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