Release of hydrogen molecules from the photodissociation of amorphous solid water and polycrystalline ice at 157 and 193 nm

The production of H2 in highly excited vibrational and rotational states (v=0-5, J=0-17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H2 (v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H2 (v=0 and 1) populations in this case were below the detection limit. The H2 products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H2 O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields H2 molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations.