Theoretical investigation of rotational rainbow structures in X–Na2 collisions using CI potential surfaces. I. Rigid-rotor X = He scattering and comparison with state-to-state experiments

Abstract

An accurate CI potential energy surface for He–Na2 is determined, which is suitable for rigid-rotor scattering calculations for collision energies below 1 eV. In the calculation of the interaction potential electron correlation effects have been considered for the bond orbital of Na2 and the 1s orbital of He together with the dispersion attraction between these orbitals using the method of self-consistent electron pairs (SCEP). A very shallow van der Waals minimum of about 0.1 meV is obtained at large internuclear distances. Rigid-rotor infinite-order-sudden (IOS) calculations have been performed for collision energies of 0.05⩽E⩽0.15 eV using an analytical representation for the potential surface constructed with the 52 original ab initio points. The differential cross sections for rotationally elastic and inelastic transitions exhibit the recently predicted rotational rainbow structures. The comparison with the state-to-state experimental data of Bergmann et al. [J. Chem. Phys. 72, 4777 (1980)] is performed for DE/E distributions at fixed laboratory scattering angles and for angular distributions of single Dj cross sections. The qualitative and quantitative agreements are satisfactory. Small deviations occur only in the classically forbidden Dj or angular region.