Multiphoton resonant processes in atoms

In this chapter, we are dealing with the processes, where transitions between discrete levels of atoms are produced by the siumultaneous absorption of n photons. They are resonant in the sense that the sum of the energies of the n photons is equal to the energy difference between the two concerned levels. But we exclude the case where an intermediate level would be resonantly excited during the process (its energy difference from the ground state beeing equal to the sum of the energies of n-1 photons, or n-2 photons, ...). Many experiments have been done with two such successive resonant processes: these involve, for example, the well-known stepwise excitation or the three-level resonance technique. Nevertheless, the detailed study of these two successive processes raises quite different problems; and we restrict our topic to resonant n-photon transitions without a resonant intermediate step. (orig./WL)