Fluence-dependent transient reflectivity measurements of a transition metal close to sub-<i>d</i>-band to Fermi surface transition

Relaxation processes in a transition metal (Ni) after ultrafast energy deposition are investigated using time-resolved transient reflectivity measurements close to the high-energy sub- d -band to Fermi surface transition of 1.3 eV. The changes in reflectivity Δ R / R 0 are measured in a picosecond time domain with femtosecond resolution at a probe photon energy of 1.56 eV in a wide range of non-damaging absorbed pump energy fluences (0.9–10.2 mJ/cm 2 ), heating electrons up to several thousand degrees. An analysis of the experimental data is presented, and the nature of the observed ultrafast, fast, and slow transients of positive and negative changes in reflectivity is discussed. The findings are expected to be of utmost importance for the verification of current theories and the further development of predictive modeling.