Nonadiabatic coupling in the 3Π3 and 4Π3 states of NaK

The excited 3Π3 and 4Π3 electronic states of the NaK molecule exhibit an avoided crossing, leading to the anomalous behavior of many features of the rovibrational energy levels belonging to each state. A joint experimental and theoretical investigation of these states has been carried out. Experimental measurements of the vibrational, rotational, and hyperfine structure of numerous levels of the 3Π3 state were recently obtained using the Doppler-free, perturbation-facilitated optical-optical double resonance technique. Additional measurements for the 4Π3 state as well as bound-free emission spectra from selected 3Π3, 4Π3, and mixed 3Π3∼4Π3 rovibrational levels are reported here. A model is also presented for calculating the mixed rovibrational level energies of the coupled 3Π3-4Π3 system, starting from a 2×2 diabatic electronic Hamiltonian. The 3Π3 and 4Π3 potential curves and the coupling between them are simultaneously adjusted to fit the observed rovibrational levels of both states. The energy levels o...