Accurate prediction of vibronic levels and branching ratios for laser-coolable linear polyatomic molecules

We report a generally applicable computational scheme to calculate vibronic levels and branching ratios for laser-coolable linear polyatomic molecules to an accuracy and completeness to be useful to guide experimental studies. The present computational scheme consists of a multi-state quasidiabatic Hamiltonian with relevant spin-vibronic perturbations, coupled-cluster calculations for adiabatic potential energy surfaces, and discrete variable representation calculations for vibronic levels and wave functions. The computed vibronic levels and branching ratios for the A²Π_1/2 → X²Σ_1/2 transitions of CaOH, SrOH, and YbOH show promising agreement with the experimental measurements. The calculations elucidate intensity borrowing mechanisms for the nominally symmetry-forbidden transitions. Based on the computed branching ratios, laser-cooling SrOH requires fewer repumping lasers than CaOH. A close inspection of computational results further reveals it beneficial to avoid Fermi resonances in designing laser-coolable molecules.