Vibrational effects in the photo-ion yield spectrum of the SiH₂⁺ molecular ion following 2p inner-shell excitation

We report on complementary theoretical and laboratory investigations of the 2p ion yield cross sections for the molecular-ion series SiH_n⁺ (n = 1, 2, 3), in the 95-108 eV photon energy range, below the L-shell threshold. The experiments used an electron cyclotron resonance (ECR) plasma molecular-ion source coupled with monochromatised synchrotron radiation in a merged-beam configuration. The experimental spectra are compared with total photoabsorption cross-sectional profiles calculated using an ab initio configuration interaction method inclusive of spin-orbit coupling and the vibrational dynamics. The experimental results show vibrationally resolved resonances for SiH₂⁺ in the 98-102 eV range. The calculations indicate twenty four core-excited states below the energy of 102 eV, of which only four contribute significantly to the observed spectrum. These states correspond to the excitation of an atomic-like 2p electron to the SiH₂⁺ (5a₁) valence orbital.