Peeling the astronomical onion

Abstract

Journal Name ARTICLEThis journal is © The Royal Society of Chemistry 20xxJ. Name., 2013, 00, 1-3 | 1 Water ice is the most abundant solid in the Universe. Understanding the formation, structure and multiplicity of physicochemical roles for water ice in the cold, dense interstellar environments in which it is predominantly observed is a crucial quest for astrochemistry as these are regions active in star and planet formation. Intuitively, we would expect the mobility of water molecules deposited or synthesised on dust grain surfaces at temperatures below 50 K to be very limited. This work delves into the thermally-activated mobility of H2O molecules on model interstellar grain surfaces. The energy required to initiate this process is studied by reflection-absorption infrared spectroscopy of small quantities of water on amorphous silica and highly oriented pyrolytic graphite surfaces as the surface is annealed. Strongly nonArrhenius behaviour is observed with an activation energy of 2 kJ mol -1 on the silica surface below 25 K and 0 kJ mol -1 on both surfaces between 25 and 100 K. The astrophysical implication of these results is that on timescales shorter than that estimated for the formation of a complete monolayer of water ice on a grain, aggregation of water ice will result in a nonuniform coating of water, hence leaving bare grain surface exposed. Other molecules can thus be formed or adsorbed on this bare surface.

Advances in the experimental exploration of water’s phase diagram

“…[74] However, Rosu-Finsen et al subsequently argued that the low-temperature endotherm is kinetic in origin and arises upon heating deep glassy states of hydrogen-disordered ice VI. [62] Consistent with the increase in volume upon hydrogen-ordering, the application of pressure suppresses the hydrogen ordering to ice XV, which then enables glassy states of ice VI to be reached at Tg(doped) along pathway (2) in Figure 4. The general effect of HCl-doping is a substantial decrease of the glass transition temperature from Tg(pure) to Tg(doped) estimated to be more than 30 K in case of ice VI.…”

“…[74], Rosu-Finsen et al have shown that low-temperature endotherms can also be found for the corresponding D2O materials and neutron diffraction data of such a sample was shown to be consistent with deep glassy hydrogen-disordered ice VI. [62] These recent findings illustrate that doping with HCl does not only enable hydrogen ordering of ices V, VI and XII. If the hydrogen ordering can be suppressed, for example by the application of pressure,…”

“…4 Schematic illustration of the various processes that can take place upon cooling pure or doped hydrogendisordered ice leading to glassy, partially hydrogen-ordered or fully hydrogen-ordered ice. Adapted from refs [9,62].…”

“…The general effect of HCl-doping is a substantial decrease of the glass transition temperature from Tg(pure) to Tg(doped) estimated to be more than 30 K in case of ice VI. [62] Increasing the pressure enables more relaxed, deeper glassy states to be reached around Tg(doped). As encountered for a wide range of other glassy materials, deep glassy states display kinetic overshoot features upon heating connected with the underlying glass transition.…”

A fibre-coupled UHV-compatible variable angle reflection-absorption UV/visible spectrometer

“…Previous studies of molecular ices have used assumed values for n where experimental values are unavailable, or used values from the liquid phase. 9,10 Typical astrochemical surface science experiments employ high-vacuum (p ≈ 10 -7 mbar) 21,22 or ultra-high vacuum (UHV, p ≈ 10 -10 mbar) 7,23 and cryogenic cooling of the order of 10-30 K. [22][23][24] It is therefore questionable to assume that an ice will behave in the same way as in the liquid phase under ambient conditions. In other cases, when ices of mixed compositions were studied, a weighted average value based upon the relative contributions of each ice component was used for n. 15 However, this does not take into account interactions within the ice, and therefore the actual refractive index will vary significantly from that derived in this way.…”