Two-center Process Research Articles

Two-center dielectronic recombination in slow atomic collisions

A free electron can form a bound state with an atomic center $A$ upon photo emission (radiative recombination). In the presence of a neighboring atom $B$, such a bound state can, under certain conditions, be also formed via resonant transfer of energy to $B$, with its subsequent relaxation through radiative decay (two-center dielectronic recombination). This two-center process is very efficient in the 'static' case where $A$ and $B$ form a weakly bound system, dominating over single-center radiative recombination up to internuclear distances as large as several nanometers. Here we study its dynamic variant in which recombination occurs when a beam of species $A$ collides with a gas of atoms $B$ and show that, even though the average distance between $A$ and $B$ in collisions is orders of magnitude larger than the typical size of a bound system, the two-center recombination can still outperform the single-center radiative recombination.

Generation Kinetics of Nonequilibrium Charge Carriers in Crystals with Deep Impurities Involving Two-Center Transitions between Band and Impurity States

A new and efficient mechanism of nonlinear photoexcitation of a transparent crystal with deep impurity centers is proposed. It is hypothesized that the energy of a quantum of light is smaller than the energy gap between the bottom of the conduction band and the impurity level, but is larger than the gap between the impurity level and the top of the valence band. The kinetics of nonlinear cascade generation of nonequilibrium charge carriers is considered taking into account two-center processes in which the energy transfer and the photon absorption occur in one and the same elementary event. The dependences of the quasi-equilibrium concentrations of nonequilibrium charge carriers in the bands and of the occupancy of impurity states on the laser-radiation intensity are obtained. It is shown that the generation process of nonequilibrium electron–hole pairs is of a threshold nature. Depending on the concentration of impurity centers, the threshold intensities can be ~105–107 W/cm2, while the setting time of the quasi-equilibrium occupancies of electronic states is ~10–0.1 ns.

Theoretical Study of 1,3-Dipolar Cycloadditions Regioselectivity of Benzyl Azide with Glycosyl-O Acetylene Using Density Functional Theory (DFT)

A theoretical study of 1,3-cycloaddition has been carried out using density functional theory (DFT) methods at the B3LYP/6-31G* level. The regioselectivity of the reaction have been clarified through different theoretical approaches: Case of a Two-Center Process (Domingo approach), HSAB principle (Gazquez and Mendez approach), and the activation energy calculations. The analysis of results shows that the reaction takes place along concerted asynchronous mechanism and the isomer meta is favored, in agreement with the experiment results. DOI: http://dx.doi.org/10.17807/orbital.v9i5.1017

Self-Scavenging of Electrons in Fe(CO)5 Aggregates Deposited on Argon Nanoparticles

We report a dramatic difference in electron-induced decomposition of aggregates of iron pentacarbonyl [Fe(CO)5] when compared to its isolated form. The aggregates are deposited on free argon nanoparticles (clusters) with the mean size of several hundreds of atoms and interact with an electron beam of 0–25 eV energy. The negatively charged products are analyzed mass-spectroscopically. These products of various Fe/CO composition always desorb from the cluster surface; no mixed anion–Ar fragments are detected. While anions containing one iron atom show strong resonances at low electron energies (<1 eV), the yields of more complex anions show broad bands at electron energies above 5 eV. We postulate a two-center process that involves electronic excitation of one aggregate constituent and attachment of the electron with low residual energy to another aggregate constituent. The dimer anion formation appears to determine completely the aggregation of larger complexes. We draw implications of the present findings...

Multiple light scattering and optomechanical forces

When off-resonant light travels through a transparent medium, light scattering is the primary optical process to occur. Multiple-particle events are relatively rare in optically dilute systems: scattering generally takes place at individual atomic or molecular centers. Several well-known phenomena result from such single-center interactions, including Rayleigh and Raman scattering, and the optomechanical forces responsible for optical tweezers. Other, less familiar effects may arise in circumstances where throughput radiation is able to simultaneously engage with two or more scattering sites in close, nanoscale, proximity. Exhibiting the distinctive near-field electromagnetic character, inter-particle interactions such as optical binding and a variety of inelastic bimolecular processes can then occur. Although the theory for each two-center process is well established, the connectivity of their mechanisms has not received sufficient attention. To address this deficiency, and to consider the issues that ensue, it is expedient to represent the various forms of multi-particle light scattering in terms of transitions between different radiation states. The corresponding quantum amplitudes, registering the evolution of photon trajectories through the material system, can be calculated using the tools of quantum electrodynamics. Each of the potential outcomes for multi-particle scattering generates a set of amplitudes corresponding to different orderings of the constituent photon-matter interactions. Performing the necessary sums over quantum pathways between radiation states is expedited by a state-sequence development, this formalism also enabling the identification of intermediate states held in common by different paths. The results reveal the origin and consequences of linear momentum conservation, and they also offer new insights into the behavior of light between closely neighboring scattering events.

One and two-center processes in high-order harmonic generation in diatomic molecules: Influence of the internuclear separation

We analyze the influence of different recombination scenarios, involving one or two centers, on high-order harmonic generation (HHG) in diatomic molecules, for different values of the internuclear separation. We work within the strong-field approximation, and employ modified saddle-point equations, in which the structure of the molecule is incorporated. We find that the two-center interference patterns, attributed to high-order harmonic emission at spatially separated centers, are formed by the quantum interference of the orbits starting at a center $C_{j}$ and finishing at a different center $C_{\nu }$ in the molecule with those starting and ending at a same center $C_{j}.$ Within our framework, we also show that contributions starting at different centers exhibit different orders of magnitude, due to the influence of additional potential-energy shifts. This holds even for small internuclear distances. Similar results can also be obtained by considering single-atom saddle-point equations and an adequate choice of molecular prefactors.

Two-center electron-electron interactions in collisions of fast Ne 7+ and Ne 6+ ions with gas atoms

Experimental and thoeretical studies of the two-center electron-electron interaction occurring during energetic ion-atom collisions are presented. Experiments were performed using the accelerator facility VICKSI at HMI, Berlin. Electron spectra from Auger and Coster-Kronig transitions following the excitation of a highly charged neon projectile have been studied by zero-degree electron spectroscopy. Various target atoms with atomic numbers ranging from 1–18 were used as exciting partners. We applied both impact parameter treatments and the plane wave Born approximation to describe screening effects and dielectronic two-center processes (“antiscreening”). We found that the 1s-2 l excitation of the Ne 7+ projectile at 8.5 MeV/u impact energy is mainly due to the screened target nucleus. In contrary, two-center processes are likely to be dominant in the 2s- nl excitation of the 6 MeV/u Ne 6+ projectile. The 2s- nl cross sections are roughly proportional to the number of electrons per target atom. This fact suggests that, in disagreement with first order theories, the 2s- nl excitation is significantly governed by the two center electron-electron interaction.

Clusters containing carbene ligands. The formation of a carbene ligand by carbon-hydrogen activation of a .pi.-complexed iminium ligand

Recently, there has been a flurry of interest in the nature of the activation of olefinic CH bonds by metal complexes. Studies have shown that the barriers to the activation of CH bonds of ..pi..-complexed olefins by mononuclear metal complexes are very large. Indeed, evidence has been presented which demonstrates that olefinic CH activation by certain mononuclear metal complexes occurs without precoordination of the olefin. By contrast olefininc CH activation by polynuclear metal complexes and on surfaces is a relatively facile process. In these cases, it is believed that the activation step is a two-center process in which the olefin is coordinated to one metal atom and the CH cleavage takes place by interaction at a vacant coordination site on a neighboring metal atom. Iminium ions, H/sub 2/C=NR/sub 2//sup +/, are isoelectronic to olefins, and they are known to form ..pi..-bonded complexes analogous to ..pi..-olefin complexes. In this communication is described the first example of a C-H activation of a ..pi..-complexed iminium ion and its conversion into a dialkylaminocarbene ligand.