High resolution infrared spectroscopy of single SF_6 molecules in helium droplets. I. Size effects in ~4He droplets

摘要

Single SF_6 molecules have been embedded in large ~4He droplets (N-bar_4 approx= 10~3-10~4 atoms) in a molecular beam and studied via infrared laser depletion spectroscopy. The rotational fine structure of the #nu#_3 transition is analyzed with an effective third order gas phase Hamiltonian of SF_6 yielding seven spectroscopic parameters as a function of the measured mean droplet size. From the intensities of the rotational lines the rotational temperature in droplets with N-bar_4>10~3 atoms is determined to be T_(rot)=0.38 (1) K consistent with theoretical estimates. Quantitative information on the efficiency of evaporative cooling of the droplets could be obtained by increasing the droplet temperature up to T_(rot)=0.55K by many successive inelastic collisions with ~4He atoms from the background gas. For small droplets the absorption maximum shows an increasing redshift with respect to the gas phase and only small downward shift with droplet sizes N-bar_4>2 * 10~3. This could not be explained quantitatively with the excluded volume model assuming a liquid drop nor by the expected decrease in the internal pressure with increasing droplet radius. The much smaller decrease in the redshift observed is shown to provide evidence that the nearest neighbor shell of ~4He next to the molecule is almost incompressible providing direct experimental evidence for the existence of a dense snowball-like shell structure of ~4He atoms around the molecule as predicted theoretically.