Formation mechanisms of oxygen atoms in the O(1D2) 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(D-1) and OH(nu= 0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O(D-1) atoms from the ice surface. One is unimolecular dissociation of H2O to H-2+O(D-1) 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. H2O2 is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3194798]