Ptychographic wavefront characterisation for single-particle imaging at X-ray lasers

TL;DR

This paper presents a high-resolution wavefront imaging technique for XFEL pulses used in single-particle imaging, enabling better understanding and correction of experimental instabilities to improve data interpretation.

Contribution

It introduces a mixed-state ptychography method for detailed wavefront characterization of XFEL pulses focused by KB mirrors, enhancing experimental stability analysis.

Findings

Local photon fluence correction is essential for SPI.

Small phase-tilt diversity likely does not affect SPI.

Wavefront profiles aid in data interpretation and experimental improvements.

Abstract

A well-characterised wavefront is important for many X-ray free-electron laser (XFEL) experiments, especially for single-particle imaging (SPI), where individual bio-molecules randomly sample a nanometer-region of highly-focused femtosecond pulses. We demonstrate high-resolution multiple-plane wavefront imaging of an ensemble of XFEL pulses, focused by Kirkpatrick-Baez (KB) mirrors, based on mixed-state ptychography, an approach letting us infer and reduce experimental sources of instability. From the recovered wavefront profiles, we show that while local photon fluence correction is crucial and possible for SPI, a small diversity of phase-tilts likely has no impact. Our detailed characterisation will aid interpretation of data from past and future SPI experiments, and provides a basis for further improvements to experimental design and reconstruction algorithms.