Coupled channel distorted wave method of atom–molecule reactive scattering: Application to p a r a to o r t h o hydrogen molecule conversion

Abstract

We present a three dimensional coupled channel distorted wave approach of the atom–molecule reactive scattering. The full entrance channel wave functions are obtained from the inelastic vibrational and rotational close-coupling approximation, and reactive cross sections are evaluated with those wave functions employing the transition matrix (T matrix) method. Therefore, in contrast to the previous adiabatic distorted wave model of the reactive scattering, the present method allows for the target molecule to be dynamically distorted following the motion of the incident atom. The formulation of the approach and efficient computational procedures for obtaining the reactive T matrix elements are presented. The reactive scattering cross sections and the rate constants of the H+H2 para to ortho hydrogen molecule conversion have been evaluated to illustrate the computational aspects of the present method. The wave function obtained from the rotational close-coupling approximation yields better results on the reactive cross section than that from the pure vibrational coupling. The wave function computed from the simultaneous vibrational and rotational close-coupling approximation results in the most accurate cross section. This indicates that in order to obtain accurate results with the T matrix method, the simultaneous vibrational and rotational distortions of the target molecule should be considered when approximating the full entrance channel wave function. The differential cross section and the rotational distribution of the product molecule calculated from the present coupled channel distorted wave scheme are qualitatively similar to those obtained from the previous adiabatic distorted wave method; however, the magnitude of the cross sections from the present theory is larger than the previous ones. This might arise from the fact that the vibrational distortion of the target in the previous adiabatic distorted wave model is not sufficiently large. The present approach is shown to account for all major features of the reactive scattering in the H3 system.