Above-Threshold Ionization with Linear and Circular Polarizations

Abstract

Ionization in the field of a strong electromagnetic wave had been studied for more than one decade when it was observed that electron spectra were not as predicted by lowest non-vanishing order perturbation theory (LOPT) [1]. Instead of one peak at EO = Nhw-E_I (where N is the minimum number of photons ho) required to overcome the ionization potential EI), the spectra consist of a number of peaks separated by hw, with significant amplitudes. Since the electrons are produced in a regime totally free of collisions, inverse Bremsstrahlung as a possible mechanism is ruled out. The only remaining possibility is an ensemble of direct transitions involving (N+S) photons and coupling the ground state to higher ionization continua. The computation of the corresponding generalized cross-sections within the framework of LOPT is more difficult than in the case of Multi photon Ionization, but the problem has been solved by several authors [2],[3],[4],[5] and it can be said that up to intensities of 10¹² W.cm^-2 and hydrogenic atoms, the agreement between theory and experiment is quite reasonable [5]. It may happen that, even at such intensities, higher-order corrections (AC-Stark shifts of intermediate bound states) must be taken into account [5]. However, the general features of Above-Threshold Ionization (ATI), as it was named [3] remain essentially perturbative in the sense that the spectra consist of a few peaks with rapidly decreasing amplitudes, each of them following the usual power law I^N+S [6].