Electronically excited states in size-selected solvated alkali metal atoms.: III.: Depletion spectroscopy of Na(NH3)n-clusters

The first electronically excited state of small Na(NH3)(n) clusters up to n = 22 is studied by means of depletion spectroscopy. A drastic decrease of the excitation energy from the 3s-->3p transition of the Na atom (16 950 cm(-1)) down to 6000 cm(-1) for the Na(NH3)(4) cluster, the closing of the first solvation shell, is observed. For larger clusters the excitation energy increases slightly toward the bulk value (6300 cm(-1)) which represents the absorption of the "solvated" electron. For all Na(NH3)(n) clusters with n greater than or equal to 3 a strong absorption peak is observed near 6600 cm(-1). By comparison with deuterated sodium-ammonia clusters this absorption can be assigned to an intramolecular vibrational overtone of the ammonia molecule. This indicates a strong coupling between electronical and vibrational excitation in the Na(NH3)(n) clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)01801-2].