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Direct measurements of photoexcited carrier dynamics in nickel are made usingfew-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy atthe nickel M$_{2,3}$ edge. It is observed that the core-level absorptionlineshape of photoexcited nickel can be described by a Gaussian broadening($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorptionspectrum. Theory predicts, and the experimental results verify that afterinitial rapid carrier thermalization, the electron temperature increase($\Delta T$) is linearly proportional to the Gaussian broadening factor$\sigma$, providing quantitative real-time tracking of the relaxation of theelectron temperature. Measurements reveal an electron cooling time for 50 nmthick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalizedcarriers, the spectral red shift exhibits a power-law relationship with thechange in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$. Rapidelectron thermalization via carrier-carrier scattering accompanies and followsthe nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermalequilibrium. Entwined with a <6 fs instrument response function, carrierthermalization times ranging from 34 fs to 13 fs are estimated fromexperimental data acquired at different pump fluences and it is observed thatthe electron thermalization time decreases with increasing pump fluence. Thestudy provides an initial example of measuring electron temperature andthermalization in metals in real time with XUV light, and it lays a foundationfor further investigation of photoinduced phase transitions and carriertransport in metals with core-level absorption spectroscopy.

Electron Thermalization and Relaxation in Laser-Heated Nickel by Few-Femtosecond Core-Level Transient Absorption Spectroscopy