Above-threshold ionization in the tunneling regime

A compact generalization of the Keldysh ionization amplitude is derived that includes rescattering. It is used for calculations of above-threshold ionization spectra with respect to energy for various emission angles for tunneling ionization of helium at high intensity, for the simple case of a zero-range potential as the binding potential. Most of the essential features of recent measurements are reproduced, that is, the onset, the extent, and the relative height of the plateau, which makes up the major part of the observed spectrum.