Isotopic Effects on the Time-Dependences of 420 nm Ice Luminescence Excited by UV Light

The kinetics of the 420 nm luminescence emitted from H2O and D2O polycrystalline Ih ices have been studied over the 77 to 162 K temperature range. In the case of both H2O and D2O ices, it was found that the luminescence rise and decay curves consisted of two luminescence components, and superimposing two first-order curves with different rate constants gave the best fit to the decay and rise curves. The mean lifetimes of the two luminescence components were 1.08 +/- 0.03 s and 2.47 +/- 0.03 s. The rate constants were found to have negligible temperature dependences, which led to activation energies well below those obtained for either activation-limited processes or even diffusion-limited processes. Furthermore, it was found that the luminescence kinetics were not affected by isotopic substitution of D for H in the ice lattice. These observations suggest that the rate-determining step in the mechanism for the production of the luminescence is a slow (probably spin-forbidden) electronic transition that can occur at two different rates due to the presence of two different types of trapping sites in the ice lattice. A possible candidate for the electronic transition is the (4)Sigma(-) -> X-2 Pi transition of excited OH. radicals and not the previously suggested and ubiquitous A2 Sigma(+) -> X-2 Pi transition of this species.