Measurements of Energy Partitioning in H 2 Formation by Photolysis of Amorphous Water Ice

Abstract

We demonstrate experimentally that photodissociation of amorphous solid water at 100 K results in formation of H2 molecules with an ortho/para ratio of gOPR = 3. Two distinct mechanisms can be identified: endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields translationally and internally hot H2. These results are in accord with predictions by molecular dynamics calculations and their astrophysical implications are discussed. © 2008. The American Astronomical Society.