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We have recently observed rich infrared spectra of helium clusters seeded with OCS, $N_{2}O$, or CO molecules. For $He_{N}-COS$ [1], transitions have been assigned up to $N=8$, for $He_{N}-N_{2}O$ up to $N=6$, and for $He_{N}-CO$ up to $N=14$. In all cases, lines due to even larger clusters are clearly present in the spectra but not yet definitively assigned. Using similar experimental conditions, we have also searched for analogous spectra using other clustering molecules such as hydrogen and neon. In the case of $(H_{2})_{N}-OCS$ clusters, we begin with a good knowledge of the binary complex, $H_{2}-OCS$ [2]. It is then possible to identify likely candidates for transitions due to the clusters with $N = 2, 3$, and 4. One limitation is that we are not sure what kind of spectrum to expect for these clusters. In the case of $H_{2}-OCS$, the spectrum due to $ortho-H_{2}$ completely dominates when normal $H_{2}$ is used in the jet expansion, and it is difficult to observe the (somewhat different) spectrum due to the para species even when using a para-enriched sample [2]. Since normal $H_{2}$ was used in the present cluster experiments, it is similarly likely that $ortho-H_{2}$ dominates, but we do not know what effect the angular momentum of each ground state $ortho-H_{2}$ will have on the cluster spectra, especially when coupling between these $J=1$ hydrogens is present for $N \geq 2$. To help to get around this problem, we have also recorded spectra of $(HD)_{N}-OCS$ clusters, since in this case we can assume that the HD molecules will all relax to the $J=0$ ground state. We will present predictions of possible microwave transitions of hydrogen - OCS clusters, since such observations could provide positive confirmations of our assignments. Spectra showing $Ne_{N}-CO$ transitions for $N=2$ and 3 will also be shown.


References [1] J. Tang, Y. Xu, A.R.W. McKellar, and W. J\""{a}ger, Science, 297, 2030 (2002). [2] J. Tang and A.R.W. McKellar, J. Chem. Phys. 116, 646 (2002).
Author Institution: Steacie Institute for Molecular Sciences, National Research Council of Canada