Effective Single-electron Research Articles

Doubly differential cross sections for ionisation in proton–helium collisions at intermediate energies: energy and angular distribution of emitted electrons

Using the two-centre wave-packet convergent close-coupling approach, we continue our study of the proton–helium collision system. This method uses a correlated two-electron wave function to describe the helium target and discretises the continuum using wave-packet pseudostates. The cross section differential in the electron-emission energy and emission angle is calculated for incident-projectile energies in the intermediate range from 70 to 300 keV, where coupling between various channels and electron–electron correlation effects are important. We also apply an alternative, simpler approach that reduces the target to an effective single-electron system. Overall, the present results from both methods agree well with the available experimental data. This positions both implementations of the two-centre wave-packet convergent close-coupling approach well to further study other doubly differential, as well as fully differential, cross sections of single ionisation in proton–helium collisions.Graphical abstract

NHC-Catalyzed Synthesis of α-Sulfonyl Ketones via Radical-Mediated Sulfonyl Methylation of Aldehydes

An N-heterocyclic carbene (NHC)-catalyzed facile assembly of α-sulfonyl ketones has been successfully developed through a radical-mediated sulfonyl methylation of readily available aldehydes. This protocol involves the effective single-electron transfer reduction of α-iodosulfones by NHC-bound Breslow intermediates, thus allowing a subsequent radical-radical coupling to afford the target compounds. Moreover, the catalytic system was found to be equally effective for difunctionalization of styrenes and 1,3-enynes via a three-component radical relay process.

Wave-packet continuum discretization approach to He2+−He collisions

The two-center wave-packet convergent close-coupling method is generalized to multiply charged ion collisions with helium and applied to the ${\mathrm{He}}^{2+}\text{\ensuremath{-}}\mathrm{He}$ scattering problem. The approach is applicable in a wide range of collision energies including low and intermediate energies, where coupling between various reaction channels and electron exchange between the fragments in the rearrangement channel are important. The target structure is treated using the configuration-interaction method within the frozen-core approximation, where one of the electrons of the target is assumed to stay in the ground state throughout the collision. This accounts for the indistinguishability and the correlations between the electrons of the target. We also use a simpler alternative method that is based on an effective one-electron target description that neglects the electron-electron correlations. In both methods, the continuum of the target atom and the hydrogenlike atom formed after electron capture by the projectile is discretized using the wave-packet approach. We present cross sections for total and state-selective electron capture, excitation, and single ionization of the target. The results are provided for the incident energies from 10 keV/u to 5 MeV/u, where one-electron processes are expected to dominate. Particular attention is focused on the intermediate-energy region where substantial deviations between various theoretical results are found. However, overall, the present results are in good agreement with experimental data and other calculations, where available. We demonstrate that both effective single-electron and two-electron methods can provide a realistic picture of all single-electron scattering processes taking place in ${\mathrm{He}}^{2+}\text{\ensuremath{-}}\mathrm{He}$ collisions in terms of the total cross sections. This creates a reliable platform for modeling differential scattering and ionization in this four-body system.

Proton-helium collisions at intermediate energies: Singly differential ionization cross sections

We investigate the four-body proton-helium scattering problem using the two-center wave-packet convergent close-coupling approach. The approach uses a correlated two-electron wave function for the helium target. The continuum is discretized using wave-packet pseudostates. Calculations of ionization cross sections differential in the electron emission energy, in the emission angle, as well as in the scattered-projectile angle are performed in the intermediate energy region where coupling between various channels and electron-electron correlation effects are important. The results agree well with experimental data, where available. Moreover, our calculations reveal an interesting interplay between direct ionization and electron capture into the continuum. In particular, we demonstrate that the ionization cross section differential in the angle of the ejected electron is dominated by electron capture into the continuum for ejection into small angles, while ejection into large angles is purely due to direct ionization. It is concluded that the two-center wave-packet convergent close-coupling approach can provide accurate singly differential cross sections for ionization in proton-helium collisions. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. Somewhat unexpectedly, the results of the effective one-electron method exhibit a very good level of agreement with the full two-electron ones for all three singly differential cross sections. Therefore, we also conclude that, at least for the purpose of calculating the singly differential cross sections for single ionization of helium, an effective one-electron treatment of the target suffices.

Effective single-electron treatment of ion collisions with multielectron targets without using the independent-event model

In ion-atom collisions involving multielectron targets the role of inner-shell electrons becomes important with increasing collision energy. Usually, processes involving the outer-shell and inner-shell electrons are treated using an independent-event model. However, the independent-event model becomes impractical when the number of shells is more than two. We develop an effective single-electron approach to ion collisions with multielectron atomic targets that overcomes this difficulty by treating all the atomic electrons on an equal footing. The approach allows the calculation of single-ionization and single-electron transfer cross sections by including the excitation of any one of the inner- or outer-shell target electrons. Accordingly, the approach does not differentiate which one of the many target electrons is captured or ionized. This is a unique feature of the proposed approach. The ground-state wave function for the target atom obtained in the multiconfiguration Hartree-Fock approach is used to calculate the probability density for the whole atom. The latter is then averaged over the spatial coordinates and spin variables of all of the target electrons except for the position of one electron from the nucleus. The obtained single-electron probability density is then used to derive a pseudopotential describing the interaction of one electron with an effective field produced by the target nucleus and the other electrons. The procedure reduces the many-body Schr\"odinger equation governing the collision system effectively into a three-body one. The reduced Schr\"odinger equation is solved using the two-center wave-packet convergent close-coupling approach. As an illustrative example we calculate integrated cross sections for single-electron transfer and ionization in proton collisions with lithium, sodium, and potassium. The results obtained agree very well with available experimental data for electron transfer. However, we find significant discrepancies with experiment for single ionization of Na and K, which warrant further experimental and theoretical investigations.

Differential study of proton-helium collisions at intermediate energies: Elastic scattering, excitation, and electron capture

In a series of papers we present a comprehensive investigation of the four-body proton-helium differential scattering problem using the two-center wave-packet convergent close-coupling approach in the intermediate energy region where coupling between various channels is important. The approach uses correlated two-electron wave functions for the helium target. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. In this paper we present calculations of angular differential cross sections for elastic-scattering, target excitation, and electron-capture processes. The results of the two-electron and effective single-electron methods exhibit a good level of agreement. They also agree well with available experimental data. It is concluded that both versions of the wave-packet convergent close-coupling approach are capable of providing a realistic differential picture of all interdependent and interconnected binary processes taking place in proton-helium collisions at intermediate energies.

Electronic structure and optical properties of quantum crystals from first principles calculations in the Born-Oppenheimer approximation.

We develop a formalism to accurately account for the renormalization of the electronic structure due to quantum and thermal nuclear motions within the Born-Oppenheimer approximation. We focus on the fundamental energy gap obtained from electronic addition and removal energies from quantum Monte Carlo calculations in either the canonical or grand-canonical ensembles. The formalism applies as well to effective single electron theories such as those based on density functional theory. We show that the electronic (Bloch) crystal momentum can be restored by marginalizing the total electron-ion wave function with respect to the nuclear equilibrium distribution, and we describe an explicit procedure to establish the band structure of electronic excitations for quantum crystals within the Born-Oppenheimer approximation. Based on the Kubo-Greenwood equation, we discuss the effects of nuclear motion on optical conductivity. Our methodology applies to the low temperature regime where nuclear motion is quantized and, in general, differs from the semi-classical approximation. We apply our method to study the electronic structure of C2/c-24 crystalline hydrogen at 200 K and 250 GPa and discuss the optical absorption profile of hydrogen crystals at 200 K and carbon diamond at 297 K.

Iron-Catalyzed Meerwein Carbooxygenation of Electron-Rich Olefins: Studies with Styrenes, Vinyl Pyrrolidinone, and Vinyl Oxazolidinone.

The arylative oxygenation of the electron-rich olefins styrene, α-methylstyrene, vinyl pyrrolidinone, and vinyl oxazolidinone was accomplished using arenediazonium salts and catalytic amounts of FeSO4 in an effective single electron transfer radical process. A broad range of aryldiazonium salts was tolerated using water, methanol, or their combination with acetonitrile to furnish the corresponding carbohydroxylated and carbomethoxylated products (42 examples), including functionalized dihydroisocoumarin and dihydrobenzofuran systems in good to excellent yields (up to 88%). The protocols developed for the Fe(II)-catalyzed carbohydroxylation were also compared to Ru(II) and Ir(III) photoredox carbooxygenations of these electron-rich olefins. The Fe(II)-catalyzed process proved to be highly competitive compared to the photoredox and the uncatalyzed processes. The proposed mechanism for the Fe(II)-catalyzed reactions involves the synergic combination with an effective Fe+2/Fe+3 redox system and a radical polar crossover mechanism featuring an unprecedented capture of the reactive N-acyliminium in the case of vinyl pyrrolidinone and vinyl oxazolidinone.

Attosecond Dynamics of sp-Band Photoexcitation.

We report measurements of the temporal dynamics of the valence band photoemission from the magnesium (0001) surface across the resonance of the Γ[over ¯] surface state at 134eV and link them to observations of high-resolution synchrotron photoemission and numerical calculations of the time-dependent Schrödinger equation using an effective single-electron model potential. We observe a decrease in the time delay between photoemission from delocalized valence states and the localized core orbitals on resonance. Our approach to rigorously link excitation energy-resolved conventional steady-state photoemission with attosecond streaking spectroscopy reveals the connection between energy-space properties of bound electronic states and the temporal dynamics of the fundamental electronic excitations underlying the photoelectric effect.

Interaction-Dependent Interfacial Charge-Transfer Behavior in Solar Water-Splitting Systems.

Dual-band-gap systems are promising for solar water splitting due to their excellent light-harvesting capability and high charge-separation efficiency. However, a fundamental understanding of interfacial charge-transfer behavior in the dual-band-gap configuration is still incomplete. Taking CdS/reduced graphene oxide (CdS/RGO) nanoheterojunctions as a model solar water splitting system, we attempt here to highlight the interaction-dependent interfacial charge-transfer behavior based on both experimental observations and theoretical calculations. Experimental evidence points to charge transfer at the CdS-RGO interface playing a dominant role in the photocatalytic hydrogen production activity. By tuning the degree of reduction of RGO, the interfacial interaction, and, thereby, the charge transfer can be controlled at the CdS-RGO interface. This observation is supported by theoretical analysis, where we find that the interfacial charge transfer is a balance between the effective single-electron- and hole-transfer probability and the surface free electron and hole concentration, both of which are related to the surface potential and tailored by interfacial interaction. This mechanism is applicable to all systems for solar water splitting, providing a useful guidance for the design and study of heterointerfaces for high-efficiency energy conversion.

Effects of low pressure plasma irradiation on electrical resistivity of perovskite oxide Eu1-xSrxMnO3

The use of perovskite type compounds, has gained a tremendous applications in various fields like memory storage devices, bolometers, superconductors, solar cells, etc. One of the very important uses of perovskite material is in the construction of bolometers. Bolometers are extensively used in fusion devices for temperature measurement and also in space application. In this paper we have presented the experimental behaviour of the electrical resistivity of the perovskite oxide Eu1-x SrxMnO3 after plasma exposure for different composition. It is found that the compound Eu1-x SrxMnO3 behaves as an insulator for all the values of x, since the effective single electron bandwidth is very narrow. The observed decrease in the resistivity below TMI may be due to the lack of delocalized carriers that contributes for the metallic nature of the sample.

Kinetic theory for strongly coupled Coulomb systems.

The calculation of dynamical properties for matter under extreme conditions is a challenging task. The popular Kubo-Greenwood model exploits elements from equilibrium density-functional theory (DFT) that allow a detailed treatment of electron correlations, but its origin is largely phenomenological; traditional kinetic theories have a more secure foundation but are limited to weak ion-electron interactions. The objective here is to show how a combination of the two evolves naturally from the short-time limit for the generator of the effective single-electron dynamics governing time correlation functions without such limitations. This provides a theoretical context for the current DFT-related approach, the Kubo-Greenwood model, while showing the nature of its corrections. The method is to calculate the short-time dynamics in the single-electron subspace for a given configuration of the ions. This differs from the usual kinetic theory approach in which an average over the ions is performed as well. In this way the effective ion-electron interaction includes strong Coulomb coupling and is shown to be determined from DFT. The correlation functions have the form of the random-phase approximation for an inhomogeneous system but with renormalized ion-electron and electron-electron potentials. The dynamic structure function, density response function, and electrical conductivity are calculated as examples. The static local field corrections in the dielectric function are identified in this way. The current analysis is limited to semiclassical electrons (quantum statistical potentials), so important quantum conditions are excluded. However, a quantization of the kinetic theory is identified for broader application while awaiting its detailed derivation.

LED-Assisted Degradation of Aromatic Organics Using Cu2O Photocatalysts

ABSTRACTIn this work, we successfully synthesized rhombic dodecahedral Cu2O nanocrystals with a size of 300 – 400 nm using a facile hydrothermal method. The as-prepared photocatalyst with narrow bandgap is activated using low power visible LED light sources and shows high efficiency in degrading aromatic organic compounds including toluene and chlorobenzene. The OH substitution leads to oxidation/ionization potential drops while the nature of the p-type Cu2O contributes to an effective single electron transfer reaction.

Intensities of K-X-ray satellite and hypersatellite target radiation in Bi83+-Xe @70 MeV/u collisions

Non-perturbative calculations of the relativistic quantum dynamics of electrons in the Bi83+-Xe collisions at 70AMeV are performed. A method of calculation employs an independent particle model with effective single-electron Dirac-Kohn-Sham operator. Solving of the single-electron equations is based on the coupled-channel approach with atomic-like Dirac-Sturm-Fock orbitals, localized at the ions (atoms). Special attention is paid to the inner-shell processes. Intensities of the K satellite and hypersatellite target radiation are evaluated. The role of the relativistic effects is studied.

Static field ionization rates for multi-electron atoms and small molecules

We present an application of the hybrid anti-symmetrized coupled channels approach to compute static field ionization rates for multi-electron atoms (He, Ne, Ar) and small molecules (H2, N2, CO). While inert gas atoms behave as effective single electron systems, molecules exhibit multi-electron effects in the form of core polarization. It is shown that at moderate field strengths, these effects can be modeled to about 10% accuracy using a few (5–6) channel ansatz. In the case of the CO molecule, description of core polarization is essential for the correct prediction of the maximum ionization direction and our converged results are in good agreement with the experimental measurements.

Non-Markovian Effects in the Spin Transfer Dynamics in Diluted Magnetic Semiconductors due to Excitation in Proximity to the Band Edge

The non-Markovian effects in the spin dynamics in diluted magnetic semiconductors found in quantum kinetic calculations can be reproduced very well by a much simpler effective single electron theory, if a finite memory is accounted for. The resulting integro-differential equation can be solved by a differential transform method, yielding the Taylor series of the solution. From the comparison of both theories it can be concluded that the non-Markovian effects are due to the spectral proximity of the excited electrons to the band edge.

Dissipative time-dependent quantum transport theory

A dissipative time-dependent quantum transport theory is developed to treat the transient current through molecular or nanoscopic devices in presence of electron-phonon interaction. The dissipation via phonon is taken into account by introducing a self-energy for the electron-phonon coupling in addition to the self-energy caused by the electrodes. Based on this, a numerical method is proposed. For practical implementation, the lowest order expansion is employed for the weak electron-phonon coupling case and the wide-band limit approximation is adopted for device and electrodes coupling. The corresponding hierarchical equation of motion is derived, which leads to an efficient and accurate time-dependent treatment of inelastic effect on transport for the weak electron-phonon interaction. The resulting method is applied to a one-level model system and a gold wire described by tight-binding model to demonstrate its validity and the importance of electron-phonon interaction for the quantum transport. As it is based on the effective single-electron model, the method can be readily extended to time-dependent density functional theory.

Site-directed electronic tunneling in a dissipative molecular environment

The ability to control electronic tunneling in complex molecular networks of multiple donor/acceptor sites is studied theoretically. Our past analysis, demonstrating the phenomenon of site-directed transport, was limited to the coherent tunneling regime. In this work we consider electronic coupling to a dissipative molecular environment including the effect of decoherence. The nuclear modes are classified into two categories. The first kind corresponds to the internal molecular modes, which are coupled to the electronic propagation along the molecular bridges. The second kind corresponds to the external solvent modes, which are coupled to the electronic transport between different segments of the molecular network. The electronic dynamics is simulated within the effective single electron picture in the framework of the tight binding approximation. The nuclear degrees of freedom are represented as harmonic modes and the electronic-nuclear coupling is treated within the time-dependent Redfield approximation. Our results demonstrate that site-directed tunneling prevails in the presence of dissipation, provided that the decoherence time is longer than the time period for tunneling oscillations (e.g., at low temperatures). Moreover, it is demonstrated that the strength of electronic coupling to the external nuclear modes (the solvent reorganization energy) controls the coherent intramolecular tunneling dynamics at short times and may be utilized for the experimental control of site-directed tunneling in a complex network.

Non-Hermitian quantum mechanics versus the conventional quantum mechanics: Effect of the relative phasing of bichromatic fields on high-order harmonic generation

We perform a dynamical symmetry analysis (DSA) of the high-order harmonic generation (HHG) spectrum of an atom interacting with a bichromatic laser field. Within the framework of the conventional Hermitian quantum mechanics (QM), the HHG spectrum calculated using a single Floquet state, or any finite number of Floquet states, is invariant under the inversion of the relative phase of the two-frequency components, $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{-}\ensuremath{\phi}$. The asymmetry with respect to the phase inversion seen in the simulated HHG spectra is obtained in the conventional QM only when the Floquet spectrum is continuous and ionization is taken into consideration. However, when the Hamiltonian is complex scaled the description is different. Even a single eigenstate of the complex scaled Floquet operator is enough to describe the breaking of the $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{-}\ensuremath{\phi}$ symmetry in the HHG spectra. We find that there is a direct correlation between the strength of the asymmetry with respect to the relative phase inversion and the magnitude of the ionization rate. For illustration purposes, the DSA is accompanied by the results obtained for a one-dimensional effective single-electron model Hamiltonian mimicking xenon atom interacting with strong laser field.

Conductance anomalies and the extended Anderson model for nearly perfect quantum wires

Anomalies near the conductance threshold of nearly perfect semiconductor quantum wires are explained in terms of singlet and triplet resonances of conduction electrons with a single weakly-bound electron in the wire. This is shown to be a universal effect for a wide range of situations in which the effective single-electron confinement is weak. The robustness of this generic behavior is investigated numerically for a wide range of shapes and sizes of cylindrical wires with a bulge. The dependence on gate voltage, source-drain voltage and magnetic field is discussed within the framework of an extended Hubbard model. This model is mapped onto an extended Anderson model, which in the limit of low temperatures is expected to lead to Kondo resonance physics and pronounced many-body effects.