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One key challenge in the analysis of ultrafast experiments is the assignment of a particular pump-induced observation to a specific degree of freedom that may cooperate or compete with another one. A seminal example is the analysis of e-ph interaction by Allen [1]. Using specific probing and resonant pumping methods, which are typically available in the experimental toolbox nowadays, very detailed microscopic understanding was developed, see, for example, the work on the interface hybrid phonons regarding energy transfer across metal-oxide interfaces [2] and hot electron injection and transport in heterostructures [3]. In this talk recent developments exploiting ultrafast soft x-ray spectroscopy carried out at the femtoslicing facility at BESSY II, Berlin, and the spectroscopy and coherent scattering (SCS) instrument at the European XFEL, Hamburg, will be presented. The experimental opportunities and related challenges [4], key observations of ultrafast spectral changes at Ni or Fe L3 and O K absorption edges in metals [5,6], charge transfer insulators [7], and films of Fe(II) spin-crossover complexes [8] will be discussed. Thereby, the manifold scientific opportunities which exploit the element and site specificity of the spectroscopy to shed light on non-equilibrium dynamics in condensed matter will be highlighted. Moreover, dichroic experiments, which provide specific access to broken symmetry ground states, will be addressed.

[1] P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987). [2] N. Rothenbach et al., Phys. Rev. B 100, 174301 (2019). [3] M. Heckschen et al., PRX Energy 2, 043009 (2023). [4] L. Le Guyader et al., J. Synchrotron Rad. 30, 284 (2023). [5] R. Y. Engel et al., Struct. Dyn. 10, 054501 (2023). [6] T. Lojewski et al., Mater. Res. Lett. 11, 655 (2023). [7] T. Lojewski et al., arXiv:2305.10145. [8] L. Kämmerer et al., arXiv:2312.01483.

Acknowledgement: Funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 278162697 – SFB 1242 is gratefully acknowledged.

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