Ac-Stark broadening of three-photon resonances in four-photon ionization of iodine atoms with broadband laser radiation

Ac-Stark-broadened three-photon resonances in four-photon ionization of iodine atoms are investigated experimentally and analyzed theoretically, including the effect of laser intensity fluctuations and spatiotemporal distribution of average intensity. It is shown that the resonance maximum cannot be used in estimating the hyperpolarizabilities of atomic levels. By applying the properly designed procedure, the difference in hyperpolarizabilities of 5p(2P1/2) and nd(…)73/2 levels is found to be α12 = 20 ± 10 A3.