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Title:Infrared spectroscopic studies of OCS trapped in solid parahydrogen: indirect evidence of large amplitude motions
Author(s):Anderson, David T.
Subject(s):Spectroscopy of Large Amplitude Motions
Abstract:The high-resolution infrared rovibrational spectroscopy of OCS clustered with multiple hydrogen molecules has previously been studied in helium nanodroplets\footnote {S. Grebenev, B. Sartakov, J.P. Toennies, A.F. Vilesov, \textit{Science }\textbf{289}, 1532-1535 (2000).} and in the gas-phase\footnote{J. Tang, Y. Xu, A.R.W. McKellar, W. J\"{a}ger, \textit{Science }\textbf{297}, 2030-2033 (2002).} in search of another substance other than helium that displays superfluidity. Para-hydrogen (pH$_{2}$) is one of the most likely candidates because it is a spinless (\textit{I}=0) composite boson with a light mass similar to helium. However, compared to helium, the pH$_{2}$-pH$_{2}$ intermolecular potential is significantly stronger and thus pH$_{2}$ solidifies at higher temperatures than the predicted superfluid transition temperature thereby blocking access to the superfluid state. Both of these previous studies reveal intriguing results linked to the microscopic details of superfluidity. We were therefore interested to characterize the IR spectrum of OCS in solid pH$_{2}$. The conventional wisdom is that because pH$_{2}$ solidifies into a quantum solid, the effects of superfluidity detected in the finite sized clusters should not be manifest in solid pH$_{2}$. However, the OCS-H$_{2}$ intermolecular potential strongly favors arranging the first ~5 pH$_{2}$ molecules in a ring around the equator of the OCS (\textit{R}=3.2 \AA). Isolation of OCS in bulk pH$_{2}$ therefore may result in a solvation site where 6 pH$_{2}$ molecules in the same basal plane form a ring around the OCS and are pulled inward decoupling them from the bulk. If the periodic barriers to motion around the ring are small, one might expect the 6 equatorial pH$_{2}$ molecules to become delocalized while still maintaining the permutation symmetry of bosons. These 6 particles-on-a-ring may only show this behavior at low temperatures when thermal excitations are minimized. Analysis of the IR spectroscopy of OCS in solid pH$_{2}$ indicates 1) the OCS molecule does not freely rotate and 2) there are at least two preferred OCS solvation sites. In principle, the measured OCS peak frequency for these two solvation sites should depend sensitively on the “structure” of the first pH$_{2}$ solvation shell and therefore provide indirect evidence of this delocalization. We are currently trying to model the effect of pH$_{2}$ delocalization on the OCS vibrational frequency to compare with experiment and test this hypothesis.
Issue Date:2016-06-21
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper/Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2017-01-26

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