Collisions*

We have measured momentum analyzed protons, emitted in fragmentation of H₂⁺, in coincidence with angular resolved neutralized projectiles for 75 keV p impact.  We extracted fully differential cross sections (FDCS) for dissociative capture.  Data were analyzed for two molecular orientations, one is perpendicular and one parallel to the momentum transfer q.  Based on the kinetic energy release (KER) the dissociative capture channel through nuclear excitation to a vibrational continuum state was selected.

 

In the scattering angle-dependence of the FDCS for the parallel molecular orientation, we observed a two-center molecular interference pattern.  Earlier, we reported that this interference pattern was phase-shifted by π.  The new data, employing improved data analysis techniques, revealed that this phase shift is not constant, but rather depends on the scattering angle q_p.  We did not find any significant dependence on the KER.

 

A phase shift was observed in previous studies of electronic dissociation for excitation to an anti-symmetric molecular state and explained by parity conservation.  But in vibrational dissociation the electron remains passive in the symmetric ground state and this explanation does not hold.

 

We have offered a hypothetical explanation for the phase shift in our data.  As the nuclear wave packet is lifted to the vibrational continuum state it can propagate either towards decreasing or increasing internuclear separation D.  The latter path leads to direct dissociation, for which we do not expect any phase shift.  But for the former path the wave packet first has to be reflected from the molecular potential wall at small D before dissociation can occur, for which a π phase leap is expected.  Depending on which path is stronger, which depends on q_p, the average phase shift can take any value between 0 and π.

 

If this explanation is correct, one would expect additional interference between the direct and the reflection paths because they are indistinguishable.  The phase angle for such an interference depends on the KER.  Indeed, in the KER dependence of the interference term we did observe an oscillating pattern.  Therefore, these data are supportive of our hypothetical explanation, but they do not provide conclusive evidence.