bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of various theoretical equations of state used in calculation of detonation properties
Authors: Charlet F. , Turkel M.-L. , Danel J.-F. , Kazandjian L.
Published: July 2002

The thermodynamic properties of detonation products are computed by allowing for the presence of several phases of gases and solids in chemical equilibrium. We briefly describe the main perturbation theories found in the literature and we discuss their reliability in including consideration of the adiabatic gamma. We also consider an equation of state of carbon in detonation products which allows a smooth transition from an effective graphite phase to an effective diamond phase. We extensively compare the most reliable calculated results with measured detonation properties of explosives made up of carbon, hydrogen, oxygen, and nitrogen (detonation velocity, kinetic energy transferred in a cylinder test, overdriven detonation, and Chapman–Jouguet pressure). Several subjects for further study of a priori calculation of detonation properties are indicated.
Optical constants of CuGaSe2 and CuInSe2
Authors: Kawashima Takahiro , Adachi Sadao , Miyake Hideto , Sugiyama Koichi
Published: July 2002

The complex dielectric functions, ε(E)=ε1(E)+iε2(E), of chalcopyrite semiconductors CuGaSe2 and CuInSe2 have been measured by spectroscopic ellipsometry in the photon energy range between 1.2 and 5.3 eV at room temperature. The measurements are carried out on the surface parallel to the optic axis c, which allow the determination of the optical properties for light polarized perpendicular (E⊥c) and parallel to the c axis (E∥c). The measured ε(E) spectra reveal distinct structures at the lowest direct gap (E0) and higher energy critical points. These spectra are analyzed on the basis of a simplified model of the interband transitions. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. Dielectric-function-related optical constants, such as the complex refractive index n*(E)=n(E)+ik(E), absorption coefficient α(E), and normal-incidence reflectivity R(E), of these semiconductors are also presented.
Hexagonal arrays of submicronic ferromagnetic wires obtained by multiple extrusion of bulk samples
Authors: Wacquant F. , Denolly S. , Nozières J.-P. , Givord D. , Mazauric V.
Published: July 2002

A new method of producing regular arrays of submicronic semi-infinite wires is briefly described here. The technique is based on the multiple extrusion of composite wires, known as the bronze route, generally used for the preparation of superconducting wires. This method has been applied to the preparation of ferromagnetic wires (Fe, Ni) embedded in a Cu matrix. Using this technique wires diameters can be continuously varied between 1 mm and 10 nm. A clear shape induced anisotropy field is observed, associated for small diameters to coercive forces as high as 400 Oe in the case of Fe. The magnetization processes are described in the framework of the classical micromagnetic theory of magnetization reversal.
Domain wall motion in micron-sized permalloy elements
Authors: Gomez R. D. , Luu T. V. , Pak A. O. , Mayergoyz I. D. , Kirk K. J. , Chapman J. N.
Published: July 2002

The magnetization reversal process in an array micron sized NiFe patterns was studied using magnetic force microscopy in the presence of external fields. The behavior of the magnetization distribution was correlated with the aspect ratio and the direction of the applied fields. Magnetizing along the hard axis was found to produce solenoidal magnetization at remanence while applying the field along the easy axis tend to form nonsolenoidal configurations. The micromagnetic evolution, which involved domain wall, crosstie, and vortex displacements, was studied and the correlations were consistent with previously reported M–H loop observations and theoretical predictions.
Quantized current jumps in silicon photoconductors at room temperature
Authors: Haddab Y. , Besse P.-A. , Popovic R. S.
Published: July 2002

We report on the observation of quantized jumps due to single-carrier trapping and detrapping at defect states in silicon photoconductors of 103 μm3 in volume. A specifically designed electrical test structure in a low-doped (2×1014 cm−3) silicon crystal was fabricated. It consists in four substrate resistances connected in a Wheatstone bridge. After the exposure to light, the bridge offset voltage recovers its equilibrium value with steps of 5–10 μV, corresponding to the emission or capture of a single carrier. Such structures also display random telegraph signals in the dark, with steps of similar amplitude. This behavior is observed with structures processed on Czochralski-grown substrates and not with those processed on float-zone substrates. Simple calculations based on quantized free carrier concentration variations corroborate the above measurements.
Magnetic properties and giant magnetoimpedance of nanocrystalline Fe92−xZr7BxCu1 ribbons
Authors: He J. , Guo H. Q. , Shen B. G. , He K. Y. , Kronmüller H.
Published: July 2002

The influence of substitution of B for Fe on the microstructure, magnetic properties, and giant magnetoimpedance (GMI) effect has been studied for the nanocrystalline soft magnetic ribbons Fe92−2Zr7BxCu1 (x=2,4,6,8,10) annealed at 873 K for 20 min. The experimental results show that the thermal stability of the amorphous precursor is enhanced and the body-centered-cubic phase grains become smaller with increasing B content. The coercive force Hc exhibits a minimum of 2.51 A/m for the ribbon with x=8, where the effective permeability reaches the maximum. The frequency and field dependence of GMI effect is very sensitive to B content. For the ribbon with x=8, The maximal GMI ratio (ΔZ/Z)ΔZ/Z=[Z(H)−Z(Hmax)]/Z(Hmax) is about 260% at f=6.5 MHz, and the maximal sensitivity reaches value up to about 0.88% (A/m)−1 (66.6% Oe−1) at f=10.5 MHz. The correlation between the magnetic properties and the GMI effect shows that the excellent soft magnetic properties and proper transverse anisotropy play a decisive role in the occurrence of the GMI effect.
Pseudo surface wave on the (101̄2) plane of sapphire
Authors: Xu Yang , Aizawa Tatsuhiko
Published: July 2002

The present article demonstrated that pseudo surface wave (PSW) should exist on a slightly anisotropic crystal sapphire. PSW velocity dispersions on the three principal planes of sapphire were numerically calculated. The existence possibility of PSW on the water-sapphire (101̄2) plane was experimentally examined by using acoustic spectromicroscopy. Numerical analysis reveals that the two-component surface wave should be activated on the sapphire (101̄2) plane along direction angle φ0=17.18631°. This also verified the existence possibility of PSW on the slightly anisotropic crystal sapphire.
Spin-polarized electronic current in resonant tunneling heterostructures
Authors: Voskoboynikov A. , Lin Shiue Shin , Lee C. P. , Tretyak O.
Published: July 2002

The spin-dependent tunneling phenomenon in symmetric and asymmetric resonant semiconductor heterostructures is employed in a theoretical study to investigate the output tunnel current polarization at zero magnetic field. A simple model of the resonant tunneling structures and a simple one-electron band approximation with spin-orbit interaction are used in this work. It is shown that asymmetry in the electron distribution at the electrode regions provides spin-polarized tunnel current. An approach to optimize this spin-dependent effect is explored theoretically. In asymmetric resonant tunneling structures, we estimate theoretically that the polarization can reach 40% with a moderate applied electric field.
Exchange biasing in polycrystalline thin film microstructures
Authors: Yu J. , Kent A. D. , Parkin S. S. P.
Published: July 2002

The effect of lateral element size, shape, and ferromagnetic layer thickness on exchange biasing in polycrystalline IrMn/CoFe thin film elements has been studied. Magnetic hysteresis loop measurements and magnetic force microscopy imaging have been used to elucidate the basic micromagnetic behavior. Magnetic imaging of micron scale elements illustrates that the magnetization reversal modes change significantly from those of unbiased ferromagnetic elements.
Measurement of collision-induced shift and broadening of the ultraviolet transitions of OH
Authors: Shirinzadeh B. , Bakalyar D. M. , Wang Charles C.
Published: July 2002

Using a cw tunable laser, absorption measurements were made to determine for the first time the shift and broadening of the ultraviolet transitions of OH induced by collisions with molecular nitrogen and oxygen. Similar measurements were also made for collisions with air and inert gases He, Ne, Ar, and Kr. Both the shift and broadening due to these collisions were found to depend linearly upon pressures in the range from 50 Torr up to 1 atm. The homogeneous linewidth for the P1(2) transition of OH in ambient air is determined to be (0.20±0.005) cm−1. Results with Ar and Kr show contributions not accounted for by the polarizability of the colliding species.
Difference frequency laser spectroscopy of the ν1 fundamental band of HOCO+
Authors: Amano T. , Tanaka Keiichi
Published: July 2002

The ν1 fundamental band of HOCO+ has been observed with a hollow-cathode discharge in a fast flowing gas mixture of H2(∼250 mTorr) and CO2(∼20 mTorr). A least squares fit of more than 500 lines including the three astrophysical millimeter-wave lines and six laboratory submillimeter-wave lines to an effective vibration-rotation Hamiltonian has yielded a set of precise spectroscopic parameters. The agreements between the present experimental rotational constants and those calculated by ab initio methods are remarkable. The band origin has been found to be 3375.374 13(8) cm−1, which is about 27 cm−1 higher than the recent ab initio value. A perturbation found in the J′2,J′−2 levels in the excited state is discussed in some detail. The rotational transition frequencies in the ground state have been given for further astrophysical and laboratory searches.
Adsorption and growth modes of Bi on Pt(111)
Authors: Paffett M. T. , Campbell C. T. , Taylor T. N.
Published: July 2002

The vapor deposition of Bi on Pt(111) at 110 and ∼600 K have been characterized by Auger electron spectroscopy (AES), thermal desorption mass spectroscopy (TDMS), low-energy electron diffraction (LEED), and changes in the work function (Δφ). At 110 K Bi growth follows a layer-by-layer mechanism. At ∼600 K Bi fills the first monolayer (θBi≂0.56) relatively uniformly, followed by 3D island growth. Bi desorption is characterized by a large, coverage-dependent desorption energy, Edes =(81−34.2 θBi ) kcal mol−1, in the first monolayer, and zero-order kinetics with constant activation energy (Edes =53–56 kcal mol−1) for the multilayer. Many LEED patterns are observed within the first monolayer for both cold and hot substrates. Structural models for these are proposed which are consistent with coverages obtained by AES. Annealed structures show continuous compression of hexagonal Bi overlayers with increasing coverage, subject to mild substrate constraints. At 110 K and θBi >0.33, uniaxial compression is instead seen, due to an unsurmounted energy barrier. Weakly repulsive lateral Bi–Bi interactions (due to dipole repulsions) dominate submonolayer growth. These results for the semimetal Bi are intermediate in behavior between alkali and transition metal overlayers on Pt(111). This is consistent with the relative strengths of the surface dipole of these adsorbed metals.
Homogeneous gas phase thermal polymerization of styrene
Authors: El-Shall M. S. , Bahta A. , Rabeony H. , Reiss H.
Published: July 2002

Self-thermal initiated gas phase polymerization in styrene vapor is investigated employing the nucleation and growth of liquid monomer droplets (in supersaturated styrene vapor) for detection. The results show rather definitively that styrene does polymerize thermally, in the vapor phase, at temperatures as low as 250 K. The same is true for α-methylstyrene which is also studied (but not so thoroughly). Phenylacetylene retards the gas phase polymerization of styrene. Polymers with degree of polymerization in the range 9 to 24 are involved in this study. The degree of polymerization increases with temperature and bimolecular termination appears to be absent. The total concentration of growing radicals is of the order of 6×103 cm−3. Thermal initiation appears to have second order kinetics. The rate constant for propagation is the same in the gas as in the liquid. At 0 °C (where we easily observe the formation of polymers), 23 million years would be required to observe the disappearance of monomer by standard methods. This explains why gas phase polymerization of styrene has not been previously conclusively demonstrated, even at high temperatures.
Infrared–ultraviolet double resonance studies of benzene molecules in a supersonic beam
Authors: Page Ralph H. , Shen Y. R. , Lee Y. T.
Published: July 2002

We have used IR excitation to selectively create populations in admixtures of the zeroth-order states comprising the ∼3000 cm−1 ‘‘C–H stretching Fermi triad’’ of benzene. UV spectra of the 260 nm Ã(1B2u)←X̃(1A1g) transition in the IR-excited molecules show several new bands, which we have assigned. Final states in the UV transitions are some vibrational levels which have not been detected before, allowing us to find several excited-state vibrational frequencies. We have determined ν′3 =1327±3 cm−1, ν19 =1405±3 cm−1, and ν′20 =3084±5 cm−1. Also, vibrational structure which was unresolved in IR spectra of the ‘‘Fermi triad’’ was resolved in the UV double resonance spectra, confirming that the C–H stretching admixture is really a tetrad. The 3048, 3079, and 3101 cm−1 states had formerly been given the labels ν″20, ν″8+ν″19, and ν″1+ν″6+ν″19, respectively. Actually, the middle level most nearly resembles ν″1+ν″6+ν″19, and the 3101 cm−1 level is strongly mixed with ν″3+ν″6+ν″15. As predicted by molecular orbital theory, excited-state C–H bending and stretching frequencies are not very different from those in the ground state. Furthermore, we suggest that the four C–H stretching frequencies increase uniformly by ∼20 cm−1 in the excited state; reexamination of the Atkinson and Parmenter 260 nm Ã←X̃ spectrum leads us to reassign ν2 from 3130 to ∼3093 cm−1, which is 19 cm−1 above ν″2. There is a Fermi resonance between the ν6+ν′20 level and another level ∼13 cm−1 lower in energy; the strength of the perturbation is ∼18 cm−1. Possibilities for the perturbing vibrational state are ν6+ν′8+ν14 and ν′6+ν13.
Generalized van der Waals theory of hard sphere oscillatory structure
Authors: Freasier B. C. , Woodward C. E. , Nordholm Sture
Published: July 2002

The recently developed generalized van der Waals (GvdW) theory, a free energy density functional theory based on cell theory and van der Waals approximations, is here applied to the prediction of hard sphere oscillatory structures at a hard wall, between two hard walls, and around a hard sphere. Three different functional forms of the crucial free volume factor are compared. The results confirm that the fine-grained GvdW theory containing a nonlocal entropy functional yields structures reproducing packing oscillations not only qualitatively but to quantitative accuracy. The error depends on the choice of free volume factor and can be made small except at high density where the range and magnitude of oscillations are overestimated. Evidence of early onset of a hard sphere freezing transition is seen.
Dynamic light scattering from concentrated water-in-oil microemulsions: The coupling of optical and size polydispersity
Authors: Yan Y. D. , Clarke J. H. R.
Published: July 2002

Dynamic light scattering from highly concentrated colloidal systems with a narrow distribution of particle sizes can be interpreted in terms of the sum of two independent modes due, respectively, to collective diffusion and polydispersity fluctuations; a general formalism has been presented for calculating the relative mode amplitudes for hard spheres in the Percus–Yevick approximation [Pusey, Fijnaut, and Vrij, J. Chem. Phys. 77, 4270(1982)]. This work extends the relative mode amplitude calculation to the general case where optical (i.e., refractive index) and size polydispersity are completely coupled such as in water-in-oil microemulsions. To develop the theory a concentric core-shell hard sphere model is adopted, in which particles possess a continuous variation in the core sizes but have constant shell thickness, thus giving rise to a distribution in the particle refractive indices. A new ‘‘measured’’ static structure factor SM(0) is derived, and applied to the calculation for the relative amplitude of the slow mode, A2/(A1+A2), as a function of solvent refractive index n0. A strong enhancement of the slow mode is predicted as the optical matching point is approached. The theory also shows that the dependence of A2/(A1+A2) on n0 is very sensitive to the extent of polydispersity, but rather insensitive to the distribution function used. Neglect of the nonuniformity in particle refractive index can cause a substantial overestimate of the size polydispersity. We have used our extended treatment to interpret the dynamic light scattering data from concentrated water-in-oil microemulsions formed from H2O, AOT, and apolar solvents using solvent composition to control the contrast in refractive index. It is found that the AOT-stabilized water microemulsion droplets have a size polydispersity of about 6.5%, which is smaller than previously thought.
Dynamics of the ultraviolet photochemistry of water adsorbed on Pd(111)
Authors: Wolf M. , Nettesheim S. , White J. M. , Hasselbrink E. , Ertl G.
Published: July 2002

UV-laser irradiation (hν=6.4 eV and 5.0 eV) of the water bilayer adsorbed on a Pd(111) surface leads to molecular desorption and to conversion of the adsorbed state as manifested in thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time-of-flight measurements show that water molecules desorb with a translational energy of about 600 K for both photon energies indicating a nonthermal process. While desorption is largely suppressed with adsorbed multilayers, conversion within the first layer still proceeds. The dependence of the desorption yield on angle of incidence and polarization of the light reveals substrate excitations as the dominant primary step. A strong variation of cross sections with isotopic substitution is observed. This is interpreted as evidence for the operation of a mechanism involving excitation onto an isotope-independent excited potential energy surface followed by rapid deexcitation to the ground state so that, of the total number of species excited, only a small mass dependent fraction actually fragments or desorbs.
Theoretical study of infrared spectra perturbed by uniform electric fields: A
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o calculations on H2O, NH3, H2CO, and C2H4
Authors: Andrés José Luis , Martí Josep , Duran Miquel , Lledós Agustí , Bertrán Juan
Published: July 2002

The changes in harmonic vibrational frequencies and line intensities with respect to different strengths of an applied electric field are computed using analytically calculated energy derivatives. The ground states of several small molecules are studied at the ab initio restricted Hartree–Fock self-consistent-field (RHF SCF) level with a double ζ plus polarization basis set. The dependence of vibrational frequencies and line intensities upon the applied field strength is discussed.
Adsorption of polymeric brushes: Bridging
Authors: Johner Albert , Joanny Jean-François
Published: July 2002

We study the adsorption of grafted polymer layers on a planar surface parallel to the grafting surface. The layer consists of two types of chains: nonadsorbed chains with a free end and adsorbed chains forming bridges between the two plates. In the limit of strong adsorption a dead zone exists in the vicinity of the adsorbing plate; its size increases with the adsorption strength. Two adsorption mechanisms are possible: adsorption of the last monomer only and adsorption of all the monomers. In both cases the adsorption regimes at equilibrium (when no external force acts on the plates) are discussed within the framework of the self-consistent mean-field theory. We also give scaling laws taking into account excluded volume correlations. Finally, we consider situations where a finite external force, either tangential or normal to the plates, is applied on the adsorbing plate. Pulling and tangential forces both reduce the fraction of bridges and eventually lead to rupture, whereas compressional forces favor bridging. For normal forces, force vs distance profiles between planes and crossed cylinders are given.
Application of a multilevel Redfield theory to electron transfer in condensed phases
Authors: Jean John M. , Friesner Richard A. , Fleming Graham R.
Published: July 2002

A quantum mechanical theory of photoinduced electron transfer, based on the Redfield theory of relaxation, is developed and applied to the standard two state–one mode system interacting with a thermal bath. Quantum mechanical treatment of the reaction coordinate allows incorporation of both finite vibrational dephasing and energy flow rates into the description of electron transfer dynamics. The field–matter interaction is treated explicitly to properly incorporate the total energy and magnitude of the vibrational coherence present in the initially prepared state. Calculation of the reduced density matrix of the system is carried out in a vibronic basis that diagonalizes the electron exchange coupling so that the method is valid for arbitrarily large coupling strength. For weak electronic coupling, we demonstrate the equivalence between the results from Redfield theory and those obtained from the standard perturbative expression (golden rule) for nonadiabatic electron transfer. We then discuss quantitatively the breakdown of the Fermi golden rule with increasing electronic coupling strength. The failure of the golden rule is seen to result from either slow energy equilibration in the reactant or product well or from quantum interference effects resulting from finite dephasing rates. For cases where the reorganization energy is large compared to the frequency of reactive motion, such that we may ignore nuclear tunneling, results from the theory show good agreement with those from the semiclassical Landau–Zener theory when motion of the reaction coordinate through the surface crossing region can be considered to be ballistic. Finally results are shown in the weak damping (coherent) limit that demonstrate interference effects between phase coherences involving states in both wells.
Rotationally resolved threshold photoelectron spectrum of the methyl radical
Authors: Blush Joel A. , Chen Peter , Wiedmann Ralph T. , White Michael G.
Published: July 2002

We report the rotationally resolved, one-photon threshold photoelectron spectrum of the methyl radical, CH3, produced by supersonic-jet, flash pyrolysis. Only rotational transitions with ΔK=0, ±2 are observed and this result is shown to be consistent with photoionization selection rules in D3h symmetry. Assignment of the threshold photoelectron spectrum results in an adiabatic ionization potential of 79 349±3 cm−1.
Impurity segregation in Lennard-Jones A/AB heterostructures. I. The effect of lattice strain
Authors: Cook Stephen J. , Clancy Paulette
Published: July 2002

The solidification kinetics of Lennard-Jones heterostructures are investigated using nonequilibrium molecular dynamics computer simulation techniques. The heterostructures are of the form A/A1−xBx. In this paper, the nature of the A and B atoms comprising the alloy differ only in size, the B atoms being 10% larger than the A atoms. The segregation of the solute B atoms from the solid into both a surface-melted thin film (wetting the solid–vapor interface) and into a bulk-liquid phase following energy input from a simulated laser pulse is studied. The segregation of the (impurity) B atoms into the liquid is found to be enhanced at the (111) orientation when the AB alloy is under strain, but enhanced on the (100) orientation when the alloy is strain free. Comparison to the solute redistribution theories of Aziz (the continuous growth model) and of Jackson, Gilmer, and Leamy shows good agreement with the simulation-derived segregation coefficients in most cases. The periodic stepwise growth model of Aziz is found to give less accurate values of the segregation coefficient. Although the evidence is not conclusive, the simulation results tend to favor a physical picture for solute redistribution in which the solute and substrate do not crystallize at the same rate.
Electric-field effects on the OH vibrational frequency and infrared absorption intensity for water
Authors: Hermansson Kersti
Published: July 2002

The variations of the anharmonic OH frequency and the infrared absorption intensity with field strength have been calculated for the uncoupled OH stretching vibration of a water molecule in a static, homogeneous electric field using ab initio methods at the MP4 level with a nearly saturated basis set. The OH frequency is found to be virtually independent of the field components perpendicular to the vibrating OH bond. For the parallel component, the frequency vs field curve is close to quadratic, with a maximum for a slightly negative (directed from H to O) field strength. The external field perturbation, defined as Vext(E∥,rOH)=Vtot(E∥, rOH)−Vfree(rOH), is found to be closely linear in rOH, except when the field strength E∥ is both large and negative. The linear external force constant is almost perfectly accounted for by the sum of two terms, −E∥⋅dμ∥free/drOH and −1/2⋅E∥⋅∂μ∥induced/∂rOH. These derivatives are quite insensitive to the choice of basis-set. The ∂μ∥induced/∂rOH derivative is approximately proportional to E∥, and gives rise to the arclike shape of the frequency vs field curve. The frequency maximum occurs where ∂μ∥tot/∂rOH≊0. It is the sign of dμ∥free/drOH which determines that the frequency maximum occurs at a negative field strength for water (but at a positive field strength for OH−, for example), i.e., that a frequency red-shift (blue-shift for OH−) occurs when the molecule is bound. The linear relationship between the infrared absorption intensity and frequency of the water OH vibration is derived.
The nonadditive interactions in the Ar2HF and Ar2HCl clusters: An ab initio study
Authors: Szczȩśniak M. M. , Chal/asiński G. , Piecuch P.
Published: July 2002

The three-body effects in the Ar2HX (X=F, Cl) are studied by means of the supermolecular Mo/ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. In both systems the nonadditive interactions are large and repulsive around the equilibrium geometry. The in-plane bending potential of H–F in the Ar2HF cluster reveals a double minimum with the barrier of ca. 2–3 cm−1. The barrier is due to the three-body interactions. In Ar2HCl the analogous potential has a single minimum, and the three-body effects make it shallower. The three-body interaction energy is dissected into its components such as exchange, polarization, and dispersion. The anisotropy of the total nonadditvity in Ar2HF is dominated by polarization and exchange effects, and, consequently, it can be well reproduced at the self-consistent field level of theory. The overall nonadditivity in Ar2HCl is quite similar in magnitude, but it displays a different composition. The most anisotropic is polarization followed by dispersion and exchange effects. The dispersion effect is the largest in magnitude. The long-range part of the polarization nonadditivity is analyzed via the multipole approximation. The calculations indicate that the multipole expansion carried out to the inverse 12 power of R correctly describes the anisotropy of this effect. In Ar2HF the effect of intrasystem correlation on dispersion nonadditivity is also investigated via the supermolecular coupled cluster calculations. These calculations provide an estimate of 13.8 cm−1 for the total three-body effect in the equilibrium Ar2HF trimer. This value may still be underestimated due to the basis-set unsaturation of dispersion nonadditivity.
Rotationally resolved photoionization of polyatomic hydrides: CH3, H2O, H2S, H2CO
Authors: Wiedmann Ralph T. , White Michael G. , Wang Kwanghsi , McKoy Vincent
Published: July 2002

Combined theoretical and experimental studies of rotationally resolved photoelectron spectra for single-photon ionization of the outermost valence orbitals of H2O, H2S, H2CO, and CH3 are reported. Agreement between calculated and measured spectra is very encouraging. Both show that photoionization dynamics is very molecular in origin for H2O, H2S, and H2CO but quite atomiclike for CH3. Parity selection rules and the angular momentum composition of the photoelectron are used to illustrate the dynamical aspects of photoionization of polyatomics as molecular symmetry changes in a group of structurally related systems.
Vibrational energy transfer of NO (X 2Π, v=2 and 1)
Authors: Wysong Ingrid J.
Published: July 2002

A two-laser (ir overtone pump and uv laser-induced fluorescence probe) technique has been used to measure vibrational relaxation rate coefficients for NO (X 2Π, v=2 and 1) with various collision partners at room temperature. The relaxation of v=2 by NO, H2, and NH3 has also been measured at 240 K. The relaxation of v=2 by NO is dominated by vibration-to-vibration (V–V) transfer and this process has not previously been measured below room temperature. The probability for V–V transfer increases with decreasing temperature, indicating that attractive forces play a key role in this process. Different physical mechanisms for NO vibrational energy transfer are discussed in light of the available data.
A contact cavity-biased method for grand canonical Monte Carlo simulations
Authors: Yau Danny H. L. , Liem Steven Y. , Chan Kwong-Yu
Published: July 2002

A modification of the cavity-biased grand canonical Monte Carlo (GCMC) proposed by Mezei is introduced here. Instead of on a fixed grid, test points of cavities are generated at the contact positions around the centers of existing particles. The increase in the probability or bias of finding a cavity is related to the radial distribution function and can hence be corrected. With this new cavity-biased implementation, an improved convergence to equilibrium is demonstrated and higher densities can be attained. Comparisons with the standard GCMC method, and the original cavity-biased scheme of Mezei are made.
Stabilized hydrogen on Cs-modified Al(111): Evidence for formation of alkali aluminum dihydride surface complex
Authors: Kondoh H. , Nozoye H.
Published: July 2002

The adsorption of hydrogen on Cs-modified Al(111) surfaces has been studied using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). A stabilized hydrogen species which desorbs simultaneously with Cs at 480 K (γ hydrogen) appears in the presence of Cs and the ratio between the number of Cs and that of the γ hydrogen is 1:2, independent of Cs coverage, which is indicative of a substantial attractive interaction between Cs and the γ hydrogen. HREEL spectra of the γ hydrogen exhibit intense loss peaks at around 800 and 1700 cm−1. Detailed analyses of these loss peaks using isotope mixtures (H+D) indicate that the γ hydrogen is directly bound to Al with a geminal structure (AlH2). From the combined results of TPD and HREELS, it is considered that a cesium aluminum dihydride complex (CsAlH2) is formed on the surface, which is supported by ab initio calculations.
Calorimetric measurement of catalytic surface reaction heat: CO oxidation on Pt{110}
Authors: Wartnaby C. E. , Stuck A. , Yeo Y. Y. , King D. A.
Published: July 2002

The technique of single crystal adsorption microcalorimetry has for the first time been applied to the study of a catalytic reaction, namely the oxidation of CO on a Pt{110} substrate at room temperature. By comparing the measured heat deposited in the crystal with that expected for the reaction, the excess energy removed by the desorbing CO2 molecules can be deduced. It is found that when CO is dosed onto a saturated oxygen overlayer, the product CO2 molecules remove only 9±17 kJ mol−1 more energy than expected for thermally accommodated molecules. However, when oxygen is dosed onto a CO overlayer, the product CO2 molecules remove 52±21 kJ mol−1 of excess energy. We suggest that the more highly excited CO2 molecules are formed by reaction of CO molecules with ‘‘hot’’ oxygen adatoms produced by the dissociation process and not thermally accommodated to the surface.
Theoretical study of the excitation spectra of five-membered ring compounds: Cyclopentadiene, furan, and pyrrole
Authors: Nakano Haruyuki , Tsuneda Takao , Hashimoto Tomohiro , Hirao Kimihiko
Published: July 2002

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions was applied to the study of the valence and Rydberg excited states in the range of 5–8 eV of five-membered ring compounds, cyclopentadiene, furan, and pyrrole. The spectra of these molecules have been studied extensively for many years but characterization is far from complete. The present approach can describe all kinds of excited states with the same accuracy. The calculated transition energies are in good agreement with corresponding experimental data. We were able to predict the valence and Rydberg excited states with an accuracy of 0.27 eV or better except for the B+2 of pyrrole. The valence excited states of five-membered ring compounds were interpreted in terms of the covalent minus states and ionic plus states of the alternate symmetry. The unobserved 1A1→A−1 transition with very weak intensity, which is hidden under the strong 1A1→B+2 transition, is also discussed. Overall, the present theory supports the assignments recently made by Serrano-Andres et al. based on the CASSCF plus second-order perturbation results. However, there remain some discrepancies in the assignment of the spectrum of pyrrole.
Jet-cooled molecular radicals in slit supersonic discharges: Sub-Doppler infrared studies of methyl radical
Authors: Davis Scott , Anderson David T. , Duxbury Geoffrey , Nesbitt David J.
Published: July 2002

A novel high-intensity source of jet-cooled molecular radicals is described based on the combination of (i) slit supersonic expansions with (ii) pulsed electric discharges. The electrode bias configuration effectively confines the discharge to a region upstream of the supersonic expansion, which results both in efficient rotational cooling (Trot≈25 K) and high radical densities (>1014/cm3). In conjunction with direct absorption laser probe methods, this discharge source provides a general technique for high-resolution IR studies of jet-cooled radicals. Performance of the slit discharge system is demonstrated on v=1←0 rovibrational transitions in jet-cooled OH radicals, which indicate sub-Doppler linewidths (Δν≈100 MHz) when probed along the slit expansion axis. The enhanced spectral resolution of the slit discharge geometry is utilized to probe the v3=1←0 asymmetric CH stretch vibration–rotation spectra of CH3 radical. Under sub-Doppler conditions, spin–rotation splittings are fully resolved and nuclear hyperfine splittings partially resolved in all of the transitions, permitting the first measure of Fermi contact interactions [af″=−65.5(9) MHz, εbb″=−354(5) MHz, af′=−65(2) MHz, εbb′=−353(2) MHz] and therefore both the sign and magnitude of spin-polarization effects for CH3 under isolated gas-phase conditions. The results permit direct comparison with high level ab initio calculations, and highlight a clear trend in spin-polarization effects between condensed and gas-phase behavior.
Nonequilibrium properties of linear polar Kihara fluids from molecular dynamics. Results for models and for liquid acetonitrile
Authors: Calero S. , Garzón B. , MacDowell L. G. , Lago S.
Published: July 2002

Molecular dynamics simulations for polar Kihara fluids are reported for linear models of different lengths at several dipole and quadrupole values and at three different thermodynamic states. Two of these states are close to the vapor–liquid equilibrium curve and the third one is at the same density as the first and at the same temperature as the second. Self-correlation functions and translational and orientational times are calculated and analyzed. Transport properties, diffusion, thermal conductivity, and shear viscosity are also reported and discussed in terms of multipolar forces. Correlation terms are used to calculate band broadening in different kinds of molecular spectra. Finally, it is shown how it is possible to discriminate between two models of acetonitrile that fit equilibrium properties equally well by using dynamic properties.
Noble gas temperature control of metal clusters: A molecular dynamics study
Authors: Westergren Jan , Grönbeck Henrik , Kim Seong-Gon , Tománek David
Published: July 2002

We use classical molecular dynamics simulations to investigate temperature control of unsupported clusters using a noble gas atmosphere. The simulations are performed using a many-body interaction scheme for the intra-cluster potential, while a pairwise Lennard-Jones potential is used to model the interaction between the noble gas and the clusters. In order to isolate different parameters determining the energy exchange efficiency, we have studied the energy transfer with respect to (i) impact parameter, (ii) cluster temperature, (iii) noble gas temperature, (iv) gas–metal interaction strength, (v) metal potential, and (vi) noble gas mass. With these results, we are able to estimate the number of collisions needed to equilibrate a cluster at a given gas temperature. Our estimates are confirmed by simulations of cluster cooling in a noble gas atmosphere.
Excitation energies from density functional perturbation theory
Authors: Filippi Claudia , Umrigar C. J. , Gonze Xavier
Published: July 2002

We consider two perturbative schemes to calculate excitation energies, each employing the Kohn–Sham Hamiltonian as the unperturbed system. Using accurate exchange-correlation potentials generated from essentially exact densities and their exchange components determined by a recently proposed method, we evaluate energy differences between the ground state and excited states in first-order perturbation theory for the helium, ionized lithium and beryllium atoms. It was recently observed that the zeroth-order excitations energies, simply given by the difference of the Kohn–Sham eigenvalues, almost always lie between the singlet and triplet experimental excitations energies, corrected for relativistic and finite nuclear mass effects. The first-order corrections provide about a factor of two improvement in one of the perturbative schemes but not in the other. The excitation energies within perturbation theory are found to be more accurate than the excitations obtained within ΔSCF while, for a two-electron system, they coincide with the ones obtained in time-dependent density functional theory within the single-pole approximation using our accurate static exchange-correlation potential and the time-dependent optimized effective potential kernel. We find that the agreement between the experimental and the perturbative excitation energies deteriorates significantly if potentials from approximate functionals such as the local density approximation and the optimized effective potential method are employed instead of the true Kohn–Sham potential.
Nonlinear and Hamiltonian extended irreversible thermodynamics
Authors: Grmela M. , Jou D. , Casas-Vázquez J.
Published: July 2002

Our aim is to formulate hydrodynamicslike theory for the fluids for which the classical hydrodynamics fails (e.g., polymeric fluids). In addition, we limit ourselves in this paper to the fluids for which the enlarged set of classical hydrodynamic fields, enlarged by the fields of the extra stress tensor and the extra energy flux, represent a dynamically closed set of state variables. We say, roughly speaking, that a set of state variables is dynamically closed if predictions calculated from the dynamical theory that uses this set of state variables agree, to some extent, with results of hydro- dynamicslike (rheological) observations. Examples of such fluids can be found in Jou et al., [Extended Irreversible Thermodynamics (Springer, Berlin, 1996)]. In this book the hydro- dynamicslike theory whose consequences are compared with results of observations is linear in the fields that extend the set of classical hydrodynamic fields. In this paper we extend the linear theory to a fully nonlinear theory. The additional physical insight that makes the extension possible is the requirement of a generalized Hamiltonian structure. This structure has been identified in all dynamical theories (on all levels of description, including, for example, kinetic theory) that describe the time evolution of externally unforced fluids (i.e., fluids that eventually reach equilibrium states at which they can be well described by equilibrium thermodynamics). A prominent new feature of the nonlinear theory is that the extra fields extending the set of classical hydrodynamical fields are not exactly the fields of the extra stress and the extra energy flux, but new fields from which the extra stress and the extra energy flux can always be calculated. The inverse of this map exists, however, always only in the linear case.
Ultrafast fluorescence quenching by electron transfer and fluorescence from the second excited state of a charge transfer complex as studied by femtosecond up-conversion spectroscopy
Authors: Iwai Shinichiro , Murata Shigeo , Tachiya M.
Published: July 2002

Photoinduced electron transfer dynamics between fluorescer (acceptor, A) and quencher (donor, D) was investigated by measuring the fluorescence quenching using femtosecond up-conversion spectroscopy. The measurements were made in a quencher concentration range of 0.15 M–1 M and also in a neat quencher solvent. Fluorescence decay at times longer than 5 ps can be explained by combining the diffusion equation with the Marcus equation of electron transfer. At higher quencher concentrations (>0.3 M), an additional component with a time constant of ∼250 fs appears. At these concentrations, the fluorescers (9-cyanoanthracene, CA and 9,10-dicyanoanthracene) and the quenchers (N,N-dimethylaniline, DMA) were found to form “weak” CT complexes. Fluorescence from the S1 state of the CA-DMA complex was detected by steady state spectroscopy. The excitation spectrum observed at the maximum intensity of this fluorescence indicates the existence of an excited S2 state of the CT complex near the energy of D⋅A* (the locally excited state of the pair). Excitation of CA at 400 nm leads to simultaneous excitation of the CT complex to the S2 state. It was concluded that the fast component is the fluorescence from the S2 state of the complex. This was confirmed by the concurrent rise of the S1 fluorescence of the CA-DMA complex. The fast decay of ∼250 fs is caused by the competition between the radiative transition S2→S0 and the nonradiative internal conversion S2→S1. The fast S2→S1 nonradiative transition can be regarded as a charge separation process.
Oxygen adsorption on Si(100)-2×1 via trapping-mediated and direct mechanisms
Authors: Ferguson B. A. , Reeves C. T. , Mullins C. B.
Published: July 2002

We present the results from a molecular beam study of the initial adsorption probability (S0) of O2 on Si(100)-2×1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trapping-mediated mechanism is dominant, exhibiting a strong increase in S0 with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are well-described by a simple precursor model, which gives a difference in activation barrier heights of (Ed−Ec)=28 meV, and a ratio of preexponentials νd/νc=24.2. Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in S0 with kinetic energy indicates that a direct chemisorption mechanism is active, with the observed energy scaling proportional to cos θi. There is also an unusual increase in S0 with surface temperature, with only a weak increase below 600 K, and a stronger increase above 600 K. The direct mechanism trends are discussed in terms of a possible molecular ion intermediate with thermally activated charge transfer. The molecular beam measurements are also used in calculating the reactivity of a thermalized gas with a clean surface. The precursor model is combined with a two-region fit of the direct adsorption data to predict chemisorption probabilities as a function of the incident conditions. These functions are then weighted by a Maxwell-Boltzmann distribution of incident angles and energies to calculate the adsorption probability for a thermal gas. These calculations indicate that the predominant mechanism depends strongly on temperature, with trapping-mediated chemisorption accounting for all of the adsorption at low temperatures, and direct adsorption slowly taking over at higher temperatures.
Interfacial tension behavior of binary and ternary mixtures of partially miscible Lennard-Jones fluids: A molecular dynamics simulation
Authors: Dı́az-Herrera Enrique , Alejandre José , Ramı́rez-Santiago Guillermo , Forstmann F.
Published: July 2002

By means of extensive equilibrium molecular dynamics simulations we have investigated the behavior of the interfacial tension γ of two immiscible symmetrical Lennard-Jones fluids. This quantity is studied as function of reduced temperature T*=kBT/ε in the range 0.6⩽T*⩽3.0. We find that, unlike the monotonic decay obtained for the liquid-vapor interfacial tension, for the liquid–liquid interface, γ(T) has a maximum at a specific temperature. We also investigate the effect that surfactantlike particles have on the thermodynamic as well as the structural properties of the liquid–liquid interface. It is found that γ decays monotonically as the concentration of the surfactantlike particles increases.
Phase behavior of gradient copolymers
Authors: Aksimentiev Aleksij , Hołyst Robert
Published: July 2002

Phase properties of gradient AB copolymer melts which consist of chains with the specified chemical distribution of A and B monomers have been studied within the Landau–Ginzburg model. All the melts with the linear distribution of the monomers exhibit only a direct continuous phase transition from disordered to the lamellar phase. The hexagonal, body-centered-cubic, double-gyroid (G), and lamellar-ordered structures have been found in the melts with the monotonic but nonlinear distribution of the monomers. The G structure has been also found in the gradient copolymer melts with the distribution function of monomers similar to the A–B–A triblock copolymers.