2009. "The Effect of the Incident Collision Energy on the Porosity of Vapor Deposited Amorphous Solid Water Films." Journal of Physical Chemistry B 113(13):4000-4007. Abstract Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mole. The effect of the incident angle and collision energy on the porosity and surface area of the vapor deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mole), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels-off at angles > 65°. This is in contrast to the nitrogen uptake data which display a maximum near 70° due to the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition-shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices.
2009. "Reactivity of Fe-0 Atoms, Clusters, and Nanoparticles with CC14 Multilayers on FeO(111)." Journal of Physical Chemistry C 113(5):1818-1829. Abstract The interaction of Fe0 atoms and clusters with CCl4 multilayers was investigated using a novel "atom dropping" method at 30 K over a FeO(111) thin film. Temperature programmed desorption experiments over a range of Fe0 and CCl4 coverages demonstrate a rich surface chemistry with several reaction products (C2Cl4, C2Cl6, OCCl2, CO, FeCl2, FeCl3) observed. X-ray photoelectron spectroscopy data show that the initial reactive interaction occurs spontaneously at 30 K, with the experimentally observed reaction products formed at higher temperature, in agreement with the results of theoretical calculations. The formation of OCCl2 and CO is concluded to occur through abstraction of O atoms from the generally inert FeO(111) substrate. The buffer layer assisted growth technique is used to show that the reactivity, and interestingly the reaction products, is determined by the size of Fe0 nanoparticles which interact with CCl4.
2009. "Reactivity of Fe-0 atoms and clusters with D2O over FeO(111)." Journal of Physical Chemistry C 113(2):4960-4969. Abstract The interaction of Fe0 atoms with D2O layers on FeO(111) has been investigated using the “atom dropping” preparation technique and a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, Auger electron spectroscopy, and infrared absorption spectroscopy. The data demonstrate that isolated Fe atoms form DFeOD insertion species upon deposition at 35 K, which then dissociate into FeOD and a surface hydroxyl above 200 K. Interestingly, even at very low Fe0 coverages the D2O is perturbed by the presence of the Fe, but only D2O desorption is observed. At higher (≥ 0.5 ML) coverages, clusters of Fe form which have molecular D2O and OD species adsorbed on the surface. Both molecular and recombinative desorption are observed in TPD. In contrast to the low coverage data, a second reaction pathway emerges at high coverage which leads to desorption of D2 and the formation of stable substoichiometric oxide. The mechanism for this minor channel is concluded to involve a reaction between two (or more) DFeOD complexes.
2009. "Reactivity of C2Cl6 and C2Cl4 multilayers with Fe0 atoms over FeO(111)." Journal of Physical Chemistry C 113(23):10233-10241. Abstract The interaction of Fe0 atoms with C2Cl6 multilayers over FeO(111) has been investigated using the “atom dropping” preparation technique and a combination of temperature programmed desorption, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. The reactivity and reaction products are strongly dependent on the Fe0 coverage. Submonolayer Fe0 doses lead to high reactivity and primarily FeCl3 and C4Cl6, whereas multilayer Fe0 doses lead to the production of FeCl2 and C2Cl4 with much lower Fe0 reactivity. The data are consistent with a model where Fe atoms form intermediate species at low coverage, which consist of a Fe atom inserted into a C-Cl bond. When two Fe atoms react with C2Cl6, a different intermediate species is formed which produces the alternative reaction pathway and the formation of C2Cl4. Similar atom dropping experiments demonstrate that C2Cl4 is also reactive towards Fe0 atoms at low Fe0 dose, leading to the production of one FeCl2 molecule per C2Cl4 molecule reacted. At higher coverages, Fe atoms form clusters which are much less reactive toward C2Cl4.
2009. "No Confinement Needed: Observation of a Metastable Hydrophobic Wetting Two-Layer Ice on Graphene." Journal of the American Chemical Society 131(35):12838-12844. Abstract The structure of water at interfaces is crucial for processes ranging from photocatalysis to protein folding. Here, we investigate the structure and lattice dynamics of two-layer crystalline ice films grown on a hydrophobic substrate - graphene on Pt(111) - with low energy electron diffraction, reflection-absorption infrared spectroscopy, rare-gas adsorption/desorption, and ab-initio molecular dynamics. Unlike hexagonal ice, which consists of stacks of puckered hexagonal "bilayers", this new ice polymorph consists of two flat hexagonal sheets of water molecules in which the hexagons in each sheet are stacked directly on top of each other. Such two-layer ices have been predicted for water confined between hydrophobic slits, but not previously observed. Our results show that the two-layer ice forms even at zero pressure at a single hydrophobic interface by maximizing the number of hydrogen bonds at the expense of adopting a non-tetrahedral geometry with weakened bonds.
2009. "Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water." Journal of Physical Chemistry B 113(13):4131-4140. Abstract Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20K on a Pt(111) substrate at various incidence angle (θBeam = 0-85o) 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(ω) and k(ω)] for porous ASW in the 1000-4000cm-1 (10-2.5 μm) range. The behaviour of the optical properties of ASW in the intramolecular OH stretching region with increasing θBeam 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 (~200cm-1) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing θBeam 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.
2007. "Adsorption, Desorption, and Diffusion of Nitrogen in a Model Nanoporous Material: II. Diffusion Limited Kinetics in Amorphous Solid Water." Journal of Chemical Physics 127(14):Art. No. 184708. doi:10.1063/1.2790433 Abstract Tykhon Zubkov, R. Scott Smith, Todd R. Engstrom, and Bruce D. Kay The adsorption, desorption, and diffusion kinetics of N2 on thick (up to ~9 mm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption (TPD). Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 mm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films, (>1 mm), N2 adsorption at 27 K results in a non-uniform distribution where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is ~7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Blocking adsorption sites near the film surface facilitates transport into the film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor-mediated and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed 2nd layer N2 species on top of the 1st layer chemisorbed layer.
2007. "Adsorption, Desorption, and Diffusion of Nitrogen in a Model Nanoporous Material: I. Surface Limited Desorption Kinetics in Amorphous Solid Water." Journal of Chemical Physics 127(18):Art. No. 184707. doi:10.1063/1.2790432 Abstract The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programmed desorption (TPD), and reflection-absorption infrared spectroscopy (RAIRS). The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness. This means that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, ~0.5 mm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.
2007. "Formation of Supercooled Liquid Solutions from Nanoscale Amorphous Solid Films of Methanol and Ethanol." Journal of Chemical Physics 127(24):Art. No. 244705. doi:10.1063/1.2819140 Abstract 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 programmed desorption (TPD) and FTIR spectroscopy are used to observe the mixing, desorption, and crystallization behavior from the initially unmixed amorphous layers. We find that after heating above Tg, the layers completely intermix to form a deeply supercooled liquid solution. Modeling of the desorption kinetics shows that the supercooled liquid films behave as ideal solutions. Deviations from ideal solution desorption behavior are observed when the metastable supercooled solution remains for longer times in regions of the phase diagram where crystallization is thermodynamically favorable. 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.
2006. "The Effect of the Incident Collision Energy on the Phase and Crystallization Kinetics of Vapor Deposited Water Films." Journal of Chemical Physics 124(11):114710 (7 pages). Abstract Molecular beam techniques are used to grow water films on Pt(111) with incident collision energies from 5 to 205 kJ/mole. The effect of the incident collision energy on the phase of vapor deposited water films and their subsequent crystallization kinetics are studied using temperature-programmed desorption and FTIR spectroscopy. We find that for films deposited at substrate temperatures below 110 K, the incident kinetic energy (up to 205 kJ/mole) has no effect on the initial phase of the deposited film or its crystallization kinetics. Above 110 K, the substrate temperature does affect the phase and crystallization kinetics of the deposited films but this result is also independent of the incident collision energy. The presence of a crystalline ice does affect the crystallization of ASW but this effect is also independent of the incident beam energy. These results suggest that the crystallization of amorphous solid water requires cooperative motion of several water molecules.
2005. "Nano-Scaffold Mediates Hydrogen Release and Reactivity of Ammonia Borane." Angewandte Chemie International Edition 44(23):3578-3582. Abstract One of the imposing barriers to realizing the promise of an energy economy based on hydrogen is onboard hydrogen storage for fuel-cell-powered vehicles. New materials that enable the release of dense, plentiful and pure hydrogen at temperatures less than 85 ºC are necessary to move the world from an oil-based economy to a hydrogen economy. We report a novel approach in which we deposit a hydrogen-rich material into a nanoporous scaffold. The role of the scaffold is to impose a nano-phase structure on the hydrogen-rich material thus providing an additional handle on the kinetics and thermodynamics of hydrogen release. We demonstrate on the example of ammonia borane infused in the nanoporous silica that the kinetics of hydrogen release is improved while the purity of hydrogen is increased in comparison with the release from bulk ammonia borane. These findings suggest that hydrogen rich materials infused in nanoscaffolds offer the most promising approach to date for onboard hydrogen storage
2005. "Water Adsorption, Desorption, and Clustering on FeO(111)." Journal of Physical Chemistry B 109(20):10362-10370. Abstract The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater) the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K.
2005. "Adsorption and Desorption of HCI on Pt(111)." Journal of Physical Chemistry B 109(32):15506-15514. Abstract The adsorption and desorption of HCl on Pt(111) is investigated by temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and low energy electron diffraction (LEED). Five peaks are identified in the TPD spectra prior to the onset of multilayer desorption. For (theta > 0.38 ML, theta = 1.5x1015 cm-2) HCl adsorbs molecularly at 20 K as an amorphous solid with two desorption peaks at 70 and 77 K. A third peak at 84 K is tentatively assigned to molecularly adsorbed HCl. Peaks at ~135 and 200 K are assigned to recombinative desorption of dissociated HCl, in agreement with earlier studies. Kinetic analysis is conducted and the results demonstrate a strong coverage dependent desorption energy for theta < 0.25 ML. The LEED data indicates that at low temperature the adsorbed HCl clusters into ordered islands with a (3 x 3) structure and a local coverage of 4/9 with respect to the Pt(111) substrate.
2004. "Interaction of CH4, CH3Cl, CH2CI2, and CCl4 with O-terminated FeO(111)." Journal of Physical Chemistry B 108(11):3644-3650. Abstract Well-ordered FeO(111) thin films are epitaxially grown on a Pt(111) substrate. A series of molecules including CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 are used as probes to test the chemical reactivity of the FeO(111) surface. The temperature programmed desorption spectra show no evidence of dissociative adsorption or chemical reaction between the substrate and the adsorbates. The desorption kinetics studies reveal that all the molecules are physisorbed and have desorption kinetics with an order between 0 and 1. Kinetic analysis is conducted, assuming both zero and first order desorption, and shows that an uncertainty in the desorption order introduces an error in determination of the terrace site desorption energies ( = 0.5 ML) of only ~ 2%. The desorption energies for the series of molecules increase with the number of chlorine atoms in the molecule. The increase of desorption energies is not linear with the molecular polarizability and the deviations from linearity are attributed to the permanent dipole in some of the molecules. We conclude that the oxygen terminated FeO(111) surface is in general unreactive toward both C-H and C-Cl bonds.
2004. "Helium Diffusion Through H2O and D2O Amorphous Ice: A Lattice Inverse Istope Effect." Physical Review Letters 92(19):Art. no. 198306. Abstract The diffusion of helium through both H2O and D2O amorphous solid water (ASW) has been measured between 55K and 110K. We find the diffusion rate is dependent on the isotopic composition of the ASW lattice. This lattice isotope effect is the "inverse" of a normal isotope effect, in that diffusion is faster in the heavier (D2O) isotope. Transition state theory calculations show that the isotope effect is due to a tight transition state results in a large zero point vibrational energy differences at the transition state predominantly due to hindered rotations of water in the lattice.
2004. "Adsorption, Desorption, and Clustering H20 on Pt (111)." Journal of Chemical Physics 120(3):1516-1523. Abstract The adsorption, desorption, and clustering behavior of H20 on Pt(111) has been investigated by specular He scattering. The data show that water adsorbed on a clean Pt(111) surface undergoes a transition from a random to a clustered structure near 60 K. The initial helium scattering cross sections as a function of temperature are found to be insensitive to H20 flux over a range of 0.005 ML/s to 0.55 ML/s indicating the clustering process is more complex than simple surface diffusion. The coarsening process of an initially random distribution of water deposited at 25 K is found to occur over a broad temperature range, 60 K< T <140 K, during thermal annealing. The desorption kinetics for submonolayer water are determined to be zero-order for surface coverages greater than 0.05 ML and temperatures between 150 K and 174 K. The zero-order desorption kinetics are consistent with a two-dimensional two-phase coexistence between H2O condensed phase and a 2-gas phase on the Pt surface
2003. "Molecular Beam Studies of Nanoscale Films of Amorphous Solid Water." Chapter 14 in Water in Confining Geometries, ed. V. Buch and J.P. Devlin, pp. 337-357. Springer-Verlag, New York, NY. Abstract In this article, we discuss the use of nanoscale thin films to explore the structural and physical properties of amorphous solid water.
2003. "The Deposition Angle-Dependent Density of Amorphous Solid Water Films." Journal of Chemical Physics 118(1):364-372. Abstract The index of refraction and thickness of amorphous solid water (ASW) films are determined using laser optical interferometry. From the film thickness, the density of ASW can be calculated directly since the molecular beam flux and the H2O condensation coefficient are both known. From the index of refraction the ASW density can also be determined using the Lorentz-Lorenz relationship. The densities determined via both methods agree within experimental uncertainty. For films deposited at 22 K using a collimated molecular beam, the index of refraction and density decrease monotonically as the deposition angle is varied from normal to oblique incidence. At normal incidence the films have an index of refraction of 1.285 and are fully dense (0.94 g/cm3). At glancing incidence (86?) the film has a refractive index of 1.05 and a density of 0.16 g/cm3, indicating a porosity exceeding 80%. The angle dependent film density is in good quantitative agreement with the results of ballistic deposition simulations of ASW film growth.
2002. "Structural and chemical characterization of aligned crystalline nanoporous MgO films grown via reactive ballistic deposition ." Journal of Physical Chemistry B 106(14):3526-3529 . doi: 10.1021/jp013801c Abstract Highly-porous (~ 90%), high-surface area (~ 1000 m2/g), thermally stable (1200 K) crystalline films of MgO are synthesized using a novel reactive ballistic deposition technique. The film consists of a tilted array of porous nanoscale crystalline filaments. Surprisingly, the individual filaments exhibit a high degree of crystallographic order with respect to each other. These films have chemical binding sites analogous to those on MgO(100). However the fraction of chemically active, high energy binding sites is greatly enhanced on the nanoporous film. This unique collection of properties makes these materials attractive candidates for chemical applications such as sensors and heterogeneous catalysts.
2002. "Structural and Chemical Characterization of Aligned Crystalline Nanoporous MgO Films Grown via Reactive Ballistic Deposition." Journal of Physical Chemistry B 106(14):3526-3529. Abstract Highly-porous (~90%), high-surface area (~1000 m2/g), thermally stable (1200K) crystalline films of MgO are synthesized using a novel reactive ballistic deposition techniques. The film consists of a tilted array of porous nanoscale crystalline filaments. Suprisingly, the individual filaments exhibit a high degree of crystallographic order with respect to each other. These films have chemical binding sites analogous to those on MgO (100). However, the fraction of chemically active, high energy binding sites is greatly enhanced on the nanoporous film. This unique collection of properties makes these materials attractive candidates for chemical applications such as sensors and heterogeneous catalysts.
2002. "Adsorption Dynamics and Desorption Kinetics of Argon and Methane on MgO(100) ." Journal of Physical Chemistry B 106(33):8360-8366. Abstract The adsorption dynamics and desorption kinetics of Ar and CH4 on MgO(100) are studied using a combination of molecular beam scattering and temperature programmed desorption (TPD). Both Ar and CH4 exhibit an initial trapping probability that decreases dramatically with increasing kinetic energy and is independent of incident angle indicating a barrier-less process obeying total energy scaling. The trapping probability for both systems increases roughly linearly with increasing adsorbate coverage in the first layer. Analysis of the TPD spectra yields desorption energies of 8.5 and 13 kJ/mole for Ar and CH4, respectively.
2002. "A Beaker without Walls: Formation of Deeply Supercooled Binary Liquid Solutions of Alcohols from Nanoscale Amorphous Solid Films." Physical Review Letters 88(24):art. no. 245505. Abstract Layered nanoscale amorphous solid films of methoanol and ethanol undergo complete intermixing prior to the onset of measurable desorption at 120 K. This intermixing precedes and inhibits crystallization. Subsequent desorption of the film is described quantitatively by a kinetic model describing evaporation from a continuously remixing ideal binary liquid solution. This occurs at temperatures below the melting point of the binary mixture indicating ideal behavior for the supercooled liquid solution. This approach provides a new method for preparing and examining deeply-supercooled solutions.
2001. "Control of Amorphous Solid Water Morphology using Molecular Beams - II: Ballistic Deposition Simulations ." Journal of Chemical Physics 114(12):5295-5303. Abstract Ballistic deposition simulations of thin film growth were performed. The results of the simulations are compared to experiments of N2 adsorption by porous amorphous solid water thin films. The simulations are in qualitative agreement with the experimental observations: The porosity of the thin films is controlled by using a collimated beam to vapor deposit the films. Films with normal or near normal growth angles (q ~ 0?) are relatively dense and smooth. Films with larger growth angles are highly porous and the average pore size increases as the growth angle increases. The simulations indicate that for growth angles greater than ~70?, adsorption into the largest pores is not possible leading to the experimentally observed maximum in N2 adsorption by porous amorphous solid water at q = 70?.
2001. "Control of Amorphous Solid Water Morphology using Molecular Beams - I: Experimental Results ." Journal of Chemical Physics 114(12):5284-5294. Abstract The adsorption of N2 was used to investigate the porosity/morphology of thin films of amorphous solid water. Molecular beams were used to vapor deposit amorphous solid water films on a Pt(111) crystal at a variety of incident growth angles. The amount of N2 adsorbed by the amorphous solid water depends very sensitively on the growth angle and thermal history of the film. For normal and nearly normal incidence growth, the water films are relatively dense and smooth and adsorb only a small amount of N2. For larger growth angles, the films are porous and adsorb large quantities of N2 with apparent surface areas as high as ~2700 m2/g. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The observations have important implications for laboratory studies which use vapor deposited amorphous solid water films as analogs for astrophysical icy bodies such as comets.
2001. "Physisorption of CO on The MgO(100) Surface." Journal of Physical Chemistry B 105(18):3747-3751. Abstract The ability to grow thin MgO(100) films of quality approaching that of vacuum cleaved MgO(100) is demonstrated using low energy electron diffraction and temperature programmed desorption of CO. The highly ordered MgO(100) surfaces are used to study adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ? 0.03 to 0.90 is observed for relative CO coverages less than 0.8 ML. This is a consequence of constant but different sticking coefficient on bare (0.47 ? 0.03) and CO covered (0.90) MgO(100) surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from the defects. At intermediate coverages (0.25 - 0.8 ML) the CO desorbs via first order desorption with the desorption energy of 17 ? 2 kJ/mol and preexponential factor of 1x10(superscript)15?2 s(superscript)-1 at 0.8 ML when the peak saturates. The desorption energy increases linearly as coverage decreases due to repulsive interactions between adsorbed CO molecules. Above Theta = 0.8 ML the adsorption occurs on fully CO covered MgO(100) surfaces and further increase in Theta is achieved by compression of the CO layer. This results in a sharp decrease in desorption energy which upon saturation of the first CO layer (Theta = 1 ML) and formation of c(4x2) ordered phase has value of ~9 kJ/mol.
2001. "Effect of Porosity on the Adsorption, Desorption, Trapping and Release of Volatile gases by Amorphous Solid Water." Journal of Geophysical Research. Planets 106(E12):33,387-33,392. Abstract We compare the adsorption, desorption, trapping, and release of Ar, N2, O2, CO, and CH4, by dense (non-porous) and highly porous amorphous solid water (ASW) films. Molecular beam deposition techniques were used to control the porosity of the vapor deposited ASW thin films. Experiments where the gas species was deposited on top and underneath of dense and porous ASW were conducted. The porous films were found to adsorb between 20 and 50 times more gas than the dense films. The desorption temperature of the adsorbed gas was also dependent on the porosity of the ASW film. The differences between desorption from porous and dense ASW films are attributed to differences in their ability to trap weakly physisorbed gases. The results were largely independent of the gas studied confirming that the adsorption and trapping of gases is dominated by the ASW porosity. These findings show that laboratory studies must account for the growth conditions and their effects on ASW morphology in order accurately predict the properties of astrophysical ices.
2000. "The Self-diffusivity of Amorphous Solid Water Near 150 K." Chemical Physics 258(3):291-305. Abstract Molecular beam techniques are used to create nanoscale thin films composed of different isotopes of amorphous solid water (ASW). The metastable ASW composites are then heated above the glass transition temperature, Tg, and the extent of isotopic intermixing is determined using temperature programmed desorption (TPD). The observed self-diffusion in the 150-160 K range is roughly a million-fold greater than that expected for crystalline ice. The magnitude and temperature dependence of the self-diffusivity are consistent with an amorphous solid that melts into a deeply supercooled liquid prior to crystallization. The overall temperature dependence for the diffusivity of liquid water, supercooled liquid water (238-273 K), and of ASW (150-160 K) is well described by the Vogel-Fulcher-Tamman equation. These results suggest that ASW above its Tg is a deeply supercooled metastable extension of normal liquid water prior to crystallizing near 160 K
2000. "The Effect of Underlying Substrate on the Crystallization Kinetics of Dense Amorphous Solid Water Films." Journal of Chemical Physics 112(13):5932-5941. Abstract The crystallization kinetics of thin, dense amorphous solid water films deposited on crystalline ice and Pt(111) substrates are investigated. A dramatic acceleration of the crystallization rate is observed for amorphous films on crystalline ice substrates. The acceleration originates from the absence of the nucleation process on the crystalline ice substrate which serves as a 2-dimensional nucleus for the growth of the crystalline phase. This contrasts with the crystallization on a Pt(111) substrate which proceeds via bulk nucleation and 3-dimensional growth. Activation energies for growth and nucleation are determined to be 56 and 140 kJ/mol, respectively using the apparent activation energies measured on crystalline ice and Pt(111) substrates. For amorphous films deposited on crystalline ice substrates, the crystallization rate decreases rapidly with increasing distance from the crystalline ice substrate most probably due to crystallization induced cracking of the films.
1999. "Controlling the Morphology of Amorphous Solid Water." Science 283:1505-1507. Abstract The morphology of amorphous solid water grown by vapor deposition depends strongly ont he angular distribution of the water molecules incident from the gas phase...
1999. "The Existence of Supercooled Liquid Water at 150 K." Nature 398:788-791. Abstract Water has long fascinated researchers due to its biological, chemical, and astrophysical importance and because of its many unusual properties.
1999. "Substrate Induced Crystallization of Amorphous Solid Water at Low Temperatures." Journal of Chemical Physics 110(12):5489-5492. Abstract We show that N2 monolayer desorption from ice surfaces is a quantitive, highly sensitive method for following the surface crystallization kinetics at low temperatures...