Changes in the morphology of interstellar ice analogues after hydrogen atom exposure

The morphology of water ice in the interstellar medium is still an open question. Although accretion of gaseous water could not be the only possible origin of the observed icy mantles covering dust grains in cold molecular clouds, it is well known that water accreted from the gas phase on surfaces kept at 10 K forms ice films that exhibit a very high porosity. It is also known that in the dark clouds H(2) formation occurs on the icy surface of dust grains and that part of the energy (4.48 eV) released when adsorbed atoms react to form H(2) is deposited in the ice. The experimental study described in the present work focuses on how relevant changes of the ice morphology result from atomic hydrogen exposure and subsequent recombination. Using the temperature-programmed desorption (TPD) technique and a method of inversion analysis of TPD spectra, we show that there is an exponential decrease in the porosity of the amorphous water ice sample following D-atom irradiation. This decrease is inversely proportional to the thickness of the ice and has a value of ?(0) = 2 × 10(16) D-atoms cm(-2) per layer of H(2)O. We also use a model which confirms that the binding sites on the porous ice are destroyed regardless of their energy depth, and that the reduction of the porosity corresponds in fact to a reduction of the effective area. This reduction appears to be compatible with the fraction of D(2) formation energy transferred to the porous ice network. Under interstellar conditions, this effect is likely to be efficient and, together with other compaction processes, provides a good argument to believe that interstellar ice is amorphous and non-porous.     

Atomic deuterium (hydrogen) adsorption on thin silver films

Atomic deuterium and hydrogen adsorption on thin silver films deposited under UHV conditions on Pyrex glass was studied by means of measurements of the resistance changes ΔR combined with thermal desorption mass spectrometry (TDMS). The roughness factor of thin Ag films of known geometry, textured as a result of sintering, was determined by means of the BET method (xenon adsorption), while their preferential crystallographic orientation (1 1 1) was estimated on the basis of XRD data. ΔR measurements were performed during various exposures of the films maintained at a constant temperature (78 or 89 K) to the flux of atomic deuterium (hydrogen) of known concentration generated on a hot tungsten filament. Every adsorption run was followed by thermal desorption. This gives a link between the ΔR measured directly in the course of adsorption and the coverage Θ determined on the basis of TDMS data, together with the BET and XRD results. It was found that at 78 K the rate of atomic deuterium (hydrogen) adsorption and recombination on the surface of sintered thin Ag films fits the Eley–Rideal (ER) mechanism, while at 89 K its overlapping with the Langmuir–Hinshelwood (LH) recombination starts to play a role. The initial sticking probability reaches 0.41 and 0.65 for D and H atoms, respectively, while the corresponding probabilities for recombination are 0.04 and 0.07. The activation energies for associative desorption of deuterium and hydrogen are 36 and 29 kJ/mol, respectively.     

Determination of the sticking coefficient and scattering dynamics of water on ice using molecular beam techniques

The sticking coefficient for D(2)O impinging on crystalline D(2)O ice was determined for incident translational energies between 0.3 and 0.7 eV and for H(2)O on crystalline H(2)O ice at 0.3 eV. These experiments were done using directed molecular beams, allowing for precise control of the incident angle and energy. Experiments were also performed to measure the intensity and energy of the scattered molecules as a function of scattering angle. These results show that the sticking coefficient was near unity, slightly increasing with decreasing incident energy. However, even at the lowest incident energy, some D(2)O did not stick and was scattered from the ice surface. We observe under these conditions that the sticking probability asymptotically approaches but does not reach unity for water sticking on water ice. We also present evidence that the scattered fraction is consistent with a binary collision; the molecules are scattered promptly. These results are especially relevant for condensation processes occurring under nonequilibrium conditions, such as those found in astrophysical systems.     

High physisorption affinity of water molecules to the hydroxylated aluminum oxide (001) surface

The adsorption mechanism of water on the hydroxylated (001) plane of α-Al(2)O(3) was studied by measuring adsorption isotherms and GCMC simulations. The experimental adsorption isotherms for three α-Al(2)O(3) samples from different sources are typical type II, in which adsorption starts sharply at low pressures, suggesting a high affinity of water to the Al(2)O(3) surface. Water molecules are adsorbed in two registered forms (bilayer structure). In the first form, water is registered at the center of three surface hydroxyl groups by directing a proton of the water. In the second form, a water molecule is adsorbed by bridging two of the first-layer water molecules through hydrogen bonding, by which a hexagonal ring network is constructed over the hydroxylated surface. The network domains are spread over the surface, and their size decreases as the temperature increases. The simulated adsorption isotherms present a characteristic two-dimensional (2D) phase diagram including a 2D critical point at 365K, which is higher than that on the hydroxylated Cr(2)O(3) surface (319 K). This fact substantiates the high affinity of water molecules to the α-Al(2)O(3) surfaces, which enhances the adsorbability originating from higher heat of adsorption. The higher affinity of water molecules to the α-Al(2)O(3) (001) plane is ascribed to the high compatibility of the crystal plane to form a hexagonal ring network of (001) plane of ice Ih.     

Sticking of CO to crystalline and amorphous ice surfaces

We present results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of crystalline ice Ih and to the surface of amorphous ice Ia. The calculations were performed for normal incidence at a surface temperature Ts = 90 K for ice Ia, and at Ts = 90 and 150 K for ice Ih. For both surfaces, the sticking probability can be fitted to a simple exponentially decaying function of the incidence energy, Ei: Ps = 1.0e(-Ei(kJ/mol)/90(kJ/mol)) at Ts = 90 K. The energy transfer from the impinging molecule to the crystalline and the amorphous surface is found to be quite efficient, in agreement with the results of molecular beam experiments on the scattering of the similar molecule, N2, from crystalline and amorphous ice. However, the energy transfer is less efficient for amorphous than for crystalline ice. Our calculations predict that the sticking probability decreases with Ts for CO scattering from crystalline ice, as the energy transfer from the impinging molecule to the warmer surfaces becomes less efficient. At high Ei (up to 193 kJ/mol), no surface penetration occurs in the case of crystalline ice. However, for CO colliding with the amorphous surface, a penetrating trajectory was observed to occur into a large water pore. The molecular dynamics calculations predict that the average potential energy of CO adsorbed to ice Ih is -10.1 +/- 0.2 and -8.4 +/- 0.2 kJ/mol for CO adsorbed to ice Ia. These values are in agreement with previous experimental and theoretical data. The distribution of the potential energy of CO adsorbed to ice Ia was found to be wider (with a standard deviation sigma of 2.4 kJ/mol) than that of CO interacting with ice Ih (sigma = 2.0 kJ/mol). In collisions with ice Ia, the CO molecules scatter at larger angles and over a wider distribution of angles than in collisions with ice Ih.     

Diffuse Reflectance IR Spectra of Molecular Hydrogen and Deuterium Adsorbed on Zinc Oxide

Diffuse reflectance IR spectroscopy is used to study hydrogen and deuterium adsorption on zinc oxide at room temperature and 77 K. At room temperature, H₂ and D₂ molecules are dissociatively adsorbed with the formation of hydrides and hydroxy groups of three types. At 77 K, diffuse reflectance spectra reveal the bands from molecular hydrogen and deuterium in addition to the dissociatively adsorbed forms. The presence of several bands of stretching H–H and D–D vibrations points to the nonuniformity of adsorption sites. This nonuniformity is also confirmed by the fact that, after heating zinc oxide from 77 K to room temperature in an atmosphere of hydrogen, only an insignificant portion of adsorbed molecular hydrogen dissociates. Most of dissociatively adsorbed hydrogen is formed without a molecular precursor. The dissociation of H₂ and D₂ most likely occurs on very active adsorption species so rapidly that the molecular precursor is not observed. The bond energy in molecular deuterium precursors of dissociation estimated from the fundamental vibration frequency and the overtone of D–D vibrations suggests moderate excitation of the bond. This agrees well with the conclusion that the dissociative adsorption of hydrogen and deuterium occurs without a molecular precursor.     

Vacuum ultraviolet photodissociation and surface morphology change of water ice films dosed with hydrogen chloride

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193 nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100-145 K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100-140 K, while the microstructure of the ice surface stayed of polycrystalline at 145 K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157 nm photodissociation of neutral HCl adsorbed on water ice films at 100-140 K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100-140 K.     

Co-adsorption of water and hydrogen on Ni(111)

We have studied the surface coverage dependence of the co-adsorption of D and D(2)O on the Ni(111) surface under UHV conditions. We use detailed temperature-programmed desorption studies and high resolution electron energy loss spectroscopy to show how pre-covering the surface with various amounts of D affects adsorption and desorption of D(2)O. Our results show that the effects of co-adsorption are strongly dependent on D-coverage. In the deuterium pre-coverage range of 0-0.3 ML, adsorption of deuterium leaves a fraction of the available surface area bare for D(2)O adsorption, which shows no significant changes compared to adsorption on the bare surface. Our data indicate phase segregation of hydrogen and water into islands. At low post-coverages, D(2)O forms a two-phase system on the remaining bare surface that shows zero-order desorption kinetics. This two phase system likely consists of a 2-D solid phase of extended islands of hexamer rings and a 2-D water gas phase. Increasing the water post-dose leads at first to 'freezing' of the 2-D gas and is followed by formation of ordered, multilayered water islands in-between the deuterium islands. For deuterium pre-coverages between 0.3 and 0.5 ML, our data may be interpreted that the water hexamer ring structure, (D(2)O)(6), required for the formation of an ordered multilayer, does not form anymore. Instead, more disordered linear and branched chains of water molecules grow in-between the extended, hydrophobic deuterium islands. These deuterium islands have a D-atom density in agreement with a (2x2)-2D structure. The disordered water structures adsorbed in-between form nucleation sites for growth of 3-D water structures. Loss of regular lateral hydrogen bonding and weakened interaction with the substrate reduces the binding energy of water significantly in this regime and results in lowering of the desorption temperature. At deuterium pre-coverages greater than 0.5 ML, the saturated (2x2)-2D structure mixes with (1x1)-1D patches. The mixed structures are also hydrophobic. On such surfaces, submonolayer doses of water lead to formation of 3-D water structures well before wetting the entire hydrogen-covered surface.     

In situ FTIR study on the formation and adsorption of CO on alumina-supported noble metal catalysts from H2 and CO2 in the presence of water vapor at high pressures

The formation and adsorption of CO from CO(2) and H(2) at high pressures were studied over alumina-supported noble metal catalysts (Pt, Pd, Rh, Ru) by in situ FTIR measurements. To examine the effects of surface structure of supported metal particles and water vapor on the CO adsorption, FTIR spectra were collected at 323 K with untreated and heat (673 K) treated catalysts in the absence and presence of water (H(2)O, D(2)O). It was observed that the adsorption of CO occurred on all the metal catalysts at high pressures, some CO species still remained adsorbed under ambient conditions after the high pressure FTIR measurements, and the frequencies of the adsorbed CO species were lower either for the heat treated samples or in the presence of water vapor. It is assumed that the CO absorption bands on atomically smoother surfaces appear at lower frequencies and that water molecules are adsorbed more preferentially on atomically rough surfaces rather than CO species.     

Ring-polymer Molecular Dynamics Simulation for the Adsorption of H2 on Ice Clusters (H2O)n (n=8, 10, and 12)

In the interstellar medium, the H₂ adsorption and desorption on the solid water ice are crucial for chemical and physical processes. We have recently investigated the probabilities of H₂ sticking on the (H₂O)₈ ice, which has quadrilateral surfaces. We have extended the previous work using classical MD and ring-polymer molecular dynamics (RPMD) simulations to the larger ice clusters, (H₂O)₁₀ and (H₂O)₁₂, which have pentagonal and hexagonal surfaces, respectively. The H₂ sticking probabilities decreased as the temperature increased for both cluster cases, whereas the cluster-size-independent profiles were observed. It is thought that the size independence of the probabilities is qualitatively understood from the similar binding energies for all the three cluster systems. Furthermore, the RPMD sticking probabilities are smaller than the classical ones because of the reduction in the binding energies owing to nuclear quantum effects, such as vibrational quantization.     

Glycine-ice nanolayers: morphology and surface energetics

Ultrathin glycine-ice films (nanolayers) have been prepared in ultrahigh vacuum by condensation of H(2)O and glycine at 110 K and 150 K on single crystalline Al(2)O(3) surfaces and have been investigated by temperature programed thermal desorption, x-ray photoelectron spectroscopy, and work function measurements. Various layer architectures have been considered, including glycine-on-ice, ice-on-glycine, and mixed glycine-ice nanolayers. Low coverages of adsorbed glycine molecules on amorphous ice surfaces suppress the amorphous-to-crystalline phase transition in the temperature range 140-160 K in near-surface regions and consequently lead to a lower desorption temperature of H(2)O molecules than from pure ice layers. Thicker glycine overlayers on ice provide a kinetic restriction to H(2)O desorption from the underlying ice layers until the glycine molecules become mobile and develop pathways for water desorption at higher temperature (>170 K). Ice overlayers do not wet glycine film surfaces, but the glycine molecules on ice are sufficiently immobile at 110 K, so that continuous glycine overlayers form. In mixed glycine-ice nanolayers the glycine phase displays hydrophobic behavior and a phase separation takes place, with the accumulation of glycine near the surfaces of the films.     

Adsorption study of acetone on acid-doped ice surfaces between 203 and 233 K

Adsorption studies of acetone on pure ice surfaces obtained by water freezing or deposition or on frozen ice surfaces doped either with HNO3 or H2SO4 have been performed using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted over the temperature range 203-233 K and freezing solutions containing either H2SO4 (0.2 N) or HNO3 (0.2-3 N). Adsorption of acetone on these ice surfaces was always found to be totally reversible whatever were the experimental conditions. The number of acetone molecules adsorbed per ice surface unit N was conventionally plotted as a function of acetone concentration in the gas phase. For the same conditions, the amount of acetone molecules adsorbed on pure ice obtained by deposition are about 3-4 times higher than those measured on frozen ice films, H2SO4-doped ice surfaces lead to results comparable to those obtained on pure ice. On the contrary, N increases largely with increasing concentrations of nitric acid in ice surfaces, up to about 300 times under our experimental conditions and for temperatures ranging between 213 and 233 K. Finally, the results are discussed and used to reestimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.     

Water adsorption on Rh(111) at 20 K: from monomer to bulk amorphous ice

The adsorption of water (D(2)O) molecules on Rh(111) at 20 K was investigated using infrared reflection absorption spectroscopy (IRAS). At the initial stage of adsorption, water molecules exist as monomers on Rh(111). With increasing water coverage, monomers aggregate into dimers, larger clusters (n = 3-6), and two-dimensional (2D) islands. Further exposure of water molecules leads to the formation of three-dimensional (3D) water islands and finally to a bulk amorphous ice layer. Upon heating, the monomer and dimer species thermally migrate on the surface and aggregate to form larger clusters and 2D islands. Based on the temperature dependence of OD stretching peaks, we succeeded in distinguishing water molecules inside 2D islands from those at the edge of 2D islands. From the comparison with the previous vibrational spectra of water clusters on other metal surfaces, we conclude that the number of water molecules at the edge of 2D islands is comparable with that of water molecules inside 2D islands on the Rh(111) surface at 20 K. This indicates that the surface migration of water molecules on Rh(111) is hindered as compared with the cases on Pt(111) and Ni(111) and thus the size of 2D islands on Rh(111) is relatively small.     

功函数测量研究氧与银和银-钯合金表面的相互作用

在超高真空条件下, 采用电子束阻挡势技术测量固体表面功函数连续变化, 并与AES, TDS等手段相配合, 研究了氧在Ag和Ag-Pd合金表面的吸附和脱附动力学。结果表明, 在Ag和Ag-Pd合金表面的两种不同的氧吸附态, 具有相反的电荷转移效应, 未解离的分子态吸附降低了表面功函数, 而解离的原子态氧吸附使表面功函数明显升高。与纯银相比, 氧在银钯合金表面吸附具有较小的粘附系数和较大的偶极矩。银钯合金组成变化时功函数和AES的连续测量表明, 表面结构从无序向有序转变和表面银偏析均为功函数降低过程。     

Structure and Growth of Two-dimensional Ices at the Surfaces Probed by Scanning Probe Microscopy

Scanning probe microscopy(SPM) stands out as one of the most powerful tools for characterizing the solid surface and the adsorbed molecules with ?ngstr?m resolution in real space. In particular, this unique technique provides an unprecedented opportunity for directly probing the low-dimensional ices at surfaces. In this perspective, we first review the recent advances of scanning tunneling microscopy(STM) imaging of various two-dimensional(2 D) ice structures on metal[1-7], insulator[8-12], graphite[13-15] surfaces and under strong confinement[10, 16-19]. We then introduce that noncontact atomic-force microscopy(AFM) with a CO-terminated tip enables atomic imaging of a genuine 2 D ice grown on a hydrophobic Au(111) surface with minimal perturbation[20], paying particular attention to the growth processes at the edges of 2 D ice. In the end, we present an outlook on the future applications of 2 D ice as well as the relation between the 2 D and 3 D ice growth.     

Adsorption and structure of water on kaolinite surfaces: possible insight into ice nucleation from grand canonical monte carlo calculations

Grand canonical Monte Carlo calculations are used to determine water adsorption and structure on defect-free kaolinite surfaces as a function of relative humidity at 235 K. This information is then used to gain insight into ice nucleation on kaolinite surfaces. Results for both the SPC/E and TIP5P-E water models are compared and demonstrate that the Al-surface [(001) plane] and both protonated and unprotonated edges [(100) plane] strongly adsorb at atmospherically relevant relative humidities. Adsorption on the Al-surface exhibits properties of a first-order process with evidence of collective behavior, whereas adsorption on the edges is essentially continuous and appears dominated by strong water lattice interactions. For the protonated and unprotonated edges no structure that matches hexagonal ice is observed. For the Al-surface some of the water molecules formed hexagonal rings. However, the a o lattice parameter for these rings is significantly different from the corresponding constant for hexagonal ice ( Ih). A misfit strain of 14.0% is calculated between the hexagonal pattern of water adsorbed on the Al-surface and the basal plane of ice Ih. Hence, the ring structures that form on the Al-surface are not expected to be good building-blocks for ice nucleation due to the large misfit strain.     

Adsorption of trinitrotoluene on uncoated silicon microcantilever surfaces

We measured the adsorption characteristics of trinitrotoluene (TNT) on piezoresistive silicon microcantilever surfaces under ambient air using a well-characterized TNT vapor generator. This allowed us to quantify the adsorption parameters and to estimate the sticking coefficient. The sticking coefficient initially increases with TNT exposure time and then levels off around 0.3. Atomic force microscopy images of silicon surfaces exposed to TNT revealed "island" formation of the adsorbate on the silicon surface. At low exposure times, mainly the number density of islands increased with exposure time; at longer exposure times, the size (in particular, height) of the islands grew, corresponding to the higher sticking coefficients. These observations can be qualitatively explained via the difference between TNT-surface and TNT-TNT interactions mediated by water molecules.     

Interaction of methanol with amorphous solid water

The interaction of methanol (MeOH) with amorphous solid water (ASW) composed of D2O molecules, prepared at 125 K on a polycrystalline Ag substrate, was studied with metastable-impact-electron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed desorption mass spectroscopy. In connection with the experiments, classical molecular dynamics (MD) simulations have been performed on a single CH3OH molecule adsorbed at the ice surface (T=190 K), providing further insights into the binding and adsorption properties of the molecule at the ice surface. Consistently with the experimental deductions and previous studies, MeOH is found to adsorb with the hydroxyl group pointing toward dangling bonds of the ice surface, the CH3 group being oriented upwards, slightly tilted with respect to the surface normal. It forms the toplayer up to the onset of the simultaneous desorption of D2O and MeOH. At low coverage the adsorption is dominated by the formation of two strong hydrogen bonds as evidenced by the MD results. During the buildup of the first methanol layer on top of an ASW film the MeOH-MeOH interaction via hydrogen-bond formation becomes of importance as well. The interaction of D2O with solid methanol films and the codeposition of MeOH and D2O were also investigated experimentally; these experiments showed that D2O molecules supplied to a solid methanol film become embedded into the film.     

Adsorption of atomic oxygen and nitrogen at beta-cristobalite (100): a density functional theory study

The adsorption of atomic oxygen and nitrogen on the beta-cristobalite (100) surface is investigated from first principles density functional calculations within the generalized gradient approximation. A periodic SiO2 slab model (6 layers relaxing 4 or 6) ended with a layer of Si or O atoms is employed throughout the study. Several adsorption minima and diffusion transition states have been characterized for the two lowest spin states of both systems. A strong chemisorption is found for either O or N in several sites with both slab endings (e.g., it is found an average adsorption energy of 5.89 eV for O (singlet state) and 4.12 eV for N (doublet state) over the Si face). The approach of O or N on top O gives place to the O2 and NO abstraction reactions without energy barriers. Atomic sticking coefficients and desorption rate constants have been estimated (300-1900 K) by using the standard transition state theory. The high adsorption energies found for O and N over silica point out that the atomic recombination processes (i.e., Eley-Rideal and Langmuir-Hinshelwood mechanisms) will play a more important role in the atomic detachment processes than the thermal desorption processes. Furthermore, the different behavior observed for the O and N thermal desorption processes suggests that the published kinetic models for atomic O and N recombination reactions on SiO2 surfaces, based on low adsorption energies (e.g., 3.5 eV for both O and N), should probably be revised.