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 spinforbidden) 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 ⁴Σ → X ²Π transition of excited OH^. radicals and not the previously suggested and ubiquitous A ²Σ⁺ → X ²Π transition of this species. Published in Russian in Kinetika i Kataliz 2006, Vol. 47, No. 5, pp. 709–721. This text was submitted by the authors in English.