Prospects for narrow-line cooling of KRb molecules in the 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 X (1)Sigma(+) -> b (3)Pi(0+) transition of KRb is well suited for realizing narrow-line laser cooling of molecules because it has highly diagonal Franck-Condon factors and a narrow line width. To confirm the prediction, we performed optical and microwave spectroscopy of ultracold (KRb)-K-41-Rb-87 molecules, and determined the line width [2 pi x 4.9(4) kHz] and Franck-Condon factors for the X (1)Sigma(+)(v '' = 0) -> b (3)Pi(0+) (v' = 0) transition [0.9474(1)]. This result opens the door towards all-optical production of polar molecules at sub-microkelvin temperatures.