Secrets of exploding clusters on the trail

Examining the dynamics of cluster explosions using intense ultraviolet (XUV) pulses has been limited to large scale research facilities such as Free Electron Lasers. A recent publication has shown that cluster research is now also possible with intense XUV pulses in a laboratory with a newly developed light source based on Higher Harmonic generation. For the first time, the formation of highly excited Rydberg atoms was demonstrated by electron-ion recombination during the expansion of clusters, which was initially triggered by an XUV pulse and provides new insights into the cluster's decomposition process.

An intense light pulse, which interacts with weakly bound van der Waals clusters consisting of thousands of atoms, can eventually lead to the explosion of the cluster and its complete decomposition. During this process, novel ionization mechanisms occur that are not observed in atoms. With a sufficiently intense light pulse, many electrons are released from their atoms, which can move within the cluster and form a plasma with the ions on a nanometer scale, a so-called nanoplasm. Finally, through collisions between the electrons, some of them can gain sufficient energy to escape the cluster. However, most of the electrons remain trapped in the cluster. It has been theoretically predicted that electrons in the nanoplasm recombine with ions to form Rydberg atoms, but there is no experimental evidence for this hypothesis. Previous experiments have been carried out on large scale research facilities such as free electron lasers, which are hundreds of meters to kilometers in size, and have already shown surprising results, e.g. the generation of very high charge states when an intense XUV pulse interacts with a cluster. However, access to these facilities is very limited and the experimental conditions are extremely challenging. Therefore, the availability of intense ultraviolet light pulses from other sources is important to gain a better understanding of the various processes that take place in clusters and other extended objects such as bio-molecules when exposed to intense XUV pulses.

Fig. 1 Time-of-flight spectrum for xenon atoms and clusters with an average size of 36000 atoms. For clusters, larger fragments such as dimers and trimers are observed. In comparison, the ratio Xe2+ / Xe+ is smaller for clusters than for atoms, which is due to recombination processes in the nanoplasm of the cluster.

Scientists at the Max Born Institute have developed a light source based on the process of Higher Harmonic Generation. An intense light pulse in the extreme ultraviolet range with a duration of 15 fs (1fs = 10-15s) interacted in the experiment with clusters consisting of argon and 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 investigations, which show a very good agreement with previously obtained results of free electron lasers. In cooperation with the theoreticians Mathias Arbeiter and Thomas Fennel of the University of Rostock it was possible to numerically simulate the ionization processes in the cluster and to reproduce the experimental results. Furthermore, using the so-called Velocity Map Imaging technique, a hitherto undiscovered distribution of very slow electrons was observed, which suggests the formation of highly excited Rydberg atoms by electron-ion recombination processes during cluster expansion. Due to the low binding energy of the electrons, the static electric field of the detector is strong enough to ionize the Rydberg atoms, resulting in the emission of very slow electrons. This process is also known as frustrated recombination and has now been experimentally demonstrated for the first time. The current results may also explain why in previous experiments with intense X-ray pulses high charge states up to Xe26+ were observed in clusters, although a large number of recombination processes are expected. Furthermore, an experiment based on a higher harmonic source in the future offers the possibility to perform excitation-query experiments in clusters and other extended objects with a temporal resolution down to the attosecond range.

Fig. 2 Left side: 2D momentum image of the electrons of Argon clusters with an average size of 3500 atoms, which shows a prominent distribution in the center, due to the ionization of Rydberg atoms with the detector field. Right side: The kinetic energy spectrum (black curve) shows good agreement with numerical simulations shown for intensities of 5x1011 W / cm2 (red), 1x1012 W / cm2 (green), and 2x1012 W / cm2 (purple).

Original publication

Rare-gas clusters in intense extreme-ultraviolet pulses from a high-order harmonic source

B. Schütte, M. Arbeiter, T. Fennel, M. J. J. Vrakking, A. Rouzée

Physical Review Letters 112 (2014) 073003/1-5