Coherent cancellation effects in the multiphoton ionization of xenon

Two experiments exploring various manifestations of the three-photon interference effect are described. In the first experiment, a transition in xenon allowed with both one and three photons is shown to exhibit a shift in the resonance wavelength which varies with the xenon density and the angle between the laser beams used for the three-photon excitation. The magnitude of the shift is shown to agree quite well with the predictions of a two-state model for the excitation which includes self-consistently the pumping rate due to the internally generated sum-frequency wave as well as that due to the laser fields. The second experiment demonstrates the suppression of the excitation of a two-and four-photon allowed transition by a three-photon interference effect occuring at an intermediate three- and one-photon allowed resonance. The degree of the suppression is measured by comparing the ionization with and without retroreflection of the near three-photon resonant laser beam. The observed suppression agrees well with the predictions of a three-state model in which the transition rate due to the internally generated third-harmonic light (plus an additional photon) is treated self-consistently. The diminution of the free third-harmonic wave which is necessary for the three-photon interference effect to occur is shown to be due mainly to a large phase mismatch between the free third harmonic and the laser field, rather than to absorption of the third harmonic. The suppression of wing absorption into the three-photon state by the three-photon interference effect is also demonstrated.