SPECTROSCOPY OF ALKALI ATOMS (LI, NA, K) ATTACHED TO LARGE HELIUM CLUSTERS

Large liquid helium clusters (He-n, n approximate to 10(4)) produced i n a supersonic jet are doped with alkali atoms (Li, Na, K) and charact erized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission sp ectra have been recorded. The observed excitation spectra are analyzed , calculating the transitions within an approach based on the hypothes is that the chromophores are trapped in a dimple on the cluster's surf ace as predicted by the theoretical calculations of Ancilotto et al. [ 9]. The results of the model calculations are in good qualitative agre ement with the experimental findings. In spite of the very weak bindin g energy (a few cm(-1)), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far fr om the alkali's gas phase absorptions. These bound-bound excitations p roduce very broad, red shifted emission spectra. At other, blue shifte d frequencies, the excited atoms desorb from the cluster's surface, gi ving rise to unshifted, free atom, emission spectra. The heavier alkal i metals (Na, K) show, compared to the calculations, an additional bro adening which is attributed to surface excitations on the helium dropl et.