Geometric and electronic structures of silicon-sodium binary clusters .2. Photoelectron spectroscopy of SinNm- cluster anions

Electronic properties of silicon-sodium cluster anions (SinNam-; 4 less than or equal to n less than or equal to 11, 1 less than or equal to m less than or equal to 3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The electron affinities (EAs) of SinNam were determined from the threshold energies in the photoelectron spectra of SinNam-. Besides, the geometries and energies of SinNa- (n=1-7) were investigated with ab initio MO (molecular orbital) calculations; Moller-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 greater than or equal to 7. The calculations for SinNa- 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 Si-7 framework. Moreover, the ionization energy (E-i) and EA of SinNam 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. (C) 1997 American Institute of Physics. [S0021-9606(97)03347-3].