Radiative lifetimes of the (1-3) 1Π states in NaCs: Experiment and theory

The radiative lifetimes of the (3) 1Π and D(2) 1Π states of the NaCs molecule have been directly measured in a thermal cell from fluorescence kinetics after modulated laser excitation. The experimental τ(3)1Πrad values of the (3) 1Π(v'(set-membership sign)[3,25];J'(set-membership sign)[25,106]) levels decrease from 29 to 21 ns as the v' values increase. The measured τ(3)1Πrad values for two rotational J'=45 and 106 levels of the v'=3 state are 29 and 25 ns, respectively, showing the effect of rotation. The experimental τ(2)1Πrad value of the D(2) 1Π(v'=0;J'=16) level is about 37 ns. The average measured effective quenching collision cross section of excited NaCs molecule with Cs atoms σ=(3.5±1.5)x10-14 cm2. The theoretical radiative lifetimes for the lowest (1-3) 1Π states have been evaluated in the framework of a pure a Hund's coupling case using the approximate sum rule over the lower-lying vibronic states. The required quasirelativistic transition dipole moments were calculated by many-body multipartitioning theory. Propagation of the systematic errors of ab initio transition moments and potential energy curves (PECs) into the τrad values is discussed. The best lifetime estimates predicted for particular rovibronic levels of the (3) 1Π and D(2) 1Π states agree with their experimental counterparts within 2-8 % which can be compared with the 1-3 % uncertainties in the measured τexpt values. The lifetimes of the B(1) 1Π and (3) 1Π states are almost exclusively determined by a single transition to the ground X 1Σ+ state while a contribution of the D(2) 1Π→A 1Σ+ transition into the τ(2)1Πrad values reaches up to 10-15%