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Journal of Chemical Physics / Volume 134 / Issue 16 / ARTICLES / Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

J. Chem. Phys. 134, 164503 (2011); http://dx.doi.org/10.1063/1.3582910 (9 pages)

Molecular dynamics simulations of D2O ice photodesorption

C. Arasa1, S. Andersson2, H. M. Cuppen3, E. F. van Dishoeck3, and G. J. Kroes4

1Leiden Observatory, Leiden University, P. O. Box 9513, 2300 RA Leiden, The Netherlands and Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
2SINTEF Materials and Chemistry, P.O. Box 4760, 7465 Trondheim, Norway and Department of Chemistry, Physical Chemistry, University of Gothenburg, 41296 Gothenburg, Sweden
3Leiden Observatory, Leiden University, P. O. Box 9513, 2300 RA Leiden, The Netherlands
4Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands

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(Received 16 December 2010; accepted 7 April 2011; published online 27 April 2011)

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D2O in an amorphous D2O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H2O, the main processes after UV photodissociation are trapping and desorption of either fragments or D2O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D2O molecule photodesorption. D2O desorption takes places either by direct desorption of a recombined D2O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D2O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D2O in the top four monolayers and are compared quantitatively with those for H2O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)10.1063/1.3422213]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5–2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D2O photodesorption probability is larger than that of H2O (by about a factor of 1.4–2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D2O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D2O ice, because the D atom formed after D2O photodissociation has a larger momentum than photogenerated H atoms from H2O, and D transfers momentum more easily to D2O than H to H2O. The total (OD + D2O) yield has been compared with experiments and the total (OH + H2O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D2O ice than when we compare with calculated yields for H2O ice.

© 2011 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. METHODS
    1. Potentials
    2. Amorphous ice surface
    3. Initial conditions and dynamics
  3. RESULTS AND DISCUSSION
    1. D atom photodesorption
    2. OD radical photodesorption
    3. D 2 O molecule photodesorption
      1. Kick-out vs. direct mechanism
      2. Trends with ice temperature
    4. Energies of the kicked out molecules
    5. Total (OD + D 2 O) photodesorption yield and comparison with experiments
  4. CONCLUSIONS

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KEYWORDS and PACS

Keywords

amorphous state, desorption, deuterium compounds, isotope effects, molecular dynamics method, photochemistry, photodissociation, photoexcitation

PACS

  • 82.50.Hp

    Processes caused by visible and UV light

  • 68.43.Nr

    Desorption kinetics

  • 82.20.Tr

    Kinetic isotope effects including muonium

  • 82.20.Db

    Transition state theory and statistical theories of rate constants

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.3582910

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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