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Title: The sticking of atomic hydrogen on amorphous water ice

Abstract

Using classical molecular dynamics, we have simulated the sticking and scattering process of a hydrogen atom on an amorphous ice film to predict the sticking probability of hydrogen on ice surfaces. A wide range of initial kinetic energies of the incident hydrogen atom (10 K-600 K) and two different ice temperatures (10 K and 70 K) were used to investigate this fundamental process in interstellar chemistry. We report here the sticking probability of atomic hydrogen as a function of incident kinetic energy, gas temperature, and substrate temperature, which can be used in astrophysical models. The current results are compared to previous theoretical and experimental studies that have reported a wide range in the sticking coefficient.

Authors:
; ; ;  [1]
  1. Department of Physics and Astronomy, and the Center for Simulational Physics, The University Georgia, Athens, GA 30602 (United States)
Publication Date:
OSTI Identifier:
22365633
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 790; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; ASTROPHYSICS; DUSTS; FILMS; HYDROGEN; ICE; KINETIC ENERGY; MOLECULAR DYNAMICS METHOD; PROBABILITY; SCATTERING; SIMULATION; SUBSTRATES; SURFACES; WATER

Citation Formats

Veeraghattam, Vijay K., Manrodt, Katie, Lewis, Steven P., and Stancil, P. C., E-mail: vijay@physast.uga.edu, E-mail: lewis@physast.uga.edu, E-mail: stancil@physast.uga.edu. The sticking of atomic hydrogen on amorphous water ice. United States: N. p., 2014. Web. doi:10.1088/0004-637X/790/1/4.
Veeraghattam, Vijay K., Manrodt, Katie, Lewis, Steven P., & Stancil, P. C., E-mail: vijay@physast.uga.edu, E-mail: lewis@physast.uga.edu, E-mail: stancil@physast.uga.edu. The sticking of atomic hydrogen on amorphous water ice. United States. doi:10.1088/0004-637X/790/1/4.
Veeraghattam, Vijay K., Manrodt, Katie, Lewis, Steven P., and Stancil, P. C., E-mail: vijay@physast.uga.edu, E-mail: lewis@physast.uga.edu, E-mail: stancil@physast.uga.edu. Sun . "The sticking of atomic hydrogen on amorphous water ice". United States. doi:10.1088/0004-637X/790/1/4.
@article{osti_22365633,
title = {The sticking of atomic hydrogen on amorphous water ice},
author = {Veeraghattam, Vijay K. and Manrodt, Katie and Lewis, Steven P. and Stancil, P. C., E-mail: vijay@physast.uga.edu, E-mail: lewis@physast.uga.edu, E-mail: stancil@physast.uga.edu},
abstractNote = {Using classical molecular dynamics, we have simulated the sticking and scattering process of a hydrogen atom on an amorphous ice film to predict the sticking probability of hydrogen on ice surfaces. A wide range of initial kinetic energies of the incident hydrogen atom (10 K-600 K) and two different ice temperatures (10 K and 70 K) were used to investigate this fundamental process in interstellar chemistry. We report here the sticking probability of atomic hydrogen as a function of incident kinetic energy, gas temperature, and substrate temperature, which can be used in astrophysical models. The current results are compared to previous theoretical and experimental studies that have reported a wide range in the sticking coefficient.},
doi = {10.1088/0004-637X/790/1/4},
journal = {Astrophysical Journal},
number = 1,
volume = 790,
place = {United States},
year = {Sun Jul 20 00:00:00 EDT 2014},
month = {Sun Jul 20 00:00:00 EDT 2014}
}
  • Accurate modeling of physical and chemical processes in the interstellar medium (ISM) requires detailed knowledge of how atoms and molecules adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H{sub 2}, D{sub 2}, N{sub 2}, O{sub 2}, CO, CH{sub 4}, and CO{sub 2} on nonporous amorphous solid water. The sticking coefficient was measured over a widemore » range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient—the King–Wells method—leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient, we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas–grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas phase.« less
  • We present both experimental and theoretical studies of the sticking of water molecules on ice. The sticking probability is unity over a wide range in energy (0.5 eV–1.5 eV) when the molecules are incident along the surface normal, but drops as the angle increases at high incident energy. This is explained in terms of the strong orientational dependence of the interaction of the molecule with the surface and the time required for the reorientation of the molecule. The sticking probability is found to scale with the component of the incident velocity in the plane of the surface, unlike the commonlymore » assumed normal or total energy scaling.« less
  • We have observed, via quadrupole mass spectrometry (QMS), stimulated production of D[sub 2] (H[sub 2]) during low-energy (5--50 eV) electron--beam irradiation of D[sub 2]O (H[sub 2]O) amorphous ice. The upper limit for the D[sub 2] (H[sub 2]) production threshold is 6.3[plus minus]0.5 eV; well below the first excited state of condensed water at 7.3 eV. The D[sub 2] (H[sub 2]) yield increases gradually until another threshold is reached at [similar to]17 eV and continues to increase monotonically (within experimental error) up to 50 eV. We assign the 6.3 eV threshold to D[sup [minus]] (H[sup [minus]])+D[sub 2]O (H[sub 2]O)[r arrow]D[sub 2]more » (H[sub 2])+OD[sup [minus]] (OH[sup [minus]]) condensed phase (primarily surface) reactions that are initiated by dissociative attachment. We associate the yield below [similar to]11 eV with the dissociation of Frenkel-type excitons and attribute the yield above [similar to]11 eV mainly to the recombination of D[sub 2]O[sup +], or D[sub 3]O[sup +], with quasifree or trapped electrons. Exciton dissociation and ion--electron recombination processes can produce reactive energetic D (H) atom fragments or D[sub 2] (H[sub 2]) directly via molecular elimination. The importance of D[sup +] (H[sup +]) interactions increases at [similar to]17 eV (dipolar threshold) and at energies [ge]21 eV where multihole and multielectron final states are energetically accessible.« less
  • The production of H{sub 2} in highly excited vibrational and rotational states (v=0-5, J=0-17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H{sub 2}(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H{sub 2}(v=0 and 1) populations in this case were below the detection limit. The H{sub 2} products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H{sub 2}O by a photolytically produced H atom yields vibrationally cold H{sub 2} products, whereas exothermic recombination of twomore » H-atom photoproducts yields H{sub 2} molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations.« less
  • Gas–grain and gas–phase reactions dominate the formation of molecules in the interstellar medium (ISM). Gas–grain reactions require a substrate (e.g., a dust or ice grain) on which the reaction is able to occur. The formation of molecular hydrogen (H{sub 2}) in the ISM is the prototypical example of a gas–grain reaction. In these reactions, an atom of hydrogen will strike a surface, stick to it, and diffuse across it. When it encounters another adsorbed hydrogen atom, the two can react to form molecular hydrogen and then be ejected from the surface by the energy released in the reaction. We performmore » in-depth classical molecular dynamics simulations of hydrogen atoms interacting with an amorphous water-ice surface. This study focuses on the first step in the formation process; the sticking of the hydrogen atom to the substrate. We find that careful attention must be paid in dealing with the ambiguities in defining a sticking event. The technical definition of a sticking event will affect the computed sticking probabilities and coefficients. Here, using our new definition of a sticking event, we report sticking probabilities and sticking coefficients for nine different incident kinetic energies of hydrogen atoms [5–400 K] across seven different temperatures of dust grains [10–70 K]. We find that probabilities and coefficients vary both as a function of grain temperature and incident kinetic energy over the range of 0.99–0.22.« less