Consequences of optical pumping and interference for excitation spectra in a coherently driven molecular ladder system

This paper presents observations and an interpretation of laser-induced excitation and fluorescence in a ladder g-e-f of three molecular levels [X Σg+1 (v″ =0, J″ =7), A Σu+1 (v′ =10, J′ =8), and 5 Σg+1 (v=10,J=9), respectively] observed in a supersonic molecular beam of Na2. The g-e coupling, by cw laser P, is strong. A weak cw laser S couples levels e and f. The basic observations are of level- f fluorescence as a function of the detuning of the S laser from resonance. The signal profile does not appear as the typical Autler-Townes doublet, but as a spectral structure, whose position, width, and shape depend upon several laser parameters. We interpret these results using a simple model of three nondegenerate quantum states coherently excited while undergoing population loss to states outside the three-level system. We invoke the mechanism of optical pumping and evolution along adiabatic states, together with Landau-Zener transition probabilities. We also present results from numerical studies, which include all quantum states, all radiative couplings, coherent and incoherent, as well as convolutions with the relevant distribution functions (velocities and Zeeman sublevels). Although no adjustable parameters are involved, excellent agreement with the experiment is found. Since successive avoided crossings of adiabatic eigenvalues occur, interference effects may be relevant. Such effects are not expected to be visible in the present experiment, for reasons that are discussed. However, we discuss conditions which would allow resolving the interference structure experimentally. We also suggest possible interesting applications of the interference to rapidly switch off Rydberg state population or to control its spatial distribution.