Internal Rotation of Methane Molecules in Large Clusters

Methane is one of the very few substances that show rotation of individual molecules in the crystalline phase. Here we explore the evolution of the rotation spectrum of methane from single molecules to clusters containing up to about 4 × 10(3) molecules. The clusters were assembled in He droplets at T = 0.38 K and studied via infrared laser spectroscopy in the ν3 region of the methane molecules. Well-resolved rotational structure in the spectra was observed in clusters containing up to about 50 molecules. We have concluded that in distinction to the crystals molecular rotation in methane clusters is confined to the surface and is enabled by low coordination of the molecules. On the contrary the molecules in the cluster's interior are in amorphous state wherein the rotation is quenched. These results demonstrate that even at very low temperature the surface of the methane clusters remains fluxional due to quantum effects.