DEFECT ACTIVITY IN AMORPHOUS ICE FROM ISOTOPIC EXCHANGE DATA - INSIGHT INTO THE GLASS-TRANSITION

The details of isotopic exchange reactions in amorphous ice (amorphous solid water, ASW) have been followed spectroscopically in an effort to establish the degree to which the exchange mechanism is analogous to that of cubic ice for which exchange is based on sequential action of mobile ion (H+) and orientational (L) point defects. Because pure amorphous ice is proton inactive below its crystallization temperature, the necessary Hf have been injected into annealed ASW, containing intact isolated D2O molecules, by the photoexcitation of dopant 2-naphthol at 80 K. The limited isotopic exchange that occurred during UV irradiation and the rate data obtained for exchange at temperatures ranging from 108 to 125 K are shown to be consistent with a point-defect model having the following specific characteristics: (1) the protons injected at 80 K, while migrating short distances before being shallowly trapped, convert some D2O to dynamically coupled (HOD)(2) units; (2) the ''L'' defects and trapped protons are immobilized below similar to 100 K, but both are thermally activated at the temperatures of the rate measurements; (3) above similar to 115 K quasi-equilibria develop between isotopomeric units as expected for an icelike point-defect mechanism having the ''L'' defects much more active than the HC ions. By contrast, the exchange data are inconsistent with the occurrence of any significant molecular diffusional motion, an observation that suggests that the molecular motion that develops at the glass transition temperature of ASW is orientational diffusion based on L-defect activity. From this view, fluidity is not expected for ASW when warmed above T-g (similar to 130 K, measured values depend on warmup rates) any more than for cubic ice above its T-g (similar to 140 K).