Ionization of low-Rydberg-state He atoms by polar molecules. I. Velocity dependence of the cross sections

The ionization of low-Rydberg-state He atoms in the n ${}_{}{}^1{}_{}{}^{}P$ state (n=14) in collisions with the polar molecules ${\mathrm{NH}}_3$, ${\mathrm{SO}}_2$, and acetone (${\mathrm{C}}_3$ ${\mathrm{H}}_6$O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\sigma }_i^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/$v_r^2$ in a large part of the studied velocity range, while a 1/$v_r$ velocity dependence is predicted by the free-electron model for the higher Rydberg levels.