Laser Spectroscopy of Alkali-Doped Helium Clusters

A beam of He_N clusters ( 10³≲N≲10⁴) is seeded with one or more chromophores by sticky collisions with a low pressure gas and is probed, thereafter, by laser-induced fluorescence spectroscopy. Using sodium, we observe the spectra of atoms, dimers, and trimers and find that they are almost unshifted from their gas phase counterparts, but show cluster-induced broadenings in the range of tens of cm^-1. Upon formation on the cluster surface, a fraction of the dimers desorbs slowly and becomes part of the beam, allowing narrow, rotationally resolved, spectra to be obtained.