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 (K(n)+, n = 9, 11, 14-2 1, and K(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 K17+, Consist of a single, broad peak, centered near hv = 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.