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 D~³Πg 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.