Deexcitation of He(2 1P) in a collision with a hydrogen molecule

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Abstract
Deexcitation cross sections of He(2 1P) by H2 have been measured using a pulse radiolysis method in a region of the mean collisional energy between 18 and 38 meV and compared with those obtained by theoretical calculations. Experimental cross‐section values and their collisional energy dependence are interpreted in terms of the Penning ionization cross section based on a long‐range dipole–dipole interaction between He(2 1P) and H2. Validity of the theoretical formula for the Penning ionization cross section, i. e., the Watanabe–Katsuura formula, is discussed. The cross section has been further calculated by means of an impact‐parameter method with experimentally simulated classical trajectories; a hydrogen molecule has been assumed to be spherical to neglect the rotational effect of H2; it is also assumed that the polarization axis of the p‐state helium rotates in order to keep collinear or perpendicular configuration with respect to the axis of He–H2. A modified form of the dipole–dipole autoionization width with an electron exchange interaction is also discussed.