Incoherent x-ray scattering in single molecule imaging

Abstract

Imaging of the structure of single proteins or other biomolecules with atomic resolution would be enormously beneficial to structural biology. X-ray freeelectron 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.

Compton spectra of atoms at high x-ray intensity

“…Then, the inelastic part of the doubly differential scattering cross section (DDSCS) is given by (see detailed derivations in Ref. [33]),…”

Atomistic three-dimensional coherent x-ray imaging of nonbiological systems

“…Here we are interested in atomic resolution reconstruction, which requires scattering at high q values. In this case, the scattering signals expressed in terms of the total differential cross section of the cluster can be regarded as the sum of the coherent, free-electron, and Compton (inelastic) scattering [34,[69][70][71] …”

“…Previous calculations on intense x-ray scattering from biomolecules consisting of mostly light elements (H, C, N, and O) demonstrate that the presence of Compton (inelastic) scattering can severely compromise the imaging resolution [34,35,74]. Specifically, the contribution to the number of scattered photons per Shannon pixel at high q corresponding to a resolution of 1.5Å is largely from Compton scattering [34].…”

“…Specifically, the contribution to the number of scattered photons per Shannon pixel at high q corresponding to a resolution of 1.5Å is largely from Compton scattering [34]. Here we investigate the relative contribution of coherent scattering, Compton scattering, and free-electron scattering to the total scattering signals in a heavier system.…”

“…The loss of the N 2 enhancement in coherent elastic scattering inherent to a crystal with N unit cells, magnifies the impact of * pho@anl.gov photon backgrounds arising from incoherent and free electron scattering and places a stricter requirement on understanding the nature of electronic damage [31][32][33][34][35]. The time scales of electronic rearrangement, Auger decay, nuclear motion, nanoplasma formation, Coulomb explosion, and ion-electron recombination are inconveniently similar and comparable to the femtosecond XFEL pulse duration, placing inherent limitations on the structural precision attainable from coherent diffractive imaging.…”