Line-shape study of two-color–three-photon ionization of Rb atoms

We report on the analysis of the line shapes in a two-color, three-photon ionization process of rubidium atoms in a magneto-optical trap. A cw and a pulsed laser are tuned almost at resonance with the 5²S_1/2-5²P_3/2 and the 5²P_3/2-6²D_5/2 transitions, respectively. The line shapes, recorded as functions of the pulsed laser wavelength, are strongly asymmetric with respect to the near resonance peak positions: in fact, they present long tails only on one side, according to the sign of the detuning of the cw laser. Moreover, the signal is overall stronger for negative than for positive detunings. Theoretical considerations based on the Autler-Townes scheme and computer simulations of the excitation-ionization dynamics show that the main source of asymmetry is the coupling of the two ionization pathways going through the 6²D_3/2 and 6²D_5/2 levels. In order to reproduce the experimental line shapes one must take into account the variability of the pulsed laser intensity and its finite coherence time.