15min:

A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada.

Recently it has been possible to study the onset of superfluid behavior in small (N 10) [1] and nanoscale (N 1000) [2] helium clusters by observing the spectrum of a probe molecule like OCS, whose vibrational and rotational motions are a sensitive indication of the local helium environment. Our previous IR and microwave spectroscopy [1] of small He_N - OCS clusters extended up to N = 8. With CO or CO₂ as the probe [3] it was possible to approach N = 20. Using a new apparatus with a partially skimmed pulsed supersonic jet expansion, it has now been possible to resolve and assign distinct IR spectra of He_N - OCS for virtually every single N-value from 1 to over 70. The observed lines remain sharp (<0.001 cm^-1) at least up to this cluster size. Analysis of these vibration-rotation spectra, in the 2062 cm^-1 region of the OCS ₁ fundamental band, show that the cluster B-values exhibit an unexpected oscillatory behavior which experimentally marks the completion of the second and third solvation shells of helium around the OCS. At N = 70, the cluster properties are already fairly close to those of large nanodroplets, meaning that we have bridged much of the gap between the individual molecule and bulk matter worlds on a one-N-at-a-time basis.

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[1] J. Tang, Y. Xu, A.R.W. McKellar, and W. Jäger, Science \textbf297, 2030 (2002).

[2] S. Grebenev, M. Hartmann, M. Havenith, B. Sartakov, J.P. Toennies, and A.F. Vilesov, J. Chem. Phys. \textbf112, 4485 (2000).

[3] J. Tang and A.R.W. McKellar, J. Chem. Phys. \textbf119, 754 (2003); \textbf121, 181 (2004).