Caustics and catastrophes in above-threshold ionization

Low-energy above-threshold ionization by a midinfrared laser field is investigated in terms of a commonly used semiclassical model. The mapping of the initial conditions (initial transverse velocity and initial phase of the field) on the final drift momenta that is mediated by Newton's equation of motion is approximated by three Coulomb interactions, which occur at the exit of the tunnel and during the subsequent hard and soft interactions that occur when the electron revisits the ion. The mapping exhibits a particular caustic structure in the area of the low-energy structure, which has the shape of the butterfly catastrophe.