It is possible that the smaller ( ) values should be associated with a more complex transition probability model than the stepladder case. However, this is just speculation at this time. To fit the data, large collision diameters (Sam) are required for the C« and C8 fluoroalkanes. Dividing Sam into and t/k can be done in several ways, and we chose to follow the trend suggested by second virial coefficient data.14 Rabinovitch and coworkers13 successfully used a constant t/k value of 160°and an increase in of 0.54 Á per CF2 group to obtain collision diameters for thermal activation studies of perfluoroalkanes and CH3NC. Their collision diameters corresponding to vam [ (2•2)*]'/ are 6.7 and 7.3A for CeFi4 and C8Fi8, respectively. The difference from our choices (8.0 and 8.9 A) could be reduced by using a larger t/k for CF3CH3; however, this would not alleviate our problem of fitting the relative rate constants in the series C^e-CeFis. 963 ConclusionsUnimolecular rate constants for elimination of HF from chemically activated CH3CF3 and CH3CH2F fit the pattern for other chloro-, bromo-, and fluoroethanes. Consequently, we conclude that the elimination reactions at 90-100 kcal mol-1 are adequately described by RRKM theory. The amount of energy removed from CH3CF3* per collision with perfluoroalkane bath gases ranged from 7 to 10 kcal mol-1; the parent molecule also followed a similar collisional deactivation36 pattern and did not show an unusually high efficiency.Acknowledgment. We wish to thank Dr. J. W. Root for sending us the data cited in ref 35 and for giving us permission to quote the results. Mr. E. E. Siefert assisted with some of the energy transfer computations. This work was supported by the National Science Foundation under Grants GP-9245 and -27536X and by the Kansas State University Bureau of General Research.

The photolysis of cyclobutanone has been investigated between 40 and 250°and in the pressure range 5.0-40.0 torr of cyclobutanone. Quantum yields for CO, C2H4, C3H6, and oC3H6 have been determined as a function of temperature, pressure, and inert gas. The yield of C3He was found to increase with decreasing cyclobutanone pressure and with shorter wavelengths, while the yield of c-C3H6 was unaffected. Added gas lowered the C3H6 yield. No effect was noted on the c-C3H6 yield. Isomerization of ci-s-to trans-butene-2 was observed.A mechanism involving formation of C3H6 from an excited triplet cyclobutanone, possibly involving a diradical intermediate, has been proposed.

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The collisional thermalization of hot H atoms produced by the excimer laser photolysis of H2S (193 nm) and HI (193 and 248 nm) has been determined by measuring the decrease in vibrational excitation of CO2 (ν3) as a function of moderator pressure. Cross sections obtained for the rare gases in this way are in reasonable agreement with those calculated for energy loss in hard-sphere collisions. These experiments also indicate that the efficiency of vibrational excitation increases as the relative translational energy increases.

Fluorescence excitation spectra, lifetimes, and quenching of S2(B 3Σ u −) in the wavelength region 280-315 nm J. Chem. Phys. 74, 4951 (1981); 10.1063/1.441748 Fluorescence lifetime studies of NO2. IV. Temperature dependence of fluorescence spectra and of collisional quenching of fluorescence Lifetimes of the B JI';-state of S2 in the presence of various quenching gases have been determined from fluorescence decay measurements made with the single-photon time correlation technique. Fluorescence was excited with a pulsed light source of wavelength 292 nm, and observed at 370 nm. He, Ar, Xe, N" S2' CF 4 , and C 2 F 6 have been used as quenching gases. At high pressures (:::: 100 torr), a single lifetime of ~ 110 nsec is observed, whereas at lower pressures there is a second component with a lifetime of ~ 5-30 nsec. From these lifetimes, bimolecular rate constants and thermally averaged cross sections for collisional quenching have been determined. The results are discussed in terms of a model in which two excited state; of S2 are either coupled with each other or with the ground state.