Direct theoretical method for the determination of peak laser intensities from Freeman resonances in above-threshold ionization

Freeman resonance is one of the most important phenomena in strong-field laser physics. The previous calculations to determine peak laser intensities were mainly based on solving the entire time-dependent Schrödinger equation, which requires tedious theoretical and numerical work. The newly obtained exact solutions to a driven two-level atom made accurate calculations of quasienergies and the Bloch-Siegert shifts with an arbitrary laser-beam intensity possible and practical. With the recent progress in the numerical calculations of driven two-level atom, we find a direct theoretical method, without solving the entire time-dependent Schrödinger equation, to calculate the peak laser intensities which can excite a ground-state electron to a resonant Rydberg state with shifted energy level followed by an above-threshold ionization. Due to the tranquility of the nonresonant Rydberg states, Freeman resonance fits the driven two-level atom theoretical model well. With accurate calculation of Bloch-Siegert shift as a function of laser-beam intensity, we determine the peak laser intensities from Freeman resonances of different Rydberg states. Some important features of Freeman resonances are also discussed with this method.