Luminescence from UV-irradiated amorphous H2O ice

The luminescence emitted by amorphous H₂O ice at 78 K when irradiated with a continuous source of 260 nm light has been examined. The emission spectrum was similar to that from crystalline ice, exhibiting two features around 340 and 420 nm that had short and long lifetimes, respectively. The rise and fall times of the 420 nm band were the same within experimental error, and the decay was equally well fit by either a biexponential decay or a second-order decay with initial concentrations in a ratio of 3.2:1. However, the identical nature of the luminescence rise and decay profiles strongly suggests biexponential kinetics. The 420 nm emission was tentatively attributed to the ⁴Σ^- → X²Π transition of excited OH radicals. A reaction mechanism that explains the preceding observations is presented, and the biexponentiality of the 420 nm decay is attributed to excited OH radicals located in two distinct environments within the ice lattice.