Ionization potentials of LinO (2 <= n <= 70) clusters: Experiment and theory

We report on experimental and theoretical investigations of the ionization potentials and structures of lithium monoxide clusters. The clusters were p roduced by a laser vaporization source, laser ionized, and mass selected by a time-of-flight mass spectrometer. Threshold photoionization spectroscopy was performed using photon energies of 3-5.52 eV and 6.4 eV. Ionization po tentials of LinO (2 less than or equal to n less than or equal to 70) were derived from the photoionization efficiency curves. The evolution of the io nization potentials as a function of cluster size shows distinct steps at n =10, 22, and 42, and a pronounced odd-even staggering up to n approximate t o 42. These steps are in agreement with the shell model for metallic cluste rs, provided that the oxygen atom localizes two of the lithium valence elec trons while leaving the other valence electrons delocalized in a metallic c luster. For the small clusters (n<6), fine structure is observed in the thr eshold spectra, possibly due to the presence of isomeric states in the clus ter beam. Geometries and relative stabilities of the smaller LinO clusters (2 less than or equal to n less than or equal to 8) were computed by means of density functional theory using the B3LYP functional, and adiabatic and vertical ionization potentials were calculated. A comparison with the exper imental ionization potentials provides evidence for the greater importance of rigid geometrical structures over metal-like characteristics for the sma ll clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)02420-4 ].