High resolution infrared spectra of helium clusters doped with carbon dioxide or nitrous oxide

Infrared laser spectroscopy is used to study low temperature ($\sim $0.2 K) helium clusters in a pulsed supersonic jet expansion. The clusters are doped with a probe molecule (CO$_{2}$ or N$_{2}$O) whose vibration-rotation spectrum is observed as a function of cluster size. Our previous work on smaller clusters (N $<$ 20) is extended to larger clusters (N $\sim $ 70) using a new apparatus with a skimmed jet nozzle. Various isotopes are studied in order to make the assignment of cluster size more secure, including the asymmetric species $^{16}$O$^{13}$C$^{18}$O. The cluster rotational parameters exhibit broad oscillations, similar to those recently observed in He$_{N}$ - OCS, which are related to the formation of superfluid helium solvation shells around the probe molecule.