Activation Energy for the Emission of 420 nm Luminescence from UV-Excited Polycrystalline H2O Ice

Temperature dependence measurements between 78 and 180 K were carried out on the decay of the 420 nm luminescence emitted by purified polycrystalline H 2 O ice, irradiated at 260 nm. Previous work at 78 K showed that a biexponential decay model fitted that luminescence decay quite well. This was also found to be the case at all of the temperatures studied in the present work, the mean lifetimes of the two decays being 0.8 ± 0.1 s and 2.5 ± 0.4 s. Activation energies were found to be negligible in both cases, being less than the experimental error of 0.003 eV. Such low values are consistent with the rate-limiting step being a light-emitting electronic transition rather than a chemical or diffusional step. The 420 nm luminescence is attributed to the spin forbidden 4 Σ - → X 2 Π transition of OH radicals from their excited state back to the ground state.