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.

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.

Using a pulsed electron beam, we investigated the dependence of micronucleus formation on the incident electron energy in AG01522 human diploid fibroblasts after nontargeted irradiations at 25 and 80 keV. Examining the dose response, we found that 25 keV electrons are more effective than 80 keV electrons at producing biological damage for a given dose. Our results demonstrating the induction of micronuclei as a function of incident electron energy offer direct support for the hypothesis that the electron track end is responsible for the biological damage occurring in the cell.

We have designed and constructed a low-cost, variable-energy low-LET electron microbeam that uses energetic electrons to mimic radiation damage produced by 7 and X rays. The microbeam can access lower regions of the LET spectrum, similar to conventional X-ray or Co-60 gamma-ray sources. The device has two operating modes, as a conventional microbeam targeting single cells or subpopulations of cells or as a pseudo broad-beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.

The growth of crystalline water films on Pt(111) is investigated using rare gas physisorption. The water monolayer wets Pt(111) at all temperatures investigated (20-155 K). At low temperatures (T< or =120 K), additional water layers kinetically wet the monolayer surface. However, crystalline ice films grown at higher temperatures (T > 135 K) do not wet the water monolayer. These results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on Pt(111).

Dosimetry calculations characterizing the spatial variation of the energy deposited by the slowing and stopping of energetic electrons are reported and compared with experimental measurements from an electron microbeam facility. The computations involve event-by-event, detailed-histories Monte Carlo simulations of low-energy electrons interacting in water vapor. Simulations of electron tracks with starting energies from 30 to 80 keV are used to determine energy deposition distributions in thin cylindrical rings as a function of penetration and radial distance from a beam source. Experimental measurements of the spatial distribution of an electron microbeam in air show general agreement with the density-scaled simulation results for water vapor at these energies, yielding increased confidence in the predictions of Monte Carlo track-structure simulations for applications of the microbeam as a single-cell irradiator. (c) 2005 by Radiation Research Society.

The electron-stimulated production of molecular hydrogen (D(2), HD, and H(2)) from amorphous solid water (ASW) deposited on Pt(111) is investigated. Experiments with isotopically layered films of H(2)O and D(2)O are used to profile the spatial distribution of the electron-stimulated reactions leading to hydrogen within the water films. The molecular hydrogen yield has two components that have distinct reaction kinetics due to reactions that occur at the ASW/Pt interface and the ASW/vacuum interface, but not in the bulk. However, the molecular hydrogen yield as a function of the ASW film thickness in both pure and isotopically layered films indicates that the energy for the reactions is absorbed in the bulk of the films and electronic excitations migrate to the interfaces where they drive the reactions.

We have measured the electron-stimulated desorption (ESD) of D(2), O(2), and D(2)O, the electron-stimulated dissociation of D(2)O at the D(2)O/Pt interface, and the total electron-stimulated sputtering in thin D(2)O films adsorbed on Pt(111) as a function of the D(2)O coverage (i.e., film thickness). Qualitatively different behavior is observed above and below a threshold coverage of approximately 2 monolayers (ML). For coverages less than approximately 2 ML electron irradiation results in D(2)O ESD and some D(2) ESD, but no detectible reactions at the water/Pt interface and no O(2) ESD. For larger coverages, electron-stimulated reactions at the water/Pt interface occur, O(2) is produced and the total electron-stimulated sputtering of the film increases. An important step in the electron-stimulated reactions is the reaction between water ions (generated by the incident electrons) and electrons trapped in the water films to form dissociative neutral molecules. However, the electron trapping depends sensitively on the water coverage: For coverages less than approximately 2 ML, the electron trapping probability is low and the electrons trap preferentially at the water/vacuum interface. For larger coverages, the electron trapping increases and the electrons are trapped in the bulk of the film. We propose that the coverage dependence of the trapped electrons is responsible for the observed coverage dependence of the electron-stimulated reactions.

The electron-stimulated production of D2 from amorphous solid D2O deposited on Pt(111) is investigated as a function of film thickness. The D2 yield has two components that have distinct reaction kinetics. Using isotopically layered films of H2O and D2O demonstrates that the D2 is produced in reactions that occur at both the Pt/amorphous solid water (ASW) interface and the ASW/vacuum interface, but not in the bulk. The energy for the reactions, however, is absorbed in the bulk of the films and electronic excitations diffuse to the interfaces where they drive the reactions.

The influence of adlayer compression on the physisorption of Ar on Pt(111) is investigated using temperature programmed desorption and modulated molecular beams. We find that the difference in coverage between the compressed and uncompressed first layers is ∼10–15%. For coverages near one monolayer, this compression causes nearly temperature independent desorption kinetics over a wide temperature range (32–41 K). We present a theory that includes the effects of the compression on the desorption kinetics and explains the observed kinetics in terms of a competition between adsorbate–substrate and adsorbate–adsorbate interactions resulting in a continuous increase in the chemical potential near the completion of each successive layer.

Highly porous (similar to 90%), high-surface area (similar to1000 m(2)/g), thermally stable (1200 K) crystalline Films of NIgO 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.

Currently, the standards for human exposure to ionizing radiation are determined by extrapolating data obtained at high doses to the low-doese region using a linear, no-threshold model.

Layered nanoscale amorphous solid films of methanol 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 mixed 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.

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 presumed to be 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 semiquantitative agreement with the results of ballistic deposition simulations of ASW film growth.

We summarize the existing experimental data for the self-diffusivity of supercooled liquid water and review two proposals for the temperature dependence of these data. These data are compared to the recently published measurements Of the self-diffusivity of amorphous solid water, We discuss the implications of these data regarding the continuity between ASW and supercooled liquid data. The advantages and limitations of using nanoscale thin films to Measure the physical properties of metastable amorphous materials are discussed.

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.

We compare the adsorption, desorption, trapping, and release of Ar, N2, O2, CO, and CH4 by dense (nonporous) and highly porous amorphous solid water (ASW) films. Molecular beam deposition techniques are used to control the porosity of the vapor‐deposited ASW thin films. Experiments where the gas species is deposited on top of and underneath dense and porous ASW are conducted. For the film thickness used in this study, the porous films are found to adsorb between 20 and 50 times more gas than the dense films. The desorption temperature of the adsorbed gas is also dependent on the porosity of the ASW film. Differences between desorption from porous and dense ASW films are attributed to differences in their ability to trap weakly physisorbed gases. The results are largely independent of the gas studied, confirming that the adsorption and trapping of gases are 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 to accurately predict the properties of astrophysical ices.