Competition Between Photoionization and Two-Photon Raman Coupling

In transitions between bound states, multiphoton processes become important at higher laser intensities when the Rabi-frequency is comparable to or larger than the width of the transition line. This has been shown, e.g., in the study of resonance fluorescence of a two-level atom [1]. Bound-free transitions, however, cannot be saturated owing to the large width of the continuum. Consequently, multiphoton processes should not be important. The only effects expected at higher laser intensities are depletion of the bound state population and possibly continuum-continuum transitions, the latter resulting in multiple peaks in the photo-electron energy spectrum [2].