Ab initiocross sections for low-energy inelastic H+Na collisions

We report ab initio results for the integral cross section of the process $\mathrm{H}+\mathrm{N}\mathrm{a}\mathrm{}\left(3s\right)\mathrm{}\to \mathrm{H}+\mathrm{N}\mathrm{a}\mathrm{}\left(3p\right)\mathrm{}$ for collision energies from the threshold (2.1 eV) to 600 eV. We achieve a reasonable agreement with the experimental data, which are available for energies above 10 eV. The main contributions to the cross section come from a rotational coupling mechanism in the NaH triplet molecular system and from a curve-crossing mechanism in the singlet system. At very low energy (2.1–2.4 eV), the process is governed by a centrifugal barrier in the exit channel leading to orbital resonances. The Landau-Zener model provides a reasonable qualitative description of the radial coupling mechanism at high energies, but fails below 10 eV.