ac Stark splitting in doubly resonant three-photon ionization with nonmonochromatic fields

We present a theory of Stark splitting in doubly resonant three-photon ionization (three-level system) with nonmonochromatic fields. The asymmetry of the peaks of the resonance curve due to Stark splitting is shown to be reversed when the laser bandwidth is larger than the width of the atomic transitions of the resonant states. It is further shown that ionization as well as the probe field have a significant effect on the features of the splitting. Similar effects are found with monochromatic fields in the presence of an additional level near the initial level (four-level system). The results of the analysis are in general qualitative agreement with existing observations.