J. Phys. Chem. 1996, 100, 16801-16807
16801
Luminescence from UV-Irradiated Amorphous H2O Ice T. I. Quickenden,* T. A. Green, and D. Lennon Department of Chemistry, The UniVersity of Western Australia, Nedlands, WA 6907, Australia ReceiVed: December 22, 1995; In Final Form: July 5, 1996X
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- f 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.
Introduction Earlier work1-4 from this laboratory has shown that, over a wide temperature range (77-270 K), H2O and D2O ices emit UV and visible region luminescence when excited with 200280 nm UV light. Crystalline H2O ice emits bands around 340 and 420 nm, and these are mainly excited by wavelengths in the vicinity of 220 and 260 nm, respectively.1,2 These two emission bands have been tentatively assigned1,2 to the matrixshifted A2Σ+ f X2Π transition of excited OH, with the splitting into two bands arising from the location of OH in respective substitutional and interstitial sites. More recently, the luminescence from crystalline H2O ice excited by 260 nm light has been resolved into short-lived and long-lived components.4 The former contained a number of bands with maxima at 351, 370, and 387 nm that were assigned to the Herzberg I A3Σu+ f X3Σg- or the Herzberg III C3∆u f X3Σg- systems of excited O2. The O2 arises from the reaction of mobile O atoms with accumulated O atoms trapped in the ice lattice. Luminescence from amorphous and polycrystalline H2O ices excited by pulsed UV light has also been examined.3 Broad band luminescence was observed in both cases and fell smoothly from a high intensity near 320 nm to zero intensity at ca. 560 nm. Significantly, no 420 nm emission band was observed with either the amorphous or the polycrystalline ice, even though the emission is present in continuously irradiated crystalline ice. The first aim of the present study was to investigate this absence and then to determine whether it is due to the transient mode of excitation used or due to the different ice morphologies involved. The second aim was to examine the kinetics of the rise and decay of the 420 nm band from amorphous ice and to develop a mechanism for this emission. Experimental Section The equipment used comprised a stainless steel high-vacuum chamber maintained at a pressure of
