X transitions of jet-cooled calcium monoalkoxide radicals: spin-ro-vibronic structure of nonlinear polyatomic molecules as candidates for direct laser cooling*

Alkaline earth monoalkoxide free radicals are promising candidates for laser cooling. Here we report a combined experimental and computational investigation of the spin-vibronic structure of the lowest electronic states of calcium methoxide [CaOCH₃ (C_3v)], ethoxide [CaOC₂H₅ (C_s)], and isopropoxide [CaOCH(CH₃)₂ (C_s)]. Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the ^~A-^~X transitions were recorded under jet-cooled conditions. The ~A₂-^~A₁ energy separation of vibronic levels is attributed to the spin-orbit (SO) interaction and, in the case of the two asymmetric tops, the difference potential between the A’ and A’’ states, both of which affect the rotational and fine structure of the nearly degenerate ^~A₁/ ^~A ₂ states. A spin-vibronic Hamiltonian has been developed in a quasi-diabatic basis for the spectral simulation, aided by ab initio calculations. The (pseudo-)Jahn-Teller and SO interactions induce off-diagonal Franck-Condon matrix elements, leading to vibronic transitions that are forbidden or negligible under the Born-Oppenheimer approximation.