Geometric and Isotopic Influences on the Fragmentation Patterns of Rapidly Ionized Methane and Ammonia*

The fragmentation branching ratios of (deuterated) ammonia and methane ionized by 19 MeV F$^{7+}$ and 4 MeV H$^{+}$ projectiles have been measured with an emphasis on dissociation channels that require bond rearrangement. For these projectiles, the collision time is approximately 10 attoseconds, a duration over which nuclear motion is negligible. As a result, the rearrangement occurs during the post-collision dissociation process and nuclear mass plays a role. Production of H$_{2}^{+}$ and H$_{3}^{+}$ ions, in coincidence with either neutral or ionic fragments, was analyzed for these eight collision systems. Statistically significant isotopic effects are observed in some ($e.g.$ H$^{+}$ + NH$_{3}^{+} \quad \to $ H$^{+}$ + N + H$_{3}^{+}$ ), but not all ($e.g$. F$^{7+}$ + NH$_{3}^{+} \quad \to $ F$^{7+}$ + N + H$_{3}^{+})$, dissociation pathways.