The H+OCS hot atom reaction: CO state distributions and translational energy from time-resolved infrared absorption spectroscopy

Abstract

Time-resolved infrared diode laser spectroscopy has been used to probe CO internal and translational excitation from the reaction of hot H atoms with OCS. Product distributions should be strongly biased toward the maximum 1.4 eV collision energy obtained from 278 nm pulsed photolysis of HI. Rotations and vibrations are both colder than predicted by statistical density of states theory, as evidenced by large positive surprisal parameters. The bias against rotation is stronger than that against vibration, with measurable population as high as v=4. The average CO internal excitation is 1920 cm−1, accounting for only 13% of the available energy. Of the energy balance, time-resolved sub-Doppler line shape measurements show that more than 38% appears as relative translation of the separating CO and SH fragments. Studies of the relaxation kinetics indicate that some rotational energy transfer occurs on the time scale of our measurements, but the distributions do not relax sufficiently to alter our conclusions. Vibrational distributions are nascent, though vibrational relaxation of excited CO is unusually fast in the OCS bath, with rates approaching 3% of gas kinetic for v=1. At 9720 cm−1 above the activation barrier, energy partitioning is consistent with incomplete IVR in an HSCO intermediate, followed by repulsive release of a large fraction of the available energy into relative translation.