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Time-dependent density functional theory (TDDFT) offers a tractable means to predict electronic excitation spectra of ices if sufficient care is taken in selecting the DFT functional, basis sets, and the number of excited states. In this work 48 functionals in conjunction with various correlation consistent basis sets were benchmarked for predicting electronic spectra. A training set of important astromolecules was chosen: carbon monoxide, water, hydrogen cyanide, ammonia, methanol, and the hydroxyl radical. Vertical excitation energies between the ground and first excited state and some higher excited states were calculated using multireference configuration interaction calculations to serve as benchmarks for gas phase TDDFT calculations. When used in conjunction with aug-cc-pVDZ quality correlation consistent basis sets, two functionals, MPW1K and ωB97, emerged as very good choices to model electronic excitations for most of the molecules in the training set. Cluster calculation predictions for the UV spectra of amorphous ices of water and ammonia were performed at the TDDFT-MPW1K/AVDZ and TDDFT-ωB97/AVDZ levels and then compared against experimental data. The calculations qualitatively reproduce the shapes of the spectra for the most part and quantitatively predict the position of the critical first absorption peak. [ABSTRACT FROM AUTHOR]

Structure determination of biological macromolecules using cryogenic electron microscopy is based on applying the phase object (PO) assumption and the weak phase object (WPO) approximation to reconstruct the 3D potential density of the molecule. To enhance the understanding of image formation of protein complexes embedded in glass-like ice in a transmission electron microscope, this study addresses multiple scattering in tobacco mosaic virus (TMV) specimens. This includes the propagation inside the molecule while also accounting for the effect of structural noise. The atoms in biological macromolecules are light but are distributed over several nanometres. Commonly, PO and WPO approximations are used in most simulations and reconstruction models. Therefore, dynamical multislice simulations of TMV specimens embedded in glass-like ice were performed based on fully atomistic molecular-dynamics simulations. In the first part, the impact of multiple scattering is studied using different numbers of slices. In the second part, different sample thicknesses of the ice-embedded TMV are considered in terms of additional ice layers. It is found that single-slice models yield full frequency transfer up to a resolution of 2.5 Å, followed by attenuation up to 1.4 Å. Three slices are sufficient to reach an information transfer up to 1.0 Å. In the third part, ptychographic reconstructions based on scanning transmission electron microscopy (STEM) and single-slice models are compared with conventional TEM simulations. The ptychographic reconstructions do not need the deliberate introduction of aberrations, are capable of post-acquisition aberration correction and promise benefits for information transfer, especially at resolutions beyond 1.8 Å.

Premelting of ice, a quasi-liquid layer (QLL) at the surface below the melting temperature, was first postulated by Michael Faraday 160 y ago. Since then, it has been extensively studied theoretically and experimentally through many techniques. Existing work has been performed predominantly on hexagonal ice, at conditions close to the triple point. Whether the same phenomenon can persist at much lower pressure and temperature, where stacking disordered ice sublimates directly into water vapor, remains unclear. Herein, we report direct observations of surface premelting on ice nanocrystals below the sublimation temperature using transmission electron microscopy (TEM). Similar to what has been reported on hexagonal ice, a QLL is found at the solid-vapor interface. It preferentially decorates certain facets, and its thickness increases as the phase transition temperature is approached. In situ TEM reveals strong diffusion of the QLL, while electron energy loss spectroscopy confirms its amorphous nature. More significantly, the premelting observed in this work is thought to be related to the metastable low-density ultraviscous water, instead of ambient liquid water as in the case of hexagonal ice. This opens a route to understand premelting and grassy liquid state, far away from the normal water triple point. [ABSTRACT FROM AUTHOR]

Laboratory experiments have confirmed that the radiolytic decay rate of astrochemical ice analogues is dependent upon the solid phase of the target ice, with some crystalline molecular ices being more radio-resistant than their amorphous counterparts. The degree of radio-resistance exhibited by crystalline ice phases is dependent upon the nature, strength, and extent of the intermolecular interactions that characterise their solid structure. For example, it has been shown that crystalline CH3OH decays at a significantly slower rate when irradiated by 2 keV electrons at 20 K than does the amorphous phase due to the stabilising effect imparted by the presence of an extensive array of strong hydrogen bonds. These results have important consequences for the astrochemistry of interstellar ices and outer Solar System bodies, as they imply that the chemical products arising from the irradiation of amorphous ices (which may include prebiotic molecules relevant to biology) should be more abundant than those arising from similar irradiations of crystalline phases. In this present study, we have extended our work on this subject by performing comparative energetic electron irradiations of the amorphous and crystalline phases of the sulphur-bearing molecules H2S and SO2 at 20 K. We have found evidence for phase-dependent chemistry in both these species, with the radiation-induced exponential decay of amorphous H2S being more rapid than that of the crystalline phase, similar to the effect that has been previously observed for CH3OH. For SO2, two fluence regimes are apparent: a low-fluence regime in which the crystalline ice exhibits a rapid exponential decay while the amorphous ice possibly resists decay, and a high-fluence regime in which both phases undergo slow exponential-like decays. We have discussed our results in the contexts of interstellar and Solar System ice astrochemistry and the formation of sulphur allotropes and residues in these settings.

We investigate via numerical simulations of the TIP4P/Ice model the isothermal decompression of high density amorphous ice (HDA) mirroring the experimental protocol followed by recent experiments [H. K. Kim, et al., Science 370, 978, 2020]. Taking advantage of the recent determination of the TIP4P/Ice liquid-liquid critical point, we decompress our samples at temperatures both below and above the critical temperature. We follow the crossover between high and low density, complementing the time evolution of the structure factor with the time evolution of microscopic descriptors. Our results support the interpretation of the experimental pathway as a structural change from high to low density states through a first-order transition line. In the simulation study we do not observe nucleation events, but rather a spinodal-like non-equilibrium transformation, which offers an explanation on why the decompression process even at sub-critical temperatures may result in a continuous time evolution of the density and of the microscopic descriptors.

Because of the negative inclination of the solid–liquid phase separation line in water, ice Ih melts on compression. We carry out molecular dynamics simulations using TIP4P/Ice water model. We find that increase in pressure drives the liquid water into a high density metastable glassy state, at relatively high pressure (greater than ∼30 kbar). The crystal-to-glass transition is characterised by a rapid approach to a zero diffusion state and absence of crystalline order in the static structure factor. The vitrification is found to occur even at high temperatures (T > 250 K). We study this novel glass transition process at four temperatures (80, 250, 300 and 320 K). The transition pressure increases with an increase in temperature, as expected. Interestingly, we find that the total rate of change of entropy with pressure undergoes a change at/near the glass transition. We discover a pressure-induced realignment of water molecules resulting in two well-separated peaks in the O–O–O angle distribution among neighbouring molecules. The difference between the positions of these two peaks undergoes a sharp change at the vitrification pressure suggesting that it can serve as an appropriate order parameter to detect the glass transition point. [ABSTRACT FROM AUTHOR]

A recent experiment [K. H. Kim, et al., Science 370, 978 (2020)] showed that it may be possible to detect a liquid–liquid phase transition (LLPT) in supercooled water by subjecting high density amorphous ice (HDA) to ultrafast heating, after which the sample reportedly undergoes spontaneous decompression from a high density liquid (HDL) to a low density liquid (LDL) via a first-order phase transition. Here we conduct computer simulations of the ST2 water model, in which a LLPT is known to occur. We subject various HDA samples of this model to a heating and decompression protocol that follows a thermodynamic pathway similar to that of the recent experiments. Our results show that a signature of the underlying equilibrium LLPT can be observed in a strongly out-of-equilibrium process that follows this pathway despite the very high heating and decompression rates employed here. Our results are also consistent with the phase diagram of glassy ST2 water reported in previous studies.

Water ice clouds form in the mesospheres of terrestrial planets in the solar system (and most likely elsewhere) by vapor deposition at low pressures and temperatures. Under these conditions a range of crystalline and amorphous phases of ice might form. The phase is important because it influences nucleation kinetics, density, vapor pressure over the solid, growth rates and particle shape. In the past, the temperature range over which these different phases exist has been defined on the basis of depositing ice at low temperature and warming it while observing phase changes. However, the direct deposition of ice at a range of temperatures relevant for the terrestrial planets has not been systematically investigated. Here we present X‐ray Diffraction (XRD) measurements of water ice deposited at temperature intervals between 88 and 145 K in a vacuum chamber. XRD patterns showed that low density amorphous ice was formed at ≤120 K, stacking disordered ice I formed from 121 to 135 K and hexagonal ice I formed at 140 and 145 K. Direct deposition results in the stable hexagonal phase at much lower temperatures than when warming stacking disordered ice. All three phases of water ice observed here are possible in clouds in the mesospheres of Earth and Mars, while on Venus only amorphous ice is likely to form. Plain Language Summary: Ice clouds made up of water ice crystals can form in the thin upper atmospheres (mesospheres) of terrestrial planets where it can be extremely cold. Under these conditions a range of crystalline (hexagonal, cubic, or stacking disordered) and amorphous (lacking long range order) phases of ice might form. The phase is important because it influences a range of cloud properties. Experimentally, rather than warming ice formed at low temperature (as has been done in the past) we deposit the ice directly. We did this because an understanding of how the structure of ice evolves on warming as well as what structure forms when ice is deposited at specific temperatures is necessary to understand the properties of these clouds. Measurements showed that the amorphous, stacking disordered and hexagonal phases of water ice (ice I) can form depending on the specific temperature. Direct deposition yields the hexagonal phase at much lower temperatures than when warming ice deposited at lower temperatures. All three phases of water ice observed here are possible in clouds in the mesospheres of Earth and Mars. In contrast, on Venus clouds form at lower temperatures and so only amorphous ice is likely to form. Key Points: Low density amorphous ice was formed at 88–120 K, stacking disordered ice I formed from 121 to 135 K and hexagonal ice I formed at 140–145 KDirect deposition results in the stable hexagonal phase at much lower temperatures than when warming stacking disordered iceAll three phases of ice are possible in clouds in the mesospheres of Earth and Mars, while on Venus only amorphous ice is likely to form [ABSTRACT FROM AUTHOR]

• Molecular dynamics simulations including the largest model of a protein in ice. • A gaussian random field approach to modelling the amorphous ice potential. • The gaussian random field and molecular dynamics models results in identical images. • Simulations with the gaussian random field model are two orders of magnitude faster. Simulations of cryo-electron microscopy (cryo-EM) images of biological samples can be used to produce test datasets to support the development of instrumentation, methods, and software, as well as to assess data acquisition and analysis strategies. To be useful, these simulations need to be based on physically realistic models which include large volumes of amorphous ice. The gold standard model for EM image simulation is a physical atom-based ice model produced using molecular dynamics simulations. Although practical for small sample volumes; for simulation of cryo-EM data from large sample volumes, this can be too computationally expensive. We have evaluated a Gaussian Random Field (GRF) ice model which is shown to be more computationally efficient for large sample volumes. The simulated EM images are compared with the gold standard atom-based ice model approach and shown to be directly comparable. Comparison with experimentally acquired data shows the Gaussian random field ice model produces realistic simulations. The software required has been implemented in the Parakeet software package and the underlying atomic models are available online for use by the wider community. [ABSTRACT FROM AUTHOR]

We here report isotope substitution neutron diffraction experiments on two variants of high-density amorphous ice (HDA): its unannealed form prepared via pressure-induced amorphization of hexagonal ice at 77 K, and its expanded form prepared via decompression of very-high density amorphous ice at 140 K. The latter is about 17 K more stable thermally, so that it can be heated beyond its glass-to-liquid transition to the ultraviscous liquid form at ambient pressure. The structural origin for this large thermal difference and the possibility to reach the deeply supercooled liquid state has not yet been understood. Here we reveal that the origin for this difference is found in the intermediate range structure, beyond about 3.6 Å. The hydration shell markedly differs at about 6 Å. The local order, by contrast, including the first as well as the interstitial space between first and second shell is very similar for both. 'eHDA' that is decompressed to 0.20 GPa instead of 0.07 GPa is here revealed to be rather far away from well-relaxed eHDA. Instead it turns out to be roughly halfway between VHDA and eHDA – stressing the importance for decompressing VHDA to at least 0.10 GPa to make an eHDA sample of good quality. [ABSTRACT FROM AUTHOR]

Here we report about the structural evolution during the conversion from high-density amorphous ices at ambient pressure to the low-density state. Using high-energy X-ray diffraction, we have monitored the transformation by following in reciprocal space the structure factor SOO(Q) and derived in real space the pair distribution function gOO(r). Heating equilibrated high-density amorphous ice (eHDA) at a fast rate (4 K min–1), the transition to the low-density form occurs very rapidly, while domains of both high- and low-density coexist. On the other hand, the transition in the case of unannealed HDA (uHDA) and very-high-density amorphous ice is more complex and of continuous nature. The direct comparison of eHDA and uHDA indicates that the molecular structure of uHDA contains a larger amount of tetrahedral motives. The different crystallization behaviour of the derived low-density amorphous states is interpreted as emanating from increased tetrahedral coordination present in uHDA. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'. [ABSTRACT FROM AUTHOR]

• Molecular dynamics calculations of amorphous ice were performed. • The surface diffusion of protons on amorphous ice was analyzed. • The mechanisms of surface diffusion of protons were determined. To investigate the surface diffusion of protons on amorphous ice, we performed molecular dynamics calculations. The diffusion mechanisms were analyzed by observing the trajectories of protons on the surface. The results show that the protons diffuse on the surface of amorphous ice via repeated formation and dissociation of H 3 O+. From the frequencies of the formation and dissociation of H 3 O+, the diffusion coefficients and activation energy were estimated in the temperature range of 40–130 K. The surface diffusion is an important factor for chemical reactions that occur on ice dust in interstellar molecular clouds. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The infrared spectra of vapor-deposited H 2 O CO 2 amorphous ice were measured. • The formation mechanisms of CO 2 clathrate hydrate from amorphous ice were analyzed. • The phase diagram of the H 2 O CO 2 system including vacuum conditions was predicted. Abstract To investigate the formation mechanisms of CO 2 clathrate hydrate under vacuum conditions, infrared spectra of vapor-deposited H 2 O CO 2 amorphous ice were measured. From analyses of variations in the spectral features with warming, remarkable changes were found at ∼100 K. The results indicate that the deposited amorphous ice is transformed into CO 2 clathrate hydrate at around ∼100 K. From the results, a phase diagram of CO 2 clathrate hydrate from vapor deposited amorphous ice was predicted. The results suggest the existence of pure CO 2 clathrate hydrates in space. [ABSTRACT FROM AUTHOR]

We review the recent research on supercooled and glassy water, focusing on the possible origins of its complex behavior. We stress the central role played by the strong directionality of the water-water interaction and by the competition between local energy, local entropy and local density. In this context we discuss the phenomenon of polyamorphism (i.e., the existence of more than one disordered solid state), emphasizing both the role of the preparation protocols and the transformation between the different disordered ices. Finally, we present the ongoing debate on the possibility of linking polyamorphism with a liquid-liquid transition that could take place in the no-man's land, the temperature-pressure window in which homogeneous nucleation prevents the investigation of water in its metastable liquid form. [ABSTRACT FROM AUTHOR]

Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high-(HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nano-meter length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid--liquid transition in the ultraviscous regime. [ABSTRACT FROM AUTHOR]

To investigate the surface structure, we performed molecular dynamics calculations of amorphous ice. The result shows that a low density layer, which forms a few hydrogen bonds with weaker strength, exists in the surface. Furthermore, the sintering processes were simulated to investigate the structure of grain boundary formed from the adsorption of two surfaces. The result indicates that a low density region exists in a boundary between amorphous ice grains. The structures of surface and interface of amorphous ice have important implications for adsorption, diffusion, and chemical reaction in ice grains of interstellar molecular clouds. [ABSTRACT FROM AUTHOR]

The description of various amorphous phases of ices has been performed on a molecular level in order to study the nature of amorphous polymorphism in a water system. The models of very high-, high- and low-density amorphous (VHDA, HDA and LDA, respectively) ices have been constructed. The molecules are located in their equilibrium states and the structure of the networks is characterized by the absence of long-range atomic order, but short-range order is still present. The thermodynamic properties of VHDA, HDA and LDA ices have been determined using the molecular dynamics and lattice dynamics methods. It has been found that at low temperatures the transformation LDA → HDA is a real phase transition in the classical sense. In the case of HDA → VHDA, the same transition has been found only in negative pressure regions. At positive pressure the transformation from the metastable phase HDA to the thermodynamically stable phase VHDA can be classified as a continuous one. [ABSTRACT FROM AUTHOR]

This work is a study of CO and H 2 O molecules as adsorbates that interact on the surface of olivine dust grains. Olivine (forsterite) is present on the Earth, planetary dust, in the interstellar medium (ISM) and in particular in comets. The composition of amorphous ice is very important for the interpretation of processes that occur in the solar system and the ISM. Dust particles in ISM are composed of a heterogeneous mixture of amorphous or crystalline silicates ( e.g. olivine) organic material, carbon, and other minor constituents. These dust grains are embedded in a matrix of ices, such as H 2 O, CO, CO 2 , NH 3 , and CH 4 . We consider that any amorphous ice will interact and grow faster on dust grain surfaces. In this work we explore the adsorption of CO-H 2 O amorphous ice onto several (100) forsterite surfaces (dipolar and non-dipolar), by using first principle calculations based on density functional theory (DFT). These models are applied to two possible situations: i ) adsorption of CO molecules mixed into an amorphous ice matrix (gas mixture) and adsorbed directly onto the forsterite surface. This interaction has lower adsorption energy than polar molecules (H 2 O and NH 3 ) adsorbed on this surface; ii ) adsorption of CO when the surface has previously been covered by amorphous water ice (onion model). In this case the calculations show that adsorption energy is low, indicating that this interaction is weak and therefore the CO can be desorbed with a small increase of temperature. Vibration spectroscopy for the most stable complex was also studied and the frequencies were in good agreement with experimental frequency values. [ABSTRACT FROM AUTHOR]

The coexistence of the amorphous structure and crystalline hexagonal structure of ice near its melting point at normal pressure is confirmed by comparing the diffraction patterns of diffuse X-ray scattering for different amorphous structures. The diffuse X-ray scattering at an ice sample maintained at a temperature of-10°C is continuously recorded for a long time. Continual variations in the nature of diffuse scattering until its complete disappearance are revealed. The disappearance of diffuse scattering indicates the metastability of amorphous ice near its melting point with a lifetime of about 15 days. [ABSTRACT FROM AUTHOR]

We have recorded dose-fractionated electron cryo-microscope images of thin films of pure flash-frozen amorphous ice and pre-irradiated amorphous carbon on a Falcon II direct electron detector using 300 keV electrons. We observe Thon rings [1] in both the power spectrum of the summed frames and the sum of power spectra from the individual frames. The Thon rings from amorphous carbon images are always more visible in the power spectrum of the summed frames whereas those of amorphous ice are more visible in the sum of power spectra from the individual frames. This difference indicates that while pre-irradiated carbon behaves like a solid during the exposure, amorphous ice behaves like a fluid with the individual water molecules undergoing beam-induced motion. Using the measured variation in the power spectra amplitude with number of electrons per image we deduce that water molecules are randomly displaced by a mean squared distance of ∼1.1 Å 2 for every incident 300 keV e − /Å 2 . The induced motion leads to an optimal exposure with 300 keV electrons of 4.0 e − /Å 2 per image with which to observe Thon rings centred around the strong 3.7 Å scattering peak from amorphous ice. The beam-induced movement of the water molecules generates pseudo-Brownian motion of embedded macromolecules. The resulting blurring of single particle images contributes an additional term, on top of that from radiation damage, to the minimum achievable B-factor for macromolecular structure determination. [ABSTRACT FROM AUTHOR]

Electron cryomicroscopy (cryoEM) has experienced a quantum leap in its capability in recent years, due to improved microscopes, better detectors and better software. It is now possible to obtain near-atomic resolution 3D density maps of macromolecular assemblies using single particle cryoEM without the need for crystals. Although this recent progress has produced some outstanding achievements, we have still only partly realised the full potential of single particle cryoEM. If one or two remaining problems can be solved, it will become an even more powerful method in structural biology that should closely approach the limit of what is theoretically possible. [ABSTRACT FROM AUTHOR]

Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion (vdW) interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment. In this work, we have used the local structure index (LSI) order parameter to analyse the local structure in such highly accurateab initioliquid water. At ambient conditions, the LSI probability distribution, P(I ), was unimodal with most water molecules characterised by more disordered high-density-like local environments. With thermal excitations removed, the resultant bimodal P(I ) in the inherent potential energy surface (IPES) exhibited a 3:1 ratio between high-density- and low-density-like molecules, with the latter forming small connected clusters amid the predominant population. By considering the spatial correlations and hydrogen bond network topologiesamongwater molecules with the same LSI identities, we demonstrate that the signatures of the experimentally observed low- and high-density amorphous phases of ice are present in the IPES of ambient liquid water. Analysis of the LSI autocorrelation function uncovered a persistence time of ∼ 4 ps – a finding consistent with the fact that natural thermal fluctuations are responsible for transitions between these distinct yet transient local aqueous environments in ambient liquid water. [ABSTRACT FROM PUBLISHER]

Layers of amorphous ice saturated with methane, ethane, propane and carbon dioxide were obtained by condensation of molecular beams of rarefied vapor and gas on a substrate cooled with liquid nitrogen. The amorphous state of such objects at low temperatures is stabilized by the high viscosity and the small value of the stationary nucleation rate of the crystal phase. Their heating in condition of high metastability is accompanied by spontaneous explosive crystallization, which leads to the formation of gas hydrates. Glass-transition and crystallization temperatures were determined by changes in their dielectric properties under heating. An increase in the gas content in layers of amorphous ice causes an increase in the crystallization temperature without any significant changes in the glass transition temperature. At atmospheric pressure in a liquid n-pentane medium the retention of gas hydrates was observed up to temperatures close to 273 K. Self-preservation ensured the retention of hydrates in a metastable state at temperatures exceeding considerably their equilibrium dissociation temperatures. Samples of gas hydrates obtained at a maximum gas flow rate during the deposition contained up to 15 mass % of methane, 12 mass % of ethane, 13 mass % of propane, and 23 mass % of carbon dioxide. [ABSTRACT FROM AUTHOR]

Ice clouds form in the summer high latitude mesopause region, which is the coldest part of the Earth's atmosphere. At these very low temperatures (<150 K) ice can exist in metastable forms, but the nature of these ices remains poorly understood. In this paper we show that ice which is grown at mesospherically relevant temperatures does not have a structure corresponding to the well-known hexagonal form or the metastable cubic form. Instead, the ice which forms under mesospheric conditions is a material in which cubic and hexagonal sequences of ice are randomly arranged to produce stacking disordered ice (ice I sd ). The structure of this ice is in the trigonal crystal system, rather than the cubic or hexagonal systems, and is expected to produce crystals with aspect ratios consistent with lidar observations. [ABSTRACT FROM AUTHOR]

Polar mesospheric clouds form in the summer high latitude mesopause region and are primarily comprised of H 2 O ice, forming at temperatures below 150 K. Average summertime temperatures in the polar mesosphere (78°N) are approximately 125 K and can be driven lower than 100 K by gravity waves. Under these extreme temperature conditions and given the relative mesospheric concentrations of CO 2 and H 2 O (~360 ppmv and ~10 ppmv, respectively) it has been hypothesised that CO 2 molecules could become trapped within amorphous mesospheric ice particles, possibly making a significant contribution to the total condensed volume. Studies of CO 2 trapping in co-deposited gas mixtures of increasing CO 2 :H 2 O ratio (deposited at 98 K) were analysed via temperature programmed desorption. CO 2 trapping was found to be negligible when the H 2 O flux to the surface was reduced to 4.8×10 13 molecules cm −2 s −1 . This corresponds to an average of 0.4 H 2 O molecules depositing on an adsorbed CO 2 molecule and thereby trapping it in amorphous ice. Extrapolating the experimental data to mesospheric conditions shows that a mesospheric temperature of 100 K would be required (at a maximum mesospheric H 2 O concentration of 10 ppmv) in order to trap CO 2 in the ice particles. Given the rarity of this temperature being reached in the mesosphere, this process would be an unlikely occurrence. [ABSTRACT FROM AUTHOR]

We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature Tg of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal Tg differences of 10 ± 2 K between H2O and D2O, sharply contrasting with other hydrogen-bonded liquids for which H/D exchange increases Tg by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecular-weight liquids. [ABSTRACT FROM AUTHOR]

We make glassy water in the form of nanofibers by electrospraying liquid water into a hyperquenching chamber. It is measured with means of differential scanning calorimetry, wide angle X-ray diffraction and Raman spectroscopy. It is found that two apparent glass transitions at Tg1 = 136 K and Tg2 = 228 K are detected and non-crystallized water is observed at temperatures up to 228 K. This finding may expand the research objects for liquid water at low temperatures. [ABSTRACT FROM AUTHOR]

Layers of ethane-saturated amorphous ice were prepared by depositing molecular beams of water and gas on a substrate cooled with liquid nitrogen. The heating of the layers was accompanied by vitrification (softening) followed by spontaneous crystallization. Crystallization of condensates under the conditions of deep metastability proceeded with gas hydrate formation. The vitrification and crystallization temperatures of the condensates were determined from the changes in their dielectric properties on heating. The thermal effects of transformations were recorded by differential thermal analysis. The crystallization of the amorphous water layers was studied by electron diffraction. Formation of a metastable packing with elements of a cubic diamond-like structure was noted. [ABSTRACT FROM AUTHOR]

Purpose: Backscattering of very low energy electrons in thin layers of amorphous ice is known to provide experimental data for the elastic and inelastic cross sections and indicates values to be expected in liquid water. The extraction of cross sections was based on a transport analysis consistent with Monte Carlo simulation of electron trajectories. However, at electron energies below 20 eV, quantum coherence effects may be important and trajectory-based methods may be in significant error. This possibility is here investigated by calculating quantum multiple elastic scattering of electrons in a simple model of a very small, thin foil of amorphous ice. Method: The average quantum multiple elastic scattering of electrons is calculated for a large number of simulated foils, using a point-scatterer model for the water molecule and taking inelastic absorption into account. The calculation is compared with a corresponding trajectory simulation. Results: The difference between average quantum scattering and trajectory simulation at energies below about 20 eV is large, in particular in the forward scattering direction, and is found to be almost entirely due to coherence effects associated with the short-range order in the amorphous ice. For electrons backscattered at the experimental detection angle (45° relative to the surface normal) the difference is however small except at electron energies below about 10 eV. Conclusion: Although coherence effects are in general found to be strong, the mean free path values derived by trajectory-based analysis may actually be in fair agreement with the result of an analysis based on quantum scattering, at least for electron energies larger than about 10 eV. [ABSTRACT FROM AUTHOR]

We analyze the heat capacity CP for low and high-density amorphous ice below the transition temperature (TC ≈ 140 K) using a power-law formula. The renormalized critical exponent αR is extracted from the observed CP data, which describes similar critical behavior for both low and high-density amorphous ice below TC. Our analysis can also describe a glass transition in the low-density amorphous ice which is made from the high-density amorphous ice at 124 K, as observed experimentally. [ABSTRACT FROM AUTHOR]

This paper reports on a thermoluminescence study of D2O ice Ih. A sample of hexagonal (Ih) ice is irradiated by a 100 MeV X-ray source at 77 K. The emission spectrum that is measured immediately after the end of the irradiation process has the thermoluminescent behaviour of amorphous ice. The kinetic transition is followed to the stable form, taking place at 85 K. The relaxation time of the transition is of the order of 5 minutes. It is concluded that, due to irradiation, a few outer layers of ice Ih are converted to the low-density amorphous form of ice, which then converts to cubic ice. Although complex to quantify, thermoluminescence appears to be, in the present study, particularly sensitive to the time evolution of irradiated samples. [ABSTRACT FROM AUTHOR]

Arguably the most important liquid in our existence, water continues to attract enormous efforts to understand its underlying structure, dynamics and thermodynamics. These properties become increasingly complex and controversial as we progress into and below the 'no man's land' where bulk water can only exist as crystalline ice. Various, so far unconfirmed, scenarios are painted for this region, including the second critical point scenario, the singularity-free scenario, and most recently the 'critical point-free' scenario [C.A. Angell, Science 319, 582 (2008)]. In this article the structural aspects of water in its supercooled, amorphous and confined states are explored, to the extent that these are known, and related to the water phase diagram. An important issue to emerge is the extent to which structural measurements on a disordered material can tell us about its phase. [ABSTRACT FROM AUTHOR]

Abstract: The dynamics of argon atom collisions with amorphous ice surfaces are investigated using molecular beam techniques and molecular dynamics simulations. The formation of an amorphous ice layer on top of crystalline ice at 110K is shown to have a strong influence on Ar scattering. Compared to crystalline ice, trapping followed by desorption is favoured over inelastic scattering, and a strongly enhanced emission of argon in the backward direction is observed. Molecular dynamics simulation with different types of amorphous and crystalline surfaces are consistent with the experimental data and show that large scale corrugation is required to reproduce the experimental findings. It is concluded that argon scattering can be used to probe changes in surface structure on the nanometer length-scale, while it is relatively insensitive to changes on the molecular level, and it thereby complements other techniques for studies of structural changes of ice surfaces. [Copyright &y& Elsevier]

Abstract: Interfacial water at a surface of nanosilica (S BET =378m2/g) with various amounts of adsorbed 1,3,5-trihydroxybenzene (phloroglucinol, PG), which as an ice-nucleator can strongly affect water structure, was studied using 1H NMR method with layer-by-layer freezing-out of bulk and interfacial water at 200< T <273K. The water/PG/silica system was also investigated using infrared (IR) and ultraviolet (UV–vis) spectroscopies, scanning electron microscopy (SEM), thermal gravimetry (TG) and TPD mass-spectrometry up to 1073K, and quantum chemical methods. The investigations reveal that the properties of interfacial water depend strongly on concentration of adsorbed PG (C PG). The 1H NMR spectra depict appearance of non-freezable water and amorphous ice (T <273K) at the interfaces of nanosilica/PG/unfrozen water/amorphous ice/bulk ice, which fills narrow gaps between PG and silica nanoparticles. The spectral and desorption data show that inter-molecular interactions in the interfacial layer become stronger in the presence of PG, and the amount of disturbed interfacial water increases with increasing C PG value. These effects are reflected in the water characteristics not only at T <273K but also at higher temperatures that are demonstrated in the dependences of the IR, UV–vis, TG/DGT, and TPD-MS graphs on the C PG value. The ordering effect of PG on water substantially increases when the PG content is higher than the monolayer coverage and the PG molecules can form oligomers. [Copyright &y& Elsevier]

Abstract: In an attempt to determine why cooling with liquid helium actually proved disadvantageous in our electron cryotomography experiments, further tests were performed to explore the differences in vitreous ice at ∼82 and ∼12K. Electron diffraction patterns showed clearly that the vitreous ice of interest in biological electron cryomicroscopy (i.e., plunge-frozen, buffered protein solutions) does indeed collapse into a higher density phase when irradiated with as few as 2–3e−/Å2 at ∼12K. The high density phase spontaneously expanded back to a state resembling the original, low density phase over a period of hours at ∼82K. Movements of gold fiducials and changes in the lengths of tunnels drilled through the ice confirmed these phase changes, and also revealed gross changes in the concavity of the ice layer spanning circular holes in the carbon support. Brief warmup–cooldown cycles from ∼12 to ∼82K and back, as would be required by the flip-flop cryorotation stage, did not induce a global phase change, but did allow certain local strains to relax. Several observations including the rates of tunnel collapse and the production of beam footprints suggested that the high density phase flows more readily in response to irradiation. Finally, the patterns of bubbling were different at the two temperatures. It is concluded that the collapse of vitreous ice at ∼12K around macromolecules is too rapid to account alone for the problematic loss of contrast seen, which must instead be due to secondary effects such as changes in the mobility of radiolytic fragments and water. [Copyright &y& Elsevier]

We have simultaneously used adsorption isotherm volumetry and Fourier transform infrared spectroscopy in order to take the investigations on amorphous ice structure a step further, especially concerning porosity and annealing-induced modifications. We have studied surface reorganization during annealing and found that the number of surface sites decreases before crystallization, their relative ratios being different for amorphous and crystalline ice. We also present results confirming that ice can have a large specific surface area and nevertheless be non-microporous. [Copyright &y& Elsevier]

We study a three-dimensional system of particles interacting via spherically symmetric pair potentials consisting of several discontinuous steps. We show that at certain values of the parameters describing the potential, the system has three first-order phase transitions between fluids of different densities ending in three critical points. [Copyright &y& Elsevier]

High-density amorphous (HDA) ice has a two-level system (TLS) optical density of states with a strength comparable to that found in many conventional glasses, whereas for the low-density amorphous (LDA) phase the TLS strength is at least 30 times smaller. Isotopic substitution resulting in 50% of the molecules having lower symmetry fails to change this strength ratio suggesting that the observed difference is not due to a breaking of the tetrahedral selection rule in the HDA phase. By doping the samples with different concentrations of LiCl, methanol, HF, NaCl, LiOH, and NaOH we have explored both the defect-induced TLS optical density of states and the glass transition characteristics in the LDA ice phase. [Copyright &y& Elsevier]

We report the observation of the boson peak around 5 meV in the inelastic neutron scattering spectrum of a low-density amorphous ice obtained by vapour deposition on a cooled substrate (at 20 K) with low flow rate (∼14 mg/h). The integrated intensity of the acoustical peak in the spectrum of the deposited amorphous ice (1–15 meV) is noticeably larger compared to that for crystalline ice-Ih (by about 5%). There was no excess scattering in the spectra of other low-density amorphous ices studied before, which were obtained either by annealing the high-density amorphous ice or by hyper-quenching of water droplets of μm size. [Copyright &y& Elsevier]

• Molecular dynamics calculations of the forsterite-ice system have been performed. • A low-density layer exists at the interface between the glassy forsterite and amorphous ice. • Intermediate structures are formed with the decomposition of water molecules. To investigate the structure of interface between forsterite glass and amorphous H 2 O ice, we performed molecular dynamics calculations. The result shows that a low-density layer for both forsterite and ice exists in the interface. Furthermore, the decomposition of water molecules was observed in the interface, although the decomposition rate decreases as the temperature decreases. The decomposition mechanisms of H 2 O are important to understand not only to the origin of water on terrestrial planets but also to the chemical evolution processes of various molecules in interstellar molecular clouds. [ABSTRACT FROM AUTHOR]

In an effort to understand the effects of H2O activity on zeolite formation, we have synthesized LTA zeolite using a combination of freezing processes and varying drying temperatures. Sodium aluminate and sodium silicate were used to form LTA zeolite, according to the IZA (International Zeolite Association) protocol. The synthesis steps were modified by adding the precursor frozen process by a rapid liquid nitrogen (−196 °C) treatment or slow conventional freezer treatment (−20 °C). The samples were subsequently sonicated and then dried at 80 °C or 40 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed on the samples immediately after the drying process as well as after 2 weeks and 1 month of aging the solid products. The results indicated that LTA zeolite does not form. The silica-alumina precursor after both freezing processes and after being dried at 80 °C showed the presence of sodalite displaying stable behavior over time. Both sets of samples dried at 40 °C and did not show the presence of zeolite immediately after the drying process. However, after 2 weeks, the liquid nitrogen–frozen precursor was characterized by the presence of EMT whereas zeolites never formed in the −20 °C samples. These results suggest that freezing processes differently control the H2O activity during the drying and aging processes in the solid state. Thus, although the precursor chemical composition is the same, the type of zeolite formed is different. [ABSTRACT FROM AUTHOR]

• Layers of methane-saturated amorphous ice with ice nanocrystals introduced into them demonstrate instability to local thermal perturbation that occurs in the places of their random accumulation. • The presence of crystal centers in a amorphous layer significantly reduces its stability and provides a transition to the explosive mode of crystallization of the metastable phase. • The realization of the spontaneous crystallization regime under conditions of deep metastability ensures the capture of gas molecules and the formation of a gas hydrate. The conditions of the emergence of self-sustaining (explosive) crystallization in amorphous substances under intense non-steady state nucleation that occur during the formation of gas hydrates in low-temperature water-gas condensates prepared by a condensation method have been analyzed. The possibility of a spontaneous occurrence of hot crystallization centers in places of random accumulation of fluctuation-induced nuclei capable of causing local heating and transition to the explosive mode of crystallization in a non-equilibrium amorphous medium has been discussed. The stability of methane-saturated amorphous ice layers with crystallization centers introduced artificially has been experimentally investigated. The presence of crystal nuclei in non-equilibrium gas-saturated amorphous condensates shifted the beginning of crystallization to the region of low temperatures. Crystallization took place in several stages. The behavior of DTA thermograms pointed to its development from different hot centers. In conditions of deep metastability, a spontaneous crystallization mode is realized, which ensures the capture of gas molecules and the formation of a gas hydrate. [ABSTRACT FROM AUTHOR]