Differential cross sections for vibrational excitation in the collision of He+with H2at moderate energies

Abstract

Differential cross sections for vibrational excitation have been measured for individual states up to nu =5, for the collision He+ on H2, in the mean energy range (100-3000 eV), in an ion-molecule crossed-beam experiment. The experimental requirements (high-energy resolution as well as very good angular resolution) are fulfilled thanks to the properties of the highly selected ion beam and of the extremely well cooled molecular beam used as a gas target. An energy spread of less than 200 meV is achieved together with an angular spread of approximately +or-0.025 degrees , both independent of the collision energy. At an impact energy of 510 eV, the cross sections are found to be peaked strongly at small angles. It is also observed that the cross sections corresponding to the different vibrational excitations ( Delta nu =1,2, . . . ,5) are of the same order of magnitude, and nearly equal for the highest excitations ( Delta nu >or=2) and for angles above 0.5 degrees in the laboratory frame. A semiclassical calculation, based on a schematic potential surface, yields a correct interpretation of the experimental data, at least for the Delta nu =0,1 processes investigated. This simple description also gives a detailed insight of the triatomic-system behaviour relative to its geometry.