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The lowest electronically excited state of small Na(H 2 O) n clusters have been investigated experimentally and theoretically. The excitation energy as determined by the depletion spectroscopy method drops from 16950 cm −1 for the sodium atom down to 9670 cm −1 when only three water molecules are attached to the Na atom. For larger clusters the absorption band shifts back towards higher energies and reaches 10880 cm −1 for n = 12. The experimental data are compared to quantum chemical calculations at the MP2 and MRSDCI levels. We found that the observed size-dependence of the transition energy is well reproduced by the interior structure where the sodium atom is surrounded by water molecules. The analysis of the radial charge distribution of the unpaired electron in these interior structures gives a new insight into the formation of the "solvated" electron.

Nam(H2O)n Clusters (n = 1 . . . 200, m = 1 . . . 50) are formed in a recently build pick-up arrangement. Preformed water clusters traverse a sodium oven, where sodium atoms are picked up. At low sodium vapour pressure (< 1 × 10 −4 mbar) pure Na(H2O)n clusters are observed in the mass spectra. At high sodium vapour pressure (> 1 × 10 −3 mbar) the water cluster pick up more than 50 Na atoms and reaction products Na(NaOH)n (n = 2, 4 . . . 50) dominate the mass spectra. The even number of NaOH units in the products indicate that also in a finite cluster the reaction occurs in pairs as in the macroscopic reaction. PACS. 33.80.Eh Autoionization, photoionization, and photodetachment -36.40.Jn Reactivity of clusters -82.80.Rt Time of flight mass spectrometry

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