Electronic spectroscopy of the A1̃2A′′/A2̃2A′−X̃2A′ transitions of jet-cooled calcium ethoxide radicals: Vibronic structure of alkaline earth monoalkoxide radicals of C s symmetry

Abstract

Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the A1̃2A′′/A2̃2A′−X̃2A′ electronic transition of the calcium ethoxide (CaOC2H5) radical have been obtained under jet-cooled conditions. An essentially constant Ã2−Ã1 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 A1̃2A″/A2̃2A′ and the B̃2A′ 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 A1̃2A′′/A2̃2A′−X̃2A′ transition contains a number of off-diagonal matrix elements that connect the vibrational ground levels to the levels of the ν8 (CO stretch), ν11 (OCC bending), ν12 (CaO stretch), ν13 (in-plane CaOC bending), and ν21 (out-of-plane CaOC bending) modes, which are used for vibrational assignments. Transitions to the ν21(a″) levels are allowed due to the pJT effect. Furthermore, when LIF transitions to the Ã-state levels of the CaOC-bending modes, ν13 and ν21, are pumped, A1̃2A′′/A2̃2A′→X̃2A′ transitions to the combination levels of these two modes with the ν8, ν11, and ν12 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.

A combined experimental and computational study on the transition of the calcium isopropoxide radical as a candidate for direct laser cooling

“…[71][72][73][74][75] The SO interaction has also been incorporated in the effective Hamiltonian to investigate spinvibronic structure of degenerate or nearly degenerate states of free radicals. 8,76,77 In our previous work on the calcium methoxide 10 and ethoxide 42 radicals, a spin-vibronic Hamiltonian was constructed using the quasi-diabatic basis that involves the A ˜and B ˜states. Experimentally recorded LIF and DF spectra of both radicals have been reproduced using the spin-vibronic Hamiltonian.…”

“…f A 2 2 A 0 À X2 A 0 electronic transition of the calcium ethoxide (CaOC 2 H 5 ) radicals under the jet-cooled conditions. 42 Aided by complete-active-space selfconsistent-field (CASSCF) and coupled-cluster (CC) calculations with the inclusion of vibronic and SO coupling, we were able to reproduce the spin-vibronic structure of experimentally obtained f A 1 2 A 00 . f A 2 2 A 0 $ X2 A 0 spectra of CaOC 2 H 5 .…”

“…A generalized spectroscopy model and the detailed analysis of symmetries and coupling between nearly degenerate 2 A 0 and 2 A 00 states, as well as the effect of such coupling on the spin-rovibronic structure and spectra, can be found in our previous publications. 42,43,46,58 In the present work, we report the LIF/DF and CRD spectra of jet-cooled CaOCH(CH 3 ) 2 radicals and spectral simulations based on quantum chemistry calculations. The experimental setup used in the present work is identical to those reported in our previous work on CaOCH 3 and CaOC 2 H 5 .…”

“…The experimental setup used in the present work is identical to those reported in our previous work on CaOCH 3 and CaOC 2 H 5 . 10,42 The only change in the present experiment is that the organic precursor used in the present work is isopropanol. Therefore, experimental details are given in the ESI.…”

Multivalent optical cycling centers in polyatomic molecules

“…A particularly promising avenue is ligand functionalization of multivalent OCCs via an MSR-type motif where R is a complex or chiral functional group. Prior experimental and theoretical studies with monovalent OCCs have found that AEM-pseudohalogen systems of the MOR, MSR, and MR-type possess properties favorable for laser cooling [18,22,26,[150][151][152]182], including cases where R is a complex organic ligand [19,27,149,222] up to as large as 60 atoms in size (i.e. fullerene) [25].…”