Collisional energy transfer in unimolecular reactions induced by vibrational overtone excitation

Abstract

The kinetics of the gas phase isomerisation of cyclobutene initiated by direct single photon excitation of C–H stretching overtones have been studied and the observed behavior has been interpreted using a treatment which combines RRKM theory and models accounting for weak intermolecular energy transfer. Results obtained from the excitation of five different overtone states spanning an energy range of 5070 cm−1 have allowed us to examine the form of the dependence on energy of the energy transfer step sizes 〈ΔE〉 deduced from numerical solutions of the collisional master equation using an exponential-down model, for the collider gases cyclobutene, SF6, CHF3, CH4, CO2, H2, CO, N2, Ar, and He. Our calculations indicate very small energy transfer step sizes, particularly for the most inefficient collider molecules. We discuss the factors which might contribute to the determination of low values for 〈ΔE〉 and the sensitivity of the master equation calculations to details of the RRKM model. Our results, when expressed in terms of absolute changes in the magnitudes of 〈ΔE〉 as a function of energy, appear to be in agreement with the recent direct studies of Hippler et al. [J. Chem. Phys. 83, 3906 (1985)] for a number of collider molecules over the energy range of the present work. Mean collision efficiency parameters calculated directly from the slopes of the Stern–Volmer plots are shown to be linearly correlated with mean values of 〈ΔE〉. In future studies of unimolecular reactions induced by overtone excitation, our results should enable full account of weak collisional energy transfer to be taken, allowing for a more rigorous comparison of experimental energy-specific rate constants with RRKM predictions.