Magneto-optical trapping of aluminum monofluoride

Magneto-optical trapping of molecules has thus far been restricted to molecules with ${}^2\mathrm{\Sigma }$ electronic ground states. These species are chemically reactive and only support a simple laser cooling scheme from their first excited rotational level. Here, we demonstrate a magneto-optical trap (MOT) of aluminum monofluoride (AlF), a deeply bound and intrinsically stable diatomic molecule with a ${}^1{\mathrm{\Sigma }}^{+}$ electronic ground state. The MOT operates on the strong A${}^1\mathrm{\Pi }\leftarrow$X${}^1{\mathrm{\Sigma }}^{+}$ transition near 227.5~nm, whose Q$(J)$ lines are all rotationally closed. We demonstrate a MOT of about $6\times {10}^4$ molecules for the $J=1$ level of AlF, more than ${10}^4$ molecules for $J=2$ and $3$, and with no fundamental limit in going to higher rotational levels. Laser cooling and trapping of AlF is conceptually similar to the introduction of alkaline-earth atoms into cold atom physics, and is key to leveraging its spin-forbidden a${}^3\mathrm{\Pi }\leftarrow$X${}^1{\mathrm{\Sigma }}^{+}$ transition for precision spectroscopy and narrow-line cooling.