Icy dust grains in the interstellar medium : their properties and impact

The behaviour of atoms and molecules on interstellar dust grain analogue surfaces has been probed with surface science techniques such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Chemical systems under investigation include O atoms, O2, CO, H2O and N2O as deposited either on a silica (SiO2) surface representative of an interstellar dust grain or on H2O surfaces. As H2O is the dominant solid state chemical species in the interstellar medium (ISM) preliminary experiments have been conducted to investigate one of its formation pathways. Such experiments involved bombarding SiO2 or H2O surfaces with O atoms, producing O2 and O3 molecules as determined by RAIRS and TPD. During the initial stages of H2O growth on dust grains in the ISM, only small quantities will be found in the solid state. The de-wetting behaviour of such quantities was investigated by directly dosing H2O onto the SiO2 surface and observing the νOH vibrational band with time and temperature. Through such experiments, H2O has been observed to de-wet even at cryogenic temperatures of 17 K with an activation energy of about 2 kJ mol-1. The effect of this in the ISM is that bare dust grain surfaces will be left exposed for other molecules, such as CO, to interact with. Such interactions may be stronger than those of the molecules directly interacting with a water surface, meaning molecules will reside in the solid state for longer times. As icy mantles develop throughout the lifetime of a molecular cloud, adsorbed CO becomes a large part of the icy mantle. Multilayers of CO were investigated and shown to exhibit a bulk potential. This charge is due to a spontaneous electric (spontelectric) field arising as certain dipolar molecules align in the solid state. To investigate the spontelectric effect in CO, N2O was first examined and shows a correlation between IR features and the direct measurement of the spontelectric potential of N2O. Such a link was found through the temperature dependent shifts in the LO-TO splitting of the N2O. CO exhibits the same temperature dependence and spontelectric parameters have been extracted to show that a spontelectric potential of 6.7 mV per monolayer is created as multilayers adsorb on a surface. The effect of this in the ISM would be to reduce the gas-phase charge and ionisation fractions in molecular clouds of the ISM which in itself has the potential to have wide-reaching implications

Thermal desorption of H₂O ice: from nanoscale films to the bulk

The desorption properties of H2O films are investigated across a wide range of film thicknesses from 53 nanometres (nm) to 101 micrometres (μm) using a quartz-crystal microbalance (QCM) and temperature-programmed desorption. Three desorption stages are observed belonging to amorphous solid water (ASW), stacking disordered ice I (ice Isd), and hexagonal ice I (ice Ih). The desorption of ASW is only detectable for the ≥10 μm films and is separated from the ice I desorption by 10–15 K with an associated desorption energy of ∼64 kJ mol−1. The desorption energy of the 53-nm film was found to be near 50 kJ mol−1 as also noted in the literature, but with increasing film thickness, the desorption energy of ice I rises, reaching a plateau around 65–70 kJ mol−1. The reason for the increased desorption energy is suggested to be due to molecules unable to desorb due to the thick covering layer of H2O and possibly re-adsorption events. Before complete desorption of ice I at around 220 K for the 101 μm film, a two-stage ice I desorption is observed with the QCM for the ≥10 μm films near 200 K. This event corresponds to the desorption of ice Isd as corroborated by X-ray diffraction patterns collected upon heating from 92 to 260 K at ambient pressure. Cubic ice is not observed as is commonly stated in the literature as resulting from the crystallization of ASW. Therefore, ice Isd is the correct terminology for the initial crystallization product of ASW

Origin of the low-temperature endotherm of acid-doped ice VI: new hydrogen-ordered phase of ice or deep glassy states?

On the basis of a low-temperature endotherm, it has recently been argued that cooling acid-doped ice VI at high pressures leads to a new hydrogen-ordered phase. We show that the endotherms are in fact caused by the glass transitions of deep glassy states related to ice VI. As expected for such endothermic overshoot effects, they display a characteristic dependence on pressure and cooling rate, they can be produced by sub-T_{g} annealing at ambient pressure, and they can be made to appear or disappear depending on the heating rate and the initial extent of relaxation. It is stressed that the existence of a new crystalline phase, as it has been suggested, cannot depend on the heating rate at which it is heated. X-ray diffraction shows that samples for which the low-temperature endotherm is present, weak or absent, as observed at a heating rate of 5 K min⁻¹, are structurally very similar. Furthermore, we show that the reported shifts of the (102) Bragg peak upon heating are fully consistent with our scenario and also with our earlier neutron diffraction study. Deuterated acid-doped ice VI cooled at high pressure also displays a low-temperature endotherm and its neutron diffraction pattern is consistent with deep glassy ice VI. Accessing deep glassy states of ice with the help of acid doping opens up a fascinating new chapter in ice research. Compared to pure ice VI, the glass transition temperature is lowered by more than 30 K by the acid dopant. Future work should focus on the deep glassy states related to all the other hydrogen-disordered ices including the ‘ordinary’ ice Ih

Structure and nature of ice XIX

Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken. Instead, hydrochloric-acid-doped ice VI undergoes an alternative type of phase transition upon cooling at high pressure as the orientationally disordered ice remains disordered but undergoes structural distortions. As seen with in-situ neutron diffraction, the resulting phase of ice, ice XIX, forms through a Pbcn-type distortion which includes the tilting and squishing of hexameric clusters. This type of phase transition may provide an explanation for previously observed ferroelectric signatures in dielectric spectroscopy of ice VI and could be relevant for other icy materials

Impact of oxygen chemistry on model interstellar grain surfaces

Temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O2) beams on amorphous silica (aSiO2) and water (H2O) surfaces (porous-amorphous solid water; p-ASW, compact amorphous solid water; c-ASW, and crystalline solid water; CSW). Altering the deposition method of O2 is shown to result in different desorption energies of O2 due to differences in O2 film morphology when deposited on the aSiO2 surface. O2 enthalpy of formation is dissipated into the aSiO2 substrate without changes in the silica network. However, on the H2O surfaces, O2 formation enthalpy release is dissipated into the H-bonded matrix leading to morphological changes, possibly compacting p-ASW into c-ASW while CSW appears to undergo amorphisation. The enthalpy release from O2 formation is, however, not enough to result in reactive desorption of O2 or H2O under the current experimental circumstances. Further to this, O2 formation on sub-monolayer quantities of H2O leads to enhanced de-wetting and a greater degree of H-bond reconnection in H2O agglomerates. Lastly, O3 is observed from the O + O2 reaction on all surfaces studied

Enabling Star Formation Via Spontaneous Molecular Dipole Orientation In Icy Solids

It is shown here how new experimental data, for the electrical properties of solid CO, can be used to fill important gaps in our understanding of the evolution of prestellar cores. Dust grains with a mantle of CO lead to a reduction in the degree of ionization in these cores by a factor of between 5 and 6. The lifetimes for expulsion of magnetic fields from cores, a process generally necessary for gravitational collapse, are reduced from current estimates of several megayears, by a similar factor. This removes a major inconsistency, since lifetimes now tally with typical ages of prestellar cores of a few hundred thousand to 106 yr, derived from observations. With the reduced timescales, cores also escape disruption by Galactic supernova remnants. Our results provide a natural mechanism for the generation of so-called magnetically supercritical cores, in which the magnetic field alone cannot prevent gravitational collapse. In addition, we find a minimum value for the density of prestellar cores of ≥(1.1 ± 0.1) × 104 H2 cm−3, in agreement with observations

Benchmarking acid and base dopants with respect to enabling the ice V to XIII and ice VI to XV hydrogen-ordering phase transitions

Doping the hydrogen-disordered phases of ice V, VI and XII with hydrochloric acid (HCl) has led to the discovery of their hydrogen-ordered counterparts ices XIII, XV and XIV. Yet, the mechanistic details of the hydrogen-ordering phase transitions are still not fully understood. This includes in particular the role of the acid dopant and the defect dynamics that it creates within the ices. Here we investigate the effects of several acid and base dopants on the hydrogen ordering of ices V and VI with calorimetry and X-ray diffraction. HCl is found to be most effective for both phases which is attributed to a favourable combination of high solubility and strong acid properties which create mobile H3O+ defects that enable the hydrogen-ordering processes. Hydrofluoric acid (HF) is the second most effective dopant highlighting that the acid strengths of HCl and HF are much more similar in ice than they are in liquid water. Surprisingly, hydrobromic acid doping facilitates hydrogen ordering in ice VI whereas only a very small effect is observed for ice V. Conversely, lithium hydroxide (LiOH) doping achieves a performance comparable to HF-doping in ice V but it is ineffective in the case of ice VI. Sodium hydroxide, potassium hydroxide (as previously shown) and perchloric acid doping are ineffective for both phases. These findings highlight the need for future computational studies but also raise the question why LiOH-doping achieves hydrogen-ordering of ice V whereas potassium hydroxide doping is most effective for the 'ordinary' ice Ih.Comment: 18 pages, 7 figures, 1 tabl

Ammonium Fluoride as a Hydrogen-disordering Agent for Ice

The removal of residual hydrogen disorder from various phases of ice with acid or base dopants at low temperatures has been a focus of intense research for many decades. As an antipode to these efforts, we now show using neutron diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice VIII with ~31% residual hydrogen disorder illustrating the long-range hydrogen-disordering effect of ND4F. The doped ice VII could be supercooled by ~20 K with respect to the hydrogen-ordering temperature of pure ice VII after which the hydrogen-ordering took place slowly over a ~60 K temperature window. These findings demonstrate that ND4F-doping slows down the hydrogen-ordering kinetics quite substantially. The partial hydrogen order of the doped sample is consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue that local ferroelectric domains must exist between ionic point defects of opposite charge. In addition to the long-range effect of NH4F-doping on hydrogen-ordered water structures, the design principle of using topological charges should be applicable to a wide range of other 'ice-rule' systems including spin ices and related polar materials.Comment: 23 pages, 4 figures, 2 table

Detailed crystallographic analysis of the ice V to ice XIII hydrogen-ordering phase transition

Ice V is a structurally highly complex material with 28 water molecules in its monoclinic unit cell. It is classified as a hydrogen-disordered phase of ice. Yet, some of its hydrogen-bonded water molecules display significant orientational order. Upon cooling pure ice V, additional orientational ordering cannot be achieved on the experimental time scale. Doping with hydrochloric acid has been shown to be most effective in enabling the phase transition of ice V to its hydrogen-ordered counterpart ice XIII. Here, we present a detailed crystallographic study of this phase transition investigating the effects of hydrochloric and hydrofluoric acid as well as lithium and potassium hydroxide doping. The magnitudes of the stepwise changes in the lattice constants during the phase transition are found to be more sensitive indicators for the extent of hydrogen order in ice XIII than the appearance of new Bragg peaks. Hydrofluoric acid and lithium hydroxide doping enable similar ordering processes as hydrochloric acid but with slower kinetics. The various possible space groups and ordered configurations of ice XIII are examined systematically, and the previously determined P21/a structure is confirmed. Interestingly, the partial hydrogen order already present in ice V is found to perpetuate into ice XIII, and these ordering processes are found to be independent of pressure. Overall, the hydrogen ordering goes along with a small increase in volume, which appears to be the origin of the slower hydrogen-ordering kinetics under pressure. Heating pressure-quenched samples at ambient pressure revealed low-temperature "transient ordering" features in both diffraction and calorimetry