bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular-dynamics simulations of steady-state growth of ion-deposited tetrahedral amorphous carbon films
Authors: Jäger H. U. , Albe K.
Published: July 2002

Molecular-dynamics calculations were performed to simulate ion beam deposition of diamond-like carbon films. Using the computationally efficient analytical potentials of Tersoff and Brenner we are able to simulate more than 103 carbon atom impacts on {111} diamond, so that steady-state film properties can be computed and analyzed. For the Tersoff potential, we achieve sp3 fractions approximately half of the experimentally observed values. For the more refined hydrocarbon potentials of Brenner the fraction of tetrahedrally coordinated atoms is much too low, even if structures with densities close to diamond are obtained. We show, that the sp3 contents calculated with Tersoff’s potential are an artifact related to the overbinding of specific bonding configurations between three- and fourfold coordinated sites. On the other hand, we can prove, that the range for the binding orbitals represented by the cutoff function is too short in Brenner’s parametrization. If an increased C–C interaction cutoff value is chosen, we achieve a distinct improvement in modeling the sp3 content of deposited ta-C films. As a result we compute sp3 fractions which lie between 52% and 95% for the C+ ion energies E=30–80 eV and are in reasonable agreement with recent experimental studies.
Formation of Ge nanoislands using a scanning tunneling microscope
Authors: Shklyaev Alexander A. , Shibata Motoshi , Ichikawa Masakazu
Published: July 2002

Germanium islands were grown on a sample surface by accumulating atoms from the surrounding area through directional surface diffusion initiated by the electric field of a scanning tunneling microscope (STM). The Ge islands grew with a constant rate determined by the tip–sample bias voltage. The parameters of tip–sample interaction were estimated from the kinetic data for island growth by using a scaling relationship among the growth rate, the dipole moment of atoms on surfaces, and the tip–sample bias voltage. The results show that continuous atom transfer with a STM occurs with a rate significantly higher for Ge than for Si.
Structural improvement in sublimation epitaxy of 4H–SiC
Authors: Syväjärvi M. , Yakimova R. , Jacobsson H. , Janzén E.
Published: July 2002

The sublimation epitaxy growth process has been studied. The structural quality of the grown layers improves compared with the substrate mainly due to a diminished domain structure misorientation. Optical microscopy shows that the as-grown surfaces are free of typical defects appearing in silicon carbice (SiC) epitaxy, whereas atomic force microcopy measurements show macrosteps. As a possible technique to produce high-quality 4H–SiC, sublimation epitaxy was performed on substrates containing a layer grown by liquid phase epitaxy which is a growth process for closing micropipes in the initial substrate. In spite of the initial surface roughness of the liquid phase epitaxy layer, the surface morphology of the sublimation grown epilayers remained smooth and the structural quality improvement was maintained. This does not occur if the initial surfaces are too rough. A suggestion for roughness reduction is presented. The growth conditions (growth rate ramp up, growth temperature, temperature gradient, source to substrate distance, and substrate surface orientation) leading to the results are presented. A model for the mechanism for structural improvement is outlined and supporting experimental observations are given.
Dielectric screening effects on electron transport in Ga0.51In0.49P/InxGa1−xAs/GaAs quantum wells
Authors: Besikci C. , Bakir A. T. , Tanatar B.
Published: July 2002

The effects of dielectric screening on the two dimensional polar optical phonon scattering and on electron transport in Ga0.51In0.49P/InxGa1−xAs/GaAs (x=0, 0.15, and 0.25) modulation doped heterostructures and high electron mobility transistors are investigated through the ensemble Monte Carlo technique. The two dimensional polar optical phonon scattering rates including and excluding dielectric screening effects are calculated using the self-consistently evaluated electronic states in the quantum well. The calculated scattering rates are compared in order to see the effects of screening on the inter- and intra-subband scattering. Screening significantly lowers the intra-subband polar optical phonon scattering rates in both lattice matched and pseudomorphic structures. This results in a considerable lowering of the critical electric field beyond which negative differential resistance is seen. Screening also modifies the dependence of transport properties on the quantum well parameters. The results of the ensemble Monte Carlo simulations of high electron mobility transistors show that the performance of the device is considerably underestimated, if screening is not included in the calculation of the polar optical phonon scattering rates.
Pulsed laser annealing for high-efficiency thin film electroluminescent devices
Authors: Mastio E. A. , Cranton W. M. , Thomas C. B.
Published: July 2002

Thin film electroluminescent (TFEL) structures composed of ZnS:Mn (phosphor) and Y2O3 (insulator) films were annealed using pulsed (∼20 ns) KrF laser irradiations under an environment of 150 psi (∼10.34 bars) of argon. The crystallinity of both layers was studied via x-ray diffraction analysis. Their degree of lattice misfit and the integrated diffraction line intensities were examined to assess the thermal effects of pulsed laser annealing. In agreement with a thermal model of the laser–matter interaction and previous results, we suggest that the pulsed laser annealing technique improves the luminescent efficiencies of thin film electroluminescent phosphors by generating an in-depth solid-state phase transition. We report that the laser processed TFEL structure exhibits better display performance than equivalent devices thermally annealed at a temperature of 450 °C as demonstrated by sharper turn-on response and also an increase in brightness by a factor greater than four times. We attribute these performance improvements to a smaller lattice misfit value, which has been previously correlated to interface states density.
Solvent molecular dynamics in regions of hydrophobic hydration
Authors: Zichi Dominic A. , Rossky Peter J.
Published: July 2002

We present a dynamical analysis of the solvent in an aqueous solution of two nonpolar atomic solutes which are constrained to an interatomic distance corresponding to a solvent-separated free energy minimum. The results are obtained from a molecular dynamics simulation using ST2 model water. Molecular mobility for solvent near the solutes is seen to be retarded, as evidenced in translational diffusion and rotational reorientation. These slower net motions are analogous to pure solvent dynamics at a temperature reduced by 10–15 °C. An analysis of intermolecular hydrogen bonding reveals that solvation shell molecules have correspondingly longer bond half-lives compared to bulk molecules, by a factor of 1.5–2.0. The spectral densities for intermolecular vibrations are computed from translational and rotational velocity autocorrelation functions for shell and bulk motions. These densities are seen to correlate well with the local binding energy distributions.
ESR and ENDOR studies of irradiated potassium dihydrogen phosphate
Authors: Wells James W. , Budzinski Edwin , Box Harold C.
Published: July 2002

Irradiated single crystals of potassium dihydrogen phosphate were studied via ESR and ENDOR techniques in order to define phosphate group structures. A precursor cation species was produced at 4.2 K which converted upon warming to a radical previously reported for 77 K irradiation. The g values and proton ENDOR results allowed detailed models for both species to be constructed. Phosphorous ENDOR resonances were also detected and provided complete hyperfine splitting tensors for both species which reflected small internal phosphate group polarization densities.
EPR study of Fe3+ in α-quartz: A reexamination of the so-called I center
Authors: Mombourquette M. J. , Tennant W. C. , Weil J. A.
Published: July 2002

10 GHz electron paramagnetic resonance studies of Fe3+ in synthetic brown α-quartz single crystals have been carried out at 295 and at 40 K. The spin-Hamiltonian parameters were determined including high-spin Zeeman terms of form BS3 and allowing (g,D) matrix noncoaxiality as well as nonuniaxial anisotropy of the g matrix. Evaluation of the results, in part using the pseudosymmetry technique of Michoulier and Gaite, suggests that the so-called interstitial model (I center) is incorrect, and that the Fe3+ ions in fact occur substitutionally at Si4+ sites. The new label [FeO4]− is proposed for the center.
Total and dissociative photoionization cross sections of N2 from threshold to 107 eV
Authors: Samson James A. R. , Masuoka T. , Pareek P. N. , Angel G. C.
Published: July 2002

The absolute cross sections for the production of N+ and N+2 have been measured from the dissociative ionization threshold to 115 Å. In addition, the absolute photoabsorption and photoionization cross sections are tabulated between 114 and 796 Å. The ionization efficiencies are also given at several discrete wavelengths between 660 and 790 Å. The production of N+ fragment ions are discussed in terms of the doubly excited N+2 states with binding energies in the range 24 to 44 eV.
Interpretation of the temperature dependent g values of the Cu(H2O)2+6 ion in several host lattices using a dynamic vibronic coupling model
Authors: Riley Mark J. , Hitchman Michael A. , Mohammed Amisa Wan
Published: July 2002

The causes of the previously reported temperature dependence of the g values of the Cu(H2O)2+6 ions in Cu2+ doped Zn(H2O)6(GeF6) and the Tutton’s salts M2Zn(H2O)6(SO4)2, where M=K+, Rb+, NH+4, and Cs+, supplemented by new experimental measurements on the K+ salt, have been investigated. The ground state dynamics of the complexes have been modeled on the cubic E×ε Jahn–Teller Hamiltonian perturbed by an orthorhombic lattice strain. For each compound, the vibronic energy levels and associated wave functions were calculated numerically, the overall g values at any temperature being given by a thermal average of the g values of the individual vibronic energy levels, because of rapid exchange between the levels. For the Tutton’s salts it was found that the low temperature g values are strongly influenced by the tetragonal component of the lattice strain, with this corresponding to an axial compression of the ligand field. The temperature dependence of the g tensors, on the other hand, was found to depend largely on the orthorhombic component of the lattice strain. For the K+ and NH+4 salts, where structural data are available, the strain parameters derived using the model are in good agreement with the geometries reported for the Zn(H2O)2+6 host complexes. For Cu2+ doped Zn(H2O)6(GeF6) the model implies a lattice strain of tetragonal symmetry corresponding to a slight elongation of the axial metal–ligand bonds. The results are compared with those reported previously by Silver and Getz, and other workers, who used a simple model involving temperature dependent equilibria between energy levels corresponding to different orientations of the Cu(H2O)2+6 ions in the host lattices to interpret the temperature dependence of the g tensors.
Theory of quantum beats in time-resolved multiphoton ionization of molecules
Authors: Nomura Y. , Fujimura Y. , Kono H.
Published: July 2002

An expression for the probability of time-resolved three-photon ionization via coherently excited resonant states of a molecular system is derived in the perturbative density matrix formalism with the aid of the Liouville space Feynman diagram. It is shown how the time evolution of the molecular coherence in the first resonant state can be detected as a function of the delay time between the pumping and probing lasers through the probing two-photon ionization process. The three-photon ionization process consists of the simultaneous and sequential processes which are classified by the Liouville space Feynman diagrams. The possibility of appearance of quantum beats in the pump–probe three-photon ionization is discussed by performing model calculations of the probability of ionization. The role of the direct and redistributed processes is discussed. The redistributed process is induced by intramolecular processes in the resonant state. The theoretical treatment is applied to the (1+2) three-photon ionization of pyrazine observed by Knee et al. It is demonstrated that the fast decay component (110 ps) of the pyrazine spectra originates from the intramolecular dephasing of the vibronic coherence created by the pumping pulse laser, and this component appears only when the redistributed process (the ionization process through triplet levels) is absent.
The analysis of intensity fluctuations for a fully resolved spectrum: Pyrazine
Authors: Kommandeur J. , Meerts W. L. , Engel Y. M. , Levine R. D.
Published: July 2002

Intensity fluctuations in the recently published fluorescence excitation spectrum of pyrazine in the 1B3u←1Ag 000 band are analyzed. A numerical study of a model Hamiltonian of a bright state coupled to a manifold of dark states is performed as an aid to the interpretation of the results. The distribution of fluctuations in the intensities computed for the model Hamiltonian is near to that expected in the chaotic limit.
Solvation dynamics in polar liquids
Authors: Rips Ilya , Klafter Joseph , Jortner Joshua
Published: July 2002

Ion and dipole solvation kinetics is studied within the framework of the mean-spherical approximation (MSA). The solvation dynamics in nonassociated polar solvents involves a hierarchy of relaxation times in accordance with Onsager’s ‘‘inverted snowball’’ picture. The average solvation time is determined by the relative solvent and solute sizes and by the dynamic screening. The dipole solvation is slower than ion solvation due to the shorter spatial range of interaction. The theoretical results are confronted with experimental data on time-resolved fluorescence shifts of dipolar probe molecules in nonassociated polar solvents. The experimental kinetic data exhibit a crossover from a short-time dipole solvation behavior to ion solvation at intermediate and long times. For associated polar solvents the Onsager picture has to be modified to account for structure breaking in the first solvation shell(s).
Photochemistry with circularly polarized light: A new method
Authors: Rubalcava Hector , Fitzmaurice Donald J.
Published: July 2002

A new method is presented which uses photoelastically modulated elliptically polarized light in chemical kinetics. The method exploits the fact that in a polarimeter using photoelastic modulation the chirality of the light is fixed and its average polarization state is about 50% circularly polarized. This method makes possible the partial resolution and racemization rate measurement of rapidly inverting molecules dissolved in achiral solvents. The necessary theory is developed, and experimental evidence supporting the theory is given. Results agree with published values for the relatively slow racemization of the cation oxolatobis (1,10-phenanthroline) chromate (III). As an advance in the measurement of faster inversion reactions, combined thermal- and photochemical racemization of rhodamine 6G was measured between 10 and 1000 ms: Preliminary results indicate a thermal relaxation time of about 6 s at 15° for the dye in an ethanol solution.
Spectroscopic observation of the b 1Σ+→X̃ 3Σ− transition of AsH
Authors: Arens M. , Richter W.
Published: July 2002

Emission lines of the b 1∑+→X̃ 3∑− transition of AsH radicals have been detected in the fluorescence of a dc-glow-discharge of arsine in hydrogen. From measurements of line positions of the (0,0), (1,1), and (2,2) Q branches and the (0,0) P and R branches, the molecular constants of the b 1∑+ state were determined: Te=14 178.0 cm−1, B0=7.2467 cm−1, D0=3.1528⋅10−4  cm−1, ωe=2213 cm−1, ωexe=47.5 cm−1, re=152.937 pm.
On the uniqueness of the Reverse Monte Carlo simulation. I. Simple liquids, partial radial distribution functions
Authors: Pusztai László , Tóth Gergely
Published: July 2002

Partial radial distribution functions of standard Monte Carlo simulations were used as input for Reverse Monte Carlo simulation, a novel method for structural modeling. From detailed comparison of the two independent (MC and RMC) particle configurations it has turned out that sufficiently close agreement in radial distribution functions involves deeper structural similarities. RMC results, i.e., particle configurations produced by RMC upon the basis of partial radial distribution functions, are therefore applicable for detailed analysis of the structure, giving new prospects for data evaluation in neutron diffraction method. As a result of extensive Reverse Monte Carlo simulations of this work some new aspects of the technique itself could be revealed.
The far infrared spectrum of C3O2
Authors: Vander Auwera J. , Johns J. W. C. , Polyansky O. L.
Published: July 2002

The far infrared spectrum of carbon suboxide, 12C316O2, has been recorded at a resolution of 0.003 cm−1 in the region from 15 to 130 cm−1 with a Bomem interferometer. Altogether 18 bands involving the excitation of up to 8 quanta in the low frequency quasilinear bending mode ν7 were analyzed rotationally. A total of 1320 far infrared lines together with 231 microwave lines taken from Karyakin et al. [J. Mol. Spectrosc. 94, 283 (1982)] have been fitted to an effective model. Improved effective constants have been obtained for all the l sublevels of v7=0 to 4 and for l=1 of v7=5. Constants have been obtained for the first time for the levels 53 and 55 and some of the l sublevels of v7=6, 7, and 8. Data for the lower lying levels were fitted to a realistic linear model and some data were also fitted to a bent model.
Time-dependent reactive scattering in the presence of narrow resonances: Avoiding long propagation times
Authors: Neuhauser Daniel
Published: July 2002

Time-dependent scattering is extended to systems possessing narrow resonances. At short times the wave function is integrated directly, and at late times the wave function is expanded in terms of the slowly decaying (complex) resonance eigenfunctions of the Hamiltonian, Ψ(X,t)≂Σn ane−iεnt−Γ nt/2Φn(X). The slowly decaying eigenfunctions are easily found via a short-time filterization approach adapted from bound-state studies, in which a random wave packet is filtered at various energies and the resulting vectors are then diagonalized. The method is exemplified for collinear reactions of H+H2, where it halves the propagation time.
A model of the B 1Σ+–D ′ 1Σ+ Rydberg–valence predissociating interaction in the CO molecule
Authors: Tchang-Brillet W.-Ü L. , Julienne P. S. , Robbe J.-M. , Letzelter C. , Rostas F.
Published: July 2002

The strong interaction between the B 3sσ 1Σ+ Rydberg state and the D′ 1Σ+ valence state of the CO molecule is shown to cause large changes in the vibrational and rotational constants of the B state, as well as predissociation of all rotational levels of B (v′=2) and a breaking off in the emission of B (v′=1) levels at J=36 in 12C 16O and J=37 in 13C 16O. A two-state diabatic model of the Rydberg–valence interaction is constructed and vibrational term values, widths, and intensities are calculated by close coupling in order to account for the strong mixing. The model separates the differences between the spectroscopic constants of the B state and those of the ground state molecular ion into two components, one due to the R-dependent quantum defect of the B state and another due to the strong Rydberg–valence perturbation. The perturbation is characterized by a constant coupling matrix element of 2900 cm−1 inside the crossing point of the two diabatic potentials, decaying to zero at long internuclear distances. Basically good agreement is found between the model and experiment for shifts in vibrational and rotational terms and for predissociation widths and relative band intensities. The second breaking off in emission in the B (v′=1) rotational series is used to estimate the height of the long-range barrier maximum in the D′ 1Σ+ state to be about 1048±19 cm−1 above the ground state dissociation limit. Comparison of predicted widths from the two channel close coupled model with those from a single channel adiabatic model shows differences on the order of a factor of 2.
Current and eigenvector analyses of chemical reaction networks at Hopf bifurcations
Authors: Hynne F. , Graae So/rensen P. , Mo/ller T.
Published: July 2002

We show how quenching data can be combined with extreme current representation of stationary states for systematic analysis and optimization of models of oscillatory chemical reactions at a Hopf bifurcation. The method is global: it considers all possible models based on a given reaction network, and does not rely on continuation techniques; all possible stationary states can be directly and systematically generated, and Hopf bifurcations can be compared with experimental (quenching) data at one or more operating points. It avoids any integration of kinetic equations. Therefore it is fast and a dense subset of all Hopf bifurcation points can be considered in a search. In this sense the method is complete. Experimentally known stationary concentrations can be fixed and reduce the dimension of the space to be searched. A scaling principle lets all Hopf bifurcation points of the current cone be treated in terms of corresponding points of the current polytope. It admits of a simple, direct calculation of points that match a given (experimental) oscillation frequency. A graphical method of testing the compatibility of a reaction network with a set of experimental quenching data may help selecting possible networks for systematic optimization. The method exploits the eigenvector property of quenching amplitudes. The paper develops the two methods; an accompanying paper applies them to two specific networks for the Belousov–Zhabotinsky reaction.
Ab initio study of intermolecular potential for dimers XO–HE (X=N,C)
Authors: Zolotoukhina Tanya N. , Kotake Susumu
Published: July 2002

The supermolecular MPPT approximation is applied to the study of the interaction energy of CO–He and NO–He dimers. The 6–311+G basis set with the addition of Sadlej’ polarization functions is shown to be appropriate for describing the spatial structure of the intermolecular potential. The necessity of the CP procedure is confirmed, and it is used to obtain correct values of the interaction energy at the MP4 level of theory. For both dimers, only one minimum of the potential surface is found in the case of the collinear spatial configuration: −0.419 kcal/mol for CO–He and −0.491 kcal/mol for NO–He. The influence of the one electron open shell at the NO–He dimer in comparison with the CO–He is estimated to be associated with a 15% increase in the interaction energy in the medium distance region.
Ab initio electron density distributions in molecules containing sulfur–sulfur bonds
Authors: Brown Anthony S. , Smith Vedene H.
Published: July 2002

The electron density distribution in H2S2, S2, S3, S4, S6, S8, 2,5-dimethyl-6a-thiathiophthene (DMT), tetramethylthiuram disulfide (TMTD), and S4N4 are examined using ab initio linear combination atomic orbital–molecular orbital methods. The inclusion of polarization functions in the basis set is found to be necessary to give a satisfactory description of the electron density distribution in these molecules. The effects of electron correlation on the electron density distribution in H2S2, S2, and S3 are assessed using configuration interaction, and are found to be less important than the use of polarization functions in the basis set. Comparisons of theoretical and experimental electron density distribution maps are made for S8, DMT, TMTD, and S4N4. The deformation density in sulfur–sulfur bonds is found to have a single peak at the bond midpoint, in contrast to the complicated double-maximum distribution commonly observed in experimental studies. The presence of deformation density peaks between across-the-ring sulfur atoms in S4N4 is confirmed, but a peak at the center of the molecule is not.
The polarized density of states of crystalline polyacetylene. Molecular dynamics analysis and comparison with neutron scattering results
Authors: Dianoux A. J. , Kneller G. R. , Sauvajol J. L. , Smith J. C.
Published: July 2002

The low-frequency (<20 meV) polarized vibrational density of states in crystalline polyacetylene has recently been derived from inelastic neutron scattering data [J. L. Sauvajol, D. Djurado, A. J. Dianoux, N. Theophilou, and J. E. Fischer, Phys. Rev. B 43, 14 305 (1991)]. The spectral profiles were found to be highly anisotropic and to vary considerably with the chain conformation (cis or trans). Here we investigate the characteristics of the low-frequency vibrations using the molecular dynamics method. Densities of states are calculated from the simulations and directly compared with the data derived from the experiments. The anisotropic and conformation-dependent profiles are mostly well reproduced. The simulation trajectories are further analyzed so as to determine the dynamical contributions to the observed spectra. The lowest-frequency vibrations parallel to the chain axes, at 1.5 meV in the cis system and 4 meV in the trans system, are found to result from whole-molecule rigid-body translations, invariant with chain length. Conversely, the low-frequency intramolecular vibrations are weakly dependent on chain conformation but vary strongly with the chain length.
The reaction of Hg(6 3P1) with hydrogen molecules studied by electron spin resonance and laser induced fluorescence
Authors: Yi Whikun , Satyapal Sunita , Shafer Neil , Bersohn Richard
Published: July 2002

Mercury atoms were excited to the 6 3P1 state with a pulsed laser and a lamp at 253.7 nm in the presence of H2, D2, HD, and mixtures of these gases. The hydrogen atom reaction products were detected by vacuum ultraviolet (VUV) laser induced fluorescence. The ratio of yields of the two body channel (mercury hydride and a hydrogen atom) and the three body channel [Hg(6 1S0) and two hydrogen atoms] were determined by combining the H/D atom ratios and a literature value of the HgD/HgH ratio from reaction with HD. Assuming that the sum of the two yields is unity, the yield of the two body channel is for H2 0.63±0.15, for D2 0.79±0.11, and for HD 0.10±0.02 (HgH+D) and 0.65±0.03 (HgD+H). The average kinetic energies calculated by combining these yields with the literature data on internal energy release in the mercury hydride molecules agree with the average kinetic energies measured from the Doppler broadened fluorescence excitation curves. When the E vector of the exciting polarized light was rotated there was no effect on the hydrogen atom spectrum proving that the hydrogen atom velocity distribution is isotropic. When the mercury atoms were excited with circularly polarized light to the J=1, MJ=1 state, the hydrogen atoms exhibited no spin polarization. Electron spin resonance (ESR) signals observed when irradiating a mixture of Hg and hydrogen molecules were shown by isotopic effects to be due to hydrogen atoms dissociated from mercury hydride molecules.
The calculation of molecular response properties using perturbed spin-coupled wave functions. I. Basic theory
Authors: Hyams Peter A. , Gerratt Joseph , Cooper David L. , Raimondi Mario
Published: July 2002

A fully coupled perturbed formalism is developed for spin-coupled wave functions for the calculation of electric and magnetic response properties of molecules, particularly as functions of the nuclear geometry. The theory is specialized to the case of polarizabilities and magnetizabilities, but is easily generalized to include gradients and second derivatives with respect to nuclear coordinates and to spin-dependent properties. For polarizabilities and similar properties, for which the perturbation is purely real, the main quantity required is the Hessian matrix. This is in any case calculated in the stabilized Newton–Raphson method during the process of minimizing the energy. For magnetic properties, or for those properties for which the perturbation is purely imaginary, a modified Hessian is needed. This last is quickly assembled from the same quantities (first-, second-, third-, and fourth-order density matrices) that are computed for the normal Hessian.
The calculation of molecular response properties using perturbed spin-coupled wave functions. II. Polarizability and magnetic susceptibility of H2 and LiH as functions of internuclear distance
Authors: Hyams Peter A. , Gerratt Joseph , Cooper David L. , Raimondi Mario
Published: July 2002

The perturbed spin-coupled formalism of the previous paper is applied to the calculation of the polarizabilities and magnetic susceptibilities of H2 and LiH. These are the first applications of the theory and as such much attention is paid to the choice of basis set, particularly as a function of internuclear distance. For H2, the results can be compared with the accurate calculations of Rychlewski [Mol. Phys. 41, 833 (1980)] and of Rychlewski and Raynes [Mol. Phys. 41, 843 (1980)]. The agreement with these is good, provided that the basis set is adequate. For LiH, there has, to our knowledge, not been any calculation of these properties as a function of R. However the value of α at large R is in excellent agreement with the value of α(Li)+α(H). A remarkable feature in LiH is that the value of αzz at ∼7a0 is ten times that for R=Re. In general we find that the determination of magnetic susceptibilities as a function of R forms a very severe test of the completeness of the basis set.
Monte Carlo treatment of the nonradiative energy transfer process for nonrandom placements of dopants in solids
Authors: Barbosa-Garcia Oracio , Struck Charles W.
Published: July 2002

The nonradiative energy transfer process from donor-to-acceptor ions is simulated for the garnet lattice using the Monte Carlo (MC) method. The probabilities of the events which occur after a donor is excited are calculated, i.e., the donor and acceptor emission transients. Two different simulation results are reported. One is obtained under the Forster and Dexter (FD) assumptions—dopants are randomly distributed in the crystal, no donor-to-donor and no acceptor-to-donor transfers occur, and the transfer is proportional to (1/R)s with s a unique integer. The second is obtained by replacing the random spatial distribution of dopants in the FD model by a nonrandom distribution. The nonrandom placements result from a short-range interaction between donors and acceptors which may be attractive or repulsive. For both distributions, the FD assumptions that s is a unique integer is relieved and transients are obtained for an arbitrary multipolar expression. The FD model was found to give a rather good approximation to the donor emission transient determinated by the MC simulations for the FD assumptions. The donor luminescence decay is faster for an attractive interaction between donors and acceptors than for the random distribution. It is slower for a repulsive interaction. Using the arbitrary multipolar expression and using random and nonrandom spatial distributions of dopants give distinguishably different decay transients. However, better discrimination among causes for some particular given transient is afforded by using different dopant levels.
The existence of nonlinear resonances in collective modes of linear macromolecules
Authors: Wozny C. E. , Sumpter B. G. , Noid D. W.
Published: July 2002

A Lissajous figure and surfaces of section are presented which show a 4:7 nonlinear resonance between two selectively excited coordinates of a system containing 900 degrees of freedom. The significance of this work is; first, plots of this nature have never before been constructed for a multidimensional system this large; and second, the nonlinear resonance exists between two collective modes of the linear macromolecule.
Identification of the core hole excited states of N2: A failure of the equivalent core approximation
Authors: Lee Kaidee , Kim Dae Young , Ma Chien-I , Hanson David M.
Published: July 2002

Monochromatic soft x-rays were used to excite the nitrogen molecule at resonances near the K-ionization edge. The absorption anisotropy parameter β was determined from the simultaneous measurement of the photofragment flux emanating from the x-ray/molecular-beam interaction region along two axes, one parallel and the other perpendicular to the electric vector of the polarized synchrotron radiation. The value of the anisotropy parameter identifies the symmetry species of a resonance and allows the core-hole excited states to be assigned from experimental data. These data force a new assignment of Rydberg states and reveal a failure of the equivalent core approximation.
Accurate spectroscopic constants for the Cd(1S,3P,1P)–H2 van der Waals complexes: A theoretical study
Authors: Ramírez-Solís Alejandro , Castillo A. Sidonio
Published: July 2002

Very accurate interaction energies and equilibrium geometries are obtained for the C2v Cd(1S, 3P, 1P)–H2 van der Waals (VDW) complexes using extensive ab initio multireference configuration interaction plus second order multireference Möller–Plesset calculations. Gaussian basis sets are of triple-ζ plus polarization quality and a relativistic effective core potential for cadmium has been used. For the interaction energies the Boys and Bernardi counterpoise (CP) correction has been used. The ground state Cd(1S)–H2 van der Waals complex was found to be very weakly bound with a 108 cm−1 well at 8.4 a.u., as compared with the estimated experimental values of De ≂50 cm−1 at 8.3 a.u. The computed (De=1580 cm−1, Re=4.9 a.u.) constants for the 1B1 Cd(1 P)–H2 exciplex are in good agreement with the recent experimentally derived values (De=1713 cm−1, Re=4.7 a.u.) obtained by Wallace et al. [J. Chem. Phys. 97, 3135 (1992)]. The well depth and equilibrium geometry for the 3B1 Cd (3P) –H2 exciplex were computed as 420 cm−1 and 5.7 a.u. while those of the 3B2 exciplex are 650 cm−1 and 4.75 a.u. The 1B2 surface did not lead to a VDW complex but instead it produced a strongly bound (De≂10 kcal/mol) bent H–Cd–H molecule responsible for the experimentally observed CdH+H and Cd+H+H reaction channels. The 3,1A1 states arising from the Cd(3,1P)+H2 asymptotes were found to be totally repulsive. In all cases the CP correction was a non-negligible fraction of the interaction energy. Finally, the energetic position and the geometry of the 1B1/3A1 surface crossing, crucial to explain the observed Cd(1B1,v′)–H2 predissociation lifetimes, are accurately determined.
Interaction of water with metal surfaces
Authors: Zhu Sheng-Bai , Philpott Michael R.
Published: July 2002

A new class of potential suitable for modeling the adsorption of water on different metal sites is described. The new potentials are simple in form and convenient for use in computer simulations. In their real space form they comprise three parts: A pairwise sum of spatially anisotropic 12-6 potentials, a pairwise sum of isotropic short range potentials, and an image potential. Two modifications of the potential are developed. In the first, the anisotropic potential acts only on the oxygen atom and not on the protons. In the second, the potential acts on all the atoms of the water molecule. In practical calculations it is convenient to transform the potential to a reciprocal space form in the manner described by Steele [Surf. Sci. 36, 317 (1973)]. Adsorption of water at top, bridge, and hollow sites on (100), (110), and (111) surfaces of Pt, Ni, Cu, and Al were studied using two fitting parameters and the results compared with previous theoretical calculations.
Divalent manganese in two different tetragonal sites of Cs2NaLaCl6
Authors: Quintanar C. , Gleason R. J. , Boldú J. L. , Muñoz P. E.
Published: July 2002

Electron paramagnetic resonance studies performed at room temperature in Mn2+ doped Cs2NaLaCl6 single crystals (elpasolite type) showed that the Mn2+ ion occupies two different tetragonal symmetry sites in a cubic crystalline lattice. Both sites possess principal Z-axes along 〈100〉 crystallographic directions with large splittings (≊1.0 cm−1 and ≊0.8 cm−1) of the spectra. Several facts are taken into account in order to establish tentative models for the structural configuration of the two sites.
Collective properties of adsorption–desorption processes
Authors: Krapivsky P. L. , Ben-Naim E.
Published: July 2002

A reversible adsorption–desorption parking process in one dimension is studied. An exact solution for the equilibrium properties is obtained. The coverage near saturation depends logarithmically on the ratio between the adsorption rate, k+, and the desorption rate, k−, ρeq≂1−1/log(k+/k−), when k+/k−≫1. A time dependent version of the reversible problem with immediate adsorption (k+=∞) is also considered. Both heuristic arguments and numerical simulations reveal a logarithmically slow approach to the completely covered state, 1−ρ(t)∼1/log(t).
Quasidiabatic states for intramolecular charge transfer. Application to the protonation of NH3
Authors: Petsalakis I. D. , Theodorakopoulos G. , Nicolaides C. A.
Published: July 2002

We have constructed ab initio adiabatic and quasidiabatic surfaces for the description of the protonation of NH3. For the diabatic states, we applied a recently developed method which is based on the propagation along the reaction coordinate, starting from the dissociation region, of the maximized nonorthonormal overlap between diabatic states calculated at successive geometries. In agreement with earlier calculations [Kaldor et al., J. Chem. Phys. 90, 6395 (1989)], it was found that the adiabatic surfaces cannot explain this charge-transfer process. On the contrary, a single diabatic potential curve correlates smoothly the ground state of NH+4 with the NH3+H+ limit.
Overtone and hot-band assignments in the 2{ν3, ν6} levels of CHD3
Authors: Roche Ch. , Champion J.-P. , Coy S. L. , Klaassen J. J. , Steinfeld J. I. , Valentin A. , Mizugai Y. , Johns J. W. C.
Published: July 2002

Assignments have been carried out for the 2ν3/(ν3+ν6)/2ν6 (triad)←ground state overtone bands and the triad←ν3/ν6 hot-band system of CHD3. A vibrational extrapolation method, using correspondences between a spherical-tensor formalism and the more conventional description of symmetric-rotor spectra, was used to facilitate the assignment and analysis of these bands. The resulting assignments have been confirmed by time-resolved double-resonance measurements.
Generalized Korteweg equations for phase separation near the critical point in binary gas–liquid mixtures
Authors: Meijer Paul H. E.
Published: July 2002

This paper deals with the definition of the order parameter in a binary gas–liquid system in the neighborhood of the critical point. The isothermal volume-composition (or V,x) diagram of a binary gas–liquid system may show one (or more) critical point(s). If a critical point is present, we determine the orientation of the tieline in the neighborhood of this point from the equation of state, using algebraic means. This orientation is needed to determine the proposed order parameter: the length of the tieline. The method is similar to the method based on the work of Korteweg, however, his calculation was restricted to the special case that the critical point lies close to the critical point of the pure component. Consequently, the character of such a critical point is mainly vapor–liquid-like in his work. For this reason Korteweg’s approach deals with a special situation and cannot be applied to the general case in which the phase separation is neither pure liquid–vapor, nor exclusively liquid–liquid, i.e., without vapor. The present method is based on a coordinate rotation in the V,x plane. It is shown that the end points of the tieline for a given pressure can be expressed in terms of the Taylor series coefficients of the equation of state evaluated around the critical point.
Viscoelasticity of lyotropic smectics
Authors: Lu C.-Y. D. , Cates M. E.
Published: July 2002

We study theoretically the contribution of thermal fluctuations to the linear rheological spectrum of lyotropic smectic A phases at low frequencies. The fluctuations are treated at the Gaussian level; their hydrodynamics is controlled by the undulation/baroclinic mode. In the layer-over-layer sliding geometry, we find an anomalous elasticity varying as ‖ω‖; the corresponding viscous drag is enhanced logarithmically. For other shear geometries we find contributions to the dissipation and elasticity that are in practice small (though formally divergent when the layer spacing is taken to zero). The connection between these results and those given previously for thermotropic smectics, and for block copolymers, is discussed. We outline the possible experimental significance of our findings for the rheology of both aligned and polydomain smectics.
Angular and velocity distributions of desorbing product carbon dioxide from two reaction sites on platinum(110)(1×2)
Authors: Ohno Yuichi , Matsushima Tatsuo , Uetsuka Hiroshi
Published: July 2002

The angular and velocity distributions of desorbing product CO2 were studied on a platinum(110)(1×2) surface over a wide range of CO coverages by means of angle-resolved thermal desorption combined with a cross-correlation time-of-flight (TOF) technique. Heating the coadsorption layer of CO and oxygen yields four CO2 formation peaks P1–CO2 (∼400 K), P2–CO2 (300 K), P3–CO2 (250 K), and P4–CO2 (170 K)]. The angular distribution of each CO2 produced at high CO coverages consists of three desorption components. Two of them show desorption collimated along the inclined terrace normal; the other shows it along the bulk surface normal. The former is assigned to reaction on the inclined terrace, and the latter mostly to reaction on the bottom of the trough. The translational temperature of each desorption component is derived by deconvoluting the TOF spectrum. This temperature reaches 1000–1500 K. The maximum translational temperature is always observed in the normal direction of each reaction site. For P2–and P3–CO2, the maximum translational temperature of the normal-directional component is lower than that of the inclined components. For P4–CO2, on the other hand, the maximum translational temperature of the normal-directional component is close to that of the inclined components. A transfer mechanism of reaction sites is proposed.
Nascent internal state distributions of ZnH(X 2Σ+) produced in the reactions of Zn(4 1P1) with some alkane hydrocarbons
Authors: Umemoto Hironobu , Tsunashima Shigeru , Ikeda Hiroyuki , Takano Kazuto , Kuwahara Kazuya , Sato Kei , Yokoyama Keiichi , Misaizu Fuminori , Fuke Kiyokazu
Published: July 2002

The reactions of Zn(4 1P1) with CH4, C2H6, C3H8, and C(CH3)4 were studied by employing a laser pump-and-probe technique. The nascent rotational and vibrational state distributions of ZnH(X 2Σ+) were determined. These distributions were compared with those predicted by statistical models. The distributions for C(CH3)4 resembled to the statistical ones, while those for simple alkanes such as CH4 were a little hotter than the statistical ones. These results suggest that the reaction proceeds via a relatively long-lived insertive complex. There was no great difference in the production yields of ZnH, although that for CH4 was the largest.
Transient electric birefringence in solutions of self-assembled rods
Authors: van der Schoot Paul , Cates Michael E.
Published: July 2002

We study, theoretically, the transient electric birefringence of semidilute and concentrated solutions of self-assembling rodlike aggregates in the fast reaction regime. The aggregates interact via a hard core excluded volume potential. We find that although the rods are very polydisperse, the zero-field relaxation is characterized by at most three exponentials depending on whether or not the assemblies have a permanent dipole moment. The static response, and one of the relaxation times, diverges at the nematic spinodal.
The effect of methane on photocurrents generated by exciting xenon below its first ionization potential
Authors: Steinberger I. T. , Krasinsky A. , Shechter R. , Gürtler P. , Meyer J.
Published: July 2002

Photocurrent excitation spectra of xenon and of the xenon–methane absorber–perturber system were studied in cells equipped with LiF windows cooled to ≊−30 °C. The cooling was needed to shift the LiF cutoff so as to enable measurements at photon energies up to the first ionization potential of Xe (12.13 eV). The spectra observed in methane–xenon mixtures were essentially similar to those in neat xenon, demonstrating that the Hornbeck–Molnar process (excitation of a xenon atom–excimer formation–ionization of an excimer) is feasible even in the presence of the molecular perturber CH4 at pressures up to ≊ 105 Pa. The primary effect of CH4 is to decrease the photocurrent, since photons in the energy range in question are absorbed by the methane molecules as well, without photoionizing them. The dependence of the current on methane concentration showed that the above mechanism is not the only one by which methane quenches the current. Moreover, the degree of quenching of the current by methane is dependent on the photon energy. The analysis of this dependence led to the assumption that an excited xenon atom and a methane molecule may form an excimer. Previous results on the Ar:Xe and Kr:Xe systems support this assumption. Peak inversions observed at high pressures in neat xenon, but absent in the CH4:Xe system, are discussed on the basis of prevailing models.
Effect of three-body forces on the shear viscosity of liquid argon
Authors: Lee Song Hi , Cummings Peter T.
Published: July 2002

We present nonequilibrium molecular dynamics simulations of pure argon at a temperature of 135 K and density of 1.034 gm/cc using the highly accurate Barker–Fisher–Watts (BFW) model for the intermolecular potential. We consider the BFW potential both with and without the Axilrod–Teller three-body interaction. We find that the three-body potential has a very small effect on the shear viscosity: The predicted value in the presence of three-body forces is 3% less than that obtained without three-body forces.
Molecular dynamics of β-carotene in solution by resonance enhanced optical Kerr effect
Authors: Ricci Marilena , Torre Renato , Foggi Paolo , Kamalov Valey , Righini Roberto
Published: July 2002

The orientational dynamics of β-carotene in n-alkane solutions is investigated by resonance enhanced optical Kerr effect. By use of this spectroscopic technique, it is possible to selectively investigate the relaxation of a probe molecule at a concentration level low enough to allow the observation of the averaged single-molecule dynamics. For delay times longer than ∼20 ps all solutions show a single exponential decay, with a time constant depending on the viscosity, that is ascribed to the β-carotene orientational relaxation. The dependence on viscosity of the measured relaxation times is compared with the predictions of different models. The purely hydrodynamic theories overestimate, by far, the solute effective volume and hence its orientational relaxation time; a much better agreement is obtained from two quasihydrodynamic models.
Electronic structure for the configurations nd
k+1(n+1)s0(n+1)p1, nd
k(n+1)s1(n+1)p1, and nd
k−1(n+1)s2(n+1)p1 (k=1 to 10) in the first and second row of transition metals
Authors: García de la Vega J. M.
Published: July 2002

Systematic numerical Hartree–Fock results for the lower states of the configurations ndk+1(n+1)s0(n+1)p1, ndk(n+1)s1(n+1)p1, and ndk−1(n+1)s2(n+1)p1 corresponding to the neutral atoms of the first and second row of transition metals are reported. Average energies of each configuration and LS energies of the corresponding lowest states are presented and discussed. The excitation energies and the relative orbital sizes for these states are also examined.