Theoretical investigation of collision induced rotational alignment in N+2–He

Abstract

Collision induced rotational alignment of N+2 ions drifting in a helium buffer gas is studied by quantum closed coupled calculations using an ab initio interaction potential obtained from multireference configuration interaction wave functions. New formulas are derived for the tensor cross sections. For a given velocity distribution of the collisional partners a set of kinetic equations is solved under steady-state conditions. The resulting alignment parameters are found to be smaller than the experimental values for the velocity distribution assumed so far in drift tube experiments. However, by modification of the anisotropy of this distribution, good agreement between the theoretical quadrupole moments of the rotational angular momentum distributions and the corresponding experimental data can be obtained. It is shown that the attractive part of the potential has a significant influence on the collision dynamics of the N+2–He system. The closed coupled m-resolved cross sections indicate that collision induced transitions between magnetic sublevels of a single rotational state contribute more to the alignment effect than transitions between different rotational states.