High resolution infrared spectroscopy of single SF6 molecules in helium droplets. II. The effect of small amounts of He-4 in large He-3 droplets

The rotationally resolved infrared spectra of single SF6 molecules embedded in large He-3 droplets have been studied as a function of mean sizes betwe en (N) over bar(3) = 2 X 10(3) and 10(5) atoms by the cluster beam depletio n technique. The observed frequency shifts and line broadening of the dopan t spectra are interpreted in terms of the small impurity of about 30-50 He- 4 atoms, which because of their lower zero point energy completely surround the SF6 molecule. For the largest droplets, the observed rotational struct ure is similar to that observed in pure He-4 droplets but the rotational te mperature as determined from the relative line intensities is T-rot = 0.15( 1) K, which is about a factor 2.5 lower than in pure He-4 droplets. The low er temperature is expected on the basis of the lower heat of evaporation of the He-3 atoms on the outside of the droplet. Mixed He-3/He-4 droplets pro duced by increasing the He-4 concentration in the source gas to 1.2 and 4% of the He-3 were found to have temperatures between that for the almost pur e He-3 droplets and that for pure He-4 droplets. (C) 1999 American Institut e of Physics. [S0021-9606(99)01907-8].