One-electron Model Research Articles

Co 2p–3d resonant photoemission spectroscopy of CoSb 3

High-resolution synchrotron radiation photoemission spectroscopy (HRPES) has been performed in the Co 2p core excitation region on thermoelectric materials of CoSb 3 with the skutterudite crystal structure. At the Co 2p 3/2–3d peak, a photoionization cross-section of the Co 3p states becomes extremely larger than that at the Co 3p–3d peak. Co 2p3d3d, 2p3p3d, 2p3s3d, 2p3p3p and 2p3s3p Auger structures and plasmon satellites are recognized. It has been confirmed that the resonant photoemission process is less important in CoSb 3. The hν-dependence of the spectra suggests that the Co 3d electrons in CoSb 3 can be treated as the one-electron model rather than the many-body particle model, which is consistent with the covalency of CoSb 3.

Temperature-induced barium de-trapping from a double-well potential inBa6Ge25

The crystal structure of barium–germanium clathrateBa6Ge25 was studied using neutron powder diffraction in the temperaturerange 20–300 K. The compound was found to be cubic (spacegroup P4132) in the entire temperature range. However, the fully ordered model of the crystal structure(no split sites) is marginal at room temperature, and clearly fails at low temperature. Amuch better description of the crystal structure below 250 K is given in terms of two splitBa sites, with random occupancies, for two out of three types of cages present in theBa6Ge25 structure. The Ba atoms were found to interact strongly with the Ge host. The separationof the split Ba sites grows with decreasing temperature, with a sudden increase on coolingthrough the 200–250 K temperature range, accompanied by an expansion of the entirecrystal structure. The ‘locking-in’ of Ba atoms into split sites was originally suggested byPaschen et al (2002 Phys. Rev. B 65 134435) as a plausible scenario behind anomalies in thetransport and magnetic properties. Our data prompt us to favour a simple model for thistransition, based on temperature-induced de-trapping of Ba from a deep double-wellpotential. The most significant of the transport anomalies, that is, the drop in electricalconductivity on cooling, can be easily explained within this model through the enhancedstructural disorder, which would affect the relaxation time for all portions of the Fermisurface. We suggest that the other anomalies (increase in the absolute value ofthe negative Seebeck coefficient, decrease in the magnetic susceptibility) can beexplained within the framework of the one-electron semi-classical model, without anyneed to invoke exotic bi-polaron-driven charge carrier interaction mechanisms.

Finite size scaling for the atomic Shannon-information entropy.

We have developed the finite size scaling method to treat the criticality of Shannon-information entropy for any given quantum Hamiltonian. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. To illustrate this approach we present a study to estimate the critical exponents of the Shannon-information entropy S approximately (lambda-lambda(c))(alpha(S) ), the electronic energy E approximately (lambda-lambda(c))(alpha(E) ), and the correlation length xi approximately mid R:lambda-lambda(c)mid R:(-nu) for atoms with the variable lambda=1/Z, which is the inverse of the nuclear charge Z. This was realized by approximating the multielectron atomic Hamiltonian with a one-electron model Hamiltonian. This model is very accurate for describing the electronic structure of the atoms near their critical points. For several atoms in their ground electronic states, we have found that the critical exponents (alpha(E),nu,alpha(S)) for He (Z=2), C (Z=6), N (Z=7), F (Z=9), and Ne (Z=10), respectively, are (1, 0, 0). At the critical points lambda(c)=1/Z(c), the bound state energies become absorbed or degenerate with continuum states and the entropies reach their maximum values, indicating a maximal delocalization of the electronic wave function.

Photoionization dynamics from [formula omitted]: a one-electron model

Quantum dynamics of photoionization from Cl 2 - is investigated by constructing a purely one-electron model Hamiltonian of the anion calibrated with the help of ab initio theoretical calculations at the MRSD-CI level. A mean-field time-dependent Fourier grid Hamiltonian method is used to investigate the modulation of the dynamics of photoionization from Cl 2 - caused by the coupling of electronic and nuclear motions. Vibrational quenching at low coupling strength is predicted to turn into vibrational enhancement of ionization at higher strengths of coupling.

Calculation of the photodetachment cross sections of the HCN- and HNC- dipole-bound anions as described by a one-electron Drude model.

The Drude model for treating the interaction of excess electrons with polar molecules is extended to calculate continuum functions and to evaluate photodetachment cross sections. The approach is applied to calculate the cross sections for photodetachment of dipole-bound electrons from HCN(-) and HNC(-). In addition, an adiabatic model separating the angular and radial degrees of freedom of the excess electron is introduced and shown to account in a qualitative manner for the cross sections.

Theoretical simulations of dynamics of excess electron attachment to acetonitrile clusters

Dynamics of the attachment of an excess electron to acetonitrile clusters has been studied using mixed quantum-classical method, in which the electron motion were quantum-mechanically propagated in the ground electronic state and the acetonitrile solvent motions were treated classically. A one-electron pseudo-potential model was employed for electron–acetonitrile interaction, while an empirical site–site model was used for acetonitrile–acetonitrile interaction. Initial geometric configurations were obtained from the standard molecular dynamics simulation at a constant temperature. An excess electron, initially in a diffuse weakly bound surface state, is solvated via the solvent reorientation within several picoseconds. It has been found that the attachment of an electron to the acetonitrile cluster involves the formation of an internal localization state except for small clusters with low temperatures, for which metastable surface states were observed during the simulation time. The electron binding energies calculated from the simulations were also compared to recent experimental results; however, we found that our model significantly overestimates the electron binding energies.

Multiple-scattering cluster model of photoelectron diffraction in magnetic solids

Photoelectron diffraction (PED) effects in magnetic solids are described within a one-electron non-relativistic model. The theory presented permits a straightforward calculation of spin-resolved PED patterns, whereby different spin arrangements in a magnetic solid may be considered. Both spin–orbit interaction and exchange interaction with the spin-polarized valence band have been taken into account in the description of the initial core state. The multiple scattering of the excited photoelectron wave is treated within a multiple-scattering cluster model. The theory may be applied to explain PED effects in magnetic solids in comparison to the nonmagnetic case.

High-order above-threshold multiphoton detachment of H−: time-dependent non-Hermitian Floquet approach

We present a nonperturbative quantum study of high-order above-threshold detachment of H− in intense laser fields using an accurate one-electron model potential and a new time-dependent non-Hermitian Floquet approach. Detailed exploration of the electron energy and angular distributions is pursued for the laser field intensities 1010–1011 W cm−2 and wavelength 10.6 µm. In accordance with semiclassical predictions, the electron energy spectrum exhibits a plateau region in the higher energy part. Transformation of the electron angular distributions in the plateau region is discussed. The computational method involves the complex-scaling generalized pseudospectral (CSGPS) spatial discretization of the Hamiltonian and non-Hermitian time propagation of the time-evolution operator by means of the split-operator technique in the energy representation. The approach is designed for effective treatment of multiphoton processes in very intense and/or low-frequency laser fields, which are generally more difficult to treat using the conventional time-independent Floquet matrix techniques.

A theoretical model of the photoabsorption spectra of carbon buckyonions.

A theoretical model has been developed to predict the photoabsorption spectra of spherical carbon buckyonions in the region dominated by the pi-plasmon feature. This model makes use of the microscopic electronic structure of the system, which is provided by an effective Huckel one-electron model. The important screening effects are treated within the random phase approximation, whose form is an extension to the dynamic case of the one derived in a previous work for the static polarizabilities of these species. A systematic analysis as a function of the buckyonion size is performed. We compare the spectra obtained in this way with those derived from a different representation of the electron motion, namely a two-dimensional spherical electron gas, and from a classical dielectric model.

A theoretical model of the static polarizability of carbon buckyonions

A theoretical approach to determine the static dipole polarizabilities of carbon buckyonions is presented. The microscopic electronic structure of the system is provided by an effective one-electron model and the screening effects are treated within the random phase approximation (RPA). The particular spherical geometry of these multishell fullerenes makes possible the use of electrostatic arguments to derive a simple expression for the RPA which gives the polarizability of the buckyonion and the dipole moment induced on each shell in terms of either the screened or unscreened polarizabilities of the isolated shells. A systematic analysis as a function of the buckyonion size is performed. The relevance of an adequate microscopic description of the electronic structure is demonstrated by contrasting the results provided by two different representations of the electron motion, namely a surface electron gas and a more realistic Hückel model. A comparison between our results and those derived from classical dielectric models is also performed.

Positronium formation for atoms with zero threshold energy

Theoretical investigations are performed for positronium formation in positroncollisions with a one-electron model atom which has zero threshold energy forpositronium formation. Investigations are also performed with positronium itselfas the target, but treating the two positrons as distinguishable. The similaritiesbetween the cross sections obtained and the value of the parameter A arisingin the previously discovered correlation between the cross section and the thresholdenergy for positronium formation, σPs = A exp(−B EPs),provide support for the previous interpretation ofA asthe positronium formation cross section for an atom with a formation thresholdenergy of zero.

Metabolic oxidation of carcinogenic arylamines by P450 monooxygenases: theoretical support for the one-electron transfer mechanism

N-oxidation by cytochrome P450 enzymes is an initial step in the metabolic activation of aromatic amine compounds. Once metabolized, these compounds are converted to DNA-reactive species which can exhibit potent mutagenic and/or carcinogenic activity. The precise mechanism of P450 enzyme oxidation is not completely understood, although various theories, involving either one-electron transfer, two-electron transfer or addition–rearrangement, have been debated. In previous studies, selection of the most probable mechanism has been based on consideration of Hückel theory charge distribution calculations and experimentally-derived enzyme metabolism data. This approach can now be improved by incorporating contemporary, ab initio quantum chemical methods to accurately determine the chemical properties of P450 aromatic amine substrates. In this work, we have re-examined the feasibility of three proposed P450 oxidation mechanisms by comparing the experimental oxidation yields and rates of 1-naphthylamine (1-NA), 2-naphthylamine (2-NA) and 2-aminofluorene (2-AF). This data has then been analyzed with respect to ab initio-calculated charge distributions and energies of reactants, oxidation products and proposed intermediates. Our analysis of theoretical and experimental data indicates that the one-electron model is more consistent with oxidation rate data for 1-NA, 2-NA, and 2-AF. We therefore conclude, in contrast to earlier studies, that the one-electron mechanism is more likely to be the pathway for P450-catalyzed aromatic amine oxidation.

Electric dipole moments and polarizabilities of single excess electron sodium fluoride clusters: Experiment and theory

In this article we present the first measurement of the electric dipole susceptibility of one excess electron NanFn−1 clusters. The static electronic polarizability and the permanent electric dipole of these clusters have been calculated with a one-electron model. Experimental values for the susceptibility are clearly related to the calculated values of the permanent dipole. The size evolution of the dipole moments is interpreted in terms of the interplay between the lattice and electron properties. This study outlines that the response of the cluster to the electric field cannot be fully understood with only equilibrium structure calculations and that the coupling between the permanent dipole and the vibrational motion of the cluster has to be taken into account.

Application of a Drude model to the binding of excess electrons to water clusters

A one-electron model potential approach for calculating the binding energies of an excess electron interacting with water clusters is presented. Drude oscillators are employed on the water monomers to describe electron–water polarization and dispersion interactions. It is demonstrated that the resulting model gives electron binding energies very close to the predictions of ab initio CCSD(T) calculations. Dispersion interactions and high-order renormalization effects are found to make large contributions to the electron binding energies.

Development of an effective single-electron model of the electronic structure of hydronium and hydronium–water clusters

A one-electron model is developed which is able to describe the electronic structure and potential-energy (PE) surface of the Rydberg molecule H3O, in particular the Rydberg-to-valence transition of the singly occupied orbital along the H3O→H2O+H reaction path. An analytic PE function for the unpaired electron is obtained by numerical inversion of the one-particle Schrödinger equation and a least-squares fitting using a Thomas–Fermi-type model. Application of this procedure to H5O2 indicates the transferability of the model parameters from H3OtoH3O–water clusters in a good approximation. The long-term goal of this work is the development of an accurate pseudopotential which can be employed in computer simulations of excess electrons and excess protons in liquid water.

Theory of dipole-bound anions.

The problem of the binding of an excess electron to polar molecules and their clusters has long fascinated researchers. Although excess electrons bound to such species tend to be very extended spatially and to have little spatial overlap with the valence electrons of the neutral molecules, inclusion of electron correlation effects is essential for quantitatively describing the electron binding. The major electron correlation contribution may be viewed as a dispersion interaction between the excess electron and the electrons of the molecule or cluster. Recent work using a one-electron Drude model to describe excess electrons interacting with polar molecules is reviewed.

Relativistic one-electron approach to the effect of pressure on the magnetism ofEuCo2P2

Nonrelativistic and four-component relativistic electronic structure calculations with the discrete variational method within density functional theory were performed for embedded clusters representing the layered compound ${\mathrm{EuCo}}_{2}{\mathrm{P}}_{2},$ with ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. A relativistic one-electron model based on Kramers degeneracy was devised to describe the observed changes in the magnetism of this compound induced by applied pressure, which include the collapse of the Eu moment and the formation of Co moments. The ${}^{151}\mathrm{Eu}$ M\ossbauer isomer shift calculated with relativistic wave functions agrees well with experiment; however, the ${\mathrm{Eu}}^{2+}{\ensuremath{-}\mathrm{E}\mathrm{u}}^{3+}$ valence change previously proposed is not predicted.

Ground state and excited state properties of ethylene isomers studied by a combined Feynman path integral-ab initio approach

Ground state and excited state properties of ethylene, C2H4, and several ethylene isomers have been studied by Feynman path integral Monte Carlo (PIMC) simulations. The PIMC treatment of the atomic nuclei has been combined with different electronic Hamiltonians in order to analyse the influence of the nuclear degrees of freedom on electronic quantities. Electronic expectation values at the minimum of the potential energy surface (PES) have been compared with PIMC based ensemble averaged values. Ensemble averaged quantities have been derived by Hamiltonians of the ab initio type and a tight-binding (TB) one-electron model. The combined influence of anharmonicities in the interatomic potential and the quantum fluctuations of the atomic nuclei lead to ensemble averaged bondlengths r g which are significantly larger than the parameters r e, at the minimum of the PES. The implications of this bond length elongation for the electronic properties of ethylene are discussed. The occupied canonical molecular orbitals (CMOs) of ethylene are destabilized under the influence of the nuclear degrees of freedom while virtual CMOs are stabilized. These shifts of one-electron energies suggest a comparison of electronic excitation energies at the minimum of the PES with PIMC based ensemble averages. The quantum fluctuations of the nuclei cause a strong redistribution in the intensities of electronic transitions. Transitions, which are dipole allowed in the planar D2h geometry of ethylene, lose intensity under the influence of nuclear quantum effects, and vice versa for electronic excitations that are dipole forbidden under D2h symmetry. This ‘vibrational borrowing’ is enhanced with decreasing atomic masses. The Feynman centroid density has been used to calculate the anharmonic vibrational wavenumbers of C2H4 and C2D4. The results of the present PIMC simulations have been employed to emphasize general problems of electronic structure calculations based on a single nuclear configuration (i.e. the configuration at the minimum of the PES).

A Drude-model approach to dispersion interactions in dipole-bound anions

A one-electron model potential for calculating the binding energy of an excess electron interacting with polar molecules and their clusters is described. The unique feature of this potential is the treatment of polarization and dispersion effects by means of a Drude model. The approach is tested by calculating the energies for binding an excess electron to HCN, (HCN)2, HNC, and (HNC)2. The model potential results are found to be in good agreement with the predictions of high-level all-electron calculations.

Spectral and temporal structures of high-order harmonic generation of Na in intense mid-ir laser fields

We present a three-dimensional quantum study of high-order harmonic generation (HHG) of the Na atom in intense mid-ir laser fields. An accurate one-electron model potential is constructed for the description of the Na atom. The time-dependent Schr\"odinger equation is solved by means of the time-dependent generalized pseudospectral method, allowing nonuniform and optimal spatial grid discretization and accurate and efficient propagation of the wave function in space and time. Excellent agreement of the HHG spectrum in the length and acceleration forms is obtained from the lowest harmonics up to the cutoff. The HHG power spectrum shows fine structures and significant enhancement of the intensities of the lower harmonics due to the strong coupling of the $3s\ensuremath{-}\mathrm{np}$ states and the $3s\ensuremath{-}3p$ multiphoton resonance. We use a wavelet transform to present a detailed time-frequency analysis of the whole range of the HHG power spectrum. The results reveal striking details of the spectral and temporal fine structures of HHG, providing insights into different HHG mechanisms in different energy regimes of Na atoms at long wavelengths.