Slow, but efficient: Low-energy electron emission from intense laser cluster interactions

When a nanoscale particle is exposed to an intense laser pulse, it transforms into a nanoplasma that expands extremely fast, and several phenomena occur that are both fascinating and important for applications. Examples are the generation of energetic electrons, ions and neutral atoms, the efficient production of X-ray radiation as well as nuclear fusion. While these observations are comparably well understood, another observation, namely the generation of highly charged ions, has so far posed a riddle to researchers. The reason is that models predicted very efficient recombination of electrons and ions in the nanoplasma, thereby drastically reducing the charges of the ions.

In a paper that was published in the current issue of Physical Review Letters, a team of researchers from the Imperial College London, the University of Rostock, the Max-Born-Institute, the University of Heidelberg and ELI-ALPS have now helped to solve this riddle. Tiny clusters consisting of a few thousand atoms were exposed to ultrashort, intense laser pulses. The researchers found that the vast majority of the emitted electrons were very slow (see Fig. 1). Moreover, it turned out that these low-energy electrons were emitted with a delay compared to the energetic electrons.

Lead scientist Dr. Bernd Schütte, who performed the experiments at Imperial College in the framework of a research fellowship and who now works at the Max-Born-Institute, says: “Many factors including the Earth’s magnetic field influence the movement of slow electrons, making their detection very difficult and explaining why they have not been observed earlier. Our observations were independent from the specific cluster and laser parameters used, and they help us to understand the complex processes evolving on the nanoscale.”

In order to understand the experimental observations, researchers around Professor Thomas Fennel from the University of Rostock and the Max-Born-Institute simulated the interaction of the intense laser pulse with the cluster. “Our atomistic simulations showed that the slow electrons result from a two-step process, where the second step relies on a final kick that has so far escaped the researchers’ attention”, explains Fennel. First, the intense laser pulse detaches electrons from individual atoms. These electrons remain trapped in the cluster as they are strongly attracted by the ions. When this attraction diminishes as the particles move farther away from each other during cluster expansion, the scene is set for the important second step. Therein, weakly bound electrons collide with a highly excited ion and thus get a final kick that allows them to escape from the cluster. As such correlated processes are quite difficult to model, the computing resources from the North-German Supercomputing Alliance (HLRN) were essential to solve the puzzle.

The researchers found the emission of slow electrons to be a very efficient process, enabling a large number of slow electrons to escape from the cluster. As a consequence, it becomes much harder for highly charged ions to find partner electrons that they can recombine with, and many of them indeed remain in high charge states. The discovery of the so-called low-energy electron structure can thus help to explain the observation of highly charged ions from intense laser cluster interactions. These findings might be important as low-energy electrons are implicated as playing a major role in radiation damage of biomolecules – of which the clusters are a model.

Senior author Professor Jon Marangos, from the Department of Physics at Imperial, says: “Since the mid-1990’s we have worked on the energetic emission of particles (electrons and highly charged ions) from laser-irradiated atomic clusters. What is surprising is that until now the much lower energy delayed electron emission has been overlooked. It turns out that this is a very strong feature, accounting for the majority of emitted electrons. As such, it may play a big role when condensed matter or large molecules of any kind interact with a high intensity laser pulse.”

Fig. 1: The electron kinetic energy spectrum from argon clusters interacting with intense laser pulses is dominated by slow electrons (orange area). The inset shows the same spectrum on a logarithmic scale, indicating the slow electrons (indicated by the red curve) and the fast electrons (indicated by the green curve).

 

 

 

Fig. 2: Atomistic simulation of the laser-induced cluster explosion. Credit: Thomas Fennel

 

 

 

 

Original publication:

Physical Review Letters 121, 063202 (2018), doi: https://doi.org/10.1103/PhysRevLett.121.063202

Low-energy electron emission in the strong-field ionization of rare gas clusters”

Bernd Schütte, Christian Peltz, Dane R. Austin, Christian Strüber, Peng Ye, Arnaud Rouzée, Marc J. J. Vrakking, Nikolay Golubev, Alexander I. Kuleff, Thomas Fennel and Jon P. Marangos

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Auger decay following near-infrared ionization of clusters

An inner-shell vacancy in an atom can decay very efficiently via the emission of an Auger electron. Thus far, Auger decay has been observed in atoms, molecules and clusters following the ionization or excitation by light with high photon energies in the extreme-ultraviolet and X-ray regime. Surprisingly, we have discovered Auger decay after the interaction of methane clusters with intense near-infrared laser pulses, even though the photon energy is not sufficient to directly generate an inner-shell vacancy. However, due to very efficient ionization avalanching in clusters, electrons from outer as well as inner shells are removed from their atoms during the laser pulse, and a nanoscale plasma is formed. Subsequent recombination of electrons to outer shells of ions and to high-lying Rydberg orbitals then results in a population inversion of the cluster atoms. By observing a clear peak in the electron spectrum, evidence was provided for the first time that Auger decay is one of the relaxation channels of the highly excited system. In the future, the observed population inversion could be exploited for the development of a table-top X-ray laser.

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Invisible laser pulses push electrons in a quantum swing

Unlike natural light sources such as the sun, lasers have special coherence properties, resulting e.g. in the intriguing observation of interference, where the overlap of two lasers can lead to darkness. The coherence of lasers can also be used to push electrons in a quantum swing inside an atom, meaning that electrons oscillate between two quantum states that lie on different energy levels. Such quantum swings are known as Rabi oscillations and have been observed with low-frequency lasers up to the ultraviolet spectral range. In this paper, we show that Rabi oscillations can be driven in an argon atom by an intense laser pulse in the extreme-ultraviolet spectral range, which is invisible for the human eye. By using a second laser pulse, the ultrafast oscillation of electrons between the ground state (the lowest energy level) and an excited state (at a higher energy level) is traced directly in the time domain, showing that it takes place within tens of femtoseconds (corresponding to 10^-14 seconds). Our results pave the way towards multi-photon coherent control techniques in the extreme-ultraviolet range, allowing extension to situations where interactions between electrons play a role, or more complex systems including molecules.

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Fast electrons at long wavelengths

Efficient electron acceleration was observed in clusters induced by a laser field at 1.8 μm that consisted of only two optical cycles. In this regime that is dominated by electronic rather than nuclear dynamics, clear signatures of direct electron emission were observed as well as rescattering of electrons that gain additional kinetic energy during laser-driven collisions with ions and with the cluster potential. The results, which were obtained at the Imperial College London, promise efficient particle acceleration in clusters at mid-infrared and terahertz wavelengths.

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Bernd Schütte receives ISUILS award

Bernd Schütte has received the 7th ISUILS Award for Young Researchers. This prize is sponsored by the Japan Intense Light Field Science Society and was awarded at the 15th International Symposium on Ultrafast Intense Laser Science, currently taking place in Cassis in the South of France. Bernd Schütte has received this award for his work on ultrafast cluster dynamics during the past few years.

Workshop impressions

The second workshop on “Ultrafast Cluster Dynamics” took place from August 23-24 at the Max-Born-Institut, and was organized together with the TU Berlin. Our 40 guests presented 14 exciting posters and 9 talks, which resulted in many lively discussions. A lot of new fascinating results have been obtained since the last workshop that took place 2 years ago in Rostock. We are already looking forward to the next edition of this workshop taking place in Freiburg in 2 years!

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Summer Workshop “Ultrafast Cluster Dynamics”

The second summer workshop on “Ultrafast Cluster Dynamics” will take place on August 23-24 at the Max-Born-Institut. Following the first edition organized by Thomas Fennel in Rostock in 2014, this workshop will be organized by Bernd Schütte in collaboration with Daniela Rupp and Maria Krikunova from the TU Berlin. We expect about 45 participants from Germany, but also from abroad. In addition to 9 talks about nonlinear cluster dynamics, we will have 14 interesting poster presentations. Please contact the organizers for more information.

Programm_UltrafastClusterDynamics