Measurement of the binding energy of ultracold $^{87}$Rb$^{133}$Cs molecules using an offset-free optical frequency comb

We report the binding energy of $^{87}$Rb$^{133}$Cs molecules in their rovibrational ground state measured using an offset-free optical frequency comb based on difference frequency generation technology. We create molecules in the absolute ground state using stimulated Raman adiabatic passage (STIRAP) with a transfer efficiency of 88\%. By measuring the absolute frequencies of our STIRAP lasers, we find the energy-level difference from an initial weakly-bound Feshbach state to the rovibrational ground state with a resolution of 5 kHz over an energy-level difference of more than 114 THz; this lets us discern the hyperfine splitting of the ground state. Combined with theoretical models of the Feshbach state binding energies and ground-state hyperfine structure, we determine a zero-field binding energy of $h\times114\,268\,135\,237(5)(50)$ kHz. To our knowledge, this is the most accurate determination to date of the dissociation energy of a molecule.