When we think of illuminating something, we naturally assume that the brighter the light source, the more luminous the resulting image will be. This notion holds true for ultra-short pulses of laser light, but only up to a certain intensity. The enigmatic phenomenon of X-ray diffraction images “darkening” at very high X-ray intensities has puzzled scientists for quite some time. This captivating observation not only deepens our fundamental understanding of the interaction between light and matter but also offers a unique perspective for the production of laser pulses with drastically shorter durations. In this article, we will explore the recent groundbreaking research conducted by a collaboration of physicists from Japanese, Polish, and German institutions that shed light on this perplexing phenomenon.
X-ray free-electron lasers (XFELs) generate exceptionally powerful X-ray pulses lasting femtoseconds, equivalent to quadrillionths of a second. These cutting-edge machines, limited to only a few locations worldwide, play a vital role in the structural analysis of matter through X-ray diffraction. In this technique, a sample is exposed to an X-ray pulse, and the resulting diffracted radiation is captured and used to reconstruct the crystal structure of the material under examination. Conventionally, one would expect that increased photon intensity would yield clearer diffraction images. However, a recent and unexpected observation challenged this intuition – as X-ray beam intensity surmounted a critical value, diffraction images weakened.
Through an intricate interplay of experimental and theoretical efforts, researchers from the RIKEN SPring-8 Centre, the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN), and the Center for Free-Electron Laser Science (CFEL) at the DESY laboratory in Hamburg embarked on a quest to elucidate this phenomenon. By firing XFEL lasers onto crystalline silicon samples at Japan’s XFEL facility, called SACLA, the team sought to uncover the underlying mechanisms through computer simulations and theoretical modeling.
The fascinating insights gained from this research unveiled a crucial explanation for the observed darkening effect. When a torrent of high-energy photons collides with a material, a massive ejection of electrons from varying atomic shells occurs, leading to rapid atomic ionization. Previously, the research group demonstrated that the initial movements of ionized atoms in the crystal lattice, which instigate the process of structural self-destruction, commenced roughly 20 femtoseconds after the light pulse impacted the sample.
The breakthrough realization of the researchers lies in the onset of phenomena transpiring in the first six femtoseconds of the X-ray-matter interaction. During this initial phase, high-energy photons stimulate not only valence electrons but also those occupying deep atomic shells near the atomic nucleus. Astonishingly, the presence of these deep shell holes dramatically reduces the atomic scattering factors, which determine the intensity of the observed diffraction signal. Dr. Ichiro Inoue from the RIKEN SPring-8 Centre, responsible for the experimental investigation, affirms that “the reason for the recently observed weakening of the diffraction signal is due to phenomena occurring earlier.”
Although the darkening effect initially appears disadvantageous, it brings new opportunities and potential applications. The variance in response among different atoms to ultrafast X-ray pulses reveals prospects for reconstructing three-dimensional complex atomic structures with enhanced accuracy. Leveraging the understanding that the final part of the pulse scarcely ionizes the material due to rapid electronic damage provides a unique advantage. By intentionally employing the material as a “scissor,” it is possible to generate laser pulses with exceedingly shorter durations than what is currently achievable. This breakthrough could revolutionize the field of imaging the microscopic quantum world.
The fascinating research conducted by a collaborative team of physicists has unraveled the mysteries surrounding the darkening of X-ray diffraction images at high intensities. Through a deep understanding of the intricate interplay between high-energy photons and atomic shells, scientists have shed light on this enigmatic phenomenon. These findings not only deepen our knowledge of light-matter interactions but also open doors to novel applications in the realm of atomic structure reconstruction and the production of ultrafast laser pulses. As we continue to explore the quantum world, new discoveries await, building upon the foundation laid by the relentless pursuit of scientific inquiry.