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Racing against one of the fastest atomic processes


When an atom or molecule absorbs a high energy X-ray it may emit one of its core electrons. This excited state is unstable and can quickly relax by filling the core hole with an outer-lying electron and emitting a second electron.

This fundamental process, known as Auger-Meitner decay, is one of the fastest relaxation processes involving X-rays and is occurring on femtosecond timescales, which means within a few millionths of a billionth of a second.

Recently, an international team of scientists headed by Aaron LaForge from the group of Nora Berrah from the University of Connecticut and Sang-Kil Son from the group of Robin Santra at CFEL, DESY in Hamburg discovered a new process at the SQS instrument of the European XFEL, which is at least as fast as the Auger-Meitner decay. Using the intense, femtosecond radiation from X-ray free-electron lasers (XFELs), argon atoms were highly ionized and nearly all of the 18 electrons removed. In this extreme state, the atoms can either absorb more X-rays or undergo Auger-Meitner decay, and it is a competition on which process is faster.

The first case can lead to a new process, dubbed XREMPI, or X-ray resonance-enhanced multiphoton ionization. The results were published in Physical Review Letters and demonstrate how XFELs can be used to measure ultrafast processes such as charge or energy transfer in molecular systems, which has applications in the photovoltaic industry. Furthermore, the study of highly-charged ions, which can be probed by this method, is of interest to quantum mechanics and astrophysics, since the heavier charged core requires more precise theoretical predictions to verify fundamental constants. More on the process soon on the webpage of the SQS instrument.

» Original publication

Source: European XFEL GmbH