High resolution infrared spectroscopy of single SF6 molecules in helium droplets. II. The effect of small amounts of He4 in large He3 droplets

The rotationally resolved infrared spectra of single SF6 molecules embedded in large He3 droplets have been studied as a function of mean sizes between N̄3=2×103 and 105 atoms by the cluster beam depletion technique. The observed frequency shifts and line broadening of the dopant spectra are interpreted in terms of the small impurity of about 30–50 He4 atoms, which because of their lower zero point energy completely surround the SF6 molecule. For the largest droplets, the observed rotational structure is similar to that observed in pure He4 droplets but the rotational temperature as determined from the relative line intensities is Trot=0.15(1) K, which is about a factor 2.5 lower than in pure He4 droplets. The lower temperature is expected on the basis of the lower heat of evaporation of the He3 atoms on the outside of the droplet. Mixed He/43He droplets produced by increasing the He4 concentration in the source gas to 1.2 and 4% of the He3 were found to have temperatures between that for the almost pure He3 droplets and that for pure He4 droplets.