SPECTROSCOPY OF ALKALI ATOMS AND MOLECULES ATTACHED TO LIQUID HE CLUSTERS

Large helium clusters, ranging in size from a few hundred to several thousand atoms, are produced in a nozzle expansion. Combining this source with a pick-up scattering cell ill which the clusters can be seeded with chromophores allows us to probe the influence of the helium environment on the atoms and molecules attached to the clusters. Using an alkali as chromophore we recorded laser induced fluorescence spectra of Na atoms and molecules attached to helium clusters. Apart from the spectrum of the Na monomer, we have found spectroscopic bands which can unambiguous be assigned to two bound Na atoms. The first of this bands is due to 1(1) Sigma(u)(+)(A) <-- 1(1) Sigma(g)(+)(X) excitations of the covalently bound singlet Na-2 molecule while the second is due to 1(3) Sigma(g)(+) <-- 1(3) Sigma(u)(+) excitations for the van der Waals bound triplet Na-2 dimer. Both bands have been vibrationally resolved. Furthermore we found very large fluorescence intensities in the region 605-635 nm which are likely due to the excitation of a species containing three Na atoms attached to a helium cluster.