Energy transfer in amorphous solid water: light-mediated expulsion of N₂O₄ guest molecules

Molecular transport and morphological change were examined in films of vapor-deposited amorphous solid water, H₂O(as). A buried N₂O₄ layer absorbs pulsed 266-nm radiation, creating heated fluid. Temperature and pressure gradients facilitate the formation of fissures through which fluid travels to vacuum. Film thickness up to 1440 Langmuirs was examined. In all cases, transport to vacuum could be achieved with a single pulse. Material that entered vacuum was detected using a time-of-flight mass spectrometer that recorded spectra every 10 μs. An amorphous solid water layer insulated the N₂O₄ layer from the high-thermal-conductivity MgO(100) substrate; this was verified experimentally and with heat transfer calculations. Laser-heated fluid strips water from fissure walls throughout its trip to vacuum. Experiments with alternate H₂O(as) and D₂O(as) layers reveal efficient isotope scrambling, consistent with water reaching vacuum via this mechanism. It is likely that ejected water undergoes collisions just above the film surface due to the high density of material that reaches the surface via fissures. Little material enters vacuum after cessation of the 10 ns pulse because cold amorphous solid water near the film surface freezes material that is no longer being heated. A proposed model is in accord with the data. ❧ A commercial program (COMSOL Multiphysics©) was used to simulate the heat transfer behavior of a multi-component system comprising solid N₂O₄ and ASW strata supported by a MgO substrate, wherein the application of heat to the amorphous solid water occurs via transfer from the N₂O₄. The N₂O₄ layer is heated, and the system is allowed to evolve. The transience of elevated temperatures is consistent with transmission Fourier-transform infrared spectroscopy of the films before and after irradiation, together leading us to conclude that crystallization does not occur in the amorphous solid water upon irradiation of the guest molecules. The time for heat to travel to the surface of an H₂O(as)/N₂O₄/H₂O(as) sample with a thick upper layer argues for material transport via fissures, and not via surface evaporation.