bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron correlation and charge transfer at the Ni/Co interface
Authors: Dhesi S. S. , Dudzik E. , Dürr H. A. , van der Laan G. , Brookes N. B.
Published: July 2002

The evolving magnetism and electronic structure at the Ni/Co interface have been studied using x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS) with circularly polarized x rays. Deposition of ultrathin Ni films on thin films of Co grown on Cu(001) results in an intensity enhancement across the Co L2.3 absorption edge. By comparison, the intensity of the Ni L2.3 edge decreases as a function of Ni film thickness. The relative changes in the Ni and Co XAS intensities are interpreted as an electronic charge transfer from the Co to the Ni. Distinct changes in the Co 2p XAS and XPS line shapes after addition of the Ni overlayer imply a modification of the Co 3d electron correlation due to the charge transfer. The change in the electronic structure is related to the interface magnetism using magnetic circular dichroism sum rule analysis.
Computational studies of the magnetic permeability of composite soft materials
Authors: Walmsley N. S. , Chantrell R. W. , Gore J. G. , Maylin M. G.
Published: July 2002

The magnetic permeability of composite soft materials is investigated using a numerical model. The model has been specifically developed to study the effect of intergranular interactions. The magnetic properties of densely packed nanocomposite granular are significantly affected by interactions between the grains. In particular, we focus on the effect of grain shape and microstructure on the permeability. The influence of packing density on the initial susceptibility is investigated.
Experimental testing of the Preisach modeling of superconducting hysteresis
Authors: Mayergoyz I. D. , Adly A. A. , Huang M. W. , Krafft C.
Published: July 2002

Extensive experimental testing of the Preisach modeling of superconducting hysteresis is reported. This testing demonstrates the remarkable accuracy of the Preisach model in predicting various branches of superconducting hysteresis for various past histories and in the wide range of temperatures.
Spin-glass behavior in ultrafine La0.7Ca0.3MnOz powders prepared by mechanical alloying
Authors: Muroi M. , Street R. , McCormick P. G.
Published: July 2002

Ultrafine La0.7Ca0.3MnOz powders with controlled oxygen stoichiometry have been synthesized by mechanical alloying at ambient temperature. It is found that high-energy ball milling of La2O3, CaO, MnO2, and Mn3O4, mixed in the stoichiometric cation ratio, yields single-phase La0.7Ca0.3MnOz powders having crystallite sizes of about 10 nm and various oxygen stoichiometries (2.68⩽z⩽3.35), adjustable by changing the MnO2/Mn3O4 ratio. The spontaneous magnetization (Ms) of the as-milled powder depends less on the nominal Mn valence (vMn) than in bulk (La, Ca)MnO3 and the maximum Ms, observed for vMn=3.3, is much smaller. Annealing in air at temperatures above 500 °C increases Ms, but the way Ms approaches the bulk value (90 emu/g) depends on vMn. These observations are discussed in terms of magnetic disorder resulting from defects induced by high-energy milling and surface effects dominant in small crystals.
In situ ion channeling study of gallium disorder and gold profiles in Au-implanted GaN
Authors: Jiang W. , Weber W. J. , Thevuthasan S.
Published: July 2002

Disorder accumulation and annealing behavior on the Ga sublattice in gallium nitride (GaN) implanted with 1.0 MeV Au2+ (60° off surface normal) at 180 or 300 K have been studied using in situ Rutherford backscattering spectrometry in a 〈0001〉-channeling geometry. Complete amorphization in GaN is attained at 6.0 and 20 Au2+/nm2 for irradiation at 180 and 300 K, respectively. A saturation in the Ga disorder at and behind the damage peak was observed at intermediate ion fluences at both 180 and 300 K. No measurable thermal recovery was found at 300 K for the full range of damage produced at 180 K. However, distinct epitaxial regrowth in the bulk and Ga reordering at surface occurred after annealing at 870 K. The implanted Au readily diffuses into the highly damaged regions at elevated temperatures, and the redistribution of the Au atoms in the implanted GaN varies with the damage profiles. A double-peak Au profile developed with the maxima located in the amorphous surface region and near the Au mean projected range. The result is interpreted as Au atom diffusion into the amorphous regime near the surface and Au trapping at irradiation-induced defects in the crystal structure. This trapping effect is also evidenced in this study by the suppressed recovery of the Au-decorated disorder in GaN.
Diffusive transport and optimization of the hybrid Hall effect device
Authors: Reijniers J. , Peeters F. M.
Published: July 2002

We study the temperature dependent Hall resistance RH of a new magnetoelectronic device consisting of a micron scale Hall cross which is subjected to the fringe field of a ferromagnetic film with in-plane magnetization, placed on top of it. External application of a weak in plane magnetic field yields reversing of the magnetization and consequently modulates RH. Starting from the continuity equation, we calculate numerically the steady state electrical potential and current distributions in the Hall device and show that the temperature dependency can be well understood in the diffusive regime. We make a detailed investigation of the influence of different parameters on the output of this device.
Degradation of field emission display phosphors
Authors: Holloway Paul H. , Trottier T. A. , Sebastian J. , Jones S. , Zhang X.-M. , Bang J.-S. , Abrams B. , Thomes W. J. , Kim T.-J.
Published: July 2002

Degradation of ZnS and Y2O2S cathodoluminescent (CL) phosphors has been studied at 1–4 keV using Auger electron spectroscopy simultaneous with CL. The data are consistent with an electron stimulated surface chemical reaction (ESSCR) which led to destruction of ZnS and formation of a surface nonluminescent ZnO layer as well as injection of oxygen point defects into the near-surface region. In the case of Y2O2S:Eu, the electron beam stimulated removal of S and formation of Y2O3:Eu in the presence of 1×10−6 Torr of oxygen. A model is presented which predicts that degradation by the ESSCR should increase with pressure in the vacuum, depend exponentially on electron dose, increase as the primary beam energy was reduced below 4 keV, and depend upon the type of gas present in the vacuum. These trends were demonstrated from the experimental data.
Luminescence and absorption in InGaN epitaxial layers and the van Roosbroeck–Shockley relation
Authors: Schenk H. P. D. , Leroux M. , de Mierry P.
Published: July 2002

Wurtzite InxGa1−xN (0.01≲x≲0.14) films have been grown by metalorganic vapor phase epitaxy on sapphire substrates. Integrated photoluminescence intensity and line shapes have been studied as functions of temperature and alloy composition x. We compare the “effective” InGaN band gap energy assessed by photothermal deflection spectroscopy with a “mean” band gap energy calculated from room temperature photoluminescence spectra utilizing the van Roosbroeck–Shockley relation and assuming a Gaussian energy dependence of the subband gap absorption coefficient. The Stokes’ shift between band gap energy and 300 K photoluminescence peak is explained by this model.
Theoretical study of a Cu+ ion impurity in a NaF host
Authors: Winter N. W. , Pitzer R. M. , Temple D. K.
Published: July 2002

The Cu+ ion impurity in a NaF host has been modeled using a finite cluster of ions to represent the crystal lattice. Several approximations to the lattice potential in the region of the cluster were compared to the exact Madelung potential. The error in the calculated nearest-neighbor distance for the pure host was found to be proportional to the error in the lattice potential. Hartree–Fock calculations were carried out for the ground 1A1g and excited 1,3Eg and 1,3T2g states of the NaF:Cu+ system. The resulting energy level structure was compared to the experimental spectra. The symmetric-stretch potential energy curve, vibrational frequencies, and Franck–Condon factors were calculated for the 1A1g and 1,3T2g states. Using a single configuration coordinate model and a semiempirical spin–orbit coupling scheme, the relative intensities and bandwidths were calculated for absorption to the 1,3T2g states and compared to experiment.
The equilibrium constant for N2O5⇄NO2+NO3: Absolute determination by direct measurement from 243 to 397 K
Authors: Cantrell C. A. , Davidson J. A. , McDaniel A. H. , Shetter R. E. , Calvert J. G.
Published: July 2002

The equilibrium constant for N2 O5 ⇄NO2 +NO3 [reaction (1)], has been determined through direct concentration measurements of N2 O5, NO2, and NO3 in a temperature controlled long path cell, using absorption spectroscopy in the infrared and visible regions from 243 to 397 K. The results give K1C =1.30×1026  exp(−21 490/RT) molecules cm−3 with an estimated overall uncertainty of 30% at the 95% confidence level. This results in an equilibrium constant near room temperature approximately one-half that of previously accepted values. Using the temperature dependence of the equilibrium constant and new data for the enthalpy of formation of gaseous N2 O5 yields an enthalpy of formation for NO3 of 15.39±0.72 kcal mol−1. In additional experiments the temperature dependent first order decay rates of N2 O5 were determined from which a value for the product of the equilibrium constant (K1C ) and the rate constant for reaction (2): NO2 +NO3 →NO+NO2 +O2 was obtained for temperatures from 298 to 396 K: K1 k2 =1.23×1013  exp(−24 300/RT) s.−1 When this is combined with the equilibrium constant of the present study the following expression is obtained for k2 : 9.5×10−14  exp(−2810/RT) cm3  molecule−1 s.−1
Markov chains in the two-dimensional Ising model: Multibody interactions and Poland’s exact solution
Authors: Katz Talbot Michael
Published: July 2002

For certain interaction parameters, one-dimensional paths between points on the two-dimensional plane-square Ising lattice gas behave like Markov chains. Poland has reduced the determination of these interactions to a single nonlinear equation with two free parameters. However, this equation neglects the effects of three-body and four-body interactions. Strengths of these interactions are computed, and conditions are given under which the lattice has only two-body interactions. Determination of such models is again reduced to a single (polynomial) equation, but with only one free parameter.
The theory of semipermeable vesicles and membranes: An integral-equation approach. III. Vesicles with internal nonpermeating ions
Authors: Zhou Yaoqi , Stell George
Published: July 2002

The system of a fluid in the presence of a spherical semipermeable vesicle (SPV) with the freely mobile nonpermeating species inside the vesicle is investigated via an integral-equation approach. This system can be used to model certain feature of a biological cell, permeable to simple ions, in which solute proteins inside the cell are unable to permeate its walls. As an illustrative example of the use of our integral equations, the analytical solution for density profiles in the mean-spherical approximation/Debye–Hückel approximation (MSA/DH) is obtained, where the MSA is used to obtain the density profiles near a membrane and the DH approximation to obtain the bulk pair correlation functions. A method which applies to nonmobile protein fixed inside a cell is also considered.
Divalent manganese in tetragonal sites of Cs2NaYCl6
Authors: Gleason R. J. , Muñoz P. E. , Boldú J. L
Published: July 2002

Electron paramagnetic resonance (EPR) studies performed at room tempertaure in Mn2+ doped Cs2NaYCl6 single crystals (elpasolite) showed that the Mn2+ ion occupies a tetragonal symmetry site in the cubic crystalline lattice. In contrast with previous results on elpasolite, where trivalent rare earth impurities occupy Oh sites, and divalent europium impurities occupy trigonal sites, this work reports, the first evidence of an impurity in a tetragonal symmetry site in this host with principal axes along 〈100〉 crystallographic directions. The unusually large total splitting of the EPR spectrum (∼10 000×10−4 cm−1) is discussed in order to establish a tentative model for the site.
High-pressure Raman study of a one-dimensional hydrogen-bonded crystal of cyanoacetylene
Authors: Aoki K. , Kakudate Y. , Yoshida M. , Usuba S. , Fujiwara S.
Published: July 2002

At room temperature, cyanoacetylene undergoes a liquid–solid transition at 70±30 MPa and further a polymerization reaction at 1.5 GPa. Raman spectra indicates that the crystal structure of the high-pressure phase is isomorphic with the monoclinic low-temperature structure (P21/m, Z=2), in which linear cyanoacetylene molecules are joined with CH NC hydrogen bonds to form infinite one-dimensional chains. The internal stretching frequency of the C–H proton donor decreases wtih increasing pressure, while that of the C 3/4 N proton acceptor increases. The librational modes associated with the rotational motions of the rigid molecules show large shifts to higher frequencies with pressure. The observed frequency shifts are attributed to the strengthening of the CH⋅⋅⋅NC hydrogen bond in the one-dimensional molecular chain.
Theoretical analysis of molecular polarizabilities and polarizability derivatives in hydrocarbons
Authors: Gough K. M.
Published: July 2002

Molecular polarizabilities and polarizability derivatives associated with CH and CC bond stretching have been calculated for CH4, C2H2, C2H4, C2H6, C3H6, C3H8, C4H8 (trans and gauche), C5H12 (all-trans) and C6H12. Self-consistent field calculations have been performed with several basis sets, principally the Dunning contractions of the Huzinaga bases with additional polarization functions. The polarizability derivative associated with a CH bond stretch varies significantly with the orientation and position of a bond within a molecule. Results are in agreement with parameters obtained from Raman trace scattering cross sections. A similar orientational dependence appears for the CC bond stretches of alkanes. There is a linear correlation between the molecular volume calculated with the proaim program and the molecular polarizability. Group polarizabilities (CH3 and CH2) are transferable.
The nearest self-adjoint operator
Authors: Maher Philip J.
Published: July 2002

In Ref. 1 Dupré, Goldstein, and Levy showed how self-adjoint approximation of square matrices arose in molecular quantum mechanics. Their main result says that (A+A*)/2=ℛA is the nearest self-adjoint matrix to the square matrix A. In this note we sharpen this result in terms of the calculus.
Ring-puckering potential function for butadiene sulphone
Authors: López Juan C. , Lister David G. , Villamañán Rosa M. , Alonso José L.
Published: July 2002

The ring puckering in butadiene sulphone has been investigated using microwave spectroscopy and ab initio computations. Microwave spectra of the ground and first eight excited states of the ring-puckering vibration have been observed. A two state analysis of the vibration–rotation interaction for the v=0 and v=1 states gives an energy separation between these states of 4.97 (3) cm−1. This separation and the vibrational dependence of the rotational constants have been used to derive the reduced potential function V(X)=4.7(X4−6.5X2) for the ring-puckering vibration. This potential function gives a barrier to ring inversion of 50 (11) cm−1 and an equilibrium dihedral angle of ≊20°. Ab initio computations using STO 3-21G* orbitals and full geometry optimization give a barrier height of 88 cm−1 and an equilibrium dihedral angle of 19.2°. The ab initio computations predict structural relaxation including a rocking of the sulphone group during the ring-puckering motion. This is supported by calculations of the vibration–rotation interaction parameter (δμac/δQ). The μa component of the electric dipole moment has been determined as 4.6 (1) D from Stark effect measurements. The ab initio computations and Stark effect measurements indicate that the transition moment 〈0‖μc‖1〉 is 1–2 D.
Ground- and excited-state dipole moments of some nitroaromatics: Evidence for extensive charge transfer in twisted nitrobenzene systems
Authors: Sinha Hemant K. , Yates Keith
Published: July 2002

Electro-optical absorption measurements have been made on four model nitroaromatics to determine the effect of twisting of the donor–acceptor single bond on the charge-transfer characteristics in the Franck–Condon excited states. Observed ground- and excited-state dipole moments of nitromesitylene, which has been treated experimentally as the nonplanar analogue of planar nitrobenzene, indicate that electronic excitation of twisted nitrobenzene results in a nearly full unit charge transfer from donor (benzene) to the acceptor (nitro) group (Δμ=18.3 D). On the other hand, in planar nitrobenzene and nitronaphthalene the charge transfer is more delocalized over the whole molecular skeleton, resulting in normal changes in dipole moment (Δμ=5–10 D). In the analogous anthracene system, i.e., 9-nitroanthracene, the charge transfer upon electronic excitation is extremely low (Δμ=1.7 D), which is reflected by its very small change in the dipole moment. Therefore, it is evident that the charge-transfer processes in the twisted molecules are quite different for different aromatic ring systems. Simple molecular-orbital calculations satisfactorily explain the reason for such differences on the basis of their highest occupied molecular-orbital (HOMO) and lowest unoccupied molecular-orbital (LUMO) characteristics. Transition moment directions have also been obtained experimentally and compared with the theoretically predicted directions based on the symmetry properties of the HOMO and LUMO. Agreement is found in all cases studied.
Instability of the B12 icosahedral cluster: Rearrangement to a lower energy structure
Authors: Kawai R. , Weare J. H.
Published: July 2002

Geometries and electronic structures of the B12 cluster have been investigated using a Car–Parrinello ab initio molecular dynamics simulation. The icosahedral structure, which is commonly found in bulk boron compounds, is locally stable, but has a few dangling bonds. On melting and annealing, this structure rearranges to a more open geometry. The new structure has a significantly lower energy despite a lower coordination. The bonds are stronger and there are no dangling bonds.
Differential delay equations in chemical kinetics. Nonlinear models: The cross-shaped phase diagram and the Oregonator
Authors: Epstein Irving R. , Luo Yin
Published: July 2002

Delayed feedback plays a key role in most, if not all chemical oscillators. We derive general results useful in the linear stability analysis of models that explicitly incorporate delay by using differential delay equations. Two models of nonlinear chemical oscillators, the cross-shaped phase diagram model of Boissonade and De Kepper and the Oregonator, are modified by deleting a feedback species and mimicking its effect by a delay in the kinetics of another variable. With an appropriate choice of the delay time, the reduced models behave very much like the full systems. It should be possible to carry out similar reductions on more complex mechanisms of oscillating reactions, thereby providing insight into the role of delayed feedback in these systems.
Excited-state structure and electronic dephasing time of Nile blue from absolute resonance Raman intensities
Authors: Lawless Mary K. , Mathies Richard A.
Published: July 2002

Absolute resonance Raman cross sections are measured for Nile blue 690 perchlorate dissolved in ethylene glycol with excitation at 514, 531, and 568 nm. These values and the absorption spectrum are modeled using a time-dependent wave packet formalism. The excited-state equilibrium geometry changes are quantitated for 40 resonance Raman active modes, seven of which (590, 1141, 1351, 1429, 1492, 1544, and 1640 cm−1 ) carry 70% of the total resonance Raman intensity. This demonstrates that in addition to the prominent 590 and 1640 cm−1 modes, a large number of vibrational degrees of freedom are Franck–Condon coupled to the electronic transition. After exposure of the explicit vibrational progressions, the residual absorption linewidth is separated into its homogeneous [350 cm−1 half-width at half-maximum (HWHM)] and inhomogeneous (313 cm−1 HWHM) components through an analysis of the absolute Raman cross sections. The value of the electronic dephasing time derived from this study (25 fs) compares well to previously published results. These data should be valuable in multimode modeling of femtosecond experiments on Nile blue.
Ab initio study of the structure, cooperativity, and vibrational properties of the H2O: (HF)2 hydrogen bonded complex
Authors: Hannachi Y. , Silvi B. , Bouteiller Y.
Published: July 2002

Ab initio self-consistent field (SCF) and MP2 calculations have been performed in order to investigate the structure, energetics, and spectroscopic properties of H2O: (HF)2. Two conformations of the 1:2 complex are energetically possible. On the one hand is a cyclic complex belonging to the C1 point group and which corresponds to the absolute minimum, and on the other hand is a bifurcated complex with C2v symmetry. In this latter complex the two hydrogen bonds are equivalent whereas in the former there are two strong H-bonds and a weaker one. The energy difference between these two conformers is ∼3 kcal mol−1. From the spectroscopical point of view the bifurcated complex is characterized by νHF frequency shifts smaller than that calculated for the dimer, in the case of the cyclic complex the two different νHF red shifts closely related to the H2O–HF and (HF)2 calculated shifts, respectively, and correspond to those experimentally observed. The cooperative effects on interaction energies and spectroscopic properties have been estimated and analyzed on the basis of the projected induced dipole moment. It is shown that the cyclic complex is favored with respect to both H2O–HF and (HF)2 precursors while the formation of the bifurcated one remains more problematic.
Direct Monte Carlo measurement of excluded volume for Pearson walks
Authors: Barrett A. J. , Benesch B. C.
Published: July 2002

The Pearson walk model of a linear polymer with excluded volume is studied by analogy with a neighbor-avoiding walk on a lattice. The pivot algorithm is used to model the excluded volume directly, with the strength of interaction controlled by bead diameter. The excluded volume per bead is shown to be nonanalytic, with two distinct excluded volume regimes corresponding to geometrically different polymer models.
A hierarchical algorithm for polymer simulations
Authors: Johnson Leslie A. , Monge Alessandro , Friesner Richard A.
Published: July 2002

A computationally efficient method to evaluate the interaction energy of a polymer configuration is presented. New polymer conformations are generated via the pivot algorithm. The method involves a hierarchical search scheme for determining intersections and nonbonded nearest-neighbor contacts. The search uses a hierarchy of spheres which enclose segments of the polymer chain. At each level of the hierarchy the spheres enclose successively smaller polymer segments. This technique is found to be very efficient in terms of CPU time. It scales as 𝒪(N), where N is the polymer length, and is independent of φ, the interaction energy parameter. The method is readily applicable to off-lattice polymer models and results for both lattice and off-lattice simulations are presented.
π-electron polarizabilities of infinite organic polymers
Authors: Ducasse L. , Villesuzanne A. , Hoarau J. , Fritsch A.
Published: July 2002

The contribution of the π electrons to the polarizabilities of finite and infinite systems have been calculated for infinite one-dimensional chains modeling organic polymers, such as polyacetylene and various polyheterocycles. In the first part, results obtained previously using a finite-field technique within the Hückel and Pariser–Parr–Pople (PPP) models applied to polymers containing up to 400 atoms have been compared to results obtained from the sum-over-states (SOS) perturbation theory. The calculated polarizabilities depend on the type of monomer following polyacetylene ≥ polythiophene ≥ polypyrrole. This result is qualitatively independent on the type of model or formalism. In the second part, we present results of the application of the Genkin–Mednis formalism within the PPP model to the calculation of the π polarizability of infinite one-dimensional chains. Numerical difficulties arise from the phase of the linear-combination-of-atomic-orbital (LCAO) coefficients corresponding to different k points. It is necessary to monitor these phases in order to obtain reliable data. The Genkin–Mednis polarizabilities are in excellent agreement with the asymptotic values obtained using the SOS method on oligomers.
A photodissociation study of HgI2
Authors: Jordan K. J. , Lipson R. H. , Yang D. S.
Published: July 2002

The photodissociation of jet-cooled HgI2 has been studied in the 244–323 nm region using a tunable ultraviolet (UV) laser. Dispersed emission, action, and multiphoton ionization spectra have been recorded. A vibrational analysis of the action spectra near 305 nm shows that the production of excited Hg atoms from HgI2 UV photodissociation proceeds through a HgI ground-state intermediate. Near 248 nm the results are less conclusive. Possible reasons for a wavelength-dependent kinetic mechanism are discussed.
Resonant coherent anti-Stokes and Stokes Raman scattering spectra of molecular impurities at low temperatures
Authors: Kukk P. , Freiberg A.
Published: July 2002

In the present paper, the results of the measurements of steady-state resonant coherent anti-Stokes Raman scattering (CARS) and coherent Stokes Raman scattering (CSRS) spectra of a tetracene impurity in the monocrystals of anthracene and a glassy matrix of polystyrene at low temperatures are presented. The inhomogeneous widths of the pure electronic lines of the samples differ over two orders of magnitude. In agreement with theory, it is shown that the widths of the spectral lines corresponding to the vibrations in both the ground and the excited electronic state of tetracene are determined by the relatively small inhomogeneous variation of the Raman frequencies and not by the much larger inhomogeneous variation of pure electronic or vibronic transition frequencies. The CARS excitation profiles reveal a shift with respect to the absorption band and hole–shape peculiarities that could be explained as the result of interference of different light waves in the sample. In the methodological part of the paper, some restrictions to the experimental possibilities determined by the resonant character of excitation have been outlined.
Scattering of NO molecules from a diamond (110) surface
Authors: Roth C. , Häger J. , Walther H.
Published: July 2002

The translational, rotational, and vibrational dependencies of NO molecules scattered from a diamond (110) surface have been investigated for a series of scattering geometries. The integral and state selected angular distributions of the scattered molecules have been measured. In the exit channel the velocities of molecules in selected states as well as the rotational population distributions in the two lowest vibrational states (ν=0,1) were analyzed for a variety of incoming energies (200 cm−1≤Ekin≤2600 cm−1), scattering angles, and surface temperatures. Molecules with a fast incoming velocity and a rotational energy of Erot≤1000 cm−1 after scattering (about 75% of all molecules) exhibit an elastic interaction with the surface (no energy exchange with the surface, Ekin+Erot=const). For molecules receiving higher rotational energy during the scattering process (Erot≥1000 cm−1) an increasing influence of the surface temperature on their final rotational and translational motion was found. For slow incoming molecules such a dependence is also observed but less pronounced. About 5×10−3 of the incoming molecules are vibrationally excited during their encounter with the surface, and exhibit, after the scattering, angular, and rotational population distributions which are similar to those of the molecules which remain in the vibrational ground state during the scattering process. Rotation of the (110) surface around its normal by 90° leads to identical scattering results: there is no detectable influence of the various surface topographies on the molecular scattering behavior within our present experimental accuracy.
Theoretical study of hydrogen and nitrogen interactions: N–H transport cross sections and collision integrals
Authors: Stallcop James R. , Bauschlicher Charles W. , Partridge Harry , Langhoff Stephen R. , Levin E.
Published: July 2002

Potential energy curves have been calculated for the X 3Σ− and 5Σ− states of NH using a complete-active-space self-consistent-field/multireference configuration-interaction procedure and extensive Gaussian basis sets. An accurate dipole moment function is computed for the X 3Σ− state. Transport cross sections have been calculated for the collisions of hydrogen and nitrogen atoms in their ground states. The mean transport cross sections are tabulated for collision energies in the range 0.0001–1 Eh; the mean collision integrals are listed for temperatures between 100 and 100 000 K. The ab initio energies for the 5Σ− state of NH are found to be consistent with the results of correlation studies and agree well with the prediction from combining relations using the energies of the highest spin states of H2 and N2.
Three-vector correlation theory for orientation/alignment studies in atomic and molecular collisions
Authors: Driessen Jan P. J. , Eno Larry
Published: July 2002

The laboratory integral cross section under crossed-beam conditions is examined for collisions between a structured and an unstructured species, where the orientation (or alignment) of the initial and final states is determined with respect to arbitrarily configured laboratory axes. Three parameters are necessary to characterize the mutual orientation of the relevant axes in this case and these parameters then also characterize the cross section. The laboratory cross section is expanded in terms of a more fundamental (angle-independent) set of cross sections as determined in the so-called collision frame, where projections of the structured species are taken with respect to the direction of the initial relative velocity vector. Drawing upon a number of symmetry relations, we count the numbers of fundamental cross sections for arbitrary initial and final angular momenta. Then we consider the explicit dependence on angular parameters which can be anticipated for the laboratory cross section. Finally, we raise a number of experimental considerations in attempting to determine the fundamental cross sections. These considerations are ‘‘fleshed out’’ by applying our cross-section analysis to a recent three-vector correlation experiment involving collisions between Ca and He.
Collisional relaxation of vibrational excitation: Effects of bath gas structure
Authors: Boering Kristie A. , Brauman John I.
Published: July 2002

Infrared multiple photon dissociation has been used to study the relaxation of vibrationally excited trifluoroacetate anion. The internal energy of the ensemble of ions was monitored by measuring the extent of photodissociation in a Fourier-transform, ion cyclotron resonance spectrometer. Bimolecular quenching rate constants are measured in the presence of excess bath gases. Comparison of the experimental collision efficiencies with calculations of purely statistical energy redistribution has been done and indicates that species which can interact chemically with a trifluoroacetate anion exhibit a marked increase in the efficiency of collisional deactivation. Systematic use of the ratio R, defined as the average energy transferred per collision derived from experiment divided by the average energy transferred calculated from a statistical model, is proposed. A correlation of R with the hypothetical, limiting statistical lifetimes of the collision complexes is observed. This suggests that both the number and nature of oscillators of the bath gases as well as the intermolecular well depths with trifluoroacetate are important in determining the extent of energy transfer. It further suggests that energy transfer is limited by the collision duration.
Interpretation of the vibrational structure of the emission and absorption spectra of C60
Authors: Negri Fabrizia , Orlandi Giorgio , Zerbetto Francesco
Published: July 2002

The vibronic intensity borrowing activity of the lowest electronically excited singlet states of C60 has been obtained through quantum chemical calculations. The vibrational structure of the UV–visible spectra is found to be dominated by false origins. The calculated intensities of the false origins of the T1g state agree with the vibrational structure observed in the fluorescence spectrum. The same false origins are recognized to be responsible for the vibrational structure of the red edge portion of the absorption spectrum. Only two bands in the spectra are assigned as combination bands involving an ag or a Jahn–Teller active mode. Absorption bands that may be associated with false origins of the states T2g and Gg which are quasidegenerate with S1 are tentatively assigned.
Alkane dissociation dynamics on Pt(110)–(1×2)
Authors: McMaster Mark C. , Madix Robert J.
Published: July 2002

Supersonic molecular beam techniques were used to study the reactive adsorption dynamics of methane and ethane on Pt(110)–(1×2). The initial dissociative sticking probability, S0, was measured as a function of surface temperature, incident translational energy, incident total vibrational energy, and incident polar angle at two azimuthal orientations. Under all experimental conditions, both alkanes dissociated via direct collisional activation. Over the range of translational energies studied here neither S0(CH4) nor S0(C2H6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl deformation, methyl rocking, C–C stretching, and torsional vibrational modes do not play a significant role in the direct dissociation of either alkane on Pt(110)–(1×2) under these experimental conditions. The C–H stretching modes were not sufficiently populated to determine the extent of their participation. Methane and ethane displayed almost identical initial reaction probabilities at a fixed incident translational energy and polar angle, similar to our findings for methane and ethane dissociation on Pt(111). However, the reactivity of both species was about a factor of 2 lower on Pt(110)–(1×2) than observed on Pt(111) at a fixed incident translational energy and polar angle. When the crystal was positioned such that the tangential velocity component of the beam was incident along the atomic rows (the [11̄0] direction) the dissociation of both alkanes exhibited normal energy scaling. When the azimuthal orientation was rotated 90° such that the tangential velocity component of the beam was directed perpendicular to the close-packed rows (the [001] direction), the initial dissociation probabilities of both alkanes appeared to scale with Ei cos0.5 θi. This is the first reported observation of non-normal energy scaling for direct alkane activation and is attributed to the corrugation of the surface microstructure.
The use of coherent phase control of multiphoton ionization to measure the refractive indices of H2 and Ar between 1100 and 1150 Å
Authors: Gordon Robert J. , Lu Shao-Ping , Park Seung Min , Trentelman Karen , Xie Yongjin , Zhu Langchi , Kumar Ashok , Meath William J.
Published: July 2002

We have used the method of coherent phase modulation, first proposed by Brumer and Shapiro for controlling the rates of chemical reactions, to measure the refractive index of a gas in the vacuum ultraviolet. In these experiments we populated a Rydberg state of HCl or CO by simultaneous absorption of one VUV photon and three UV photons. Molecular ions were generated by absorption of an additional UV photon and detected with a time-of-flight mass spectrometer. The phase difference for the UV and VUV beams was altered by passing them through a cell containing either H2 or Ar. By varying the pressure in the cell we could control this phase difference and thereby modulate the ion signal. From the modulation frequency of the signal it was possible to determine the difference between the refractive indices of the gas at the UV and VUV wavelengths. Using reliable refractive indices in the UV, we extracted from our data values for the VUV indices. These values are in quantitative agreement with a calculation that uses dipole oscillator strengths constructed using constrained oscillator strength methods, taking into account the dispersion resulting from individual rotational transitions. The experimental method can be used very close to an optical transition of the refractive medium, provided that the refractive index does not vary appreciably across the bandwidth of the VUV laser.
The barrier to dissociation in the B̃ 2A1′ state of the methyl radical
Authors: Botschwina P. , Schick E. , Horn M.
Published: July 2002

Making use of internally contracted multireference configuration interaction and a large basis set of 213 contracted Gaussian-type orbitals, the barrier height to dissociation in the B̃ 2A1′ state of the methyl radical was obtained to be 2417 cm−1, in very good agreement with a recent experimental estimate of 2200 cm−1. The equilibrium bond lengths in the X̃ 2A1″ and B̃ 2A1′ states are 1.076 and 1.109 Å, respectively.
Orientational dynamics on glassformer 2 [Ca(NO3)2]⋅3[KNO3]: A study by transient optical Kerr effect
Authors: Ricci Marilena , Foggi Paolo , Righini Roberto , Torre Renato
Published: July 2002

The fast orientational dynamics of molten 2[Ca(NO3)2]⋅3[KNO3] is investigated by time- resolved optical Kerr effect (OKE) with subpicosecond resolution in the temperature range 380–620 K. Apart from the instantaneous electronic response and a very fast decay, the OKE intensity profiles show clearly the presence of a nonexponential decay. The simplest function which fits the experimental data is a biexponential with time constants ranging from 120 to 15 and 10 to 3 ps, respectively. The results are discussed in terms of an integrated correlation time τc that does not show a diverging behavior approaching the glass transition. At high temperatures τc follows a typical Arrhenius law with activation energy of 1.5 kcal/M. Different possible interpretations of the observed behavior (α–β bifurcation, domain size effect) are discussed. The main conclusion is that rotational motions are decoupled from translational ones even for temperatures well above the liquid–glass transition.
Vibrational effects for the dispersion-energy and dispersion-polarizability coefficients for interactions between H, He, and H2
Authors: Bishop David M. , Pipin Janusz
Published: July 2002

The effects of zero-point and ‘‘pure’’ vibration on the dispersion-energy and the dispersion-polarizability coefficients for interactions between H and H2, He and H2, and H2 and H2, are investigated. It is found that calculations of these coefficients which use the vibrationally averaged internuclear separation for H2, differ from the properly vibrationally averaged values by the order of one percent. So-called pure vibrational contributions are found to be negligible.
Conformational analysis of jet cooled tryptophan analogs by molecular mechanics: Comparison with experiment
Authors: Sipior Jeffrey , Sulkes Mark
Published: July 2002

The possibility of predicting the geometries of jet cooled conformers of tryptophan analogs via molecular mechanics, employing MM2 (1987), was studied by carrying out computations on six different analog molecules. For three of the analogs, tryptamine in particular, the computationally predicted gas phase conformer geometries could be compared with experimentally determined geometries obtained from jet cooled samples. Some conformer geometries agreed qualitatively with experiment but the match did not extend to all cases. Most notably, the MM2 calculations failed to predict the eclipsed conformers of tryptamine that were deduced experimentally via analysis of rotational constants. They similarly failed to predict a planar geometry conformation found for 3-indole acetic acid. These two disagreements prompted a reconsideration of the possible geometries that yield the best match to the experimentally determined rotational constants for various jet cooled conformers. For tryptamine conformers D/E it was found that likely geometries might in fact be more than 30° from eclipsed structures. Some differences were also found for the Cα–Cβ and Cβ–Cγ dihedral angle values assigned to tryptamine conformers A/B and conformer F. Nonetheless, the overall agreement of the MM2 calculations with the refined tryptamine conformer geometries remains no more than qualitative. Reanalysis of the possible geometries of conformer A of 3-indole acetic acid that are consistent with the experimental rotational constants yielded agreement with the previous planar geometry assignment. It was also possible to use the molecular mechanics data to predict relative conformer populations and from this relative peak sizes of 0–0 transitions in the jet cooled excitation spectra. Considering all six cases, the match with experiment is again at best partial. The agreement is not good enough for reliable association of definite computed geometries with particular peaks in an experimental excitation spectrum.
Pressure induced phase transitions in pentacene doped para-terphenyl probed by changes in the impurity absorption spectrum
Authors: Baer Bruce J. , Chronister Eric L.
Published: July 2002

The pressure dependence (0–10 kbar) of the low temperature (15 K) absorption spectrum of pentacene impurities in a para-terphenyl host crystal is used to investigate the pressure and/or temperature induced triclinic to monoclinic phase change of the para-terphenyl host crystal. Due to the characteristic absorption spectrum of pentacene in these two crystal phases, the pentacene impurity proves to be a sensitive probe of the structure of the surrounding para-terphenyl matrix.
Molecular dynamics in perfluoro-n-eicosane. IV. Oscillatory and diffusive rotational motion
Authors: Kimmig Martin , Steiner Ralf , Strobl Gert , Stühn Bernd
Published: July 2002

Rotations of the molecules about their long axis constitute an important part in the complex pattern of internal motions in the rhombohedral phase of perfluoro-n-eicosane (C20F42). Dynamical studies were performed by quasielastic and inelastic neutron scattering. Spectra were determined in a range of strong diffuse scattering. They show a superposition of librations (δE≂1–2 meV) and rotational diffusion (δE≂10–100 μeV). Reorientation rates exhibit a slowing down on approaching the transition to the low temperature phase (Tc=200 K). As indicated by this slowing down, by the nonexponential relaxation function, and also an intensity enhancement at the Bragg-positions, rotations occur correlated between neighboring molecules. The state of motion can be simulated by molecular dynamics calculations performed for a two-dimensional lattice of coupled rotators.
Kinetics of surface roughening via pit growth during the oxidation of the basal plane of graphite. II. Theory and simulation
Authors: Pakula T. , Tracz A. , Wegner G. , Rabe J. P.
Published: July 2002

Analytical expressions for the description of the oxidation kinetics of the basal planes of the graphite surface are proposed. They are developed from an ansatz that extends former theories of phase transformations by nucleation and growth to multilayer systems in which the kinetics of transformation in subsequent layers is recurrently related. The results from analytical theory are compared with computer simulations obtained for an adequate model. Correspondence of these results to the structural changes observed in the oxidation of the graphite surface, as revealed by scanning tunneling microscopy, is discussed.
Vibrational energy relaxation of Si–H stretching modes on the H/Si(111)1×1 surface
Authors: Gai Huadong , Voth Gregory A.
Published: July 2002

The vibrational relaxation rate of an excited Si–H stretching mode on the ideally H-terminated H/Si(111)1×1 surface is calculated using Bloch–Redfield theory combined with classical molecular dynamics. The calculated lifetime is found to be in good agreement with the experimental result. Analysis of the power spectrum of the fluctuating force along the Si–H bond suggests that the dominant energy relaxation pathway for the v=1 stretching state is through kinetic coupling to a state with three Si–Si–H bending quanta plus one phonon. The v=2 state is predicted to relax at a considerably higher rate.
The electronic structure of the PtH molecule: Fully relativistic configuration interaction calculations of the ground and excited states
Authors: Visscher L. , Saue T. , Nieuwpoort W. C. , Faegri K. , Gropen O.
Published: July 2002

Fully relativistic all-electron self-consistent field calculations based on the Dirac–Coulomb Hamiltonian have been performed on the three lowest lying states of the PtH molecule. The resulting four-component Dirac–Hartree–Fock (DHF) molecular spinors are subsequently used in relativistic configuration interaction (CI) calculations on the five lower states of PtH. Spectroscopic properties are obtained by fitting the potential curve to a Morse function and show good agreement with experimental data. The effect of relativistic corrections to the Coulomb electron–electron interaction is investigated at the DHF level and is found to be insignificant for the molecular spectroscopic properties investigated by us. The CI wave functions are found to have only one dominant configuration, indicating a lack of static correlation. Dynamic correlation in the d shell is, however, important for the spectroscopic properties of PtH. The results conform with a bonding scheme in which the three lower and two upper states of PtH are assigned 5d3/245d5/25σ1/22 and 5d3/235d5/26σ1/22 electronic configurations, respectively. The configurations are only approximate and are perturbed by 5d participation in bonding. The stability of the Pt–H bond is explained in terms of the relativistic stabilization of the 6s orbital in analogy with the electron affinity of the platinum atom.
A theoretical study of the influence of the surface on the electronic structure of CdSe nanoclusters
Authors: Hill Nicola A. , Whaley K. Birgitta
Published: July 2002

The effect of the cluster surface on the ground state electronic properties of CdSe nanoclusters is calculated using a tight-binding method. The calculation of optical matrix elements between selected surface and interior states is described. We find that the density of states is strongly dependent on the nature of the cluster surface and that there is a significant probability of an optical transition into unoccupied surface states.