Delayed onset and directionality of x-ray-induced atomic displacements observed on subatomic length scales

TL;DR

This study investigates ultrafast atomic displacements in Al₂O₃ caused by intense x-ray pulses, revealing delayed and directional atomic movements driven by electron excitation, and demonstrates high-resolution structural analysis feasibility with femtosecond pulses.

Contribution

It provides new insights into the timing and directionality of x-ray-induced atomic displacements and shows that femtosecond x-ray pulses can enable precise structural analysis during irradiation.

Findings

Atoms remain stationary for ~20 fs after pulse peak.

Directional atomic displacements occur following initial delay.

High-resolution structural analysis is feasible with ultrashort x-ray pulses.

Abstract

Transient structural changes of Al$_2$O$_3$ on subatomic length scales following irradiation with an intense x-ray laser pulse (photon energy: 8.70 keV; pulse duration: 6 fs; fluence: 8$\times$10$^2$ J/cm$^{2}$) have been investigated by using an x-ray pump x-ray probe technique. The measurement reveals that aluminum and oxygen atoms remain in their original positions by $\sim$20 fs after the intensity maximum of the pump pulse, followed by directional atomic displacements at the fixed unit cell parameters. By comparing the experimental results and theoretical simulations, we interpret that electron excitation and relaxation triggered by the pump pulse modifies the potential energy surface and drives the directional atomic displacements. Our results indicate that high-resolution x-ray structural analysis with the accuracy of 0.01 \AA is feasible even with intense x-ray pulses by making the pulse duration shorter than the timescale needed to complete electron excitation and relaxation processes, which usually take up to a few tens of femtoseconds.