OPTICAL-RESPONSE OF SMALL CLOSED-SHELL CS-CLUSTER IONS

The absolute photoabsorption cross sections of Cs-9(+) and Cs-21(+) ha ve been measured at two temperatures (evaporative ensemble, 115 K). Th e maximum of the observed resonance is nearly independent of the tempe rature and agrees within 5% with the Mie plasmon resonance as calculat ed from the bulk electron density of cesium. This is in contrast to th e sodium case, where the plasmon is redshifted by about 20%. For n = 9 and 21 the integrated oscillator strength amounts to 89% (Cs-9(+)) an d 73% (Cs-21(+)) per valence electron in the energy range 1.5 eV < hv < 3.0 eV.