SINGLE-ELECTRON AND MULTIPLE-ELECTRON EFFECTS IN THE SI 1S PHOTOABSORPTION SPECTRA OF SIX4 (X=H,D,F,CL,BR,CH3,C2H5,OCH3,OC2H5) MOLECULES - EXPERIMENT AND THEORY

New or improved photoabsorption spectra of gas-phase silicon molecules SiX4 (X=H,D,F,Cl,Br,CH3,C2Hd5,OCH3, OC2H5) have been measured near the silicon 1s edge in the 1830 1900-eV photon energy range. On the basis of configuration-interaction calculations performed on the core-equivalent species, we show that for SiH4 and SiF4 the two lowest resonances are due to excitation of the Si 1s electron into unoccupied orbitals, both having a mixed-valence Rydberg character. For all other systems, there is a unique intense transition into a pure low-lying valence orbital, and all Rydberg states have a negligible oscillator strength. Vibronic coupling effects are found in the intensity of a symmetry-forbidden valence transition, but only for X=F,Cl,Br. In addition, we interpret multiplet features found in the immediate vicinity above the ionization edge, as double core-valence vacancy-excited states. Possible overlap of such states with shape resonance in SiX4 (X=F,OCH3,OC2H5) is discussed. © 1990 The American Physical Society.