bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled GaAs/AlGaAs quantum-well electroabsorption modulators for low-electric-field optical modulation
Authors: Debbar Nacer , Hong Songcheol , Singh Jasprit , Bhattacharya Pallab , Sahai Rajeshwar
Published: July 2002

Experimental and theoretical studies are presented for exciton transitions in p-i-n GaAs/AlGaAs multiple coupled-quantum-well structures where each quantum well consists of two identical wells with a thin barrier. Electroabsorption and photocurrent studies are carried out to identify how the excitonic peaks respond to transverse electric fields. With a careful choice of the dimensions of the coupled quantum well, it is seen that the lowest heavy-hole exciton peak moves at a rate ∼2.5 faster than in a square well. Thus strong modulation is obtained at much lower electric fields. The nature of the higher-energy transitions is also studied.
Optimization of process parameters for the deposition of improved a-Ge:H by dc magnetron sputtering
Authors: Drüsedau T. , Schröder B.
Published: July 2002

A detailed study of the influence of hydrogen and argon partial pressures, substrate temperature, and dc power on the composition and the optoelectronic properties of hydrogenated amorphous germanium (a-Ge:H) is presented. The sputtered a-Ge:H has a typical Ge density of 4×1022 atoms/cm3 and contains up to 4×1021 H atoms/cm3. It is chemically stable and shows no sign of postoxidation in the IR spectra over a period of one year. A low midgap absorption according to α(hν=0.6 eV)<10 cm−1 and Urbach energy E0<50 meV is observed for the best films which are prepared at the lowest argon pressure p(Ar)=0.5 mTorr and a dc power of Pdc<100 W, i.e., a growth rate of 1 μm/h or less. The ratio of photo- to dark conductivity reaches a maximum value of σph/σD=0.3 for σD=10−4 (Ω cm)−1 and correspondingly a normalized photoconductivity of ημτ=10−5 cm2/V is achieved. The increased dark conductivity of the a-Ge:H films is explained in terms of an unintentional n-type doping effect.
Electron relaxation time measurements in GaAs/AlGaAs quantum wells: Intersubband absorption saturation by a free-electron laser
Authors: Duboz J. Y. , Costard E. , Rosencher E. , Bois P. , Nagle J. , Berset J. M. , Jaroszynski D. , Ortega J. M.
Published: July 2002

The intersubband absorption saturation in GaAs/AlGaAs quantum wells as a function of the incident power has been measured, using picosecond micropulses with a power density up to 1 GW/cm2 delivered by a free-electron laser. The absorption in a sample with a bound-to-bound transition was compared to the absorption in a sample with a bound-to-resonant transition, and it was found that the electron relaxation time in the bound-to-bound transition is about four times shorter than for the bound-to-resonant transition.
Analysis of nonideal Schottky and p-n junction diodes—Extraction of parameters from I–V plots
Authors: Lyakas M. , Zaharia R. , Eizenberg M.
Published: July 2002

We propose two novel methods of determining nonideal Schottky and p-n junction diodes parameters from I–V plots. The series resistance Rs, saturation current Is, as well as the bias-dependent ideality factor n(V), can be obtained from two successive I–V measurements—one solely of the diode and the other with an external resistance added in series with the measured diode. Our analysis confirms that the methods produce accurate and reliable results even when the conventional techniques fail, such as when we have strongly varying function n(V) in the presence of series resistance and an experimental noise.
The thermal conductivity of polycrystalline diamond films: Effects of isotope content
Authors: Belay Kalayu , Etzel Zongyin , Onn David G. , Anthony T. R.
Published: July 2002

The thermal conductivities of two polycrystalline diamond films made by chemical-vapor-deposition process have been measured from a temperature of 15–550 K by using a standard steady-state technique. While one of the samples was of natural abundance composition (98.9% 12C and 1.1% 13C) the second sample was composed of 50% of each carbon isotope. Around room temperature the thermal conductivity of the natural abundant diamond film for heat flow parallel to the plane was found to be more than three times higher than that of the sample that contained equal amounts of each 12C and 13C isotopes. The results are interpreted in terms of the Klemens–Callaway model for thermal conductivity of solids as a function of temperature.
Shock compaction of SmCo5 particles
Authors: Chau R. , Maple M. B. , Nellis W. J.
Published: July 2002

Fine ferromagnetic particles (1–10 μm) of SmCo5 were compacted using shock pressures of 1.7–27 GPa (17–270 kbar). One-piece disks were produced with pressures in the range 2–9 GPa. X-ray-diffraction measurements revealed no changes in the composition of the compacted SmCo5 for shock pressures below 15 GPa. Magnetic properties of the shocked samples were measured with a superconducting quantum interference magnetometer. In the pressure range 2–9 GPa, the shocked specimens exhibit high intrinsic coercivity Hci. Below 2 GPa, lower values of coercivity were observed, presumably caused by incomplete bonding of particles. Above 10 GPa the intrinsic coercivity drops suddenly and levels off at higher pressures. The remnant magnetization Br, exhibits a broad peak at 6 GPa and then falls off rapidly at higher pressures. The saturation magnetization Ms, has a peak near 10 GPa that correlates with the drop in coercivity. The hardness coefficient a, derived from a fit of the phenomenological ‘‘law of approach’’ formula to the hysteresis curve, shows a linear increase with pressure near 8 GPa. This suggests an increase of inclusions in the samples and correlates with the drop in Hci and the peak in Ms. There is a non-negligible linear contribution to the magnetization given by κH. The nonzero value of κ suggests the possibility of incomplete saturation or paramagnetic impurities. Maximum values in coercivity Hci, remnant magnetization Br, and saturation magnetization Ms are found at about 9 GPa.
Mössbauer study of the surface crystallization of the amorphous and nanocrystalline Fe81Zr7B12 alloy
Authors: Kopcewicz M. , Grabias A.
Published: July 2002

The combined conversion electron emission and γ-transmission Mössbauer spectroscopy is used to investigate the differences in the bulk and surface crystallization of the amorphous Fe81Zr7B12 alloy in which the nanocrystalline bcc-Fe was formed due to annealing. The measurements were performed for Fe81Zr7B12 ribbons in the as-quenched state and after 1 h annealing at 430, 500, 550, 600, and 780 °C. The clear differences in the surface and bulk crystallization behaviors were detected. Not only does the surface crystallization start at lower annealing temperature than the bulk one, but additional phases were detected at the surface which do not form in the bulk. The enhanced surface crystallization was attributed to the boron depletion of the surface region which leads to the decrease of the crystallization temperature at the surface.
Energy states of Ge-doped SiO2 glass estimated through absorption and photoluminescence
Authors: Fujimaki Makoto , Ohki Yoshimichi , Nishikawa Hiroyuki
Published: July 2002

The energy states of oxygen-deficient type defects in the vacuum ultraviolet region are discussed based on the experimental results of vacuum ultraviolet absorption, temperature dependence of the photoluminescence (PL) intensities at 4.3 and 3.1 eV, and lifetimes of the PLs. It was found that the oxygen-deficient type glass has a large absorption tail above 6 eV in addition to an absorption band around 5 eV and that the 3.1 eV PL intensity scarcely depends on temperature when excited above 6 eV. It was also found that the lifetime of the 4.3 eV PL is 9 ns and that of the 3.1 eV PL is 113 μs irrespective of the excitation photon energy. The obtained results are explainable by assuming that electrons excited into the conduction band by photons above 6 eV contribute to the PLs.
Thermal properties of molybdenum below 1 K with application to ultracryogenic resonant-mass gravitational wave detectors
Authors: Duffy W. , Dalal S. , Quiazon M.
Published: July 2002

Measurements of the thermal conductivity and specific heat of commercial 99.95% purity annealed polycrystalline molybdenum in the temperature range of 0.1–1 K are reported. These thermal data and other published data on molybdenum and aluminum–magnesium alloys are used to calculate and compare cool-down and steady state thermal response of 1 kHz ultracryogenic resonant-mass gravitational wave detectors made of these materials. At temperatures below 100 mK, the Mo is shown to be far superior to the 5056 Al–Mg alloy with regard to temperature homogeneity in the resonant mass at the expected levels of residual heat leak.
Two-dimensional model of photon recycling in direct gap semiconductor devices
Authors: Parks Joseph W. , Brennan Kevin F. , Smith Arlynn W.
Published: July 2002

The effects of photon recycling are examined in a general, fully numerical, two-dimensional model accounting for the detailed geometry of the device and the spectral content of the recombined excess carriers. The primary component of this model is a three-dimensional ray tracing algorithm which encompasses effects such as wavelength dependent absorption and index of refraction, the angular dependence of transmissivity between layers in a heterostructure device, and multiple reflections within a device. This ray tracing preprocessing step is used to map all of the possible trajectories and absorption of various wavelengths of emitted light from each originating node within the device. These data are integrated into a macroscopic device simulator to determine the spatial and temporal location of the reabsorbed radiation within the geometry of the device. By incorporating the ray tracer results with the total quantity and spectral content of recombined carriers at each node within the simulation, the recycled generation rate can be obtained. To demonstrate the use of this model, the effects of photon recycling on the carrier lifetime in an InP/InGaAs double heterostructure photodiode are presented. Good agreement between the experimentally measured lifetime and that predicted using photon recycling is obtained.
Dynamic compression of an Fe–Cr–Ni alloy to 80 GPa
Authors: Duffy Thomas S. , Ahrens Thomas J.
Published: July 2002

Wave profiles were measured in an Fe–Cr–Ni alloy (stainless steel 304) shock compressed to Hugoniot stresses between 7 and 80 GPa. A single-stage propellant gun was used to generate shock states and time histories were recorded by velocity interferometry. The particle velocity measurements are generally consistent with impedance match calculations to ±2%. Unloading wave velocities were obtained from analysis of the release wave profiles. Using Eulerian finite strain theory and under the assumption of fully elastic initial release, the first and second pressure derivatives of the longitudinal modulus are given by: 7.9(0.5) and −0.16(0.06) GPa-1, where the numbers in parentheses are one standard deviation uncertainties. The first and second pressure derivatives of the adiabatic bulk modulus are: 6.4(1.0) and −0.17(0.08) GPa-1. The unloading wave velocities are generally consistent with extrapolated trends from low-pressure ultrasonic data as well as with higher stress shock measurements on an alloy of similar composition. From 1 bar to 80 GPa, Poisson’s ratio, ν, increases with Hugoniot stress, σ (in GPa), according to the relation: ν=0.29 + 0.0008σ. The Hugoniot elastic limit of 304 steel was found to be 0.35(0.12) GPa, and the initial yield stress is 0.21(0.07) GPa. The elastic precursor velocity was 5.8(0.1) km/s. Numerical simulations of the wave profiles using a constitutive model that incorporates a Bauschinger effect and stress relaxation reproduced the main features observed in the profiles. Release adiabats were also calculated from the measured wave profiles. The shear stress at unloading was determined to vary with stress according to the relation: τ0+τc=0.149+0.018σ, where σ is given in GPa.
Magnetic and structural properties of high coercivity Sm(Co, Ni, Cu) sputtered thin films
Authors: Prados C. , Hadjipanayis G. C.
Published: July 2002

The effects of heat treatments on the structural and magnetic properties of Sm(Co, Ni, Cu) sputtered thin films were investigated. Crystallization of the initially amorphous magnetic films produces a huge enhancement of coercivity (from 100 Oe to more than 40 kOe). The crystallized structure consists of exchange coupled precipitates in the nanometers range. The pinning of the magnetization reversal at the high anisotropy grains is proposed as the origin of such a magnetic behavior. By tuning the coercivity through the annealing conditions, these systems could be used in a number of applications, from longitudinal recording media to thin film permanent magnet.
Effect of oxygen stoichiometry on the electrical property of thin film La0.5Sr0.5CoO3 prepared by pulsed laser deposition
Authors: Liu J.-M. , Ong C. K.
Published: July 2002

A measurement of electrical conductivity and Hall effect has been made for thin film La0.5Sr0.5CoO3−δ (LSCO) prepared on (001) SrTiO3 substrates by pulsed laser deposition under reduced oxygen pressures. A considerable effect of oxygen stoichiometry on electrical resistivity of LSCO thin films has been observed. The semiconducting behavior was observed as long as the sample was slightly oxygen deficient. The Hall effect measurements revealed a change ranging from four to six orders of magnitude in the carrier density, from 1019 to 1015 cm−3 at room temperature, and 1020–1012 cm−3 at ∼80 K for the films prepared inbetween 1 bar and 1 μbar O2. A chemical equilibrium approach of the interaction between oxygen vacancies and hole carriers was used to explain the measured data. The results were compared with the microstructural measurements, yielding an empirical relation between the lattice expansion along c-axis and oxygen vacancies for LSCO films.
Ultraviolet-reduced reduction and crystallization of indium oxide films
Authors: Imai Hiroaki , Tominaga Atsushi , Hirashima Hiroshi , Toki Motoyuki , Asakuma Naoko
Published: July 2002

Structural changes stimulated by ultraviolet (UV) irradiations of sol–gel-derived indium oxide thin films were investigated. Illumination of incoherent UV photons (4.9 eV) from a low-pressure mercury lamp resulted in formation of crystalline indium metal. Irradiation of coherent UV beams from an ArF excimer laser (6.4 eV) and from the fourth harmonics of a Nd:YAG laser (4.7 eV) was found to be effective in the crystallization of indium oxide, accompanied by a decrease in the sheet resistance. The lowest resistance without a reduction of transmission in the visible region was achieved with a 6.4 eV laser beam at a fluence over 10–20 mJ cm−2 shot−1. The results of x-ray photoelectron spectroscopy revealed that charge transfer from O2− to In3+ was induced by the incoherent and the coherent UV photons. The partial reduction with the incoherent illumination and the crystallization with the laser irradiation are tentatively assumed to be due to electronic excitations in the amorphous network.
Microstructure and high temperature magnetic properties of Sm(Co, Cu, Fe, Zr)z (z=6.7–9.1) permanent magnets
Authors: Liu J. F. , Zhang Y. , Dimitrov D. , Hadjipanayis G. C.
Published: July 2002

The effect of ratio z on the high temperature magnetic properties of Sm(Co, Fe, Cu, Zr)z magnets have been examined in a range of ratio z values from 6.7 to 9.1. Reasonably high coercivity at room temperature has been achieved in all the magnets. It was found that the lower the ratio z, the smaller the temperature coefficient of coercivity. When the ratio z=7.0, a temperature coefficient of coercivity of −0.03%/°C can be achieved, which is more than eight times smaller than the magnet with z=8.5. A coercivity of more than 10 kOe has been obtained at 773 K for the newly developed magnets, which is believed to be the highest coercivity at 773 K ever reported. Transmission electron microscope studies showed that the cell size decreases with the decrease of the ratio z, while the density of lamella phase remains almost the same. This suggests that smaller cell size leads to a smaller temperature coefficient of intrinsic coercivity.
Uniaxial magnetic anisotropy of amorphous Fe–B films deposited electrochemically in a magnetic field
Authors: Fujita Naoyuki , Inoue Mitsuteru , Arai Ken’Ichi , Izaki Masanobu , Fujii Toshitaka
Published: July 2002

To attempt the formation of induced uniaxial magnetic anisotropy of electrodeposited amorphous Fe–B (a-Fe–B) films, a direct current magnetic field was applied in the film plane during deposition. The easy axis of the uniaxial anisotropy was parallel to the direction of the applied magnetic field. It was found that the magnitude of the applied magnetic field necessary for the formation of induced uniaxial anisotropy is as small as 10 Oe. The composition, deposition rate, and coercivity of the films were found to be independent of the magnitude of the applied magnetic field. From scanning electron microscopy and atomic force microscopy micrographs, no topological change of the film surface due to the application of a magnetic field was observed, indicating that the anisotropy rises from a microscopic origin such as atom pair ordering as in vacuum evaporated or sputtered amorphous Fe–B films.
Electrical properties of surface conductive layers of homoepitaxial diamond films
Authors: Jiang Nan , Ito Toshimichi
Published: July 2002

The electrical properties of surface conductive layers of the undoped diamond films via various subsequent treatments and of the as-grown N-doped diamond films have been investigated in the present study. Hall effect measurements reveal that the sheet carrier density of the surface conductive layers almost remains constant with varying temperature, and the temperature dependence of sheet resistivity is mainly determined by that of the Hall mobility, which varies exponentially with reciprocal temperature. The activation energies deduced for the mobility are demonstrated to monotonously decrease when increasing the sheet carrier densities. Based on these experimental results, the origin of the surface conductive layers is discussed.
Carbon clustering kinetics in detonation wave propagation
Authors: Viecelli J. A. , Ree F. H.
Published: July 2002

Much of the nonideality of insensitive carbon rich explosives such as TATB is thought to be caused by a late-time slow release of energy associated with diffusion limited growth of carbon clusters. We have adapted Shaw and Johnson’s approximate analytic solution of the Smoluchowski rate equations for the evolution of the cluster size distribution function for use in hydrodynamic calculations and in Wood and Kirkwood approximate theory for the detonation velocity-curvature relationship. Solutions for the effect of carbon cluster growth on the structure of the reaction zone are obtained, and the results are compared with available experimental data.
A study of the failure mechanism of a titanium nitride diffusion barrier
Authors: Lee Hoo-Jeong , Sinclair Robert , Li Pamela , Roberts Bruce
Published: July 2002

TiN has been popularly used as a diffusion barrier between Al and Si to prevent “spiking.” It has, however, been reported that spiking still occurs through TiN at temperatures higher than 500 °C. In this study, we investigated the mechanism of spiking through TiN using high resolution transmission electron microscopy and electron dispersive spectroscopy (EDS). We found TiN to be saturated with Al upon annealing at 550 °C. Si also diffuses through TiN and dissolves into Al. Spikes form upon 550 °C annealing at the Si substrate. EDS analysis revealed the phase of the spikes to be Al3Ti containing a considerable amount of Si. These results indicate that spiking through TiN is due to the formation and growth of Al3Ti after the Al saturation at the bottom of TiN. We discuss these results based on the Al–Ti–Si and Al–Ti–N ternary phase diagrams.
Photoluminescence studies of CdS films grown by close-spaced vapor transport hot walls
Authors: Mejı́a-Garcı́a C. , Escamilla-Esquivel A. , Contreras-Puente G. , Tufiño-Velázquez M. , Albor-Aguilera M. L. , Vigil O. , Vaillant L.
Published: July 2002

Semiconducting films of CdS grown by a modified close-spaced vapor transport hot-wall technique were studied by photoluminescence (PL) in the range of 10–300 K. The films were grown with an Ar pressure of 100 mTorr, the substrate temperature (Tsu) varied between 500 and 700 °C, while the source temperature (Tso) was kept at 750 °C. With these parameters it was possible to obtain different mean grain sizes. Several luminescence bands were observed, one located near the band-gap energy around 2.5 eV at 10 K. This band shows a strong temperature dependence and has an excitonic origin. Two other bands were detected around 2.4 and 2.1 eV at 10 K, and are denoted in the literature as green and yellow emission bands, respectively. The lower energy band with a mean width of 150 meV is accompanied by the corresponding optical phonon replica separated by exactly the LO-phonon energy of 38 meV at low temperatures. The nature of these bands and the photoluminescence process as a function of the temperature as well as of the grain size will be analyzed and discussed in this work.
Improved potential energy surfaces for the interaction of H2 with Ar, Kr, and Xe
Authors: Le Roy Robert J. , Hutson Jeremy M.
Published: July 2002

A combined analysis of discrete infrared and microwave spectra, elastic and inelastic differential cross section measurements, and virial coefficient data has been used to determine improved potential energy surfaces for the H2–Ar, –Kr, and –Xe systems. Key improvements over previous surfaces for these species are an improved delineation of the diatom bond length dependence of the potential anisotropy, and the first experimental determination of a distinct P4(cos θ) anisotropy for an atom–diatom system. The effective anisotropy strength seen by bound state properties (such as transition frequencies) is found to increase from H2–Ar to H2–Kr to H2–Xe, although that seen by properties sensitive to the short-range potential (such as rotational predissociation and rotational inelasticity) decreases along this series. This reflects the lack of conformality of the various potentials; however, both these and analogous trends predicted for properties such as vibrational frequency shifts and vibrational inelasticity are readily understood in terms of the detailed properties of these surfaces.
The liquid–liquid phase diagram of sulfur+biphenyl
Authors: Anderson E. M. , Greer S. C.
Published: July 2002

We have studied the liquid–liquid phase diagram of sulfur+biphenyl. Larkin, Katz, and Scott [J. Phys. Chem. 71, 352 (1967)] reported a liquid–liquid phase separation with an upper critical solution point (UCSP) at 381 K, complete miscibility between 381 K and a lower critical solution point (LCSP) at 494 K, and a polymerization line between the polymerization point of pure sulfur at 432 K and a point on the high temperature coexistence curve near the LCSP. We found that the phase transition temperatures for the higher temperature miscibility gap increased with time. We attribute this shift to chemical reaction between the sulfur and the biphenyl. Therefore, the transition temperatures which we obtain are necessarily upper limits to the true transition temperatures, but are in agreement with the results of Larkin et al. We find that the polymerization line meets the coexistence curve at 0.730±0.005 mole fraction sulfur and 498±2 K, whereas the LCSP, as indicated by the ratio of volumes of coexisting phases, is at 0.750±0.005 mole fraction sulfur and 496±3 K. Such a shift of the LCSP away from the end of the polymerization line is consistent with proximity to a nonsymmetrical tricritical point [Wheeler, J. Chem. Phys. 81, 3635 (1984), and references therein]. If we describe the shape of a branch of the high temperature coexistence curve by ‖x−xc ‖/xc =Bt β, where x is the mole fraction of sulfur, xc is the mole fraction at the critical point, and t is the reduced temperature ‖T−Tc ‖/Tc (with T the temperature and Tc the critical temperature), then the polymer-rich branch for sulfur+biphenyl is described well with β=0.25, the value appropriate for a nonsymmetrical tricritical point. The polymer-poor branch could not be described by any of the values of β thought to be possible (i.e., 0.25, 0.325, 0.50, 0.67, or 1.00). We searched carefully for a three-phase region near the LCSP and found none.
The effects of substrate roughness on ultrathin water films
Authors: Garoff S. , Sirota E. B. , Sinha S. K. , Stanley H. B.
Published: July 2002

Real surfaces are rough and chemically heterogeneous on many length scales and wetting phenomena on these surfaces are affected by this inhomogeneous nature. We have used x-ray reflectivity to examine the structure of static precursing films which precede the macroscopic meniscus of water on glass. We have examined the thickness of the film, the roughness of the interfaces bounding the film, and the conformality or correlations between the roughness features of these bounding interfaces. The wetting film consists of three basic components: the bulk meniscus, a thin (≤100 Å) precursing wetting film which has a sudden termination, and beyond that, a much thinner film characteristic of the glass surface in contact with water vapor only. The roughness and chemical heterogeneity of the solid manifest themselves in the roughness of the water/vapor interface of the film, in the irregularity of the terminating boundary of the precursor wetting film, and in the hysteretic pinning of that boundary. The roughness of the glass surface is observed to impose additional roughness to the water/vapor interface beyond that which would exist on a bulk interface. These results are discussed in terms of the disjoining pressure which governs the behavior of such films.
The application of wave packets to reactive atom–diatom systems: A new approach
Authors: Neuhauser Daniel , Baer Michael
Published: July 2002

This work describes a new approach for the study of atom–diatom reactive collision employing the time-dependent wave packet Schrödinger equation. The method makes use of the projection operator formalism to form a coupled system of time dependent Schrödinger equations and of optical potentials to circumvent the necessary use of products coordinates. Two versions are presented. As an example we applied the method to the collinear reactive H+H2 system for which are calculated both transition probabilities and rate constants.
Examination of the Br+HI, Cl+HI, and F+HI hydrogen abstraction reactions by photoelectron spectroscopy of BrHI−, ClHI−, and FHI−
Authors: Bradforth S. E. , Weaver A. , Arnold D. W. , Metz R. B. , Neumark D. M.
Published: July 2002

The photoelectron spectra of the ions BrHI−, ClHI−, and FHI−, along with their deuterated counterparts, are presented. These spectra provide information on the transition state region of the potential energy surfaces describing the exothermic neutral reactions X+HI→HX+I(X=Br, Cl, F). Vibrational structure is observed in the BrHI− and ClHI− spectra that corresponds to hydrogen atom motion in the dissociating neutral complex. Transitions to electronically excited potential energy surfaces that correlate to HX+I(2P3/2,2P1/2) products are also observed. A one-dimensional analysis is used to understand the appearance of each spectrum. The BrHI− spectrum is compared to a two-dimensional simulation performed using time-dependent wave packet propagation on a model Br+HI potential energy surface.
Real-time clocking of bimolecular reactions: Application to H+CO2
Authors: Scherer N. F. , Sipes C. , Bernstein R. B. , Zewail A. H.
Published: July 2002

An experimental methodology is described for the real-time clocking of elementary bimolecular reactions, i.e., timing the process of formation and decay of the collision complex. The method takes advantage of the propinquity of the potential reagents in a binary van der Waals (vdW) ‘‘precursor’’ molecule. An ultrashort pump laser pulse initiates the reaction, establishing the zero-of-time (e.g., by photodissociating one of the component molecules in the vdW precursor, liberating a ‘‘hot’’ atom that attacks the nearby coreagent). A second ultrashort, suitably tuned, variably delayed probe laser pulse detects either the intermediate complex or the newly born product. From an analysis of this temporal data as a function of pump and probe wavelengths, the real-time dynamics of such a ‘‘van der Waals-impacted bimolecular (VIB)’’ reaction can be determined. Chosen as a demonstration example is the VIB reaction H+CO2→HOCO‡→HO+CO, using the HI⋅CO2 vdW precursor. The pump laser wavelength was varied over the range 231–263 nm; the probe laser detected OH in two different quantum states. The measured rates of formation and decay of the HOCO‡ complex are characterized by time constants τ1 and τ2; τ2 spanned the range 0.4–4.7 ps, varying with the available energy. The dynamics of the HOCO‡ decay are discussed.
Electric field dependence of reactivity of state-selected and oriented methylhalides
Authors: Bulthuis J. , Milan J. B. , Janssen M. H. M. , Stolte S.
Published: July 2002

The energies and eigenfunctions of hyperfine levels belonging to the J=‖K‖=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos ϑ), where ϑ is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos ϑ, can be characterized by two moments only: 〈P1〉 the orientation and 〈P2〉 the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions with Ca atoms, down to a field strength of a few volts per centimeter.

At lower field strengths the orientation falls off more rapidly than predicted by the theory. This behavior is tentatively attributed to the variation in the direction of the electric field, and hence in the axis of quantization, across the reaction volume, at low field strengths. This may, in addition, cause a breakdown of the adiabatic approximation. In the case of methyliodide, quantitative agreement between the theoretical and the experimental asymmetry parameters of photodissociation is obtained if the latter ones are provisionally corrected for systematic errors. Contrary to earlier preliminary conclusions, the results show that within experimental error the adiabatic approximation is applicable. Up until now, no experimental data has been available for methylbromide. The calculations clearly demonstrate the inadequacy of restricting theoretical calculations to limiting cases, i.e., weak field and strong field, particularly because the Stark curves reveal the presence and the precise position of avoided crossings. Dependent on the field strength used for state selection and focusing, these crossings may, in principle, have a marked effect on the orientation obtained, given that the final orientation field at the reaction site remains the same. In addition, knowledge of the detailed Stark curves suggests a novel and feasible way to reverse the orientation of the molecules without interference with the detection system, which may be particularly useful if ionic products are formed.
The structure of interlayer water in vermiculite
Authors: Skipper N. T. , Soper A. K. , McConnell J. D. C.
Published: July 2002

Neutron diffraction, in conjuction with substitution of deuterium (D) for hydrogen (H), has been used to determine the structure of interlayer water in sodium- and nickel-substituted vermiculites. We have measured the intensities of the (00l) Bragg reflections as a function of relative humidity and H/D content, up to l=30. Difference analysis has then been used to obtain separate density profiles, ρ(z), for both the hydrogen atoms and the oxygen atoms plus the clay sheets. Ni–vermiculite was studied at 84% relative humidity, while Na–vermiculite was studied at both 88% and 30% relative humidity. At these values the layer spacings are 14.40, 14.96, and 11.78 Å, respectively. We find that each interlayer nickel ion is coordinated octahedrally to 6.0 water molecules. All of these water molecules are oriented to form a strong hydrogen bond to the adjacent clay surface. We also find that extra water is located close to the clay layers. This additional water is situated within the hexagonal rings of SiO4 and AlO−4 tetrahedra, which comprise the clay surfaces. In the 14.96 Å phase of Na–vermiculite there are an average of 4.9 interlayer water molecules per cation. About half of these water molecules are oriented to form a hydrogen bond to one of the clay surfaces. Additional water is found close to the clay surface, occupying the same hexagonal ring sites as in 14.40 Å Ni–vermiculite. In the 11.78 Å phase of Na–vermiculite there are an average of 2.1 water molecules per interlayer cation. The oxygen atoms of these interlayer water molecules are found close to halfway between the layers, while the hydrogen atoms are directed towards one of the adjacent clay sheets.
Collective and single-chain correlations near the block copolymer order–disorder transition
Authors: Barrat Jean-Louis , Fredrickson Glenn H.
Published: July 2002

The concentration pair correlation function and the order–disorder transition in a diblock copolymer melt are studied in the framework of a self-consistent one-loop approximation. In contrast with earlier work, no simplifying assumption on the wave vector dependence of the correlation functions is made. This allows us to calculate the shift in the peak of the fluctuation spectrum recently observed in neutron scattering experiments. Quantitative agreement with experimental results is found. We also present a new calculation of how the collective concentration fluctuations influence single chain statistics in the copolymer melt. It is shown that even in the disordered phase, strong deviations from Gaussian statistics can be expected for finite molecular weights. A local segregation between monomers of the two different blocks results in an increase in the copolymer radius of gyration.
Chemical association in simple models of molecular and ionic fluids. II. Thermodynamic properties
Authors: Zhou Yaoqi , Stell George
Published: July 2002

A simple interpolation scheme (SIS) suggested in an earlier paper is used to obtain the excess Helmholtz free energy and the equation of state for the shielded sticky-shell model and the shielded sticky-point model of associative fluids. It is found that the equation of state under the SIS for a fully associated homonuclear dumbbell fluid is quite accurate as long as L*=L/σ≥0.8. For ionic association, the excess Helmholtz free energy in this work reduces to that of Bjerrum’s theory in the ideal limit. An analytical equation of state for associative ions is evaluated.
Nonisothermal homogeneous nucleation
Authors: Wyslouzil B. E. , Seinfeld J. H.
Published: July 2002

Classical homogeneous nucleation theory is extended to nonisothermal conditions through simultaneous cluster mass and energy balances. The transient nucleation of water vapor following a sudden increase in saturation ratio is studied by numerically solving the coupled mass and energy balance equations. The ultimate steady state nucleation rate, considering nonisothermal effects, is found to be lower than the corresponding isothermal rate, with the discrepancy increasing as the pressure of the background gas decreases. After the decay of the initial temperature transients, subcritical clusters in the vicinity of the critical cluster are found to have temperatures elevated with respect to that of the background gas.
Highly excited vibrational eigenstates of nonlinear triatomic molecules. Application to H2O
Authors: Choi Seung E. , Light John C.
Published: July 2002

Highly accurate quantum-mechanical calculations are presented for highly excited vibrational states of H2O. The vibration Hamiltonian operator Ĥvib for a nonlinear triatomic molecule is given in Radau coordinates. A direct product basis is chosen, and the Hvib matrix is evaluated in the discrete variable representation (DVR) for the symmetrized Radau coordinates. Vibrational eigenstates are computed from the DVR Hvib via the successive diagonalization/truncation technique. A comparison of the computed eigenvalues with those observed demonstrate the accuracy of our model. Highly excited vibrational states, up to 30 000 cm−1 above the zero-point energy, are reported for the potential energy surface (PES) given by Jensen [J. Mol. Phys., 133, 438 (1989)]. Using natural orbital expansions, the eigenfunctions of vibrational states are analyzed to understand the origins of the dynamical mixing of the vibrational modes. The local/normal mode transitions, Fermi resonances, Darling–Dennison interactions, and the mode separabilities are investigated. Statistical studies on the energy level spacings are presented for two different types of PES.
Electronic energy transfer in Ba 6s8p 1P1 rare gas collisions
Authors: Smedley J. E. , Marran D. F. , Peabody M. R. , Marquis C. N.
Published: July 2002

Electronic energy transfer in collisions of Ba 6s8p 1P1 with He, Ne, Ar, Kr, and Xe is studied using time- and wavelength-resolved fluorescence techniques in a vapor cell. Total deactivation rate constants at 833 K are measured using time-resolved emission on the 6s8p 1P1→6s5d 1D2 transition at 408.2 nm, and the average deactivation cross sections are found to be (210±9) Å2 (He), (72±2) Å2 (Ne), (89±3) Å2 (Ar), (101±5) Å2 (Kr), and (185±6) Å2 (Xe). Seven near-resonant product states are significantly populated by energy transfer from the 6s8p 1P1 level in collisions with He, with the majority of the transfer occurring to the 6s7d 3D multiplet. Individual state-to-state forward rate constants for transfer from the 6s8p state to the product states are obtained from the intensities of the product state emission lines, taken at relatively low helium densities and evaluated using an approximate kinetic model. In addition, the Einstein A coefficient of the 6s8p 1P1→6s5d 1D2 408.2 nm line is found to be (3.3±0.4)×106 s−1.
Simulations of solvent effects on confined electrolytes
Authors: Zhang Lianrui , Davis H. Ted , White Henry S.
Published: July 2002

Monte Carlo simulations are reported for the solvent primitive model (SPM) of the electrical double layer at electrically charged and neutral surfaces. Both solvent and ions are modeled as hard spheres with interactions governed by hard sphere and Coulomb interactions. Density profiles of solvent and ions and electrostatic potential profiles are presented for 1:1 electrolyte concentrations of 1 and 2 M at 300 K. Comparison of results at charged and neutral walls indicates that the density distribution of a dense solvent near a wall induces significant layering of ions and decrease of potentials in the double layer. This layering effect cannot be captured in simulations of the conventional primitive model (PM) of the double layer. The solvent induced steric ordering of electrolyte ions at a neutral wall can create a space charge layer.
Vibrational corrections for some electric and magnetic properties of H2, N2, HF, and CO
Authors: Bishop David M. , Cybulski Sl/awomir M.
Published: July 2002

The effects of vibration on certain electric and magnetic properties of H2, N2, HF, and CO are reported. These properties include electric field gradients, generalized Sternheimer shielding constants, electric-field-gradient polarizabilities, nuclear shielding constants, and shielding polarizabilities. The calculations were based on both electron correlated and uncorrelated methods. Pure vibrational effects, where appropriate, were investigated as well as conventional vibrational averaging. It is found that in many cases vibration plays a very significant role.
Variational transition-state theory calculation using the direct dynamics method: NH3+H→NH2+H2 reaction
Authors: Espinosa-García J. , Corchado J. C.
Published: July 2002

The title reaction has been studied following the ‘‘direct dynamics’’ method, which uses information on electronic structure energy and energy derivatives calculated ab initio along the minimum energy path. With this information, the rate constants were calculated for the temperature range 200–2000 K, using the variational transition-state theory and different approximations to calculate the tunneling effect. With respect to the construction of the minimum energy path, some possible sources of error were analyzed and quantified, namely, the number of points calculated along the minimum energy path (MEP), and the scale factor in the Hessian matrix. With respect to the comparison with experimental values, several factors were also analyzed, namely, the correlation energy, the influence of spin projection and the basis set superposition error on the height of the barrier, the influence of anharmonicity on the final rate constants, and different ways to scale the potential energy curve. The conclusion was that the variational effects are small for this reaction, and for T≥500 K the tunneling effect is also small. Good agreement was found with the experimental values when using the PMP-SAC4/A1 curve and the Hessian matrix scaled by 0.9.
Benchmark full configuration interaction calculations on the helium dimer
Authors: van Mourik Tanja , van Lenthe Joop H.
Published: July 2002

Full configuration interaction calculations are presented for the helium dimer employing large basis sets. Using the best basis, which contains up to h-type basis functions and several closely spaced sets of bond functions, the interaction energy was calculated for a variety of internuclear distances in the range 4.0 to 12.0 bohr. The best calculated values for the He2 interaction energy are −10.947 K at 5.6 bohr (the van der Waals minimum) and +294.90 K at 4.0 bohr (on the repulsive wall). The interaction energy at 4.0 bohr differs significantly from the most recent semiempirical potential of Aziz and Slaman [J. Chem. Phys. 94, 8047 (1991)], indicating that this potential is too attractive around 4.0 bohr. Using a more generally accessible basis, containing only up to f-type basis functions and only one set of bond functions, the interaction energy was calculated to be −10.903 K at 5.6 bohr and +294.96 K at 4.0 bohr. These results show that functions of higher than f symmetry and bond functions distributed over several centers are necessary for obtaining highly accurate results, particularly at the van der Waals minimum. Our results may be used to benchmark more approximate methods. The CCSD(T) method is estimated to underestimate the full CI interaction energy by 0.33 K at 5.6 bohr and by 2.0 K at 4.0 bohr.
The structure of casein aggregates during renneting studied by indirect Fourier transformation and inverse Laplace transformation of static and dynamic light scattering data, respectively
Authors: Bauer R. , Hansen M. , Hansen S. , O/gendal L. , Lomholt S. , Qvist K. , Horne D.
Published: July 2002

Aggregation of casein micelles after addition of the proteolytic enzyme chymosin has been studied by static and dynamic light scattering at three different concentrations of casein corresponding to dilutions 1:100, 1:500, and 1:1000 of native milk. The static light scattering data have been analyzed by an indirect Fourier transformation method which gives the distance distributions as a function of time. From these curves radius of gyration and an average number of casein micelles in the aggregates have been derived as a function of time. The dynamic light scattering experiments give the hydrodynamic radius as a function of time after the addition of rennet. The initial radius of gyration for the intact casein micelles is 140 nm. The corresponding hydrodynamic radius is also 140 nm. This shows that the casein micelles are not solid spheres. Inspection of a plot of relative mass versus radius of gyration for the aggregates appearing after the addition of chymosin shows that two processes take place. First extended linear aggregates are built up to a relative mass of the aggregates of about 10 and then restructuring of aggregates occurs such that increasingly compact objects are formed. Whereas the first process exhibits a relatively fast growth in size, the aggregates grow slowly in size during the second process. Further evidence of the formation of linear aggregates followed by more dense aggregates was obtained by forming the ratio between the radius of gyration and the hydrodynamic radius.

This ratio increases to values of about 2.5 (indicating that linearly extended molecules are present followed) by a decrease to about 1. The log–log plot of mass versus radius of gyration is linear up to relative masses of about 10 with a slope of about 2. This extends up to sizes of 1 μm in diameter. The slope then increases to values indicating branching and thereby the formation of more compact aggregates. For relative masses below 10 and sizes below 1 μm sedimentation is unlikely to occur and information about the mechanism of aggregation can be obtained. The aggregation number as a function of time has been analyzed in terms of Smoluchowski’s equations with a rate constant including both functionality and a changing barrier height as a function of the extent of proteolysis. The functionality obtained from Smoluchowski’s equations is about 2.1.
A molecular dynamics study of dielectric friction
Authors: Kurnikova M. G. , Waldeck D. H. , Coalson R. D.
Published: July 2002

A molecular dynamics study of the friction experienced by the dye molecule resorufamine rotating in a polar solvent is performed. The validity of simple continuum theories of dielectric friction is tested. It is found that the Alavi–Waldeck theory gives reasonable results for the zero frequency dielectric friction coefficient while the Nee–Zwanzig theory requires an unphysically small cavity radius. A procedure for evaluating the time dependent friction kernel from torques and angular velocities, which enables the contributions to the friction from the van der Waals and Coulomb forces to be evaluated separately, is suggested. This study of a realistic system shows that electrostatic interactions can enhance friction by at least two physical mechanisms. First is a contribution to the friction which arises solely from retardation of the solvent reaction field. Second is a contribution arising from local structural changes of the solvent which are driven by the electrostatic field, i.e., a change in the local viscosity.
On potential energy surfaces and relaxation to the global minimum
Authors: Doye Jonathan P. K. , Wales David J.
Published: July 2002

By analyzing the dynamics of model potential energy surfaces we systematically investigate the processes involved in passing from a high energy state to the global minimum and how the probability of reaching the global minimum depends upon the topography and topology of the potential energy surface (PES). Relaxation to the global minimum is easiest for PES’s consisting of a single funnel (a set of convergent pathways which lead to the global minimum) with low barriers and a significant potential energy gradient towards the global minimum. The presence of additional funnels on the surface can severely reduce the rate of relaxation to the global minimum. Such secondary funnels act most efficiently as kinetic traps when they terminate at a low energy minimum, have a steep potential energy gradient and are wide (i.e., have a large configurational entropy) compared to the primary funnel. Indeed, it is even possible to construct PES’s for which the system relaxes to the minimum at the bottom of a secondary funnel rather than the global minimum and then remains in this metastable state over a long time scale. Our results for these model PES’s are discussed in the context of theoretical and experimental knowledge of the dynamics of proteins, clusters, and glasses.