Plutonium Chemistry - American Chemical Society

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11 Plutonium Hexafluoride Gas Photophysics and Photochemistry JAMES V. BEITZ, CLAYTON W. WILLIAMS, and W. T. CARNALL

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Argonne National Laboratory, Chemistry Division, Argonne, IL 60439

Little has been published concerning the photodynamics of PuF gas, although this compound was first synthesized in 1942. We recently reported the first observation of fluorescence from electronically excited PuF and found its behavior to differ significantly from that of UF or NpF . The photophysics of PuF excited at 532 nm and at 1064 nm has now been observed in detail using laser-induced fluorescence techniques. The fluorescence emission spectra recorded are the same at both excitation wavelengths with the fluorescence intensity peaking at about 2300 nm. The fluorescence decay of PuF gas excited at 532 nm was found to be laser-fluence dependent and a mechanism is proposed which accounts for this observation. Net photodecomposition of PuF was rapid at a fluence of 5 J/cm at 532 nm (7 ns pulse width). 6

6

6

6

6

6

6

2

Our recent observation (1) of e l e c t r o n i c s t a t e fluorescence from l a s e r - e x c i t e d PuF6(g) and NpF6(g) has marked the beginning of systematic studies of the photophysics and photochemistry of transuranic hexafluorides and has provided the key to f u r t h e r e x p l o r a t i o n of the complex v i b r o n i c s t r u c t u r e c h a r a c t e r i s t i c of these compounds. While the photochemistry of UF6 has been the object of numerous i n v e s t i g a t i o n s , only a few remarks are found i n the l i t e r a t u r e concerning the e l e c t r o n i c state photochemistry of the transuranic hexafluorides · Given the dense e l e c t r o n i c energy l e v e l s t r u c t u r e s of PuF6 and NpF6 and t h e i r r e l a t i v e i n s t a b i l i t y i n comparison with UF6, we can a n t i c i p a t e that t h e i r photochemistry will involve a rich and complex s e t of interactions. The work we report here deals p r i m a r i l y with PuF6(g). We turn f i r s t to a very b r i e f review of PuF6 studies 0097-6156/83/0216-0155$06.00/0 © 1983 American Chemical Society

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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related to i t s synthesis, energy p h o t o p h y s i c s and p h o t o c h e m i s t r y .

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R e v i e w o f R e l a t e d PuF6

levels,

thermodynamics,

Studies

In l i g h t of the subject of this Symposium i t i s p a r t i c u l a r l y a p p r o p r i a t e t o n o t e t h a t PuF6 was f i r s t s y n t h e s i z e d i n 1942 on a m i c r o g r a m s c a l e a s p a r t o f t h e M a n h a t t a n P r o j e c t a l t h o u g h i t was n o t i s o l a t e d as a p u r e compound (2_) · S t e i n d l e r (3,4) has reviewed e a r l y PuF6 s t u d i e s . By 1950 F l o r i n h a d synthesized PuF6 on a gram s c a l e and i t was no l o n g e r a laboratory curiousity. Many o f t h e p h y s i c a l p r o p e r t i e s o f PuF6 were measured by W e i n s t o c k , Malm and c o - w o r k e r s i n t h e 1 9 5 0 ' s . These w o r k e r s were u n a b l e t o r e c o r d t h e Raman s p e c t r u m o f PuF6 due t o p h o t o d e c o m p o s i t i o n (_5_). They n o t e d t h a t " l i g h t o f t h e i n t e n s i t y o f o r d i n a r y room i l l u m i n a t i o n " decomposed P u F g , a n d t h e y r e a s o n a b l y assumed t h a t t h e p r o d u c t s were PuFi+(s) and F ( g ) (6) · S t e i n d l e r and coworkers found that PuF6 c o u l d be s y n t h e s i z e d u s i n g u l t r a v i o l e t i r r a d i a t i o n o f PuFi+(s) and F ( g ) (7) . The k e y t o t h i s seeming c o n t r a d i c t i o n p r o b a b l y l i e s i n t h e t h e r m o d y n a m i c s t a b i l i t y o f P u F s ( s ) , a compound o f t e n p o s t u l a t e d to e x i s t b u t whose s y n t h e s i s h a s n e v e r b e e n r e p o r t e d . Other more g e n e r a l r e v i e w s w h i c h d e a l i n p a r t w i t h PuF6 p r o p e r t i e s i n c l u d e t h o s e o f Rand ( 8 ) , O e t t i n g ( 9 ) , D. Brown (10) and K e l l e r ( 11). Brown's ( 12) and C l e v e l a n d ' s ( 13) books a l s o c o n t a i n u s e f u l information concerning PuF6. O ' D o n n e l l and c o - w o r k e r s have r e c e n t l y compared t h e c h e m i c a l r e a c t i v i t y o f PuF6 i n a n ­ h y d r o u s HF t o t h a t o f o t h e r h e x a f l u o r i d e s i n t h e same medium ( 14). L i t t l e c a n be g l e a n e d f r o m t h e l i t e r a t u r e c o n c e r n i n g g a s phase c h e m i c a l r e a c t i o n s o f PuF6. 2

2

E l e c t r o n i c E n e r g y L e v e l s and Thermodynamics A b s o r p t i o n band s t r u c t u r e due t o e l e c t r o n i c s t a t e s o f PuF6 extends from the i n f r a r e d into t h e vacuum u l t r a v i o l e t and c o n t a i n s c o n t r i b u t i o n s from 5f e l e c t r o n s t a t e s , charge t r a n s f e r and d i s s o c i a t i v e s t a t e s . I t s complex v i b r o n i c s t r u c t u r e has long challenged s p e c t r o s c o p i s t s . M. F r e d r e c o r d e d t h e o p t i c a l a b s o r p t i o n s p e c t r u m o f PuF6 i n 1 9 5 4 , n o t i n g t h e p r e s e n c e o f vibronic structure (15). S t e i n d l e r and Gunther published a c c u r a t e m o l a r a b s o r p t i v i t y d a t a f o r PuF6 i n 1963 ( 1 6 ) . Kugel et a l . discovered r e s o l v a b l e isotope s h i f t s i n the e l e c t r o n i c a b s o r p t i o n s p e c t r a o f PuF6 gas a t room t e m p e r a t u r e i n 1976 and d e s c r i b e d a t h e o r e t i c a l model u s e d t o p r e d i c t t h e e n e r g y l e v e l s t r u c t u r e o f i t s 5f e l e c t r o n s t a t e s (15). A t h e o r e t i c a l model f o r t h i s s t r u c t u r e h a s a l s o been p r e s e n t e d by B o r i n g and Hecht (17). Relativistic c a l c u l a t i o n s concerning PuF6 have been c a r r i e d o u t by K o e l l i n g , E l l i s and B a r t i e t t ( 1 8 ) a n d , c o n c e r n i n g Pu , by D e s c l a u x a n d Freeman (19). These theoretical a s s i g n m e n t s o f t h e e l e c t r o n i c e n e r g y l e v e l s t r u c t u r e o f PuF6 + 6

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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AL.

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have y e t t o be c o n f i r m e d e x p e r i m e n t a l l y . No t h e o r y has b e e n reported which successfully p r e d i c t s the occurrence of the i s o t o p e s h i f t s f o u n d by K u g e l e t a l . ( 1 5 ) . F u g e r has b r i e f l y r e v i e w e d the c u r r e n t s t a t e o f c r i t i c a l assessments of a c t i n i d e thermodynamics ( 2 0 ) . He l a r g e l y f o l l o w s Rand's earlier assessment in determining the enthalpy of f o r m a t i o n o f PuFi* and P u F 6 . As F u g e r n o t e d , i t i s s u r p r i s i n g t h a t t h e h e a t of f o r m a t i o n o f s u c h a t e c h n o l o g i c a l l y i m p o r t a n t compound as PuF$ s h o u l d be b a s e d s o l e l y on v a l u e s f o r t h e e q u i l i b r i u m between PuF6, PuFi+ and F 2 . W h i l e P u F 6 ( g ) a t room temperature is thermodynamically unstable with respect to PUFL^S) and F 2 ( g ) , i t i s q u i t e s t a b l e a t room t e m p e r a t u r e with r e s p e c t t o f o r m i n g PuFi+(g) and F 2 ( g ) . From a photochemical p o i n t of view, the l a c k of thermodynamic i n f o r m a t i o n c o n c e r n i n g PuF5 i s p a r t i c u l a r l y u n f o r t u n a t e s i n c e i t i s l i k e l y t h a t t h e first dissociation limit of PuF6(g) corresponds to forming P u F 5 ( g ) and an F atom. I n F i g u r e 1 t h e e l e c t r o n i c e n e r g y l e v e l s t r u c t u r e s o f UF6, NpF6 and PuF6 g a s e s a r e shown t o g e t h e r w i t h the r e g i o n s i n w h i c h t h e s e compounds show c o n t i n u o u s o p t i c a l a b s o r p t i o n and their known o r e s t i m a t e d t h e r m o d y n a m i c d i s s o c i a t i o n l i m i t s f o r gas phase p r o d u c t s . The d a t a i n F i g u r e 1 a r e p r e s e n t e d i n t e r m s o f t h e t r a d i t i o n a l s p e c t r o s c o p i c u n i t of e n e r g y , t h e cm"" (1 cm" = 19.86484 χ 1 0 " J per molecule or atoms). S i n c e UF6 i s a 5f° system i t has no low-lying electronic states. Continuous o p t i c a l a b s o r p t i o n i n UF6, b e g i n n i n g i n t h e riear-ultraviolet, is b e l i e v e d to a r i s e from charge t r a n s f e r bands. The f i r s t of t h e s e bands has an o r i g i n a t a b o u t 24647 cm"" b a s e d on Andrews and co-workers' matrix isolation studies (21). The UF6 d i s s o c i a t i o n l i m i t shown i s b a s e d on H i l d e n b r a n d ^ s " w o r k (22). The r e g i o n s i n w h i c h NpF6 shows c o n t i n u o u s a b s o r p t i o n a r e shown i n F i g u r e 1 and are based largely on Steindler and G e r d i n g ' s measurements ( 2 3 ) w i t h p r e f e r e n c e g i v e n i n s e t t i n g t h e lowest energy at which e l e c t r o n i c s t a t e a b s o r p t i o n occurs to unpublished NpF6 f l u o r e s c e n c e work by Beitz. T h e r e i s no thermodynamic information concerning the first dissociation l i m i t o f NpF6. The v a l u e shown i s a r o u g h i n t e r p o l a t i o n o f t h e measured UF6 value and our crude estimate of the first d i s s o c i a t i o n l i m i t o f PuF6. The p o s i t i o n s o f t h e 5f s t a t e s o f NpFe a r e b a s e d on t h e work o f Goodman and F r e d ( 2 4 ) and o f H u t c h i n s o n and W e i n s t o c k ( 2 5 ) . The r e g i o n s i n w h i c h PuF6 shows c o n t i n u o u s a b s o r p t i o n as w e l l as t h e 5f s t a t e e n e r g i e s shown i n F i g u r e 1 a r e b a s e d p r i m a r i l y on S t e i n d l e r and G u n t h e r ' s measurements ( 1 6 ) . For a c u r r e n t t h e o r e t i c a l i n t e r p r e t a t i o n o f t h e 5f s t a t e s t r u c t u r e o f PuF6 see J . B l a i s e e t a l . ( t h i s v o l u m e ) . The f i r s t d i s s o c i a t i o n l i m i t of PuF6 i s c r u d e l y e s t i m a t e d b a s e d on t h e e x t r e m e l y s m a l l PuF6 gas fluorescence photon yield but long fluorescence l i f e t i m e we have f o u n d ( s e e b e l o w ) . In a d d i t i o n , the f a c t t h a t PuF6 i s a b e t t e r f l u o r i n a t i n g a g e n t t h a n m o l e c u l a r fluorine 1

2 I +

1

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1

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0

J

1

UF

6

(5f°)

NpF (of) 1

6

PuF

6

2

(5f )

F i g u r e 1. E l e c t r o n i c energy l e v e l diagram f o r gas phase a c ­ tinide hexafluorides. The r e g i o n s i n w h i c h a g i v e n h e x a f l u o r i d e e x h i b i t s c o n t i n u o u s a b s o r p t i o n a r e shown shaded w i t h diagonal l i n e s . 5f e l e c t r o n s t a t e s a r e shown a s s h o r t h o r i ­ zontal lines. The thermodynamic d i s s o c i a t i o n l i m i t s a n d r e ­ s u l t a n t g a s p h a s e p r o d u c t s a r e shown t o t h e r i g h t o f t h e e n e r g y l e v e l diagram f o r each h e x a f l u o r i d e . U F ^ i g ) , a 5f° s y s t e m , has no l o w - l y i n g e l e c t r o n i c l e v e l s a n d i s t h e r m o d y n a m i c a l l y more s t a b l e t h a n N p F ^ ( g ) o r P u F ^ ( g ) . F o r t h e s e r e a s o n s UF^ i s u n l i k e l y t o be a good m o d e l compound f o r t r a n s u r a n i c h e x a f l u o r i d e photochemistry studies.

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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s u g g e s t s t h a t t h e f i r s t d i s s o c i a t i o n l i m i t o f PuF6 l i e s b e l o w t h e d i s s o c i a t i o n l i m i t o f m o l e c u l a r f l u o r i n e (12800 cm"" ). The second d i s s o c i a t i o n l i m i t , c o r r e s p o n d i n g t o m o l e c u l a r p r o d u c t s , i s b a s e d on Rand's a n a l y s i s ( 8 ) o f t h e e n t h a l p i e s o f f o r m a t i o n o f P u F i t ( g ) and P u F 6 ( g ) . The s t r i k i n g f e a t u r e o f F i g u r e 1 i s t h e d i s s i m i l a r i t y o f UF6 and e i t h e r NpF6 o r PuF6. UF6, p o s s e s s i n g greater thermodynamic s t a b i l i t y and no l o w - l y i n g e l e c t r o n i c s t a t e s , i s l i k e l y t o be a p o o r model compound f o r t r a n s u r a n i c h e x a f l u o r i d e p h o t o p h y s i c s and p h o t o c h e m i s t r y . Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch011

1

E x p e r i m e n t a l Methods A Q - s w i t c h e d NdrYAG l a s e r (7 ns p u l s e d u r a t i o n , Quanta-Ray DRC-1A) o p e r a t e d a t 10 Hz was u s e d as a l i g h t s o u r c e . The 1064 nm f u n d a m e n t a l was f r e q u e n c y d o u b l e d t o 532 nm f o r some experiments. I n a l l e x p e r i m e n t s r e p o r t e d h e r e a g e o m e t r y was u s e d w h i c h f o c u s e d t h e l a s e r beam i n f r o n t o f t h e e n t r a n c e window o f t h e sample c e l l s u c h t h a t t h e l a s e r beam was d i v e r g i n g as i t p a s s e d t h r o u g h t h e sample c e l l . I n t h i s geometry t h e l a s e r beam was a b o u t 3 mm i n d i a m e t e r a t t h e r e g i o n v i e w e d by the l i g h t d e t e c t i o n system. P u F e was s y n t h e s i z e d u s i n g P u 0 2 f r o m ANL s t o c k s i n a n a p p a r a t u s s i m i l a r t o t h a t d e s c r i b e d by S t e i n d l e r e t a l . ( 2 6 ) w i t h t h e a d d i t i o n o f a f l u o r i n e gas r e c i r c u l a t i o n s y s t e m . Prior t o f i l l i n g a sample c e l l t h e PuF6 was p u r i f i e d as d e s c r i b e d by Malm and W e i n s t o c k ( 2 7 ) . The a l l q u a r t z sample c e l l s were c o n s t r u c t e d w i t h c y l i n d r i c a l b o d i e s made o f 8 mm i . d . " w a t e r free" quartz (GE 214) t o m i n i m i z e optical absorbance at 2700 nm. The laser beam passed coaxially through the cylinder. The sample c e l l s were baked o u t u n d e r vacuum and were p a s s i v a t e d u s i n g s m a l l amounts o f PuF6 p r i o r t o b e i n g f i l l e d and s e a l e d o f f . The t e m p e r a t u r e o f a s i d e a r e on t h e sample c e l l was v a r i e d t o o b t a i n t h e d e s i r e d PuF6 p r e s s u r e b a s e d on t h e vapor pressure-temperature r e l a t i o n s h i p g i v e n by W e i n s t o c k , Weaver and Malm ( 2 8 ) . The s i d e arm was a l s o u s e d t o condense o u t a l l t h e PuF6 i n t h e c e l l when b a c k g r o u n d f l u o r e s c e n c e was being checked. T y p i c a l l y t h e m a i n body o f t h e sample c e l l was h e l d a t 300 K. 2 i + 2

F l u o r e s c e n c e was c o l l e c t e d p e r p e n d i c u l a r t o t h e l a s e r beam p r o p a g a t i o n d i r e c t i o n ( i . e . 90 d e g r e e l i g h t c o l l e c t i o n ) u s i n g calcium fluoride lenses. I n f r a r e d f l u o r e s c e n c e was m o n i t o r e d u s i n g a 3 mm d i a m e t e r I n f r a r e d A s s o c i a t e s InSb photovoltaic d e t e c t o r c o o l e d t o 77 Κ and a c u r r e n t mode p r e a m p l i f i e r w h i c h p r o v i d e d an o v e r a l l 1/e r e s p o n s e t i m e o f 1 m i c r o s e c o n d . A g a l l i u m a r s e n i d e p h o t o m u l t i p l i e r (RCA 31034) was u s e d i n t h e s p e c t r a l r e g i o n b e t w e e n 560 nm and 920 nm and a c o o l e d S - l photomultiplier (RCA 7102) was u s e d f r o m 920 nm t o 1100 nm. C i r c u l a r v a r i a b l e i n t e r f e r e n c e f i l t e r s (CVF) were i n s t a l l e d i n a h o u s i n g e q u i p p e d w i t h a s y n c h r o n o u s motor d r i v e and were u s e d a s

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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a scanning mono chroma t o r f o r wavelengths from 1370 nm t o 4600 nm. The OCLI CV-1.3/2.5 a n d CV-2.3/4.6 CVF's p r o v i d e d a s p e c t r a l b a n d p a s s w h i c h v a r i e d o v e r t h e r a n g e o f 1% t o 3% o f t h e wavelength s e l e c t e d . The CVF's made p o s s i b l e 1:1 i m a g i n g a t f/1.4 light collection thereby improving fluorescence light c o l l e c t i o n e f f i c i e n c y by a f a c t o r o f 9 i n c o m p a r i s o n w i t h t h e 20 cm f . l . , f / 4 . 2 , h o l a g r a p h i c g r a t i n g monochromator u s e d . The g r a t i n g monochromator ( I S A H20FIR) was u s e d between 560 nm a n d 1400 nm a t t y p i c a l l y 32 nm s p e c t r a l b a n d p a s s . F l u o r e s c e n c e l i f e t i m e s were measured u s i n g a 2048 c h a n n e l d i g i t a l t r a n s i e n t r e c o r d e r (AEL PTR-9200) w h i c h was c o n n e c t e d t o a m u l t i c h a n n e l a n a l y z e r (Tracor Northern 570A). This provided "add-to-memory" s i g n a l a v e r a g i n g w i t h a b e s t t i m e r e s o l u t i o n o f 10 ns p e r c h a n n e l . T y p i c a l l y 2048 l a s e r s h o t s were a v e r a g e d together. The m u l t i c h a n n e l analyzer output was f e d t o a computer where a l i n e a r l e a s t s q u a r e s a n a l y s i s was c a r r i e d o u t to obtain the l i f e t i m e of the f l u o r e s c i n g s t a t e . Fluorescence emission spectra were recorded using a boxcar i n t e g r a t o r (PAR 162/164) w i t h t y p i c a l l y a 5 m i c r o s e c o n d g a t e w i d t h d e l a y e d 25 m i c r o s e c o n d s f r o m t h e l a s e r p u l s e and a n o u t p u t t i m e c o n s t a n t o f 100 s e c o n d s . The pump l a s e r i n t e n s i t y was s i m u l t a n e o u s l y recorded a n d was u s e d t o c o r r e c t t h e r e c o r d e d fluorescence intensity f o r the small changes i n laser intensity which occurred during a fluorescence emission scan. Experimental

Results

Fluorescence Emission Spectra. We r e c e n t l y r e p o r t e d t h e f i r s t o b s e r v a t i o n o f f l u o r e s c e n c e from e l e c t r o n i c a l l y e x c i t e d PuF6 gas n o t i n g t h a t t h e o b s e r v e d f l u o r e s c e n c e peaked a t a b o u t 2300 nm ( J _ ) . We have now measured t h e f l u o r e s c e n c e emission s p e c t r u m o f PuF6 e x c i t e d b o t h a t 1064 nm a n d a t 532 nm. The s p e c t r u m shown i n F i g u r e 2 was c o r r e c t e d f o r t h e w a v e l e n g t h dependence o f t h e i n f r a r e d d e t e c t i o n s y s t e m d e s c r i b e d above by recording t h e spectrum of a tungsten lamp whose filament t e m p e r a t u r e was measured w i t h a n o p t i c a l p y r o m e t e r . From t h i s d a t a t h e s p e c t r a l response f u n c t i o n (29) o f t h e d e t e c t i o n system was determined and t h e f l u o r e s c e n c e spectrum accordingly corrected. The e m i s s i o n s p e c t r u m o f a l o w p r e s s u r e m e r c u r y a r c was r e c o r d e d t o o b t a i n a w a v e l e n g t h c a l i b r a t i o n . To w i t h i n t h e i n t e n s i t y u n c e r t a i n t y shown i n F i g u r e 2, t h e f l u o r e s c e n c e e m i s s i o n s p e c t r u m o f PuF6 i s unchanged i n s w i t c h i n g f r o m 1064 nm e x c i t a t i o n t o l o w f l u e n c e 532 nm e x c i t a t i o n ( c i r c a 0.3 J / c m ) . L a s e r f l u e n c e i s a measure o f t h e l a s e r p u l s e e n e r g y p e r u n i t c r o s s - s e c t i o n a l a r e a o f t h e l a s e r beam. The l a s e r e n e r g y was measured u s i n g a c a l i b r a t e d volume absorber c a l o r i m e t e r and t h e l a s e r beam d i a m e t e r was e s t i m a t e d from " b u r n " p a t t e r n s on t h e r m a l l y s e n s i t i v e p a p e r . The w a v e l e n g t h r a n g e o f 1370 nm t o 4600 nm was s e a r c h e d when u s i n g 1064 nm e x c i t a t i o n a n d t h e r a n g e 560 nm t o 4600 nm was s e a r c h e d when 2

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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11.

ΒΕίτζ ET AL.

PuFe Gas Photophysics and Photochemistry

4000

4500

wavenumber

161

5000

( cm

- 1

)

F i g u r e 2. The f l u o r e s c e n c e i n t e n s i t y o f P u F g ( g ) e x c i t e d a t 1064 nm i s shown a s a f u n c t i o n o f t h e e n e r g y o f t h e e m i t t e d p h o t o n s . The s p e c t r a l b a n d p a s s and i n t e n s i t y u n c e r t a i n t y a r e indicated. To w i t h i n e x p e r i m e n t a l e r r o r , t h e same e m i s s i o n s p e c t r u m i s f o u n d when 532 nm e x c i t a t i o n i s u s e d .

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u s i n g 532 nm e x c i t a t i o n . The o n l y P u F 6 f l u o r e s c e n c e emission f o u n d was i n t h e 4500 cm" r e g i o n as shown i n F i g u r e 2. When 532 nm e x c i t a t i o n was u s e d , s c a t t e r e d l a s e r l i g h t was f o u n d t o give r i s e t o weak f l u o r e s c e n c e f r o m some o f t h e l o n g p a s s o p t i c a l f i l t e r s used t o s e p a r a t e g r a t i n g o r d e r s .

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1

Photodecomposition. A greyish-white f i l m of s o l i d m a t e r i a l s l o w l y f o r m e d a l o n g t h e b o t t o m o f t h e sample c e l l i n P u F 6 c e l l s irradiated a t 1064 nm. A s i m i l a r f i l m formed considerably faster i n sample c e l l s irradiated a t 532 nm w i t h t h e f i l m f o r m i n g on t h e e n t r a n c e f a c e o f t h e c e l l s a s w e l l as a l o n g t h e walls of the c e l l . F i l m f o r m a t i o n was e v i d e n t a f t e r l e s s t h a n a m i n u t e o f i r r a d i a t i o n a t a l a s e r f l u e n c e o f 5 J / c m a t 532 nm. The exact chemical composition o f t h e f i l m h a s n o t y e t been determined. I t i s known t h a t i t contains p l u t o n i u m and fluorine. Most l i k e l y i t i s amorphous PuFi+. A t p r e s e n t we c a n not r u l e out the p o s s i b i l i t y that the f i l m contains other plutonium f l u o r i d e s such as P u i f F i 7 , PU2F9 or PUF5. It is evident that net photodecomposition of PuF6 occurs using l i g h t of a photon energy l e s s than t h e f i r s t d i s s o c i a t i o n l i m i t o f 2

UF6.

Fluorescence L i f e t i m e s . We r e c e n t l y r e p o r t e d t h e f i r s t measurement o f t h e f l u o r e s c e n c e lifetimes of electronically e x c i t e d P u F 6 , u s i n g 1064 nm and 532 nm e x c i t a t i o n ( 1 ) . We have e x t e n d e d o u r measurements u s i n g 532 nm l i g h t t o h i g h e r l a s e r f l u e n c e a n d have f o u n d t h a t t h e f l u o r e s c e n c e d e c a y o f P u F 6 excited a t 532 nm i s n o t s i n g l e e x p o n e n t i a l . This "none x p o n e n t i a l " behavior i s evident a t lower l a s e r f l u e n c e o n l y i n a s l i g h t c u r v a t u r e i n p l o t s o f t h e r e s i d u a l s t h a t r e s u l t from fitting t h e data t o a s i n g l e e x p o n e n t i a l decay model. Plots were made o f t h e o b s e r v e d f l u o r e s c e n c e d e c a y s a s l o g [ f l u o r , i n t e n s i t y ] v e r s u s t i m e f o l l o w i n g t h e 532 nm l a s e r p u l s e . The p l o t s s i g n i f i c a n t l y departed from t h e s t r a i g h t l i n e behavior e x p e c t e d f o r a s i n g l e e x p o n e n t i a l d e c a y a s l a s e r f l u e n c e was increased. F l u o r e s c e n c e i n t e n s i t y d e c a y e d most r a p i d l y a t s h o r t times. Even a t t h e s e higher fluences, the near-infrared f l u o r e s c e n c e i n t e n s i t y o f P u F 6 was so weak t h a t t h e l i f e t i m e s had t o be measured u s i n g t h e InSb d e t e c t o r a n d o n l y a l o n g p a s s f i l t e r t o b l o c k s c a t t e r e d 532 nm l a s e r l i g h t . The o b s e r v e d fluorescence r i s e t i m e s corresponded t o t h e d e t e c t i o n system response time. A t 1064 nm, l a s e r f l u e n c e s i n t h e r a n g e o f 0.15-1.4 J / c m were u s e d d u r i n g t h e c o u r s e o f t h i s w o r k . A t 1064 nm, t h e observed fluorescence d e c a y s were w e l l - f i t i n a l l cases by assuming a s i n g l e e x p o n e n t i a l decay. As we p r e v i o u s l y r e p o r t e d (JL), t h e f l u o r e s c e n c e l i f e t i m e o f P u F 6 g a s a t 300 Κ e x c i t e d by 1064 nm l i g h t i s 204 ± 12 m i c r o s e c o n d s i n d e p e n d e n t o f P u F 6 g a s pressure over t h e range s t u d i e d ( 1 8 t o 109 t o r r , 1 torr = 133.322 p a s c a l ) . At the highest PuF6 pressure used about 2

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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ΒΕίτζ ET AL.

PuFq

Gas Photophysics and Photochemistry

163

120,000 h a r d s p h e r e c o l l i s i o n s o c c u r d u r i n g t h e l i f e t i m e o f t h e e m i t t i n g PuF6 e x c i t e d e l e c t r o n i c s t a t e . The e m i t t i n g s t a t e , assigned as a 5f e l e c t r o n e x c i t e d s t a t e (see below), i s remarkably metastable with respect to c o l l i s i o n a l l y induced energy t r a n s f e r . At 532 nm, t h e r a n g e o f l a s e r f l u e n c e s u s e d was 0.1-5.0 J / c m . As p r e v i o u s l y r e p o r t e d (_1_), e x c i t a t i o n a t 532 nm r e s u l t e d i n a s h o r t e r l i f e t i m e (86 ± 4 m i c r o s e c o n d s ) e v e n a t t h e l o w e s t f l u e n c e u s e d ( 0 . 1 J / c m ) , where, w i t h e x t e n s i v e signal averaging, a final signal-to-noise ratio o f about 20 was obtained. As n o t e d above t h e o b s e r v e d f l u o r e s c e n c e d e c a y s a t 532 nm became i n c r e a s i n g n o n - e x p o n e n t i a l with increasing laser fluence· The s i g n i f i c a n t l y f a s t e r PuF6(g) f l u o r e s c e n c e d e c a y r a t e found using 532 nm e x c i t a t i o n i s unlikely t o be due t o a thermally induced effect (e.g. pyrolysis). The optical a b s o r p t i o n c o e f f i c i e n t s o f PuF6(g) a t 532 nm i s a t most t w i c e a s l a r g e a s a t 1064 nm ( 1 5 ) . A s s u m i n g t h e 1064 nm a b s o r p t i o n c o e f f i c i e n t remains independent of l a s e r f l u e n c e , then use of 1.4 J / c m l a s e r f l u e n c e a t 1064 nm w o u l d r e s u l t i n d e p o s i t i o n o f a b o u t 7 t i m e s a s much e n e r g y p e r u n i t sample volume as w o u l d be d e p o s i t e d by 0.1 J / c m l a s e r f l u e n c e a t 532 nm. W h i l e d i f f e r e n t e n e r g y d e g r a d i n g p r o c e s s e s may o c c u r a t t h e s e two w a v e l e n g t h s , shorter wavelength excitation will more strongly favor d i s s o c i a t i o n o f PuF6 and hence may e v e n r e s u l t i n a s m a l l e r immediate h e a t i n g e f f e c t p e r absorbed photon. The n e t r e s u l t i s t h a t any sample h e a t i n g e f f e c t p r e s e n t a t 532 nm e x c i t a t i o n a t 0.1 J / c m laser fluence should be s u b s t a n t i a l l y larger at 1.4 J / c m laser fluence using 1064 nm excitation. The observation that t h e measured fluorescence lifetime using 1064 nm e x c i t a t i o n was i n d e p e n d e n t o f l a s e r f l u e n c e t h e r e f o r e provides evidence that thermal e f f e c t s a r e unimportant i n t h i s work. The "add-to-memory" s i g n a l a v e r a g i n g method c u r r e n t l y available t o us d i s t o r t s f l u o r e s c e n c e i n t e n s i t y versus time p l o t s when t h e f l u o r e s c e n c e i n t e n s i t y i s a n o n - l i n e a r f u n c t i o n o f i n c i d e n t l a s e r e n e r g y and t h e l a s e r e n e r g y v a r i e s f r o m s h o t to shot. F o r t h i s r e a s o n we have n o t a t t e m p t e d d e t a i l e d k i n e t i c m o d e l l i n g o f t h e observed f l u o r e s c e n c e i n t e n s i t y decay curves r e c o r d e d a t h i g h 532 nm l a s e r f l u e n c e . 2

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2

2

2

2

2

Fluorescence Photon Y i e l d s . We p r e v i o u s l y e s t i m a t e d t h e experimental fluorescence photon y i e l d o f PuF6 e x c i t e d a t 1064 nm a s w e l l as a t 532 nm as b e i n g 10 "ί ** ) ( 1 ) . We d e f i n e t h e e x p e r i m e n t a l f l u o r e s c e n c e p h o t o n y i e l d (FPY) as : 1

FPY

1

= ( f l u o r , photons emitted)/(pump l a s e r photons absorbed) ( 1 )

T h i s e s t i m a t e d FPY was b a s e d on t h e r e p o r t e d m o l a r a b s o r p t i v i t y o f PuF6 a t t h e pump l a s e r w a v e l e n g t h , t h e p r e s s u r e o f PuF6 i n

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the optical region o f t h e sample cell, the geometrical fluorescence l i g h t c o l l e c t i o n s o l i d angle, the transmission of t h e o p t i c a l f i l t e r s u s e d , t h e e s t i m a t e d quantum e f f i c i e n c y o f t h e InSb d e t e c t o r , t h e g a i n o f t h e a m p l i f i e r s u s e d a n d a n i n t e g r a t i o n o f t h e observed f l u o r e s c e n c e i n t e n s i t y s i g n a l as a function of time. A l t h o u g h t h e u n c e r t a i n t y i n t h e e s t i m a t e d FPY is large, this estimate i s valuable i n assessing the p h o t o p h y s i c s and p h o t o c h e m i s t r y o f PuF6(g)»

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Discussion We w i l l f i r s t consider p o s s i b l e assignments f o r the f l u o r e s c i n g s t a t e s i n l a s e r - e x c i t e d P u F 6 ( g ) b a s e d on a v a i l a b l e e n e r g y l e v e l s t r u c t u r e a n d thermodynamic i n f o r m a t i o n . We w i l l t h e n c o n s i d e r some o f t h e i m p l i c a t i o n s o f t h e l o n g - l i v e d PuF6 fluorescence we have observed i n terms of p o t e n t i a l photochemical separation processes. E m i s s i o n S p e c t r u m A s s i g n m e n t - 1064 nm E x c i t a t i o n . The f l u o r e s c e n c e e m i s s i o n s p e c t r u m o b s e r v e d u s i n g 1064 nm e x c i t a t i o n i s shown i n F i g u r e 2. We n o t e t h a t t h e l a r g e s t e n e r g y gap between PuF6 e l e c t r o n i c s t a t e s e n e r g e t i c a l l y a c c e s s i b l e u s i n g t h i s e x c i t a t i o n w a v e l e n g t h o c c u r s between t h e g r o u n d s t a t e a n d t h e 5 f s t a t e a t a b o u t 4550 cm" ( s e e F i g u r e 1 ) . The o b s e r v e d emission spectrum corresponds w e l l t o that expected f o r the 4550 cm" s t a t e and i t s v i b r a t i o n a l l e v e l s f l u o r e s c i n g t o t h e g r o u n d s t a t e and i t s v i b r a t i o n a l l e v e l s i f one assumes t h a t t h e next higher 5f s t a t e contributes little t o the observed fluorescence spectrum. The f l u o r e s c e n c e s p e c t r u m shown i n F i g u r e 2 c o n f i r m s t h e v i b r o n i c nature o f t h e 4550 cm" t r a n s i t i o n s i n c e t h e peak s e p a r a t i o n c o r r e s p o n d s w e l l t o 2v6 a s w o u l d be e x p e c t e d f r o m S t e i n d l e r and G u n t h e r ' s a b s o r p t i o n measurements ( 1 6 ) . Based on c a l c u l a t i o n s o f K u g e l e t a l . ( 1 5 ) o f PuF6 e n e r g y l e v e l s t r u c t u r e and t h e i r s t a t e a s s i g n m e n t s , P u F 6 ( g ) f l u o r e s c e s f r o m i t s l o w e s t lying Ι % state. I t s h o u l d be n o t e d a g a i n , h o w e v e r , t h a t t h e s t a t e a s s i g n m e n t s g i v e n by K u g e l e t a l . have n o t y e t been s y s t e m a t i c a l l y confirmed. The different relative intensities of the v i b r o n i c components of the fluorescence emission spectrum shown i n Figure 2 i n comparison w i t h the i n t e n s i t i e s of the corresponding peaks i n S t e i n d l e r and Gunther's a b s o r p t i o n spectrum a r e r e a d i l y u n d e r s t o o d i n t e r m s o f t h e arguments p r e s e n t e d by E i s e n s t e i n and Pryce (30) concerning t h e 7540 cm" absorption band o f NpF6(g). The argument i n e s s e n c e i s t h a t t h e weaker band i n e a c h c a s e c a n be i n t e r p r e t e d a s a " h o t " b a n d . The s m a l l s h o u l d e r e v i d e n t a t 4550 cm" i n S t e i n d l e r and G u n t h e r ' s P u F 6 ( g ) a b s o r p t i o n s p e c t r u m , w h i c h m i g h t be i n t e r p r e t e d a s a d i r e c t , magnetic d i p o l e allowed, transition, i s not evident i n the f l u o r e s c e n c e s p e c t r u m shown i n F i g u r e 2. T h i s p r o b a b l y r e s u l t s 1

1

1

3

1

1

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from t h e lower s p e c t r a l r e s o l u t i o n o f t h e f l u o r e s c e n c e spectrum which would tend t o obscure such a f e a t u r e . An a l t e r n a t e a s s i g n m e n t o f t h e e m i t t i n g s t a t e i n PuF6 f o l l o w i n g 1064 nm e x c i t a t i o n w o u l d be t h e 5 f s t a t e a t a b o u t 9755 c m " . F l u o r e s c e n c e from t h i s s t a t e t o lower l y i n g l e v e l s w o u l d be e x p e c t e d t o o c c u r c e n t e r e d a r o u n d 9755, 5205, 4970 and 3960 cm"" , i n p o o r agreement w i t h t h e s i n g l e o b s e r v e d band c e n t e r e d a t about 4550 cm"* . This a l t e r n a t e assignment i s therefore rejected. 1

1

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1

N o n - R a d i a t i v e Decay C h a n n e l s - 1064 nm E x c i t a t i o n . We t u r n now t o a c o m p a r i s o n o f t h e o b s e r v e d f l u o r e s c e n c e p h o t o n y i e l d d e f i n e d by E q u a t i o n 1 and t h e e x p e c t e d f l u o r e s c e n c e quantum y i e l d o f t h e 4550 cm"" s t a t e w h i c h i n d i c a t e s t h a t s e v e r a l n o n radiative decay channels may be open following 1064 nm excitation o f PuF6(g)» The f o l l o w i n g relationship between f l u o r e s c e n c e quantum y i e l d ( F Q Y ) , o b s e r v e d f l u o r e s c e n c e l i f e t i m e and t h e 4550 cm"" s t a t e ' s r a d i a t i v e o r s p o n t a n e o u s e m i s s i o n l i f e t i m e i s expected to hold (31): 1

1

FQY

= (observed f l u o r , l i f e t i m e ) / ( r a d i a t i v e l i f e t i m e )

(2)

1

S i n c e t h e 4550 cm"" s t a t e i s t h e f i r s t e x c i t e d s t a t e o f PuF6, i t s r a d i a t i v e l i f e t i m e c a n be d e t e r m i n e d t o be a r e a s o n a b l e a p p r o x i m a t i o n by i n t e g r a t i n g t h e o p t i c a l a b s o r p t i o n s p e c t r u m o f PuF6 o v e r t h e w a v e l e n g t h r a n g e where a b s o r p t i o n due t o t h e 4550 cm"" s t a t e o c c u r s . Some u n c e r t a i n t y a r i s e s s i n c e o p t i c a l a b s o r p t i o n from the next h i g h e r s t a t e undoubtedly o v e r l a p s t h a t due t o t h e 4550 cm"" s t a t e . U s i n g S t e i n d l e r and G u n t h e r ' s q u a n t i t a t i v e PuF6 a b s o r p t i o n s p e c t r u m ( 1 6 ) , t h e o b s e r v e d PuF6 f l u o r e s c e n c e e m i s s i o n band h a l f w i d t h a s f o u n d f r o m F i g u r e 2, and i n i t i a l a n d f i n a l state d e g e n e r a c i e s b a s e d on t h e PuF6 s t a t e a s s i g n m e n t s o f K u g e l e t a l . ( 15), t h e r a d i a t i v e l i f e t i m e o f t h e 4550 cm" s t a t e i s e s t i m a t e d to be 120 m i l l i s e c o n d s . U s i n g t h i s v a l u e and t h e o b s e r v e d lifetime (204 m i c r o s e c o n d s ) i n E q u a t i o n 2 r e q u i r e s t h a t t h e 4550 cm"" s t a t e ' s f l u o r e s c e n c e quantum y i e l d be a b o u t 1.7 χ 10"~ . S i n c e t h e upper l i m i t o f t h e e x p e r i m e n t a l f l u o r e s c e n c e photon y i e l d (1 χ Î O " ^ * ' ) d i f f e r s little from the v a l u e r e q u i r e d by E q u a t i o n 2, i t may be t h a t most i n i t i a l l y e x c i t e d PuF6 m o l e c u l e s n o n - r a d i a t i v e l y decay t o t h e 4550 cm*" s t a t e . I f the t r u e f l u o r e s c e n c e p h o t o n y i e l d l i e s n e a r t h e m i d d l e r a n g e o r lower l i m i t of the e x p e r i m e n t a l f l u o r e s c e n c e photon y i e l d then o t h e r n o n - r a d i a t i v e decay p r o c e s s e s must o c c u r w h i c h r e s u l t i n o n l y a s m a l l f r a c t i o n o f t h e a b s o r b e d pump l a s e r p h o t o n s g i v i n g rise to population of the emitting state. As shown s c h e m a t i c a l l y i n F i g u r e 3 one s u c h n o n - r a d i a t i v e p r o c e s s may be d i s s o c i a t i o n o f PuF6 i n t o t h e r a d i c a l s PuF5 and F atom. A f u r t h e r consequence o f a s s i g n i n g t h e e m i t t i n g s t a t e a s the 4550 cm" s t a t e i s t h a t t h i s s t a t e t h e n must be assumed t o 1

1

1

1

3

1

1

1

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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166

PLUTONIUM CHEMISTRY

PuF, + F 5 ^

8H

Est. Fluorescence Quantum Yield: 10

eu

0

•

J

5f

ale

2

States

F i g u r e 3. E n e r g y d i a g r a m f o r 1064 nm e x c i t a t i o n o f P u F g ( g ) . The 5 f e l e c t r o n s t a t e s o f P u F ^ a r e shown a t t h e l e f t . The s o l i d arrows i n d i c a t e photon a b s o r p t i o n o r e m i s s i o n proces­ ses. The wavy a r r o w s i n d i c a t e n o n r a d i a t i v e p r o c e s s e s b y w h i c h e x c i t e d s t a t e s o f PuFg a r e l o s t . Comparison o f observed f l u o r e s c e n c e p h o t o n y i e l d s v e r s u s t h e f l u o r e s c e n c e quantum y i e l d e x p e c t e d f o r t h e 4550 cm" s t a t e i n d i c a t e t h a t t h e P u F ^ s t a t e i n i t i a l l y p o p u l a t e d f o l l o w i n g 1064 nm e x c i t a t i o n may d i s s o c i a t e a s shown. 1

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

11.

Β Ε ί τ ζ ET AL.

PuFe Gas Photophysics and Photochemistry

167

undergo e f f i c i e n t n o n - r a d i a t i v e decay i n l i g h t o f i t s e v i d e n t l y s m a l l f l u o r e s c e n c e quantum y i e l d . I t i s n o t known w h e t h e r t h e primary n o n - r a d i a t i v e decay process i s c o l l i s i o n l e s s conversion of 5f e l e c t r o n i c energy i n t o i n t e r n a l energy o r a " u n i m o l e c u l a r " reaction (32) i n v o l v i n g bimolecular collisions activating e x c i t e d PuFg m o l e c u l e s t o a d i s s o c i a t i v e s t a t e . I f such a "unimolecular r e a c t i o n i s o c c u r r i n g t h e n e v e n 18 t o r r p r e s s u r e must be s u f f i c i e n t t o r e a c h t h e h i g h p r e s s u r e l i m i t i n w h i c h t h e " u n i m o l e c u l a r " r e a c t i o n becomes p s e u d o - f i r s t o r d e r s i n c e e v e n a t 18 t o r r the observed fluorescence decays were single exponential.

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1 1

E m i s s i o n S p e c t r u m A s s i g n m e n t - 532 nm E x c i t a t i o n . Since the same P u F 6 ( g ) f l u o r e s c e n c e e m i s s i o n s p e c t r u m i s f o u n d u s i n g 1064 nm o r 532 nm e x c i t a t i o n , we b e g i n by a s s u m i n g t h a t t h e e m i t t i n g s t a t e f o l l o w i n g 532 nm e x c i t a t i o n o f PuF6 i s t h e same s t a t e a s when PuF6 i s e x c i t e d a t 1064 nm ( i . e . t h e 4550 cm" state). The p r i n c i p a l d i f f i c u l t y w i t h t h i s a s s i g n m e n t i s t h e f a c t t h a t the observed f l u o r e s c e n c e decay r a t e i s over a f a c t o r of 2 f a s t e r when 532 nm r a t h e r t h a n 1064 nm e x c i t a t i o n i s u s e d (1). I n a d d i t i o n t h e observed f l u o r e s c e n c e decay r a t e i s none x p o n e n t i a l and l a s e r - f l u e n c e dependent a t 532 nm e x c i t a t i o n . F i g u r e 4 shows s c h e m a t i c a l l y some o f t h e r a d i a t i v e and non r a d i a t v e processes w h i c h may o c c u r when PuF6 i s e x c i t e d a t 532 nm. I f t h e p r o c e s s e s shown i n F i g u r e 4 w i t h a " ? " do o c c u r , t h e n t h e f a s t e r f l u o r e s c e n c e d e c a y r a t e as w e l l as i t s n o n exponential character and l a s e r - f l u e n c e dependence c a n be u n d e r s t o o d i n terms o f t h e p h o t o c h e m i c l mechanism described below. S i n c e t h e p r o p o s e d p h o t o c h e m i c a l mechanism q u a l i t a t i v e l y accounts the observations found u s i n g 532 nm e x c i t a t i o n , we t e n t a t i v e l y a s s i g n the fluorescence emission spectrum observed f o l l o w i n g 532 nm e x c i t a t i o n as a r i s i n g f r o m t h e f i r s t e x c i t e d s t a t e o f P u F 6 , t h e s t a t e a r o u n d 4550 c m " . 1

1

Non-Radiative Decay C h a n n e l s - 532 nm E x c i t a t i o n . A mechanism i n w h i c h t h e e m i t t i n g s t a t e o f PuF6 u n d e r g o e s a bimolecular r e a c t i o n w i t h a photodecomposition product would a c c o u n t f o r t h e f l u o r e s c e n c e decay b e h a v i o r f o u n d when P u F 6 ( g ) is excited at 532 nm. The significantly increased net p h o t o d e c o m p o s i t i o n o b s e r v e d on s w i t c h i n g f r o m 1064 nm t o 532 nm e x c i t a t i o n a l s o a r g u e s t h a t an a d d i t i o n a l n o n - r a d i a t i v e d e c a y channel h a s been opened when u s i n g the shorter wavelength excitation. S i n c e f l u o r e s c e n c e i s o b s e r v e d when u s i n g 532 nm e x c i t a t i o n , i t i s l i k e l y t h a t t h e i n i t i a l l y p o p u l a t e d PuF6 s t a t e branches to several non-radiative decay channels s u c h as d i s s o c i a t i o n and c o l l i s i o n l e s s d e g r a d a t i o n o f e l e c t r o n i c e n e r g y i n t o v i b r a t i o n a l and r o t a t i o n a l e n e r g y . One o r more o f t h e s e c h a n n e l s w o u l d l e a d t o p o p u l a t i o n o f t h e e m i t t i n g PuF6 s t a t e . I f PuFi+(g) i s formed v i a d i s s o c i a t i o n o f t h e PuF6 s t a t | i n i t i a l l y populated, t h e n a b i m o l e c u l a r r e a c t i o n between PuF6

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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PLUTONIUM CHEMISTRY

and PuF*f(g) t o p r o d u c e P u F s ( g ) w o u l d a c c o u n t f o r t h e n o n e x p o n e n t i a l and l a s e r - f l u e n c e dependent f l u o r e s c e n c e intensity decay observed. Here PuF6 d e n o t e s PuF6(g) i n i t s 4550 cm" state. The r e a c t i o n between PuF6(g) i n i t s g r o u n d e l e c t r o n i c state and g r o u n d state P u F ^ g ) would be e x p e c t e d t o be e x o t h e r m i c b o t h i n c o m p a r i s o n w i t h t h e known t h e r m o d y n a m i c s o f the corresponding u r a n i u m s p e c i e s ( 2 2 , 2 3 ) and f r o m t h e PuF6 dissociation limits shown i n F i g u r e 1. However such a bimolecular reaction involving non-radical reactants i n their g r o u n d e l e c t r o n i c s t a t e s w o u l d be e x p e c t e d t o have a s i g n i f i c a n t a c t i v a t i o n energy. The a c t i v a t i o n e n e r g y f o r t h e PuF6(g) + P u F i f ( g ) r e a c t i o n w o u l d most l i k e l y be a c o n s i d e r a b l e f r a c t i o n o f t h e e n e r g y r e q u i r e d t o b r e a k a Pu-F bond i n PuF6. T h e r e f o r e t h e r e a c t i o n o f PuF6(g) + PuFit(g) i n t h e i r g r o u n d s t a t e s w o u l d be e x p e c t e d t o proceed relatively slowly a t room temperature. I f the electronic energy o f PuF6 i s a v a i l a b l e t o overcome this a c t i v a t i o n e n e r g y b a r r i e r , t h e n t h e r e a c t i o n o f PuF6 + PuFi* w o u l d become s u f f i c i e n t l y r a p i d t o l a r g e l y account f o r the l a s e r - f l u e n c e d e p e n d e n t f l u o r e s c e n c e d e c a y s we have o b s e r v e d following 532 nm e x c i t a t o n o f PuF6(g). I n the proposed mechanism, t h e m e a s u r e d f l u o r e s c e n c e l i f e t i m e o f PuF6 f o l l o w i n g 532 nm excitation would be e x p e c t e d t o be s h o r t e n e d in c o m p a r i s o n w i t h 1064 nm e x c i t a t i o n and t o become n o n - e x p o n e n t i a l with increasing laser fluence due t o t h e i n c r e a s i n g r a p i d r e m o v a l o f PuF6 v i a r e a c t i o n w i t h PuFi+(g). Dimerization of subhexavalent plutonium f l u o r i d e s , subsequent f o r m a t i o n o f h i g h e r aggregates and possibly disproportionation reactions are processes w h i c h may c o n t r i b u t e t o formation of the s o l i d photodecomposition product. The p r o p o s e d p h o t o c h e m i c a l r e a c t i o n mechanism shown i n F i g u r e 4 a c c o u n t s f o r t h e o b s e r v a t i o n s we have r e p o r t e d and i s in accord with presently available thermodynamic and spectroscopic data. However, t h i s mechanism must be r e g a r d e d a s subject to revision a s more i n f o r m a t i o n becomes a v a i l a b l e concerning the p r o p e r t i e s of plutonium f l u o r i d e s i n gas and s o l i d phases. We have e x c l u d e d b i m o l e c u l a r energy t r a n s f e r r e a c t i o n s s u c h a s PuF6(g) + PuF5(g) PuF6(g) + P u F s ( g ) f r o m c o n s i d e r a t i o n as important n o n - r a d i a t i v e decay channels s i n c e e n e r g y t r a n s f e r between 5 f s t a t e s seem u n l i k e l y t o be f a c i l e d u r i n g t h e s h o r t t i m e s p a n o f a g a s phase c o l l i s i o n .

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1

Research O p p o r t u n i t i e s . The p r e s e n c e o f a l o n g - l i v e d f l u o r e s c i n g s t a t e f o l l o w i n g e i t h e r 532 nm o r 1064 nm e x c i t a t i o n of PuFe(g) p r o v i d e s a v a l u a b l e o p p o r t u n i t y t o study t h e e x t e n t to which e l e c t r o n i c energy i n a 5f e l e c t r o n s t a t e i s a v a i l a b l e i n p h o t o c h e m i c a l and e n e r g y t r a n s f e r r e a c t i o n s . Such g a s phase b i m o l e c u l a r r e a c t i o n s w o u l d o c c u r i n a weak i n t e r a c t i o n l i m i t g o v e r n e d by v a n d e r W a a l s ' f o r c e s . Seen f r o m t h e p e r s p e c t i v e o f potential photochemical separations i n fluoride volatility

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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11.

ΒΕίτζ ET AL.

PuFe Gas Phoîophysics and Photochemistry

O

15000-

rQ Ë

10000-

CD %

5000-

e

5f STATES

Π

? + PuF -> 2PuF

4

R

5

F i g u r e 4. E n e r g y d i a g r a m f o r 532 nm e x c i t a t i o n o f P u F ^ C g ) . The 5 f e l e c t r o n s t a t e s o f PuFg a r e shown a t t h e l e f t . The s o l i d arrows i n d i c a t e photon a b s o r p t i o n o r emission p r o c e s ­ ses. The wavy a r r o w s i n d i c a t e n o n r a d i a t i v e p r o c e s s e s by w h i c h e x c i t e d s t a t e s o f P u F ^ may be l o s t . The l a s e r - f l u e n c e dependent f l u o r e s c e n c e d e c a y f o u n d a t t h i s e x c i t a t i o n wave­ l e n g t h c a n be e x p l a i n e d i n t e r m s o f a b i m o l e c u l a r r e a c t i o n between P u F ( g ) i n i t s 4550 cm" s t a t e and P u F ^ ( g ) t o f o r m PuF^Cg). I t i s assumed t h a t P u F ^ ( g ) i s formed v i a d i s s o c i a ­ t i o n o f t h e i n i t i a l l y p o p u l a t e d PuF s t a t e . 1

6

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

169

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PLUTONIUM CHEMISTRY

processing, for example, the long-lived excited electronic states found in both NpF6(g) and PuF6(g) may pose significant problems·

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Conclusions Laser-induced fluorescence has proven to be the key to these pioneering studies of transuranic hexafluoride electronic state photophysics and photochemistry. This is a research area of unique opportunity in which fundamental and technical research interests strongly converge. As we have noted many of the properties of PuF6 and NpF6 are poorly understood and should be the subject of further fundamental investigation. Little is known concerning the gas phase reaction rates and mechanisms of mixtures containing transuranic hexafluorides. The thermodynamic dissociation limits of PuF6(g) are not well-established which is only in part due to the present lack of a synthesis for PuFs. There is no thermodynamic information concerning NpF6. While NpF6(g) and PuFe(g) have been the subject of several spectroscopic investigations, our work again points up the lack of proven electronic state assignments and the difficulty in making such assignments posed by the predominantly vibronic nature of their 5f transitions. Given the powerful techniques that can now be brought to bear on these problems, we stand on the threshold of a new era and confidently look forward to achieving a systematic and predictive understanding of the electronic state properties of all the actinide hexafluorides. Acknowledgment We wish to thank one of our reviewers for his informed and constructive criticism. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Nuclear Sciences, U. S. Department of Energy under contract number W-31-109-ENG-38. Literature Cited 1. 2. 3. 4. 5.

Beitz, J. V.; Williams, C. W.; Carnall, W. T. J. Chem. Phys. 1982, 76, 2757-2758. Seaborg, G. T. Univ. of Chicago Met. Lab. Report CN-125 (1942). Steindler, M. J. Argonne Nat. Lab. Report ANL-6753 (1963). Steindler, M. J. U.S. AEC Div. Tech. Info. Report CONF-680610 (1968); pp. 2-17. Malm, J. G.; Weinstock, B. and Claassen, H. H. J. Chem. Phys., 1955, 23, 2192-2193.

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

11. BEITZ ET AL. 6. 7. 8.

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9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

PuFe Gas Photophysics and Photochemistry

Malm, J. G.; Weinstock, Β.; Weaver, Ε. Ε. J. Phys. Chem. 1958, 62, 1506-1508. Trevorrow, L. Ε . ; Gerding, T. J.; Steindler, M. J. Inorg. Nucl. Chem. Let. 1969, 5, 837-839. Rand, M. H. Atomic Energy Review 1966, 4 (Special Issue 1), 7-51. Oetting, F. L. Chem. Rev. 1967, 67, 261-297. Brown, D. "Gemlins Handbuch der Anorganischen Chemie, Transurane, Teil C: Die Verbindungen", G. Koch, ed.; Verlag Chemie: Weinheim, 1972; pp. 100-128. Keller, C.; Chemiker-Zeitung 1982, 106, 137-142. Brown, D. "Halides of the Lanthanides and Actnides"; John Wiley and Sons: London, 1968. Cleveland, J. M. "The Chemistry of Plutonium"; Gordon and Breach: New York, 1970. Burns, R. C.; O'Donnell, T. A. ; Randall, C. H. J. Inorg. Nucl. Chem. 1981, 43, 1231-1238. Kugel, R.; Williams, C.; Fred, M.; Malm, J. G.; Carnall, W. T.; Childs, W. J.; Goodman, L. S. J. Chem. Phys. 1976, 65, 3486-3492. Steindler, M. J.; Gunther, W. H. Spectrochim. Acta 1964, 20, 1319-1322. Boring, M.; Hecht, H. G. J. Chem. Phys. 1978, 69, 112-116. Koelling, D. D.; E l l i s , D. E.; Bartlett, J. Chem. Phys. 1976, 65, 3331-3340. Desclaux, J. P.; Freeman, A. J. J. Magn. Magn. Mater. 1978, 8, 119-129. Fuger, J. in "Actinides in Perspective" Ν. M. Edelstein, ed.; Pergamon press: Oxford, 1982; pp. 409-431. Miller, J. C.; Allison, S. W.; Andrews, L. J. Chem. Phys. 1979, 70, 3524-3530. Hildenbrand, D. L. J. Chem. Phys. 1977, 65, 4788-4794. Steindler, M. J.; Gerding, T. J. Spectrochim. Acta 1966, 22, 1197-1200. Goodman, G. L.; Fred, M. J. Chem. Phys. 1959, 30, 849-850. Hutchison Jr., C. Α.; Weinstock, B. J. Chem. Phys. 1960, 32, 56-61. Steindler, M. J.; Steidl, D. V.; Steunenbeg, R. K. Nuclear Sci. and Eng. 1959, 6, 333-340. Weinstock, B.; Malm, J. G. J. Inorg. Nucl. Chem. 1956, 2, 380-394. Weinstock, B.; Weaver, Ε. E.; Malm, J. G. J. Inorg. Nucl. Chem. 1959, 11, 104-114. Rutgers, G. A. W. J. Res. Natl. Bur. Stand. Sec. A 1972, 76A, 427-436. Eisenstein, J. C.; Pryce, M. H. L. Proc. Roy. Soc. (London) 1960, A255, 181-198. Carnall, W. T. in "Organometallics of the f-Elements" T. J. Marks and R. D. Fischer, eds.; D. Reidel Pub. Co.: Dordrecht, 1979; pp. 281-307.

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

172 32· 33.

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Robinson, R. J.; Holbrook, K. A. "Unimolecular Reactions"; John Wiley and Sons: London, 1972; pp. 13-27· Hildenbrand, D. L. Lawrence Berkeley Lab. Report LBL-12441 (1981); pp. 256-257. December 21,1982

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RECEIVED

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.