Velocity dependence of the cross sections for Penning and associative ionization of H atoms by He(2 1S) metastable atoms

The velocity dependence of the Penning and associative ionization cross sections for the He(2 1S) + H interacting system is measured for the relative velocity range 1900--5300 m/sec in a crossed-beam experiment by a time-of-flight technique. We observe that the total ionization cross section decreases monotonically with increasing kinetic energy, and that associative ionization is a minor process, contrary to some theoretical predictions made for the He(2 1S) + H system