Si2pphotoabsorption inSiH4andSiD4: Molecular distortion in core-excited silane

Abstract

We report on a high-resolution photoabsorption study of ${\mathrm{SiH}}_{4}$ and ${\mathrm{SiD}}_{4}$ in the excitation region below the Si $2p$ threshold. The spectra can be separated into a lower region from valence excitations and an upper region from Rydberg excitations. In the valence region, intense excitations of vibrational bending modes were observed, reflecting a deviation of the core-excited molecules from the ${T}_{d}$ symmetry of ground-state silane. The derived geometry of the core- to -valence-excited molecules was found to be similar to that of the equivalent-cores molecule ${\mathrm{PH}}_{4}.$ For excitations to higher Rydberg states, symmetric-stretching vibrational modes were exclusively observed, while for the lowest Rydberg states, additional bending-mode vibrational excitations were found to contribute. The latter observation can be explained by a mixing of Rydberg and valence character, causing an increase of the deviation from ${T}_{d}$ symmetry with increasing valence character of the core-excited state. The spectral shape of the ${2p}_{3/2}^{\ensuremath{-}1}5s$ Rydberg state was analyzed by a four-mode Franck-Condon fit, providing insight into the structure of the core-excited molecule. The high instrumental resolution ($\ensuremath{\Delta}E=15\mathrm{meV}$ full width at half maximum) allowed the derivation of the natural widths \ensuremath{\Gamma} of the Rydberg states, which were found to be identical within the limits of error (with $\ensuremath{\Gamma}=50\ifmmode\pm\else\textpm\fi{}5\mathrm{meV}$).