Nonresonant frequency tripling of intense dye laser radiation in negative dispersive Kr ( \lambda = 1203-1235 Å ) is efficiently enhanced by phase matching with the positive dispersive tare gas Ar or Xe. Phase matching of Kr with Ar improved the efficiency of the conversion of 3648 Å to L_{\alpha} = 1216 Å by more than two orders of magnitude. At L_{\alpha}, 16 W ( 4 \cdot 10^{10} photons/pulse) could be generated with 1.5 MW pulses of a frequency doubled dye laser. The optimum conversion is limited by absorption of the VUV in the rare gas mixture and by dielectric breakdown. In mixtures of Kr and Xe, the dependence of the VUV on the density of the two gas components indicates a phase difference between their contributions to the induced third-order polarization. Measurements of the VUV output of different Kr-Xe mixtures provide a determination of the ratio of the third-order nonlinear susceptibilities χ Kr /χ Xe and of the wave vector mismatches per atom C Kr /C Xe at wavelengths \lambda = 1205-1230 Å Besides frequency tripling in Kr, third-harmonic generation of blue dye laser light ( \lambda = 4208-4406 Å) has been investigated in Xe phase matched with Kr. The peak power of the tunable VUV at \lambda = 1403-1469 Å exceeded 30 W ( 8.7 \cdot 10^{10} photons/pulse).

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