2301
Communications to the Editor '
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Flgure 3. Time-resolved iodine laser pulses.
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Flgure 1. Dependence of the absorption of initial laser energy and temperature inhomogeneity in the measuring cell on the product of absorption coefficient (p) and thickness of the heated layer ( d ) with the laser beam passing twice through the cell. os 2 , I!(1 t o r
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Flgure 4. Relaxation signal of a glycine buffered aqueous solution of phenolphthalein at pH 9.6, temperature 298 K registered with a Bomation 8100. (X) computed points for the relaxation trace.
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Flgure 2. Experimental arrangement of the iodine laser T-jump: M, mirrors; AOM, acousto-optical mode locker; G, Glanprism; P, Pockels cell; SG, spark gap, MC, measuring cell; L, lenses; MO, monochromator; PM, photomultiplier: AM, amplifier; OSC, oscilloscope; and REU, registration unit.
6% at an absorption of 49% laser energy if a single reflection is allowed and the water layer is 0.2 cm thick. The experimental arrangement is shown schematically in Figure 2. The laser can be operated in two different ways. As an oscillator-amplifier chain with two amplification stages using acousto-optical mode locking for obtaining a train of short pulses and a Pockels cell for single pulse selection. This system produces 1-3 J in 3 ns. If the amplifiers are operated separately as oscillators in the multimode version they produce 1-30 J in about 3 ps. Higher energies could be achieved if the system is scaled up.6 Figure 3 shows time-resolved measurements of the laser pulses from the oscillator-amplifier chain in the TEM,, mode and from an oscillator in the multimode operation obtained with a valvo XA 1003 vacuum photodiode on a Tektronix 7904 oscilloscope and on a Biomation 8100, respectively. This versatile iodine laser being part of a temperature-jump system has been tested by measuring the protonation of tropaeolin 0 and phenolphthalein in aqueous solution.' As an example Figure 4 shows the relaxation signal obtained from phenolphthalein with a single heating pulse. Three advantages of this kind of laser T-jump technique can be stated from our results. There is enough energy available for T-jumps of some Kelvin with characteristic time constants of 2 ps or 2 ns in a volume of 0.1 mL containing solvents such as H20, D20, and alcohols. No additives are required to assist in heat
transfer. Shock waves and different relaxation amplitudes inside the measuring cell can be avoided so that the observation of relaxation times from nanoseconds to seconds at different temperatures is possible. Acknowledgment. The authors thank Dr. S. Witkowski and Professor Dr. H. Gerischer for the encouraging support of this work and the Deutsche Forschungsgemeinschaft for a research grant. References and Notes (1) E. F. Caklin, Chem. Br., 11, 4 (1975); G. 0.Hammes in "Techniques of Chemistry", Vol. VI, Part 11, A. Welssberger, Ed., Interscience,
New York, N.Y., 1974. (2) E. M. Eyring and B. C. Bennion, Annu. Rev. Phys. Chem., 19, 129 (1968). (3) D. M. Goodall and R. C. Qeenhow, Chem. Phys. Lett., 9, 583 (1971). (4) J. V. Beitz, G. W. Flynn, D. H. Turner, and N. Sutln, J . Am. Chem. SOC.,92, 4130 (1970). (5) D. H. Turner, G. W. Flynn, N. Sutin, and J. V. Beltz, J . Am. Chem. Soc., 94, 1154 (1972); G. W. Flynn and N. Sutin in "Chemical and Biochemical Appllcations of Lasers", Vol. I, C. B. Moore, Ed., Academic Press, New York, N.Y., 1974, Chapter 10. (6) K. Hohla, 0.Brederlow, W. Fuss, K. L. Kompa, J. Raeder, R. Volk, S. Witkowski, and K.J. Witte, J . Appl. Phys., 46, 808 (1975). (7) M. C. Rose and J. Stuehr, J . Am. Chem. Soc., 90, 7205 (1968). Fritz-Haber-Ins titut der Max-Planck-Gesellschaft D-1000 Berlin 33, West Germany Max-Planck-Institut, fu? Plasmaphysik, 0-8046 Garching Projektgruppe fur Laserforschung D- 1000 Berlin 33, West Germany
J. F. Holzwarth" A. Schmldt H. Wolff R. Volk
Received July 1 1, 1977
Laser Induced Decomposition of Fluoroethanes Pubiication costs assisted by Kansas State Unlversity
Sir: The use of intense infrared radiation to promote selective chemical proce~sesl-~ has led us to examine the high-power, pulsed C 0 2 laser induced unimolecular deThe Journal of Physical Chemistty, Voi. 81, No. 24, 1977
2302
Communications to the Editor
composition of fluoroethanes. We previously have studied the unimolecular decomposition of these molecules by chemical activation technique^.^ These molecules are well suited to C02laser photolysis since the characteristic C-F stretching frequency overlaps the OOol-lOoO and 00°1-0200 C02 transitions. The excitation process must be multiphoton since the threshold energies5 for HF elimination from fluoroethanes are -60 kcal mol-l vs. the 3 kcal mol-l COB photon energy. This multiphoton requirement raises the problem of absorption and retention of this energy during the 150-11s laser pulse while the molecules are simultaneously undergoing approximately 107/Torr collisions per second with other molecules. The multiphoton absorption mechanism, which is not well understood,6 will be accepted as fact7 and the major concern of the present investigation is the demonstration that multiphoton laser photolysis can initiate the HF elimination reaction without a significant contribution from intermolecular transfer of the initially absorbed energy, which ultimately leads to bulk heating of the reaction mixture. The latter possibility has several different regimes of energy relaxation; in this work identification of the different regimes is not attempted and reactions resulting from intermolecular transfer of energy will be termed a “thermal” component. A means of monitoring a “thermal” component is the addition of a second molecule to the reaction mixture which itself does not absorb the laser energy sufficiently to react but which does possess a unimolecular reaction pathway with a threshold energy comparable to that for HF elimination. The decomposition of the monitor molecule, relative to that for the fluoroethanes, provides a measure of the extent of the “thermal” reaction. The excitation of a molecule possessing two distinctly different decomposition pathways allows an examination of the question of the intramolecular selectivity of the reactions initiated by laser excitation. In the ideal situation, the energy absorption gives a localized energy distribution close to one reaction path of the two competing processes. In the present work we have studied C2H5F,CH3CF3,and CH2FCH2Br. In principle, the first two molecules offer competing channels of HF elimination vs. C-C bond rupture. However, the threshold energy for C-C bond rupture is 25 kcal mol-l higher than for HF elimination and a strong discrimination would result just because of the minimum energy requirement. The CH2FCH2Br molecule is much better because the threshold energies for the HF and HBr elimination pathways differ5by only -6 kcal mol-l. We wish to report experiments for which intermolecular energy relaxation was minimized and the HF elimination reactions of CzHBFand CH3CF3are truly laser induced: N
CH,CF, t mhv
+
CH,CF, t HF
kuni = 10’‘‘ exp(-68700/RT)
CH,CH,F t rnhv .+ CH,CH, t H F kuni = 1013*4 exp(- 59900/RT)
After finding proper conditions for these reactions, CH2FCHzBrwas examined to ascertain whether or not the absorbed energy was statistically distributed prior to decomposition: CH,FCH,Br t mhv
--f
-+
HF t CH,CHBr HBr t CH,CHF
The experiments were done by placing small cells, fitted with NaCl windows, at various positions in the path of the TEA Lumonics-103 laser beam, which was mildly focused with a 40-cm focal length lens. After a small number of irradiations ( 10) the contents of the cell were analyzed N
The Journal of Physical Chemlstv, Vol. 81, No. 24, 1977
“i 0’ z a‘
4
”
0.5
p
:
1.0
2
3
5
10
20
P/torr Figure 1. Pressure dependence of the “thermal” and laser induced reactions. The ordinate, P/M, is the ratio of the yield of the pumped molecule to the yield of monitor molecule. All of these experiments were done by placing the cell 25 cm behind the focusing lens; the energy denslty was 2.5 J/crn2 for photolysis with the R(16) line of the 00’1-10’0 transition: (0) CHBCFBwith 25% cyclopropane; (H) CH3CF, with 50% CH3CHPF.
by gas chromtography with a H2-flamedetector. Care was taken to ensure that starting materials were pure. Further experimental conditions are cited in figure captions and elsewhere.Ea The extent and pressure dependence of the “thermal” contribution was monitored by adding either CH3CH2For cyclopropane to the cell for CH3CF3(irradiated with the R(16) line of the 00’1-10’0 band) and by adding CH3CF3 to C2H5F(irradiated with the P(18) line of the 00°1-0200 band). If intermolecular energy transfer is extensive, formation of CzH4 or C3H6will be observed from irradiation of CH3CF3 in the presence of fluoroethane or cyclopropane. Similarly, CH3CF3 will give C2H2F2if the “thermal” contribution is significant for irradiation of C2HBF. All monitor molecules were shown not to react when irradiated alone under the conditions used for Figure 1. Cyclopropane gave no reactionEbupon irradiation with either of the lines mentioned above, but CH3CF3was the preferred monitor for CzH5F because of ease of analysis. The pressure dependence of the competition between the laser and “thermal” components is illustrated for CH3CF3 in Figure 1. For pressures k l Torr, intermolecular energy transfer is sufficiently rapid that “thermal” decomposition of the monitoring molecule is significant. However, the rapidly diminishing yields of either propene or ethene for pressures 51 Torr shows that CH3CF3elimination can be directly induced by absorption of laser energy. At higher pressures the specific laser decomposition of CH3CF3may be augmented by “thermal” processes. Since the monitoring reactions have lower threshold energies than CH3CF3,the “thermally” induced reactions of C2H5Fand cyclopropane will occur to a somewhat greater extent than for CH3CF3. The results for photolysis of CH3CHzFwith 12% added CH3CF3as a thermal monitor are qualitatively similar to those shown in Figure 1. The dependence of percent conversion per flash (expressed in terms of volume of sample actually irradiated) on the incident laser energy was investigated for CzHBF and CH3CF3at a total pressure of 0.6 Torr; the relative concentrations of the monitoring molecules are given in the figure caption. The incident energy was varied by attenuating the laser beam with layers of poly(viny1 chloride) film without changing the focusing optics. The extent of reaction varied linearly with the log of the energy d e n ~ i t ythe ; ~ slopes of the lines in Figure 2 are 6 f 1and 9 f 1 for CH3CF3and C2H5F,respectively. At the lower energy densities, contribution from the “thermal” com-
Communications to the Editor
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2303
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Log(p(E) joule/cm2) Figure 2. Dependence of the conversion per flash (in the Irradiated volume) upon the lncdenf laser energy density: (0)pumping CH3CH2F with the P(18) line of the 00’1-02’0 transition with 12% CH3CF3;(W) pumping CH3CF3with the R(16) line of the OOo-lOrrO transition with 50% CH3CH2F;(A)pumping CH3CF, with 25% cyclopropane. The slope of the lines drawn through the data points are 9 f 1 for CPHSF and 6 f 1 for CH3CF3. The total pressure for these experiments was 0.6 Torr.
ponent was negligible; however, at the maximum energy a minor “thermal” contribution was identified by the monitoring molecules. Thus, the slopes are upper limits to the true values for the laser induced reaction. Even at the highest energy, HF eliminationlo was still the only reaction observed for C2HsFor CH3CF3. Based upon the sensitivity of our detection system, the practical threshold for observation of reaction was 2 J/cm2. Variation of the wavelength of irradiation by selection of different C02lines only showed effects consistent with the change in energy density illustrated in Figure 2. The question of intramolecular relaxation of the absorbed laser energy was studied by irradiating bromofluoroethane with the P(36) line of the 00’1-02’0 band. If intramolecular energy randomization were minimal, excitation by absorption of laser energy would be expected to excite the molecule to favor HF elimination. In contrast the RRKM statistical4 yields would greatly favor HBr because of the lower threshold energy. Experiments at 0.6 Torr using 3.1 J/cm2 showed a pronounced favoring for HBr elimination over HF elimination; the measured ratio was bout 101. This ratio is consistent with an RRKM calculationll if the mean excitation energy in the CH2FCH2Br molecule was 75 kcal mol-’. If randomization has occurred, the RRKM calculated k~ values also can be used to estimate the mean energy in CH3CH2Fand CH3CF3. At 1 Torr the collision rate is lo7 s-l which can be taken as the lower limit to the rate For this limit the molecules must have -70 and -85 kcal mol-’ for C2H5F and CH3CF3,respectively?* Providing that these reactions occur via a RRKM mechanism, these molecules evidently acquire energy from the laser field in significant excess of the threshold energy. Acknowledgment. We thank Dr. Wayne C. Danen (Kansas State University) for useful suggestions and discussions. This work was supported by the National Science Foundation (MPS 75-02793).
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References and Notes (1) R. V. Ambartzumian and V. S. Letokov, Acc. Chem. Res., 10, 61 (1977). (2) D. F. Dever and E. Grunwald, J. Am. Chem. Soc., 98, 5055 (1976). (3) (a) I. Glatt and A. Yogev, J. Am. Chem. Soc., 98, 7087 (1976); (b) A. Yogev and R. M. J. Benmair, Chem. Phys. Left., 46, 290 (1977). (4) (a) H. W. Chang, N. L. Craig, and D. W. Setser, J . Phys. Chem., 76, 954 1972); (b) K. C. Kim, D. W. Setser, and B. E. Holmes, ibld.,
77, 725 (1973); (c) P. J. Marcoux, E. E. Seifert, and D. W. Setser, Int. J. Chem. Kinet., 7, 473 (1975); J. Phys. Chem., in press (1977). (a) E. Tschulkow-Roux and W. J. Qulring, J. Phys. Chem., 75, 295 (1971); (b) P. Cadman, M. Day, A. W. Kirk, and A. F. TrotmanDickenson, Chem. Commun., 203 (1970); (c) S.W. Benson and H. E. O’Neal, “Kinetic Data on Gas Phase Unimolecular Reactions”, National Bureau of Standards, Washington, D.C., 1970, p 89. (a) D. S.Frankel. J. Chem. Phvs.. 65. 1696 (1976) (b) S.Makamel and J. Jortner, lbM., 65, 5204i1976);‘ (c) M.‘F. ododman, J. Stone, and D. A. Dows, ibid., 65, 5052, 5062 (1976). M. J. Cosalola, P. A. Schulz, Y. T. Lee, and Y. R. Shen, Phys. Rev. Lett., 38,-17 (1977). (a) W. C. Danen, W. D. Munslow, and D. W. Setser, J . Am. Chem. Soc., 99, 6961 (1977). (b) Our finding of no laser induced decomposition for cyclopropane differs from reports in the literature, E. Grunwald, Chem. Eng. News, 54 (48), 18 (1976); M. L. LeSieckl and W. A. Guillory, J . Chem. Phys., 66, 4317 (1977), presumably because of our less severe focusing conditions. M. C. Gower and K. W. Billman, Appi. Phys. Lett., 30, 514 (1977); Opt. Commun., 20, 123 (1977). W. Braum and W. Tsang, Chem. Phys. Lett., 44,354 (1976). These authors observed mainly elimination reactions from alkyl chlorides and bromaes, but for the ioddes radicals were formed from C-I bond rupture. T. H. Richardson, unpublishedresults; the threshold energles used in the calculatlon were 60 and 54 kcai/mol for HF and HBr eiiminatlon, respectively. Depattment of Chemistw Kansas State University Manhattan, Kansas 66506
T.
H. Rlchardson D. W. Setser’
Received July 11, 1977
Kinetics of the CI 4- C2H2Reaction. Stratospheric Implication Publicatlon costs assisted by Centre Natlonal de la Recherche Sclentifique, France
Sir: Termination of C1 atom chains in the stratosphere would essentially occur via C1 t CH,
-+
CH, t HCl
(1)
which would result, at least temporarily, in the interruption of ozone depletion in the earth’s strat0sphere.l Another hydrocarbon, acetylene, has been recently suggested by Lee and Rowland2as a species which would also remove C1 atoms. To estimate the effect of C2H2, these authors have studied the kinetics of the C1+ C2H2 reaction by a competitive method. We have also performed in 1972 a kinetic study of this reaction which led to an absolute rate constant determ i n a t i ~ n . This ~ study was made in relation to the inhibition of hydrocarbon flames by chlorinated compounds4 in order to explain the cl emical mechanism of the inhibition. The Cl + C2Hzreaction was studied by the flow discharge-mass spectrometer method. Typical experimental conditions for the rate constant measurements were as follows: pressure, 0.50-2.00 Torr (diluent, helium); temperature, 295-500 K; (C,H,), 0.1-5 X 1014mol cmM3; . rate constant for the (Cl), 1-3 X 1015atoms ~ m - ~The elementary reaction C1 + C,H,
+
products
(2)
was found to be temperature independent in the 295-500 K range: kz = (2.0 f 0.5) X cm3 molecule-1 s-l. The products of the overall reaction were also identified. The main products HC1, C2H2C1,, and C2HCl were simultaneously detected in the flow reactor; when the excess of C1 atoms was increased, other chlorinated compounds were also observed as CzHzC14and C2Cl4. Stoichiometry The Journal of Physical Chemistry, Vol. 81, No. 24, 1977
