The rotational spectrum of single OCS molecules in liquid 4He droplets

Single OCS molecules have been embedded in large 4He droplets (N̄=1×103–8×103 atoms) and their infrared spectra in the vicinity of the ν3-fundamental at 2062 cm−1 have been studied using coaxial laser depletion spectroscopy. Sharp lines corresponding to the P- and R-branches with a linewidth of 160 MHz or greater are observed. From the line intensities a droplet temperature of 0.37±0.02 K is obtained and from the line positions the rotational constants B for the ground and excited states and an average centrifugal distortion constant D are determined. The former are about a factor of 2.8 smaller and the latter four orders-of-magnitude larger than for the free molecule. The decrease in B is attributed to an attached nonsuperfluid component which has the same effect as a ring of about six 4He atoms in an equatorial plane around the waist of the OCS molecule, which is carried along in the end-over-end rotation. The widths of the individual lines show an interesting asymmetry which is oppositely sloped for the P- and R-branches and increases with the rotational quantum number. Several possible explanations for the line shapes and widths are presented.