SI L-EDGE AND K-EDGE X-RAY-ABSORPTION NEAR-EDGE SPECTROSCOPY OF GAS-PHASE SI(CH3)(X)(OCH3)(4-X) - MODELS FOR SOLID-STATE ANALOGS

Both the L- and K-edge x-ray-absorption near-edge spectra for the five compounds Si(OCH3)(x)(CH3)(4-x) (x =0 to 4) are reported. The spectra of the two end members of the series, Si(OCH3)(4) and Si(CH3)(4), are compared to the spectra of their respective solid-state analogs, SiO2 , and SiC. The L-edge spectra of gaseous Si(OCH3)(4) and solid SiO2 ar e qualitatively identical while the L-edge spectra of gaseous Si(CH3)( 4) and its solid-state counterpart SiC, show strong similarities. MS-X alpha calculations were performed for the two species Si(OCH3)(4) and Si(CH3)(4) for the Si 2p, 2s, and 1s regions and used to assign the s pectra for both compounds. By comparison of molecular-orbital diagrams of Si(OCH3)(4) and SiO2, the first two transitions in the L-edge spec tra of SiO2 have been assigned to transitions from the Si 2p to orbita ls of a(1)() and a(1)(*) symmetry. In addition to SiO2, the L-edge sp ectra of the isoelectronic species PO43-, SO42-, and ClO4- are also as signed in a similar manner. Compared to the L edge, the K-edge region of the gas-phase compounds is found to have less in common with the sp ectra of the solid-state analogs. In the K-edge region, peaks from the extended band structure and/or multiple scattering are evident in the solid-state compounds. These peaks are not present, as expected, in t he gas-phase models or in the theoretical calculations on the gas-phas e compounds. These multiple-scattering peaks are not seen in the L-edg e solid-state spectra because of the strong shape resonances peaks in this region.