Scattering and reaction processes in powerful laser fields

Abstract

This chapter provides an overview on various scattering and reaction processes taking place in a powerful laser field. These processes can be either laser-induced or laser-assisted. In the case of molecules, the additional freedoms of motion lead in the presence of a laser field to a variety of more complicated effects, but the treatment of these processes also causes more computational difficulties. The general approach for solving the problems is adapted from the S-matrix theory, originally conceived for the solution of problems of quantum electrodynamics. This method turns out to be very efficient in the present case of treating atomic processes in powerful laser fields, since at high laser field intensities a number of approximations can be made, in particular, the "quasi-free" electron approximation in which it is assumed that an ionized or scattered electron is interacting with the laser field only, while the atomic binding effects can be neglected to a very large extent. Moreover, the unitarity of the S-matrix presents an intuitive link between various different laser-induced processes like, for example, the connection between above threshold ionization (ATI) and the concomitant generation of higher harmonics (HHG). Another approximation turns out to be very useful and, in many cases, leads to sufficiently good results.