Present Theoretical Calculations Research Articles

Dipole model for the linear response of adsorbed overlayers

AbstractThis paper present theoretical calculations of reflectance anisotropy spectra (RAS) for overlayers of alkali atoms adsorbed on a crystalline metallic surface. We assume that the overlayer is formed by an ordered twodimensional (2D) array of adatoms that respond to the local electric field like point‐like harmonic oscillators. We consider overlayers with several rational coverages, assuming that the adsorbates occupy high symmetry sites which form a Bravais lattice that is commensurate with the substrate [1,2]. RAS spectra are obtained for the five 2D Bravais lattices. We found that RAS can be used to observe ordered phases when the ordered phase has a rectangular, centered‐rectangular or oblique symmetry. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modified Wagner model for the active-to-passive transition in the oxidation of Si3N4

Si3N4 is used as the coating material of space shuttle structures which receive very high thermal fluxes during the atmospheric re-entry phase. Two main regimes govern the oxidation of Si3N4: the passive oxidation, with the formation of a protective silica layer leading generally to a mass gain, and the active oxidation, with vaporization of SiO leading to a mass loss of the sample. To ensure that silicon nitride will efficiently protect a material in given oxidizing environments, its own oxidation behaviour must be previously known. Therefore, the active-to-passive transition of Si3N4 oxidation is a problem of deep scientific importance and of wide technological relevance. In this paper, a modified Wager model for the active-to-passive transition in the oxidation of Si3N4 is presented, which includes the non-equilibrium effects caused by the mass transfer. The present theoretical calculations satisfactorily explained the reported experimental and theoretical data. The influence of flow rate on the active-to-passive transition boundary has been explained using our model. The rate controlling mechanism of the oxidation at the active-to-passive transition point is proposed.

Theoretical Investigation for the Active‐to‐Passive Transition in the Oxidation of Silicon Carbide

The oxidation of silicon carbide at high temperatures from 673.15 to 2173.15 K is investigated by using thermodynamic equilibrium calculations and a mass transfer model. The dominant reaction of the active‐to‐passive transition and five other dominant reactions, which are in six different temperature regions, have been determined according to the main equilibrium products. Then, a modified Wagner's model has been developed to determine the active‐to‐passive transition boundary by combining mass transport and thermodynamic calculations. The present theoretical calculations satisfactorily explained the reported experimental and theoretical data. The influence of flow rate on the active‐to‐passive transition boundary has been explained using our model. The rate controlling mechanism of the oxidation at the active‐to‐passive transition point is proposed.

Fully relativistic distorted-wave calculations for electron impact excitation of highly chargednickel-like ions

A fully relativistic distorted-wave method and the corresponding procedure REIE06 have been developed recently to study the electron impact excitation processes of highly charged ions. In this study，we calculated the collision strengths，cross sections and rate coefficients for electron impact excitation of highly charged Nickel-like Gd36+，Rn58+—U64+(Z=86—92)ions from the ground state to 106 low-lying excitated states. In the calculations，the relativistic effects，electron correlation effects，Breit interactions and high partial wave contributions are considered systematically. Based on the calculations，we studied the contribution of Breit interaction to collision strengths，the cross sections connected with the strongest 4d-4p (J=0—1) X-ray laser transitions along the Ni-like sequence，and the effects of strong configuration interaction on cross sections from the ground state 3d101S0 to the (3d3/24d5/2)1，(3d3/24d3/2)1 and (3p3/24p1/2)1 states. A comparison was made between the present results and previous theoretical calculations for collision strengths of Gd36+ and U64+ ions，and rate coefficients of Gd36+ ion，good agreement was obtained.

Spontaneous and Piezoelectric Polarization Effects on the Frequency Response of Wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors

ABSTRACTWe present theoretical calculations for the effect of spontaneous and piezoelectric polarization on the base resistance and frequency response of wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors. Heterojunction Bipolar Transistors ( HBTs ) built using wide band gap semiconductors with AlGaN emitter and SiC base/collector hold the promise of high-power and high-frequency operation due to lower impact ionization coefficients and higher breakdown voltages. Further, Silicon Carbide has an indirect bang gap, and a large lifetime of minority carriers compared to most other compound semiconductors, which tend to have direct band gaps. This reduces the base recombination factor when the base is made from SiC, and helps to achieve higher overall current gain. Spontaneous and piezoelectric polarizations of the order of 1013 electrons per cm2 exist in wurtzite wide band-gap semiconductors. This has a non-trivial effect on band profile, charge transport and overall device characteristics since the polarization-induced charges are of the same order of magnitude as the total dopant charge content of critical device layers, and can significantly affect the amount of mobile charge depletion or accumulation in these layers. We calculate the effect of this polarization for both very thin pseudomorphic emitters and for relaxed emitter structures. We present calculations for the cases of Si-face and C-face SiC, since the signs of polarization-induced charges are different for the two cases. The intrinsic base resistance near emitter flat-band conditions is changed by a factor of 10 depending on the alloy composition of the emitter and the polarity of growth. The maximum frequency of oscillation under emitter flat-band conditions can also be modulated by the polarization-induced charges by up to 60%. Our calculations show that the technologically less prevalent C-face SiC can give a higher advantage for frequency response, especially when the emitter thickness is larger than the critical thickness.

Structural and Thermodynamic Analysis of the First Mononuclear Aqueous Aluminum Citrate Complex Using DFT Calculations.

Structural and thermodynamic properties of the mononuclear Al/citrate complexes have been theoretically investigated aiming to understand the coordination mechanism at an atomic level. GGA-DFT/PCM calculations have been performed for the different conformations and tautomers arising from the Al(3+) and citric acid (H3L) interaction in aqueous solution. The Gibbs reaction energies were estimated based on the reaction of the trigonal planar Al(OH)3 and H3L to form different Al-citrate complexes. The estimated Gibbs free reaction energies for the [AlL], [AlHL](+), and [Al(OH)L](-) species are in good agreement with the experimental values. In these species, the Al(3+) center is coordinated by two carboxylic and the tertiary hydroxyl groups of the citrate. Conversely to what has been proposed based on the experiments, the present theoretical calculations indicate that the citric acid hydroxyl group remains protonated upon the coordination of Al(3+). In fact, our model turns out to be more consistent with the relative pKa values of citrate protonation groups and with the hydrolysis constant of the H2O bound to Al(3+) leading to better agreement with the available experimental data.

Comparing the electron and hole mobilities in the α and β phases of perylene: role of π-stacking

Computational studies are carried out on α and β polymorphs of perylene molecular crystals to estimate their hole and electron mobilities (μhole and μelectron). For both the polymorphs, it is found that the LUMO bandwidhs are larger than the HOMO bandwidth resulting in larger electron conductance, as observed experimentally and which has remained unexplained until now. For a microscopic understanding, we perform quantum chemical calculations for the hole and electron transfer matrix elements (Hmnhole and Hmnelectron) on some selected unique nearest neighbour pairs of molecules in the crystal geometries. This along with calculations for the hole and electron reorganization energies (λhole and λelectron) within the embedded cluster method reveal that for both the crystals, μelectron exceeds μhole by two orders of magnitude. The electron mobility for the α-phase (μelectron = 67.2 cm2 V−1 s−1) is found to be three times that for the β-phase. The major driving force for preferential electron conductance in α-perylene is the slipped parallel π-stacking arrangement of the molecules at short intermolecular distances (d = 3.9 A) in the crystal. We suggest that experimental strategies that further enhance the percentage of such specific π-stacking dimers in molecular assemblies have the potential to further increase μelectron. The present theoretical calculations provide a unified understanding of the parameters that optimize an organic crystal for enhanced electron and hole mobilities.

Theoretical investigation of two-photon absorption allowed excited states in symmetrically substituted diacetylenes by ab initio molecular-orbital method

Symmetrically substituted diacetylene compounds, which shows large two-photon absorption (TPA) cross sections, have been theoretically investigated by the ab initio molecular-orbital method employing several theoretical models including the configuration interaction with single excitation (CIS), random phase approximation (RPA), and time-dependent density-functional theory (TDDFT) methods. The calculated excited energies are overestimated by CIS or RPA, whereas underestimated by TDDFT with the B3LYP parametrization for both one-photon absorption (OPA) and TPA allowed states. The lowest OPA state is well described by the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transition. On the other hand, lower TPA allowed states can be represented as the superposition of the HOMO-LUMO+1 and HOMO-1-LUMO transitions, giving rise to two TPA allowed states. The absorption intensity for the lower TPA state of the diacetylenes molecules is discussed in terms of the alternancy symmetry and its breaking. The symmetry property is differently manifested for neutral and dicationic diacetylenes. Introduction of charges breaks the alternancy symmetry, which gives rise to an increase in the TPA cross sections at the lower frequency. The upper TPA state is calculated to show huge TPA cross sections, which reproduces the enhancement of the TPA cross section experimentally observed for one of the diacetylenes at the higher-frequency region. The enhancement is discussed employing an index defined as the ratio of the transition polarizability and its static limit, which represents the degree of influence of one-photon resonance on the TPA intensity. The huge TPA cross sections are found to be due to a near-resonance effect. The present theoretical calculation approves the previously proposed assumption based on the four-state (dual three-state) model, which consists of the ground, one OPA allowed, and two TPA allowed states.

Spin asymmetries in elastic and inelastic scattering from rubidium

Experimental and theoretical results are presented for the A2 spin asymmetry for elastic and inelastic scattering of spin-polarized electrons from rubidium, at incident electron energies of 15, 20, 30, 50 and 80 eV. Theoretical calculations are performed within a semi-relativistic Breit–Pauli R-matrix (close-coupling) model. The experimental data reveal significant asymmetries at all energies. The experimental results are reasonably well described by the present theoretical calculations and by other calculations using a relativistic distorted-wave approach, and provide a clear relativistic signature.

Experimental results of [formula omitted] photoproduction at SPring-8/LEPS

Photon beam asymmetries for the γ → p → K + Λ and γ → p → K + Σ 0 reactions have been measured for the photon energy range from 1.5 GeV to 2.4 GeV and for the angular range of 0 ° < Θ cm K + < 60 ° at the SPring-8/LEPS facility. The observed asymmetries are found to be positive and increase with increasing the photon energy. Present theoretical calculations do not reproduce the data. To extend our knowledge on the K + photoproduction mechanism including the effect of nucleon resonances, further theoretical development is required to fit both the cross section and asymmetry data.

Differential and total (e,2e) cross sections of simple polyatomic molecules

In this paper, we present a theoretical approach to calculate differential and total ionization cross sections of polyatomic molecules by fast electron impact. More exactly, we have studied the ionization of ammonia (NH(3)) and methane (CH(4)) molecules, and previous results concerning the H(2)O molecule ionization are reported for comparison. The calculations are performed in the distorted wave Born approximation without exchange by employing the independent electron model. The molecular target wave functions are described by linear combinations of atomic orbitals. To describe the interaction between the inactive target electrons and the slow ejected electron, we have introduced a distortion via an effective potential calculated for each molecular orbital. The present theoretical calculations agree well with a large set of existing experimental data in terms of multiple differential and total cross sections.

Evolution of the high power THz source program at Jefferson Lab

We report the evolution of the high power THz source program at Jefferson Lab. The source is based on coherent synchrotron radiation in which short bunches of relativistic electrons radiate when traversing a dipolar magnetic field. In our first accelerator we produced 20 W of broadband THz light. Our upgraded accelerator with higher current and improved THz extraction optics will considerably enhance the output power to >100 W. In this paper we describe the source in some detail and present theoretical calculations for the upgrade.

Intersubband absorption in n‐type GaAs/AlGaAs (001) quantum wells: A tight‐binding study

AbstractWe present theoretical calculations of the intersubband absorption in the conduction band of GaAs/AlGaAs (001) quantum wells (QWs), performed using the sp3d5s* empirical tight‐binding model and a Green’s functions approach. The absorption spectra for both in‐plane (TE) and perpendicular (TM) polarizations are calculated. The obtained TE/TM absorption ratio is in excellent agreement with recent experimental results. We find that the asymmetry of the QW potential has no significant effect on the TE/TM ratio. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

New contribution to study on Stark broadenings of 6965 Å of Ar(I) spectral lineshapes in dense cold plasmas including levels of like and unlike parentage interactions

In the present paper are considered again and reexamined the conclusions given in a previous publication about significant departures between theoretical predictions and experimental data on 6965.43 A ̊ Ar(I) line, in high density plasmas. In particular, in the estimation of the contributions of interacting levels of like and unlike parentage, the latter was overestimated. This point is improved here, in the light of our recent published studies devoted to He(I) lines where the main role of the electron screening upper cut-off ρ max due to the ion field distribution is evidenced. If the currently admitted situation is the appropriateness of the U–F approximation for the one-electron atoms and the N–N approximation for the many electrons atoms, the use of the N–N approximation for He(I) was satisfying: so, we set the possible improvement of theoretical line profile calculation devoted to argon by introducing the N–N approximation. Then, the present theoretical calculation shows the complementarity of these two parameters: the electron screening upper cut-off ρ max and the variable rate of interaction between like and unlike parentage levels, which seems linked to the lowering of the ionization level. Then for every experimental conditions, very good fits are obtained.

X-ray laser without inversion in a three-level ladder system

A scheme for obtaining laser in the deep x-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in ${\mathrm{Ar}}^{+8}$ system. In this system, conventional lasing with population inversion has been obtained in a neonlike fashion. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. This will be selected to best match the frequency needed for the transition from lower level of the present x-ray lasing to the ground state. Numerical calculations in the bare state picture exhibit possible gain at resonance, and at the Rabi side bands, for appropriate choice of system parameters, i.e., the strength of the driving field, the strength of the incoherent pump that provides the required energy needed for the inversionless amplification and the deexcitation rate. The latter rate is detrimental to lasing as it increases. It is quite interesting to note that the system exhibits surprisingly high stability for gain without inversion even for quite high rate of the incoherent deexcitation processes. Real and imaginary parts of the coherences were also obtained for detuned drive and probe fields. These provide insight into the role played by the quantum interference. The present theoretical calculations give high confidence in the quantum interference process as a candidate for realizing very short wavelength laser.

On parametric x-ray radiation

The spectral-angular and angular distributions of parametric x-ray radiation generated in the vicinity of the maximum at small angles to the direction of charged-particle motion are studied under the conditions of Laue and Bragg diffraction geometries. Detector parameters at which the dynamical maxima of parametric x-ray radiation can be observed against the background of transition radiation are indicated. The spectral-angular and angular distributions of parametric x-ray radiation are analyzed for the case of backward geometry. The results of the present theoretical calculations are found to be in good agreement with experimental data.

Influence of Interaction between Neighboring Oxygen Ions on Phase Stability in Cubic Zirconia

The electronic structures of undoped c‐ and t‐ZrO2 were calculated by a first‐principles molecular orbital method. A preliminary analysis revealed that experimental energy‐loss near‐edge structure profiles obtained in ZrO2–8 mol% Y2O3 could be satisfactorily explained from the present theoretical calculation. The calculation suggests that the stability of t‐ZrO2 could be described by the interaction between neighboring oxygen ions rather than the covalency of Zr—O bonds. The effect of dopant cations on the stability of cubic‐zirconia solid solutions can be estimated semiquantitatively in terms of the repulsive Coulomb force between neighboring oxygen ions.

Elastic cross-section calculations for electron collisions with XY 4 (X=Si, Ge; Y=H, F, Cl, Br, I) molecules

We report calculations of integral cross-section and differential cross-section for intermediate- and high-energy (20–2000 eV) elastic electron collisions with tetrahedral compounds of silicon and germanium: SiH 4, SiF 4, SiCl 4, SiBr 4, SiI 4, GeH 4, GeF 4, GeCl 4, GeBr 4 and GeI 4. The calculations have been carried out using an independent atom model with static-polarization model potential. The present results are compared with available experimental and theoretical data. For collision energies higher than 100 eV, results of present theoretical calculations are in satisfactory agreement with experimental data.

Molecular structures of carotenoids as predicted by MNDO-AM1 molecular orbital calculations

Semi-empirical molecular orbital calculations using AM1 Hamiltonian (MNDO-AM1 method) were performed for a number of biologically important carotenoid molecules, namely all- trans-β-carotene, all- trans-zeaxanthin, and all- trans-violaxanthin (found in higher plants and algae) together with all- trans-canthaxanthin, all- trans-astaxanthin, and all- trans-tunaxanthin in order to predict their stable structures. The molecular structures of all- trans-β-carotene, all- trans-canthaxanthin, and all- trans-astaxanthin predicted based on molecular orbital calculations were compared with those determined by X-ray crystallography. Predicted bond lengths, bond angles, and dihedral angles showed an excellent agreement with those determined experimentally, a fact that validated the present theoretical calculations. Comparison of the bond lengths, bond angles and dihedral angles of the most stable conformer among all the carotenoid molecules showed that the displacements are localized around the substituent groups and hence around the cyclohexene rings. The most stable conformers of all- trans-zeaxanthin and all- trans-violaxanthin gave rise to a torsion angle around the C6–C7 bond to be ±48.7 and −84.8°, respectively. This difference is a key factor in relation to the biological function of these two carotenoids in plants and algae (the xanthophyll cycle). Further analyses by calculating the atomic charges and using enpartment calculations (division of bond energies between component atoms) were performed to ascribe the cause of the different observed torsion angles.

Determination of Time Delay between Blood and Interstitial Adipose Tissue Glucose Concentration Change by Microdialysis in Healthy Volunteers

For the development and use of subcutaneous glucose sensors it is important to know the time lag between changes in blood glucose and subcutaneous interstitial glucose concentration. To determine the time lag we inserted a microdialysis probe into the abdominal subcutaneous adipose tissue of healthy volunteers (n = 19) and performed oral glucose tolerance tests (n = 39) over a 7-day period. After correction for the microdialysis system time lag, we compared the change in dialysate glucose concentration with the capillary blood glucose concentration. We found no significant delay time between a change in capillary blood glucose concentration and subcutaneous interstitial fluid glucose concentration using the Mann-Whitney test. The substantial interindividual variation of glucose recovery and the changing recovery in time makes it difficult to draw unambiguous conclusions about the exact physiological time lag. Based on the present experimental findings and theoretical calculations of glucose transport in adipose tissue, the physiological lag time is short and negligible compared to the system delay time of a glucose sensor.