  Direct measurements of photoexcited carrier dynamics in nickel are made using
few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at
the nickel M$_{2,3}$ edge. It is observed that the core-level absorption
lineshape of photoexcited nickel can be described by a Gaussian broadening
($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorption
spectrum. Theory predicts, and the experimental results verify that after
initial 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 the
electron temperature. Measurements reveal an electron cooling time for 50 nm
thick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalized
carriers, the spectral red shift exhibits a power-law relationship with the
change in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$. Rapid
electron thermalization via carrier-carrier scattering accompanies and follows
the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal
equilibrium. Entwined with a <6 fs instrument response function, carrier
thermalization times ranging from 34 fs to 13 fs are estimated from
experimental data acquired at different pump fluences and it is observed that
the electron thermalization time decreases with increasing pump fluence. The
study provides an initial example of measuring electron temperature and
thermalization in metals in real time with XUV light, and it lays a foundation
for further investigation of photoinduced phase transitions and carrier
transport in metals with core-level absorption spectroscopy.
