PROBING THE NONMETAL METAL-TRANSITION OF DIVALENT ELEMENTS - DIRECT EXPERIMENTAL-OBSERVATION OF DISCRETE ELECTRONIC SHELLS IN CADMIUM CLUSTERS

By measuring size specific photoelectron spectra of neutral cadmium cl usters Cd(x) (5 less-than-or-equal-to x less-than-or-equal-to 46) in a molecular beam, we find direct experimental evidence for the existenc e of discrete electronic shells and subshells. The formation and filli ng of shells has been observed at 10 eV and 8.3 eV photon energy. The photoionization mass spectra of neutral cadmium clusters also clearly exhibit pronounced electronic shell closings with 20 (Cd10), 40 (Cd20) , 70 (Cd35), 92 (Cd46) and 138 (Cd69) electrons. Ionization potential measurements indicate a rapid convergence towards the values expected for clusters with metallic interaction.