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Posted: May 09, 2012
Researchers get up close to double ionisation
(Nanowerk News) Normally, when an intense laser pulse interacts with an atom it generates agitation on the micro-scale and this interaction produces a single ionisation, where one electron is ejected from the atom. Sometimes, however, two electrons can be removed from the atom at the same time, which results in the more complex process of double ionisation.
The study was supported by the ATTOFEL ('Ultrafast Dynamics using ATTosecond and XUV Free Electron Laser Sources') project which was funded by a EUR 3,601,028 Marie Curie Initial Training network (ITN) grant under the 'People' Theme of the EU's Seventh Framework Programme (FP7) and the Laserlab-Europe ('The Integrated Initiative of European Laser Research Infrastructures') project, a European Consortium of major Laser Research Infrastructures, funded in part by EUR 8,650,000 under the Integrated Infrastructure Initiative of the FP7's 'Capacities' Theme.
The researchers say the double ionisation process resembles a billiard game, where, following a collision, one ball sets another ball in motion.
The strong laser light first ejects an electron from the atom, accelerates it away from and then back towards the atomic core. During the collision the electron transfers part of its energy onto a second electron, which is promoted into an excited state of the core. Soon after, the electric field of the laser pulse also liberates the second electron from the atomic core.
As non-sequential double ionisation usually consists of many such re-collision and excitation events, it is usually tricky to accurately interpret experimental results. To overcome this obstacle the team successfully confined non-sequential double ionisation to a single re-collision and excitation event which enabled them to trace this process at attosecond time scales.
To achieve this, the scientists sent a four femtosecond-long laser pulse onto argon atoms (a femtosecond is a millionth of a billionth of a second). The light wave of this pulse essentially consisted of two wave maxima and two wave minima (i.e. two cycles). Due to the action of the laser field, most atoms were singly ionised. However, every thousandth atom underwent non-sequential double ionisation.
After ionisation of the first electron just after the first wave maximum, it took approximately 1.8 femtoseconds for it to come back to the atomic core and excite the second electron. The electron stayed in this excited state for about 400 attoseconds before the laser field released it from the core just before the second wave maximum.
One of the study authors, Boris Bergues from the Max Planck Institute of Quantum Optics, comments: 'We were surprised to see that the second electron leaves the atomic core 200 attoseconds before the maximum of the second cycle.'
The team therefore successfully challenged the assumption that the second electron escapes the atomic core at the maximum of a cycle by getting to grips with the inner quantum dynamics of a laser-driven multi-electron system.
This type of research at attosecond time scales is essential for deepening our fundamental understanding of matter-light interactions, and further application of this experimental technique to the study of molecules might help shed light on more complex multiple electron processes in the course of chemical reactions.