Experimental Evidence for the Competition between Resonantly Enhanced Multiphoton Ionization and Third-Harmonic Generation in Xenon

Competition between third-harmonic generation and multiphoton ionization in xenon is confirmed in a direct experiment using counterpropagating circularly polarized laser beams to suppress third-harmonic emission. Three-photon resonantly enhanced multiphoton ionization through the $6s$ and $6s^{\prime }$ levels in xenon, previously reported to disappear at pressures above a few Torr, gives rise to sharp intense ionization signals under these conditions.