Formation mechanisms of oxygen atoms in the O((1)D(2)) state from the 157 nm photoirradiation of amorphous water ice at 90 K.

Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O((1)D) and OH(v=0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O((1)D) atoms from the ice surface. One is unimolecular dissociation of H(2)O to H(2)+O((1)D) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H(2)O(2) is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice.