OH product energy partitioning and absolute reactive cross section of the reaction H + H 2 O → OH + H 2

Abstract

Using 193 nm photolysis of HBr, we have studied the reaction H+H 2 O→OH+H 2 at a center of mass energy of 2.52 eV. Laser induced fluorescence has been used to measure the nascent rotational quantum-state distribution and the translational energy of the OH product molecules. Within the experimental uncertainty no vibrational excitation of the OH products could be found ( v =1/ x =0<0.1). The degree of OH( v =0) rotational excitation is also very low. We found that only 4% of the total available reaction energy is channeled into OH rotation. The results of sub-Doppler measurements of the OH line profiles show that most of the available energy in the reaction ends up as c.m. translational energy of the products (67%). The population of two spin orbit multiplet states shows a slight preference for the low lying 2 Π 3/2 state. The ratio of the lambda doublet state population shows a preference for the (A′) state at high rotational quantum numbers. Together with the very low vibrational and rotational excitation of the OH products, this suggests that many of the reactive collisions are in character planar stripping collisions, with the OH bond acting as a spectator during the reaction. Using the H 2 O 2 photolysis at 193 nm as a reference source of well defined concentrations of OH radicals we determined an absolute reactive cross section: σ(2.52 eV)=(0.26±0.09) 2 . The experimental results show excellent agreement with predictions of dynamic calculations using a ab initio potential energy surface.