bluuebunny/crossref_metadata_2025 huggingface dataset:
Alignment and orientation of nonpolar molecules utilizing the laser-induced ac-Stark effect
Authors: Linskens A. F. , Dam N. , Reuss J. , Sartakov B.
Published: July 2002

The laser-induced ac-Stark effect influences the M sublevels of a molecule differently. This provides for a spectroscopical means of isolating individual M sublevels and thereby of producing aligned, or even oriented, excited molecules. We present an experimental study of this phenomenon in the spherical top SF6 using narrow bandwidth CO2 lasers. One high power CO2 laser induces an ‖M‖-dependent ac-Stark shift of the magnetic sublevels of the J=4 level of the vibrational ground state. A weak probe laser induces a two-photon transition starting from the same level and produces aligned, vibrationally excited molecules under appropriate experimental conditions. A theoretical description is provided in the dressed state picture. In principle, our triple-resonance technique is able to produce aligned or oriented molecules in a well-defined (vibrationally) excited state in any molecule possessing a similar three level system.
Surface-induced ordering for confined random block copolymers
Authors: Gutman Lorin , Chakraborty Arup K.
Published: July 2002

Motivated by practical issues that pertain to polymer adhesion, we consider the equilibrium behavior of a dilute solution of ideal A–B random block copolymers confined between two solid surfaces. We develop a general theory for the situation wherein the A–A, B–B, and A–B intersegment interactions are different, and furthermore, the A and B segments interact differently with the solid surfaces. Random block copolymers constitute a class of materials wherein a quenched disorder (the sequence distribution) is carried by the fluid whose statistical properties are of interest. In our theory, we perform quenched disorder averages using the replica trick. The nonlocal terms in our action functional are decoupled by introducing a set of random fields. The resulting equations for the propagator are analyzed within the framework of eigenfunction expansions. Since we consider long chains in confined geometries, we invoke the ground state approximation. We also carry out the functional integrals over the random fields using saddle points.

Our theory does not treat the segment–surface interactions within a mean field approximation. Our analysis leads to a set of nonlinear self-consistent-field equations. We have solved our general equations numerically for a particular problem. In order to isolate and highlight the effects of dissimilar segment–surface interactions, we consider a case wherein the intersegment interactions are all alike (of the excluded volume type), while the A segments are attracted to the solid surfaces and the B units are repelled. For this specific problem we find that, above a threshold value of the fraction of attractive segments, significant microphase ordering is induced by the surface. This leads to damped oscillations in the composition profile. This onset of significant surface-induced composition fluctuations is accompanied by an ‘‘adsorption–desorption transition’’ which corresponds to a qualitative change in the shape of the total segment density profile. These and other results are discussed and the experimentally testable consequences of our predictions are elucidated. Our results are in agreement with recent simulation studies. We suggest specific experiments that may shed further light on the physical phenomena revealed by our calculations.
Absorption spectrum calculations for a system having a few quantum and many ‘‘classical’’ degrees of freedom
Authors: Blake Nick P. , Metiu H.
Published: July 2002

We examine here several methods for calculating the photon absorption spectrum in a system consisting of an electron coupled to a large number of heavy particles. Past work has shown that calculations of the absorption cross section which treat the heavy particles classically and the electron quantum mechanically lead to qualitative errors. It has been suggested that a Gaussian wave packet description of the nuclear motion will remove these qualitative errors; moreover it will give accurate results in the case when the system has many ‘‘heavy’’ degrees of freedom or the spectrum is taken with low frequency resolution. Here we examine several implementations of this suggestion and show that it fails if a time-dependent Hartree method is used for computing the dynamics but it works very well if one treats the electron motion adiabatically or one uses the time-dependent Hartree equations with an adiabatic initial electronic state. Both methods lead to very efficient computational schemes in which the electronic wave function is calculated once, at one nuclear configuration, per time step.
The thermal conductivity of argon in the critical region
Authors: Tiesinga B. W. , Sakonidou E. P. , van den Berg H. R. , Luettmer-Strathmann J. , Sengers J. V.
Published: July 2002

We have measured the thermal conductivity of argon at temperatures from 302 K down to 150.8 K and at densities up to 25 mol L−1. The data were obtained with a steady-state method and we employed a guarded parallel-plate apparatus designed especially for investigating the thermal conductivity of fluids in the critical region. To interpret the data in the critical region a scaled crossover equation of state for argon in the critical region has been constructed. Equations for the thermal conductivity and viscosity of argon as a function of density and temperature are presented.
HF vibrational redshift for the icosahedral Ar12HF van der Waals cluster is the same as in an Ar matrix: Quantum five-dimensional bound state calculations
Authors: Liu Suyan , Bačić Zlatko , Moskowitz Jules W. , Schmidt Kevin E.
Published: July 2002

Quantum 5D bound state calculations for the fully solvated Ar12HF cluster, with the Ar atoms frozen at their icosahedral equilibrium geometry, gave the HF vibrational redshift of 42.46 cm−1. This value is equal to that measured for HF in an Ar matrix, 42.4 cm−1.
Rotation operator approach to spin dynamics and the Euler geometric equations
Authors: Zhou Jinyuan , Ye Chaohui , Sanctuary B. C.
Published: July 2002

The rotation operator approach proposed previously is applied to spin dynamics in a time-varying magnetic field. The evolution of the wave function is described, and that of the density operator is also treated in terms of a spherical tensor operator base. It is shown that this formulation provides a straightforward calculation of accumulated phases and probabilities of spin transitions and coherence evolutions. The technique focuses, not on the rotation matrix, but on the three Euler angles and its characteristic equations are equivalent to the Euler geometric equations long known to describe the motion of a rigid body. The method usually depends on numerical calculations, but analytical solutions exist in some situations. In this paper, as examples, a hyperbolic secant pulse is solved analytically, and a Gaussian-shaped pulse is calculated numerically.
Near-resonant electronic energy transfer in the electronic quenching of NO A 2Σ+ by hydrocarbons and ammonia
Authors: Furlanetto M. R. , Thoman J. W. , Gray J. A. , Paul P. H. , Durant J. L.
Published: July 2002

We report rate coefficients for the electronic quenching of NO A 2Σ+ v=0 by several hydrocarbon fuel gases, methane, ethane, propane, ethene, and ethyne, and by ammonia over a wide temperature range (300–2300 K). High temperature data is obtained behind reproducible shock waves. High-temperature quenching of NO by many species has previously been explained by a charge-transfer (harpoon) model. However, we find such a model unable to explain a portion of the quenching behavior reported here. Instead, we propose that a near-resonant electronic energy transfer mechanism is active.
Molecular reorientation cross sections in collisions of He(1S)+D2(B 1Σ+u), HD(B 1Σ+u)
Authors: Odutola J. Adeola , Lester William A.
Published: July 2002

Reorientation cross sections for He–D2(B) and He–HD(B) are reported for the energy range of 0.302×10−3–0.300 eV. The single surface scattering model and coupled-channel method used for the He–H2(B) [J. Chem. Phys. 99, 2632 (1993)] are also used for these systems. At 295 K the calculated reorientation cross section for He–D2(B) is 45.4 Å2 and the Boltzmann average is 39.5 Å2. The Boltzmann average value is to be compared with the measurement by Pibel and Moore of 27.6±10.8 Å2 [J. Chem. Phys. 93, 4804 (1990)]. The corresponding cross section calculated for He–HD(B) is 29.5 Å2 and the Boltzmann average is 29.3 Å2. The latter is in good agreement with the experimental determination of 33.2±5.0 Å2. Because of the rotational transition that is open for the HD target, many more open channels had to be considered to achieve convergence compared to the H2 and D2 targets.
Transition state vibrational level thresholds for the dissociation of triplet ketene
Authors: Kim Sang Kyu , Lovejoy Edward R. , Moore C. Bradley
Published: July 2002

Rate constants for the unimolecular dissociation of ketene (CH2CO) and deuterated ketene (CD2CO) have been measured at the threshold for the production of CH2 (X̃ 3B1) or CD2 (X̃ 3B1) and CO (X̃ 1Σ+) by photofragmentation in a cold jet. The rate constant increases in a stepwise manner as energy increases. This is in accord with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds at each energy level for vibrational motion orthogonal to the reaction coordinate. The first step in rate constant and/or photofragment excitation (PHOFEX) spectrum gives accurate values for the barrier to dissociation above the zero-point energy of the products, 1281±15 cm−1 for CH2CO and 1071±40 cm−1 for CD2CO. The measured rate constants are fit by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The vibrational frequencies at the transition state obtained from the fits are compared with ab initio results. Vibrational motions at the transition state orthogonal to the reaction coordinate are also revealed in CO product rotational distributions. Calculations using an impulsive model which includes vibrational motions at the transition state reproduce the experimental dependence of the PHOFEX spectra on the CO J state quite well. The small dependence of rate constant on jet temperature (4–30 K) indicates that the Ka quantum number for rotation about its symmetry axis is conserved in the energized ketene molecule.
A molecular dynamics simulation study with a combined quantum mechanical and molecular mechanical potential energy function: Solvation effects on the conformational equilibrium of dimethoxyethane
Authors: Liu Haiyan , Müller-Plathe Florian , van Gunsteren Wilfred F.
Published: July 2002

Solvent effects on the conformational equilibria of dimethoxyethane (DME, CH3–O–CH2–CH2–O–CH3) have been studied using molecular dynamics simulation with a combined molecular mechanical and semiempirical quantum mechanical potential energy function. The potential of mean force around the central O–C–C–O dihedral angle of DME in water was determined using umbrella sampling, with the DME molecule treated by the modified intermediate neglect of diatomic differential overlap (MNDO) method and water molecules described using the extended simple point charge model. This potential of mean force shows the same bias as the gas phase potential energy surface calculated by MNDO: the heights of the barriers are underestimated and the stability of the gauche conformation with respect to the trans conformation is overestimated. However, the calculated stabilization of the gauche state by solvation is in good agreement with experimental results, suggesting a quite reasonable description of this system by the combined potential energy function. The solute–water atom pair distributions and the averaged percentage of solute–water hydrogen bonding does not change much when the central dihedral angle changes, but in the gauche conformation one water hydrogen can be hydrogen bonded with the two solute oxygen atoms simultaneously. The interactions between DME and its explicit water environment favor the gauche over the trans conformation much more than predicted by the reaction field model, although the overall relative stabilities predicted by the latter model are not so biased due to compensation of errors. The trans conformation of the outer C–O–C–C dihedral angles is most favored during the simulations, which is qualitatively different from the reaction field prediction that the trans–gauche+–gauche− would be the most stable conformer in solution.
Application of genetic algorithm to the calculation of bound states and local density approximations
Authors: Zeiri Yehuda , Fattal Eyal , Kosloff Ronnie
Published: July 2002

A novel method, based on genetic algorithms, has been developed and applied to the solution of differential equations. The new approach is based on the use of real numbers to form the candidate solutions which are improved iteratively by a suitable breeding process. The algorithm was tested in the calculation of the bound states of a double well potential and in the nonlinear density functional calculation. Comparison of the results with those obtained using the direct relaxation method shows excellent agreement.
Semiclassical theory of molecular nonlinear optical polarization
Authors: Sepúlveda Miguel Angel , Mukamel Shaul
Published: July 2002

The calculation of linear and nonlinear optical response as well as nonadiabatic curve crossing processes depends on the time evolution of the electronic coherences (off-diagonal elements of the density matrix). Unlike their diagonal counterparts, the off-diagonal elements do not have an obvious classical limit. A semiclassical approximation for the nonlinear optical response function, which reveals the classical orbit structure underlying the electronic coherences between Born–Oppenheimer surfaces, is developed. The resulting numerical propagation, which applies to arbitrary anharmonic potentials, is based on integrating the time-dependent Schrödinger equation using the semiclassical time evolution operator, Van Vleck propagator. Using the present formalism it is possible to describe semiclassically multiphoton processes involving several Born–Oppenheimer surfaces.
A graphical approach to quantum molecular time scale generalized Langevin equation theory
Authors: McDowell H. Keith , Clogston A. M.
Published: July 2002

A graphical approach for obtaining the algebraic and topological structure of time derivatives of time correlation functions is proposed in the context of quantum molecular timescale generalized Langevin equation (MTGLE) theory. A sequence of approximate MTGLE procedures is introduced using the graphical approach, and computations on a single-vibron Hamiltonian model are carried out using the procedures. In particular a new unlinked procedure for obtaining estimates of higher-order time derivatives is revealed by the graphical approach. The graphical approach is shown to be a new and powerful tool to be used with MTGLE theory.
Density functionals for polymers near surfaces
Authors: McMullen William E. , Trache Maria
Published: July 2002

We derive a gradient expansion for the external field necessary to weakly perturb the density profile of noninteracting polymer chains near a surface. This result can be used as part of a more general methodology for analyzing the long-wavelength adsorptive properties of polymer solutions and melts. The coefficients appearing in the expansion are determined from the solutions of a hierarchy of linear, Fredholm integral equations that contain information about the reference (unperturbed) state. We illustrate the calculation of the two lowest-order coefficients for reflecting and absorbing reference boundary conditions. A similar gradient expansion of the intrinsic free energy functional suggests the need to exercise care in obtaining variational forms for the grand free energy from the Legendre transform of the grand partition function. Extremely simple symmetry arguments show that a proper application of gradient expansion techniques leads to a vanishing of the linear gradient contributions to the free energy.
Vibrational relaxation in the 3B1 state of SO2
Authors: Bae S. C. , Lee K. , Kim G. H. , Ku J. K.
Published: July 2002

The vibrational relaxation rate constants of the four low-lying vibrational levels in the 3B1 state of the SO2 molecule have been investigated by a laser induced phosphorescence method in pure SO2. The relaxation rate constants for the 3B1(0,1,0) level have also been studied in Ar, O2, N2, and CO2. The low-lying vibrational levels of the 3B1 state are populated from ground state SO2(X 1A1) by direct pumping, and the phosphorescence emissions from the laser excited as well as the collisional product levels are monitored. The state-to-state relaxation rate constants for the upper two levels are assigned by the kinetic simulations of the phosphorescence time profiles observed from each level. The magnitudes of the relaxation rate constants are in the range of 2.3–3.0×10−11 cm3 molecule−1 s−1 in SO2 and 0.26–1.3×10−11 cm3 molecule−1 s−1 in other gases.
Vibrational mode effects, scattering dynamics, and energy disposal in reaction of C2H+2 with methane
Authors: Chiu Yu-hui , Fu Hungshin , Huang Jui-tsen , Anderson Scott L.
Published: July 2002

The effects of collision energy and mode-selective vibrational excitation on the reaction of C2H+2 with CH4 and CD4 have been measured, along with the corresponding product velocity distributions. Two distinct reaction mechanisms are active in the energy range below 5 eV. At low energies, a long-lived C3H+6 complex forms efficiently, then decomposes primarily to C3H+5+H and C3H+4+H2. The RRKM lifetime of this complex is estimated to range between ∼10 ns and ∼10 ps over the experimental energy range, and this is sufficient time to allow substantial H-atom scrambling. Complex formation is strongly inhibited by collision energy, weakly inhibited by CC stretching, and enhanced by bending excitation. Competing with the complex-mediated mechanism is a direct H-atom abstraction reaction, producing C2H+3+CH3 with little atom scrambling. This reaction is shown to have a ∼150 meV activation barrier and is strongly enhanced by collision energy, becoming the dominant channel above 0.4 eV. CC stretching provides a weaker enhancement than collision energy, while bending enhances the reaction ∼10 times more efficiently. As collision energies increase, the C2H+3 product is increasingly forward scattered with an increasing fraction of the available energy going into recoil. Energy put into reactant vibration mostly is retained as internal energy of the products. Over the collision energy range from 0.4 to 2.8 eV, the collision time in the direct reaction varies from ≥1.3 ps to ≤70 fs.
Statistical thermodynamics of protein folding: Comparison of a mean-field theory with Monte Carlo simulations
Authors: Hao Ming-Hong , Scheraga Harold A.
Published: July 2002

A comparative study of protein folding with an analytical theory and computer simulations, respectively, is reported. The theory is based on an improved mean-field formalism which, in addition to the usual mean-field approximations, takes into account the distributions of energies in the subsets of conformational states. Sequence-specific properties of proteins are parametrized in the theory by two sets of variables, one for the energetics of mean-field interactions and one for the distribution of energies. Simulations are carried out on model polypeptides with different sequences, with different chain lengths, and with different interaction potentials, ranging from strong biases towards certain local chain states (bond angles and torsional angles) to complete absence of local conformational preferences. Theoretical analysis of the simulation results for the model polypeptides reveals three different types of behavior in the folding transition from the statistical coiled state to the compact globular state; these include a cooperative two-state transition, a continuous folding, and a glasslike transition. It is found that, with the fitted theoretical parameters which are specific for each polypeptide under a different potential, the mean-field theory can describe the thermodynamic properties and folding behavior of the different polypeptides accurately. By comparing the theoretical descriptions with simulation results, we verify the basic assumptions of the theory and, thereby, obtain new insights about the folding transitions of proteins. It is found that the cooperativity of the first-order folding transition of the model polypeptides is determined mainly by long-range interactions, in particular the dipolar orientation; the local interactions (e.g., bond-angle and torsion-angle potentials) have only marginal effect on the cooperative characteristic of the folding, but have a large impact on the difference in energy between the folded lowest-energy structure and the unfolded conformations of a protein.
Ab initio study of the vibronic and spin–orbit coupling in the X 2Πu state of C2H
+
2
Authors: Perić Miljenko , Thümmel Helmar , Marian Christel M. , Peyerimhoff Sigrid D.
Published: July 2002

A variational approach for treating the Renner–Teller effect in tetra-atomic molecules [Perić et al., Mol. Phys. 55, 1159 (1985)] is extended to account for the effect of spin–orbit coupling. The approach is applied to compute the spin–orbit splittings of the vibronic levels in the X 2Πu state of C2H+2. The bending potential curves employed in a previous study [Perić and Peyerimhoff, J. Chem. Phys. (in press)] are improved by carrying out ab initio calculations at a higher level of sophistication. The results of the computations enable a reliable interpretation of recent experimental findings [Pratt et al., J. Chem. Phys., 99, 66 (1993)].
Optical theorem and the inversion of cross section data for atom scattering from defects on surfaces
Authors: Hamburger D. A. , Gerber R. B.
Published: July 2002

The information content and properties of the cross section for atom scattering from a defect on a flat surface are investigated. Using the Sudden approximation, a simple expression is obtained that relates the cross section to the underlying atom/defect interaction potential. An approximate inversion formula is given, that determines the shape function of the defect from the scattering data. Another inversion formula approximately determines the potential due to a weak corrugation in the case of substitutional disorder. An optical theorem, derived in the framework of the Sudden approximation, plays a central role in deriving the equations that conveniently relate the interaction potential to the cross section. Also essential for the result is the equivalence of the operational definition for the cross section for scattering by a defect, given by Poelsema and Comsa, and the formal definition from quantum scattering theory. This equivalence is established here. The inversion result is applied to determine the shape function of an Ag atom on Pt(111) from scattering data.
Quantum mechanical study of the unimolecular dissociation of HO2: A rigorous test of RRKM theory
Authors: Dobbyn Abigail J. , Stumpf Michael , Keller Hans-Martin , Hase William L. , Schinke Reinhard
Published: July 2002

Three-dimensional quantum mechanical calculations are carried out, in a time-independent scattering approach, to study the unimolecular dissociation HO2→H+O2. The dissociation cross section is governed by narrow resonances with widths that vary over five orders of magnitude. The unimolecular dissociation rates strongly fluctuate about an average which agrees surprisingly well with the Rampsberger–Rice–Kassel–Marcus (RRKM) theory.
Correlations of NO rotation and translation produced in the photofragmentation of 2-chloro-2-nitrosopropane via two dissociation channels
Authors: Tomer Jeffrey L. , Wall Mark C. , Reid Brian P. , Cline Joseph I.
Published: July 2002

Correlated rotational and translational product state distributions of the NO X 2Π1/2,3/2 (v=0) product of the dissociation of 2-chloro-2-nitrosopropane in a molecular beam following Ã 1A″←X̃ 1A′[n(N),π*(N=0)] excitation at 600 and 650 nm are measured by resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry in a molecular beam. The NO speed distribution appears bimodal and the average speed increases with NO rotational quantum number. The slow component of the NO velocity distribution is attributed to dissociation on the S0 surface and the fast component to dissociation across a barrier along the reaction coordinate on the T1 surface. A two-channel dynamical model based on statistical phase space theory is used to model the correlated NO rotational and translational distributions. The experimental data are consistent with a C–N bond energy of 12 900±200 cm−1 on the S0 surface and a barrier height of 1500±200 cm−1 on the T1 surface. The high rotational excitation of NO products originating on the T1 surface can be attributed to impulsive recoil of NO from a bent C–N–O geometry atop the T1 barrier.
Ar and CH4 van der Waals complexes of 1- and 2-fluoronaphthalene: A perturbed spherical top attached to a surface
Authors: Champagne B. B. , Pfanstiel J. F. , Pratt D. W. , Ulsh R. C.
Published: July 2002

We compare and contrast the low and high resolution S1←S0 fluorescence excitation spectra of four van der Waals complexes, Ar–1FN, CH4–1FN, Ar–2FN, and CH4–2FN (where 1FN and 2FN are 1- and 2-fluoronaphthalene, respectively) in the gas phase. Whereas the Ar and CH4 complexes exhibit comparable low resolution spectra, their high resolution spectra are significantly different. The CH4–1/2FN complexes exhibit origin bands that are each split into three distinct subbands with different intensities and separations of less than 1 cm−1. No such splittings are observed in Ar–1/2FN. The relative intensities of the three subbands in both CH4 complexes are 1:2:2. These are identical, within experimental error, to the total statistical weights of the J=0, 1, and 2 rotational levels of CH4. Both Ar and CH4 are weakly attached to 1/2FN at a distance of ∼3.5 Å above the aromatic plane. This distance decreases slightly (∼0.1 Å) on S1←S0 excitation. Thus, the splittings observed in CH4–1/2FN are attributed to ‘‘surface-induced’’ perturbations of the normally isotropic rotational motion of methane whose magnitudes depend on the electronic structure of the surface to which it is attached. A model is proposed that accounts for these observations. Comparison of the numerical predictions of this model with the experimental results shows that the rotational motion of the attached CH4 is nearly the same as that of the free molecule.
Dynamic fluorescence Stokes shift of an acetylene-bridged donor–acceptor compound in alcohols at low temperatures
Authors: Khundkar L. R. , Bartlett J. T. , Biswas M.
Published: July 2002

The time-dependent fluorescence Stokes shift of p,p’-cyano thiomethyl diphenylacetylene in three different alcohols (1-propanol, 1-butanol, and 1-octanol) at room temperature was measured using time-correlated single photon counting and the spectral reconstruction method. These measurements were also performed with the probe in 1-propanol at low temperatures. The observed behavior is consistent with the dynamics of solvation of a polar excited state. As expected, the average solvation time increases with decreasing temperature and is correlated with solvent bulk viscosity down to 180 K. Apparent deviations observed at lower temperatures are attributed to the finite emission lifetime (≊1 ns) of the probe. A significant fast component (<100 ps) is observed in the dynamic Stokes shift even at temperatures close to the freezing point of 1-propanol. Different mechanisms that could result in such a fast component are discussed.
Liquid densities and structural properties of molecular models of water
Authors: Wallqvist A. , Åstrand P.-O.
Published: July 2002

We have calculated the density of model water potentials—employing an effective pairwise additive potential (RER) and an ab initio-derived many-body potential including polarizabilities (NEMO)—as a function of temperature at constant pressure. It is found that neither model can quantitatively reproduce the density of liquid water between 250 and 373 K. More disturbingly, neither potential exhibits a density maximum. Modifying the effective potential to give the correct density at different temperatures reveals subtle changes in the importance of different water configurations. These configurational changes are shown as possible causes of the anomalous density behavior of liquid water.
Mechanical route to the pressure of a fluid adsorbed in a random porous medium
Authors: Dong W.
Published: July 2002

By using different methods, we show how to derive the correct mechanical route to the pressure of a fluid adsorbed in a random porous medium. Some discussions are also made on the thermodynamic consistency of the mechanical route.
Melts of polymeric fractals and D-dimensional manifolds: Saturation vs screening
Authors: Haronska P. , Vilgis T. A.
Published: July 2002

This paper discusses the behavior of melts of polymeric fractals in their dense state. It is shown that melts of polymeric fractals of spectral dimension D behave differently when D exceeds a critical value, i.e., DS=2d/(2+d). Here d is the dimension of space. For larger connectivities the fractals are saturated, whereas fractals with smaller connectivity behave similarly to melts of linear polymer chains. In the latter case the polymers interpenetrate each other to a great extent, screen their excluded volume, and retain their ideal shape. This conclusion is drawn by studying screening processes in melts of polymeric manifolds of arbitrary connectivity. It is shown that for systems above the critical spectral dimension a screening length exists, i.e., in other words, a concentration, for which the screening condition can never be satisfied. It is shown that this fact corresponds to saturation. Below the critical connectivity dimension the classical excluded volume screening—comparable to the case in linear polymer chains—takes place. The condition on the spectral dimension is equivalent to a condition in terms of the Gaussian fractal dimension df=2D/(2−D), i.e., the melt saturates when the embedding space dimension is less or equal than the Gaussian fractal dimension, i.e., d≤df.
Molecular dynamics, density functional theory of the metal–electrolyte interface
Authors: Price David L. , Halley J. W.
Published: July 2002

Quantitative, predictive theories for metal–electrolyte interfaces require an atomic-scale representation of the interface, which must include an accurate statistical description of a polar fluid in contact with a solid surface; and also a description of the electronic density and structure of a metal surface in contact with a fluid. Such a complex system presents a difficult computational problem, and has been dealt with in the past essentially by parts; either by molecular dynamics calculations of the fluid structure, or density functional calculations of the metal–surface electronic structure. A complete and self-consistent determination of the surface structure would, however, involve a simultaneous calculation of both the atomic and electronic structure of the interface. This suggests a combination of these two calculational techniques, and it is just this sort of molecular dynamics and density functional combination which comprises the Car–Parrinello, and related, methods. We have developed a Car–Parrinello type combination of molecular dynamics and density functional methods, suitable for application to the metal–electrolyte interface. We briefly describe this calculation and discuss our initial results for a fairly simple metal–water interface.
Optothermal detection of nonradiative relaxation channels in electronically excited molecules
Authors: Miller C. Cameron , Hewett Kevin B. , Shen Meihua , Philips Laura A.
Published: July 2002

Optothermal detection has been used to observe nonradiative relaxation channels in aniline, p-bromoaniline, and trans-stilbene. p-Bromoaniline has no detectable fluorescence due to a heavy atom effect which increases the rate of intersystem crossing to the triplet state. An optothermal spectrum of p-bromoaniline was observed with the origin at 32 625 cm−1. For trans-stilbene, the differences between the laser excitation spectrum and the optothermal spectrum of the S1 state clearly show the onset of isomerization at ∼1250 cm−1 above the origin. Absolute quantum yields of fluorescence, Franck–Condon factors, nonradiative rates, and radiative rates have been obtained for a series of vibronic transitions. For low energy vibrational states, there is good agreement between the current study and previous work. For vibrational energies above the barrier of isomerization, predicted quantum yields do not agree with our experimental results.
Dissociation dynamics of a field-evaporated diatomic ion: Rovibrational coupling, isotope effects, and fractal field thresholds
Authors: Yuan Jian-Min , Wei Ching-Ming , Tsong Tien T.
Published: July 2002

We investigate the classical dissociation dynamics of field-evaporated HeRh2+, in which we consider explicitly the couplings between the vibrational, rotational, and translational degrees of freedom of the diatomic ion. We show that dissociation starts with rapid rotations driven by the large electric field gradient. Through rovibrational coupling rotational energy is converted into vibrational energy via cycles of sudden increase in interatomic distance and settling-down to rapid vibrational motion. During any of such rotational–surge–rapid–vibration cycles the molecule may dissociate and the dissociation threshold varies drastically with the isotopes of He as observed experimentally. When compared to experimental data our model provides information about the range parameter of the interaction potential of the diatomic ion. We show also that the boundary separating the dissociating and nondissociating trajectories exhibits fractal behavior as a function of the field strength. Furthermore, using ensembles of trajectories we have found that the dissociation fraction is a highly fluctuating function of the field strength near the dissociation threshold and that the time-of-flight spectra consist of four to five pronounced peaks on a noisy background.
On O− emission from cesium-coated surfaces
Authors: Wang Yicheng
Published: July 2002

A new model is proposed to explain the observed O− emission from cesium-coated surfaces on exposure to oxygen, whereby hyperthermal O resulting from thermal O2+Cs(s) collisions is the intermediate product leading to O− formation. The velocity of O is estimated on the basis of the potential-energy-surface crossing model of Gadzuk and Holloway [J. Chem. Phys. 82, 5203 (1985)]. The mechanism of O− formation is also discussed.
Effect of substrate potential strength on the melting temperature of a hexane monolayer adsorbed on graphite
Authors: Velasco E. , Peters Günther H.
Published: July 2002

In this note we present molecular dynamics (MD) simulation results of the melting transition of a hexane monolayer adsorbed on graphite. The present study was motivated by the recent simulation results of the same system by Hansen et al. [J. Chem. Phys. 98, 4128 (1993)]. Using a United Atom model for the representation of the inter- and intramolecular interactions, and the standard Steele expression for the monolayer-substrate potential, the latter authors obtained a melting temperature for the monolayer-substrate potential, the latter authors obtained a melting temperature for the monolayer which is at odds with experiment by 25%. In contrast, we show that when Steele’s method to model adsorbate–substrate interactions, based on Lorentz–Berthelot’s rule, is used, the transition temperature is in good agreement with experiment. The weaker surface potential also causes a slight increase in the width of the tilt distribution of the molecular axes and a decrease in the number of gauche defects.
Ab initio study of the three lowest-lying (X 1Σ+, 3Σ+, and 1Σ+) electronic states of AgF
Authors: Ramirez-Solis A. , Schamps J.
Published: July 2002

Ab-initio pseudopotential two-configuration self-consistent field followed by extensive variational and perturbational second order Mo/ller–Plesset multireference configuration interaction calculations using localized molecular orbitals were performed to characterize the structure and adiabatic potential energy curves of the three lowest (X 1Σ+, 3Σ+, and 1Σ+) purely electronic states of the AgF molecule. Spin-orbit interactions were introduced semiempirically in a second step. The very strong coupling of the neutral Ag(4d105s1)F(2s22p5) and ionic Ag+(4d95s1)F−(2s22p6) configurations at rather short internuclear distance for both excited 3,1Σ+ states is responsible for the appearance of very shallow minima, thus leading to a limited number of stable vibrational levels for these excited states as suggested previously for the AO+ state. In contrast with the CuF molecule, where only the ionic configuration Cu+(3,1D)F−(1S) is present in the 3,1Σ+ states, this coupling of ionic and neutral structures in AgF is explained by the relative positions of the valence orbital energies of the neutral Cu and Ag atoms with respect to the 2p level of the halogen atom. These results lead to the assignment of the observed AO+–X 1Σ+ transition as a 1Σ+–1Σ+ type transition.

The very recently observed aΩ1 and A′Ω1 states are shown to be, respectively, the Ω=O− and Ω=1 spin–orbit components of the 3Σ+ state, which justifies the relabeling of aΩ1 into a aΩO−. The calculated spin–orbit-induced splitting between these two components is in excellent agreement with the observed one after reconsidering spectroscopic data. For all these states the calculated spectroscopic constants are in good agreement with available experimental data. The fourth experimental state, BO+, is probably not correlated with the 3Π valence state as previously suggested but it could rather correspond to a Rydberg ionic state involving the Ag+(4d95p)F−(2s22p6) structure.
Optical measurements of the phase diagram of Langmuir monolayers of fatty acid–alcohol mixtures
Authors: Fischer Birgit , Teer Ellis , Knobler Charles M.
Published: July 2002

The surface pressure-temperature diagram of Langmuir monolayers of mixtures of heneicosanoic acid with heneicosanol has been determined from 10 °C to 40 °C by direct observation with Brewster angle microscopy. The measurements focused on the way in which the boundary between the L′2 and L2 phases change with composition. As previously observed by Shih et al. [J. Chem. Phys. 101, 9132 (1994)], it moves to lower pressure and higher temperature with increasing concentration of alcohol. We have discovered that the boundary between the Ov and L2 phases, which had not been studied before, moves in the opposite direction and that the L′2 and Ov phase regions merge, an unanticipated result.
Ab initio potential energy surfaces for the two lowest 1A′ states of H+3
Authors: Ichihara Akira , Yokoyama Keiichi
Published: July 2002

Three-dimensional potential energy surfaces of H+3 in the two lowest 1A′ electronic states have been calculated by the full configuration interaction method with a [8s6p2d1f] Gaussian-type basis set. The features of the avoided crossing of two surfaces as well as the energy minimum of the 1A′ ground state have been produced by the potential calculation at 680 different spatial geometries. These surfaces should be useful for the detailed studies of charge transfer and chemical reaction in the H+ and H2 collisions.
The predissociation dynamics of ammonia: A theoretical study
Authors: Seideman Tamar
Published: July 2002

A quantum mechanical model of the ammonia predissociation dynamics is presented. The effects of an excited state barrier and well, a deep conical intersection between the ground and excited states, and strong angular dependence of the potential energy surfaces are explored. The predissociation rate is sensitive to both the vibrational (v2′) and the rotational (J′,K′) level of the initially excited metastable state. The product state distribution following excitation within the v2′=0 band reflects the shape of the ground bend resonance and is broad, extending to the energetic limit. The photofragment angular distributions following excitation of magnetic–rovibronic state-selected parent molecules depend sensitively on the photon frequency. Their structure varies markedly with the internal state of the accompanying NH2 fragment. The results are traced to the complex excitation dynamics, which prepare a frequency-dependent superposition of rotational states, and to the strong forces exerted on the dissociating system in the region of the conical intersection.
Rotational analysis of the threshold photoelectron spectra of room temperature and jet-cooled CO2
Authors: Wiedmann Ralph T. , White Michael G. , Lefebvre-Brion Hélène , Cossart-Magos Claudina
Published: July 2002

The rotational structure associated with the X̃ 2Πg (000) ground ionic state of CO+2 has been investigated by threshold photoelectron spectroscopy and semiempirical calculations based on the distribution of Rydberg state levels at n=2000. The calculations are in very good agreement with the rotationally resolved spectra for the X̃ 2Πg,3/2 and X̃ 2Πg,1/2 spin–orbit levels for both room temperature and supersonically cooled ground state CO2. Intensity anomalies are partly explained by decay mechanisms involving rotational and spin–orbit autoionization.
Infrared laser spectroscopy of uracil in a pulsed slit jet
Authors: Viant Mark R. , Fellers Raymond S. , McLaughlin Ryan P. , Saykally Richard J.
Published: July 2002

A new high-temperature pulsed slit jet source has enabled the first observation of a rotationally resolved vibrational spectrum of a nucleotide base. The spectrum, centered at 1703.888 cm−1, has been assigned to the fundamental out-of-phase mixed carbonyl stretching vibration of the diketo tautomer of uracil, clarifying an ambiguous assignment from low-resolution studies.
Applications of the perturbative hypernetted-chain equation to the one-component plasma and the one-component charged hard-sphere systems
Authors: Kang Hong Seok , Ree Francis H.
Published: July 2002

The perturbative hypernetted-chain (PHNC) equation developed recently has been applied to the one-component plasma (OCP) and the one-component charged hard-sphere (OCCHS) systems in a uniform compensating background. Computed thermodynamic properties and pair correlation functions show that the PHNC gives excellent agreement with computer simulations and that it is as accurate as (or, in some cases, superior to) the reference-hypernetted chain and the hypernetted-chain-mean spherical equations, representing the two best currently available theories. The PHNC also predicts the OCP screening function at short range in close agreement with computer simulations and is superior to other theoretical results. Reliability of the radial distribution function at the hard-sphere contact distance for the OCCHS is also discussed.
Vibrational energy transfer in linear hydrocarbon chains: New quantum results
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

In this paper we report quantum calculations of the survival probability in linear hydrocarbon chains. We have performed both adiabatic gauge transform calculations and calculations that include corrections beyond the adiabatic approximation. We have managed to perform intermediate steps of the calculations analytically. We require the initial basis set expansion and final summations to be performed numerically. The corrections beyond the adiabatic approximation are shown to be small for this system for multiple time step calculations and large for single time step calculations. We have proved an identity that allows the extension of the calculations for  HC2 to longer chains at little computational cost. In particular, we have proved that the quantum solution for any linear hydrocarbon chain can be obtained from the solution of a problem with 3 degrees of freedom. We have performed multi-step adiabatic calculations for  HC2 and  HC6 that converge at up to 35–40 fs. We have devised a simple diagrammatic scheme that summarizes our method in a very compact form. Finally, we propose an alternative strategy of calculation that might lead to very fast solutions of the quantum dynamics of this system.
Ab initio potential energy surfaces and quantum scattering studies of NO(X 2Π) with He: Λ-doublet resolved rotational and electronic fine-structure transitions
Authors: Yang Moonbong , Alexander Millard H.
Published: July 2002

New ab initio potential energy surfaces (PES’s) are presented for the interaction of He with the NO radical in its ground (X 2Π) electronic state, determined within the coupled electron pair approximation (CEPA) with a large atomic orbital basis set. The dynamics of the collisions of NO with He are then investigated, in particular the coupling between nuclear motion (rotation and translation) and the internal electronic motion of the open-shell partner. State-to-state integral and differential cross sections are calculated using full close coupling and coupled states methods. These cross sections are compared with the results of the two separate measurements at different initial collision energies, 508 and 1186 cm−1 (63 and 147 meV). Excellent agreement is obtained in both cases. Also comparisons with previous calculations, based on an earlier local density potential energy surface, are made at 508 and 2420 cm−1 (63 and 300 meV).
A discrete theory for ion–molecule cluster formation in the gas phase
Authors: Gong S. L. , Jervis R. E.
Published: July 2002

A theory was developed to treat the formation of ion–molecule clusters in the gas phase as a discrete process and it includes ion–dipole and dipole–dipole interactions as a basis for depicting the electrostatic force between the central ion and the clustering molecules. All the energy of the interactions is summed over the total number of clustering molecules around a central ion. The total free energy of cluster formation is comprised of the electrostatic energy of the cluster formation, the free energy of vaporization of the condensing molecules, and the work done in changing the surface area through condensation of additional molecules about each ion cluster. A minimum in the total free energy with respect to the number of neutral molecules corresponds to the most stable ion–molecule cluster. Comparison between the theoretical predictions and experimental observations for both stable and radioactive ion clusters showed good agreement.
Theoretical study of Xe monolayer adsorbed on NaCl(100): Structures and phase diagrams
Authors: Ramseyer C. , Girardet C.
Published: July 2002

Calculations based on semiempirical potentials are performed to interpret the structure of the Xe monolayer adsorbed on a clean (100) surface of NaCl. The rare gas atoms are located along Na troughs in which the corrugation remains less than 10 meV, while it can reach 25 meV outside the troughs. The competition between this corrugation and the lateral Xe–Xe interactions favors the occurrence of several (n×2) high order commensurate structures (exhibiting distorted hexagonal geometries) in a very narrow range of potential energy values. The behavior of the free energy with temperature and coverage shows four high order commensurate phase transitions below 80 K with a concomitant discrete variation of the mean lattice parameter aM of the structures from 4.30 to 4.46 Å. Special attention is brought to the study of the lattice parameter behavior between 40 and 80 K which was investigated independently by means of thermal desorption, photoelectron spectroscopy and high resolution electron diffraction experiments. Two sets of high order commensurate phases are determined within this temperature range; one set corresponds to structures with aM=4.365 Å for 31≤T≤55 K while the other gives structures with aM=4.41 Å for 55≤T≤78 K. These results are quite consistent with electron diffraction measurements which show an evolution from a perfect hexagonal phase (aM=4.37 Å) to a distorted hexagonal structure (aM=4.42 Å) around T=55 K.