A study of multiphoton resonances in Kr and Ar under intense laser field

Multiphoton ionization and vacuum ultraviolet (vuv) light generation through 3‐ and 4‐photon resonances in Kr and Ar are investigated as a function of gas pressure and laser fluence. The observed spectral broadening and shifting in the MPI spectra near 3‐ and 4‐photon resonances are interpreted as a result of the ac Stark effect and the competing harmonic generation processes. 4‐photon resonances was observed to enhance and deplete third harmonic generation under certain pressure conditions. In addition, vuv light at wavelength shorter than the third harmonic wavelength was produced when several 4‐photon resonances in Ar were excited.