We report measurements of seventh-harmonic generation in xenon, using a femtosecond Ti:sapphire laser operating at ∼763 nm, at intensities of as much as 2 × 1014 W/cm2. Several atomic levels in xenon are nearly resonant with the seventh harmonic; as a consequence, the phase mismatch exhibits strong variations as a function of frequency. As a result of the tight-focusing condition, these variations are mapped directly onto the spectrum of the harmonic radiation. We have measured the pressure and the intensity dependences of the harmonic signal, and we report an unusual recurrence of the signal at moderately high pressures, which may also be related to resonance effects. The tight-focusing geometry, in which the variations in the phase mismatch may most easily be observed, affords the opportunity for investigations of the spectroscopy of bound states in the presence of extremely high fields.
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