2-LEVEL SYSTEM IN A STRONG LASER FIELD - A COMPARATIVE-ANALYSIS BETWEEN THE LANDAU-LIFSHITZ SOLUTION AND AN EXACT NUMERICAL ONE

We numerically solve the problem of a two-level system in a near-resonance time-dependent electric field of arbitrary amplitude. The obtained radiation spectrum of the dipole moment is compared both to that of a Landau-Lifshitz (LL) textbook solution [Quantum Mechanics (Pergamon, New York, 1987), p. 145] and to ap off-resonance analytical solution [Dakhnovskii and Bavli (unpublished)] obtained recently for both optical and tunneling initial conditions. We show that the LL solution is valid at low field intensities while the off-resonance analytic solution qualitatively describes the intermediate- and high-intensity regimes.