Reexamination of high-energy above-threshold ionization (ATI): An alternative strong-field ATI model

The multiphoton strong-field phenomenon of high-energy above-threshold ionization (or high-order ATI) of an isolated atomic system exposed to an intense monochromatic linearly polarized laser field is considered analytically and studied numerically within the framework of an alternative nonrelativistic strong-field approach developed earlier for theoretical treatment of high-order harmonic generation process. The related proposed alternative strong-field ATI model is fully quantum mechanical and mainly based on the Keldysh approximation combined with making use of the essential states method (along with the pole approximation). Applying together these two methods allows for representation of the total ATI amplitude in a closed and compact analytical (the so-called factorized) form quite transparent for interpretation and available for direct numerical calculations. To demonstrate the model applicability, a number of certain photoelectron spectra produced by various atomic species (mostly noble gas atoms and negative ions) have been calculated numerically and shown to reproduce the well-known conventional semiclassical rule for the extent of high-energy plateau and position of its cutoff energy. All the calculated photoelectron spectra as well as the specific details of respective angular distributions (viz., their sidelobes or so-called rings) corresponding to a certain photoelectron peak within a high-energy plateau region (formed primarily by rescattered photoelectrons) demonstrate a very credible behavior within a broad and most interesting region of the problem parameters. Moreover, they all are also in an excellent or fairly good accordance with typical ones measured in standard ATI experiments or calculated by other authors within different (analytically more sophisticated or computationally very demanding) approaches and methods developed earlier.