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Prospects for Narrow-line Cooling of KRb Molecules in Rovibrational Ground State

We propose and experimentally investigate a scheme for narrow-line cooling of KRb molecules in the rovibrational ground state. We show that the spin-forbidden $\mathrm{X^1Σ^+} \rightarrow \mathrm{b^3Π_{0^+}}$ transition of KRb is ideal for realizing narrow-line laser cooling of molecules because it has highly diagonal Franck-Condon factors and narrow linewidth. In order to confirm the prediction, we performed the optical and microwave spectroscopy of ultracold $^{41}$K$^{87}$Rb molecules, and determined the linewidth ($2π\times$ 4.9(4) kHz) and Franck-Condon factors for the $\mathrm{X^1Σ^+} (v''=0) \rightarrow \mathrm{b^3Π_{0^+}} (v'=0)$ transition (0.9474(1)). This result opens the door towards all-optical production of polar molecules at sub-microkelvin temperatures.