# Ultrafast heat transfer in single palladium nanocrystals seen with an X-ray free-electron laser

## TL;DR

This study reveals ultrafast, heterogeneous heat transfer and strain dynamics in individual palladium nanocrystals following laser heating, using X-ray free-electron laser measurements to observe structural evolution.

## Contribution

It provides direct experimental evidence of transient strain states and heat distribution in single nanocrystals with femtosecond resolution, supported by a detailed lattice model.

## Key findings

- Observation of split Bragg peaks indicating heterogeneous strain
- Identification of threshold laser fluence for strain heterogeneity
- Correlation between strain dynamics and heat distribution

## Abstract

We report transient highly strained structural states in individual palladium (Pd) nanocrystals, electronically heated using an optical laser, which precede their uniform thermal expansion. Using an X-ray free-electron laser probe, the evolution of individual 111 Bragg peaks is measured as a function of delay time at various laser fluences. Above a laser fluence threshold at a sufficient pump-probe delay, the Bragg peak splits into multiple peaks, indicating heterogeneous strain, before returning to a single peak, corresponding to even heat distribution throughout the lattice expanded crystal. Our findings are supported by a lattice displacement and strain model of a single nanocrystal at different delay times, which agrees with the experimental data. Our observations have implications for understanding femtosecond laser interactions with metals and the potential photo-catalytic performance of Pd.

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Source: https://tomesphere.com/paper/2508.21670