Multiphoton ionization of atoms in strong fields

Most of the features observed in strong field multiphoton ionization cannot be interpreted by perturbation theory applied at the minimum non-vanishing order. When it is realistic to assume that only one electron is involved in the process, experimental results are well reproduced by calculations in which some discrete quasi-resonant states are treated on the same footing as the initial state. These methods are not applicable to complex atoms such as noble gases. We present a simple picture of multiphoton processes which allows an interpretation of the most salient experimental features in multiphoton stripping and electron-energy spectroscopy of noble gases. 11 references, 2 figures