Interaction of NaI with solid water and methanol

The interaction of NaI with amorphous solid water (ASW) and methanol (MeOH) has been investigated with metastable impact electron spectroscopy (MIES), UPS(HeI), and temperature programmed desorption (TPD). We have studied the electron emission from the ionization of the highest-lying states of H(2)O, CH(3)OH, and of 5pI. We have prepared NaI layers on ASW (MeOH) films at about 105 K and annealed them up to about 200 K. Surface segregation of iodide is observed in ASW, as predicted for NaI aqueous solutions. On the other hand, surface segregation is not observed in MeOH, again as predicted for the interaction of NaI with liquid methanol. Electronic properties (ionization potentials, optical band gaps) and water binding energies are reported and are analyzed on the basis of available DFT results for hydrated NaI clusters.     

Interaction of benzene with amorphous solid water adsorbed on polycrystalline Ag

The interaction of benzene with polycrystalline Ag and amorphous solid water (D(2)O) deposited thereupon at 124 K was investigated. Metastable impact electron spectroscopy, Reflection-absorption infrared spectroscopy, and temperature programmed desorption were utilized to obtain information on the electronic structure and the relative contribution to the bonding properties of the aromatic molecules among themselves and with D(2)O. On Ag, the benzene molecular plane is oriented parallel to the surface in the first layer. The second layer is tilted with respect to the first one. A total work function decrease of 0.8 eV takes place during the buildup of the first two layers. On amorphous solid water, the orientational distribution of the benzene molecular planes is initially peaked at an angle parallel to the water surface. During the completion of the first adlayer a coverage-induced reorientation takes place, inducing a tilt of the benzene molecules of the first adlayer. Still larger benzene exposures appear to lead to the formation of three-dimensional benzene clusters. Films produced by codepositing benzene and D(2)O or by postdepositing D(2)O layers on benzene films display "volcano like" benzene desorption during ice crystallization.     

Interaction of NaCl with solid water

The interaction of NaCl with solid water, deposited on tungsten at 80 K, was investigated with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) (He I). We have studied the ionization of Cl(3p) and the 1b(1), 3a(1), and 1b(2) bands of molecular water. The results are supplemented by first-principles density functional theory (DFT) calculations of the electronic structure of solvated Cl(-) ions. We have prepared NaCl/water interfaces at 80 K, NaCl layers on thin films of solid water, and H(2)O ad-layers on thin NaCl films; they were annealed between 80 and 300 K. At 80 K, closed layers of NaCl on H(2)O, and vice versa, are obtained; no interpenetration of the two components H(2)O and NaCl was observed. However, ionic dissociation of NaCl takes place when H(2)O and NaCl are in direct contact. Above 115 K solvation of the ionic species Cl(-) becomes significant. Our results are compatible with a transition of Cl(-) species from an interface site (Cl in direct contact with the NaCl lattice) to an energetically favored configuration, where Cl species are solvated. The DFT calculations show that Cl(-) species, surrounded by their solvation shell, are nevertheless by some extent accessed by MIES because the Cl(3p)-charge cloud extends through the solvation shell. Water desorption is noticeable around 145 K, but is not complete before 170 K, about 15 K higher than for pure solid water. Above 150 K the NaCl-induced modification of the water network gives rise to gas phase like structures in the water spectra. In particular, the 3a(1) emission turns into a well-defined peak. This suggests that under these conditions water molecules interact mainly with Cl(-) rather than among themselves. Above 170 K only Cl is detected on the surface and desorbs around 450 K.     

Interaction of formic acid with solid water

The interaction of formic acid (HCOOH) with solid water, deposited on tungsten at 80 K, was investigated. We have prepared and annealed formic acid (FA)/water interfaces (FA layers on thin films of solid water and H(2)O adlayers on thin FA films). Metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (He I and II) were utilized to study the electron emission from the 10a' to 6a' molecular orbitals (MOs) of FA, and the 1b(1), 3a(1), and 1b(2) MOs of H(2)O. These spectra were compared with results of density-functional theory calculations on FA-H(2)O complexes reported in Ref. 14 [A. Allouche, J. Chem. Phys. 122, 234703(2005), (preceding paper)]. Temperature programmed desorption was applied for information on the desorption kinetics. Initially, FA is adsorbed on top of the water film. The FA spectra are distorted with respect to those from FA monomers; it is concluded that a strong interaction exists between the adsorbates. Even though partial solvation of FA species takes place during annealing, FA remains in the top layer up to the desorption of the water film. When H(2)O molecules are offered to FA films at 80 K, no water network is formed during the initial stage of water exposure; H(2)O molecules interact individually via H bonds with the formic acid network. Experiment and theory agree that no water-induced deprotonation of the formic acid molecules takes place.     

Interaction of acetic acid with solid water

The interaction of acetic acid (AA, CH(3)COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, was investigated. We have prepared acid/water interfaces at 80 K, namely, acid layers on thin films of solid water and H(2)O adlayers on thin acid films; they were annealed between 80 and 200 K. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy UPS(HeII) were utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest bound MOs of the acids, and of the molecular water. Temperature-programmed desorption (TPD) provided information on the desorption kinetics, and Fourier-transformed infrared spectroscopy (FTIR) provided information on the identification of the adsorbed species as well as on the water and acid crystallization. The results are compatible with the finding of ref 1 (preceding paper), made on the basis of DFT calculations, that AA adsorbs on ice as cyclic dimers. Above 120 K, a rearrangement of the AA dimers is suggested by a sharpening of the spectral features in the IR spectra and by spectral changes in MIES and UPS; this is attributed to the glass transition in AA around 130 K. Above 150 K the spectra transform into those characteristic for polycrystalline polymer chains. This structure is stable up to about 180 K; desorption of water takes place from underneath the AA film, and practically all water has desorbed through the AA film before AA desorption starts. There is no indication of water-induced deprotonation of the acid molecules. For the interaction of H(2)O molecules adsorbed on amorphous AA films, the comparison of MIES with the DFT results of ref 1 shows that the initial phase of exposure does not lead to the formation of a top-adsorbed closed water film at 80 K. Rather, the H(2)O molecules become attached to or incorporated into the preexisting AA network by H bonding; no water network is formed in the initial stage of the water adsorption. Also under these conditions no deprotonation of the acid can be detected.     

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.     

Electron stimulated reactions of methyl iodide coadsorbed with amorphous solid water

The electron stimulated reactions of methyl iodide (MeI) adsorbed on and suspended within amorphous solid water (ice) were studied using a combination of postirradiation temperature programmed desorption and reflection absorption infrared spectroscopy. For MeI adsorbed on top of amorphous solid water (ice), electron beam irradiation is responsible for both structural and chemical transformations within the overlayer. Electron stimulated reactions of MeI result principally in the formation of methyl radicals and solvated iodide anions. The cross section for electron stimulated decomposition of MeI is comparable to the gas phase value and is only weakly dependent upon the local environment. For both adsorbed MeI and suspended MeI, reactions of methyl radicals within MeI clusters lead to the formation of ethane, ethyl iodide, and diiodomethane. In contrast, reactions between the products of methyl iodide and water dissociation are responsible for the formation of methanol and carbon dioxide. Methane, formed as a result of reactions between methyl radicals and either parent MeI molecules or hydrogen atoms, is also observed. The product distribution is found to depend on the film's initial chemical composition as well as the electron fluence. Results from this study highlight the similarities in the carbon-containing products formed when monohalomethanes coadsorbed with amorphous solid water are irradiated by either electrons or photons.     

Interaction of methanol with ruthenium

The interaction of methanol with a clean (110) ruthenium surface has been studied using temperatures programmed desorption methods. Methanol dissociates upon adsorption at 300 K and yields H₂(g) and chemisorbed CO as the dominant products. Randomization of evolved hydrogen was shown to occur during methanol adsorption and also upon subsequent thermal desorption using isotopically labeled methanol, CH₃OD. In addition to hydrogen and CO, small amounts of H₂CO, CH₃OH, CO₂, and H₂O, are also observed upon thermal desorption. In contrast with a previous study of formaldehyde on Ru(110), no detectable CH₄ product is found upon methanol desorption.     

Ultrafast electron-induced desorption of water from nanometer amorphous solid water films

Laser-induced desorption of water molecules from nanometer amorphous solid water films supported on a single-crystal platinum substrate is reported. A femtosecond laser pulse creates hot substrate electrons, which are injected into the water layer, resulting in significant desorption at the water-vacuum interface. The dependence of the desorption yield on film thickness and results for isotopic spacer and capping layers reveal that the desorbing water originates from relatively deep down into the water layer, i.e., from several nanometers below the surface. This is proposed to be the result of cooperative electronic effects resulting from the high electron densities in the thin water film, which cause a transient destabilization of the water H-bonded network. Motion of excited water molecules through the layer is enabled by mixing within the layer on ultrafast timescales during the desorption process.     

Interaction of trivalent f-element cations with iodide in methanol + water solvents

The stability constants (β ₁) of iodide ion-pairs of trivalent f-block element ions (lanthanoids Ce, Eu, Gd, Tb and Tm, and actinoid Am) were determined in the vicinity of pH 2.5 of mixed methanol/water solvent solutions of an ionic strength of 1.00 mol·dm⁻³ at 298±1 K. The values were less than 2. From the variation in distance between Eu³⁺ and I⁻, which was calculated using a Born-type equation for Gibbs’ free energy derived from β ₁(Eu), the Eu³⁺-I⁻ interaction was shown to be solvent-shared ion-pair formation when the mole fraction of methanol (X _MeOH)≤0.40. In contrast, it was suggested that the interaction of Am³⁺-I⁻ changed from solvent-separated ion-pair to solvent-shared ion-pair with increasing X _MeOH when X _MeOH≤0.10, but remained as solvent-shared ion-pair in the range 0.16≤X _MeOH≤0.40. Furthermore, β ₂(Am) was measured in the range 0.31≤X _MeOH≤0.40. It was also shown that the β ₁ values of lanthanoids at X _MeOH = 0.40, except for that of Gd(III), decreased with increasing atomic number.     

Electron-stimulated production of molecular oxygen in amorphous solid water

The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films codosed with H(2)O(2) is investigated. Layered films of H(2)(16)O and H(2)(18)O are used to investigate the reaction mechanisms for O(2) production and the spatial profile of the reactions within the films. The O(2) yield is dose-dependent, indicating that precursors are involved in the O(2) production. For temperatures below approximately 80 K, the O(2) yield at steady state is relatively low and nearly independent of temperature. At higher temperatures, the yield increases rapidly. The O(2) yield is enhanced from H(2)O(2)-dosed water films, but the experiments show that H(2)O(2) is not the final precursor in the reactions leading to O(2). Instead, a stable precursor for O(2) is produced through a multistep reaction sequence probably involving the reactions of OH radicals to produce H(2)O(2) and then HO(2). The O(2) is produced in a nonthermal reaction from the HO(2). For relatively thick films, the reactions leading to O(2) occur at or near the ASW/vacuum interface. However, the electronic excitations that initiate the reactions occur over a larger range in the film. A kinetic model that qualitatively accounts for all of the observations is presented.     

Photochemistry of ethyl chloride caged in amorphous solid water

Caging and photo-induced decomposition of ethyl chloride molecules (EC) within a layer of amorphous solid water (ASW) on top of clean and oxygen-covered Ru(001) under ultra-high vacuum (UHV) conditions are presented. The caged molecules were estimated to reside 1.5 +/- 0.2 nm above the solid surface, based on parent molecule thermal decomposition on the clean ruthenium. Dissociative electron attachment (DEA) of the caged molecules following 193 nm laser irradiation, result in initial fragmentation to ethyl radical and chloride anion. It was found that photoreactivity on top of the clean ruthenium surface (Ru) is twenty times faster than on the oxygen-covered surface (O/Ru), with DEA cross sections: sigma(Ru) = (3.8 +/- 1) x 10(-19) cm(2) and sigma(O/Ru) = (2.1 +/- 0.3) x 10(-20) cm(2). This difference is attributed to the higher work function of oxygen-covered ruthenium, leading to smaller electron attachment probability due to mismatch of the ruthenium photo-electron energy with the adsorbed EC excited electron affinity levels. EC molecules fragmented within the cage, result in post-irradiation TPD spectra that reveal primarily C(4)H(8), C(3)H(5) and C(3)H(3), without any oxygen-containing molecules. Unique stabilization of the photoproducts has been observed with the first layer of water molecules in direct contact with the substrate, desorbing near 180 K, a significantly higher temperature than the desorption of fully caged molecules. This study may contribute for understanding stratospheric photochemistry and processes in interstellar space.     

Compaction of microporous amorphous solid water by ion irradiation

We have studied the compaction of vapor-deposited amorphous solid water by energetic ions at 40 K. The porosity was characterized by ultraviolet-visible spectroscopy, infrared spectroscopy, and methane adsorption/desorption. These three techniques provide different and complementary views of the structural changes in ice resulting from irradiation. We find that the decrease in internal surface area of the pores, signaled by infrared absorption by dangling bonds, precedes the decrease in the pore volume during irradiation. Our results imply that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, which can explain the absence of dangling bond features in the infrared spectrum of molecular clouds.     

Formation of supercooled liquid solutions from nanoscale amorphous solid films of methanol and ethanol

Molecular beam techniques are used to create layered nanoscale composite films of amorphous methanol and ethanol at 20 K. The films are then heated, and temperature programed desorption and infrared spectroscopy are used to observe the mixing, desorption, and crystallization behavior from the initially unmixed amorphous layers. We find that the initially unmixed amorphous layers completely intermix to form a deeply supercooled liquid solution after heating above T(g). Modeling of the desorption kinetics shows that the supercooled liquid films behave as ideal solutions. The desorption rates from the supercooled and crystalline phases are then used to derive the binary solid-liquid phase diagram. Deviations from ideal solution desorption behavior are observed when the metastable supercooled solution remains for longer times in regions of the phase diagram when thermodynamically favored crystallization occurs. In those cases, the finite lifetime of the metastable solutions results in the precipitation of crystalline solids. Finally, in very thick films at temperatures and compositions where a stable liquid should exist, we unexpectedly observe deviations from ideal solution behavior. Visual inspection of the sample indicates that these apparent departures from ideality arise from dewetting of the liquid film from the substrate. We conclude that compositionally tailored nanoscale amorphous films provide a useful means for preparing and examining deeply supercooled solutions in metastable regions of the phase diagram.     

The nucleation rate of crystalline ice in amorphous solid water

The kinetics of crystalline ice nucleation and growth in nonporous, molecular beam deposited amorphous solid water (ASW) films are investigated at temperatures near 140 K. We implement an experimental methodology and corresponding model of crystallization kinetics to decouple growth from nucleation and quantify the temperature dependence and absolute rates of both processes. Nucleation rates are found to increase from approximately 3x10(13) m(-3) s(-1) at 134 K to approximately 2x10(17) m(-3) s(-1) at 142 K, corresponding to an Arrhenius activation energy of 168 kJ/mol. Over the same temperature range, the growth velocity increases from approximately 0.4 to approximately 4 A s(-1), also exhibiting Arrhenius behavior with an activation energy of 47 kJ/mol. These nucleation rates are up to ten orders of magnitude larger than in liquid water near 235 K, while growth velocities are approximately 10(9) times smaller. Crystalline ice nucleation kinetics determined in this study differ significantly from those reported previously for porous, background vapor deposited ASW, suggesting the nucleation mechanism is dependent upon film morphology.     

Infrared spectroscopy and optical constants of porous amorphous solid water

Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20 K on a Pt(111) substrate at various angles (theta(Beam) = 0-85 degrees ) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film-substrate and film-vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(omega) and k(omega)] for porous ASW in the 1000-4000 cm(-1) (10-2.5 microm) range. The behavior of the optical properties of ASW in the intramolecular OH stretching region with increasing theta(Beam) is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference ( approximately 200 cm(-1)) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing theta(Beam) in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects.     

Comparative study of the interaction of pyridine with polycrystalline Ag and amorphous solid water

The interaction of pyridine (C5H5N) with polycrystalline Ag and amorphous solid water (D2O) is compared. Metastable impact electron spectroscopy (MIES) and reflection-absorption infrared spectroscopy (RAIRS) were utilized to obtain information on the structure of the pyridine-Ag and pyridine-water interfaces. On polycrystalline Ag, C5H5N adsorbs with its molecular axis perpendicular to the surface whereby a work function decrease of 1.5 eV takes place during the build up of the first layer. In the second layer the molecular axis is tilted with respect to the surface normal. On amorphous solid water, C5H5N is initially adsorbed on top with its ring plane oriented preferentially near parallel with respect to the surface, reflecting the contribution of two different interactions to the bonding, the formation of a pi-hydrogen bond, and competitive bonding via the nitrogen lone pair. Coverage-driven reorientation takes place during the completion of the first monolayer and increases the average tilt angle. We have followed the growth of pyridine films up to the third layer which, according to RAIRS, shows clear signs of condensation. No embedding of pyridine species into the underlying water film can be noticed when heating up to desorption. The exposure of a pyridine film at 124 K to D2O molecules does not lead to on top adsorption. Instead, D2O becomes initially embedded into the pyridine film, and RAIRS indicates solvation of the pyridine species.     

Transport in amorphous solid water films: implications for self-diffusivity

Thermal desorption spectroscopy is employed to examine transport mechanisms in structured, nanoscale films consisting of labeled amorphous solid water (ASW, H(2)(18)O, H(2)(16)O) and organic spacer layers (CCl(4), CHCl(3)) prior to ASW crystallization (T approximately 150-160 K). Self-transport is studied as a function of both the ASW layer and the organic spacer layer film thickness, and the effectiveness of these spacer layers as a bulk diffusion "barrier" is also investigated. Isothermal desorption measurements of structured films are combined with gas uptake measurements (CClF(2)H) to investigate water self-transport and changes in ASW film morphology during crystallization and annealing. CCl(4) desorption is employed as a means to investigate the effects of ASW film thickness and heating schedule on vapor-phase transport. Combined, these results demonstrate that the interlayer mixing observed near T approximately 150-160 K is inconsistent with a mechanism involving diffusion through a dense phase; rather, we propose that intermixing occurs via vapor-phase transport through an interconnected network of cracks/fractures created within the ASW film during crystallization. Consequently, the self-diffusivity of ASW prior to crystallization (T approximately 150-160 K) is significantly smaller than that expected for a "fragile" liquid, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.     

Characterization of porosity in vapor-deposited amorphous solid water from methane adsorption

We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that the multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.     

Electron-stimulated sputtering of thin amorphous solid water films on Pt(111)

The electron-stimulated sputtering of thin amorphous solid water films deposited on Pt(111) is investigated. The sputtering appears to be dominated by two processes: (1) electron-stimulated desorption of water molecules and (2) electron-stimulated reactions leading to the production of molecular hydrogen and molecular oxygen. The electron-stimulated desorption of water increases monotonically with increasing film thickness. In contrast, the total sputtering--which includes all electron-stimulated reaction channels--is maximized for films of intermediate thickness. The sputtering yield versus thickness indicates that erosion of the film occurs due to reactions at both the water/vacuum interface and the Pt/water interface. Experiments with layered films of D2O and H2O demonstrate significant loss of hydrogen due to reactions at the Pt/water interface. The electron-stimulated sputtering is independent of temperature below approximately 80 K and increases rapidly at higher temperatures.