## 3 Results  
Figure 3 shows the relative absorption for both field directions, \(\frac{\Delta H^{+}(t)}{H_{0}^{+}}\) obtained using 5 different XUV wavelengths. For all these probing energies, we observe an increase of the relative absorption for majority electrons (\(H^{+}\), dark green), while the relative minority electron absorption (\(H^{-}\), light green) increases for 64.6, 65.7 and 66.3 eV and decreases for 66.7 and 68 eV. Thus, the evolution of the minority electron (\(H^{-}\), light green) absorption with time (and, to a lesser extent, the majority electron adsorption) depends on the probing energy. Note that these changes result from a modification of available states before and after optical excitation. For probing energies below the Fermi level (66.2 eV for Ni [44]) the absorption increase corresponds for example to a depletion of electrons (i.e., more free states available), similarly, for probing energies above the Fermi level (namely 66.7 eV and 68 eV), a decrease of absorption indicates that the initial free states are now occupied by excited electrons. In addition, Figure 3 shows that transient absorption changes for different applied fields do not systematically start at \(t=0\): the same dynamics are observed for both field directions during the first tens of fs after optical excitation for \(E=\) 64.6, 65.7, and 66.3 eV (highlighted grey regions). At first glance, this is in line with the recent observation of a delay between ultrafast electronic and magnetic dynamics [30; 34]. For \(E=\) 65.7 eV, a closer look at the first 100 fs reveals a crossing between \(H^{+}\) and \(H^{-}\) curves. As we will explain in the discussion section, we believe that this is the signature of a MCD spectrum shift. Figure 3: Relative absorption time traces for both field directions: \(H^{+}\) shown in dark green and \(H^{-}\) shown in light green using XUV probing energies equal to (**a**) 68 eV (**b**) 66.7 eV (**c**) 66.3 eV (**d**) 65.7 eV and (**e**) 64.6 eV. In order to compare the results obtained for different probing energies and to further quantify the electronic and magnetic dynamics, we have plotted the relative absorption and the relative MCD in Figure 4a,b, respectively. Figure 4a shows \(\frac{\Delta\mu(t)}{\mu_{0}}\) (vertical axis) as a function of the delay (left axis) and probing energies (right axis). As a reminder, the static absorption curve is shown in the background (dotted black line) with the corresponding probing energies (dotted colored line). Figure 4b shows \(\frac{\Delta MCD(t)}{MCD_{0}}\) which is directly proportional to the relative magnetization changes as a function of the time delay for all XUV energies. Several conclusions can be drawn from these graphs: (i) The relative absorption dynamics depend strongly on the probing energy. In fact, the maximum of the relative transient absorption change observed at 65.7 eV is one order of magnitude stronger than for all other energies scrutinized in this experiment. (ii) The relative magnetization curves display more subtle differences. The characteristic demagnetization times and rates show a weak dependence on the probing energy. As already highlighted, we see that for 66.3 and 64.6 eV, the onset of the demagnetization process is delayed with respect to \(t=0\). For a probing energy of 65.7 eV, we see a surprising increase and subsequent decrease of the magnetization in less than 100 fs. Note that this behavior is seen independently of the helicity of the incoming XUV pulses and has been observed in other Ni samples probed during the same experiment (data not shown). Figure 4: Time- and energy-dependent absorption and MCD data: (**a**) Relative absorption time traces as a function of probing photon energies and time delay. (**b**) Relative MCD time traces for the five probing energies. (**c**) Calculated absorption, as a function of energy, resulting from a shift of the absorption spectra to lower energies (dark and grey lines).