Si L- and K-edge x-ray-absorption near-edge spectroscopy of gas-phase Si(${\mathrm{CH}}_3$${)}_{\mathit{x}}$(${\mathrm{OCH}}_3$${)}_{4\mathrm{-}\mathit{x}}$: Models for solid-state analogs

Both the L- and K-edge x-ray-absorption near-edge spectra for the five compounds Si(${\mathrm{OCH}}_3$ ${)}_{\mathit{x}}$(${\mathrm{CH}}_3$ ${)}_{4\mathrm{-}\mathit{x}}$ (x=0 to 4) are reported. The spectra of the two end members of the series, Si(${\mathrm{OCH}}_3$ ${)}_4$ and Si(${\mathrm{CH}}_3$ ${)}_4$, are compared to the spectra of their respective solid-state analogs, ${\mathrm{SiO}}_2$ and SiC. The L-edge spectra of gaseous Si(${\mathrm{OCH}}_3$ ${)}_4$ and solid ${\mathrm{SiO}}_2$ are qualitatively identical; while the L-edge spectra of gaseous Si(${\mathrm{CH}}_3$ ${)}_4$ and its solid-state counterpart, SiC, show strong similarities. MS-Xα calculations were performed for the two species Si(${\mathrm{OCH}}_3$ ${)}_4$ and Si(${\mathrm{CH}}_3$ ${)}_4$ for the Si 2p, 2s, and 1s regions and used to assign the spectra for both compounds. By comparison of molecular-orbital diagrams of Si(${\mathrm{OCH}}_3$ ${)}_4$ and ${\mathrm{SiO}}_2$, the first two transitions in the L-edge spectra of ${\mathrm{SiO}}_2$ have been assigned to transitions from the Si 2p to orbitals of ${\mathit{a}}_1^{\mathrm{*}}$ and ${\mathit{t}}_2^{\mathrm{*}}$ symmetry. In addition to ${\mathrm{SiO}}_2$, the L-edge spectra of the isoelectronic species ${\mathrm{PO}}_4^{3\mathrm{-}}$, ${\mathrm{SO}}_4^{2\mathrm{-}}$, and ${\mathrm{ClO}}_4^{\mathrm{-}}$ are also assigned 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 spectra 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 the gas-phase models or in the theoretical calculations on the gas-phase compounds. These multiple-scattering peaks are not seen in the L-edge solid-state spectra because of the strong shape resonances peaks in this region.