Energy exchange in highly ionized nanoparticles

Excited atoms often decay by the emission of radiation, a process known as fluorescence. Another scenario can occur when an excited atom is surrounded by other excited atoms, ions, and electrons. Such a situation is reached when an intense laser pulse interacts with a nanoscale object. In this case, an excited atom can decay by transferring the surplus energy to another particle in the environment. Scientists at the Max Born Institute in Berlin, the University of Rostock and the University of Heidelberg have now found evidence for such an energy exchange that takes place between electrons trapped in a nanocluster. They have observed a hitherto undiscovered peak in the electron spectrum that occurs after ionization of a nanocluster by a near-infrared (NIR) laser pulse. The researchers attribute this signal to the relaxation of one electron from a Rydberg state and the simultaneous transfer of excess energy to a second electron that can escape the cluster. The results obtained, which have now been published in Nature Communications, are of a universal nature. It is expected to play an important role in other nanosystems, such as Play biomolecules.

Interatomic Coulomb decay (known as ICD) describes the relaxation of an excited atom by transferring the excess energy to an adjacent atom that is ionized. This effect has attracted considerable attention in recent years as it could be a source of radiation damage in biological systems. At the same time it was proposed to use ICD for novel cancer therapies. So far, ICD has been observed after clusters of high-energy photons have been ionized or excited in the extreme ultraviolet (XUV) or X-ray region. In contrast, ICD was not expected to be triggered by low-energy photons in the NIR range.

The ionization of a cluster with an intense NIR laser pulse introduces highly complex dynamics. A so-called nanoplasm is formed, consisting of a large number of electrons and ions interacting with each other. It has been observed that the recombination of electrons and ions leads to the generation of Rydberg atoms and ions, which can decay by fluorescence. However, in a strongly ionized cluster, Rydberg atoms may also relax by correlated electronic decay (CED), similar to the ICD, i. without the emission of radiation. CED means that an electron can relax from a Rydberg state to the ground state and transfer the excess energy to a second electron that is either in the same atom, in the nanoplasm, or in a Rydberg state of a nearby atom (see Figure 1). , With the help of this extra energy, the second electron can escape the cluster. "Although you can basically expect CED in nanoplasmas, the effect was neither experimentally observed nor predicted by theoretical models," explains Dr. Bernd Schütte from the Max Born Institute. "The big challenge in the experiment was to find suitable conditions that allow us to directly observe correlated electronic decay."

Fig. 1 (a) Correlated electronic decay in clusters: An electron in a Rydberg state can relax into the ground state and thereby transfer the excess energy (a) to a second electron in a Rydberg state of the same atom, (b) to a quasi-free electron in the environment, or (c) an electron occupying a Rydberg state in a second atom.

Recently, researchers were rewarded for their search and found evidence of CED in the electron spectrum of argon clusters ionized by intense NIR laser pulses. Their findings have now been published in Nature Communications. The emergence of a peak in the energy spectrum of emitted electrons near the atomic ionization potential (see Figure 2) could be identified as characteristic of an electronic decay process involving bound atomic states. Surprisingly, the scientists found that the exchange of energy between electrons occurs nearly 100 picoseconds after the cluster is ionized. This is much slower than typical ICD processes that run on time scales of 100 femtoseconds.

Fig. 2 Measured electron spectrum after the ionization of argon clusters by intense NIR pulses. The gray area represents thermal emission of electrons. In addition, a peak structure (blue area) with a prominent peak near the ionization potential of atomic argon (15.76 eV) appears. This structure can be explained by correlated electronic decay.

Support for this explanation was achieved by modeling the complex dynamics that take place in the cluster by the group of Prof. Thomas Fennel from the University of Rostock. "The tricky aspect of the experiment is that the charged and expanding cluster disturbs the electrons emitted by CEDs. Electrons emitted in early expansion phases will have lost their specific characteristics stemming from the bound states, "explains Fennel. The ICD expert Dr. Alexander Kuleff of the University of Heidelberg adds: "Our calculations show that ICD occurs between low-excited argon atoms on a time scale of 200 femtoseconds. However, the process slows down significantly when higher Rydberg states are involved. This is in good agreement with the experiment, which suggests that the observed electrons are indeed emitted by higher Rydberg orbitals. "

Although the first experiments were carried out on clusters of intense NIR laser pulses as early as the 1990s, it took a long time to observe correlated electronic decay in expanding nanoplasmas for the first time. One reason why this effect could not be revealed in previous experiments is that it can be directly observed only in a very small range of laser intensities and cluster sizes. However, after understanding the dynamics involved, the researchers were able to show that CED is of universal importance. The process was observed in all investigated clusters, including atomic krypton and xenon clusters as well as molecular oxygen clusters. "CED takes place as soon as a nanoplasm is generated in the cluster and excited atomic states are populated by recombination," explains Dr. Arnaud Rouzée of the Max Born Institute. He adds, "One may therefore expect CED to be important for experiments in which intense XUV and X-ray laser pulses interact with nanoscale objects, including biomolecules." Further experiments are in preparation to clarify the general meaning of correlated electronic Decay in highly excited complex systems.

Original publication

Observation of correlated electronic decay in expanding clusters triggered by near-infrared fields

B. Schütte, M. Arbeiter, T. Fennel, G. Jabbarai, A. I. Kuleff, M. J. J.Vrakking, A. Rouzée

Nature Communications 6 (2015) 8596/1-7