bluuebunny/crossref_metadata_2025 huggingface dataset:
Interference of three electron waves by two biprisms and its application to direct visualization of electromagnetic fields in small regions
Authors: Hirayama Tsukasa , Lai Guanming , Tanji Takayoshi , Tanaka Nobuo , Tonomura Akira
Published: July 2002

We present a method for the interference of three electron waves and its application to direct visualization of pure phase objects such as electromagnetic microfields. Using a transmission electron microscope equipped with a field-emission electron gun and two electron biprisms, an object wave and two reference waves at either side of the object wave are superposed to produce a new type of interference pattern. In this pattern, equal-phase lines of the object wave are directly displayed as intensity modulation of periodic interference fringes. An electric field around a latex particle, induced by electron-beam irradiation, has been observed. The electric charge of the particle is estimated, from observed phase shift, to be 6.4×10−17 C, which is equal to about 400 electrons. A change of the electric field around charged alumina particles at high temperatures has been observed dynamically. Magnetic flux lines emerging from a barium ferrite particle are also visualized.
An evidence of trap activation for positive temperature coefficient of resistivity in BaTiO3 ceramics with substitutional Nb and Mn as impurities
Authors: Miki Toshikatsu , Fujimoto Akira , Jida Shin’suke
Published: July 2002

The mechanism of the electrical resistivity increase above the Curie temperature TC [so-called positive temperature coefficient of resistivity (PTCR) effect] in BaTi1−x−yNbxMnyO3 ceramics is studied with the temperature-dependent resistivity and electron paramagnetic resonance (EPR) data and the calculated energy levels of various Mn-associated centers. The activation energy of trapped electrons at grain boundary interface states is deduced from the resistivity data. The deduced value is found to be nearly equal to the activation energy of Mn2+ and/or singlet EPR signals. This indicates that the EPR signals arising at the cubic phase are due to trapped-electron centers at grain boundaries, and that the centers responsible for the EPR signals contribute to the rise of grain boundary barrier above TC. The PTCR mechanism is separately discussed for mode I of abrupt resistivity jump at TC and mode II of moderate resistivity increase at higher temperatures. From the analyses of resistivity and EPR data, we found that mode II PTCR is well explained by the Heywang model, but mode I PTCR is hardly explained only by the effect of spontaneous polarization proposed by Jonker. Defect models of Mn–VO, Mn–VO–Mn, and Mn–O–Nb–VO type are proposed for Mn associated centers and VBa–VO or VO–VTi–VO type for cation-vacancy-associated centers, taking the local electroneutrality requirement for stable defects and the electronic energy change of substitutional Mn by phase transitions into account. Changes of the electronic energy levels of such defects by phase transitions successfully explain not only the PTCR and EPR data but also the degraded PTCR by incomplete or overabundant oxidation. A part of the Mn–VO centers formed at low Mn contents release electrons at the tetragonal phase forming Mn3+–VO with an effective charge of +1 to act as a charge compensator for negatively charged acceptors at the grain boundary, but the Mn–VO centers hardly act as acceptor-type electron-trapping centers at the cubic phase. At high Mn concentrations, the Mn–VO–Mn type di-Mn center acts as an acceptor-type electron-trapping center at both tetragonal and cubic phases. A trap activation occurs in one type of Mn–O–Mn–VO centers at the tetragonal-to-cubic transition. We conclude that the trap activaiton of the Mn interface states by the tetragonal-to-cubic transition significantly contributes to the PTCR effect of BaTiO3 ceramics with a high content of Mn.
Effect of the surface upon misfit dislocation velocities during the growth and annealing of SiGe/Si (001) heterostructures
Authors: Stach E. A. , Hull R. , Tromp R. M. , Reuter M. C. , Copel M. , LeGoues F. K. , Bean J. C.
Published: July 2002

We have measured the velocity of misfit dislocation threading segments in real time during ultrahigh vacuum (UHV) chemical vapor deposition heteroepitaxial growth of thin SiGe epilayers on Si (001) using ultrahigh vacuum transmission electron microscopy. We observe no measurable difference in dislocation velocities during growth and during post-growth annealing of samples with an atomically clean surface, in contrast to previous observations in the InGaAs/GaAs (001) system. However, dislocations are seen to move approximately three times slower during growth and post-growth UHV annealing than during annealing of samples which have a native oxide present on the surface. We have used post-growth depositions of arsenic and oxygen to investigate the effect of surface condition on dislocation velocities, and discuss possible causes for the increase in dislocation velocities in the presence of a native oxide. These systematic studies suggest a hitherto unappreciated interaction between moving dislocations and the surface in this system.
Magnetoelectrolysis of copper
Authors: Hinds G. , Coey J. M. D. , Lyons M. E. G.
Published: July 2002

The effect of a uniform static magnetic field on processes occurring at Cu/acidified CuSO4 interfaces is examined by analysis of potentiodynamic polarization curves. The mass transport controlled deposition current is increased by as much as a factor of 2 in a 0.6 T field, independent of field direction and electrode orientation. The extent of current enhancement is greatest at low pH(<1), where it is associated with diffusional rather than migrational transport of the copper ions.
Role of the A-site size and oxygen stoichiometry in charge ordering commensurability of Ln0.50Ca0.50MnO3 manganites
Authors: Barnabé A. , Hervieu M. , Martin C. , Maignan A. , Raveau B.
Published: July 2002

The commensurability of the charge ordering process of the Ln0.50Ca0.50MnO3 manganites (Ln=Pr, Nd, Sm, Eu, and Gd) has been studied by electron diffraction and lattice imaging versus temperature. For the as-synthesized phases, it is shown that an incommensurate state at low temperature (92 K) is favored as the size of the lanthanide increases. A large transition width (ΔT ranging from 40 to 110 K) is observed which is closely related to the size of Ln3+, and also to the size mismatch between Ln3+ and Ca2+ cations. The most important result concerns the effects of oxygen stoichiometry, varied via different annealings carried out at low temperature (T⩽600 °C) under Ar/H2 flow, air or O2 at PO2=100b. It is shown that very tiny deviations in the oxygen stoichiometry, which are not measurable using conventional techniques, affect significantly the transition width and commensurability, keeping the magnetic properties nearly unchanged. It is demonstrated that oxidized samples are systematically incommensurate whereas stoichiometric or the less oxidized ones are commensurate. A structural model is proposed to account for the modulated distribution of the additional [Mn4+]∞ layers, with respect to the 1:1 ordering that should be obtained for the perfectly stoichiometric Ln0.50Ca0.50MnO3 phase. The concomitant influence of oxygen stoichiometry, cationic size, and mismatch effect upon the charge ordering phenomena in these oxides is discussed.
New rare-earth permanent magnets with an intrinsic coercivity of 10 kOe at 500 °C
Authors: Liu J. F. , Ding Y. , Zhang Y. , Dimitar D. , Zhang F. , Hadjipanayis G. C.
Published: July 2002

Sm ( Co bal Fe 0.1 Cu x Zr 0.033 ) z magnets with higher Cu content were found to have higher intrinsic coercivity Hci at both room temperature and higher temperatures up to 500 °C. For the magnets with a ratio z=8.5, the intrinsic coercivity reaches a maximum of about 40 kOe at room temperature when the Cu content x reaches 0.088. But for the magnets with a ratio z=7.0, the intrinsic coercivity continues to increase with increasing Cu content x up to 0.128. Microstructure analysis showed that the cell size decreases while the density of lamella phase increases with increasing Cu content. A record high intrinsic coercivity of 10.8 kOe at 500 °C was obtained on a Sm(CobalFe0.1Cu0.128Zr0.033)z magnet.
Perpendicular magnetic anisotropy of Co–Ag granular thin films
Authors: Chen Y. J. , Suzuki T. , Wong S. P. , Sang H.
Published: July 2002

The perpendicular magnetic anisotropy constant in giant magnetoresistive granular Co22Ag78 thin films is found to increase and then decrease with annealing temperature, the maximum being about 5×105 ergs/cc at an annealing temperature of about 600 K. The observation of domain patterns by magnetic force microscopy is consistent with this result. The origin of the perpendicular magnetic anisotropy is not clear, but suggested to be in an anisotropic Co–Co particle distribution and also due to a surface anisotropy of Co particles.
Charge trapping and defect segregation in quartz
Authors: Stevens Kalceff M. A. , Thorogood G. J. , Short K. T.
Published: July 2002

Irradiation induced charging of wide band gap materials may significantly influence the development of radiation damage and associated defect migration. Charge trapped at irradiation induced and/or pre-existing defects induces a localized electric field within the irradiated volume of specimen. The powerful combination of cathodoluminescence microanalysis and electric force microscopy allows direct monitoring of the development of the irradiation induced charge distribution and its effect on the microscopic spatial segregation of defects. These techniques have been used to demonstrate the important influence of the induced local field on the microscopic defect structure of quartz.
Charge transport in thin interpoly nitride/oxide stacked films
Authors: De Salvo B. , Ghibaudo G. , Pananakakis G. , Guillaumot B. , Reimbold G.
Published: July 2002

In this work, charge transport through interpoly thin nitride/oxide stacked films, including nitride/oxide dual- and oxide/nitride/oxide tri-layer films, was studied. Extensive experimental results, concerning current conduction in single oxide layer, single nitride layer, nitride/oxide dual-layer, and oxide/nitride/oxide tri-layer films are presented. An effective investigation of the various mechanisms that can explain current conduction and charge trapping in these dielectrics was performed. To this aim, different approaches to transport modeling, namely, a classical current continuity model, a transmission model, and a two-step trap assisted model are proposed. The gains and trade offs offered by each model are pointed out. A comprehensive model for the conduction mechanisms in thin nitride/oxide stacked films is proposed.
Internal sheaths in electronegative discharges
Authors: Kouznetsov I. G. , Lichtenberg A. J. , Lieberman M. A.
Published: July 2002

In three component electronegative discharges a parameter regime can be found in which the positive ions reach the local ion sound velocity at a position where the negative ion density may be significant compared to the electron density. For this regime a quasineutral electronegative core breaks down and a space charge region forms. Solutions in the space charge region are obtained in collisionless and collisional cases, neglecting ionization and positive-negative ion recombination. The structure of the non-neutral region is shown to vary significantly with the ratio of the negative ion and electron densities at the core edge, the ratio of ion and electron temperatures, and the ratio of the electron Debye length to the ion mean free path. If the first ratio is not too large the non-neutral region displays potential oscillations on the electron Debye length spatial scale, which damp away on the scale of the ion–neutral mean free path. The non-neutral region then terminates within the plasma. The change in electric potential across this region is several times the negative ion temperature, which is sufficient to confine the negative ions to the core. The non-neutral region merges with a quasineutral halo containing essentially only positive ions and electrons. If the negative ion density is sufficiently high compared to the electron density the electropositive halo disappears and the non-neutral region extends from the ion sound velocity threshold to the wall. For intermediate values of the negative ion density, a space charge double layer forms between the electronegative and electropositive regions.
Growth of III2VI3 compound semiconductor (GaxIn1−x)2Se3 single crystals with giant optical activity for visible and infrared light
Authors: Ye Jiping , Nakamura Yoshio , Nittono Osamu
Published: July 2002

High grade single crystals of (GaxIn1−x)2Se3 showing giant optical activity for visible and infrared light have been successfully grown with an iodine transport technique. Two kinds of single crystals with reverse rotatory directions to each other have been obtained; one is the dextrorotatory with space group P61 and another is the levorotatory with P65. The composition x is controlled from 0.03 to 0.70. The morphology is hexagonal plate and the size is up to 3 mm in diameter and hundreds of microns in thickness. The crystals for different compositions x possess different fundamental optical absorption edges from 610 to 720 nm. The crystal habit facets are the (001) and {110} planes. These crystals have a polarity and their growth direction is limited to the [00-1] direction. The lattice defects such as antiphase domain boundary, stacking fault, inversion twin, and reversion screw almost have not been observed. It is found that these growth characteristics are strongly related to the crystal structure in which the structural vacancies are ordered in screw form along the c axis.
Debye function analysis of structure in diffraction from nanometer-sized particles
Authors: Hall B. D.
Published: July 2002

The special features of diffraction from particles in the nanometer size range are discussed and the problem of structure determination is considered. A direct method of structure analysis, known as Debye Function Analysis, is presented and evaluated. The method is capable of obtaining information about both the size and structure of domains in a sample, and can identify noncrystalline structures. Numerical simulations of observations are used to investigate the limitations of the technique, and also provide a general method for quantifying uncertainties in parameters estimated by Debye Function Analysis.
Magnetic and transport properties of epitaxial and polycrystalline chromium dioxide thin films (invited)
Authors: Gupta A. , Li X. W. , Xiao Gang
Published: July 2002

The magnetic and transport properties of epitaxial and polycrystalline chromium dioxide (CrO2) thin films have been investigated. They are grown epitaxially on single crystal TiO2 (100) substrates, and with multiple grain orientations on polycrystalline TiO2 substrates, by chemical vapor deposition. The films have a Curie temperature (TC) of 390–395 K, with the epitaxially grown CrO2 (100) films exhibiting in-plane uniaxial magnetic anisotropy. While the epitaxial samples display metallic characteristics, the polycrystalline films are semiconducting with a dominant grain boundary contribution to the resistance at low temperatures. The magnetoresistance (MR) properties have also been measured with the magnetic field applied in the plane. For the epitaxial films, the MR is negative at temperatures near TC and is positive at low temperatures. A negative MR is also observed near TC for the polycrystalline samples. However, unlike the epitaxial films, the MR is found to be negative also at low temperatures, with a significant low field component. The latter is attributed to spin-polarized transport of electrons across grain boundaries.
Magnetic and structural properties of ultrafine Ni–Zn–Cu ferrite grown by a sol–gel method
Authors: Kim Woo Chul , Park Seung Iel , Kim Sam Jin , Lee Seung Wha , Kim Chul Sung
Published: July 2002

Ultrafine Ni0.65Zn0.35Cu0.2Fe1.8O4 particles were fabricated by a sol–gel method. The magnetic and structural properties of the powders were investigated with x-ray diffraction, vibrating sample magnetometer, and Mössbauer spectroscopy. Ni–Zn–Cu ferrite powders that were fired at and above 823 K have only a single phase spinel structure and behave ferrimagnetically. Powders annealed at 523, 623, and 723 K have a typical spinel structure and are simultaneously paramagnetic and ferrimagnetic in nature. The magnetic behavior of Ni–Zn–Cu ferrite powders fired at and above 623 K showed that an increase of the annealing temperature yielded a decrease of the coercivity and an increase of the saturation magnetization. The maximum coercivity and the saturation magnetization of Ni–Zn–Cu ferrite powders were Hc=96 Oe and Ms=68 emu/g. Mössbauer spectra of powder annealed at 1223 K were taken at various temperatures ranging from 12 to 675 K. As the temperature increased toward TN, a systematic line broadening effect in the Mössbauer spectra was observed and was interpreted as originating from the different temperature dependencies of the magnetic hyperfine fields at various iron sites. The isomer shifts indicated that the iron ions were ferric at the tetrahedral [A] and the octahedral site [B]. The Néel temperature was determined to be TN=675±2 K.
Magnetic dead layers in NiFe/Ta and NiFe/Si/diamond-like carbon films
Authors: Leng Qunwen , Han Hua , Mao Ming , Hiner Craig , Ryan Francis
Published: July 2002

NiFe, Ta films were fabricated by ion beam deposition (IBD) and diamond-like carbon (DLC) films by ion beam chemical vapor deposition (IB-CVD) and filtered cathodic arc (FCA) process. Magnetic dead layers at interfaces of Ta/NiFe/Tn and NiFe/Si/DLC trilayer films were determined by characterizing magnetic flux loss using a B–H loop tracer. Dependence of magnetic dead layer on ion beam voltage and thicknesses of Ta, DLC, and Si layers was investigated. It is found that the thickness of magnetic dead layer increases monotonously with increasing ion beam voltage for Ta and DLC film depositions. The magnetic dead layer of 4–6 Å thick forms at Ta/NiFe and NiFe/Ta interfaces at an ion beam voltage of 1000 V, which can be attributed to the atomic intermixing of incoming energetic adatoms with atoms of grown films at interfaces. Direct ion beam deposition of the DLC film in NiFe/Si/DLC layered structure gives rise to a magnetic thickness loss of 12–18 Å. Transmission electron microscopy cross-sectional observations have confirmed the formation of an amorphous-like interfacial layer, as a result of carbonization or silicidation of NiFe at interfaces of the trilayer film.
Temperature dependent chemical ordering in FePt(001) and FePt(110) films
Authors: Schwickert M. M. , Hannibal K. A. , Toney M. F. , Best M. , Folks L. , Thiele J.-U. , Kellock A. J. , Weller D.
Published: July 2002

The growth temperature is one of the critical parameters in the formation of the chemically ordered L10 phase in FePt. To uniquely determine the influence of growth temperature, while other growth parameters remain fixed, we present the “temperature wedge method” where a calibrated temperature gradient of several hundred Kelvin is established across the substrate during co-deposition of Fe and Pt. Samples were deposited in a temperature range of 250 °C–600 °C simultaneously on MgO(001) and MgO(110) substrates. The Fe≈50Pt≈50 composition was verified by Rutherford backscattering. The chemical order parameter, S, at different positions along the samples, i.e., at different growth temperatures, was determined by x-ray diffraction measurements, and it was found that at all temperatures S(001)>S(110). Perpendicular and in-plane magneto-optic Kerr hysteresis loop measurements indicate large magnetic anisotropy at the “hot ends” of the samples, corresponding to large values of S.
Magnetic properties of disordered Fe0.9−xMn0.1Alx alloys
Authors: Restrepo J. , Pérez Alcázar G. A. , González J. M.
Published: July 2002

We have examined the characteristics of the magnetic order present at room and lower temperatures in disordered samples corresponding to the Fe0.9−xMn0.1Alx (0.1⩽x⩽0.5) series prepared by arc melting, high temperature homogenization, and quenching. To that purpose we have measured the compositional variation of the lattice parameters, the Mössbauer spectra, the high field magnetization, and the saturation coercivity and also the compositional and temperature dependences of the magnetic susceptibility and the low field magnetization. Our results are summarized in a magnetic phase diagram including ferromagnetic, paramagnetic, pure spin glass, and reentrant spin glass regions.
A theoretical and experimental study of the infrared line shape from resonance to the wings for uncoupled lines
Authors: Houdeau J. P. , Boulet C. , Robert D.
Published: July 2002

In a theoretical approach of the infrared line shape in the gas phase, each of the following effects is carefully examined: initial orientational correlations between colliding molecules, lines coupling, and finite duration of collisions. For the case where only the last effect is significant, a model is proposed which is convenient for calculating involved molecular systems. Application to the CO2-broadened HCl lines leads to a new and clear understanding of the pressure dependence of the infrared line shape, in the resonance regions as well as in the troughs between lines. In particular, a consistent description of the observed super-Lorentzian behavior in these troughs is obtained.
Through bond interaction of two radical centers: Analysis of the spin polarization and related mechanisms in linear π diradicals
Authors: Karafiloglou Padeleimon
Published: July 2002

Linear π diradicals are treated as a through-bond interaction of two radical centers. The ferromagnetic or antiferromagnetic electron coupling is examined by means of an analytic study of the interaction of two methylene groups (the radical centers) through a butadiene (the bridge). The interaction of each radical center with the bridge is treated at the Hartree–Fock level; the obtained NBMO’s (or magnetic orbitals) A and B are of two types: (i) A and B are of the same parity (i.e., with common delocalization coefficients on the same AO’s of the bridge) for the first class of polyenes with m=m*±2 (m and m* are the unstarred and starred positions). (ii) A and B are of a different parity for the second class of polyenes with m=m*. The method of the direct calculation of the singlet–triplet transition energy is used at configuration interaction level, in order to elucidate the ferromagnetic or antiferromagnetic behavior of the various electronic mechanisms. Numerical calculations show that double spin polarization (DSP) controls the whole singlet–triplet energy difference; this is then analyzed exhaustively in the framework of RHF plus CI methods; a clear physical interpretation is then given in the language of UHF methods. The two spin densities induced in the bridge from the two radical centers are: (i) in phase, for the triplet state of the first class of polyenes and for the singlet state of the second class; the two spin polarizations are additive and the spin fluctuations very high; (ii) out of phase for the singlet state of the first class of polyenes and for the triplet state of the second class; the two spin polarizations are competitive without high spin fluctuations. The analytical study generalizes our conclusions for the ferromagnetic or antiferromagnetic behavior of any dimethylene linear-polyene system.
Observation of n
s 3Σ+u(1u, 0−u)→m
p 3Πg Rydberg transitions of Ar2 (n=4; 5≤m≤15) and Kr2(n=5; 6≤m≤16) in absorption
Authors: Killeen K. P. , Eden J. G.
Published: July 2002

Rydberg series of Ar2 and Kr2 have been observed in the visible and ultraviolet (350≤λ≤560 nm) absorption spectra of the lowest-lying metastable states of the dimer (ns 3Σ+u(1u, 0−u); Ar: n=4, Kr: n=5). The most prominent series for Ar2 and Kr2 are assigned to ns 3Σ+u → mp 3Πg (Ar: 5≤m≤15; Kr: 6≤m≤16) Rydberg transitions of the diatomic molecule and adiabatic ionization potentials are determined from an extrapolation of the series to their limits (m → ∞) to be 116 356±340 cm−1 (14.43±0.04 eV) for Ar2 and 104 616±150 cm−1 (12.97±0.02 eV) for Kr2. The dissociation energies of the A2Σ+u[1( (1)/(2) )u] dimer ion states of Ar2 and Kr2 are, therefore, 1.33±0.04 and 1.03±0.02 eV, respectively. All of the observed Rydberg states have an A2 Σ+u ion core. Binding energies of the lowest observed 3Πg states, were determined to be (1.331±0.005) eV for Ar2 and (1.030±0.001) eV for Kr2. The quantum defects for the Ar2 and Kr2 series (1.656±0.006 and 2.621±0.004, respectively) and ωe−2ωexe for the ns 3Σ+u metastable state of Ar2 (303±20 cm−1) and Kr2 (172±10 cm−1) were also measured and the vibrational frequencies are consistent with previously reported values. Absorption peaks which are attributed to ns 3Σ+u→m′p 3Σ+g Rydberg transitions of Ar2 (6≤m′≤10) and Kr2 (7≤m′≤10) are also reported.
Hyperfine structure and electric dipole moment of BaF X 2Σ+
Authors: Ernst W. E. , Kändler J. , Törring T.
Published: July 2002

Molecular-beam laser-microwave double-resonance spectroscopy was applied to study rotational transitions in the X 2Σ+, v=0 state of BaF at high resolution. From the analysis of the fluorine hyperfine structure (hfs) the magnetic hfs parameters were determined as b=63.509 (32) MHz and c=8.224 (58) MHz (statistical standard deviation in parentheses). Additional Stark effect measurements yielded and electric dipole moment μ=3.170(3) D. The magnetic hfs is interpreted as an effect of fluorine electron spin polarization. Mixing coefficients for Ba+ hybridization are estimated and compared with the charge distribution derived from the measured dipole moment in terms of an ionic bonding model.
Selection rules for surface Raman spectroscopy: Experimental results
Authors: Hallmark Vickie M. , Campion Alan
Published: July 2002

We report the results of an experimental survey determining the contributing factors for Raman spectral activity of molecules adsorbed on metal surfaces. The systems investigated are benzene, pyrazine, and s-triazine physically adsorbed onto the (111) and (110) faces of single crystal silver. We consider the relative importance of three factors: symmetry reduction, image dipoles (metal optics), and quadrupole polarizability contributions. The experimental data may be explained satisfactorily by involving both the adsite geometry and image effects in the theoretical selection rules. Quadrupole interactions appear neither necessary nor sufficient to predict the proper surface spectra.
Propensities for collision induced electronic transitions in a diatomic molecule
Authors: Katayama Daniel H. , Dentamaro Anthony V.
Published: July 2002

An optical–optical double resonance (OODR) technique is used to determine propensities for collision induced electronic relaxation by helium atoms from a specific A 2Πui (v=4, J) rotational level to the X 2Σ+g (v=7) manifold of N+2. The propensities for collisional transfer from this specific level to the nearly degenerate (∼0.04 cm−1 separation) spin components of the X(v=7) state are resolved by scanning the probe laser through the B 2Σ+u –X 2Σ+g (5,7) band whose upper state is perturbed. Although ΔJ≈0 transfers are preferred, the results show the propensities to be quite different and strongly dependent on the A(v=4,J) level initially populated by the pump laser. The observation of these propensities for collisional electronic energy transfer through a large energy gap of approximately 1760 cm−1 demonstrates the remarkable fact that this process occurs as fast or faster than rotational energy transfer through gaps of ∼10 cm−1. These results are found to be in qualitative agreement with theoretical relative cross sections derived by Alexander and Corey for inelastic collision induced transitions between 2Π and 2Σ electronic states of a diatomic molecule.
The fractal nature, graph invariants, and physicochemical properties of normal alkanes
Authors: Rouvray D. H. , Pandey R. B.
Published: July 2002

The variation in the physicochemical properties of linear normal alkane molecules with the graph invariants carbon number and the Wiener index is investigated. It is demonstrated that the variation can be interpreted in terms of the fractal dimension of these species. A general formula expressing the precise nature of the relationship has been derived.
Intermolecular bonding and vibrations of the carbazole⋅B complexes (B=H2O, D2O, NH3)
Authors: Honegger Evi , Bombach Rolf , Leutwyler Samuel
Published: July 2002

Carbazole⋅B complexes (B=H2O, D2O, NH3) were synthesized and cooled in pulsed supersonic nozzle beams. The intermolecular hydrogen-bond vibrations and dissociation energies were studied by several laser-spectroscopic techniques (fluorescence excitation and emission, resonance-two-photonionization with mass-specific detection). The following results were obtained for both S0 and S1 electronic states: (1) determination of the structural symmetry of the complexes, (2) measurement of the intermolecular stretching (νσ) and bending (νβ) frequencies, (3) determination of stretching force constants, (4) hydrogen-bond dissociation energies D0 for carbazole⋅H2O/D2O, and (5) electronic spectral shifts δν̃ relative to the bare carbazole molecule. The latter are large (500–710 cm−1) and reflect an increase of the hydrogen-bond energy by ≈40% upon electronic excitation. Fermi resonance couplings between the intermolecular νσ and an intramolecular b1 vibration of carbazole are observed and partially analyzed. To complement the experimental work, extensive ab initio quantum chemical calculations of the same complexes were performed at the Hartree–Fock level. The calculated complex structures are consistent with the experimental information. Intermolecular potential-energy curves for the stretching vibrational coordinate were calculated for a number of increasingly flexible basis sets (STO-3G, 4-31G, 6-31G, 4-31G*); anharmonic vibrational frequencies were then obtained numerically. Excellent agreement with the experimental intermolecular stretching frequencies was found for all three complexes using the 4-31G* basis set. Good agreement with experiment was also found for the calculated 4-31G* hydrogen-bond dissociation energy.
The effect of critical fluctuations on chemical equilibrium
Authors: Tveekrem J. L. , Cohn R. H. , Greer S. C.
Published: July 2002

We have studied the temperature dependence of the extent of dimerization of NO2 dissolved in a mixture of perfluoromethylcyclohexane+carbon tetrachloride near the mixture liquid–liquid critical point. We used the static dielectric constant as a measure of the extent of dimerization. We find that the dimerization decreases near the critical temperature by about 4% and that the functional form of the temperature dependence can be described by a critical exponent of (1−α), where α is the exponent for the heat capacity at constant pressure and composition and has the value 0.11. Such a temperature dependence is in agreement with the theoretical analysis of J. C. Wheeler and R. G. Petschek [Phys. Rev. A 28, 2442 (1983)].
Molecular dynamics of a model S
N2 reaction in water
Authors: Bergsma John P. , Gertner Bradley J. , Wilson Kent R. , Hynes James T.
Published: July 2002

Molecular dynamics are computed for a model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water and are found to be strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier. There are significant deviations from the simple picture of passage over a free energy barrier in the reaction coordinate, and thus, a marked departure from transition state theory occurs in the form of barrier recrossings. Factors controlling the dynamics are discussed, and, in particular, the rate of change of atomic charge distribution along the reaction coordinate is found to have a major effect on the dynamics. A simple frozen solvent theory involving nonadiabatic solvation is presented which can predict the outcome of a particular reaction trajectory by considering only the interaction with the solvent of the reaction system at the gas-phase transition barrier. The frozen solvent theory also gives the transmission coefficient κ needed to make the transition state theory rate agree with the outcome of the molecular dynamics trajectories. This theoretical κ value, which is the implementation for the SN2 reaction of the van der Zwan–Hynes nonadiabatic solvation transmission coefficient, is in good agreement with the trajectory results. In contrast, a Kramers theory description fails dramatically.
Experimental study of the cold mercury dimer
Authors: Zehnacker A. , Duval M. C. , Jouvet C. , Lardeux-Dedonder C. , Solgadi D. , Soep B. , d’Azy O. Benoist
Published: July 2002

The mercury dimer is among the most weakly bound metal dimers and has been extensively studied.1 The ground state O+g dissociation energy has been considered to lie between 0.55 eV (440 cm−1) and 0.091 eV (730 cm−1). We report here a spectroscopic study of Hg2 in a supersonic jet. The first optical transition, 1u←O+g, was characterized by its fluorescence excitation spectrum and the binding energy of the ground state has been measured precisely through the threshold of collision induced dissociation of Hg2 1u to Hg(1S0)+Hg(3P0).
The thermodynamic properties of electrolyte solutions: Some formal results
Authors: Kusalik P. G. , Patey G. N.
Published: July 2002

The Kirkwood–Buff approach is used to obtain exact determinate expressions for the thermodynamic properties of electrolyte solutions. The solvent is treated at a molecular level and the thermodynamic functions are expressed in terms of ion–ion, ion–solvent, and solvent–solvent correlation functions. The equations obtained are particularly useful when used in conjunction with integral equation theories. The low concentration limiting behavior of the microscopic expressions is examined and it is shown that the Debye–Hückel limiting law for the activity coefficient can be readily extracted from the molecular theory. Also the partial molecular volume of the salt is considered in some detail and microscopic relationships are given for the infinite dilution value.
A surface enhanced resonance Raman study of cobalt phthalocyanine on rough Ag films: Theory and experiment
Authors: Zeman E. J. , Carron K. T. , Schatz G. C. , Van Duyne R. P.
Published: July 2002

We present a combined experimental/theoretical study of the coverage and frequency dependence of surface (enhanced) resonance Raman scattering [S(E)RRS] of cobalt phthalocyanine (CoPc) on CaF2 roughened silver films. The experimental spectra indicate a rather strong coverage dependence at very low coverage for excitation at or close to the molecular resonance frequency, with a peak at 0.07 monolayer (ML) followed by a rapid decrease above that. This coverage dependence differs strongly with observations on smooth films, where a much weaker dependence is observed. At very low coverage on the rough Ag films S(E)RRS enhancements comparable to SERS are observed. To model this coverage dependence, we consider the electromagnetic interactions between the adsorbate and substrate, with the substrate modeled as a metal spheroid and the adsorbate as a layer with variable coverage. Two models for this layer are considered, an effective medium model in which the layer is taken to have a coverage dependent dielectric constant, and a coupled dipole model in which both the molecules and metal are taken to be polarizable dipoles. The dependence of field enhancement and S(E)RRS intensity is studied for these two models as a function of frequency, coverage, and Stokes shift. It is found that although there are differences between the two models, the coverage dependence is similar in both, with peak intensities at about 0.1 ML for reasonably prolate spheroids. These models also demonstrate that the drop in intensity above 0.1 ML arises from damping of the plasmon resonance by the adsorbed layer. Interadsorbate interactions are found to play a role in determining the coverage dependence of the S(E)RRS intensity that is secondary to this damping. The large enhancements seen below 0.1 ML suggest that excited state quenching by the surface is not important for this nonfluorescent molecule.
On the Born and Markov approximations: Phonon relaxation and coherent excitation of adsorbed molecules
Authors: van Smaalen Sander , George Thomas F.
Published: July 2002

The phonon relaxation of the vibrational adbond of an adsorbed molecule and a phonon-damped adbond irradiated by a laser are studied. In the first half of the paper, approximations are made within the Zwanzig projection operator formalism in order to arrive at a master equation for the reduced density operator of a small subsystem (the adbond) in contact with a reservoir (the phonons). The conditions of validity for the Born and Markov approximations are derived. It is shown that the master equation is only valid for times t≫τc, where τc is the characteristic time of the reservoir. These results are then applied to the phonon relaxation of the vibrational adbond of physisorbed molecules. It is shown that for CO adsorbed on Ni or Cu (a strongly bound physisorbed system) the Born and Markov approximations are not justified. For the weakly bound system Ar on W, numerical results show that these approximations can be made. Finally, an adbond interacting with both laser radiation and lattice vibrations is considered. This system can be regarded as a subsystem (the adbond) in contact with two reservoirs, where the conditions for validity of the Markov approximation is then seen to be more severe than when each reservoir is considered independently. For the phonons, these conditions can never be matched. However, for an initial state given by an adbond in equilibrium with the lattice vibrations, the conditions for validity of the approximations prove to be the same as for the phonons and the laser considered independently.
Optical emission studies of atomic, molecular, and particulate carbon produced from a laser vaporization cluster source
Authors: Rohlfing Eric A.
Published: July 2002

We perform optical emission studies on the species generated by the laser vaporization of graphite into a pulsed helium flow. With short flow channels prior to free-jet expansion the emission at short downstream distances is dominated atomic carbon lines and three band systems of C2: d 3∏g–a 3∏u (Swan), C 1 ∏g–A 1∏u (Deslandres–d’Azambuja), and D 1∑+u –X 1∑+g (Mulliken). Partially rotationally resolved spectra of the Δv=−1 Swan bands are analyzed using a band contour approach to obtain rotational temperatures and rough vibrational distributions for C2 d 3∏g. The direct emission spectrum from the short-channel source becomes a weaker, structureless continuum when viewed at long downstream distances. Similar long-lived continua are induced by a low-fluence laser at 266, 355, or 532 nm when it crosses the expansion emanating from a source with a long flow channel. We attribute these continua to the incandescence of hot carbon particles, T=2500–4000 K. The continua are fit to a model in which the Planck blackbody function is modified by the particle emissivity to determine particle temperatures and obtain crude estimates of particle sizes. These data provide the first direct evidence that carbon particles are produced in a laser-vaporization cluster source.
Femtosecond study of geminate electron–hole recombination in neat alkanes
Authors: Bowman Robert M. , Lu Hong , Eisenthal Kenneth B.
Published: July 2002

The first measurement of the geminate electron–hole recombination times of two neat octane isomers has been studied using femtosecond techniques. The recombination times were found to be 2.2±1.0 ps in n-octane and 470±40 fs in iso-octane, thus showing that the rates are strongly dependent on the structure of the alkane solvents.
Coherent radiative control of unimolecular reactions: Selective bond breaking with picosecond pulses
Authors: Seideman Tamar , Shapiro Moshe , Brumer Paul
Published: July 2002

We show that the principle of coherent control of chemical reactions can be implemented, in a two pulse sequence, to control the relative yield of reaction products in unimolecular dissociation. Experimental control parameters in this implementation are seen to be the intensity of the pulses and the delay time between them, with required delay times in the experimentally accessible picosecond range. As an example, application is made to control the production of D+H2 vs HD+H in the dissociation of a model collinear molecule with DH2 masses. In addition, plots of the evolving superposition state are presented in order to emphasize relevant characteristics of excitation with slow pulses.
Polarizabilities and hyperpolarizabilities of F2
Authors: Maroulis George , Thakkar Ajit J.
Published: July 2002

The dipole (α), quadrupole (C), and dipole–octopole (E) polarizabilities, the dipole–dipole–quadrupole (B) and second dipole (γ) hyperpolarizabilities, and the quadrupole (Θ) and hexadecapole (Φ) moments are calculated for the ground state of F2 at its equilibrium bond length. These properties are obtained from fourth-order Mo/ller–Plesset perturbation theory energies of F2 in the presence of various configurations of point charges; the perturbation theory uses the self-consistent-field wave function for F2 perturbed by charges as the zeroth-order wave function. Our best vibrationless estimates for the isotropic averages are ᾱ=8.485 e2 a20 E−1h, C̄=23.588 e2 a40 E−1h, B̄=−84 e3 a40 E−2h, and γ̄=512 e4 a40 E−3h. The ᾱ value is in good agreement with experiment, whereas our vibrationally corrected estimate of Θzz ≂0.74 e a20 should be more accurate than the existing experimental estimates.
Dissociative attachment reactions in electron stimulated desorption from condensed O2 and O2-doped rare-gas matrices
Authors: Sanche L. , Parenteau L. , Cloutier P.
Published: July 2002

Desorption of the ions O−, O−2,O−3 (and/or O2⋅O−) induced by electron impact on pure O2 multilayer films and Ar, Kr, and Xe matrix films containing O2 is reported. In addition to these anions, the ionic complexes M⋅O− (M=Ar and Kr) are also observed to desorb from Ar and Kr matrices, respectively. In the range 4–16 eV, the incident electron energy (Ei) dependence of the yields (i.e., the yield functions) of all the diatomic and triatomic anions exhibit features which can be correlated with the O− yield function; indicating that, these anions are produced by dissociative attachment reactions whose first step involves the formation of O−2 quasibound states. From analysis of all yield functions and variations of the anion yields as a function of O2 concentration in the matrices, we find that the simplest dissociative transient state, which can propel in vacuum an M⋅O− or O2⋅O− ion, must have the configuration M⋅O2⋅O−*2. To explain the formation of O−2 and O−3 ions below Ei≂6 eV, the existence of an electronically excited O−4 state decaying into the limits O−2+O2 and O−3+O must be postulated. At higher energies, O−2 can be formed by the reaction of O− (produced by dissociating O−2 states) with other O2 molecules (e.g., O−+O2→O−3→O−2+O). Both transient anion, M⋅O2O−*2,O−*4 result from initial electron capture by an O2 molecule in a dimeric configuration.
Relativistic effects in gold chemistry. I. Diatomic gold compounds
Authors: Schwerdtfeger Peter , Dolg Michael , Schwarz W. H. Eugen , Bowmaker Graham A. , Boyd Peter D. W.
Published: July 2002

Nonrelativistic and relativistic Hartree–Fock (HF) and configuration interaction (CI) calculations have been performed in order to analyze the relativistic and correlation effects in various diatomic gold compounds. It is found that relativistic effects reverse the trend in most molecular properties down the group (11). The consequences for gold chemistry are described. Relativistic bond stabilizations or destabilizations are dependent on the electronegativity of the ligand, showing the largest bond destabilization for AuF (86 kJ/mol at the CI level) and the largest stabilization for AuLi (−174 kJ/mol). Relativistic bond contractions lie between 1.09 (AuH+) and 0.16 Å (AuF). Relativistic effects of various other properties are discussed. A number of as yet unmeasured spectroscopic properties, such as bondlengths (re), dissociation energies (De), force constants (ke), and dipole moments (μe), are predicted.
Electrodeposition and diffusion-limited aggregation
Authors: Halsey Thomas C. , Leibig Michael
Published: July 2002

We discuss pattern formation in electrodeposition under the conditions of ‘‘secondary current distribution,’’ in which ionic transport is determined by electrical migration and large concentration gradients of the depositing metallic cations appear only in the double layer near the cathode. If the charge in the double layer is sufficiently small, a linear approximation to the deposition kinetics will be adequate to describe the physics. We show that in this case the deposition can be modeled by a random walk algorithm closely related to the ‘‘diffusion-limited aggregation’’ algorithm of Witten and Sander. Depending upon the electrochemical regime, this algorithm reduces either to diffusion-limited aggregation or to the ‘‘Eden model.’’ We treat intermediate cases by a scaling ansatz. We also explore experimental consequences of these results.
Dynamical branching during fluorination of the dimerized Si(100) surface: A molecular dynamics study
Authors: Weber Thomas A. , Stillinger Frank H.
Published: July 2002

Collections of classical trajectories have been numerically generated for individual F2 molecules impinging at normal incidence on a Si(100) surface at 0 K dimerized in a p(2×1) pattern. A linear combination of two-atom and three-atom interaction functions represents the potential energy. Trajectories fall into four categories: (a) non-reactive F2 rebound, (b) monofluorination at a surface dangling bond with energetic expulsion into the vacuum of the remaining F atom, (c) difluorination of a pair of dangling bonds, and (d) monofluorination with retention of the second F in a weakly bound Si–F⋅⋅⋅F surface complex. Surface patterns for difluorination, (c), indicate absence of surface diffusion during this mode of chemisorption. Increasing either the translational kinetic energy or the vibrational excitation of the incident F2 appears to enhance its surface reactivity.
Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. IV. CH4→H−+CH+3 and CD4→D−+CD+3
Authors: Mitsuke Koichiro , Suzuki Shinzo , Imamura Takashi , Koyano Inosuke
Published: July 2002

Ion-pair formation from photoexcited methanes, CH*4 →H− +CH+3 and CD*4 →D− +CD+3, has been studied by measuring the efficiency curves of H− and D− using synchrotron radiation. Ion pairs are produced both by direct dissociation and predissociation of the npt2 (n=3–5) Rydberg state converging to the Ã 2A1 state of CH+4/ CD+4. The 4pt2 and 5pt2 Rydberg states are observed here for the first time.
Accurate multireference configuration interaction calculations of the potential energy function and the dissociation energy of N2
Authors: Werner Hans-Joachim , Knowles Peter J.
Published: July 2002

The potential energy function of the N2 molecule is calculated using the internally contracted multireference CI method (CMRCI) and complete active space SCF (CASSCF) reference wave functions. A full CI calculation in a DZP basis set is used to estimate the errors associated with the CMRCI wave function. The dependence of the computed spectroscopic constants and the dissociation energy on the basis set is also investigated. Uncontracted and segmented basis sets are compared with ANO (atomic natural orbital) and other generally contracted basis sets. It is found that the energy optimized ‘‘correlation consistent’’ basis sets of Dunning yield substantially better results than ANO basis sets of the same size. In the largest calculations, which included up to h type basis functions and also accounted for core–core and core–valence correlation effects, the remaining errors are 0. 0003 Å, 8 cm−1, and 0.7 kcal/mol for re, ωe, and De, respectively. The inclusion of an i type basis function reduces the error in the dissociation energy to 0.3 kcal/mol (0.013 eV).