Greg A. Kimmel

Low‐energy electron‐stimulated production of molecular hydrogen from amorphous water ice

We have observed, via quadrupole mass spectrometry (QMS), stimulated production of D2 (H2) during low‐energy (5–50 eV) electron–beam irradiation of D2O (H2O) amorphous ice. The upper limit for the D2 (H2) production threshold is 6.3±0.5 eV; well below the first excited state of condensed water at 7.3 eV. The D2 (H2) yield increases gradually until another threshold is reached at ∼17 eV and continues to increase monotonically (within experimental error) up to 50 eV. We assign the 6.3 eV threshold to D− (H−)+D2O (H2O)→D2 (H2)+OD− (OH−) condensed phase (primarily surface) reactions that are initiated by dissociative attachment. We associate the yield below ∼11 eV with the dissociation of Frenkel‐type excitons and attribute the yield above ∼11 eV mainly to the recombination of D2O+, or D3O+, with quasifree or trapped electrons. Exciton dissociation and ion–electron recombination processes can produce reactive energetic D (H) atom fragments or D2 (H2) directly via molecular elimination. The importance of D+ (H...

Effect of porosity on the adsorption, desorption, trapping, and release of volatile gases by amorphous solid water

We compare the adsorption, desorption, trapping, and release of Ar, N 2 , O 2 , CO, and CH 4 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.

Crystalline ice growth on Pt(111) and Pd(111): Nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental 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 both Pt(111) and Pd(111).

Electron-stimulated production of molecular hydrogen at the interfaces of amorphous solid water films on Pt(111)

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.

Electron-stimulated production of molecular oxygen in amorphous solid water on Pt(111): Precursor transport through the hydrogen bonding network

The low-energy, electron-stimulated production of molecular oxygen from thin amorphous solid water (ASW) films adsorbed on Pt(111) is investigated. For ASW coverages less than approximately 60 ML, the O(2) electron-stimulated desorption (ESD) yield depends on coverage in a manner that is very similar to the H(2) ESD yield. In particular, both the O(2) and H(2) ESD yields have a pronounced maximum at approximately 20 ML due to reactions at the Pt/water interface. The O(2) yield is dose dependent and several precursors (OH, H(2)O(2), and HO(2)) are involved in the O(2) production. Layered films of H(2) (16)O and H(2) (18)O are used to profile the spatial distribution of the electron-stimulated reactions leading to oxygen within the water films. Independent of the ASW film thickness, the final reactions leading to O(2) occur at or near the ASW/vacuum interface. However, for ASW coverages less than approximately 40 ML, the results indicate that dissociation of water molecules at the ASW/Pt interface contributes to the O(2) production at the ASW/vacuum interface presumably via the generation of OH radicals near the Pt substrate. The OH (or possibly OH(-)) segregates to the vacuum interface where it contributes to the reactions at that interface. The electron-stimulated migration of precursors to the vacuum interface occurs via transport through the hydrogen bond network of the ASW without motion of the oxygen atoms. A simple kinetic model of the nonthermal reactions leading to O(2), which was previously used to account for reactions in thick ASW films, is modified to account for the electron-stimulated migration of precursors.

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.

Electron-stimulated reactions in thin D2O films on Pt(111) mediated by electron trapping

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.

Growth rate of crystalline ice and the diffusivity of supercooled water from 126 to 262 K

Significance Water is ubiquitous, but its physical properties are anomalous compared with most liquids. Because the anomalies become enhanced upon cooling, understanding the behavior of deeply supercooled water is critical. Unfortunately, experiments below ∼236 K at ambient pressure are difficult due to uncontrolled crystallization. Using a pulsed-laser–heating technique, we have determined the crystalline-ice growth rate and liquid-water diffusivity for temperatures between 180 and 262 K in ultrahigh-vacuum conditions. The fact that both of these quantities are smoothly varying rules out the hypothesis that water’s properties have a singularity at or near 228 K. However, the results are consistent with a previous prediction for the diffusivity that assumed no thermodynamic transitions occur in the supercooled region. Understanding deeply supercooled water is key to unraveling many of water’s anomalous properties. However, developing this understanding has proven difficult due to rapid and uncontrolled crystallization. Using a pulsed-laser–heating technique, we measure the growth rate of crystalline ice, G(T), for 180 K < T < 262 K, that is, deep within water’s “no man’s land” in ultrahigh-vacuum conditions. Isothermal measurements of G(T) are also made for 126 K ≤ T ≤ 151 K. The self-diffusion of supercooled liquid water, D(T), is obtained from G(T) using the Wilson–Frenkel model of crystal growth. For T > 237 K and P ∼ 10−8 Pa, G(T) and D(T) have super-Arrhenius (“fragile”) temperature dependences, but both cross over to Arrhenius (“strong”) behavior with a large activation energy in no man’s land. The fact that G(T) and D(T) are smoothly varying rules out the hypothesis that liquid water’s properties have a singularity at or near 228 K at ambient pressures. However, the results are consistent with a previous prediction for D(T) that assumed no thermodynamic transitions occur in no man’s land.

Temperature Independent Physisorption Kinetics and Adsorbate Layer Compression for Ar Adsorbed on Pt (111)

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.

Kinetic and internal energy distributions of molecular hydrogen produced from amorphous ice by impact of 100 eV electrons

Abstract We have measured the quantum-resolved translational and internal (vibrational and rotational) energy distributions of the D 2 desorbates produced during electron (100 eV) irradiation of D 2 O amorphous ice using resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The D 2 desorbates have very little translational energy (∼ 20–50 meV) but are vibrationally ( ν = 0–4) and rotationally ( J = 0–12) excited. The rotational state distribution of the D 2 does not change as the ice temperature increases from 88 to 145 K. However, we find that the D 2 yield increases monotonically in this temperature range, with the yield at 145 K approximately double than that at 88 K. Although the mobilities of defects, charge carriers, and radicals are known to be temperature dependent, these results suggest that the final states leading to D 2 production are independent of temperature. We suggest that the dominant mechanisms for production of D 2 at 100 eV incident electron energy are dissociative recombination of holes (D 2 O + or D 3 O + ) with quasi-free or trapped electrons and dissociation of excitons at the vacuum-surface interface. These dissociation events can produce D 2 directly via molecular elimination or indirectly via reactive scattering of the energetic D atom fragments.