| Abstract
| - Singlet and triplet excited states of the title compounds wereinvestigated experimentally, using electron-energy-loss spectroscopy (EELS), and theoretically, using densityfunctional calculations that includeconfiguration interaction (DFT/SCI). Both the triplet and thesinglet spectra are well rationalized by thetheory, permitting an assignment of the observed, broad bands andproviding strong indication that DFT/SCIis suitable for the description of excited states in moleculescontaining third-row elements. The low-lyingtransitions in tetramethylsilane are found to be 4s and 4p Rydberg; thelowest valence states are higher, at 9.8eV (triplet) and 10.5 eV (singlet). The lowest triplet band inhexamethyldisilane is found to be valence, theupper orbital having π symmetry. The lowest singlet state is 4s,albeit with zero oscillator strength. Thelowest observed singlet band is 4pxy Rydbergwith substantial π* valence admixture. The calculateddensityof excited states is high in tris(trimethylsilyl)silane, andRydberg−valence mixing is prevalent. A high densityof states is found also for tetramethoxysilane, the low-lyingtransitions being all Rydberg originating fromthe oxygen lone pair orbitals. Excitation functions for selectedvibrational and electronically excited statesare presented. The former provide the electron attachmentenergies. The latter indicate large cross sectionsfor triplet excitation near threshold and, thus, imply substantialyield of triplet states under typical plasmaconditions. The He I photoelectron spectrum of tetramethoxysilaneis also presented.
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