Autler-Townes effect in a sodium molecular-ladder scheme

We report results from studies of the Autler-Townes (AT) effect observed in sodium molecules from a molecular beam. A relatively weak laser field P couples an initially populated rovibronic level g in the electronic ground state (here X Σg+1, v″ =0, J″ =7) to a selected excited rovibronic level e (here A Σu+1, v′ =10, J′ =8), which in turn is coupled by a relatively strong laser field S to a more highly excited level f (here 5 Σg+1, v=10, J=9), a scheme we idealize as a three-state ladder. The AT effect is seen by scanning the frequency of the P field while recording fluorescence from both the e and f levels in separate detection channels. We present qualitative theoretical considerations showing that, when the P field is weak, the ratio of doublet component areas in the excitation spectrum from level f can be used to determine the lifetime of this level. We obtain a value of 17±3 ns. When the P field is stronger, such that its Rabi frequency is larger than the decay rate of level e, the fraction of f -level population that decays to the intermediate electronic state A Σu+1 can be deduced from the AT spectrum. When supplemented with values of Franck-Condon and Hönl-London factors, our measurements give a value for the branching ratio (the fraction returning to level e) of re =0.145 with a statistical error of ±0.004. The use of a strong P field on the g-e transition and a weak S field as a probe on the e-f transition results in complex line shapes in the excitation spectrum of level f, not showing the familiar Autler-Townes doublet structure.