A GLOBAL A-STATE POTENTIAL SURFACE FOR H2O - INFLUENCE OF EXCITED-STATES ON THE O(D-1)-2 REACTION(H)

In this article a global potential energy surface for the 1A'' state o f H2O based on application of the reproducing kernel Hilbert space int erpolation method to high quality nb initio results is presented. The resulting 1A'' surface is used in conjunction with a previously determ ined 1A' surface to study the O(D-1) + H-2(HD,D-2) reaction dynamics, with emphasis on the influence of the 1 A '' excited state on measurab le properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of abou t 2 kca/mol on the 1A'' surface, and even at 5 kcal/mol, the 1A '' rea ctive cross section is only a small fraction (similar to 20%) of the b arrierless 1A'. However, the 1A'' surface populates very specific prod uct vibrational states (v'= 3-4) and lives strongly backward peaked di fferential cross sections, so certain types of measurements are quite sensitive to the presence of this excited stare. In particular, better agreement is found with experimental vibrational and angular distribu tions with 1A '' included, especially at translational energies of 4 k cal/mol and above. A rough estimate of the influence of the 2A' surfac e indicates that this state also makes an important contribution to vi brational 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 depe ndence of thermal rate coefficients for O(D-1) + H-2 at temperatures o f 1000 K and above. (C) 1997 American Institute of Physics.