Bridging the experimental observables in the pump-probe experiment with the ab-initio methods*

The interaction of coherent pulsed lasers with magnetically ordered materials induces various elementary excitations and drives the material out of equilibrium at timescales as short as the driving laser itself. The pump-probe experiment on magnetic materials can measure the temporal evolution of the magnetic moment at femtosecond timescales. Bridging the macroscopic experimental observables to the quantum mechanical microscopic picture is of fundamental importance to understand the interplay of different degrees of freedom and correctly interpret the experimental measurements. To do this, we devise the cutting-edge TDDFT to present a mixed scheme between the time evolution of the time-dependent Kohn Sham (TDKS) equation and the linear response to transiently calculate the dielectric tensor. By introducing the electron correlations from the Hubbard model to the quasi-particle TDKS Hamiltonian, we find electron correlations play a fundamental role in interpreting the transient shift of the transient absorption spectra. We apply our scheme to Co and Ni and achieve very good agreement with the state-of-the-art pump-probe experiments at the L edge. We further extend our conclusions to the transient MOKE experiments measuring the magneto-optical functions as a probe for the magnetization dynamics via the discrete energies from the high harmonic generation laser sources and discuss possible contradicting results at energy windows below and above the absorption edge