X-ray photoemission and absorption spectroscopies as tools for ultrafast measurement of the "effective temperature" of nonequilibrium electrons.

Despite the fact that pump-probe spectroscopy is extensively applied to study the relaxation processes of nonequilibrium electrons, there remains a need to develop a robust and relatively simple method for effective thermometry of the nonequilibrium state. From our theoretical approach, we find a strong correlation between the shape-independent integrated weights of main and satellite peaks of the time-resolved X-ray photoemission (tr-XPS) and X-ray absorption (tr-XAS) spectra with the probabilities of the correlated thermal occupancies of electrons. This allows us to probe the energy of the electrons in the nonequilibrium state directly and therefore can serve as an effective ultrafast thermometer. Our finding gained an interest among experimentalists that has grown into collaboration. Since the model we apply in our study (the Falicov-Kimball model) is one of the simplest models of the strongly correlated electron systems, it is challenging to find the appropriate real material. Eventually, CeO₂ compound was found as the best candidate for both tr-XPS and tr-XAS ultrafast experiments which are planned in near future.