Infrared spectra in the 3 μm region of ethane and ethane clusters in helium droplets.

Ethane and ethane clusters (N ≈ 102–104) were studied inside helium droplets with infrared laser spectroscopy. The spectra were measured in the 2880–3000 cm–1 range, which covers the ν5, ν8+11, and ν7 vibrational bands of ethane. Partially resolved rotational fine structure in the spectrum of the monomer reveals solvent-induced band origin blue shifts that are each approximately 1 cm–1. The effective BHe and AHe rotational constants were found to be reduced by 52% and 16% in comparison to their gas phase values, respectively. Spectra of the clusters show the same three bands shifted toward low frequency by approximately 10 cm–1 because of intermolecular interactions in the clusters. The spectra of the ethane clusters are dominated by the ν7 band, whereas the relative intensities of the ν5 and ν8+11 bands are about a factor of 5 weaker than for single molecules or for solid ethane, the spectrum of which is also reported here.