Spectroscopy of molecules in helium droplets

The work reviewed in the talk explores the possibilities of using molecules immersed in liquid helium as probes to study superfluidity on the microscopic level. For this purpose liquid He-4, He-3 and mixed He-4/He-3 droplets consisting of 10(3)-10(5) atoms have been doped with single molecules or a small defined number of molecules which form clusters in the droplet interior. The dopants were investigated with high-resolution optical spectroscopy. The electronic and rovibrational spectra show unusually sharp spectral lines and unexpected features such as a distinct gap between the zero phonon line and the phonon wing as well as the free rotations of single molecules. Both are not observed in colder nonsuperfluid He-3 droplets and therefore are considered to be new microscopic manifestations of superfluidity. Spectroscopic studies of small p-H-2 and o-D-2 clusters surrounding a single chromophore molecule formed in the interior of the He droplets are also reported. (C) 2000 Elsevier Science B.V. All rights reserved.