bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation transfer in disordered two-dimensional and anisotropic three-dimensional systems: Effects of spatial geometry on time-resolved observables
Authors: Baumann J. , Fayer M. D.
Published: July 2002

A unified treatment of dipole–dipole excitation transfer in disordered systems is presented for the cases of direct trapping (DT) in two-component systems and donor–donor transfer (DD) in one-component systems. Using the two-particle model proposed by Huber we calculate the configurational average of Gs(t), the probability of finding an initially excited molecule still excited at time t. For the isotropic three-dimensional case treated by Huber excellent correspondence is found with the previously reported infinite diagrammatic approximation. The anisotropy of the dipole–dipole interaction is included in the averaging procedure. Two regimes of orientational mobility are considered: the dynamic and static limit, rotations being much faster or slower, respectively, than the energy transfer. The following geometrical distributions are investigated: (a) Infinite systems of one, two, and three dimensions which lead to Förster-like decays. Two orientational distributions are considered for monolayers: dipoles confined to the plane or oriented isotropically. (b) Bilayers and multilayers. The averaging procedure for transfer from one layer to another is outlined in detail. The main parameters determining the decay of Gs(t) are the surface concentration and the ratio of the layer separation and the Förster radius. In a stack with a small number of layers, which is a finite system in one of the dimensions, an average over positions of the initially excited donor is included. At low surface concentration the decay gradually changes from two- to three-dimensional character as one increases the number of layers. This fractal-like behavior is solely due to the presence of excluded volumes and the finite nature of the system. Experimental observables are considered in detail. An analysis including a general formalism is presented to determine the loss of polarization memory if an excitation is transferred to a random distribution within the given geometrical constraints. It follows that after one transfer step, in the worst case, less than 10% of the initial anisotropy is conserved if the appropriate observation geometry is chosen. The anisotropy decay, which is manifested in a transient grating or fluorescence depolarization experiment, is

thus a useful observable for Gs(t) in DD transfer.
An EXAFS investigation of Cu–ZnO methanol synthesis catalysts
Authors: Sankar G. , Vasudevan S. , Rao C. N. R.
Published: July 2002

Cu K-edge EXAFS of calcined and reduced Cu–ZnO methanol synthesis catalysts (with a maximum copper content of 33 at wt. %) have been studied. The calcined catalysts contain two species, Cu2+ in a CuO-like phase and Cu2+ occupying substitutional sites in the ZnO lattice. The reduced catalysts contain three species, Cu0, Cu1+ in a Cu2O-like phase, and Cu1+ in an interstitial site in the ZnO lattice. In carrying out the EXAFS analysis of these multiphasic catalysts, we have employed an additive relation for the EXAFS function and made use of residual spectra for identifying unknown species.
On the large-gradient behavior of the density functional exchange energy
Authors: Becke A. D.
Published: July 2002

A coordinate-space model of the exchange hole density, previously introduced by the author, is extrapolated to the case of very strongly inhomogeneous systems (i.e., large density gradients). As a result, we propose a new gradient-corrected exchange energy functional for application to atomic and molecular problems. The model provides theoretical estimates of the parameters in the functional, and these compare well with empirical values deduced from a least squares fit to exact atomic data.
A full CI treatment of the 1A1–3B1 separation in methylene
Authors: Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

Full CI calculations are presented for the 1A1–3B1 separation in CH2 as a calibration for other levels of theory. Compared to the full CI result, the SCF/SDCI separation, after applying the Davidson correction for higher excitations, is found to be in error by only 0.38 kcal/mol, while the error in a CASSCF/MRSDCI treatment is less than 0.01 kcal/mol.
A lattice field theory for polymer systems with nearest-neighbor interaction energies
Authors: Bawendi M. G. , Freed Karl F. , Mohanty Udayan
Published: July 2002

We generalize a lattice field theory that formally provides an exact description of the statistical mechanical entropy of nonoverlapping flexible polymers to enable treatment of nearest-neighbor interaction energies. The theory is explicitly solved within an extended mean field approximation for a system of polymer chains and voids, and we also provide mean field results for polymer–solvent–void and binary blend–void mixtures. In addition to recovering the Flory–Huggins mean field approximation for these systems, our extended definition of the mean field approximation contains a set of corrections to Flory–Huggins theory in the form of an expansion in powers of the nearest-neighbor interaction energies.
The dielectric relaxation of mixtures of water and primary alcohol
Authors: Mashimo Satoru , Kuwabara Shinichi , Yagihara Shin , Higasi Keniti
Published: July 2002

Dielectric measurements over a microwave frequency range 10 MHz–15 GHz were carried out by the use of new time domain reflectometry equipment on the mixtures of water with five primary alcohols, viz., methanol, ethanol, and n-propanol in the concentration range 0≤x≤1 and n-butanol and amyl alcohol in the range 0≤x≤0.5 at room temperature; x being the mole fraction of water. The systems of water and two alcohols of low molecular weight are characterized by a single relaxation with a distribution parameter of the unity or near to it. The molecular reorientation in the mixtures as well as water and these alcohols is a cooperative process involving a large number of molecules with the hydrogen-bond linkages (O–H⋅⋅⋅O). Dielectric behavior of the mixtures of water and methyl or ethyl alcohol is due to the structure of a hydrogen-bonded network being microscopically homogeneous. Microscopic heterogeneity occurs in the mixtures of water and higher alcohols.
Morphological studies of micelle formation in block copolymer/homopolymer blends
Authors: Kinning David J. , Thomas Edwin L. , Fetters Lewis J.
Published: July 2002

The micellar structure of poly(styrene-butadiene) diblock copolymer/polystyrene homopolymer blends has been investigated by small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) as a function of copolymer concentration, copolymer block molecular weights, and homopolymer molecular weight. The SAXS from blends exhibiting spherical micelles, as evidenced by TEM, has been modeled using a polydisperse Percus–Yevick hard sphere fluid model to obtain the interparticle interference contribution to the scattered intensity. By combining this modeling with quantitative TEM and SAXS invariant analysis, the structure of the micelles as a function of the molecular parameters can be completely characterized. In addition, a transition from liquid-like to an ordered simple cubic arrangement of micelles has been observed upon increasing the copolymer concentration.
Thermophysical properties of the lanthanide sesquisulfides. I. Schottky functions and magnetic and electronic properties of γ-La2S3, γ-Ce2S3, γ-Nd2S3, and γ-Gd2S3
Authors: Westrum Edgar F. , Burriel Ramón , Gruber John B. , Palmer P. E. , Beaudry B. J. , Plautz W. A.
Published: July 2002

Heat capacity measurements of four lanthanide sesquisulfides La2S3, Ce2S3, Nd2S3, and Gd2S3, prepared in the γ phase, have been obtained between 6 and 350 K by adiabatic calorimetry. The total heat capacity has been resolved into lattice, electronic, magnetic, and Schottky components. The Schottky contributions agree well with the calculated values based on the observed splitting of the ground-state manifold of the rare earth ions occupying sites of S4 symmetry in the Th3P4 structure. The observed splitting is obtained from an analysis of the hot bands in the absorption spectrum and from direct observation of the Stark levels in the far infrared. The Stark levels (all doublets) for Ce2S3 (2F5/2) are 0, 185, and 358 cm−1; for Nd2S3(4I9/2), they are 0, 76, 150, 180, and 385 cm−1. For La2S3, which has no Schottky or magnetic contributions to the heat capacity, the thermal data can be extrapolated to 0 K. The entropy for La2S3 at 298.15 K (as S0/R) is 19.51. Schottky and magnetic ordering at lower temperatures in Ce2S3, Nd2S3, and Gd2S3 preclude such extrapolation techniques. Therefore the entropy at 298.15 K for these compounds {S0–S0(7 K)}/R, is 21.34, 22.38, and 20.05, respectively.
Order–disorder transition as a function of surface coverage for n-hexadecanoic acid chemisorbed on aluminium
Authors: Sondag A. H. M. , Raas M. C.
Published: July 2002

Fourier transform infrared multiple specular reflectance (FTIR-MSR) spectroscopy has been applied to study the chemisorption of n-hexadecanoic or palmitic acid [CH3(CH2)14COOH] on aluminium as a function of surface coverage. The intensity of the symmetric carboxylate stretching vibration has been used to estimate the surface coverage. The intensities and band positions of the CH stretching modes are presented as a function of surface coverage. A sudden transition at about 80% of monolayer coverage is observed for the intensity as well as the position of the CH2 stretching modes. The intensity changes correspond to a change from a more or less planar orientation of the polymethylene chains at low surface coverage to an almost perpendicular orientation at high surface coverage. The asymmetric CH3 stretching vibration exhibits a shift opposite to and smaller than what is observed for the CH2 vibrations. No significant changes are observed for the position of the symmetric CH3 stretching vibration. However, for the symmetric carboxylate stretching vibration a sudden shift of the band position is observed of the same magnitude, but opposite to the shifts of the CH2 vibrations.
Superradiance in molecular aggregates
Authors: Spano Frank C. , Mukamel Shaul
Published: July 2002

Using a reduced equation of motion for the density matrix which accounts for spontaneous emission and superradiance, we analyze the fluorescence and transient grating (TG) decays from a dilute, optically thin distribution of molecular aggregates. We find that the fluorescence is a limited form of superradiance, where cooperativity is restricted to the number N of two level systems which make up the aggregate. The dependence of the linear aggregate decay rate on the amount of inhomogeneous broadening, which is randomly distributed according to Gaussian line shape of width (1/e)2σ, is calculated. It is shown that σ must be comparable in magnitude to the nearest neighbor dipole–dipole coupling V and not the superradiant decay rate, in order to quench the superradiance.
On the equation of state of athermal lattice chains: Test of mean-field and scaling theories in two dimensions
Authors: Dickman Ronald
Published: July 2002

A recently devised method for determining the pressure in lattice simulations is applied to two-dimensional, athermal chains of 40, 80, and 160 segments, over the full range of fluid densities, from dilute solution to dense melt. The results are used to test Bawendi and Freed’s correction to Flory–Huggins mean-field theory, and the des Cloizeaux scaling law. The scaling of the mean-square end-to-end distance with density is also discussed.
On the theory of concentration depolarization of fluorescence in one- and two-component systems for multipole interactions in one, two, and three dimensional medium
Authors: Sienicki K. , Itagaki H. , Mattice W. L.
Published: July 2002

A theoretical analysis of the emission anisotropy of one- and two-component systems for multipole interactions in one, two, and three dimensional medium is presented. The developed model explores an alternative extension of the Huber et al. theory [Phys. Rev. B 16, 4642(1977)] for higher concentrations where pair approximation is not fulfilled. A detailed analysis of emission anisotropy for three dimensional medium and dipole–dipole interactions has been presented. Inclusion of the possibility of return of the excitation to the originally excited donor molecule from more and more remote donor molecules lead to a substantial increase in the fluorescence emission anisotropy. Numerical analysis shows that experimental results of the emission anisotropy for rhodamine B in the presence of malachite green are well described by the equations presented here for a two-component system. A comparison with models developed by other authors has been presented.
Wave packet theory of dynamic absorption spectra in femtosecond pump–probe experiments
Authors: Pollard W. Thomas , Lee Soo-Y. , Mathies Richard A.
Published: July 2002

The large spectral width of ultrashort optical pulses makes it possible to measure the complete time-resolved absorption spectrum of a sample with a single pulse, offering simultaneously high resolution in both the time and frequency domains. To quantitatively interpret these experiments, we start with the usual perturbative density matrix theory for the third-order susceptibility of a multilevel system. However, the theory is formulated in terms of four-time correlation functions which are interpreted as the time-dependent overlap of bra and ket vibrational wave packets propagating independently on the ground and excited electronic state potential surfaces. This approach captures the critical distinction between electronic population decay and pure dephasing processes, while retaining the intuitive physical picture offered by the time-dependent wave packet theories of molecular spectroscopy. A useful simplification is achieved by considering the absorption of the probe pulse as the first-order spectroscopy of the nonstationary state created by the pump pulse. In this case, the dynamic spectrum is obtained through the Fourier transform of the time-dependent overlap of the initial wave packet propagating on its potential surface and a second wave packet, created by the probe pulse, which evolves simultaneously on the final surface. Calculations for model systems using harmonic surfaces and δ-function pulses are presented to illustrate the application of this theory and to clarify the unique spectral behavior of the nonstationary states created in femtosecond pump–probe experiments. Finally, we demonstrate the practical application of the theory for anharmonic surfaces and finite pulses by analyzing the dynamic spectroscopy of the excited state torsional isomerization of the bacteriorhodopsin chromophore.
A theoretical investigation of HSO/HOS and their positive ions
Authors: Plummer Patricia L. Moore
Published: July 2002

The formation and eventual fate of sulfur-containing aerosols play a central role in global pollution. An understanding of the oxidation paths for sulfur species and of the formation and stability of radical and ionic intermediates is required for optimum control of acid deposition. To gain insight into these processes ab initio calculations were performed for ground and first excited electronic states of the isomers HSO/HOS and for the ground and first two excited states for their positive ions, HSO+/HOS+. A variety of basis sets were used for calculations at the self-consistent field (SCF) level. (Full details available in supplementary material for this article.) These calculations included full optimization of the geometry and examination of the potential surface for transition states. Post Hartree–Fock calculations were performed and included Moller–Plesset calculations through fourth order with quadruple excitations as well as configuration interaction including all double excitations. The effect of geometry optimization at the MP4 level and the effect of freezing the core orbitals in the calculations were examined. Five transition states for the isomerization of HSO to HOS were located for the ground and excited electronic states of the neutral and ion systems. For all transition states in both systems, the geometry resembles an isosceles triangle. The barriers to rearrangement are quite high, ranging from ∼20 to ∼60 kcal/mol depending on the state. Two of the barriers were approximately symmetric while three were asymmetric with a lower barrier for HSO to HOS isomerization.
Photon- and electron-induced chemistry of chlorobenzene on Ag(111)
Authors: Zhou X.-L. , White J. M.
Published: July 2002

The surface chemistry, induced by UV photons and low energy electrons, of chlorobenzene (C6H5Cl) on Ag(111) has been studied. C6H5Cl adsorbs molecularly on Ag(111) at 100 K and desorbs molecularly at 170 K (physisorbed) and 230–240 K (chemisorbed) with no thermal decomposition. The adsorption is accompanied by a surface work function change (ΔΦ) of −0.65 eV at one monolayer and −0.9 eV at multilayer coverages. Both UV photons and low energy electrons induce the decomposition of adsorbed C6H5Cl. The energy threshold for the decomposition is 3.5–3.8 eV for photons and ∼5 eV for electrons. For photons and <12 eV electrons, the decomposition involves only C–Cl bond cleavage, producing surface phenyl groups and Cl atoms. Phenyl groups recombine and desorb as biphenyl at 390–400 K during post-irradiation temperature programmed desorption (TPD). They do not dehydrogenate. However, for electron energies higher than ∼12 eV, the decomposition involves cleavage of C–H, C–Cl, and, probably, C–C bonds. In this case, post-irradiation TPD shows the desorptions of H2, HCl, AgCl, benzene, biphenyl, and C6H5Cl with C left on the surface. The cross section for the photodissociation is 3×1021 cm2 at 254 nm and decreases with increasing wavelength. The cross section for the electron-induced decomposition (EID) is 8×1017 cm2 (upper limit) at 50 eV and decreases with electron energy. The photodissociation on the surface is ascribed to direct photon induced n→σ* transition and/or photoexcited electron attachment processes. Compared to the gas phase, the photodissociation threshold is red shifted. This is qualitatively explained by direct excitation process assuming an effective stabilization of the excited C6H5Cl by Ag(111) (strong adsorbate–substrate interactions) and/or ‘‘hot’’ electrons. EID by <12 eV electrons probably involves dissociative electron attachment and electronic excitation processes; EID by >12 eV electrons also involves ionization processes.
Spatial dependence of time-dependent friction for pair diffusion in a simple fluid
Authors: Straub John E. , Berne Bruce J. , Roux Benoît
Published: July 2002

Molecular dynamics simulations are used to determine the time-dependent friction for pair diffusion in an isotropic Lennard-Jones fluid as a function of the separation between two diffusing particles. A numerical method proposed by Straub, Borkovec and Berne is used. It is found that both the initial value and the detailed time-dependence of the friction are dependent on the interparticle separation. The dependence of the pair diffusion coefficient on separation is determined. Comparisons are made with various hydrodynamic and collision theories. The rate constant for diffusion controlled reactions is discussed.
Study of collisional effects on band shapes of the ν1/2ν2 Fermi dyad in CO2 gas with stimulated Raman spectroscopy. II. Simultaneous line mixing and Dicke narrowing in the ν1 band
Authors: Lavorel B. , Millot G. , Saint-Loup R. , Berger H. , Bonamy L. , Bonamy J. , Robert D.
Published: July 2002

An experimental (SRS) and theoretical analysis for the ν1 component of the ν1/2ν2 Fermi dyad of CO2 has been performed for densities lying from 0.01 to 50 amagat at 295 K, and from 0.01 to 20 amagat at 500 K. At subatmospheric pressure, both line mixing and Dicke narrowing take place for this component due to the very weak Q line spacings. A simple method to account for both diffusional narrowing (due to velocity changing collisions) and collisional narrowing (due to energy transfers) on isotropic Raman Q-branch profile is proposed. This method is based on the transformation of the collapsed Q-branch profile as a sum of individual Lorentzian plus dispersive components whose parameters are density-dependent. Such an exact transformation permits to easily introduce the averaging effect of velocity changing collisions on each component, and then on the collapsed Q-branch itself. In the present study, the Galatry soft collision model is used to define a generalized complete profile for each Lorentzian plus dispersive component. Such a procedure allows us to take into account the velocity changing collision’s effects not only on isolated lines (the well-known Dicke narrowing) but also on the line couplings resulting from collisionally induced rotational energy transfers. The present analysis permits an accurate description of the observed modifications on the SRS profile of the ν1 band of CO2 (1388 cm−1) as a function of density. The straightforward extension to other spectroscopies (linear and nonlinear) is suggested.
Laser deposition of carbon clusters on surfaces: A new approach to the study of Fullerenes
Authors: Meijer Gerard , Bethune Donald S.
Published: July 2002

We have accumulated large pure carbon clusters (Fullerenes) on a surface. These clusters were produced by laser vaporization of graphite in a static rare-gas atmosphere. Their presence on the surface was confirmed by an isotope scrambling experiment using a laser-desorption/laser-ionization mass spectrometric detection scheme.
Potential energy surface for unimolecular dissociation and rearrangement reactions of the ground electronic state of HFCO
Authors: Kamiya Kenshu , Morokuma Keiji
Published: July 2002

The potential energy surface of the HFCO molecule in its electronic ground state has been investigated with ab initio method, at levels up to MP4(SDTQ)/6-311G**//MP2/6-31G*. At the highest level, the barrier height for molecular dissociation (HFCO→HF+CO) was calculated to be 46.9 kcal/mol with a zero-point energy correction, in good agreement with an experimental estimate and a recent theoretical result. The intrinsic reaction coordinate (IRC) for molecular dissociation was traced and the coupling between the IRC and normal modes as well as that among the normal modes was analyzed along the IRC. The analysis is consistent with the mode specificity of recently observed quasistable vibrational states of HFCO above the dissociation limit. Almost all possible stationary points on the potential surface of the HFCO system have been located, including the rearrangement and atomic dissociation products and transition states, as well as van der Waals complexes. These are compared with the H2CO system. All the species that have an FO bond are unstable or nonexistent, reflecting the weakness of the bond.
A three-photon fluorescence excitation study of the S(A1g)→S1(B2u) transition in neat liquid benzene
Authors: Shreve A. P. , Albrecht A. C.
Published: July 2002

The three-photon fluorescence excitation spectrum of the S0(A1g)→S1(B2u) transition in neat liquid benzene is reported. For both linear and circular incident polarization, the electronic origin and its associated ν1 Franck–Condon progression are observed. The appearance of the electronic origin in the neat liquid is contrasted with the results of previous studies demonstrating the surprising absence of the group-theoretically allowed origin in the three-photon spectra of benzene vapor and dilute benzene solutions.
Density functional Gaussian-type-orbital approach to molecular geometries, vibrations, and reaction energies
Authors: Andzelm J. , Wimmer E.
Published: July 2002

We present the theory, computational implementation, and applications of a density functional Gaussian-type-orbital approach called DGauss. For a range of typical organic and small inorganic molecules, it is found that this approach results in equilibrium geometries, vibrational frequencies, bond dissociation energies, and reaction energies that are in many cases significantly closer to experiment than those obtained with Hartree–Fock theory. On the local spin density functional level, DGauss predicts equilibrium bond lengths within about 0.02 Å or better compared with experiment, bond angles, and dihedral angles to within 1–2°, and vibrational frequencies within about 3%–5%. While Hartree–Fock optimized basis sets such as the 6-31 G** set can be used in DGauss calculations to give good geometries, the accurate prediction of reaction energies requires the use of density functional optimized Gaussian-type basis sets. Nonlocal corrections as proposed by Becke and Perdew for the exchange and correlation energies are found to be essential in order to predict bond dissociation energies and reaction energies within a few kcal/mol. The computational efficiency of the present method together with its accuracy, which is comparable to correlated Hartree–Fock based methods, promises a great usefulness of the DGauss approach for the study of large and complex molecular structures.
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o studies of dissociation pathways on the ground state potential energy surface for HFCO and HClCO
Authors: Francisco J. S. , Zhao Y.
Published: July 2002

Reaction pathways for the decomposition of HFCO and HClCO on the ground state potential energy surface have been studied by using ab initio methods. Heats of reaction and barrier heights have been computed by using Mo/ller–Plesset perturbation theory. Spin projections have been applied to free radical dissociation pathways for annihilation of spin contamination. The favorable dissociation path predicted is molecular elimination of HX to yield CO. The substitution effects on decomposition pathways of HFCO and HClCO are also examined.
Monte Carlo simulation of liquid n-alkanes. I. Intramolecular structure and thermodynamics
Authors: Almarza N. G. , Enciso E. , Bermejo F. J.
Published: July 2002

The conformational properties of liquid n-alkanes (ranging from n-pentane to n-decane) have been investigated using Monte Carlo computer simulation techniques. The method of simulation combines the ‘‘reptation’’ method with a scheme of preferential sampling, which leads to an improvement of the simulation efficiency. The change of internal properties and structure as an effect of the density is studied.
Femtosecond coherent transient infrared spectroscopy of CO on Cu(111)
Authors: Owrutsky J. C. , Culver J. P. , Li M. , Kim Y. R. , Sarisky M. J. , Yeganeh M. S. , Yodh A. G. , Hochstrasser R. M.
Published: July 2002

Femtosecond infrared coherent transients have been measured for the stretch vibration of CO on Cu(111). The free induction decay exhibits a dephasing time (T2) of 2±0.3 ps (and 2±0.1 ps assuming a single exponential decay between 2 and 3 ps). The decay was best fit by exponential relaxation, thereby suggesting that the CO vibrational band is almost entirely homogeneously broadened. The surface sum frequency spectrum was also measured at two coverages (0.10 and 0.45 L) using spectrally narrowed pulses. Interferences were observed leading to a determination of the relative phase and amplitude of the resonant and nonresonant second-order susceptibility in this system. The magnitude of the nonresonant susceptibility was only weakly dependent on coverage, suggesting that the nonresonant polarizability originates in the bulk Cu. Time and frequency domain results were in good agreement.
Open-shell coupled-cluster theory
Authors: Jayatilaka Dylan , Lee Timothy J.
Published: July 2002

An efficient formulation of our recently proposed [Chem. Phys. Lett. 199, 211 (1992)] open-shell singles and doubles coupled-cluster (OCCSD) method is presented. This formulation is in terms of spatial orbital one- and two-electron integrals. Our new OCCSD method is based on ‘‘symmetric spin orbitals’’ and is thus symmetric (or antisymmetric) in the spin indices. It therefore contains about half the number of independent parameters in the coupled-cluster wave function compared to other open-shell CCSD methods. It is shown that the formulation presented here contains less than half the number of n6 steps (where n is the number of molecular orbitals) of other recently proposed open-shell CCSD methods. A new approach by which amplitudes in our method may be compared with amplitudes in a previous OCCSD method is examined.
Density functional description of transition structures using nonlocal corrections. Silylene insertion reactions into the hydrogen molecule
Authors: Sosa Carlos , Lee Chengteh
Published: July 2002

The insertion reactions of SiH2 , SiHF, and SiF2 into the hydrogen molecule have been investigated using density functional methods. Local spin density (LSD) calculations were performed with the LSD exchange functional and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (B–P); Becke and the correlation energy functional of Lee, Yang, and Parr (B–LYP); and Perdew and Wang (PW) generalized gradient exchange-correlation energy functional. Reactants, transition structures, and products were fully optimized at the LSD and NLSD levels. For each of these reactions, relative energies have been calculated with density functional methods and also at the quadratic configuration interaction with single, double, and triple excitations [QCISD(T)]/6-31G(2df,pd) level. Vibrational frequencies were also computed with local and nonlocal approximations as well as at the second-order Mo/ller–Plesset (MP2)/6-31G(d,p) level. As previously reported in the Hartree–Fock case, density functional methods also predict a dramatic increase in the barrier height with fluorine substitutions. The best estimated barrier heights for SiH2+H2 , SiHF+H2 , and SiF2+H2 are −3.52±3, 16.07±3, and 50.49±2 kcal/mol, respectively.
The optical dielectric function of polarizable liquids
Authors: Leegwater Jan A. , Mukamel Shaul
Published: July 2002

We report numerical simulations of the density of states and the frequency dependent dielectric function for optical frequencies of a polarizable Lennard-Jones atomic fluid from gas to liquid densities. Our simulations at high densities can be successfully interpreted in terms of fluctuations around a fcc solid reference model. The mean spherical approximation, which is based on a resummed density expansion, is shown to give reasonable predictions for intermediate densities but is shown to fail at liquid densities. We discuss the transition from the gas phase limit to the liquid phase theories, and show that for liquids nuclear motions do not strongly affect the line shape, whereas local field effects do.
High resolution double-resonance spectroscopy on Rydberg states of CO
Authors: Drabbels Marcel , Heinze Johannes , ter Meulen J. J. , Meerts W. Leo
Published: July 2002

The Rydberg states L 1Π(v=0), L′ 1Π(v=1), K 1Σ+(v=0), W 1Π(v=0), and W′ 1Π(v=2) have been studied in a 2+1 double-resonance detection scheme with a resolution of 0.005 cm−1. Accurate molecular constants have been derived for these states. From the observed linewidths of individual rotational transitions, predissociation rates for the Rydberg states have been deduced. For the first time a clear J and e/f dependence for the predissociation of CO has been observed. One of the states causing the observed predissociation could be identified as the D′ 1Σ+ state.
Accurate quantum calculation for the benchmark reaction H2+OH→H2O +H in five-dimensional space: Reaction probabilities for J=0
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

A time-dependent wave packet method has been employed to compute initial state-specific total reaction probabilities for the benchmark reaction H2+OH→H2O+H on the modified Schatz–Elgersman potential energy surface which is derived from ab initio data. In our quantum treatment, the OH bond length is fixed but the remaining five degrees of freedom are treated exactly in the wave packet calculation. Initial state-specific total reaction probabilities for the title reaction are presented for total angular momentum J=0 and the effects of reagents rotation and H2 vibration on reaction are examined.
J-aggregate formation of a water-soluble porphyrin in acidic aqueous media
Authors: Ohno Osamu , Kaizu Youkoh , Kobayashi Hiroshi
Published: July 2002

J aggregate of a water-soluble porphyrin, 5,10,15,20-tetra(4-sulfophenyl)porphyrin (H2TPPS44−), formed in acidified aqueous solutions, exhibits sharp and intense absorption bands at 491 and 707 nm. These characteristic transitions, J bands, are of linear oscillators polarized in the long axis of rodlike aggregate. The molecules in the aggregate stack so as to lift the degeneracy of the porphyrin planar oscillator excited states. Measurements of flow-induced linear dichroism, circular dichroism, magnetic circular dichroism, as well as polarized fluorescence excitation spectra provide evidence not only of linear oscillator character of the intense J band at 491 nm, but also of presence of another diffuse absorption band around at 420 nm polarized in the short axis of the aggregate, which is the counterpart of the 491 nm band of porphyrin Soret origin. Extrinsic circular dichroism is induced upon addition of L-tartaric acid or by mechanical swirling flow in the period of aggregate growth. Resonance Raman spectrum of the aggregate is rather similar to that of the monomeric diacid except the polarization. The observed shifts of Raman peaks to lower frequency are ascribed to a deformation of porphyrin moiety and/or hydrophobic interaction between component molecules in the aggregate. The sharp and intense J bands polarized in the long axis of aggregate cannot be well described without taking into account the participation of interporphyrin charge resonance excited states, whereas the broadbands polarized in the short axis are exclusively ascribed to exciton resonance excited states. The porphyrin J band is characterized as sustaining an exchange narrowing by fast migration of excitation over the whole system of chromophores in a linear array of the porphyrin planar oscillators in slipped face-to-face stacking.
The chemical and electronic structure of the interface between aluminum and polythiophene semiconductors
Authors: Dannetun P. , Boman M. , Stafström S. , Salaneck W. R. , Lazzaroni R. , Fredriksson C. , Brédas J. L. , Zamboni R. , Taliani C.
Published: July 2002

We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core-level x-ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl-substituted poly-3-octylthiophene and (ii) the α-sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon–aluminum bonds are formed in the α positions of the thiophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper π levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals.
The reaction ensemble method for the computer simulation of chemical and phase equilibria. I. Theory and basic examples
Authors: Smith W. R. , Triska B.
Published: July 2002

We present a new efficient Monte Carlo method for the molecular-based computer simulation of chemical systems undergoing any combination of reaction and phase equilibria. The method requires only a knowledge of the species intermolecular potentials and their ideal-gas properties, in addition to specification of the system stoichiometry and thermodynamic constraints. It avoids the calculation of chemical potentials and fugacities, as is similarly the case for the Gibbs ensemble method for phase equilibrium simulations. The method’s simplicity allows it to be easily used for situations involving any number of simultaneous chemical reactions, reactions that do not conserve the total number of molecules, and reactions occurring within or between phases. The basic theory of the method is presented, its relationship to other approaches is discussed, and applications to several simple example systems are illustrated.
Equilibrium and dynamic properties of polymethylene melts from molecular dynamics simulations. I. n-Tridecane
Authors: Smith Grant D. , Yoon Do Y.
Published: July 2002

Employing an explicit atom (EA) model of polymethylene, we have carried out molecular dynamics simulations of n-tridecane (C13H28) melts at experimental densities to compute both equilibrium and dynamic properties. The calibrated EA model reproduces quite well the experimental results of pressure, x-ray diffraction patterns, and self-diffusion constants at different temperatures. A united atom (UA) model that reproduces the experimental pressures also yields good agreement with experimental x-ray diffraction patterns and self-diffusion data, and the calculated degree of intermolecular orientational correlation is in good agreement with predictions of the EA model. However, the UA model yields significantly more extended chain dimensions than a previously investigated model, and, most importantly, significantly enhanced local chain dynamics compared to the EA model, as monitored by the chain vector reorientation and local torsional dynamics. The EA model simulations yield C–H vector orientational correlation times associated with each carbon of n-tridecane, in excellent agreement with experimental values deduced from C13-NMR T1 spin–lattice relaxation times. The C–H vector reorientation was found to be closely related to conformational jumps. These jumps, whose rates closely follow torsional correlation times, appear to occur mostly as unconcerted individual transitions for these short chains.
Scanning tunneling microscopy (STM) studies of the chemical vapor deposition of Ge on Si(111) from Ge hydrides and a comparison with molecular beam epitaxy
Authors: Wintterlin J. , Avouris Ph.
Published: July 2002

Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and tip-induced desorption are used to study the mechanism of the chemical vapor deposition (CVD) of Ge on Si(111)-7×7 from GeH4 and Ge2H6. The chemical and structural changes that follow the dissociation of the hydride molecules are investigated as a function of the substrate temperature, and the structure and growth mode of the resulting films are compared to those of films generated by molecular beam epitaxy (MBE). At room temperature, only Ge2H6 reacts appreciably with the Si(111) surface. The reaction proceeds via a mobile precursor state, but does not exhibit a site selectivity with respect to surface sites of the 7×7 unit cell. For temperatures in the neighborhood of 400 °C, continuous film growth is observed for both GeH4 and Ge2H6, despite the fact that hydrogen is present at the surface. The film growth mode undergoes a qualitative change in a relatively narrow temperature range ∼400 °C. At 370 °C, the film grows in the form of monolayers with a hydrogen-stabilized 1×1 structure. This is in contrast to growth by MBE which involves 7×7 and 5×5 reconstructed bilayers. At the same time, a highly site-selective substrate etching process favoring the center-adatom sites is observed and characterized. This etching process leads to a chemically induced Si–Ge intermixing at the growing interface. At 430 °C, the CVD film growth mode involves bilayers with 7×7 and 5×5 reconstructions, but in addition, areas with a new √3 ×√3R30° reconstruction which is not observed in MBE are formed. Short-lived GeH species acting similarly to column III elements are probably the building blocks of this structure. We show that all of the above observations can be understood by considering the effect of hydrogen on the film growth process. The ability of the hydrogen to influence the growth process can, in turn, be tuned by varying its surface concentration and residence time by the substrate temperature.
Proton transfer in solution: Molecular dynamics with quantum transitions
Authors: Hammes-Schiffer Sharon , Tully John C.
Published: July 2002

We apply ‘‘molecular dynamics with quantum transitions’’ (MDQT), a surface-hopping method previously used only for electronic transitions, to proton transfer in solution, where the quantum particle is an atom. We use full classical mechanical molecular dynamics for the heavy atom degrees of freedom, including the solvent molecules, and treat the hydrogen motion quantum mechanically. We identify new obstacles that arise in this application of MDQT and present methods for overcoming them. We implement these new methods to demonstrate that application of MDQT to proton transfer in solution is computationally feasible and appears capable of accurately incorporating quantum mechanical phenomena such as tunneling and isotope effects. As an initial application of the method, we employ a model used previously by Azzouz and Borgis to represent the proton transfer reaction AH–B■A−–H+B in liquid methyl chloride, where the AH–B complex corresponds to a typical phenol–amine complex. We have chosen this model, in part, because it exhibits both adiabatic and diabatic behavior, thereby offering a stringent test of the theory. MDQT proves capable of treating both limits, as well as the intermediate regime. Up to four quantum states were included in this simulation, and the method can easily be extended to include additional excited states, so it can be applied to a wide range of processes, such as photoassisted tunneling. In addition, this method is not perturbative, so trajectories can be continued after the barrier is crossed to follow the subsequent dynamics.
Molecular dynamics free energy calculation in four dimensions
Authors: Beutler Thomas C. , van Gunsteren Wilfred F.
Published: July 2002

A new method for the calculation of free energy differences by molecular dynamics simulations of molecular systems is presented. The main characteristic of the method is that the three-dimensional physical space is augmented by a further dimension. This allows the system to circumvent energy barriers in three-dimensional space. When simulating physical (three-dimensional) states, the dynamics along the 4th coordinate is uncoupled from the dynamics described with the original three coordinates. In the method presented here, free energy differences between physical states are determined using pathways leading over states where the dynamics along all four dimensions are coupled. Due to the barrier lowering in four dimensions, an increase of efficiency can be expected when low free energy physical states are separated by high barriers. The method is tested by application to an atomic liquid.
Comment on ‘‘The electron density and chemical bonding: A reinvestigation of Berlin’s theorem’’ [J. Chem. Phys. 94, 2977 (1991)]
Authors: Hinze Juergen
Published: July 2002

With the reinvestigation [H. Silberbach, J. Chem. Phys. 94, 2977 (1991)] of Berlin’s theorem [Th. Berlin, J. Chem. Phys. 19, 208 (1951)] the author purports to show that the theorem is wrong, and that ‘‘the notion of binding and antibinding regions in a diatomic molecule cannot be maintained.’’ It is shown, using the universal inversion symmetry responsible for parity conservation, that Berlin had obtained the correct result, and that binding and antibinding regions are well defined in a diatomic molecule.
Collision induced absorption spectra and line broadening of CsRg system (Rg=Xe, Kr, Ar, Ne) studied by the symmetry adapted cluster-configuration interaction (SAC-Cl) method
Authors: Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

The collisionally induced absorption process and the broadening of the 6P resonance line of the Cs–Rg system (Rg=Xe, Kr, Ar, and Ne) are studied theoretically by the symmetry adapted cluster-configuration interaction (SAC-CI) method. The potential energy curves and the transition moments of the CsRg system correlating to the 6S, 6P, 5D, and 7S states of the Cs atom are investigated. The reduced absorption coefficients are calculated using the quasistatic approximation and the results agree well with the experimental data. The monotonic dependence of the spectral peaks on the rare gas species is due to the similar monotonic dependence of the avoided crossing point between the 7sΣ and 5dΣ states. The absorption intensities decrease as the rare gas atom is substituted from Xe to Ne in agreement with the experimental observation. The intensities of the 6sΣ–5dΣ transitions are calculated to be larger than those of the 6sΣ–7sΣ ones, since the former transitions are induced at larger internuclear distances than the latter.
Internuclear distance measurements in solid state nuclear magnetic resonance: Dipolar recoupling via rotor synchronized spin locking
Authors: Sun B.-Q. , Costa P. R. , Kocisko D. , Lansbury P. T. , Griffin R. G.
Published: July 2002

The dipolar coupling in rotating solids, averaged to zero when the rotation is about the magic angle, can be reintroduced in the spin-locking interaction frame by rotor-synchronized 90° phase shifts of an applied spin-locking field. This new technique is an interaction frame analogue of previously proposed experiments. Because the spin-locking fields both truncate and play the role of the chemical shift terms in the Hamiltonian, this technique is insensitive to the chemical shifts as well as chemical shift anisotropies of the coupled spins. Through numerical simulations of the magnetization exchange trajectory (e.g., the mixing time dependence of the cross-peak intensity connecting two coupled spins), the through-space distance between two spin nuclei can be estimated. This technique can also be applied to obtain two-dimensional correlation spectra, or alternatively it can be used as a double-quantum filter. Preliminary experiments demonstrate that the cross peaks in the two-dimensional correlation spectra obtained with this technique have a ‘‘negative–positive–negative’’ pattern to the first, second, etc., coupled neighbor. This spectral feature is useful in the assignment of complex magic angle spinning spectra.
Molecular dynamics simulation of the orthobaric densities and surface tension of water
Authors: Alejandre José , Tildesley Dominic J. , Chapela Gustavo A.
Published: July 2002

Molecular dynamics simulations have been performed to study the liquid–vapor equilibrium of water as a function of temperature. The orthobaric densities and the surface tension of water are reported for temperatures from 316 K until 573 K. The extended simple point charge (SPC/E) interaction potential for water molecules is used with full Ewald summation. The normal and tangential components of the pressure tensor were calculated and are presented at 328 K. The nature of the long-range contribution to the surface tension has been studied in detail. At 328 K the calculated surface tension is 66.0±3.0 mN m−1 in comparison with the experimental value of 67 mN m−1. The simulated surface tensions between 316 K and 573 K are in good agreement with experiment. The orthobaric densities are in better agreement with experimental values than those obtained from the Gibbs ensemble calculation for the SPC model of water.