Effect of quantum interference on the suppression of the ac Stark shifting of a multiphoton resonance

We report detailed experimental results in xenon that confirm theoretical predictions [Payne et al., Phys. Rev. A 48, 2334 (1993)] on the interference-based suppression of ac Stark shifting of even-photon resonances under circumstances where the shift is produced through strong coupling to a state that is also coupled back to the ground state. In the copropagating-beam configuration and in the pressure range of 0.5--250 Torr, the multiphoton ionization spectral line shape is unchanged regardless of the presence of the second laser field, i.e., the ac Stark shift introduced by the second laser is totally suppressed due to a complete destructive interference between two excitation pathways. In the counterpropagating-beam configuration, however, the ac Stark shift persists in the range of the pressure studied. By choosing a different set of energy levels where no four-wave-mixing field is permitted, we show that the very same ac Stark shift introduced by the second laser field persists even with copropagating-beam configuration, as predicted by theory.