bluuebunny/crossref_metadata_2025 huggingface dataset:
Comment on “Quasisaddles as relevant points of the potential energy surface in the dynamics of supercooled liquids” [J. Chem. Phys. 116, 10297 (2002)]
Authors: Doye Jonathan P. K. , Wales David J.
Published: February 2003

Recently, the properties of supercooled liquids have been studied by mapping instantaneous configurations onto minima of the gradient squared. It was originally suggested that this mapping would probe higher-order saddle points of the potential energy surface. However, it was subsequently shown that the majority of the minima of this function are only local minima and so do not correspond to saddles. In this comment, we provide a critique of the suggestion made by Angelani et al. [J. Chem. Phys. 116, 10297 (2002)] that although these minima are not true saddles, they are almost saddles (hence the term quasisaddles). This issue has important implications for the interpretation of the results obtained by this approach.
Reply to “Comment on ‘Quasisaddles as relevant points of the potential energy surface in the dynamics of supercooled liquids’ ” [J. Chem. Phys. 118, 5263 (2002)]
Authors: Angelani L. , Di Leonardo R. , Ruocco G. , Scala A. , Sciortino F.
Published: February 2003

We reply to the Comment on the paper “Quasisaddles as relevant points of the potential energy surface in the dynamics of supercooled liquids” [J. Chem. Phys. 116, 10297 (2002)]. While we agree with J. P. K. Doye and D. J. Wales on their improved enumeration of zero curvature directions at quasisaddle points, we criticize their statement that this has important implications for the interpretation of our results. Indeed, we show here that the temperature dependence of the order of quasisaddles and true saddles are numerically coincident, providing a strong support to our previous interpretation.
Molecular dynamics simulations of Hg2+ in aqueous solution including N-body effects
Authors: Kritayakornupong Chinapong , Rode Bernd M.
Published: February 2003

Pair-potential, three-body corrected and combined ab initio quantum mechanics/molecular mechanics molecular dynamics (QM/MM-MD) simulations have been performed for Hg2+ in water. The ion’s hydration structure was evaluated in terms of radial distribution functions, coordination numbers, and angular distributions, together with changes in ligand structure upon binding of water molecules. An octahedral solvate complex with coordination number of 6/22 for first/second shell results from QM/MM-MD simulations, with average Hg2+–O distance 2.42 Å for the first shell. The pair-potential simulation gives strongly wrong results, the inclusion of three-body effects corrects many, but not all of the errors. The hydration enthalpy of −553±3 kcal/mol obtained from the QM/MM-MD simulation seems a reasonable value for dilute solution, when compared with experimental estimations.
Al
0.95
Ga
0.05
As
0.56
Sb
0.44
for lateral oxide-confinement layer in InP-based devices
Authors: Reddy M. H. M. , Buell D. A. , Huntington A. S. , Asano T. , Koda R. , Feezell D. , Lofgreen D. , Coldren L. A.
Published: February 2003

We report a lateral oxide-confinement layer for InP-based devices using lattice-matched AlGaAsSb. The confinement-layer-induced excess loss at different widths was extracted after de-embedding the losses due to carrier diffusion, nonradiative recombination, and changes in internal injection efficiency. The results show that AlGaAsSb oxide acts as an excellent confinement layer, and shows no excess loss down to a width of 4 μm.
Bright blue phosphors in ZnO–WO3 binary system discovered through combinatorial methodology
Authors: Hayashi Hideki , Ishizaka Akitoshi , Haemori Masamitsu , Koinuma Hideomi
Published: February 2003

We screened bright blue phosphor materials for a ZnO–WO3 binary system. The film was deposited by combinatorial pulsed laser deposition at room temperature and then annealed. The film composition changes continuously in a full-range from ZnO to WO3 on a single substrate. We obtained two results from the distribution of blue emission-intensity observed by cathodoluminescence of the film: (1) not only did the film have a W composition of 50 mol % (stoichiometric ZnWO4 as the well-known blue phosphor material) but also the W composition region from 45 to 60 mol % had the most emissive phase, and (2) two emissive phases other than the ZnWO4, which have not been described in the phase diagram, were found in the W composition region from 10 to 20 mol % and from 65 to 75 mol %.
Fatigue-free La-modified PbTiO3 thin films prepared by pulsed-laser deposition on Pt/Ti/SiO2/Si substrates
Authors: Dong Zhenggao , Shen Mingrong , Cao Wenwu
Published: February 2003

Fatigue-free 14 mol % La-modified PbTiO3 (PLT) thin films were grown on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition and crystallized by furnace annealing at 600 °C. The 220-nm-thick PLT film capacitors with a Pt top electrode showed excellent ferroelectric properties. The remanent polarization (2Pr) and the coercive field (2Ec) were about 20 μC/cm2 and 70 kV/cm, respectively, and the PLT capacitors did not show any noticeable fatigue up to 3×109 read/write switching cycles at a frequency of 1 MHz and switching voltage of 5 V. By comparing the microstructures, electric, and dielectric properties with those of pure PbTiO3 thin films, the suppression of oxygen vacancies and/or charged defects, and the coral-like microstructures developed in PLT films were attributed to its fatigue-free feature.
Formation of the accumulation layer in polymer field-effect transistors
Authors: Bürgi L. , Friend R. H. , Sirringhaus H.
Published: February 2003

We present an experimental study of charge transfer in polymer thin-film field-effect devices. The rearrangement of the charge-carrier density in the transistor channel upon a gate-voltage swing has been monitored in real time and space by means of noncontact scanning potentiometry. The experimental results are in excellent agreement with a simple theory, in which the charging currents are assumed to be driven by drift in the self-induced electric field. The charge density exponentially approaches its final value with a time constant given by L2/μ|Vg|π2, where L is the characteristic device dimension, μ the field-effect mobility, and Vg the final gate voltage.
Layer-by-layer growth of Ag on a GaN(0001) surface
Authors: Wu Kehui , Xue Q. Z. , Bakhtizin R. Z. , Fujikawa Y. , Li X. , Nagao T. , Xue Q. K. , Sakurai T.
Published: February 2003

A drastic change of the Ag growth mode on the GaN(0001) surface, from Stranski–Krastanov (SK) growth at low Ag flux (∼0.8 ML/min) to layer-by-layer growth at a high flux (∼60 ML/min), was observed. Based on this finding, an approach to obtain a flat epitaxial Ag film on the GaN(0001) surface, by using the high Ag flux, was demonstrated. In addition, an unreconstructed Ag-terminated GaN(0001)-1×1 surface was obtained by annealing the Ag film-covered GaN(0001) surface, and its structure was explained by T1-site adatom model.
Studies of field-induced nonequilibrium electron transport in an InxGa1−xN (x≅0.6) epilayer grown on GaN
Authors: Liang W. , Tsen K. T. , Ferry D. K. , Kim K. H. , Lin J. Y. , Jiang H. X.
Published: February 2003

Field-induced electron transport in an InxGa1−xN (x≅0.6) sample grown on GaN has been studied by subpicosecond Raman spectroscopy. Nonequilibrium electron distribution and electron drift velocity due to the presence of piezoelectric and spontaneous fields in the InxGa1−xN layer have been directly measured. The experimental results are compared with ensemble Monte Carlo calculations and reasonable agreements are obtained.
Photoluminescence quenching in Er-doped compounds
Authors: Zanatta A. R.
Published: February 2003

The temperature-induced quenching in the photoluminescence intensity (IPL) of Er3+ ions in different semiconductor hosts is discussed in this letter. Based on a compilation of several IPL(T) experimental curves, corresponding to either crystalline or amorphous Er-doped samples, and after a critical data analysis, it was possible to determine a temperature of quenching Tq such that IPL(Tq)=0.95 IPL (lowest T). These experimental Tq values have been analyzed in terms of certain host characteristics such as optical band gap and phonon frequency. As a result of this study it was possible to state that: (i) Tq strongly depends on the optical band gap and atomic structure of all studied semiconductor hosts; (ii) the host phonon frequency plays a minor role in the quenching of IPL; and (iii) based on the partial ionic character of each semiconductor host, both the energy and the localization of the photon-generated electron–hole pairs determine the IPL quenching. Moreover, the present phenomenological model is consistent with the influence that codoping and thermal anneals have on the IPL enhancement of Er-doped compounds.
Current-free double-layer formation in a high-density helicon discharge
Authors: Charles Christine , Boswell Rod
Published: February 2003

A strong, current-free, electric double-layer with eΦ/kTe∼3 and a thickness of less than 50 debye lengths has been experimentally observed in an expanding, high-density helicon sustained rf (13.56-MHz) discharge. The rapid potential decrease is associated with the “neck” of the vacuum vessel, where the glass source tube joins the aluminum diffusion chamber, and is only observed when the argon gas pressure is less than about 0.5 mTorr. The upstream electron temperature Te appears 25% greater than the downstream Te, and there is a density hole on the downstream edge. This experiment differs from others in that the potentials are self-consistently generated by the plasma itself, and there is no current flowing through an external circuit. The plasma electrons are heated by the rf fields in the source, provide the power to maintain the double-layer, and hence accelerate ions created in the source out into the diffusion chamber.
Widely tunable L-band erbium-doped fiber laser with fiber Bragg gratings based on optical bistability
Authors: Mao Qinghe , Lit John W. Y.
Published: February 2003

We propose and demonstrate a mechanism to widely tune L-band erbium-doped fiber lasers with ordinary commercial tunable fiber Bragg gratings. The function is based on the dual-wavelength bistability in linear overlapping laser cavities. The laser may be switched between two wavelengths located, respectively, in the short- and long-wavelength regions of the L-band by triggering the pump with a typical switching time of about 11 ms. The two wavelengths can be independently tuned to give the laser a total tuning range of 33 nm and an output dynamic range of 7 dB. Nearly constant output powers with high optical signal-to-noise ratios are achieved across the whole tuning range.
Self-organized phase gratings in photoreactive polymer liquid crystals
Authors: Ono Hiroshi , Emoto Akira , Kawatsuki Nobuhiro , Hasegawa Takako
Published: February 2003

Optical phase gratings have been prepared in a photo-cross-linkable polymer liquid crystal substituted with a 4-methoxycinnamoyloxybiphenyl side group by the use of linearly polarized ultraviolet interference light and subsequent annealing. The diffraction was almost invisible after irradiation, and was successfully enhanced during the subsequent annealing. Recorded phase grating involves both linear birefringence and surface-relief modulation, which is generated by thermally enhanced reorientation of the mesogenic groups and molecular migration. The sign of the induced birefringence was dependent on the degree of photoreaction and the reversion of the reorientation direction of mesogenic groups has been observed.
Tunable microfluidic optical fiber gratings
Authors: Kerbage C. , Eggleton B. J.
Published: February 2003

We demonstrate periodic refractive index gratings in optical waveguides formed by microfluidic plugs that are infused into the airholes of a microstructured optical fibers. The periodic microfluidic plugs, created in the cladding, cause resonant coupling between copropagating optical fiber modes, which results in wavelength-dependent attenuation. We also demonstrate the ability to tune the resonant wavelength by compressing the microfluidic structure. These microfluidic resonant structures provide an alternative method for creating and tuning long-period gratings in optical fibers. This also represents an example of a resonant microfluidic structure and establishes potential strategies for enhanced tunable photonic crystal devices.
Stimulated emission and ultrafast carrier relaxation in InGaN multiple quantum wells
Authors: Özgür Ümit , Everitt Henry O. , Keller Stacia , DenBaars Steven P.
Published: February 2003

Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different QW In compositions x. SE threshold energy densities (Ith) increased with increasing x-dependent QW depth. Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of Ith on x. Carriers are captured from the barriers to the QWs in <1 ps, while carrier recombination rates increased with increasing x. For excitation above Ith, an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing Ith.
Enhanced effect of magnetic anisotropy on free clusters
Authors: Xie Yuannan , Blackman John A.
Published: February 2003

We propose a simple model to estimate the effective moments of free clusters in a magnetic field. Compared with supported clusters, the effect of magnetic anisotropy is significantly enhanced, in good agreement with experimental results. The controversy of relations between the effective moment and cluster temperature presented in experiments is explained within our model. We can evaluate the magnetic anisotropy energies of free clusters from Stern–Gerlach experimental results.
Mechanisms of temperature performance degradation in terahertz quantum-cascade lasers
Authors: Indjin D. , Harrison P. , Kelsall R. W. , Ikonić Z.
Published: February 2003

Electron transport in a terahertz GaAs/AlGaAs quantum-cascade laser is calculated using a fully self-consistent intersubband scattering model. Subband populations, carrier transition rates, and current densities are calculated and all relevant intra- and interperiod electron–electron and electron–LO-phonon scattering mechanisms are included. Employing an energy balance equation that includes the influence of both electron–LO-phonon and electron–electron scattering, the method also enables evaluation of the average electron temperature of the nonequilibrium carrier distributions in the device. In particular, the influence of the lattice temperature on the degradation of population inversion and device performance is investigated. The threshold currents, electric-field-current-density characteristics, and temperature-dependent performance are in good qualitative and quantitative agreement with measurement in a recent experimental realization [Köhler et al., Nature (London) 417, 156 (2002)]. Calculations indicate that an important mechanism limiting its operating temperature is the increase of leakage current from the injector to low levels in the active region, and this feature should be improved in future designs.
In situ growth of blue-emitting thin films of cerium-doped barium chloride hydrate at low temperatures
Authors: Hao Jianhua , Lou Zhidong , Cocivera Michael
Published: February 2003

Blue emission was observed from thin films of barium chloride hydrate doped with cerium. The films were deposited by spray pyrolysis of aqueous solutions with substrate temperatures between 250 and 450 °C. The cathodoluminescence (CL) spectrum consists of two peaks at 443 and 485 nm due to 4f-5d transitions of cerium ion. The dependence of the emission band on deposition temperature and Ce/Ba ratio is discussed. The CL luminance and luminous efficiency at 5 kV were 120 cd/m2 and 0.48 lm/W, respectively, for the films deposited at temperatures as low as 250 °C. The results indicate barium chloride hydrate doped with cerium is a blue phosphor that exhibits relatively efficient luminescence after processing at low temperatures.
Improved interface stability of phase-change recording media
Authors: Kuiper A. E. T. , van Schijndel M. , Tamminga Y.
Published: February 2003

During writing, the phase-change layer in a medium for optical recording is heated to above its melting temperature of typically 600 °C. Although this situation lasts for only 10–100 ns, it is repeated many times during consecutive overwrite cycles. As a result, the phase-change layer gradually reacts with the adjacent ZnS:SiO2 isolation layers, which deteriorates the overwrite performance. Applying Rutherford backscattering spectrometry, we observed that thin caplayers of AlOxNy, HfOxNy, Si3N4, or In–SnOx, introduced at both interfaces of the phase-change layer, suppress this reaction adequately. Such layers appear also very beneficial for the inherently unstable interface between the Ag mirror and the ZnS:SiO2 dielectric layer.
Giant magnetothermopower associated with large magnetoresistance in Ag2−δTe
Authors: Sun Young , Salamon M. B. , Lee M. , Rosenbaum T. F.
Published: February 2003

We have probed the temperature and magnetic-field dependence of the thermopower and resistance of a p-type silver chalcogenide, Ag2−δTe. The application of a magnetic field causes not only a large magnetoresistance but also a giant magnetothermopower effect. The maximum change of thermopower is as high as 470 μV/K in a 7 T magnetic field. Both the magnetoresistance and the magnetothermopower show a pronounced peak and nearly linear behavior near the sign change of the thermopower. Bandcrossing and quantum confinement due to disorder appear to play key roles in the heightened response to field.
Electrochemical route for the fabrication of alkanethiolate-capped gold nanoparticles
Authors: Zhang P. , Kim P. S. , Sham T. K.
Published: February 2003

Nearly monodispersed gold nanoparticles (NPs) of a few nanometers were fabricated and immobilized simultaneously on silicon surface by the galvanostatic reduction of HAuCl4 in the presence of dodecanethiol. X-ray absorption spectra at both sulfur K edge and gold L3 edge confirm the existence of the alkanethiolate-protected Au NPs and reveal their structural and bonding characteristics. Alkanethiolate-capped Au NPs were also successfully fabricated, with the same technique, on porous silicon, a functional substrate with high surface area. The hybrid thiol-capping/electrodeposition method reported here offers an attractive approach to the fabrication of highly stable gold NPs on various functional substrates with a simple, fast, and easily controlled experimental procedure.
Illumination-induced recovery of Cu(In,Ga)Se2 solar cells after high-energy electron irradiation
Authors: Jasenek A. , Rau U. , Weinert K. , Schock H. W. , Werner J. H.
Published: February 2003

Cu ( In , Ga ) Se 2 / CdS / ZnO solar cells irradiated with a 1 MeV electron fluence of 1018 cm−2 degrade to about 80% of their initial conversion efficiency. Illumination with white light at an intensity of 100 mW cm−2 for 3 h at room temperature restores more than 90% of the preirradiation efficiency. The healing process is more efficient if the device is kept under open-circuit conditions during illumination than for short-circuit conditions. Injecting minority carriers by voltage bias in the dark, instead of illumination, does not cause enduring device recovery. This behavior of Cu(In,Ga)Se2 is in contrast to illumination-induced defect healing processes reported for other semiconductor materials, like GaAs, InP, or GaP.
Measuring electronic structure of wurtzite InN using electron energy loss spectroscopy
Authors: Mkhoyan K. A. , Silcox J. , Alldredge E. S. , Ashcroft N. W. , Lu H. , Schaff W. J. , Eastman L. F.
Published: February 2003

The electronic structure of wurtzite InN has been investigated by electron energy loss spectroscopy (EELS). Spectra of the nitrogen K edge and the indium M4,5 edge have been measured and were compared with calculated partial, N 2p and In 5p conduction band density of states in InN. Excellent agreement on the relative positions of the characteristic peaks were obtained. From low-loss EELS the bulk plasmon energy at 15.5 eV, location of the In 4d deep valence states at about 16.3 eV below the conduction band maximum and strong interband transitions with 6.2 eV excitation energy are also found.
Rates of reaction of butyl radicals with molecular oxygen
Authors: Lenhardt Terence M. , McDade Charles E. , Bayes Kyle D.
Published: February 2003

Rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature. The radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer. The radical concentrations were kept low to avoid complications from radical–radical reactions. Radical lifetimes were long, up to 50 msec, thus assuring that thermalized radicals were being studied. The rate constants, in units of 10−11 cm3 molecule−1 sec−1, are: n-butyl (0.75±0.14); s-butyl (1.66±0.22); t-butyl (2.34±0.39); 3-hydroxy s-butyl (2.8±1.8). No pressure dependence of the rate constants was observed over the range 1 to 4 Torr. In the absence of O2, the butyl radicals decay mainly by loss on the quartz surface of the reaction cell, with sticking coefficients in the range of 10−2 to 10−3. The Adiabatic Channel Model can predict the approximate absolute values of these rate constants using reasonable molecular parameters, but it fails to reproduce the observed trend of rate constants with radical ionization potential.
The gas phase structure of transition metal dihydrides
Authors: Demuynck Jean , Schaefer Henry F.
Published: February 2003

ESR and infrared spectroscopic measurements on matrix isolated MnH2 and CrH2 have recently suggested that these simple molecules may be bent. This result would be the opposite of that found experimentally for the transition metal dihalides MX2, known to be linear. Here the geometrical structure of MnH2 has been investigated by molecular electronic structure theory. A large contracted Gaussian basis set [Mn(14s11p6p/9s8p3d), H(5s1p/3s1p)] was used in conjunction with self-consistent field and configuration interaction methods. These suggest that the 6A1 ground state of MnH2 is linear. Further studies of the 3A1 state (one of several low-lying states) of TiH2 also favor linearity, although this potential energy surface is extremely flat with respect to bending. Thus it appears probable that most MH2 molecules, like the related MX2 family, are linear.
Determination of the particle size required for bulk metallic properties
Authors: Baetzold R. C. , Mason M. G. , Hamilton J. F.
Published: February 2003

Small transition-metal clusters have been prepared by vacuum deposition and characterized by transmission electron microscopy. The valence bandwidth has been measured by x-ray photoelectron spectroscopy (XPS) and compared with the theoretical prediction of extended Hückel and self-consistent-field Xα scattered-wave molecular-orbital calculations. The semiempirical extended Hückel calculations agree much better with the experimentally determined values.
Nonequilibrium thermodynamics of fuel cells: Heat release mechanisms and voltage
Authors: Wilemski Gerald
Published: February 2003

Nonequilibrium thermodynamics is used to analyze the spatial distribution of heat release mechanisms occurring in fuel cells operating under load in nonisothermal steady states. Novel contributions to heat release in the bulk electrolyte are found which are analogous to Peltier and Thomson effects in metallic conductors. Expresions for the heat release at individual electrodes are presented. An equation for the voltage of these cells is also derived.
Observation of the isotope effect in the ν2 fundamental of germane
Authors: Magerl G. , Schupita W. , Bonek E. , Kreiner W. A.
Published: February 2003

Ten ro–vibrational transitions in the ν2 fundamental of germane, GeH4, have been investigated with a tunable sideband laser. The isotope effect, i.e., the fivefold occurence of each transition due to the natural germanium isotopes, has been clearly resolved. From the observation of a combination difference the ground state centrifugal distortion constant Dt is determined to 67.6±0.4 kHz for two molecular isotopes separately. The dependence of other molecular constants on the isotope mass is shown. The worst-case error in absolute frequency measurement is about 18 MHz while the relative accuracy is believed to be better than 5 MHz for most of the lines observed.
On the intermolecular interaction in π-exciplexes of aromatic hydrocarbons with amines
Authors: Amalia M. , Tavares F.
Published: February 2003

Comparison of the energy of exciplex band maxima of aromatic hydrocarbons with dimethylaniline and diethylaniline has been used to study the nature of the intermolecular interaction in these exciplexes. For the interpretation of the results an approximate method is used to compare the zeroth-order energies of the first locally excited, LE, and charge-transfer, CT, states of the complexes. It led to the conclusion that even for strongly polar exciplexes the covalent part of the wave function can not be neglected; its weight depends on the energy gap between the first LE state and the ’’pure’’ CT state of the complex, and greatly increases when there is a near degeneracy between the first excited singlet state of the donor and acceptor molecules.
Collisional relaxation of highly excited vibrational levels of the I2 X 1Σ+g state using an I2 optically pumped laser
Authors: Koffend J. Brooke , Wodarczyk F. J. , Bacis R. , Field R. W.
Published: February 2003

A new technique for the study of energy transfer processes using a cw optically pumped laser (OPL) is presented. Population removal rates from individual vibration–rotation levels of a nonfluorescing state may be measured. The rate measured is that out of the lower level of the lasing transition. A limiting case of a simple kinetic model, involving seemingly contradictory requirements of high OPL circulating power and low pump power, is described and experimentally realized. Rate coefficients are extracted by observing the change in spontaneous emission (side fluorescence) intensity from the OPL gain medium with and without OPL oscillation. Removal rates from several highly excited vibrational levels of I2 X 1Σ+g are measured using an Ar+ laser pumped I2 BO+u–X 1Σ+g(v′=43, v″=42, 64, and 76–83) laser.
Potential energy characteristics and energy partitioning in chemical reactions: A
b i
n
i
t
i
o MO study of H2CCH2F→H2CCHF+H reaction
Authors: Kato Shigeki , Morokuma Keiji
Published: February 2003

Geometries, relative energies, and normal coordinates for the reactant, the transition state (saddle point), and the product were determined for the unimolecular dissociation reaction H2CCH2F→H2C=CHF+H, by the use of the energy gradient technique in the ab initio SCF method with a split valence basis set. The saddle point is located near the dissociation product. The RRKM theory including a tunneling correction was used to calculate the average lifetime of the reactant complex. Nonstatistical internal and translational energy distributions observed experimentally in the reaction product were interpreted by the analysis of the potential energy characteristics: the deformation and interaction contributions to the reverse activation barrier and the relationship between the transition state normal coordinates and the product normal coordinates.
On the FeO valence band photoemission spectra
Authors: Grenet G. , Jugnet Y. , Duc Tran Minh , Kibler M.
Published: February 2003

New XPS and UPS spectra of an FeO single crystal are reported and interpreted in the framework of a simplified LCAO–MO approach. Configuration interaction, interference processes, and covalency effects are, for the first time, simultaneously introduced in photoemission analyses of valence band structures.
A laser flash photolysis-resonance fluorescence kinetics study of the reaction Cl(2P)  + CH4→CH3 + HCl
Authors: Ravishankara A. R. , Wine P. H.
Published: February 2003

The technique of laser flash photolysis-resonance fluorescence is employed to study the kinetics of the reaction Cl(2P)  + CH4→CH3 + HCl over the temperature range 221–375 K. Chlorine atoms are produced by photolysis of Cl2 at 355 nm. At temperatures ? 241 K the apparent bimolecular rate constant is found to be dependent upon the identity of the chemically inert gases in the reaction mixture. For Cl2/CH4/He reaction mixtures (total pressure =50 Torr) different bimolecular rate constants are measured at low (k1L) and high (k1H) methane concentrations. For Cl2/CH4/CCl4/He and Cl2/CH4/Ar reaction mixtures, the bimolecular rate constant (k1) is independent of methane concentration with k1?k1L. k1 and k1L are in good agreement with previous results obtained using the flash photolysis-resonance fluorescence technique while k1H is in good agreement with previous results obtained using the discharge flow-resonance fluorescence and competitive chlorination techniques. At 298 K the measured bimolecular rate constant is independent of the identity of the chemically inert gases in the reaction mixture and in good agreement with all previous investigations. The low temperature results obtained in this investigation and all previous investigations can be rationalized in terms of a model which assumes that the Cl(2P1/2) state reacts with CH4 much faster than the Cl(2P3/2) state. Extrapolation of this model to higher temperatures, however, is not straightforward.
Monte Carlo simulations of water clusters around Zn++ and a linear Zn++⋅CO2 complex
Authors: Clementi E. , Corongiu G. , Jönsson B. , Romano S.
Published: February 2003

As a step towards the elucidation of the reaction mechanism of the enzyme carbonic anhydrase, we have considered the solvation of Zn++ and a linear Zn++⋅CO2 complex, using Monte Carlo techniques. The simulated temperature of the system is 300 K. The coordination numbers for the first and second solvation shell are computed as 6 and 16, respectively, for the case of Zn++. When placed in the first solvation shell the CO2 molecule displaces one water molecule, apparently without altering the remaining water structure in the first shell.
Precipitation of formaldehyde from solid xenon solutions. Infrared spectra of dissolved and aggregated formaldehyde
Authors: Maier William B. , Holland Redus F. , Freund Samuel M. , Beattie Willard H.
Published: February 2003

Optically clear solutions of ?0.01 mole % formaldehyde in solid xenon have been grown in cells of 1.3 cm thickness. Infrared absorption spectra have been obtained, and near the melting point of xenon the absorption features are fairly similr to those found for formaldehyde dissolved in liquid xenon. As the temperature of the solid solution is lowered,the formaldehyde precipitates from solution and the spectra change considerably. Absorption spectra of aggregated CH2O and CD2O have been measured; the mode corresponding to the CO stretch shows considerable structure.
Conductivity of α-silver iodide in the microwave range
Authors: Gebhardt K. F. , Soper P. D. , Merski J. , Balle T. J. , Flygare W. H.
Published: February 2003

The Ag+ ion conductivity in α-AgI has been measured in the frequency range 8 to 40 GHz. The conductivity has been found to be frequency-independent in this range with a value of 1.2 (Ω cm)−1 at 429 K. The data obtained here are combined with other recent measurements to interpret the ionic transport in terms of motion between the tetrahedral sites.
Thermodynamic interpretation of sound waves attenuation data for chemically reacting systems
Authors: de la Selva S. M. T. , García-Colín L. S.
Published: February 2003

Sound attenuation data for chemically relaxing solutions are commonly interpreted as the sum of two terms, one exclusively relaxational and the other exclusively hydrodynamical, linear in the frequency. It is shown here from irreversible thermodynamics that this is not the case. The interpretation used until now is shown to be recovered as an approximation in which the bulk and shear viscosities together with the viscoreactive coefficient are all assumed to be zero. A procedure to be applied to attenuation data to obtain the values of the bulk viscosity and the viscoreactive coefficient of the system is offered.
Gas phase infrared spectra of ammoniated ammonium ions
Authors: Schwarz Harold A.
Published: February 2003

Infrared spectra of 2–5 μ are reported for NH4(NH3)n+, n=0 to 4. The ν3 mode is found at 3335 cm−1 in NH4+, 2867 cm−1 in NH4(NH3)4+. The ν3′ modes in NH4(NH3)3+ are found at 2682 and 3365 cm−1 and the ν1′ mode is found at 2790 cm−1. The overtone band 2ν4 is observed at 3087 cm−1 in the n=4 species and 3000 cm−1 in the n=3 species. Isotope effects are reported. Less detailed spectra are reported for n=1 and n=2 species.
Raman investigation of vitreous and molten boric oxide
Authors: Walrafen G. E. , Samanta S. R. , Krishnan P. N.
Published: February 2003

Raman data have been obtained for vitreous and molten B2O3 from −196 to 1594 °C. A ΔH° of 6.4±0.4 kcal/mole boroxol was obtained from the temperature dependence of ratios of integrated Raman intensities. This heat is thought to correspond to the transformation of BO3 triangles in boroxol rings, to BO3 triangles in a random network. Vitreous B2O3 is composed predominantly of boroxol rings, but the boroxol ring concentration decreases with increasing temperature in the melt, and becomes small at temperatures above 1600 °C.
Fluorescence kinetics of cadmium and cadmium–mercury molecules
Authors: McGeoch M. W.
Published: February 2003

The molecular fluorescence of Cd2 and CdHg has been studied experimentally using optical excitation at 266 nm. The 460 nm, 150 nsec band of CdHg(b) is described and related to the cadmium molecular system. A model is proposed to describe the decay of both the pure cadmium and cadmium–mercury systems which involves cadmium trimer, dimer reservoir, and dimer radiating species. Energy spacings and equilibrium constants are derived for these states.
Discussion of the origin of secondary photon and secondary ion emission during energetic particle irradiation of solids. I. The collision cascade
Authors: Wright R. B. , Gruen D. M.
Published: February 2003

Secondary photon and secondary ion emission during energetic particle irradiation of solid surfaces is assumed to arise due to excitation and de-excitation of sputtered particles originating from a collision cascade induced by the incident projectile. The excitation is postulated to occur by two alternative mechanisms: path (a), where excitation occurs at or very near the surface of the solid due to atom–atom or atom–electron collisions; and path (b), where excitation occurs as the sputtered particle leaves the solid, but is still under its influence so that electron exchange processes are permitted. Once the excited and/or ionized sputtered particle is formed nonradiative de-excitation processes are then included in the discussion which allow the excited and/or ionized particle to be de-excited and/or neutralized. The result of these nonradiative de-excitation processes is shown to provide a possible channel for the formation of new excited ’’daughters’’ by the de-excitation of the initial excited ’’parent’’. Depending on the initial excitation probability of the parent the new excited daughters are shown to contribute to various energy regions of the excited and/or ionized secondary particle energy distribution. A mathematical formalism is developed based on the neutral sputtered atom energy and velocity distributions assuming a collision cascade origin for these sputtered particles. By including various models for the excitation probability, and the survival probability for excited particles once formed to not undergo nonradiative de-excitation the resulting energy and velocity distributions of the sputtered excited and/or ionized secondary particles are calculated. These distributions are found to be a function of the emission angle depending on the model assumed for the initial excitation. From this formalism the total excited secondary particle yield may be calculated. The mathematical expressions herein developed may be used to interpret present experimental data in such a manner that the initial assumptions regarding path (a) or path (b) excitation may or may not be substantiated. The formalism also suggests new experiments which can be performed to further test the excitation processes involved in excited and/or ionized secondary particle emission and thereby increase the understanding of the origin of these phenomena.
Resonance Raman spectra of FeBr4− in various solvents and excited in the 4A1 4E d–d band
Authors: Mejean T. , Forel M. T. , Bourgeois M. T. , Jacon M.
Published: February 2003

The resonance Raman excitation profiles (REP) of the ν1 and ν3 modes of the FeBr4− ion solved in CHCl3, CH3CN, and CH3NO2 and excited in the d–d band has been experimentally measured. Two theoretical methods based on the harmonic oscillator model and on the reflection approximation are proposed. The introduction of spin–orbit coupling and vibronic coupling in the Raman tensor are necessary to account for the observed calculated profiles, and the influence of a Jahn–Teller effect in the charge transfer band is revealed in the REP of the ν3 mode.
Electron photodetachment of thiomethoxyl and deuterothiomethoxyl anions: Electron affinities, vibrational frequencies, and spin–orbit splitting in CH3S⋅̄ and CD3S⋅̄
Authors: Janousek Bruce K. , Brauman John I.
Published: February 2003

The cross section for electron photodetachment has been measured for the thiomethoxyl anion (CH3S−) and the deuterothiomethoxyl anion (CD3S−) using an ion cyclotron resonance spectrometer in conjunction with a tunable dye laser. After accounting for the effective rotational broadening, these spectra yield the electron affinities, EA(CH3S) =1.861±0.004 eV and EA(CD3S) =1.858±0.006 eV. A doubling of features is observed in the photodetachment spectra which arises from transitions to the 2E3/2 and 2E1/2 spin–orbit states of the final-state radicals. From these spacings we obtain a spin–orbit coupling constant (A) in CH3S of −280±50 cm−1 and in CD3S of −260±50 cm−1. The spectra also contain transitions to excited vibrational states of the radicals for which we obtain vibrational frequencies of 770±50 cm−1 for the symmetric C–S stretching motion in CH3S (660±60 cm−1 in CD3S) and 1360±70 cm−1 for the methyl umbrella motion (1100±50 cm−1 in CD3S).
A consequence of the motional Stark effect in two photon spectroscopy
Authors: Harris Robert A.
Published: February 2003

The effect of the presence of an external magnetic field on the two photon spectroscopy of an ideal gas of atoms or molecules is examined. It is then shown that the resulting ’’motional Stark effect’’, gives rise to a line shape which is not a delta function when the two photons have equal and opposite wave vectors. (AIP)
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o calculation of the rate constant for the OH+H2→H2O+H reaction
Authors: Schatz George C. , Walch Stephen P.
Published: February 2003

The rate constants for OH+H2 reactions are calculated, using the reagent and saddle point parameters from table I and 6.2 kcal/mole barrier estimate. (AIP)
Reply to a Comment by Milonni on ‘‘A classical/semiclassical theory for the interaction of infrared radiation with molecular systems’’
Authors: Miller William H.
Published: February 2003

The classical model for describing the interaction of molecules with infrared radiation, proped, by the author is further explored and Milonni’s comments are taken into consideration. (AIP)
Excitation and deactivation of O(6s5S) in collisions with N2 at above-thermal energies
Authors: Sheridan J. R. , Enzweiler J. A.
Published: February 2003

Parameters in the theoretical equation for growth of O(6s5S) states in an atomic oxygen beam have been determined by fitting the growth equation to values measured for the intensity of 5436 Å emission from the beam as a function of target pressure. In this manner, cross sections for excitation and deactivation of the 6s 5S state in collisions of oxygen atoms with N2 were determined for beam energies from 3.85 to 19.65 keV. A correlation in the energy dependence for the two cross sections was observed.
Theoretical studies of positron complexes with atomic anions
Authors: Kurtz Henry A. , Jordan Kenneth D.
Published: February 2003

The LCAO–SCF Hartree–Fock method is employed to study the ground and excited states of [H−;e+], [Li−;e+], [F−;e+], and [Na−;e+]. The nature of the binding of the positron in each of these species as well as the perturbation on the electron orbitals due to the addition of the positron is explored in detail.
An experimental study of resolution of proton chemical shifts in solids: Combined multiple pulse NMR and magic-angle spinning
Authors: Ryan L. M. , Taylor R. E. , Paff A. J. , Gerstein B. C.
Published: February 2003

High-resolution nuclear magnetic resonance spectra of protons in rigid, randomly oriented solids have been measured using combined homonuclear dipolar decoupling (via multiple pulse techniques) and attenuation of chemical shift anisotropies (via magic-angle sample spinning). Under those conditions, isotropic proton chemical shifts were recorded for a variety of chemical species, with individual linewidths varying from about 55 to 110 Hz (1–2 ppm). Residual line broadening was due predominately to (i) magnetic-field instability and inhomogeneity, (ii) unresolved proton–proton spin couplings, (iii) chemical shift dispersion, (iv) residual dipolar broadening, and (v) lifetime broadening under the multiple pulse sequences used. The magnitudes of those effects and the current limits of resolution for this experiment in our spectrometer have been investigated. The compounds studied included organic solids (4, 4′-dimethylbenzophenone, 2, 6-dimethylbenzoic acid, and aspirin), polymers (polystyrene and polymethylmethacrylate), and the vitrain portion of a bituminous coal.
Ultrahigh single-step deuterium enrichment in CO2 laser photolysis of trifluoromethane as measured by carbon–isotope labeling
Authors: Herman Irving P. , Marling Jack B.
Published: February 2003

Extremely high single-step deuterium enrichment factors (β) were measured in the CO2 laser photolysis of trifluoromethane near 10.3 μ by irradiating a prepared mixture of 12CDF3/13CHF3 and determining the 13C content in the product C2F4. With 30 J/cm2 peak focal fluence incident on 200 mTorr CHF3/20 Torr Ar, β was ≳ 20 000 for λ?10.25 μ and decreased monotonically toward shorter wavelengths. β was observed to decrease at higher laser fluences.
Quenching kinetics for the HgBr* (B 2Σ1/2) and HgI* (B 2Σ1/2, C 2Π1/2) states
Authors: Roxlo C. , Mandl A.
Published: February 2003

Measurements of the rates of collisional quenching of HgBr* (B 2Σ1/2) and HgI* (B 2Σ1/2) by He, Ne, N2, Ar, and Xe at densities up to about 2 amagat are reported in this paper. The quenching rates of the HgI* (C 2Π1/2) state by those gases are also determined and the branching ratio to the B state is found to be unity within experimental error. Additional quenching rates of HgI* (B) by CF3I and I2 are included.
Experimental study of nucleation in binary mixtures: The methanol–water and n-propanol–water systems
Authors: Flageollet C. , Cao M. Dinh , Mirabel P.
Published: February 2003

The upward diffusion cloud chamber was used to study the homogeneous nucleation of the MeOH–H2O and n-PrOH–H2O mixtures over a wide range of water activities (i.e., relative humidities). The results are compared with the predictions of theory. Good agreement between the experimental results and the classical theory of homogeneous nucleation was found for alcohol rich mixtures. However, at high water concentrations, the experiments require much more alcohol than theoretically predicted. This is the same conclusion as was obtained previously for the EtOH–H2O mixture. It is shown that this disagreement can be explained by the fact that, for the alcohol–water systems, the use of macroscopic surface tension is not relevant to binary nucleation experiments.
Structure of a dilute aqueous solution of argon. A Monte Carlo simulation
Authors: Alagona Giuliano , Tani Alessandro
Published: February 2003

A Monte Carlo simulation of a dilute aqueous solution of argon has been performed in the canonical (T,V,N) ensemble. The argon–water pair potential energy function has been obtained, with a best fitting procedure, from a set of 85 ab initio SCF energy values calculated with an extended basis set. Both energetic and structural evidences supporting the hypothesis of water structure promotion by nonpolar solutes are provided. Water molecules close to argon are oriented in a way similar to that of a crystalline clathrate cage. This analogy however is restricted to a local environment, which suggests that only parts of a real clathrate cage are present at a given time in this solution.
Sub-Doppler spectroscopy of BO2 from backscattering
Authors: Schulz W. , Weyer K. G. , Beaudet R. A. , Walther H.
Published: February 2003

Line narrowing occurring in backscatter fluorescence has been investigated for doing Doppler free molecular spectroscopy. The theory has been put into a suitable form. The P–R line spacing in the (0,0,0)2Π3/2← (1,0,0)2Π3/2 band of the A 2Πu←X 2Πg system of BO2 have been measured.
Inhibition and enhancement of positronium formation in aqueous solutions from the Doppler broadening of annihilation peaks
Authors: Duplâtre G. , Abbé J. Ch. , Maddock A. G. , Haessler A.
Published: February 2003

The Doppler broadened positron annihilation line shapes have been determined in aqueous solutions of various inhibiting and enhancing agents of positronium (Ps) formation : H+aq, NO−3, S2−, SCN− and the halide ions. The results confirm that the inhibition is made through electron scavenging by H+aq and NO−3, and through positron capture, to form a bound state, by the other ions. The characteristics and yields of the various positron-containing species in the solutions are inferred from the deconvoluted experimental line shapes. On referring to the lifetime spectroscopy experimental data, it is found that the yields of positron bound states are lower than expected, and the formation of excited states is hypothesized to account for the discrepancy.
Interpretation of the ESR spectrum from a mobile peroxide on a MgO surface
Authors: Schlick Shulamith , Kevan Larry
Published: February 2003

It is shown that the reported spectra of (CO2−O2)− on MgO at 77 K and room temperature can be reinterpreted in terms of a single radical species undergoing intramolecular rotational motion of the O-O group. The ’’extra’’ lines in the spectrum at 77 K arise from averaging of g-anisotropy and can be simulated by the modified Bloch equations. (AIP)