[Paper Review] Enhanced ultrafast X-ray diffraction by transient resonances

Stephan Kuschel, Phay J. Ho+16 more|arXiv (Cornell University)|Jul 12, 2022

Advanced X-ray Imaging Techniques4 citations

TL;DR

This study demonstrates that transient resonances in xenon nanoparticles can enhance ultrafast X-ray diffraction images by up to 10× beyond predictions of linear models. By tuning ultrashort X-ray pulses to excite core-hole resonances, the effective scattering cross-section increases by over an order of magnitude, enabling unprecedented brightness and potential for sub-femtosecond, sub-nanometer resolution in single-shot X-ray imaging of non-equilibrium dynamics.

ABSTRACT

Diffraction-before-destruction imaging with single ultrashort X-ray pulses has the potential to visualise non-equilibrium processes, such as chemical reactions, at the nanoscale with sub-femtosecond resolution in the native environment without the need of crystallization. Here, a nanospecimen partially diffracts a single X-ray flash before sample damage occurs. The structural information of the sample can be reconstructed from the coherent X-ray interference image. State-of-art spatial resolution of such snapshots from individual heavy element nanoparticles is limited to a few nanometers. Further improvement of spatial resolution requires higher image brightness which is ultimately limited by bleaching effects of the sample. We compared snapshots from individual 100 nm Xe nanoparticles as a function of the X-ray pulse duration and incoming X-ray intensity in the vicinity of the Xe M-shell resonance. Surprisingly, images recorded with few femtosecond and sub-femtosecond pulses are up to 10 times brighter than the static linear model predicts. Our Monte-Carlo simulation and statistical analysis of the entire data set confirms these findings and attributes the effect to transient resonances. Our simulation suggests that ultrafast form factor changes during the exposure can increase the brightness of X-ray images by several orders of magnitude. Our study guides the way towards imaging with unprecedented combination of spatial and temporal resolution at the nanoscale.

Motivation & Objective

• To investigate whether transient resonances in Xe nanoparticles can enhance X-ray diffraction image brightness beyond linear models.
• To determine the impact of ultra-short, resonant X-ray pulses on diffraction signal intensity in single-particle imaging.
• To resolve the paradox of increased brightness despite expected sample damage from high-intensity X-ray pulses.
• To develop a model explaining how transient resonances increase scattering cross-sections during ultrafast exposure.
• To guide future experiments toward higher spatial and temporal resolution in X-ray free-electron laser (FEL) imaging.

Proposed method

• Performed coherent X-ray diffraction imaging (CDI) on individual 100 nm Xe nanoparticles using ultrashort X-ray pulses from an FEL.
• Varied X-ray pulse duration and intensity near the Xe M-shell resonance to probe transient resonance effects.
• Conducted Monte Carlo simulations to model electron dynamics, core-hole formation, and scattering cross-section evolution during pulse exposure.
• Used statistical analysis of full experimental data sets to validate observed brightness enhancements.
• Calculated resonant scattering cross-sections using quantum mechanical matrix elements for 3d→4f transitions in neutral and core-excited Xe ions.
• Compared experimental diffraction images with predictions from linear models and bleaching-corrected simulations.

Experimental results

Research questions

• RQ1 Can transient resonances in Xe nanoparticles enhance X-ray diffraction image brightness beyond linear scaling with intensity?
• RQ2 What is the role of ultra-short pulse duration in maximizing brightness enhancement via transient resonances?
• RQ3 How do transient resonances alter the effective scattering cross-section during a single FEL pulse?
• RQ4 To what extent do core-hole dynamics and orbital restructuring contribute to increased scattering?
• RQ5 Can transient resonances mitigate or reverse the detrimental effects of sample bleaching in ultrafast X-ray imaging?

Key findings

• X-ray diffraction images from 100 nm Xe nanoparticles exposed to few-femtosecond and sub-femtosecond pulses were up to 10 times brighter than predicted by linear models.
• The brightness enhancement is attributed to transient resonances that increase the effective scattering cross-section by more than one order of magnitude during the pulse.
• Monte Carlo simulations confirmed that ultrafast form factor changes due to core-hole excitation and orbital restructuring significantly amplify scattering intensity.
• The enhancement occurs when X-ray pulses are tuned to excite transient resonances in the Xe M-shell, particularly via 3d→4f transitions in core-excited Xe+∗ ions.
• The effect is most pronounced at ultra-short pulse durations (<10 fs), where the transient resonance lifetime overlaps with the pulse width.
• This mechanism offers a pathway to overcome the brightness limitations imposed by sample bleaching in single-shot, ultrafast X-ray imaging.