Final State Effects in Above-Threshold Ionization

Abstract

With the goal in mind to separate final state effects due to the interaction of the ejected electron with the laser field from dynamical effects associated with the process of ionization, we investigate the following simple model: We assume an effective interaction that just lifts the electron into the continuum via absorption of the minimum number N of photons which is necessary to overcome the ionization potential (defined in the absence of the field). The effective interaction is left unspecified except its matrix element is assumed to be proportional to IN/2 with I the intensity of the laser field. As soon as the electron is free we assume it only feels the laser field and, consequently, is described by the so-called Volkov solution which provides an exact solution for an electron in an external plane wave field. For simplicity, we also adopt the long wavelength approximation for the laser field. For the initial atom and the final ion we take the unperturbed wave functions and we also disregard the recoil imparted to the ion. Altogether, this is essentially the Keldysh approximation. It turns out that boundary conditions, i.e. the way the electrons leaves the laser pulse, are of vital importance.1 In order to explain the experimentally observed absence of intensity-dependent shifts in the electron spectra we have to assume that the electron leaves the electron pulse on the side rather than being passed over by the pulse. Under these conditions, we also obtain the total suppression of the low-lying peaks of the electron distribution with increasing intensity which is impressively born out by recent experiments.2,3 The mechanism can be ascribed to the ponderomotive potential which adds to the ionization potential. When the electron leaves the pulse on one side the energy corresponding to the ponderomotive potential is converted into kinetic energy.4