The interaction of an intense laser pulse with a nanometer-scale particle results in the generation of an expanding nanoplasma. In the past, nanoplasma dynamics were typically described by classical phenomena, like the thermal emission of electrons. In contrast, a new study on the interaction of intense near-infrared (NIR) laser pulses with molecular oxygen clusters now demonstrates that phenomena, which can only be described quantummechanically, play an important role. For the first time, evidence of efficient formation of autoionizing states in nanoplasmas is found. Autoionization of so called superexcited states of atomic oxygen is directly observed on a nanosecond time scale, whereas indirect signatures are visible for decay processes occurring on shorter time scales. Autoionization is found to take place in various systems and is expected to be important also in the interaction of finite systems with intense extreme-ultraviolet (XUV) and X-ray pulses from novel free-electron laser sources.

Following the interaction of intense NIR laser pulses with clusters, the recorded electron spectra typically show a smooth distribution. In the past, the absence of discrete state signatures in these spectra led to the conclusion that the dynamics of charged particles during the cluster expansion can be well described by fully classical behavior. As a consequence, simulations that model the interaction of intense lasers with clusters, nanoparticles or large molecules, often make use of quasiclassical approaches. With the advent of novel laser sources and time-resolved techniques during the last year, this picture began to falter. Recently, extensive formation of excited atoms in nanoplasmas driven by electron-ion recombination processes was reported. When an atom with 2 electrons in excited states is formed, it may decay via an electron correlation effect, where one electron is released into the continuum, while the second electron relaxes to a lower bound state. However, since the electrons emitted via such autoionization processes exchange kinetic energy with the cluster environment, they had not been observed in experiments so far.

In a collaboration led by scientists from the Max-Born-Institut, the first evidence of autoionization following intense NIR laser-cluster interactions is now reported. In the current issues of Physical Review Letters [114, 123002 (2015)] Bernd Schütte, Marc Vrakking and Arnaud Rouzée, and their colleagues Jan Lahl, Tim Oelze and Maria Krikunova from the TU Berlin present results obtained from oxygen clusters. This system was chosen, because oxygen atoms have previously been shown to exhibit long-lived autoionizing states. In the present study, clear peaks were observed in the electron spectrum from oxygen clusters ionized by intense NIR pulses (Fig. 1). These peaks could be assigned to well-known autoionizing states, and it was shown that they decay on a nanosecond time scale, when the cluster has already significantly expanded. Therefore, the influence of the environment on the electrons emitted via autoionization was negligible. The observed autoionization contributions were found to be very sensitive on the intensity of the NIR laser pulse. At higher intensities, the autoionization peaks were blurred out, but still visible. These results indicate that autoionization plays an important role in many experiments that study the interaction of intense laser pulses with nanometer-scale systems, even when these processes cannot be directly observed in the electron spectrum. Previously, it was demonstrated that the observed nanoplasma dynamics following intense XUV and NIR ionization of clusters are similar, and therefore, the current results are expected to be highly relevant as well for experiments at novel free-electron lasers. The experimental findings of autoionization are also important for improving theoretical models of nanoplasmas in the future in order to gain a better understanding of the underlying microscopic processes.

The presented results demonstrate that a description of nanoplasma dynamics by classical approaches is insufficient. Quantum phenomena like autoionization play an important role during the expansion of clusters following the interaction with intense light pulses.

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During the interaction of an intense extreme-ultraviolet (XUV) laser pulse with a cluster, many ions and free electrons are created, leading to the formation of a nanoscale plasma. In experiments using XUV/X-ray free electron lasers (FELs) it was previously demonstrated that only a small fraction of these electrons can leave the cluster, while the majority of the electrons remain trapped within the cluster and may therefore recombine with ions. In a novel approach using a laboratory-scale XUV source, we have now measured the time scale of these electron-ion recombination processes leading to a strong formation of excited atoms, which is in the picosecond range. The results show that it is even possible to follow the laser-induced cluster expansion process up to nanosecond times.

The formation of a large number of charges in a cluster by an intense laser pulse can lead to the generation of a transient nanoplasma consisting of free electrons and ions. In the past, fascinating processes could already be observed in nanoplasmas, including nuclear fusion or the creation of neutral atoms with very high kinetic energies. While nanoplasmas are routinely generated during the interaction of clusters with intense XUV pulses from free-electron lasers, a detailed understanding of the processes inside the plasma is challenging. Theoretical models have predicted that the majority of electrons remains trapped in the cluster and may eventually recombine with ions such that both transient species cannot be observed in usual experiments. However, an experimental investigation of these dynamics is crucial, since processes in clusters are complex and manifold, and their detailed prediction is difficult. A promising route towards a better understanding of the different mechanisms in nanoplasmas is the development of time-resolved experiments. In this context, intense high-order harmonic generation (HHG) sources that can deliver light pulses down to the attosecond regime are very interesting. This laboratory-scale XUV sources provide a straightforward way to carry out pump-probe experiments on clusters and can significantly improve the possibilities for the understanding of cluster dynamics.

In an international collaboration led by researchers from the Max-Born-Institut, the first pump-probe experiment on clusters using an intense HHG source was now performed. In the current issue of Physical Review Letters [112, 253401 (2014)] Bernd Schütte, Marc Vrakking and Arnaud Rouzée and their colleagues Filippo Campi from the University of Lund and Mathias Arbeiter and Thomas Fennel from the University of Rostock present the results of these investigations. The development of a technique allowing the Reionization of Excited Atoms from Recombination (REAR) makes it possible for the first time to infer information on ion charge states prior to recombination. By using near-infrared (NIR) probe pulses, a surprisingly extensive formation of excited atoms was observed and could be shown to originate from recombination between electrons and ions. It was demonstrated that in the nanoplasma electrons released by means of photo-ionization only remain quasi-free for a short time up to 10 picoseconds before they undergo a recombination process with the surrounding ions. More information about these processes was obtained by generating special clusters that consist of a xenon core and an argon shell. These investigations showed that recombination preferentially takes place in the xenon core of the cluster. The wavelength of the ionizing pulse interacting with the cluster was shown not to be important: excited atom formation attributed to recombination processes was also observed when using NIR or blue pump pulses instead of XUV pulses. This demonstrates the general implications of the current findings for the explanation of previous experiments carried out in different wavelength regimes. Moreover, the cluster expansion dynamics could be traced up to the nanosecond range by using the REAR technique.

Our results show the remarkable versatility of intense HHG pulses for the study of dynamic processes in clusters. In the future, the investigation of other extended systems like biomolecules will benefit from the availability of these laboratory-scale XUV light sources.

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The investigation of cluster explosion dynamics under intense extreme-ultraviolet (XUV) pulses has so far been limited to large scale facilities likefree-electron lasers. In a recent publication it was shown that the research on clusters is now also possible with intense XUV pulses obtained in a laboratory-scale environment with a newly developed light source that makes use of the high-order harmonic generation process. For the first time, the formation of high-lying Rydberg atoms by electron-ion recombination during the cluster expansion initially triggered by an intense XUV pulse was identified, giving new insight into the cluster dissociation process.

An intense light pulse interacting with a weakly bound van der Waals cluster consisting of thousands of atoms can eventually lead to the explosion of the cluster and its complete disintegration. During this process, novel ionization mechanisms occur that are not observed in atoms. With a light pulse that is intense enough, many electrons are removed from their atoms that can move within the cluster, where they form a plasma with the ions on the nanometer scale, a so called nanoplasma. Due to collisions between the electrons, some of them may eventually gain sufficient energy to leave the cluster. A large part of the electrons, however, will remain confined to the cluster. It was theoretically predicted that electrons and ions in the nanoplasma recombine to form Rydberg atoms, however, an experimental proof of this hypothesis is still missing. Previous experiments were carried out at large scale facilities like free-electron lasers that have sizes from a few hundred meters to a few kilometers showing already surprising results such as the formation of very high charge states when an intense XUV pulse interacts with the cluster. However, the accessibility to such sources is strongly limited, and the experimental conditions are extremely challenging. The availability of intense light pulses in the extreme-ultraviolet range from an alternative source is therefore important to gain a better understanding of the various processes occurring in clusters and other extended systems such as biomolecules exposed to intense XUV pulses.

Scientists from the Max-Born-Institut have developed a new light source that is based on the process of high-order harmonic generation. In the experiment, an intense pulse in the extreme-ultraviolet range with a duration of 15 fs (1fs=10^-15 s) interacted with clusters consisting of argon or xenon atoms. In the current issue of Physical Review Letters (Vol. 112-073003 publ. 20 February 2014) Bernd Schütte, Marc Vrakking and Arnaud Rouzée present the results of these studies, which are in very good agreement with previously obtained results from free-electron lasers: the formation of a nanoplasma was inferred by measuring the kinetic energy distributions of electrons formed in the cluster ionization process, showing a characteristic plateau up to a maximum kinetic energy given by the kinetic energy resulting from photoionization of an individual atom. In collaboration with the theoreticians Mathias Arbeiter and Thomas Fennel from the University of Rostock, it was possible to numerically simulate the ionization processes in the cluster and to reproduce the experimental results. In addition, by using the velocity map imaging technique, a yet undiscovered distribution of very slow electrons was observed and attributed the formation of high-lying Rydberg atoms by electron-ion recombination processes during the cluster expansion. Since the binding energies of the electrons are very small, the DC detector electric field used in the experiment was strong enough to ionize these Rydberg atoms, leading to the emission of low energy electrons. This process is also known as frustrated recombination and could now be confirmed experimentally for the first time. The current findings may also explain why in recent experiments using intense X-ray pulses, high charge states up to Xe²⁶⁺ were observed in clusters, although a large number of recombination processes is expected to take place. Moreover, the opportunity to carry out this type of experiment with a high-order harmonic source makes it possible in the future to perform pump-probe experiments in clusters and other extended systems with a time resolution down to the attosecond range.