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Impurity rotations in quantum versus classical solids: O2 in solid hydrogens

Summary: Impurity rotations in quantum versus classical solids:
O2 in solid hydrogens
Z. Li and V. A. Apkarian
Department of Chemistry, University of California, Irvine, California 92697-2025
Received 4 December 1996; accepted 24 April 1997
Molecular dynamics simulations based on pseudopotentials are used to characterize the difference
between impurity rotations in classical versus quantum solids. The method is first applied to the pure
solids and demonstrated to faithfully reproduce static and dynamical properties, in the form of pair
distributions and phonon density of states of solid H2 D2 . Then the rotations of molecular oxygen
in the ground X(3
g ) and electronically excited state A (3
u) is investigated. Where the
substitutional impurity is small, O2(X), in the classical solid, the cavity remains nearly spherical and
the molecule undergoes rotation-translation coupled motion. In contrast, in the quantum solid, the
lattice locally distorts around the impurity and forces librations with occasional reorientational hops
as rotation-distortion coupled motion. These effects are amplified in the excited O2(A ) state, in
which due to the larger molecular bond length, the angular anisotropy of the guest­host interaction
is larger. Now, in the classical solid a small cage distortion forces the molecule into large amplitude
librations. The molecule, however, reorients occasionally, when the lattice fluctuations lead to a
nearly spherical cage geometry. In the quantum host, O2(A ) becomes a strict librator, due to a large


Source: Apkarian, V. Ara - Department of Chemistry, University of California, Irvine


Collections: Chemistry