Collision spectroscopy of the He-, He/sup +/-H/sub 2/(D/sub 2/) systems. A triatomic extension of the molecular-orbital-promotion model

An experimental investigation of the He- and He/sup +/-H/sub 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-H/sub 2/ case, selective population of n = 2 singly and doubly excited states of H/sub 2/ is observed. In contrast with this finding, the whole Rydberg series H/sub 2/ /sup +asterisk/(nllambda), H/sub 2/ ((1ssigma,nllambda), He((1s,nl) and the associated ionization continua are observed in soft (Etheta< or approx. =2 keV deg) He/sup +/-H/sub 2/ collisions. More violent He/sup +/-H/sub 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 2psigma/sub u/ orbital of the molecule.