Spectroscopy of alkali atoms and molecules in superfluid helium

We report on the first laser spectroscopy of alkali atoms (Cs and Rb) and molecules (${\mathrm{Na}}_2$ and ${\mathrm{Li}}_2$) in superfluid helium. General features of the observed spectra can be explained in terms of the spherical atomic bubble model. The excitation and emission spectra for the ${\mathit{D}}_2$- lines, however, have double peaked profiles, indicating the importance of the nonspherical density distribution of surrounding helium atoms. The emission spectra for the molecules show well-resolved vibrational structures in the electronic ground states.