Plasma excitations in charged sodium clusters

Photoabsorption cross sections for free, singly charged sodium clusters (14--48 atoms) have been measured by recording the light-induced evaporation. 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 {}10 nm. The evaporation residues are separated in an electrostatic field and detected by a position-sensitive microchannel plate. The spectra are dominated by surface-plasma oscillations of the valence electrons with a cross section that exhausts {}70--100 % of the dipole sum rule. Over the mass range considered, the mean energies of the resonances are constant ({}2.64 eV), and redshifted compared to the corresponding experimental bulk-plasmon energy but slightly blueshifted compared to the resonance energy of neutral clusters. A double structure of the resonance peak is observed for nonmagic clusters. It is discussed and interpreted in terms of spheroidal deformations.