SUPPRESSION OF A 4-PHOTON RESONANCE BY 4-WAVE-MIXING NEAR AN INTERMEDIATE 3-PHOTON RESONANCE

We use resonance ionization involving two colors in single-pass and retroreflected geometries to study the suppression of excitation of a four-photon resonance by an interference involving four-wave sum-frequency generation occurring near an intermediate three-photon resonance. The 6p[1/2](J = 0) state of xenon is excited by three blue photons plus one infrared photon. One beam, focused to I1 congruent-to 7 X 10(9) W/cm2, is nearly three-photon resonant with the 6s[3/2](J = 1) resonance; the second, unfocused beam (I2 congruent-to 2 X 10(7) W/cm2) is tuned to maintain exact four-photon resonance with the 6p[1/2](J = 1) state. One additional blue photon ionizes from the four-photon level. Third-harmonic light is generated by the first, nearly three-photon-resonant beam by four-wave sum-frequency mixing. One third-harmonic photon plus one photon from the second laser form a second excitation pathway to the four-photon level; destructive interference between the two pathways results in a reduction in the observed ionization. The degree of reduction depends on the xenon density and the detuning (both positive and negative) from three-photon resonance. This interference may be overcome by retroreflecting the nearly three-photon-resonant beam to provide an additional excitation pathway which generates no third harmonic. The study provides detailed information on the pressure-dependent range from exact resonance over which the familiar three-photon cancellation effect is operative; it shows that the interference occurs in high- and low-energy regions about three-photon resonance; and it establishes that interference is produced for large values of the phase mismatch even if the absorption of the interfering field is low. We compare our measurements of the reduction in ionization with predictions published by us elsewhere [M. G. Payne, J. C. Miller, R. C. Hart, and W. R. Garrett, Phys. Rev. A 44, 7684 (1991)]; excellent agreement is observed. We also report the observation of suppression of pressure-induced wing absorption into the three-photon state observed in a classic "hybrid" resonance mode of excitation.