Lorenz S. Cederbaum

Intermolecular Coulombic decay (ICD) is a widely spread phenomenon in nature and the laboratory in which the excess energy of a donor is utilized to ionize a nearby acceptor. If the excess energy is insufficiently large to enable ICD, two (or more) donors can collectively transfer their combined excess energy to ionize the acceptor. Experiments show that this collective ICD is, surprisingly, operative in gases. Recently, it has been demonstrated that standard ICD can take place efficiently at large distances between the donors and acceptors due to retardation. Here, we derive the theory of collective ICD including retardation. Quantum electrodynamics perturbation theory is used, and it is shown that the theory can be substantially simplified and the process made more amenable to interpretation by introducing two-body interaction potentials that include retardation. Explicit formulas for the rate of collective ICD are derived and interpreted by expressing the rate in terms of measurable quantities and geometric factors. It is demonstrated that the change in the permanent dipole moments of the species upon excitation is a relevant ingredient in collective ICD.

Hydration critically reshapes ultrafast electronic relaxation in biorelevant molecules, yet its mechanistic role remains poorly understood. Here, we investigate intermolecular Coulombic decay (ICD) following medium-energy (E0 = 103 eV) electron impact ionization of hydrated pyridine, a prototypical nucleobase analogue. Combining multiparticle momentum coincidence spectroscopy with high-level electronic structure calculations, we identify water-initiated ICD and find evidence consistent with an additional pyridine-initiated ICD pathway, in which inner-valence ionization leads to ionization of the neighboring molecule. Water vacancies decay exclusively via the ICD, establishing a highly efficient direct channel. In contrast, pyridine vacancies compete with Auger decay, reducing the efficiency of the pyridine-to-water ICD pathway. As in pyridine dimers, heterocycle-initiated ICD is dominated by N 2s vacancy states. Our results reveal a dual role of water: it opens a highly efficient direct ICD pathway via O 2s vacancy states and acts as an energy-accepting partner following biomolecular inner-valence ionization. These results reveal how microsolvation governs ultrafast relaxation and shapes radiation-induced damage pathways in DNA-related systems.

Excited atoms and molecules can utilize their excess energy to ionize a neighboring system by a process named interatomic and intermolecular Coulombic decay (ICD). ICD is ultrafast, in the femtosecond regime, and has many modes of appearance. Ample applications of ICD have been reported spanning a wide range of fields and it is expected to be ubiquitous in nature. Essentially all the investigations on ICD were for weakly bound systems, like clusters and fluids. We demonstrate that, unexpectedly, ICD can be efficiently active in atomic and molecular gases in spite of the very large distances between the units. We uncover the underlying mechanism which is due to retardation and differs from the standard dipole–dipole interaction prevailing in weakly bound systems. The dynamics of ICD in gases is analyzed. The results considerably broaden the impact of ICD and open the gateway to new kinds of applications.

When an electronically excited atom or molecule is embedded in a chemical environment as, e.g., in a liquid or a loosely bound cluster, it can de-excite through mechanisms where neighboring atoms or molecules are actively participating in the decay: either by donating or accepting energy or electrons. For such nonlocal decay channels, nuclear dynamics play a crucial role as they have a direct impact on the decay efficiency itself. Here, we present a detailed study of the electron-transfer-mediated decay in a loosely bound triatomic prototype system, combining experimental results from a 5-fold coincidence measurement and theoretical modeling of the decay process. Depending on the decay time, we find that certain classes of molecular geometries are favored for this type of decay. Our findings provide an intuitive picture of how electron-transfer-mediated decay proceeds. In particular, our results confirm a roaming-like behavior of the atoms of the trimer prior to its decay. Our combined theoretical and experimental approach enables a comprehensive tracing of the real-space properties of the decaying system in the time domain.

Reduction of ions is an important process in many areas of science, including surface physics and radiation biology. In weakly bound matter, such as clusters or liquids, neutralization can occur with the explicit involvement of atoms or molecules in the local environment of the ion. Among the possible pathways, electron-transfer-mediated decay (ETMD) is particularly efficient in such systems. In ETMD, charge and energy are transferred to neighboring species, leading to the reduction of the multiply ionized atom. Although studied for more than a decade, important aspects of this nonlocal process remain unclear, including the role of nuclear dynamics and the influence of geometry. Here, we investigate the NeKr₂ trimer after core ionization of the Ne atom. We perform fully dimensional nuclear dynamics combined with ETMD decay rate calculations and introduce a new 1/R⁶ asymptotic expression for the decay width. We assess the competition between ETMD, which leads to Coulomb explosion, and direct dissociation without ETMD. We quantify the efficiency of ETMD in asymmetric configurations of the trimer, where charge transfer occurs along the shorter Ne-Kr distance, while energy transfer proceeds along the longer one. The generality of the ETMD mechanism makes it highly relevant in weakly bound environments such as clusters and liquids, where its efficiency increases substantially with the number of neighbors.

Highly charged cations are omnipresent species after the interaction of high-energy or high-intensity light with matter. When embedded in environments, the mechanism and outcome of the redistribution of the cation’s charge are crucial for the further fate of the whole system. Generally, ground-state cations can decay by charge transfer, proceeding radiatively, through nuclear dynamics, or by electron-transfer-mediated decay (ETMD). ETMD causes electron emission from a remote neighbor and appears ubiquitous in loosely bound systems. It remained unclear how multiply charged ions decay if conventional ETMD channels are closed. Here, we show that a yet undiscovered variant of ETMD is possible, where multiple electrons are collectively transferred. Explicitly, we observe ETMD in which two electrons from two distinct neighbors (partially) neutralize a multiply charged ion, and an electron from a fourth site is emitted. According to the established nomenclature, we suggest naming the process ETMD(4).

We consider energy transfer from dark states of the donor. Starting from the general full relativistic expressions for the various asymptotic contributions, we derive the leading terms describing the energy transfer for small and intermediate sized excess energies.

Low-energy-electron emission from resonant Auger final states via intermolecular Coulombic decay (RA-ICD) has previously been described as a promising scenario for controlling radiation damage for medical purposes, but it has so far only been observed in prototypical atomic and molecular van der Waals dimers and clusters. Here, we report the experimental observation of RA-ICD in an aqueous solution. We show that for solvated Ca²⁺ ions, the emission can be very efficiently controlled by tuning the photon energy of exciting X-rays to inner-shell resonances of the ions. Our results provide the next step from demonstrating RA-ICD in relatively simple prototype systems to understanding the relevance and potential applications of ICD in real-life scenarios.