H+H2O reaction dynamics: state distribution for the OH product

Abstract

A method has been developed to study the dynamics of high barrier reactions by combining hot-atom production by laser photolysis and fast-product detection by laser induced fluorescence. The nascent OH rotational, vibrational and fine structure state distributions produced in the endothermic reaction\(H + H_2 O \to \underline {OH(K,\upsilon ,f)} + H_2\) have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. OH rotational excitation is low and corresponds to a partitioning of only around 3% of the total reaction energy to OH rotation. OH could only be observed in the vibrational ground state, i.e.σ(v=1)/σ(v=0)≦0.1. The OH spin doublets are produced statistically, but the partitioning in the λ-doublets is very non-statistical with a strong preference for the π+-component (σ(π+)/σ(π−)=3.2±1.0) demonstrating the exit channel of the reaction to be preferentially planar. By measuring reactant and product densities at short times, an absolute total cross-section of 0.24±0.1[A2] has been obtained.