Two-Colour X-Ray Pulses Capture the Fastest Nanoscale Movies Ever Recorded

For decades, one of the most tantalising promises of ultrafast science has been the ability to watch matter transform on its own natural timescale. Molecules vibrate, bonds break, and nanoparticles rearrange themselves in femtoseconds, quadrillionths of a second, far too quickly for any conventional camera to register. Now an international research team, including members of Professor Daniela Rupp’s group in the Department of Physics at ETH Zurich, has taken a decisive step toward that goal by solving a stubborn experimental bottleneck: how to record two X-ray snapshots of the same nanoscale object when no detector on Earth is fast enough to capture them separately. Their solution, published in two papers in Nature Communications, effectively turns a single detector exposure into a two-frame movie of nanomatter in motion.

The underlying experimental capability has only recently become available. At the European XFEL in Schenefeld, Germany, scientists can now perform two-colour X-ray diffraction experiments in which independently tunable pairs of X-ray pulses are produced with a highly controllable temporal separation. In principle, this means the first pulse can take a snapshot of a sample in its initial state and the second pulse, arriving femtoseconds later, can take a second snapshot of the same object after it has begun to evolve. On paper, two-frame X-ray movies of nanoscale dynamics seem within reach. In practice, however, a considerable hurdle stood in the way: the diffraction patterns from both pulses land on the detector at essentially the same time, superimposed on a single image like a double-exposure photograph. The two frames are there, but they are hopelessly entangled.

Disentangling those overlapping signals is precisely what the team achieved. Working at the Small Quantum Systems (SQS) instrument at the European XFEL, the researchers fired pairs of X-ray pulses with photon energies of about 1 and 1.2 kiloelectronvolts and temporal separations of up to 750 femtoseconds onto isolated, free-flying helium nanodroplets. The scattered X-rays were recorded on a pn-junction charge coupled device, or pnCCD, a detector that measures the electric charge created on each pixel by incoming photons. Dr Michael Meyer, lead scientist at the SQS, noted that this endstation is ideally suited to an experiment conceived to show that two-colour X-ray imaging of nanoscale dynamics works. The choice of helium nanodroplets was deliberate: they are well-characterised, structurally simple targets on which the separation methods could be rigorously tested before being applied to more complex systems.

The first of the two approaches relies on the remarkable properties of the pnCCD detector itself. Because the detector offers high photon-energy resolution and very low readout noise, the researchers were able to perform an individual pixel count analysis that assigned each pixel to single- and multiple-photon hits originating from either of the two X-ray pulses, and even to identify mixed-photon events where photons from both colours contributed to the same pixel. Combined with an image post-processing algorithm, this pixel-based colour separation cleanly pulled the two overlapping diffraction patterns apart. The outcome showed good agreement with simulated results, giving the team confidence that the recovered patterns faithfully represented the two time-delayed snapshots rather than artefacts of the analysis.

The flexibility of this pixel-based strategy is one of its most attractive features. Linos Hecht, a PhD candidate in Rupp’s group and first author of both publications, explained that the method can be applied to any kind of target system and is especially good at extracting valuable high-resolution information from the diffraction patterns. Dr Yevheniy Ovcharenko, a scientist at the SQS and principal investigator of the study, added that these findings open the door to observing rapid changes in nanoparticles, comparable to what is already possible with small molecules. In other words, the technique is not tied to a particular sample geometry or composition; it is a general-purpose tool for separating two-colour diffraction data, which is exactly what a maturing field needs as it moves from proof-of-principle demonstrations to real scientific applications.

Encouraged by the performance of the colour separation strategy, the team went a step further with an algorithmic approach they termed Dichography. Conventional single-colour diffraction imaging already benefits from a mature suite of reconstruction algorithms that convert diffraction patterns into real-space images of the sample. Dichography extends these well-established algorithms to the two-colour case, making it possible to recover two time-delayed snapshots of a given sample directly from the superimposed diffraction pattern, without requiring the pixel-level sorting step. To demonstrate the method on a target with higher structural complexity than plain helium droplets, the researchers imaged helium droplets doped with xenon, a system in which heavy xenon atoms form distinct filamentary structures inside the light helium matrix.

The results presented in the second paper show how the Dichography algorithm can reconstruct two views of xenon filaments within a nanodroplet from its two-colour diffraction pattern. From a two-colour diffraction pattern, two snapshots of the xenon-doped helium droplet can be reconstructed, and these images can in turn inform a potential three-dimensional rendering of the nanodroplet. Dr Alessandro Colombo, the member of Rupp’s group who led this investigation, did not mince words about the significance: to his knowledge, these are the fastest nanoscale movies ever recorded, if by movie we mean multiple frames of the same object. That qualifier matters. Previous ultrafast studies have often compared snapshots of different particles or relied on pump-probe schemes that average over many identical repetitions. Here, the two frames belong to one and the same nanoscale object, captured 750 femtoseconds or less apart.

Two-colour X-ray imaging remains experimentally demanding, and the authors are candid about the constraints. Dichography requires high brightness, because the diffraction signal from a single nanoparticle is inherently weak, and it requires similar scattering contributions from the two X-ray pulses so that neither frame dominates the recorded pattern. Colombo believes that continued machine development at facilities like the European XFEL will help make Dichography more readily applicable, improving the stability and tunability of two-colour pulse pairs. The broader picture, however, is one of remarkable convergence. As Professor Marcel Mudrich from the University of Kassel, the experimental project leader, summarised, the community now has free-electron lasers that can produce two-colour pairs of X-ray pulses, detectors that allow images to be distinguished based on the colour of the scattered light, and analysis tools to reconstruct the shape of individual particles from the recorded X-ray snapshots. Each of these ingredients existed separately; the new work binds them into a functioning imaging pipeline.

The significance of this advance extends well beyond helium droplets decorated with xenon. Nanoscale objects, from catalyst particles to aerosols to biological assemblies, undergo structural changes that determine their function, and those changes often unfold on femtosecond timescales when triggered by light or other stimuli. A reliable two-frame X-ray movie capability means researchers can, for the first time at this spatial resolution, capture the before and after states of a single nanoparticle within one experimental shot, rather than inferring dynamics from ensemble averages. The team’s stated ambition is to build a complete, powerful toolbox for studying X-ray driven dynamics, and Colombo expressed excitement about the new science that will be unlocked by the ability to film ultrafast structural changes in nanomatter. With the separation problem solved by two complementary routes, one rooted in detector physics and pixel statistics, the other in advanced phase-retrieval algorithms, femtosecond cinematography of the nanoworld has moved from aspiration to demonstrated reality, and the first reel has already been screened.

Subject of Research: Two-colour X-ray diffraction imaging for recovering femtosecond-resolved snapshots of ultrafast dynamics in nanoparticles

Article Title: Making X-ray movies for ultrafast dynamics in nanomatter

Article References: Making X-ray movies for ultrafast dynamics in nanomatter. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: X-ray diffraction, European XFEL, ultrafast dynamics, nanoparticles, helium nanodroplets, Dichography, pnCCD detector, femtosecond imaging, ETH Zurich, Nature Communications, free-electron laser, phase retrieval