Determination of generalized multiphoton ionization cross sections from ponderomotive shifts

We present a recently developed method for determining the of absolute valu es of generalized multiphoton ionization cross-sections from the ponderomot ive shifts observed in high-resolution photoelectron above-threshold ioniza tion (ATI) spectra. As examples of applications of the method, the generali zed four-photon non-resonant ionization cross-sections of Ar and Xe ionized with 248-nm, 0.5-ps and 399-nm, 60-fs laser pulses, respectively, are dete rmined. The applicability of the method is evaluated, and limitations are d iscussed. The method provides an increased accuracy of typically better tha n one order of magnitude in the measurements of generalized cross-sections.