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Title:
Methane clathrate: CH4 quantum rotor state dependent rattling potential
Authors:
Prager, M.; Press, W.
Affiliation:
AA(Institut für Festkörperforschung, Forschungzzentrum Jülich, D-52425 Jülich, Germany), AB(Institut für Festkörperforschung, Forschungzzentrum Jülich, D-52425 Jülich, Germany)
Publication:
Journal of Chemical Physics, Volume 125, Issue 21, pp. 214703-214703-7 (2006). (JChPh Homepage)
Publication Date:
12/2006
Origin:
AIP
PACS Keywords:
Molecular solids, Potential energy surfaces for ground electronic states, Rotation, vibration, and vibration-rotation constants, Potential energy surfaces for excited electronic states
DOI:
10.1063/1.2400031
Bibliographic Code:
2006JChPh.125u4703P

Abstract

In methane hydrate the dominant peak in the density of states above 3 meV represents a rattling mode of the guest molecule CH4 in the large ice cages. This mode shifts from 6.7 meV at T=4.5 K to T=30 K to 7.14 meV with conversion of CH4 guest molecules into the tunneling ground state. The less symmetric angular density distribution PsiPsi* in the excited rotational state compared to the ground state allows the methane to fit better in the orientation dependent cage potential surface. This leads to a larger average distance to the cage-forming molecules with a weaker potential and a reduced rattling energy. A two state single particle model with characteristic rattling energies of 5.20 meV for pure T-methane and 7.3 meV for pure A-methane weighted by the population factors can fit the data.
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