Multiphoton ionization of the noble gases by an intense1014-W/cm2dye laser

The absolute yield of multiply charged ions of the noble gases argon, krypton, and xenon as a function of laser intensity in the range of ${10}^{13}$≤I≤5×${10}^{14}$ W/${\mathrm{cm}}^2$ is reported. The actual numbers of ions produced using a well-characterized tunable picosecond dye laser were measured. These absolute measurements allow a direct, quantitative comparison of the data with theory. Both lowest-order perturbation theory and the ‘‘tunneling ionization’’ theory of Keldysh are used to model the experiment. Ionization rates obtained from the Keldysh theory are in quantitative agreement with the experimental results for production of the first charge state only. Reasons for the inability of the Keldysh theory to describe the production of multiply charged ions are discussed.