Similarity between the angular distributions of the first- and second-step electrons in sequential two-photon atomic double ionization

Angular distributions of electrons produced in sequential two-photon atomic double ionization are considered theoretically by comparing the results obtained for the first and the second electron. It is shown analytically that the angular distributions of the two emitted electrons in many cases are similar for ionization from a closed-shell atom. Numerical examples of this counter-intuitive similarity are presented. In addition, examples are given demonstrating the difference between the angular distributions of the first photoelectron in sequential two-photon double ionization and in conventional one-photon single ionization.