Knowledge Bank

We have obtained rotationally-resolved IR spectra of van der Waals clusters composed of one OCS molecule and many helium atoms ($n=1$ to about 20). The observations are made in the region of the $\nu 1$ fundamental band of OCS, near $2062cm-1$, using a rapid-scan infrared diode laser spectrometer to probe a pulsed pinhole or slit supersonic jet expansion from a cooled ($T=230$ to 280 K) nozzle with fairly high backing pressures ($P=6$ to 30 atmospheres). For $OCS-Hen$ clusters with $n\le 8$, the spectra have been assigned and confirmed by microwave observations [1]. For larger clusters, something changes: perhaps the effective onset of superfluid-type behavior. The spectra are still quite strong but not yet assigned in detail. As a function of n, the O-C vibrational frequency first shifts to the blue, as we already knew from the binary complex, OCS-He [2]. Once the first `solvation ring' of about 5 He atoms around the OCS becomes filled, the vibrational frequency starts to turn around and shift to the red, in accord with observations on larger helium nanodroplets (with $n\approx 10,000$).