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

TL;DR

This study uses ultrafast XUV transient absorption spectroscopy to measure electron thermalization and relaxation dynamics in laser-heated nickel, revealing rapid thermalization, electron cooling times, and spectral shifts related to electron temperature changes.

Contribution

It demonstrates real-time measurement of electron temperature and thermalization in metals using core-level transient absorption spectroscopy, a novel application in this context.

Findings

Electron thermalization time ranges from 34 fs to 13 fs, decreasing with higher pump fluence.

Electron cooling time in nickel films is approximately 640 fs.

Spectral red shift follows a power-law relationship with electron temperature change, ω_s ∝ ΔT^{1.5}.

Abstract

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.