In addition to being suitable for laser cooling and trapping in a magneto-optical trap (MOT) using a relatively broad ($sim$5 MHz) transition, the molecule YO possesses a narrow-line transition. This forbidden transition between the $X^{2}Sigma$ and $A^{2}Delta_{3/2}$ states has linewidth $sim$2$pitimes$160 kHz. After cooling in a MOT on the $X^{2}Sigma$ to $A^{2}Pi_{1/2}$ (orange) transition, the narrow (red) transition can be used to further cool the sample, requiring only minimal additions to the first stage system. The narrow line cooling stage will bring the temperature from $sim$1 mK to $sim$10 $mu$K, significantly advancing the frontier on direct cooling achievable for molecules.
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