Narrowline Laser Cooling of YO

The unique energy level structure of YO provides a metastable A^{' 2}Δ_3/2 excited state with a narrow transition linewidth Γ= 2π x 7 kHz to the electronic ground state. The existence of a near-degenerate Λ doublet within this excited state enables full molecular polarization at electric field strengths ~10 mV/cm. High precision optical Stark spectroscopy constrains the parity doublet splitting to within 7 kHz. By applying a weak electric field, a quasi-rovibrational closure scheme is implemented by addressing an isolated parity-preserving excited state. This novel cycling scheme enables the first narrowline laser cooling of a diatomic molecule with a demonstrated 20% reduction in radial temperature.