Coulombic Capture Research Articles

Interatomic Coulombic electron capture beyond the virtual photon approximation.

Via the interatomic Coulombic electron capture (ICEC) process, an electron can be captured by an atom or a molecule, while the binding and excess energy is transferred, via a long-range Coulomb interaction, to a neighboring atom or molecule. The transferred energy can be used to ionize or electronically excite the neighboring species. When the two species are asymptotically far apart, an analytical formula for the ICEC cross sections can be derived. The latter can then be estimated using only the energies and the photoionization cross sections of each species. In this work, we develop an analytical model that allows us to predict the ICEC cross sections when the size of the involved species is comparable to the distance between the two entities. Using abinitio R-matrix results for various systems, we show that the new model reduces the error of the asymptotic formula by two orders of magnitude on average while only using parameters that can be taken from the properties of each species.

Virtual photon exchange vs electron transfer in interparticle Coulombic electron capture.

We have investigated Interparticle Coulombic Electron Capture (ICEC) using an abinitio approach for two systems, H+ + H2O and H + H2O+. In this work, we have determined the contribution of virtual photon exchange and electron transfer to the total ICEC cross section as a function of the distance between the charged and neutral particles. Furthermore, we have shown that the relative orientation of the electron acceptor and neighbor systems affects the magnitude of the ICEC cross sections by at least two orders at relatively small distances. This geometry dependence, present even for distances as large as 10 a0, is due to the electron transfer contribution. The relative magnitude of each contribution to ICEC seems to depend on the system studied. By replacing the projectile electron with a positron, we have confirmed that electron transfer also takes place in positron collisions and that the charge of the projectile has a noticeable effect on the process, particularly at low scattering energies.

Resonantly enhanced interatomic Coulombic electron capture in a system of three atoms

In interatomic Coulombic electron capture, the capture of a free electron at an atomic center is accompanied by the radiationless transfer of the excess energy to a neighboring atom of different species, leading to ionization of the latter. We show that this interatomic process can be strongly enhanced by the presence of an additional third atom, provided the energy of the free-bound capture transition in the first atom is resonant to a dipole-allowed excitation energy in this assisting atom. The relation of the resonantly enhanced three-center electron capture with other processes is discussed, and its dependencies on the incident electron energy and the spatial geometry of the triatomic system are illustrated.

Water-assisted electron capture exceeds photorecombination in biological conditions.

A decade ago, an electron-attachment process called interatomic Coulombic electron capture has been predicted to be possible through energy transfer to a nearby neighbor. It has been estimated to be competitive with environment-independent photorecombination, but its general relevance has yet to be established. Here, we evaluate the capability of alkali and alkaline earth metal cations to capture a free electron by assistance from a nearby water molecule. We introduce a characteristic distance rIC for this energy transfer mechanism in equivalence to the Förster radius. Our results show that water-assisted electron capture dominates over photorecombination beyond the second hydration shell of each cation for electron energies above a threshold. The assisted capture reaches distances equivalent to a fifth to seventh solvation shell for the studied cations. The far reach of the assisted electron capture is of significant general interest to the broad spectrum of research fields dealing with low-energy electrons, in particular radiation-induced damage of biomolecules. The here introduced distance measure will enable quantification of the role of the environment for assisted electron attachment.

Electron attachment to a proton in water by interatomic Coulombic electron capture: An R -matrix study

Interatomic Coulombic electron capture (ICEC) is an environment-enabled electron capture process in which a free electron can efficiently attach to a quantum system by transferring the excess energy to a neighbor thus ionizing it. Using the ab initio $R$-matrix method, we investigate the electron attachment to a proton in the neighborhood of a water molecule. The corresponding ICEC cross sections exhibit clear Fano profiles, resulting from interferences between the ICEC final states and resonant states. These Fano interferences, observed in the total ICEC cross sections, were discussed in our recent work on large system-neighbor separations [A. Molle et al., Phys. Rev. A 103, 012808 (2021)]. In the present study, we report on the ICEC cross sections at shorter distances which are relevant in biological and biochemical contexts. Furthermore, we investigate the partial ICEC cross sections and demonstrate that the ionization of a water molecule via ICEC is substantially different from that due to direct photoionization. Finally, we show that the distortion of the equilibrium geometry of the water molecule due to the presence of the proton influences the ICEC process.

Fano interferences in environment-enabled electron capture

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to a quantum system by transferring the excess energy of the electron capture to a neighbor ionizing it. Using the ab initio $R$-matrix method, we show that Fano profiles, resulting from interferences between the ICEC final states and resonant states, appear in the ICEC cross sections even at extremely large system-neighbor separations. We identify several types of resonant states depending on their decay pathways which may involve long-range electron and energy transfer. ICEC is a fundamental process and the interferences lead to substantial enhancement or decrease of the cross sections. The present investigation is therefore of general relevance in many contexts wherever electron capture in an environment takes place.

Electron collisions with molecules and molecular clusters

State-of-the art computational studies of electron collisions with molecules and small molecular clusters are illustrated with results obtained from the application of the R-matrix method and the UKRMol/UKRMol+ suites. High-level calculations of electronic excitation cross sections and core-excited resonances, mainly of core-excited shape character, show excellent agreement with experiment for mid-size molecules like pyrimidine and thiophene. Simpler calculations are paving the way for an in-depth understanding of the effect of hydration on resonance formation: how the shift in resonance energy depends on the characteristics of the hydrogen bond and the resonance being studied. Finally, applications of the software to a little studied process, interatomic coulombic electron capture are also illustrated.Graphical abstract

Interatomic coulombic electron capture in slow atomic collisions

In the process of radiative recombination (RR)/attachment the capture of a free electron by an atomic species A proceeds via photoemission. In the so called interatomic coulombic electron capture, a capture of a free electron by A proceeds via its interaction with a neighboring species B that results in electron emission from the latter. This process can outperform RR/attachment when A and B form a (weakly) bound system. Here we consider this process in slow collisions between A and B and show that also in this dynamical situation it can successfully compete with RR/attachment if the density of species B is sufficiently large.

Interatomic Coulombic electron capture from first principles

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to an ion, atom or molecule by transferring the excess energy to a neighboring species. Absolute cross sections are necessary to evaluate the relative importance of this process. In this work, we employ the R-matrix method to compute ab initio these cross sections for a singly charged neon ion embedded in small helium clusters. Our results show that the ICEC cross sections are several orders of magnitude higher than anticipated and dominate other competing processes. Electron energy loss spectra on an absolute scale are provided for the Ne+@He20 cluster. Such spectra exhibit an unambiguous signature of the ICEC process. The finding is expected to stimulate experimental observations.

Electron-correlation driven capture and release in double quantum dots

We recently predicted that the interatomic Coulombic electron capture (ICEC) process, a long-range electron correlation driven capture process, is achievable in gated double quantum dots (DQDs). In ICEC an incoming electron is captured by one quantum dot (QD) and the excess energy is used to remove an electron from the neighboring QD. In this work we present systematic full three-dimensional electron dynamics calculations in quasi-one dimensional model potentials that allow for a detailed understanding of the connection between the DQD geometry and the reaction probability for the ICEC process. We derive an effective one-dimensional approach and show that its results compare very well with those obtained using the full three-dimensional calculations. This approach substantially reduces the computation times. The investigation of the electronic structure for various DQD geometries for which the ICEC process can take place clarify the origin of its remarkably high probability in the presence of two-electron resonances.

Interatomic Coulombic electron capture in atomic, molecular, and quantum dot systems

The interatomic Coulombic electron capture (ICEC) process has recently been predicted theoretically for clusters of atoms and molecules. For an atom A capturing an electron e() it competes with the well known photorecombination, because in an environment of neutral or anionic neighboring atoms B, A can transfer its excess energy in the ultrafast ICEC process to B which is then ionized. The cross section for e() + A + B → A − + B + + e( � ) has been obtained in an asymptotic approximation based on scattering theory for several clusters (1, 2). It was found that ICEC starts dominating the PR for distances among participating species of nanometers and lower. Therefore, we believe that the ICEC process might be of importance in the atmosphere, in biological systems, plasmas, or in nanostructured materials. As an example for the latter, ICEC has been investigated by means of electron dynamics in a model potential for semiconductor double quantum dots (QDs) (3). In the simplest case one QD captures an electron while the outgoing electron is emitted from the other. The reaction probability for this process was found to be relatively large.

Environment assisted electron capture

Electrons scattering off isolated atoms or positive atomic ions may be captured in the non-resonant photorecombination or resonant dielectronic recombination processes with the excess energy carried off by a photon. We demonstrate that in a medium a new interatomic process becomes important, where in the electron capture by an atom or atomic ion the energy is transferred to a neigbouring species ionising it. We discuss different manifestations of this interatomic Coulombic electron capture and estimate its efficiency for a number of realistic systems.

Interatomic Coulombic electron capture

In a previous publication [K. Gokhberg and L. S. Cederbaum, J. Phys. B 42, 231001 (2009)] we presented the interatomic Coulombic electron capture process---an efficient electron capture mechanism by atoms and ions in the presence of an environment. In the present work we derive and discuss the mechanism in detail. We demonstrate thereby that this mechanism belongs to a family of interatomic electron capture processes driven by electron correlation. In these processes the excess energy released in the capture event is transferred to the environment and used to ionize (or to excite) it. This family includes the processes where the capture is into the lowest or into an excited unoccupied orbital of an atom or ion and proceeds in step with the ionization (or excitation) of the environment, as well as the process where an intermediate autoionizing excited resonance state is formed in the capturing center which subsequently deexcites to a stable state transferring its excess energy to the environment. Detailed derivation of the asymptotic cross sections of these processes is presented. The derived expressions make clear that the environment assisted capture processes can be important for many systems. Illustrative examples are presented for a number of model systems for which the data needed to construct the various capture cross sections are available in the literature.

Aerosol penetration properties of an electret filter with submicron aerosols with various operating factors

This study was undertaken to determine the effects of using an electret filter on aerosol penetration. Various factors, including particle size (0.05 to 0.5 μ m), aerosol charge state (neutral and single charge), face velocity (0.1, 0.3, 0.5 and 1.0 m/s), and relative humidity (RH 30% and RH 70%), were examined to assess their effects on aerosol collection characteristics. The results presented here demonstrate that the electric fields of the electret and discharged filter were −1.53 × 104 and −1.3 × 102 (V/m). The penetration through the electret filter with singly charged aerosol and neutral aerosol ranged from 0.4% to 13% and 14% to 29%, respectively. According to these results, the coulombic capture force was dominant for the smaller aerosol and the dielectrophoretic capture mechanism was considered important for the larger aerosol. The level of penetration through the electret filter increased with increasing face velocity and relative humidity. The temperature did not affect the penetration through the electret. Furthermore, from the regression analysis conducted during the operating conditions of this work, the aerosol charge was shown to exert the greatest influence on aerosol penetration.

Filtration characteristics of a fibrous filter pretreated with anionic surfactants for monodisperse solid aerosols

This study elucidates the effects of using an anionic surfactant-pretreated filter as an electret filter on the aerosol penetration. Anionic surfactants (sodium oleate, SO; sodium dodecyl sulfate, SDS) were used to pretreat polypropylene fibrous filters to make them negatively charged. Various factors, including the particle size (0.05– 0.5 μ m ), the aerosol charge state (Boltzmann-equilibrium charge, neutral and singly charge), the face velocity (0.1, 0.3, 0.5 and 1.0 m / s ), the species of aerosol (sodium chloride, NaCl and aluminum oxide, Al 2 O 3 ), the relative humidity (RH 30% and 70%) and the concentration of surfactant (0.01, 0.05 and 0.08 M) were considered to evaluate their effects on the aerosol collection characteristics. Experimental results from our study demonstrate that the aerosol penetrations through anionic surfactant-pretreated filters (ASPFs) were lower than through the untreated filter. The 0.3 μ m -aerosol penetrations through an untreated, 0.01, 0.05 and 0.08 M SO-pretreated filters with Boltzmann-equilibrium charged aerosol were about 78%, 53%, 45% and 40%. The penetration of 0.3- μ m aerosol of 0.01, 0.05 and 0.08 M SDS-pretreated filters with Boltzmann-equilibrium charged aerosol were, 62%, 58% and 51%. Pretreating the filter with anionic surfactant did not change the structure of the filter and the mechanical capture force. The electric field measured by an electrofieldmeter of the ASPFs was larger than that of untreated filter obviously. These findings imply that pretreatment with anionic surfactant made the filters charged. The surface charge of the ASPFs increased with the anionic-surfactant concentration. The increase of the collection efficiency of the ASPFs is in the range of 5–30%, and that depends on the aerosol size and surfactant concentration. Comparing between the aerosol penetration through the ASPFs with singly charged aerosol and that through the untreated filter with neutral aerosol indicates that the penetration through the ASPFs with singly charged aerosol decreases by a factor of 1.3–12.5. In contrast with the penetration through the ASPFs to that through the untreated filter with neutral aerosol, the penetration reduction factor of the ASPFs is in the range of 1.1–1.9. As the results shown, the Coulombic capture force is dominant for the smaller aerosol ( < 0.1 μ m ) and the dielectrophoretic capture mechanism works at larger aerosol size ( > 0.2 μ m ) . Additionally, the penetrations through the ASPFs increased with the face velocity. ASPFs performed better when the tested aerosol had a larger dielectric constant ( k ) such as the penetration through 0.05 M SO-pretreated filter with 0.3 μ m Al 2 O 3 aerosol ( k = 9.6 ) was 47% and that with NaCl aerosol ( k = 5.9 ) was about 53%. RH has no effect on the aerosol penetration through the ASPFs.

Experimental Study of Electrostatic Capture Mechanisms in Commercial Electret Filters

ABSTRACT Electret filters are widely used in applications requiring high-filtration efficiency and low-pressure drop. These filters rely on electrostatic particle capture mechanisms in addition to the conventional mechanical capture mechanisms. This study reports experimental data collected on the performance of three types of commercially available fibrous electret filters: corona-charged fibrillated split-fiber media (type A), triboelectrically charged mixed-fiber media (type B) and corona-charged meltblown media (type C). The filtration efficiency of these filters was measured as a function of particle size (i.e., 0.05 to 0.5 μm) and charge state (i.e., singly charged and neutral) for two face velocities. The same experiments also were performed on discharged electret filters to obtain information on the pure mechanical capture mechanisms. The magnitude of the effective surface electric field for each tested electret filter was estimated from a simplified Coulombic capture model, giving information on ...

Recombination in germanium: Voltage-decay experiments on induced-junction devices and validity of the SRH model

Measurements of open-circuit voltage decay following the application of a dark forward-current pulse have been made on induced pn-junction devices in moderately-doped germanium. Minority-carrier lifetime has been measured over a wide temperature range, as well as over a wide range of injection level. All of the results are accurately explained by the SRH model with a single acceptorlike trap level at an energy 0.128 ± 0.008 eV below the conduction band edge, i.e. above the equilibrium Fermi level in the n-type material. The dependence of the hole capture cross section on temperature is further quantitatively explained by the theory for coulombic capture of Rose, without adjustable parameters.

Concerning the dependence of photoconductivity on photogeneration rate in intrinsic amorphous semiconductors

A simplified treatment of recombination, quasi-Fermi levels and photoconductivity in pure (undoped) amorphous semiconductors is presented assuming a symmetrical Cohen-Fritzsche-Ovshinsky distribution of localized bandgap states, with donorlike states of density decreasing exponentially with increases in energy from the valence band (mobility) edge: and acceptorlike states of exponentially decreasing density with decreases in energy from the conduction band edge. Recombination proceeds by Shockley-Read-Hall capture processes, but due account is taken of the dependence of cross sections on whether Coulombic or neutral capture is present in each case The ratio of photoconductivity to dark conductivity ( σ ph/ σ dark) is calculated for parameters expected to correspond to (hdrogenated) intrinsic amorphous silicon. The dependence of this ratio upon the photogeneration rate ( G ph) involves a single adjustable parameter, which represents the rate of decay of the density of gap states with energy away from the band edges. Comparison of experimental data for σ ph/ σ dark vs. G ph with this model provides for an estimate of the gap-state energy distribution.

Electron Injection Studies of Radiation Induced Positive Charge in MOS Devices

Avalanche injection and internal photo-emission techniques have been used to study the capture of electrons by positive charges introduced into the oxide layers of MOS capacitors. These two techniques have been used to study the positive charge in Al gate and poly-Si gate capacitors. The electron capture crosssection of this charge has been found to depend on the composition of the interface; positive charge at the Si interface tends to have a coulombic electron capture cross-section (~ 4×10-13cm2), while that at the alumninum interface has a non-coulombic electron capture crosssection (≤ 10-14cm2). The location of the positive charge induced by radiation under positive or negative bias has been determined in a completely non-destructive manner by photocurrent-voltage experiments. It has been found that under a positive irradiation bias, positive space charge accumulates within ~ 50 A of the Si-SiO2 interface, while under a negative bias, the space charge is within ~ 50 Å of the Al-SiO2 interface. In both cases there is evidence for some charge at the other interface introduced by the irradiation.