Photodissociation dynamics of vibrationally excited hydrogen molecules examined with reaction microscopy in kinematically complete VUV + NIR experiments

Abstract

Using the Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) technique, we investigated the dissociation of vibrationally excited H₂ and D₂ molecules after two-color vacuum ultraviolet (VUV) + near-infrared (NIR) absorption. The Advanced Light Source synchrotron provided horizontally polarized VUV radiation to photoionize H₂ (or D₂), leaving H₂⁺ (or D₂⁺) ions in specific vibrational states. A synchronized 1030-nm NIR, horizontally polarized, 12 ps-long laser pulse with an intensity of ≈ 3 × 10¹¹ W/cm² then dissociated these ions at a time delay that was controlled with picosecond-scale precision. We report on electron emission in the molecular frame, retrieved from the recoil ion labframe momentum. For some vibrational states, we observed an asymmetric photoelectron angular distribution that depends on the VUV/NIR time delay. We observed dissociation of lower energy vibrational states that should be forbidden. We also investigate the dependence of this asymmetry on the angle between the molecular axis and the polarization direction of the synchrotron radiation for one-color VUV-only dissociation, and the dependence of dissociation on the angle between the molecular axis and the polarization directions of the synchrotron and laser radiation for the two-color dissociation. We believe this asymmetry might be related to photoelectron retroaction with the dissociating ion in both the one-color and two-color dissociation, and that the dissociation of lower energy vibrational states could be due to a dressed potential energy surface with a light induced conical intersection.