bluuebunny/crossref_metadata_2025 huggingface dataset:
Endohedral formation, energy transfer, and dissociation in collisions between Li+ and C60
Authors: Bernshtein V. , Oref I.
Published: July 2002

Quasiclassical trajectory calculations were performed on Li+ ion collisions with a C60 molecule. The probabilities of endohedral formation and escape from the cage are reported. It is found that endohedral formation depends on the relative translational energy and it is independent of the internal energy. The average energy transferred per collision of a Li+ with a fullerene molecule is reported and its dependence on the relative translational energy is given. The collisional energy transfer probability density function, P(E′,E), is calculated for two translational energies and the results are used to calculate the degree of dissociation of the fullerene molecule following a collision with Li+. Details of the intramolecular vibrational energy redistribution, IVR, are reported. It is found that following an exciting collision, energy relaxes by moving from one moiety to another within the molecule. Initial partial relaxation can be as fast as ∼67 fs but total redistribution of energy takes ∼1.5 ps.
Protonation effects on the photophysical properties of poly(2,5-pyridine diyl)
Authors: Monkman A. P. , Halim M. , Samuel I. D. W. , Horsburgh L. E.
Published: July 2002

We have recently shown that poly(2,5-pyridine diyl) (PPY) can be synthesized to yield a polymer with high photoluminescence quantum yield (PLQY) in the solid state, and that it is an excellent electron transport material. To explore the photophysical properties of PPY further, we have used a range of acidic “dopants” to protonate the nitrogen sites on each ring and made observations on how this affects the optical properties of the resultant protonated PPY films. In general, we find that sulphonic acids have the greatest effect, causing perturbations to both the ground-state and excited-state properties of the PPY. These changes occur with only moderate reduction of the PLQY, whereas nonsulphonic acids cause a larger reduction in PLQY without significantly affecting the ground- or excited-state energy levels. These aspects of the photophysics of PPY can be described using a simple ring torsion argument. This model can also account for the observed shifts between solution state and solid-state emission wavelengths.
Effective pair potentials for charged colloidal particles
Authors: González-Mozuelos P. , Carbajal-Tinoco M. D.
Published: July 2002

A general formalism for ionic fluids is presented. We report general and exact expressions for the distribution functions which directly satisfy the Stillinger–Lovett moment conditions. General and exact expressions are also provided for the effective pair potentials among charged colloidal particles. These general expressions are a direct consequence of the multicomponent Ornstein–Zernike (OZ) equations and the asymptotic behavior of the direct correlation functions in charged systems. The effective pair potentials show two distinct parts: short-ranged and electrostatic. For the primitive model (PM) with pointlike small ions the electrostatic component is reduced, for sufficiently large distances, to a screened Coulomb potential with renormalized charges. The general expression of the effective electrostatic interaction gives a clear insight of the specific conditions of validity of the Derjaguin–Landau–Verwey–Overbeek (DLVO) model and of the possible directions in which this model may be improved. We also present results for the PM with pointlike small ions obtained from an implementation of the Zerah–Hansen (ZH) approximation complemented with a new minimization criterion. These results are compared with experimental data to test the accuracy of the predictions of the ZH scheme against those obtained from the DLVO model.
Dynamical structure of water in alkali halide aqueous solutions
Authors: Ujike Tomoko , Tominaga Yasunori , Mizoguchi Kohji
Published: July 2002

The low-frequency Raman spectra of alkali halide AX (A=Na, K, Rb; X=Cl, Br) aqueous solutions with various temperatures and concentrations are investigated. We have found that at the supercooled temperature region the relaxation time of the KX and RbX aqueous solutions becomes shorter than that of pure water. Since in these solutions the viscosity decreases with increasing concentration, this result suggests that the relaxation time of the electrolytic solutions in the Raman region is strongly correlated with the viscosity. Moreover, through comparing the spectral profiles of the relaxation mode among AX aqueous solutions we demonstrate that the pattern of the concentration dependence of the relaxation mode parameters such as the activation energy changes between NaX and (KX, RbX) aqueous solutions.
Deterministic identifiability of photophysical kinetic models with transients via the method of similarity transformation
Authors: Molski Andrzej , Boens Noël
Published: July 2002

The method of similarity transformation is shown to be applicable to the problem of deterministic identifiability of photophysical kinetic models with transients. A two-state excited-state model of irreversible intermolecular association with transients is demonstrated to be identifiable from two decay traces at two observation wavelengths and one co-reactant concentration, when either only the species that associates with the co-reactant is directly excited or the excited species spectra are separated such that each species can be monitored individually. When transients are absent, two decays at one co-reactant concentration do not guarantee the recovery of the model parameters, which indicates that transient effects change identifiability criteria compared to the case with time-invariant rate constants.
Integral equation theory of single-chain polymers: Comparison with simulation data for hard-sphere and square-well chains
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

A systematic comparison of computer simulation data for linear hard-sphere and square-well chains with the results of single-chain integral equation is reported. The single-chain integral equation is derived from the polymer Kirkwood hierarchy for site–site or pair distribution functions. Quantities compared include radius of gyration, end-to-end distance, and internal energy. We examine chain lengths up to 1000 sites for hard-sphere chains. The radius of gyration and end-to-end distance from the theory are found to agree quantitatively with Monte Carlo simulation data. Results for square-well chains with the range λ=1.5 are compared with Monte Carlo and constant temperature molecular dynamics simulation data for chains having up to 64 sites. The radius of gyration and internal energy generally deviate from simulation data by about 10% for reduced temperatures greater than 1. The values of the radius of gyration at reduced temperatures below 1 are larger than those from simulations.
The microwave spectrum of the rubidium monoxide RbO radical
Authors: Yamada Chikashi , Hirota Eizi
Published: July 2002

The rotational spectra of 85RbO in the ground- and the first excited vibrational states and of 87RbO in the ground vibrational state were observed in the millimeter- and submillimeter-wave region. The RbO radical was generated in a high-temperature absorption cell by the reaction of N2O with Rb vapor, which was produced in the cell by heating a mixture of RbCl and lithium metal to 500–550 °C. Analysis of the observed spectra yielded the rotational and centrifugal distortion constants, spin–rotation interaction constants, and hyperfine interaction constants. The bond length and the vibrational frequency were calculated from the rotational and centrifugal distortion constants to be 2.254 193 1 (15) Å and 387.22 (20) cm−1, respectively, for 85RbO. The ground electronic state of RbO was confirmed to be 2Σ, and the low-lying 2Π state was estimated to be located at 650–700 cm−1 above the ground state by using a second-order perturbation expression with the vibrational energy in the denominator for the spin–rotation interaction constant. The observed hyperfine interaction constants indicated that the RbO radical is essentially an ionic molecule.
Formation of He* by ion–ion neutralization reactions of He+ with C6F5X− (X=F, Cl, Br, CF3) in a helium flowing afterglow
Authors: Tsuji Masaharu , Nakamura Masafumi , Nishimura Yukio , Oda Erika , Oota Hidefumi , Hisano Masahiro
Published: July 2002

The ion–ion neutralization reactions of He+ with C6F5X− (X=F, Cl, Br, CF3) leading to He* have been spectroscopically studied in a helium flowing afterglow. Although both singlet and triplet He* states were formed when X=F and Cl, only triplet He* states were produced when X=Br and CF3. More than 99% of the product He* atoms were formed in the low-lying He(3s, 3p, or 3d) states for all the reactions, and their electronic-state distributions were similar. The electronic-state populations decreased rapidly with increasing excitation energy of He*. They were represented by effective electronic temperatures of 0.070–0.19 eV. The observed electronic-state distributions were compared with those predicted from a simple statistical theory.
Reactive and inelastic collisions of H atoms with vibrationally excited water molecules
Authors: Lendvay G. , Bradley K. S. , Schatz G. C.
Published: July 2002

The dynamics of the collisions of H atoms with vibrationally excited H2O were studied using classical mechanical reactive and quantum mechanical nonreactive scattering calculations. The classical trajectory calculations were performed with the I5 potential surface of Isaacson. These results show the expected behavior for an endoergic reaction with a late barrier, with the cross section exhibiting a high threshold when the water is unexcited, and a much lower threshold if the asymmetric stretch of water is highly excited. Qualitatively this matches experimental results, although the threshold energy for reaction of water in the ground vibrational state is too low to reproduce the measured rate coefficients. The rate coefficient is higher than for ground state water by six orders of magnitude when the asymmetric stretch mode is excited by four quanta. However the rate for reaction from this excited state is still two orders of magnitude smaller than the total reactive+inelastic rate coefficient obtained in recent measurements by Smith and co-workers. Quantum scattering calculations of the vibrational energy transfer rate coefficients show that the pure stretch excited states can have very high deactivation rate coefficients, resulting from transitions to states that are separated by a small energy gap (<50 cm−1) from the initial state. The calculated rate coefficients for reactive+inelastic transitions are therefore dominated by inelastic scattering, and the results we obtain are in good agreement with the Smith data.
Characterization of water octamer, nanomer, decamer, and iodide–water interactions using molecular dynamics techniques
Authors: Dang Liem X.
Published: July 2002

The lowest minimum-energy structures for the water octamer, nanomer, and decamer and the hydration of iodide were characterized using molecular dynamics techniques and polarizable potential models of Dang and Chang [J. Chem. Phys. 106, 8149 (1997)]. The calculations predicted the two lowest-energy cubic isomers, D2d and S4, for the water octamer. The lowest minimum-energy structures for the nanomer and decamer were derived from the octamer by insertion of one and two water molecules, respectively, into the cubic isomers. Our potential models provided an excellent description of the hydration of iodide in water clusters and in solution at room temperature. At 0 K, the lowest energy-minimum structures predicted by our calculations are in excellent agreement with the available optimized structures obtained from accurate electronic structure theory calculations for similar systems. In all cases, the surface states are dominant and the polarizability plays an important role in the hydration of iodide in water clusters at 0 K.
Molecular alignment from femtosecond time-resolved photoelectron angular distributions: Nonperturbative calculations on NO
Authors: Althorpe Stuart C. , Seideman Tamar
Published: July 2002

We study numerically the ability of time-resolved photoelectron angular distributions to provide useful information regarding the alignment of wave packets and hence a new view on excited state dynamics. The calculations employ a recently developed theory of pump–probe photoelectron spectroscopy [J. Chem. Phys. 107, 7859 (1997)] which treats both laser pulses nonperturbatively. Taking the NO system as a prototypical example, we first describe the alignment dynamics in the course of the perpendicular Π→Σ transition. The observation of alignment perpendicular to the field polarization which converts upon turn-off of the pulse to alignment parallel to the field is explained in terms of the phase relation between the wave packet components. Considering next the ionization stage, we find only weak dependence of the photoelectron angular distribution on the field intensity in the range typically employed in gas-phase femtosecond experiments. Our results illustrate the utility of time-resolved photoelectron angular distributions as a probe in pump–probe studies.
“Two-step” model of molecular diffusion in silicalite
Authors: Kärger Jörg , Demontis Pierfranco , Suffritti Giuseppe B. , Tilocca Antonio
Published: July 2002

The influence of the particle “memory” on long-range diffusion in the channel network of silicalite is taken into account by considering pairs of subsequent steps between the channel intersections. It is shown that in this case the correlation rule between the principal elements of the diffusion tensor has to be modified by including an additional term, which takes account of the deviation of molecular propagation from complete randomness. The obtained relations are discussed in terms of molecular dynamics simulations of ethane in silicalite.
Helfrich free energy for aggregation and adhesion
Authors: Blokhuis E. M. , Sager W. F. C.
Published: July 2002

We present a theoretical study of the shape and free energy of a vesicle (or microemulsion droplet) adhered to a substrate (other droplet) based on the expression for the surface free energy by Helfrich. Analytical formulas are presented for the shape and free energy when the rigidity constant for bending, k, is small; i.e., when (k/σ)1/2, with σ the surface tension, is small compared to the typical dimension of the vesicle (k/σ)1/2≪V1/3, with V the vesicle volume. These formulas are compared with numerical solutions of the shape equations such as those first provided in the work by Seifert and Lipowsky. Results are presented when the exact formulas are applied to study the onset of microemulsion droplet aggregation, e.g., dimer formation, in terms of the usual coefficients in the Helfrich free energy expression, such as the rigidity constant for bending and the spontaneous curvature.
Reversible molecular dynamics for rigid bodies and hybrid Monte Carlo
Authors: Matubayasi Nobuyuki , Nakahara Masaru
Published: July 2002

A time-reversible molecular dynamics algorithm is presented for rigid bodies in the quarternion representation. The algorithm is developed on the basis of the Trotter factorization scheme, and its structure is similar to that of the velocity Verlet algorithm. When the rigid body is an asymmetric top, its computationally inconvenient Eulerian equation of motion is integrated by combining the computationally convenient solutions to the Eulerian equations of motion for two symmetric tops. It is shown that a larger time step is allowed in the time-reversible algorithm than in the Gear predictor–corrector algorithm. The efficiency of the hybrid Monte Carlo method for a molecular system is also examined using the time-reversible molecular dynamics algorithm in the quarternion representation.
Quantum mechanical study of time-dependent energy transfer between perturbers in a Scheibe aggregate
Authors: Engelhard S. , Faisal F. H. M.
Published: July 2002

In this paper time-dependence of coherent energy transfer in a model oxacyanine (S9) Scheibe aggregate (or J-aggregate) in the presence of thiacyanine (S11) perturber chromophores is studied quantum mechanically. The mixed Scheibe aggregate is assumed to have a hexagonal two-dimensional (2D) brick layer structure. It is shown that the present model, based on the extended dipole-dipole resonance coupling energy among the monomer “bricks”, can account for the observed red-shift of the J-band (of the aggregate) with respect to the band of the monomers, for monomer of the same brick dimensions as estimated by Kuhn and co-workers, experimentally. Quantum propagation of the energy of an initial excitation in the aggregate is found to occur as a (probability) wave that can be strongly affected by the presence of the perturber chromophores. It is found that if a perturber is excited, a part of the excitation energy can be trapped at the site. More interestingly, it may also oscillate between two or more perturbers placed in the aggregate. This coherent oscillation of energy (that has been called a “sloshing” effect) has been at first predicted for a quantum mechanical 1D molecular chain, is shown to occur also in the present 2D system of a mixed S9/S11-Scheibe aggregate. Under suitable conditions, the effect can be used to control the excitation transfer between the perturbing chromophores. For example, it is shown that more than 60% of the excitation energy can be transferred in this way within 1.5 ps between two perturber chromophores situated 80 Å from each other. The density of states of the mixed aggregate and the excitation spectrum under various initial excitation conditions are investigated. It is suggested that the sloshing effect can be detected by resolving the associated strongly excited doublet line structure in the excitation spectrum of the perturbed aggregate.
The steady state nuclear Overhauser effect revisited
Authors: Karthik G.
Published: July 2002

The steady state of a two spin system coupled to an isotropic environment and to each other through a dipolar interaction and under irradiation by a monochromatic, circularly polarized radio frequency field is determined ab initio using thermodynamic arguments. This steady state is used to describe the well known nuclear Overhauser effect in liquids. The steady state is also derived from the Solomon–Bloch set of equations used to describe the driven spin. It is shown that in the limit of weak driving, the two solutions coincide.
Dynamics in propylene carbonate and propylene carbonate containing LiPF6
Authors: Fontanella John J. , Wintersgill Mary C. , Immel Jeffrey J.
Published: July 2002

Electrical conductivity and differential scanning calorimetry studies (DSC) have been carried out on 1 M LiPF6 in propylene carbonate (PC) from 187 to 296 K. The electrical conductivity data are analyzed in terms of Vogel–Tammann–Fulcher (VTF), Williams–Landel–Ferry (WLF), and Bendler–Shlesinger (BENSH) formalisms. In addition, literature data for viscosity and relaxation times for PC (not containing salt) are analyzed using the same formalisms. Large, systematic variations are found in both the VTF and WLF fitting parameters with temperature interval. The large variation of the parameters with temperature demonstrates the failure of both formalisms and shows that care must be taken when utilizing the VTF/WLF parameters to draw conclusions concerning the behavior of systems. One conclusion which can be reached is that the VTF results indicate a significant difference between the molecular motions in PC and PC-containing LiPF6. Another is that the WLF parameter C1 is about 11 for low temperature data, referenced to the DSC glass transition temperature, Tg, which shows that for PC and PC-based liquids the value of all three dynamical quantities at Tg is about 11 decades smaller than the value predicted for very high temperatures. Finally, all data are found to be better fit by the BENSH equation.
Dissociation lifetimes of alkali halide dianions
Authors: Sommerfeld T. , Child M. S.
Published: July 2002

LiF 3 2− is the smallest dianion that has been predicted to be detectable in a mass spectrometer, but, so far all attempts to prove its existence have failed. In this communication we investigate the lifetimes of its vibrational states using one- and three-dimensional model Hamiltonians for the LiF32− dissociation process. Based on our findings we propose approximations to the number of states showing a long lifetime on a mass spectrometric time scale and compare the respective values for dianionic alkaline and alkaline earth fluoride systems. Further influences on the observable ion currents are briefly discussed and alternative experiments are suggested.
Analysis of the mechanical behavior of poly(trimethylene terephthalate) in an amorphous state under uniaxial extension–compression condition through atomistic modeling
Authors: Jang Seung Soon , Jo Won Ho
Published: July 2002

The mechanical behavior of poly(trimethylene terephthalate) (PTT) in an amorphous state was simulated under uniaxial extension–compression conditions through atomistic modeling. The elastic properties obtained from the well-defined structure were comparable with those obtained from experiments. Typical stress–strain curves showing yielding and plastic behaviors were obtained under both extension and compression conditions. It was observed that the stress under uniaxial compression is higher than the stress under uniaxial extension with the same strain. The absence of structural defects in samples for simulation leads us to conclude that the difference of stress levels between two conditions is not intrinsically related to structural defects. Various analyses of chain conformation such as bond orientation, the dihedral angle, and Voronoi volume tessellation shows that the PTT chain under extension undergoes conformational changes different from those under the compression condition. When the change in the energetic state of PTT is calculated as a function of strain, it is revealed that the dihedral angle torsion energy and the van der Waals interaction energy play more important roles in deformation under extension than under compression.
Energy and momentum transfer of He atoms scattered from a lithium fluoride crystal surface
Authors: Legge H. , Manson J. R. , Toennies J. P.
Published: July 2002

Both the energy transfer and the normal and tangential force coefficients for scattering of a nearly monoenergetic beam of He atoms (incident energy Ei=63 meV) from a clean single crystal LiF(001) surface have been measured under wind tunnel conditions in the range of surface temperatures from 300 to 720 K. Expressions are derived for the differential reflection coefficient for a realistic He-LiF(001) potential with the inclusion of multiphonon processes, and these expressions are used to calculate the average energy and momentum transfers as well as their higher moments. The comparisons between theory and experiment indicate the presence of a moderate degree of surface roughness. The measured recovery temperature depends on incident beam angle and is significantly larger than expected for complete or partial accommodation, in good agreement with the theoretical calculations.
Multicanonical jump walking: A method for efficiently sampling rough energy landscapes
Authors: Xu Huafeng , Berne B. J.
Published: July 2002

The multicanonical sampling and jump walking methods are combined to provide a new, effective means of overcoming quasiergodicity in Monte Carlo simulations. In this new method, configurations generated during a long multicanonical sampling are stored infrequently and a modified jump walking procedure is implemented using this set of configurations to sample phase space at low temperature. Multicanonical jump walking, as this new method is called, is compared with regular jump walking and with straight multicanonical ensemble sampling on two systems: a one-dimensional random potential and an Ar13 cluster. It is shown that for the same number of MC steps, the multicanonical jump walking method more efficiently samples the phase space than either the regular jump walking or the pure multicanonical ensemble sampling method.
Molecular theory of thermomechanical coupling in cholesteric liquid crystals
Authors: Sarman Sten
Published: July 2002

A cholesteric liquid crystal lacks a center of inversion and it is consequently different from its mirror image. The low symmetry allows linear cross couplings between thermodynamic forces and fluxes that are polar vectors and pseudovectors, respectively. This makes it possible for a temperature gradient, which is a polar vector to induce a director angular velocity, which is a pseudovector. The reverse is also possible; the torque conjugate to the director angular velocity can drive a heat current. This is the basis for the Lehman effect where a temperature gradient parallel to the cholesteric axis causes the local director to rotate. We use linear response theory to derive Green–Kubo relations and nonequilibrium molecular dynamics simulation algorithms for the transport coefficient that couples the temperature gradient and the director angular velocity. The theory is completely general and can consequently be used to find relations for any linear cross coupling coefficient between a polar vector and a pseudovector.
Effect of reagent rotation in O(1D)+H2 (v=0,j): A sensitive probe of the accuracy of the ab initio excited surfaces?
Authors: Lee Shih-Huang , Liu Kopin
Published: July 2002

By exploiting two different sources for the target H2 beam, normal H2 and para H2, the effect of the reagent rotation in the title reaction was deduced. It was found that while the H2 rotation has little influence on reactivity at low collision energies, it has position impact in promoting the reaction at higher collision energies. The latter finding is opposite to the previous theoretical prediction based on the ab initio RKHS excited potential energy surfaces.
The vibrational energy pattern in acetylene. V. C213H2
Authors: Di Lonardo G. , Fusina L. , Venuti E. , Johns J. W. C. , El Idrissi M. I. , Liévin J. , Herman M.
Published: July 2002

A total of 134 vibrational levels with assigned rotational structure have been gathered in the ground electronic state of C213H2. Most of these measurements are updated or new compared to the previously published data. Altogether, they cover the range up to 23 670 cm−1. 118 out of the 119 levels observed below 13 000 cm−1 have been simultaneously fitted using the so-called cluster model, already used to deal with the vibrational energy levels in other isotopomers of acetylene [El Idrissi et al., J. Chem. Phys. 110, 2074 (1999), and references therein]. Twenty-nine vibrational constants have been determined, including the off-diagonal parameters K3/245, K1/244, K1/255, K11/33, K14/35, and r45, with a rms of the fit equal to 0.52 cm−1. The same three constants of the motion as in C212H2 emerged, Ns=v1+v2+v3, Nr=5v1+3v2+5v3+v4+v5 and k=l4+l5. The energies of the levels above 13 000 cm−1 calculated with the obtained parameters compare reasonably well with the experimental values. For all levels the predicted rotational constants compare satisfactorily with the experimental results.
A theoretical study of the linear OCuO species
Authors: Deng Ke , Yang Jinlong , Yuan Lanfeng , Zhu Qingshi
Published: July 2002

Density functional calculations are performed to study the linear OCuO molecule in the neutral, cationic, and anionic charge states. The equilibrium bond lengths, vibrational frequencies, and electronic configurations are obtained. A theoretical assignment for the features in the photoelectronic spectrum is given at the local spin-density approximation level. Our results compare well with the available experimental results and show that the ground state of the OCuO molecule is the doublet (2Πg).
Damped oscillatory motion and internal friction in bulk polyatomic gases
Authors: de Lange O. L. , Pierrus J.
Published: July 2002

A modified version of Rüchardt’s experiment is used to measure the bulk moduli and relaxation times for damped oscillations in five gases with low γ=Cp/Cv (CHClF2, CHCl2F, C2Cl2F4, C2F6, C4F8). The results support the theoretical prediction that the internal friction is small for gases with γ≈1 (and not too large a thermal diffusivity). Two types of damped oscillation are observed: (i) Initial oscillations which are intermediate between the adiabatic and isothermal extremes. These oscillations occur if the initial amplitude exceeds a threshold value which increases with the volume of gas. (ii) The ensuing oscillations in the tail. For the latter, the measured relaxation times are consistent with isothermal values. However, the bulk moduli are appreciably lower than the corresponding isothermal values, and the disparity increases as the atomicity of the gas increases.
Perturbation theory approach to tunneling: Direct and resonance transmission in super-exchange models
Authors: Galperin Misha , Segal Dvira , Nitzan Abraham
Published: July 2002

In this paper we examine, within simple models, different approaches to computing tunneling probabilities in super-exchange models of electron transfer. The relationship between tunneling calculations that use scattering theory type formalisms and approaches based on standing waves, which are more closely related to electron transfer between bound donor and acceptor states, is established. Transmission probabilities computed by using truncated basis representations are compared to exact analytical or numerical results for one- and two-dimensional models. We find that while resonance tunneling is well approximated by truncated basis approaches, computing deep tunneling using such basis sets can lead to large errors. Implications for calculations of bridge assisted electron transfer are discussed.
A statistical rate theory description of CO diffusion on a stepped Pt(111) surface
Authors: Torri Massimo , Elliott Janet A. W.
Published: July 2002

The statistical rate theory approach is used to describe far-from-equilibrium diffusion of carbon monoxide on a stepped Pt(111) surface at low total coverages. Under nonequilibrium conditions, migration of adsorbates from terraces to steps, where adsorbates are more strongly bound, can occur. An expression for the molecular transport rate between terraces and steps is derived in terms of an equilibrium exchange rate, and the instantaneous chemical potentials of the molecules adsorbed on the terraces and along the steps. The theory contains no free parameters. Both the equilibrium exchange rate and the chemical potentials are obtained in the framework of a lattice gas model. The time evolution of the populations of steps and terraces is calculated and used to fit the available experimental data in order to evaluate the activation barrier of diffusion on terraces and the associated prefactor.
Relative mixing coefficients of zero-order B11(n,0,0) levels in the B11–A21 hybrid wave functions of the vibronic bands of SO2 in the 32 184–34 040 cm−1 region
Authors: Dastageer A. , Hegazi E. , Hamdan A. , Al-Adel F.
Published: July 2002

The relative mixing coefficients of the zero-order B11(n,0,0) vibrational levels present in the hybrid A11–B11 wave functions are estimated according to the observed Franck–Condon patterns in dispersed fluorescence spectra. Non-Condon effects and spin–orbit coupling have been ignored in the treatment. The data cover nearly 50 cold vibronic bands in the 32 895–34 040 cm−1 region in addition to 9 vibronic bands in the 32 185–32 559 cm−1 region.
Fluorescence quenching by reversible excimer formation: Kinetic study on the basis of generalized, non-Markovian rate equations
Authors: Naumann Wolfgang
Published: July 2002

The kinetics of fluorescence quenching by reversible excimer formation has been studied theoretically on the basis of generalized, non-Markovian rate equations derived from a diffusion-kinetic hierarchy approach in the low-reactant density limit. It is demonstrated that, in contrast to the case of reversible excitation transfer [W. Naumann, J. Chem. Phys. 110, 3926 (1999)], compact rate kernel expressions can be derived not only for contact encounters but also for more realistic, longer-range reactivities. Given as functionals of so-called phenomenological excimer formation and dissociation coefficients, the rate kernel expressions allow a critical assessment of approximations based on time-scale separation arguments. An effective excimer formation coefficient is defined, which leads to simple and physically transparent formulas for the Laplace transforms of the excited monomer and excimer concentrations. The effect of the back reaction on the total fluorescence yield is discussed. The study also includes a critical comparison to analogous results obtained for the case of reversible energy transfer quenching.
Orbital transformations and configurational transformations of electronic wavefunctions
Authors: Atchity Gregory J. , Ruedenberg Klaus
Published: July 2002

Transformations among molecular orbitals are often expedient or illuminating, and sometimes essential in quantum chemical contexts. In order to express the many-electron wavefunction in terms of the corresponding transformed configurations, full CI calculations used to be repeated in the transformed orbital basis. The configurational transformations can however be obtained directly, as shown by Malmqvist, by a factorization into single orbital transformations. In the present paper, a direct transformation method is presented that is based on the factorization of orbital transformations in terms of Jacobi rotations. Compared to the repetition of a CI calculation, both direct re-expansion methods drastically reduce the computational effort and increase the numerical accuracy. They are, moreover, applicable to wavefunctions whose original construction is not accessible.
Electronic states of the copper silicide and its ions
Authors: Turski Przemysław , Barysz Maria
Published: July 2002

Potential energy curves and spectroscopic parameters of the ground and exited states of SiCu, SiCu+, and SiCu− are presented. The calculations were performed by high-level correlated methods including the relativistic correction for the lowest states. The present results are compared with recent theoretical and experimental studies of SiCu and its ions and support the earlier theoretical conclusions concerning the assignment of the electronic ground state of SiCu. According to calculations presented in this paper the lowest energy states of SiCu, SiCu+, and SiCu−, are 2Πr, 1Σ+, and 3Σ−, respectively.
Two-pulse laser control for selective photofragment orientation
Authors: Machholm Mette , Henriksen Niels E.
Published: July 2002

The nuclear wave packet dynamics in the potential well of a bound molecule can be controlled by an intense infrared (IR)-laser pulse. The phase of the nuclear wave packet motion is shown to depend on the phase of the laser field and the initial orientation of the molecule. We demonstrate, for diatomic heteronuclear molecules, that these spatial effects can be used to control the angular distribution of photofragments by selective dissociation of molecules with a given initial orientation from a sample of randomly oriented molecules.
GAUSSIAN-2 calculations of the electron affinities of PCln and POCln
Authors: Miller Thomas M. , Viggiano A. A. , Morris Robert A. , Stevens Miller Amy E.
Published: July 2002

The results of GAUSSIAN-2 calculations are presented for the total energies and zero-point energies of neutral and anionic PCln and POCln (n=1–3), from which adiabatic electron affinities (EA) are determined for the ground-state neutral species. The calculations show that EA(POCl2)>EA(Cl), by 0.11 eV, and that EA(PCl3) deduced from collisional ionization experiments is probably too low by 0.4 eV.
A Δ-self-consistent-field study of the nitrogen 1s binding energies in carbon nitrides
Authors: Johansson Åsa , Stafström Sven
Published: July 2002

Binding energies of the N 1s level in hard and elastic CNx films are investigated by means of theoretical studies of model molecules. Results for the model systems are obtained from ab initio Hartree–Fock calculations, where the core electron binding energies are determined using the delta self-consistent-field method. The theoretical results are compared with experimental x-ray photoelectron spectroscopy data in order to understand the microstructure of the CNx films. Both sp2 and sp3 bonded nitrogen have been identified. The presence of nitrogen bonded to graphite edges is also likely.
Large ionic clusters in concentrated aqueous NaCl solution
Authors: Degrève Léo , da Silva Fernando Luı́s B.
Published: July 2002

The stability of the local structure of aqueous 1.0 M NaCl solution at 293 K was investigated by molecular dynamics simulations. The mean and maximum life-times of the ion pairs were determined to be 0.13, 0.14 and 0.27 ps for negatively charged, neutral and positively charged ions pairs, respectively. The stability of non-neutral ion pairs was studied from the structure of the anion–cation radial distribution function and other structural functions. We found that non-neutral ions pairs are stabilized by at least one counter-ion forming in this way large ionic clusters that include the hydration molecules. About 25% of the ions are included in neutral clusters formed by a minimum of four ions. The existence of these large clusters obfuscates the commonly accepted chemical reaction scheme for the interconvention of ion pairs in aqueous solutions.
The influence of polymer on the diffusion of a spherical tracer
Authors: Berezhkovskii A. M. , Bezrukov S. M. , Bicout D. J. , Weiss G. H.
Published: July 2002

We analyze how the addition of a small number of polymer molecules influences the diffusion constant of a spherical tracer, whose radius is small compared to the size of the polymer. We show that the polymer chain can be regarded as a two-dimensional object which is an impenetrable obstacle for the tracer. It is also shown that the diffusion constant of the tracer, in contrast to the solution viscosity, is independent of chain length, depending only on the monomer concentration.
The effect of coupled nonreactive modes on laser control of quantum wave packet dynamics
Authors: Karmacharya Rakesh , Gross Peter , Schwartz Steven D.
Published: July 2002

The laser control of reactions in solution has recently been a topic of growing interest. The theoretical possibility for such control has now been established. This paper investigates two closely related issues regarding potential practical implementation of condensed phase control schemes. A previous study from our group was able to demonstrate control assuming that a laser field coupled only to a reaction coordinate. The assumption that the laser will not couple to the condensed phase environment is a drastic simplification, and we investigate in this paper how relaxing this simplification may affect the possibility of control. To investigate this phenomenon, we study two cases: that of a laser coupled only to a reaction coordinate which is in turn coupled to a “bath” mode, as compared to the case in which the laser is coupled both to the reaction coordinate and the environmental mode. In another closely related investigation, we study the effect of uncertainty introduced into the controlling pulse. The exact potential of a chemical reaction in solution cannot be known to perfect accuracy. Our results give insights into the challenges which will face attempts at condensed phase chemical reaction control, and point strongly to the need for adaptive algorithms for laser control pulse generation.
Quantum beats and ultrafast solvation dynamics
Authors: Zolotov B. , Huppert D. , Fainberg B. D.
Published: July 2002

The beats during solvation dynamics have been studied theoretically and experimentally, using the heterodyne optical Kerr effect and pump–probe spectroscopy. We showed that the solvation process increases the beat contrast due to the hole-burning effect. Owing to the dipole activity of the intramolecular vibration responsible for the beats, a dipole–dipole coupling between the intramolecular vibration and the solvation coordinate is available. We have calculated a correlation function of the perturbation of nuclear motion during electronic transition, when such a coupling exists. Taking into account the linkage between the intramolecular vibration and the solvation coordinate yields better agreement between theory and the experimental data. We have evaluated the value of the linkage. The evaluation conforms with the values obtained by the computer fit of the experimental signals.
Reaction dynamics of O(1D)+HD. II. Effects of excited surfaces
Authors: Hsu Yen-Tsung , Liu Kopin , Pederson Lisa A. , Schatz George C.
Published: July 2002

To explore the effects of excited surfaces on the title reaction, an in-depth crossed-beam investigation was carried out at two collision energies, 3.7 and 4.6 kcal/mol. In combination with the results obtained at the lower energy, 2.05 kcal/mol (the preceding paper), the extent of abstraction contribution to total reactivity was deduced from the doubly differential cross-section measurements and its dominant dynamical features were revealed for the first time. The former was found to be in reasonable agreement with our previous estimate based on the excitation function data. We also present quasiclassical trajectory results based on accurate potential energy surfaces for the 1 1A′ and 1 1A″ surfaces. The trajectory results give approximately the same breakdown into insertion and abstraction components as in the experiments, and some aspects of the product distributions, such as the angular distributions, are in rough agreement. However, other aspects of the product distributions, such as the abstraction component of the product translational distributions, are in serious disagreement. Possible deficiencies in the potential surfaces and dynamics that could be responsible for these results are considered.
Possibilities for a density matrix theory
Authors: Nooijen Marcel
Published: July 2002

Two possible routes are considered to arrive at a one-particle reduced density matrix formulation of electronic structure theory. In the first scheme, an extended Fock matrix H is defined that has twice the dimension of the one-particle basis set. The corresponding Green’s function, defined as the upper left block of (ω1−H)−1, yields the exact one-particle density matrix and energy. The poles of the Green’s function are precisely the ionization potentials and electron affinities of the extended Koopmans theorem. In the second scheme, a generalized Fock equation [F(ρ),ρ]=X is derived that is satisfied by the exact non-idempotent one-particle density matrix. The antisymmetric matrix X on the right-hand side is obtained from the irreducible part of the two-particle reduced density matrix, while F is the usual Fock matrix defined using the correlated one-matrix. The generalized Fock equation is a necessary condition but does not determine ρ uniquely. Alternatively, the one-matrix can be obtained from the irreducible part of the two-matrix directly, using a sum rule. The analysis leads to some additional desiderata and separability properties that may be imposed on traditional wave function based approaches. Possibilities for practical computational schemes are addressed briefly.
N
14
nuclear magnetic resonance of polycrystalline solids with fast spinning at or very near the magic angle
Authors: Khitrin A. K. , Fung B. M.
Published: July 2002

The N14 nuclear magnetic resonance spectra of polycrystalline samples of KNO3, NH4NO3, and several amino acids have been studied under the condition of magic angle spinning. These compounds have N14 quadrupolar coupling constants of about 1 MHz, but it is possible to obtain peaks with linewidths of a few hundred Hz and good signal/noise ratio. The spectra contain information on both isotropic chemical shifts and quadrupolar coupling constants. The effect of very small deviations from the magic angle (⩽0.05°) is studied, and the corresponding spectral features are analyzed qualitatively and quantitatively. The use of a weak pulse or a weak composite pulse causes an increase in the signal intensity, and an analysis is given to explain these unusual results.
A nine-dimensional perturbative treatment of the vibrations of methane and its isotopomers
Authors: Wang Xiao-Gang , Sibert Edwin L.
Published: July 2002

The vibrations of methane isotopomers with Td, C3v, and C2v symmetry are studied by means of high order Van Vleck perturbation theory. The vibrational states up to 9000 cm−1 are investigated by combining the ab initio force field of Lee, Martin and Taylor [J. Chem. Phys. 95, 254 (1995)] with a fourth order perturbative treatment based on curvilinear normal coordinates. Implementation of the perturbation theory using both analytical and numerical expression of the kinetic energy operator is considered. The quadratic and select cubic and quartic force constants are refined via a nonlinear least squares fit to experimental data The fit force constants reproduce 130 experimental band centers with a root mean squares deviation of 0.70 cm−1. The choice of polyad quantum number is discussed with respect to different molecules. The convergence of the energy levels is discussed by carrying out the perturbation calculation up to eighth order.
Evolution of the structure and mechanical stability of self-assembled alkanethiol islands on Au(111) due to diffusion and ripening
Authors: Barrena E. , Ocal C. , Salmeron M.
Published: July 2002

An atomic force microscope (AFM) study of the structure and stability of islands of alkanethiol (C16H33SH) grown on Au(111) is presented. Ripening processes involving long-range molecular diffusion takes place at ambient temperature in the scale of days. In the partially covered surface, ripening is manifested by an enlargement of the islands as well as by molecular densification. On the well packed islands, the c(4×2) superstructure is imaged. Another manifestation of island densification is a substantial increase in mechanical stability, as shown by the resistance of the film to penetration by the tip. The threshold pressure for molecular displacement and tip penetration was found to increase by almost 70% relative to the initial value before ripening.
The asymptotic exchange potential in Kohn–Sham theory
Authors: Tozer David J.
Published: July 2002

The asymptotic behavior of the exchange potential is derived from the long-range form of the exchange-only Kohn–Sham equations. This is then used to correct the asymptotic potential of a conventional continuum exchange functional. For atomic systems the influence of the correction on the electronic energy can be rigorously determined using a well-known virial relationship; its effect is to make the continuum exchange more like Hartree–Fock exchange. For molecular systems the model nature of the potential leads to problems with translational non-invariance. Molecular energies are therefore not considered, although optimized molecular structures can still be determined, providing internal coordinates are used. The effect of the asymptotic correction is to slightly shorten the bond lengths, again indicating an increased Hartree–Fock character. Possible implications for functional development are considered.
Core correlation effects in weak interactions involving transition metal atoms
Authors: Urban Miroslav , Sadlej Andrzej J.
Published: July 2002

In our calculations of weak interactions in transition metal complexes some rather unusual contributions due to excitations from relatively inert subvalence electronic shells have been observed for copper interacting with water and ammonia. These contributions are analyzed in terms of cluster amplitudes which indicate that the copper–molecule interaction considerably increases the near degeneracy of atomic states. The t1 amplitude associated with the 3d→4s excitation rapidly increases with the decrease of the copper–molecule distance. This, in consequence, leads to the increasing role of the 3p shell polarization. This effect is essentially absent in the case of silver complexes.