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 ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2.$ For ultrahigh intensities, the photoelectron continuum dynamics will be relativistic. However, the measured charge-state yields for ${\mathrm{Ar}}^{9+}$ to ${\mathrm{Ar}}^{16+}$ 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.

• Received 21 September 2000

DOI:https://doi.org/10.1103/PhysRevA.63.042712