## 4 Discussion  
The optical properties of transition metals are largely defined by the features of their electronic structure. Contrary to simple metals, such as Al, Mg, or Zn, transition metals are characterized by the presence of \(d\)-bands that translate into a high-density \(d\)-block in the DOS [5]. The \(d\)-block represents highly localized electron states filled with bound-like electrons. The metallic elements used in this study are representative to capture the electronic and optical characteristics for a broad variety of transition metals. During the laser excitation, the electrons thermalize and redistribute almost instantly (on the femtosecond time scale) within the DOS [4, 31]. The \(d\)-block is affected by laser-induced Fermi smearing of electrons extending the energy range available for optical transitions and thus strongly impacting the permittivity. Particularly, the position of the \(d\)-block relative to the Fermi level defines the strength of the interband component of the permittivity as a function of the photon energy. The \(d\)-block also serves as a reservoir for populating upper free-like states, and its electronic filling degree thus defines the direct transitions within and from the \(d\)-states. The other part of the DOS corresponds to spatially and energetically delocalized \(sp\)-states and mainly defines the intraband component. Figure 5: Imaginary part of the permittivity (**dashed line**) and contributions of each Drude–Lorentz term (**solid lines**) at different electron temperatures for the six transition metals. Drude term (0-order oscillator) reaches maximum at zero frequency and is followed by Lorentzians describing the 1 st-, 2 nd-, 3 rd and 4 th-order oscillators with a higher order corresponding to a higher central frequency. The 4 th-order oscillator appears only for W, but for other metals, it is out of the shown frequency range. In Au, the \(d\)-block is completely filled and separated from the Fermi level by about 2 eV. This favors significant interband transitions from the \(d\)-band to the \(sp\)-band in the visible range for photon energy higher than 2 eV with intra-\(sp\)-band transitions prevailing below 2 eV. This is almost the case for Ni as well, with the Fermi level being close to the top of the \(d\)-block and intraband transitions dominating in the infrared range. For Cr, W, Ti, and Fe, the \(d\)-states are partially filled, fostering interband contribution for a wide range of photon energies with the intraband transitions inside the \(d\)-band at low photon energies. The estimated relative contribution of the intraband process to the total optical transitions is presented in Figure 4. As expected, for photons in the infrared and near-infrared region at \(\hbar\omega<1\) eV, intraband transitions are largely dominant. For higher photon energies \(\hbar\omega>1\) eV, interband transitions dominate over all spectra for all metals, although at a higher electron temperature, the intraband role in the optical response is strengthened. For Au and Ni, a high electron temperature leads to the loss of the localized \(d\)-electrons and the increase in the density of delocalized \(sp\)-electrons that results in the shift of the \(d\)-block towards low energies. For Cr, Fe, Ti, and W, the effect of electronic heating induces an inverse electron flow, and the \(d\)-block shifts towards higher energies. The modification of the DOS manifested mainly by the shift of the \(d\)-block impacts the interband contribution of the permittivity with a large effect related to the shift of the oscillator positions (Figure 5). The Fermi-Dirac smearing at a high electron temperature significantly populates \(sp\)-states while decreasing the \(d\)-band filling, leading to the increase in the intraband fraction, as shown in Figure 4. The opposite behavior of Fe is remarkable as due to the DOS modification, the \(d\)-block becomes completely filled upon electron heating, fostering interband transitions under a strong nonequilibrium.