When we think of illuminating something with a brighter light source, we expect the resulting image to be brighter as well. This rule holds true for ultra-short pulses of laser light, including X-rays. However, recent research has revealed a counterintuitive effect – at very high X-ray intensities, diffraction images “darken” instead of becoming brighter. This puzzling phenomenon has not only deepened our understanding of light-matter interaction but also opened up new possibilities for generating shorter laser pulses. In this article, we will explore the explanation behind this unexpected effect and its potential applications.
Experimental and theoretical physicists from Japanese, Polish, and German research institutions have collaborated to shed light on the darkening of X-ray diffraction images. Using X-ray free-electron lasers (XFELs), powerful X-ray pulses with femtosecond durations, they conducted experiments on silicon crystals and analyzed the results. XFELs are currently only operational in a few locations worldwide and are used to investigate the structure of matter through X-ray diffraction.
Traditionally, it was assumed that increasing the number of photons in the beam would result in a clearer diffraction image. However, the researchers discovered that beyond a critical X-ray intensity of tens trillions of watts per square centimeter, the diffraction signal unexpectedly weakened. This effect was previously unexplained. Professor Beata Ziaja-Motyka, who specializes in theoretical modeling and computer simulations, remarks, “Our research is the first attempt to explain this unexpected effect.”
Computer simulations supported the theoretical research carried out by the physicists. They found that when high-energy photons bombard a material, there is a massive release of electrons from atomic shells, leading to rapid ionization of the atoms. The researchers had previously shown that structural self-destruction of the sample occurs approximately 20 femtoseconds after the light pulse hits the sample. Dr. Ichiro Inoue, responsible for the experimental study, states, “We are now convinced that the reason for the recently observed weakening of the diffraction signal is due to phenomena occurring earlier, in the first six femtoseconds of the interaction.”
During the initial phase of the X-ray-matter interaction, the incoming high-energy photons excite both valence electrons and electrons occupying deep atomic shells, located close to the atomic nucleus. However, the presence of deep shell holes in atoms significantly reduces their atomic scattering factors, which determine the intensity of the observed diffraction signal. Prof. Ziaja-Motyka explains, “Our research shows that before any structural damage occurs, rapid electronic damage takes place. As a result, the final part of the pulse no longer ionizes the material.”
At first glance, the darkening of diffraction images may seem unfavorable as it leads to decreased brightness. However, this effect could be exploited in multiple ways. The differential response of different atoms to ultrafast X-ray pulses could aid in more accurate reconstruction of three-dimensional atomic structures from diffraction images. Additionally, this phenomenon could be used to produce laser pulses with even shorter durations. The material that the high-intensity X-ray pulse passes through acts as a “scissors,” effectively cutting off a significant part of the already ultra-short pulse and generating pulses that are shorter than what has been achieved so far.
The discovery of the darkening of X-ray diffraction images at high intensities marks a significant advancement in our understanding of light-matter interaction. It opens up new possibilities for improving the accuracy of structural analysis and the production of shorter laser pulses. Further research and experimentation in this field could lead to breakthroughs in imaging the quantum world. As we continue to delve into the mysteries of light and matter, we uncover knowledge that transforms the way we approach fundamental scientific principles.