Single- and multiple-electron effects in the Si 1s photoabsorption spectra of Si$X_4$ (X=H,D,F,Cl,Br,${\mathrm{Ch}}_3$,$C_2$$H_5$,${\mathrm{OCH}}_3$,${\mathrm{OC}}_2$$H_5$) molecules: Experiment and theory

New or improved photoabsorption spectra of gas-phase silicon molecules Si$X_4$ (X=H,D,F,Cl,Br,${\mathrm{CH}}_3$,${\mathrm{C}}_2$Hd5,${\mathrm{OCH}}_3$, ${\mathrm{OC}}_2$ ${\mathrm{H}}_5$) 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 ${\mathrm{SiH}}_4$ and ${\mathrm{SiF}}_4$ 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 Si$X_4$ (X=F,${\mathrm{OCH}}_3$,${\mathrm{OC}}_2$ ${\mathrm{H}}_5$) is discussed.