Ultracold molecules offer a rich internal structure of long-lived rotational and hyperfine states easily accessible with microwave fields, illustrated up to N = 6 for 87Rb133Cs prepared initially in the hyperfine ground state of N = 0 at ~181 G. The microwave spectra (right) are labelled to match the transitions (left). The top panel shows Rabi oscillations for the N = 5 -> 6 transition. The nuclear spins of 87Rb (i=3/2) and 133Cs (i=7/2) result in 32 x (2N + 1) hyperfine states per total angular momentum, N.
