Analytical approach to Coulomb focusing in strong-field ionization. I. Nondipole effects

The role of the Coulomb potential of the atomic core for the creation of caustics in the photoelectron momentum distribution for tunneling ionization in a linearly polarized strong laser field, commonly termed Coulomb focusing, is investigated within classical theory beyond the dipole approximation. Coulomb focusing is addressed by analytical calculation of Coulomb momentum transfer to the tunneled electron due to rescatterings, while applying perturbation theory and classifying the recollisions either as fast or as slow. With the help of the derived analytical treatment, we analyze the origin of the counterintuitive energy-dependent bend of the Coulomb focusing cusp in the photoelectron momentum distribution in a linearly polarized laser field in the nondipole regime, and its scaling with the field parameters. The high-order recollisions are shown to be responsible for a decrease of the bend of the cusp at very low energies in this regime.