# Tracking the time-evolution of the electron distribution function in Copper by femtosecond broadband optical spectroscopy

## TL;DR

This study investigates the ultrafast evolution of the electron distribution in copper using broadband optical spectroscopy, testing the validity of multi-temperature models in describing electron-phonon interactions.

## Contribution

It demonstrates that the electron distribution remains athermal for hundreds of femtoseconds and shows how to accurately extract electron-phonon coupling constants using two-temperature model analysis.

## Key findings

- Electron distribution is athermal over several 100 fs after excitation.
- Substantial energy transfer to the lattice occurs during this period.
- Electron-phonon coupling constants can be accurately determined from quasi-thermal electron states.

## Abstract

Multi-temperature models are nowadays often used to quantify the ultrafast electron-phonon (boson) relaxations and coupling strengths in advanced quantum solids. To test their applicability we study the time evolution of the electron distribution function, f(E), in Cu over large range of excitation densities using broadband time-resolved optical spectroscopy. Following intraband optical excitation, f(E) is found to be athermal over several 100 fs, while substantial part of the absorbed energy already being transferred to the lattice. We show, however, that the electron-phonon coupling constant can still be obtained using the two-temperature model analysis, provided that the data are analyzed over the time-window, when the electrons are already quasi thermal, and the electronic temperature is determined experimentally.