Nonlinear reversal of photo-excitation on the attosecond time scale improves ultrafast x-ray diffraction images

TL;DR

This paper demonstrates that intense sub-femtosecond X-ray pulses can partially reverse radiation damage in molecules by stimulating emission, enabling improved ultrafast imaging and spectroscopy.

Contribution

It introduces a method using sub-fs X-ray pulses to outrun ionization cascades and reverse damage, enhancing imaging contrast and resolution.

Findings

X-ray diffraction increases with sub-fs pulses

Average charge state decreases with sub-fs pulses

Stimulated emission cycles electrons between states

Abstract

The advent of isolated and intense sub-femtosecond X-ray pulses enables tracking of quantummechanical motion of electrons in molecules and solids. The combination of X-ray spectroscopy and diffraction imaging is a powerful approach to visualize non-equilibrium dynamics in systems beyond few atoms. However, extreme x-ray intensities introduce significant electronic damage, limiting material contrast and spatial resolution. Here we show that newly available intense subfemtosecond (sub-fs) x-ray FEL pulses can outrun most ionization cascades and partially reverse x-ray damage through stimulated x-ray emission in the vicinity of a resonance. In our experiment, we compared thousands of coherent x-ray diffraction patterns and simultaneously recorded ion spectra from individual Ne nanoparticles injected into the FEL focus. Our experimental results and theoretical modeling reveal that x-ray diffraction increases and the average charge state decreases in particles exposed to sub-fs pulses compared to those illuminated with 15-femtosecond pulses. Sub-fs exposures outrun most Auger decays and impact ionization processes, and enhance nonlinear effects such as stimulated emission, which cycle bound electrons between different states. These findings demonstrate that intense sub-fs x-ray FEL pulses are transformative for advancing high-resolution imaging and spectroscopy in chemical and material sciences, and open the possibilities of coherent control of the interaction between x-rays and complex specimen beyond few atoms.