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Infrared laser induced chemistry: Reactions of difluoromethane
Volume 49, number 2
CHEMICAL PHYSICS LFTTERS
15 July 1977
INFRARED LASER INDUCED CHEMISTRY: REACTIONS OF DIFLUOROMETHANE S.T. L I N and A.M. R O N N Chemistry Department, City University o f New York at Brooklyn College, Brooklyn. New York 11210, USA Received 11 March 1977
Reactions of CH2F2 excited in its C - F stretching mode by a TEA CO 2 laser have been investigated. Chemical reactions of the excited species in the neat show evidence of the difluorocarbene radical involvement. Reactions with molecular chlorine display at least two distinct mechanisms at different pressure regimes. Comparisons are made with other uninlolecular dissociations and bimolecular reactions induced by multiphoton absorptions from similar laser sources.
1. Introduction Laser e n h a n c e d or laser i n d u c e d chemical reactions c o n s t i t u t e o n e o f the m o s t significant scientific develo p m e n t s in recent y e a r s [ 1 - - 3 ] . While m o s t o f the limelight was c a p t u r e d b y a n u m b e r o f strikingly successful i s o t o p e s e p a r a t i o n e x p e r i m e n t s in a variety o f m o l e c u l a r and a t o m i c species, there remains a w e a l t h o f c h e m i c a l reactions to be e x p l o r e d a n d e x p l o i t e d utilizing these t e c h n i q u e s [4]. A l t h o u g h the n u m b e r o f p o t e n t i a l l y interesting reactions is literally unlimited this p a p e r deals w i t h the reactions o f d i f l u o r o m e t h a n e . T h e particular interest in C H 2 F 2 was g e n e r a t e d s u b s e q u e n t to similar w o r k on laser driven m e t h y l halide reactions t h a t were explored in detail at this l a b o r a t o r y [5]. T h e l a t t e r w e r e originally c h o s e n for investigation f o r their simplicity and t h o r o u g h k n o w l e d g e o f t h e kinetic routes f o l l o w e d b y excited species during energy transferring collisions [6]. T h e m u l t i p h o t o n laser i n d u c e d reactions o f the m e t h y l halides with chlorine g e n e r a t e d a n u m b e r o f conclusions w h i c h in t u r n d e m a n d e d f u r t h e r explorat i o n in similar y e t less s y m m e t r i c molecules. It was thus t h a t the reactions o f d i f l u o r o m e t h a n e w i t h chloride a n d in the neat w e r e c h o s e n for s t u d y w i t h a specific focus o n the o f f - r e s o n a n t behavior, the efficiency in t e r m s o f p h o t o n utilization to m o l e c u l e s reacted, the t h r e s h o l d to these reactions a n d the particular
m e c h a n i s m or m e c h a n i s m s responsible for p r o d u c t formation.
2. E x p e r i m e n t a l A R o g o w s k i t y p e T E A C O 2 - N 2 - H e laser capable o f a I - - 2 J o u t p u t per line at r e p e t i t i o n rates o f 1--5 pps was used as the e x c i t a t i o n source. T h e laser outp u t was directed into glass cells o f various sizes ranging f r o m a 10 c m length and 2.5 c m i.d. to a t w o liter flask fitted wdth 5 c m windows. T h e b e a m was either used directly or focussed w i t h a 5 inch focal length ZnSe lens. Pulse lengths o f 0 . 5 - - 2 / a s were used t h r o u g h o u t . Gases were M a t h e s o n Research grade a n d used w i t h o u t further purification save for f r e e z e t h a w cycles. P r o d u c t analysis was carried o u t utilizing m e d i u m resolution infrared s p e c t r o s c o p y and mass s p e c t r o m e t r y . P r o d u c t a n d r e a c t a n t peaks were c o m p a r e d p r i o r to and s u b s e q u e n t to laser e x c i t a t i o n o f C H 2 F 2 for a preset n u m b e r o f shots at p r e d e t e r m i n e d frequencies and p o w e r levels.
3. Results As this investigation closely paralleled the m e t h y l halide w o r k m e n t i o n e d previously, e x p e r i m e n t a l con255
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CHEMICAL PHYSICS LETTERS
product formation was detectable when the laser power level was less than 100 MW/cm2. At his and higher powers conversion efficiency was at best 20% of initial reactants_ No dimerization reaction of the neat CH, F (or other halides) were observed and products were identical at pressures ranging from 200 mtorr to 60 torr of CH3F and Cl, in the bimolecular case. The speculation then regarding the nature of the operating mechanism was that either (i) or (ii) or both were operative and that the !arge off-resonance behavior was due to the very high level density generated by the very high electric field (275 kV/cm) of the high power
ditions were chosen so as to compare both sets of data in the interest of establishing similar or dissimilar trends. A concise review of the methyl halide-chlorine reactions is thus included below to facilitate comparison. The direct excitation of the C-F stretching mode in CH, F with the P(20) 9.6p CO, laser line resulted in reactions depicted schematically below.
CH?F* + Ci, + CH?CIF + HCI. It wx sllow11 that the mechanism was not a radical mechanism but rather followed one or two preferred
laser. Thus an a priori set of hypotheses
existed before the investigations of CH,F, began. It was conjectured that reaction (a) will predominate over (b)
paths.
(i) CH,F*
+ Cl, +- CH, F f 2Cl- ,
Cl- + CH3F + CH,FCl or
15 July 1977
-
+ H- ,
CH,F2 + a,
5 >
CHF,Cl CH,FCl
+ HCl, A/Y= -27 kcal/mole, + FCl, M=+32 k&/mole
_
aSit did in the CH3F case despite the fact that the C-F vibration was laser driven in both cases. Off-resonant behaviour was anticipated to be somewhat more restrictive since no first order mixing was possible with the C 2v symmetry of the difluoromethane. Additionally, the threshold power for initiation was expected to be lower since the addition of a fluorine weakens the carbon hydrogen bond significantly (~23 kcal) [7]. The experimental results presented below essentially follow all the anticipated trends. Table 1 refers to data taken with samples of
(ii)CH3F’+C1,-+C1,+CH,F-+H-, Cl, +H-+HCl+Cl-, Cl- i- CH,F- -+ CH2FCI. The reaction product was limited to CH,FCl when the CH3F/C17 ratio was held at 1 : 1 and successive chlorination occurred as the ratio imzreased in favor of chlorine. Off-resonant pumping up to 250 cm-l of the known absorption of CHjF yielded significant and identical products_ Finally it was shown that no Table
1
Reactants ratio
Products
Number of
Condition
Escitation source
a2/CH2F2
R(18) 9.6p
5:l
CHF2Cl
1200
R(18) 9.6p line
5:l
CHF2CI
15
R(18) 9.6p line
5:
CHF2Cl CI=*Cl*
100
focussed small cell
R(18) 9.6y line
5:
4500
focussed small cell
R(18) 9.6p line
5:l
1200
unfocussed large cell
shots
line
1
unfocussed small cell focussed small cell
C2F4C12
1
C2 F4Cl2 CF2C12 CHF2Cl CHF‘zCl CF2a2 C2F4a2
(very small)
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15 July 1977
CHEMICAL PHYSICS LETTERS
2 torr CH,F, and 10 torr of Cl,. Excitation was provided on the-resonant R( 18) 9.6p CO, laser line and products were analysed for by mass and infrared spectroscopy and the listings are from top to bottom in order of decreasing abundance. The reaction to form CHF,Cl is clearly very efficient as product formation is ;ery obvious after 15 laser shots. No geometric power threshold is obvious since the reaction is driven in the unfocussed geometry towards the single product. It was attempted to drive the reaction with a lower power Q-switch laser, however no product formation was detectable at power levels of 10 kW. The tabulated experiments refer only to experiments in high scavenger ratio and, as can be seen, multiple chlorination proceeds as a function of laser pulses. New products namely C,F,Cl, appear at 100 pulses at low concentrations and dominate at a high number of pulses. The implication being that the initial product CHF,Cl rereacts subsequently to its formulation via either direct laser excitation (there are several near resonances within a few cm-l of the R( 18) 9.6p) or via collisions with colli~onally excited CH,F, molecules. The existence of a -CF,Cl radical is certainly suggestible at this point since its dimer becomes a major product at long excitation times. The reaction proceeds at CH2F2 and chlorine pressures ranging from 200 mtorr to 10 torr each with identical product distributions and proportional yields yet distinction must be made between the operational mechanism at total pressure of less than 1 torr and that at higher pressures_ We defer this discussion until the results of the pure CHZF2 experiments are presented in fig. 1 and below. The reactions of CH,F, in the neat were investigated over the pressure range of 200 mtorr to 25 torr. In all cases above the pressure of 2 torr the major reaction product was C2F4 accompanied by a significant amount of SiF4 from the reaction of the produced fluorinated radicals with the glass cell. Reference to fig. 1 which is a reproduction of the infrared spectrum of pure CH, F, at 15 torr as a function of laser shots at the near resonance frequency of the R(18) 9.6p line shows ciearly the production of C2F4 which peaks at approximately 200 shots. Subsequently the disappearance of the C2F4 and the parent peaks and the predominance of SiF4 is very clear at 2000 shots. This behaviour of the product’s subsequent reaction
0
1
100
400
2000
1600
‘
600
Fig. 1. Successive infrared spectra of CHzr2 taken at 15 torr pressure [S]. Arrow denotes !ocation of CzF4 peak [9]. SiF4 peak’s growth is evident at 1030 cm-’ [lo]. All other peaks are parent peaks of CHtF2. In the five spectra the number of laser shots is indicated on the left. parallels
that of CHF-,Cl successive chlorination and eventual dimerizatioi The strong implication of the existence of the CF, radical evidenced by the copious formation of C,F, was further substantiated by scavenging with 0, with a consequent formation of CF,O as the major product.
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CHEMICAL PHYSICS LETTERS
It should be noted that no formation of C2H4 or the mixed fluorine hydrogen dimer C,H,F, were detected while the species C2HzF4 and C2F4 dominated. The former at lower pressures and the latter at pressures above 2 torr. The off-resonance behaviour for both the neat and the chlorine experiments showed a similar trend to that observed in our CH,X work. Products were observable when the laser was turned away from the resonance line by 10 cm-r to the R(34) 9.6p line as well as 30 cm-l to the P(20) 9.6fl transition. No strong dependence of product concentration or nature on the laser frequency within the boundaries listed above was detected. A 100 cm-l offset resulted in no reaction whatsoever.
4. Discussion
The energy transfer behavior of CH,F, has been studied previously under low power excitation with the R( 1g) 9.6p laser line [6]. Qualitatively the V-V transfer scheme is very similar to CH, F’s behavior in that the C-F stretching vibrations are excited within I or 2 gas kinetic collisions at least to the u = 3 of that ladder. Subsequently the excitation transfers to the C-H stretching vibration at a lower rate in approximately 100 collisions. The V-T]R relaxation rate of the total manifold follows substantially slower at a rate of 45 nis-l torr- 1 _While this information is quite useful for the basic understanding of relaxation kinetics it becomes clear that a photon flux of some 1019 presents quite a different situation in terms of both the absorption efficiency and the subsequent interand intramolecular V-V energy transfer rates_ The low power excitation model would, for example, predict that the C-F vibrational ladder will be at a much higher vibrationai temperature than the C-H ladder within the time regime of the laser pulse (0.5-2~). Therefore one could legitimately doubt that reactions along the C-H coordinate would be catalyzed or induced at all. Yet, in the series of methyl halide reactions as well as in this work, essentially all products were uniquely those that proceeded along the C-H channel. The self-consistency of the experimental results within this set of reactions as well as the increased efficiency of H atom removal to form radicals such as CHF2 and CF, point toward a much more 258
15 July 1977
rapid excess energy deposition within the entire manifold and particularly in the C-H bond. It is worth mentioning that a number of polyatomic dissociation reactions that have been studied previously all bear out the experimental observation that, when the molecular level density is high enough and the photon flux sufficiently high enough, unimolecular dissociation occurs in a time shorter than the laser pulse. Unfortunately little polyatomic ener,oy transfer work whether V-V, E-V, or V-E has been reported on recently and yet it is precisely these data that are all important in predicting and understanding specific laser driven chemical reactions. The lack of such data does not however detract from the experimentai observations that point clearly towards exceedingly rapid V-V equilibration times. Specifically we point out that in all methyl halides reactions and this work the major products formed invariably were those that were thermodynamically favored. In spite of the mode specific excitation, bond stability and thermodynamic considerations seem to dominate the chemistry that occurs subsequent to the laser pulse in these polyatomic species. Thus the mechanisms discussed below presuppose substantial V-V equilibration and possibly even V-E transfer and thus conform to the experimental observations. In the previous discussion the implication was made that two mechanisms operate in the two different pressure regimes and that CF, and CHF, radicals were present. The following mechanisms are suggested: (a)
CH,F,
+ n(M)+
-CHF,
+Cl,
-CHF,
-+CHF,Cl+
-H+-Cl+HCl (b)
+ -H , high pressure regime >I torr; -Cl ,
,
CH2F2 + n(hv)+
CH2F;
\
low pressure regime
CH,F;+C12+CH,F2+2ClCH2F2+
-Cl-+CHFZCl+-H
-H+ -Cl+HCl
,
,
The latter mechanism is consistent with the observation that at low pressures the chlorine dependence is very critical. No dimerization reactions were discerned at pressures below 1 torr and therefore either mechanism (b) or the alternate (c)
Volume
(c)
49, number
CH,F,
CHEMICAL
2
+ n(hv) + CH,F;
,
CH,F;tCl,+Cl,+-CHF,+H-. Cl, + H- + HCl + Cl* , -Cl + - CHF,
-+ CHF, Cl
must predominate over the unimolecular decomposition of CH,F, at low pressures_ At pressur& larger than 1 torr dimerization predominated in mixtures high in difluoromethane. Chlorine begins to compete efficiently at 1 : 1 ratios at pressures higher than 5 torr in large vessels (500 cm3). In the smaller 50 cm3 cells chlorine begins to compete efficiently at ratios of 4 : 1 and total mixture pressures of 10 torr. The observed products however are, as was noted earlier, independent of reactant pressures. Thus mechanism (a) in which either CH,F, or Cl, compete for the created radical is definitely favored. The reduced stabil@ of the C-H bond relative to CH, F argues favorably for this mechanism as well. As in all free radical reactions, dependence is noted on cell wass proximity and relative size. Smaller cells (i.e., closer walls) favor the dimerization reactions and higher yields of SiF, while larger cells yield lesser dimers and higher concentrations of the chlorinated species (CHF2C1 or CF2ClZ). This behavior is of course confmed to the higher pressure regime. No such dependence was noted at pressures below 1 torr where only the chlorinated product predominated.
5. Conclusions The observations summarized in this paper are consistent with previous work both performed here and elsewhere. Although low power energy transfer information “predicts” a more highly reactive C-F channel the C-H channel is favored almost exclusively as the products are thermodynamically preferred. The exis-
PHYSICS
LETTERS
15 July 1977
tence of a unimolecular mechanism for decomposition of CH,F, prior to its reaction with C!, at pressures above 1 t&r is shown as is a collisionally controlled mechanism at lower pressures. The reaction efficiency is quite high and at higher pressure proceeds appreciably cn one shot of a 1 J pulse. The products, however, unless removed continue to react aided by either existing laser overlaps or via collisional transfer from high lying states in CH2F2.
Acknowiedgement We gratefully acknowledge support by the National Science Foundation under contract CHE 73-05099 A02 and by the City University of New York Faculty Research Award Program.
References
111 AM. Ronn, Spcctry. Letters 8 (1975) 303.
121 J.T_ Knudtson and E.M. Eyring. Ann. Rev. Phys. Chem. 131
24 (1974) 255. N.G. Basov, A-N. Oraevsky and A.V. Pankratov, in: Chemrcal and biochemica! applications of lasers, cd. C.B. Moore (Academic Press, New York, 1974).
[41 A.L. Robinson, Science 193 (1976) 1230. [51 B.L. Earl and A.M. Ronn, Chem. Phys. Letters 41 (1976) 29. [61 L.A. Gamss and A.M. Ronn, Chem. Phys. 9 (1975) 319. 171 J.L. Franklin, J.G. Dillard, H.M. Rosenstock, J.T. Herron, K. Draxl and F.H. Field, Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions, NSRDS-NBS 26, Office of Standard Reference Data, National Bureau of Standards, U.S. Department of Commerce,Washington, D.C. (1969). [81 E-K. Plyler and W.S. Benedict, J. Res. Natl. Bur. Std. US 47 (1951) 202. PI J.R. Nielsen, H.H. Claassen and D.C. Smith, J. Chem. Phys. 18 (1950) 812. Acta 20 (1964) t101J. Heicklen and V. Knight, Spectrochim. 295.
259
CHEMICAL PHYSICS LFTTERS
15 July 1977
INFRARED LASER INDUCED CHEMISTRY: REACTIONS OF DIFLUOROMETHANE S.T. L I N and A.M. R O N N Chemistry Department, City University o f New York at Brooklyn College, Brooklyn. New York 11210, USA Received 11 March 1977
Reactions of CH2F2 excited in its C - F stretching mode by a TEA CO 2 laser have been investigated. Chemical reactions of the excited species in the neat show evidence of the difluorocarbene radical involvement. Reactions with molecular chlorine display at least two distinct mechanisms at different pressure regimes. Comparisons are made with other uninlolecular dissociations and bimolecular reactions induced by multiphoton absorptions from similar laser sources.
1. Introduction Laser e n h a n c e d or laser i n d u c e d chemical reactions c o n s t i t u t e o n e o f the m o s t significant scientific develo p m e n t s in recent y e a r s [ 1 - - 3 ] . While m o s t o f the limelight was c a p t u r e d b y a n u m b e r o f strikingly successful i s o t o p e s e p a r a t i o n e x p e r i m e n t s in a variety o f m o l e c u l a r and a t o m i c species, there remains a w e a l t h o f c h e m i c a l reactions to be e x p l o r e d a n d e x p l o i t e d utilizing these t e c h n i q u e s [4]. A l t h o u g h the n u m b e r o f p o t e n t i a l l y interesting reactions is literally unlimited this p a p e r deals w i t h the reactions o f d i f l u o r o m e t h a n e . T h e particular interest in C H 2 F 2 was g e n e r a t e d s u b s e q u e n t to similar w o r k on laser driven m e t h y l halide reactions t h a t were explored in detail at this l a b o r a t o r y [5]. T h e l a t t e r w e r e originally c h o s e n for investigation f o r their simplicity and t h o r o u g h k n o w l e d g e o f t h e kinetic routes f o l l o w e d b y excited species during energy transferring collisions [6]. T h e m u l t i p h o t o n laser i n d u c e d reactions o f the m e t h y l halides with chlorine g e n e r a t e d a n u m b e r o f conclusions w h i c h in t u r n d e m a n d e d f u r t h e r explorat i o n in similar y e t less s y m m e t r i c molecules. It was thus t h a t the reactions o f d i f l u o r o m e t h a n e w i t h chloride a n d in the neat w e r e c h o s e n for s t u d y w i t h a specific focus o n the o f f - r e s o n a n t behavior, the efficiency in t e r m s o f p h o t o n utilization to m o l e c u l e s reacted, the t h r e s h o l d to these reactions a n d the particular
m e c h a n i s m or m e c h a n i s m s responsible for p r o d u c t formation.
2. E x p e r i m e n t a l A R o g o w s k i t y p e T E A C O 2 - N 2 - H e laser capable o f a I - - 2 J o u t p u t per line at r e p e t i t i o n rates o f 1--5 pps was used as the e x c i t a t i o n source. T h e laser outp u t was directed into glass cells o f various sizes ranging f r o m a 10 c m length and 2.5 c m i.d. to a t w o liter flask fitted wdth 5 c m windows. T h e b e a m was either used directly or focussed w i t h a 5 inch focal length ZnSe lens. Pulse lengths o f 0 . 5 - - 2 / a s were used t h r o u g h o u t . Gases were M a t h e s o n Research grade a n d used w i t h o u t further purification save for f r e e z e t h a w cycles. P r o d u c t analysis was carried o u t utilizing m e d i u m resolution infrared s p e c t r o s c o p y and mass s p e c t r o m e t r y . P r o d u c t a n d r e a c t a n t peaks were c o m p a r e d p r i o r to and s u b s e q u e n t to laser e x c i t a t i o n o f C H 2 F 2 for a preset n u m b e r o f shots at p r e d e t e r m i n e d frequencies and p o w e r levels.
3. Results As this investigation closely paralleled the m e t h y l halide w o r k m e n t i o n e d previously, e x p e r i m e n t a l con255
Volume 49, number 2
CHEMICAL PHYSICS LETTERS
product formation was detectable when the laser power level was less than 100 MW/cm2. At his and higher powers conversion efficiency was at best 20% of initial reactants_ No dimerization reaction of the neat CH, F (or other halides) were observed and products were identical at pressures ranging from 200 mtorr to 60 torr of CH3F and Cl, in the bimolecular case. The speculation then regarding the nature of the operating mechanism was that either (i) or (ii) or both were operative and that the !arge off-resonance behavior was due to the very high level density generated by the very high electric field (275 kV/cm) of the high power
ditions were chosen so as to compare both sets of data in the interest of establishing similar or dissimilar trends. A concise review of the methyl halide-chlorine reactions is thus included below to facilitate comparison. The direct excitation of the C-F stretching mode in CH, F with the P(20) 9.6p CO, laser line resulted in reactions depicted schematically below.
CH?F* + Ci, + CH?CIF + HCI. It wx sllow11 that the mechanism was not a radical mechanism but rather followed one or two preferred
laser. Thus an a priori set of hypotheses
existed before the investigations of CH,F, began. It was conjectured that reaction (a) will predominate over (b)
paths.
(i) CH,F*
+ Cl, +- CH, F f 2Cl- ,
Cl- + CH3F + CH,FCl or
15 July 1977
-
+ H- ,
CH,F2 + a,
5 >
CHF,Cl CH,FCl
+ HCl, A/Y= -27 kcal/mole, + FCl, M=+32 k&/mole
_
aSit did in the CH3F case despite the fact that the C-F vibration was laser driven in both cases. Off-resonant behaviour was anticipated to be somewhat more restrictive since no first order mixing was possible with the C 2v symmetry of the difluoromethane. Additionally, the threshold power for initiation was expected to be lower since the addition of a fluorine weakens the carbon hydrogen bond significantly (~23 kcal) [7]. The experimental results presented below essentially follow all the anticipated trends. Table 1 refers to data taken with samples of
(ii)CH3F’+C1,-+C1,+CH,F-+H-, Cl, +H-+HCl+Cl-, Cl- i- CH,F- -+ CH2FCI. The reaction product was limited to CH,FCl when the CH3F/C17 ratio was held at 1 : 1 and successive chlorination occurred as the ratio imzreased in favor of chlorine. Off-resonant pumping up to 250 cm-l of the known absorption of CHjF yielded significant and identical products_ Finally it was shown that no Table
1
Reactants ratio
Products
Number of
Condition
Escitation source
a2/CH2F2
R(18) 9.6p
5:l
CHF2Cl
1200
R(18) 9.6p line
5:l
CHF2CI
15
R(18) 9.6p line
5:
CHF2Cl CI=*Cl*
100
focussed small cell
R(18) 9.6y line
5:
4500
focussed small cell
R(18) 9.6p line
5:l
1200
unfocussed large cell
shots
line
1
unfocussed small cell focussed small cell
C2F4C12
1
C2 F4Cl2 CF2C12 CHF2Cl CHF‘zCl CF2a2 C2F4a2
(very small)
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Volume 49, number 2
15 July 1977
CHEMICAL PHYSICS LETTERS
2 torr CH,F, and 10 torr of Cl,. Excitation was provided on the-resonant R( 18) 9.6p CO, laser line and products were analysed for by mass and infrared spectroscopy and the listings are from top to bottom in order of decreasing abundance. The reaction to form CHF,Cl is clearly very efficient as product formation is ;ery obvious after 15 laser shots. No geometric power threshold is obvious since the reaction is driven in the unfocussed geometry towards the single product. It was attempted to drive the reaction with a lower power Q-switch laser, however no product formation was detectable at power levels of 10 kW. The tabulated experiments refer only to experiments in high scavenger ratio and, as can be seen, multiple chlorination proceeds as a function of laser pulses. New products namely C,F,Cl, appear at 100 pulses at low concentrations and dominate at a high number of pulses. The implication being that the initial product CHF,Cl rereacts subsequently to its formulation via either direct laser excitation (there are several near resonances within a few cm-l of the R( 18) 9.6p) or via collisions with colli~onally excited CH,F, molecules. The existence of a -CF,Cl radical is certainly suggestible at this point since its dimer becomes a major product at long excitation times. The reaction proceeds at CH2F2 and chlorine pressures ranging from 200 mtorr to 10 torr each with identical product distributions and proportional yields yet distinction must be made between the operational mechanism at total pressure of less than 1 torr and that at higher pressures_ We defer this discussion until the results of the pure CHZF2 experiments are presented in fig. 1 and below. The reactions of CH,F, in the neat were investigated over the pressure range of 200 mtorr to 25 torr. In all cases above the pressure of 2 torr the major reaction product was C2F4 accompanied by a significant amount of SiF4 from the reaction of the produced fluorinated radicals with the glass cell. Reference to fig. 1 which is a reproduction of the infrared spectrum of pure CH, F, at 15 torr as a function of laser shots at the near resonance frequency of the R(18) 9.6p line shows ciearly the production of C2F4 which peaks at approximately 200 shots. Subsequently the disappearance of the C2F4 and the parent peaks and the predominance of SiF4 is very clear at 2000 shots. This behaviour of the product’s subsequent reaction
0
1
100
400
2000
1600
‘
600
Fig. 1. Successive infrared spectra of CHzr2 taken at 15 torr pressure [S]. Arrow denotes !ocation of CzF4 peak [9]. SiF4 peak’s growth is evident at 1030 cm-’ [lo]. All other peaks are parent peaks of CHtF2. In the five spectra the number of laser shots is indicated on the left. parallels
that of CHF-,Cl successive chlorination and eventual dimerizatioi The strong implication of the existence of the CF, radical evidenced by the copious formation of C,F, was further substantiated by scavenging with 0, with a consequent formation of CF,O as the major product.
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It should be noted that no formation of C2H4 or the mixed fluorine hydrogen dimer C,H,F, were detected while the species C2HzF4 and C2F4 dominated. The former at lower pressures and the latter at pressures above 2 torr. The off-resonance behaviour for both the neat and the chlorine experiments showed a similar trend to that observed in our CH,X work. Products were observable when the laser was turned away from the resonance line by 10 cm-r to the R(34) 9.6p line as well as 30 cm-l to the P(20) 9.6fl transition. No strong dependence of product concentration or nature on the laser frequency within the boundaries listed above was detected. A 100 cm-l offset resulted in no reaction whatsoever.
4. Discussion
The energy transfer behavior of CH,F, has been studied previously under low power excitation with the R( 1g) 9.6p laser line [6]. Qualitatively the V-V transfer scheme is very similar to CH, F’s behavior in that the C-F stretching vibrations are excited within I or 2 gas kinetic collisions at least to the u = 3 of that ladder. Subsequently the excitation transfers to the C-H stretching vibration at a lower rate in approximately 100 collisions. The V-T]R relaxation rate of the total manifold follows substantially slower at a rate of 45 nis-l torr- 1 _While this information is quite useful for the basic understanding of relaxation kinetics it becomes clear that a photon flux of some 1019 presents quite a different situation in terms of both the absorption efficiency and the subsequent interand intramolecular V-V energy transfer rates_ The low power excitation model would, for example, predict that the C-F vibrational ladder will be at a much higher vibrationai temperature than the C-H ladder within the time regime of the laser pulse (0.5-2~). Therefore one could legitimately doubt that reactions along the C-H coordinate would be catalyzed or induced at all. Yet, in the series of methyl halide reactions as well as in this work, essentially all products were uniquely those that proceeded along the C-H channel. The self-consistency of the experimental results within this set of reactions as well as the increased efficiency of H atom removal to form radicals such as CHF2 and CF, point toward a much more 258
15 July 1977
rapid excess energy deposition within the entire manifold and particularly in the C-H bond. It is worth mentioning that a number of polyatomic dissociation reactions that have been studied previously all bear out the experimental observation that, when the molecular level density is high enough and the photon flux sufficiently high enough, unimolecular dissociation occurs in a time shorter than the laser pulse. Unfortunately little polyatomic ener,oy transfer work whether V-V, E-V, or V-E has been reported on recently and yet it is precisely these data that are all important in predicting and understanding specific laser driven chemical reactions. The lack of such data does not however detract from the experimentai observations that point clearly towards exceedingly rapid V-V equilibration times. Specifically we point out that in all methyl halides reactions and this work the major products formed invariably were those that were thermodynamically favored. In spite of the mode specific excitation, bond stability and thermodynamic considerations seem to dominate the chemistry that occurs subsequent to the laser pulse in these polyatomic species. Thus the mechanisms discussed below presuppose substantial V-V equilibration and possibly even V-E transfer and thus conform to the experimental observations. In the previous discussion the implication was made that two mechanisms operate in the two different pressure regimes and that CF, and CHF, radicals were present. The following mechanisms are suggested: (a)
CH,F,
+ n(M)+
-CHF,
+Cl,
-CHF,
-+CHF,Cl+
-H+-Cl+HCl (b)
+ -H , high pressure regime >I torr; -Cl ,
,
CH2F2 + n(hv)+
CH2F;
\
low pressure regime
CH,F;+C12+CH,F2+2ClCH2F2+
-Cl-+CHFZCl+-H
-H+ -Cl+HCl
,
,
The latter mechanism is consistent with the observation that at low pressures the chlorine dependence is very critical. No dimerization reactions were discerned at pressures below 1 torr and therefore either mechanism (b) or the alternate (c)
Volume
(c)
49, number
CH,F,
CHEMICAL
2
+ n(hv) + CH,F;
,
CH,F;tCl,+Cl,+-CHF,+H-. Cl, + H- + HCl + Cl* , -Cl + - CHF,
-+ CHF, Cl
must predominate over the unimolecular decomposition of CH,F, at low pressures_ At pressur& larger than 1 torr dimerization predominated in mixtures high in difluoromethane. Chlorine begins to compete efficiently at 1 : 1 ratios at pressures higher than 5 torr in large vessels (500 cm3). In the smaller 50 cm3 cells chlorine begins to compete efficiently at ratios of 4 : 1 and total mixture pressures of 10 torr. The observed products however are, as was noted earlier, independent of reactant pressures. Thus mechanism (a) in which either CH,F, or Cl, compete for the created radical is definitely favored. The reduced stabil@ of the C-H bond relative to CH, F argues favorably for this mechanism as well. As in all free radical reactions, dependence is noted on cell wass proximity and relative size. Smaller cells (i.e., closer walls) favor the dimerization reactions and higher yields of SiF, while larger cells yield lesser dimers and higher concentrations of the chlorinated species (CHF2C1 or CF2ClZ). This behavior is of course confmed to the higher pressure regime. No such dependence was noted at pressures below 1 torr where only the chlorinated product predominated.
5. Conclusions The observations summarized in this paper are consistent with previous work both performed here and elsewhere. Although low power energy transfer information “predicts” a more highly reactive C-F channel the C-H channel is favored almost exclusively as the products are thermodynamically preferred. The exis-
PHYSICS
LETTERS
15 July 1977
tence of a unimolecular mechanism for decomposition of CH,F, prior to its reaction with C!, at pressures above 1 t&r is shown as is a collisionally controlled mechanism at lower pressures. The reaction efficiency is quite high and at higher pressure proceeds appreciably cn one shot of a 1 J pulse. The products, however, unless removed continue to react aided by either existing laser overlaps or via collisional transfer from high lying states in CH2F2.
Acknowiedgement We gratefully acknowledge support by the National Science Foundation under contract CHE 73-05099 A02 and by the City University of New York Faculty Research Award Program.
References
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