The effect of reagent translation on the reaction dynamics and the absolute reaction cross section of H+H2O→OH+H2

Abstract

With H atoms from ultraviolet laser photolysis of H2S and HI, the influence of the translational excitation of the reagents on the reaction dynamics and the absolute value of the reaction cross section of H+H2O→OH+H2 has been studied in the center of mass (c.m.) energy range from the reaction threshold up to 2.2 eV. To determine the OH product rotational fine-structure distributions, the nascent OH radicals were detected with quantum state resolution by laser-induced fluorescence (LIF). It was found that at all c.m. collision energies, the OH radicals are produced exclusively in the vibrational ground state. The measured OH(v=0) rotational fine-structure distributions can be described by Boltzmann distributions, with rotational temperatures which increase only slightly with increasing collision energy. Near the threshold, the OH fine structure rotational temperatures are almost equal; at higher collision energies, the rotational temperature of the OH(A′) fine structure distribution is about a factor of 1.5 higher than the rotational temperature of the corresponding OH(A″) fine-structure distribution, leading to preferential population of the symmetric Π(A′) state at high rotational quantum numbers. To investigate the influence of the reagents’ translational energy on the reactivity, absolute reaction cross sections were measured at different collision energies. Using a calibration method to measure absolute number densities of nascent OH product radicals under single-collision conditions, the following absolute reaction cross sections were obtained: σR(1.0 eV)=(0.03±0.02) Å2, σR(1.5 eV)=(0.16±0.05) Å2, σR(1.8 eV)=(0.18±0.06) Å2, σR(2.2 eV)=(0.25±0.07) Å2. The experimental absolute reaction cross sections and OH rotational distributions are compared to the results of recent quasiclassical and quantum scattering calculations on an ab initio potential energy surface.