Interaction of He+2 ions with hydrogen molecules

Cross sections for all elementary reactions involving a change in charge state in He+2-H2 collisions have been measured for He+2 kinetic energies in the range E=1.2--100 keV. Measurements were carried out by distinguishing an individual collision by a coincidence method and by simultaneously analyzing the charge states of the fast and slow particles. Furthermore, in the same event, the electronic states of the particles after the collision were determined by analyzing the kinetic energies of the resulting ions. The elementary reactions involving the formation of He+ ions in the ground and excited states were studied. The reactions involving transitions in the hydrogen molecule to the 1ssigma/sub g/ and 2psigma/sub u/ states of H2+ ions, and reactions in which wto protons are formed, were also studied. At E>15 keV, the largest cross section is that corresponding to one-electron capture: He+2+H2→He++H2+ (this cross section is sigma=8.3 x 10-16 cm2 at E=50 keV). In this reaction, 90--98% of the He+ ions are formed in excited states with principal quantum number n=2. At E<10 keV, the probability for the reaction falls off sharply because capture of the electron to the 1s state of the He+ ion predominates, accompanied by the simultaneous dissociation of the H2+ ion: He+2+H2→He+(1s)+H+H+H0+e-. The cross section for this exothermic capture with dissociation (the energy released is ΔEapprox. =+36.3--3.8 eV) increases with decreasing energy E. At E>15 keV, an endothermic pathway is predominant: →He+(2s,2p)+H++H+0+e- (the energy expended, ΔE, is more than 3.2 eV). The existence of two capture reactions with dissociation - exothermic and endothermic - leads to a minimum in the cross section for this reaction, at Eapprox. =15 keV. Ionization reactions and ionization with dissociation have the smallest cross sections