Analysis of long-lived effects in high-repetition-rate stroboscopic transient X-ray absorption experiments on thin films

TL;DR

This study investigates long-lived effects in high-repetition-rate stroboscopic X-ray absorption experiments on thin films, revealing their origins and proposing correction methods to improve measurement accuracy.

Contribution

It provides a detailed analysis of long-lived excitations in Ni and NiO thin films at high repetition rates, linking effects to temperature increase and charge trapping, and suggests ways to mitigate these effects.

Findings

Long-lived effects in Ni linked to local temperature increase.

Long-lived effects in NiO linked to charge carrier trapping.

Proposed correction methods to minimize long-lived effects.

Abstract

Time-resolved X-ray absorption spectroscopy (tr-XAS) has been shown to be a versatile measurement technique for investigating non-equilibrium dynamics. Novel X-ray free electron laser (XFEL) facilities like the European XFEL offer increased repetition rates for stroboscopic XAS experiments through a burst operation mode, which enables measurements with up to 4.5 MHz. These higher repetition rates lead to higher data acquisition rates but can also introduce long-lived excitations that persist and thus build up during each burst. Here, we report on such long-lived effects in Ni and NiO thin film samples that were measured at the European XFEL. We disentangle the long-lived excitations from the initial pump-induced change and perform a detailed modelling-based analysis of how they modify transient X-ray spectra. As a result, we link the long-lived effects in Ni to a local temperature increase, as well as the effects in NiO to excited charge carrier trapping through polaron formation. In addition, we present possible correction methods, as well as discuss ways in which the effects of these long-lived excitations could be minimized for future time-resolved X-ray absorption spectroscopy measurements.