Energy exchange in highly ionized nanoparticles

Excited atoms often decay via the emission of radiation, a process that is known as fluorescence. A different scenario can emerge when an excited atom is surrounded by other excited atoms, ions and electrons. Such a situation is achieved when an intense laser pulse interacts with a nanoscale object. In this case, an excited atom can decay by transferring its excess energy to another particle in the environment. Researchers from the Max-Born-Institut in Berlin, the University of Rostock, and the University of Heidelberg found evidence for such an energy exchange involving electrons that are trapped within a nanocluster. They observed a so far unidentified peak in the electron spectrum following the ionization of a nanocluster by a near-infrared (NIR) laser pulse. The researchers attributed this signal to the relaxation of one electron from an excited Rydberg atom and the simultaneous transfer of the excess energy to a second electron that can escape from the cluster. The obtained results, which were published in Nature Communications, are of universal nature and expected to play an important role in other nanoscale systems including biomolecules.

Interatomic Coulombic decay (ICD) describes the relaxation of an excited atom by transferring its excess energy to a neighboring atom that gets ionized. This effect has received significant attention in recent years, as it may be a source of radiation damage in biological systems. At the same time, it was proposed to exploit ICD for novel cancer therapies. So far, ICD has been observed following the ionization or excitation of clusters by high-energy photons in the extreme-ultraviolet (XUV) and X-ray range. In contrast, it had not been expected that ICD could be induced by low-energy photons in the NIR regime.

The ionizaton of a cluster by an intense NIR laser pulse triggers highly complex dynamics. A so called nanoplasma is formed that consists of a large number of ions and electrons interacting with each other. Recombination of electrons and ions has been found to result in the generation of Rydberg atoms and ions, which can decay via fluorescence. However, in a strongly ionized cluster, Rydberg atoms may also relax via correlated electronic decay (CED) processes similar to ICD, i.e. without the emission of radiation. In CED, one electron can relax from a Rydberg state to the ground state and transfer its excess energy to a second electron, which is either located in the same atom, in the nanoplasma, or which is in a Rydberg state of a nearby atom (see Figure 1). Using this additional energy, the second electron can escape from the cluster. “Even though CED may be expected in nanoplasmas, the effect had neither been observed in experiments nor had it been predicted by theoretical models.”, explains Dr. Bernd Schütte from the Max-Born-Institut. “The major challenge in the experiment was to find suitable conditions that allow a direct observation of correlated electronic decay.”

Just recently, the researchers were rewarded for their search and found evidence of CED in the electron spectrum from argon clusters ionized by an intense NIR laser pulse. Their results have now been published in Nature Communications. The emergence of a peak in the energy spectrum of emitted electrons that is close in energy to the atomic ionization potential (see Figure 2) was found to be the signature of an electronic decay process involving bound atomic states. Surprisingly, the scientists found that the energy exchange between electrons takes place almost 100 picoseconds after the cluster is ionized. This is much slower than for typical ICD processes that proceed on 100 femtoseconds timescales.

Support for this explanation was obtained by modeling the complex dynamics taking place in the expanding clusters 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 via CED. Electrons that have been emitted in early expansion stages will have lost their specific bound-state signatures.”, explains Fennel. The ICD expert Dr. Alexander Kuleff from the University of Heidelberg adds “Our calculations show that ICD between lowly excited argon atoms takes place on a timescale of 200 femtoseconds, but the process significantly slows down, when higher Rydberg states are involved. This is in good agreement with the experiment, which suggests that the observed electrons are emitted from higher Rydberg orbitals.”

Although the first experiments on clusters with intense NIR laser pulses were already performed in 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 only be directly observed in a very small range of laser intensities and cluster sizes. However, after having understood the involved dynamics, the researchers could show that CED has a universal nature. The process was observed in all the investigated clusters, which include atomic krypton and xenon clusters as well as molecular oxygen clusters. “CED takes place as soon as a nanoplasma is born within the cluster and excited states are populated by recombination”, explains Dr. Arnaud Rouzée from the Max-Born-Institut, adding “CED is therefore expected to be important also for experiments, in which intense XUV and X-ray laser pulses that interact with nanoscale objects, including biomolecules.” Further experiments are under way in order to elucidate the overall significance of correlated electronic decay in highly excited complex systems.

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