7 Threshold Behavior for Collision-Induced Dissociation* Ε. K. PARKS, S. H. SHEEN, G. DIMOPLON, and S. WEXLER
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Chemistry Division, Argonne National Laboratory, Argonne, IL 60439
A fundamental collisional process at elevated energies i s the dissociation of a molecule i n t o two or more fragments v i a a colli sion w i t h a third body. This so called c o l l i s i o n - i n d u c e d dissoci ation (CID) has been observed e x t e n s i v e l y i n ion-molecule reac tions (1) and, i n more recent y e a r s , between neutral species (2-4). CID o f the alkali and t h a l l i u m h a l i d e s to ion p a i r s has been stud ied i n t h i s l a b o r a t o r y f o r the l a s t few y e a r s , and the t h r e s h o l d behavior f o r the process has been determined (4,5). T y p i c a l r e a c t i o n s observed are +
-
M + CsCl --> M + Cs + Cl +
–
--> CsM + Cl for the monomer and +
-
M + Cs Cl --> M + Cs Cl + Cl 2
2
2
+
--> M + Cs + CsCl-2 f o r the dimer. The experiments i n v o l v e a c c e l e r a t i n g the M spe cies by the seeded jet method, with H as the p r o p e l l a n t , and c r o s s i n g the beam o f M w i t h a beam o f the cesium o r t h a l l i u m h a l i d e . The ions formed are then mass analyzed (by a TOF method) and counted. Absolute cross s e c t i o n s are obtained from the ion yields and the measured intensities o f the two beams. The behavior o f the cross s e c t i o n as a f u n c t i o n o f energy f o r a few o f the above r e a c t i o n s i s shown i n Figures 1, 2 and 3 with M = Xe, K r , and SF r e s p e c t i v e l y . In Figure 1, both the CsXe and CID channels are shown. The CsXe product (from rearrangement i o n i z a t i o n ) shows a "step" behavior characteristic of reactions l e a d i n g to o p p o s i t e l y charged ions (but no a d d i t i o n a l particles). The CID channel e x h i b i t s a power law rise i n the t h r e s h o l d r e g i o n and a l e s s r a p i d rise at higher e n e r g i e s . The dashed l i n e s i n the 2
+
6
+
*Work performed under the auspices o f USERDA. 94
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Figure 1. Cross sections for Cs* and CsXe* formation in Xe-CsCl collisions as functions of the center-of-mass kinetic energy
f i g u r e s are the assumed cross s e c t i o n s , and the s o l i d l i n e s are the r e s u l t of c o n v o l u t i n g the l a t t e r with the various energy d i s t r i b u t i o n s i n the experiments. For CID, the power law σ
= V tot- ο>Χ·β1 E
W
Ε
was assumed to be o p e r a t i v e i n the t h r e s h o l d r e g i o n , where E i s the t h r e s h o l d energy, E £ i s the t o t a l energy ( k i n e t i c plus i n t e r n a l ) , and E i i s the i n i t i a l r e l a t i v e k i n e t i c energy. This f u n c t i o n a l i t y has been obtained by a number of authors ( 6 . , 7 J from s t a t i s t i c a l theory f o r an atom-diatom r e a c t i o n , w i t h values o f η of approximately 1 . 8 - 1 . 9 . While A i s i n general a f u n c t i o n of the i n t e r n a l energy of the diatom, i t was assumed constant i n our a n a l y s i s . Values of A , E and η f o r the various systems s t u d i e d are given i n Table I. 0
t 0
r e
0
0
0
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Figure 2.
CHEMISTRY
Cross sections for Cs CV formation in Kr-Cs Cl collisions as a function of the center-of-mass kinetic energy 2
2
2
Experiments on the l a r g e number of systems i n Table I demon s t r a t e t h a t Eq. (1) gives a good d e s c r i p t i o n of CID up to about 1 eV above t h r e s h o l d , and appears to be v a l i d even f o r r e a c t i o n s which are d e c i d e d l y not s t a t i s t i c a l ( d i s s o c i a t i o n of TIF, f o r ex ample, i s probably a very dynamically c o n s t r a i n e d process ( 5 j ) . This suggests the increase i n the number of continuum s t a t e s with i n c r e a s i n g energy (which determines the f u n c t i o n a l i t y of Eq. (1)) dominates the t h r e s h o l d behavior i r r e s p e c t i v e o f the r e a c t i o n mechanism. The s p e c i f i c ( n o n - s t a t i s t i c a l ) e f f e c t of i n t e r n a l energy of the diatom on the CID cross s e c t i o n has been s t u d i e d only f o r T1C1 (4J. I f the observed e f f e c t i s taken i n t o account i n the c o e f f i c i e n t A , the power η must be r a i s e d from ^1.6 to ^1.8 to f i t the experimental d a t a . I f we assume a s i m i l a r e f f e c t f o r the other atom-diatom r e a c t i o n s , then most of the η values i n the upper s e c t i o n of Table I are i n reasonable agreement with the 1.8-1.9 value obtained from s t a t i s t i c a l theory. The two high values f o r TIBr (2.0 and 2.3) r e s u l t from a low t r a n s i t i o n p r o b a b i l i t y to the i o n i c s u r f a c e . I f t h i s i s s p e c i f i c a l l y taken i n t o account i n the c o 0
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e f f i c i e n t A , both powers are lowered to ^1.6 (excluding the i n t e r n a l energy c o r r e c t i o n ) . The high values of η f o r T l I are pro bably due to the same cause; however CID i n T i l i s a l s o dynamical l y constrained (_3), making s i m i l a r c o r r e c t i o n s to the powers f o r t h i s molecule i n t r a c t a b l e . For the atom-dimer r e a c t i o n s , the same threshold a n a l y s i s leads to higher powers of η ranging from 2.1 to 2 . 7 . The high 0
Figure 3.
Cross sections for Cs* formation in SF -CsCl collisions as a function of the center-of-mass kinetic energy 6
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powers are most l i k e l y due to the presence of e n e r g e t i c a l l y a c c e s s i b l e i n t e r n a l s t a t e s o f the product molecular i o n s . As the c o l l i s i o n energy i n c r e a s e s , an i n c r e a s i n g number o f i n t e r n a l s t a t e s becomes a c c e s s i b l e , and thus the cross s e c t i o n , based on a s t a t i s t i c a l model, would r i s e more r a p i d l y than i n the atom-diatom c a s e , where only the ground s t a t e s o f the products are a c c e s s i b l e . This p o i n t o f view i s c o n s i s t e n t with the r e s u l t s f o r S F 5 given i n Table I. Since the S F 6 can absorb energy i n the c o l l i s i o n , the powers obtained f o r S F 6 would be expected to be higher than f o r analogous c o l l i s i o n s i n v o l v i n g the i n e r t gases. Indeed, the i n crease i n power from monomer to dimer, which r e f l e c t s the a d d i t i o n a l i n t e r n a l s t a t e s of the CS2C1 p r o d u c t - i o n , i s approximately the same f o r S F 6 c o l l i s i o n s ( 2 . 4 -> 3 . 5 ) as f o r the i n e r t gas c o l l i s i o n s ( 1 . 5 •> 2 . 5 ) . +
Table I:
σ = A (E ot-Eo) /Erel Ao
System Xe+CsCl+Xe+Cs++ClKr+CsCl Ar+CsCl Xe+Tl F Kr+TlF Xe+TlCl Kr+TlCl Xe+TlBr Kr+TlBr Xe+TlI Kr+TlI Xe+Cs2Cl 2->-Xe+Cs2Cl ++C1 " Kr+Cs2Cl2 Ar+Cs2Cl2 Xe+Tl2F2 Kr+Tl2F2
Xe+Tl2Cl2 Xe+Tl2Br2 Kr+Tl2Br2 Xe+Tl212 SF6+CsCl-»SF6+Cs++ClSF +Cs2Cl2^SF +Cs Cl +Cr +
6
6
2
0
n
t
(cm2-evO-O
·1θ1 )
4.02 6.64 6.44 0.152 0.114 1.41 1.48 0.063 0.0077 0.00092 0.00118 0.482 1.00 2.17 0.139 0.306 2.30 6.17 6.03 2.30 0.600 0.0501
6
Eo (eV) 4.85±.08 4.85+.08 4.85±.08 7.15+.08 7.23±.08 6.31+.08 6.25+.08 6.17±.08 6.20+.08 6.40+.08 6.40+.08 5.55±.l 5.60+.1 5.30±.l 8.1 ± . 2 8.1 ± . 2 6.6 ± . 3 6.1 +.15 6.2 +.15 5.45±.2 4.75±.08 5.65+.15
η 1.5 ±. 1.4 + . 1.4 ± . 1.85±. 1.95±. 1.6 ±. 1.45±. 2.0 +. 2.3 +. 2.1 +. 2.1 ± . 2.65+. 2 ?.. 70+. 2 2.70±. 2 2.6 ±. 2 2.7 +. 2 2.4 ±. 3 2.4 ±. 2 2.1 ±. 2 2.45± 3 2.4 +. 1 3.5 +. 1
Literature Cited 1. 2.
McClure, G.W. and Peek, J.M., "Dissociation in Heavy P a r t i c l e Collisions" (Wiley-Interscience, New York, 1972). T u l l y , F . P . , Lee, Y.T. and Berry, R . S . , Chem. Phys. Lett. (1971) 9, 80.
Brooks and Hayes; State-to-State Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
7. PARKS ET AL. 3. 4. 5. 6. 7.
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Parks, Ε.Κ., Hansen, N.J. and Wexler, S., J. Chem. Phys. (1973) 58, 5489. Parks, E.K., Wagner, A. and Wexler, S., J. Chem. Phys. (1973) 58, 5502. Parks, E.K., Kuhry, J.G. and Wexler, S., J. Chem. Phys. (in press). Rebick, C. and Levine, R . D . , J. Chem. Phys. (1973) 58, 3942. Wagner, A. and Parks, E.K., (unpublished r e s u l t s ) .
Brooks and Hayes; State-to-State Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
