On the mechanism for vibrational autoionization in H2

The cross sections for the reactions of H2+ with He and Ne to form HeH+ and NeH+ are known to depend strongly on the H2+ vibrational level. When the H2+ reactant ions are produced by autoionization of Rydberg states, the measurement of these cross sections provides a means for studying the final vibrational state distribution of H2+ ions resulting from the autoionization process. Relative cross sections for hydride ion formation were determined for approximately 100 autoionizing Rydberg states in para-H2 between the H2+2𝒥g+,v=0 and v=6 convergence limits. The relative cross sections for formation of HeH+ and NeH+ (and thus the final vibrational state distributions of H2+) are nearly the same for all the autoionizing Rydberg states between any two H2+ vibrational convergence limits v and v+1 regardless of the initial vibrational level of the autoionizing state. Combining this observation with elementary considerations, we show that vibrational autoionization proceeds predominantly (≳ 95%) via a Δv=1 mechanism when this channel is energetically allowed. When autoionization via Δv=1 is forbidden, autoionization proceeds predominantly (≳ 75%) via the minimum change in Δv. In cases when autoionization proceeds via Δv≳1, the fraction of ions left in the highest available vibrational level decreases slightly with increasing initial vibrational level.