Kinetic and internal energy distributions of molecular hydrogen produced from amorphous ice by impact of 100 eV electrons

Abstract

We have measured the quantum-resolved translational and internal (vibrational and rotational) energy distributions of the D 2 desorbates produced during electron (100 eV) irradiation of D 2O amorphous ice using resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The D 2 desorbates have very little translational energy (∼ 20–50 meV) but are vibrationally ( ν = 0–4) and rotationally ( J = 0–12) excited. The rotational state distribution of the D 2 does not change as the ice temperature increases from 88 to 145 K. However, we find that the D 2 yield increases monotonically in this temperature range, with the yield at 145 K approximately double than that at 88 K. Although the mobilities of defects, charge carriers, and radicals are known to be temperature dependent, these results suggest that the final states leading to D 2 production are independent of temperature. We suggest that the dominant mechanisms for production of D 2 at 100 eV incident electron energy are dissociative recombination of holes (D 2O + or D 3O +) with quasi-free or trapped electrons and dissociation of excitons at the vacuum-surface interface. These dissociation events can produce D 2 directly via molecular elimination or indirectly via reactive scattering of the energetic D atom fragments.