High resolution spectrum of the B 1Π–X 1Σ+ transition of the NaRb molecule was measured with the technique of the Doppler-free laser polarization spectroscopy. Molecular constants of the B 1Π(v=0–12) and X 1Σ+(v=0–6) states of 23Na85Rb were determined. The energy levels of the B 1Π state were found to present many irregularities due to perturbations. The resonance fluorescence spectrum following an excitation to a strongly perturbed level was measured. The fluorescence to the (1)3Σ+ state, which consists of discrete lines followed by a continuum band, was observed in addition to the fluorescence lines to rovibrational levels of the X 1Σ+ state. The perturbing state to the B 1Π(v=8,J=15–21) levels is identified as the (1)3Π state by comparing the observed fluorescence spectra with the selection rules for perturbations and radiative transitions. The dissociation limit of the (1)3Σ+ state, which separates into the Na(3s2S1/2)+Rb(5s2S1/2) atoms, was deduced from the spectrum. The dissociation energies of the X 1Σ+, (1)3Σ+, and B 1Π states were determined to be 5030±2, 182±2, and 1319±2 cm−1, respectively.

A number of the rovibrational levels of the B 1Π state are found to be strongly perturbed. The energy shifts of the e and f levels of the B 1Π state are studied, and the perturbations between the B 1Π and (2) 3Σ+ states and between the B 1Π and (1) 3Π1 states are identified. Forty-seven centers of perturbation are observed, and the molecular constants of the (2) 3Σ+ and (1) 3Π1 states are estimated. The hyperfine splitting of the (2) 3Σ+ state is observed. Each rovibrational level splits primarily into four lines, and each of the four lines splits into six lines. The splitting is identified as originating from the magnetic dipole interaction primarily with the nuclear spin of 23Na (I=3/2) and secondarily with the nuclear spin of 85Rb (I=5/2).

The sub-Doppler high resolution excitation spectrum of NaRb was measured by selectively monitoring the fluorescence intensity from perturbed levels to the (1)3Σ+ lower state. Many transitions were observed to the (2)3Σ+ state perturbed by B 1Π. Energy shifts of the perturbed levels were analyzed and the spin–orbit coupling constant between the (2)3Σ+ and B 1Π states was determined, together with molecular constants of each state. Fully resolved hyperfine splittings, induced by the coupling of electron spin with both 23Na and 85Rb nuclear spins, were observed in the transitions. By analysis of the observed splittings, the mechanism of perturbation was confirmed as due to Fermi contact interaction and hyperfine constants of the 23Na85Rb (2)3Σ+ state were determined. The ratio of electron spin densities at 23Na and 85Rb in the (2)3Σ+ state is estimated to be 0.84:0.16.