In this chapter, we focus our attention on the multiphoton ionization (MPI) of atoms interacting with intense laser fields with wavelengths in the infra-red to visible part of the spectrum. Section 8.1 is devoted to multiphoton single ionization. We begin by giving an overview of key early experiments, in particular those exploring the phenomenon of “above-threshold ionization” (ATI). We then discuss general features of ATI spectra and consider how these features can be understood within the framework of the semi-classical model. We conclude the section by examining how two-color processes can be used to study MPI. In Section 8.2, we analyze multiphoton double ionization, a process which has attracted considerable attention due to the prominent role played by electron correlation effects. Detailed reviews of atomic multiphoton ionization and ATI have been given by Joachain [1], DiMauro and Agostini [2], Protopapas, Keitel and Knight [3], Joachain, Dörr and Kylstra [4], Kylstra, Joachain and Dörr [5], Dörner et al. [6], Becker et al. [7] and Lewenstein and L'Huillier [8].

Multiphoton single ionization

As noted in Section 1.3, multiphoton ionization (MPI) was first observed in 1963 by Damon and Tomlinson [9] and also investigated in 1965 by Voronov and Delone [10] and Hall, Robinson and Branscomb [11]. In the following two decades, a number of experiments were performed to study various aspects of MPI, and results were obtained concerning the dependence of the ionization yields on the laser intensity, absolute MPI cross sections and the resonantly enhanced multiphoton ionization (REMPI) phenomenon.