Quantum-Mechanical Calculation of Cross Sections for Inelastic Atom-Atom Collisions. I. Inelastic2S3→2P3Collisions between Metastable and Ground-State Helium Atoms

A quantum-mechanical two-state close-coupling calculation of the inelastic cross section for the $2{}_{}{}^3{}_{}{}^{}S\to 2{}_{}{}^3{}_{}{}^{}P$ excitation has been carried out for collisions between $2{}_{}{}^3{}_{}{}^{}S$ metastable and ground-state helium atoms in the center-of-mass (c.m.) energy range 5-500 eV. The calculations were carried out in both the diabatic and adiabatic representations using two single-configuration valence-bond molecular wave functions and the corresponding adiabatic linear combinations, respectively. Both representations, as expected, gave identical results. In the diabatic representation, the coupling matrix element was found to be extremely large, resulting in a strong avoided crossing of the corresponding adiabatic potential curves and very small inelastic cross sections for energies $E\lesssim 100$ eV (c.m.), but increasing to a value of about 4 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 500 eV (c.m.), The semiclassical Stueckelberg-Landau-Zener approximation was found to be very poor at all energies considered. The JWKB distorted-wave approximation was found to be fairly good at energies $E\gtrsim 400$ eV (c.m.).