Rapid formation of H⁺₃from ammonia and methane following 4 MeV proton impact

Abstract

Bond rearrangement, specifically the formation of H+2 and H+3 after ionization of methane and ammonia by fast (4 MeV) protons, is studied in both the common and deuterated isotopes of those molecules. Our coincidence time-of-flight measurements show that the relative probability of H+2 and H+3 production from ammonia is higher for the lighter isotope, contradicting the common intuition that the rearrangement occurs on the timescale of the dissociation. The isotopic effects in methane were much smaller. The relative probability of bond rearrangement leading to H+2 increases with the number of hydrogen atoms in the target. Unexpectedly, however, formation of H+3 is less likely from a methane target than from ammonia. We examined this result by calculating the ionic potential energy surface in reduced coordinates, corresponding to a symmetric stretch of a H+3 triangle away from the remaining C–H complex or nitrogen atom. From both the experiment and the model calculation, we find evidence to support the hypothesis that the bond rearrangement in these collisions proceeds as a two-step process in which a sudden ionization is followed by a slow molecular dissociation.