E5lEnT / K FLH, Eq. (2.7), and a strong collision broadening factor Fsc, Eq. (4.1). The detailed dependence of the broadening factor Fsc (ko/k,) on the reduced pressure ko/k, has been expressed by Eq. (6.1) for sufficiently narrow fall-off curves at low temperatures T(where 1 L F:Zt 1 0.4), by Eqs. (6.2) and (6.4) for broader fall-off curves at higher T (where 1 1 FZZ, L 0.2), or by Eqs. (6.3) -(6.6) for still higher T and/or better precisions. The amplitude F:Zt of the broadening factor has been represented by Eqs. (5.1)-(5.8).Concluding this work we demonstrate the results from Eqs.(5.1) -(5.5) and (6.3) -(6.6) by a comparison with RRKM calculations, for various examples from Table l, in Figs. 7 and 8. The agreement in most cases is quite satisfactory. For better precision, detailed statistical calculations are required. It should be finally noted that the even simpler empirical representation of the temperature dependence of the center broadening factor in the form of Eq. (5.9) in F,,,, should not only include F:$ but also the weak collision contribution. Then, the agreement between Eq. (5.9) and results from Fig. 8 generally still improves. This will be demonstrated in part I1 [3]. ReaktionskinetikThe master equation of thermal unimolecular reactions in the fall-off range has been solved for a number of representative molecular systems. (ko/k,) are derived and represented empirically. Weak collision efficiencies / 3, for the low pressure range are calculated for very high temperatures. Combined with earlier representations (part I) of strong collision broadening factors Fsc (ko/km), compact empirical expressions for the rate coefficient in the full fall-off range are proposed. These expressions are useful for data representation and modeling of complex reaction systems which involve isomerization, dissociation and recombination reactions. Weak collision broadening factors Fwc

A quasiclassical trajectory study of energy transfer in benzene-benzene collisions A classical trajectory study of collisional energy transfer in thermal unimolecular reactions Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene ͑HFB͒ molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient ''supercollisions'' are also observed for all substrate-bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi-steady-state in rotational energy after only a few collisions.

Complete and detailed experimental transition probability density functions P(E′,E) have been determined for the first time for collisions between a large, highly vibrationally excited molecule, toluene, and several bath gases. This was achieved by applying the method of kinetically controlled selective ionization (KCSI) (Paper I [J. Chem. Phys. 112, 4076 (2000), preceding article]). An optimum P(E′,E) representation is recommended (monoexponential with a parametric exponent in the argument) which uses only three parameters and features a smooth behavior of all parameters for the entire set of bath gases. In helium, argon, and CO2 the P(E′,E) show relatively increased amplitudes in the wings—large energy gaps |E′−E|—which can also be represented by a biexponential form. The fractional contribution of the second exponent in these biexponentials, which is directly related to the fraction of the so-called “supercollisions,” is found to be very small (<0.1%). For larger colliders the second term disappears completely and the wings of P(E′,E) have an even smaller amplitude than that provided by a monoexponential form. At such low levels, the second exponent is therefore of practically no relevance for the overall energy relaxation rate. All optimized P(E′,E) representations show a marked linear energetic dependence of the (weak) collision parameter α1(E), which also results in an (approximately) linear dependence of 〈ΔE〉 and of the square root of 〈ΔE2〉. The energy transfer parameters presented in this study form a new benchmark class in certainty and accuracy, e.g., with only 2%–7% uncertainty for our 〈ΔE〉 data below 25 000 cm−1. They should also form a reliable testground for future trajectory calculations and theories describing collisional energy transfer of polyatomic molecules.

No abstract

No abstract