Geometric and electronic structures of silicon-sodium binary clusters. II. Photoelectron spectroscopy of Si_nNa_m- cluster anions

Electronic properties of silicon-sodium cluster anions (Si_nNa_m ^-,; 4≤n≤11, 1≤m≤3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The electron affinities (EAs) of Si_nNa_m were determined from the threshold energies in the photoelectron spectra of Si_nNa_m ^-. Besides, the geometries and energies of Si_nNa^- (n=1-7) were investigated with ab initio MO (molecular orbital) calculations; Møller-Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. By an Na adsorption on Si_n, the EA decreases for n=4-6, but increases for n≥7. The calculations for Si_nNa^- show that the Si_n framework for n=4-6 accepts at least two excess electrons. For Si₇Na^-, however, the charge transfer from the Na atom scarcely occurs to avoid distorting the close-packed Si₇ framework. Moreover, the ionization energy (E_i) and EA of Si_nNa_m, show several patterns with the number of the Na atoms, which reflect the charge transfer mechanism in the cluster reflecting the nature of the electron-accepting orbitals of the corresponding Si_n and the adsorption types of the Na atoms.