3563
J. Phys. Chem. 1083, 87,3563-3572
Electronic Energy Transfer in the Infrared Multiphoton SiF,-Sensitized Decomposition of CFZHCI Jerry Kramer GTE Laboratories, Inc., Wallham, Massachusetts 02254 (Received: November 29, 1983)
CF2,produced in the IR multiphoton SiF,-sensitized decomposition of CF,HCl, has been temporally detected by laser-inducedfluorescence. The onset of decomposition, under the pressure conditionsemployed, corresponded to as few as 2-3 hard-sphere collisions. The sensitized reaction was studied as a function of CF2HC1and SiF, pressure and at different COPlaser fluences. SiF4 luminescence, following inverse electronic relaxation of vibrationally excited SiF4into an excited electronic state of SiF4,was measured concurrently with the CF, LIF signal. The induction time for CF2formation correlated with the induction time for SiF4luminescence, but was always longer. The experimental results strongly suggest that the dominant dissociation mechanism is electronic energy transfer from an excited electronic state of SiF, to CF2HC1.
Introduction The collisionless nature of IR multiphoton decomposition was firmly established by the molecular beam studies of Lee et al.' At higher reactant pressures, collisions influence the multiphoton excitation process. Examples of collisional phenomena observed include rotational-hole filling, vibrational up-pumping, and loss of isotopic selectivity. The ultimate collisional process in IR multiphoton chemistry is the sensitized decomposition of acceptor molecules which do not absorb the IR laser radiation. Grunwald and co-workers have modeled the timedependent temperature rise of Brz produced by collisional energy transfer from CF2HClor SiF4 (at pressures of lo's of torrs) excited by unfocused COz laser radiation.2 The currently accepted mechanism for IR multiphoton sensitized decomposition, which derives from Grunwald's results, the lack of isotopic selectivity in sensitized reactions, and the general thermal character of sensitized decomposition products, is degradation of the sensitizer vibrational energy to heat (V T energy transfer) followed by thermal decomposition of the acceptor m ~ l e c u l e . ~ In this paper we describe the time dependence of the decomposition products produced in the SiF4-sensitized decomposition of CF2HC1. The onset of decomposition, under the pressure conditions employed, corresponded to as few as 2-3 hard-sphere collisions. This initial, highly efficient energy transfer process is consistent with inverse electronic relaxation4 (IER) of vibrationally excited SiF4 into an excited electronic state of SiF4, followed by electronic energy transfer to CF2HC1at early reaction times as the dominant mechanism. A pioneering experiment in IR multiphoton chemistry was the observation of collisionless IR multiphoton excitation in SiFkS Under focused conditions, Isensor et al. observed a prompt luminescence which followed the temporal profile of the CO, laser, followed in time by a strong collisional luminescent component at higher pressures which corresponded to the a-band of the SiF radical. Using gated electronics, Dolzhikov and co-workers have shown that the prompt luminescence is characterized by
-
(1)M. J. Coggiola, P. A. Schulz, Y. T. Lee, and Y. R. Shen, Phys. Rev. Lett., 38, 17 (1977). (2)E. Grunwald, C. M. Lonzetta, and S. Popok, J.Am. Chem. SOC., 101,5063 (1979). (3)W.C. Danen and J. C. Jang, 'Laser Induced Chemical Processes", J. I. Steinfeld, Ed., Plenum Press. New York. 1981.ChaDter 2. (4)A. Nitzan and J. Jortner, J: Chem. Phys., 71; 3524 (1979). (5)N.R. Isenor, V. Merchant, R. S. Hallsworth, and M. C. Richardson, Can. J. Phys., 51, 1281 (1973). 0022-3654/83/2087-3563$0 1 .ti010
a broad featureless continuum extending from 330 to 630 nm and confirmed that the collisional component corresponded to the SiF emission around 440 nm.6 Yahav and Haas have also observed the same broad emission spectrum and have shown that the intensity is proportional to the SiF4pressure up to about 0.5 torr, proving the collisionless nature of the prompt emission.' They also determined that the luminescence had an incubation period of order 100 ns, which depended on the total COz pulse energy. Both Dolzhikov and co-workers and Yahav and Haas have argued that the prompt emission must come from an excited electronic state of SiF4 produced in an IER process. This excited molecular state was postulated to correlate with the ionic fragments SiF3+and F- and be shifted with respect to the ground state. Thus, the Franck-Condon factor would prevent direct W excitation from the ground state to the excited state and emission to the rapidly varying attractive part of the ground-state potential would yield a broad continuum spectrum. In addition to its interesting photophysics, SiF4has had an extensive role in IR multiphoton chemistry as a sensitizer.Q' SiF4strongly absorbs CO, laser radiation at 1025 cm-' and is efficient at transferring that energy to acceptor molecules. The Si-F bond strength, at 144 kcal/mol, is very high, preventing decomposition of the sensitizer, except at very high fluences. CF2HCl has become a prototypic molecule in IR multiphoton chemistry. Both King and Stephenson, and van den Bergh and their respective co-workers, have studied the decomposition of CFzHCl by monitoring the CF2 radical product by laser-induced fluorescence (LIF) or kinetic absorption spectroscopy, respe~tively.'&'~ The chemistry of CFzHCl is particularly simple. nhu
CF2HClCFZ + CFZ
CF, kz 4
+ HC1
(1)
CzF4
(2) ~
(6) V. S. Dolzhikov, V. N. Lokhman, N. V. Chekalin, and A. N. Shibanov, Sou. J. Quantum Electron., 8, 373 (1978). (7)G. Yahav and Y. Haas, Chem. Phys. Lett., 83,493 (1981). (8)K. J. Olszyna, E. Grunwald, P. M. Keehn, and S. P. Anderson, Tetrahedron Lett., 19, 1609 (1977). (9) D. Garcia and P. M. Keehn, J. Am. Chem. SOC.,100,6111(1978). (10)J. C.Stephenson and D. S. King, J. Chem. Phys., 69,1485(1978). (11)J. C. Stephenson, D. S. King, M. F. Goodman, and J. Stone, J. Chem. Phys., 70,4496 (1979). (12)D. S.King and J. C. Stephenson, Chem. Phys. Lett., 66,33(1979). (13)R. Duperrex and H. van den Bergh, J. Chem. Phys., 71, 3613 (1979). (14)R. Duperrex and H. van den Bergh, J.Mol. Struct., 61,291(1980).
0 1983 American Chemical Society
3564
The Journal of Physical Chemistry, Vol. 87, No.
CF2 + H C l A CFzHCl
18, 1983
(3)
The activation energy for the three-centered elimination of HC1 in reaction 1is 53 kcal/mol.15 The dimerization of CF2 via reaction 2 is very slow (k, = 3.7 X lo-', cm3/ (molecules s)) and can be ignored for most of the time domains studied.16 The back-reaction, which has an activation energy of at least 6 kcal/mol, is also very s10w.15 LIF of the CF2 radical has been well characterized by King, Schenck, and Stephens_on17 Wavelengths and Franck-Condon factors for the A-X transition in_CFzhave been determined. The radiative lifetime of the_A state of CF2 is 61 ns. Low vibrational levels of the A state are inefficiently quenched by atomic and diatomic foreign gases as well as SF6.18 The quenching rats constants @crease with vibrational excitation. For the A(0,6,0) and A(0,5,0) states of CF2 quenching by CF2Brzis highly efficient with a rate constant of 8.2 X lo6 (s torr)-'.19 Using CFzHCl as an acceptor molecule for SiF, sensitization and detecting the CF, product has particular advantages. Only energy which is transferred to CFzHCl as internal energy is probed. In addition, energy in excess of the activation energy for dissociation (53 kd/mol) must be accumulated before the CF, fragment is observed. This point is particularly important under pressure conditions such that the appearance time for CF2 is comparable to the hard-sphere collision time. When SiF, and CFzHCl mixtures are used, it is possible to pump either molecule independently. Thus, the influence of SiF, on the direct multiphoton decompmition of CFPHClcan be investigated independent of the sensitized reaction. With mildly focused COzlaser radiation, high excitation levels of SiF, can be attained. The perpendicular dye laser beam used for LIF allows us to probe the reaction in a volume of constant known fluence even though the CO, fluence changes through the cell.
Experimental Section A low pressure flow cell was constructed from a 12.7 cm long X 5.5 cm i.d. Pyrex cylinder with quartz windows and four side arms. One set of side arms, attached to opposite sides of the cylinder and perpendicular to the cylinder axis, had NaCl windows secured with stainless steel O-ring fittings for passage of the C02 laser radiation. Dye laser radiation traversed a second set of side arms with quartz windows which were orthogonal to both the cylinder axis and the C02 laser beam. Laser-induced fluorescence was collected through the large aperture window along the cylinder axis perpendicular to both laser beams. The SiF, and CFzHCl flow rates, in the range 15-80 scm3/min, were regulated with Tylan FC 260 mass flow controllers. The flow controllers were calibrated with bubble flow meters, directly for CF2HC1and indirectly with CFzClz(which has the same conversion factor as SiF4)for SiF,. The ratio of SiF4 to CF2HCl was determined from the respective flow rates. The total pressure in the cell, monitored with an MKS Baratron 0-100-torr head, was adjusted with a throttle valve. The flow rates were sufficient for a complete change of gas in the cell between each (15) G. R. Barnes, R. A. Cox, and R. F. Simmons, J . Chem. SOC.B, 1176 (1971). (16)R. I. Martinez, R. E. Huie, J. T. Herron, and W. Braun, J . Phys. Chem., 84,2344 (1980). (17)D. S. King, P. K. Schenck, and J. C. Stephenson, J. Mol. Spectrosc., 78, 1 (1979). (18)D. L. Akins, D. S. King, and J. C. Stephenson, Chem. Phys. Lett., 65,257 (1979). (19)F. B. Wampler, J. J. Tiee, W. W. Rice, and R. C. Oldenborg, J. Chem. Phys., 71,3926 (1979).
Kramer
laser pulse, but the liner flow rate was very slow with respect to the measurement time. The multimode output from a Tachisto 215G grating tuned TEA pulsed C02laser consisted of a mode-locked spike (50-11s fwhm) followed by a weaker tail which extended to -500 ns. The IR radiation was focused by a 30-cm Ge meniscus lens into the center of the low-pressure flow cell. A Ge beam splitter diverted a small fraction of the radiation through a 10-cm focal length BaFz lens into a Laser Precision RjP-735 pyroelectric probe. A Laser Precision Rj-7200 energy meter measured the energy from the pyroelectric probe and was calibrated with a Scientech 38-0102 volume absorbing disk calorimeter and energy indicator. Beam dimensions were determined with a scanning pinhole, and laser energy was attenuated with CaFz flats. In the sensitized experiments, the P42 line of the 9-pm band at 1025 cm-' was used to irradiate a SiF4 vibration at 1031 cm-l where CFzHCl is transparent. The R26 line of the 9-pm band at 1082 cm-' was used to directly pump a CFzHC1vibration centered at 1108 cm-' even in the presence of SiF,. A Lambda-Physik excimer (EMG-101) pumped-dye (FL-2000) laser containing Coumarin 485 and doubled with a KDP crystal was focused with a 30-cm quartz lens into the center of the low-pressure _flowsystem. The UV radiation was tuned to excite the A X transition in CFZ.,O A second RjP-735 pyroelectric probe and an EG&G FND-100 Q fast silicon photodiode with a Suprasil window monitored the relative doubled dye laser energy by using quartz beam splitters. The doubled dye was attenuated, when necessary, with metallic coated quartz neutral density filters and steered with aluminum mirrors. The laser-induced fluorescence was collected with a 7.5-cm focal length fl quartz condensing lens and passed through a 1/2-m Jarrel-Ash monochrometer operated in second order. The signal from an RCA 1P28AVIPMT was sent to a PAR Model 162 boxcar averager with two Model 165 gated integrators and the digital storage option. The PMT signal was normalized to the UV laser intensity. The boxcar averager was typically used in the sum mode, and each experimental point is the sum of 10 shots. The firing of the dye laser was delayed with respect to the COz laser by a Cordin Model 437 variable time delay generator. Zero delay, which corresponds to both laser pulses starting at the same time, was found by detecting the C02and dye laser pulse shapes with a Laser Precision kT-1510 fast pyroelectric detector and the FND-100 Q fast silicon photodiode, respectively. The waveforms were plotted on a strip chart recorder after processing by the boxcar averager. The jitter between the two pulses, determined with an oscilloscope, was = h15 ns. The flow cell could be moved along a rail for optical alignment. A He-Ne laser defined the LIF optical axis, and a pin was used to ensure that the COPand dye laser beam intersected. The beam diameter of the focused CO, laser was larger than the focused beam of the dye laser. Ground electronic statepibratio_nallevels of CF2 were monitored by exciting the AIB1 XIAl transition in CF2.
-
-
(20) Direct UV photodissociation of vibrationally excited CF,HCl by the UV probe laser could not affect the number of CF2radicals measured in any consequential manner. The absorption cross section of CF,HCl is cm2/molecule (base e) at 160 nm and monotonically decreases to at 220 nm with no measurable absorption at 261.7 nm. (H. Okabe, 'Photochemistry of Small Molecules", Wiley, New York, 1978,p 308.) Even if CFzHCl had 23 kcal/mol of vibrational energy, which could correspond to a red shift in the W absorption spectrum from 220 to 261.7 nm, only about 1 in of the UV photons would be absorbed. The number of potential CF, radicals produced by this mechanism would be many orders of magnitude less than formed by sensitized decomposition and measured by LIF.
le
SiF,-Sensitized Decomposition of CF,HCI
The Journal of phvsicai Chemistry, Vol. 87, No. 18, 1983 3565
TABLE I: CF,HC1 Pressure Dependence of the SiF,-Sensitized Decomposition of CF,HCl at 1025 cm-' CF,
production rate, molecules/(cm3s)
CF,$naX ,c molecules/cm3
CF,HCl press., torr
SiF, press., torr
CO, energy, mJ
fluence,a J/cm2
0.50 1.0 3.0
1.0 1.0 1.0
164 164 168
43 43 44
1.0 x loZoe 2.0 x 10," 4.1 X lozof
5.8 x 1013e 1.0x 1014 2.1 x 1014f
1.8 x 10-3e 3.1 x 10-3 6.3 x 1 0 - ~ f
0.50 1.0 3.0
1.0 1.0 1.0
90 87 86
24 23 23
9.1 x 1019 e 1.2 x 10'" 3.3 x 10," f
4.8 x 1013 e 6.3 x 1013 1.6 x 1oi4f
1.5 x 10-3e 1.9 x 10-3 4.8 x f
CF,,,/SiF4
a The focused beam radius was determined by fitting the intensity to the equation ZIR = ZIR" exp(-r2/RIR2). The radius The maximum CF, yield in the The slope of CF, vs. time. was very sensitive to the adjustment of the output coupler. The fraction of electronically excited SiF, molecules_which reacted, assumin CF, was only profirst 2 p s of reaction. duced by SiF4*. e Multiply by 0.89 Lo correct for CF,HCl quenching of the A(0,2,0) state of CF,. YMultiply by 1.48 to correct for CF,HC1 quenching of the A(0,2,0) state of CF,.
TABLE 11: SiF, Pressure Dependence of the SiF,-Sensitized Decomposition of CF,HCl at 1025 cm-I CF, production CF,HCl SiF, co2 fluence,a rate, CF2Jn€LXlC CF2m€LX/SiF4 press., torr press., torr energy, mJ J/cmZ molecules/(cm3s ) molecules/cm3 1.0 1.0 1.0 1.0
0.50 1.0 3.0 5.0
167 182 186 176
84 92 94 89
1.3 X 10'" 2.1 x lo2" 2.9 X 10," 2.5 X 10"
1.0 1.0 1.0 1.0
0.50 1.0 3.0 5.0
84 83 85 88
27 27 28 28
1.4 X 1.8 X 2.3 X 3.1 X
10," 10'" 10'"
10,"
1.0x 1.1x 6.8 X 4.0 x 7.0 x 8.5 x 8.6 X 1.2 x
loi3 1013
6.4 x 3.3 x 7.0 x 2.4 x
1013 1013 10'' 1014
4.3 x 10-3 2.6 x 10-3 8.8 x io-, 7.3 x lo-,
1014 1014
10-3 10-3 10-4
10-4
The focused beam radius was determined by fitting the intensity to the equation IIR = IIR"exp(-rz/RIRZ). The radius The maximum CF, yield in the The slope of CF, vs. time. was very sensitive to the adjustment of the output coupler. The fraction of electronicallv excited SiF, molecules which reacted, assuming CF2 was only profirst 2 us of reaction. duced by SiF,*. a
This electronic spectrum is characterized by lo-ng progressions in the u2 mode. The X(O,O,O), X(O,J,O), X(0,2,0), and X(0,3,0) states were all excited to the A(0,2,0) state a t 261.7, 266.4, 271.1, and 276.1 -nm, respeztively. Fluorescence was observed from the 40,2,0) X(0,2,0) tranjition, except in the case of the X(0,2,0) state where the A(0,2,0) X(O,l,O) transition was monitored. Relative Franck-Condon factors were-determined from the intensities of transitions from the A(0,2,0) state to the first four vibrational levels of the ground state divided by the appropriate frequencies raised to the fourth power. The wavelength dependence of the spectrometer and PMT were measured with a calibrated D2 source. The CF2LIF signal was related to the CF2concentration indirectly. IR radiation (R26) was passed through an ~7-mm circular aperture and then focused by a 50-cm Ge meniscus lens. The beam diameter was essentially constant over a length greater than 10 cm. In the flow cell containing 1.0 torr of CF2HCl the maximum CF2 LIF signal, corresponding to excitation of the X(O,O,O) state, was determined. A measurement of the vibrational temperature of CF2 at the same deJay time determined the fraction of CF2molecules in the X(O,O,O) state. In a second experiment a static cell of known volume and 10 cm in length was filled with 1.0 torr of CF2HCl and irradiated. With IR spectroscopy a plot of In PIP, vs. the number of laser shots was linear. The slope, multiplied by the ratio of the total cell volume to the irradiated volume, was equal to the fraction of molecules decomposed per p ~ 1 s e . l A ~ calibration factor was obtained by dividing the LIF signal by the fraction of molecules in the X(O,O,O) state and equating this number with the CF2HClconcentration (3.26 X lo1, molecules/cm3) times the fraction dissociated. The SF,-sensitized decomposition of CF2HC1was investigated briefly. A static cell with the same dimensions as the flow cell was filled with 1.0 torr of SF, and 1.0 torr
-
-
of CF2HC1. SF6was irradiated with the P20 line of the 10-pm band at 944 cm-'. Because of the very long lifetime of CF2in the cell, the C02laser repetition rate was limited to 0.05 Hz to allow sufficient time for the CF2 radicals to fully recombine.
Results Absolute CF2 yields were determined by LIF as a function of the delay time between the C02 pump laser and the doubled dye probe laser. In Figure 1, an oscilloscope trace is shown of the COPlaser pulse (tuned to the P42 line and detected by a fast pyroelectric detector) added to the LIF signal (detected by a PMT) at a delay of 190 ns. Boxcar averaging of the CF2 LIF signals was used to generate temporal CF2 curves for the following parameters: SiF4pressure; CF2HClpressure; and fluence. Typical data are shown in Figure 2. From the temporal CF2 curves, the slope of the CF2 yield vs. time was determined as well as the maximum CF2 yield. For each electronically excited SiF4molecule which reacts, one CF2 radical will be formed. The fraction of electronically excited SiF4molecules which react will be equal to or less than the maximum CF2yield divided by the SiF4pressure. (We argue in the discussion that electronic energy transfer from SiF4* is the dominant mechanism for CF2 production.) The above data and calculated values are given in Table I as a function of the CF2HClpressure and in Table I1 as a function of the SiFa pressure. The data in Tables I and 11were obtained by pumping SiF4with the 9-pm P42 line of the C02 laser at 1025 cm-'. With only CF2HCl present, no CF2was detected with the P42 line. In Table 111, the results of directly pumping CF2HC1, with and without SiF4, with the 9-pm R26 line of the C02laser at 1082 cm-' are presented. While investigating the fluence dependence of the CF2 yield from the SiF4-sensitized decomposition of CF2HC1,
The Joumal of Byslcai Chemistry, Vd. 87, No. 18, 1983
5566
Kramer
TABLE 111: Effect of SiF, on the Direct Multiphoton Decomposition of CF,HCI at 1082 cm-* CF, producCF,HCl SiF, CO, tion rate, press., press., energy, fluence,= molecules/ torr torr mJ J/cm2 (cm's) k,C s-' 1.0 0 190 90 1.4 x io= 4.4 x 104 1.0 1.0 189 89 1.8 X 10" 5.5 X lo4 1.0 3.0 180 85 2.7 X 10,' 8.4 X lo4 1.0 0 92 43 1.5 X lo2' 4.6 X lo4 1.0 1.0 88 42 1.5 X 10,' 4.6 X lo4 1.0 3.0 98 46 2.4 X loa 7.2 X lo4 The focused beam radius was determined by fitting the intensity to the equation Im = I~R' exp (-r2/Rm2). The slope of CF, vs. time. k is defined as the CF, production rate divided by the CF,HCl pressure. Flgure 1. The positive COP laser pulse, detected with a fast pyroelectric detector, was added to the negatlve CF2 LIF signal, detected with a PMT, in a single sweep. The 190ns delay between the start of the C02 laser pulse and the LIF signal was determined by the delay generator. Noise 90 ns after the start of the C02laser pulse is pkk-up from the firing of the excimer laser. 28
I
. . c 4-
.
C '
2-:
ported in the literature?,' The continuum radiation has been ascribed to an excited electronic state of SiF,, while the peak at 440 nm has been assigned to SiF emission. The induction times for luminescence and detection of CF2 were compared on a single shot basis to eliminate jitter. The COPlaser pulse was displayed on an oscilloscope, and either the broad-band luminescence or the LIF signal were added to the COPlaser. The delay generator was adjusted to provide the minimum delay which still produced an observable LIF signal. The induction time for LIF was always longer (11kcal/mol of SiF, be transferred per collision. The induction time for appearance of CF2 at 0.5, 1.0, and 3.0 torr of CFzHC1,all with 1.0 torr of SiF,, is essentially the same. The calculated temperature rise for all three CF2HCl pressures is 1020 f 20 K and yet, because of the increased heat capacity of the system with 3 torr of CF2HC1,64 kcal/mol of SiF, must be transferred in the same time period to raise the temperature 1000 K. For three collisions, this would correspond to >20 kcal/mol per collision. The similar induction times at the three different CFzHCl pressures and the magnitude of energy transfer required per collision are both incompatible with a thermal reaction. Vibrational energy transfer data from molecules in highly excited vibrational levels is primarily derived from chemical activation and thermal decomposition experim e n t ~ .The ~ ~ vibrational levels attained in the chemical and thermal experiments most closely resemble the vibrational levels reached by IR multiphoton pumping, although the distributions may differ. Molecules excited above the dissociation limt, either by chemical reaction or heat, react in a unimolecular reaction. From the competition between reaction and stabilization by collisions with inert gas molecules, the average energy transferred, which depends on the inert gas collision partner, ranged from 0.5 to 12-15 kcal/mol. The low average energy transferred corresponded to atomic collision partners and the higher values to polyatomics. Although these experimentS do not measure the internal energy of the quenching partner, theoretical models are consistent with most of the vibrational energy being transferred as thermal energy.27 More recent experiments with vibrationally excited azulene have shown an N 1.5% yield of vibrationally excited COz by V -, V energy transfer.2s The largest average energy transferred per collision in chemical activation experiments is 12-15 kcal/mol. Even if we assume that all this vibrational energy goes into heat, the magnitude is insufficient to account for the observed induction times. We attempted to investigate V -,V energy transfer by determining the temporal evolution of CF2 using SF6as a sensitizer. SF6has no known electronic state below the SF,-F dissociation energy of 91 kcal/mol and, therefore, electronic energy transfer from SF, would be impossible. CFz was observed with SF6 as a sensitizer, but the CF2 temporal dependence differed from SiF,. Using chemiluminescence from SF,/CH, mixtures we have been able to relate the formation of CFz to the production of F atoms from multiphoton decomposition of SF,. At our lower limit for detection of CFz, F atoms were present. With increasing C02 laser energy a correlation was observed between growth in the F atom concentration and growth in the CFz yield. We would expect V V energy transfer from SF6to CF2HClto be most important at a laser energy (density) just below the threshold for multiphoton decomposition, since decomposition will dissipate the vibrational energy. As the COz laser energy (density) is raised above threshold the probability of multiphoton
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(26) 'JANAF Thermochemical Tables", 2nd ed, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 37 (1971). (27) D. C . Tardy and B. S. Rabinovitch, Chem. Reu., 77, 369 (1977). (28) J. R. Barker, M. J. Rossi, and J. R. Pladziewicz, Chem. Phys. Lett., 90,99 (1982).
3571
decomposition increases and the vibrational energy stored in SF6should decrease. Based on the above arguments and experimental results, the major contribution to the formation of CFzusing SF6as a sensitizer during the first 2 p s of reaction should be from F atom attack on CF2HC1, rather than V -,V energy transfer. At low levels of vibrational excitation, current theories of V -,V energy transfer predict a probability of transfer for resonant single quantum transition^.^^ The of N probability decreases as a function of energy mismatch, and for each additional quantum change, the probability is lower. For those molecules which interact via long-range dipole-dipole interaction, the probability of a single quantum energy transfer approaches 1 and again falls off with energy mismatch. The vibrational frequency of the pumped SiF4 mode at 1031 cm-l lies 77 cm-l below the 1108-cm-' band of CFzHC1. The closest resonance is between the 391-cm-' band of SiF, and the 400-cm-' band of CFzHC1.26Even if we assume that the largest single SiF, vibrational quantum at 1031 cm-' ( - 3 kcal/mol) were transferred to CF2HClwith unit efficiency, at least 18 such collisions would be required for the internal energy of CFzHCl to exceed the activation energy for dissociation. Thus, we conclude that electronic energy transfer from SiF4*provides a much better explanation for our results than V -,V or V T energy transfer. Another possible explanation for our results is that SiF, collisionally excites CFzHCl into the quasicontinuum by V V processes and that the tail of the C02 laser pumps CFzHC1to dissociative levels. Were this mechanism to be operative, we would expect that the time to maximum CF2 would be determined by the length of the C02laser tail. However, with increasing SiF4pressure, the time to maximum CF2 gets shorter. Even if the formation of SiF* decreased the number of vibrationally excited SiF, molecules, the yield of CFz might decrease, but not the time to maximum CF2yield if the C02laser tail dissociated the CF2HC1. Thus, we conclude that electronic energy transfer from SiF4*is still the mechanism most consistent with our experimental results. A mechanism involving electronic energy transfer in the SiF4-sensitizeddecomposition of CF2HClcomplements the accepted mechanism of V T energy transfer. At delay times greater than approximately 10 p s , a second component in the CFz signal is present which increases rapidly with SiF, pressure. We attribute this component to thermal dissociation of CFzHCl following V T energy transfer. Vibrational energy is initially retained in those vibrationally excited SiF4molecules which do not relax into the electronic state. In addition, broad-band luminescence from electronically excited SiF4* leaves SiF4 in excited vibrational levels. At sensitizer pressures of 2 5 torr typically used in sensitized reactions and the higher acceptor pressures, many collisions during the COz laser pulse decrease the population of highly excited vibrational levels which decreases the occurrence of inverse electronic relaxation. At higher sensitizer and acceptor pressures, the thermal reaction will dominate the electronic reaction in magnitude, and the thermal induction time will become much shorter. In addition, sensitized reactions are often carried out at lower fluences by unfocused COz laser beams, which also decreases the occurrence of inverse electronic relaxation. Other Systems. Differences in the distribution of chloropropene isomers have been observed in the focused vs. unfocused SiF,-sensitized decomposition of 1,2-di-
-
-
-
-
(29) J. T. Yardley, "Introduction t o Molecular Energy Transfer",Academic Press, New York, 1980, Chapter 5.
3572
J. Phys. Chem. 1983, 87, 3572-3578
c h l o r ~ p r o p a n e .The ~ ~ ratio of isomers was the same for unsensitized focused decomposition at a different COz laser wavelength and the focused SiF,-sensitized decomposition. The ratios of the reaction rates for formation of the different isomers were calculated as a function of energy (photolytic mechanism) and temperature (thermal mechanism). The focused SiF,-sensitized ratios (and the focused unsensitized ratios) were consistent with 1,2-dichloropropane reacting with an energy content of 117 kcal/mol or at a temperature of 2000 K. From arguments about the calculated reaction time, the photolytic mechanism was preferred. Similarly, the ratios of isomers with unfocused SiF,-sensitized decompositon was attributed to a thermal reaction at 1100 K. For the focused SiF,-sensitized decomposition, a mechanism of vibrational excitation of 1,2-dichloropropane into the quasicontinuum by SiF, followed by C 0 2laser pumping was suggested. Based on the present work, an alternative mechanism would be electronic energy transfer from an excited electronic state of SiF,. It is surprising, however, that with focused radiation the isomer ratios do not change with pressure up to 32 torr at a SiF4/1,2-dichloropropaneratio of 2.
The sensitization of acceptor molecules by IR multiphoton pumped SiF4in an excited electronic state should not be unique to SiF,. Inverse electronic relaxation has been observed in a number of IR multiphoton pumped molecules. The possibility of other molecules acting as electronic state sensitizers will depend on the energy and lifetime of the electronic state. The photophysical properties of BC13bear a strong resemblence to SiF4.31 BC13 exhibits a prompt broad-band luminescence from 440 to 660 nm. As with SiF,, the emission does not correspond to any known transition of the parent molecule or its fragments and could well arise from a Franck-Condon shifted electronic state. Collisionally induced emission from an excited electronic state of BC1 is observed in BCl, comparable to the SiF emission in SiF,.
(30) W. Tsang, J. A. Walker, and W. Braun, J. Phys. Chem., 86,719 (1982).
(31) V. N. Bourimov, V. S. Letokhov, and E. A. Ryabov, J . Photochem., 5 , 49 (1976).
Kinetics of CI(,P,) and CH,CICHCI,
Acknowledgment. I thank Sherman McCutcheon, 111, for his excellent technical assistance and Dr. Wayne Sharfin for his help in setting up the initial experiments. I also thank one of the referees for suggesting SF, as a sensitizer. Registry NO.CF2,2154-59-8;SiF4,7783-61-1;CFZHCl, 75-45-6.
Reactions with the Chloroethanes CH3CH,CI, CH,CHCI,, CH2CICH2CI,
P. H. Wine* and D. H. Semmes Molecular Sciences Group, Engineering Experiment Station, Georgia Instnute of Technology,Atlanta, Georgla 30332 (Received: December 10, 1982)
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The kinetics of the reactions Cl('PJ) + RHCl RC1+ HCl were investigated over the temperature range 257426 K for RHCl = CH3CH2C1( k 2 ) ,CH3CHC12(k3),CHzCICHzCl(k,), and CHzCICHClz(k5). c1(2PJ)was produced by 355-nm pulsed laser photolysis of Clzand monitored by time-resolved resonance fluorescence spectroscopy. The data are adequately described by the following Arrhenius expressions (units are cm3molecule-' s-l, errors are 2u and refer to precision only): k z = (2.34 f 0.42) X lo-" exp[-(310 f 56)/T], k3 = (8.19 f 1.84) x exp[-(554 f 71)/T], k4 = (2.21 f 0.51) X lo-" exp[-(793 f 73)/T], and k5 = (4.88 f 1.41) X exp[-(786 f 88)/77. Under some experimental conditions evidence for Cl('PJ) regeneration via a secondary reaction was observed. At 258 f 1 K, deviations of Cl('PJ) temporal profiles from first-order behavior were attributable to the reactions RC1+ clz RClz + c1(2PJ)(k,). By modeling the observed Cl('PJ) temporal profiles, we found the rate constants kj to lie in the range (5-12) X lo-', cm3molecule-' s-l for all RC1 investigated. The reactivity trends observed in reactions of Cl('PJ) with C2H,C16,, x = 3-6, are discussed.
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Introduction Reactions involving abstraction of a hydrogen atom by ground-state atomic chlorine, Cl('PJ), have been studied extensively by kineticists for many years. Early work, which employed end product analysis techniques to determine relative rate coefficients for reactions of Cl('PJ) with hydrogen, alkanes, and chlorinated alkanes, was motivated by the desire to test the ability of various theories to predict the rate coefficients for a series of related reactions.'V2 In recent years, the controversy concerning
the extent of chlorine-catalyzed destruction of stratospheric ozone3 has led to renewed interest in reactions which convert c1(2PJ) into the relatively stable reservoir species HC1; this has motivated the application of modern "direct" kinetic techniques in numerous investigations of c1(2PJ) reactions with hydrogen-containing atmospheric constituents such as Hz, hydrocarbons, H02, and H2COe4 A number of Cl('PJ) + RH reactions have also been investigated as initiation reactions in model systems for (3) F. S. Rowland and M. J. Molina, Reu. Geophys. Space Phys., 13,
(1) G. C. Fettis and J. H. Knox, Prog. React. Kinet., 2 , l (1964), and references therein. (2) H. S. Johnston and P. Goldfinger, J.Chem. Phys., 37, 700 (1962). 0022-365418312087-3572$01.50/0
1 (1975).
(4) "Chemical Kinetic and Photochemical Data for Use in Stratospheric Modelling", Evaluation No. 5, J P L Publication 82-57, Jet Propulsion Laboratory, Pasadena, CA, 1982, and references therein.
0 1983 American Chemical Society
