Ionization dynamics of helium dimers in fast collisions with He++

Ultrafast temporal evolution of interatomic Coulombic decay in NeKr dimers

We investigate interatomic Coulombic decay in NeKr dimers after neon inner-valence photoionization [Ne⁺(2s^-1)] using a synchrotron light source. We measure with high energy resolution the two singly charged ions of the Coulomb-exploding dimer dication and the photoelectron in coincidence. By carefully tracing the post-collision interaction between the photoelectron and the emitted ICD electron we are able to probe the temporal evolution of the state as it decays. Although the ionizing light pulses are 80 picoseconds long, we determine the lifetime of the intermediate dimer cation state and visualize the contraction of the nuclear structure on the femtosecond time scale.

Probing the Helium Dimer by Relativistic Highly Charged Projectiles

We study the fragmentation of He_{2} dimers into He^{+} ions by relativistic highly charged projectiles. We demonstrate that the interaction between an ultrafast projectile with an extremely extended object-the helium dimer-possesses interesting features that are absent in collisions with "normal" molecules. We also show that such projectiles, due to their enormous interaction range, can accurately probe the ground state of the dimer and even be used for a determination of its binding energy.

Interatomic and Intermolecular Coulombic Decay

Interatomic or intermolecular Coulombic decay (ICD) is a nonlocal electronic decay mechanism occurring in weakly bound matter. In an ICD process, energy released by electronic relaxation of an excited atom or molecule leads to ionization of a neighboring one via Coulombic electron interactions. ICD has been predicted theoretically in the mid nineties of the last century, and its existence has been confirmed experimentally approximately ten years later. Since then, a number of fundamental and applied aspects have been studied in this quickly growing field of research. This review provides an introduction to ICD and draws the connection to related energy transfer and ionization processes. The theoretical approaches for the description of ICD as well as the experimental techniques developed and employed for its investigation are described. The existing body of literature on experimental and theoretical studies of ICD processes in different atomic and molecular systems is reviewed.

Heavy N+ ion transfer in doubly charged N2Ar van der Waals cluster

Van der Waals clusters are weakly bound atomic/molecular systems and are an important medium for understanding micro-environmental chemical phenomena in bio-systems. The presence of neighboring atoms may open channels otherwise forbidden in isolated atoms/molecules. In hydrogen-bond clusters, proton transfer plays a crucial role, which involves mass and charge migration over large distances within the cluster and results in its fragmentation. Here we report an exotic transfer channel involving a heavy N⁺ ion observed in a doubly charged cluster produced by 1 MeV Ne⁸⁺ ions: (N₂Ar)²⁺→N⁺+NAr⁺. The neighboring Ar atom decreases the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_2^{2 + }$$\end{document}N22+ barrier height and width, resulting in significant shorter lifetimes of the metastable molecular ion state \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_2^{2 + }$$\end{document}N22+(\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\mathrm{X}}^{1}}{\Sigma _{{\mathrm{g}}}^{+}}$$\end{document}X1Σg+). Consequently, the breakup of the covalent N⁺−N⁺ bond, the tunneling out of the N⁺ ion from the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_2^{2 + }$$\end{document}N22+ potential well, as well as the formation of an N−Ar⁺ bound system take place almost simultaneously, resulting in a Coulomb explosion of N⁺ and NAr⁺ ion pairs.

There are multiple ways by which energy and charge transfer occur in weakly bound systems. Here the authors reveal a heavy ion N⁺ transfer in a doubly charged Van der Waals cluster produced in collisions of the highly charged Ne⁸⁺ ion with N₂Ar, leading to fragmentation of N⁺ and NAr⁺ via Coulomb explosion.

Charge-Exchange-Driven Low-Energy Electron Splash Induced by Heavy Ion Impact on Condensed Matter

Low-energy electrons (LEEs) are of great relevance for ion-induced radiation damage in cells and genes. We show that charge exchange of ions leads to LEE emission upon impact on condensed matter. By using a graphene monolayer as a simple model system for condensed organic matter and utilizing slow highly charged ions (HCIs) as projectiles, we highlight the importance of charge exchange alone for LEE emission. We find a large number of ejected electrons resulting from individual ion impacts (up to 80 electrons/ion for Xe⁴⁰⁺). More than 90% of emitted electrons have energies well below 15 eV. This "splash" of low-energy electrons is interpreted as the consequence of ion deexcitation via an interatomic Coulombic decay (ICD) process.

Quantum size effect affecting environment assisted electron capture in quantum confinements

Ultrafast inter-Coulombic electron capture (ICEC) has been established as an important energy-transfer process in open paired-quantum-dot systems which can mediate between entrapment of free-moving electrons and release of trapped ones elsewhere by long-range electron-electron interaction within nanowires. Previous studies indicated ICEC enhancement through population and secondary decay of two-center resonance states, the latter known as inter-Coulombic decay (ICD). This study investigates the quantum-size effect of single- and double-electron states in an established model of a quasi-one-dimensional nanowire with two embedded confinement sites, represented by a pair of Gaussian wells. We analyze the ICEC related electron flux density as a function of confinement size and are able to clearly identify two distinct capture channels: a direct long-range electron-electron impulse and a conversion of kinetic energy to electron-electron correlation energy with consecutive ICD. The overlay of both channels makes ICEC extremely likely, while nanowires are a strong candidate for the next miniaturization step of integrated-circuit components.

Damaging Intermolecular Energy and Proton Transfer Processes in Alpha-Particle-Irradiated Hydrogen-Bonded Systems

Although the biological hazard of alpha-particle radiation is well-recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionized states produced by an alpha-particle impact. Two such processes were identified in the present study by considering an acetylene dimer, a biologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well-established intermolecular Coulombic decay of inner-valence-ionized states. The other is a novel relaxation mechanism of dicationic states involving intermolecular proton transfer. Both processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge-separated states. These processes are general and predicted to occur also in alpha-particle-irradiated nucleobase pairs in DNA molecules.

Interatomic Coulombic decay as a new source of low energy electrons in slow ion-dimer collisions

We provide the experimental evidence that the single electron capture process in slow collisions between O^{3+} ions and neon dimer targets leads to an unexpected production of low-energy electrons. This production results from the interatomic Coulombic decay process, subsequent to inner-shell single electron capture from one site of the neon dimer. Although pure one-electron capture from the inner shell is expected to be negligible in the low collision energy regime investigated here, the electron production due to this process overtakes by 1 order of magnitude the emission of Auger electrons by the scattered projectiles after double-electron capture. This feature is specific to low charge states of the projectile: similar studies with Xe^{20+} and Ar^{9+} projectiles show no evidence of inner-shell single-electron capture. The dependence of the process on the projectile charge state is interpreted using simple calculations based on the classical over the barrier model.

Atomic site-sensitive processes in low energy ion-dimer collisions

Electron capture processes for low energy Ar(9+) ions colliding with Ar(2) dimer targets are investigated, focusing attention on charge sharing between the two Ar atoms as a function of the molecular orientation and the impact parameter. A preference for charge-asymmetric dissociation channels is observed, with a strong correlation between the projectile scattering angle and the molecular ion orientation. The measurements here provide clear evidence that projectiles distinguish each atom in the target and that electron capture from near-site atoms is favored. Monte Carlo calculations based on the classical over-the-barrier model, with dimer targets represented as two independent atoms, are compared to the data. They give new insight into the dynamics of the collision by providing, for the different electron capture channels, the two-dimensional probability maps p(b), where b is the impact parameter vector in the molecular frame.

Enhanced production of low energy electrons by alpha particle impact

Radiation damage to living tissue stems not only from primary ionizing particles but to a substantial fraction from the dissociative attachment of secondary electrons with energies below the ionization threshold. We show that the emission yield of those low energy electrons increases dramatically in ion-atom collisions depending on whether or not the target atoms are isolated or embedded in an environment. Only when the atom that has been ionized and excited by the primary particle impact is in immediate proximity of another atom is a fragmentation route known as interatomic Coulombic decay (ICD) enabled. This leads to the emission of a low energy electron. Over the past decade ICD was explored in several experiments following photoionization. Most recent results show its observation even in water clusters. Here we show the quantitative role of ICD for the production of low energy electrons by ion impact, thus approaching a scenario closer to that of radiation damage by alpha particles: We choose ion energies on the maximum of the Bragg peak where energy is most efficiently deposited in tissue. We compare the electron production after colliding He(+) ions on isolated Ne atoms and on Ne dimers (Ne(2)). In the latter case the Ne atom impacted is surrounded by a most simple environment already opening ICD as a deexcitation channel. As a consequence, we find a dramatically enhanced low energy electron yield. The results suggest that ICD may have a significant influence on cell survival after exposure to ionizing radiation.