Scientists blast iron with powerful X-rays, then watch its electrons rearrange themselves

Free-electron X-ray lasers, such as Linac Coherent Light Source (LCLS) at the Department of Energy’s SLAC National Accelerator Laboratory, produce intense X-ray pulses that allow researchers to visualize biological objects, such as proteins and others. molecular machines, high resolution. But these powerful beams can destroy delicate samples, triggering changes that can affect the result of an experiment and invalidate the results.

To combat this, the researchers use a method called probe before destruction, which allows them to collect precise information from samples the instant before they are ejected, generating images that preserve information about the molecular structure of biological particles such as cells, proteins. And viruses. But until recently, it wasn’t clear how much this method could be trusted to measure electron behavior, as powerful X-rays can affect electrons much faster than atoms. This could limit the applicability of the technique to ultra-fast chemical processes, such as those involved in catalysis.

Now, a team led by SLAC scientists Roberto Alonso-Mori, Dimosthenis Sokaras and Diling Zhu has found a way to get a precise idea of ​​how to adjust the X-ray beam to make sure the electronic structure is not damaged before measuring it. providing more confidence in the results of XFEL experiments. In one, the team observed how electrons behaved in the first few femtoseconds, or millionths of a billionth of a second, after a sample of iron was detonated with intense laser pulses. Their findings, recently published in Scientific reports, demonstrate how specific properties of the X-ray beam, such as the length or intensity of the pulse, can affect the outermost electrons of an atom, which are those that participate in the creation and breaking of bonds during chemical reactions.

The results will enable scientists to set up probe-pump experiments, in which a laser pulse initiates a reaction in a sample and an X-ray pulse immediately measures the electron rearrangement. By varying the time between the laser and the X-ray pulses, researchers can create a series of images and merge them into a stop-motion movie of these small and fast movements, offering insights into the chemical reactions triggered by light.

“These experiments are a key tool in our team’s research agenda,” says Sokaras. “The ability to carefully access the ‘acceptable’ range of LCLS conditions will allow us to perform pump-probe studies that are both reliable and unprecedented.”

The team worked closely with the LCLS accelerator assembly to deliver even shorter-than-usual X-ray pulses to study how electrons rearranged themselves in the first femtoseconds of the explosion. An electron beam camera, the XTCAV, was instrumental in accurately measuring the length of X-ray pulses.

Alonso-Mori says, “The study validates methods that have been used at LCLS in recent years, resolving the debate on whether the data collected is already altered in the first femtoseconds by the intense pulses of X-rays.” “

To follow up on this research, the team hopes to probe the electronic structure with even greater intensity, taking advantage of recent advances in X-ray beam formation and control.

“This can be used to further understand the early stages of hot and dense matter formation processes in XFELs,” says Zhu, “which offer insight into the formation and evolution of planetary systems.”