Associative ionization mechanisms in collisions between He(5P3) and He(11S) atoms at thermal energies

Homonuclear associative ionization, also known as the Hornbeck-Molnar process, has been investigated in a crossed-beam experiment for the collision He($5^3$P)+He with use of a time-of-flight technique. Excited states were produced from metastable He($2^3$S) with use of a tunable cw source of coherent light. The density distribution of the He($5^3$P) atoms in the interaction zone was investigated by a highly sensitive photoionization method. The combination of these techniques with a crossed-beam geometry allowed the kinetic-energy dependence of the cross section to be investigated with a very good resolution in the range 20–200 meV. The experimental curve exhibits structures similar to that calculated by Cohen for the excited states $3^3$P and $4^3$P. The interpretation of our results has been performed by an extension of Cohen’s model to the present case, considering bound-bound transitions between quasidiabatic states of ${\mathrm{He}}_2^{\mathrm{*}}$, and assuming a 100% ionization rate for the states penetrating into the continuum. The shape of the experimental curve is well reproduced by the contribution due to channels of ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ symmetry, which is found to be preponderant, and the validity of Cohen’s model is thus demonstrated. Two different processes are found to occur in different regions of the kinetic-energy range separated by a narrow transition zone. The limits of the latter are the diabatic threshold [crossing energy of the repulsive entrance channel ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ $5^3$P with the ionization limit ${\mathrm{He}}_2$ ${\mathrm{}}^{+}$(${\mathrm{X}}^2$ ${\mathrm{\Sigma }}_{\mathrm{u}}^{+}$)], and the asymptotic energy of the first attractive channel ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ $6^3$D.