Polar dissociation of vibrationally de-excited H3+ and D3+ in collision with He

Abstract

A discharge ion source under high pressure was used to produce de-excited H3+ and D3+ beams. The energy spectra of the positive and negative ion fragments were studied in coincidence over the energy range 5.5-9.8 keV, when the molecular ion beams, in their ground state, are excited, they undergo polar dissociation in collisions with an He target. Accurate energy distributions of the ionic fragments were obtained by transforming the measured laboratory spectra to the projectile centre-of-mass (CM) frame of reference. From the transformation to the CM frame, the energy shared by the positive (H+,D+) and negative (H-,D-) fragments was evaluated; it was found that the energy taken by the positive fragments was about 25 times larger than that of the corresponding negative fragments, and it is shared equally by the positive fragments. The energy threshold for the positive fragments resulting from the polar dissociation channel is in agreement with that derived from the potential energy curves for H3+ with D3h symmetry for transitions from v = 0 in the Franck-Condon region.