Multiply differential study of vibrational dissociative capture in p + D2 collisions

We measured fully momentum-analyzed projectiles and D+ fragments from p + D2 collisions in coincidence and extracted fully differential cross sections (FDCS) for vibrational dissociative capture as a function of projectile scattering angle θp. The data show a pronounced molecular two-center interference pattern caused by indistinguishable projectile diffraction from the two atomic centers. Earlier p + H2 studies revealed a similar pattern but with a phase shift relative to theory. For dissociation via electronic excitation to the 2pσu state, such a shift can be linked to a symmetry change in the molecule that requires a compensating switch in projectile symmetry. In vibrational dissociation, characterized by small KER and no symmetry change, that explanation does not apply.

A key difference emerges between H2 and D2. For H2, the phase shift depends on θp, while for D2 it is constant at π. We propose two nuclear paths in vibrational breakup: a direct path where the nuclei separate immediately, and a reflected path where they first approach smaller internuclear distance D and reflect from a potential barrier, introducing a π phase leap. The θp-dependent shift in H2 indicates a reflected-path dominance at small θp and a direct-path dominance at large θp. For D2, the reflected path dominates for all θp. This is consistent with KER-resolved FDCS: H2 shows oscillations from interference between reflected and direct paths, while D2 shows no such structure because the direct channel is negligible.