bluuebunny/crossref_metadata_2025 huggingface dataset:
On-the-fly localization of electronic orbitals in Car–Parrinello molecular dynamics
Authors: Iftimie Radu , Thomas Jordan W. , Tuckerman Mark E.
Published: January 2004

The ab initio molecular-dynamics formalism of Car and Parrinello is extended to preserve the locality of the orbitals. The supplementary term in the Lagrangian does not affect the nuclear dynamics, but ensures “on the fly” localization of the electronic orbitals within a periodic supercell in the Γ-point approximation. The relationship between the resulting equations of motion and the formation of a gauge-invariant Lagrangian combined with a gauge-fixing procedure is briefly discussed. The equations of motion can be used to generate a very stable and easy to implement numerical integration algorithm. It is demonstrated that this algorithm can be used to compute the trajectory of the maximally localized orbitals, known as Wannier orbitals, in ab initio molecular dynamics with only a modest increase in the overall computer time. In the present paper, the new method is implemented within the generalized gradient approximation to Kohn–Sham density-functional theory employing plane wave basis sets and atomic pseudopotentials. In the course of the presentation, we briefly discuss how the present approach can be combined with localized basis sets to design fast linear scaling ab initio molecular-dynamics methods.
Hydrates of the most stable gas-phase mono- and di-protonated glycine derivatives: Origin of no reservation energy bond in glycine-2H2+
Authors: Ai Hongqi , Bu Yuxiang
Published: January 2004

Series of hydrates of the most stable glycine-H+/2H2+ in the gas phase are presented at the B3LYP level. The results show that only the amino hydrogens and hydroxyl hydrogens can be monohydrated for the glycine-H+, and the amino hydrogens are preferred. The H6(O4) of glycine-2H2+ is the best site for a water molecule to attach, i.e., the corresponding hydrate is the most stable one among its isomers. Calculations reveal that the binding energies of hydrated hydrogens decrease relative to their counterparts in the isolated glycine-H+/2H2+ complexes and they are positive values and without proton transfer except those of monohydrated glycine-2H2+ complexes with the combination modes of H3O+…(glycine-H+). The complex H3O+…(glycine-H+) is formed by the combination of a H2O molecule and one hydroxyl-site proton of glycine-2H2+, and with the proton transfer to H2O. Here the interaction between the proton of H3O+ and the glycine-H+ mainly depends on an electronic one instead of an initial covalent one of the isolated glycine-2H2+. The generation of the bond between the H3O+ and the glycine-H+ makes the energy of the complex higher than the energy sum of its two separated species (or two reactants of the complex), just like the case of M+…(glycine-H+) bond (M=Li,Na). The observation can explain satisfactorily why the combinations of both a proton and an alkali ion or two alkali ions to a glycine molecule can make the corresponding complex hold reservation energy bond(s), while the combination of two protons and a glycine in our previous work cannot [H. Ai et al., J. Chem. Phys. 117, 7593 (2002)]. For the glycine-2H2+, monohydration at the any site of its amino hydrogens can make the binding strength of any other neighboring proton (hydrogens) stronger relative to its counterpart in the isolated glycine-2H2+. Further hydration, especially at the site of either of hydroxyl hydrogens, would disfavor the reservation energy of the system.
The open-shell interaction of He with the B 3Πu(0+) state of Br2: An ab initio study and its comparison with a diatomics-in-molecule perturbation model
Authors: de Lara-Castells M. P. , Buchachenko A. A. , Delgado-Barrio G. , Villarreal P.
Published: January 2004

The interaction of He with Br2 in electronically excited B 3Πu state is investigated using spin-unrestricted single and double coupled-cluster approach with noniterative perturbative treatment of triple excitations. Internal electrons of the Br atom are described by effective core pseudopotentials. The validity of this approach is analyzed by comparing the lowest Σ+2 and Π2 electronic states of the HeBr molecule with those obtained in all electron calculations [J. Chem. Phys. 115, 10438 (2001)]. In this context, we examine the performance of different basis sets and saturation with bond functions. The comparison of theoretical blue-shifts with the experiment provides confidence about the present ab initio calculations. In addition, He–Br results of ab initio calculations at the same level are used to obtain approximate He–Br2 (3Πu) interactions in the framework of the diatomics-in-molecule first order perturbation theory (IDIM-PT1) [J. Chem. Phys. 104, 9913 (1996)]. Overall, the IDIM-PT1 model results show a good agreement with the ab initio ones, being the main difference the sensitivity to the elongation of the Br–Br bond.
Relativistic spin-orbit effects on hyperfine coupling tensors by density-functional theory
Authors: Arbuznikov Alexei V. , Vaara Juha , Kaupp Martin
Published: January 2004

A second-order perturbation theory treatment of spin-orbit corrections to hyperfine coupling tensors has been implemented within a density-functional framework. The method uses the all-electron atomic mean-field approximation and/or spin-orbit pseudopotentials in incorporating one- and two-electron spin-orbit interaction within a first-principles framework. Validation of the approach on a set of main-group radicals and transition metal complexes indicates good agreement between all-electron and pseudopotential results for hyperfine coupling constants of the lighter nuclei in the system, except for cases in which scalar relativistic effects become important. The nonrelativistic Fermi contact part of the isotropic hyperfine coupling constants is not always accurately reproduced by the exchange-correlation functionals employed, particularly for the triplet and π-type doublet radicals in the present work. For this reason, ab initio coupled-cluster singles and doubles with perturbative triples results for the first-order contributions have been combined in the validation calculations with the density-functional results for the second-order spin-orbit contributions. In the cases where spin-orbit corrections are of significant magnitude relative to the nonrelativistic first-order terms, they improve the agreement with experiment. Antisymmetric contributions to the hyperfine tensor arise from the spin-orbit contributions and are discussed for the IO2 radical, whereas rovibrational effects have been evaluated for RhC, NBr, and NI.
The reduced density matrix method for electronic structure calculations and the role of three-index representability conditions
Authors: Zhao Zhengji , Braams Bastiaan J. , Fukuda Mituhiro , Overton Michael L. , Percus Jerome K.
Published: January 2004

The variational approach for electronic structure based on the two-body reduced density matrix is studied, incorporating two representability conditions beyond the previously used P, Q, and G conditions. The additional conditions (called T1 and T2 here) are implicit in the work of Erdahl [Int. J. Quantum Chem. 13, 697 (1978)] and extend the well-known three-index diagonal conditions also known as the Weinhold–Wilson inequalities. The resulting optimization problem is a semidefinite program, a convex optimization problem for which computational methods have greatly advanced during the past decade. Formulating the reduced density matrix computation using the standard dual formulation of semidefinite programming, as opposed to the primal one, results in substantial computational savings and makes it possible to study larger systems than was done previously. Calculations of the ground state energy and the dipole moment are reported for 47 different systems, in each case using an STO-6G basis set and comparing with Hartree–Fock, singly and doubly substituted configuration interaction, Brueckner doubles (with triples), coupled cluster singles and doubles with perturbational treatment of triples, and full configuration interaction calculations. It is found that the use of the T1 and T2 conditions gives a significant improvement over just the P, Q, and G conditions, and provides in all cases that we have studied more accurate results than the other mentioned approximations.
Improved interatomic potentials for silicon–fluorine and silicon–chlorine
Authors: Humbird David , Graves David B.
Published: January 2004

Improved sets of empirical interatomic potentials for silicon–fluorine and silicon–chlorine are presented. The Tersoff–Brenner potential form has been reparameterized using the density-functional theory (DFT) cluster calculations of Walch. Halogenated silicon cluster energetics computed with DFT are, on average, within several tenths of an eV of the energies of the corresponding clusters with the reparameterized empirical potential for both Si–F and Si–Cl. Using the reparameterized Tersoff–Brenner potentials, molecular-dynamics simulations of F and Cl atom exposure to undoped silicon surfaces are in excellent agreement with published data on etch probability, halogen coverage at steady state, and etch product distributions.
Exciton migration dynamics in a dendritic molecule: Quantum master equation approach using ab initio molecular orbital configuration interaction method
Authors: Nakano Masayoshi , Takahata Masahiro , Yamada Satoru , Yamaguchi Kizashi , Kishi Ryohei , Nitta Tomoshige
Published: January 2004

We investigate the exciton migration dynamics in a dendritic molecular model composed of π-conjugation linear-leg units (acetylenes and diacetylene) and a benzene ring (branching point) using the quantum master equation approach with the ab initio molecular orbital (MO) configuration interaction (CI) method. The efficient migration of exciton from short-length linear legs (acetylenes) to long-length linear leg (diacetylene) via a benzene ring is observed. As predicted in previous studies, the exciton (electron and hole) distributions are relatively well localized in each generation segmented by the meta-branching point (meta-substituted benzene ring) though the electron and hole distributions are delocalized and are somewhat spatially different from each other within each generation. It is found that the excitons localized in the generation composed of short linear legs occupy in higher-lying exciton states, while those in the generation composed of long linear legs do in lower-lying ones. These features suggest the decoupling of π-conjugation at the meta-branching point. On the other hand, the relaxation effect between exciton states is found to be caused by the exciton–phonon coupling, in which the existence of common configurations (electron-hole pairs) in CI wave functions between adjacent exciton states (having primary distributions on short and long linear-leg regions, respectively) is important for the relaxation between their exciton states. This feature indicates the importance of partial penetration of π-conjugation through the meta-substituted benzene ring in excited states for such exciton migration.
Photobleaching time distribution of a single tetramethylrhodamine molecule in agarose gel
Authors: Ko D.-S.
Published: January 2004

In this study, the temporal distribution of the photobleaching of tetramethylrhodamine molecules embedded in agarose gel is measured using a single molecule detection technique equipped with a confocal microscope and a multichannel scaler. The result shows the exponential photobleaching time distribution at long time scales and the average photobleaching time of 46.1±3.8 ms for a given excitation intensity of about 8 kW/cm2.
Aperiodic nanophotonic design
Authors: Gheorma Ioan L. , Haas Stephan , Levi A. F. J.
Published: January 2004

The photon scattering properties of aperiodic nanoscale dielectric structures can be tailored to closely match a desired response by using adaptive algorithms for device design. We show that broken symmetry of aperiodic designs provides access to device functions not available to conventional periodic photonic crystal structures.
Elastic properties of LaSnxNi5−x compounds
Authors: Yeheskel O. , Nattrass C. E. , Leisure R. G. , Jacob I. , Bowman R. C.
Published: January 2004

Ultrasonic methods were used to measure the room-temperature elastic moduli of polycrystalline LaSnxNi5−x compounds for 0⩽x⩽0.5. These materials are of great importance for their hydrogen storage properties. The samples, prepared by a hot isostatic pressing method, had near-theoretical densities with calculated porosities ranging from 0 to 1.5%. The porosity-corrected moduli decreased with increasing x. Poisson’s ratio was approximately constant at 0.314 for all the compounds. The Debye temperature, calculated from the RT polycrystalline moduli, decreased from 359 to 344 K as x increased from 0 to 0.5. The results were used to calculate the elastic interaction energy of an interstitial hydrogen atom with the strain fields of all the other interstitial hydrogen. This energy was in turn used to calculate the critical temperature below which phase separation occurs in LaMxNi5−xHy compounds (M=Sn or Al). It was found that the critical temperature decreases with increasing x, confirming in a more general way a conclusion drawn for a specific case from earlier thermodynamic measurements. It is suggested that the lowering of the critical temperature plays a role in limiting the width of the plateaus in pressure-composition isotherms for the two-phase regions of these compounds. This suggestion implies a relation between the elastic properties and the maximum hydrogen capacity.
Self-focused distributed Bragg reflector laser diodes
Authors: Yanson D. A. , Rafailov E. U. , Sokolovskii G. S. , Kuchinskii V. I. , Bryce A. C. , Marsh J. H. , Sibbett W.
Published: January 2004

We report a theoretical and experimental investigation of a special type of tapered distributed Bragg reflector laser diode incorporating confocal gratings. The devices exhibit single-longitudinal mode operation with a laterally focused beam and will be suitable for applications requiring both spectrally and spatially enhanced beam quality.
Ion-channeling and Raman scattering study of damage accumulation in silicon
Authors: Johnson B. C. , McCallum J. C.
Published: January 2004

Damage was introduced into Si(100) using 245 keV Si+ ions implanted to a wide range of doses with implant temperatures of −195, 25 or 100 °C. The accumulation of this damage was monitored with Rutherford backscattering and ion channeling (RBS-C) and by following the intensity and lineshape variation of the first-order (1-O) Raman peak of silicon. For all implant temperatures the RBS-C data showed the expected trend with dose. For −195 °C and room temperature implants, the decrease in intensity of the 1-O Raman peak shows a similar trend to the RBS-C data, but in each case the threshold dose is about a decade lower than its RBS-C counterpart. On implantation at 100 °C the sensitivity of the Raman spectra to low damage concentrations is more dramatic and decreases continuously over the full dose range, from 5×1012 to 2×1016 Si/cm2, examined in this study. This suggests that the intensity of the 1-O Raman peak is particularly sensitive to the types of defect structures that are stable in silicon during irradiation at elevated temperatures. The phonon confinement model is discussed in light of these results.
Lattice Boltzmann study on size effect with geometrical bending on phonon heat conduction in a nanoduct
Authors: Jiaung Wen-Shu , Ho Jeng-Rong
Published: January 2004

Phonon heat transport based on the Boltzmann transport equation (BTE) in a free standing, bent duct with characteristic dimension down to the nanoscale is investigated through the lattice Boltzmann (LB) method. Both the thermal excited transverse and longitudinal phonons are considered. The collision term in BTE is approximated by the relaxation time approximation. Both diffusive and specular phonon scatterings at duct surfaces are considered. An analytical expression for thermal conductivity suitable for an infinitely long, straight duct with constant properties is derived. Results show that the size effect depends strongly on the Knudsen number. For large Knudsen number, heat transport is mainly dominated by the ballistics that results in strong size effect, and vice versa. For the bent duct more phonons take the passageway near the inner corner of the bending region where higher local thermal conductivity is expected. Although the specular boundary scattering introduces no change in the bulk quantities for a straight duct, it, however, brings in the geometric influence as the duct is bent. Compared to the straight duct, the bent duct has the supremacy in conducting heat as the Knudsen number is small. Conversely the straight duct presents higher conductivity when the Knudsen number is large. By the present LB method, jumps in macroscopic quantities, occurring at boundary and wall surface, can be calculated naturally and straightforwardly.
Grain boundary dependency of nonlinear current–voltage characteristics in Pr and Co Doped ZnO Bicrystals
Authors: Sato Yukio , Oba Fumiyasu , Yodogawa Masatada , Yamamoto Takahisa , Ikuhara Yuichi
Published: January 2004

Pr and Co doped ZnO bicrystals with various types of grain boundaries were fabricated to investigate their current–voltage characteristics. In addition, their grain boundary structures and chemical compositions were investigated using high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy. It was found that all the boundaries are atomically joined, but have some precipitates of Pr6O11. Co dopants are distributed over the grain interiors and boundaries, while Pr dopants segregate at the fabricated grain boundaries except for a highly coherent Σ1 boundary. The Σ1 boundary with no Pr segregation shows an almost Ohmic characteristic, and coincidence-site lattice and random boundaries with clear Pr segregations show nonlinear characteristics. The change in the nonlinearity is attributed to the difference of the Pr concentration that is associated with the grain boundary structure.
Magnetotransport properties of hard magnetic pinned multilayers
Authors: Rabe M. , May U. M. , Siegle H. , Hütten A. , Kämmerer S. , Justus M. , Reiss G.
Published: January 2004

Hard magnetic (HM) thin films have been vertically integrated below giant magnetoresistive (GMR) multilayer sensors in order to shift the operating point of an applicable GMR sensor. The shift in maximum magnetoresistance (MR) peaks was found to be dependent on the thickness of the HM layer. As a result of different bias geometries, which have been tested, current shunting effects mainly reduce the maximum MR amplitudes. A strong microstructural influence on the magnetotransport has been found; meander-shaped microstructures with different stripe widths have been microfabricated in order to investigate the influence of shape anisotropy and stray field geometry on the magnetotransport. As a result, the bias strength, as well as the shape of the HM hysteresis, varies with the underlying microstructured pattern geometries and the angle of applied field. Comparing the major MR loops of meanders with 1.5, 5, and 10 μm structure width of HM/GMR systems, different MR behavior has been found. It was determined that the MR loops are shifted by different field values simultaneously with a change in the MR loop shape. The effect depends on the orientation of applied field with respect to the meander structure, as well as on the structure width. A creeping effect, which demagnetizes the hard layer, has not been observed during sine-shaped switching cycles. The MR response signal of biased multilayers can be understood using numerical calculations.
Laser dynamics in self-pulsating quantum dot systems
Authors: Summers Huw D. , Matthews Daniel R. , Smowton Peter M. , Rees Paul , Hopkinson Mark
Published: January 2004

We have studied self-pulsation in InGaAs quantum dot lasers with an emission wavelength in the 1 μm band. The use of saturable absorption to produce internal optical feedback in semiconductor lasers is well established and leads to the phenomenon of self-pulsation. The characteristics of this self-sustaining oscillation in the optical intensity are determined by the optical characteristics of the amplifying and absorbing media. These experiments therefore provide a direct measure of the intrinsic dynamics of the dot laser system free of any external parasitics. At room temperature, pulsation is observed up to a drive current of 1.5 Ith with a maximum pulsation frequency of 700 MHz. The self-pulsation is strongly temperature dependent, and cannot be maintained below a temperature of 150 K. Studies of the optical gain and carrier lifetime within the lasers indicate that the dynamic characteristics are controlled by the interaction of the quantum dots with the two-dimensional wetting layer surrounding them. The relatively low pulsation frequency results from the strong saturation of the gain with increasing injection at room temperature, while the thermal switch-off of pulsation is due to changes in the absorber recovery time.
High electric-field effects on short-channel polythiophene polymer field-effect transistors
Authors: Xu Yifan , Berger Paul R.
Published: January 2004

The field-effect mobility (FEM) in polythiophene (PT) polymer field-effect transistors (PFETs) increases with reduced channel lengths during high driving forces across the source and drain, which is contradictory to the decrease in mobility caused by short-channel effects in amorphous Si thin-film transistors. The longitudinal electric-field (across source and drain) dependence of the FE mobility is believed to create the rise in mobility once the longitudinal electric field exceeds a critical value of 105 V/cm. The high longitudinal electric field also modulates the influence of the gate bias upon the FEM in PT PFETs. With increased longitudinal electric field, the correlation between FEM and gate bias is largely enhanced.
Molecular-scale noncontact atomic force microscopy contrasts in topography and energy dissipation on c(4×2) superlattice structures of alkanethiol self-assembled monolayers
Authors: Fukuma Takeshi , Ichii Takashi , Kobayashi Kei , Yamada Hirofumi , Matsushige Kazumi
Published: January 2004

Alkanethiol self-assembled monolayers formed on Au(111) surfaces were investigated by noncontact atomic force microscopy (NC-AFM). Dodecanethiol monolayers prepared at 78 °C were imaged by NC-AFM, which revealed that the film is composed predominantly of two different phases of c(4×2) superlattice structures. The obtained molecular-scale NC-AFM contrasts are discussed in comparison with previously reported scanning tunneling microscopy images. We found that the energy dissipation image exhibits a clear difference in its molecular-scale contrast between the two phases. Possible origins of the difference are discussed in relation to the fluctuation and/or stability of the packing structures.
Surface tension of associating fluids by Monte Carlo simulations
Authors: Tapia-Medina Carlos , Orea Pedro , Mier-y-Terán Luis , Alejandre José
Published: January 2004

Canonical Monte Carlo (NVT-MC) simulations were performed to obtain surface tension and coexistence densities at the liquid–vapor interface of one-site associating Lennard-Jones and hard-core Yukawa fluids, as functions of association strength and temperature. The method to obtain the components of the pressure tensor from NVT-MC simulations was validated by comparing the equation of state of the associative hard sphere system with that coming from isothermal–isobaric Monte Carlo simulations. Surface tension of the associative Lennard-Jones fluid determined from NVT-MC is compared with previously reported results obtained by molecular dynamics simulations of a pseudomixture model of monomers and dimers. A good agreement was found between both methods. Values of surface tension of associative hard-core Yukawa fluids are presented here for the first time.
Excluded volume entropic effects on protein unfolding times and intermediary stability
Authors: Chapagain Prem P. , Gerstman Bernard S.
Published: January 2004

The dynamics of protein folding result from both enthalpic and entropic contributions to the free energy. In this paper we focus on entropic volume exclusion effects. We carry out computer simulations using a model that allows us to independently change the size or biochemical properties of amino acid residues. To determine the importance of excluded volume effects, we investigate the effects of changing the size of side chains on the unfolding dynamics of a model four-helix bundle protein. In addition, we also investigate the effects of changing the thickness of the chain’s backbone. This has relevance to the behavior of synthetic polymers where the size of the constituent units can be varied. We find that entropic excluded volume effects are crucially important for stabilizing the organized native state relative to the molten globule.
First principles study of oxygen vacancy migration in tantalum pentoxide
Authors: Ramprasad R.
Published: January 2004

First principles total energy calculations were performed in order to determine oxygen vacancy migration energies in Ta2O5. A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. O vacancies in the chosen model of Ta2O5 can be broadly classified into ‘cap’ and ‘in-plane’ sites based on their location in the lattice. The cap type of vacancies display the largest barriers both for migration to a neighboring cap site or to a neighboring in-plane site, thus behaving as oxygen vacancy ‘sinks’. A lowering of the barriers to migration is generally seen when the vacancies are doubly positively charged. All calculations were performed within the local density approximation of density functional theory, and the elastic band method was used in the estimation of migration barriers.
Ce/GaN(0001) interfacial formation and electronic properties
Authors: Xiao Wende , Guo Qinlin , Wang E. G.
Published: January 2004

X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and low-energy electron diffraction have been used to study the growth, interfacial reaction, and Fermi level movement of Ce on a n-type GaN(0001)-(1×1) surface. The results demonstrate that Ce grows in a layer-by-layer-like mode and reacts with the substrate at the interface, leading to formation of metallic Ga at room temperature. With increasing Ce coverage, a downward Fermi level movement is observed, and the resultant Schottky barrier height is 1.3 eV. Annealing promotes further diffusion and thus interfacial Ce–Ga exchange reaction.
Magnetite particles studied by Mössbauer and magneto-optical Kerr effect
Authors: Kalska B. , Paggel J. J. , Fumagalli P. , Rybczyński J. , Satula D. , Hilgendorff M. , Giersig M.
Published: January 2004

We present results of transmission electron microscopy, Mössbauer spectroscopy, magneto-optical Kerr effect, and magnetization measurements on magnetite-based nanoparticles M0.5Fe2.5O4 (M=Fe, Co, Mn, and Ni). Room-temperature Mössbauer spectra show ferrimagnetic behavior of the particles and suggest a preferential occupation of the B (octahedral) magnetite site by Ni and Mn cations. Magnetization measurements reveal a homogeneous magnetic-size distribution irrespective of the substitution elements. The influence of the chemical nature of the substituent on the magneto-optical Kerr spectrum is investigated. Magneto-optical magnetization loops reveal that particles in the frozen solution have the same magnetically active size as those deposited on Al substrates.
Structure-related optical properties of luminescent hetero-opals
Authors: Gaponik N. , Eychmüller A. , Rogach A. L. , Solovyev V. G. , Sotomayor Torres C. M. , Romanov S. G.
Published: January 2004

Light-emitting hetero-opals have been synthesized by successive crystallization of one thin opal film on top of the other from beads of two different diameters and selective impregnation of one of films with luminescent CdTe nanocrystals. A strong spectral and spatial anisotropy of the reflectance and the photoluminescence spectra has been observed and attributed to the photonic bandgap mismatch in opal films. Changes of the emission anisotropy with excitation power have been associated with the stimulation of CdTe nanocrystal emission in a host opal film. An interface-related transmission minimum has been revealed and tentatively explained as the consequence of the distractive interference of optical modes of hetero-opal constituents. The idea of the interface resonance has been supported by the observation of the emission modification in this frequency range.
Iron detection in crystalline silicon by carrier lifetime measurements for arbitrary injection and doping
Authors: Macdonald D. H. , Geerligs L. J. , Azzizi A.
Published: January 2004

An existing technique for accurate measurement of iron in silicon, which was previously restricted to low injection and a narrow doping range, has been extended to arbitrary injection and doping levels. This allows contactless lifetime measurement techniques to be used for very sensitive and rapid iron detection under a wide range of conditions. In addition, an easily measured and unambiguous “fingerprint” of iron in silicon has been identified. It is based on the invariant nature of the excess carrier density at which the injection-dependent lifetime curves, measured before and after iron–boron pair dissociation, cross over. This characteristic crossover point lies in the narrow range of 1.4 to 2.0×1014 cm−3, provided only that the boron concentration is below 5×1016 cm−3. To demonstrate the value of these techniques, they have been applied to photovoltaic-grade cast multicrystalline silicon wafers.
Influence of substrate temperature on epitaxial copper phthalocyanines studied by photoemission spectroscopy
Authors: Ellis Trinity S. , Park Kenneth T. , Hulbert Steven L. , Ulrich Mark D. , Rowe Jack E.
Published: January 2004

We report the formation of heteroepitaxial copper phthalocyanine (CuPc) overlayers on a “5×20” reconstructed Au(001) substrate, with thicknesses ranging from 0.8 to 26.9 Å. The adsorbed CuPc at room temperature forms a quasiepitaxial overlayer, incommensurate with the substrate lattice, aligned with the 〈110〉 and 〈11̄0〉 axes of the Au(001) surface. At elevated substrate temperatures (ET) between 200 and 250 °C, the deposition of CuPc results in a 13.8 Å square overlayer structure commensurate with the substrate lattice. The electronic structures of CuPc overlayers grown at both RT and ET are also investigated. While the films grown at both temperatures generally exhibit lowering of the vacuum level, consistent with the presence of an interfacial dipole, it is the ET-grown CuPc overlayer that exhibits a sharp decrease of the surface vacuum level upon completion of a monolayer. The experimental CuPc valence band energies are compared with the calculated ionization potentials of a CuPc molecule. Using the tunability of the light source and the dependence of the photoionization cross sections on photon energy for various atomic shells, the CuPc molecular orbitals are assigned with either Cu d states or Pc-derived valence orbitals.
Calculation of electric field and dielectrophoretic force on spherical particles in chain
Authors: Techaumnat B. , Eua-arporn B. , Takuma T.
Published: January 2004

This article presents an analytical method for calculating electric field and dielectrophoretic force in three-dimensional arrangements of spherical particles. The analytical method is based on the method of images that utilizes the multipole re-expansion and the fundamental solutions for several arrangements of a multipole. It is capable of calculating electric field for various conditions of particles and energization. The method needs much less memory than the already proposed one. The calculation results show that force on a dielectric particle chain in a dielectric fluid depends on the number of particles and the chain direction. However, the maximal attractive and repulsive forces reach their saturation values at about 32 and 12 particles, respectively. When the lower particle of a two-particle chain is in contact with a plate electrode, the dielectrophoretic force on the chain becomes higher on the whole, and it always attracts the chain to the electrode. As a result, the particle chain is stabilized for a wider range of the angle between the chain and the applied field. Neglecting the interaction between the electrodes and particles usually gives adequate accuracy in the force calculation, unless the electrodes are not very close to particles.
Spectroscopic analysis of the Er3+ (4f11) absorption intensities in NaBi(WO4)2
Authors: Sardar Dhiraj K. , Russell Charles C. , Yow Raylon M. , Gruber John B. , Zandi Bahram , Kokanyan Edvard P.
Published: January 2004

A spectroscopic analysis is performed on Er3+ (4f11) ions doped in NaBi(WO4)2 (NBWO) in order to assess this material for its potential as a near-infrared laser. The Judd–Ofelt model is applied to the room-temperature absorption intensities of Er3+ (4f11) in NBWO to obtain the three phenomenological intensity parameters: Ω2=5.50×10−20 cm2, Ω4=1.00×10−20 cm2, and Ω6=0.71×10−20 cm2. The intensity parameters are then used to determine the radiative decay rates (emission probabilities), radiative lifetimes, and branching ratios for the Er3+ transitions from the excited state multiplet manifolds to the lower lying manifold states. Using the radiative decay rates for the Er3+ (4f11) transitions between the corresponding excited states and the lower lying states, the radiative lifetimes of eight excited states of Er3+ are determined in this host. Using the measured room temperature fluorescence lifetime of approximately 4.75 ms and the radiative lifetime of 5.63 ms as predicted by the Judd–Ofelt model for the I13/24→I15/24 (1.52 μm) transition of Er3+ in NBWO the quantum efficiency is determined to be approximately 84% for this laser material.
In situ stress measurements during the growth at different temperatures of Ag–Cu(111) multilayers
Authors: Bocquet F. , Bigault T. , Alfonso C. , Labat S. , Thomas O. , Charai A.
Published: January 2004

We studied, at different temperatures, the stress development during the growth of Ag–Cu(111) multilayers. Multilayers, with a nanometric period, were UHV evaporated on Si(111) and stress was monitored in situ by measuring substrate bending. We show that whatever the evaporation temperature may be, the Ag on Cu(111) growth does not develop any representative stress. This quasi-unstressed Ag on Cu(111) growth could be consistent with an intrinsic relaxation resulting from a strong atomic reorganization during early stage (1 monolayer deposited) of interface formation. On the contrary, the Cu on Ag(111) growth develops a temperature-dependent tensile stress. The measured stress at room temperature is 1 GPa and decreases to 0.5 GPa at 110 °C for a 3 nm deposited thickness. This temperature-dependent stress could be related to a weak three-dimensional temperature-dependent growth mode during an early stage of film deposition. Our results also indicate, at 35 °C, a clear stress relaxation for thickness larger than 3 nm, while no relaxation is observed at 110 °C. We show that the Matthews–Blakeslee model is not adequate to describe this relaxation.
Two-dimensional modeling of etched strained-silicon quantum wires
Authors: Curatola G. , Iannaccone G.
Published: January 2004

We present two-dimensional simulations of different types of strained-silicon quantum wires obtained by selective etching on silicon germanium heterostructures. Such structures are promising both for emerging ballistic devices in silicon compatible technology and for innovative nanoscale field-effect transistors. Numerical modeling has been performed with a procedure designed to solve the Poisson–Schrödinger equation for electrons and holes, that takes into account the effect of strain on the band structure, conduction band anisotropy, and the effect of states at the exposed surfaces. We show that the simulations provide insights into the capability to control the wire via an external gate voltage, and into the dependence of wire properties on geometry and surface states.
Shear-induced migration in flowing polymer solutions: Simulation of long-chain DNA in microchannels
Authors: Jendrejack Richard M. , Schwartz David C. , de Pablo Juan J. , Graham Michael D.
Published: January 2004

We simulate dilute solution dynamics of long flexible polymer molecules in pressure driven flow in channels with widths of roughly 0.1–10 times the polymer bulk radius of gyration. This is done using a self-consistent coarse-grained Langevin description of the polymer dynamics and a numerical simulation of the flow in the confined geometry that is generated by the motions of polymer segments. Results are presented for a model of DNA molecules of ∼10–100 μm contour length in micron-scale channels. During flow, the chains migrate toward the channel centerline, in agreement with well-known experimental observations. The thickness of the resulting hydrodynamic depletion layer increases with molecular weight at constant flow strength; higher molecular weight chains therefore move with a higher average axial velocity than lower molecular weight chains. In contrast, if the hydrodynamic effects of the confining geometry are neglected, depletion of concentration is observed in the center of the channel rather than at the walls, contradicting experimental observations. The mechanisms for migration are illustrated using a simple kinetic theory dumbbell model of a confined flexible polymer. The simple theory correctly predicts the trends observed in the detailed simulations. We also examine the steady-state stretch of DNA chains as a function of channel width and flow strength. The flow strength needed to stretch a highly confined chain away from its equilibrium length is shown to increase with decreasing channel width, independent of molecular weight; this is fairly well explained using a simple blob picture.
State-resolved dynamics of oxygen atom recombination on polycrystalline Ag
Authors: White M. G. , Beuhler R. J.
Published: January 2004

Rotationally resolved, velocity distributions for desorbed O2 molecules formed by O-atom recombination on the surface of a polycrystalline Ag surface are reported. Surface O atoms are generated by oxygen permeation through a 0.25-mm-thick Ag foil heated to 1020 K. Desorbing O2 molecules are probed by (2+1) resonant multiphoton ionization via the C 3Πg (3sσ), v′=2←←X 3Σg−, v″=0 transition and time-of-flight mass spectrometry. Measured velocity distributions are near Maxwell–Boltzmann and yield average translational energies which are significantly lower than the surface temperature (〈Et〉/2kB∼515 K) and essentially independent of rotational excitation. Comparison of the observed C–X (2,0) resonantly enhanced multiphoton ionization spectrum with spectral simulations suggests that the v″=0 rotational state distribution is more consistent with the surface temperature, but spectral congestion and apparent intensity perturbations prevent a more quantitative analysis. The calculated, sticking curves show a small barrier energy barrier (∼10 meV) beyond which sticking decreases. These observations are consistent with low energy desorption and adsorption pathways involving a weakly bound molecular O2 precursor.
Enhancements in dissociative electron attachment to CF4, chlorofluorocarbons and hydrochlorofluorocarbons adsorbed on H2O ice
Authors: Lu Q.-B. , Sanche L.
Published: January 2004

We report that the absolute cross sections for dissociative attachment of ∼0 eV electrons to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are strongly enhanced by the presence of H2O ice. The absolute cross sections for CFCl3, CHF2Cl, and CH3CF2Cl on water ice are measured to be ∼8.9×10−14, ∼5.1×10−15, and ∼4.9×10−15 cm2 at ∼0 eV, respectively. The former value is about 1 order of magnitude higher than that in the gas phase, while the latter two are 3–4 orders higher. In contrast, the resonances at electron energies ⩾2.0 eV are strongly suppressed either for CFCs and HCFCs or for CF4 adsorbed on H2O ice. The cross-section enhancement is interpreted to be due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of CFCs or HCFCs followed by its dissociation. This study indicates that electron-induced dissociation is a significant process leading to CFC and HCFC fragmentation on ice surfaces.
Laser control of product electronic state: Desorption from alkali halides
Authors: Beck Kenneth M. , Joly Alan G. , Dupuis Nicholas F. , Perozzo Peggy , Hess Wayne P. , Sushko Peter V. , Shluger Alexander L.
Published: January 2004

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin–orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P1/2), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.
Application of the modified Shepard interpolation method to the determination of the potential energy surface for a molecule–surface reaction: H2+Pt(111)
Authors: Crespos C. , Collins M. A. , Pijper E. , Kroes G. J.
Published: January 2004

We have used a modified Shepard (MS) interpolation method, initially developed for gas phase reactions, to build a potential energy surface (PES) for studying the dissociative chemisorption of H2 on Pt(111). The aim was to study the efficiency and the accuracy of this interpolation method for an activated multidimensional molecule–surface reactive problem. The strategy used is based on previous applications of the MS method to gas phase reactions, but modified to take into account special features of molecule–surface reactions, like the presence of many similar reaction pathways which vary only slightly with surface site. The efficiency of the interpolation method was tested by using an already existing PES to provide the input data required for the construction of the new PES. The construction of the new PES required half as many ab initio data points as the construction of the old PES, and the comparison of the two PESs shows that the method is able to reproduce with good accuracy the most important features of the H2+Pt(111) interaction potential. Finally, accuracy tests were done by comparing the results of dynamics simulations using the two different PESs. The good agreement obtained for reaction probabilities and probabilities for rotationally and diffractionally inelastic scattering shows clearly that the MS interpolation method can be used efficiently to yield accurate PESs for activated molecule–surface reactions.
Molecular dynamics of oligofluorenes: A dielectric spectroscopy investigation
Authors: Papadopoulos P. , Floudas G. , Chi C. , Wegner G.
Published: January 2004

The molecular dynamics were investigated in a series of “defect-free” oligofluorenes up to the polymer by dielectric spectroscopy (DS). The method is very sensitive to the presence of keto “defects” that when incorporated on the backbone give rise to poor optical and electronic properties. Two dielectrically active processes were found (β and α process). The latter process (α) displays strongly temperature dependent relaxation times and temperature- and molecular weight-dependent spectral broadening associated with intramolecular correlations. The glass temperature (Tg) obeys the Fox–Flory equation and the polymer Tg is obtained by DS at 332 K. The effective dipole moment associated with the α process is 0.27±0.03 D.
State-to-state rotational rate constants for CO+He: Infrared double resonance measurements and simulation of the data using the SAPT theoretical potential energy surface
Authors: Smith Tony C. , Hostutler David A. , Hager Gordon D. , Heaven Michael C. , McBane George C.
Published: January 2004

An extensive data set of 54 time-resolved pump-probe measurements was used to examine CO+He rotational energy transfer within the CO v=2 rotational manifold. Rotational levels in the range Ji=2–9 were excited and collisional energy transfer of population to the levels Jf=1–10 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions and ab initio theoretical calculations that employed the SAPT potential energy surface of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)]. Fitting laws based on the modified exponential gap (MEG), statistical power exponential gap (SPEG), and energy corrected sudden with exponential power (ECS-EP) models all yielded acceptable simulations of the kinetic data, as did the theoretical rate constants. However, the latter were unique in their ability to reproduce both our kinetic data and the pressure broadening coefficients for CO+He. These results provide an impressive demonstration of the quality of the symmetry adapted perturbation theory (SAPT) potential energy surface.
Measurements and simulations of high energy O(3P)+Ar(1S) angular scattering: Single and multi-collision regimes
Authors: Braunstein M. , Brunsvold Amy L. , Garton Donna J. , Minton Timothy K.
Published: January 2004

We present differential angular cross sections for O(3P)+Ar(1S) scattering at collision energies near 90 kcal mol−1 (∼8 km s−1 relative velocity) from molecular beam measurements and high-level theoretical calculations. Beams of hyperthermal O(3P) are now being used to investigate novel gas-phase and gas-surface chemistries, and the comparison of theory and measurements on this simple system will be a stringent test of the experimental methodology. Potential energy curves were generated for O(3P)+Ar(1S) using a large cc-pVQZ basis within a valence multi-configuration plus perturbation theory treatment. These curves were then used in quantum scattering calculations to generate differential cross sections. Agreement between experiment and theory is excellent. In addition to these comparisons, the cross sections were used in direct simulation Monte Carlo calculations to investigate effects of increasing the Ar flux above the “single-collision” regime. As the Ar flux increases, the observed differential angular cross sections change in two ways. In addition to the main “single-scatter” peak along the incident O-atom beam direction, a secondary O-atom peak appears in the direction of the incident Ar beam, and the multiple-scattered O-atom translational energy starts to reflect the energy of the relatively slow moving Ar beam.
Ionic excitation in dense, two-component plasmas: Effect of energy level shifts
Authors: Kitamura Hikaru
Published: January 2004

Collisional excitation rates for an ion immersed in dense electron–ion two-component plasmas are formulated by taking into account shifts of ionic energy levels due to static and dynamic plasma perturbations. The theory is based on the equations of motion for density matrices combined with the statistical theory of plasma density fluctuations. Through separation of the time scales associated with electron- and ion-density fluctuations, the electron-induced excitation rates are derived, where energy level shifts arising from quasistatic electric microfields by screened plasma ions as well as those arising from time-averaged spherical plasma polarization are considered. As a numerical example, the transition rates among the 2s and 2p fine-structure levels of a Ne9+ ion in dense hydrogen plasmas are calculated. It is demonstrated that, at sufficiently high plasma densities, the ion microfields cause Stark splittings and affect the excitation rates significantly through modifications of the generalized oscillator strengths.