Influence of multipole effects on the cross section and alignment following inner-shell ionization of atoms by a linearly polarized photon

Highlights: • The fully relativistic method for treating the PI process is developed. • Good agreement with theory and experiments. • Multipole effects on the cross section and alignment after PI are assessed. - Abstract: On the basis of the fully relativistic Dirac-Fock treatment of single ionization by a linearly polarized photon with regard to the E1, M1, E2, M2, and E3 multipoles of the radiative field as well as their interference, we have assessed the influence of multipole effects on the cross sections and alignment parameters A₂₀ and A₂₂ of the residual ions, taking the 3p_3/2 and 3d_3/2 vacancies of selected Zn and Zn-like Kr⁶⁺, Cd¹⁸⁺, and Xe²⁴⁺ ions as examples. Starting from the most general method of density matrix theory, we carried out specific analytic expressions in terms of the reduced matrix elements of multipole fields, corresponding to different ion-core states. It is shown that the multipole contributions to the photoionization cross sections can be considerable, the character of which becomes more evident as the incident energy and/or atomic number increases. These dramatic influences also lead to a remarkable increase in the alignment parameter A₂₀ of the residual ions, yet virtually independent of the nuclear charge for A₂₂. The present results in Coulomb gauge and Babushkin gauge excellently agreed with each other, suggesting that there is a high degree of convergence achieved in this study. Comparison of our results with experimental data and other theoretical predictions, when available, is made.