Electron thermalization and relaxation in laser-heated nickel by few-femtosecond core-level transient absorption spectroscopy

Thermalization and relaxation of photoexcited carriers play an important role in ultrafast demagnetization and photoinduced phase transitions. However, direct observation of carrier thermalization in metals is highly challenging due to its fast timescale. Here, the authors track the thermalization and relaxation of photoexcited electrons in nickel in real time with few-femtosecond core-level transient absorption spectroscopy. In addition, a method to extract carrier temperature from the core-level absorption spectrum is proposed and verified by the experimental results. This enables further applications of core-level spectroscopy to explore element- and, hence, domain-specific carrier dynamics in metallic alloys and multilayers.