Control of amorphous solid water morphology using molecular beams. II. Ballistic deposition simulations

Kimmel, Greg A.; Dohnálek, Zdenek; Stevenson, Kip P.; Smith, R. Scott; Kay, Bruce D.

doi:10.1063/1.1350581

Cited by 120 publications

(148 citation statements)

References 15 publications

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“…(4) H 2 O RAIR spectra for sub-ML coverages on amorphous silica at temperatures below 30 K confirm the ballistic deposition mechanism that many have postulated for growth in this temperature regime (Zondlo et al 1997;Kimmel et al 2001). At temperatures above 40 K, the weak O-H stretching spectra strengthen in a manner consistent with increasing ease of diffusion and growth of H 2 O cluster and islands.…”

Section: O N C L U S I O Nsupporting

confidence: 55%

Probing model interstellar grain surfaces with small molecules

Collings

Frankland

Lasne

et al. 2015

Monthly Notices of the Royal Astronomical Society

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Temperature-programmed desorption and reflection-absorption infrared spectroscopy have been used to explore the interaction of oxygen (O 2 ), nitrogen (N 2 ), carbon monoxide (CO) and water (H 2 O) with an amorphous silica film as a demonstration of the detailed characterization of the silicate surfaces that might be present in the interstellar medium. The simple diatomic adsorbates are found to wet the silica surface and exhibit first-order desorption kinetics in the regime up to monolayer coverage. Beyond that, they exhibit zero-order kinetics as might be expected for sublimation of bulk solids. Water, in contrast, does not wet the silica surface and exhibits zero-order desorption kinetics at all coverages consistent with the formation of an islanded structure. Kinetic parameters for use in astrophysical modelling were obtained by inversion of the experimental data at sub-monolayer coverages and by comparison with models in the multilayer regime. Spectroscopic studies in the sub-monolayer regime show that the C-O stretching mode is at around 2137 cm −1 (5.43 μm), a position consistent with a linear surface-CO interaction, and is inhomogenously broadened as resulting from the heterogeneity of the surface. These studies also reveal, for the first time, direct evidence for the thermal activation of diffusion, and hence de-wetting, of H 2 O on the silica surface. Astrophysical implications of these findings could account for a part of the missing oxygen budget in dense interstellar clouds, and suggest that studies of the sub-monolayer adsorption of these simple molecules might be a useful probe of surface chemistry on more complex silicate materials.

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Section: O N C L U S I O Nsupporting

confidence: 55%

Probing model interstellar grain surfaces with small molecules

Collings

Frankland

Lasne

et al. 2015

Monthly Notices of the Royal Astronomical Society

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“…Two forms of solid water were grown, CI at a surface temperature of 140 K and ASW at T S = 120 K. At these growth conditions, the ice is not porous. [30][31][32] High translational energy beams of CF 4 (Aldrich, 99.9%), SF 6 (Aldrich, > 99.75%), Xe (Airgas, 99.995%), or Kr (Praxair, Research Grade) were made by mixing <0.5% (by volume) of the gas with either H 2 or He. The mixture was expanded through a 10 or 15 μm pinhole with several hundred psi of backing pressure.…”

Section: A Thermal Desorptionmentioning

confidence: 99%

Molecular interactions with ice: Molecular embedding, adsorption, detection, and release

Gibson

Langlois

et al. 2014

The Journal of Chemical Physics

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The interaction of atomic and molecular species with water and ice is of fundamental importance for chemistry. In a previous series of publications, we demonstrated that translational energy activates the embedding of Xe and Kr atoms in the near surface region of ice surfaces. In this paper, we show that inert molecular species may be absorbed in a similar fashion. We also revisit Xe embedding, and further probe the nature of the absorption into the selvedge. CF4 molecules with high translational energies (≥3 eV) were observed to embed in amorphous solid water. Just as with Xe, the initial adsorption rate is strongly activated by translational energy, but the CF4 embedding probability is much less than for Xe. In addition, a larger molecule, SF6, did not embed at the same translational energies that both CF4 and Xe embedded. The embedding rate for a given energy thus goes in the order Xe > CF4 > SF6. We do not have as much data for Kr, but it appears to have a rate that is between that of Xe and CF4. Tentatively, this order suggests that for Xe and CF4, which have similar van der Waals radii, the momentum is the key factor in determining whether the incident atom or molecule can penetrate deeply enough below the surface to embed. The more massive SF6 molecule also has a larger van der Waals radius, which appears to prevent it from stably embedding in the selvedge. We also determined that the maximum depth of embedding is less than the equivalent of four layers of hexagonal ice, while some of the atoms just below the ice surface can escape before ice desorption begins. These results show that energetic ballistic embedding in ice is a general phenomenon, and represents a significant new channel by which incident species can be trapped under conditions where they would otherwise not be bound stably as surface adsorbates. These findings have implications for many fields including environmental science, trace gas collection and release, and the chemical composition of astrophysical icy bodies in space.

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“…Extensive experimental studies demonstrate that porous ASW films undergo an irreversible densification upon annealing, 1 and that the destruction of the porous network is complete by 110 K. 32,33 The re-arrangement of water molecules during densification by annealing implies the diffusion of H2O molecules on the surface. In the specific case of interest here, that is porous ice films kept at 10 K, it would be perhaps more appropriate to speak of restricted local migration rather than diffusion.…”

Section: Origin Of the Decrease Of The Porositymentioning

confidence: 99%

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

Accolla

Congiu

Dulieu

et al. 2011

Phys. Chem. Chem. Phys.

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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 H2 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 H2 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 φ0 = 2 × 10 16 D-atoms cm -2 per layer of H2O. 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. 1.IntroductionAmorphous solid water (ASW) is reputed to be the most abundant form of water in the Universe, thanks to its propensity for forming, molecule after molecule, as a deposit on interstellar dust particles. 1,2 The accretion of icy mantles (predominantly constituted by ASW) on silicate and carbonaceous dust grains takes place in interstellar molecular clouds. Some of them are cold (~10 K) and dense (10 4 -10 6 cm -3 ) regions where gaseous species heavier than hydrogen and helium freeze out onto the grains. Beside H2O and CO, other species constitute the inventory of icy mantles such as CO2, CH3OH, CH4, H2CO and NH3, 3,4 as a result of the gasgrain interactions occurring on the surface of interstellar dust. The most important and abundant molecular species as well, H2, is formed likewise thanks to reactions occurring on amorphous water ice in molecular clouds 5,6 . In addition, a significant fraction of the gas phase species is constituted by atomic hydrogen whose abundance is mostly governed by the destruction of H2 due to cosmic rays. It has been evaluated that, in dense clouds, the average number density ratio of atomic hydrogen component compared to the molecular one is ~ 0.1%. 7 In a dark cloud, atomic hydrogen thus represents the third most abundant gas-phase species, after H2 and He.Along with atoms and molecules available in the gas-phase and the condensed/adsorbed speci...

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Control of amorphous solid water morphology using molecular beams. II. Ballistic deposition simulations

Cited by 120 publications

References 15 publications

Probing model interstellar grain surfaces with small molecules

Probing model interstellar grain surfaces with small molecules

Molecular interactions with ice: Molecular embedding, adsorption, detection, and release

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

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