INFRARED SPECTRA OF HYDROGEN CLUSTERS SEEDED WITH CARBON DIOXIDE
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Date
2008
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Ohio State University
Abstract
It is now possible to probe cold ($<$0.5 K) helium clusters in the size range $N$ $\sim$ 2 to 70 by means of the vibration-rotation spectrum of an embedded infrared chromophore molecule such as CO$_2$,} \textbf{128}, 044308 (2008).} often with atom-by-atom resolution. To some extent, hydrogen clusters can also be studied in this way, as shown by our previous work in which CO,} \textbf{122}, 094314 (2005).} OCS,} \textbf{121}, 3087 (2004).} and N$_2$O} \textbf{123}, 114314 (2005).} were the chromophores. Here we extend the study of hydrogen clusters to the case of CO$_2$ as the probe. The symmetry of CO$_2$ provides an important difference compared to the other probe molecules. This has the effect of eliminating half of the rotational levels (for the normal C$^{16}$O$_2$ or C$^{18}$O$_2$ isotopomers) and of accentuating the differences between $para$H$_2$ and $ortho$H$_2$ clusters. As in the case of (H$_2$)$_N$-OCS and (H$_2$)$_N$-N$_2$O, we find that (H$_2$)$_N$-CO$_2$ cluster transitions are relatively easy to identify up to about $N$ = 7, but difficult to follow above this point. However, in contrast to the previous work there is intriguing evidence for a series of weak but regularly-spaced transitions which may extend to $N$ $\sim$ 15 or beyond.
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A.R.W. McKellar, J. Chem. Phys.S. Moroni, M. Botti, S. De Palo, and A.R.W. McKellar, J. Chem. Phys.J. Tang and A.R.W. McKellar, J. Chem. Phys.J. Tang and A.R.W. McKellar, J. Chem. Phys.
Author Institution: Steacie Institute for Molecular Sciences, National Research; Council of Canada, Ottawa, Ontario K1A 0R6, Canada
Author Institution: Steacie Institute for Molecular Sciences, National Research; Council of Canada, Ottawa, Ontario K1A 0R6, Canada