New or improved photoabsorption spectra of gas-phase silicon molecules SiX4 (X = H, D,F,Cl,Br,CH3, C2HS, OCH3, OC2H5) have been measured near the silicon 1s edge in the 1830-1900-eV photon energy range. On the basis of con6guration-interactioncalculations 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,C1,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.

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