Photoassociative Ionization of Heteronuclear Molecules in a Novel Two-Species Magneto-optical Trap

We describe the experimental observation of ultracold ${\mathrm{NaCs}}^{+}$ molecular formation in a novel two-species magneto-optical trap. We interpret our observations in terms of a photoassociative ionization pathway similar to that observed in a sodium trap: $\mathrm{Na}+\mathrm{Cs}+ħ{\omega }_{\mathrm{Na}}+ħ{\omega }_{\mathrm{Cs}}\to {\mathrm{NaCs}}^{+}{+e}^{-}$. We have directly measured the ${\mathrm{NaCs}}^{+}$ production rate as a function of the excitation laser frequency and intensity elucidating the role of ground-state hyperfine levels. We have observed resonance structures in the frequency-dependent ion rate which we associate with the molecular character of the process.