Multiphoton processes in an intense laser field: Harmonic generation and total ionization rates for atomic hydrogen

Abstract

We report results of large-scale nonperturbative (Floquet) calculations of rates for harmonic generation and total ionization of H(1s) by fields whose intensity ranges from about ${10}^{12}$ to 3×${10}^{13}$ W/${\mathrm{cm}}^2$ and whose wavelength ranges from 265 to 1064 nm. At long wavelengths and moderate to high intensities, perturbation theory yields estimates of total ionization rates that are orders of magnitude too large, and estimates of harmonic-generation rates that exhibit a qualitatively incorrect behavior with respect to order. We have studied the influence of resonances on the ionization rates, and we illustrate the resonance enhancement of the ionization yield for a realistic pulse. Finally, we address several formal aspects of Floquet theory, including the convergence of the induced dipole moment, gauge invariance, and the normalization of the wave function.