Abstract

A competition between resonant multiphoton ionization (MPI) and third harmonic generation (THG) involving three-photon resonances was first reported by Miller et al.¹ Recently, a similar competing process involving four-photon resonances was observed in the multiphoton ionization of Hg². In this paper we report a similar observation in the three- and four-photon excitation spectra of Kr and Ar. Both the multiphoton ionization and the VUV radiation generated in the focal volume are measured over a broad range of gas pressures (0.01-100 Torr) and laser fluences. The 5s, 5f, and 6p′ levels of Kr and the 4s, 4p′, and 7p levels of Ar are investigated. The spectral lines are observed to broaden and to shift from line center as a function of increasing gas pressure and laser intensity. The effect observed in three-photon resonant lines is explained by the occurrence of a competition between MPI and THG.¹ A similar effect observed in the four-photon resonances is interpreted as the result of THG enhanced by the four-photon levels—a five-photon process. However, it is not clear why the four-photon spectral line-width becomes narrower when the gas pressure is further increased; this is discussed.

© 1986 Optical Society of America