Experimental photoelectron spectra, with high resolution in both kinetic energy and intensity, have been obtained and compared to a high-precision integration of the Schrödinger equation for photoelectron kinetic-energy yields in argon. We find exceptional quantitative agreement between data and calculation over a wide range of kinetic energies and peak laser intensities. In this paper we conclusively show that the single active electron model describes the physics of high-intensity photoionization to a high degree of accuracy. Further-more, while multiple electrons may be ionized, multielectron effects appear to be completely absent from above-threshold-ionization photoelectron spectra.

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Journal Phys. Rev. A
Nandor, M. J, Walker, M. A, van Woerkom, L. D, & Muller, H. G. (1999). Detailed comparison of above-threshold-ionization spectra from accurate numerical integrations and high-resolution measurements. Phys. Rev. A, 60, 1771–1774.