Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser by Engel, RY, Alexander, O, Atak, K, Bovensiepen, U, Buck, J, Carley, R, Cascella, M, Chardonnet, V, Chiuzbaian, GS, David, C, Döring, F, Eschenlohr, A, Gerasimova, N, de Groot, F, Guyader, LL, Humphries, OS, Izquierdo, M, Jal, E, Kubec, A, Laarmann, T, Lambert, C-H, Lüning, J, Marangos, JP, Mercadier, L, Mercurio, G, Miedema, PS, Ollefs, K, Pfau, B, Rösner, B, Rossnagel, K, Rothenbach, N, Scherz, A, Schlappa, J, Scholz, M, Schunck, JO, Setoodehnia, K, Stamm, C, Techert, S, Vinko, SM, Wende, H, Yaroslavtsev, AA, Yin, Z, Beye, M