bluuebunny/crossref_metadata_2025 huggingface dataset:
Intensity-dependent minority-carrier lifetime in III-V semiconductors due to saturation of recombination centers
Authors: Ahrenkiel R. K. , Keyes B. M. , Dunlavy D. J.
Published: July 2002

The minority-carrier lifetime has been measured by time-resolved photoluminescence in a variety of III-V epitaxial material including GaAs and AlxGa1−xAs. In cases where Shockley–Read–Hall recombination is dominant, the measured lifetimes are dependent upon the intensity of the excitation source. These lifetime effects can be described by a Shockley–Read–Hall model that includes the injection dependence of the recombination. As the lifetimes increase with the injection level, we describe the effects as the saturation of recombination centers.
The role of impurity scattering in the quantum waveguide
Authors: Mao J. M. , Huang Y. , Zhou J. M.
Published: July 2002

The effect of elastic scattering due to impurities in the quantum waveguide is investigated with an exact calculation. We treat it as a one-dimensional scattering problem and neglect any width effect. The method is applied to the quantum modulated transistor and Aharonov–Bohm ring. It is found that the overall transmission of the structure is distorted due to the scattering, the extent of distortion is not only dependent on the impurity potential strength but also on impurity positions. The cutoff frequency of the transistor affected by the scattering is also discussed.
Effect of gas-phase collisions in pulsed-laser desorption: A three-dimensional Monte Carlo simulation study
Authors: Sibold Dieter , Urbassek Herbert M.
Published: July 2002

The gas flow of particles laser desorbed from an elemental target into a vacuum is studied by Monte Carlo simulation. Pulsed desorption off a finite area is modeled; this is possible by using a three-dimensional simulation algorithm. We monitor the temporal evolution of the desorption cloud and global features of the flow, such as the number of collisions occurring in the gas, and the fraction of particles backscattered to the surface. The angle and energy distribution of the desorbed particles is investigated as a function of the number of monolayers desorbed, and the laser spot width. Our results show the formation of a desorption jet, in which fast particles are focused towards the jet axis, while slow particles leave the jet at oblique angles. Many features of the particle flux may be fitted by so-called elliptical distributions. However, these represent the velocity distribution of particles at oblique angles only poorly. Finally, we demonstrate the differences which exist between our three-dimensional simulation and previous, one-dimensional treatments. These studies are of fundamental interest for the characteristics of thin films deposited from the desorbed material.
Magnetic properties of RNi4B (R=Y, Gd): Magnetic and specific-heat measurement
Authors: Hong N. M. , Thuy N. P. , Schaudy G. , Holubar T. , Hilscher G. , Franse J. J. M.
Published: July 2002

YNi4B is a paramagnetic compound with the susceptibility equal to 2.7×10−6 emu/g at 5 K. From the low-temperature specific heat, values of 458 K for the Debye temperature and 14.1 mJ/K2 mol for the coefficient of the electronic specific heat are obtained. GdNi4B is ferromagnetic below Tc=35.0 K where the λ-type anomaly is observed in the specific heat. Magnetic and specific-heat measurements provide strong evidence that the 3d band in YNi4B and GdNi4B is completely filled.
Dependence of electrical properties on film thickness in Pb(ZrxTi1−x)O3 thin films produced by metalorganic chemical vapor deposition
Authors: Sakashita Yukio , Segawa Hideo , Tominaga Kouji , Okada Masaru
Published: July 2002

Strongly c-axis oriented Pb(ZrxTi1−x)O3 (PZT) thin films with tetragonal perovskite structure (0.45≤x≤0.52) were epitaxially grown on (100)Pt/(100)MgO substrates using metalorganic chemical vapor deposition. Film thickness could be varied by altering the growth time. The electrical properties of PZT thin films sharply change below a thickness of 0.5 μm: the dielectric constant and remanent polarization decrease while the coercive field increases. These phenomena are explained by a model in which a layer with low dielectric constant exists in series with the normal PZT layer. The origins of this layer are systematically studied and found to be the intrinsic stress produced by the coalescence of crystal grains.
Surface electron-diffraction patterns of β-FeSi2 films epitaxially grown on silicon
Authors: Mahan John E. , Thanh V. Le , Chevrier J. , Berbezier I. , Derrien J. , Long Robert G.
Published: July 2002

Semiconducting β-FeSi2 is drawing much current research interest because of hoped-for silicon-based optoelectronics applications. The study of heteroepitaxial film growth on silicon depends heavily upon several transmission and reflection electron-diffraction techniques. Because of the complicated crystal structure of this material, the possibility of competing heteroepitaxial relationships, the propensity for formation of epitaxial variants by rotation twinning, and the uncertainty in the crystalline surface nets, the analysis of experimental diffraction patterns is complicated. A theoretical reference for a number of fundamental electron-diffraction patterns is provided and they are illustrated with a broad range of experimentally obtained patterns from the surfaces of epitaxial films. In situ transmission reflection high-energy electron diffraction (RHEED) (transmission electron diffraction with conventional RHEED instrumentation), from rough but epitaxial films, is of great utility and quite feasible with epitaxial systems such as this one, which exhibit a tendency toward islanding. The possibilities for experimentally distinguishing, with this technique, the competing epitaxial relationships on Si(111) are clarified; it is found that the β-FeSi2(110) matching face is certainly present in these samples and the (101) may be also. An experimental determination of the two-dimensional space groups of the (100), (110), and (101) faces is also presented—in the first and third cases the surface unit meshes are different from the simple projections of the bulk crystalline unit cell.
Epitaxial growth and surface acoustic wave properties of lithium niobate films grown by pulsed laser deposition
Authors: Shibata Yoshihiko , Kaya Kiyoshi , Akashi Kageyasu , Kanai Masaki , Kawai Tomoji , Kawai Shichio
Published: July 2002

The deposition of lithium niobate (LiNbO3) films on sapphire substrates (001) and (110) using pulsed excimer-laser ablation has been investigated. The effect of several experimental parameters. Li/Nb ratio of the target, the substrate temperature, and the pressure and composition of the gas during the deposition, on the resulting film quality are examined. Under optimized conditions, epitaxial LiNbO3 films of high quality are deposited. The as-grown LiNbO3 films are sufficiently piezoelectric to fabricate surface acoustic wave (SAW) devices without poling. Rayleigh and Love-type waves are observed on the films and these waves show high velocities of propagation (5300 to 6600 m/s) and good temperature coefficients (−34 to −80 ppm/deg). The chemical composition, microstructure, crystallographic orientation, crystallinity, and SAW properties of the films are evaluated.
Artifacts in magnetic resonance imaging from metals
Authors: Bennett L. H. , Wang P. S. , Donahue M. J.
Published: July 2002

Metallic biomedical implants, such as aneurysm clips, endoprostheses, and internal orthopedic devices give rise to artifacts in the magnetic resonance image (MRI) of patients. Such artifacts impair the information contained in the image in precisely the region of most interest, namely near the metallic device. Ferromagnetic materials are contraindicated because of the hazards associated with their movement during the MRI procedure. In less-magnetic metals, it has been suggested that the extent of the artifact is related to the magnetic susceptibility of the metal, but no systematic data appear to be available. When the susceptibility is sufficiently small, an additional artifact due to electrical conductivity is observed. We present an initial systematic study of MRI artifacts produced by two low susceptibility metals, titanium (relative permeability μr≊1.0002) and copper (μr≊0.99998), including experimental, theoretical, and computer simulation results.
High level injection phenomena in P–N junctions
Authors: Manifacier J. C. , Ardebili R. , Popescu C.
Published: July 2002

To model high injection phenomena in P–N junction devices it is usually necessary to use numerical analysis. This is because the standard procedure of dividing the structure into neutral zones and depletion layer (regional approximation) fails. In a recent paper, Yue et al. [J. Appl. Phys. 77, 1611 (1995)] proposed an extension of the Shockley theory, retaining the form of the conventional diffusion current-only model for the conduction mechanism. Their solution was based on the resolution of the ambipolar transport equation in the base. It is shown here by a numerical simulation of the complete structure, within the framework of the drift-diffusion model, that both the exponential current dependence: Jα exp(eVa/2kT) as well as the Yue et al. approximation are valid only in the limiting case of a strongly extrinsic short base diode.
Photostimulated luminescence of BaFBr:Eu2+ phosphors
Authors: Chen Wei , Su Mianzeng , Song Jiaqing , Lin Jinhua
Published: July 2002

After x-irradiation for 10 s, luminescence from BaFBr:Eu2+ phosphors by photostimulation of longer wavelength than F absorption bands was observed and assigned to the surface states or intrinsic defects of the powders. It is found that the luminescence by photostimulation into F bands can be reduced via electron migration from F centers into the surface states or intrinsic defects, thus reducing the x-ray storage or image stability. Surface passivation can lower these defects and improve the phosphors or imaging plate quality.
Analyzing atomic force microscopy images using spectral methods
Authors: Fang S. J. , Haplepete S. , Chen W. , Helms C. R. , Edwards Hal
Published: July 2002

Various statistical quantities (such as average, peak-to-valley, and root-mean-square roughness) have been applied to characterize surface topography. However, they provide only vertical information. Because spectral analysis provides both lateral and longitudinal information, it is a more informative measurement than all these commonly used statistical quantities. Unfortunately, a standard method to calculate power spectral density (PSD) is not available. For example, the dimensions of PSD are often denoted as either (length)3 or (length)4. This may lead to confusion when utilizing spectral analysis to study surface morphology. In this paper, we will first compare the definitions of PSD commonly used by various authors. Using silicon surface roughness measurements as examples, we will demonstrate the advantages of spectral methods on atomic force microscopic (AFM) image analysis. In this context, we study the effects of typical AFM imaging distortions such as image bow, drift, tip-shape effects, and acoustic noise. As a result, we will provide a procedure to obtain accurate and reproducible AFM measurements.
Dual electroluminescence of an amino substituted 1,3,5-triphenylbenzene
Authors: Rommens J. , Vaes A. , Van der Auweraer M. , De Schryver F. C. , Bässler H. , Vestweber H. , Pommerehne J.
Published: July 2002

The electroluminescent properties of 5′-[4-[bis(4-ethylphenyl)amino]phenyl] -N,N,N′,N′-tetrakis(4-ethylphenyl) -[1,1′:3′,1″-terphenyl]-4,4″-diamine (pEFTP) were investigated in a vapor deposited layer and dispersed in a polymer matrix. Blue–violet electroluminescence was observed, after applying voltages beyond 12 V for the single layer devices and 22 V for the double layer devices. The electroluminescence (EL) spectrum shows two maxima, of which one corresponds to that of the photoluminescence spectrum. Either direct radiative recombination of the hole and the electron residing at two neighboring molecules or phosphorescence is causing this red shifted electroluminescence maximum. Transient electroluminescence measurements allow us to estimate the mobilities of the charge carriers in the different transport layers. The occurrence of an EL overshoot after switching off a voltage pulse, confirms the importance of detrapping and interfacial phenomena in the radiative recombination in pEFTP.
Effects of annealing on modal properties and second-harmonic generation in periodically poled LiTaO3 channel waveguides
Authors: Wong Victor , Gupta Mool C.
Published: July 2002

We provide a systematic investigation of the effects of annealing on modal properties and second-harmonic generation in periodically domain-reversed LiTaO3 channel waveguides. Prism-coupling studies show complex spatial mode distributions and strong dependence of waveguide modal properties on index profiles. Second-harmonic measurements observe the recovery of the optical nonlinear coefficient by annealing. The optimum conversion efficiency is obtained in a very narrow window of annealing time. A considerably large shift in quasi-phase-matching wavelength is also documented. Measurement results are correlated with theoretical modeling.
Pockel’s electro-optic coefficients of lithium tetraborate Li2B4O7
Authors: Méndez A. , Garcı́a-Cabañes A. , Diéguez E. , Cabrera J. M.
Published: July 2002

Pockel’s electro-optic coefficients r31 and r33 of Li2B4O7 have been measured with an improved interferometric (Mach–Zehnder) method. The effective rc coefficient has also been measured by a polarimetric setup which, within the experimental error, has given the same value as that of the interferometric method. Attempts to measure the r42 coefficient with either the interferometric or the polarimetric method have given a value below the detection limit of our setups. All coefficients are given in terms of the published values of the piezoelectric coefficient d13.
Organic semiconductor interfaces: Discrimination between charging and band bending related shifts in frontier orbital line-up measurements with photoemission spectroscopy
Authors: Schlaf R. , Merritt C. D. , Crisafulli L. A. , Kafafi Z. H.
Published: July 2002

Gaq 3 is a promising luminescent organic semiconductor for applications in organic light emitting diodes. The frontier orbital alignment at the tris (8-hydroxyquinolinato) gallium (Gaq3)/Pt organic Schottky contact was determined by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. A Gaq3 thin film was deposited in several steps on a previously Ar+ sputtered pure Pt foil. After each growth step, the sample was characterized by XPS and UPS. The combination of XPS and UPS measurements allows the precise evaluation of the interface dipole independent from the simultaneously occurring band bending at the interface and charging artifacts. The measurements show that the Pt/Gaq3 interface has a strong dipole of 0.71 eV indicating the transfer of negative charge from Gaq3 to Pt. Due to the large work function difference between Pt and Gaq3, strong band bending occurred. At Gaq3 coverages higher than 128 Å strong charging shifts occurred in the overlayer related emission lines which were identified by measuring the high binding energy cutoff (secondary edge) of both the XP and UP spectra. Due to the several magnitudes difference between the x-ray and ultraviolet source photon intensities, differences between the high binding energy cutoff positions of both measurements allow to pinpoint charging shifts with high sensitivity. In order to investigate the possible influence of radiation damage to the Gaq3 film, a single growth step control film of Gaq3 of the same thickness as in the multi-step orbital line-up procedure was deposited. The comparison of XPS peak intensities and positions suggest that no significant chemical alteration occurred during the photoemission spectroscopy measurements of the multi-step deposition experiment. However, the doping level of the layer may have changed due to the radiation exposure during the measurements.
Strain distribution in self-assembled InP/GaInP quantum dots
Authors: Jin-Phillipp N. Y. , Phillipp F.
Published: July 2002

Local strain on an atomic scale, as well as the size and the shape of InP quantum dots (QDs) embedded in GaInP, have been measured directly from high-resolution electron microscopy images. There is a strong spatial variation in strain within a QD. A large part of the misfit strain is distributed in the GaInP matrix surrounding the QDs, especially in the case of small spacing between the QD layers. The strain distribution varies significantly with the thickness of the GaInP spacer layers between QD layers. The compressive strain within the QDs decreases with decreasing spacing between the QD layers. This contributes to an increasing red shift of photoluminescence energy peak positions of QDs of multi-layers with decreasing thickness of the spacer layers between the QD layers in comparison with that of single layer QDs.
Photoluminescence and photostimulated luminescence of Tb3+ and Eu3+ in zeolite-Y
Authors: Chen Wei , Sammynaiken Ramaswami , Huang Yining
Published: July 2002

Tb 3+ and Eu3+ were codoped in zeolite-Y. Their photoluminescence and photostimulated luminescence were investigated. Due to the dissipation of excitation energy by OH vibrations, the luminescence from the hydrated zeolite containing Tb3+ and Eu3+ prepared at room temperature is very weak. However, the luminescence is enhanced greatly when the sample was treated at 800 °C. Strong photostimulated luminescence of both Tb3+ and Eu3+ was detected in the sample prepared at 800 °C. The photostimulated luminescence of Tb3+ is due to the recombination of electrons with Tb4+ ions, while the photostimulated luminescence of Eu3+ is caused by energy transfer from Tb3+ to Eu3+. The existence of Tb4+ and Eu2+ ions is due to charge transfer from Eu3+ to Tb3+. The occurrence of photostimulated luminescence and discrete emission lines in blue (434 nm), green (543 nm), and red (611 nm) colors indicate that this material has potential applications in white light-emitting devices and erasable optical storage.
Classical theory of collisional depolarization and rotational relaxation in open-shell diatomic molecules
Authors: Corey Gregory C. , Smith Andrew D.
Published: July 2002

The semiclassical S-matrix theory of Miller and Marcus is utilized to derive an exact classical mechanical expression for the effective cross sections governing collisional depolarization and rotational relaxation in open-shell molecules with Σ electronic symmetry. These cross sections are expressed as a sum of products of classical dynamical terms (which may be evaluated by conventional Monte Carlo techniques) and geometrical terms that describe the quantum mechanical coupling of the angular momentum vectors in the open-shell molecule. All effects on rotational transitions due to the nonzero electronic spin angular momentum in the open-shell molecule are accounted for through the geometrical terms. Recently, sophisticated molecular beam and laser double resonance techniques have allowed the state-resolved study of rotational energy transfer in collisions involving open-shell molecules in their ground electronic state. The present extension of classical scattering theory provides the theoretical framework for an exact classical mechanical calculation of the collision dynamics in such experiments.
van der Waals potentials from the infrared spectra of rare gas–HF complexes
Authors: Fraser G. T. , Pine A. S.
Published: July 2002

High-resolution infrared spectra of the Ar–HF, Kr–HF, and Xe–HF van der Waals molecules have been recorded in the vicinity of the H–F stretching fundamentals, ν1, under thermal equilibrium conditions at T≂211 K with a tunable difference-frequency laser. Rotational structure has been observed up to or approaching rotational predissociation, permitting us to model the effective radial van der Waals potentials for these complexes. These potentials provide good estimates for the binding energies, D0, and the van der Waals stretching frequencies, ν3, in the ground (v1=0) and excited (v1=1) states of the molecules. For v1=0 in Ar–HF, Kr–HF, and Xe–HF, we find D0=102, 133, and 181 cm−1 and ν3=39.2, 41.1, and 43.4 cm−1, respectively. The ν3 modes characterized by the model potentials aid in the assignment of the ν1+ν3−ν3 hot bands observed in our spectra. The band centers for the ν1 fundamentals are all down shifted in frequency from the isolated HF monomer by Δν=−9.654, −17.518, and −29.185 cm−1 for the Ar, Kr, and Xe complexes, respectively, indicating that the van der Waals bonds are some 10% to 15% stronger in the excited vibrational state. This increased vibrational attraction also results in a contraction of the van der Waals radial coordinate manifest in the larger rotational constants observed for ν1; ΔB/B0=+0.35%, +1.00%, and +1.75% for Ar–, Kr–, and Xe–HF. We have also observed the Q branch of the ν1+ν2 stretch–bend combination band in Ar–HF some 70.2 cm−1 above the ν1 fundamental with a large negative ΔB/B0=−2.00% implying a strong anisotropy in the potential.
Theoretical study of OH+: Potential curves, transition moments, and photodissociation cross sections
Authors: Saxon Roberta P. , Liu B.
Published: July 2002

CASSCF/CI calculations have been performed for the three lowest 3Σ− and three lowest 3Π states of OH+. The Rydberg nature of the excited states at small internuclear distances has been carefully investigated by obtaining an optimized Rydberg orbital and enlarging the active space in the CI calculation. The identification of the Rydberg states was found to depend sensitively on the procedure used to characterize the states. Transition dipole moments between the ground and excited states were evaluated and used to calculate photodissociation cross sections.
The Raman-active O–H and O–D stretching vibrations and Raman spectra of gaseous formic acid-d1 and -OD
Authors: Bertie John E. , Michaelian Kirk H. , Eysel Hans H. , Hager Darcy
Published: July 2002

In keeping with current theoretical activity concerning the OH and OD stretching bands of the carboxylic acids, we report the Raman spectra of gaseous formic acid-OD and formic acid-d1 for the first time. We emphasize the OH and OD stretching bands, which can be studied cleanly in these isotopomers but not in normal or perdeuterated formic acid. The spectra of the dimers and monomers below 2000 cm−1 are assigned, and current knowledge of the vibrations of the molecules is summarized. The Raman spectra allow the estimation of the energies of the Bu combination levels that may be in Fermi resonance with the infrared active Bu, OH or OD stretching fundamental, as well as those of the Ag overtone and combination levels that may interact with the Raman-active stretching fundamental. We conclude that the sharp features on the Raman OH and OD stretching bands are due to overtone and combination transitions, that the stretching modes cause the underlying broad scattering, namely three broad bands, centered at 2430, 2270, and 2080 cm−1 for (HCOOD)2 and at 3240, 3074, and 2880 cm−1 for (DCOOH)2. We further conclude that the higher and lower frequency broad bands are due to sum and difference transitions with the hydrogen bond modes, which lie between 60 and 240 cm−1. The infrared OD and OH stretching bands of (HCOOD)2 and (DCOOH)2 are consistent with this interpretation. The Raman OD stretching band of HCOOD ⋅ HCOOH coincides with that of (HCOOD)2 apart from the overtone and combination transitions. The centers of Raman intensity, corrected for instrument, wave number, and temperature dependencies, of the OD stretching bands of (HCOOD)2 and HCOOH ⋅ HCOOD are 2300 ±5 and 2305 ±10 cm−1, respectively, and that of the OH stretching band of (DCOOH)2 is 3035±10 cm−1. This data and the near coincidence of the infrared and Raman bands of (HCOOD)2 show that the vibrational coupling of the two OD bonds in the dimer is not unusually large. Comparison of the infrared and Raman OH stretching bands indicates a larger coupling between OH oscillators. These results agree with earlier result for acetic acid.
Dynamical effects in electron transfer reactions. II. Numerical solution
Authors: Nadler Walter , Marcus R. A.
Published: July 2002

In part I a reaction–diffusion equation was introduced for the description of electron transfer reactions which are induced by fluctuations in both the solvent polarization and in the intramolecular vibrational coordinates. We analyze the model employing a generalized moment expansion for the time behavior of the survival probability Q(t), i.e., for the fraction of molecules that have not transferred their electron at time t. Numerical and, in the narrow reaction window limit, analytical solutions are given for the average survival times τ. When the contribution of the intramolecular coordinates is appreciable an approximate power-law behavior τ∝ταL, with 0<α≤1, is found for the dependence of τ on the solvent dielectric relaxation time τL, in the large τL regime. Within the framework of the generalized moment description Q(t) is approximated as a superposition of several optimized exponential functions. In the small and intermediate τL regimes it is found that a single- or bi-exponential description, respectively, is sufficient. Simple formulas for such approximations in terms of the average survival times are given. Furthermore it is demonstrated that in the large τL regime a truly multiexponential time behavior for the survival probability is encountered which, over a certain range of time, can appear to be algebraic, i.e., Q(t) ∝t−γ. The relation of these results to experimental data is discussed.
Adiabatic approximation and nonadiabatic corrections in the discrete variable representation: Highly excited vibrational states of triatomic molecules
Authors: Light J. C. , Bačić Z.
Published: July 2002

An adiabatic approximation for the calculation of excited vibrational (J=0) levels of triatomic molecules is developed using the discrete variable representation (DVR). The DVR is in the large amplitude bending motion coordinate which is taken to be the adiabatic degree of freedom. We show that the adiabatic treatment in the DVR has some major advantages over the usual formulation in the finite basis representation (FBR), namely improved accuracy and broader range of applicability. An adiabatic rearrangement of the full Hamiltonian matrix in the DVR-ray eigenvector (REV) basis is defined, such that the diagonal blocks provide the rigorous matrix representation of the adiabatic bend Hamiltonian; their diagonalization yields bending level progressions corresponding to various stretching states. The off-diagonal blocks contain all nonadiabatic coupling matrix elements. The nonadiabatic corrections to the adiabatic vibrational levels are readily taken into account via second-order perturbation theory. One unique feature of our approach is that, in contrast to the FBR formulation, evaluation of the adiabatic and nonadiabatic matrix elements does not require evaluation of derivatives of the stretching wave functions with respect to the adiabatic variable. This approach is tested on the two-mode LiCN/LiNC (fixed CN distance) and the three-mode HCN/HNC. The adiabatic vibrational levels are in good agreement with accurate variational results. When corrected by second-order perturbative treatment, many levels are given very accurately (to within 0.1%) even for energies above the isomerization barriers. More localized states are better represented in the adiabatic approximation then delocalized vibrational states.
Molecular dynamics simulation for chemically reactive substances. Fluorine
Authors: Stillinger Frank H. , Weber Thomas A.
Published: July 2002

Molecular dynamics computer simulation has been utilized to study physical and chemical properties of the highly reactive element fluorine in its fluid phases. The underlying model approximates the energy of the ground electronic state for an arbitrary collection of fluorine atoms with a combination of two and three atom interactions. The classical simulation employed 1000 atoms subject to periodic boundary conditions. Diatomic molecules spontaneously form and are stable at low temperatures, but dissociation and atom exchange reactions occur at high temperatures. Steepest-descent quenching on the potential energy hypersurface reveals the presence of a temperature-independent inherent structure for the low-temperature undissociated liquid. Dissociation is found to be strongly enhanced at high density owing to relatively strong solvation by diatomics of chemically unbonded fluorine atoms. Slow cooling of the fluid from well above the critical temperature, at one-eighth of the triple-point density, produced a condensation phenomenon driven by the weak van der Waals attractions that operate between intact diatomic molecules.
Highly excited rovibrational states using a discrete variable representation: The H+3 molecular ion
Authors: Tennyson Jonathan , Henderson James R.
Published: July 2002

A formulation of the rovibrational problem in Jacobi coordinates is presented which employs a discrete variable representation for the angular internal coordinate. Rotational excitation is implemented via a two-step procedure and symmetry (for AB2 systems) included using a computationally efficient method. Energies for the lowest 180 vibrational states of H+3 are presented and their wavefunctions analyzed graphically. J=1←0 excitation energies are presented for the lowest 41 vibrational states. The significance of the regular states in the high-energy regime of H+3 is discussed.
Equilibrium distributions of isomer groups in homologous series of hydrocarbons at a specified partial pressure of molecular hydrogen
Authors: Alberty Robert A.
Published: July 2002

The equilibrium distribution of isomer groups within a homologous series at a specified partial pressure of molecular hydrogen is given by a geometric distribution when the standard Gibbs energies of formation of the successive isomer groups in the homologous series are given by A+Bn, where n is the carbon number of the isomer group and A and B are functions of temperature only. This distribution depends only on A, the specified partial pressure of molecular hydrogen, and the pressure on the homologous series. This applies to the usual homologous series, where the increment between successive isomer groups is CH2, and to the benzenoid polycyclic aromatic hydrocarbons, where the increment is C4H2. In some homologous series (e.g., alkenes and the coronene series of polycyclic aromatic hydrocarbons), where the successive isomer groups have the same H/C ratio, the equilibrium distribution within the homologous series is, of course, independent of the specified pressure of molecular hydrogen, but the distribution can be calculated in the same way.
Fourier transform emission spectroscopy at 13 μm: Vibration–rotation spectrum of SiS
Authors: Frum C. I. , Engleman R. , Bernath P. F.
Published: July 2002

The vibration–rotation emission spectrum of the high temperature molecule SiS was recorded with a Fourier transform spectrometer. The observation of a high resolution emission spectrum at 13 μ (750 cm−1) is unusual but we believe this technique is generally applicable to high temperature species. Seven bands (1–0 through 7–6) of the main isotopic form, 28Si32S, were observed along with three bands (1–0, 2–1, and 3–2) for each of the minor species, 29Si32S, 30Si32S, and 28Si34S. Mass-reduced Dunham parameters, including Watson’s correction due to the breakdown of the Born–Oppenheimer approximation, were derived from the data.
Molecular dynamics investigation of deeply quenched liquids
Authors: Yang Ju-xing , Gould Harvey , Klein W. , Mountain Raymond D.
Published: July 2002

Molecular dynamics simulations of homogeneous crystalline nucleation in systems of 1300 particles have been performed as a function of quench depth for the Lennard-Jones, r−12, and r−6 potentials. We observe that the nucleating droplet is spatially asymmetric, has a layered structure, and is ramified for deep quenches. The initial growth of the droplet occurs by the addition of layers until the droplet becomes sufficiently large to be characterized by a crystalline close-packed structure. We also observe that the time lag between the time of formation of the nucleating droplet and the time of release of latent heat is a nonmonotonic function of quench depth. The results for deep quenches are interpreted as evidence for the influence of a pseudospinodal.
Harmonic force field: An approximate relationship between the exact nonrelativistic and the Hartree–Fock limit values of the force constants
Authors: Pupyshev Vladimir I. , Panchenko Yurii N. , Bock Charles W. , Pongor Gábor
Published: July 2002

A theoretical background to the scaling procedure used for correcting molecular force constants computed at the Hartree–Fock (HF) level is presented, in which scaling is considered as an empirical simulation of the effect of electron correlation. Using a variational formalism for the analytical first and second derivatives, it is shown that a successful scaling requires (i) relatively large exact excitation energies; (ii) a singlet-stable solution for the HF ground state; and (iii) molecular orbitals that can be well localized for the ground electronic state. The relationship between the exact nonrelativistic and HF limit values of the quadratic force constants has been investigated when the above conditions are satisfied. A single multiplicative (scale) factor is required at this limit and its value is approximately C20 near the ‘‘exact’’ equilibrium geometry, where C0 is the coefficient of the HF determinant in a complete configuration interaction expansion. This approach requires a moderate size of the molecules investigated. A specific numerical example is considered.
Network defects and molecular mobility in liquid water
Authors: Sciortino Francesco , Geiger Alfons , Stanley H. Eugene
Published: July 2002

As a step toward elucidating the connection between the structure and mobility of liquid water, we analyze quenched molecular dynamics configurations at different densities. We find that the mobility is directly related to the existence of ‘‘topological defects’’ of the tetrahedral network. The defects act as catalysts, providing lower energy pathways between different tetrahedral local arrangements.
Core-valence separation in the spin-coupled wave function: A fully variational treatment based on a second-order constrained optimization procedure
Authors: Karadakov Peter B. , Gerratt Joseph , Cooper David L. , Raimondi Mario
Published: July 2002

The theory of the spin-coupled (SC) wave function with core-valence separation, in which the core electrons are confined to a closed shell of doubly-occupied orbitals and the valence electrons are described with the complete set of features of the SC formalism, is developed to produce an efficient approach which makes possible its fully variational determination. The simultaneous optimization of the core orbitals, valence orbitals, and spin-coupling coefficients is achieved through a second-order nonlinear elimination constrained minimization algorithm which exhibits excellent convergence properties. It is no longer necessary to introduce an ad hoc preselection of core and valence orbitals−this is carried out by the minimization procedure itself which makes an optimum choice from a variational point of view. The only important item left to personal judgment is the selection of the number of core and valence electrons in the problem under investigation. Simplifications such as ‘‘freezing’’ of a part of the core orbitals are discussed alongside with the verification of the theoretical work and program code by emulating, under an appropriately modified set of constraints, the generalized valence-bond (GVB) wave function with perfect pairing and strong-orthogonality restrictions. It is demonstrated that it is possible, similar to the Hartree–Fock (HF) method, to transform the core orbitals into canonical form and to associate with each one of them an energetical quantity analogous to the HF orbital energy. The essential features of the approach are illustrated by a SC study of the process H2CO(1A1)→CH2(X̃ 3B1)+O(3P), involving the breaking of the C■O double bond. The results prove that only a fully variational SC wave function with core-valence separation is capable of providing a uniform description of the change in the physical properties of the system upon dissociation, which can be achieved by including in the SC part of the wave function just the four electrons immediately involved in the carbon–oxygen double bond.
Efficient elimination of basis set superposition errors by the local correlation method: Accurate ab initio studies of the water dimer
Authors: Saebo/ Svein , Tong Wen , Pulay Peter
Published: July 2002

The water dimer has been studied by accurate ab initio calculations. The main purpose of the calculations was to investigate the magnitude of, and how to eliminate the basis set superposition errors at different levels of theory. At the Hartree–Fock level the superposition errors are insignificant with the largest basis sets, and the counterpoise method works well with all the basis sets used in this study. At the correlated level superposition errors are still significant even for very large basis sets, and the standard counterpoise technique leads to overcorrection. The most important result of the present study is that the local correlation methods gives essentially the correct result for the correlation contribution to the association energy even with modest basis sets. The association energy at the MP4(SDQ) level is predicted to be 4.8 kcal/mol. The correlation contribution to the association energy is 1.2 kcal/mol which can be decomposed into an attractive intermolecular contribution of 1.8 kcal/mol and a repulsive intramolecular contribution of 0.6 kcal/mol. Ionic terms contribute about 30% to the dispersion force at the equilibrium distance. If the effect of triple substitutions is taken into account the association energy is estimated to be around 5.1 kcal/mol.
On the determination ofD(CaCl) from translational energy threshold measurements
Authors: Menéndez M. , Garay M. , Verdasco E. , González Ureña A.
Published: July 2002

By using the crossed-beam technique the translational energy threshold for the Ca(3PJ)+HCl→CaCl (A 2Π)+H chemiluminescent reaction was found to be E0=350±60 meV. By means of energy balance relations, a value of D00 (CaCl)=4.2±0.09 eV was obtained. The potentiality of the present method, based on translational energy measurements, to determine dissociation energies is addressed.
Broadening of vibrational lines by attractive forces: Ultrafast Raman echo experiments in a CH3I:CDCl3 mixture
Authors: Muller Laura J. , Vanden Bout David , Berg Mark
Published: July 2002

A definitive demonstration of inhomogeneous vibrational line broadening in a liquid is made from Raman echo measurements of the sym-methyl stretching vibration of CH3I in a 50% mixture with CDCl3. The lifetime of the inhomogeneity is found to be 4–7 ps. The source of the inhomogeneity is identified as concentration fluctuations within the first solvation shell. The range and time scale of the interaction are consistent with the predictions of Schweizer and Chandler [J. Chem. Phys. 76, 2296 (1982)] for an attractive force interaction.
Atom-spherical top van der Waals complexes: A theoretical study
Authors: Hutson Jeremy M. , Thornley Alice E.
Published: July 2002

The theory of the vibration-rotation states of atom-spherical top van der Waals complexes is developed. The exact close-coupled equations are closely analogous to those encountered in atom+spherical top scattering. The structure of the coupled equations is investigated, and close-coupling calculations of the bound states of Ar–CH4 are presented for two different intermolecular potentials. The role of symmetry in the complex is discussed, and the energy levels are interpreted using a model in which the CH4 molecule undergoes hindered rotation in the field of the Ar atom. Correlation diagrams are presented, showing how the free-rotor levels are converted into near-rigid vibrational energy levels as the anisotropy of the intermolecular potential increases. The effect of higher-order anisotropic terms is investigated, and correlation diagrams are given for complexes of tetrahedral, octahedral, and icosahedral molecules. The role of monomer vibrational angular momentum is investigated.
Spectroscopy and dynamics of rare gas–spherical top complexes. The infrared spectrum of the ν3 band of argon–silane
Authors: Randall Robert W. , Ibbotson Jonathan B. , Howard Brian J.
Published: July 2002

The infrared spectrum of Ar–SiH4, a rare gas–spherical top van der Waals complex, has been recorded in the vicinity of the silane ν3 triply degenerate stretching vibration ∼2189 cm−1. A complex, dense spectrum is observed, and this is due to the observation of 12 bands with origins corresponding to transitions between different internal rotor states of the silane molecule within the complex. From the analysis of the rotational structure within each band, the average argon–silane separation is determined to be 4.043 Å in the ground vibrational state and 4.046 Å in the excited vibrational state with v3=1. Using a model developed previously and a very simple form for the intermolecular potential, these 12 bands have been assigned and an effective anisotropic intermolecular potential for the internal rotation of the silane molecule within the complex has been determined. The vibrational anisotropy responsible for lifting the threefold degeneracy of the ν3 vibration is found to be fairly small, of the order of 1 cm−1, while the rotational anisotropy is much larger, of the order of 90 cm−1.
Static dipole polarizabilities of N2, O2, F2, and H2O
Authors: Spelsberg Dirk , Meyer Wilfried
Published: July 2002

Standard ab initio methods are used to determine static dipole polarizabilities as the static limit of perturbational and derivative methods to calculate dynamic polarizabilities. The performance of multireference configuration-interaction wavefunctions in the perturbational procedure is found to be very promising for its application in dynamic polarizabilties calculations. Results of the perturbational and derivative approach based on wavefunctions of different structure are compared and improved dipole polarizabilities are obtained.
Locating transition states using double-ended classical trajectories
Authors: Matro A. , Freeman D. L. , Doll J. D.
Published: July 2002

In this paper we present a method for locating transition states and higher-order saddles on potential energy surfaces using double-ended classical trajectories. We then apply this method to 7- and 8-atom Lennard-Jones clusters, finding one previously unreported transition state for the 7-atom cluster and two for the 8-atom cluster.
Quartic anharmonic resonances in acetylenes and haloacetylenes
Authors: Borro Alexander F. , Mills Ian M. , Venuti Elizabetta
Published: July 2002

Formulas are derived for the quartic anharmonic resonance coefficients observed to be important between C–H stretching and the combination of one quantum of C≡C stretching and two quanta of H–C≡C bending in a number of acetylene molecules. Examples of this resonance are ν3 with ν2+ν4+ν5 in 12C2H2, ν1 with ν2+2ν5 in 13C2H2, and ν1 with ν2+2ν4 in monofluoroacetylene and monochloroacetylene. The coefficients characterizing the resonances in these examples, which we denote K3,245, K1,255, and K1,244, arise from cubic and quartic terms in the anharmonic force field, in the normal coordinate representation, through second order and first order perturbation treatments respectively, where the second order resonances are calculated by a Van Vleck resonance formalism. The experimentally determined values of these coefficients are compared with values calculated from model anharmonic force fields.
An explicitly correlated coupled cluster calculation of the helium–helium interatomic potential
Authors: Klopper Wim , Noga Jozef
Published: July 2002

Explicitly correlated coupled cluster (CCSDT-1a-R12) results were obtained for the He2 interatomic potential from a new, integral-direct implementation. With the new code, Gaussian basis sets as large as 11s8p6d5f4g3h could be employed, and the potential energy curve was calculated over a wide range using a basis of the type 11s8p6d5f4g.This curve is very close to represent the basis set limit of the CCSDT-1a approach. At the internuclear separation R=5.6 a0, the CCSDT-1a limiting value for the interaction energy is −10.68 K. As the effect of quadruple substitutions can be estimated as −0.32 K, this limiting value is perfectly consistent with the accurate quantum Monte Carlo calculation of Anderson et al. [J. Chem. Phys. 99, 345 (1993)], who reported a well depth of −11.01±0.10 K. On the other hand, however, CCSDT-1a-R12 calculations of the He2 potential energy curve strongly indicate that the most recent semiempirical potentials available in the literature are slightly too repulsive for short (R≤4.0 a0) interatomic distances.
The delayed ionization and atomic ion emission of binary metal metallocarbohedrenes TixMyC12 (M=Zr, Nb; 0⩽y⩽4; x+y=8)
Authors: Cartier S. F. , May B. D. , Castleman A. W.
Published: July 2002

Investigations of the photophysical character of binary metal containing metallocarbohedrene clusters (Met-Cars), reveal the phenomena of delayed ionization. Additionally, the onset of a channel corresponding to delayed atomic ion emission is observed at high laser fluence, becoming progressively more significant as the fluence is raised. The fluence dependence of both the delayed electron and delayed atomic ion emission channels at 355 and 532 nm are reported, and the Met-Car delayed ion yield estimated. At moderate fluences, the yield of the delayed electron emission channel leading to the Met-Car ion is found to be on the order of 70% of the total Met-Car signal. All of the experimental findings point to thermionic emission being the operative mechanism for the delayed ionization, while the delayed atomic ion emission is evidently a new mechanism [B. D. May, S. F. Cartier, and A. W. Castleman, Jr., Chem. Phys. Lett. 242, 265 (1995)] which may be due to the collective electronic excitation of these cluster materials.
Comment on ‘‘Constant pressure molecular dynamics algorithms’’ [J. Chem. Phys. 101, 4177 (1994)]
Authors: Melchionna S. , Ciccotti G. , Holian B. L.
Published: July 2002

In this comment we point out the differences between the NPT ensemble techniques proposed in references [S. Melchionna, G. Ciccotti, and B. L. Holian, Mol. Phys. 78, 533 (1993)] and [W. G. Hoover, Phys. Rev. A 31, 1695 (1985)], i.e., the revised form and the original form of the Hoover NPT dynamics. We show that our dynamics generates exactly the isobaric–isothermal distribution function for a system with (N−1) particles (similarly to what happens with the molecular dynamic (MD) microcanonical ensemble in which the total momentum is conserved). Moreover we show numerically that in the original Hoover dynamics, i.e., the one leading to an incorrect distribution function, the Nosè variable thermalizes the center-of-mass velocity while that is exactly what does not happen with our dynamics which produces the correct isobaric distribution.