o r e , most C r ( M l ) c o o r d i n a t i o n compounds show 9 phosphorescence o n l y . Therefore i t is not s u r p r i s i n g that most of the data pub l i shed on l i f e t i m e s and quantum y i e l d s are concerned e x c l u s i v e l y 4 5 wi th t h i s type of e m i s s i o n . ' S ince l i f e t i m e s are measured q u i t e e a s i l y and w i t h good accuracy when compared to abso lu te emiss ion quantum 10 yields, they are the most important diagnostic tool in studying rate constants. Many systematic studies of the lifetime of phosphorescence 1 7-22 have been described in the literature. In particular, much atten-1 9-22 tion has been focussed on its temperature dependence. From these studies thermally activated processes are detected and activation ener-gies are obtained which in turn may be used for their characterization. In addition, such data allow a correlation with results from emission 12 20 and absorption spectra. ' Thus, from the temperature dependence studies of the lifetime and the mechanism governing the temperature dependence, important information can be obtained on the fate of elec-tronic excitation energy in transition metal compounds. The phosphorescence lifetimes of C r ( l l l ) complexes are strongly dependent on temperature and a few investigations have been made in the 1 9 attempt to clarify the mechanism. Targos and Forster described the temperature dependence of the lifetime as a function of three terms k = l / x = k + k + kL (10) p p o a b with k being temperature-independent, k slightly temperature depend-O 3 ent and k^  being strongly temperature dependent. The rate constant k^ 2 and part of k Q were interpreted as intersystem crossing from the to the ground state. The radiative transition comprises the rest of k Q. The last term, k^, which is overwhelmingly dominant at relatively high temperatures, although not established then, was postulated to 2 represent the thermally activated intersystem crossing from the E back k 2 k to the T^g state, k_^  . Such crossing, E^ - - -> T^^, certainly does occur in compounds which have rather small energy separation between the 11 2 k / +3 23 2k Eg and the 1 s t a t e s , l i k e [Cr (urea)^] . The temperature v a r i a t i o n of k_^ then is g iven by the Boltzmann f a c t o r , exp ( -AE/kT) , where AE represents the energy d i f f e r e n c e between the z e r o - v i b r a t i o n a 1 k 2 l e v e l s of the T_ and E s t a t e s . Fur ther ev idence f o r such a c r o s s -2g g 1 2 ing came from s t u d i e s of ruby and emera ld . At h igh temperatures , delayed f l u o r e s c e n c e was observed and the c a l c u l a t e d a c t i v a t i o n energ ies of k_^ are kOO cm ' f o r emerald and 1200 cm ' f o r ruby. The c o r r e l a t i o n k 2 of AE w i t h the T 2 g ( v = 0 ) - Eg(v=0) s e p a r a t i o n , however, is not unambig-uous f o r both compounds. The va lue of AE f o r emera ld , a l though of k reasonable magnitude, puts the o r i g i n of the T^^ s t a t e at a s p e c t r a l - 1 25 2 l i n e at 151^0 cm ass igned by Wood to the Tj l e v e l . The (v= 0 ) - ^ E g(v= 0 ) s e p a r a t i o n fo r ruby i s about 2300 cm ' 2 6 and thus the va lue f o r AE obta ined from the l i f e t i m e data below 500°K is too 2 s m a l l . If the a c t i v a t i o n energy f o r E^ depopu la t ion above 500°K i s computed, a r e s u l t c o n s i s t e n t w i t h the s e p a r a t i o n i s o b t a i n e d . ' ' The involvement of k ^ was f u r t h e r demonstrated by recent work of Camassei 20 and F o r s t e r in a s e r i e s of C r ( l l l ) complex ions in va r ious c r y s t a l l i n e hosts that f l u o r e s c e on ly at high temperature . A c t i v a t i o n energ ies f o r k_^ were computed from the l i f e t i m e data but a c o r r e l a t i o n to the 4 2 T 2 g ( v = 0 ) - Eg(v=0) s e p a r a t i o n was not p o s s i b l e because the exact s e p a r -a t i o n s are not known. A l l the above-mentioned compounds have small Dq v a l u e s . The study was extended to complexes w i t h a high l i gand f i e l d 21 22 s t rength by Zander and Chen . The a v a i l a b l e evidence support therm-2 a l l y a c t i v a t e d reverse in te rsys tem c r o s s i n g as a major path of E^  d e a c t i v a t i o n at h igher temperatures . Delayed f l u o r e s c e n c e , however, was not observed . 12 5. PURPOSE OF THIS WORK From the fo rego ing i t is obvious that the proposed mechanism 2 of the temperature dependence of the E^ phosphorescence has to be f u r t h e r conf i rmed . Th is is important f o r photochemists in t h e i r study of the r e a c t i v i t y of e x c i t e d s t a t e s as w e l l as fo r s p e c t r o s -4 c o p i s t s . The p o s i t i o n of the z e r o - v i b r a t i o n a 1 l e v e l of the T „ 2 g s t a t e , not a v a i l a b l e from s p e c t r o s c o p i c s t u d i e s in many c a s e s , cou ld be est imated from the a c t i v a t i o n energy of k_^. Furthermore, except 27 f o r the R - l i n e s of ruby , the temperature dependence of has not yet been s t u d i e d f o r any complex. Equation 7 o f f e r s a way to c a l c u -l a t e the dependence from the in teg ra ted a b s o r p t i o n c o e f f i c i e n t /e(v)dv of 2 E + V . 9 2 9 It was f e l t that an oc tahedra l molecu lar complex MA^ w i t h monoatomic l igands A such as h a l i d e s would be the s i m p l e s t system f o r a study of t h i s k i n d . A molecu lar complex r e t a i n s i t s i d e n t i t y in the s o l i d s t a t e and in s o l u t i o n , as opposed to i o n i c complexes ( t h i s term has been used by Fors ter ' ' ) l i k e ruby, fo r example. The p r o p e r t i e s of an i o n i c complex are i n t i m a t e l y r e l a t e d to those of the host l a t t i c e . The e l e c t r o n i c s t a t e s u s u a l l y are coupled s t r o n g l y to perturbed l a t t i c e modes, hence a s a t i s f a c t o r y a n a l y s i s of the v i b r o n i c spectrum of a C r ^ + 2 7 i o n i c complex has not been p r e s e n t e d . S ince i n t r a - l i g a n d v i b r a t i o n s of po lyatomic l i gands a l s o couple to e l e c t r o n i c t r a n s i t i o n s of the d - e l e c t r o n s , the v i b r a t i o n a l s t r u c t u r e of a b s o r p t i o n and emiss ion spec t ra of the oc tahedra l MAg w i th monatomic 13 l igands should be the l e a s t compl icated and e a s i e s t to r e s o l v e . In 4 t h i s case i t might a l s o be p o s s i b l e to l o c a t e the o r i g i n s of 2 and E s t a t e s from a v i b r a t i o n a l a n a l y s i s of the s t r u c t u r e . In 9 a d d i t i o n , from a good reso lved s t r u c t u r e r e l i a b l e va lues f o r the in teg ra ted a b s o r p t i o n c o e f f i c i e n t may be obta ined even at h igh temperature, which is important f o r the study of the temperature dependence of k,.. For t h i s work the oc tahedra l complex Cs2MnF^ was chosen. Th is complex is molecu lar and has monoatomic l i g a n d s . In a d d i t i o n k 2 28 the T^g and E^ t r a n s i t i o n s are s u f f i c i e n t l y separated fo r a proper a n a l y s i s as opposed to the case f o r ICrF,]^ , f o r e x a m p l e . 2 9 The f i r s t step was to determine the fundamental f requenc ies of the ground s t a t e . They are used in the v i b r a t i o n a l a n a l y s i s of the a b s o r p t i o n and emiss ion s p e c t r a and thus are important . Such data are prov ided by the IR and Raman s t u d i e s desc r ibed in Chapter I I I . In Chapter IV a v i b r a t i o n a l a n a l y s i s of the e l e c t r o n i c absorp -t i o n and emiss ion spec t ra is c a r r i e d out and, where p o s s i b l e , o r i g i n s are a s s i g n e d . A s u i t a b l e host was sought to study [MnF^p in o p t i c a l l y d i l u t e c r y s t a 1 s . The r e s u l t s of the temperature dependence of emiss ion quantum y i e l d , l i f e t i m e and in teg ra ted o s c i l l a t o r s t rength are presented in Chapter V a long w i t h a d i s c u s s i o n of the p o s s i b l e primary processes F i n a l l y , in Chapter V I , the c o n c l u s i o n s of t h i s study are sum-marized and suggest ions f o r f u r t h e r work are made. of [MnF6J 2-At the beginning of this study, only the reflectance spectrum 29 k+ of K^MnF^ and a very low resolution emission spectrum of Mn in 30 the MgGeFg host had been described in the li terature. However, while this work was in progress, several publications on the complex 2- ~i] 32 33 [MnF ]^ appeared in a rapid succession which described the IR, ' ' 32 3H 35 31 35 36 37 35 the Raman, ' ' the reflectance ' ' ' and the emission spectrum High resolution spectra of single crystals have not yet been reported i n the 1i terature. 15 II E X P E R I M E N T A L 1. PREPARATION Potass ium hexaf1uoromanganate(I V) was prepared by reducing potassium permanganate w i th ether in h y d r o f l u o r i c a c i d . The crude compound was p u r i f i e d by repeated c r y s t a l l i z a t i o n s from k8% HF (reagent grade, F i s h e r S c i e n t i f i c Co.) u n t i l no f u r t h e r change in the emiss ion spectrum or l i f e t i m e could be d e t e c t e d . Rubidium and cesium hexaf1uoromanganate(I V) were syn thes i zed from the potassium s a l t by c a t i o n exchange. S i n g l e c r y s t a l s were obta ined from concentrated s o l u t i o n s of the complexes in k8% HF by slow evaporat ion of the so l ven t at 40°C in p la t inum d i s h e s . The s i n g l e c r y s t a l s were about 2 mm in diameter and 1.5 mm t h i c k . The potassium and rubidium s a l t s form y e l l o w ho lohedra l hexagonal pyramids. The cesium s a l t c r y s t a l l i z e s in y e l l o w oc tahedra . S i n g l e c r y s t a l s of cesium hexaf1uorogermanate(I V) doped w i t h Mn ions were grown from a concentrated s o l u t i o n in k8% HF c o n t a i n i n g t races of Cs^MnF^. By slow evaporat ion of the so lvent w e l l developed s i n g l e c r y s t a l s in the form of cubes were o b t a i n e d , some about 3 mm in d i ameter. k+ A s e r i e s of s i n g l e c r y s t a l s w i t h d i f f e r e n t Mn content was p r e -pared. The mole r a t i o ( in %) v a r i e d from 0.1 to about 10. The Mn 39 content was determined by an lodometr ic t i t r a t i o n . For a n a l y s i s k+ c r y s t a l s c o n t a i n i n g about k% of Mn were used. About 500 mg w i th 16 equal o p t i c a l d e n s i t y to t h i c k n e s s r a t i o were d i s s o l v e d in concent rated h y d r o c h l o r i c a c i d and the c h l o r i n e l i b e r a t e d was passed through a p o t -assium iod ide s o l u t i o n . The iod ine formed was then t i t r a t e d w i th n/100 t h i o s u l f a t e s o l u t i o n from a m i c r o b u r e t t e . Cesium hexaf1uorogermanate(I V) was prepared by g e n t l y heat ing a mixture of germanium t e t r a c h l o r i d e (Pen insu la r Chem-Research, Inc. ) in aqueous HF c o n t a i n i n g cesium f l u o r i d e (A lpha - Inorgan ics ) in excess . The crude compound was p u r i f i e d by repeated c r y s t a l l i z a t i o n s from 48% HF. K^GeF^ was prepared s i m i l a r l y . Pure Cs^CeF^ forms c o l o r l e s s we l l dev-eloped cubes whereas the K^GeF^ c r y s t a l l i z e s in t h i n hexagonal p l a t e s . Attempts to grow s i n g l e c r y s t a l s of C s ^ S i F ^ , Cs2SnF^ and Cs^TiF^ were not s u c c e s s f u 1 . 2 . IR SPECTRA The IR spect ra in the region 2000 - 500 cm were taken on a P e r k i n g - E l m e r 421 and from 250 - 2000 cm ' on a P e r k i n g - E l m e r 457. The s p e c t r a were obta ined in three d i f f e r e n t media: p e l l e t s of the very f i n e l y powdered sample were e i t h e r fused w i th p o l y e t h y l e n e , pressed w i t h KBr or mul led w i t h Nujol us ing AgCl windows. 3 . RAMAN SPECTRA The s p e c t r a were recorded on a Cary Model 81 Raman Spect rophoto -o meter. The samples were e x c i t e d by the 6328 A l i n e of a He/Ne l a s e r of 17 Spect ra P h y s i c s Model Ml 25 which was operated at 80 mW output power. A s p e c i a l dewar was employed to o b t a i n the low temperature s p e c t r a . k. THE DEWAR The dewar was s p e c i a l l y c o n t r u c t e d to a l l o w s imultaneous a b s o r p -t i o n , emiss ion and l i f e t i m e s t u d i e s over a wide temperature range. In a d d i t i o n , i t s dimensions were chosen in such a way that i t f i t t e d i n t o the c e l l compartment of the Cary \k spect rophotometer . The outer metal c y l i n d e r of the dewar was equipped w i t h two q u a r t z windows at 180° f o r a b s o r p t i o n measurements and two a d d i t i o n a l q u a r t z windows at 45° to the d i r e c t i o n of a b s o r p t i o n to permit s i m u l -taneous emiss ion and l i f e t i m e s t u d i e s by f r o n t i l l u m i n a t i o n of the sample. Th is is i l l u s t r a t e d in F i g . 3-A copper b l o c k , t i g h t l y screwed to the bottom of the inner c a n , served as a sample h o l d e r . The temperature of the copper b lock was v a r i a b l e from 80°K to about 700°K. Temperatures below 300°K were obta ined by c i r c u l a t i n g c o l d n i t r o g e n gas through the inner c a n . The temperature was c o n t r o l l e d by the f low ra te of the gas generated from l i q u i d n i t r o g e n in a 50 l i t r e dewar. High temperatures up to 700°K and over were obta ined w i t h two heat ing elements (Gee Henley L t d . , 25 W), both in c l o s e contact w i t h the copper b l o c k . The sample was at tached to the copper b lock w i t h v a s e l i n e or w i t h s i l i c o n e rubber (RTV, General E l e c t r i c ) and heated or cooled by 18 DET gure 3> Schematic of the setup f o r a b s o r p t i o n , emiss ion and l i f e t i m e measurements. Legend: AMP = a m p l i f i e r ; BS = beam s p l i t t e r ; D = dewar; DET = d e t e c t o r ; F = f i l t e r ; FL = f l a s h lamp; L = quar tz l e n s e s ; M = monochromator; MA = mercury a r c ; Ql = quar tz iod ine lamp; PMP = p h o t o m u l t i p l i e r ; PT = phototube; S = sample; SF = s o l u t i o n f i l t e r . 19 c o n d u c t i o n . To improve thermal contact the m a t e r i a l was mixed w i th copper d u s t . These mixtures d id not show any d e t e c t a b l e emiss ion under s t rong u l t r a v i o l e t r a d i a t i o n in the temperature range s t u d i e d . Care was taken to c a r r y out temperature changes very s low ly to a l l o w thermal e q u i l i b r a t i o n . Dur ing measurement the temperature was kept constant to w i t h i n 2 degrees or b e t t e r over a s u f f i c i e n t per iod of time to scan the spectrum. A d i f f e r e n t dewar was employed to record the a b s o r p t i o n and emiss ion s p e c t r a at a temperature c l o s e to that of l i q u i d h e l i u m . k] This dewar was s p e c i a l l y designed f o r low temperature s t u d i e s . A Sylphon be l low at the top of the dewar a l lowed a v e r t i c a l d i s p l a c e -T i ment of the sample ho lder so that two d i f f e r e n t c r y s t a l s cou ld be s tud ied dur ing the same exper iment . The samples were at tached to the ho lder by GE7031 cement and cooled by c o n d u c t i o n . The a c t u a l temperature of the sample was approx imate ly 10°K. 5. ABSORPTION SPECTRA The complexes were s tud ied as s i n g l e c r y s t a l s and in s o l u t i o n . The absorp t ion spectrum of K^MnF^ d i s s o l v e d in 20% HF was measured w i t h a Cary )k spectrophotometer . Quartz c e l l s were employed which were coated on the i n s i d e w i th f l u o r o c a r b o n grease (KEL -F , No. 90) f o r p r o t e c t i o n . The a b s o r p t i o n spec t ra of s i n g l e c r y s t a l s in the temperature range from 90°K to about 700°K were measured w i t h the same ins t rument . 20 In most cases the re fe rence beam had to be a t tenuated w i t h n e u t r a l d e n s i t y f i l t e r s made of w i r e mesh. Depending on the o p t i c a l d e n s i t y requ i red , the t h i c k n e s s of the c r y s t a l s v a r i e d from 30 u to about 1.5 mm• The temperature was monitored w i t h a copper - cons tan tan thermo-couple next to the c r y s t a l in connect ion w i t h a potent iometer (Rub icon) . The a b s o r p t i o n spectrum of s i n g l e c r y s t a l s of Cs^MnF^ at 10°K was measured w i t h a d i f f e r e n t high r e s o l u t i o n appara tus . A b lock diagram is shown in F i g . 3-The l i g h t of a q u a r t z - i o d i n e lamp (SYL, 100 W) was focussed onto the sample conta ined in a c r y o s t a t . The t r a n s m i t t e d l i g h t was analyzed by a Czerny -Turner g r a t i n g type monochromator (Spex Inst ruments , 0.75 m, o o r e c i p r o c a l l i n e a r d i s p e r s i o n 10 A/mm, b lazed at 7500 A , 1200 grooves per mm) and detected by a RCA 1P28 p h o t o m u l t i p l i e r (S~5 c h a r a c t e r i s t i c s ) powered by a s t a b i l i z e d high v o l t a g e source (Nuclear E n t e r p r i s e s L t d . ) . The s i g n a l was a m p l i f i e d (4l4 Micro -microammeter , K e i t h l e y I n s t r . ) and d i s p l a y e d on a paper char t recorder ( B r i s t o l Dynamaster) . The wavelength marker of the Spex monochromator was checked w i t h neon 1 i nes. 6. EMISSION SPECTRA The complexes were i n v e s t i g a t e d as s i n g l e c r y s t a l s , in p o l y c r y s -t a l l i n e form and in s o l u t i o n . The s o l v e n t was 20% HF which forms a r i g i d g l a s s at l i q u i d n i t r o g e n temperature . 21 The general schematic diagram of the emiss ion apparatus is shown in F i g . 3-The sample was i r r i d i a t e d w i t h the 366 l i n e of a po in t source mercury arc (PEK 110, 100 W) powered by a s t a b i l i z e d DC power supply (PEK, Model 401) . The 366 nm l i n e was i s o l a t e d w i th a monochromator (Bausch 6 Lomb, Model 33-86-25) equipped w i th a UV g r a t i n g (33"86 -Ol), a f i l t e r s o l u t i o n c o n s i s t i n g of a 3 cm layer of a concent ra ted aqueous s o l u t i o n of CuSO^ and a blue f i l t e r (Corning 7"54) . Large s l i t widths were used, 5.36 mm on the entrance and 3 mm on the e x i t s l i t . The r e l a t i v e i n t e n s i t y of the e x c i t a t i o n l i g h t was monitored through a beam s p l i t t e r , a phototube (RCA 935) and a 10 mV recorder to avoid e r r o r s caused by f l u c t u a t i o n s of the i n t e n s i t y of the e x c i t a t i o n l i g h t . A short f o c a l length lens ( 1 inch in d iameter , 27 mm f o c a l length) was placed in to the dewar c l o s e to the sample to c o l l e c t the emi t ted 1 i g h t . The l i g h t was then passed through an orange f i l t e r (Corning 3_67) or a sharp cut y e l l o w f i l t e r (Corning 3~73) and analyzed by the d e t e c t i n g system which has been a l ready desc r ibed (Sect ion 5, page 2 0 ) . The emiss ion spec t ra so obta ined were c o r r e c t e d f o r s p e c t r a l r e s -ponse of the d e t e c t i n g system. The u n i t , c o n s i s t i n g of the Spex mono-chromator and the RCA 1P28 p h o t o m u l t i p l i e r , was c a l i b r a t e d w i t h a s t a n d -ard q u a r t z - i o d i n e lamp ( L - 1 0 1 , E l e c t r o O p t i c s , w i th P-101 power s u p p l y ) . The spectrum of the lamp was recorded under the same c o n d i t i o n s used to 22 o b t a i n the s p e c t r a of the complexes, except that the s l i t s of the monochromator were rep laced by a p i n h o l e . The c o r r e c t i o n f a c t o r s were computed by comparing the spectrum of the lamp as seen by the d e t e c t i n g system w i t h that s u p p l i e d by the manufacturer . The r e s u l t s are d i s p l a y e d in F i g . 4. As the i r r a d i a n c e c h a r a c t e r i s t i c s of the s tandard lamp depend s t r o n g l y on input power, the c a l i b r a t i o n of the ammeter was checked a g a i n s t a l a b o r a t o r y s t a n d a r d . A d i f f e r e n t dewar was employed to study the e m i s s i o n (and a b s o r p -t i o n ) at 10°K. Th is c r y o s t a t was equipped w i t h two q u a r t z windows at 180°. There fo re the emiss ion s p e c t r a were recorded by back i l l u m i n a t i o n . A l though t h i n c r y s t a l s were employed, approx imate ly 50 microns t h i c k n e s s , the emiss ion was p a r t i a l l y s e l f - a b s o r b e d which r e s u l t e d in a l oss of emiss ion i n t e n s i t y . The samples were e x c i t e d w i t h the 436 nm l i n e of a mercury a r c , i s o l a t e d w i t h a 433 nm i n t e r f e r e n c e f i l t e r (Fa . Schott ) and a 3 cm l a y e r of a concent ra ted aqueous s o l u t i o n of CuSO^. The emiss ion spectrum of K^MnF^ was obta ined w i t h a d e t e c t i n g system of lower r e s o l u t i o n . The emi t ted l i g h t was passed through an orange f i l t e r (C3"67) and recorded w i t h a J a r r e l l Ash monochromator (Model 8 2 - 4 0 0 , d i s p e r s i o n o 33 A/mm, 500 micron s l i t s ) and a coo led P h i l i p s 150 CVP p h o t o m u l t i p l i e r (SI response) powered from a Kepco ABC DC high v o l t a g e s u p p l y . 23 F igure k. P r o f i l e of the standard lamp ( l ) , response of the d e t e c t i n g system ( l l ) and curve of c o r r e c t i o n f a c -tors ( I I I ) . 2k 7. LIFETIME MEASUREMENTS The l i f e t i m e of the complexes were measured by the s i n g l e pu lse method. Acco rd ing to t h i s method, a s i n g l e high i n t e n s i t y l i g h t f l a s h i s generated f o r e x c i t a t i o n which i s shor t compared to the l i f e t i m e of the sample. The important elements of t h i s apparatus are d e p i c t e d in F i g . 3: a shor t f l a s h , generated by a high pressure n i t r o g e n f l a s h lamp, is focussed onto the sample conta ined in the dewar and the decay a n -a l y z e d by a d e t e c t i n g system of f a s t response. Operated at 5 atmospheres and 15 kV the lamp generated a f l a s h \k of approx imate ly 200 nsec h a l f - h e i g h t w idth and an i n t e n s i t y of 5 x 10 photons per p u l s e . The d e t a i l s of c o n s t r u c t i o n and f u r t h e r c h a r a c t e r -i s t i c s w i l l be d e s c r i b e d in the next s e c t i o n . The f l a s h was f i l t e r e d through a 3 cm layer of a concent ra ted aqueous s o l u t i o n of CuSO^ to remove the red par t of the e m i s s i o n which l i m i t e d the e x c i t a t i o n l i g h t to the range from 300 nm to about 500 nm. S o l u t i o n s were s t u d i e d in a t i g h t l y capped, f l a t t e n e d quar t z c e l l of approx imate ly 2 mm path length and 1 ml c a p a c i t y . The temperature was monitored in s i t u by a smal l q u a r t z c l a d copper - cons tan tan thermo-couple c l o s e to the i r r i d i a t e d part of the sample. The s o l u t i o n s were deoxygenated before measurements by purg ing the s o l u t i o n s at room temp-e r a t u r e w i t h pure n i t r o g e n ( L i q u i d A i r of Canada L t d . ) of L grade p u r i t y which had been passed through a t r a i n of washing b o t t l e s c o n t a i n i n g 25 vanadium ( l I ) c h l o r i d e . Mo lecu la r oxygen appears to quench the phos-phorescent s t a t e of complexes in s o l u t i o n . Th is has been demonstrated 42 at least , f o r a s e r i e s of C r ( l l l ) complexes. To e l i m i n a t e s c a t t e r e d l i g h t from the e x c i t a t i o n source the emiss ion was f i l t e r e d through a 2 cm layer of a concent rated aqueous s o l u t i o n of I^C^O^, an orange f i l t e r (C3_67) and a Bausch S Lomb mono-chromator equipped w i t h a v i s i b l e g r a t i n g (33-86-02, 1350 grooves per mm, b lazed at 500 nm, r e c i p r o c a l l i n e a r d i s p e r s i o n 6.4 nm/mm). The entrance and the e x i t s l i t s were set at 3«^ and 6 mm, r e s p e c t i v e l y . The decay was detected w i th a RCA 8645 p h o t o m u l t i p l i e r (S -20 c h a r a c t e r -i s t i c s ) connected to a Kepco high vo l tage supp l y . The s i g n a l was measured across a 1000 load r e s i s t o r (ICR, 2%, metal f i l m , 1/2 W). The p h o t o m u l t i p l i e r was connected d i r e c t l y to the Type L p l u g - i n u n i t of a T e k t r o n i x 453B o s c i l l o s c o p e . The response time of the combined u n i t ( p h o t o m u l t i p l i e r , l o a d , BNC connecto r , o s c i l l o -scope) was c a l c u l a t e d to be 35 nsec which is n e g l i g i b l e compared to the width of the e x c i t a t i o n pu lse (200 n s e c ) . A smal l c a p a c i t o r was p laced in p a r a l l e l w i th the p h o t o m u l t i p l i e r and the o s c i l l o s c o p e as a no ise f i l t e r . Depending on the l i f e t i m e of the samplejthe s i z e of the c a p a c i t o r ranged from 50 pf to 0.003 u f . As a consequence, the response time of the d e t e c t i n g system was mainly d e t e r -mined by the noise f i l t e r . The s i z e of the f i l t e r was chosen so that the response time was at l e a s t ten times f a s t e r than the decay rate to be measured. Under these c o n d i t i o n s the f i l t e r had a n e g l i g i b l e e f f e c t on the decay curve of the sample. The t races on the o s c i l l o s c o p e were photographed w i th a Du Mont 26 camera (Type 302) and Polaroid Type 410 film (10,000 ASA). To improve accuracy of the analysis the decay curves were enlarged by approximately six times with a delineascope. The liftimes obtained in this way were reproducible to within 5% or better. 8, THE FLASH LAMP The flash lamp was designed to study the weak emission of trans-ition metal complexes with decay times less than 1 usee. The require-ments called for a polychromatic source of: sufficiently high intensity together with a rapid decay rate. Therefore, the design considerations were: (l) a small point light source, and (2) a s t r ic t coaxial arrange-ment for the lamp with small length to keep the se1f-inductance of the electrical c i rcui t to a minimum. A point 1 ight source was approximated by using a short electrode spacing ( 1 mm or less) and high pressures (above 1 atm) at which a dense and confined discharge occurs. At high pressure, the radiative efficiency is high and the decay characteristics, because of a high 44 col l i s ion rate, are good. A cross section drawing of the lamp is shown in Fig. 5. The lamp housing (2.5 cm diam. and 8 cm long) was mounted directly on a flat coaxial disk capacitor (Tobe Deutschmann ESC 247F), which has a low inductance (about 1 nH) and can store up to 10 J (0.05 pF at 20 kV) . The electrodes (1.5 mm diam. tungsten rods), welded into brass rods 6 mm in diameter, were insterted through brass cones sealed 27 Figure 5 . Cross sectional view of the flash lamp. 28 wi th A r a l d i t e cement to the female B-14 j o i n t s of a shor t s i l i c a tube. The tube was blown out s l i g h t l y f o r added s t reng th and to minimize the depos i t of e l e c t r o d e m a t e r i a l onto the inner s u r f a c e . With t h i s a r range -ment, the lamp has been operated f o r severa l years at 6 atm and 15 kV (5 J e l e c t r i c a l energy) wi thout b r e a k i n g . The lamp bulb had to be c leaned only once a year . The lamp could be evacuated and p r e s s u r i z e d through the upper brass tube. The upper e l e c t r o d e (cathode) had a rounded end w h i l e the anode 45 was p o i n t e d , an arrangement found best f o r a s t a b l e d i s c h a r g e . The e l e c -t rode spac ing was a d j u s t a b l e by a threaded f i t t i n g on the cathode assembly sealed w i th O - r i n g s . A t i g h t f i t t i n g Te f lon tube , 2 mm t h i c k , i n s u l a t e d the anode e l e c t r i c a l l y from the grounded brass hous ing . To prevent r i n g -i n g , a set of damping r e s i s t o r s c o n s i s t i n g of 12 low inductance carbon r e s i s t o r s (12 fi, 1/2 W) in p a r a l l e l , connected the anode to the c a p a c i t o r . The c a p a c i t o r was charged w i t h a standard high vo l tage DC supp l y . Discharge was i n i t i a t e d by c l o s i n g a manual high vo l tage swi tch w i t h s t a i n l e s s s t e e l contact s u r f a c e s , as show in F i g . 5- The lamp was f i l l e d w i th L grade n i t rogen ( L i q u i d A i r of Canada L imi ted) taken d i r e c t -ly from the tank. N i t rogen was chosen because of i t s high breakdown p o t -e n t i a l and i t s high e f f i c i e n c y of e l e c t r i c a l to r a d i a t i v e energy convers ion r e l a t i v e to other gases . The s p e c t r a l composi t ion of the l i g h t from the f l a s h lamp operated at 5 atm and 15 kV w i th an e l e c t r o d e spac ing of 0.75 mm was recorded w i t h a medium H i l g e r spectrograph (0.1 mm s l i t s ) on HP3 l l f o r d p l a t e s . The spectrum c o n s i s t e d of many l i n e s c l o s e l y spaced in the u l t r a v i o l e t w i t h 29 an u n d e r l y i n g continuum. The i n t e n s i t y of the source in the o p t i c a l 46 arrangement used was measured w i th a f e r r i o x a l a t e a c t i n o m e t e r . A 1 cm c e l l . w a s p laced d i r e c t l y over the 1 cm diam. opening in the lamp hous ing , 1.5 cm from the center l i n e of the lamp. With 0 .006 M s o l u -t i o n of potassium f e r r i o x a l a t e , a l l l i g h t below 400 nm was absorbed, w h i l e at 450 nm the c e l l t ransmi ts about 80%. The average l i g h t o u t -14 put from the lamp operated at 5 atm and 15 kV was 5 x 10 quanta per pu lse up to c a . 420 nm. Some t y p i c a l decay curves are d i s p l a y e d in F i g . 6. In g e n e r a l , the shape was found to be dependent on the pressure and a p p l i e d v o l t a g e . For example, at 1.5 atm and 7 kV w i th an e l e c t r o d e spacing of 0.75 mm, the decay was f i r s t order over three l i f e t i m e s . The decay l i f e t i m e was 130 nsec , the h a l f - h e i g h t w idth 165 nsec , and , a f t e r 900 nsec , the i n t e n s i t y of the t a i l was below ]% of the maximum. At 5 atm and 15 kV, the h a l f - h e i g h t width was 200 nsec and the t a i l was somewhat more intense ( less than ]% a f t e r 1.4 u s e e ) . In each c a s e , the r i s e t i m e was a p p r o x i -mately 60 nsec . The decay curves were obta ined w i th an RCA 8645 photo -m u l t i p l i e r w i th 1 kfi load connected d i r e c t l y to the Type L p l u g - i n u n i t of a T e k t r o n i x 453B o s c i l l o s c o p e . To minimize RF pickup by the d e t e c t i n g system, the lamp base, i n c l u d i n g the c a p a c i t o r , was enc losed in a Faraday cage made of copper mesh. 30 1 < n 1 1 \ Figure 6. Decay curves (200 nsec/div) with (a) 1.5 kV at 7 atm and (b) 15 kV at 5 atm (electrode spacing in both cases 0.75 mm). 31 III F U N D A M E N T A L F R E Q U E N C I E S O F T H E G R O U N D S T A T E If not o therwise p e r t u r b e d , the ground s t a t e v i b r a t i o n a l f requenc ies c o n s i s t of combinat ions and overtones of a c e r t a i n number of f requenc ies which are c a l l e d the fundamental f r e q u e n c i e s . As ide from the i n t e r e s t in the fundamental v i b r a t i o n a l f requenc ies of the ground s t a t e i t s e l f , the knowledge of these f requenc ies i s important fo r a v i b r a t i o n a l a n a l y s i s of the s t r u c t u r e in emiss ion and a b s o r p t i o n spect r a . The fundamentals of the ground s t a t e are determined from Raman and IR s p e c t r a . An i n t e r p r e t a t i o n of these s p e c t r a requ i res p r i o r knowledge of the number of fundamentals to be expected and t h e i r a c t i v -i t y (IR or Raman a c t i v e ) . Th is in fo rmat ion i s obta ined from the s t r u c -ture of the molecule by symmetry c o n s i d e r a t i o n s . The X - ray s t r u c t u r e s of the K, Rb and Cs s a l t s of H^nF^. and 47 H^GeFg were s tud ied by Bode et_ a_l_. The p e r t i n e n t r e s u l t s a r e : Cs^MnF^ and Cs^GeF^ are both cub ic and belong to the space group Fm3m (0^) . The s i x f l u o r i d e anions form a regu la r octahedron w i t h the t e t r a v a l e n t c a t i o n s at a p e r f e c t oc tahedra l s i t e . The room temperature s t r u c t u r e s of K^MnF^ and Rb^MnF^ are hexagonal and belong to the space group C6mc (cj^)• In t h i s c a s e , the octahedron formed by the f l u o r i d e anions is t r i g o n a l l y 4+ d i s t o r t e d and the s i t e symmetry of the Mn ion is C^ v -4+ In the mixed c r y s t a l s Cs^GeF^iMn , the manganese c a t i o n s u b s t i t u t e s 32 4+ the germanium ion e x a c t l y and the s i t e symmetry of the Mn ion i s Accord ing to group theory the fundamental f requenc ies of an ( i s o l a t e d ) oc tahedra l MA, spec ies l i k e [ M n F ^ ~ (A i s a monoatomic l igand) c o n s i s t s of three Raman-act ive v i b r a t i o n s , and of symmetry a jg> e g a n d t2g' r e s p e c t i v e l y , two IR a c t i v e v i b r a t i o n s and v^, both of symmetry t ^ u , and one i n a c t i v e v i b r a t i o n of symmetry t 2 u - With the except ion of v^, a l l of the fundamentals are doubly or t r i p l y degenerate . In the s o l i d , however, the spec ies are not i s o l a t e d from each other and molecu lar i n t e r a c t i o n s compl i ca te the v i b r a t i o n a l spectrum. In c o n t r a s t to gases and l i q u i d s , e x t e r n a l modes appear in a d d i t i o n to the i n t e r n a l modes. The i n t e r a n l modes are the v i b r a t i o n s of the MA^ u n i t perturbed by intermolecu 1ar fo rces and/or d i f f e r e n t s i t e symmetries. The e x t e r n a l modes a r i s e from the motion of the e n t i r e molecules or ions r e l a t i v e to one another and are l a t t i c e v i b r a t i o n s . Compared w i t h the i n t e r n a l modes, they are of lower i n t e n s i t y and have low f requenc ies (of the order of 100 cm ^ or l e s s ) . Accord ing 82 to P o l l a c k , the l a t t i c e v i b r a t i o n s of c r y s t a l s of t h i s type c o n s i s t of a t rans1 a t iona1 mode of symmetry t j ( a c o u s t i c a l l a t t i c e mode) and two l a t t i c e modes of symmetry t ^ and due to the v i b r a t i o n s of the c a t i o n r e l a t i v e to the[MA^| complex. Except fo r the mode which is Raman-act i ve , the l a t t i c e v i b r a t i o n s are r a r e l y observed because of t h e i r low f requency . Presented in t h i s chapter are the fundamental modes of the 2-complex [MnF^] as determined from IR and Raman s p e c t r a . 33 1. RESULTS The Raman spec t ra of the K, Rb and Cs s a l t s of h^MnF^ in the form of s i n g l e c r y s t a l s and as powders were obta ined at room tempera-ture and as powders at about 100°K. The spec t ra f o r the Cs and the K complexes are dep ic ted in F i g s . 7 and 8. No d i f f e r e n c e was noted in the spectrum of a powder and a s i n g l e c r y s t a l at room temperature . The s p e c t r a (Raman and absorpt ion ) f o r the K and Rb s a l t s were i d e n t i c a l and t h e r e f o r e Rb„MnF, was not s t u d i e d f u r t h e r . 2 6 The IR spectrum of I^MnF^ i s shown in F i g . 9- The IR frequency of K^MnF^ determined in three d i f f e r e n t media (po lye thy lene and KBr p e l l e t s , Nujol mul l ) agree w i t h i n 3>%. Wi th in exper imental e r r o r no d i f f e r e n c e in band p o s i t i o n s was observed in the IR s p e c t r a of the Cs and K complexes. 2. DISCUSSION i) Raman Spectrum In the Raman spectrum of an oc tahedra l h e x a f l u o r i d e complex l i k e K^NiF^, fo r example, three dominant bands at 562, 520 and 310 cm ' are observed , which have been ass igned to the three fundamental v i b r a t i o n s , and v ^ , r e s p e c t i v e l y . " ^ Although N i ( l V ) is very s i m i l a r to Mn(lV) (with respect to v a l e n c y , mass, rad ius) the Raman 2-spectrum of [MnF^] d i f f e r s c o n s i d e r a b l y : there are more bands w i t h remarkably h igher i n t e n s i t y , e s p e c i a l l y in the spectrum of the Cs s a l t . 3h INTENSITY *~ 0 F igure 7. Raman spectrum of p o l y c r y s t a l 1 i n e Cs„MnF, . 2 6 S o l i d l i n e : 300°K, s e n s i t i v i t y 2 , s l i t 1 cm" 1 (0.1 cm" 1 ) Dashed l i n e : 100°K, s e n s i t i v i t y 2 , s l i t 5 cm" 1 35 INTENSITY •924 •906 •867 0 F igure 8. Raman spectrum of po l yc rys ta1 I ine K MnF. . 2 6 S o l i d l i n e : 300°K, s e n s i t i v i t y 200, s l i t 3.8 cm" 1 Dashed l i n e : 100°K, s e n s i t i v i t y 2000, s l i t 10.0 cm 37 Furthermore, the Raman spectra of Cs„MnF, and K„MnF, d i f f e r in band 2 o Z b positions and i n t e n s i t i e s (three comparatively narrow bands at 600, 507 and 310 cm ' in the l a t t e r are not to be seen at a l l in the Raman spectrum of the Cs complex) which was not observed in the spectra of the same s a l t s of N i ( I V ) . 5 0 , 5 1 High i n t e n s i t i e s (on an absolute basis) can be caused by a 52 resonance Raman e f f e c t or may be due to resonance emission. Such overlapping emission bands are observed, for example, in the Raman 53 spectrum of some C r ( l l l ) complexes and resonance emission (phosphor-escence) most probably is responsible for the observed anomalies in the Raman spectra of s o l i d hexaf1uoromanganates. The complexes are strong phosphors and they emit even at room temperature. The emission of the cesium s a l t is e s p e c i a l l y intense with the same emission quantum y i e l d at low (85°K) and high (300°K) temperatures. The emission properties of the hexaf1uoromaganates(IV) w i l l be described in de t a i l in the next chapter. For comparison, emission spectra, relevant parts of the absorp-tion spectra together with the Raman spectra are displayed in Figs. 10 and 11. Two d i s t i n c t features should be noted: (1) to almost every band in the Raman spectrum there corresponds a s i m i l a r band in the emission spectrum and (2) some absorption bands are very close to or overlap the exc i tat ion l i n e . From both emission and absorption studies, these p a r t i c u l a r bands 2 4 were i d e n t i f i e d as "hot bands" of the spin-forbidden t r a n s i t i o n s E -«- A„ . g 2g 38 kK 15-5 16.0 16.5 t 1 1 F igure 10. Comparison of Raman, emiss ion and a b s o r p t i o n spectrum of Cs2MnF^. Lower p a r t : Raman spectrum at 300°K ( s o l i d l i n e ) and 100°K (dotted 1 i ne) . Upper p a r t : Emission spectrum at 10°K (dashed l i n e ) and at 300°K (dotted l i n e ) , a b s o r p t i o n spectrum at 300°K ( s o l i d l i n e ) 39 kK 15-5 16.0 16.5 400 0 400 800 -1 cm F igure 11. Comparison of Raman, emiss ion and a b s o r p t i o n spectrum of K.MnF, . 2 0 Lower p a r t : Raman spectrum at 300°K ( s o l i d l i n e ) and 100°K (dotted 1i ne ) . Upper p a r t : Absorp t ion spectrum at 300°K ( s o l i d l i n e ) and emiss ion spectrum at 85°K (dashed l i n e ) . 40 In t he s p e c t r a o f the Cs complex, o n l y the o r i g i n o f t h i s t r a n s i t i o n a t 16030 cm ' i s common t o both a b s o r p t i o n and e m i s s i o n a t 10°K ( c f . T a b l e 1). H i g h e r t e m p e r a t u r e s p o p u l a t e h i g h e r l y i n g v i b r a t i o n a l l e v e l s f rom w h i c h e l e c t r o n i c t r a n s i t i o n s may o c c u r , thus hot bands appear a t 15681 and 15784 cm" 1 a t 300°K. The l a t t e r band i s r e l a t i v e l y broad ( a p p r o x . 35 cm 1 h a l f - h e i g h t w i d t h ) and o v e r l a p s d i r e c t l y t he e x c i t -a t i o n l i n e a t 15803 cm '. The r e s u l t i n g e m i s s i o n i s superi m p o s e d on the ( c o m p a r a t i v e l y weak) Raman l i n e s and o b s c u r e s t h e Raman s p e c t r u m , hence t h e a n o m a l i e s d e s c r i b e d above a r e o b s e r v e d . L o w e r i n g the temp-e r a t u r e t o 100°K has no s i g n i f i c a n t e f f e c t on the o b s e r v e d Raman sp e c -trum. The hot band (now s h i f t e d t o 1579^ cm ') i s s t i l l p r e s e n t and o v e r l a p s the e x c i t a t i o n l i n e . A d i f f e r e n t s i t u a t i o n i s e n c o u n t e r e d f o r the K s a l t . The i n t e n s i t y o f t h e e m i s s i o n i s c o n s i d e r a b l y lower due t o q u e n c h i n g by 2+ t r a c e s o f Mn and t h e r e i s l e s s o v e r l a p w i t h the e x c i t a t i o n l i n e . 2+ (The t r a c e s o f Mn were r e a d i l y d e t e c t e d i n t h e e m i s s i o n s p e c t r u m but c o u l d not be removed by f u r t h e r r e c r y s t a 1 1 i z a t i o n s . ) The hot band n e a r e s t t o the e x c i t a t i o n l i n e i s a t 15842 cm ' and i s s h i f t e d t o I5858 cm ' when the t e m p e r a t u r e i s lowered t o 85°K. As a conse-quence, the pho s p h o r e s c e n c e i n t e n s i t y i s d e c r e a s e d and the t h r e e narrow l i n e s ( h a l f - h e i g h t w i d t h a p p r o x i m a t e l y 5 cm ' ) , b e f o r e b a r e l y d i s c e r n i b l e from e m i s s i o n bands a t room t e m p e r a t u r e , a p p e a r , s l i g h t l y s h i f t e d and w i t h r e l a t i v e l y enhanced i n t e n s i t y . They were i d e n t i f i e d as the fundamental Raman f r e q u e n c i e s Vj-600, v^=S07 and v^=310 cm \ The r e l a t i v e i n t e n s i t i e s f o l l o w the common p a t t e r n I(v.) > I ( v 0 ) , I ( v c ) 41 54 w h i c h i s c h a r a c t e r i s t i c o f most o c t a h e d r a l MA^ c o m p l e x e s . The a s s i g n m e n t s made a r e s u m m a r i z e d i n T a b l e I and T a b l e I I . In t h e same t a b l e s , t h e a b s o l u t e band p o s i t i o n s a r e compared w i t h t h o s e o b t a i n e d f r o m e m i s s i o n and a b s o r p t i o n s p e c t r a . In g e n e r a l , t h e t o t a l l y s y m m e t r i c mode, a ] g » c a n be d i s t i n g u i s h e d f r o m t h e a s y m m e t r i c modes e and t . i n s o l u t i o n by t h e d i f f e r e n c e i n t h e d e p o l a r i z a t i o n 9 2g r a t i o . The a ^ mode i s p o l a r i z e d and t h e r a t i o i s p < 0 . 7 5 , w h e r e a s t h e e and t_ modes a r e d e p o l a r i z e d w i t h p - 0 . 7 5 -g 2g W h i l e t h i s work was i n p r o g r e s s , t h e m i s s i n g d a t a were r e p o r t e d 32 by A s p r e y et_ a_L The d e p o l a r i z a t i o n r a t i o f o r t h e mode was f o u n d t o be 0 . 3 and f o r t h e e and t „ modes 0 . 8 . In a d d i t i o n , o n l y t h r e e g 2g bands were d e t e c t e d i n t h e Raman s p e c t r u m o f t h e s o l u t i o n , c e n t e r e d a t 610 ( v j ) , 488 (Vy) and 240 (v,.) . T h i s r e s u l t i s e x p e c t e d i f one c o n -2 -s i d e r s t h a t t h e e m i s s i o n o f MnF^ i n s o l u t i o n ( d i l u t e d HF) i s v e r y weak a t room t e m p e r a t u r e and t h e r e f o r e does not i n t e r f e r e w i t h t h e Raman 1i n e s . The same r e p o r t d e s c r i b e s the Raman s p e c t r u m o f s o l i d C s „ M n F , Z o o b t a i n e d ( i n d e p e n d e n t l y ) w i t h t h e same e x c i t a t i o n s o u r c e . The same pheno -menon was o b s e r v e d and t h e band p o s i t i o n s a r e i n good a g r e e m e n t . How-e v e r , t h e a u t h o r s based t h e i n t e r p r e t a t i o n o f t h e s p e c t r u m e n t i r e l y on a r e s o n a n c e Raman e f f e c t . i i ) IR S p e c t r u m 50 The IR s p e c t r u m i s v e r y s i m i l a r t o K ^ N i F ^ and no d i f f e r e n c e was o b s e r v e d between t h e s p e c t r u m o f t h e K and Cs s a l t s w i t h i n e x p e r i -m e n t a l e r r o r . The s p e c t r u m c o n s i s t s o f two s t r o n g p e a k s a t 3^0 and TABLE 1 Observed f requenc ies ( in cm ) in the Raman spectrum of Cs2MnF^ comparec to f r e q u e n c i e s observed in emi ss ion and a b s o r p t i o n spect ra (Laser e x c i t a t i o n frequency 15803 cm ') 1 2 3 4 5 6 7 8 9 +932 16735 16752 Emi ss ion +870 16673 16660 16686 1 1 +835 16638 16621 16645 16633 1 1 +790 sh 16593 16582 16580 1 1 +685 16483 16488 16485 1 1 +557 16360 16349 16369 16359 +435 16238 16234 16259 16242 " +266 16069 16067 16083 16072 +226 16024 16034 16028 16030 + 167 15970 15964 15969 - - 15784 15793 15801 -- I l l 15692 15681 15694 15696 Emi ss ion -340 sh - 3 3 5 sh 15453 15468 v 5 -396 -389 15407 15414 15406 15414 Emi ss ion -450 sh -415 sh 15353 15388 15378 15382 " -503 -503 15290 15290 15294 " -587 -587 15216 15216 15214 -611 -610 15192 15193 15191 -695 -693 15108 15110 15106 -850 sh - 8 4 8 14953 14955 " -895 -892 14908 14911 14908 -975 -971 14828 14832 -1010 -1007 14793 14796 1 1 Legend: Raman spectrum ( r e l a t i v e p o s i t i o n s ) at 300°K ( l ) and 100°K ( 2 ) , a b s o l u t e p o s i t i o n s at 300°K (3) and 100°K ( 4 ) , absorpt ion spectrum of a s i n g l e c r y s t a l at 300°K (5) and 85°K ( 6 ) , emiss ion spectrum at 300°K (7) and 10°K ( 8 ) , and assignments of Raman bands (9) sh - shoulder 43 TABLE I I Observed f requenc ies ( in cm ) in Raman spectrum of I^MnFg compared to f requenc ies observed in emiss ion and a b s o r p t i o n spec t ra (Laser e x c i t a t i o n frequency 15803 cm ') 1 2 3 4 5 6 7 + 1066 16869 16861 Emi ss ion +966 16769 16767 " +891 16694 16681 +732 16535 M6540 sh +612 16415 16410 +486 16289 16283 16303 +345 16148 16147 +270 16073 16071 15842 15733 16067 15848 15723 --310 -311 15493 15492 v 5 -348 -342 15455 15461 15456 Emi ss i on -383 sh - 3 9 3 15420 15411 11 -471 -466 15332 15337 11 -507 -507 15294 15294 V 2 -600 -604 15203 15199 V l - 6 6 7 15136 Emi ss i on -867 14943 11 -906 sh 14897 v l + v 5 -924 sh 14879 Emi ss ion Legend: Raman spectrum ( r e l a t i v e pos i ton) at 300°K ( l ) and 100°K (2), abso lu te p o s i t i o n s at 300°K (3) and 100°K ( 4 ) , a b s o r p t i o n spectrum of a s i n g l e c r y s t a l at 300°K ( 5 ) , emiss ion spectrum at 85°K ( 6 ) , and assignments of Raman bands (7) sh - shoulder kk 620 cm 1 which were ass igned to the two i n f r a r e d - a c t i v e fundamental f requenc ies and v ^ , r e s p e c t i v e l y , and a weaker band at 1130 cm ' which was ass igned to the combinat ion V^ + V2 ( c a ^ C L J ^ ated 1127 cm ^). In a d d i t i o n , a shoulder appears at approx imate ly 7^ 0 cm \ Th is band is probably the combinat ion V g + V 2 a n c ' knowing the frequency of v ^ , would p lace at about 230 cm \ The fundamental ' s comparat i ve ly narrow (approx imately 25 cm ' h a l f - h e i g h t width) whereas the fundamental is much broader (over 100 cm ' ) . Th is is a general phenomenon f o r the h e x a f l u o r i d e 1 50,55 complexes. The fundamental is i n a c t i v e and does not appear d i r e c t l y in e i t h e r the Raman or IR spectrum. B e s i d e s . f r o m combinat ions , can be determined from the emiss ion spectrum by a v i b r a t i o n a l a n a l y s i s which is the subject of the next c h a p t e r . The fundamental , determined in t h i s way, is 229 cm ' which supports the assignment made above. Thi completes the a n a l y s i s of the (interna1) fundamental f requenc ies of the groundstate of the hexaf1uoromanganese(I V) complexes. The f requenc ies the o p t i c mode l a t t i c e v i b r a t i o n s were too low to be observed . i i i ) Summary 2-The fundamental i n t e r n a l f requenc ies of [MnF^] were determined from Raman and IR s p e c t r a . The observed f requenc ies of the fundamental are summarized in Table III and compared to fundamentals of other t e t r a va len t hexaf1uor ides . 0 2 k The 6328 A He/Ne l a s e r l i n e s t r o n g l y e x c i t e s the E -»• A phos-phorescence of the complex. The Raman l i n e s are r e a d i l y d i s t i n g u i s h e d 45 at low temperatures from the luminescence due to t h e i r much s m a l l e r h a l f - h e i g h t w i d t h . 46 TABLE I I I Fundamental f requenc i es of some hexaf1uor ides w i th t e t r a v a l e n t cat i ons Complex V l V2 V3 v4 v5 v6 References [ M n F 6 ] 2 " 600 507 620 340 310 225 t h i s work [ N i F , ] 2 - 562 520 658 345 310 (220) a 50, 51 [ P d F 6 ] 2 " 578 558 52 [ P t F , ] 2 - 600 576 571 281 210 (I84) a 50, 56 [ S i F 6 ] 2 " 663 477 741 483 408 (347) a 57 [ G e F 6 ] 2 " 627 454 600 350 318 (289) a 57, 58 [ S n F 6 ] 2 - 592 477 559 300 252 (252) a 57 a c a l c u l a t e d f requenc ies hi IV E L E C T R O N I C A N D V I B R O N I C S T A T E S The i n t e r p r e t a t i o n of d e a c t i v a t i o n mechanisms of an e x c i t e d molecule requ i res knowledge of the e x c i t e d s t a t e s i n v o l v e d . Such knowledge i s obta ined from a b s o r p t i o n and emiss ion s t u d i e s at d i f f -erent temperatures . Presented in t h i s chapter are the r e s u l t s f o r high r e s o l u t i o n 2-spect ra of s i n g l e c r y s t a l s of [MnF^] at d i f f e r e n t temperatures as we l l as the arguments to support the ass ignments . 1. RESULTS The emiss ion and a b s o r p t i o n s p e c t r a of the Cs , Rb and K s a l t s of H^MnF^ were measured in s o l u t i o n , as powders and in the form of s i n g l e c r y s t a l s at d i f f e r e n t temperatures . The spec t ra of s i n g l e k+ c r y s t a l s of Cs^GeF^ and K^GeF^ doped w i th Mn were a l s o i n v e s t i g a t e d and the r e l a t i v e emiss ion quantum y i e l d s and the o s c i l l a t o r s t rength recorded as a f u n c t i o n of temperature. i) Absorp t ion Spectra S o l u t i o n Spectrum: The broad fea tu res of the e l e c t r o n i c t r a n s -2-i t i o n s of the complex [MnF^] were obta ined from study of the absorp -t i o n spectrum of K^MnF^. The spectrum was measured in s o l u t i o n of 20% aqueous HF at room temperature from 700 to 300 nm. In the c o n c e n t r a t i o n -2 -3 range s t u d i e d , from 10 to 10 M, Beer ' s law was obeyed. The spectrum 48 i s dep ic ted in F i g . 12. It c o n s i s t s of two broad bands of medium i n t e n s i t y at 27600 cm ' and 21100 cm ' and a weak band at a p p r o x i -mately 16250 cm \ The maximum e x t i n c t i o n c o e f f i c i e n t s are 3 2 . 0 , 55 .3 and 0 .9 M 'cm \ r e s p e c t i v e l y . R e f l e c t a n c e Spectrum: The t a i l of a s t rong band is to be seen at 300 nm. It was not p o s s i b l e to l o c a t e the p o s i t i o n of the peak. R e c e n t l y , the r e f l e c t a n c e spectrum of Cs„MnF, was r e i n v e s t i -Z b 37 gated by R e i s f e l d , Matw iyo f f , and Asprey . They found a peak at 39000 cm ' and ass igned the band to a charge t r a n s f e r t r a n s i t i o n . That the so l vent has only a smal l e f f e c t on the p o s i t i o n s of the braod bands is shown by comparison of the s o l u t i o n a b s o r p t i o n spectrum w i t h the ref1ectancespectrum*" of a powdered c r y s t a l l i n e sample in F i g . 12. However, the weak band at 16250 cm ' is reso lved in the r e f l e c t a n c e spectrum i n t o a s e r i e s of sharp bands. The p o s i t i o n s of these bands are not l i s t e d because they cou ld be more a c c u r a t e l y determined from the absorp t ion spectrum of s i n g l e c r y s t a l s where an even b e t t e r r e s o l u t i o n of the weak band was found. Spectrum of Pure S i n g l e C r y s t a l s : The a b s o r p t i o n spect ra of s i n g l e c r y s t a l s of K.MnF, , Rb MnF, and Cs.MnF, were measured at 300°K Z b Z 0 Z b and 85°K. The th i ckness of the s i n g l e c r y s t a l s was approx imately 0 . 5 mm. The s p e c t r a show the i n f l u e n c e of c a t i o n s i z e and s t r u c t u r e . The bands f a l l h igher in energy 60 cm ' f o r potassium and approx imately 40 cm ' h igher f o r rubidium when compared to the cesium s a l t . Except fo r the s h i f t , the s p e c t r a of I^MnF^ were i d e n t i c a l . T h e r e f o r e , on ly the band p o s i t i o n s K^MnF^ are l i s t e d . The spectrum at 85°K i s dep ic ted in The r e f l e c t a n c e spectrum was obta ined on a S p e c t r o n i c 600 w i t h a tungsten l i g h t source . MgO was used as a r e f e r e n c e . [ M _ 1 c m ~ ' ] 40 20 \— 1 5 . 0 2 0 . 0 2 5 . 0 3 0 . 0 F igure 12. Absorp t ion and r e f l e c t a n c e spectrum of K.MnF. at 300°K. L fa r e f l e c t a n c e , absorp t ion kK 35.C 50 F i g . 13- The spectrum of Cs^MnF^ was a l s o measured at l i q u i d he l ium temperature . The band p o s i t i o n s are g iven and the spectrum shown in F i g . 14. Spectrum of O p t i c a l l y D i l u t e C r y s t a l s : A b s o r p t i o n s p e c t r a 4+ were measured of Mn o p t i c a l l y d i l u t e d in s i n g l e c r y s t a l s of Cs^GeF^. The spectrum at 80°K w i t h a guest c o n c e n t r a t i o n of k% (mole r a t i o ) is shown in F i g . 15. The spectrum r e v e a l s no s i g n i f i c a n t d i f f e r e n c e ex -cept f o r a s h i f t to the b lue by about 10 cm ' when compared w i t h Cs2MnF^. The spectrum is n e v e r t h e l e s s t a b u l a t e d f o r comparison w i t h the emiss ion spectrum which cou ld be measured in more d e t a i l than the 4+ cor responding spectrum of Cs^MnF^. Because the exact Mn c o n c e n t r a -t i o n and the t h i c k n e s s of the c r y s t a l was known, i t was p o s s i b l e to c a l c u l a t e the o s c i l l a t o r s t r e n g t h of the a b s o r p t i o n bands from Equat ion (1). The t o t a l o s c i l l a t o r s t r e n g t h was determined as a f u n c t i o n of temperature from 80°K up to 600°K. The r e s u l t s are shown in F i g . 16. In the a b s o r p t i o n s p e c t r a of s i n g l e c r y s t a l s at 80°K v i b r a t i o n a l f i n e s t r u c t u r e was a l s o observed on the two broad bands centered at 21100 cm 1 and 276OO cm 1 which i s shown f o r Cs„MnF, in F i g s . 17 and 19, 2 b r e s p e c t i v e l y . The band at 21100 cm ' which e x h i b i t s f i n e s t r u c t u r e even at room temperature (see F i g . 23) was a l s o s t u d i e d at l i q u i d he l ium temperature . The spectrum is shown in F i g . 18. The s t r u c t u r e i s l ess w e l l reso lved f o r K^MnF^ and i t was on ly observed f o r the broad band at 21100 cm \ The spectrum at 85°K i s shown in F i g . 20. As b e f o r e , the presence of c o n c e n t r a t i o n e f f e c t s in the absorp -51 OPTICAL DENSITY 0.25 0.5 0.75 F igure 13. Absorp t ion spectrum of K^MnF^ at 85°K, s i n g l e c r y s t a l . Thickness of s i n g l e c r y s t a l : 0.54 mm. 52 OPTICAL DENSITY F igure ]h. Absorp t ion spectrum of Cs2MnF^ at I0°K, s i n g l e c r y s t a l . Th ickness of c r y s t a l : 0 .55 mm. 53 OPTICAL DENSITY 0.05 0.1 0.15 Figure 15- Absorp t ion spectrum of Mn*+:Cs0GeF,(4%) at 80°K, s i n g l e 2 6 c r y s t a l . Th ickness of c r y s t a l : 2.55 mm. F i g u r e 1.7. Absorp t ion spectrum of Cs MnF^ at 80°K, s i n g l e c r y s t a l . Th ickness of c r y s t a l : 30 u. 56 OPTICAL DENSITY 22931 22862 22779 22517 22422 21882 21815 21492 21441 C -*—2TT8T 21368 3 21331 21008 JE> 20938 20627 20640 20614 20687 20859 20799 20730 F igure 18. Absorp t ion spectrum of Cs MnF& at 10°K, s i n g l e c r y s t a l . Thickness of c r y s t a l : 30 u. 57 Figure 19. D e t a i l s of the absorp t ion spectrum of Cs„MnF. at 85°K, 2 D s ing le c r y s t a l . Figure 20. Details of the absorption of K MnF, at 85°K, single crystal . 2 6 Thickness of crystal : 30 u. 59 t i o n s p e c t r a of Cs^MnF^ was tes ted by comparison w i t h the a b s o r p t i o n 4+ spectrum of Mn :Cs2GeF^ (mole r a t i o k%) at 8 0 ° K . Except f o r a s h i f t by 2 2 cm ' to h igher energ ies no s i g n i f i c a n t d i f f e r e n c e was no ted . S i n g l e c r y s t a l s of Cs^MnF^ and K^MnF^ were examined w i t h p o l a r -ized l i g h t under the mic roscope . Only H^MnF^ showed a marked d i c h r o i s m w i t h l i g h t p o l a r i z e d w i t h respect to the t r i g o n a l a x i s . U n f o r t u n a t e l y , the smal l s i z e and the b r i t t l e nature of the m a t e r i a l d i d not permit c u t t i n g and p o l i s h i n g the c r y s t a l s in such a way s u i t a b l e f o r the study of p o l a r i z e d a b s o r p t i o n s p e c t r a . i i ) Emiss ion Spect ra When i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t , c r y s t a l l i n e h e x a f l u o r o -manganates(IV) show an in tense red emiss ion even at room temperature . A s i m i l a r in tense emiss ion was observed in a r i g i d - g l a s s s o l u t i o n in 20% HF at l i q u i d n i t r o g e n temperature . Spectrum of Pure S i n g l e C r y s t a l s : The emiss ion s p e c t r a of Cs„MnF,. I b as a s i n g l e c r y s t a l , in p o l y c r y s t a 1 1 i n e form or in a r i g i d - g l a s s s o l u t i o n are i d e n t i c a l . The spectrum c o n s i s t s of a s e r i e s of sharp bands and the s t r u c t u r e i s w e l l reso l ved at room temperature . On c o o l i n g to l i q u i d n i t r o g e n temperature and below the on ly change in the spectrum was a n a r -rowing of the l i n e s and a decrease in the i n t e n s i t y of the hot bands. The spectrum at l i q u i d he l ium temperature is shown in F i g . 2 1 . The bands were recorded by back i l l u m i n a t i o n . Spectrum of O p t i c a l l y D i l u t e S i n g l e C r y s t a l s : Except f o r a blue -1 k+ s h i f t of approx imate ly 1 0 cm , the same spectrum was observed f o r Mn : Cs„GeF, {k% mole r a t i o ) . However, s i n c e f r o n t i l l u m i n a t i o n of the c r y s t a l 60 RELATIVE EMISSION INTENSITY > 16030 > 15969 15801 15696 ==- 15414 15382 15294 > 15214 15191 15106 > 14908 F igure 2 1 . Emission spectrum of Cs MnF, at 10°K, s i n g l e c r y s t a l . 61 was used, a d d i t i o n a l bands were recorded which were too weak to be observed w i th the less s e n s i t i v e apparatus (see page22). The spectrum at 80°K is d e p i c t e d in F i g . 22 and the band p o s i t i o n s are g iven in Table IV. The i n t e n s i t i e s of the bands were c o r r e c t e d f o r the s p e c t r a l response of the d e t e c t i n g system. Emiss ion Quantum Y i e l d s : The emiss ion spectrum and the r e l a t i v e quantum y i e l d s of Cs^MnF^ and i t s o p t i c a l l y d i l u t e form (k%) were s t u d i e d as a f u n c t i o n of temperature in the range from 80°K up to 600°K. Even at t h i s h igh temperature v i b r a t i o n a l f i n e s t r u c t u r e was s t i l l o b s e r v a b l e . At about 350°K, a broad s t r u c t u r e l e s s band of r e l a t i v e l y low i n t e n s -i t y appeared, centered at 5^5 nm. Th is band had a h a l f - h e i g h t width of approx imately 2500 cm ' and was p a r t i a l l y over lapped by the more in tense red e m i s s i o n . The i n t e n s i t y of t h i s band was s t r o n g l y dependent on temp-e r a t u r e . R a i s i n g the temperature above 350°K caused a sharp increase in the i n t e n s i t y of the band u n t i l i t f i n a l l y reached a maximum at kS0°K. Above t h i s temperature the i n t e n s i t y decreased . At 590°K the band was s t i l l v i s i b l e as is shown in F i g . 23 . Above 600°K the i n t e n s i t y of both the red and the green emiss ion became too weak to be measured. The temperature dependence of the emiss ion quantum y i e l d s of the red e m i s s i o n , Q , and the green e m i s s i o n , Q .^, are summarized in F i g . 2k. k+ The r e s u l t s were the same f o r both Cs.MnF, and Mn :Cs„GeF , , but Z D Z D were r e p r o d u c i b l e only a f t e r the s i n g l e c r y s t a l s were f i n e l y ground and thoroughly d r i e d at 100°C in an o i l pump vacuum over ^2^5 ^ o r a P e r ' ° d of 5 hours .^ By t h i s procedure the water in the c r y s t a l l i n e m a t e r i a l At h igh temperatures (above 400°K) water which had been int roduced dur ing p r e p a r a t i o n reacts w i t h the complex under fo rmat ion of MnO^ and HF. During the heat ing process the presence of HF was tes ted f o r , w i th a p iece of g l a s s which covered the compound. 62 Figure 22. Emission spectrum of Mn :Cs 2GeF 6 (h%) at 80°K, single c r y s t a l . 63 TABLE IV Deta i1s of the v i b r o n i c s t r u c t u r e in the emiss ion spectrum of Mn:Cs 2 GeF 6 (h%) at 80°K, s i n g l e c r y s t a l # Pos i t ion [cnrf 1] 1ntens i ty # Pos i t ion [cm" 1 ] 1 ntens i ty 0 16043 4.83 28 15225 0 .29 1 15999 0.47 29 15200 0 .45 2 15979 0.78 30 15175 0, .05 3 15953 0.33 31 15138 0, • 07 4 15941 0.28 32 15118 0 .18 5 15901 0.31 33 15103 0, .12 6 15884 0.62 34 15049 0, .02 7 15860 0.54 36 14998 0. .01 8 15815 100.00 37 14926 0, . 1 1 9 15781 1 .08 39 14897 0, .22 10 15754 . 0.99 40 14844 0. .03 11 15739 1.34 41 14816 0. .22 12 15708 59.2 42 14797 0. .02 13 15684 1.12 43 14771 0. ,01 14 15648 0.47 44 14742 0. .01 15 15634 0.59 45 14728 0. .01 16 15615 0.37 46 14693 0. ,01 17 15556 0.10 1* 16079 0. • 29 18 15531 0. 1 1 2" 16102 0. • 31 19 15511 0.15 3* 16129 0. • 09 20 15469 1.34 4* 16150 0. ,06 21 15436 2.21 5* 16187 0. ,06 22 15405 7-75 f 16225 0. 03 23 15358 0.21 8* 16263 2. ,88 24 15332 0 .16 9* 16303 0. ,02 25 15307 1.44 10* 16334 0. ,01 27 15251 1 .08 12* 16374 0. .34 hot bands x JO2 • 0 100 200 300 400 500 600 Figure 2k. Quantum yields of phosphorescence Q and delayed P 1 fluorescence Q^. as a function of temperature. 66 used was small and no further decomposition at high temperatures was noted. The emission y i e l d was strongly dependent on the degree of such decomposition. No reproducible results were obtained for single crysta1s. The quantum yi e l d s of the c r y s t a l l i n e compound were corrected for changes with temperature in both the absorption and emission spectrum. Raising the temperature causes a broadening and a red s h i f t of the spectral bands. In addition, hot bands appear in both spectra. In the c a l c u l a t i o n of the quantum yi e l d s the changes in the emission spectrum were properly accounted for by correcting the intensity of each individual band for spectral response of the detecting system, including the hot bands. The quantum y i e l d s , and Q^ ., were then com-puted by integration. The integration was done graphically by cutting and weighing the paper chart. This method allowed a separation of the p a r t i a l l y overlapping spectra of the green and the red emission at higher temperatures. These data were corrected for the temperature dependence of the absorption spectrum. Because the samples were pressed powders, a d i r e c t measurement of the absorption was not possible. The correction factors were therefore computed from the absorption spectrum of a single c r y s t a l of MniCs^GeF^ at d i f f e r e n t temperatures by c a l c u l a t i n g the intensity r a t i o I ( T ) / I ( T Q ) which changes with temperature. The results obtained in this way, depicted in Fig. 24, represent quantum yi e l d s of both the red and the green emission as a function of temperature r e l a t i v e to the a r b i t r a r y assignment of unity for the quantum 67 y i e l d of the red emiss ion at 300°K. The a b s o l u t e quantum y i e l d s were est imated from a comparison of the i n t e n s i t i e s of the emiss ion spectrum of Cs 2MnF^ w i th that of C r (en) 1 ^ + and C r t N H ^ C l ^ . The a b s o l u t e emiss ion quantum y i e l d s of these complexes at l i q u i d n i t r o g e n temperature 18 are 0.009 and 0.003, r e s p e c t i v e l y . The emiss ion of Cs^MnF^ was found to be more intense by a f a c t o r of at l e a s t two orders of magnitude which p laces the a b s o l u t e quantum y i e l d of t h i s complex c l o s e to un i t y at l i q u i d n i t r o g e n temperature . Experiments were a l s o c a r r i e d out to measure the r e l a t i v e emiss ion y i e l d s of C s ^ n F ^ in s o l u t i o n (20% HF) . No r e p r o d u c i b l e r e s u l t s were o b t a i n e d . Below -130°C 20% HF forms a r i g i d c l e a r g l a s s which sof tens above t h i s temperature to turn i n t o an opaque and m i l k y - w h i t e s o l u t i o n whereby s o l i d C s ^ n F ^ is p r e c i p i t a t e d . The p r e c i p i t a t e d s o l i d was most probably r e s p o n s i b l e fo r the large s c a t t e r of the r e s u l t s making them u n s u i t a b l e f o r a n a l y s i s . Emiss ion of Pure I^MnF^: Compared to the cesium s a l t the emiss ion of s o l i d K2MnFg was c o n s i d e r a b l y weaker and an apparatus of lower r e s o -l u t i o n had to be used to record the spectrum. Although a s e r i e s of sharp bands s i m i l a r to that of the cesium s a l t were observed at 85°K the spec t ra d i f f e r c o n s i d e r a b l y in many aspects besides o v e r a l l i n t e n s i t y . The most prominent are : the i n d i v i d u a l bands have a d i f f e r e n t i n t e n s i t y d i s t r i b u -t i o n and the h a l f - h e i g h t width is approx imate ly twice of that of the cesium s a l t which i s 10 cm ' at 85°K. In a d d i t i o n , an intense band of asymmetric shape and 900 cm ' h a l f - h e i g h t width was observed centered at 690 nm. This band is shown in F i g . 25- Only t h i s asymmetric band was observed in the t en = ethy1enediamine 68 emiss ion spectrum of the c r u d e , unrecrys ta11 i zed potass ium complex. However, i t d i s a p p e a r s comple te l y in a r i g i d s o l u t i o n where, in t u r n , the sharp bands appear w i t h c o n s i d e r a b l y enhanced i n t e n s i t y . More-o v e r , no band of the type as d e s c r i b e d above was observed in the . . k+ emiss ion spectrum of s o l i d Mn : K^GeF^ where the emiss ion i n t e n s i t y was comparable to that of the Cs s a l t . Emiss ion of O p t i c a l l y D i l u t e K^MnF^: The spectrum of t h i s complex, o p t i c a l l y d i l u t e d in K^GeF^ (0.1% mole r a t i o ) , was s t u d i e d o in more d e t a i l . The high r e s o l u t i o n spectrum (2 A s l i t width) at 80°K i s d e p i c t e d in F i g . 26 and the d e t a i l s are g iven in Table V. o The c o m p a r a t i v e l y low r e s o l u t i o n spectrum (13 A s l i t ) of K„MnF. in a 2 o r i g i d g l a s s at 85°K i s shown in F i g . 25. 2. DISCUSSION The type of t r a n s i t i o n s in the [MnF^] ion are best c h a r a c t e r -ized from the a b s o r p t i o n spectrum of K^MnF^ ' n s o l u t i o n . The spectrum is d e p i c t e d in F i g . 12 and c o n s i s t s of three bands at 27600, 21100 and 16250 cm ' w i t h an o s c i l l a t o r s t r e n g t h of 6.9 x 10 \ 6.6 x 10 ^ and 8 x 10 7 , r e s p e c t i v e l y . From t h e i r i n t e n s i t i e s the bands are ass igned k k k k 2 k as the d -d t r a n s i t i o n s T, •<- A „ , T „ -<- A_ , and E •*- A_ , r e s -lg 2g' 2g 2g' g 2g p e c t i v e l y , assuming an oc tahedra l microsymmetry. From the r e f l e c t a n c e spectrum a l o n e , the same assignments were proposed by J^rgensen fo r 29 I^MnF^. The o b s e r v a t i o n of f u r t h e r d -d t r a n s i t i o n s is obscured by 69 gure 2 5 . D e t a i l s of the emiss ion spectrum of K MnF, at 85°K. 2 6 S o l i d l i n e : s i n g l e c r y s t a l . Dashed l i n e : r i g i d s o l u t i o n (20% HF). 70 Figure 2 6 . Emiss ion spectrum of Mn :K^GeF^ ( 0;U) at 80°K, s i n g l e c r y s t a l . 71 TABLE V Deta i I s of the v i b r o n i c s t r u c t u r e in the emiss ion spectrum of M n ^ K ^ e F ^ (0 .]%) at 80°K, s i n g l e c r y s t a l # Pos i t ion [cm"l] 1 ntens i ty # P o s i t i o n Intensi [cm"'] 1 16079 9 19 15467 10.6 2 16068 16 20 15457 10.0 3 15991 3-8 21 15427 10.0 4 15981 5-8 22 15418 22.4 5 15936 2.9 23 15407 9.2 6 15926 3-3 24 15336+ 2.0 7 15869 sh 33-6 25* 16157 1 .2 8 15859 sh 60.0 26* 16167 0.9 9 15848 91.2 27* 16209 0.8 10 15838 85.8 28* 16219 0.6 11 15823 33-9 29* 16279 2.3 12 15813 26.7 30* 16290 2.0 13 15770 37-4 31* 16324 0.4 14 15760 80.0 32" 16336 0.3 15 15746 15-5 33* 16376 0.3 16 15736 sh 28.3 34* 16385 0.2 17 15731 100.0 35* 16416 0.1 18 15720 70.4 36* 16427 0.1 hot bands not reso lved 72 the t a i l of the intense charge transfer band starting at 320 nm. The well-resolved vibrational structure in the spectra of single crystals allows a more detailed analysis. The spin-allowed and the spin-forbidden transitions w i l l be discussed separately. . 2 2 4 i) T, , E •<-»• A „ transitions 1 g-*——9 2g In octahedral symmetry the d-d transitions are parity-forbidden. This rule is relaxed by several mechanisms which account for their appearance: 1) magnetic dipole transitions, 2)vibrationa1ly induced transitions, and 3) static distortions which lower the site symmetry. 2 4 Cs^MnF^ has cubic symmetry and the 0-0 transition of Eg"~*" ^ is made allowed only by a magnetic dipole mechanism. From inspection of Fig. 27, which shows the relevant part of the absorption spectrum and the emission spectrum, respectively, only the band at 16030 cm ' is common to both spectra. This band is the 0-0 transition of 2 4 Eg ^2g' ~^^e o s c ' l ' a t o r strength of this transition calculated -9 from the absorption spectrum of a single crystal at 85°K is 4.2 x 10 . — Q This value compares well with that of 7.2 x 10 found for the 0-0 3+ transition of Cr :Mg0 where the magnetic dipole character was estab-59 lished by Zeeman-effect studies. The intensities of the vibrational satel l i tes are comparatively much stronger and a vibronic mechanism must account for their appearance. In general, evidence for the participation of this mechanism is the increase of the osci l lator strength with increasing temperature accord-ing to the relat ionship.^ f = f coth (hv/2kT) (11) 73 16.0 _ _ L _ o A. 16.5 _ J _ 12 kK 21 17.0 l_ 17-5 L ABSORPTION +^3 T ^ ^ +v 6 +vk +v. 31 22 27 A *• /V J 34 JL 37 44 41 47 8 JL +v 6 12 22 25 27 EMISSION 16.0 15.5 15.0 kK F igure 27. 4+ Absorp t ion and emiss ion s p e c t r a of Mn : C s 2 G e F 6 (k%) at 80°K, s i n g l e c r y s t a l . (Numbers r e f e r to assignments made in Table V I ) . 7h where f is the o s c i l l a t o r s t r e n g t h , v the frequency of the v i b r a t i o n and T the abso lu te temperature . From t h i s equat ion the t o t a l o s c i l l a -tor s t r e n g t h and the o s c i l l a t o r s t rength of a s i n g l e v i b r a t i o n (v 6 = 228 cm ) were c a l c u l a t e d as a f u n c t i o n of temperature . ( Included in the t o t a l o s c i l l a t o r s t rength are the hot bands which were c a l c u l a t e d from the equat ion above m u l t i p l i e d by the a p p r o p r i a t e Boltzman f a c t o r s . ) The r e s u l t s so obta ined are compared w i t h the exper imental s t rengths in F i g . 16. The good agreement i s s t rong evidence f o r the presence of a v i b r o n i c mechanism. By t h i s mechanism c e r t a i n v i b r a t i o n s couple w i t h the 0-0 t r a n s i -t i o n to g i ve r i s e to the observed v i b r a t i o n a l s t r u c t u r e . The symmetry of these v i b r a t i o n s can be determined by group t h e o r e t i c a l methods. For an octahedra l complex the v i b r a t i o n s p r e d i c t e d are the ant i symmetr i c t j u (v^ and v^) and (v^) v i b r a t i o n s . In Table VI ant i symmetr ic f u n d -amentals and ant isymmetr ic combinat ions of fundamentals are compared to the v i b r a t i o n s as observed in the emiss ion and a b s o r p t i o n spectrum of a s i n g l e c r y s t a l of Mn :Cs2GeFg (4%) at 80°K. The p e r t i n e n t r e s u l t s a r e : 1) the part of the v i b r a t i o n a l s t r u c t u r e which is r e s p o n s i b l e fo r 30% of the t o t a l emiss ion i n t e n s i t y and 93% of the t o t a l a b s o r p t i o n i n t e n s i t y can be s a t i s f a c t o r i l y accounted fo r by the fundamental f requenc ies and combinat ions thereof p r e d i c t e d by group theory 2) there is a good m i r r o r image r e l a t i o n s h i p between the a b s o r p t i o n and the emiss ion spectrum. The two spec t ra are compared in F i g . 27. The numbers which d e s i g -nate the bands r e f e r to the assignments made in Table V I . The 0-0 band 75 TABLE Vl Comparison and assignment of f requenc ies as observed in emiss ion (v_) and a b s o r p t i o n (v.) of ^E -«-»- S\„ E A g 2g # l 6 0 4 3 - v E l 6 0 4 3 - v A C a l c . Assignment V e - 1 6 0 4 3 v A - l 6 0 4 3 (Stokes) (Ant i s tokes) (An t i s tokes ) (Stokes) 1 44 36 2 64 59 59 3 90 L a t t i ce 86 102 ) Modes 107 5 142 144 6 7 159 183 J 182 8 228 229 228 v 6 220 220 9 262 260 10 289 291 11 304 304 (v 5 ) 12 335 335 v 4 331 330 13 359 14 395 15 409 16 428 17 487 18 512 508 (v 2 ) 19 532 532 V v 5 20 574 21 608 608 V 3 V v 5 601 22 638 639 627 23 685 684 3 v 6 24 711 76 TABLE VI ( cont . ) # 1 6 0 4 3 ~ V E 1 6 0 4 3 - V a C a l c . Assignment v E ~ l 6 0 4 3 v A " l 6 0 4 3 (Stokes) (An t i s tokes ) ( A n t i s t o k e s ) (Stokes) 25 26 736 736 , V V 2 2 T l g ( 0 - 0 ) 720 (16832) 27 792 28 818 818 V v i 813 29 843 843 V V 2 30 868 31 905 912 V 3 + V 5 32 925 925 V v l 925 33 940 943 V 2 v 5 34 35 994 988 3 v 6 + v 5  Tlg+V6 (17056) 36 1045 1035 37 1117 1116 v 3 + v 2 1107 38 2 T , +v. lg 4 V 3 V 5 (17164) 39 1146 1140 40 1199 1198 V V 1 41 1227 42 1246 1247 V 3 v 5 1234 43 1272 1274 3 v 6 + v l 44 1301 1292 3 V 2 v 5 45 1315 1309 3 v 4 + v 5 1308 46 47 48 1350 1351 ? V 2 v 2 T, +v, - ig 3 T, +v,+v r lg 4 5 1335 (17432) (17465) 49 1515 v l 4 +2v ] 1500 50 1520 V 3 + 3 V 5 1515 51 1613 3v/+2v5 1605 52 1624 v + 2 v 2 1628 77 2 4 - 1 of E A„ t r a n s i t i o n i s located at 16043 cm . The three intense 9 2g l i n e s in the emiss ion spectrum (8, 12, 21) are r e a d i l y ass igned to the v^, and v i b r a t i o n a l l e v e l s of the ground s t a t e , and t h e i r f r e q u e n -c i e s of 228 cm \ 335 cm 1 and 608 cm 1 are in good agreement w i th the f requenc ies obta ined from the IR spectrum. The f requenc ies of the sym-met r i c v i b r a t i o n s v^, and were determined from combinat ions and are 590 cm ' , 508 cm 1 and 304 cm \ r e s p e c t i v e l y . They are in good agreement w i th the f requenc ies obta ined from the Raman spectrum. 2 The f requenc ies of the fundamentals of the E^ e x c i t e d s t a t e were determined from the a b s o r p t i o n spectrum and are summarized in Table X together w i th the f requenc ies o f . t h e ground s t a t e fundamentals as determined from the IR, Raman and emiss ion s p e c t r a . On e i t h e r s i d e of the 0 -0 band at 16043 cm ' in the emiss ion 4+ spectrum of Mn -.Zs^leF^ there i s a s e r i e s of weak bands which could 35 not be exp la ined on the present model. The same was noted by F l i n t in the emiss ion spectrum of Cs^MnF^ at 80°K. Based on t h e i r m i r r o r -image r e l a t i o n s h i p (the i n t e n s i t i e s of the hot bands have to be c o r r e c t e d fo r the a p p r o p r i a t e Boltzmann f a c t o r s ) and on t h e i r low f requenc ies the bands were t e n t a t i v e l y ass igned as l a t t i c e modes. However, there are only two l a t t i c e v i b r a t i o n s of the c o r r e c t symmetry to be v i b r o n i c a l l y 50 a c t i v e . The bands at 17056, 17164, 17^32 and 17465 cm" 1 in the a b s o r p t i o n 2 spectrum could not be ass igned to t r a n s i t i o n s a s s o c i a t e d w i t h the E^ s t a t e . S ince they do not have a counterpar t in the emiss ion spectrum they were ass igned to v i b r o n i c t r a n s i t i o n s to the T^  s t a t e . The bands 78 d i s p l a y an i n t e n s i t y p a t t e r n which i s very s i m i l a r to the c o r r e s p o n d -2 ing t r a n s i t i o n s to the s t a t e . C a l c u l a t i o n s based on the spac ings of the l i n e s above, p l a c e the o r i g i n at 16835 cm ' r i g h t on top of a very weak band at 16832 cm \ Th is band was ass igned as the 0 - 0 t r a n -2 4 s i t i o n of T ^ <- f^2g' Th is assignment i s conf i rmed f u r t h e r by the appearance of a hot band at 1659^ cm ' when the temperature i s r a i s e d to 300°K. The d i s t a n c e of t h i s hot band to the 0 - 0 band of T „ i s 2g equal to the frequency of of the ground s t a t e c o n s i d e r i n g a red s h i f t of approx imate ly 10 cm \ The i n t e r p r e t a t i o n of the a b s o r p t i o n spectrum of K^MnF^ i s more d i f f i c u l t . The p o r t i o n s of the spectrum which o v e r l a p w i t h the emiss ion 2 4 spectrum inc lude the o r i g i n of the E -«-»- t r a n s i t i o n and v i b r a t i o n a l hot bands. From the v a r i a t i o n of i n t e n s i t y oj; the bands w i t h temperature the o r i g i n was l o c a t e d at 16098 cm Cont rary to the case of the cesium s a l t the 0 - 0 band i s the most in tense one of the spectrum and has an o s c i l l a t o r - 8 s t r e n g t h of 4 . 5 x 10 , which i s ten t imes more in tense than the s t r e n g t h of the cor respond ing bond of the cesium s a l t U n l i k e C s ^ n F ^ in I^MnF^ the center 2 -of symmetry of the [MnF^] octahedron is dest royed by a s t a t i c t r i g o n a l 4+ d i s t o r t i o n and the s i t e symmetry of Mn i s lowered to C^v* Th is mechanism would account f o r the observed inc rease in i n t e n s i t y of the 0 - 0 l i n e . The v i b r a t i o n a l p r o g r e s s i o n s which s t a r t from t h i s o r i g i n are very s i m i l a r to the p r o g r e s s i o n s observed in the a b s o r p t i o n spectrum of Cs„MnF, . Z b But most of the bands are c o n s i d e r a b l y broader ( h a l f - h e i g h t width is 20 cm ' or more) and in a d d i t i o n , there are some bands which have no 2 counte rpar t in the spectrum of the Cs s a l t . In C^ v symmetry the E l e v e l 79 is s p l i t by the combined e f f e c t of the t r i g o n a l f i e l d and s p i n o r b i t c o u p l i n g . The s p l i t t i n g can approach 100 cm ' ? Moreover, the t r i g o n a l component may a l s o s p l i t the degeneracy of the fundamental v i b r a t i o n s v^, (both of symmetry t ^ ) , v^(t^g) and v ^ ( t 2 u ^ " e x a m P ' e > ' n K^GeF^ the fundamental is s p l i t by the t r i g o n a l f i e l d i n t o two com-_ ] rO ponents of symmetry a^ and e which are separated by 2 3 cm . The band a s s o c i a t e d w i th in the a b s o r p t i o n spectrum is located at 16443 cm 1 and is s p l i t by approx imate ly 7 cm ' (the s p l i t t i n g i s not complete ly 2 r e s o l v e d ) . Two cases are p o s s i b l e : the E s t a t e or the v i b r a t i o n v^ , wi th the degeneracy l i f t e d , can be r e s p o n s i b l e f o r the observed s p l i t t i n g . An unequivocal a n a l y s i s is not p o s s i b l e wi thout p o l a r i z e d emiss ion and absorp t ion s p e c t r a . In F i g . 28 the f o l l o w i n g assignments are proposed: the weak band - 1 2 at 16064 cm is ass igned as the second component of the E, s p l i t by 34 cm \ The intense l i n e s at 16312, 16435 and 16710 cm 1 are a t t r i b u t e d 2 to t r a n s i t o n s to the v , v, and v v i b r a t i o n a l l e v e l s of the intense E 6 4 i o r i g i n at 16098 cm , w i th the v i b r a t i o n s s p l i t by 10 cm 1 or l e s s . The 2 cor responding t r a n s i t i o n s fo r the second component of E were only p a r -2 t i a l l y r e s o l v e d . A s i m i l a r assignment is proposed f o r the Tj s t a t e . In g e n e r a l , t h i s s t a t e i s s p l i t by the t r i g o n a l f i e l d and s p i n o r b i t coup l ing i n t o three components. In the a b s o r p t i o n spectrum of K^MnF^, three r e l a t i v e l y sharp bands were located at 16793, 16816 and 16888 cm 1 which had no counterpar t in the spectrum of Cs2MnFg. They were ass igned 2 as the three components of the s p l i t T^. In the same F i g u r e , the a b s o r p t i o n spectrum of K.MnF^ is compared 2 o 80 16.0 16.5 1 17-0 kK 17.5 r v6 vl< Pi 0 I IN V A A ABSORPTION 85°K V6 Vk H O H I AA u6 v 5 l v l . 16.0 V EMISSION 80°K J A 15.5 15.0 kK <~ F igure 28. Absorp t ion spectrum of K ^ n F ^ and emiss ion spectrum of M n ^ G e F ^ (0 .1%) , s i n g l e c r y s t a l s . (Assignments) 81 4+ with the emission spectrum of Mn iK^GeF^ (0.1 mole rat io) . In 4+ this latter host, Mn encounters a similar environment as in K^MnF^. 2 The components of the 0-0 transition E were located at 16079 and 16068 cm \ respectively, sp l i t by 11 cm \ Vibrations are assigned in Fig. 28 to the fundamentals v^, v^, and v,.. The f i r s t three are sp l i t by 25, 15 and 30 cm \ respectively, while v c is assumed not p to be spli t. The emission spectrum of K^MnF^ was not well resolved because of its low intensity. A mirror-image relationship to absorption was observed, The spectrum consists of five bands which comprise a hot band at 16290 cm the origin at approximately 16100 cm and three more bands at 15880, 15780 and 15480 cm ' which were assigned as the transitions to v^, and vibrational levels of the ground state, respectively. The spectrum is shown in Fig. 25. The intensities cannot be directly com-pared with the intensities of the absorption spectrum because they were not corrected for the SI response of the photomultiplier. The broad band at 14600 cm 1 in the emission spectrum of solid K^MnF^ cannot be assigned to a transition within the Mn(lV) ion. But its shape, position and lifetime (0.4 msec at 80°K) can be compared with the emission spec-trum of KMnF .^ In this complex manganese is divalent (3d^) and is oct-ahedral ly surrounded by six fluoride ions. The emission spectrum con-sists of one broad band of asymmetric shape, centered at 16400 cm 1 with 61 2+ a lifetime of 0.3 msec at 70°K. Mn has a similar environment when 2+ present as an impurity in K^MnF^. Traces of Mn in the lat t ice of suit-able hosts are known to be strong phosphors and the emission band has an 82 6 3 2+ asymmetric shape. Small t races of Mn were doubt less formed dur ing p r e p a r a t i o n and r e c r y s t a l l i z a t i o n of the potassium complexes. The band in F i g . 25 at 14600 cm ' i s thus l i k e l y to be an impur i t y emiss ion a r i s i n g from the t r a n s i t i o n S", ->• 6 A , of the M n 2 + in the l a t t i c e of lg lg 2+ K^MnF^. However, the e x c i t e d l e v e l s of Mn cannot be populated d i r e c t l y , 2 4+ but only by an energy t r a n s f e r process depopu la t ing the E s t a t e of Mn : Mn( lV ) ( 2 E) + M n ( l l ) ( 6 A ] ) + M n ( l V ) ( 4 A 2 ) + MndOcVj) (12) Such an energy t r a n s f e r is s p i n - a l l o w e d . Comparable energy t r a n s f e r processes by e x c i t o n t rapp ing and e x c i t o n m i g r a t i o n are wel l -known f o r aromat ic hydrocarbons, e . g . the mixed c r y s t a l system of anthracene -64 naphthacene. In a r i g i d g l a s s , such energy t r a n s f e r is not p o s s i b l e 2+ and the emiss ion of Mn i s not observed . 2 -The assignments made fo r the emiss ion bands of the [MnF^] ion 30 are p a r t i a l l y in c o n f l i c t w i t h those made by Kemeny and Haake , and 33 F l i n t . In the emiss ion spectrum of K.„MnF, , F l i n t observed a s e r i e s L o of sharp bands (which agree in p o s i t i o n and assignment to the ones descr ibed above) and a broad emiss ion band centered at 14300 cm ' . The d e s c r i p t i o n of t h i s band matches the one d iscussed above. The 4 4 band, however, was ass igned as the -> A^ f l u o r e s c e n c e . In a sub-35 2 4 sequent paper on the E -> A„ t r a n s i t i o n in Cs 0 MnF, a s i m i l a r g iq l b band was desc r ibed at 13000 cm ' which was ass igned to the same t r a n -s i t i o n . As a consequence, the emiss ion of the complexes Cs 2 MnF^, 4+ 4+ Mn tCs^GeF^ and Mn iK^MnF^ was r e i n v e s t i g a t e d w i t h a more r e d - s e n s i -t i v e p h o t o m u l t i p l i e r (SI c h a r a c t e r i s t i c s ) . The r e s u l t was n e g a t i v e . No such emiss ion was detected fo r these complexes. On the other hand, 83 4+ Kemeny and Haake who obta ined the emiss ion s p e c t r a of Mn in a magnesium f1uorogermanate mat r i x (again a s e r i e s of sharp l i n e s s t a r t -ing at 16030 cm 1 and going to lower energ ies were observed) ass igned 4 4 the emiss ion to ->• IK^ f l u o r e s c e n c e . That assignment is incompat-i b l e w i t h the data presented here . \ 4 k 4 11) T. , T_ -<- A» t r a n s i t i o n s 1 g~ 2g 2g A s i m i l a r approach was attempted to ana lyze the v i b r a t i o n a l s t r u c t u r e of the s p i n - a l l o w e d t r a n s i t i o n s . However, w i th the data a v a i l a b l e a d e t a i l e d a n a l y s i s of the v i b r a t i o n a l s t r u c t u r e i s d i f f i -c u l t . J a h n - T e l l e r i n s t a b i l i t y and s p i n - o r b i t c o u p l i n g (the s i n g l e e l e c -4+ - 1 t ron s p i n - o r b i t c o u p l i n g parameter fo r the f ree Mn ion i s 415 cm ) have to be taken i n t o c o n s i d e r a t i o n . A thorough a n a l y s i s is f u r t h e r compl icated by the f a c t that the ground s t a t e v i b r a t i o n a l f requenc ies cannot be d i r e c t l y compared w i t h the f requenc ies of the e x c i t e d s t a t e 4+ because of d i f f e r e n t geometr ica l c o n f i g u r a t i o n s . The presence of Mn p a i r l i n e s in the spec t ra of Cs^MnF^ was checked by comparison w i t h the 4+ corresponding s p e c t r a of Mn : Cs„GeF. (4%). /. o 4 4 V i b r a t i o n a l s t r u c t u r e was observed in the T, -> A„ t r a n s i t i o n lg 2g of C^MnFg in the absorp t ion spectrum at 85°K (data are a v a i l a b l e at t h i s temperature o n l y ) . The v i b r a t i o n a l i n t e n s i t i e s are s t r o n g l y depend-ent on temperature and a v i b r o n i c i n t e n s i t y mechanism must be a c t i v e . The assignments are summarized in Table VII and the r e s u l t s d i s p l a y e d in F i g . 29. Regular spacings of 540 and 450 cm 1 were observed s t a r t i n g from the band centered at 25850 cm ' . These spacings were ass igned to the v i b r a t i o n a l modes and r e s p e c t i v e l y , which correspond to a 8k TABLE VI I k D e t a i l s and assignment of the T a b s o r p t i o n band at 85°K of Cs.MnF, # P o s i t i o n [cm ] Assignment 1 25850 2 26390 25850 + v l 3 26760 25850 + 2 V 2 27480 25850 + 3 V1 5 27700 25850 + S 6 28090 25850 + 5 V 2 7 28550 25850 + 5 V1 8 29100 25850 + 6v, 9 29630 25850 + 7 v , 86 decrease of 10% r e l a t i v e to the f requenc ies of the ground s t a t e . Whereas the mode is symmetric and leaves the symmetry of the com-p lex unchanged, the mode causes a te t ragona l d i s t o r t i o n . The presence of t h i s mode in a p rogress ion suggests that a J a h n - T e l l e r d i s t o r t i o n i s p r e s e n t . 4 4 The r i c h v i b r a t i o n a l s t r u c t u r e in the T „ A» should 2g 2g permit a more complete study (the labe l T „ w i l l be used in the 2g remainder of the tex t a l though t h i s s t a t e is probably s p l i t by s p i n -o r b i t c o u p l i n g and/or a J a h n - T e l l e r e f f e c t ) . Again i t was noted that the i n t e n s i t i e s of the bands are s t r o n g l y temperature dependent sug -g e s t i n g that a v i b r o n i c i n t e n s i t y mechanism is o p e r a t i n g . As above, p rogress ions of regu la r spacings of 546 ± 22 and 443 ± 13 cm ' were observed which o r i g i n a t e at the f i r s t seven bands of the spectrum s t a r t i ng at 20627 cm 1 . 4 4 Th is band cou ld be an o r i g i n of the T „ •<- A„ t r a n s i t i o n 2g 2g c o n s i d e r i n g i t s i n t e n s i t y and p o s i t i o n : the band is weak and has an -6 o s c i l l a t o r s t r e n g t h of approx imately 10 which i s in the range ex -_r Cr pected fo r a s p i n - a l l o w e d magnetic d i p o l e t r a n s i t i o n (f^lO ) At h igher temperatures (up to 150°K) hot bands appear on the low energy s i d e of t h i s band which are located at 20447, 20350 and w i th v a n i s h i n g i n t e n s i t y at 20068 cm \ The spacings between these bands are 97 and 282 cm 1 , r e s p e c t i v e 1 y , which is almost e x a c t l y the spacing between the ground s t a t e v i b r a t i o n a l f requenc ies and (107 cm ' ) , and and (273 cm ^). These bands then must have a common o r i g i n located at approx imately 20680 cm 1 at 85°K. C o n s i d e r i n g the uncer -87 t a i n t i e s in the p o s i t i o n s t h i s o r i g i n may very w e l l be the band which was reso lved at 20627 cm ' at 10°K. Under increased r e s o l u t i o n , how-ever , the band s p l i t s in to three components at 20614, 20627 and 20640 cm \ w i th the f i r s t and the l a s t appear ing as s h o u l d e r s . The assignment above is f u r t h e r supported from emiss ion s t u d i e s at h igh temperatures . The purpose of t h i s study was t w o f o l d : 1) to ga in in fo rmat ion about the f a t e of e l e c t r o n i c e x c i t a t i o n energy (which w i l l be d iscussed in the next chaper) and 2) to look f o r p o s s i b l e r a d i a t i o n from the T^^ s t a t e at temperatures high enough to s u f f i c i e n t l y populate t h i s s t a t e . The r e s u l t s of t h i s study were a l ready p resented . A green emiss ion was observed at temperatures above 360°K. The emiss ion has a maximum at 18350 cm ' and is c l o s e to the s p i n - a l l o w e d ^T^ absorp -t i o n band centered at 21400 cm ' as shown in F i g . 23. This emiss ion 4 must be the thermal a c t i v a t e d f l u o r e s c e n c e from the T „ s t a t e to the 2g ground s t a t e . The band is b road , s t r u c t u r e l e s s and has a h a l f - h e i g h t width of 2400 cm ' which g e n e r a l l y is the case fo r the f l u o r e s c e n c e of C r ( l l l ) c o m p l e x e s . 6 6 ' ^ 7 The Stokes s h i f t of 3000 cm ' compares we l l - 1 3 - 6 7 w i th the va lue of 2900 cm found f o r [CrF Q ] • From the emiss ion peak at 18350 cm ' and the center of a b s o r p t i o n at 21400 cm ' the 4 4 o r i g i n of the 1^^ A 2 t r a n s i t i o n can then be est imated at approx-imately 20000 cm ' . In a d d i t i o n , the energy s e p a r a t i o n between the 4 2 T 2 and s t a t e can be est imated from the temperature dependence of Q^ . in a p l o t of log Q^ . a g a i n s t the r e c i p r o c a l temperature ( a b s o l u t e ) . The energy s e p a r a t i o n was c a l c u l a t e d from the i n i t i a l s lope of t h i s p l o t 88 2 (where Q^ . was dependent on ly on the thermal a c t i v a t i o n v i a the s ta te ) and was found to be 4540 cm 1 which compares we l l w i t h the expected d i f f e r e n c e of 4600 cm A l t o g e t h e r , there is enough con -v i n c i n g ev idence to i n t e r p r e t the l i n e at 20627 cm 1 as an o r i g i n of 4 4 the T- +• A„ t r a n s i t i o n . 2 9 2g Fur ther assignments were made based on a s imple s p i n - o r b i t 4 coup l ing model . The T^^state which is t w e l v e - f o l d degenerate , is s p l i t by the a c t i o n of s p i n - o r b i t c o u p l i n g in to the three s p i n s t a t e s T^, Tg and g separated by 3/4A and 5/kX, r e s p e c t i v e l y . The p a r a -meter X is p r o p o r t i o n a l to the s p i n - o r b i t c o u p l i n g constant E, (X = 5/3 fo r a 3d ion) and can be determined through t h i s r e l a t i o n s h i p from the express ion ^ ? 9 f ( 2 E ) A 2 ( 2 E , V ) v(V ) c2 3 2 i g 2g_ ( ] 3 ) 4 f ( V ) v ( 2 E ) 2g g 2 2 4 where f ( E ) is the t o t a l o s c i l l a t o r s t reng th of E A„ , 9 g 2g ' 2 2 4 A ( E , ^2g^ ' S t ' 1 e s c l u a r e °^ t n e e n e r g y d i f f e r e n c e between the 0 -0 2 4 4 t r a n s i t i o n s of T and ^2g' V^ ^2g^ ' S t ' i e e n e r g y °^ t p e 0 - 0 t r a n s i t i o n 4 4 4 of T^g ^ 2 g ' ^ "^2g^ ' S t o t a ^ o s c i l l a t o r s t reng th of t h i s t r a n s -2 2 4 i t i o n , and v( E ) the 0 -0 t r a n s i t i o n of E <- A , . The f o l l o w i n g g g 2g numerical va lues obta ined from the a b s o r p t i o n and emiss ion spectrum of 2 - 7 Cs.MnF, were s u b s t i t u t e d in to the equat ion above: f ( E ) = 3-5 x 10 , Z b g A ( 2 E , V ) = 4.6 x 10 3 c m " 1 , v ( V ) = 21.4 x 10 3 cm _ 1 , g 2g 2g f(2*T2 ) = 3-8 x 1 0 _ Z + and v ( 2 E ) = 16.0 x 10 3 c m " 1 , l ead ing to £ ( c a l c . ) zg g cm and hence, X = 81 cm . The r a t i o of the s p i n - o r b i t c o u p l i n g cons tant c a l c u l a t e d in t h i s way and the value fo r the f r e e ion (415 cm 89 from Ref. 68) is 0.58 and compares well with the ratio of 0.57 of the electron repulsion parameter B for the complex and the free ion. (the B value for the complex calculated from the strong f ield quartet 69 - ] matrix was found to be 609 cm ; the value for the free ion is 1064 _ j Q g cm ). The predicted split t ings of the spin states are then, f ina l ly , 61 and 102 cm 1 respectively. Thus starting from the band at 20627 cm \ which was already characterized as a no-phonon tran-s i t ion, two bands should appear 61 and I63 cm 1 to the blue of this origin. Three bands are observed within the required distance, centered at 20681, 20730 and 20799 cm"'. The f i r s t band is 53 cm"1 to the no-phonon l ine, at 20627 cm ' and is of comparable intensity. In contrast, -1 4 the band at 20799 cm is too strong to be an origin of the ^ • ^ e band at 20730 cm 1 is not resolved and appears as a shoulder on the ta i l of the intense l ine. This band is weak and considering the uncertainty in its position, is close to the calculated one. On the basis of this spin-orbit coupling model the two lines at 20681 and 20730 cm ' (exact position not resolved) were assigned as the no-phonon spin states of the 4 -1 T_ in addition to the line at 20627 cm 2g The bands at 20799, 20859, 20938 and 21008 cm"1 were not yet discussed. They are part of the seven bands which are origins for series involving the and vibrational modes. The distances of the f i r s t and third band to the no-phonon line at 20627 cm ' and of the second and fourth band to the no-phonon line at 20681 cm 1 are 172, 311, 178, 327 cm \ respectively. The vibration with the frequency 90 311 (327) cm 1 is most probably the fundamental w i th the frequency k smal le r than that in A^^ by approx imate ly the same amount as f o r the 4 -1 fundamentals v, and v„ in the T „ s t a t e . The v i b r a t i o n 172 (178) cm I 2 2g i s ass igned to the fundamental v^. The decrease of about 20% i s some-what la rge when compared to the ground s t a t e f requency . The assignments are summarized in Table V I M and shown in F i g . 30. k The assignment f o r the v i b r a t i o n a l s t r u c t u r e in the T^ a b s o r p t i o n spectrum of K^MnF^ i s less d i f f i c u l t . The s t a t i c t r i g o n a l f i e l d of the c r y s t a l o v e r r i d e s the e f f e c t s of s p i n - o r b i t c o u p l i n g and J a h n - T e l l e r 4 4 -1 i n s t a b i l i t y . The o r i g i n of T^ A^ is s p l i t by 300 cm . P rogress ions i n v o l v i n g the symmetrical mode only were observed. However, a s e r i e s of smal l peaks s t a r t i n g at 23216 cm 1 could not be i n t e r p r e t e d on t h i s b a s i s . A s e r i e s of the fundamentals v^, and could be i d e n t i f i e d o r i g i n a t i n g at 23216 and 23319 cm They were 2 2 assigned as the 0-0 t r a n s i t i o n s of the T^ s t a t e . L i k e the Tj s t a t e , the t h r e e f o l d degeneracy is l i f t e d by the t r i g o n a l f i e l d and s p i n - o r b i t 2 c o u p l i n g . The t h i r d component of the T^ s t a t e , however, could not be i d e n t i f i e d . The assignments are summarized in Table IX and d i s p l a y e d in F i g . 31. i i i ) Summary For Cs 2MnF^ bands at 16030, 16832 and 20627 cm" 1 were ass igned as 2 2 4 o r i g i n s of e l e c t r o n i c s t a t e s E , T. and T „ . The e l e c t r o n i c s t a t e s of 9 ig 2g K^MnF^ are s p l i t by a t r i g o n a l f i e l d or the combined e f f e c t of the t r i g o -nal f i e l d and s p i n - o r b i t c o u p l i n g . Band ass igned to o r i g i n s were located fo r 2 E at 16098 and 16064(?) c m " ' , f o r 2J ] at 16793, 16816 and 16888 cm" 1 2 -1 and fo r T^ at 23216 and 23319 cm . The f requenc ies of the fundamental v i b -r a t i o n s of the ground and e x c i t e d s t a t e s are summarized in Table X. 91 TABLE VIII D e t a i l s and assignment of s t r u c t u r e of T absorp t ion spectrum of Cs„MnF, # P o s i t i o n [cm ] P o s i t i o n [cm ] Assignment (10°K) (85°K) 1 20068 I - v 3 2 20350 I - vk 3 20447 I - v 6 20614 sh 4 20627 V (I) 2g 20640.sh 5 20681 V ( I I ) L 9 6 20730 sh 4 T 2 g ( I I I ) 7 20799 20768 I + 172 8 20859 20826 II + 172 9 20938 20969 I +311 10 21008 11+311 11 21181 sh I + v j 12 21331 21304 I + 172 + v ] 13 21368 1 + 311 + v 2 14 21441 21404 1 1 + 3 1 1 + v 2 15 21492 21459 I +311 + Vj 16 21570 sh I I + 311 + Vj 17 21709' sh I + 2\>j 18 21815 21791 III + ?.Vj 19 21882 21834 I + 172 + 2v ] 20 21949 I I + 172 + 2v 21 22017 sh I + 3 H + 2v, 92 TABLE VI I I (cont.) # Pos i t i on [cm 1 ] (10°K) Position [cm (85°K) Ass i gnment 22 22237 sh 1 + 3 v 2 23 22306 22292 II + 3v, 24 22381 22336 111 + 3v,• 25 22422 1 + 172 + 3v, 26 22517 22495 1 1 + 172 + 3v, 27 22779 22743 1 + 4 v £ 28 22862 22831 II + 4 v 2 29 22931 22910 1 + 172 + 4 v , 30 23272 23245 11 + 311 + 5vk 31 23386 23354 M + 5 v , 32 23485 23447 1 + 172 + 5 v , 33 23815 23781 1 + 7 v 2 34 23929 23923 II + 6 v 1 35 24067 24067 1 + 311 + 7 v 2 36 24301 v] = 546 ± 22 cm v„ = 443 ± 13 cm" 2 sh - shoulder 93 94 TABLE IX D e t a i l s and assignments of the s t r u c t u r e of 4 T^ a b s o r p t i o n spectrum of K^MnF^ # Pos i t ion [cm 1 ] Ass ignment 1 20145 20363" " v 6 2 20903 20363 + v ] 3 21230 20663"" + v 21446 20363 + 2 V , 5 21739 20663 + 2 V ] 6 21970 20363 + 3v ] 7 22247 20663 + 3v ] 8 22487 20363 + 4 V ] 9 22800 sh 20663 + 4 V ] 10 22980 20363 + 5v ] 11 23216 2 T 2 (1) 12 23319 sh 2 T 2 (2) 13 23455 23216 + v 6 14 23562 23319 + v 6 , 23216 15 23619 sh 23319 + vk 16 23825 23216 + v 3 17 23926 sh 23319 + v 3 18 24087 20363 + 7v, 19 24400 sh 20663 + 7vj , - proposed o r i g i n s of T sh - shoulder v,= -531 ± 30 cm" 1 95 TABLE X 2- - ] The fundamental f requenc ies of [MnF^] ( in cm ) V i b r a t i o n Symmetry Ground s t a t e f requenc ies \ E x c i t e d s t a t e s f requenc ies 2 E 2 T l q \ \ g 'g 2g lg Raman IR Emi ss ion (from absorp t ion ) v l ig 600 (590) a (593) a 546±22 540 V2 e g 507 (510) 3 (508) a (507) a 443±13 450 V3 *l u 620 608 627 600 v4 C l u 340 335 330 332 316±5 V5 l2g 310 ( 3 0 4 )a (308) a (301) a v6 *2u (233) a 228 220 224 172 (?) a from combinat ions °-° 21.0 22.0 23.0 24.0 F i g u r e .31. A b s o r p t i o n s p e c t r u m o f K^nF^ a t 85°K, s i n g l e c r y s t a l . (Numbers r e f e r t o a s s i g n m e n t s made i n T a b l e 97 V T E M P E R A T U R E D E P E N D E N C E O F T H E P H O S P H O R E S C E N C E L I F E T I M E In t h i s c h a p t e r , r e s u l t s are desc r ibed of the temperature dependence of the l i f e t i m e in the i n t e r v a l from 80 to 600°K. The r e s u l t s are d i s c u s s e d in c o n j u n c t i o n w i t h the data on the tempera-ture dependence of the emiss ion quantum y i e l d s and the i n t e g r a t e d a b s o r p t i o n c o e f f i c i e n t from Chapter IV. The primary photophysica1 2 -processes in [MnF,] are d i s c u s s e d . 1. RESULTS L i f e t i m e s were measured of Cs^MnF^ in s o l u t i o n , o p t i c a l l y d i l u t e d in s i n g l e c r y s t a l s of Cs2GeFg, and in pure form. 4+ F igure 32 shows the temperature dependence of Mn :Cs GeF, 2. b (0.1% mole r a t i o ) in the range from 80 to 600°K. The decays were pure ly e x p o n e n t i a l . A s i n g l e c r y s t a l was used, approx imate ly 2 mm in diameter and 1 mm t h i c k . The r e s u l t s f o r pure Cs^MnF^ are a l s o conta ined in F i g . 32. A s i n g l e c r y s t a l of approx imate ly the same s i z e was employed and the l i f e t i m e measured in the temperature i n t e r v a l from 80 to 400°K. The decays were not e x p o n e n t i a l . The l i f e t i m e s were c a l c u l a t e d from the t a i l of the decay curves where an exponent ia l c h a r a c t e r was approached. The l i f e t i m e s of pure Cs„MnF, are c o n s i d e r a b l y s h o r t e r 99 than the l i f e t i m e s o f the o p t i c a l l y d i l u t e compound. The d i f f e r e n c e in l i f e t i m e and t h e n o n e x p o n e n t i a l i t y o f t h e decay must be caused by 4+ a c o o p e r a t i v e e f f e c t o f c o u p l e d Mn ions i n t h e u n d i l u t e d c r y s t a l . T h i s i s r e f l e c t e d i n the l i f e t i m e s o f c r y s t a l s w i t h d i f f e r e n t g u e s t c o n c e n t r a t i o n s . For example, a t 80°K the l i f e t i m e o f the pure Cs„MnF, I b 4+ c r y s t a l i s 4.6 msec. As the Mn c o n c e n t r a t i o n d e c r e a s e s , the l i f e t i m e 4+ i n c r e a s e s t o 13.7 msec f o r Mn :Cs 2GeF^ w i t h a 4.4% guest c o n c e n t r a t i o n and i s 15-5 msec f o r a c r y s t a l w i t h a 0.1% c o n c e n t r a t i o n . The decay of 4+ Mn :Cs 2GeF^ (4%) s t i l l i s s l i g h t l y n o n e x p o n e n t i a 1. The n a t u r a l l i f e -2 time o f the E s t a t e was c a l c u l a t e d from the o s c i l l a t o r s t r e n g t h t o be 9 17-3 msec a t t h a t t e m p e r a t u r e . -2 F i g u r e 33 shows the l i f e t i m e s o f a 2 x 10 molar s o l u t i o n o f Cs^MnF^ i n deoxygenated 20% HF i n the t e m p e r a t u r e i n t e r v a l from 100° t o 185°K. The e n v i r o n m e n t a l e f f e c t o f the s o l v e n t i s c l e a r l y i n d i c a t e d . Compared t o the n a t u r a l l i f e t i m e at 80°K, f o r example, the l i f e t i m e o f the s o l u t e i s s h o r t e r by a f a c t o r o f 6 a t t h i s t e m p e r a t u r e . The decay was a n a l y s e d by a f i r s t o r d e r p r o c e s s and was found to be s t r i c t l y expo-n e n t i a l . The l a r g e s c a t t e r o f the e x p e r i m e n t a l p o i n t s a t a p p r o x i m a t e l y l40°K was caused by the inhomogeneity o f the s o l v e n t . T h i s problem was a l r e a d y d i s c u s s e d i n the l a s t c h a p t e r . A t t e m p t s were made t o measure the l i f e t i m e o f the d e l a y e d f l u o r -e s c e n c e . However, the i n t e n s i t y was found t o be too weak and the l i f e -time c o u l d not be measured. In F i g u r e 34 the e x p e r i m e n t a l and t h e o r e t i c a l v a l u e s o f the n a t u r a l l i f e t i m e of the phosphorescence o f Cs^MnF^ a r e d e p i c t e d as a 100 120 140 160 180 °K > F igure 33. Phosphorescence l i f e t i m e of C s ^ n F ^ in s o l u t i o n (20% HF) as a f u n c t i o n of temperature (c = 2 x 10 2 M ). 101 F igure 34. The natu ra l l i f e t i m e of phosphorescence of Cs MnF, as c a l c u l a t e d 2 o from the o s c i l l a t o r s t reng th ( • e x p e r i m e n t a l , c a l c u l a t e d ) . 102 f u n c t i o n of temperature . The exper imental va lues of the n a t u r a l l i f e t i m e were obta ined from the i n t e g r a t e d a b s o r p t i o n c o e f f i c i e n t 2 h of the Eg A^g t r a n s i t i o n at d i f f e r e n t temperatures ( i n c l u d i n g the hot bands) us ing Equation 7- The t h e o r e t i c a l va lues were c a l -c u l a t e d as a f u n c t i o n of temperature from the sum f ( t o t a l ) = Z f . + E f . (]k) i 1 j J where f ( t o t a l ) is the t o t a l o s c i l l a t o r s t r e n g t h , f . the o s c i l l a t o r s t rength of the i ^ Stokes band der i ved from the r e l a t i o n s h i p f . = f coth(hv ./kT) (15) t h and f . the o s c i l l a t o r s t reng th of the j a n t i - S t o k e s band as c a l -c u l a t e d from the express ion f . = f . c o t h ( h v V k T ) exp( -hv ./kT) (16) J j o J J Th is exp ress ion was tes ted fo r the hot band r e l a t e d to the f u n d -amental and i t f i t t e d the exper imental va lues ra ther w e l l , as demonstrated in F igure 16. f . and f . ( f . i s equal to the o s c i l l a -j o i o io ^ tor s t reng th of the i**1 v i b r o n i c band at 0°K) are constants and were obta ined s e p a r a t e l y f o r each i n d i v i d u a l v i b r o n i c band from a f i t to the e x p e r i m e n t a l l y observed o s c i l l a t o r s t rength at at l e a s t two d i f -f e r e n t temperatures . 2 . DISCUSSION i) The Temperature Dependence of the Phosphorescence L i f e t i m e In F igure 35 the r e c i p r o c a l of the natu ra l l i f e t i m e , k,., as 103 °K <r 1/T x 10 Figure 35. Quantum y i e l d s and l i f e t i m e s of emiss ion of Mn :Cs 2 GeF (4% and 0.1% mole r a t i o , r e s p e c t i v e l y ) . 104 obta ined from the i n t e g r a t e d a b s o r p t i o n c o e f f i c i e n t of the 2 4 . . Eg •<- A^g t r a n s i t i o n us ing Equat ion 7, i s p l o t t e d a g a i n s t tempera-ture and compared to the r e c i p r o c a l exper imental l i f e t i m e , T e X p > and the quantum y i e l d s of phosphorescence, Q^, and delayed f l u o r e s -cence, Q_^ . Two d i s t i n c t fea tu res should be noted : l ) the temperature dependence of k g f o l l o w s that of up to 300°K and 2) whereas the natu ra l l i f e t i m e decreases monotonously w i t h temp-e r a t u r e , the exper imental l i f e t i m e along w i th the quantum y i e l d s f o r phosphorescence and delayed f l u o r e s c e n c e decrease sharp l y at tempera-tures above 400°K. The high quantum y i e l d of phosphorescence at l i q u i d n i t r o g e n temperature suggests that the n o n - r a d i a t i v e processes k^, k^ and k ^ are smal l at t h i s temperature . Furthermore, the decay is most ly r a d i a t i v e up to about 400°K as judged from the i n v a r i a n c e of phosphor-escence quantum y i e l d w i th temperature . S ince v i b r o n i c t r a n s i t i o n s are r e s p o n s i b l e f o r at l e a s t 30% of the t o t a l phosphorescence i n t e n s i t y , the increase in k,_ in t h i s temperature range must r e s u l t from v i b r o n i c -a l l y induced i n t e n s i t y and the temperature dependence of k^ should f o l l o w that d e s c r i b e d by the o s c i l l a t o r s t r e n g t h . Th is is a c t u a l l y observed. Above 350°K delayed f l u o r e s c e n c e is d e t e c t e d . The emiss ion is very weak and at 450°K, f o r example, the y i e l d c o n s t i t u t e s only ]% of the t o t a l emiss ion i n t e n s i t y . Delayed f l u o r e s c e n c e i s a the rmal l y a c t i -vated process and as such has an a c t i v a t i o n energy which should be equal 4 2 to the energy s e p a r a t i o n of the no-phonon ^ 2q a n c ' ^g s t a t e s > From i n s p e c t i o n of F igure 35 , a p l o t of log Q^ . aga ins t 1/T does not g ive a 105 s t r a i g h t l i n e due to the p a r t i c i p a t i o n of severa l competing processes which a l l must be dependent on temperature . Below 450°K, however, the e f f e c t of the competing processes is small and the a c t i v a t i o n e n e r -gy es t imated from the s lope of the p l o t at 400°K is of the r i g h t magni -tude (approx imately 13-0 k c a l / m o l e ) . The f a c t that delayed f l u o r e s c e n c e i s a c t u a l l y observed sets an upper l i m i t fo r k^ and k^ w i th respect to k^. k^  es t imated from the 4 - 1 in teg ra ted a b s o r p t i o n c o e f f i c i e n t us ing Equat ion 7, i s 5 x 10 sec _ 3 at 400°K, f o r example. At t h i s temperature Q. = 1 and Q f = 2 x 10 . If the abso lu te quantum y i e l d were u n i t y (and i t must be c l o s e to u n i t y f o r Cs 2MnFg) t h i s does not mean that the sum (k 2 + k^) << k^  . If the exper imental u n c e r t a i n t y of i s , say , 10% and the a c t u a l va lue 0p = 0.9 [the d i f f e r e n c e of 10% a c t u a l l y being r a d i a t i o n 1 ess d e a c t i v a -t i o n v i a (k 2 + k^)] then (k 2 + k^) could exceed k^  by a f a c t o r of as much as 50 and ought to be cons idered in an a n a l y s i s . F a i l u r e to observe d i r e c t f l u o r e s c e n c e is caused by a very r a p i d 2 i n te rsys tem c r o s s i n g to the E^ s t a t e . The rate constant f o r t h i s p r o -3 c e s s , k^, must be l a r g e r than kj at l e a s t be a f a c t o r of 10 from the f a c t that the s m a l l e s t f l u o r e s c e n c e quantum y i e l d detected e x p e r i m e n t a l l y _ 3 g _ j is about 10 . The va lue of k^ as computed from these data i s 10 sec and i s of the same order of magnitude as that f o r many o r g a n i c systems. 7*^ U n l i k e k_^ which i s a the r mal l y a c t i v a t e d process w i th an a c t i v a t i o n 4 energy equal to the energy s e p a r a t i o n of the no-phonon l e v e l s of the T 2 2 and E s t a t e ( in t h i s case 13-2 k c a l ) , k^ i s g e n e r a l l y assumed to occur 106 by a t u n n e l l i n g mechanism and as such has no a c t i v a t i o n energy . Above k50°K the exper imental l i f e t i m e and the quantum y i e l d s of both phosphorescence and f l u o r e s c e n c e decrease s h a r p l y . In terna l convers ion or a ( r e v e r s i b l e ) photochemical process o c c u r r i n g e i t h e r 2 2 2 from the E^ (or T ^ ) or T ^ s t a t e , must be r e s p o n s i b l e fo r t h i s deact i vat i o n . Accord ing to recent t h e o r i e s of r a d i a t ion less t r a n s i t i o n s by 71 72 73 Robinson and Frosch , S iebrand , and L i n , the temperature depend-ence of r a d i a t i o n 1 ess d e a c t i v a t i o n i s s m a l l . For most aromat ic com-74 75 pounds of the a c t i v a t i o n energ ies range from 0 .9 to 2.k Kca l/mole . ' In c o n t r a s t , a very s t rong temperature dependence of c e r t a i n , r a d i a t i o n l e s s t r a n s i t i o n s , though at high temperatures , as proposed by 1 2 K i s l i u k and Moore. They observed a s i m i l a r sharp and abrupt decrease in both the l i f e t i m e and emiss ion quantum y i e l d of ruby and emerald at temperatures above 700°K. 2k The same argument was used by Ding le to account fo r the sudden decrease of the l i f e t i m e and quantum y i e l d s of [ C r ( u r e a ) , ] (N0_)_ at temp-o 5 3 e ra tures above 25°K. In t h i s study the l i f e t i m e and quantum y i e l d of i, three d i f f e r e n t bands - the o r i g i n of the 1 s t a t e ( ? ) , the o r i g i n of 2 2 the Eg s t a t e and a band above the E^ being in rap id e q u i l i b r i u m w i t h t h i s s t a t e - were s e p a r a t e l y monitored w h i l e temperature was changed from 1.5 to 300°K. The quantum y i e l d of the v i b r o n i c band showed a s i m i l a r temperature dependence when compared to that of the delayed f l u o r e s c e n c e of Cs^MnF^: f i r s t , the i n t e n s i t y of the band inc reases w i th i n c r e a s i n g temperature accord ing to Boltzmann s t a t i s t i c s , a 107 maximum o c c u r s , and at s t i l l h igher temperatures the i n t e n s i t y decreases a b r u p t l y together w i t h the l i f e t i m e . D ing le i n t e r p r e t e d the rap id quenching of luminescence and l i f e t i m e s to an i n t r a m o l e c -u l a r mult iphonon p r o c e s s . Other d e a c t i v a t i o n processes l i k e photochemical processes should a l s o be c o n s i d e r e d . An example f o r t h i s type of r e a c t i o n i s the complex [ C r ( e n ) ^ ] l ^ . Under i r r a d i a t i o n w i t h UV l i g h t at low temperatures (below 130°K) the complex changes c o l o r from y e l l o w to d v 76 23 red . R e c e n t l y , the photochemistry of t h i s s o l i d was s t u d i e d more c l o s e l y and the f o l l o w i n g mechanism was proposed: [ C r ( e n ) 3 ] l 3 ^ [Cr( e n ) 2 e n 1 , 2 I ] l 2 (17) kT 2-In [MnF^] a r e v e r s i b l e photochemical process of the k ind [ M n F 6 ] 2 " ^ > [ M n F 5 ] 1 _ + F~ (18) 2 could be proposed. If i t occurs from the E g s t a t e the a c t i v a t i o n energy of t h i s process i s 25.8 kca l/mole and, o t h e r w i s e , i s 12.6 kca l/mole i f the T 2 g i s c h e m i c a l l y r e a c t i v e . Very l i t t l e i s known however, about photochemical processes in s o l i d s . It i s assumed that a photochemical process of the k ind p r o -posed in Equat ion 17 i s r e s p o n s i b l e f o r the quenching at h igh temp-e r a t u r e s . However, the exact nature of the process need not be known f o r the a n a l y s i s . From the fo rego ing d i s c u s s i o n i t i s necessary to c o n s i d e r on ly three d e a c t i v a t i o n processes in order to d e s c r i b e the temper-a t u r e dependence of the phosphorescence l i f e t i m e . In terms of ra te 108 c o n s t a n t s , the processes are k,., k_^ and k^ (or k^) and the o v e r a l l ra te constant i s k = k_ + k . + k, (19) exp 5 ~k 7 o r , in numerical terms, k =1 - 3 1 + 3 x 10 9 exp(-13-2 x J_03) + 1 0 1 5 exp(-24.9 x IO3) (20) 6 X P T RT RT o The frequency f a c t o r s f o r k ^ and k^ were obta ined by f i t t i n g the r e l a t i o n above to the exper imental d a t a . The r e s u l t s are d i s p l a y e d in F igure 36-At low temperatures the exper imental va lues seem to be somewhat too s m a l l . Th is i s not s u r p r i s i n g c o n s i d e r i n g the f a c t that the l i f e -times of Cs^MnFg were measured in form of a s i n g l e c r y s t a l of f i n i t e t h i c k n e s s (approx. 1 mm). Depending on the t h i c k n e s s of the c r y s t a l s the emiss ion can be p a r t i a l l y reabsorbed which tends to pro long the observed decay. Th is was observed , f o r example, fo r ruby, e s p e c i a l l y 27 at low temperatures . The lengthening of the l i f e t i m e from s e l f -absorp t ion can be avoided by the use of very t h i n c r y s t a l . U n f o r t u n a t e l y , the b r i t t l e nature of the m a t e r i a l of Cs^MnF^ d i d not permit c u t t i n g c r y s t a l s w i th a t h i c k n e s s less than 1 mm. The r e s u l t s f o r Cs„MnF, d i s s o l v e d in 20% HF are a l s o conta ined 2 6 in F igure 36 and, on compar ison, are q u i t e d i f f e r e n t . The l i f e t i m e s are c o n s i d e r a b l y s h o r t e r and, in a d d i t i o n , decrease sharp l y above temp-e ra tu res as low as 140°K. The a c t i v a t i o n energy c a l c u l a t e d from the Ar rhen ius p l o t i s 7.2 kcal/mole and i s less than that found f o r Mn :Cs2GeFg. S ince no s i g n i f i c a t s p e c t r a l change was observed in the 1 0 9 5 . 0 4 . 0 3 . 0 2 . 0 F igure 3 6 . Temperature dependence of the l i f e t i m e of Mn :Cs„MnF, (0.!%)(•) and 2 2 6 Cs 2 MnF 6 in deoxygenated 2 0 % HF ( 2 x 1 0 M) (O) . • , 0 - exper imental c a l c u l a t e d 110 a b s o r p t i o n and emiss ion spec t ra of Cs^MnF^ in s o l u t i o n , the energy of 7-2 kcal/mole i s too smal l to represent the a c t i v a t i o n energy of reverse in te rsys tem c r o s s i n g . The v a l u e , however, is in the range t y p i c a l f o r the a c t i v a t i o n energy of photoaquat ion of t r a n s i t i o n metal complexes in s o l u t i o n and, in t h i s c a s e , may s imply represent 2 -the a c t i v a t i o n energy of the aquat ion of the MnF^ complex a c c o r d -ing to the r e a c t i o n [ M n F 6 ] 2 " + H 2 0 •+ [ M n F ^ O ] 1 " + F _ (21) Th is process occurs q u i t e r a p i d l y at room temperature therm-a l l y w i th p r e c i p i t a t i o n of hydrated MnO^. On c o o l i n g the sample the process i s slowed down; no decomposit ion was noted at the temperature of l i q u i d n i t r o g e n even under prolonged i r r a d i a t i o n w i t h uv l i g h t . At t h i s temperature the so lvent forms a r i g i d g l a s s . 130°K seems to be the l i m i t i n g temperature f o r the decomposit ion p r o c e s s . Below t h i s temperature the so lvent i s not f l u i d but a r i g i d g l a s s . As a l ready mentioned the environmental e f f e c t of the so l vent HF is r e f l e c t e d in the shor ten ing of the l i f e t i m e . Such e f f e c t s of c r y s t a l l i n e and n o n c r y s t a l l i n e hosts on the n o n r a d i a t i v e t r a n s i t i o n p r o b a b i l i t i e s of the guest (or s o l u t e ) were s t u d i e d in d e t a i l by 1 9 Targos and F o r s t e r . It appears that hosts which are not isomorphous w i th the guest s t r o n g l y enhance the n o n r a d i a t i v e t r a n s i t i o n s , in p a r t i c u l a r the n o n c r y s t a l l i n e environment of s o l v e n t s . Cs„MnF, i s isomorphous w i t h Cs 0 GeF , and no environmental e f f e c t 2 o z b on the t r a n s i t i o n p r o b a b i l i t i e s was noted. In c o n t r a s t , the emiss ion of Cs_MnF, as a s o l u t e in HF is c o n s i d e r a b l y quenched as judged from 111 the shor ten ing of the l i f e t i m e , becomes a s i g n i f i c a n t pathway fo r the d i s s i p a t i o n of e l e c t r o n i c e x c i t a t i o n energy . The temp-e r a t u r e dependence of the l i f e t i m e of Cs MnF, in HF can t h e r e f o r e Z b be d e s c r i b e d as the sum k = k_ + k, + k., (22) exp 5 6 7 or in numerical terms, k = 1/x + 310 + 1 0 1 3 exp(-7 .2 x 103/RT) (23) exp o The r e s u l t i s shown in F igure 3D. The e f f e c t of v i s c o s i t y need not be cons idered s i n c e k^ i s the important term at temperatures above l40°K. 2 -i i ) The Pr imary Photophysica1 Processes of [MnF^] In t h i s s e c t i o n are summarized the severa l processes which 2 -determine the f a t e of e l e c t r o n i c e x c i t a t i o n energy of the [MnF^] ion when i t is present as an impur i ty in the isomorphous host Cs^GeF^. The processes and t h e i r ra te constants are dep ic ted in F igure 37-E x c i t a t i o n i n t o the q u a r t e t man i fo ld i s f o l l o w e d by a rap id 4 i n t e r n a l convers ion w i t h i n severa l picoseconds to the T^^ l e v e l at 59 ' 0 k c a l / m o l e . Th is process is independent of e x c i t a t i o n wavelength 4 The T „ s t a t e has a n a t u r a l l i f e t i m e of 20 microseconds . D i r e c t 2g f l u o r e s c e n c e is not observed because of the f a s t e r i n t e r s y s t e m c r o s s -4 ing rate in to the doublet mani fo ld which dep letes the T^g l e v e l w i th 2 severa l nanoseconds. At low temperatures the Tjg s t a t e at 48 .1 kcal/mole i s r a p i d l y d e a c t i v a t e d by i n t e r n a l convers ion to the zeroth 112 113 2 v i b r a t i o n a l l eve l of the E s t a t e at 45.8 kca l/mole and phosphores-2 cence from the E^ s t a t e occurs w i t h a quantum y i e l d c l o s e to or equal 2 to u n i t y . The low temperature l i f e t i m e of the E^ s t a t e which would be the r a d i a t i v e l i f e t i m e i f 0 = 1 , i s 18 m i l l i s e c o n d s . On r a i s i n q p 3 the temperature above 350°K the T 2 g s t a t e is g r a d u a l l y populated by reverse in te rsys tem c r o s s i n g and delayed f l u o r e s c e n c e is observed . 4 In ternal convers ion from the T 2 to the ground s t a t e is longer than 20 microseconds and the d e a c t i v a t i o n s are pure ly r a d i a t i v e up to 450°K. Above 450°K the t o t a l emiss ion quantum y i e l d drops sharp l y and a photo -chemical process is a c t i v a t e d . The o r i g i n of t h i s process is not e n t i r e l y c l e a r and i t may 2 4 occur e i t h e r from the E or from the T „ s t a t e . Above 600°K the 9 2g photochemical d e a c t i v a t i o n dominates a l l o ther processes and emiss ion is no longer observed . If t h i s process is indeed c h e m i c a l , i t must a l s o be r e v e r s i b l e in the dark . Above 300°K reverse in te rsys tem c r o s s i n g and photochemical r e a c t i o n are r e s p o n s i b l e f o r the temperature dependence of the phos-phorescence l i f e t i m e . Below 300°K the e f f e c t of the v i b r o n i c mech-anism dominates and the change of the t r a n s i t i o n p r o b a b i l i t y of the v i b r o n i c bands on ly account f o r the temperature dependence. 114 VI C O N C L U S I O N S 1. SPECTRA As expected , v i b r a t i o n a l s t r u c t u r e was observed on a l l t r a n s i t i o n s . O r i g i n s were located as weak bands c o n s i s t e n t w i t h magnetic d i p o l e t r a n s i t i o n s . A d e t a i l e d a n a l y s i s of the w e l l -it reso lved v i b r a t i o n a l s t r u c t u r e of the T „ s t a t e i s d i f f i c u l t 2g because of s p i n - o r b i t c o u p l i n g and J a h n - T e l l e r s p l i t t i n g s . The c o n t r i b u t i o n of the magnetic d i p o l e i n t e n s i t y to the t o t a l i n t e n -s i t y is n e g l i g i b l e . The t o t a l i n t e n s i t y i s almost e n t i r e l y v i b r a -t i o n a l l y induced. The temperature dependence of the t o t a l i n t e n s i t y was ra ther w e l l desc r ibed over a wide i n t e r v a l by equat ion 11, page 72. Th is e q u a t i o n , though in a mod i f ied form, d i d apply a l s o f o r v i b r o n i c hot bands. A m i r r o r - i m a g e r e l a t i o n s h i p was found f o r the emiss ion and 2 4 absorp t ion spectrum of Eg"*-*" ^2g t 1 " 3 1 1 5 ' 1 ' 0 1 1 , The v i b r a t i o n a l s t r u c -ture cou ld be w e l l c o r r e l a t e d w i t h the fundamental i n t e r n a l f requenc ies of the ground s t a t e obta ined from IR and Raman s p e c t r a . it The assignment of an o r i g i n of the T^^ s t a t e was conf i rmed by the p o s i t i o n of the f l u o r e s c e n c e spectrum at high temperatures . D i r e c t f l u o r e s c e n c e was not observed. 115 2 . TEMPERATURE DEPENDENCE OF THE PHOSPHORESCENCE LIFETIME i) The Rate Constant k 1, Throughout the l i t e r a t u r e Equat ion 7 (page 8) i s w i d e l y used to c a l c u l a t e k^, however, a thorough study of i t s a p p l i c a b i l i t y to the weak t r a n s i t i o n metal ions has yet not been done and t h e r e f o r e the data c a l c u l a t e d from t h i s equat ion should be cons idered as e s t i -mations . The e q u a l i t y of the c a l c u l a t e d and exper imental va lue of k^ f o r Cs2MnF^ w i t h i n exper imental e r r o r must be t h e r e f o r e f o r t u i t o u s . In c o n j u n c t i o n w i t h Equat ion 11, the temperature dependence of k^ can never the less be c a l c u l a t e d . R e l a t i v e to o ther rate c o n s t a n t s , k^ i s on ly weakly dependent on temperature ( for example, in the temperature range from 0 to 600°K, k c increases on ly t h r e e f o l d ) and the general assumption that k^ is temperature independent i s j u s t i -f i e d f o r approximate c a l c u l a t i o n s . i i ) The Rate Constant k_^ k_^ i s thermal l y a c t i v a t e d and has t h e r e f o r e an a c t i v a t i o n energy which is equal to the s e p a r a t i o n of the zero v i b r a t i o n a l l e v e l s k 2 of the T2^ and E^ s t a t e s . Only i f k ^ represents the dominant temp-e r a t u r e dependent process can the a c t i v a t i o n energy of k_^ be d i r e c t l y obta ined from a measurement of the temperature dependence of phosphor-escence. In t h i s case the observed a c t i v a t i o n energy can be equated to the energy s e p a r a t i o n of the two s t a t e s invo lved and, knowing the 2 k l o c a t i o n of the no-phonon t r a n s i t i o n of E , the lowest o r i g i n of T . 9 2g can be l o c a t e d . 116 3. SUGGESTIONS FOR FURTHER WORK The de te rminat ion of the pr imary processes in t r a n s i t i o n metal compounds is at best an educated guess s i n c e the number of unknowns exceeds by f a r that of the o b s e r v a b l e s . The f o l l o w i n g problems must be s o l v e d : 1) The de te rminat ion of p r e c i s e quantum y i e l d s . 2) The a p p l i c a b i l i t y of Equat ion 7 to t r a n s i t i o n s of t r a n s i t i o n metal compounds. Equat ion 7 has been tes ted f o r o rgan ic compounds by com-par ing the c a l c u l a t e d value of the natu ra l f l u o r e s c e n c e l i f e t i m e T q w i t h the va lue obta ined e x p e r i m e n t a l l y from l i f e t i m e and abso lu te quantum y i e l d measurements v i a the r e l a t i o n 'o - ? <2*> where T I S the exper imental l i f e t i m e and 0 i s the quantum y i e 1 d . 1 ^ ' 1 5 ' 1 ° A few t r a n s i t i o n metal complexes are known from which f l u o r -escence has been o b s e r v e d . ^ ' 9 ' ^ 7 For these complexes the a p p l i c a b i l -i t y of Equat ion 7 can be tes ted s i m i l a r l y as has been desc r ibed f o r o r g a n i c compounds. The dete rminat ion of abso lu te quantum y i e l d s of t r a n s i t i o n metal complexes i s made d i f f i c u l t by the f a c t that in most compounds, emiss ion i s observed at low temperatures o n l y . Recent r e s u l t s by Wasges t ian 7 ^ and C h e n 7 9 i n d i c a t e , however, that some complexes emit q u i t e s t r o n g l y in deoxygenated nonaqueous s o l v e n t s . In t h i s case accurate quantum y i e l d s can be o b t a i n e d . 117 F luorescence i s not u s u a l l y observed fo r complexes w i t h 9 la rge l i g a n d - f i e l d s p l i t t i n g s but f l u o r e s c e n c e must always o c c u r . It may go undetected in s t e a d y - s t a t e experiments because of i t s cont inuous broad spectrum and low i n t e n s i t y as i s the case f o r 3- 80 81 f l u o r e s c e n c e of [Cr(CN)g] and azulene . In the l a t t e r report a very s e n s i t i v e apparatus was desc r ibed which was capable of meas-~6 ur ing the low f l u o r e s c e n c e quantum y i e l d of 10 f o r a z u l e n e . 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