bluuebunny/crossref_metadata_2025 huggingface dataset:
Segment dynamics in entangled polymer melts
Authors: Wilson Jeffrey D. , Loring Roger F.
Published: July 2002

We calculate the dependence on time and on molecular weight of the mean-squared displacement of a polymer segment in a dense fluid of linear chain molecules. Time scales are considered that range from times sufficiently short that a segment behaves as a free Brownian particle to times over which terminal diffusion occurs. We employ a stochastic model that has formed the basis of our earlier studies of the self-diffusion coefficient in monodisperse and polydisperse melts. A macromolecule is represented by a freely jointed chain that moves through space by two mechanisms—a local conformational change and a cooperative slithering motion. The local motions are blocked by dynamical obstacles, whose relaxation rate is determined self-consistently from the dynamics of the chain. Calculations of polymer properties are exactly mapped onto the solution of random walk problems with dynamical disorder, which are treated within the dynamical effective medium approximation. Our results are shown to share common features with recent molecular dynamics and dynamical Monte Carlo simulations of polymer melts. A procedure is suggested for assigning values to our model parameters in order to mimic specific experimental systems or other theoretical models.
Long time scale molecular dynamics subspace integration method applied to anharmonic crystals and glasses
Authors: Space Brian , Rabitz Herschel , Askar Attila
Published: July 2002

A subspace dynamics method is presented to model long time dynamical events. The method involves determining a set of vectors that span the subspace of the long time dynamics. Specifically, the vectors correspond to real and imaginary low frequency normal modes of the condensed phase system. Most importantly, the normal mode derived vectors are only used to define the subspace of low frequency motions, and the actual time dependent dynamics is fully anharmonic. The resultant projected set of Newton’s equations is numerically solved for the subspace motions. Displacements along the coordinates outside the subspace are then constrained during the integration of the equations of motion in the reduced dimensional space. The method is different from traditional constraint methods in that it can systematically deduce and remove both local and collective high frequency motions of the condensed phase system with no a priori assumptions. The technique is well suited to removing large numbers of degrees of freedom, while only keeping the very low frequency global motions. The method is applied to highly anharmonic Lennard-Jones crystal and glass systems. Even in these systems with no intramolecular degrees of freedom or obvious separation of time scales, the subspace dynamics provides a speed up of approximately a factor of 5 over traditional molecular dynamics through use of a larger integration time step. In the cases illustrated here a single set of subspace vectors was adequate over the full time interval, although this is not expected to be true for all systems.
Time-resolved anisotropic coherent anti-Stokes Raman scattering: A new probe of reorientational dynamics
Authors: Wan Chaozhi , Johnson Carey K.
Published: July 2002

A formalism for the time-dependent anisotropic third-order susceptibility induced by a linearly polarized excitation pulse has been derived to describe the time dependence of coherent anti-Stokes Raman scattered (CARS) and Raman time-resolved experiments. The third-order susceptibility induced in a randomly oriented molecular system contains nine independent molecular parameters—three isotropic and six anisotropic—as compared to three independent parameters in an isotropic system. Methods of time-resolved anisotropic anti-Stokes Raman scattering (TRA CARS) are discussed as an approach to reorientational measurements. The dependence of the TRA CARS or TRA Raman signal on vibration and polarization component can in principle provide more rotational information than fluorescence depolarization or linear dichroism measurements and may allow more complete characterization of rotational dynamics. Results are reported for three types of TRA CARS experiments to demonstrate the capabilities of TRA CARS to study the rotational motion of molecules in liquids.
The nature of the chemical bond in simple oxides: A theoretical journey from the ionic model to the ab initio configuration interaction approach
Authors: Illas F. , Lorda A. , Rubio J. , Torrance J. B. , Bagus P. S.
Published: July 2002

Ab initio cluster model wave functions of increasing complexity have been obtained for alkaline-earth oxides MgO to BaO. Using a wave function corresponding to the superposition of the electronic densities of the cations and anions obtained in a Madelung field, an ab initio version of the ionic model is obtained. This simple ionic model is improved with self-consistent field (SCF) and large multireference configuration interaction (CI) wave functions. Analysis of these different types of wave functions shows that the ground state of these oxides is strongly ionic with the ideally ionic configuration having a weight of ≊95% in the total CI wave function. With all the criteria that we have used, the degree of charge transfer from O2− to M2+ is always very small. Furthermore, the instantaneous electron–electron interactions (correlation effects) treated in the CI wave function have been found to be mainly intra-atomic and especially important for the 2p electrons of O2−. Point charges were used to represent the contribution to the Madelung field made by the atoms not explicitly included in the cluster; they were chosen to reproduce the Madelung field arising when a fully ionic crystal is assumed. Sets of scaled point charges which correspond to a smaller Madelung field were also used. The cluster model results were not significantly changed when the point charges were reduced by as much as a factor of 2 from the values for the fully ionic crystal. This is strong evidence that the ionicity of the crystals results from chemical forces and is not due to the use of an assumed Madelung field external to the cluster.
Stability of MX2−3 ions in the gas phase and when do ionic molecules have large ionization potentials
Authors: Scheller M. K. , Cederbaum L. S.
Published: July 2002

The present work aims at theoretical consideration of the geometrical and electronic structures of a homologous series of free MXmq−-type alkali halides (M=Li, Na, K; X=F, Cl; m=1,2,3; q=0,1,2) in order to get insight into their kinetic and electronic stability. At the ab initio Hartree–Fock self-consistent field (HF-SCF) level of theory, the lowest energy fragmentation channel leading to the decomposition of the dianions MX32− into MX2− and X− has been investigated. The potential energy surface was found to exhibit a broad, but flat energy barrier to fragmentation. These findings have been confirmed using results from configuration interaction calculations and the molecular dianions are predicted to be long-lived species formally existing in a metastable state. The stability of the gas-phase MX32− dianions and of the MX2− fragmentation products with respect to autodetachment of an extra electron has been investigated using ab initio HF-SCF and Green’s function methods. The inclusion of many-body effects by the latter was found to decrease the vertical binding energy of the extra electrons with respect to the Koopmans’ theorem HF-SCF result, but the extra electrons remain bound. At the ab initio level, the compounds studied in this work are predicted to be extremely ionic species. The variation of the properties through a series of MXmq− species has been investigated. Motivated by the overall large ionic character—in particular of the MX32− molecules—the properties of the systems have been studied using a previously derived theoretical approach, referred to as ionic model, based on (classical) electrostatic terms. The versatility of the ionic model scheme has been extended here to obtain information on the tendency of an ionic molecule to accomodate an additional electron.
Electron propagator theory of BO2 and BO−2 electronic structure
Authors: Ortiz J. V.
Published: July 2002

Electron propagator calculations on the vertical and adiabatic ionization energies of BO2− yield structural information on ground and excited states of BO2. Analytical gradient techniques for BO2 geometry optimizations and renormalized self-energies for the evaluation of vertical ionization energies are employed. The calculated adiabatic electron affinity of BO2 is 4.65 eV. Geometry optimizations and force constant calculations arrive at D∞h structures for all states. Harmonic vibrational frequencies are in reasonable agreement with experimental spectra. Adiabatic excitation energies of BO2 are inferred from the anion’s adiabatic ionization energies. Potential and electric field expectation values of the Feynman–Dyson amplitudes provide a one-electron picture of how removal of an electron leads to bond length changes.
Can corundum be described as an ionic oxide?
Authors: Sousa Carmen , Illas Francesc , Pacchioni Gianfranco
Published: July 2002

The electronic structure of bulk corundum(α-Al2O3) has been studied by using ab initio cluster model Hartree–Fock self-consistent-field (SCF) and configuration interaction (CI) wave functions. An array of optimized point charges surrounding the cluster model is used to represent the Madelung field. Different analyses of the energetic chemical bond contributions and of the wave functions have been performed. These include constrained space orbital variations (CSOV), use of projection operator techniques, the decomposition on the ground state SCF wave function into ionic and charge transfer components, and finally, the effect of the electronic correlation. These analyses enabled us to determine the relationship between each chemical contribution to the energy stabilization of corundum and its effect on the corresponding wave function. All these different analyses are consistent and indicate that the oxygen–aluminium bond is highly ionic in nature, with a net atomic charge for Al not less than +2.7 e.
Intensity of CH- and NH-stretching transitions in the overtone spectra of cyclopropylamine
Authors: Niefer Bernadette I. , Kjaergaard Henrik G. , Henry Bryan R.
Published: July 2002

Vapor phase room temperature overtone spectra of cyclopropylamine are measured both with conventional absorption spectroscopy for the lower overtones and with intracavity laser photoacoustic spectroscopy for the higher overtones. Spectra are recorded in both the CH-stretching (ΔvCH=2–7) and NH-stretching (ΔvNH=2–6) regions. The relative intensities of XH-stretching peaks are explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the XH-stretching modes are neglected. The dipole moment function is expanded in the XH-stretching coordinates, with the expansion coefficients determined from ab initio molecular orbital calculations. The high number of peaks throughout the spectra makes assignment difficult, but allows the overlap of spectra from different experimental conditions. Thus it is possible to approximately determine the absolute intensities for the higher overtone spectra that are obtained from the laser experiments. The simple intensity calculation, which contains no adjustable parameters, can account very well for the magnitude of the intensities and for the relative intensities of the pure local mode peaks for all XH oscillators in the overtone spectral region from ΔvCH=3 to 7.
Quantum dynamics of Renner–Teller vibronic coupling: The predissociation of HCO
Authors: Goldfield Evelyn M. , Gray Stephen K. , Harding Lawrence B.
Published: July 2002

A Hamiltonian model and parity-adapted wave packet representation are developed to describe a rotating triatomic system with two Renner–Teller coupled potential surfaces, and HCO predissociation is studied. New configuration interaction calculations on HCO are performed to determine its excited Ã(2A″) potential surface, and Bowman, Bittman, and Harding’s X̃(2A′) ground potential surface is employed. The properties of many resonances, correlating with stretch/bend excitations on the A″ surface, are determined. Resonance energies and decay constants are in good agreement with experimental results of Houston and co-workers, but CO rotational product distributions tend to be hotter and narrower than experiment, particularly for pure bend excitations. Wave packet dynamics involves growth of amplitude on the A′ surface near collinear geometries via Renner–Teller coupling, and subsequent adiabatic evolution to determine product distributions. The wave packets probe a previously untested part of the A′ surface, and point to the need for improvements of this surface.
Density-scaling Monte Carlo study of subcritical Lennard-Jonesium
Authors: Valleau John P.
Published: July 2002

In density-scaling Monte Carlo (DSMC) one covers a substantial range of density in a single simulation, obtaining the free energy as well as the usual ensemble averages; a further advantage is the avoidance of quasiergodicity. The method is applied to the subcritical Lennard-Jones fluid. At kT/ε=1.2, the density range 0.02≤ρσ3≤0.80 was studied in four overlapping DSMC runs; data was extracted at intervals of 0.005, i.e., at a total of 157 densities. Free energies, energies, and pressures are found with striking precision, and reasonable estimates are obtained for heat capacity, compressibility, and kinetic pressure. The liquid–gas phase transition is thus characterized with a precision not otherwise possible.
Cumulative reaction probabilities using Padé analytical continuation procedures
Authors: Ryaboy Victor , Lefebvre Roland , Moiseyev Nimrod
Published: July 2002

New computational techniques for calculation of cumulative reaction probabilities, N(E), are suggested. They are based on the expression of N(E) through the imaginary part of the Green function G [Seideman and Miller, J. Chem. Phys. 96, 4412 (1992)]. We use three methods to overcome numerical problems arising from branch cuts of G located along the real positive energy axes, addition of constant imaginary part iε to the Hamiltonian, addition of unoptimized optical potentials of the form iλ‖s‖ or iλ‖s‖2, and complex rotation of the reaction coordinate s→s⋅exp(iϑ). When N(E,u) is calculated on a grid of values of the numerical parameter u (u being ε, λ, or ϑ), Padé analytical continuation to their zero values gives correct energy dependence of N(E). The method makes it possible to save computer time by using unoptimized parameters of the optical potential or of the complex scaling when calculating N(E,u). Test calculations on a one dimensional Eckart barrier and a model H+H2(ν=1) potential which supports a quasibound state have shown high accuracy and convergence of the method with respect to Padé input parameters.
A femtosecond-time-scale photolysis study of vapor-phase GaCl
Authors: Glownia J. H. , Gnass D. R. , Walkup R. E. , Ratzlaff E. H. , Sorokin P. P.
Published: July 2002

Time-resolved absorption spectra are recorded for Ga atoms produced by the photolytic action of 300 fs, 248.5 nm laser pulses on gas-phase GaCl molecules. Photodissociation of GaCl at 248.5 nm produces both ground-state (2P1/2) and spin–orbit-excited (2P3/2) Ga atoms, resulting in strong transient-absorption features at 403.3 and 417.2 nm, respectively. The experimental spectra are analyzed to obtain the kinetic energy of the photofragments, which is estimated to be ≂0.08 eV for both channels. The analysis suggests that the dissociation limits are (1) Ga 2P1/2+Cl* 2P1/2 for the 403.3 nm channel; and (2) Ga* 2P3/2+Cl 2P3/2 for the 417.2 nm channel. From these results, a new estimate for the dissociation energy of GaCl is obtained: D00=4.80±0.03 eV.
Theoretical study of the ground states of the rare-gas hydrides, HeH, NeH, and ArH
Authors: Partridge Harry , Schwenke David W. , Bauschlicher Charles W.
Published: July 2002

Potential energy curves for HeH, NeH, and ArH are determined using large basis sets and high levels of theory. The computed potential energy curves are in excellent agreement with the experimentally determined model potentials for HeH and NeH, but differ with the model potentials deduced for ArH by Tang and Toennies. We derive a potential based on the ab initio calculations and our best estimate of the well depth; using this potential we are able to reproduce the results of the orbiting resonance experiments of Toennies and co-workers.
Wavelength dependence of the photodissociation and photodesorption of CD3I adsorbed on the TiO2(110) surface
Authors: Garrett Simon J. , Holbert Victor P. , Stair Peter C. , Weitz Eric
Published: July 2002

The ultraviolet photodissociation and photodesorption of CD3I adsorbed on the TiO2(110) surface at ∼100 K has been investigated at 257, 275, 302, and 351 nm using modulated continuous-wave laser irradiation followed by resonantly enhanced multiphoton ionization of fragments expelled from the adsorbate layer. Photodissociation at these wavelengths produces CD3 radicals. Nonthermal photodesorption also contributes to removal of CD3I from the adsorbate layer, becoming a major mechanism at 351 nm. Similar processes are observed at both 1 and 25 monolayer (ML) coverages. The cross section for CD3I depletion from the monolayer is qualitatively similar to the gas phase CD3I absorption profile, decreasing by ∼3 orders of magnitude between 257 and 351 nm. Depletion cross sections, S(λ), for CD3I are 3±2×10−18 cm−2, 8±3×10−19 cm−2, 1±0.5×10−19 cm−2, and 3±1×10−21 cm−2 for 257, 275, 302, and 351 nm irradiation, respectively. The depletion cross section for 25 ML CD3I coverage is approximately an order of magnitude less than for 1 ML coverage with S(λ) calculated to be 3±2×10−19 cm−2, 1.5±0.7×10−19 cm−2, 1.5±0.7×10−20 cm−2, and 2±0.8×10−22 cm−2 for 257, 275, 302, and 351 nm radiation, respectively. We find no correlation between substrate absorption and the wavelength dependence of photodissociation or photodesorption suggesting that direct excitation of the adsorbate molecule is the dominant dissociation mechanism. The lack of substrate involvement may be due to poor coupling of the CD3I adsorbate and TiO2 substrate electronic structures.
A perturbed-mean-field approach to the decay rates of excited vibrational states in extended systems: An application to I2(Ne)n
Authors: Roitberg Adrian E. , Gerber R. Benny , Ratner Mark A.
Published: July 2002

A method for calculating decay rates of vibrational modes in large polyatomic systems is proposed and tested. The high frequency excited vibration is treated quantum mechanically, and the soft modes are described classically. The initial state is described by the hybrid quantum/classical self-consistent-field (SCF) approximation. The formalism is based on a golden-rule expression. The driving potential is the difference between the full Hamiltonian and the mean field Hamiltonian (SCF) causing the decay of the initial state to final mixed quantum/classical SCF states. These states are calculated using an extension of the usual static mean-field techniques to systems with mixed quantum and classical degrees of freedom. The formalism for obtaining the mean-field states and calculating the decay rates is presented, and the method is applied to a diatomic molecule treated quantum mechanically, embedded in a 1D model for a rare gas cluster treated classically. The dependence of the eigenenergies of the quantum and the decay rates with temperature is studied. The influence on the system size is also presented and compared with the prediction of the isolated binary collision model. The effect of a change in the linear density of the cluster on the eigenenergies of the vibrational mode is presented.
Sub-Doppler, infrared laser spectroscopy of the propyne 2ν1 band: Evidence of z-axis Coriolis dominated intramolecular state mixing in the acetylenic CH stretch overtone
Authors: McIlroy Andrew , Nesbitt David J. , Kerstel Erik R. Th. , Pate Brooks H. , Lehmann Kevin K. , Scoles Giacinto
Published: July 2002

The eigenstate-resolved 2ν1 (acetylenic CH stretch) absorption spectrum of propane has been observed for J′=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full-width at half-maximum resolution of 6×10−4 cm−1 (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm−1 wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν1 to the near resonant bath states with an average matrix element of 〈V2〉1/2=0.002 cm−1 that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum.

The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z-axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z-axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z-axis Coriolis coupled to the dark states. Z-axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν1 normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν1 do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.
The proton-transfer surface of CH3OHF−
Authors: Wladkowski Brian D. , East Allan L. L. , Mihalick Jennifer E. , Allen Wesley D. , Brauman John I.
Published: July 2002

Diverse aspects of the potential surface for the proton-transfer reaction CH3OH+F−→CH3O−+HF have been investigated by means of high-level ab initio electronic structure methods based on single-reference wave functions, namely, Mo/ller–Plesset perturbation theory from second through fourth order (MP2–MP4), the configuration interaction and coupled-cluster singles and doubles methods (CISD and CCSD), and CCSD theory augmented by a perturbative correction for connected triple excitations [CCSD(T)]. The one-particle Gaussian basis sets for (C,O,F;H) ranged in quality from [4s2p1d;2s1p] to [14s9p6d4f;9s6p4d], including as many as 482 atomic orbitals for the CH3OHF− system. The ion–molecule complex on the proton-transfer surface is a tight, hydrogen-bonded structure of CH3OH⋅F− character, exhibiting a nearly linear -OHF− framework, an elongated O–H distance of 1.07(1) Å, and a small interfragment separation, r(H–F)=1.32(1) Å. Improved structural data for F−⋅H2O are obtained for calibration purposes. A large fluoride affinity is found for the CH3OHF− adduct, D0=30.4±1 kcal mol−1, and a bonding analysis via the Morokuma decomposition scheme reveals considerable covalent character. The harmonic stretching frequencies within the -OHF− moiety are predicted to be 421 and 2006 cm−1, the latter protonic vibration being downshifted 1857 cm−1 relative to ω1(O–H) of free methanol.

A systematic thermochemical analysis of the reactants and products on the CH3OHF− surface yields a proton-transfer energy of 10.6 kcal mol−1, a gas-phase acidity for methanol of 381.7±1 kcal mol−1, and D0(CH3O–H)=104.1±1 kcal mol−1, facilitating the resolution of previous inconsistencies in associated thermochemical cycles. A minimum-energy path in geometric configuration space is mapped out and parametrized on the basis of constrained structural optimizations for fixed values of an aptly chosen reaction variable. The evaluation of numerous energy points along this path establishes the nonexistence of either a proton-transfer barrier, an inflection region, or a secondary minimum of CH3O−⋅HF type. The mathematical considerations for a classical multipole analysis of reaction path asymptotes are outlined for ion–dipole systems and applied to the CH3OHF− surface with due concern for bifurcations in the exit channel for the proton-transfer process. A global analytic surface for vibrational stretching motion in the -OHF− moiety of the CH3OHF− system is constructed, and a suitable dynamical model is tested which involves an effective, triatomic hydrogen pseudobihalide anion, [-OHF]−. Converged variational eigenstates of [-OHF]− to one-half its dissociation limit are determined using vibrational configuration interaction expansions in terms of self-consistent-field modals. The fundamental stretching frequencies of the CH3OHF− complex predicted by the [-OHF]− model are 504 (+84) and 1456 (−549) cm−1, the corresponding anharmonicities appearing in parentheses.
Computer simulation of the folding of coiled coils
Authors: Rey Antonio , Skolnick Jeffrey
Published: July 2002

A simple model capable of providing possible folding pathways of two stranded, coiled coil peptides is described and simulated using an off-lattice dynamic Monte Carlo algorithm. Short sequences of very regular repetitive blocks of amino acids are studied. The regularity of the sequence is enhanced by a simplified interaction scale between pairs of residues. Following the transition from two isolated chains in a random conformation to the folded dimeric structure, the main features capable of obtaining a parallel, in-register, unique conformation, are examined. These include the geometrical representation of the model, the cooperative development of secondary and tertiary structures, and the role of tertiary interactions stabilizing the coiled coil geometry. The influence of introducing disulfide bridges in certain locations of the sequence is also discussed.
The fluorescence excitation spectrum of 1-naphthoic acid at rotational resolution: S and S1 potential energy surfaces along the R–COOH torsional coordinate
Authors: Jagannathan Surya , Pratt David W.
Published: July 2002

Several vibronic bands that appear within 300 cm−1 of the electronic origin in the S1←S0 fluorescence excitation spectrum of 1-naphthoic acid (1NA) and carboxyl-deuterated 1NA have been examined at full rotational resolution. The data show that all bands belong to the s-cis isomer of 1NA. They also show that all bands are torsional in nature; i.e., that they involve displacements along either the S0 or the S1 carboxyl torsional coordinate, φ, or both. Unambiguous assignments of the bands follow from the observed inertial defects, from which the torsional potential energy surfaces of both electronic states are derived. In S0 s-cis-1NA, the naphthalene and carboxyl groups are not coplanar. The s-cis minima are at φ=±17°; the barrier to planarity is 13.2 cm−1. In contrast, S1 s-cis-1NA is a completely planar molecule, with φ=0°. The barrier for s-cis to s-trans isomerization is ∼1450 cm−1 in the S0 state and ∼2200 cm−1 in the S1 state. The validity of the two derived potential energy surfaces of s-cis-1NA has been tested by comparing the observed inertial defects with those computed using the torsional eigenfunctions. Excellent agreement is obtained, confirming the one-dimensional approach employed.
Recovery of the Landau matrix elements from the classical Fourier components: The one-dimensional dissociating oscillator
Authors: Karni Y. , Nikitin E. E.
Published: July 2002

The recently suggested method of recovering the Landau exponent of the quasiclassical matrix elements from the attributes of classical motion is illustrated by way of an example of dissociating anharmonic oscillators. For a Morse oscillator, in which case the exact analytical results are available, the so-called improved semiclassical approximation that incorporates the Landau exponential yields quite accurate matrix elements for classically strongly forbidden events. This provides a firm support for the method of estimation of quasiclassical matrix elements between distant states from the information on the motion of system in the classically allowed region.
Quantum mechanical study of H(g)+Cl–Au(111): Eley–Rideal mechanism
Authors: Jackson Bret , Persson Mats , Kay Bruce D.
Published: July 2002

Beams of atomic hydrogen are known to react with halogens adsorbed on metal surfaces, and an Eley–Rideal mechanism has been suggested. The collinear reaction of gas phase H with Cl atoms adsorbed on an Au(111) surface is explored using time dependent quantum techniques. Probabilities for reaction and reflection are computed as a function of incident H energy and Cl–metal vibrational state. The time evolution is examined and a long lived intermediate is found to exist in which the Cl is bound to both the H and the metal. The probability for Eley–Rideal reaction is found to be small, on the order of a few to several percent, due primarily to the mass mismatch. The vibrational distribution of the product HCl and the temperature dependence of the reaction probability are in agreement with experiment.
Velocity distributions of photochemically desorbed molecules
Authors: Zimmermann Frank M. , Ho W.
Published: July 2002

The standard picture of desorption induced by electronic transitions (DIET) is analyzed for high excited state quenching rates. Simple dynamical considerations are found to explain the velocity distributions characterizing a large number of photodesorption and electron stimulated desorption systems. Without invoking any thermalization processes, the model predicts a Maxwell–Boltzmann velocity distribution, thus providing a theoretical justification for this distribution’s widespread use as an empirical fitting formula for velocity distributions of nonthermally desorbed species.
Vibrational autoionization in H2 above the v+=3 ionization limit
Authors: Stephens J. A. , Greene Chris H.
Published: July 2002

We report multichannel quantum defect theory calculations of vibrational branching ratios and photoelectron angular distributions for photoionization of an ortho/para equilibrium mixture of H2 at 300 K, between wavelengths 762.5 and 765.0 Å. We compare our results with the recent synchrotron measurement of these observables by Dehmer et al. [J. Chem. Phys. 97, 7911 (1992)], and the total photoionization cross sections measured by Dehmer and Chupka [J. Chem. Phys. 65, 2243 (1976)]. Except in the region of autoionizing resonances which compete with a dissociation channel, semiquantitative agreement between calculations and experiment is obtained for branching ratios and angular distributions for the v+=0–2 levels, and for the v+=3 branching ratio. However, the level of agreement between the present calculations and the synchrotron measurements is significantly worse than that obtained for the total photoionization cross section.
Contaminated torsional tunneling splittings in five normal-mode vibrations of propene
Authors: Ainetschian A. , Fraser G. T. , Ortigoso J. , Pate B. H.
Published: July 2002

Sub-Doppler infrared spectra of five normal-mode vibrations of propene between 900–1100 cm−1 reveal strongly perturbed torsional tunneling splittings due to nonresonant anharmonic couplings to vibrational states in which the torsional mode is excited. The present results suggest that perturbed tunneling splittings will be a general feature for vibrationally excited states. Consequently, care should be taken when using tunneling splittings to obtain the vibrational dependence of a tunneling barrier. Instead, tunneling splittings in excited states can be used as a probe of long-range vibrational–torsional state mixing in molecules.
Structure of complex systems using electronic excitation transport: Theory, Monte Carlo simulations, and experiments on micelle solutions
Authors: Finger Karin U. , Marcus Andrew H. , Fayer M. D.
Published: July 2002

Monte Carlo (MC) simulations and an analytical theory are presented to describe electronic excitation transport (EET) among static chromophores constrained to lie on the surfaces of spherical micelles. Both donor–trap (DT) and donor–donor (DD) EET are examined for two types of systems: probe molecules on the surfaces of isolated (low concentration) micelles, and probes on the surfaces of interacting (concentrated) micelles. The EET dynamics are described by the function, 〈G s(t)〉, the probability of finding the excitation on the originally excited chromophore. For the isolated micelle calculations, the excitation dynamics depend on the distribution of probes on a single hard sphere surface. For the interacting micelle calculations, the hard sphere structure is accounted for by using the radial pair distribution function, g(r). Both single micelle and many micelle DT calculations do not involve approximations. Consequently, the DT expressions agree exactly with the MC calculations. For the DD calculations, a first order cumulant approximation is used to obtain analytically tractable solutions to 〈G s(t)〉. Padé approximants of the cumulant solution, accurate over a broad range of chromophore number and Förster interaction strengths, are used to describe DD EET on isolated micelles. For DD EET in many micelle systems, the first order cumulant approach is shown to be a suitable method for intermicelle structural studies. Both the cumulant and MC calculations are simultaneously compared to time resolved flourescence depolarization measurements performed on octadecylrhodamine B(ODRB)/triton X-100/water systems made in previous investigations.
Two-color study of Autler–Townes doublet splitting and ac Stark shift in multiphoton ionization spectra of CO
Authors: Xu Su , Sha Guohe , Jiang Bo , Sun Weizhong , Chen Xiangling , Zhang Cunhao
Published: July 2002

By using a two-color scheme, we have given a closer scrutiny to the Autler–Townes doublet splitting and ac Stark shift in the 2+2-photon ionization spectra of CO Ion A 1(v′=4)←X 1Σ+(v″=0) transition in the presence of another strong laser field, which is resonant with the transition C 1Σ+(v′=0)←A 1Π(v′=4)P10 branch. All the observations of ac Stark shift/broadening as a function of the perturbing light frequency and its spatial homogeneity strongly support our previous theoretical interpretation for one-color studies. A qualitative interpretation based on the dressed state theory, which is being quantified, for the Autler–Townes splitting as well as the ac Stark effect has been given. In particular, the magnitude of the Autler–Townes splitting is found to vary with the square root of the perturbing light intensity, in line with the dressed state theory.
Quantum mechanical and quasiclassical calculations for the H+D2→HD+D reaction: Reaction probabilities and differential cross sections
Authors: Aoiz F. J. , Bañares L. , D’Mello M. J. , Herrero V. J. , Rábanos V. Sáez , Schnieder L. , Wyatt R. E.
Published: July 2002

A detailed comparison of quasiclassical trajectory (QCT) and quantum mechanical (QM) reaction probabilities and differential cross sections for the H+D2→HD+D reaction at the collision energies of 0.54 and 1.29 eV has been carried out using the same potential energy surface. The theoretical simulation of the recently published experimental results is also reported. The comparisons made here demonstrate the level of agreement between QCT and QM approaches, as well as between theory and experiment for this reaction.
Variational transition state theory for multidimensional activated rate processes in the presence of anisotropic friction
Authors: Berezhkovskii Alexander M. , Frishman Anatoli M. , Pollak Eli
Published: July 2002

Variational transition state theory (VTST) is applied to the study of the activated escape of a particle trapped in a multidimensional potential well and coupled to a heat bath. Special attention is given to the dependence of the rate constant on the friction coefficients in the case of anisotropic friction. It is demonstrated explicitly that both the traditional as well as the nontraditional scenarios for the particle escape are recovered uniformly within the framework of VTST. Effects such as saddle point avoidance and friction dependence of the activation energy are derived from VTST using optimized planar dividing surfaces.
The effect of control field and measurement imprecision on laboratory feedback control of quantum systems
Authors: Tóth Gábor J. , Lőrincz András , Rabitz Herschel
Published: July 2002

Recent theoretical studies suggest the feasibility of laboratory adaptive feedback optimal control (AFOC) of chemical processes with the use of ultrashort laser pulses. The feedback process is introduced to provide robustness to laboratory error and electric field design uncertainties. Adaptive approaches for laboratory applications have been suggested. To foster laboratory implementation a number of questions still need to be answered. The paper addresses the problem of AFOC in the presence of laboratory errors and uncertainties both in adjusting the control parameters and in measuring the results. Through simulations we show that there exist optimization methods which are robust against systematic errors and certain optimization methods are also fairly robust against uncertainties. These results suggest that the effect of inevitable laboratory errors can be overcome. Furthermore the results suggest that noise is not a major problem and optimization in the laboratory is feasible.
Nonadditivity effects in the molecular interactions of H2O and HF trimers by the symmetry-adapted perturbation theory
Authors: Tachikawa Masanori , Iguchi Kaoru
Published: July 2002

Nonadditivity contribution to the three-body interaction energy is studied in terms of the symmetry-adapted perturbation theory for many-electron systems. Each component of energy, particularly the second-order exchange dispersion and exchange induction contributions, is given by a combination of electrostatic interaction energies in Longuet–Higgins representation of the intermolecular charge distribution. The formulas of these energies are derived with the Hartree–Fock approximation and by taking triple-electronic exchanges among three monomers into account. Numerical calculation has been performed for the cyclic planar H2O and HF trimers, considering only single-electronic exchanges between molecules. The three-body effect of the second-order exchange energy has been found to be repulsive, while the main part of attractive contribution is due to the induction. The ratio of three-body energy to two-body one for the dispersion is much smaller than that for the induction, though the latter decreases more rapidly than the former as the angle between monomers increases. As a result, the three-body contribution lowers the total interaction energy slightly near the van der Waals minimum in both trimers. The contribution of two-body energies is also shown.
Monte Carlo studies of interpenetrating polymer network formation
Authors: Schulz Michael , Frisch Harry L.
Published: July 2002

We present a lattice Monte-Carlo simulation of polymer network formation using the bond fluctuation method. We apply this procedure to simulate the formation of a class of simultaneous interpenetrating polymer networks (IPNs) which has been experimentally studied consisting of one network crosslinked with a trifunctional monomer (PCU) and another crosslinked with a tetrafunctional monomer (the free radical vinyl polymer network). The numerical simulation reveals essentially all features found experimentally for this class of IPNs. The system composition and initial morphology, at least for reactions which are relatively fast compared to the uphill diffusion process causing phase separation, is in good agreement with a mean field [random-phase approximations (RPA)] theory.
High level ab initio study on the ground state potential energy hypersurface of the HCO+–COH+ system
Authors: Yamaguchi Yukio , Richards Claude A. , Schaefer Henry F.
Published: July 2002

The ground state potential energy hypersurface of the HCO+–COH+ system has been reinvestigated systematically using high level ab initio electronic structure theory. The geometries and physical properties of the two equilibrium and one isomerization transition state structures were determined at the self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with nine basis sets. First, the optimized geometries of the three stationary points at twenty eight (28) levels of theory were discussed. Second, the characteristics and responses of the molecular orbitals (MOs) with respect to the normal coordinates have been elucidated via energy derivative analysis technique. Third, dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities were described. Finally, the relative energies among the stationary points have been compared. At the highest level of theory, CCSD(T) using triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the linear HCO+ molecule is found to be 39.7 kcal/mol [37.7 kcal/mol with zero-point vibrational energy (ZPVE) correction] lower in energy relative to the linear COH+ molecule. At the same level of theory the activation energy for the isomerization reaction HCO+→COH+ is predicted to be 76.9 kcal/mol (72.6 kcal/mol with the ZPVE correction). Once the COH+ species is formed, therefore, it should be reasonably stable with respect to the isomerization reaction at low temperatures.
Supermolecular approach to many-body dispersion interactions in weak van der Waals complexes: He, Ne, and Ar trimers
Authors: Chal/asiński Grzegorz , Szczȩśniak Mal/gorzata M. , Kendall Rick A.
Published: July 2002

Using the diagrammatic many-body perturbation theory, various three-body dispersion terms that appear in the intermolecular Mo/ller–Plesset perturbation theory (MPPT) are identified and classified with regard to the effects of intramonomer electron correlation on the dispersion term. Via the connection with the supermolecular MPPT, it is demonstrated how the leading dispersion nonadditivities arise within supermolecular calculations that employ MPPT or coupled cluster formalisms. The numerical calculations for He3, Ne3, and Ar3 in triangular geometries fully confirm theoretical predictions. The calculated values of dispersion nonadditivity clearly show that the coupled cluster theory with single, double, and noniterative triple excitations provides the proper framework for the efficient inclusion of the intramonomer correlation effects in dispersion nonadditivity. The convergence of the two-body and three-body terms is shown to be very similar if we compare the three-body terms of an order higher than the two-body terms. This pattern is used to provide the estimates of the total nonadditivities in the three trimers within a few percent accuracy.
Calculations on rotationally and diffractionally inelastic molecule-surface scattering for arbitrary angles of incidence: A new wave packet technique
Authors: Kroes G. J. , Mowrey R. C.
Published: July 2002

The close-coupling wave packet (CCWP) method has been adapted for performing calculations on molecule-surface scattering with arbitrary angles of incidence. The method used involves a slight modification of the fast Fourier transform (FFT) technique for evaluating the action of the translational kinetic energy operator on the wave function, employing the shifting theorem of Fourier analysis. We present and compare results of CCWP and close-coupling (CC) calculations on the He+LiF and H2+LiF systems using simple model potentials. The results presented establish the validity of the proposed technique and may be useful as benchmarks.
Laser-reduced fluorescence spectroscopy on predissociated CO triplet states
Authors: Mellinger A. , Vidal C. R.
Published: July 2002

Triplet states of the CO molecule in the region between 86 000 and 94 000 cm−1 have been investigated in a 1+1 double-resonance experiment using a detection scheme based on laser-reduced fluorescence (LRF). The strongly predissociated (3pσ) j 3Σ+ Rydberg state at 90 833 cm−1 with an average lifetime of 4.6 ps has been observed spectroscopically at high resolution for the first time, providing a clear identification of this state. Both the accidental predissociation of the Jf=6 level of CO (E 1Π, v=0) and the value of the lambda-doubling constant q of the c 3Π state can be attributed to j 3Σ+. In addition, a valence state around 91 960 cm−1 has been identified as 3Π. Accurate molecular constants have been determined for both states.
Rydberg states near the ionization continuum: Autoionization in ammonia
Authors: Cramb D. T. , Wallace S. C.
Published: July 2002

Previously undetected autoionizing Rydberg levels of ammonia which exist in the energy region between the first ionization continuum and the first excited vibrational level of the NH+3 ion have been measured. Jet cooled ammonia molecules in high lying Rydberg levels were detected by a two-color, three-photon excitation resonantly enhanced multiphoton ionization process, 2+1′. The Rydberg series can be assigned to a continuation of the (a2″)(nsa1′) series to which the Ã, D̃′, and Ẽ″ states belong. A vibronic autoionization mechanism based on the electrostatic and linear terms [V0 and (∂V/∂Q)Q, respectively] in the potential energy expansion has been suggested.
Electronically excited states in size selected solvated alkali metal atoms. I. First observation in NaNH3 by resonant two color two photon ionization
Authors: Nitsch C. , Hüglin Chr. , Hertel I. V. , Schulz C. P.
Published: July 2002

Resonant two color two photon ionization is used to investigate the first electronically excited state of NaNH3 as a first example of size selected, solvated alkali metal atoms systems (alkali⋅solventn). The observed spectrum shows a complex vibrational and rotational structure. The 0-0 transition is found at 12 220 cm−1 (1.52 eV), in good agreement with recent theoretical calculations. With increasing laser power ionic fragments of the molecule are seen, indicating possible new avenues to study the dynamics of charge transfer processes in this type of model system.
Photodissociation dynamics in quantum state-selected clusters: A test of the one-atom cage effect in Ar–H2O
Authors: Plusquellic David F. , Votava Ondrej , Nesbitt David J.
Published: July 2002

High resolution IR overtone pumping with an injection seeded optical parametric oscillator (OPO) is used in conjunction with excimer laser photolysis to investigate the state-resolved dynamics of quantum state-selected van der Waals clusters in a slit supersonic expansion. The narrow band IR light source (160 MHz, 5 mJ) preselects a specific upper state via the internal rotor band of Ar–H2O which correlates to the ‖03〉−←‖00〉+, 000←101 transition in H2O monomer. At fixed UV photolysis and probe wavelength, scanning the high resolution OPO yields the ‖03〉−←‖00〉+, Σ(000)←Σ(101) overtone action spectrum of Ar–H2O complexes. Conversely, at fixed IR pump wavelength, the state distribution of the OH photoproduct from photolysis of quantum state selected Ar–H2O clusters can be probed by laser induced fluorescence on the A 2Σ+←X 2Π (0,0) band. The OH distributions from H2O monomer vs Ar–H2O photolysis from the same internal rotor state are remarkably similar, though significant anomalies are observed for specific K rotational levels.