Si 2p photoabsorption in SiH₄ and SiD₄:  Molecular distortion in core-excited silane

We report on a high-resolution photoabsorption study of SiH₄ and SiD₄ 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 PH₄. 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^-15s 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 (ΔE=15meV full width at half maximum) allowed the derivation of the natural widths Γ of the Rydberg states, which were found to be identical within the limits of error (with Γ=50±5meV).