bluuebunny/crossref_metadata_2025 huggingface dataset:
Doppler-free optical-optical double resonance polarization spectroscopy of the Cs2 (2)3Π+u, (2)3Π1u, C 1Πu, and 1u[(2)3Σu++(2)3Πu] states
Authors: Kasahara Shunji , Otsuka Kyoko , Baba Masaaki , Katô Hajime
Published: July 2002

High resolution spectra of the (2)3Πu[Ω=0+ and 1](v,J)←X 1Σg+(v″,J″) transitions of the Cs2 molecule in the 15 400−16 000 cm−1 region are measured by the technique of Doppler-free optical-optical double resonance polarization spectroscopy (OODRPS). Molecular constants of the (2)3Π0+u(v=0−18) and (2)3Π1u(v=0−13) levels are determined, and potential energy curves are constructed by the RKR method. The RKR potentials are found to be in very good agreement with the ab initio potentials computed by Spies. OODRPS spectra of the C 1Πu(v′=19−21,J′)←X 1Σg+ transitions are measured in order to expand the data on the C 1Πu state. We reconsider the perturbation of the C 1Πu(v′⩾13,J′) levels, and the perturbing state is identified as the 1u[(2)3Σu++(2)3Πu] state. The molecular constants of the perturbing state are determined to be Te=16 332.3 cm−1, ωe=21.9 cm−1, and Be=0.007 94 cm−1, and the potential is constructed by the RKR method. Energy shifts of both e and f levels of the C 1Πu state centered at the same J and the hyperfine splittings are identified as the results of spin-orbit coupling between the C 1Πu and 1u[(2)3Σu++(2)3Πu] states. Another type of energy shifts observed in only f level of the C 1Πu state is identified as the results of the L-uncoupling interaction between the (2)3Σu+[0u−(c)] state and the C 1Πu state which is mixed with the (2)3Πu[1u(a)] state by spin-orbit coupling. Line broadenings observed in the C 1Πu(v′⩾16) levels are identified as originating from the predissociation, which arises from interactions with the repulsive part of the 1u(a) state which has the potential hill with a peak of 16 420 cm−1 at about 6.6 Å.
Systematic design and evaluation of multiple-pulse experiments in nuclear magnetic resonance spectroscopy using a semi-continuous Baker–Campbell–Hausdorff expansion
Authors: Hohwy M. , Nielsen N. C.
Published: July 2002

We show that an explicit solution to a semi-continuous analog to the Baker–Campbell–Hausdorff (BCH) problem can be derived by an appropriate combination of the Magnus and BCH expansions. The resulting semi-continuous BCH (scBCH) expansion forms a valuable tool for solving the time-dependent Schrödinger equation for Hamiltonians with complicated, piecewise continuous time dependence. Such Hamiltonians are typical in multiple-pulse coherent spectroscopy. Using the scBCH expansion we derive a number of general formulas, including relations for permuted pulse sequences. These formulas simplify calculation of the effective Hamiltonian for advanced multiple-pulse experiments and allow for evaluation of this to considerably higher order than is possible using the Magnus expansion. This is important for the detailed analysis and systematic design of multiple-pulse experiments which emphasize some interactions while effectively suppressing others. The scBCH expansion is applied to problems of homonuclear dipolar decoupling in solid-state NMR and broadband heteronuclear decoupling in liquid-state NMR. Improved high-order pulse sequences for on- and off-resonance decoupling are introduced and existing recursive expansion strategies are evaluated within the presented theoretical framework.
Coloring a Lorentz gas
Authors: Nielsen Steve , Kapral Raymond
Published: July 2002

A catalytic site is introduced into a two-dimensional Lorentz gas system consisting of three disks arranged in an equilateral triangle to model reactive dynamics. This system is studied at a microscopic level using an N-cylinder description where the exact dynamics is replaced by a symbolic dynamics which is a generating partition. The Kolmogorov–Sinai entropy and its finite and colored varieties are discussed. These are then related to the colored escape rate, a macroscopic property. Lastly, escape is eliminated by extending the three disk system to an infinite lattice, and the color correlation function is studied. For large catalytic regions the Poisson process rate law expression breaks down.
Optimizing electrostatic affinity in ligand–receptor binding: Theory, computation, and ligand properties
Authors: Kangas Erik , Tidor Bruce
Published: July 2002

The design of a tight-binding molecular ligand involves a tradeoff between an unfavorable electrostatic desolvation penalty incurred when the ligand binds a receptor in aqueous solution and the generally favorable intermolecular interactions made in the bound state. Using continuum electrostatic models we have developed a theoretical framework for analyzing this problem and have shown that the ligand-charge distribution can be optimized to produce the most favorable balance of these opposing free energy contributions [L.-P. Lee and B. Tidor, J. Chem. Phys. 106, 8681 (1997)]. Herein the theoretical framework is extended and calculations are performed for a wide range of model receptors. We examine methods for computing optimal ligands (including cases where there is conformational change) and the resulting properties of optimized ligands. In particular, indicators are developed to aid in the determination of the deficiencies in a specific ligand or basis. A connection is established between the optimization problem here and a generalized image problem, from which an inverse-image basis set can be defined; this basis is shown to perform very well in optimization calculations. Furthermore, the optimized ligands are shown to have favorable electrostatic binding free energies (in contrast to many natural ligands), there is a strong correlation between the receptor desolvation penalty and the optimized binding free energy for fixed geometry, and the ligand and receptor cannot generally be mutually optimal. Additionally, we introduce the display of complementary desolvation and interaction potentials and the deviation of their relationship from ideal as a useful tool for judging effective complementarity. Scripts for computing and displaying these potentials with GRASP are available at http://mit.edu/tidor.
O
−
formation from O2 via Rydberg–Rydberg electron transfer
Authors: Nagesha K. , Pinnaduwage L. A.
Published: July 2002

We report the first clear evidence of dissociative electron attachment involving electron capture by a Rydberg molecule from another Rydberg molecule. We observed the formation of O− from excimer-laser-irradiated O2 in the presence of toluene (or benzene). Results indicate that O− is formed via electron capture by Rydberg states of O2 molecules from high Rydberg states of the hydrocarbon molecules.
Photodissociation dynamics of HNF. I. Ab initio calculation of global potential energy surfaces, vibrational energies, and wave functions
Authors: Fang Wei-Hai , Peyerimhoff Sigrid , Beck Christian , Flöthmann Heiner , Schinke Reinhard , Suter Hans Ulrich , Huber J. Robert
Published: July 2002

Three-dimensional potential energy surfaces for the X̃ 2A″ and Ã 2A′ states of HNF are reported in the present paper. The ab initio calculations are carried out at the multireference configuration interaction (MRD–CI) level of theory employing a large basis set. The X̃ 2A″ potential surface possesses a deep potential well. Both surfaces have a bent equilibrium, at approximately 100 deg for the ground state and at about 125 deg for the excited one. The two electronic states become degenerate at the linear geometry. Variational calculations for the vibrational energies and the corresponding wave functions have been performed on three-dimensional fitted potential energy surfaces. The first 101 levels of the X̃ 2A″ state and the lowest 51 levels of the Ã 2A′ manifold are reported, and their vibrational modes are assigned on the basis of the nodal structure of the corresponding wave functions. The Ã 2A′ vibrational states consist of well-defined polyads with polyad quantum number P=3ν1+ν2+ν3, where ν1−ν3 are the H–N stretching, bending, and N–F stretching quantum numbers, respectively. The calculated barrier height, vertical and adiabatic excitation energies, as well as the dissociation limits, agree satisfactorily with the available experimental data. This underlines that the overall accuracy of the potential energy surfaces is good.
The low-frequency vibrational modes of c(4×2) CO on Pt(111)
Authors: Graham Andrew P.
Published: July 2002

Two, previously unobserved, low-frequency vibrational modes of c(4×2) CO/Pt(111), measured using high-resolution helium atom scattering, are reported. The modes, at frequencies of ℏω=8 meV and ℏω=10 meV, are nearly dispersionless, similar to the previously known low-frequency mode at ℏω=7 meV, which was also observed in the present measurements. From the intensity dependence on wave vector and relative intensity to the ℏω≈7 meV mode, we suggest that the new modes are also polarized parallel to the Pt(111) surface and, further, that they are associated with the bridge bonded CO molecules.
Interpolated potential energy surface and dynamics for the reactions between N(4S) and H3+(1A1′)
Authors: Bettens Ryan P. A. , Collins Michael A.
Published: July 2002

An ab initio potential energy surface for the quartet electronic state of NH3+ has been constructed at the MP2/6-31G(d,p) level of theory. The accuracy of this surface has been verified by comparison with high levels of theory. Classical simulations of the collision of N(4S) and H3+(1A1′) showed no reaction to form NH2++H at thermal energies. The possibility of reaction via surface hopping to the doublet electronic state has been investigated by calculation of the quartet–doublet energy gap at the MRCI/6-311+G(2df,p) level of theory. No evidence of surface crossing could be found for configurations accessible at thermal energies.
A molecular beam study of nonlinearity in the CO-induced surface restructuring of Ir{100}
Authors: Ali T. , Klötzer B. , Walker A. V. , King D. A.
Published: July 2002

The kinetics of CO chemisorption on both the (1×5) and (1×1) surfaces of Ir{100}, including the CO-induced surface restructuring process, have been studied by measuring the sticking probability as a function of the surface temperature and beam flux. Due to competition between desorption from the (1×5) phase and growth of (1×1) islands, the sticking probability on the initial (1×5) surface is strongly flux-dependent at surface temperatures Ts in the range 480⩽Ts⩽510 K. It is shown that this is due to a strongly nonlinear dependence of the (1×1) growth rate on the local CO coverage on the (1×5) substrate, with an apparent reaction order of around 5. Desorption energies and pre-exponentials of desorption for CO from both the (1×1) and (1×5) surfaces have been determined by means of a modified lifetime measurement technique. Equilibrium coverages as well as isothermal desorption rates of CO were determined for both surface phases. The zero coverage desorption energy of CO from the (1×1) substrate is 196±5 kJ/mol and from the (1×5) surface it is around 150 kJ/mol. This difference in adsorption energies is the driving force for the CO-induced (1×5) to (1×1) phase transition. TEAS data show that the local CO coverage on the growing (1×1) islands during the phase transformation is 0.5 ML.
Correlation and dimerization effects on the physical behavior of the NR4[Ni(dmit)2]2 charge transfer salts: A density matrix renormalization group study of the quarter-filling
t–J
model
Authors: Doublet Marie-Liesse , Lepetit Marie-Bernadette
Published: July 2002

The present work studies the quasi-one-dimensional Ni(dmit)2-based compounds within a correlated model. More specifically, we focus our attention on the composed influence of the electronic dimerization-factor and the repulsion, on the transport properties and the localization of the electronic density in the ground-state. Those properties are studied through the computation of the charge gaps (difference between the ionization potential and the electro-affinity, IP-EA) and the long- and short-bond orders of an infinite quarter-filled chain within a t–J(t,U) model. The comparison between the computed gaps and the experimental activation energy of the semiconductor NH2Me2[Ni(dmit)2]2 allows us to estimate the on-site electronic repulsion of the Ni(dmit)2 molecule to 1.16 eV.
A reciprocal space based method for treating long range interactions in ab initio and force-field-based calculations in clusters
Authors: Martyna Glenn J. , Tuckerman Mark E.
Published: July 2002

A new reciprocal space based formalism for treating long range forces in clusters is presented. It will be shown how the new formalism can be incorporated into plane-wave based density function theory calculations, standard Ewald summation calculations, and smooth particle-mesh Ewald calculations to yield accurate and numerically efficient descriptions of long range interactions in cluster systems.
Dissociative sticking of CH4 on Ru(0001)
Authors: Larsen J. H. , Holmblad P. M. , Chorkendorff I.
Published: July 2002

In this study the CH4 dissociation probability on Ru(0001) is found for various translational and vibrational energies. The absolute sticking values are determined from King and Wells experiments and carbon uptake curves. The carbon amount is determined from the recombination signal of carbon with oxygen obtained after the beam exposure when heating in an oxygen atmosphere. The measured sticking coefficient of CH4 is strongly enhanced both by increasing the translational and the vibrational energy of the CH4 molecule. A model is applied to the data and an estimate of the thermal activation energy for CH4 dissociation is found to be in good agreement with previous bulb experiments.
Calculation of ab initio dynamic hyperpolarizabilities of polymers
Authors: Otto Peter , Gu Feng Long , Ladik Janos
Published: July 2002

The coupled Hartree–Fock (CHF) equations in second order are derived to calculate dynamic polarizabilities and hyperpolarizabilities for infinite periodic chains. The analytical expressions for the second derivatives of the perturbed crystal orbitals with respect to the quasimomentum k are developed. The first and second derivatives are required on behalf of the definition of the perturbation operator describing the effect of the time-dependent electric field on the electronic structure of the polymer. The computer program has been applied to calculate the tensor elements of the second-harmonic generation and the optical rectification for the model chain poly(water) and the conjugated π-electron system poly(carbonitrile), respectively. The CHF-results are compared with uncoupled Hartree–Fock (UCHF) calculations.
Monte Carlo method and the nonequilibrium structure and non-Newtonian viscosity of a sheared simple liquid
Authors: Farhat Hikmat , Eu Byung Chan
Published: July 2002

The Monte Carlo method is applied to study the nonequilibrium steady-state structure and non-Newtonian viscosity of a sheared simple liquid. The simulation is performed in-plane Couette flow geometry on the basis of the nonequilibrium statistical mechanical formalism provided by the nonequilibrium ensemble method. The results are compared with those by the nonequilibrium molecular-dynamics simulation results and also those by the integral equation method. The present Monte Carlo method is comparable in accuracy with the molecular-dynamics simulation method for the quantities compared.
Neutron diffraction and computer modeling studies of expanded liquid Cs-Tl
Authors: Leichtweiß O. , Hochgesand K. , Biermann C. , Winter R.
Published: July 2002

We report on neutron diffraction measurements on the equiatomic liquid alloy Cs–Tl over a wide temperature interval, ranging from 730 to 1700 K, and at pressures up to 100 bar. In the crystalline state, this semiconducting alloy contains octahedrally coordinated polyanions Tl66−. The static structure factor S(Q) of molten Cs–Tl at T=730 K shows a prepeak at Q=0.72 Å−1, indicating intermediate-range correlations and thus the existence of large Tl clusters. With increasing temperature the prepeak shifts towards smaller Q values and its height decreases. At T⩾1300 K the prepeak in S(Q) vanishes and a strong increase of small-angle scattering is observed, suggesting a loss of coherence between Tl clusters and a tendency towards microphase separation. Analysis of the pair distribution functions n(r) indicates that thallium polyanion clusters may be present in liquid Cs–Tl even up to high temperatures. Based on the experimental diffraction patterns reverse Monte Carlo (RMC) simulations have been carried out. They show that the prepeak and the strong increase of small-angle scattering observed in the experimental total Faber–Ziman structure factors occur predominantly in the partial structure factor STlTl(Q). The RMC results support a network-like picture for the arrangement of Tl atoms. With increasing temperature the network disintegrates and the tendency of Tl atoms to form clusters becomes a more general tendency towards microphase separation.
Effect of long-range forces on the interfacial profiles in thin binary polymer films
Authors: Werner A. , Müller M. , Schmid F. , Binder K.
Published: July 2002

We study the effect of surface fields on the interfacial properties of a binary polymer melt confined between two parallel walls. Each wall attracts a different component of the blend by a nonretarded van der Waals potential. An interface which runs parallel to the surfaces is stabilized in the center of the film. Using extensive Monte Carlo simulations we study the interfacial properties as a function of the film thickness, the strength of the surface forces, and the lateral size over which the profiles across the film are averaged. We find evidence for capillary wave broadening of the apparent interfacial profiles. However, the apparent interfacial width cannot be described quantitatively by a simple logarithmic dependence on the film thickness. The Monte Carlo simulations reveal that the surface fields give rise to an additional reduction of the intrinsic interfacial width and an increase of the effective interfacial tension upon decreasing the film thickness. These modifications of the intrinsic interfacial properties are confirmed by self-consistent field calculations. Taking account of the thickness dependence of the intrinsic interfacial properties and the capillary wave broadening, we can describe our simulation results quantitatively.
Pragmatic analysis for the range of validity of the lattice cluster theory
Authors: Freed Karl F. , Dudowicz Jacek
Published: July 2002

While Quinn and Gujrati [B. Quinn and P. D. Gujrati, J. Chem. Phys. 110, 1299 (1998), preceding paper] introduce several mathematical criteria to assess the validity of the lattice cluster theory (LCT) and any theory for the thermodynamics of polymer systems, their main criticisms of the LCT apply either to temperatures far below those sensible in view of the derivation of the LCT, which retains only the first few terms from a high-temperature expansion of the free energy, or to regions of the phase diagram for which a mean-field theory is well known to be inadequate. Additional complaints with the LCT stem from the occurrence of a physically obvious extraneous root that is an inevitable consequence of truncating a high-temperature expansion and that is readily rejected based on elementary physical grounds. Moreover, the “pathologies” of the LCT alleged by Quinn and Gujrati for the incompressible polymer–solvent system are largely irrelevant to all applications of the lattice cluster theory to binary polymer blends. Finally, we discuss how simple physical considerations and comparisons with Monte Carlo computations provide far better insights into the practical limits of validity of the LCT than the purely mathematical criteria of Quinn and Gujrati as applied for unphysically low, dimensionless temperatures.
Molecular association between water and dimethyl sulfoxide in solution: A molecular dynamics simulation study
Authors: Borin Ivana A. , Skaf Munir S.
Published: July 2002

A molecular dynamics simulation study of the local structures and H-bond distribution for water–dimethyl sulfoxide (DMSO) mixtures over the entire composition range is presented. Analysis of several site-site pair distribution functions reveals that two well-defined kinds of aggregates characterize the molecular association between water and DMSO in solution. One of them, already identified through recent neutron diffraction experiments and computer simulations, consists of two water molecules H-bonded to the oxygen atom of a DMSO molecule, such that the angle between the two H-bonds is nearly tetrahedral. The other complex features a central water molecule and two DMSO, making H-bonds to water hydrogens. According to the simulation data, these molecular aggregates coexist with each other in the mixture, but their proportions change with composition. 1DMSO-2water complexes predominate over 2DMSO-1water aggregates for water-rich mixtures (water mole fractions >50%), whereas the opposite is true for DMSO-rich mixtures. The present simulations also seem to indicate that an association between a pair of DMSO molecules through their oxygen atoms is made possible by the formation of the 2DMSO-1water complexes. Implications of the existence of these aggregates to the mobility and other dynamic properties of these mixtures are briefly discussed.
A molecular theory of the homogeneous nucleation rate. II. Application to argon vapor
Authors: Senger B. , Schaaf P. , Corti D. S. , Bowles R. , Pointu D. , Voegel J.-C. , Reiss H.
Published: July 2002

The molecular theory of the homogeneous nucleation rate based on the n/v-Stillinger cluster, and developed in the preceding paper (paper I), is applied to the condensation of supersaturated argon vapor, in a preliminary calculation of the rate of nucleation for a single set of conditions (temperature=85 K, pressure=2500 Torr). Free energies are obtained by means of Monte Carlo simulation. Upper and lower bounds differing by only two orders of magnitude are obtained. Since the best current measurements of vapor phase nucleation rates are accurate to within about a single order of magnitude, this result is considered promising. The direction of future work to improve the accuracy of the predicted rate is clear, and considerable improvement should be possible. These directions are discussed in the paper. Also, the essentially non ad hoc nature of the n/v-Stillinger cluster is demonstrated by the appearance of a range of connectivity distances (in a predicted location) within which the calculated nucleation rate is insensitive to the connectivity distance. As in the case of paper I, subtle features of the nucleation process, unnoticed in earlier theories, are revealed.
Asymptotic convergence rates of Fourier path integral methods
Authors: Eleftheriou Maria , Doll J. D. , Curotto E. , Freeman David L.
Published: July 2002

The asymptotic rates of convergence of thermodynamic properties with respect to the number of Fourier coefficients, kmax, included in Fourier path integral calculations are derived. The convergence rates are developed both with and without partial averaging for operators diagonal in coordinate representation and for the energy. Properties in the primitive Fourier method are shown to converge asymptotically as 1/kmax whereas the asymptotic convergence rate is shown to be 1/kmax2 when partial averaging is included. Properties are shown to converge at the same rate whether full partial averaging or gradient partial averaging is used. The importance of using the proper operator to optimize convergence rates in partial averaging calculations is emphasized.
Jastrow corrected time-dependent self-consistent field approximation
Authors: Wilkie Joshua , Ratner Mark A. , Gerber R. B.
Published: July 2002

An improved approximation to the time-dependent Schrödinger equation is developed by correcting the time-dependent self-consistent field ansatz with a Jastrow prefactor defined via a set of variationally determined time-dependent parameters and a linearly independent set of prespecified spatial functions. The method is applicable in any number of dimensions, conserves norm and energy, is without parametric singularities, possesses an internal estimate of the accuracy, and has computational costs that scale algebraically with the number of degrees of freedom. The new formalism is applied to a two-dimensional double well potential to demonstrate the improved accuracy of the method. An extension of the method to electronically nonadiabatic problems is also presented.
Electron-ion recombination rate constant in dense gaseous argon and krypton
Authors: Wojcik Mariusz , Tachiya M.
Published: July 2002

The rate constants of bulk electron-ion recombination in dense gaseous argon and krypton have been calculated by means of computer simulation. The electron scattering in those systems was modeled by the low-pressure energy-dependent cross sections for elastic collisions with gas atoms. The applicability of the assumed cross sections to the considered systems was verified by the calculation of the electron mobility in an external electric field. The recombination rate constants determined from our simulation are two orders of magnitude lower than the experimental results. Possible reasons for this disagreement are discussed.
Isotopic probing of very weak intermolecular forces: Microwave and infrared spectra of CO-He isotopomers
Authors: McKellar A. R. W. , Xu Yunjie , Jäger Wolfgang , Bissonnette C.
Published: July 2002

The pure rotational transition (J,j,l)=(101)←(000) of the weakly bound CO-He complex has been observed in the 17 GHz region for five isotopomers, namely C12O-16He4, C13O-16He4, C12O-18He4, C13O-18He4, and C13O-17He4, using a pulsed-jet cavity Fourier-transform microwave spectrometer. Hyperfine structure due to the quadrupolar O17 (I=5/2) nucleus has been observed and analyzed to yield the quadrupole coupling parameters. A new microwave-terahertz double resonance spectrometer has been used to carry out an experiment on C13O-16He4 in which a submillimeter-wave pump transition (J,j,l)=(110)←(000) and a microwave signal transition (211)←(110) were detected. Infrared spectra of C13O-16He4 and C12O-18He4, have been obtained in the 2100 cm−1 region of the C-O stretch using a tunable diode laser spectrometer and a long-path (200 m), low-temperature (46 K) equilibrium gas cell. The combined data are sufficient to construct essentially complete experimental energy level schemes for the bound states of the C12O-16He4, C13O-16He4, and C12O-18He4 forms of the complex. These energies are compared with calculated levels derived from two intermolecular potential models, V(3,3,3) and XC (fit). It is shown that the new spectra, in particular those of C13O-He16, discriminate between the two models and indicate that V(3,3,3) provides a better representation of the potential in the region of the attractive well probed by the bound states. A quasibound state of CO-He was observed for the first time, and its width due to predissociation was approximately determined, providing a sensitive measure of the depth of the potential.
Homogeneous nucleation of supersaturated cesium vapor
Authors: Rudek Markus M. , Katz Joseph L. , Uchtmann Hermann
Published: July 2002

The homogeneous nucleation of supersaturated cesium vapor was investigated in a thermal diffusion cloud chamber operating in both the upward and the downward mode. In the upward operating mode, critical supersaturations were measured in the temperature range, 446–492 K. By operating the chamber in the downward mode, it was possible to circumvent experimental difficulties which arise at low pressures (due to phoretic effects) for the measurements made in the upward mode. Our previously published measurements on cesium were obtained in the upward mode in the temperature range, 421–554 K. These earlier measurements agree perfectly (to within their scatter) with the measurements presented here in the temperature range where they overlap, i.e., 421–492 K. The use of the downward mode enabled the extension of the temperature range of the measurements by 132 K towards lower temperatures. This makes cesium the substance whose homogeneous nucleation has been measured, in a thermal diffusion cloud chamber, over the largest range of temperatures, i.e., 289–554 K. The measured critical supersaturations were compared to the predictions of the Internally Consistent version of Classical Nucleation Theory and to the predictions of the Classical Theory.
Ab initio and molecular-dynamics studies on rare gas hydrides: Potential-energy curves, isotropic hyperfine properties, and matrix cage trapping of atomic hydrogen
Authors: Kiljunen Toni , Eloranta Jussi , Kunttu Henrik
Published: July 2002

Ground-state potential-energy curves and distance dependent isotropic hyperfine coupling (IHC) constants for ground-state H–RG (=Ne, Ar, Kr, Xe) are obtained at CCSD(T) (coupled-cluster single double triple) and MP4(SDQ) (fourth-order Moller–Plesset single double quadruple) levels, respectively, with an augmented basis set aug-Stuttgart (RG)/aug-cc-pVQZ (H). The obtained Rm and ε are for NeH: 3.45 Å and −1.36 meV; ArH: 3.65 Å and −3.48 meV; KrH: 3.75 Å and −4.32 meV; XeH: 3.90 Å and −5.22 meV. The computed pair potentials are utilized in classical molecular-dynamics simulations of H–RG lattices. Along the classical trajectory, the many-body perturbation on the H atom hyperfine coupling constant is computed by pair-wise addition of the individual RG–H contributions obtained from the present quantum-chemical calculations. The computed IHC shifts are compared with electron paramagnetic resonance (EPR) spectra obtained in low-temperature matrix isolation experiments. For most cases this theoretical treatment agrees very well with the experiment and confirms the previous site assignments. However, for H–Xe, the theory would suggest stability of both interstitial Oh and substitutional sites, whereas only one site is observed in the experiment. Based on the present calculations this site can be assigned as a nearly undistorted substitutional site.
A three-dimensional potential energy surface for He+Cl2 (B 3Π0u+): Ab initio calculations and a multiproperty fit
Authors: Williams Jason , Rohrbacher Andreas , Seong Jeonghee , Marianayagam Neelan , Janda Kenneth C. , Burcl Rudolf , Szcześniak M. M. , Chałasiński Grzegorz , Cybulski Sławomir M. , Halberstadt Nadine
Published: July 2002

High quality ab initio calculations for the interaction of He with the B 3Π0u+ state of Cl2 for three r(Cl–Cl) distances, and for the He(1S)+Cl(2P) interaction are used to obtain a three-dimensional potential energy surface for the system. The surface was used to calculate HeCl2 excitation spectra, predissociation lifetimes, and product state distributions for comparison with experimental data, and yields a remarkably good agreement. The largest discrepancy is in the dependence of the lifetime on the excited state vibrational level. The calculated lifetimes are too short for the lowest measured vibrational levels. To investigate how the surface could be modified to obtain even better agreement, a microgenetic algorithm was used to adjust the potential parameters to improve the fit. The adjusted surface has a softer repulsive wall for small Cl–Cl separations which helps to lengthen the excited state lifetimes and yields better agreement with the data. Also, the shape of the well region is adjusted somewhat in the fitting process, which yields a stronger dependence of lifetime on vibrational level.
Third-order nonlinear optical response of energy transfer systems
Authors: Yang Mino , Fleming Graham R.
Published: July 2002

The third-order nonlinear optical response of energy transfer systems is theoretically investigated. A system composed of two chromophores having the same electronic transition energies is considered. The dynamics of energy transfer between the two chromophores is assumed to occur via a hopping (incoherent) mechanism. We introduce new types of pathways incorporating the hopping processes occurring while the system is in population states and reconstruct a third-order response function which is computationally viable. The nuclear propagators in the electronic population states are written as convolution integrals between those of the nonreactive two-state system weighted by some factors for the energy transfer. The response function is given by multitime correlation functions and these are analyzed by the cumulant expansion method. Based on this approach, the three-pulse photon echo peak shift for several models of energy transfer systems is discussed. It is shown that the rephasing capability of the induced signal is reduced by the memory loss due to resonant energy transfer. A previous model which incorporates resonant energy transfers in an intuitive way is reviewed and modified to supplement the loss of dynamic correlation of nuclear motion within the framework of the theory. The response function obtained by our new approach gives a more accurate description than the existing theory and a comparative discussion is given. The effect of inhomogeneity in rate constants on the third-order signal is discussed and the temperature dependence of the echo signal is examined.
Hyper-Rayleigh scattering of centrosymmetric molecules in solution
Authors: Yaliraki S. N. , Silbey R. J.
Published: July 2002

We study theoretically the harmonic light scattering (HLS) of centrosymmetric molecules in solution. Since HLS is inherently absent for centrosymmetric molecules, the intensity and line shape are obtained by taking into account two distinct physical processes: solute vibrational transition and solvent effective field contribution. The intensity is expressed in each case as the Fourier transform of the relevant time correlation functions. The implications for experiments, which use HLS to determine the first hyperpolarizability β of optically interesting molecules, are discussed.
The 1Πu← X 1Σg+ electronic spectrum of C5 in the gas phase
Authors: Motylewski Tomasz , Vaizert Olga , Giesen Thomas F. , Linnartz Harold , Maier John P.
Published: July 2002

The origin and three vibronic bands of the 1Πu←X 1Σg+ electronic transition of linear C5 have been observed in the gas phase. The carbon chain is produced in a slit nozzle employing both discharge and ablation techniques. Cavity ring down spectroscopy is used to measure the electronic transition. The origin band is found at 510.94(1) nm, shifted 29 cm−1 to the red of the value in a neon matrix. Intramolecular processes lead to broadening and irregularities in the rotational structure. The relation to astronomical observations is discussed.
On the scaling of semiclassical initial value methods
Authors: Brewer Mark L.
Published: July 2002

The semiclassical initial value method of Walton and Manolopoulos, the cellularized frozen Gaussian approximation for the calculation of bound–bound Franck–Condon spectra, has been applied to a series of model problems with up to ten coupled degrees of freedom. The number of classical trajectories required to converge these spectra is found to increase exponentially with the number of degrees of freedom in the model problems. On comparison with earlier applications of the method to the threshold photodetachment spectra of the ArnI− (n=2–6) anions, it appears that the number of trajectories required to converge a Franck–Condon spectrum semiclassically is directly proportional to the number of final quantum states which contribute to the spectrum.
Theory and analysis of sodium dimer Rydberg states observed by all-optical triple resonance spectroscopy
Authors: Chang Edward S. , Li Jing , Zhang Jianming , Tsai Chin-Chun , Bahns John , Stwalley William C.
Published: July 2002

For the nf (l=3) series of Na2, quantum defects are calculated from theoretical values of the core quadrupole moment and polarizabilities. They compare favorably with those inferred from our preliminary report of high resolution all-optical triple resonance spectroscopy [Chem. Phys. Lett. 236, 553 (1995)] and from the full report of such spectra given here. The spectrum is the simplest when the higher intermediate state has J′=0 which requires a final state J=1. We predict the stroboscopic effect should first occur when n=52 for the nf series, as we observe, rather than at n=69 for the np series. Our data thus confirm that the strongest series is the nf. Hence the ionization potential is not 39 478.75±0.04 cm−1 as previously reported, but rather 39 478.101±0.013 cm−1, implying a molecular ion dissociation energy of D00(Na2+)=7914.038±0.014 cm−1.
Microwave spectrum of the NI radical in the X 3Σ− ground state
Authors: Sakamaki Toru , Okabayashi Toshiaki , Tanimoto Mitsutoshi
Published: July 2002

The rotational spectrum of the NI radical in the X 3Σ− ground electronic state has been observed by a source modulated spectrometer with a free space cell. The NI radical was generated in the cell by a dc (direct current) glow discharge in a mixture of iodine vapor with N2 and He. A rotational transition consisted of a group of congested lines resulting from the hyperfine interactions due to both iodine and nitrogen nuclei. The molecular constants including the spin–spin and spin–rotation coupling constants and the magnetic and nuclear quadrupole hyperfine interaction constants were determined by the least-squares analysis. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 76.8% and 23.4% on the nitrogen and the iodine atoms, respectively. The quadrupole coupling constant was related with the difference in electronegativity of the two bonded atoms.
Interaction-induced pair hyperpolarizabilities by spherical irreducible tensors
Authors: Bancewicz Tadeusz
Published: July 2002

Starting from the electrostatic part of the intermolecular multipole interaction energy we derived irreducible spherical tensor formulas for the first-order long-range, interaction-induced first and second pair hyperpolarizabilities ΔβLM and ΔγLM for arbitrary shape monomers. For atoms a general relation is obtained for the dipolei–2k-pole hyperpolarizability tensor Z(i+k) of arbitrary order, between its irreducible spherical components Z00[((((11)a11)a2…)ai−21)kk] and the Cartesian counterpart Zzz…z(i+k). For isotropic systems the expressions for Δβzzz and Δγzzzz are in full agreement with the Cartesian tensor results of Buckingham, Concannon and Hands [J. Phys. Chem. 98, 10455 (1994)] and Li et al. [J. Chem. Phys. 105, 10954 (1996)]. Our irreducible spherical tensor results for ΔβLM and ΔγLM are very desirable when dealing with molecular rotations, e.g., in spectral line shape calculations of interaction-induced hyper-Rayleigh and/or hyper-Raman light scattering. Finally we show how our spherical tensor formula for ΔβLM can be used for calculation of, one molecule forbidden, hyper-Rayleigh and hyper-Raman (A1 vibration) pair hyperpolarizability Δβzzz for molecules of octahedral symmetry Oh.
Dynamic scaling concepts applied to numerical solutions of Smoluchowski’s rate equation
Authors: Odriozola G. , Schmitt A. , Callejas-Fernández J. , Martínez-Garcı́a R. , Hidalgo-Álvarez R.
Published: July 2002

Smoluchowski’s equation is widely applied to describe the time evolution of the cluster-size distribution during aggregation processes. Analytical solutions for this equation, however, are known only for a very limited number of kernels. Therefore, numerical methods have to be used for describing the time evolution of the cluster-size distribution. In this work, we present a novel self-consistent method for solving Smoluchowski’s equation for any homogeneous kernel. The method considers dynamic scaling to be valid but does not need to assume a given form for the scaling distribution Φ(x). Moreover, the scaling distribution Φ(x) is obtained as a natural result of the algorithm. Due to the implementation of dynamic scaling concepts, the algorithm converges almost immediately with a minimal calculation effort. Comparing calculated size distributions with the corresponding analytical solutions shows the validity of the method. The method is then used to fit experimental data for diffusion limited aggregation. For this purpose, a fitting procedure is developed which allows us to fit the corresponding parameters for any given homogeneous kernel. As an application, a full comparison between the experimental data and the numerically obtained cluster-size distributions for the constant and the Brownian kernel was carried out.
The mapping of the conditional pair density onto the electron density
Authors: Bader R. F. W. , Heard G. L.
Published: July 2002

This paper shows that the Fermi hole of a reference electron can be so strongly localized to a given region of space, as to cause the conditional pair density for same-spin electrons to approach the one-electron spin density outside the region of localization and for a closed-shell system, the conditional pair density for both spins will approach the total density. Correspondingly, the Laplacian of the conditional pair density, whose local concentrations indicate the positions where the density of the remaining electrons are most likely to be found for a fixed position of a reference pair, approaches the Laplacian of the density. The Laplacian of the conditional pair density generated by a sampling of pair space by an α,β pair of reference electrons, exhibits a homeomorphism with the Laplacian of the electron density. This homeomorphism approaches an isomorphic mapping of one field onto the other, as the reference electron pair becomes increasingly localized to a given region of space. Thus the local charge concentrations (CCs) displayed by the Laplacian of the electron density, the local maxima in L(r)=−∇2ρ(r), signify the presence of regions of partial pair condensation, regions with greater than average probabilities of occupation by a single pair of electrons, as has been previously surmized on empirical grounds. This paper establishes a mapping of the essential aspects of electron pairing, determined in six-dimensional space, onto the three-dimensional space of the electron density. The properties of the conditional pair density enable one to determine which CCs of L(r) are coupled and represent the same localized pair of electrons. It is found that the pattern and properties of the electron localization domains predicted by the Laplacian of the conditional pair density differ in important aspects from those predicted by ELF, the electron localization function.
Phase coexistence properties for the polarizable point charge model of water and the effects of applied electric field
Authors: Svishchev Igor M. , Hayward Tina M.
Published: July 2002

Extensive efforts are currently devoted to the development of water models for computer simulations that explicitly incorporate molecular polarizability. Liquid–vapor coexistence properties for the polarizable point charge (PPC) model of water are examined in this contribution with the aid of molecular dynamics calculations. An accurate analytical equation of state for the high-temperature states of the PPC model is presented, including its critical region. The liquid–vapor coexistence curve and the critical point parameters for the PPC water are extracted from its equation of state. The critical temperature, pressure, and density for the PPC water are found to be within 10% of their experimental values and its critical compression factor is within 2%. The effect of applying an external electric field to the system on the coexistence properties is investigated. A shift of the critical point of water to higher temperature and pressure is observed, a result similar to that of the introduction of an electrolyte to water. A large applied electric field is capable of inducing phase separation of a subcritical fluid on the time scale of a typical simulation run.
Electron-induced “localized atomic reaction” (LAR): Chlorobenzene adsorbed on Si(111) 7×7
Authors: Lu P. H. , Polanyi J. C. , Rogers D.
Published: July 2002

Electron-induced reaction of chlorobenzene (ClPh) adsorbed on silicon [Si(111)7×7] is shown by scanning tunneling microscopy (STM) to result in “localized atomic reaction” (LAR), imprinting Cl as chemically-bound Cl–Si on the surface. Voltage pulses of −4 V from the STM tip give LAR restricted to the site of electron impact. Delocalized electron impact imprints the self-assembled pattern of ClPh(ad) on the surface as Cl–Si. The imprint is found to be on the same area of the unit cell as ClPh(ad), but at adjacent atomic sites. The occurrence of LAR is ascribed to a concerted reaction; this can only occur if the new bond (Cl–Si) is directly adjacent to the old one (Cl–Ph).
The point charge model of nuclear quadrupoles: How and why does it work
Authors: Kellö Vladimir , Sadlej Andrzej J.
Published: July 2002

The point charge model of nuclear quadrupoles is analyzed beyond the multipole approximation. The effect of the short-range expansion terms is shown to be responsible for the contamination of the calculated electric field gradients. Several regularities observed in earlier numerical calculations are elucidated and their analysis helps to devise proper computational strategies. The use of the point charge nuclear quadrupole model is strongly advocated in approximate relativistic calculations of electric field gradients at heavy nuclei.
Photochemistry of hydrogen peroxide in Kr and Xe matrixes
Authors: Khriachtchev Leonid , Pettersson Mika , Jolkkonen Santtu , Pehkonen Susanna , Räsänen Markku
Published: July 2002

UV photolysis of hydrogen peroxide (H2O2) in various rare-gas matrixes is comparatively studied. The photorecovery of H2O2 from the tight H2O⋯O complex is observed in Kr and Xe matrixes, in addition to this reaction in an Ar matrix found previously. The similarity of spectral position and efficiency of the photorecovery reaction in various rare-gas solids indicates its fundamental character, supports charge-transfer excitation of H2O⋯O as its origin, and preserves promises to find this photoreaction in media of environmental importance. In UV photolysis of H2O2, the relatively small concentration of isolated OH radicals is found in a Kr matrix, and no OH radicals appear in a Xe matrix, and this trend is discussed in terms of delayed cage exit. Moreover, additional species photogenerated from H2O2 in a Xe matrix as well as the absence of OH radicals might be connected with participation of some hidden intermediates (HOXeOH, HXeOOH, etc.) in the dynamics, thus, catalyzing new photodissociation channels. Among the photolysis products, the loose H2O//O complex is suggested to be stabilized in Kr and Xe matrixes. This loosely bound complex is quasistable and decomposes at relatively low temperatures (below 20 K) quantitatively forming the known tight H2O⋯O structure. This low-temperature process offers one additional example of short-range atomic mobility introduced recently in the literature.
Digitally filtered molecular dynamics: The frequency specific control of molecular dynamics simulations
Authors: Phillips Stephen C. , Essex Jonathan W. , Edge Colin M.
Published: July 2002

A new method for modifying the course of a molecular dynamics computer simulation is presented. Digitally filtered molecular dynamics (DFMD) applies the well-established theory of digital filters to molecular dynamics simulations, enabling atomic motion to be enhanced or suppressed in a selective manner solely on the basis of frequency. The basic theory of digital filters and its application to molecular dynamics simulations is presented, together with the application of DFMD to the simple systems of single molecules of water and butane. The extension of the basic theory to the condensed phase is then described followed by its application to liquid phase butane and the Syrian hamster prion protein. The high degree of selectivity and control offered by DFMD, and its ability to enhance the rate of conformational change in butane and in the prion protein, is demonstrated.