Quantum resonances and strong field ionization

The phenomenon of subthreshold ionization of quantum systems in an intense monochromatic field is discussed. A particularly simple system exhibiting this phenomenon is studied both analytically and numerically. The spectral properties of the quasi-energy-operator of the system are emphasized. The notion of doorway states is defined, and their importance in the mechanism of ionization is shown. The relationship between the quantum and the corresponding classical motion is emphasized. The quantum analog of the classical nonlinear resonance is recognized in the structure of some eigenfunctions of the Floquet operator. The aim of this research is twofold: reliable quantum estimates are sought to predict the onset of subthreshold ionization, which cannot be obtained from standard perturbation theory; and a precise theoretical framework is a necessary requirement in discussing the existence of chaos in the quantum mechanics of these systems