Electronic Decay Processes Research Articles

Electronic-decay-process spectra including nuclear degrees of freedom

In the field of chemistry, where nuclear motion has traditionally been a focal point, we now explore the ultrarapid electronic motion spanning attoseconds to femtoseconds, demonstrating that it is equally integral and relevant to the discipline. The advent of ultrashort attosecond pulse technology has revolutionized our ability to directly observe electronic rearrangements in atoms and molecules, offering a time-resolved insight into these swift processes. Prominent examples include Auger-Meitner decay and interparticle Coulombic decay. However, the real challenge lies in interpreting these observations, where theoretical models are indispensable. Building upon the analytical framework introduced by Fasshauer and Madsen [], which analyzed the spectra of electrons emitted during electronic decay processes from a purely electronic perspective, we extend this theoretical base to include nuclear dynamics, utilizing the Born-Oppenheimer approximation to deepen our understanding of the intricate interaction between electronic and nuclear motion in these processes. We illustrate the impact of incorporating nuclear degrees of freedom in several theoretical cases characterized by different numbers of vibrational bound states in both the electronic resonance and the electronic final state. This approach not only clarifies complex spectral features and unusual peak shapes but also demonstrates a method for extracting the energy differences between multiple vibrational resonance states through their distinctive interference patterns. Published by the American Physical Society 2025

Time, momentum, and energy resolved pump-probe tunneling spectroscopy of two-dimensional electron systems

Real-time probing of electrons can uncover intricate relaxation mechanisms and many-body interactions in strongly correlated materials. Here, we introduce time, momentum, and energy resolved pump-probe tunneling spectroscopy (Tr-MERTS). The method allows the injection of electrons at a particular energy and observation of their subsequent decay in energy-momentum space. Using Tr-MERTS, we visualize electronic decay processes, with lifetimes from tens of nanoseconds to tens of microseconds, in Landau levels formed in a GaAs quantum well. Although most observed features agree with simple energy-relaxation, we discovered a splitting in the nonequilibrium energy spectrum in the vicinity of a ferromagnetic state. An exact diagonalization study suggests that the splitting arises from a maximally spin-polarized state with higher energy than a conventional equilibrium skyrmion. Furthermore, we observe time-dependent relaxation of the splitting, which we attribute to single-flipped spins forming skyrmions. These results establish Tr-MERTS as a powerful tool for studying the properties of a 2DES beyond equilibrium.

Interatomic Coulombic decay in small helium clusters.

Interatomic Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process wherein an excited atom transfers its excess energy to a neighboring species leading to the ionization of the latter. In helium clusters, ICD can take place, for example, after simultaneous ionization and excitation of one helium atom within the cluster. After ICD, two helium ions are created and the system undergoes a Coulomb explosion. In this work, we investigate theoretically ICD in small helium clusters containing between two and seven atoms and compare our findings to two sets of coincidence measurements on clusters of different mean sizes. We provide a prediction on the lifetime of the excited dimer and show that ICD is faster for larger clusters. This is due to (i) the increased number of neighboring atoms (and therefore the number of decay channels) and (ii) the substantial decrease of the interatomic distances. In order to provide more details on the decay dynamics, we report on the kinetic-energy distributions of the helium ions. These distributions clearly show that the ions may undergo charge exchange with the neutral atoms within the cluster, such process is known as frustrated Coulomb explosion. The probability for these charge-exchange processes increases with the size of the clusters and is reflected in our calculated and measured kinetic-energy distributions. These distributions are therefore characteristics of the size distribution of small helium clusters.

Luminescence color regulation of carbon quantum dots by surface modification

Labelling various biological molecules with multi-color carbon quantum dots (CQDs) in the same scene has great potential application for investigating the relationship between those molecules. It is still a great challenge to regulate the color of CQDs. Ample –NH2 groups on the surface of green carbon quantum dots (GCQDs) were characterized and confirmed by 1H nuclear magnetic resonance (1H NMR) data. As–NH2 groups had high organic reactivity, the surface of GCQDs was designed and modified with several organic functional groups through a simple surface modification strategy of organic reactions. The surface band gap states of GCQDs were tuned gradually by the corresponding modified functional groups, and the emission colors of GCQDs were tuned from green to yellow-green, then to orange, and finally to red. All the emission spectra for these multicolor carbon quantum dots (M-CQDs) were excitation independent, and the lifetimes for the M-CQDs were mainly determined by the second electron decay process from the surface defect band states. The red-shift colors for the M-CQDs were ascribed to the emissions from their surface defect band states, and the luminescence mechanism had been investigated. This work opens up a novel synthesis strategy to regulate the emission colors of carbon quantum dots.

Nonradiative relaxation of charge carriers at molecule-metal interfaces: Nonadiabatic molecular dynamics study

Nonadiabatic molecular dynamics simulations in combination with time-dependent Kohn-Sham density functional theory are used to study the nonradiative relaxation of charge carriers in 4-mercaptobenzonitrile (NCOPE1), 4-(mercaptomethyl) benzonitrile (NC-PT1), 4′-mercapto-[1,1′-biphenyl]-4-carbonitrile (NC-BP0) and 4-((4-mercaptophenyl) ethynyl) benzonitrile (NCOPE2) molecules absorbed on Au(111) surface. The fastest hot electron decay (∼30 fs) is obtained for the NCOPE1 molecule due to strong electron-phonon couplings. However, the relaxation kinetics strongly depends on the properties of the initially populated state (e.g., orbital-overlap/hybridization between the organic molecules and the substrate), on the proximity of neighboring electronic states (i.e., intraband gaps) and on the electron-phonon nonadiabatic couplings. The obtained orbital- and size-dependent relaxation dynamics is in qualitative agreement with the results of core-hole clock experiments, which indicates the possibility of modeling the interfacial electron transfer by the decay process of hot electrons.

Impact of cavity on interatomic Coulombic decay

The interatomic Coulombic decay (ICD) is an efficient electronic decay process of systems embedded in environment. In ICD, the excess energy of an excited atom A is efficiently utilized to ionize a neighboring atom B. In quantum light, an ensemble of atoms A form polaritonic states which can undergo ICD with B. Here we investigate the impact of quantum light on ICD and show that this process is strongly altered compared to classical ICD. The ICD rate depends sensitively on the atomic distribution and orientation of the ensemble. It is stressed that in contrast to superposition states formed by a laser, forming polaritons by a cavity enables to control the emergence and suppression, as well as the efficiency of ICD.

Direct Observation of Photoexcited Electron Dynamics in Organic Solids Exhibiting Thermally Activated Delayed Fluorescence via Time‐Resolved Photoelectron Emission Microscopy

AbstractSolid‐state films exhibiting thermally activated delayed fluorescence (TADF) can offer high internal electroluminescence (EL) quantum efficiency, even in a simplified device structure. However, the exciton dynamics in solid‐state TADF films, particularly for the unexpected exciton loss processes such as concentration quenching, have not yet been clarified. The dynamics of photoexcited electrons in the TADF process of a 2,4,5,6‐Tetra (9H‐carbazol‐9‐yl) isophthalonitrile (4CzIPN) solid film are observed via time‐resolved photoelectron emission microscopy (TR‐PEEM) and the results are compared with the conventional time‐resolved photoluminescence (TR‐PL) technique. The initial decay process of the photoexcited electrons probed via TR‐PEEM is thoroughly traced in the TR‐PL signal, while unusual long‐lived electrons are detected only through the use of TR‐PEEM. These results indicate that the excitons of 4CzIPN spontaneously dissociate into free carriers within the exciton lifetime, which seems to be a common process that contributes to the exciton loss in polar organic solids.

Tracking the Local Structure Change during the Photoabsorption Processes of Photocatalysts by the Ultrafast Pump-Probe XAFS Method

The birth of synchrotron radiation (SR) facilities and X-ray free electron lasers (XFELs) has led to the development of new characterization tools that use X-rays and opened frontiers in science and technology. Ultrafast X-ray absorption fine structure (XAFS) spectroscopy for photocatalysts is one such significant research technique. Although carrier behavior in photocatalysts has been discussed in terms of the band theory and their energy levels in reciprocal space (k-space) based on optical spectroscopic results, it has rarely been discussed where photocarriers are located in real-space (r-space) based on direct observation of the excited states. XAFS provides information on the local electronic and geometrical structures around an X-ray-absorbing atom and can address photocarrier dynamics in the r-space observed from the X-ray-absorbing atom. In this article, we discuss the time dependent structure change of tungsten trioxide (WO3) and bismuth vanadate (BiVO4) photocatalysts studied by the ultrafast pump-probe XAFS method in the femtosecond to nanosecond time scale with the Photon Factory Advanced Ring (PF-AR) and the SPring-8 Angstrom Compact free-electron LAser (SACLA). WO3 shows a femtosecond decay process of photoexcited electrons followed by a structural change to a metastable state with a hundred picosecond speed, which is relaxed to the ground-state structure with a nanosecond time constant. The Bi L3 edge of BiVO4 shows little contribution of the Bi 6s electron to the photoabsorption process; however, it is sensitive to the structural change induced by the photoexcited electron. Time-resolved XAFS measurements in a wide range time domain and with varied wavelengths of the excitation pump laser facilitate understanding of the overall details regarding the photocarrier dynamics that have a significant influence on the photocatalytic performance.

Influence of non-equilibrium electron dynamics on photoluminescence of metallic nanostructures

A microscopic model is still strongly needed to understand the intrinsic photoluminescence (iPL) of metallic nanostructures. In this paper, a phenomenological model concerning the electron dynamics at the excited states, including the electron–phonon (e-p) and electron–electron (e-e) interactions, is developed. This model shows that the dynamics of non-equilibrium electrons at the excited states influence the iPL features significantly. Two main aspects determine the iPL process of metallic nanostructures: the photonic density of states relating to the Purcell effect caused by the surface plasmon resonances, and the electrons transition factor. This model takes into account the contribution of the e-p and e-e interactions to the dynamic electron distribution. The decay process of the non-thermal electrons at the excited states helps understanding most of the iPL features of metallic nanostructures. The calculated and experimental results coincide well regarding the spectral shape, temperature-dependent anti-Stokes emission, and nonlinear behaviors, and time-resolved spectra. The results presented in this paper provide a concise, intuitive, and comprehensive understanding of the iPL of metallic nanostructures.

Multiplet structure for perovskite-type Ba0.9Ca0.1Ti0.9Zr0.1O3 by core–hole spectroscopies

Core–hole spectroscopies such as x-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HR-EELS) in combination with multiplet calculation are important tools to elucidate the electronic structure of transition metal compounds. This work presents a comparison between the electronic structure obtained by XPS and HR-EELS for polycrystalline perovskite-type Ba0.9Ca0.1Ti0.9Zr0.1O3. Raman analysis suggests that Ca2+ cations could partly occupy the Ti4+ cations in the B-site of a perovskite structure with the tetragonal phase. A multiplet structure was determined by XPS for Ti 2p and by HR-EELS for Ca L2,3 and Ti L2,3 edges. Octahedral (Oh) symmetry in the crystal field (CF) effects reproduces the local distortion of TiO6 octahedra. The charge transfer (CT) effects were also considered to reproduce L3-edge EELS shoulders and the satellite in the Ti 2p XPS region. CF and CT parameters, 10 Dq, (charge transfer energy) Δ, and (Coulomb repulsion energy) Udd, are reported for future reference. The broadening of the Ti L2-edge suggests the presence of the Coster–Kronig electron decay process. Multiplet calculation in Oh symmetry for the Ca L2,3-edge could support the Raman interpretation.

Effect of spin-orbit coupling on decay widths of electronic decay processes.

Auger-Meitner processes are electronic decay processes of energetically low-lying vacancies. In these processes, the vacancy is filled by an electron of an energetically higher lying orbital, while another electron is simultaneously emitted to the continuum. In low-lying orbitals, relativistic effects can not, even for light elements, be neglected. At the same time, lifetime calculations are computationally expensive. In this context, we investigate which effect spin-orbit coupling has on Auger-Meitner decay widths and aim for a rule of thumb for the relative decay widths of initial states split by spin-orbit coupling. We base this rule of thumb on Auger-Meitner decay widths of Sr4p-1 and Ra6p-1 obtained by relativistic FanoADC-Stieltjes calculations and validate it against Auger-Meitner decay widths from the literature.

Time-resolved spectroscopy of interparticle Coulombic decay processes

We report theory for time-resolved spectator resonant Interparticle Coulombic Decay (ICD) processes. Following excitation by a short extreme ultraviolet pulse, the spectrum of the resonant ICD electron develops. Strong-field ionization is imagined to quench the decay at different time delays and to initiate regular ICD. In this latter process, the ICD electron signal can be measured without interference effects. The typical lifetimes of ICD processes allow for the observation of oscillations of the time- and energy-differential ionization probability. We propose to utilize this oscillation to measure lifetimes of electronic decay processes.

Ultrafast electron and spin dynamics of strongly correlated NdNiO3

Electron and spin dynamics of strongly correlated NdNiO3 are investigated across the metal–insulator transition with ultrafast optical spectroscopy at different temperatures, pump laser fluences, and laser polarizations. Transient differential reflectivity measurements on the insulating phase NdNiO3 show two characteristic electronic decay processes. The slow decay process, strongly dependent on coherent phonon excitation, is attributed to the electron–phonon interaction, while the fast decay process, to the electron-spin interaction. The temporal evolution of Kerr rotation reveals that net spin polarization can be induced by circularly polarized light in insulating NdNiO3 via electron excitation from the Ni t to e orbital, and two net spin polarizations with opposite directions are observed given the antiferromagnetic ordering in insulating NdNiO3. At the insulator-to-metal transition, the transient differential reflectivity reverses its sign; the metallic phase NdNiO3 also shows two characteristic electronic decay processes, a typical metallic decay processes and a slow decay process; the slow decay process is attributed to the interaction between electrons and the low frequency coherent phonons (2.45 THz). Our experiments demonstrate that the phase transition involves several processes upon continuous heating: the electron-spin interaction disappears at 9 K below the transition temperature; at the transition temperature, NdNiO3 transitions into the metallic phase and the antiferromagnetic ordering transitions into the paramagnetic ordering; at the transition temperature, the structure of NdNiO3 changes from a monoclinic structure () to a high temperature orthorhombic () structure. Such a structure change results in the change in phonon vibrations, and subsequently, leads to the disappearance of the electron–phonon interactions due to the insulating phase NdNiO3 and the emergence of other types of electron–phonon interactions due to the metallic phase.

Correlated electronic decay in expanding clusters triggered by intense XUV pulses from a Free-Electron-Laser

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. The dynamics of intense laser-cluster interaction is encoded in electron kinetic energy spectra, which contain signatures of direct photoelectron emission as well as emission of thermalized nanoplasma electrons. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core - Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. By applying the terahertz field streaking principle we demonstrate that CED-electrons are emitted at least a few picoseconds after the ionizing XUV pulse has ended. Following the recent finding of CED in clusters ionized by intense near-infrared laser pulses, our observation of CED in the XUV range suggests that this process is of general relevance for the relaxation dynamics in laser produced nanoplasmas.

Non-nearest neighbour ICD in clusters

Interatomic Coulombic decay (ICD) is an electronic decay process of excited, ionized systems. It has been shown to occur in a multitude of small and large systems. The effects of more than one possible decay partner are discussed in detail illustrated by simulated ICD electron spectra of NeAr clusters and pure Ne clusters. Hereby, the mostly underestimated contribution of decay with non-nearest neighbours is highlighted. In the neon clusters, the lifetime of the bulk atoms is found to be in excellent agreement with experiment (Jahnke et al 2004 Phys. Rev. Lett. 93 173401) while the lifetimes of the surface atoms differ significantly. Hence, the experimental lifetime can not purely be explained by the effect of the number of neighbours. We propose the possibility to investigate the transition from small clusters to the solid state by using the ICD electron spectra to distinguish between icosahedral and cuboctahedral cluster structures.

Relaxation Processes in Aqueous Systems upon X-ray Ionization: Entanglement of Electronic and Nuclear Dynamics.

The knowledge of primary processes following the interaction of high-energy radiation with molecules in liquid phase is rather limited. In the present Perspective, we report on a newly discovered type of relaxation process involving simultaneous autoionization and proton transfer between adjacent molecules, so-called proton transfer mediated charge separation (PTM-CS) process. Within PTM-CS, transients with a half-transferred proton are formed within a few femtoseconds after the core-level ionization event. Subsequent nonradiative decay of the highly nonequilibrium transients leads to a series of reactive species, which have not been considered in any high-energy radiation process in water. Nonlocal electronic decay processes are surprisingly accelerated upon proton dynamics. Such strong coupling of electronic and nuclear dynamics is a general phenomenon for hydrogen-bonded systems, however, its probability correlates strongly with hydration geometry. We suggest that the newly observed processes will impact future high-energy radiation-chemistry-relevant modeling, and we envision application of autoionization spectroscopy for identification of solution structure details.

Correlated electronic decay following intense near-infrared ionization of clusters

We report on a novel correlated electronic decay process following extensive Rydberg atom formation in clusters ionized by intense near-infrared fields. A peak close to the atomic ionization potential is found in the electron kinetic energy spectrum. This new contribution is attributed to an energy transfer between two electrons, where one electron decays from a Rydberg state to the ground state and transfers its excess energy to a weakly bound cluster electron in the environment that can escape from the cluster. The process is a result of nanoplasma formation and is therefore expected to be important, whenever intense laser pulses interact with nanometer-sized particles.

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

When an excited atom is embedded into an environment, novel relaxation pathways can emerge that are absent for isolated atoms. A well-known example is interatomic Coulombic decay, where an excited atom relaxes by transferring its excess energy to another atom in the environment, leading to its ionization. Such processes have been observed in clusters ionized by extreme-ultraviolet and X-ray lasers. Here, we report on a correlated electronic decay process that occurs following nanoplasma formation and Rydberg atom generation in the ionization of clusters by intense, non-resonant infrared laser fields. Relaxation of the Rydberg states and transfer of the available electronic energy to adjacent electrons in Rydberg states or quasifree electrons in the expanding nanoplasma leaves a distinct signature in the electron kinetic energy spectrum. These so far unobserved electron-correlation-driven energy transfer processes may play a significant role in the response of any nano-scale system to intense laser light.

Relativistic decay widths of autoionization processes: The relativistic FanoADC-Stieltjes method

Electronic decay processes of ionized systems are, for example, the Auger decay or the Interatomic/ Intermolecular Coulombic Decay. In both processes, an energetically low lying vacancy is filled by an electron of an energetically higher lying orbital and a secondary electron is instantaneously emitted to the continuum. Whether or not such a process occurs depends both on the energetic accessibility and the corresponding lifetime compared to the lifetime of competing decay mechanisms. We present a realization of the non-relativistically established FanoADC-Stieltjes method for the description of autoionization decay widths including relativistic effects. This procedure, being based on the Algebraic Diagrammatic Construction (ADC), was adapted to the relativistic framework and implemented into the relativistic quantum chemistry program package Dirac. It is, in contrast to other existing relativistic atomic codes, not limited to the description of autoionization lifetimes in spherically symmetric systems, but is instead also applicable to molecules and clusters. We employ this method to the Auger processes following the Kr3d(-1), Xe4d(-1), and Rn5d(-1) ionization. Based on the results, we show a pronounced influence of mainly scalar-relativistic effects on the decay widths of autoionization processes.

Communication: electron transfer mediated decay enabled by spin-orbit interaction in small krypton/xenon clusters.

In this work we study the influence of relativistic effects, in particular spin-orbit coupling, on electronic decay processes in KrXe2 clusters of various geometries. For the first time it is shown that inclusion of spin-orbit coupling has decisive influence on the accessibility of a specific decay pathway in these clusters. The radiationless relaxation process is initiated by a Kr 4s ionization followed by an electron transfer from xenon to krypton and a final second ionization of the system. We demonstrate the existence of competing electronic decay pathways depending in a subtle way on the geometry and level of theory. For our calculations a fully relativistic framework was employed where omission of spin-orbit coupling leads to closing of two decay pathways. These findings stress the relevance of an adequate relativistic description for clusters with heavy elements and their fragmentation dynamics.