Amorphous Solid Water: Pulsed Heating of Buried N₂O₄

Molecular transport and morphological change were examined in films of amorphous solid water (ASW). 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 (ultrahigh) vacuum. Film thickness up to 2400 monolayers 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 ASW layer insulated the N₂O₄ layer from the high-thermal-conductivity MgO 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 and D₂O 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, as evidenced by complex temporal profiles extending past 1 ms. Little material enters vacuum after cessation of the 10 ns pulse because cold ASW near the film surface freezes material that is no longer being heated. A proposed model is in accord with the data