Deexcitation cross sections of He(2thinsp;1P) by H2have 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 crosshyphen;section values and their collisional energy dependence are interpreted in terms of the Penning ionization cross section based on a longhyphen;range dipolendash;dipole interaction between He(2thinsp;1P) and H2. Validity of the theoretical formula for the Penning ionization cross section, i. e., the Watanabendash;Katsuura formula, is discussed. The cross section has been further calculated by means of an impacthyphen;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 thephyphen;state helium rotates in order to keep collinear or perpendicular configuration with respect to the axis of Hendash;H2. A modified form of the dipolendash;dipole autoionization width with an electron exchange interaction is also discussed.
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