Relativistic calculations of two-color two-photon K-shell ionization J. Fan, J. Hofbrucker, A. V. Volotka and S. Fritzsche Eur. Phys. J. D, 76 2 (2022) 18 Published online: 04 February 2022 DOI: 10.1140/epjd/s10053-021-00334-x