Incoherent x-ray scattering in single molecule imaging

TL;DR

This study investigates how incoherent x-ray scattering impacts the quality of single-molecule imaging with x-ray free-electron lasers, highlighting the challenge of background noise and potential improvements at higher photon energies.

Contribution

The paper provides an ab initio analysis of incoherent x-ray scattering effects on single-molecule imaging and suggests that higher photon energies can enhance imaging quality.

Findings

Incoherent scattering significantly degrades high-resolution imaging.

Background signal becomes dominant at high x-ray fluence.

Higher photon energies may improve coherent scattering signals.

Abstract

Imaging of the structure of single proteins or other biomolecules with atomic resolution would be enormously beneficial to structural biology. X-ray free-electron lasers generate highly intense and ultrashort x-ray pulses, providing a route towards imaging of single molecules with atomic resolution. The information on molecular structure is encoded in the coherent x-ray scattering signal. In contrast to crystallography there are no Bragg reflections in single molecule imaging, which means the coherent scattering is not enhanced. Consequently, a background signal from incoherent scattering deteriorates the quality of the coherent scattering signal. This background signal cannot be easily eliminated because the spectrum of incoherently scattered photons cannot be resolved by usual scattering detectors. We present an ab initio study of incoherent x-ray scattering from individual carbon atoms, including the electronic radiation damage caused by a highly intense x-ray pulse. We find that the coherent scattering pattern suffers from a significant incoherent background signal at high resolution. For high x-ray fluence the background signal becomes even dominating. Finally, based on the atomic scattering patterns, we present an estimation for the average photon count in single molecule imaging at high resolution. By varying the photon energy from 3.5 keV to 15 keV, we find that imaging at higher photon energies may improve the coherent scattering signal quality.