Measurements of Energy Partitioning in H2 Formation by Photolysis of Amorphous Water Ice
Ashfold, Michael N. R.
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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.