bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarized light scattering from sickle hemoglobin polymers
Authors: Kim-Shapiro Daniel B. , Hull Patricia G.
Published: July 2002

The Mueller scattering matrix for sickle cell hemoglobin polymers is calculated using the coupled-dipole approximation method. The complex polarizability of the fiber is calculated using the absorption spectrum of hemoglobin to obtain the imaginary part and a Kramer–Kronig transform to obtain the real part. An anisotropy in the polarizability is calculated based on previous work using linear dichroism. The results of the polarized light scattering calculations are compared to previous measurements of total intensity light scattering and circular intensity differential scattering (CIDS) made on sickle red blood cells. Calculations of CIDS are found to be very sensitive to structural and optical parameters but reasonable agreement between experimental measurements and calculations are obtained. Further measurements and calculations should provide new structural information concerning the sickle fiber.
Line shape, transport and relaxation properties from intermolecular potential energy surfaces: The test case of CO2–Ar
Authors: Roche Christine F. , Dickinson Alan S. , Ernesti Andreas , Hutson Jeremy M.
Published: July 2002

Two new potential surfaces for CO2–Ar, obtained principally from the spectra of van der Waals complexes, are tested against measurements of pressure broadening and of transport and relaxation properties, none of which was used in the determination of the surfaces. Pressure broadening of both infrared and Raman lines is considered. The coupled states (CS) approximation is used for all line shape calculations. Thermally averaged infrared and Raman cross sections at 523, 296, 160 and 77 K (infrared) and 295 K (Raman) show good agreement with the experimental data available. Generalized transport and relaxation cross sections are obtained via full classical trajectory and classical CS calculations. Properties tested include diffusion, viscosity and nuclear spin relaxation. They provide a different test of the surfaces and agree well with experiment.
Equilibrium structure, fluctuations, and spectroscopy of a solvated electron in methanol
Authors: Turi László , Mosyak Alex , Rossky Peter J.
Published: July 2002

Structural and spectroscopic properties of the equilibrium excess electron solvated in methanol are evaluated via adiabatic quantum molecular dynamics simulations employing an electron-methanol pseudopotential based on that of Zhu and Cukier et al. [J. Chem. Phys. 98, 5679 (1993)]. The solvated electron is localized in a roughly spherical ground electronic state of 2.5 Å radius. The electron is surrounded by approximately six or seven methanol molecules in the first solvent shell with mostly O–H bond orientation. Due to the strong electron-methanol coupling, solvent fluctuations result in substantial fluctuations of energy, size, and shape of the electron. The influences of radial fluctuations and asymmetric distortions of solvent cavities are illustrated on both the solvation structure and the optical spectrum. The optical-absorption spectrum of the solvated electron is dominated by transitions to three clearly nondegenerate p-like excited states. The fluctuations influence the s-p energy gap and modulate the p-p energy splitting leading to the calculated broad, featureless optical band. Comparison between systems containing flexible and rigid methanol molecules is also reported. Unlike in water, characteristic fluctuations of approximately 20-ps time scale can occur in methanol. The structural and optical consequences of these long timescale fluctuations are examined.
On the Longuet-Higgins phase and its relation to the electronic adiabatic–diabatic transformation angle
Authors: Baer Michael
Published: July 2002

In this Note a proof is provided that, within certain well defined approximations, the Longuet-Higgins phase [H. C. Longuet-Higgins, U. Opik, M. H. L. Pryce and R. A. Sack, Proc. R. Soc. London, Ser. A 244, 1 (1958)], known also as the geometrical phase, and the electronic two-state adiabatic-diabatic transformation angle are identical.
Initial conditions and paths in semiclassical tunneling
Authors: Guo Yin , Li Shenmin , Thompson Donald L.
Published: July 2002

The choice of initial conditions and tunneling paths for semiclassical tunneling are investigated for a two-dimensional double-well system which is often used for modeling hydrogen tunneling in unimolecular isomerization processes. Tunneling effects are incorporated in a standard trajectory simulation. The exact quantum mechanical calculations are performed for comparison. We find that tunneling splittings are in close agreement for initial conditions obtained by using adiabatic switching and normal-mode sampling methods. These results suggest that the approximate normal-mode sampling can be used to compute accurate level splittings. We also find that simple straight-line tunneling paths work well from the sudden to intermediate regimes. However, the method is less accurate for the intermediate cases.
Interplay of geminate and bimolecular reactions in multistage electron and energy transfer
Authors: Burshtein A. I. , Frantsuzov P. A.
Published: July 2002

Within the universal integral formalism of encounter theory, we described (a) the bimolecular charge recombination that follows the geminate one beginning with photoionization and (b) bimolecular quenching of singlet and triplet excitations parallel to an inter-system crossing (ISC). In both cases the new features appear which represent either the geminate ion pair density (positive) or pair vacancies density (negative), which accounts for singlets quenched before ISC. It was justified that the shortage of near-contact excitations resulting from static singlet quenching may be built into the initial conditions for subsequent evolution of triplets and lead to the “anti-Smoluchowski time behavior” of their annihilation rate constant. However, only an integral theory can account for the same effect when singlet quenching is diffusion accelerated.
A study of aqueous glutamic acid using the effective fragment potential method
Authors: Day Paul N. , Pachter Ruth
Published: July 2002

Glutamic acid in the gas phase and in aqueous solution was studied with ab initio quantum chemistry calculations. The effective fragment potential (EFP) has been used to study the effects of up to ten water solvent molecules on the neutral and zwitterionic isomers of glutamic acid, as well as to model the reaction path connecting these two isomers. The results with one and two water molecules have been compared to full ab initio “super molecule” calculations. This comparison shows that the EFP successfully models the effects of solvent molecules on structure and energetics. In the calculations including zero, one, or two water molecules, the neutral isomer was shown to be more stable than the zwitterion. On the other hand, in the EFP calculations with ten water molecules, where Monte Carlo methods were used to assist in finding the lowest energy conformations, the zwitterion was found to be more stable, which is the expected aqueous phase result.
Dynamics of quantum particles by path-integral centroid simulations: The symmetric Eckart barrier
Authors: Ramírez Rafael
Published: July 2002

The path-integral centroid approach has been applied to study the dynamical properties of a flux of protons impinging on a symmetric Eckart barrier. The mean transmission coefficient, transmitted flux, and kinetic energy of transmitted particles have been calculated by path-integral centroid simulations as a function of temperature, and compared to exact results obtained from the solution of the Schrödinger equation. The studied temperatures cover the crossover from a classical regime, where the barrier crossing is thermally activated, to a quantum regime, where the barrier crossing is dominated by tunneling of low energy particles. We show, in agreement with previous studies, that the centroid density is a central quantity to derive dynamical properties. Moreover, we find that the equilibrium internal energy obtained for the centroid fixed at the barrier top, reproduces closely the difference between the mean kinetic energy of transmitted and incident particles, and it can be used to define a velocity (pre-exponential) factor that improves previous approximations to the transmitted flux, in the whole temperature range above and below the classical-quantum crossover.
Vibrational lifetimes and vibrational line positions in polyatomic supercritical fluids near the critical point
Authors: Urdahl R. S. , Myers D. J. , Rector K. D. , Davis P. H. , Cherayil B. J. , Fayer M. D.
Published: July 2002

Picosecond infrared pump–probe experiments are used to measure the vibrational lifetime of the asymmetric (T1u) CO stretching mode of W(CO)6 in supercritical CO2, C2H6, and CHF3 as a function of solvent density and temperature. As the density is increased at constant temperature from low, gaslike densities, the lifetimes become shorter. However, in all three solvents, it is found that within a few degrees of the critical temperature (Tr≡T/Tc≈1.01), the lifetimes are essentially constant over a wide range of densities around the critical value (ρc). When the density is increased well past ρc, the lifetimes shorten further. At higher temperature (Tr=1.06) this region of constant vibrational lifetime is absent. Infrared absorption spectra of W(CO)6 and Rh(CO)2acac in supercritical CO2, C2H6, and CHF3 acquired for the same isotherms show that the vibrational spectral peak shifts follow similar trends with density. The peak positions shift to lower energy as the density is increased. Near the critical point, the peak positions are density independent, and then redshift further at densities well above ρc. It is shown that critical fluctuations play a dominant role in the observed effects. Theoretical calculations ascribe the density independence of the observables to the cancellation of various rapidly changing quantities near the critical point. The theory’s calculation of density independence implicitly involves averages over all local densities and does not involve any form of solute–solvent clustering.
Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. I. Σ←Π Q branches
Authors: Rodrigues R. , Khalil B. , Le Doucen R. , Bonamy L. , Hartmann J.-M.
Published: July 2002

Experimental and theoretical results on the influence of line mixing on the shape of infrared CO2 Q branches of importance for atmospheric applications are presented. Two Q branches of Σ←Π symmetry, which lie near 618 and 720 cm−1 and belong to the 1000II←0110I and 1000I←0110I bands, have been studied for many conditions of temperature (200–300 K), total pressure (0.5–10 atm), and mixture (with He, Ar, O2, and N2). The theoretical approach used is based on the Energy Corrected Sudden approximation; its parameters have been deduced from both line-broadening data and measured absorption by the Q branches. Comparisons between experimental and computed spectra demonstrate the quality of the model, regardless of the conditions. Detailed analysis of the influences of the Q-lines spectral spacing, temperature, total pressure, and collision partner are presented. They show that significantly larger line-mixing effects are obtained when CO2–He is considered with respect to CO2–(Ar,O2,N2). This is analyzed in terms of the relative contributions of the short- and midrange interaction forces and of propensity rules resulting from the coupling of angular momenta.
Dipole–dipole interaction and anisotropy effects in the itinerant oscillator model
Authors: Coffey W. T.
Published: July 2002

The itinerant oscillator model whereby a typical molecule of a polar fluid may exhibit librational motion under the influence of the field of its large cage of neighboring dipoles is examined when the restriction of rotation about a common fixed axis is removed, both cage and tagged molecule now being free to rotate about a common fixed point of themselves. It is shown that the equations of motion of the system cannot, in general, be decomposed into the equations of motion of the tagged molecule relative to the cage and that of the cage alone on account of the Coriolis torques. If, however, one can make the assumption that the cage is much more massive than the tagged molecule so that in a short time (essentially a time less than the Debye relaxation time of the cage so that the cage represents a slowly relaxing local structure) after the removal of an external uniform field the cage remains virtually at rest relative to the tagged molecule, then the dipole autocorrelation function of the tagged molecule is approximately the product of the autocorrelation function of the cage and the autocorrelation functions of the motion of the tagged molecule relative to the cage. The behavior of the model, with a cage–dipole interaction potential including both the permanent and induced dipole terms in the noninertial limit is discussed using the above assumption. It is shown that the inclusion of the induced dipole term essentially creates an asymmetric bistable interaction potential in which a relaxation process indicative of a slow overbarrier (activation) process coexists with the relatively fast relaxation modes in the wells of the potentials [as predicted by Polemino and Freed, Adv. Chem. Phys. 83, 89 (1993) in their numerical analysis of the model]. The conditions under which the fast relaxation processes may come to dominate the overall relaxation behavior are discussed by analogy with superparamagnetic relaxation [cf. Coffey, Crothers, and Kalmykov, Phys. Rev. E 55, 4812 (1997)].
Structure and dynamics of the S3 state of CS2
Authors: Sadeghi Raymond R. , Gwaltney Steven R. , Krause Jeffrey L. , Skodje Rex T. , Weber Peter M.
Published: July 2002

We present a detailed theoretical investigation of the structure and dynamics of the S3 excited state of CS2. The excited state is treated using equation-of-motion coupled-cluster (EOM-CC) theory, and analytic gradients are used in determining the vibrational frequencies. Information from the structure calculations is used to construct a model potential surface. Time-dependent wave packet methods are used to generate the transition state spectrum corresponding to collinear CS2. The spectrum is comprised of transition state resonances which are assigned via a spectral quantization method. The energies and lifetimes of the spectral features are related to recent experimental measurements.
Photodissociation of CH2. VI. Three-dimensional quantum dynamics of the dissociation through the coupled 2A″ and 3A″ states
Authors: Kroes Geert-Jan , van Hemert Marc C. , Billing G. D. , Neuhauser Daniel
Published: July 2002

We present quantitative results on photodissociation of CH2(X̃ 3B1) through the coupled 2A′′ and 3A′′ states. A three-dimensional, hybrid quantum dynamical method was used, employing hyperspherical coordinates. The diabatic potential energy surfaces (PES’s) used in the dynamics were derived from ab initio calculations. A small product fraction (2.7%) was computed for the CH(A 2Δ)+H channel, in agreement with experiment and approximate dynamical calculations. The dissociation proceeds mostly on a A2-like diabatic surface, into CH(a 4Σ−)+H(93.3%) and C(3P)+H2(4.0%). Resonances of widths in the range 0.1–10 meV affect the photodissociation. Pre-exciting a vibrational mode of CH2(X̃ 3B1) prior to photodissociation does not alter the picture, except if the antisymmetric stretch mode is excited: In this case the product fractions for the C(3P)+H2 and CH(A 2Δ)+H channels collapse to values of 1% or lower, and the resonances disappear. Model calculations show that the large product fraction found for CH(a 4Σ−)+H is due to the initial motion on the “bright”B1-like surface, which biases the outcome of the dissociation in favor of CH(a 4Σ−)+H.
Chaotic dynamics and approximate semiclassical quantization: Assigning highly excited vibrational eigenstates
Authors: Davis Michael J.
Published: July 2002

The classical dynamics of highly chaotic vibrational dynamics is studied via the nature of resonance zones. It is demonstrated that the flow into and out of the resonance zones is complicated with some trajectories spending less than one period outside before reentering, referred to here as “virtual recrossing.” Despite the strong chaos, an approximate semiclassical quantization method based on the resonance zone structure can be used to generate eigenvalues to within a few percent of the quantum values well into the chaotic regime. The semiclassical quantization allows for the assignment of highly excited, strongly mixed vibrational eigenstates.
Quasielastic neutron scattering study on rotational aromatic groups of a thermotropic copolyester (vectra)
Authors: Koizumi Satoshi
Published: July 2002

The rotational motion of aromatic groups which constitute a thermotropic copolyester, vectra was investigated in terms of the quasielastic neutron scattering (QENS) originating from incoherent scattering of hydrogen. Vectra which is a random copolyester of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with a faction of 73/27 (mol/mol), gives rise to the nematic phase transition at Tm=558 K from semicrystalline phase, i.e., a mixture of crystalline and amorphous phases (Tg=393 K). In the region above Tm, the elastic incoherent scattering cross section due to hydrogen, obtained for optical geometry in which q vector is normal to the molecular axis of vectra, was successfully decomposed into (i) the Debye–Waller factor due to vibrational hydrogen, (ii) the elastic incoherent structure factor (EISF) for the rotational aromatic groups with hydrogen and (iii) the QENS for the rotational aromatic groups with hydrogen. The QENS spectra and EISF thus obtained were well reproduced by a free rotational model for the aromatic groups, in which residing sites for hydrogen on the rotational circle is infinite. On the other hand, in the solid (semicrystalline) phase below Tm, the QENS spectra and EISF became less q dependent with decreasing temperature; these observations agree qualitatively with a more restricted motion, e.g., a jump rotation having finite residing sites for hydrogen.
Theoretical studies of the available volume for adsorption in a random quenched and depleted disordered medium
Authors: Van Tassel Paul R.
Published: July 2002

The use of a removable template during synthesis of a disordered porous material can result in a material of greatly differing pore structure. We present here a theoretical study of the influence of a template on the material’s available volume. We model the material as a medium formed by quenching and depleting an equilibrium fluid and develop a theoretical method by which the available volume for adsorption in this model material may be investigated. This method makes use of the known relationship between the direct correlation function and the available volume. An analysis of the diagrams contributing to the available volume shows that two types of field points are present: those corresponding to the density of the medium following depletion (ρ) and those corresponding to the density of the original quenched fluid (ρq). A perturbation term to the available volume is derived that accounts for the diagrams containing ρq field points. The resulting expression contains only the direct and total correlation function; these quantities are known exactly for a hard sphere system within the Percus–Yevick approximation. Using this approach, we show that for materials of a given density, those formed by depletion can possess available volumes enhanced by up to an order of magnitude.
Ab initio study of HCl and HF interaction with crystalline ice. I. Physical adsorption
Authors: Bussolin Giovanni , Casassa Silvia , Pisani Cesare , Ugliengo Piero
Published: July 2002

A quantum-mechanical ab initio study is presented concerning the physisorption of HCl and HF on ice surfaces, modeled with periodic, proton-ordered thin films. Three methods are adopted: (1) Periodic two-dimensional calculations concerning ice surfaces, both clean and covered with ordered overlayers; (2) embedded cluster calculations, concerning two-dimensional ordered structures interacting with a single molecule; (3) molecular cluster calculations, simulating a portion of the surface. The combined and interactive use of these techniques has permitted us to recognize some deficiencies of molecular cluster models of ice and to correct for them. The energy of physisorption of HCl on a perfect basal surface of ice is estimated to be about 8 kcal/mol, on prismatic faces about 11 kcal/mol. Adsorption energies of HF are larger by 4 to 5 kcal/mol.
Ab initio molecular orbital studies on the structure, energies, and photodissociation of the electronic excited states of C2H
Authors: Cui Qiang , Morokuma Keiji
Published: July 2002

High level ab initio calculations have been carried out to study seven electronic states of C2H. The calculated equilibrium structure, energetics and vibrational frequencies for the 3 2A′ state at the CASPT2/PVTZ level are in good agreement with those obtained experimentally by Hsu et al. The transition dipole moments from the ground state C2H to electronic excited states depend sensitively on the H–C–C bending angle and often peak at nonlinear configurations. Based on this and the dissociation behavior of the excited states, we predict A 1Πu and c 3Σu+ C2 fragments to be rich in population, the former of which is experimentally detected recently by Jackson et al. The ground X 1Σg+ and the a 3Πu state C2 are expected to be formed via the nonadiabatic process 3 2A′→2 2A′ or 4 2A′→3 2A′→2 2A′, which is in accord with the experimentally observed lifetime pattern by Hsu et al. No reverse barrier for CC–H dissociation was found on the X and A electronic states of C2H in the linear configuration. The 2 2A′ state, however, develops a distinct “barrier” (not a true saddle point) along dissociation coordinate when the H–C–C is significantly bent, due to the interaction with upper electronic state. Since the 2 2A′ state energetically prefers a linear dissociation, we suspect that the upper bound of the CC–H bond energy measured by Hsu et al. is not severally affected by this “barrier.”
A semi-empirical potential for simulations of transition metal clusters: Minima and isomers of Nin (n=2–13) and their hydrides
Authors: Curotto E. , Matro Alexander , Freeman David L. , Doll J. D.
Published: July 2002

A potential energy surface (PES) for bare, mono and di-hydrogenated nickel clusters is constructed using the extended-Hückel approximation. The parameters are optimized and good agreement with theoretical and experimental results is obtained without including a posteriori coordination dependent terms. The global minimum and the first few low-lying isomers of several nickel clusters are investigated using a variety of minimization techniques. The difference in energy between isomers is much smaller than the Ni-Ni dissociation energy. Both geometric and optical isomers are found for many cluster sizes. In some cases symmetric nuclear configurations give rise to orbital degeneracies in the adiabatic surface which lead to distortions. The hydrogen atom is most frequently found on the surface. All isomers of NinH2 contain a dissociated hydrogen molecule. The results are in good agreement with quantitative and qualitative experimental findings on this system.
Stability of H, D, N14, and N15 atoms in solid ammonia above 100 K
Authors: DeMarco L. G. , Brill A. S. , Crabb D. G.
Published: July 2002

The measurements reported below quantify the stability and decay of hydrogen, deuterium, and nitrogen atoms in frozen ammonia above 100 K. The decay of H atoms is observed on a time scale of minutes in the range of 100–110 K and follows first-order kinetics. Analogous decays of D and N atoms are observed in the ranges 105–120 and 140–160 K, respectively. Activation energies for the decay processes range from 0.1 to 0.4 eV.
Model protein conformations via pair correlation functions, distance matrix, and embedding algorithm
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

A method of constructing three-dimensional structures of model protein conformations is presented. The method consists of self-consistent field integral equations for pair correlation functions of constituent units in a heteropolymer chain and the use of the distance matrix and the embedding algorithm for constructing conformations. The pair correlation functions obey integral equations that are derived from the Kirkwood hierarchy by applying closure approximations; they appear as a generalized form of the liquid-state Percus–Yevick integral equation. Model protein sequences that exhibit the formation of secondary-like patterns and tertiary-like structures are examined. These structural features are formed at low temperatures and they are stabilized by strong hydrogen bonding forces. To obtain the structure in three dimensions, the method of distance geometry is used to refine the distance matrix of a folded structure which is then embedded in the three-dimensional space by an embedding algorithm.
Coarsening of cubic lattice domains observed in a nonionic surfactant system
Authors: Imai M. , Kato T. , Schneider D.
Published: July 2002

We have investigated the coarsening process of domains having a cubic lattice in a nonionic surfactant system using a time-resolved small-angle neutron scattering technique. The obtained time evolution of scattering patterns showed growing large domains with strong diffraction intensity fluctuation and shrinking small domains obeying dynamical scaling law. This bimodal domain coarsening suggests an anomalous domain coarsening. During the coarsening the cubic lattice was distorted about 10%, which is probably due to the adjustment of the adjacent lattice at boundary. This elasticity effect probably brings about the observed anomalous domain coarsening.
Stochastic potential for a periodically forced nonlinear oscillator
Authors: Vance William , Ross John
Published: July 2002

We investigate stationary and nonstationary probability densities for a weakly forced nonlinear physical or chemical system that displays self-oscillations in the absence of forcing. The period and amplitude of forcing are taken as adjustable constraints. We consider a homogeneous reaction system described by a master equation. Our method of solution is based on the Wentzel–Kramers–Brillouin (WKB) expansion of the probability density with the system size as the expansion parameter. The first term in this expansion is the stochastic potential (eikonal). In the absence of forcing, the probability density is logarithmically flat on the limit cycle. With periodic forcing, the phenomenon of phase locking can occur whereby a stable cycle, which is close to the unforced cycle, adopts a constant relative phase to the forcing. A saddle cycle also exists and has a different constant relative phase. For such phase-locked solutions, the distribution over the relative phases is peaked on the stable cycle and exhibits a logarithmically flat region (a plateau) that originates on the saddle cycle. This plateau is due to a nonzero relative phase slippage: large fluctuations from the stable cycle over the saddle cycle are overwhelmingly more probable in a certain relative phase direction, which depends upon the location of the parameters within an entrainment region. This distribution of relative phases is logarithmically equivalent to that of a Brownian particle in a periodic potential with a constant external force in the strong damping and weak noise limits. For parameter values outside of an entrainment region (for which a quasiperiodic solution exists), the distribution in relative phase is logarithmically flat. For this regime, we investigate the evolution of an initially localized density and show that the width grows proportionally with the square root of time. The proportionality factor depends upon both the position (phase) on the cross section of the peak of the density and the distance in parameter space from the boundary of the entrainment region. For parameter values that approach the boundary of an entrainment region, this proportionality factor tends to infinity. We also determine an expression for the first order correction to the stochastic potential for both entrained and quasiperiodic solutions. A thermodynamic interpretation of these results is made possible by the equality of the stochastic potential with an excess work function.
Dynamic path integral methods: A maximum entropy approach based on the combined use of real and imaginary time quantum Monte Carlo data
Authors: Kim Dongsup , Doll J. D. , Freeman David L.
Published: July 2002

A new numerical procedure for the study of finite temperature quantum dynamics is developed. The method is based on the observation that the real and imaginary time dynamical data contain complementary types of information. Maximum entropy methods, based on a combination of real and imaginary time input data, are used to calculate the spectral densities associated with real time correlation functions. Model studies demonstrate that the inclusion of even modest amounts of short-time real time data significantly improves the quality of the resulting spectral densities over that achievable using either real time data or imaginary time data separately.
Theoretical studies of the effect of hydrogen–hydrogen interactions on the structural and dynamical properties of metal/hydrogen clusters
Authors: Chen B. , Gomez M. A. , Doll J. D. , Freeman David L.
Published: July 2002

Using a combination of ground state, equilibrium, and dynamical Monte Carlo methods, we examine the role of hydrogen-hydrogen interactions on selected structural and time-dependent properties of hydrogen containing metal clusters. Equilibrium simulations include studies of the classical and quantum-mechanical geometries and energetics for embedded atom potential models of both the ground states and low-lying structural isomers of NinH2 and PdnH2 clusters (4⩽n⩽9). In addition to these time-independent investigations, we utilize dynamical path integral methods to characterize the effects of hydrogen-hydrogen interactions on the hydrogen vibrational lineshapes in these systems.
Theoretical study of spectroscopical properties of Na and Na2 in argon clusters and matrices
Authors: Groß M. , Spiegelmann F.
Published: July 2002

We present a calculation of the structure and the optical transitions of sodium atoms and dimers embedded in argon clusters and matrices. We studied several different systems: A single sodium atom in a dodecahedral argon cluster, a Na atom in a substitutional site of a fcc (face-centered-cubic) Ar lattice containing 63 atoms and a sodium dimer in a 9-atom vacancy of the same fcc lattice (Na2@Ar54). For optimizing the system geometry in its ground state, we use a simplified tight-binding scheme of a metal cluster dressed by the metal-matrix and matrix-matrix van der Waals interactions. A procedure closer to ab initio methodology is then applied using e-Na+ and e-Ar semi-local pseudopotentials and core-polarization operators to determine the electronic structure of the metal valence electrons in the environment of the rare-gas atoms. The electronic transitions and oscillator strengths are obtained by a full two-electron configuration interaction (CI) treatment in the case of Na2@Ar54. The A1Σu+→X1Σg+ transition is redshifted in comparison to the free Na2 dimer. This phenomenon does not appear in the case of a matrix-isolated atom, where all lines are blueshifted.
Determination of lateral interaction energies in AgAu alloy films on Ru(0001) by Monte Carlo simulations
Authors: Michels J. , Wandelt K.
Published: July 2002

A lattice gas model with nonadditive lateral interactions is used to describe Ag, Au and AgAu bilayer films on a Ru(0001) substrate. By Monte Carlo simulations of adsorbate configurations and thermal desorption spectra of the Ag, Au and AgAu films, the microscopic interaction parameters of the gold and silver adatoms in the alloy film on Ru(0001) are obtained. The model reproduces experimental thermal desorption spectra of Ag, Au and AgAu films on Ru(0001) and also shows segregation of silver in the second monolayer of the alloy in agreement with experimental results. Based on perturbation theory, the configurational Gibbs free energy G of the AgAu monolayer alloy is calculated as a function of temperature and composition.
Six-wave mixing spectroscopy: Resonant coherent hyper-Raman scattering
Authors: Yang Mino , Kim Joohyun , Jung Younjoon , Cho Minhaeng
Published: July 2002

A theoretical description of the six-wave mixing spectroscopy is presented. The six-wave mixing spectroscopic signals in time and frequency domains are expressed in terms of the fifth-order nonlinear response functions. Particularly, the resonant coherent hyper-Raman scattering with wave vector of 4k1−k2 and frequency of 4ω1−ω2, where 2ω1≈ω2, is discussed by invoking the optical Bloch approximation. The coherent anti-Stokes hyper-Raman scattering (CAHRS) susceptibility is found to be very similar to that of CARS. The dephasing-induced vibrational resonance in the excited state is also manifest in this experimental configuration. By making a direct comparison of CAHRS signal with that of CARS, it is found that the ratio of CAHRS to CARS susceptibilities provides a crucial information on the permanent dipole moment of the excited state in either gas or condensed phases. It is briefly discussed that this method can be of use in estimating the molecular hyperpolarizability.
Response to “Comment on ‘The hyperpolarizability of trans-butadiene: A critical test case for quantum chemical models’ ” [J. Chem. Phys. 108, 4355 (1998)]
Authors: Norman Patrick , Luo Yi , Jonsson Dan , Ågren Hans
Published: July 2002

We show that the Comment of Kirtman et al. [J. Chem. Phys. 108, 4355 (1998), preceding paper] does not explain the conspicuous fact that the low-order random phase approximation gives the best result for the hyperpolarizability of butadiene, and call for new high-level calculations to resolve the controversy.
The protonated water dimer: Brueckner methods remove the spurious C1 symmetry minimum
Authors: Valeev Edward F. , Schaefer Henry F.
Published: July 2002

The H5O2+ system has been studied using a variety of coupled cluster methods based on a Brueckner reference determinant with levels of correlation up to double and perturbatively treated connected triple excitations [B–CCD(T)]. Basis sets as large as the triple-ζ plus double polarization basis augmented with f functions on oxygen and d functions on hydrogen [TZ2P(f,d)] were used. Harmonic vibrational frequencies were also predicted. In contrast with previous high-level ab initio studies, a stationary point of C1 symmetry was not found. An absence of imaginary vibrational frequencies at all levels of theory for the stationary point of C2 symmetry proves it to be the global minimum, lying only ∼0.4 kcal/mol lower in energy than the transition state of Cs symmetry.
Monte Carlo simulations of symmetric polymer blends. I. Characterization of the interface
Authors: Olaj Oskar Friedrich , Petrik Thomas , Zifferer Gerhard
Published: July 2002

A new method is presented for the simulation of the interface between nearly immiscible amorphous polymer phases. Strictly speaking, two independent interfaces are developed whose positions remain fairly in place during the whole simulation process. The interface is characterized by its thickness, by the excess number of contacts between segments of different type in the interface region (a quantity being characteristic of cavities that contain the other type of polymer) as well as by the fractal dimension and the relative surface enlargement (compared to a totally flat interface) similar to the methods introduced earlier for polymer surfaces. Further quantities that are used for a proper description of the interface are density profiles of polymer segments as well as of end segments and centers of mass. A variation of the repulsive energy between the two types of polymers on the one hand and a variation of the (average) chain length on the other hand reveals that the roughness of the interface decreases with increasing immiscibility (increasing energy parameter) and decreasing chain length (when the product of energy parameter and chain length is kept constant).
Quantum mechanical elucidation of reaction mechanisms of heavy-light-heavy systems: Role of potential ridge
Authors: Nobusada Katsuyuki , Tolstikhin Oleg I. , Nakamura Hiroki
Published: July 2002

A new idea to elucidate quantum reaction dynamics of heavy-light-heavy (HLH) systems is proposed on the basis of the hyperspherical elliptic coordinate approach. This coordinate system has a big advantage of nicely expressing good vibrational adiabaticity in the HLH systems. Taking this advantage, the concept of potential ridge is introduced, for the first time, in three-dimensional reactions. The potential ridge is proved to be very useful to extract some important avoided crossings which dominate the reaction dynamics. In fact, qualitative features of the reaction dynamics can be interpreted in terms of nonadiabatic transitions at those important avoided crossings near the potential ridge. Examples are: (i) onset of reaction for a specified initial rotational state, and (ii) major reactive transition for a specified initial rotational state. Avoided crossings to the left of the potential ridge are also useful to interpret certain aspects of reactions accompanying vibrational transitions. The new idea mentioned above is applied to a typical HLH reaction O(3P)+HCl→OH+Cl with the use of two types of potential energy surfaces.
Molecular femtosecond excitation described within the Gaussian wave packet approximation
Authors: Braun M. , Metiu H. , Engel V.
Published: July 2002

We consider the ultrashort pulse excitation of molecules within bound electronic states. Using ultraviolet (UV) electronic and intense pulse infrared (IR)-excitation as numerical examples we compare the approximative Gaussian wave packet (GWP)-method applied to the coherent excitation of molecules to numerically exact solutions of the time-dependent Schrödinger equation. The results imply that the short-time vibrational dynamics of molecules upon femtosecond excitation can be described accurately within the GWP-method. This especially holds for the calculation of femtosecond pump/probe signals, which are much less sensitive to the errors of the method than the wave function is.
Cluster velocity distributions in a vapor at equilibrium
Authors: Soto Rodrigo , Cordero Patricio
Published: July 2002

We present the microscopic description of the vapor using the concept of cluster. Taking into consideration nonideal contributions, the distribution functions of every cluster species are obtained. From these distribution functions it is possible to derive kinetic “temperatures” associated with each cluster species and it is shown that the internal kinetic temperature and the kinetic temperature associated with the center of mass of the clusters are different from the thermodynamic temperature of the system as a whole. Molecular dynamic simulations show that the internal temperatures are smaller than the thermodynamic one, which is smaller than the kinetic temperatures for all cluster sizes. For the case of monomers more precise predictions can be made and they are in excellent agreement with our simulations.
Collisions of O(1D) with HF, F2, XeF2, NF3, and CF4: Deactivation and reaction
Authors: Sorokin V. I. , Gritsan N. P. , Chichinin A. I.
Published: July 2002

The reactions of O(1D) atoms with fluorides have been investigated by time-resolved laser magnetic resonance (LMR). O(1D) atoms were produced by the dissociation of ozone with an excimer laser (KrF, 248 nm). By monitoring Cl atoms (when HCl or Cl2 is added) or FO radicals, the rate constants for total removal of O(1D) (×10−11 cm3/s) by HF(5.1±1.0), F2(0.81±0.20), XeF2(16±3.0), NF3(1.2±0.25), and CF4(<0.016) have been determined at 298 K. Reaction rate constants (×10−11 cm3/s) have been obtained for HF(1.5±0.3), F2(0.81±0.2), XeF2(14±3.0), and NF3(1.0±0.3). The deactivation of O(1D) by HX (X=F, Cl, Br) is discussed. Ab initio calculations have been performed with the aim of qualitative comparison of deactivation of O(1D) by F2 and Cl2.
Freezing of NO gateway emission by a magnetic field and very long field-free lifetimes of perturbed NO(a 4Π) levels
Authors: Mo Y. , Ottinger Ch. , Shen G.
Published: July 2002

The collision-induced emission from perturbed NO(B 2Π/a 4Π) (“gateway”) levels, previously studied by us using a beam/target gas cell configuration, was re-examined by crossing the NO(a 4Π) beam with a target gas jet. Moving the observation point along the primary beam, spatially resolved NO(B,v=0 and 3) gateway emission profiles were recorded. Two types of measurements were made: (a) Applying a magnetic field at the collision zone, the B, v=0 emission was quenched within the field, but reappeared at the field exit. This “freezing” confirms the mechanism of the gateway quenching as formulated earlier. (b) In the absence of a magnetic field, anomalously long radiative lifetimes were determined from both the v=0 and v=3 emission profiles. This direct observation of the long-lived eigenstate resulting from the perturbation is consistent with earlier measurements on the other, short-lived component.
Quantum mechanical calculation of resonance tunneling in acetylene isomerization via the vinylidene intermediate
Authors: Germann Timothy C. , Miller William H.
Published: July 2002

Microcanonical rate constants for the acetylene isomerization reaction have been computed using the direct cumulative reaction probability methodology of Seideman and Miller [J. Chem. Phys. 96, 4412 (1992); 97, 2499 (1992)] and Manthe and Miller [J. Chem. Phys. 99, 3411 (1993)]. Two- and three-degree-of-freedom calculations are reported using a normal mode Hamiltonian based on the geometry of the vinylidene intermediate. Due to the vinylidene well, numerous resonances are found in the isomerization rate. Little coupling is found between the CH2 rock reaction coordinate and the other normal modes, so that the resonances are readily assignable as normal mode progressions. Qualitatively similar results are obtained using two different potential energy surfaces, or different reduced dimensionality sets of coordinates.
Ab initio relativistic all-electron calculation of the Ar–I2 ground state potential
Authors: Kunz Christian F. , Burghardt Irene , Heß Bernd A.
Published: July 2002

Correlated relativistic all-electron supermolecular ab initio calculations of the ground state potential of the Ar–I2 molecule are presented. The role of differential intramonomer spin–orbit and correlation effects in the interaction energy is investigated and found to be only of minor importance. Two energetically very similar minima of the Ar–I2 complex are found, corresponding to a linear and a T-shaped geometry of the monomers. The comparatively large isomerization barrier for the two conformations indicates the existence of two stable isomers at very low temperatures.
Raman resonance de-enhancement in the excitation profile of CS2
Authors: Wu Ming , Ray Mark , Sedlacek Arthur J.
Published: July 2002

The total differential Raman cross section of the symmetric vibrational mode of CS2 (652 cm−1) in liquid phase has been measured as a function of excitation wavelength from the visible to the ultraviolet. The resulting excitation profile shows a strong preresonance enhancement when the excitation wavelength is less than 300 nm. The cross section measured at 240 nm is about three orders-of-magnitude larger than the ν4 dependence for Raman scattering. The observed preresonant effect appears to be dominated by the B21(Σu+1)←Σg+1 transition. A minimum in the excitation profile occurs at a wavelength that is associated with the peak of the near-UV absorption band (∼320 nm). The observed dip in the profile is ascribable to a quantum interference between the B21(Σu+1) and the two Renner–Teller components, B21 and A21(Δu1). The transition from the ground state to the lower electronic state is electronically forbidden, but it becomes vibronically allowed due to the Renner–Teller interaction. This may be the first observation of Raman resonance de-enhancement due to the interference involving three excited states.
A mechanism of photodissociation of diphenylmethane to a diphenylmethyl radical in solution
Authors: Fujiwara M. , Yamasaki A. , Mishima K. , Toyomi K.
Published: July 2002

The photodissociation of diphenylmethane by excitation to the S1 state at 266 nm in n-heptane solution is studied by nanosecond fluorescence and absorption spectroscopy. The formation of the diphenylmethyl radical is identified by its fluorescence, which is induced by excitation at 308 nm, and by its absorption. The growth rate of (3.7±0.4)×107 s−1 for the radical is equal to the decay rate of (3.8±0.4)×107 s−1 for the precursor fluorescence. The quantum yield of the radical is of the order of ∼10−3. Neither dissociation to the radical nor intersystem crossing to the T1 state is thermally activated, whereas activated internal conversion to the S0 state is observed. The formation of the radical depends linearly on the photolysis pulse fluence. The data are consistent with a mechanism that the molecule undergoes intersystem crossing from thermally equilibrated levels of the S1 state to vibrationally excited levels of the T1 state at which it dissociates in competition with vibrational relaxation. The mechanism is explained in terms of electronic coupling between the precursor and product states. The S1 state does not correlate adiabatically to the ground state of the C–H bond fission products, so intersystem crossing to the T1 state precedes dissociation. In the T1 state, avoided crossing between the ππ* (benzene) configuration and the σσ* (C–H) repulsive configuration results in the adiabatic potential energy surface which evolves to the ground state of the C–H bond fission products allowing rapid dissociation.
Study of hyper-Rayleigh scattering and two-photon absorption induced fluorescence from crystal violet
Authors: Chui T. W. , Wong K. Y.
Published: July 2002

Measurement of second-order nonlinear optical response of molecules by hyper-Rayleigh scattering (HRS) is often affected by the presence of two-photon excited fluorescence. For crystal violet we found experimentally that both HRS and two-photon absorption induced fluorescence give the same depolarization ratio for the signal. This shows that the depolarization measurement is generally not useful for distinguishing HRS from an unwanted fluorescence contribution. A theoretical explanation of the equality of the depolarization ratio for the two processes is given. Spectral study shows that signals from HRS and two-photon excited fluorescence for crystal violet are well separated to allow the measurement of β by standard HRS technique. A modified external reference method is developed to deal with the measurement of β by HRS for molecules that absorb the second harmonic light. The resulting β value for crystal violet is 1140×10−30 esu, which is considerably smaller than the literature value but still amounts to a factor of 33 compared to that of p-nitroaniline.
Smoothing the landscapes of protein folding: Insights from a minimal model
Authors: Betancourt Marcos R.
Published: July 2002

This work addresses the consideration of the energy landscape roughness in protein sequence design. The proteins are modeled by 2D lattice chains, initially designed to maximize the energy gap between the folded and unfolded states. Additional optimization and control of the folding properties is achieved by specific sequence mutations that alter the energetic and geometric roughness of the landscape. It is found that mutations that reduce the energetic roughness at the expense of increasing the native-state energy generally lead to a fast folding and stable protein at lower temperatures. Such mutations are also found to modify the geometric roughness (related to nucleation effects) creating variations in the folding time that depends specifically on each sequence and can lead in many cases to a reduction of the total landscape roughness. An additional reduction of the geometric roughness is achieved by adding local bond-angle propensities to selected sequence sites.