Electron–spin dynamics studied on their natural timescale

With the help of extremely short light pulses and coincidence technology, researchers from several Swedish universities have succeeded in following the dynamic process of when the electron's spin – its rotation around its own axis – controls how an atom absorbs light. Göran Wendin, at the Department of Microtechnology and Nanoscience – MC2, at Chalmers and Raimund Feifel, at the Department of Physics at the University of Gothenburg, are two of the contributors. The new results were recently published in the scientific journal Nature Communications.
Picture of Göran Wendin.Professor Göran Wendin (to the right) is one of the driving forces within the Wallenberg Centre for Quantum Technology (WACQT), which is led by Chalmers and aims to build a Swedish quantum computer within twelve years. He is involved in several research projects, including this, financed by Knut and Alice Wallenberg Foundation (KAW).
"In fact, my contribution goes all the way back to my doctoral thesis from 1972 which explained the photo absorption cross section of the 4d-shell in xenon in the range 70-140 eV, studied by the present KAW collaboration," explains Göran Wendin.
 
In the new study, the researchers have used attosecond light pulses and coincidence techniques to follow – in real time – how the electron spin (i.e. the angular momentum of the electron around its own axis) influences the absorption of a photon in a  many-electron quantum system, the xenon atom. An attosecond is a billionth of a billionth of a second.
 
The study is using xenon, a heavy rare gas element that exists in small amounts in the atmosphere of the Earth. It is known to absorb soft x-rays of specific wavelength unusually efficiently.
Physicists have named the effect a giant resonance and explained that it is caused by a strong collective response of the electron cloud when the atom is exposed to the x-rays. Especially intriguing is that the electron spin has a pronounced effect on the light absorption in this system.
 
The present analysis combines precision in both time and energy to show that the strong absorption is explained by an excited state living less than 50 attoseconds. The influence of the electron spin, however, is due to a ten times longer-lived nearby state, which can be reached by a change of electron spin (called spin flip).
The spin-flipped state serves as a switch and determines the state of the remaining ion. The results provide new insight into the complex electron-spin dynamics of photo-induced phenomena and might be of considerable interest to applied science such as spintronics.
 
Photo of Göran Wendin: Johan Bodell
 

Read the article in Nature Communications >>>

 

More information >>>

Anne L’Huillier, Department of Physics, Division of Atomics Physics, The Lund Attosecond Science Center (LASC), Lund University, 0705-317529, anne.lhuillier@fysik.lth.se
Eva Lindroth, Department of Physics, Stockholm University, 0736-795034, eva.lindroth@fysik.su.se
Göran Wendin, Quantum Technology Laboratory, Wallenberg Centre for Quantum Technology (WACQT), Department of Microtechnology and Nanoscience – MC2, Chalmers, 031-7723189, goran.wendin@chalmers.se
Raimund Feifel, Department of Physics, University of Gothenburg, 0708-381689, raimund.feifel@physics.gu.se

More background >>>

Inspired by his supervisor, legendary Chalmers Professor Stig Lundqvist (1925-2000), Göran Wendin in his thesis applied the many-body theories developed for collective excitations in atomic nuclei to the electron dynamics in heavy atoms. The point was that independent-electron models did not work – it was all pretty collective, and it explained the experimental data from the pioneering work with synchrotron radiation. Actually, Wendin was the one who in 1973 introduced the name "giant dipole resonance" to describe the phenomenon.
 
While working in France 1981-83, Göran Wendin came in contact with Anne L’Huillier at the research institute Commissariat à l’Energie Atomique (CEA) in Saclay, France. Anne was doing her PhD work in the pioneering high-intensity laser group, and she wanted to do calculations for multiphoton ionization of rare-gas atoms, including xenon. Wendin became her theory supervisor, and they collaborated during the following 5 years and published a number of papers together.
 
After that, Anne L’Huillier took off and became one of the world-leading experimentalists in the field, and she became deeply involved in the development to understand and make use of high-harmonic radiation for attosecond spectroscopy. The Nobel Prize for this kind of work was awarded to Gérard Mourou and Donna Strickland in 2018. 

If one sends intense infrared femtosecond laser pulses on a metal substrate, one can generate a comb of up to 100 overtones with 2 eV distance, covering a range of 200 eV, including the region of the xenon giant resonance. With these experiments one has a stopwatch ticking with attosecond resolution, meaning that one can follow electrons on their flight out of the atom.
 
Related work that Göran Wendin did around 1975-85 had analyzed the giant resonance in term of atomic effective potentials, and that turned out to be very useful when relating the theoretical calculations of Eva Lindroth and her group at Stockholm University to experiment. The good old models for collective excitations – "atomic plasmons" – still provide the background for understanding the results of modern attacks on the atoms with optical and free-electron lasers.

Published: Thu 05 Nov 2020.