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 les s 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 spectro scopy using a magnetic-bottle type electron spectrometer. The electron affinities (EAs) of SinNam were determined from the threshold energie s in the photoelectron spectra of SinNam-. Besides, the geometries and energies of SinNa- (n=1-7) were investigated with ab initio MO (molec ular orbital) calculations; Moller-Plesset perturbation and coupled cl uster 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 increase s for n greater than or equal to 7. The calculations for SinNa- show t hat 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 o ccurs to avoid distorting the close-packed Si-7 framework. Moreover, t he ionization energy (E-i) and EA of SinNam show several patterns with the number of the Na atoms, which reflect the charge transfer mechani sm in the cluster reflecting the nature of the electron-accepting orbi tals of the corresponding Si-n and the adsorption types of the Na atom s. (C) 1997 American Institute of Physics. [S0021-9606(97)03347-3].