Attosecond Spectroscopy Probes Atomic Cores
It is possible to infer the lifetime of core holes by looking at the energy spectral width of the photons or electrons that are emitted following excitation of an atom, but until recently, physicists had been unable to observe the reorganization process directly. Now researchers at Technische Universität Wien in Austria and Universität Bielefeld in Germany have reported an approach for time-resolved atomic inner-shell spectroscopy, with which they detected reorganization of strongly bound electronic systems following excitation.
An attosecond spectroscopy technique enables researchers to measure the complex reorganization of electrons in the inner shell of an atom. In the process, an x-ray pulse photoexcites the sample, which ejects a photelectron (1), thereby leaving a hole in the inner shell. A weaker-bound electron fills the vacancy (2) and transfers its energy to a third electron. This Auger electron is emitted from the atom with a characteristic delay (3) and is probed by a delayed laser pulse.
After excitation, systems with many bodies relax to lower-energy states by rearranging their molecular, atomic or nuclear structures. In many cases, it is relatively straightforward to observe these processes with pump-probe spectroscopy. For example, ultrafast time-resolved spectroscopy with femtosecond laser pulses is employed to observe atomic motion in molecules. It is also possible to identify electronic motion in weakly bound Rydberg states.
However, observing strongly bound electrons following excitation is not so readily achieved with femtosecond optical techniques. The excitation is followed by a complex reorganization of the atomic inner shell that results in the creation of a "core hole," the temporal dynamics of which range from a few attoseconds to a few femtoseconds.
"In order to gain the most direct view of the process, we aimed for a time-based tracking of the evolution of the radiationless relaxation of the electron cloud, the so-called 'Auger-decay,' " explained Markus Drescher, the lead author of a paper describing the new work. This requires a pulsed source with attosecond resolution and with photon energies of approximately 90 eV for inner-shell excitation, dedicated x-ray optical components for guiding and filtering, and a technique that can monitor the emission of Auger electrons, he said.
The researchers developed the necessary tools for this approach in a previous study. In the current work, they applied these tools to the time-resolved study of Auger decay in krypton. Attosecond soft x-ray pulses were generated as high harmonics of intense, few-cycle laser pulses approximately 7 fs in duration. Bandpass filtering at 96 ±1.5 eV provided x-ray pulses 900 as in duration. A core hole was created less than a femtosecond following excitation of the sample, and the subsequent decay was observed on a timescale of a few femtoseconds using a 750-nm probe pulse and a time-of-flight electron-energy spectrometer. The measured hole lifetime agreed with that deduced indirectly from measurements of the spectral linewidth of the Auger electrons.
Initial applications of the technique will be in fundamental atomic research. but the results might influence other fields. "A better understanding of core-relaxation dynamics will aid in the development of more efficient pumping schemes for x-ray lasers, where an inversion can be sustained for only very short periods of time," Drescher said. "[Also], the secondary electrons produced by the decay of photoexcited atoms are to a large extent responsible for the radiation damage to tissues after absorption of x-rays. Taking undistorted x-ray microscopic images of small objects would be possible if the exposure times were shorter than the electronic relaxation. The object would be destroyed after exposure, but the snapshot would show its condition before."
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