Fig. 1, a) XAS spectrum of the Tb M₅ edge 150 fs after optical excitation (orange) in comparison to the spectrum for the unpumped sample (blue). The vertical line indicates the energy at which we recorded the pump-probe-delay trace (Fig. 2). b)N_4,5 edge RIXS signal measured with 147.2-eV pulses for the unpumped Tb sample (blue) and 300 fs after pump-pulse excitation (orange). The hatched area marks the energy-loss window, over which the data were integrated to study the pump-probe-delay dependence (Fig. 2). c) and d)Simulation of the Tb M₅ XAS and N_4,5 RIXS signal based on atomistic calculations. In XAS, changes of the spectral shape and in RIXS a shift of spectral weight to lower energy losses is caused by excitation of the 4f initial state.

Fig. 2 Relative change of the XAS (black dots) and RIXS signal (green markers) as a function of pump-probe delay. The XAS signal was measured at a photon energy of 1236 eV (vertical lines in Fig 1a. The RIXS signal is integrated over the energy loss region 2.7 – 3.2 eV as marked in Fig 1b. For comparison with XAS data the y-scale is inverted. The 5d6s electron temperature (blue solid line) has been calculated by the two-temperature model. The black solid line depicts the temperature evolution of those electrons, that can actually induce the 4f excitation.

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Magnetism in rare-earth 4f metals evolves from indirect coupling of the localized 4f states, mediated by the spin-polarized 5d valence electrons. Optical pulses only excite the 5d states; the cross section for excitations within the 4f shell is very small. Consequently, 4f magnetization dynamics must stem from coupling to other degrees of freedom. In the past 4f-spin lattice coupling has been identified as the main driver of 4f magnetization dynamics.

With time-resolved X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) experiments at European XFEL and FLASH, respectively, Nele Thielemann Kühn et al. [1] could show that optical pumping causes 4f electronic excitations in Tb metal. Spectral changes in XAS and shifts of spectral weight in RIXS indicate that the 4f⁸ initial state has changed (Fig. 1). From atomistic calculation they identified the ⁷F₆ → ⁷F₅ transition to be dominant. The temporal evolution of the pump-effect observed in XAS and RIXS (Fig. 2) follows the temperature of the directly excited 5d electrons. The 4f excitations arise from direct interaction between 5d and 4f electrons via inelastic 5d-4f electronic scattering.

Excitations out of the 4f ground state quench the atomic magnetic moment and further affect the spin and orbital quantum numbers, which define the coupling of the 4f ions to their surroundings. The findings provide basic insights to atomic spin and orbital dynamics, as well as to non-equilibrium 5d-4f coupling mechanisms. The electronic 5d-4f scattering channel might be exploited to optically control material parameters, such as magneto-crystalline anisotropy..

[1] N. Thielemann-Kühn, T. Amrhein, W. Bronsch, S. Jana, N. Pontius, R. Y. Engel, P. S. Miedema, D. Legut, K. Carva, U. Atxitia, B. E. van Kuiken, M. Teichmann, R. E. Carley, L. Mercadier, A. Yaroslavtsev, G. Mercurio, L. Le Guyader, N. Agarwal, R. Gort, A. Scherz, S. Dziarzhytski, G. Brenner, F. Pressacco, R.-P. Wang, J. O. Schunck, M. Sinha, M. Beye, G. S. Chiuzbăian, P. M. Oppeneer, M. Weinelt, and C. Schüßler-Langeheine: Optical control of 4f orbital state in rare-earth metals - Sci. Adv. 10, eadk9522 (2024) - DOI: 10.1126/sciadv.adk9522