Theoretical investigation of the autoionization process in molecular collision complexes: Computational methods and applications to He*(2(3)S)+H(1(2)S)

The first complete ab initio treatment is applied to the autoionization process in the He*(2s(3)S) + H(1s) collisional complex. The autoionizing resonance state is defined through Feshbach projection based on orbital occupancy, and the corresponding potential is determined from multireference-configuration interaction (MR-CI) calculations with an accuracy of about 10 meV. The energy-dependent coupling with the continuum is derived from a compact (L-2) molecular orbital (MO) without any phase information being lost. This ''Penning MO'' is projected onto the states of the continuum electron for energies that comply with the resonance condition thus providing the l-dependent coupling elements in local approximation. The continuum electron functions are calculated within the static-exchange approximation for up to 25 coupled angular momentum channels. The nuclear dynamics calculation is based on a complex Numerov algorithm and uses a converged set of seven complex coupling matrix elements. Weighting with experimental collision energy distributions finally gives the angle-dependent, as well as the angle-integrated, electron spectra for Penning and associative ionization processes. The results are discussed with respect to previous, either partial or model studies, and are compared with the recent most detailed experimental study of the angular-dependent Penning ionization electron spectra. The close agreement of theory and experiment demonstrates the adequacy of the local complex potential approach, as well as the importance of electron angular momentum transfer so far neglected in theoretical treatments. (C) 1997 American Institute of Physics.