Photoabsorption spectra of tetramethylsilane in the energy region 6-11.5 eV

Assignments of photoabsorption spectra of tetramethylsilane (TMS) recorded in 6-11.5 eV region using synchrotron radiation are reexamined. The broad band spectra up to first ionization potential at 10.6 eV is explained to consists of mixed valence-Rydberg character arising from first two occupied molecular orbitals to 4^s, 4^p, 3^d, 5^s and 5^p Rydberg states with the help of computational calculations, using TDDFT methodology. TMS is found to have two stable conformers viz. staggered and eclipsed, where the eclipsed conformer lies higher in energy by 6.88 kcal/mol. Presence of a broad continuum devoid of vibronic features is attributed to large change in geometry of molecule, high density of states due to triply degenerated highest occupied molecular orbitals and unimolecular dissociation of molecule as it is excited to its molecular ionization limit. Results from theoretical computations help in relating observed photoabsorption spectra of TMS on the basis of geometry changes in the ground and electronically excited states on excitation of molecule up to its first ionization potential. (author)