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

Single OCS molecules have been embedded in large He-4 droplets ((N) over ba r = 1 x 10(3)-8 x 10(3) atoms) and their infrared spectra in the vicinity o f the nu(3)-fundamental at 2062 cm(-1) have been studied using coaxial lase r depletion spectroscopy. Sharp lines corresponding to the P- and R-branche s with a linewidth of 160 MHz or greater are observed. From the line intens ities a droplet temperature of 0.37 +/- 0.02 K is obtained and from the lin e positions the rotational constants B for the ground and excited states an d an average centrifugal distortion constant D are determined. The former a re about a factor of 2.8 smaller and the latter four orders-of-magnitude la rger than for the free molecule. The decrease in B is attributed to an atta ched nonsuperfluid component which has the same effect as a ring of about s ix He-4 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 indi vidual lines show an interesting asymmetry which is oppositely sloped for t he P- and R-branches and increases with the rotational quantum number. Seve ral possible explanations for the line shapes and widths are presented. (C) 2000 American Institute of Physics. [S0021-9606(00)00801-1].