Multiphoton ionization and collisions in strong laser fields

Two elementary atomic processes are investigated: multiphoton ionization of the hydrogen atom and multiphoton free–free transitions under the influence of high-intensity laser radiation. The first process is investigated using the S-matrix formalism allowing for the Coulomb field of ions and for the field of laser radiation acting on the final states of the ejected electrons. It is shown that a simple model allowing for field fluctuations makes the results qualitatively very close to the actual experimental observations. The second process is discussed by generalization of the well-known results on the balance between absorption and emission in a wide range of field intensities and for different numbers of exchanged photons.