State-selected reactive scattering. II. He+H+2→HeH++H

The endoergic reaction He+H+2→HeH++H is investigated in a molecular-beam experiment as a function of H+2 vibrational energy at c.m. collision energies between 0.3 and 1.9 eV. Reactant ions generated by resonantly enhanced four-photon ionization are impulsively accelerated to collide with a beam of He. Time-of-flight velocity distributions of HeH+, measured at one laboratory angle, yield the differential cross section at c.m. angles θ=0° and 180°. A shift from ‘‘He rebound’’ to ‘‘H+ stripping’’ behavior accompanies the enhancement in the cross section as the H+2 vibrational energy increases, which matches previous studies at higher collision energy. Small-impact-parameter events produce HeH+ with less recoil velocity (more internal energy) than those at large impact parameters. Within the limits of sensitivity and resolution (ΔE≊0.15 eV), definitive resonance features in the collision energy dependence of dσ/dω are not evident. Improvements in the technique to enable such observations are suggested.