bluuebunny/crossref_metadata_2025 huggingface dataset:
The conversion of isolated oxygen atoms to a fast diffusing species in Czochralski silicon at low temperatures
Authors: McQuaid S. A. , Johnson B. K. , Gambaro D. , Falster R. , Ashwin M. J. , Tucker J. H.
Published: July 2002

New measurements of oxygen out-diffusion in Czochralski silicon confirm previous reports that it occurs over an anomalously long range at low temperatures (T<700 °C). The surface concentration is shown to decrease with increasing anneal time at ∼430 °C while the depth to which the concentration is depleted remains almost constant. The rate of decrease of the surface concentration is correlated with the rate of loss of isolated oxygen atoms in the bulk. All these observations are explained if isolated oxygen atoms are converted slowly during the anneal to an oxygen-containing fast diffusing species (Ofds) which diffuses over long distances before being trapped. High concentrations (0.6–1.7×1017 cm−3) of substitutional carbon in the crystal increase the trapping rate of Ofds but this rate tends to decrease with increasing anneal time, tending to the value for carbon-free material. Estimates of the rate of formation of Ofds are close to the expected rate of dimerization in a dilute solution at the lowest temperatures (T<450 °C), but tend to be below the expected rate at higher temperatures. There is no clear indication of Ofds dissociation over the whole temperature range (T⩽700 °C). These observations suggest that the formation of Ofds is more complicated than one would expect if it were the O2 dimer. Alternatives such as the oxygen vacancy complex should be considered.
RuO 2 -based thick film resistors studied by extended x-ray absorption spectroscopy
Authors: Meneghini C. , Mobilio S. , Pivetti F. , Selmi I. , Prudenziati M. , Morten B.
Published: July 2002

RuO 2 -based thick film resistors were studied by extended x-ray absorption fine structure. A bimodal distribution of RuO2 particle size has been determined by comparing the obtained results with x-ray diffraction data. The partial dissolution of RuO2 particles into the glassy matrix is suggested to be one of the principal mechanisms responsible for the electrical conduction in these materials.
Low temperature heat capacities of Ti3Al1.1C1.8, Ti4AlN3, and Ti3SiC2
Authors: Ho J. C. , Hamdeh H. H. , Barsoum M. W. , El-Raghy T.
Published: July 2002

For the binary Ti–Al system, an ordering transformation in Ti3Al has been shown to result in a significant lowering of the electronic heat-capacity coefficient, γ, by removing electrons from conducting states. When γ is normalized to a per Ti atom basis, the same tendency is found in low temperature calorimetric studies of the conducting ternary carbides Ti3Al1.1C1.8, Ti4AlN3, and Ti3SiC2 reported herein. As a consequence of C- or N-induced covalent-like bond formation, the Debye temperatures in these ternaries are in excess of 700 K.
Polarized photoluminescence and order parameters of “in situ” photopolymerized liquid crystal films
Authors: Sánchez C. , Villacampa B. , Cases R. , Alcalá R. , Martı́nez C. , Oriol L. , Piñol M.
Published: July 2002

Oriented liquid crystal polymer films with different degrees of cross-linking have been obtained by “in situ” photopolymerization of different mesogenic samples at various temperatures. These samples are mixtures of a naphthalene based monoacrylate and a crosslinking agent (diacrylate). Photoluminescence has been obtained by adding a cyanoterphenyl chromophore. The order parameters of the polymer films have been derived from dichroic measurements. An increase of the dichroism has been found when the diacrylate content and/or the polymerization temperature decrease. The dichroism is also bigger if the chromophore is in a “guest-host” configuration than if it is covalently attached to the polymer backbone. The dichroic ratios do not change under annealing up to 200 °C but some photodegradation of the luminescence is observed in the naphthalene containing films.
Structural properties of polycrystalline Co–Cr–X (X=Pt and/or Ta)/Cr-alloy magnetic recording media
Authors: Takahashi Yoshio , Yajima Yusuke , Kojima Syuichi , Hosoe Yuzuru
Published: July 2002

The grain size distributions of {11.0} textured Co–Cr–Pt and Co–Cr–Pt–Ta magnetic layers with Cr-alloy underlayers have been investigated by high-resolution transmission electron microscopy. The grain size distribution of the Co–Cr–Pt layer can be best described by the Rosin–Rammlar function. On the other hand, the Co–Cr–Pt–Ta layer is best described by the log–normal function which has a longer tail in the large-grain-size region than the Rosin–Rammlar function has. The size distribution tended to broaden as the average grain size of the magnetic layer decreased.
Temperature dependence of magnetic viscosity and irreversible susceptibility for aligned Stoner–Wohlfarth particle
Authors: Harrell J. W. , Khapikov A. F.
Published: July 2002

The temperature dependence of the magnetic viscosity S and the irreversible susceptibility χirr has been calculated for distributions of noninteracting, aligned Stoner–Wohlfarth particles. Significant differences are shown to exist for volume and anisotropy field distributions. For a volume distribution, Smax is independent of temperature and χirr,max∼T−1/2, below the superparamagnetic temperature. For an anisotropy field distribution Smax≈c1T1/2+c2T and χirr,max≈c3+c4Smax, where the c’s are positive constants. Results obtained using approximate analytical expressions that have been previously published have been compared with numerical calculations.
Photoreflectance study in the E1 and E1+Δ1 transition regions of CdTe
Authors: Kaneta Akio , Adachi Sadao
Published: July 2002

Photoreflectance (PR) measurements have been carried out to determine the E1 and E1+Δ1 critical-point (CP) parameters in CdTe at temperatures between T=77 and 300 K using a He–Ne laser as modulation light source. The measured PR spectra give very weak, but distinct, structures at ∼3.5 eV(E1) and ∼4 eV(E1+Δ1). When an Ar+-ion laser is illuminated as bias light, the spectra measured for T⩽150 K become very strong. This effect is considered to be due to a bias-laser-induced band bending. The experimental PR spectra can be successfully explained by an excitonic model of the interband transitions. The temperature dependence of the CP parameters (energy, amplitude, and broadening parameter) are analyzed using the Varshni equation and an empirical expression of Bose–Einstein type.
Tight-binding calculations of energy gaps in (001)-(InAs)n(InSb)m strained superlattices
Authors: Lin-Chung P. J. , Yang M. J.
Published: July 2002

Tight-binding calculations of electronic structures for (001)-(InAs)n(InSb)m strained layer superlattices are presented. The dependences of the superlattice band gap on the band offsets between InAs and InSb are examined for three different types of biaxial strains. It is found that the band gap depends strongly on the band offset, and that for m=1 the ordering lowers the band gap with respect to the random alloy. A comparison with the photoluminescence data for the energy gaps of (n×1) strained-layer superlattices is discussed. In addition, the electronic structures of strained InAs quantum wells are calculated, and interpretations are provided for the observed type-I to type-II band alignment transition at n=5 in a quantum well formed by (n×1) strained layer superlattices and AlSb barriers. Changes of energy gaps with layer thicknesses in strained layer superlattices with n=m and n=8−m are also studied.
A reverse bias, tip-insulator-semiconductor tunnel diode model accounting for the delineation of a p/p+ junction using scanning tunneling microscopy
Authors: Lin Hai-An , Jaccodine Ralph J. , Freund Michael S.
Published: July 2002

In this article, a tunneling-generation-avalanche model has been proposed to explain the reverse bias current–voltage behavior observed at a tip/air/p-type silicon junction. Based on this model, under conditions where the applied bias is more negative than the flat band voltage, the current will be dominated by generation processes, which has significant doping density dependence. Since mechanically cut tips, used in this work, can have complicated geometries, geometric effects, such as extended gates and concentration of the electrical field must be taken into account. By taking these factors into account, good agreement between theory and experiments can be achieved. Finally, in the presence of illumination, p/p+ junctions can be delineated successfully by taking advantage of the generation process. These results demonstrate that scanning tunneling microscopy can be used as a powerful tool for characterizing semiconductor devices.
Simulation of electrochemical deposition process by a multiparticle diffusive aggregation model
Authors: Mizuseki Hiroshi , Kawazoe Yoshiyuki
Published: July 2002

A model based on multiparticle diffusive aggregation is introduced to examine the generated pattern of metal leaf crystals in electrochemical deposition. We simulated the behavior of ions in the solution during electrochemical deposition from two points of view on crystal growth. The first model assumes that the ion in the solution is affected by the force from other ions in order to consider the concentration of ions. The second model is that the ion is affected by the force which corresponds to the applied external voltage from the electrode. Several specific patterns of the crystal growth under an electric field were obtained by a multiparticle Monte Carlo model. The results of the present simulation show that the patterns strongly depend on the force applied to the ions and on their concentration.
High-energy electron irradiation effects in C60 films
Authors: Khan Aurangzeb , Kojima Nobuaki , Yamaguchi Masafumi , Narayanan K. L. , Goetzberger Oliver
Published: July 2002

We report here the high-energy (1 MeV) electron irradiation effects on C60 films. Fourier transform infrared spectroscopy, Raman spectroscopy, and x-ray diffraction results are consistent and provide clear evidence of the nondestruction of C60 molecules upon irradiation. The presence of C60 signature peaks in the spectra of all three techniques indicates that the C60 films have not been transformed into a hitherto proposed structure of amorphous carbon for low-energy irradiation. A very short collision time mechanism has been proposed to explain the observation.
Precise noise characterization of perpendicular recording media
Authors: Takeo Akihiko , Takahashi Yuka , Tanaka Yoichiro , Miura Kenji , Muraoka Hiroaki , Nakamura Yoshihisa
Published: July 2002

Perpendicular recording is one of the technology candidates to extend the areal density growth trend with the superior thermal stability and the high recording resolution. As we have experienced with longitudinal recording, noise originating from perpendicular recording also has a strong impact on the channel performance. In this study, the noise characteristics of the readout waveforms of a tri-layer perpendicular media and single-pole write/MR read heads were investigated using precise time-domain analysis. Waveform fluctuations were obtained by subtracting the ensemble-averaged signal from each measured noise-contaminated waveform. The time-domain analysis makes clear the dominant type of transition-related noise such as transition jitter, pulse breathing or dc noise, etc. It was revealed that the noise from perpendicularly recording medium concentrates at the center of written transitions. The dominant transition noise source was found to be the transition position jitter. The level of transition jitter of the perpendicular media used in this study was found to be comparable to that of 5 Gbpsi class longitudinal media. MFM measurements revealed that magnetization fluctuates in large size of about 80 nm, which results in the fluctuation of the transition centerline. Reducing transition jitter should be the focus of developing future high performance perpendicular recording media.
Write driver design considerations for high-frequency rigid disk recording
Authors: George Peter K. , Jury Jason C. , Judy Jack H.
Published: July 2002

Prior reported PSPICE modeling work has demonstrated subnanosecond field rise-times for a head driven by an ideal current source. Results including the flexure interconnection have demonstrated 1 ns field rise-times without pulse distortion. Three write driver designs are explored. A simple H-bridge, an H-bridge with fly-over diodes, and an H-bridge with clamped transistors to improve switching speed are compared. The driver models are implemented using BJTs with state-of-the-art industry MOSIS process model parameters for PSPICE. The simulations show that for the low inductances considered and the relatively high supply voltage employed, the fly-over diodes are not required. The H-bridge with clamped transistors is the fastest driver (50 Ω resistive load) but suffers from a limited current capability for the small transistor areas required for high speed. The driver designs are restricted by termination requirements which generally do not correspond to optimum switching times for the 50 Ω resistive load used for preliminary evaluation. Finally, by reducing the interconnection length to 25 mm, stacking the head turns, and using the simple H-bridge driver, it is demonstrated that a field rise-time of less than 0.2 ns can be achieved. The resulting waveform is presented and the implications discussed.
Massive spectral weight transfer and colossal magneto-optical effect in doped manganites
Authors: Alexandrov A. S. , Bratkovsky A. M.
Published: July 2002

We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction (∼1 eV) is much stronger than any relevant magnetic interaction. This suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. As these materials are cooled below the Curie temperature, the colossal magnetoresistance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by the exchange interaction with localized Mn spins during the transition. The corresponding current carrier density collapse in doped manganites leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition, in agreement with available experimental observations.
Magnetization-controlled spin transport in DyAs/GaAs layers
Authors: Mao J. M. , Zudov M. E. , Du R. R. , Lee P. P. , Sadwick L. P. , Hwu R. J.
Published: July 2002

Electrical transport properties of DyAs epitaxial layers grown on GaAs have been investigated at various temperatures and at magnetic fields up to 12 T. The measured magnetoresistances show two distinct peaks at fields around 0.2 and 2.5 T which are believed to arise from the strong spin-disorder scattering occurring at the phase transition boundaries induced by the external magnetic field. An empirical magnetic phase diagram is deduced from the temperature dependence of magnetoresistance, and the anisotropic transport properties are also presented for various magnetic field directions with respect to the current flow.
The magnetic properties of nanocrystalline Fe-Co(Cr)–Hf–N thin films
Authors: Kim K. H. , Kim Y. H. , Kim J. , Han S. H. , Kim H. J.
Published: July 2002

We have investigated the effect of Co(Cr) substitution on magnetic properties of as-deposited Fe–Hf–N films. 4πMs increases with increasing Co content (3–16 at. %) and decreases with increasing Cr content. The film Fe56Co16Hf10N18(at. %) shows excellent soft magnetic properties, with 4πMs∼16.8 kG, Hc∼0.25 Oe, and effective permeability ∼4200 at 100 MHz. In Fe71Cr3Hf9N17, the permeability is about 5200 at 100 MHz, while the values of 4πMs and Hc are ∼14.6 kG and 0.5 Oe, respectively. In order to estimate the magnetic anisotropy, the angular dependency of the resonance magnetic field and linewidth were observed by ferromagnetic resonance (FMR) at 9.84 GHz. The Fe56Co16Hf10N18 film shows a single resonance peak, whereas the as-deposited Fe–Hf–N films shows weak secondary peaks.
Scattering times in AlGaN/GaN two-dimensional electron gas from magnetoresistance measurements
Authors: Braña A. F. , Diaz-Paniagua C. , Batallan F. , Garrido J. A. , Muñoz E. , Omnes F.
Published: July 2002

The diagonal and nondiagonal components of the transverse magnetoresistance have been measured, over a wide magnetic field range, in modulated doped Al0.25Ga0.75N/GaN heterostructures. The diagonal component shows electron–electron interaction in the whole magnetic field range, Shubnikov–de Hass (SdH) oscillations superimposed at high magnetic field, and weak localization at very low magnetic field. The SdH oscillations are evidence of the existence of a two-dimensional electron gas (2DEG) in the heterostructure. Only one kind of carriers is present with an electron density of 1.01×1017 m−2, an effective mass of 0.23m0 and a quantum scattering time τq=0.05 ps. From the diffusive electron–electron interaction, an impurity scattering time τee=0.044 ps, a Hartree factor F=0.25 and the Drude scattering time τ0=0.26 ps, were obtained. The weak localization yields two scattering times, an elastic scattering time τe=0.023 ps independent of the temperature, and an inelastic scattering time, τi, with a temperature dependence following the 1/τi∝T ln T law expected for the impurity contribution of the electron–electron interaction in 2D. The τq/τ0 ratio gives the dominant scattering mechanism, which in our case is 0.19. The remote ionized impurities alone do not explain this obtained ratio, while the introduction of the interface roughness could explain it.
Raman spectrum and structure of silica aerogel
Authors: Walrafen G. E. , Hokmabadi M. S. , Holmes N. C. , Nellis W. J. , Henning S.
Published: July 2002

Raman spectra have been obtained from silica aerogel, a porous low-density material having grain sizes near 70 Å. The Raman spectra are qualitatively similar to those from fused silica, thus indicating that the aerogel is amorphous. However, a greatly intensified peak (relative to ordinary fused silica) occurs near 478 cm−1 and is assigned to small rings, e.g., six (3-SiO) or eight (4-SiO) membered. Such rings may be more important in the aerogel than in fused silica, but the observed absence of the 600 cm−1 ring peak indicates that only one ring type, e.g., eight membered, is predominant. Other marked frequency and intensity changes in the Si–O–Si bending and Si–O stretching region, compared to fused silica, were also observed.
A procedure for calculating thermodynamic functions of cavity formation from the pure solvent simulation data
Authors: Lee B.
Published: July 2002

It is shown that the thermodynamic quantities for the process of inserting a cavity into a liquid are given by the ensemble average of the function ζ̄ and the correlation in the fluctuation of this function with the thermodynamic quantity of interest. The function ζ̄ is defined for each configuration of the system as the fraction of volume, at each point of which a cavity of given size can be placed. The algorithm for computing this volume is already known. As a by-product of this derivation, it is pointed out that the entropy change upon cavity insertion is made of two parts—that associated with simple restriction of the solvent configurations due to the presence of the cavity and another that results from the reorganization of the solvent around the cavity. The latter part precisely cancels the change in enthalpy, so that the free energy of cavity formation is entirely given by the entropy change associated with the simple restriction in the configuration of the solvent. A potentially useful approximation scheme is presented for the function ζ̄, by means of which all thermodynamic functions of cavity formation can be expressed in terms of various fluctuation properties of the solvent and a few cavity size dependent parameters.
Kinetics of the isotope exchange reaction of 18O with NO and O2 at 298 K
Authors: Anderson S. M. , Klein F. S. , Kaufman F.
Published: July 2002

The rates of 18O isotope exchange reactions were measured at 298 K using a discharge-flow, modulated molecular beam mass spectrometer apparatus whose detection limit for NO and O was in the 109–1010 cm−3 range. The NO exchange is very fast, k1, x=(3.7±0.5)×10−11 cm3 s−1, and, assuming statistical breakdown of a long-lived complex, the rate constant for the formation of the NO°2 complex, k1, c f is (7.4±1.0)×10−11 cm3 s−1. The slower O2 exchange was measured three different ways, yielding three rate constants whose average is k2, x=(2.9±0.5)×10−12 cm3 s−1. The results are compared with earlier isotope exchange experiments and discussed in the context of measured and calculated high pressure recombination and vibrational relaxation rate constants.
Crystal structure and charge density of 4-methylpyridine (C6H7N) at 120 K
Authors: Ohms U. , Guth H. , Treutmann W. , Dannöhl H. , Schweig A. , Heger G.
Published: July 2002

4-Methylpyridine (C6H7N) has been studied at 120 K by combined x-ray and neutron diffraction. It crystallizes in the space group I41a: a=7.618(5) Å, c=18.635(10) Å, and Z=8. The crystal structure was solved by direct methods and refined to RF =0.059 (x-ray data) and RF =0.035 (neutron data), respectively, after reducing the intensities of the meroedrically twinned crystals. The resulting X–N synthesis agrees well with the theoretically calculated (4-31G+BF) dynamic electron deformation density.
Approximate relationships between density power integrals, moments of the momentum density, and interelectronic repulsions in diatomic molecules
Authors: Pathak Rajeev K. , Sharma B. S. , Thakkar Ajit J.
Published: July 2002

Moments, 〈pk〉, of the momentum distribution and density power integrals, Ik=∫ρ1+k/3dτ in which ρ is the charge density, with −2≤k≤4 are computed for 35 ground state diatomic molecules at equilibrium using self-consistent-field wave functions of nearly Hartree–Fock quality. These data are then used to test various relationships between 〈pk〉, Ik, and the interelectronic repulsion Vee. Empirical bounds to 〈pk〉 in terms of Ik are found. Moreover, empirical approximations to 〈pk〉 in terms of Ik for −1≤k≤4 are given; these have median errors not exceeding 2% for the 35 diatomics under consideration. Gradient-free density functional approximations, in both position and momentum space, to Vee with median errors of less than 5% are presented.
Complex periodic oscillations and Farey arithmetic in the Belousov–Zhabotinskii reaction
Authors: Masel/ko J. , Swinney Harry L.
Published: July 2002

Our experiments on the manganese-catalyzed Belousov–Zhabotinskii reaction in a stirred flow reactor reveal many sequences of distinct multipeaked periodic states. In the parameter ranges studied the waveform for each periodic state consists of an admixture of small and large amplitude oscillations. No chaos is discernible, and in many cases the transitions from one periodic state to another occur without any observable hysteresis. Two types of sequences were studied in detail, one with waveforms consisting of concatenations of two basic patterns and another with waveforms consisting of concatenations of three basic patterns. The sequences of states with two patterns are described well by Farey arithmetic, which provides rational approximations of irrational numbers. These states can be characterized by a firing number, the ratio of the number of small amplitude oscillations to the total number of oscillations per period. For our data this ratio is a monotone stepwise-increasing function of flow rate, and the steps have a fractal dimension. The relationship between the observed sequence and the Farey arithmetic and the observation of a fractal dimension for the steps in the firing number suggest that the states formed by concatenating two patterns can be interpreted in terms of frequency locking on a 2 torus in phase space. The sequences of states with three basic patterns are described by a generalized Farey arithmetic that provides rational approximations for pairs of irrational numbers that are mutually irrational; this suggests that these states can be interpreted in terms of frequency locking on a 3 torus. A piecewise-linear two-dimensional map is shown to yield a phase diagram in qualitative accord with the measured phase diagram for these sequences.
Diffusion limited growth of crystalline domains in phospholipid monolayers
Authors: Miller A. , Möhwald H.
Published: July 2002

The growth kinetics of two-dimensional crystalline phospholipid domains at the air/water interface is studied following a stepwise increase in density of interfacial molecules. In addition to pressure relaxation the development of domain size and shape and impurity distribution could be followed by fluorescence microscopy. All experimental findings agree, within a factor of 3, with results of simple model calculations. These consider the growth kinetics to be controlled by two-dimensional diffusion of impurities from the solid/lipid boundary (constitutional supercooling) and do not use any freely adjustable, but independently measurable parameters.
Photodissociation of CF2BrCH2I at 248, 266, and 308 nm
Authors: Minton Timothy K. , Nathanson Gilbert M. , Lee Yuan T.
Published: July 2002

The technique of photofragmentation translational spectroscopy has been used to study the photodissociation of CF2BrCH2I at excitation wavelengths of 248, 266, and 308 nm. The primary photofragments are CF2BrCH2 and either I(2P1/2) or I(2P3/2), although some C–Br bond fission does occur at 248 and 266 nm. A large fraction of the CF2BrCH2 radical product contains enough internal excitation after the primary process to undergo secondary dissociation into CF2CH2 and Br. Secondary dissociation is also observed to take place at 248 and 266 nm via absorption of a photon by the CF2BrCH2 photofragment. By observing the threshold for the spontaneous secondary dissociation process, the reaction enthalpy for CF2BrCH2I→CF2CH2+Br+I, was determined to be 67.5±2 kcal/mol, which leads to: Δ H○f,0 (CF2BrCH2I)=−92.6±2 kcal/mol. The c.m. translational energy distributions were derived for both the I(2P1/2) and I(2P3/2) dissociation channels resulting from primary C–I bond breakage. The I(2P1/2)/I(2P3/2) branching ratios are 3.3, 9.0, and 0.5 for excitation wavelengths of 248, 266, and 308 nm, respectively. The translational energy distributions also reveal that a major fraction of the CF2BrCH2 product radicals are formed with high internal energies, averaging around 50% of the excess energy. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state channels at both wavelengths proceeds via a parallel transition—i.e., the transition moment must be nearly parallel to the C–I bond.
Asymptotic expansions of the electron momentum densities of the atoms from hydrogen through lawrencium
Authors: Thakkar Ajit J. , Wonfor Alison L. , Pedersen Ward A.
Published: July 2002

The first three coefficients in each of the small p Maclaurin and large p asymptotic expansions of the spherically averaged electron momentum densities of the ground states of the 103 neutral atoms from hydrogen through lawrencium, 73 atomic cations and 41 atomic anions are calculated from nonrelativistic self-consistent-field wave functions. These coefficients should be useful in the analysis of experimental Compton profiles. An analysis of the periodic behavior of these coefficients is given.
High resolution Fourier spectroscopy and laser spectroscopy of Cs2: The 2 1Σ+g, (C) 2 1Πu, (D) 2 1Σ+u, 3 1Σ+g, and (E) 3 1Σ+u electronic states
Authors: Amiot C. , Demtröder W. , Vidal C. R.
Published: July 2002

More than 8000 completely resolved rotational lines in the E 1Σ+u →X 1Σ+g, E 1Σ+g →(2)1Σ+g, E 1Σ+u →(1)1Πg, E 1Σ+u →(3)1Σ+g, D 1Σ+u →(2)1Σ+g, C 1Σu →X 1Σ+g, and D 1Σ+u →X 1Σ+g transitions of Cs2 have been accurately measured with the techniques of Fourier transform spectroscopy, Doppler-free polarization spectroscopy, and optical optical double resonance. The wave numbers of all these rotational lines were determined with an accuracy of 1 to 5×10−3 cm−1. A thorough and simultaneous analysis of all the measured data yields molecular constants, potential energy curves, and dissociation energies for six different excited electronic states of the Cs2 molecule.
The isotropic–nematic phase transition and other properties of a solution of semiflexible polyelectrolytes
Authors: Jan Vroege Gert
Published: July 2002

We extend previous work [G. J. Vroege and T. Odijk, Macromolecules 21, 2848 (1988)] on the formation of nematic liquid crystals from uncharged semiflexible polymers to the charged case. The polyelectrolytes are modeled as long slender worm-like cylinders interacting via both hard-core and electrostatic repulsions in the second virial approximation. Analogously to rod-like polyelectrolytes it is possible to describe the result of charge by introducing an effective diameter and a twisting effect. We then calculate numerically the orientational distribution function as a function of concentration. We determine the effect of charge on the phase transition and—in the nematic phase—its influence on the order parameter and the global persistence length (directly connected to the splay elastic modulus). Although the phase transition concentrations are clearly dependent on charge, the properties of the nematic phase at the transition are not—in contrast to the result for rod-like polyelectrolytes.
Direct calculation of the reactive transition matrix by ℒ2 quantum mechanical variational methods with complex boundary conditions
Authors: Sun Yan , Yu Chin-hui , Kouri Donald J. , Schwenke David W. , Halvick Philippe , Mladenovic Mirjana , Truhlar Donald G.
Published: July 2002

We present a new formalism of the generalized Newton variational principle for the calculation of quantum mechanical state-to-state reaction probabilities. The reformulation involves solving directly for the transition matrix rather than the reactance matrix so that calculations may be carried out for individual columns of the transition matrix without obtaining solutions for all possible initial channels. We compare the convergence of calculations with real and complex boundary conditions for H+H2→H2+H, O+H2→OH+H, and O+HD→OH+D and OD+H.
Optical impulsive excitation of molecular pseudorotation in Jahn–Teller systems
Authors: Cina Jeffrey A. , Romero-Rochin Victor
Published: July 2002

We analyze the use of vibrationally abrupt nonresonant laser pulses to prepare coherently pseudorotating states in a model Jahn–Teller molecule. Our derivation of impulsive excitation invokes the dynamical adiabatic phase for the perturbed electronic ground state. Polarization selection between two Raman active distortion coordinates allows creation of an orbit of arbitrary eccentricity. Repetition of the pulse pair at the pseudorotational frequency amplifies the nuclear motion. Timing of a resonant pulse of given polarization, or choice of polarization for a given delay, transfers the moving wave packet to either or both Jahn–Teller branches of an electronic excited state.
Theoretical study of the homonuclear tetramers and pentamers of the group IB metals (Cu, Ag, and Au)
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

We find the optimal structures of the homonuclear tetramers and pentamers of Cu, Ag, and Au to be planar trapezoidal. However, on the basis of spin densities deduced from electron spin resonance experiments, it has been suggested that these pentamers have distorted trigonal bipyramidal structures. This apparent discrepancy is reconciled by the fact that Mulliken populational analyses indicate that the open-shell spin density distribution on the planar structures are also consistent with experiment. Since the vertical electron attachment energies for both the tetramers and pentamers are in good agreement with experiment, the negative ions probably also have planar trapezoidal structures. The trends in ionization potentials, atomization energies, and electron affinities with cluster size are discussed.
Role of nuclear tunneling in aqueous ferrous–ferric electron transfer
Authors: Bader J. S. , Kuharski R. A. , Chandler D.
Published: July 2002

By computer simulation and also by analytical methods we have computed the nuclear tunneling enhancement of the rate for ferrous–ferric exchange in water. The model we have examined is the one studied earlier where we treated water as a classical liquid [R. A. Kuharski, J. S. Bader, D. Chandler, M. Sprik, M. L. Klein, and R. W. Impey, J. Chem. Phys. 89, 3248 (1988)]. But now we treat water quantum mechanically and find that the tunneling enhancement is a factor of 60 in the rate constant. Further, as observed experimentally, we find that the isotope shift on the rate when changing from D2O to H2O is approximately a factor of 2. The computer simulation aspects of these calculations employ path integral methods and a novel partitioning of the free energy associated with electron transfer. Our results show that it is insufficient to quantize only the atoms composing the ligands. The quantum dynamics of water molecules beyond the first solvation shell prove quite significant.
Quantum confinement effects in semiconductor clusters
Authors: Rama Krishna M. V. , Friesner R. A.
Published: July 2002

The band gaps, band structure, and excited-state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials. In addition, the sensitivity of the exciton energies to the size, shape, crystal structure, and lattice constant of the unit cell are investigated. The calculated exciton energies of CdS clusters are in excellent agreement with experiment over a wide range of cluster sizes. Also, the exciton states of small CdS clusters are sensitive to whether their crystal structure is zinc blende or hexagonal. Such a sensitivity is absent in large CdS clusters. Furthermore, small GaAs clusters are shown to exhibit anomalous redshift of their absorption spectra, in sharp contrast to CdS and large GaAs clusters whose spectra always shift to blue with decreasing cluster size. Finally, the lowest-energy non-Franck–Condon transition in GaP clusters always shifts to blue with decreasing cluster size, whereas the higher-energy Franck–Condon transition in small clusters exhibits the anomalous redshift. These novel findings reveal that (1) the optical spectroscopy of semiconductor clusters is strongly material and crystal structure dependent; (2) the spectroscopy of small clusters is dramatically different from those of large clusters and bulk; and (3) these effects cannot be explained, even qualitatively, using the effective-mass approximation.
Fractal and multifractal analysis of the sensitivity of catalytic reactions to catalyst structure
Authors: Gutfraind Ricardo , Sheintuch Moshe , Avnir David
Published: July 2002

The effects of catalyst surface morphology and surface active-sites distribution on diffusion limited reactions are studied. The catalysts structures are deterministic and random Cantor sets (CS) and Devil’s staircases (DS). The reaction is an Eley–Rideal process in which a random walker approaches the surface from the bulk and reacts with an active site upon collision with it. Two types of scaling analysis are performed: Fractal time scaling of product formation that provides a global analytical tool of the process; and multifractal analysis with which the complex distribution of reaction probabilities of the active sites are analyzed. Some of the main findings are (a) the fractal time scaling theory that has been applied successfully for connected fractal objects, also describes well the performance of disconnected, dust-type fractal objects such as found in most real catalysts and as modeled by the DS and CS; (b) the effective diffusion coefficient for reactions with disconnected active sets is smaller than for connected sets; we provide a method for calculating it; (c) despite the structural similarity of the DS and CS, they react with different efficiencies. It is shown that this difference can be explained in terms of kv, the prefactor of the neighboring-volume/yardstick-sized relation; (d) by performing the analyses on objects with various sizes, we provide evidence that corroborates the specific scaling assumptions of the multifractal formalism for the studied structures; and (e) significant differences between the τ(q) and f (α) multifractal spectra of the four objects are found and analyzed.
Chemical association in simple models of molecular and ionic fluids. III. The cavity function
Authors: Zhou Yaoqi , Stell George
Published: July 2002

Exact equations which relate the cavity function to excess solvation free energies and equilibrium association constants are rederived by using a thermodynamic cycle. A zeroth-order approximation, derived previously by us as a simple interpolation scheme, is found to be very accurate if the associative bonding occurs on or near the surface of the repulsive core of the interaction potential. If the bonding radius is substantially less than the core radius, the approximation overestimates the association degree and the association constant. For binary association, the zeroth-order approximation is equivalent to the first-order thermodynamic perturbation theory (TPT) of Wertheim. For n-particle association, the combination of the zeroth-order approximation with a ‘‘linear’’ approximation (for n-particle distribution functions in terms of the two-particle function) yields the first-order TPT result. Using our exact equations to go beyond TPT, near-exact analytic results for binary hard-sphere association are obtained. Solvent effects on binary hard-sphere association and ionic association are also investigated. A new rule which generalizes Le Chatelier’s principle is used to describe the three distinct forms of behaviors involving solvent effects that we find. The replacement of the dielectric-continuum solvent model by a dipolar hard-sphere model leads to improved agreement with an experimental observation. Finally, equation of state for an n-particle flexible linear-chain fluid is derived on the basis of a one-parameter approximation that interpolates between the generalized Kirkwood superposition approximation and the linear approximation. A value of the parameter that appears to be near optimal in the context of this application is obtained from comparison with computer-simulation data.
Theoretical study of small aluminum phosphide and magnesium sulfide clusters
Authors: Al-Laham Mohammad A. , Trucks Gary W. , Raghavachari Krishnan
Published: July 2002

Electronic structures and stabilities of small AlnPn and MgnSn clusters (n=1–3) are explored by means of accurate quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. Ionic factors are clearly dominant for MgS clusters. Thus, both Mg2S2 and Mg3S3 have planar ground state geometries where charge alternation is utilized effectively. AlP clusters, on the other hand, behave intermediate between the ionic MgS clusters and the covalent Si clusters. Thus, while the ground state structures of Al2P2 and Al3P3 are both analogous to those of the isoelectronic silicon clusters Si4 and Si6, other low-lying minima which are similar to those of MgS clusters are also present. The hybridization and bonding in the different structures are discussed.
A theoretical study of linear carbon cluster monoanions, C−n, and dianions, C2−n (n=2–10)
Authors: Watts John D. , Bartlett Rodney J.
Published: July 2002

A large number of carbon cluster monoanions, C−n, have now been detected by negative ion photoelectron spectroscopy. In addition, evidence for carbon cluster dianions, C2−n, as small as C2−7 has been obtained mass spectrometrically. In this research we report results of theoretical calculations of structures and energetics of formation of linear carbon cluster monoanions and dianions containing up to ten carbon atoms. A number of different electronic states have been investigated. Self-consistent field (SCF) theory, many-body perturbation theory, and coupled-cluster theory including triple excitations have been used with basis sets containing polarization and diffuse functions. Considerably larger basis sets have also been used in calculations on some of the smaller species. For the monoanions, the observed electron detachment energies and the even–odd alternation thereof are well reproduced by the calculations. For the dianions, the even numbered species are found to be more easily formed than the odd numbered species, in accord with the intensity pattern observed in the mass spectrometric experiments, and with the availability of partially occupied π orbitals. C2−10 is established to be vertically and adiabatically stable to electron loss, while C2−8 is found to be vertically stable but adiabatically unstable to electron loss. Improved calculations may be sufficient to make C2−8 also stable to adiabatic electron loss. C2−7 and C2−9 are both found to be unstable to vertical electron loss, although both have negative highest occupied molecular orbital (HOMO) eigenvalues and C2−9 is stable to vertical electron loss at the SCF level. The geometry changes resulting from the addition of two electrons are significant, especially for the even numbered clusters. Addition of two electrons to the partially occupied π orbitals of the latter leads to strong single–triple bond alternation, which may be rationalized by noting that the dianions are products of double deprotonation of HC2nH. Such an ‘‘accordion’’ mechanism may have a role in the ability of carbon clusters to conduct electricity.
An ab initio investigation of clusters NanCln
Authors: Ochsenfeld Christian , Ahlrichs Reinhart
Published: July 2002

We report results of a systematic theoretical investigation on sodium chloride clusters NanCln, for n=1–6, 8, 9, 12, 15, 18, and 32. For the smaller clusters a variety of isomers has been treated by self-consistent-field (SCF), second-order Mo/ller–Plesset perturbation theory (MP2), and the coupled-pair functional (CPF) method, partly within the effective core potential (ECP) approximation. Stationary geometries are characterized as local minima by SCF force field calculations up to n=9. The energetically most stable isomers of even the smaller clusters show a marked preference for geometries which are fragments of the solid state lattice. The results obtained for the larger clusters allow for an extrapolation of structural and energetic properties of the solid in good agreement with experiment.
Absorption spectra of Xe/F2 and Kr/F2 collision complexes. Short-range potential approach
Authors: Dubov V. S.
Published: July 2002

A theory is developed which makes it possible to calculate the absorption spectrum of the collision complex atom A/molecule BC. The potential surface of the ground (initial) state is covalent (A+BC). When absorbing a photon, the molecular system goes over to the ionic state (A++BC−). The absorption spectrum corresponding to the reaction A+BC+ℏω→A++BC− is analyzed. The short-range potential approach is developed and employed to calculate the absorption spectrum. The main problem considered in the article is finding the dipole moment of the transition as a function of the nuclear coordinates which determine the relative position of the atoms in the system atom–diatom. The theory developed uses the data about separate particles A, BC, A+, and BC− alone. It is applied to obtain the absorption spectra of Xe/F2 and Kr/F2 collision complexes. The results found have been compared with the experimental data available and good agreement has been stated. The sensitivity of the simulated spectra to the variation of the parameters of the task has been analyzed. A conclusion is drawn that the theory is perspective for the study of transition state spectra.
Off-specular reflection from flat interfaces with density or compositional fluctuations
Authors: Wu Wen-li
Published: July 2002

The reflection or scattering from a flat interface with density or compositional fluctuations has been investigated. An integral equation to relate the scattered intensities and the interface structure was proposed using a two-potential approach. The reflection geometry was taken into consideration explicitly in this derivation. This integral equation was applied to relate the off-specular scattering intensities to the interface structure in neutron or x-ray reflectivity measurements. Some numerical calculations based on model interfaces with certain structural features are conducted; and the results demonstrate the possibility of using the information from off-specular scattering to narrow the selections among the many model potentials or interface structures allowed by the specular reflectivity results.
The methyl torsional levels of solid acetonitrile (CH3CN)—A neutron scattering study
Authors: Trevino S. F. , Choi C. S. , Neumann D. A.
Published: July 2002

Neutron powder diffraction has been used to obtain the thermal parameters for the deuterium atoms and to confirm the crystal structure of acetonitrile-d3 at 4 K. Inelastic neutron scattering from both isotopic species is used to determine the energies of the first and second rotational levels of the methyl group. These four levels are reasonably reproduced by a threefold potential with V3=125 meV. The activation energy derived from this potential is in agreement with that previously obtained from the temperature dependence of T1 in proton magnetic resonance measurements.
Chemiluminescent channels in reactions of Ba(1P1) with water, alcohols, and ethers
Authors: de Pujo P. , Sublemontier O. , Visticot J.-P. , Berlande J. , Cuvellier J. , Alcaraz C. , Gustavsson T. , Mestdagh J.-M. , Meynadier P.
Published: July 2002

The present paper investigates chemiluminescence in reactions of excited barium atoms (6s6p 1P1) with water and a series of alcohols and ethers. The electronically excited product molecule from the reaction with H2O (and D2O) is BaOH (and BaOD) in the A 2Π, A′ 2Δ, and B 2Σ levels. The product molecule is always Ba–ORx in reactions with alcohols ROH, whatever the size of the alcohol (methanol to butanol), and whatever its class (primary to tertiary). By comparison, no chemiluminescence was observed when the reactant was dimethyl and diallyl ether although allowed energetically. The nature of the product molecules in reaction with alcohols, and the absence of reaction with ethers that were found here are remarkably close to what was found by Davis et al. (submitted to J. Chem. Phys.) for the formation of ground state products in reactions of Ba(6s5d 1D2) with water, methanol, and dimethyl ether. The present work thus allows us to extend the model of Davis et al. for the reactivity of Ba(6s5d 1D2) as a powerful and general description of the reactivity of electronically excited barium atoms—the close approach of excited barium to water or alkyl alcohol molecules induces migration of the H atom that is bonded to oxygen. The reaction and the nature of the reaction product is determined by the departure of this H atom. The present paper also provides as side results determination of the Ba–OCH3 binding energy 3.8±0.1 eV, and spectroscopic information on several barium monoalkoxides.