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

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M-2,M-3 edge. It is observed that the core-level absorption line shape 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 +/- 80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of omega(s) proportional to 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 quasithermal 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.

Automated summary

Direct measurements of photoexcited carrier dynamics in nickel were conducted using XUV transient absorption spectroscopy. The study found that after initial rapid carrier thermalization, the increase in electron temperature is linearly proportional to the Gaussian broadening factor, allowing real-time tracking of electron temperature relaxation. Additionally, electron thermalization time was observed to decrease with increasing pump fluence.