Multiphoton ionization spectra and tunable fifth-harmonic production near five-photon resonances in Xe and Ar

Through multiphoton ionization measurements, we have observed the interference of five-photon and internally generated fifth-harmonic excitation processes in J=1 even-odd parity resonant transitions in Xe and Ar. In agreement with theory, these five-photon resonances are not observable in single-pass laser beams, but are convincingly observed in counterpropagating beams. In the negatively dispersive regions of the 5d[3/2]0J=1 level of Xe, tunable fifth-harmonic light is generated by phase matching with various buffer-gas mixtures. The fifth-harmonic profiles are in good agreement with calculated phase-matching curves.