Electronic spectroscopy of the ÃA/ÃA-X̃A transitions of jet-cooled calcium ethoxide radicals: Vibronic structure of alkaline earth monoalkoxide radicals of C symmetry.

Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the ÃA/ÃA-X̃A electronic transition of the calcium ethoxide (CaOCH) radical have been obtained under jet-cooled conditions. An essentially constant Ã-Ã energy separation for different vibronic levels is observed in the LIF spectrum, which is attributed to both the spin-orbit (SO) interaction and non-relativistic effects. Electronic transition energies, vibrational frequencies, and spin-vibrational eigenfunctions calculated using the coupled-cluster method, along with results from previous complete active space self-consistent field calculations, have been used to predict the vibronic energy level structure and simulate the recorded LIF/DF spectra. Although the vibrational frequencies and Franck-Condon (FC) factors calculated under the Born-Oppenheimer approximation and the harmonic oscillator approximation reproduce the dominant spectral features well, the inclusion of the pseudo-Jahn-Teller (pJT) and SO interactions, especially those between the ÃA/ÃA and the B̃A states, induces additional vibronic transitions and significantly improves the accuracy of the spectral simulations. Notably, the spin-vibronic interactions couple vibronic levels and alter transition intensities. The calculated FC matrix for the ÃA/ÃA-X̃A transition contains a number of off-diagonal matrix elements that connect the vibrational ground levels to the levels of the ν (CO stretch), ν (OCC bending), ν (CaO stretch), ν (in-plane CaOC bending), and ν (out-of-plane CaOC bending) modes, which are used for vibrational assignments. Transitions to the ν(a″) levels are allowed due to the pJT effect. Furthermore, when LIF transitions to the Ã-state levels of the CaOC-bending modes, ν and ν, are pumped, ÃA/ÃA→X̃A transitions to the combination levels of these two modes with the ν, ν, and ν modes are also observed in the DF spectra due to the Duschinsky mixing. Implications of the present spectroscopic investigation to laser cooling of asymmetric-top molecules are discussed.