A global A-state potential surface for H2O: Influence of excited states on the O(1D)+H2 reaction

Abstract

In this article a global potential energy surface for the 1A′′ state of H2O based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio results is presented. The resulting 1A′′ surface is used in conjunction with a previously determined 1A′ surface to study the O(1D)+H2(HD,D2) reaction dynamics, with emphasis on the influence of the 1A′′ excited state on measurable properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of about 2 kcal/mol on the 1A′′ surface, and even at 5 kcal/mol, the 1A′′ reactive cross section is only a small fraction (∼20%) of the barrierless 1A′. However, the 1A′′ surface populates very specific product vibrational states (v=3–4) and gives strongly backward peaked differential cross sections, so certain types of measurements are quite sensitive to the presence of this excited state. In particular, better agreement is found with experimental vibrational and angular distributions with 1A′′ included, especially at translational energies of 4 kcal/mol and above. A rough estimate of the influence of the 2A′ surface indicates that this state also makes an important contribution to vibrational and angular distributions at high energies. The 1A′′ and 2A′ surfaces also measurably affect the energy dependence of the integral cross sections at energies above 2 kcal/mol, and the temperature dependence of thermal rate coefficients for O(1D)+H2 at temperatures of 1000 K and above.