A beam of ${\mathrm{He}}_N$ clusters ( ${10}^3\lesssim N\lesssim {10}^4$) 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 ${\mathrm{cm}}^{-1\mathrm{}}$. 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.

• Received 21 October 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3592