``Nonrelativistic'' ionization of the L-shell states in argon by a ``relativistic'' 1019 W/cm2 laser field

The field ionization cross sections for the L-shell states in argon are presented as measured with pulsed-laser radiation at an intensity of up to 10¹⁹ W/cm². For ultrahigh intensities, the photoelectron continuum dynamics will be relativistic. However, the measured charge-state yields for Ar⁹⁺ to Ar¹⁶⁺ compare favorably to numerical solutions of the nonrelativistic Schrödinger equation and a widely used Ammosov-Delone-Krainov/WKB tunneling ionization model. The results are interpreted within a two-step, strong-field ionization model, where the initial tunneling ionization process is dominated by nonrelativistic effects while the photoelectron continuum dynamics are strongly relativistic.