Surface-plasmon absorption in singly and doubly charged potassium clusters

We report the optical-absorption spectra of hot, singly, and doubly charged potassium cluster ions (${\mathrm{K}}_{\mathit{n}}^{+}$, n=9,11,14–21, and ${\mathrm{K}}_{\mathit{n}}^{2+}$, n=odd, 39–53) in the 1.8–2.2 eV range. The cluster ions are produced by a modified liquid-metal ion source. Within experimental uncertainty, the spectra of all clusters, with the exception of ${\mathrm{K}}_{17}^{+}$, consist of a single, broad peak, centered near hν=2.0 eV. The results are interpreted in terms of the surface-plasmon absorption of small, liquid-metal droplets. The lack of structure in the observed spectra is attributed to thermal broadening of the individual contributions to the plasmon. A model, based on the liquid-drop model, is introduced to account for the thermal shift and broadening of the surface plasmon.