The Visible Spectrum and Ionization Energy of Jet-Cooled Linear SiCCSi

The C̃³Σ_u^- ← X̃³Σ_g^- transition of linear SiCCSi has been examined in detail using resonant two-color two-photon ionization (R2C2PI), laser-induced fluorescence (LIF), and single-vibronic-level emission (SVLE) spectroscopy. Twenty-three ground-state vibrational levels, including fundamental frequencies of totally symmetric modes and first overtones of IR-active nontotally symmetric modes, are assigned with confidence, providing a foundation for infrared laboratory surveys. In addition, a nearly complete assignment of the excitation spectrum, including many previously unidentified hot bands, has been made for transitions detectable by fluorescence. With the C̃-state vibrationless level serving as the intermediate, we determine an adiabatic ionization energy (AIE) of 7.477(1) eV. B3LYP/aug-cc-pVQZ calculations give an AIE and ground-state vibrational frequencies in close accord with experiment but provide a relatively poor description of the C̃-state, possibly because of a vibronic interaction with the close-lying ${\tilde{D}}^3{\Pi }_g$ state. The change in the Si-C bond length and rotational constant that we infer by fitting the Franck-Condon activity in the Si-C stretching mode is in excellent agreement with previous rotationally resolved analysis (both yielding B'/B″ ∼ 0.966) but departs significantly from theory (B'/B″ = 0.954); furthermore, the calculated C̃-state Si-C stretch frequency is 25% too high. Measurement of the C̃ ← X̃ transition using 193 nm radiation (6.42 eV) for ionization results in remarkably different relative intensities for the same spectral features in comparison with LIF and near-threshold R2C2PI, as well as a relative insensitivity to the delay between resonant and ionizing photons. This difference is reconciled in terms of internal conversion from the C̃-state to highly vibrationally excited levels of dark electronic states located ∼6.4 eV below the cation zero-point level, thereby admitting favorable overlap with commensurately vibrationally excited levels of the cation ground state at 193 nm.