Collisional Energy Transfer. Thermal Unimolecular Systems in the Low-Pressure Region

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Abstract
Collisional activation—deactivation efficiencies, β, for thermal unimolecular reactions in the second‐order region were computed on a stochastic model by use of an iterative procedure. Four assumed collisional transition probability models were used: stepladder, Gaussian, Poisson, and exponential; detailed balance and completeness were observed. The collisional efficiencies increase with increase of average energy removed per collision, 〈ΔE〉, and with decrease in the average excess energy of the molecules, 〈E+〉, above the critical energy for reaction. Efficiency is defined as a product, β=γNγP, and varies with inert gas dilution. Calculations were made for the nitrous oxide, nitryl chloride, methyl isocyanide, cyclopropane, and 1,2‐dimethylcyclopropane systems over a range of temperatures. This provides a large variation in the internal vibrational‐energy densities and critical energies in question. For a particular transition probability model, β may be expressed as a quasiuniversal function of the reduced parameter, E′ = 〈ΔE〉/〈E+〉. Experimental data may thus be readily related to a corresponding value of 〈ΔE〉. The relation between the various probability models is discussed. Various deductions made are reminiscent of those previously encountered in work on chemical activation systems. The range of validity of the conventional strong‐collision assumption for thermal systems is made explicit.