Multiphoton Ionization of Atomic Hydrogen Using Perturbation Theory

Abstract

Publisher Summary This chapter discusses multiphoton ionization of atomic hydrogen using perturbation theory. As atoms possess discrete negative energy spectra, there are nonresonant and resonance-enhanced multiphoton processes. In the case of nonresonant multiphoton ionization, no intermediate states have the same energy as atomic states in discrete spectrum. When one or more intermediate states coincide with atomic states, one has resonance-enhanced multiphoton ionization process. In strong fields atoms may absorb not only the minimum energetically necessary number of photons but also one or more excess photons. One-step multiphoton ionization process in which extra photons are absorbed is called above-threshold ionization (ATI). The energy of the extra absorbed photons is converted entirely into the kinetic energy of the emitted electron. Therefore photoelectron energy spectra must be studied, which became possible only with the development of methods of photoelectron spectroscopy with very high energy resolution. In the case of ATI by monochromatic light the energy spectrum of photoelectrons consists of equally spaced peaks. The Sturmian transition matrix elements for atomic hydrogen are presented. The chapter discusses the method of analytical continuation of the Sturmian transition matrix elements in the case of ionization with an excess photon is first described in detail.