Experimental studies of vapor-deposited water-ice films using grazing-angle FTIR-reflection absorption spectroscopy

Grazing-angle Fourier transform infrared reflection absorption spectroscopy was used to monitor the free OH stretch (or "dangling bond") in vapor-deposited H₂O-ice films between 94 and 120 K. Ice film thicknesses and the sensitivity of our instrument to water-ice molecules were determined by optical interference using a helium-neon laser. These calibrations indicate that the dangling bond signals observed in the present study are indicative of the surfaces of micropores present within the amorphous ice bulk. The largest dangling bond signal (corresponding to the largest number of micropores) was observed at 94 K under conditions of fast ice growth while the smallest signal was observed at 120 K under conditions of slow growth. The temperature and pressure dependence of the dangling bond signal during film growth was used to estimate a barrier to diffusion (E_dif) for H₂O on amorphous ice. We measured 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. The decay of the dangling bond over time (corresponding largely to the collapse of the micropores) was monitored in ice films roughly 100 nm thick. With initial deposition 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 decreased the decay rate.