Formation of ultracold LiCs molecules

We present the first observation of ultracold LiCs molecules. The molecules are formed in a two-species magneto-optical trap and detected by two-photon ionization and time-of-flight mass spectrometry. The production rate coefficient is found to be in the range $10^{-18}unit{cm^3s^{-1}}$ to $10^{-16}unit{cm^3s^{-1}}$, at least an order of magnitude smaller than for other heteronuclear diatomic molecules directly formed in a magneto-optical trap.