Cesium oligomers are formed on Helium nanodroplets which are doped with one or a few Cs atoms. The monomer absorption of the first electronic p ← s transition upon laser excitation is probed. Spectra employing laser-induced fluorescence, beam depletion and resonant photo-ionization are compared. In particular, mass-resolved photo-ionization allows us to specifically probe excitation induced processes such as e.g. the formation of cesium-helium exciplexes. Absorption spectra of Cs dimers and

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... ers are recorded in the spectral region accessible by a Ti:Sapphire laser. Assignment of dimer spectra is achieved by comparison with model calculations based on ab initio potentials. Electronic absorption lines of Cs trimers are attributed to transitions in the quartet manifold.