PLASMA EXCITATIONS IN CHARGED SODIUM CLUSTERS

Photoabsorption cross sections for free, singly charged sodium cluster s (14-48 atoms) have been measured by recording the light-induced evap oration. The measurements cover the region from 366 nm (3.4 eV) to 605 nm (2.0 eV) in 15 steps with a spectral resolution of congruent-to 10 nm. The evaporation residues are separated in an electrostatic field and detected by a position-sensitive microchannel plate. The spectra a re dominated by surface-plasma oscillations of the valence electrons w ith a cross section that exhausts almost-equal-to 70-100% of the dipol e sum rule. Over the mass range considered, the mean energies of the r esonances are constant (congruent-to 2.64 eV), and redshifted compared to the corresponding experimental bulk-plasmon energy but slightly bl ueshifted compared to the resonance energy of neutral clusters. A doub le structure of the resonance peak is observed for nonmagic clusters. It is discussed and interpreted in terms of spheroidal deformations.