Deexcitation of He(2 1P) in collisions with rare gas atoms

Abstract

Deexcitation (Penning ionization) cross sections of He(2 1P) by Kr and Xe have been obtained in a region of mean collisional energy between 18–38 meV by a pulse radiolysis method. Fairly large cross sections of above 100 Å2 and their collisional energy dependence are interpreted by the Penning ionization cross sections based on a long-range dipole–dipole interaction. Validity of the theoretical formula for the Penning ionization cross section (the Watanabe–Katsuura formula) is discussed. Two kinds of the cross sections have been further calculated by means of the impact parameter method with experimentally simulated classical trajectories; in one procedure, the polarization axis of the p-state helium has been assumed to rotate in order to keep collinear or perpendicular configuration with respect to the interatomic axis, in the other procedure, the polarization axis is fixed in a certain direction. The classical motion of the particles have been shown to cause considerable influence on the absolute values and the collisional energy dependence of the cross sections. The influence has increased accordingly to the attractive force of the interatomic potential, i.e., in order of Ar<Kr<Xe. Modified form of the dipole–dipole autoionization width with the electron exchange interaction is also discussed. It has also been suggested that rotation of the p-state atomic polarization depends strongly on the van der Waals interaction with the target atoms. The effect of the rotation has been shown to be most prominent for Xe but small for Ar.