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 $mathrm{X^1Sigma^+} rightarrow mathrm{b^3Pi_{0^+}}$ 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 ($2pitimes$ 4.9(4) kHz) and Franck-Condon factors for the $mathrm{X^1Sigma^+} (v=0) rightarrow mathrm{b^3Pi_{0^+}} (v=0)$ transition (0.9474(1)). This result opens the door towards all-optical production of polar molecules at sub-microkelvin temperatures.