Collision spectroscopy of the He-, ${\mathrm{He}}^{+}$-${\mathrm{H}}_2$(${\mathrm{D}}_2$) systems. A triatomic extension of the molecular-orbital-promotion model

An experimental investigation of the He- and ${\mathrm{He}}^{+}$-${\mathrm{H}}_2$ collisions is performed in the 200-eV - 3-keV energy range. The direct and charge-exchange processes are identified by their characteristic energy losses. The relative probabilities of the main processes are determined as a function of the scattering angle. For the He-${\mathrm{H}}_2$ case, selective population of $n=2\mathrm{}$ singly and doubly excited states of ${\mathrm{H}}_2$ is observed. In contrast with this finding, the whole Rydberg series $\mathrm{H}_2^{}{}_{}{}^{+*}\left(nl\lambda \right)$, $\mathrm{H}_2^{}{}_{}{}^{*}(1s\sigma ,nl\lambda )$, ${\mathrm{He}}^{*}\mathrm{}(1s,nl)\mathrm{}$ and the associated ionization continua are observed in soft ($E\theta \lesssim 2$ keV deg) ${\mathrm{He}}^{+}$-${\mathrm{H}}_2$ collisions. More violent ${\mathrm{He}}^{+}$-${\mathrm{H}}_2$ collisions, however, show trends similar to those of the neutral system. An interpretation is attempted within a triatomic extension of the electron-promotion model. In addition to the well-known diabatic II and rotational coupling mechanisms, a purely triatomic curve-crossing process is shown to account for the population of the $2p{\sigma }_u$ orbital of the molecule.