3977
J. Phys. Chem. 1984, 88, 3977-3981
Infrared Multiphoton Dissociation of Pentafluoroethane: Two-Channel Dissociation Process and Secondary Photolysis of Radical Products Shuji Kato, Yoshihiro Makide,* Kazuo Takeuchi,? and Takeshi Tominaga Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan (Received: September 27, 1983; In Final Form: April 10, 1984)
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The dissociation yield and branching ratio in C 0 2 laser-induced multiphoton dissociation (MPD) of C2HF5were investigated. In order to distinguish the two primary dissociation pathways (C2HF5 C2F4 + HF, E, = 7 1.6 kcal/mol; C2HF5 CF3 + CHF2, E, = 93.5 kcal/mol), Br2 was employed as an excellent scavenger of radicals and C2F4. The scavenged products were CBrF,, CHBrF2, CBr2F2,and C2Br2F4.The yield of C2Br2F4originating from HF elimination was much smaller than those of CBrF, and CHBrF2 from C-C bond rupture. The pulse energy dependence of the product distribution demonstrates that the primarily produced radicals were further photolyzed within the laser pulse (CF3 + nhv CF2 + F, and CHF, + n’hv CF, + H) to yield CBr2F2. The secondary photolysis of the radicals was also confirmed by real-time monitoring of infrared emission from HF’ and DF’ generated in the MPD of C2DF5in the presence of H, as an F atom scavenger. In the MPD of neat C2HF5,the formation of C2F4 was unexpectedly enhanced with increasing pulse energy; this was explained by assuming that C2F4was mainly formed via recombination of CF2 radicals originating from the secondary photolysis of primarily produced radicals.
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Introduction Infrared multiphoton dissociation (IRMPD) of polyatomic molecules has been the subject of numerous studies during the past ten While most works have been intended to elucidate its mechanisms from physicochemical point of view, its application to practical purposes such as laser isotope separation has also been extensively inve~tigated.~-~ One such application may be found in our successful isotope separation of tritium by C 0 2 laser-induced multiphoton dissociation of trifluoromethane-t (CTF3).6 Furthermore, we have extended our survey to haloethanes in search of other candidates for tritium isotope separation with much lower threshold fluence for MPD and found pentafluoroethane-t to be another promising molecule for tritium isotope separation.’ As the molecular complexity increases, multiple reaction paths can be accessible. The IRMPD of pentafluoroethane is considered to proceed via two dissociation pathways. In the shock-tube pyrolysis of C2HF5,Tschuikow-Roux et aL8 have assumed two primary pathways for decomposition, a,B-elimination of molecular hydrogen fluoride and C-C bond rupture: CF3CHF2 log A = 13.6 CF3CHF2 log A = 16.6
-+
C2F4 + HF
(1)
E , = 71.6 kcal/mol
CF3 + CHF2
(2)
E, = 93.5 kcal/mol
Compounds well defined by conventional pyrolysis measurements may be suitable for studies of multichannel decomposition in IRMPD.1,2 In the C2HF5system, the branching ratio may be remarkably changed with the degree of excitation, since the A factor for the radical formation (q2) is much larger than that for H F elimination with lower E, (eq 1). Consequently, C2HF5 can be a good molecule for such study. From this viewpoint, we have investigated in this paper the pulse energy dependence of dissociation yields and product distribution in the IRMPD of C2HFS. However, the observed pulse energy dependence of the product distribution was contrary to the data of thermal decomposition study; the yield of C2F4 seemingly derived from the lower E, channel (eq 1) was found to increase with the increase in pulse energy. We have therefore employed the radical scavenging technique to distinguish the two primary pathways (eq 1 and 2), since it is particularly useful when the true primary processes are hidden by secondary processes such as radical-radical and radical-substrate interactions, or secondary ‘The Institute of Physical and Chemical Research, Wako, Saitama 351, Japan.
0022-3654/84/2088-3977$01.50/0
photolysis of radicals. We have used Br, as radical scavenger which is highly effective9J0 but not too reactive to initiate chain reactions like Cl,. With this Br2 scavenging technique, we have disclosed the primary dissociation processes and the secondary photolysis of the radical products in IRMPD of C2HF5.
Experimental Section Pentafluoroethane (C2HF5) was obtained from PCR Inc. and purified by trap-to-trap distillation before use. No impurities were found by gas chromatographic analysis. Bromine (Wako Pure Chemical, Ltd.) was degassed at -78 OC and used without further purification. Octafluorocyclobutane (c-C4F8)was kindly supplied by Daikin Kogyo Co., Ltd., and purified by trap-to-trap distillation. A TEA C 0 2 laser (Lumonics 103-2) with a lasing gas of He and C 0 2 (=lOO-ns pulse duration) was used for multiphoton excitation with R(20) line of the 00°1-0200 band at 1078.6 cm-’, which corresponds to the red-side wing of the IR peak of a’ fundamental of C2HF5at 11 11 cm-l,ll The pulse energy was measured by a pyroelectric detector (Lumonics 20D) which had been calibrated with a Scientech 364 disk calorimeter. Shot-toshot variation of the pulse energy was within 3%. The transverse intensity profile of the laser output was measured by scanning a 0.5-mm slit along horizontal and vertical axes and observing the intensity with the pyroelectric detector. The observed intensity profile of 17 mm X 27 mm rectangular laser output was nearly uniform, not Gaussian, which is typical of multimode oscillation of the laser. Sample pressures were measured by MKS Baratron manometer. All of the experiments were performed with a C2HF5pressure ~
(1) J. I. Steinfeld, Ed., “Laser-Induced Chemical Processes”, Plenum, New York, 1981. (2) P. A. Schulz, Aa. S. Sudbs, D. J. Krajnovich, H. S. Kwok, Y. R. Shen, and Y. T. Lee, Annu. Rev. Phys. Chem., 30, 319 (1979). (3) V. S. Letokhov and C. B. Moore in “Chemical and Biochemical Applications of Lasers”, Vol. 3, C. B. Moore, Ed., Academic Press, New York, 1977, pp 1-165. (4) C. D. Cantrell, S. M. Freund, and J. L. Lyman in “Laser Handbook”, Vol. 3, M. L. Stitch, Ed., North-Holland, Amsterdam, 1979, pp 485-576. ( 5 ) V. S. Letokhov, Phys. Today, 33, 34 (1980). (6) Y. Makide, S. Hagiwara, 0. Kurihara, K. Takeuchi, Y. Ishikawa, S. Arai, T. Tominaga, I. Inoue, and R. Nakane, J . Nucl. Sci. Technol., 17, 645 (1980); Y. Makide, S. Hagiwara, T. Tominaga, K. Takeuchi, and R. Nakane, Chem. Phys. Lett., 82, 18 (1981). (7) Y. Makide, S. Kato, T. Tominaga, and K. Takeuchi, Appl. Phys. B, 28, 341 (1982); 32, 33 (1983). (8) E. Tschuikow-Roux, G. E. Millward, and W. J. Quiring, J . Phys. Chem., 75, 3493 (1971). (9) R. J. S. Morrison and E. R. Grant, J . Chem. Phys., 71,3537 (1979). (10) R. J. S. Morrison, R. F. Loring, R. L. Farley, and E. R. Grant, J . Chem. Phys., 75, 148 (1981). (1 1) J. R. Nielsen, H. H. Claassen, and N. B. Moran, J . Chem. Phys., 23, 329 (1955).
0 1984 American Chemical Society
3978 The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 of 100 mtorr, which can be regarded as a condition nearly free from collisions between substrate molecules within a 100-ns laser pulse (since mean time between collisions is about 1 p s ) . In preliminary experiments, the relative amount of Br2 necessary to prevent radical-radical recombinations (e.g., CF3 + CF, C2F6) was determined. A 50-fold excess of Br2 (5 torr) was found to be enough in the photolysis of 100 mtorr of C2HF5. Irradiations of C2HF5were performed in a Pyrex cell (2-cm i.d., 65-cm length) equipped with KC1 windows and a small cold finger with a greaseless stopcock. By virtue of a fairly low pressure of C2HF5adopted, the extinction of the laser beam along the cell axis was negligible. Pentafluoroethane was once condensed into the cold finger cooled at -196 "C. Then Br2 was introduced into the cell and completely mixed. Laser pulses at 0 . 5 - H ~repetition rates attenuated with CaF2 and/or BaF, flats and passed through an aperture (14-mm diameter) were focused into the center of the photolysis cell by a BaF2 lens (52-cm focal length). The spot size of the laser beam was measured to be about 0.029 cm2 by a burn pattern on thermal sensing paper, which corresponds to a laser beam divergence of =3.5 mrad. Irradiation pulse numbers were varied between 30 and 9000 depending on pulse energy conditions, so that the conversion of C2HF5in the Br,-scavenging experiments was kept below 10%. In this conversion range, the products increased linearly with the number of laser pulses, affirming that all the products were of primary origin.12 The irradiated sample was once condensed in an evacuated sample loop cooled at -196 OC. After removing Br2 by passing it through a precolumn containing anhydrous potassium ferrocyanide (60-80 mesh),13 the gas sample was analyzed by a gas chromatograph equipped with a Porapak-Q or Porapak-T separation column (80-100 mesh, 5 mm-i.d., 3-m long) and a thermal conductivity detector (TCD). The absence of any adsorption of irradiated products in the precolumn was carefully checked. No chemical reaction of products with anhydrous potassium ferrocyanide was observed either. Each peak in a gas chromatogram was identified by comparison with the known compound or estimated on the basis of the correlation between atomic composition and gas chromatographic retention time of halogenated hydroc a r b o n ~ . ~The ~ ~IR ] ~and GC-MS measurements were also employed for identification of the irradiation products.
Kat0 et al. 1
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Results and Discussion Pulse Energy Dependence of Dissociation Yield. Irradiation in tightly focused geometry is not appropriate for the observation of the fluence dependence of branching ratio in IRMPD of C2HF5, as is discussed later. On the other hand, unfocused-beam experiments were not feasible because the dissociation rate is too slow. Therefore a mildly focused irradiation geometry was adopted with a 52-cm focal-length lens as an optimum condition. Figure 1 shows the pulse energy (Eo)dependence of the dissociation rate constant (d), which is defined by
d = -(l/t) In (1 -X)
(3)
where X i s the dissociated fraction of C2HFSafter irradiation of t pulses. The relation d a E2.O was observed in the sufficiently low Eo region (Eo 5 0.1 J) irrespective of Br2 addition. The dissociation rate constant d was decreased slightly by the addition of Br, presumably due to collisional deexcitation. The slope of the plot of log d vs. log Eo was found to decrease and approach (12) The secondary photolysis of the scavenged products (CBrF,, CHBrF2, CBr2F2,and C2Br2F4)was assumed to be virtually negligible at about 10% C2HFSdissociation. While these bromides have absorption peaks around 1100 cm-' which are relatively close to the laser frequency, the assumption was confirmed by simulating the progress of photolysis with a difference equation analysis based on their dissociation rate constants in IRMPD measured under the same condition. (13) W. E. Harris, W. H. McFadden, and R. G. McIntosh, J . Phys. Chem., 63, 1784 (1959). (14) Y.Makide, T. Fukumizu, and T. Tominaga, Bunseki Kuguku, 25, 1 (1976). (15) Y.Kanai, Y., Makide, and T. Tominaga, Nippon Kuguku Kuishi, 663 (1980).
6
c
2
2
4
0.1 1.o Pulse energy, E o ( J ) Figure 1. Dissociation rate constant (4 for C,HF, as a function of laser pulse energy (Eo). Laser frequency: R(20), 1078.6 cm-'. Sample: 100 mtorr of CzHF, containing 0 (0),5 (A), and 20 torr (0) of Br,. n is the slope of log d vs. log Eo plot.
0
I
-.-.-.-.,
1 1 ' 1 1 '
1
,
0.1 1.0 Pulse energy, Eo(J) Figure 2. Pulse energy (Eo)dependence of the relative yield of irradiation products: CBrF3 (0),CHBrF, (A),CBr,F, ( O ) , and C,Br,F, (m). The relative yield was defined as the ratio of the pressure of the product to the decrement of the C,HF, pressure. Sample: 100 mtorr of C2HF, containing 20 torr of Br,.
asymptotically 1.5 with the increase of Eo. Such a trend is anticipated by our model proposed previously.I6 Briefly, let the intrinsic dependence of dissociation probability (9) on fluence (9) be defined by q =
(a/@$'
for 9 < 9,
(4)
q = 1 for 9 2 9, (5) where @, is the critical fluence for saturation of dissociation. W h e n the pulse energy Eo is sufficiently low and the focal fluence (af) is smaller than GC,dissociation occurs in the range of (4) mostly within a focal volume (Vf) where laser fluence is nearly uniform. Consequently, the reaction volume (V, = V,d, where V, is the cell volume) is expected to be in proportion to E,". The value of 9, was calculated as 8.1 J/cm2 for 100 mtorr of C2HFSon the basis of our deconvolution model. When af exceeds a,, saturation occurs within Vf and the reaction volume extends to the wings beyond Vf resulting in the gradual decrease in the slope of the curve (log d vs. log Eo) to 1.5. (16) K. Takeuchi, I. Inoue, R. Nakane, Y.Makide, S.Kato, and T. Tominaga, J . Chem. Phys., 76, 398 (1982).
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 3979
IRMPD of C2HF5
Focal f luence,
TABLE I: Distribution of Products Observed in IRMPD of CzHFs with Brla
pulse energy, J
focal fluence, J/cm2
CBrF,
0.08 0.10 0.18
2.9 3.7 6.8 21
77 70 64 52
0.55
re1 yield,b% CHBrF2 CBr2F2 C2Br2F4 52 54 9 63 53 7 79 102
48
34
0.8
“Br, pressure 20 torr. bRatio of the pressure of product to the de-
crement of the C2HF5pressure.
E
Radical Scavenging with Br,. In the photolysis of C2HF5with Br2, four products CBrF,, CHBrF,, CBr2F2,and CZBr2F4(1,2dibromotetrafluoroethane) were usually detected by the gas chromatographic analysis. Table I and Figure 2 show the pulse energy dependence of the relative yields of the products. At the lowest pulse energy of 0.075 J ( ~ 2 . 9J/cm2 in focal fluence), the major products are CBrF, and CHBrF,, which can be the scavenged products of the radical species CF, and CHF,, respectively, from the higher activation energy channel (eq 2).
Br2
CF, CHF,
Br2
CBrF,
(6)
CHBrF,
(7)
The yield of C2Br2F4was substantially low. These observations indicate that dissociation via the higher activation energy channel is dominant in IRMPD of C2HF5. In contrast to the result from the thermal decomposition study,* the relative yields of CBrF, and CHBrF2 decreased and that of CBr2F2 increased with an increase in pulse energy. The relative yield of C2BrZF4remained below 10% over the pulse energy range studied. The pulse energy dependency of the product distribution was similar a t different Brzpressures (5 and 20 torr). Since it is unlikely that the higher activation channel is suppressed at higher pulse energy, the decrease of the CBrF, and CHBrF, yields at higher pulse energy should be accounted for by other processes. A probable explanation assumes secondary photolysis of the radicals within the 100-ns laser pulse as is discussed in the next section. The origin of small amounts of C2BrzF4is ascribed to the gas-phase bromination of the double bond in CzF4: C2F4 Br, C,Br2F4 (8)
+
-
AH = -38.35 kcal/mol
l7
This reaction proceeds so rapidly even at room temperature that the CzF4 produced is completely converted to C2Br,F4 in the presence of Br,. Other possible reactions of CzF4 with Br2, e.g.
were also considered, because the C2F4molecule can be formed in vibrationally excited states in the laser field. For simulating the reaction of vibrationally excited C2F4 with Br,, the IRMPD experiment of octafluorocyclobutane (c-C4Fs) has been carried out. Octafluorocyclobutane (1 torr) was irradiated by the TEA CO, laser at 1OP (14) 949.4 cm-’ in the presence of 5 torr of Br,, where its MPD has been reported to yield two CzF, molecules.18
nhu
c-C4Fs E , = 74.3 kcal/mol
2CZF4
(9)
AH = 49.9 kcal/mol’g
The only product observed was 1,2-dibromotetrafluoroethane (CBrF2CBrF2). Thus C2Br2F4observed in the photolysis of C2HF5 in the presence of Br2 corresponds to the amount of C2F4 derived (17) J. R. Lacher, L. Casali, and J. P. Park, J . Phys. Chem., 60, 608 (1956). (18) J. M. Preses, R. E. Weston, Jr., and G. W. Flynn, Chem. Phys. Lett., 46, 69 (1977). (19) A. Lifshitz, H. F. Carroll, and S. H. Bauer, J . Chem. Phys., 39, 1661 ( 1963).
I
1.0
4 6
9,( J/cm2)
----
0.4
0.1 Pulse energy, E o ( J )
1.o
Figure 3. Dependence of product ratio (R)upon pulse energy (E,) and focal fluence (af),where R = [CBrF3]/0.5([CBrF3]+ [CHBrF,] + [CBr,F,]). Sample: 100 mtorr of C2HF5containing 5 (A) and 20 torr (0) of Br,. from eq 1 while the formation of CBr2F2via C2F4 intermediate is negligible. Therefore we can confirm that the IRMPD of C2HFS proceeded mainly (>90%) via the higher activation energy channel (Le., C-C rupture). Besides the dissociation mechanisms described above, the possibility of a purely thermal reaction of C2HF5with Br2 was also examined since the bromination of CzHFs is slightly exothermic.20 However, our calculation has revealed that the maximum temperature attainable in the focal region is at most 410 ‘C with 5 torr of Br, (or less with 20 torr of Br,), assuming even significantly high absorbed energy such as 70 kcal/mol above the lower activation energy channel and its adiabatic thermalization within the focal volume. At such temperatures, the only possible thermal reaction is hydrogen abstraction of C2HFswith Br, yielding C2BrFs.20 The products observed in the IRMPD of C2HF5with Br2 were almost exclusively CBrF,, CHBrF,, CBr2F2, and C2Br2F4.While C2BrF5was found under certain conditions, its amount was always less than 3%. Therefore, we have concluded that the contribution of the thermal reaction is almost negligible under these experimental conditions. Secondary Photolysis of Radicals. The yield of CBr2F2in the IRMPD of CzHF5with Br, increased with an increase in pulse energy as shown in Figure 2. Since the vibrationally excited C2F4 and Br, cannot form CBr2F2as shown before, these results can be explained in terms of the secondary photolysis of radicals within the laser pulse:
CF3 CHF,
- +- + mhu
m’hv
CF2
CF,
F
H
Br2
Br2
CBr2F2 CBr,F,
(11)
These processes are expected to depend on the pulse energy as well. We will discuss the processes quantitatively; if we assume that CBr2F2originates from reactions 10 and 11, the sum of the products ([CBrF,] + [CHBrF,] + [CBr,F,]) corresponds to twice the dissociation yield of C2HF5via C-C rupture. On the other hand, [CBrF3] corresponds to the amount of CF3 radicals which has not undergone secondary dissociation. Therefore, the product ratio R defined as R =
[CBrFd 0.5([CBrF3] + [CHBrF,]
+ [CBr,F,])
(12)
represents the fraction of CF, radicals which has not further decomposed. Figure 3 shows the plot of R vs. pulse energy. Since (20) J. W. Coomber and E. Whittle, Trans. Faraday SOC.,63,608 (1967).
3980 The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 Focal fluence,
( J/cm2 )
10
1
01 01
a 0.01
A/
0.1 Pulse energy, Eo( J )
1.o
Figure 4. Dependence of dissociation probability of CF3 (4) upon pulse energy (Eo)and focal fluence (+f). Sample: 100 mtorr of C2HFScontaining 5 (A)and 20 torr (0)of Br2.
R approached unity with a decrease in the pulse energy, it is suggested that the primarily produced radicals can be observed without suffering further dissociation in the sufficiently low pulse energy region. The pulse energy where R asymptotically approached unity was about 0.05 J (Figure 3) which corresponds to a focal fluence of =2 J/cm2. The secondary photolysis of radicals appears to be a general phenomenon rather than an exceptional one. For example, CF3,21,22 C2F5,23C3F7,24SFs,25*26 CC12F,27and CF3OZ8have been reported to undergo secondary photolysis within a laser pulse. The infrared spectrum of CF3 isolated in an Ar matrix shows its v1 absorption at 1087 cm-1.29 Considering the red shift generally observed in IRMPD,30-32the secondary photolysis of CF3 can be expected to occur appreciably at a laser frequency of 1078.6 cm-’. Wiirzberg et aL2* have studied the IRMPD of CBrF, with time-resolved infrared emission measurements. In the photolysis of CBrF3 with H I scavenging gas, they observed the emissions from HBr* and HF*, the latter indicating that primarily produced CF3 has decomposed to CF, + F within a laser pulse. About 3% of CF3 products was estimated to have undergone the secondary photolysis at 1083.5 cm-’. They also suggested the fluence dependence of CF3 decomposition. Our results observed in the IRMPD of C2HFs are consistent with their results. Since the ratio R represents the fraction of retained CF3 radicals, (1 - R) is equivalent to q, the dissociation probability of (21) E. Wiirzberg, L. J. Kovalenko, and P. L. Houston, Chem. Phys., 35, 317 (1978). (22) A. B. Horwitz, J. M. Preses, R. E. Weston, Jr., and G. W. Flynn, J . Chem. Phvs.. 74. 5008 (19811. (23) P.‘A.’Hackett, E: Weinberg, M.Gauthier, and C. Willis, Chem. Phys. Lett., 82, 89 (1981). (24) M. J. Rossi, J. R. Barker, and D. M. Golden, J. Chem. Phys., 76,406 ~
(1982).
(25) E. R. Grant, M. J. Coggiola, Y. T. Lee, P. A. Schulz, Aa. S.Sudb0, and Y. R. Shen, Chem. Phys. Lett., 52, 595 (1977). (26) J. M. Preses, R. E. Weston, Jr., and G. W. Flynn, Chem. Phys. Lett., 48, 425 (1977). (27) Aa. S. Sudb0, P. A. Schulz, E. R. Grant, Y. R. Shen, and Y. T. Lee, J . Chem. Phys., 70, 912 (1979). (28) F. Zhang, J. S. Francisco, and J. I. Steinfeld, J . Phys. Chem., 86, 2402 (1982). (29) D. E. Milligan and M. E. Jacox, J . Chem. Phys., 48, 2265 (1968). (30) J. L. Lyman, G. P. Quigley, and 0. P. Judd in “Multiple-Photon Excitation and Dissociation of Polyatomic Molecules”,C. D. Cantrell, Ed., “Topics in Current Physics”, Springer, Berlin, in press. (31) R. V. Ambartzumian and V. S. Letokhov in “Chemical and Biochemical Applications of Lasers”, Vol. 3, C. B. Moore, Ed., Academic Press, New York, 1977, pp 167-316. (32) W. Fuss and J. Hartmann, J . Chem. Phys., 70, 5468 (1979).
Kat0 et al. CF, averaged over the mildly focused irradiation zone. Figure 4 shows that 4 is nearly proportional to Eo5in the region of Eo below 0.1 J. Although 4 has been spatially averaged, this value shows an intrinsic fluence dependence when the decomposition of the parent C2HF5occurs within a focal volume V,. When the pulse energy Eo was increased and the dependence of the dissociation rate constant of C2HFs approached Eo15 , 4 approached a limiting value between 0.4 and 0.5. From the value of 0, for C2HFs, the threshold fluence for complete depletion of CF3 originating from C2HF5is estimated to be 13-1 5 J/cm2 by the same deconvolution method mentioned before. The secondary photolysis of CF3 has been reported in the cases of CBrF321and CC1F3.22 Our analysis of the results of infrared emission of CBrF, (Figures 5 and 9 in ref 21) revealed that the rate constants for the production of CF, and subsequent CF2 were proportional to about 2.4- and 7.2-th power, respectively, of the laser fluence in the range below 4 J/cm2. Then an apparent fluence dependence of the secondary photolysis of CF3 is obtained as about the fifth power, which is consistent with our results observed in the sufficiently low Eo region. More recently, Horwitz et have investigated the secondary photolysis of CF3 originating from the photolysis of CC1F3 at 1090 cm-I over a wide fluence range (0-100 J/cmZ). Their results have shown that the LIF signal of CF2 starts to level off at a fluence as low as 10 J/cm2 and saturates at about 20 J/cmz. The value of 0cfor the photolysis of CF, appears to be fairly low and is similar to our case for C2HFs photolysis. The minor discrepancy between their results and ours may have arisen from the difference in laser frequency and the internal energy of CF3 produced. We also investigated the secondary photolysis of radicals in IRMPD of C2HFSby time-resolved emission measurements: a (1:lO) mixture of C2DF5/H2or C2HFs/D2(total pressure of 20 torr) was irradiated at 1078.6 cm-I and the emission from vibrationally excited hydrogen fluoride was monitored by an InSb (77 K) infrared detector. In the photolysis of a C2DFS/H2 mixture, the DF* emission attributed to the molecular elimination from C2DFs was rather weak, and a strong emission from KF* was observed instead. Furthermore, the time profile of HF* emission intensity was very sharp compared with that of DF*, suggesting the prompt production of HF*. These observations suggest strongly that an F atom was produced within the laser pulse by the secondary photolysis of CF3. The mechanism for secondary photolysis of CHF2 is somewhat ambiguous. The relative yield of CHBrF2 was dependent on the pulse energy in a manner similar to CBrF,. Such dependence cannot be expected by any other mechanism such as a collisional process (e.g., C2HF,* CHF, C2HFS CF2 H). While the infrared spectrum of CHFz isolated in an Ar matrix has absorptions at 1164 and 1173 ~ r n - ’ far , ~ ~remote from the laser frequency, the radical species formed in the quasi-continuum state could absorb infrared photons and compensate for some mismatching between their linear absorptions and laser frequency, as was suggested for the secondary photolysis of SF5.25The release of a hydrogen atom from CHF, was also suggested experimentally; when the MPD products of a C2TFs/C2HFsmixture were analyzed by radio-gas chromatography in the manner as reported previously,’ a small amount of tritiated hydrogen (HT) was detected together with other major radioactive products, indicating the release of hydrogen atoms in their MPD.34 Multiphoton Dissociation of Neat C2HF5. Major products detected in the irradiation of neat C2HFSwere C2F4, CHF,, C2F6, CH2F2,C2HF3,C3F8,C,F,, C2H2F4,C3HF7,and C4FI0.GC-MS analyses revealed some other minor products with higher molecular weights, of which the total amount was negligible (
