FORMATION, RELAXATION AND QUENCHING OF XeF, KrF AND XeCℓ

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F O R E A T I O N ,

RELAXATION AND

Q U E N C H I N G

OF XeF,

K r F

AND

X e C L

D.W. S e t s e r , B.C. B r a s h e a r s a n d T.D. D r e i l i n g

Chemistry Department, Kansas State University, Manhattan, Kansas, U.S.A. 66506

La relaxation reactive des deux premiers niveaux excites des atomes de gaz rares, ~ ~ ( n p ' n+l s.

'P2 et 'P,)

,

par les halogsnes produit des molii- cules d'halogenure de gaz rares dans les Gtats B et C, avec des distributions vibrationnelles deve- loppees. L'interpretation des spectres d'emission lies-dissociatifs permets la determination des distributions initiales. Des collisions entre des molecules d'halogenure de gaz rares et les atomes He. Ne et Ar, du gaz tampon produisent des trans- ferts entre les Ctats B et C et une relaxation vibrationnelle. Pour les niveaux de v ClevE, la reaction de transfert est plus rapide que la relaxation vibrationnelle, mais pour les niveaux de v faible c'est l'inverse. Ceci peut conduire 5 une distribution vibrationnelle de Boltzmann i 300 K mais avec une distribution hors dt6quilibre pour les Etats Electroniques des niveaux B et C de la mol6cule d'halogenure de gaz rares. La modSlisa- tion de la variation des distributions Slectroni- que et vibrationnelle de 1'halogEnure de gaz rares avec la pression donne une vue etat par Stat des processus de relaxation. La photodissociation de XeF2 produit XeF(B) dans les bas niveaux v. Obser- vation de l'intensite d'emission, en regime sta- tionnaire, 5 partir des niveaux XeF(B) et XeF(C) peuples par la photodissociation de XeF,, en prii- sence de riiactants additionnels permets la &para- tion des constantes de vitesse pour le transfert B-C et pour le quenching de XeF(B) et XeF(C) avec une distribution vibrationnelle 2 300 K. Les deux exp6riences. relaxation reactive et photodissociation, indiquent que pour XeF. XeCL et KrF l'gtat (C, 2 = 312) est sit& au-dessous de 116tat (B, f2 = 112). Ceci a une importante implication pour

l'utilisation des halogEnures de gaz rares pour les lasers i puissance.

ABSTRACT

Reactive quenching of the first two excited states of rare gas atoms, ~ ~ ( n ~ ' n+l s. 'P, and 'P~), by halogens yields rare gas halide molecules in the B and C states and with high vibrational distributions. Interpretation of the bound-free emission spectra permits assignment of the initial distributions. Subsequent collisions of the rare gas halide molecules with He, Ne and Ar bath gas atoms result in transfer between the B and C states and vibrational relaxation. For high v levels the transfer rate is faster than vibrational relaxation, but the reverse holds for lower v levels.

This can lead to a 300 K Boltzmann vibrational distribution but a nonequilibrium electronic state distribution of rare gas halide molecules in the B and C states. Modeling of the variatiqn of the rare gas halide electronic and vibrational distributions with pressure gives a state-to-state view of relaxation processes. Photolytic dissociacion of XeF2 yields XeF(B) in low v levels. Observation of the steady-state emission intensity from XeF(B), and XeF(C) from photodissociation in the presence of added reagents permits assignment of rate cons- tants for B-C transfer and for quenching of XeF(B) and XeF(C) with 300 K Boltzmann vibrational distributions. Both the reactive quenching and photodissociation experiments indicate that the (C.R= 312)

state is lower state for XeF.

pl icat ions for cules for high

in energy than the (B, R = 112) XeCE and KrF. This has important im- utilization of rare gas halide mole- power lasers.

I. INTRODUCTION

The three lower states of the rare gas halide molecules X(1/2), A(312) and A(112) correlate to,the ground state rare gas. R~('S~), and halogen, X('P )

12

and X('P ),atoms ; the first set of excited states B( 112) and C(3/2), correlate to 'fa R~+('P ) and

3 1 2

X-(IS ) ions. The strong transitions are the B(112)-

X(1/2) emission, which is the common laser band, and the C(312)-A(312) emission, which is frequently referred to as the broad band transition. For heavy halogens the B( 112)-A(112) transition may overlap the C(3/2)-A(312) band and this must be given at- tention. The B and C state are coulombic in nature and differ in that the half-filled p-orbital of

~ g + lies along and perpendicular to the internuclear axis, respectively ; they are very close in energy. The ab initio calculations' ordered the B(112) state slightly below the C(312) state at the equilibrium internuclear distance for B. Howe- ver, at shorter internuclear distances the C state was the lower energy state because of the strong configuration interaction between the A, B and D states. Recent experiments 2-4 have demonstrated that the C state actually is lower in energy than the B state, at least for XeF. XeCl. KrF, KrCl and ArF. The energy separation is largest for XeF,

2. 720 cm-'. This presumably is because of the stronger configuration interaction, which also leads to a bound5 XeF(X.112) state (De(XeF(X)) =

1175 cm-I), for Xe and F relative to other rare gas halogen pairs. Another important general point is the nature bf the RgX(X) and RgX(A) potentials which are flat and repulsive, respectively, throughout the Franck Condon regions, which results in a large separation between the B-X and C-A emission bands. A final point of interest for this discus- sion is that the radiative lifetimes of the B and C states are typically 10 and 100 nsec, respectively. A diagram of the XeCL potential curves is shown in Figure I.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980331

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JOURNAL DE PHYSIQUE

F i g u r e 1 : Comparison of XeCR emission s p e c t r a from r e a c t i o n s of x ~ ( ~ P ~ ) , lower t r a c e , and x ~ ( ~ P ~ ) , upper t r a c e , w i t h CR2 a t 0.2 t o r r argon p r e s s u r e . The XeCR p o t e n t i a l s a r e from Hay and Dunning w i t h m o d i f i c a t i o n t o show t h e XeCR(C) p o t e n t i a l lower i n energy t h a n t h e B p o t e n t i a l a t small i n t e r n u - c l e a r d i s t a n c e . The weak sharp f e a t u r e a t % 235 nm i s t h e XeCR(D-X) t r a n s i t i o n s ; t h e weak f e a t u r e a t 260 nm i s cR; emission. Except f o r t h e somewhat lower s i g n a l / n o i s e r a t i o f o r x ~ ( ~ P ~ ) c a s e , t h e two s p e c t r a a r e v i r t u a l l y t h e same.

The most important RgX formation p r o c e s s e s f o r e l e c t r i c a l l y d r i v e n l a s e r s i s e i t h e r r e a c t i v e quenching o r ion-ion combination.

~ ~n+l s ) ( + nX2 3 ~RgX(B,C, high v) ~ + X (1)

R ~ + + X- + M -f RgX(B,C, high v)

+

M (2)

Reacttve quenching i s w e l l c h a r a c t e r i z e d , see sec- t i o n 11. Ion-ion combination has not been i s o l a t e d and s t , l d i e d i n t h e l a b o r a t o r y ; b u t , t h e t h e o r e t i -

' c a l t r e a t m e n t s 6 f o r the r a t e c o n s t a n t s seem r e l i a - b l e . Presumably ion-ion combination w i l l y i e l d mo- l e c u l e s i n both B and C s t a t e s with high v i b r a t i o - n a l d i s t r i b u t i o n s . Vacuum U.V. p h o t o l y s i s of XeF2 o r KrF2 provides a source of XeF(B) and KrF(B) i n low v i b r a t i o n a l l e v e l s 7

.

XeF,

+

KV + XeF(B, low v) + F (3)

Since ( I ) and (2) give high v l e v e l s , one needs t o understand t h e r e l a x a t i o n and quenching p r o c e s s e s of RgX(B) and RgX(C) throughout t h e manifold of v i b r a t i o n a l l e v e l s . R e l a x a t i o n i s d e f i n e d t o in- clude v i b r a t i o n a l r e l a x a t i o n and t r a n s f e r between t h e B and C p t a t e s . Quenching r e f e r s t o c o l l i s i o n a l t r a n s f e r t o the RgX(X) o r RgX(A) s t a t e s . ,

The g e n e r a l importance of t h e RgX(C) s t a t e has been r e a l i z e d only r e c e n t l y . C l e a r l y t h e C s t a t e must be included i n any r e a l i s t i c modeling of l a s e r s involv- i n g t h e B-X t r a n s i t i o n , and L o r e n t s and co-workers 8 have demonstrated t h a t t h e XeF(C,3/2

-

A,3/2) t r a n - s i t i o n i t s e l f can be made t o l a s e . They u t i l i z e d t h e vacuum p h o t o d i s s o c i a t i o n of XeF, a s t h e XeF

*

source. The f o c a l p o i n t of t h i s d i s c u s s i o n w i l l be t h e r e l a t i o n s h i p between t h e RgX(B) and RgX(C) s t a t e s . Our work only d e a l s w i t h heavy p a r t i c l e c o l l i s i o n s , but t h e e f f e c t s of e n e r g e t i c e l e c t r o n s on t h e B and C s t a t e s a l s o i s o f p r a c t i c a l i m p o r t a n - ce. The work t h a t w i l l be d i s c u s s e d i s of p r a c t i c a l importance t o l a s e r s ; however, t h e o p p o r t u n i t y t o observe t h e s t a t e - t o - s t a t e k i n e t i c s of t h e i o n i c r a r e gas h a l i d e molecules i s of more b a s i c i n t e r e s t . I n t h e remainder of t h e d i s c u s s i o n , t h e s t a t e - t o - s t a t e k i n e t i c s w i l l be emphasized r a t h e r t h a n ap- p l i c a t i o n s t o l a s e r s .

11. RARE GAS HALIDE FORMATION PROCESSES

A. Reactive Quenching of R ~ ( ~ P ~ ) o r R ~ ( ~ P ~ ) The r e a c t i o n s of x ~ ( ~ P , ) , K ~ ( ~ P ~ ) and A ~ ( ~ P ~ ) w i t h a v a r i e t y of halogen donors X 2 and RX (R = polyatomic group) have been thoroughly s t u d i e d by t h e f l o w - i n g a f t e r g l o w technique 7-1 I . T o t a l quenching r a t e c o n s t a n t s , branching f r a c t i o n s f o r r a r e gas h a l i d e formation (TRgX), e l e c t r o n i c s t a t e r a t i o s and v i - b r a t i o n a l d i s t r i b u t i o n s have been e s t a b l i s h e d . Very r e c e n t l y t h e r e a c t i o n s of t h e x ~ ( ~ P ~ ) and K ~ ( ~ P ~ ) resonance s t a t e atoms have been i n v e s t i g a t e d by

t h e resonance s e n s i t i z a t i o n technique 1 2 . Low pres- s u r e s p e c t r a from t h e r e a c t i o n s of x ~ ( ~ P ~ ) and X e ( 3 ~ , ) atoms w i t h CR, a r e shown i n Figure 1 . The appearance of t h e s p e c t r a a r e v i r t u a l l y i d e n t i c a l

( c l o s e i n s p e c t i o n r e v e a l s s l i g h t l y more XeCR(D)

and s l i g h t l y more v i b r a t i o n a l energy f o r t h e x ~ ( ~ P ) r e a c t i o n s ) . Within our experimental u n c e r t a i n t y

t h e r e s u l t s from t h e resonance s t a t e atom r e a c t i o n s g e n e r a l l y appear t o be t h e same a s f o r t h e ( 3 ~ 2 ) m e t a s t a b l e s t a t e s of K r and Xe. A summary o f r e s u l t s from some common donor molecules i s g i v e n i n Table I. The v i b r a t i o n a l d i s t r i b u t i o n s 1 0 y 1 3 were o b t a i n - ed by computer s i m u l a t i o n of t h e bound f r e e s p e c t r a ; two samples a r e shown i n F i g u r e 2. G e n e r a l l y t h e v i b r a t i o n a l d i s t r i b u t i o n s f o r RgX(B) and RgX(C) a r e t h e same. D e s p i t e t h e f a c t t h a t HCR i s t h e f a - vored f u e l f o r t h e XeCR l a s e r , t h e TXeCR f o r reac- t i o n with HCR(v=O) i s v e r y small and some o t h e r formation p r o c e s s must be important i n t h e l a s e r

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1 1 s t u d i e s can be s u m a r i z e d a s f o l l o w s

.

Table 1 . Summary of r e s u l t s f o r r e a c t i v e quenching

Reaction K~()P~) + P2 K~('P~) + NP3 K~()P~) + OF2 K ~ O P ~ ) + CP30€

xe('p2) r F~

x ~ ( ~ P ~ ) r CI ~ xc(]p2) + SC12 x ~ ( ~ P ~ ) r C C I ~ xet3p2) c0c12 x ~ ( ~ P ~ ) + HCI

a . Taken from r e f e r e n c e 9 , 10 and 1 1 ; f o r t h e ( 3 ~ ) resonance s t a t e s t h e r a t e c o n s t a n t s may be

% 16 73 l a r g e r . The u n c e r t a i n t y i n

r

^{i s}

+

¹⁰% ; branching f r a c t i o n s f o r the (3E',) Rf; atoms a r e n o t well e s t a b l i s h e d but t h e t r e n d s a r e t h e same a s f o r t h e ( 3 ~ 2 ) atom r e a c t i o n s .

b. For x ~ ( ~ P , ) r e a c t i n g with NF,, OF2 and CF,OF,

rXeF

⁼^{1 . 1 ,} 0.92 and 0.40, r e s p e c t i v e l y .

k B I

2m3

^aec-l

72 29 53 42 75 72 52 63 49 56

c . These a r e t h e low p r e s s u r e l i m i t s .

d. These a r e t h e s t e a d y - s t a t e d i s t r i b u t i o n s . Cor- r e c t i o n f o r t h e v a r i a t i o n of l i f e t i m e s with v l e - v e l g i v e s an i n i t i a l d i s t r i b u t i o n of s l i g h t l y lower <f >. The l i n e a r s u r p r i s a l type d i s t r i b u t i o n denote3 by the A v a l u e i s used t o r e p r e s e n t a type of d i s t r i b u t i o n

7

we do not mean t h a t t h e i n i t i a l d i s t r i b u t i o n h a s a l i n e a r s u r p r i s a l i n t h e i n f o r - m a t i o n - t h e o r e t i c sense.

PRg/

0.91 0.57~

0.94~

0.51b 1.0 1.0 0.42 0.13 0.24

*0.01

Figure 2 : Comparison of s i m u l a t e d (upper) and ex- p e r i m e n t a l (lower) s p e c t r a f o r XeCR(B-X) t r a n s i - t i o n : lower s e t = 0.25 t o r r of A r , upper s e t = 1.3 t o r r of A r .

( 1 ) T o t a l thermal quenching c r o s s - s e c t i o n s i n v a r i a - b l y a r e l a r g e , n e a r t h e g a s k i n e t i c v a l u e s .

wc

1.4 1.2 1.1.5 1.1.5 1.4 1.3 1.2 1.2 1.0 not mas.

rured

t i o n from diatomic halogens a r e n e a r l y u n i t y f o r x e c 3 p 2 ) and K ~ ( ~ P ~ ) but somewhat l e s s t h a n u n i t y f o r A ~ ( ~ P , ) .

( 3 ) Only t h e h a l i d e s of Groups V I and V and a few .carbon compounds (CCR,

,

C B r +

,

CF ,I) have

rRgX

va-

l u e s of s i g n i f i c a n c e . A good c o r r e l a t i o n e x l s t s between compounds which d i s s o c i a t i v e l y a t t a c h t h e r - mal e l e c t r o n s t o give X- and

rRgX.

<fy>

0.65 0.40 0.60 0.60 0.72 0.77 0.69 0.67

0.20 ( i ) For a given polyatomic r e a g e n t ,

rRgX

^de-

c l i n e s w i t h i n c r e a s i n g energy of Rg

*

(ii) For a given Rg

* , r

d e c l i n e s with i n c r e a s - RgX

i n g complexity of RX.

-

Vibrational distributiond Av = -6

t r s p e r o ~ d r ; A,,

-

^-6.5

A,,

-

^-6.5

70% Av = r 3OX Flat 752 A,,

-

^-15 ^{25% Flat}

802 A, = -9 20% flat iv

-

^-8

A"

-

^0.3

( 4 ) For n e a r l y a l l r e a g e n t s RgX(B)/RgX(C) i s between 1.0 and 2.0

( 5 ) The RgX v i b r a t i o n a l d i s t r i b u t i o n s mzinly depend upon t h e p r o p e r t i e s of the a n i o n , X; o r RX-, i n ac- cord w i t h t h e models developed t o e x p l a i n t h e energy d i s p o s a l f o r t h e e l e c t r o n t r a n s f e r r e a c t i o n s of a l k a l i metal atoms.

Reaetlon is slightly endoerglc

Most of t h e s e f e a t u r e s can be explained i n q u a l i t a - t i v e terms by t h e ionic-covalent curve-crossing mechanism developed f o r a l k a l i metal atom r e a c t i o n s . This mechanism i s presented i n d i a g r a m a t i c form i n Figure 3. The t o t a l quenching c r o s s - s e c t i o n s a r e l a r g e and determined by o r b i t i n g r a d i u s on t h e l o - wer a d i a b a t i c p o t e n t i a l . The branching f r a c t i o n s may be l e s s t h a n u n i t y because t h e lower a d i a b a t i c p o t e n t i a l , i . e . v ( R ~ + , RX-)

,

i s imbedded i n a continuum of X: o r RX* p o t e n t i a l s and t h e i n t e r a c t i o n between v ( R ~ + , RX-) and V(Rg, RX*) i s l i k e l y t o be s t r o n g f o r some of t h e a l t e r n a t i v e channels. This l i k e l i h o o d i n c r e a s e s with i n c r e a s i n g d e n s i t y of s t a t e s , e . g . h i g h e r energy o r more complex RX mo- l e c u l e s . Since t h e n a t u r e of t h e e x i t channel po- t e n t i a l depends on t h e p r o p e r t i e s of t h e n e g a t i v e i o n , < f

v

> s t r o n g l y c o r r e l a t e s w i t h t h e n a t u r e of RX-. There i s no fundamental e x p l a n a t i o n of why B and C a r e formed i n approximately e q u a l p r o p o r t i o n s . Apparently t h e s e two p o t e n t i a l s a r e c l o s e i n energy throughout t h e t r a v e r s a l of t h e e x i t channel and t h e s t a t e s a r e s t r o n g l y mixed i f t h e r e i s a l a r g e amount of e x c e s s energy. There i s l i t t l e dependence of t h e B and C s t a t e r a t i o upon-.whether t h e r a r e gas atom i s i n t h e 3 ~ 2o r 3 ~ 1 - - s t a t e ; however, no experimental t e s t s have been made f o r t h e

3 ~ ,01. l p l s t a t e s .

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JOURNAL DE PHYSIQUE

R g

+

X2 Entrance Channel

F i g u r e 3 : Schematic p o t e n t i a l d i a g r a m f o r Rg + RX r e a c t i v e quenching. The h e a v i l y l i n e d a r e a o v e r t h e e n t r a n c e channel r e p r e s e n t s t h e continuum of RX* o r (x:) e x c i t e d s t a t e s t h r o u g h which t h e i o n - p a i r po-

t e n t i a l must p a s s t o r e a c h t h e R ~ X * +X e x i t chan- n e l . The d o t t e d a r e a i s t h e R ~ X * v i b r a t i o n a l d i s -

t r i b u t i o n . For ArCL, A r B r , A r I , K r I and KrBr ; some of t h e R ~ X * l e v e l s a r e p r e d i s s o c i a t e d t o Rg + X*

and t h i s i s i n d i c a t e d i n t h e upper l e f t hand c o m e r .

B. P h o t o d i s s o c i a t i o n of XeF2 and KrF,

I n c o n t r a s t t o r e a c t i v e quenching, vacuum u l t r a v i o - l e t p h o t o d i s s o c i a t i o n 7 o f XeF, o r KrF, g i v e s

2

90 % XeF(B) o r KrF(B) i n a v e r y low v i b r a t i o n a l d i s t r i - b u t i o n , s e e F i g u r e 4. Although t h e quantum y i e l d h a s n o t been measured, i t must be h i g h and may be u n i t y . A f t e r p a s s i v a t i o n of t h e s t a i n l e s s s t e e l c e l l w i t h F, f o l l o w e d by f l o w i n g XeF, t h r o u g h t h e c e l l f o r s e v e r a l h o u r s , t h e f l u o r e s c e n c e s i g n a l from XeF, i s v e r y c o n s t a n t b e c a u s e XeF i s reformed.

I n c o n t r a s t q u a n t i t a t i v e s t u d i e s w i t h KrF, a r e much more d i f f i c u l t because o f t h e i n i t i a t i o n o f t h e c h a i n r e a c t i o n d e s t r o y i n g KrF,.

KrF, + k, + KrF(B) + F KrF(B) + K r + F + hv F

+

KrF, ⁺F2 + K r + F

T h e r e f o r e , s t e a d y - s t a t e p h o t o l y t i c s t u d i e s w i t h KrF2 must be done by c o n t i n u o u s f l o w of KrF, through

II

^{A r to}¹³⁵⁰^torr

SF6 to 253 torr

F i g u r e 4 : XeF e m i s s i o n s p e c t r a from p h o t o d i s s o c i a - t i o n (147 nm) o f XeF,, t h e t o p s p e c t r a i s w i t h o n l y XeF, p r e s e n t i n t h e c e l l ; f o r t h e c e n t e r spectrum

1350 t o r r o f Ar h a s been added t o t h e XeF2, f o r t h e bottom s p e c t r u m 253 t o r r o f SF6 was added t o t h e XeF,. The s t r u c t u r e d e m i s s i o n i s t h e XeF(B-X) t r a n - s i t i o n and t h e broad band i s t h e XeF(C-A) t r a n s i - t i o n .

t h e c e l l . The p h o t o d i s s o c i a t i o n method p r o v i d e s a n i m p o r t a n t a l t e r n a t i v e method t o r e a c t i v e quenching f o r g e n e r a t i n g XeF(B) and KrF(B) and f o r s t u d y o f t h e k i n e t i c s o f t h e B s t a t e i n low v l e v e l s . The p h o t o d i s s o c i a t i o n method, u s i n g e i t h e r p u l s e d 14,15 o r s t e a d y - s t a t e t e c h n i q u e , have p r o v i d e d t h e b e s t measurements of r a d i a t i v e l i f e t i m e s and quenching r a t e c o n s t a n t s .

The d i s s o c i a t i v e s t a t e of XeF, r e s u l t s from promo- t i o n of an e l e c t r o n from t h e I O U t o t h e 70 o r b i -

g

t a l . The l a t t e r i s a s t r o n g l y a n t i b o n d i n g o r b i t a l and l e a d s t o d i s s o c i a t i o n . Based upon t h e XeF pro- d u c t s t a t e d i s t r i b u t i o n , t h i s p o t e n t i a l h a s no r e -

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quenching. This s u g g e s t s t h a t t h e r e a c t i v e quench- i n g p o t e n t i a l does not i n v o l v e i n s e r t i o n of xe+

between t h e F2 atoms but r a t h e r xe+ r e a c t s w i t h a

-

t e r m i n a l F- i n t h e same way t h a t an a l k a l i metal atom r e a c t s w i t h X 2 .

t h e IB-X/IC-A r a t i o s ( s e e Figure 5) and v i b r a t i o n a l d i s t r i b u t i o n s ( s e e Figure 6) can be o b t a i n e d vs

111. RELAXATION OF RgX(B) AND RgX(C) IF HIGH VIBRA- TIONAL LEVELS.

The primary d a t a a r e XeF, XeCR and KrF s p e c t r a from r e a c t i v e quenching o b t a i n e d i n t h e presence of va- r i o u s p r e s s u r e s of He, Ne and A r , s e e Figure 5.

F i g u r e 5 : V a r i a t i o n of t h e XeF i n t e n s i t y r a t i o s from x ~ ( ~ P ~ ) + F2 ; IB-X/IC-A vs p r e s s u r e of A r (upper) and I /I vs p r e s s u r e - ' (lower). The

C-A B-X

r e s u l t s from x ~ ( ~ P ~ ) + CR, a l s o i s shown i n t h e lower f i g u r e . The i n t e n s i t y r a t i o s must be m u l t i - p l i e d by t h e l i f e t i m e r a t i o s t o g e t s t e a d y - s t a t e c o n c e n t r a t i o n r a t i o s .

The p h o t o s e n s i t i z a t i o n technique i s e s p e c i a l l y u s e f u l because p r e s s u r e s from 0.1 t o 3000 t o r r can be i n v e s t i g a t e d . The flowing a f t e r g l o w technique i s

Figure 6 : The s t e a d y - s t a t e XeCR(B) v i b r a t i o n a l d i s - t r i b u t i o n s from x ~ ( ~ P ~ ) o r ( 3 ~ ) + CR2 f o r t h e a r - gon p r e s s u r e i n d i c a t e d i n t h e $ i g u r e . The lowest p r e s s u r e d i s t r i b u t i o n i s t h e i n i t i a l d i s t r i b u t i o n from r e a c t i v e quenching, s e e Table 1 . The d i s t r i - b u t i o n i n d i c a t e d by t h e d o t t e d curve i s t h e XeCR(B) d i s t r i b u t i o n r e s u l t i n g from c o l l i s i o n a l t r a n s f e r from XeCR(C) i n t h e i n i t i a l v i b r a t i o n a l d i s t r i b u - t i o n . The i n i t i a l XeCR(C) d i s t r i b u t i o n i s t h e same a s t h a t f o r XeCR(B) ; n o t e t h e l a r g e l o s s of v i b r a - t i o n a l energy t h a t accompanies C-B t r a n s f e r .

p r e s s u r e . The v i b r a t i o n a l d i s t r i b u t i o n s a r e o b t a i n - ed by computer s i m u l a t i o n ( s e e Figure 2 f o r an exam- p l e ) . In c o n v e r t i n g from IB-X/IC-A t o a steady- s t a t e c o n c e n t r a t i o n r a t i o , t h e emission r a t i o must be m u l t i p l i e d by T ~ / T ~ , which i s % 10. Although t h e l i f e t i m e s i n c r e a s e by a f a c t o r of % 3 f o r l e v e l s a t v = 100, t h e r a t i o of l i f e t i m e s s t a y s approximately c o n s t a n t . The r e l a x a t i o n can be d i s c u s s e d b e s t by c o n s i d e r i n g t h r e e p r e s s u r e regimes, which f o r X ~ F * ( o r XeCR) i n A r a r e 0-5 t o r r , 5-50 t o r r and 50-2000 t o r r .

I n t h e 0-5 t o r r regime t h e IB-X i n c r e a s e s a t t h e expense of I t h e B s t a t e v i b r a t i o n a l d i s t r i b u -

C-A

'

t i o n s change a s shown i n F i g u r e 6 but only modest s h i f t s o c c u r i n t h e v i b r a t i o n a l p o p u l a t i e n s of t h e C s t a t e . Since t h e l i f e t i m e s of t h e B z t a t e l e v e l s a r e s h o r t e r t h a n t h e time between c o l l i s i o n s f o r t h i s p r e s s u r e range, t h e s e e f f e c t s must be a s s o c i a - ted! w i t h c o l l i s i o n s w i t h t h e XeF(C) ( o r XeCR(C)

.

-

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c3-200

JOURNAL DE PHYSIQUE

s t a t e ) . E v i d e n t l y m o l e c u l e s i n t h e h i g h v l e v e l s of t h e C s t a t e m a i n l y a r e t r a n s f e r r e d t o l o w e r l e v e l s i n t h e B s t a t e r a t h e r t h a n b e i n g c o l l i s i o n a l l y r e - l a x e d w i t h i n t h e C s t a t e . From e x a m i n a t i o n o f t h e v a r i a t i o n of t h e v i b r a t i o n a l d i s t r i b u t i o n o f t h e XeCR(B) s t a t e w i t h p r e s s u r e , t h e d i s t r i b u t i o n f o r m o l e c u l e s b e i n g t r a n s f e r r e d from C t o B was o b t a i n - ed ( s e e F i g u r e 6 ) . A f t e r t h e s i m u l a t i o n o f t h e C-A s p e c t r a have been c o m p l e t e d , t h e v i b r a t i o n a l r e l a - x a t i o n w i t h i n t h e C s t a t e a l s o c a n b e a s s i g n e d .

I n t h e i n t e r m e d i a t e p r e s s u r e regime t h e I / I B-X C-A r a t i o i n c r e a s e s s l o w l y b u t t h e v i b r a t i o n a l r e l a x a - t i o n i n b o t h B and C s t a t e s i s q u i t e r a p i d . Under- s t a n d i n g t h e p r o c e s s e s i n t h i s r e g i m e must a w a i t a f u l l model : however, most l i k e l y t r a n s f e r b e t - ween B and C and v i b r a t i o n a l r e l a x a t i o n w i t h i n e a c h m a n i f o l d o f l e v e l s o c c u r s i m u l t a n e o u s l y . T h e s e two p r o c e s s e s have a s t r o n g dependence upon v i b r a t i o n a l l e v e l and a t some i n t e r m e d i a t e e n e r g y v i b r a t i o n a l r e l a x a t i o n b e g i n s t o dominate o v e r t r a n s f e r . A t 10 t o r r , t h e XeCR(B) v i b r a t i o n a l d i s t r i b u t i o n s have r e l a x e d t o a p p r o x i m a t e l y a v e r y h i g h t e m p e r a t u r e Boltzmann d i s t r i b u t i o n . F o r s u c h a t e m p e r a t u r e t h e e q u i l i b r i u m r a t i o s h o u l d be 2. 1.5, which d o e s ap- p r o x i m a t e l y c o r r e s p o n d t o t h e o b s e r v e d s t e a d y - s t a t e c o n c e n t r a t i o n r a t i o i n F i g u r e 6. With f u r t h e r i n - c r e a s e o f p r e s s u r e from 10-100 t o r r , v i b r a t i o n a l r e l a x a t i o n t o a 3 0 0 ' ~ d i s t r i b u t i o n o c c u r s w i t h l i t - t l e change i n e l e c t r o n i c s t a t e p o p u l a t i o n . T h u s , t h e B/C r a t i o c h a r a c t e r i s t i c o f t h e h i g h v i b r a t i o - n a l t e m p e r a t u r e i s f r o z e n o u t .

With i n c r e a s e o f p r e s s u r e above 50 t o r r , t h e IC-A/

IB-X r a t i o i n c r e a s e s b e c a u s e t h e h i g h t e m p e r a t u r e e l e c t r o n i c s t a t e d i s t r i b u t i o n i s r e l a x i n g t o a t r u e 300 K Boltzmann d i s t r i b u t i o n . F o r XeCR and XeF t h e l i m i t i n g i n t e n s i t y r a t i o a p p e a r s t o be 0 . 3 and 3 . 2 r e s p e c t i v e l y , which c o r r e s p o n d t o e n e r g y s e p a r a - t i o n s o f 230 and 720 cm l , r e s p e c t i v e l y , w i t h t h e C s t a t e b e i n g lower. T h e s e r e l a x a t i o n p r o c e s s e s a r e summarized below f o r a r a r e g a s b a t h g a s ; t h e sub- s c r i p t s , v", v ' and v r e p r e s e n t h i g h , i n t e r m e d i a t e and 300 K Boltzmann l e v e l s , r e s p e c t i v e l y .

kz:B = g a s k i n e t i c i n m a g n i t u d e v"

k z > % = " l a r g e 1 ' v 1

k v l C , v

5

k V ' B,v > 5 k ~ : ~

ke-B % 1 / I 00 k e l B ( s e e n e x t s e c t i o n )

The v a l u e s f o r t h e t r a n s f e r r a t e c o n s t a n t s f o r t h e l e v e l s a r e a s s i g n e d from p h o t o l y t i c s t u d i e s i n t h e n e x t s e c t i o n . Modeling s h o u l d g i v e r a t e c o n s t a n t s f o r t h e i n t e r m e d i a t e l e v e l s a s s o o n a s t h e C s t a t e d i s t r i b u t i o n s v s p r e s s u r e h a v e been e v a l u a t e d . The- r e i s no a c c e p t e d e x p l a n a t i o n a s t o why t h e t r a n s - f e r r a t e c o n s t a n t s have s u c h a s t r o n g dependence upon v i b r a t i o n a l e n e r g y . P o s s i b l y i t is b e c a u s e t h e t r a n s i t i o n s f o r h i g h v l e v e l s o c c u r a r t h e ou- t e r t u r n i n g p o i n t s when t h e p o t e n t i a l c u r v e s a r e n e a r l y c o n c i d e n t ; t h i s s e p a r a t i o n i s l a r g e r f o r l o w e r v l e v e l s .

I V . RELAXATION AND QUENCHING OF XeF(B) AND XeF(C) P h o t o l y s i s o f XeF2 w i t h t h e 147 nm Xe r e s o n a n c e li- n e o r t h e CO 4 t h p o s i t i v e b a n d s (160-180 nm) g i v e s XeF(B) i n low v i b r a t i o n a l l e v e l s ( v ' = 0-15). The y i e l d o f XeF(C) o r t h e B-A e m i s s i o n from,XeF(B) was t o o low t o b e measured. The s u b s e q u e n t r e l a x a t i o n and q u e n c h i n g p r o c e s s e s i n t h e p r e s e n c e o f r e a g e n t , Q , a r e d e f i n e d below.

F o r most r e a g e n t s v i b r a t i o n a l r e l a x a t i o n i s f a s t e r t h a n t r a n s f e r o r q u e n c h i n g , s o t h e above scheme ap-

(7)

S t e a d y - s t a t e a n a l y s i s g i v e s t h e r a t i o o f e m i s s i o n i n t e n s i t i e s a s

Some s a m p l e s p e c t r a w e r e shown i n F i g u r e 4 a n d p l o t s o f I / I v s [QIe1 a r e shown i n F i g u r e 7 .

B C

F i g u r e 7 : V a r i a t i o n o f I B - X / I C - A vs ( r e a g e n t p r e s - s u r e ) " f o r A r , N 2 a n d SF, a s r e a g e n t s ; XeF(B) w a s g e n e r a t e d by p h o t o l y s i s o f XeF2.

S i n c e T - ' i s known, v a l u e s f o r c a n b e a s s i g n e d

B 3

f r o n t h e s l o p e s o f t h e s e p l o t s

.

^{I f} ^>^k:, ^{t h e}

i n t e r c e p t c a n be u s e d t o o b t a i n k / k

C-B B-C' t h e 3 0 0 K e q u i l i b r i u m c o n s t a n t , K

.

The i n t e r c e p t w a s t h e

e q

same f o r a l l t h e r a r e g a s e s ( e x c e p t X e ) , N t r CF,.

a n d SF,, w h i c h i m p l i e s t h a t t h e i n t e r c e p t c o r r e s - T

p o n d s t o (kC-B/kg-C) ($). U s i n g t h e i n t e r c e p t va- l u e s a n d T C = 150 n s e c Band T~ = 15 n s e c I ' I 4 g i v e s K = 0 . 0 3 1 2 0 . 0 0 4 . From K a n d k B-C, t h e v a l u e s

e q eq

o f c a n b e o b t a i n e d by d e t a i l e d b a l a n c e . I f k z

2

t h e i n t e r c e p t a n d k n o w l e d g e o f a n d K g i v e k:. I f k Q i s l e s s t h a n k t h e n kQ c a n

eq

c

C-B'

c

n o t be d e t e r m i n e d f r o m t h e I B / I C vs [Q] p l o t .

I n f o r m a t i o n a l s o c a n b e o b t a i n e d f r o m a p l o t o f t h e r a t i o o f t h e e m i s s i o n i n t e n s i t y o f XeF(B) i n t h e a b s e n c e o f a d d e d b a t h g a s , I;, t o t h e i n t e n s i t y i n

a n a l y s i s g i v e s

I

Og

~ : I Q

I

^{T E '}^IQ

I

⁺kbC k Q [ ~

l2

- - - = I + + ( 8 )

I B T ;I T - I T i 1 + ( k

B C-B + k;) T ; ~ [ Q I

F o r many c a s e s kB-C k:[Q12 < ^T? [ Q ] a n d t h e t e r m i n v o l v i n g [Q12 c a n be n e g l e c t e d . F o r c a s e s i n w h i c h a n d kc a l r e a d y a r e e s t a b l i s h e d ,

i . e . t h o s e r e a g e n t s w i t h kQ

c 4

f i t t l n g o f t h e a b o v e e q u a t i o n e s t a b l i s h e s a l l f o u r r a t e c o n s t a n t s T h i s i s t h e c a s e f o r CCLF,, CHF,, N F 3 , CF

,

F 2 a n d Xe. I f k:

t

t h e n t h e e x c h a n g e o f m o l e c u l e s i n t h e B a n d C s t a t e i s r a p i d r e l a t l v e t o q u e n c t i i n g a n d f i t t i n g t h e d a t a f r o m p l o t s o f 1:/1, v s [ Q ]

U "

o n l y c a n be u s e d t o g e t t h e sum. k: + k: K e q - I = k: ⁺ k: ( 3 2 ) . I f i t i s a s s u m e d t h a t kQ = k z , t h e n

B

v a l u e s f o r t h e s e r a t e c o n s t a n t s c a n be e s t i m a t e d The r a t e c o n s t a n t s a r e l i s t e d i n T a b l e 2 .

T a b l e 2 . R a t e C o n s t a n t s a f o r X e F ( B , 1 / 2 ) a n d XeF(C, 3 1 2 ) .

a ) The l i s t e d u n c e r t a i n t i e s a r e t h e s t a n d a r d d e v i a - t i o n s .

b ) Q u a l i t a t i v e m e a s u r e m e n t s o f I ~ / I ~ f o r COLland HF s u g g e s t t h a t kQ _{B X}I x lo-'' c m 3 m o l e s " s e c

.

( c n 3 m ~ e c u l ; ' s e c - l ) l . Z ( L 0 . 5 ) lo-"

1.2Cf0.4) * lo-"

L . 2 ( t 0 . 8 ) x 3 . 3 ( t 0 . 6 ) lo-13

3 . 0 ( ? 2 . 0 ) lo-"

l . Z ( t 0 . 3 ) l o - ' ' r a w a s :L ( e l s.e 1s k j ( c ) 9 . r as k$ ( c )

**a as :4 (F)

*.p.

..

k i ( c ) sror as kg cc)

C ) The k y v a l u e s w e r e o b t a i n e d by a s s u m i n g t h a t k: = k: ; t h e e x p e r i m e n t s o n l y g i v e kQ B + k Q C K - I e q

(cm3 wlecul;' S ~ E - ' )

9 . 0 ( ! 2 . 0 ) x

9 . 6 1 . 1 0

3 . 5 ( ? 2 . 0 ) x l o - ' ' P . Z ( t 3 . 0 )

.

l 9 ( 1

2 . 5 ( 2 0 . 6 ) X 1 0 - l 1 8.8 r l o - ' '

!.0 x 2.1 * lo-'' 6 . 5 x lo-"

7.4 " 1 0 - I ' 1 . 9 r 1 0 - I '

~ c r s e n r ~

C C I F 3 CHFI HF, CFq F 2 xed N2 Y r SF,, A r HI NC

( s e e t e x t ) .

d ) I n c o n t r a s t t o o t h e r s t u d i e s , 1 5 , we f i n d no e v i - d e n c e f o r t h r e e - b o d y q u e n c h i n g o f XeF b y Xe.

( < n 3 m o ~ e c u ~ e . ' k ~ -s e c - l ) ~ 2 . 1 ( ! O . L ) r l o - ' ' Z . O ( t 0 . 3 ) X l o - "

5 . l ( t 0 . 3 ) " l o - "

2 . 7 0 2 0

Z . L ( t 1 . 0 ) x lo-"

4 . 7 ( ! 0 . 3 ) X lo-"

2 . 6 ( ! 0 . 1 ) x lo-"

3 O I I '

l . e ( t 1 . 0 ) x l o - "

7 0 2 0

2 . 0 ( . 0 . I )

.

7 . 6 ( $ 0 . ~ ) * lo-13

Q u a l i t a t i v e e x p e r i m e n t s a l s o w e r e d o n e f o r HF a n d C02 ; t h e s e r e a g e n t s r e a d i l y q u e n c h XeF(B) a n d XeF(C) a n d k i a n d k; must b e

5 lo-"

cm3 m o l e c - ' s e c " . T h i s s u g g e s t s t h a t t y p i c a l t a n k g a s i m p u r i - t i e s r e a d i l y q u e n c h x ~ F * .

A f e w comments s h o u l d be made a b o u t t h e r a t e c o n s - t a n t s o f T a b l e 2 . F i r s t l y t h e r e s u l t s w e r e d e r i v e d

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c3-202 J O U R N A L DE PHYSIQUE

b y a s s i g n i n g a l l o f t h e b r o a d b a n d e m i s s i o n t o X e F (C-A). I f t h e XeF(B-A) e m i s s i o n m a k e s a s i g n i f i c a n t c o n t r i b u t i o n t h e n t h e r a t e c o n s t a n t s a n d K w i l l

e q b e a l t e r e d . T h e c a l c u l a t e d b r a n c h ~ n g f r a c t i o n f o r X e F ( B

-

A . 1 1 2 ) i s o n l y 0 . 0 3 w h i c h i s s i m i l a r t o t h e experimental uncertainty i n t h e i n t e n s i t y m c a q u r e - m e n t s . S e c o n d l y , t h e r e w a s n o c v i d c n c c ( f r o m e i t h e r t h e f i t o f t h e k i n e t i c d a t a o r f r o m t h e a p p e a r a n c e o f t h e s p c c t r a ) f o r t h r e e - b o d y q u e n c h i n g o f X ~ F *

f o r a n y o f t h e g a s e s : t h e d a t a f o r t h e r a r e g a s e s e x t e n d u p t o t h r e e a t m o s p h e r e s . T h i r d l y . f o r N F , . C F , , F 2 a n d p o s s i b l y Xe t h e q u e n c h i n g r a t e r o n s t a n t ! f o r t h e B a n d C s t a t e s s e e m t o d i f f e r _-- ; w i t h k i b e i n g l a r g e r t h a n k c . T h i s s h o u l d b e r e g a r d e d a s Q a p r e l i m i n a r y c o n c l u s i o n .

J . E . V c l a a c o a n d D.W. S e t s e r , J . Chem. P h y s . 6 8 . 5 1 8 9 ( 1 9 7 8 )

6 . ? I . R . F l a n n e r y a n d T . Y a n g , App. P h y s . L e t t .

31.

3 2 7 , 3 5 6 ( 1 9 7 8 ) ; Chem. P h y s . L e t t . 5 5 , 1 4 3 ( 1 9 7 8 )

7 . H.C. B r a s ~ e ~ r s . D.k. S e t s e r a n d 3. D e s M a r t c a u . Chem. P h y s . L e t t .

48.

8 4 ( 1 9 7 7 )

8 . W.K. B i s c h e l . H.H. N a k a n o , D . J . E c k s t r o m . R . M . H i l l , D.L. H u e s t i s a n d D.C. L o r c n t s , App. P h y s . L e t t .

34.

5 6 5 ( 1 9 7 9 )

9 . J . E . V c l a z c o , J.H. K o l t s a n d D.W. S e t s e r , J . Chem. P h y s . 6 9 , - 4 3 5 7 ( 1 9 7 8 )

1 0 . J . H . K o l t s . J . E . V e l a z c o a n d D.W. S e t s e r , J . Chem. P h y s . I n P r e s s . ( 1 9 7 9 )

I I . D.W. S e t s e r , T.D. D r e i l i n g , H.C. B r a s h e a r s a n d .l.H. K o l t s , F a r a d a y D i s c . Chem. S o c . 6 2 . I n I ' r e s s . ( 1 9 7 9 )

1 2 . H.C. B r a s h e a r s a n d D . W . S e t s e r . J . P h y s . Chem.

T o b e s u b m i t t e d . ( I 9 7 9 )

1 3 . K. T a m a g a k e a n d D.W. S e t s e r . J . Chem. P h y s .

57,

S i n c e t h e e n e r g y s e p a r a t i o n b e t w c c n X e F ( C ) a n d L 3 7 0 ( 1 9 7 7 )

X e F ( B ) i s o n l y 7 2 0 cm

' ,

t h e m a g n i t u d e a n d d c p e n - 1 4 . J . G . E d e n a n d S.K. S c a r l e s , A p p . P h y s . L e t t . - 3 0 , d e n c e u p o n c o l l i s i o n p a r t n e r f o u n d f o r a r c 2 8 7 ( 1 9 7 7 )

1 5 . J . C . E d e n a n d R . W . W a y n a n r . J . Chem. P h y s .

68.

n o r m a l . On t h e o t h e r h a n d , t h e m a g n i t u d e o f k z ( o r

2 8 5 0 ( 1 9 7 8 ) . k Q ) s e e m s surprisingly t a r g c f o r E-T o r E-V t r a n s -

B

f e r w h i c h m u s t b e t h e c a s e f o r a l l t h e a t o m s a n d m o l e c u l e s i n T a b l e 2 . e x c e p t f o r F 2 . b e c a u s e t h e y h a v e n o e x c i t e d e l e c t r o n i c s t a t e s b e l o w 3 . 5 7 e V . T h e i n c r e a s e i n q u e n c h i n g r a t e c o n s t a n t u p o n s u b s - t i t u t i o n o f o n e F a t o m i n C F b b y a C P o r H a t o m i s v e r y l a r g e a n d s u g g e s t s t h e i m p o r t a n c e o f s t r o n g

i n t e r a c t i o n s w i t h X e F ( C ) v i a d l p o l e - d i p o l e o r d i -

? o l e i n d u c e d d ~ p o l e a t t r a c t i v e t e r m s . A p o s s i h l e e x p l a n a t i o n o f why k: a p p e a r s t o b e l a r g e r t h a n k Q

C ' may b e t h e e x t e n s i v e c o n f i g u r a t i o n i n t e r a c t i o n b e t - w e e n t h e X e F ( B ) a n d XeF(X) s t a t e s . e s p e c i a l l y a t s h o r t r a n g e .

ACKNOWLEDGMENT

T h i s w o r k w a s s u p p o r t e d b y t h e U.S. D e p t . o f E n e r g y ( E ( I 1 - 1 ) - 2 8 0 7 ) a n d b y t h e A d v a n c e d R e s e a r c h P r o j e c t s A g e n c y o r t h e U.S. D e p t . o f D e f e n s e ( m o n i t o r e d b y O.N.R. c o n t r a c t N 0 0 0 1 L - 7 6 - C - 0 3 8 0 ) .

REFERENCES

I . P . J . Hay a n d T.H. D u n n i n g . J . Chem. P h y s .

69,

2 2 0 9 ( 1 9 7 8 ) ;

66,

1 3 0 6 ( 1 9 7 7 )

2 . D. K l i g l e r . H.H. N a k a n o , D.L. H u e s t i s . N.K.

E i s h e l . R.M. H i l l a n d C.K. R h o d e s , App. P h y s . L e t t .

2.

3 9 ( 1 9 7 8 )

3 . H.C. B r a s h e a r s a n d D.W. S e t s e r . A p p . P h y s . L e t t . 3 3 , 8 2 1 ( 1 9 7 8 )

-

4 . J . H . K o l t s a n d D.W. S e t s e r , J . P h y s . Chem.

82.

1 7 6 6 ( 1 9 7 8 )

5 . P.C. T e l l i n g h u i s e n , J . T e l l i n g h u i s e n , J.A. C o x o n