Electronic Spectroscopy Of The (A)Over-Tilde(1)(2)A ''/(A)Over-Tilde(2)(2)A ' - (X)Over-Tilde(2)A ' Transitions Of Jet-Cooled Calcium Ethoxide Radicals: Vibronic Structure Of Alkaline Earth Monoalkoxide Radicals Of C-S Symmetry

Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the (A) over tilde (2)(1)A ''/(A) over tilde (2)(2)A'-(X) over tilde (2)A' electronic transition of the calcium ethoxide (CaOC2H5) radical have been obtained under jet-cooled conditions. An essentially constant (A) over tilde (2)-(A) over tilde (1) 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 (A) over tilde (2)(1)A ''/(A) over tilde (2)(2)A'-(X) over tilde (2)A' and the (B) over tilde (2)A' 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 (A) over tilde (2)(1)A ''/(A) over tilde (2)(2)A'-(X) over tilde (2)A' transition contains a number of off-diagonal matrix elements that connect the vibrational ground levels to the levels of the nu(8) (CO stretch), nu(11) (OCC bending), nu(12) (CaO stretch), nu(13) (in-plane CaOC bending), and nu(21) (out-of-plane CaOC bending) modes, which are used for vibrational assignments. Transitions to the nu(21)(a '') levels are allowed due to the pJT effect. Furthermore, when LIF transitions to the A;-state levels of the CaOC-bending modes, nu(13) and nu(21), are pumped, (A) over tilde (2)(1)A ''/(A) over tilde (2)(2)A'-(X) over tilde (2)A' -> (X) over tilde (2)A' transitions to the combination levels of these two modes with the nu(8), nu(11), and nu(12) 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. Published under an exclusive license by AIP Publishing.