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.
All Science Journal Classification (ASJC) codes
- General Physics and Astronomy
- Physical and Theoretical Chemistry