Quantum Chemistry Calculations and Spectroscopic Predictions of Cyclic MgC(3)H (C(2v)) as a Candidate Molecule for Laser Cooling.

We report quantum chemistry calculations on the lowest electronic states (A, B, B, and A) of the lowest-energy conformer of MgCH, namely, its planar cyclic form c-MgCH (), an elusive and intriguing molecule with both astrochemical and fundamental significance. Its equilibrium geometries and vibrational frequencies were calculated at the CASSCF(3,6)/cc-pVTZ level of theory, and the Franck-Condon factors (FCFs) between the ground and excited electronic states have been determined, based on which the excitation and emission spectra of vibronic transitions are predicted. Calculations suggest that c-MgCH () is an excellent candidate for direct laser cooling of asymmetric-top molecules. Because the B - B transition is electric-dipole-forbidden, the B - A system can be used for photon cycling with exceptionally high efficiency. Pumping/repumping schemes for laser cooling have been proposed, and the predicted cooling efficiencies have been reported.