Recent Developments in Semiclassical Floquet Theories for Intense-Field Multiphoton Processes

Abstract

Publisher Summary This chapter discusses the recent developments in semiclassical Floquet theories and their applications to multiphoton excitation, ionization, and dissociation processes in intense laser fields. The theory of multiphoton processes can be formulated in a fully quantum-mechanical or semiclassical formalism. In the former approach, both the system and the field are treated quantum mechanically, while in the semiclassical approach, the system is described by a time-dependent Schrӧdinger equation in which the effect of the radiation field is represented by an effective Hamiltonian consistent with Maxwell's equations. The Floquet matrix methods involving the time-independent Hermitian Floquet Hamiltonian provide nonperturbative ab initio techniques for the treatment of bound–bound multiphoton transitions Floquet matrix formalism is a non-perturbative approach applicable to multiphoton processes involving arbitrary high field strengths. It provides a simple physical picture for the intensity-and time-dependent multiphoton phenomena in terms of avoided crossings of a few number of real or complex quasi-energy levels. It also offers simplicity in numerical computations—mainly an eigenvalue problem. In the case of complex quasi-energy formalism, it takes into account self-consistently all the intermediate level shifts and broadenings and multiply coupled continua. Only square-integrable functions are required, and no asymptotic boundary conditions need to be enforced in multiphoton ionization/ multiphoton dissociation multiphoton ionization (MPI)/multiphoton dissociation (MPD) calculations.