Permanent dipole transitions remain elusive in HD+ strong-field dissociation

Abstract

A pertinent question in strong-field molecular physics is: what role does the permanent electric dipole moment of heteronuclear molecules play in their dissociation dynamics? Recently, Kiess et al (2008 Phys. Rev. A 77 053401) reported the first evidence for direct two-photon dissociation of an HD+ beam involving its permanent dipole moment, using 790 nm, 100 fs pulses. However, the measurement was convoluted by the fact that the H+ (H) and D+ (D) fragments could not be well resolved. Using high resolution coincidence 3D momentum imaging, which distinguishes all fragments, we find new evidence that challenges the previous findings. Specifically, we find that the small peak observed and assigned earlier to direct two-photon dissociation is instead due to one-photon dissociation of the v= 8 vibrational state by bond softening. Our vibrationally resolved spectra covering the intensity interval 3 × 1013 − 5 × 1014 W cm−2 show that one-photon dissociation of HD+ dominates, with no clear evidence of permanent dipole transitions.