bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption in the visible region of LiNbO3 sequentially implanted with Ag and Cu ions
Authors: Saito Y. , Kitahara A.
Published: July 2002

Ag and Cu ions were implanted sequentially into LiNbO3 at different ratios of Cu and Ag ions, and in two different energy regimes: kiloelectron volt and megaelectron volt. Optical absorption spectra were measured in the visible region at each stage in the process of annealing the sample. For the samples implanted sequentially with Ag and Cu ions at the low energy of 20 keV, only one peak appeared between 430 and 630 nm, the position of which depended on the ratios of Ag and Cu ions. For the samples implanted at high energies of 3 or 4.2 MeV, two large and distinct absorption peaks were observed at 480 and 590 nm. When the samples were heated to high temperatures, the peak at 480 nm ascribed to colloids of Ag shifted gradually to longer wavelengths and reached 530 nm, whereas the red shift of the peak ascribed to colloids of Cu was approximately 15 nm. Cu atoms diffuse out of the doubly implanted layer faster and at lower temperatures compared with Ag, even when Cu ions were implanted deeper than the Ag.
Third-order nonlinear optical response of metal dielectric composites
Authors: Gao Lei , Li Zhen-Ya
Published: July 2002

Effective linear and third-order nonlinear optical properties of metal/dielectric composites are studied. We take into account the interaction between metal particles and dielectric matrix by introducing an interfacial factor I via a simple model. Explicit expressions for the effective linear dielectric function εe(0) and cubic nonlinear susceptibility χe(3), which are dependent on I and the mean radius a of metal particles (or the annealing temperature TA), are derived with Taylor expansion methods. We find that the interfacial factor I plays an important role in determining both the linear and nonlinear optical properties. Our theoretical results are in agreement with the observed dependence of the linear absorption coefficient α and the nonlinear susceptibility χe(3) on the mean radius a (or TA).
Pressure effect on crystallization of metallic glass Fe72P11C6Al5B4Ga2 alloy with wide supercooled liquid region
Authors: Jiang J. Z. , Olsen J. S. , Gerward L. , Abdali S. , Eckert J. , Schlorke-de Boer N. , Schultz L. , Truckenbrodt J. , Shi P. X.
Published: July 2002

The effect of pressure on the crystallization behavior of metallic glass Fe72P11C6Al5B4Ga2 alloy with a wide supercooled liquid region has been investigated by in situ high-pressure and high-temperature x-ray diffraction measurements using synchrotron radiation. In the pressure range from 0 to 2.4 GPa, the crystallization temperature, Tx, increases with pressure, p, having a slope of 30 K/GPa while a minimum in Tx was detected in the range from 2.4 to 3.9 GPa. With a further increase of p, Tx is independent of p. The Tx vs p behavior can be qualitatively explained by the suppression of atomic mobility and changes of the Gibbs free energy of various phases with pressure.
Broadband permeability measurement of ferromagnetic thin films or microwires by a coaxial line perturbation method
Authors: Adenot A.-L. , Acher O. , Pain D. , Duverger F. , Malliavin M.-J. , Damiani D. , Taffary T.
Published: July 2002

A new method to measure the microwave permeability of ferromagnetic thin film or microwires from 50 MHz up to 18 GHz is presented. In contrast with all previous permeameters based on perturbation techniques, our permeameter does not require any reference sample for calibration, which is a definite advantage, as it is very difficult to have a reference magnetic sample of known permeability. In our technique, a network analyzer detects the small perturbation of the impedance of a coaxial line cell loaded by the magnetic sample. Satisfactory sensitivity is achieved, even for magnetic volume fraction as low as 10−4. To assess the validity of this new technique, comparisons are performed with three other methods on different samples, in particular on ferromagnetic wires. It is shown that in the case of very soft magnetic materials, the shape of the sample affects the permeability measured with a conventional permeameter.
Spectroscopy and diode-pumped laser oscillation of Yb3+, Ho3+-doped yttrium scandium gallium garnet
Authors: Diening A. , Kück S.
Published: July 2002

We report on the laser oscillation of Yb3+, Ho3+-doped yttrium scandium gallium garnet around 1.2 and 3 μm under laser diode excitation. At 1208 nm output powers of up to 135 mW and slope efficiencies up to 7.6% with respect to the incident pump power were achieved at room temperature under continuous wave excitation. Quasicontinuous wave excitation led to laser emission at 2844 nm with pulse energies up to 10 m.J. The laser relevant spectroscopic data, i.e., lifetimes, absorption, and emission cross sections are given and the excited state absorption spectrum is presented.
Keyhole modeling during laser welding
Authors: Fabbro R. , Chouf K.
Published: July 2002

A model, which generalizes the concept of keyhole formation due to sideways melt displacement, allows us to study the keyhole geometry as a function of the main operating parameters such as welding speed, laser incident intensity, or sample material. It is based on a drilling velocity whose combination with the welding velocity causes the inclination of the front keyhole wall. This front inclination is shown to be stationary and stable all along the front side. The penetration depth results from the product of this drilling velocity and a characteristic time defined as the beam diameter divided by the welding speed. By using a ray-tracing procedure, the dynamics and the complete keyhole geometry can be determined by taking into account the possible multiple reflections inside the keyhole and a simplified description of the closure process of the rear keyhole wall. It is shown that for usual conditions of laser irradiation, this rear keyhole wall cannot be stationary all along its surface and only an adequate laser intensity distribution can make it stationary. The interest of elongated focal spots or twin spots is then demonstrated. At high welding velocity the front wall is inclined and is composed of several layers resulting from the successive reflections. The rear wall fluctuates around an apparent equilibrium, and corresponding fluctuations occur at maximum penetration depth. At low welding speeds, the keyhole appears to be more symmetric as a consequence of the multiple reflections between the rear and the front keyhole walls.
Ilmenite-like stacking defect in nonstoichiometric lithium niobate crystals investigated by Raman scattering spectra
Authors: Kong Yongfa , Xu Jingjun , Chen Xiaojun , Zhang Cunzhou , Zhang Wanlin , Zhang Guangyin
Published: July 2002

The Raman spectra of three pure lithium niobate (LiNbO3) crystals with different content of Li and several doped LiNbO3 (Mg, In and Ti) crystals have been investigated. A weak peak at about 738 cm−1 reported as a very strong Raman peak of ilmenite phase LiNbO3 was observed for nonstoichiometric LiNbO3 crystals. The dependence of this peak on Li content and doping concentration in crystal suggested that there is a new kind of defect structure which has ilmenite-like stacking and direct relationship with antisite NbLi4+ ion. This result experimentally proved the existence of ilmenite-like stacking defect in nonstoichiometric LiNbO3 for the first time. This defect is in good agreement with the Li vacancy model and has a different structure from that proposed by Smyth [Ferroelectrics 50, 93 (1983); Proc. IEEE, p. 115 (1986)].
Frequency dependence of giant magnetoimpedance effect in CuBe/CoFeNi plated wire with different types of magnetic anisotropy
Authors: Kurlyandskaya G. V. , Barandiarán J. M. , Muñoz J. L. , Gutiérrez J. , Vázquez M. , Garcia D. , Vas’kovskiy V. O.
Published: July 2002

A giant magnetoimpedance effect (GMI) has been measured in a CuBe 0.1 mm diameter wire, electroplated with a Co6Fe20Ni74 layer 1 μm thick. Heat treatments after the electrodeposition were performed under the dc or ac field. The GMI has been measured in the frequency range 0.25 to 9 MHz, using ac driving current from 5 to 35 mA. The maximum GMI ratio depends strongly on the heat treatments, the intensity and the frequency of the driving current, reaching the value of 165% for optimal conditions. Results are discussed, taking into account the anisotropy distribution in the magnetic layer induced by the heat treatments under the field and a simple rotation model.
Total secondary-electron yield of metals measured by a dynamic method
Authors: Pintão Carlos Alberto Fonzar , Hessel Roberto
Published: July 2002

The secondary electron emission of dielectrics usually is measured by the pulse method, in which the dielectric is irradiated with short pulses of electrons. Attempts to use a dynamic method, in which the dielectric is irradiated continuously, have failed because the dielectric becomes charged and this charge interferes with the emission process. The dynamic method can, however, be applied to metals where volume charges are prevented. This article reports dynamic measurements of the total secondary emission yield from stainless steel, platinum, and aluminum and compares them with results from the current pulse method. In order to apply the dynamic method to metals a simple but important change in the setup was introduced: a dielectric slab was placed between the electrode and the metallic sample, which permitted the sample surface potential and therefore the energy of the incident electrons to change continuously. Unlike for dielectrics, the emission curves for metals are identical when obtained by the two methods. However, for a sample with deliberately oxidized surfaces the total secondary emission yield is smaller when measured with the dynamic method as compared with the pulse method, just as happens for dielectrics.
Explosive change in crater properties during high power nanosecond laser ablation of silicon
Authors: Yoo J. H. , Jeong S. H. , Greif R. , Russo R. E.
Published: July 2002

Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreement with measured crater depths. Induced transparency is demonstrated to play an important role in the formation of a deep superheated liquid layer, with subsequent explosive boiling responsible for large-particulate ejection.
The effect of intramolecular paramagnetic–diamagnetic exchange on the magnetic field effect of radical pair recombination
Authors: Schulten Klaus
Published: July 2002

Diamagnetic–paramagnetic exchange has a profound influence on the magnetic field dependence of the geminate recombination between the partners of a radical pair generated in a pure spin state. We consider the situation that the exchange takes place between two groups which are part of one molecule. We outline the proper theory of the spin dynamics, discuss the limits of slow and of fast exchange, and for these two situations provide an estimate of the B1/2 value which characterizes the magnetic field dependence at intermediate fields. The theory provides the basis for a novel method of measuring the rates of electron transfer between two molecular sites with a defined geometry.
Rovibrational molecular populations, atoms, and negative ions in H2 and D2 magnetic multicusp discharges
Authors: Péalat M. , Taran J-P. E. , Bacal M. , Hillion F.
Published: July 2002

Coherent anti-Stokes Raman scattering is applied to the study of rovibrational populations in magnetic multicusp H2 and D2 discharges. This subject is of interest to negative hydrogen ion formation by volume plasma processes. The populations of high-lying rotational states (J≳5) in the vibrational levels v=0, 1, and 2 are found to be significantly higher than expected from the Boltzmann law. In H2 the net populations of the first four vibrational levels follow approximately the Boltzmann law, with the vibrational temperature of 2390 K (in a 90 V-10 A discharge at 55 μbar). In similar discharge conditions, the population of the state v=3 in D2 is higher than expected from the Boltzmann law. In the presence of the discharge a deficiency in H2 and D2 molecule density was observed and was attributed to the possible presence of H and D atoms. This was verified by an independent measurement of the atomic fraction and temperature. The density of negative ions, measured by the photodetachment technique, is also reported.
Calculation of pressure broadening parameters for the CO–He system at low temperatures
Authors: Green Sheldon
Published: July 2002

Theoretical pressure broadening parameters were computed for the 0–1 and 1–2 rotational transitions of CO in He at very low temperatures and compared with the recent experimental measurements at 4.2 K. The interaction potential was taken from extensive SCF-CI calculations, molecular collision dynamics were described by essentially exact converged close coupling calculations, and pressure broadening cross sections were obtained from the collisional S matrices within the accurate Fano–Ben Reuven framework. Resonances at low collision energies give rise to an increase in the thermally averaged cross sections at low temperatures. Although previous calculations for this system at higher temperatures (77–300 K) were in good accord with experiment, at 4.2 K predicted values are about two times larger than experiment; possible sources of this discrepancy are discussed.
Multiple-quantum dynamics in solid state NMR
Authors: Baum J. , Munowitz M. , Garroway A. N. , Pines A.
Published: July 2002

Recently developed solid state multiple-quantum NMR methods are applied to extended coupling networks, where direct dipole–dipole interactions can be used to create coherences of very high order (∼100). The progressive development of multiple-quantum coherence over time depends upon the formation of multiple-spin correlations, a phenomenon which also accompanies the normal decay to equilibrium of the free induction signal in a solid. Both the time development and the observed distributions of coherence can be approached statistically, with the spin system described by a time-dependent density operator whose elements are completely uncorrelated at sufficiently long times. With this point of view, we treat the distribution of coherence in a multiple-quantum spectrum as Gaussian, and characterize a spectrum obtained for a given preparation time by its variance. The variance of the distribution is associated roughly with the number of coupled spins effectively interacting, and its steady growth with time reflects the continual expansion of the system under the action of the dipolar interactions. The increase in effective system ‘‘size’’ is accounted for by a random walk model for the time development of the density operator. Experimental results are presented for hexamethylbenzene, adamantane, and squaric acid. The formation of coherence in systems containing physically isolated clusters is also investigated, and a simple method for estimating the number of spins involved is demonstrated.
Inherent structure theory of liquids in the hard-sphere limit
Authors: Stillinger Frank H. , Weber Thomas A.
Published: July 2002

Atomic pair correlation functions for liquids provide an image of temperature-dependent short-range order. If the thermal ensemble of atomic configurations is mapped (by steepest descent on the potential hypersurface) onto potential energy minima, the pair correlation function from the resulting transformed configurations exhibits substantial image enhancement, revealing short-range order in a much more vivid fashion. Previous studies of model atomic liquids have demonstrated that at fixed density, mapped short-range order is virtually independent of the initial temperature, and thereby amounts to an ‘‘inherent structure’’ for the liquid. The present paper investigates steepest-descent mapping and inherent structure for hard spheres, construed as the infinite-n limit for pair potentials (a/r)n. Methods used are both analytical and simulational, the latter involving molecular dynamics for n=12 and 24. Results show that inherent structures in the hard-sphere limit are randomly packed configurations, where particle radii have been inflated to the point of jamming.
Molecular dynamics of the crystal–fluid interface. V. Structure and dynamics of crystal–melt systems
Authors: Broughton J. Q. , Gilmer G. H.
Published: July 2002

Crystal–melt interfaces at the triple point have been investigated by molecular dynamics techniques. The face–centered-cubic (111), (100), and (110) orientations were simulated using a modified Lennard-Jones potential between particles. Comparisons of structural and dynamic properties are made. Profiles measured through the interfaces show that the (111) and (110) systems have density oscillations of constant period, whereas the period of the (100) oscillations expands on the liquid side of the interfaces. Radial and angular correlation functions through the interface show an abrupt transition from crystal to liquid structure, with an implied interface thickness of several atomic diameters. The lateral diffusion coefficients of the three orientations are approximately equal, but the excess stresses are different.
The effect of pressure on the Raman spectra of solids. III. Sodium formate, NaHCOO
Authors: Heyns A. M.
Published: July 2002

The pressure dependence of the Raman bands of NaHCOO has been studied at 298 K and three different phases were identified up to pressures of 30 kbar. Phase II is the one stable at ambient conditions, and when heated to 500 K at ambient pressures it undergoes a transition to phase I. This transition has also been studied and the frequencies of the Raman-active librations of NaHCOO I resemble those of aqueous solutions of formate ions. From the shifts in the librations which accompany the I–II transition it can be concluded that the Na+–HCOO− interactions are much weaker in phase I than phase II. The Raman spectrum of KHCOO II at ambient conditions is also reported and resemblances between this spectrum and that of NaHCOO III are discussed.
Shape resonances in the photoionization of CF4
Authors: Stephens J. A. , Dill Dan , Dehmer J. L.
Published: July 2002

Calculations of photoionization cross sections and photoelectron angular distributions have been performed for all occupied orbitals of CF4 using the multiple-scattering model. Results are compared with very recent experiments which employ synchrotron radiation to measure these quantities, namely the measurements of Truesdale et al. for the carbon K shell, and measurements of Carlson et al. and Novak et al. for the five outermost valence levels. The calculations predict intense shape resonances below 3 eV in continua of a1 and t2 final state symmetry. Qualitative agreement is attained on comparing much of the theory with experiment, notably the five outer valence levels, which serves to establish a one-electron picture of the photoionization dynamics of CF4.
Multiple transition states in unimolecular reactions
Authors: Chesnavich Walter J.
Published: July 2002

We report the results of a variational TST calculation on the reaction CH+4 → CH+3+H in the zero angular momentum limit using a simple model to account for the smooth conversion of the transitional modes from vibrations of the reagent to rotational-orbital modes of the products as a function of energy and position along the reaction coordinate. The calculations show that the simultaneous existence of multiple transition states depends strongly on the behavior of the transitional modes. Specifically, if the transitional modes loosen early in the decomposition process the orbiting transition state is the dominant transition state, and in extreme cases the only transition state, until energies well above threshold are reached. If, on the other hand, the loosening occurs late in the decomposition, the oribiting transition state, although present, may always be dominated by the ‘‘free energy bottleneck’’ tight transition state. Since the long range potential only mildly affects the orbiting transition state in the zero angular momentum limit, our results suggest that multiple transition states can also arise in neutral systems.
Magnetic field modulated infrared laser spectroscopy of molecular ions: The ν1 band of DCO+
Authors: Kawaguchi Kentarou , McKellar A. R. W. , Hirota Eizi
Published: July 2002

The ν1 (C–D stretch) fundamental band of the DCO+ molecular ion has been studied using a tunable diode laser spectrometer and a liquid-nitrogen-cooled hollow-cathode absorption cell with magnetic field modulation. A total of eight lines were measured, and their analysis yielded the ν1 band origin, 2584.5618(14) cm−1, and the change in rotational constant, α1=226.3(11) MHz. The relatively small value of α1 as compared to theory and to the isoelectronic molecules DN+2 and DCN suggests the possibility of mixing between the (v1v2l2v3)=(1000) and (0400) vibrational states. The observation of ν1 for DCO+ means that 11 of the 12 fundamental bands have been observed for the isoelectronic family of molecular ions HCO+, DCO+, HN+2, and DN+2, leaving only ν2 of DCO+ still undetected.
Densities of supercooled H2O and D2O in 25 μ glass capillaries
Authors: Hare D. E. , Sorensen C. M.
Published: July 2002

We have measured the densities of supercooled H2O to −34.2 °C, and D2O to −19.2 °C in 25 μ i.d. glass capillaries. Because of the small size of our capillaries, some surface energy effects appear to be present in our data. Despite this we found that reasonably accurate values of the thermal expansivity could be obtained from our data. Our expansivity results support the recent contention of Leyendekkers and Hunter that previous estimates of the anomalous expansivity of supercooled water have been overestimated. The results of a power-law analysis of our expansivity were ambiguous due to the poorly known background expansivity.
Photofragmentation dynamics of acetone of 193 nm: State distributions of the CH3 and CO fragments by time- and wavelength-resolved infrared emission
Authors: Donaldson D. J. , Leone Stephen R.
Published: July 2002

The photolysis of acetone at 193 nm is known to produce two methyl radicals and CO following excitation of a 1(n,3s) Rydberg transition. Vibrational excitation is detected in both products immediately following the dissociating laser pulse by observing the resulting infrared emission. Vibrational distributions are obtained for CH3(ν3) and for CO. These are, for CH3(ν3): v=1/2/3=0.73±0.05/0.14±0.05/0.13±0.05 and for CO: v=1/2/3=0.75±0.05/0.16±0.05/0.09±0.05. An approximate rotational temperature of 1500 K can be used to fit the CH3(ν3) emission spectrum. The CO is formed with very high, non-Boltzmann rotational excitation. This result strongly suggests that the three-body dissociation occurs via a two-step mechanism, rather than a rigorously concerted process. The high rotational excitation is most likely imparted by the kinematics in the breakup of a bent acetyl fragment.
Mean field theory of nematic order for grafted rods
Authors: Halperin A. , Alexander S. , Schechter I.
Published: July 2002

We consider rod-like polymers which are grafted on a flat surface, such that the grafting sites are mobile. A liquid interface supporting a monolayer of amphiphilic copolymers consisting of rod-like and coil-like parts may realize this model. Using the Onsager approach we find that alignment increases with surface concentration in a continuous fashion. We present diagrams of surface pressure and of nematic order parameter vs surface concentration.
Semiclassical quantization using classical perturbation theory: Algebraic quantization of multidimensional systems
Authors: Fried Laurence E. , Ezra Gregory S.
Published: July 2002

The method of algebraic quantization, a semiclassical analog of Van Vleck perturbation theory, is applied to multidimensional resonant, nonresonant, and nearly resonant systems. perturb, a special purpose program written in C, is utilized to implement classical perturbation theory efficiently to high order. States corresponding to both regular and chaotic classical regimes are quantized, and accurate eigenvalues obtained in both cases. Various quantization rules are compared, and a novel symmetry preserving rule is given which leads to good agreement with quantum mechanics. The method is able to reproduce purely quantum mechanical splittings to very good accuracy. Algebraic quantization combined with Padé resummation is used to determine energy eigenvalues for a resonant system with five degrees of freedom.
Vibrational distributions and rate constants from reactions of oxygen atoms with HI, GeH4, SiH4, H2Se, and H2S
Authors: Agrawalla B. S. , Setser D. W.
Published: July 2002

The OH(v≥0) distributions from the title reactions have been measured in a flowing-afterglow reactor using infrared chemiluminescence and laser-induced fluorescence techniques, which give the OH(v≥1) and OH(v≤1) distributions, respectively. The measured OH(v=0) relative population confirmed previous estimates for OH(v=0) populations based on extrapolations of linear surprisals using a three-body prior. The 〈fV(OH)〉 values closely resemble the 〈fV(HF)〉 and 〈fV(HCl)〉 values from the corresponding F and Cl atom reactions, suggesting similar dynamics for H abstraction by O(3P), F(2P), and Cl(2P) atoms. The room temperature rate constants for OH formation are 4.2±0.5×10−12 (GeH4), 2.1±0.8×10−12 (H2Se), 1.2±0.4×10−12 (SiH4), and ≤3.8×10−14(H2S)cm3 molecule −1 s−1, which are 2–3 orders of magnitude lower than for the corresponding F(2P) and Cl(2P) atom reactions. Formation of OH is not the major product channel from O+PH3; however, for certain conditions there are fast secondary reactions that can lead to strong OH chemiluminescence.
Study of low-energy electron attachment to C6F6 using K(n
d ) Rydberg atoms
Authors: Marawar R. W. , Walter C. W. , Smith K. A. , Dunning F. B.
Published: July 2002

An investigation of electron attachment to perfluorobenzene at subthermal energies using K(nd) Rydberg atoms is reported. The data indicate that electron capture occurs through two channels, one resulting in formation of long-lived C6F−6 ions, the other yielding C6F−6 ions that undergo rapid autodetachment. The data are compared to results obtained in free-electron studies using swarm and threshold photoelectron spectroscopy techniques.
On the molecular theory of aqueous electrolyte solutions. I. The solution of the RHNC approximation for models at finite concentration
Authors: Kusalik P. G. , Patey G. N.
Published: July 2002

This paper describes a theoretical study of the thermodynamic, dielectric, and structural properties of model aqueous electrolyte solutions. The model considered consists of hard sphere ions immersed in a hard polarizable dipole tetrahedral–quadrupole solvent with water-like parameters. The calculations involve the solution of the reference hypernetted-chain (RHNC) approximation for ion–solvent mixtures at finite concentration and some details of the general method are discussed. The influence of the molecular polarizability of the solvent particles is treated at the self-consistent mean field (SCMF) level and, surprisingly, the mean dipole moment of the solvent is found to be nearly independent of the salt concentration. Numerical results are reported for model alkali halide solutions and other 1:1 electrolytes, and comparisons are made with experimental results at 25 °C. The agreement obtained between theory and experiment is variable depending upon the particular property and solution considered. In addition to the explicit numerical results for aqueous electrolytes several general analytical results are also given. The most interesting of these are expressions for the low concentration large separation limiting behavior of the ion–solvent and solvent–solvent radial distribution functions.
Ordered structure and critical phenomena of a semidilute solution of polymer mixtures under shear flow
Authors: Hashimoto Takeji , Takebe Tomoaki , Suehiro Shoji
Published: July 2002

A semidilute solution of a polymer mixture with a nearly critical composition was studied by using the small-angle light scattering method as a model system to study ordered structures and critical phenomena of mixtures under shear flow. The experimental studies revealed that there are five regimes in the shear-rate dependence of the concentration fluctuations of the solution, as discussed in the text, including the regime in which the ‘‘shear-induced homogenization’’ takes place. The results were interpreted, as much as possible, with the theories developed for binary mixtures of simple liquids and, when necessary, with a theory proposed here for polymer systems. Unique features of polymer dynamics in comparison with the dynamics for the critical mixtures of simple liquids were presented in the text, e.g., the shear-rate (S) dependence of the cutoff wave number being proposed to be given by qc,S∼(Dcξ20)−1/4 S1/4 rather than qc,S∼(6πη/kBT)1/3S1/3 for the simple liquids.
Predesorption of CO from the sodium chloride (100) surface: Study by the complex coordinate method
Authors: Ben Ephraim A. , Folman M. , Heidberg J. , Moiseyev N.
Published: July 2002

The rotational predesorption resonances of CO adsorbed on NaCl(100) were calculated by the quantum mechanical complex coordinate method, the CO bond length being varied adiabatically. It is shown that the internal vibrational excitation of adsorbed CO by an infrared laser does not have a strong effect on the resonance positions and lifetimes, i.e., predesorption states, and the desorption of CO from the NaCl(100) surface is probably a rotational predissociation phenomenon.
Interatomic potentials for van der Waals complexes of group 13 metal atoms: InAr, InKr, and InXe
Authors: Callender C. L. , Mitchell S. A. , Hackett P. A.
Published: July 2002

Fluorescence excitation and emission spectra of the van der Waals molecules InAr, InKr, and InXe, formed by laser vaporization of a metal target followed by supersonic expansion, have been recorded. Observed vibrational progressions indicate that the interatomic potentials for the X 2Π1/2,3/2 and B 2Σ+ states can be approximated by Morse functions. Isotopic splittings are observed in the excitation spectra and allow the vibrational numbering in the excited B 2Σ+ state to be assigned. The dissociation energies of the three states are estimated for each molecule and are found to correlate well with the rare gas polarizabilities.
Forward and reverse energy transfer in the presence of energy migration and correlations
Authors: Sienicki K. , Mattice W. L.
Published: July 2002

A theoretical model of donor–acceptor kinetics in the presence of energy migration and forward and reverse energy transfer taking into account correlations between molecules in the solution is presented. The model has been developed on the base of Markoffian and non-Markoffian theory of energy transfer recently proposed by Gochanour, Andersen and Fayer [J. Chem. Phys. 70, 4254 (1979)] and Loring, Andersen and Fayer [J. Chem. Phys. 76, 2015 (1982)]. It has been shown that correlations are an important factor in the case of forward and reverse energy transfer. This non-Markoffian behavior can be investigated experimentally.
Energies and electric dipole moments of the low lying electronic states of the alkaline earth monohalides from an electrostatic polarization model
Authors: Törring T. , Ernst W. E. , Kändler J.
Published: July 2002

A simple electrostatic polarization model is applied to the low lying electronic states A 2 Π, B 2 ∑+, and A′ 2 Δ of the alkaline earth monohalides which correlate to the electronic d state of the free metal ion. The number of fit parameters can be greatly reduced using relations which are derived from the well known angular part of the free ion wave function. The model predicts energies and electric dipole moments for all Ca, Sr, and Ba monohalides in good agreement with experimental data. The model can also be applied to the C state confirming the highly ionic character of this state.
The decay of triplet pyrazine in supersonic jets
Authors: Sneh Ofer , Dünn-Kittenplon Dana , Cheshnovsky Ori
Published: July 2002

The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecule was explored. Decay rates between 7×102 –2.5×104 s−1 were found in the 1500 cm−1 range of excess vibrational energy from the origin of the T1 state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T1 →S0 intersystem crossing of pyrazine.
Dynamic light scattering study of nonexponential relaxation in supercooled 2Ca(NO3)2⋅3KNO3
Authors: Sidebottom D. L. , Sorensen C. M.
Published: July 2002

We have performed depolarized dynamic light scattering (photon correlation) measurements on the ionic glass forming mixture 2Ca(NO3)2⋅3KNO3 above its glass transition in the temperature range 68–86 °C. We find the relaxation function is a stretched exponential. The average relaxation time follows an Arrhenius temperature dependence and can be related to motions on a molecular scale. The width parameter β does not vary with temperature and is β=0.40±0.05. Combination of our β data with other literature values shows that β evolves with temperature over a larger range. At high temperature, β crosses over from β=1 to β<1 at a temperature which matches well with where the viscosity first deviates from a power law behavior. β then evolves downward with lowering temperature and then settles at 0.4 roughly 30 °C above the glass transition Tg. This behavior corresponds well with recent theory relating the glass transition to the ramification of phase space but it implies that the percolation of phase space occurs well above Tg.
Effect of variation of dielectric constant on the magnetic field modulation of exciplex luminescence: Nontypical behavior of alcohols
Authors: Nath Deb Narayan , Basu Samita , Chowdhury Mihir
Published: July 2002

The effect of variation of dielectric constant (ε) on the relative magnetic field effect on singlet luminescence (Δφ/φ) has been studied using a typical exciplex system at a saturating field. The study indicates strong specificity in the perturbation of the magnetic field effect by alcoholic solvents. In constrast to alcohols where relative singlet magnetic field effect is of the order of 1% only, the magnetic field effect in non-alcoholic medium reaches as high as 4.5%. Moreover, ε variation in alcohols yields curves which are distinctly different from those in nonalcoholic media.
Reaction rate oscillation of CO oxidation on Pt(210)
Authors: Ehsasi M. , Rezaie-Serej S. , Block J. H. , Christmann K.
Published: July 2002

We report on the occurrence of kinetic instabilities and regular oscillations in the rate of CO2 formation as it results from the CO oxidation reaction carried out in ultrahigh vacuum and high vacuum on a platinum (210) surface. While the surface properties are characterized and controlled by combined low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and ΔΦ measurements, the rate of reaction is monitored by measurements of the partial pressures of the reactants and/or of the work-function change of the Pt(210) surface. We have followed the reaction kinetics as a function of ‘‘external’’ parameters (PCO, PO2, T) and have investigated the conditions under which oscillations occur. Likewise, the influence of some of the ‘‘internal’’ parameters (sample cleanliness, gas purity, surface structure) has been worked out. Finally, possible mechanisms for the occurrence of periodic oscillations are discussed, including the local oscillation as well as the process of synchronization.
Rotational energy transfer in OH (A 2Σ+, v′=0): A method for the direct determination of state-to-state transfer coefficients
Authors: Jörg Andreas , Meier Ulrich , Kohse-Höinghaus Katharina
Published: July 2002

We have determined state-to-state rate coefficients for rotational and fine structure transitions of OH (A 2Σ+, v′=0) in thermal collisions with He and Ar at 300 K. The temporal evolution of single fluorescence lines within the A–X, 0–0 band of OH were measured, exciting either the F2(4) or F2(5) state by a nanosecond laser pulse. The OH radical was produced in a discharge flow cell, containing predominantly He or Ar, at various pressures between 1 and 6 mbar. The time resolution in the experimental setup was sufficient to evaluate the rotational energy transfer coefficients directly from the time dependence of two fluorescence lines. The observed average rate coefficients for collisions of OH (A, v′=0) with Ar are approximately 3 times larger than those with He. The two rare gases show different qualitative behavior. Whereas the almost isoenergetic transitions with ΔJ=1 and ΔN=0 are favored in collisions with Ar, those with ΔJ=ΔN=−2 are favored in collisions with He. In addition, a strong preference for transitions conserving the parity of the OH, a propensity rule, previously reported for rotational relaxation in the A state of OH, was found for collisions with He but not for collisions with Ar. Our experimental results for He and for Ar are in good agreement with recent quantum mechanical calculations of the energy transfer coefficients.
Variational transition state theory for activated rate processes
Authors: Pollak Eli
Published: July 2002

A variational transition state theory is formulated for the decay rate of a particle trapped in a metastable potential well and coupled to a heat bath. Rigorous upper bounds are derived for the transmission coefficient and the rate constant. The variational theory leads to minimization of the flux of an effective two degree of freedom Hamiltonian whose parameters depend on the system potential and the time dependent friction kernel. An explicit solution for the canonical variational dividing surface in the presence of nonlinearities in the system potential is provided. The Kramers expression for the rate in the spatial diffusion limit and its generalization to memory friction, is shown to give upper bounds provided that the nonlinearity in the system potential is positive definite. However, the variational result can still lead to substantially lower bounds for the rate. An application of VTST to a symmetric cusped double well potential provides a new expression for the rate, valid for arbitrary friction kernels and damping strength.
Transport and turnstiles in multidimensional Hamiltonian mappings for unimolecular fragmentation: Application to van der Waals predissociation
Authors: Gillilan Richard E. , Ezra Gregory S.
Published: July 2002

A four-dimensional symplectic (Hamiltonian) mapping of the type studied by Gaspard and Rice is used to model the predissociation of the van der Waals complex He–I2. Phase space structure and unimolecular decay in this mapping are analyzed in terms of a general approach recently developed by Wiggins. The two-dimensional area preserving map obtained by restricting the 4D map to the T-shaped subspace is studied first. Both the Davis–Gray theory and the analog of the alternative RRKM theory of Gray, Rice, and Davis for discrete maps are applied to estimate short-time decay rates. A four-state Markov model involving three intramolecular bottlenecks (cantori) is found to give a very accurate description of decay in the 2D map at short to medium times. The simplest version of the statistical Davis–Gray theory, in which only a single intermolecular dividing surface is considered, is then generalized to calculate the fragmentation rate in the full 4D map as the ratio of the volume of a four-dimensional turnstile lobe and a four-dimensional complex region enclosed by a multidimensional separatrix. Good agreement with exact numerical results is found at short times. The alternative RRKM theory is also applied, and is found to give a level of agreement with the Davis–Gray theory comparable to the 2D case. When the height of the barrier to internal rotation in the van der Waals potential is increased, however, it is found that volume-enclosing turnstile no longer exist in the 4D phase space, due to the occurrence of homoclinic tangency. The implications of this finding for transport theories in multimode systems are briefly discussed.
The chemical potential in terms of n-particle direct correlation functions
Authors: Attard Phil
Published: July 2002

A simple derivation of an expression for the chemical potential involving higher order direct correlation functions is given, and the Percus–Yevick approximation is extracted.
The jet-cooled fluorescence excitation spectrum and ring-bending potential-energy function and conformation of 2-cyclopenten-1-one in the S1(n,π*) electronic excited state
Authors: Cheatham C. M. , Laane Jaan
Published: July 2002

The jet-cooled fluorescence excitation spectra of 2-cyclopenten-1-one and its 5,5-d2 isotopomer have been recorded in the 370–340 nm region. The electronic origin for the undeuterated species occurs at 27 210 cm−1 for the S1(n,π*) electronic excited state. The vibrational frequencies for the three carbonyl motions and the nine ring modes were observed for the excited state. Bands at 67, 158, and 256 cm−1 for the d0 species, at 63, 147, and 240 cm−1 for the 5-d1 isotopomer, and at 59, 138, and 227 cm−1 for the d2 species were assigned to the ring-puckering motion in the S1 state. A single one-dimensional potential-energy function accurately fits the data for all three isotopomers. This function is nearly purely quartic in character and shows the ring to be planar in the electronic excited state. However, it has become less rigid, and this is ascribed to a decrease in initial angle strain within the ring. The C=O and C=C stretching frequencies occur at 1418 and 1357 cm−1 for the d0 molecule. The ring-twisting frequency for the S1 state occurs at 274 cm−1. Previous electronic absorption measurements had resulted in a misassignment for this motion.
Photothermal compression of colloidal crystals
Authors: Rundquist Paul A. , Jagannathan S. , Kesavamoorthy R. , Brnardic Charles , Xu S. , Asher Sanford A.
Published: July 2002

Dyed sulfonated polystyrene spheres (83 nm diameter) self-assemble into regular crystalline arrays that Bragg diffract visible light. Absorption of high intensity radiation (>25 W/cm2) by the dye results in a local compression of the crystalline array due to local heating. The Kossel ring pattern is used to probe the variation in the lattice parameter and the dynamics of the thermally induced compression. The thermally induced compression derives from the negative temperature dependence of the interparticle interaction potential.
The dipole moment of AuH
Authors: Sadlej Andrzej J.
Published: July 2002

A finite-field perturbation approach is used to study the relativistic and relativistic-correlation contributions to electric properties of AuH. The relativistic terms in the molecular Hamiltonian are approximated by the so-called Cowan–Griffin perturbation operator whose first-order contributions are evaluated in a large basis set of Gaussian functions. The electron correlation contribution is accounted for by using the many body perturbation theory methods. Both the relativistic and electron correlation effects as well as the mixed relativistic-correlation contributions are shown to be important in accurate predictions of the dipole moment of AuH. According to the present study the dipole moment of the 1Σ+ ground electronic state of AuH calculated at the experimental equilibrium bond distance of 2.880 bohr is found to be in the range between 0.6–0.7 a.u. The quadrupole moment of AuH is shown to have positive sign in the relativistic-correlated treatment while its negative value is predicted in nonrelativistic calculations. The parallel component of the dipole polarizability tensor is found to be equal to about 44. a.u. It is also shown that the electric property values of AuH are strongly affected by the electron correlation effects involving the next-to-valence shell of the Au atom. The valence approximation gives quite unrealistic results for both the electron correlation and relativistic contributions to electric properties of AuH.
Comparison of experimental and theoretical integral cross sections for D+H2(v=1, j=1)→HD(v′=1, j′)+H
Authors: Kliner Dahv A. V. , Adelman David E. , Zare Richard N.
Published: July 2002

We have measured the nascent HD(v′=1, j′) product rotational distribution from the reaction D+H2(v, j) in which the H2 reagent was either thermal (v=0, j) or prepared in the level (v=1, j=1) by stimulated Raman pumping. Translationally hot D atoms were obtained by uv laser photolysis of DBr or DI. Photolysis of DBr generated D atoms with center-of-mass collision energies (Erel) of 1.04 and 0.82 eV, which corresponded to the production of ground state Br and spin–orbit-excited Br*, respectively. The Erel values for DI photolysis were 1.38 and 0.92 eV. Quantum-state-specific detection of HD was accomplished via (2+1) resonance-enhanced multiphoton ionization and time-of-flight mass spectrometry. Vibrational excitation of the H2 reagent results in substantial rotational excitation of the HD(v′=1) product and increases the reaction rate into v′=1 by about a factor of 4. Although the quantum-mechanical calculation of Blais et al. [Chem. Phys. Lett. 166, 11 (1990)] for the D+H2(v=1, j=1)→HD(v′=1, j′)+H product rotational distribution at Erel=1.02 eV is in qualitative agreement with experiment, it does not quantitatively agree with the measured distribution. Specifically, the calculated distribution is too hot by 2–3 rotational quanta, and the predicted enhancement in the v′=1 rate with reagent vibrational excitation is too large by 67%±9.
Inertial axis reorientation in the S1←S electronic transition of 2-pyridone. A rotational Duschinsky effect. Structural and dynamical consequences
Authors: Held A. , Champagne B. B. , Pratt D. W.
Published: July 2002

Rotationally resolved fluorescence excitation spectra of two vibronic bands in the S1←S0 electronic transition of 2-hydroxypyridine (2HP), and of the corresponding bands in the hydroxy-deuterated molecule, have been obtained. A comparison of the rotational constants of the two molecules shows that the two bands both originate in the zero-point vibrational level of the planar keto tautomer of 2HP, 2-pyridone (2PY), and terminate in different zero-point levels of 2PY that have different out-of-plane equilibrium geometries at nitrogen. Additionally, all four bands exhibit ‘‘anomalous’’ rotational line intensities that are shown to result from an in-plane inertial axis reorientation which occurs on absorption of the photon. Likely atomic displacements that are responsible for this ‘‘rotational’’ Duschinsky effect, which may have significant dynamical consequences in 2PY and other molecules, are discussed.