ELECTRON-IMPACT IONIZATION-POTENTIALS OF GOLD AND SILVER CLUSTERS MN-N, N LESS-THAN-OR-EQUAL-TO 22

Small noble metal clusters were produced by a gas aggregation technique and were investigated mass spectrometrically using electron ionizing energies ranging from 100 eV down to the threshold region. For energies above approximately 20 eV electronic effects are displayed, that characterize a stable cationic distribution, i.e. enhanced stability is found for odd numbered clusters posessing an even number of electrons, furthermore, shell closing appears at n = 3, 9, 21 ... On the other hand, the electronic effects found while investigating the threshold energy region have to be attributed to neutral clusters, i.e. even numbered clusters are more stable. - The vertical ionization potentials in this region clearly demonstrate the manifestations of shell effects as well as distinct even-odd oscillations. For both Ag and Au a dramatical drop in the ionization potential is observed from dimer to trimer. The overall size-dependence of the ionization energies differs from that of alkali clusters due to the presence of d-electrons.