EXPERIMENTAL STUDIES OF VAPOR-DEPOSITED WATER-ICE FIRMS USING GRAZING-ANGLE FTIR-REFLECTION ABSORPTION-SPECTROSCOPY

Grazing-angle Fourier transform infrared reflection absorption spectro scopy was used to monitor the free OH stretch (or ''dangling bond'') i n vapor-deposited H2O-ice films between 94 and 120 K. Ice film thickne sses and the sensitivity of our instrument to water-ice molecules were determined by optical interference using a helium-neon laser. These c alibrations indicate that the dangling bond signals observed in the pr esent study are indicative of the surfaces of micropores present withi n the amorphous ice bulk. The largest dangling bond signal (correspond ing to the largest number of micropores) was observed at 94 K under co nditions of fast ice growth while the smallest signal was observed at 120 K under conditions of slow growth. The temperature and pressure de pendence of the dangling bond signal during film growth was used to es timate a barrier to diffusion (E-dif) for H2O on amorphous ice. We mea sured an upper limit of E-dif = 4.2 (+/- 0.5) kcal mol(-1), consistent with a theoretically derived value of E-dif = 2.5-3 kcal mol(-1). bon d over time (corresponding largely to the collapse of the micropores) was monitored in ice films roughly 100 nm thick. With initial depositi on rates of 2 nm s(-1), the decay took 125 and 175 min at 118 and 112 K, respectively. Faster deposition rates and colder temperatures decre ased the decay rate.