Observation and analysis of the hyperfine structure of near-dissociation levels of the NaCscΣ+3state below the dissociation limit3

We report photoassociation (PA) of ultracold Na and Cs atoms in a dual-species magneto-optical trap. Trap loss spectroscopy of the ultracold polar NaCs molecules formed by PA, which carries information about relative PA transition strengths, has been experimentally obtained by using highly sensitive modulation spectroscopy technique. The fine and hyperfine effects at near-dissociation levels of NaCs molecular $c\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Sigma}}^{+}$ state are observed and modeled. The interaction Hamiltonian is described in terms of the Hund's case ($a$) coupling scheme. The molecular hyperfine structure of near-dissociation levels is simulated within a simplified model of four interacting vibrational levels belonging to different initially unperturbed electronic states. The results of the simulation infer that the interaction parameters of the observed near-dissociation levels are close to the asymptotic parameters of the pair of atoms. The coupling of the electronic states is essential for forming the hyperfine structure.