bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation trapping in liquid solutions
Authors: Burshtein A. I. , Lukzen N. N.
Published: July 2002

Reversible transfer of photogenerated short-lived excitations to energy acceptors with relatively stable excited states was considered. When it occurs in the course of bimolecular encounters in liquid solutions, it leads to energy conservation for a longer time. The conventional rate description of such a process even within the most sophisticated non-Markovian differential encounter theory is shown to be impossible due to divergency of the rate constants. The only alternative is the original integral encounter theory. It provides a proper kinetic description of delayed luminescence during encounter via energy donor as well as quantum yield of energy trapping by stable products (ions) escaped from encounters. The reasons are revealed why the usual reduction of integral kinetic theory to the differential one is inappropriate for either contact or remote reversible energy transfer between metastable states. The energy trapping via reversible ionization of the excited state is an extreme case that may be approached only within integral theory by means of which the ion accumulation kinetics and charge separation quantum yield were obtained.
Nonadiabatic molecular dynamics simulation of ultrafast pump-probe experiments on I2 in solid rare gases
Authors: Batista V. S. , Coker D. F.
Published: July 2002

Recent experimental studies of both A and B state photoexcitation of I2 and the ensuing many-body dynamics in rare gas matrices by Apkarian and co-workers are simulated using the methods we presented in an earlier work combining nonadiabatic molecular dynamics with semiempirical diatomics-in-molecules (DIM) excited state electronic structure techniques. We extend our DIM methods to compute the ion pair states of the I2-rare gas crystal system and use these states together with a model of the configurational dependence of the electronic dipole operator matrix elements to calculate the time resolved probe absorption signals in these pump - probe experiments using a simple golden rule result. Our computed signals are in remarkable agreement with experiments and we use our calculations to provide a detailed microscopic analysis of the channels to predissociation and recombination underlying these experiments.
Collisional energy transfer between Ar and normal and vibrationally and rotationally frozen internally excited benzene-trajectory calculations
Authors: Bernshtein V. , Oref I.
Published: July 2002

Quasiclasical trajectory calculations of energy transfer between an exited benzene molecule and an argon atom were performed. Values of average energy transferred per collision, 〈ΔE〉, were calculated. Three cases were investigated. (a) Collisions with unconstrained “normal” initial conditions. (b) Collisions where the rotations of the benzene molecule are initially “frozen.” (c) Collisions where the out-of-plane vibrations of the benzene molecule are initially “frozen.” The distributions of 〈ΔE〉 vs collision durations and the values of 〈ΔE〉 for collisions with frozen degrees of freedom are different than those obtained in normal collisions. This indicates the effects these modes have on the energy transfer process. The effect of rotations was found to be the largest. This indicates the predominant role rotations play in the energy transfer process. The effect of out-of-plane vibrations on the efficiency of energy transfer corroborates quantum mechanical calculations which show that out-of-plane motions are particularly efficient in energy transfer [Clary, Berenshtein, Oref, Gilbert Faraday Discussions 102 (1995)]. One in every 800 trajectories with normal initial conditions was found to be a supercollision. For frozen out-of-plane vibration the number dropped to one in 1500 and for frozen rotations it dropped even further to one in 4000. This shows the effect these wide angle motions have on the production of supercollisions. An impact parameter “window” was created in the initial conditions which enable an enhanced production of supercollisions by a factor of 4 thus helping to create a “bank” of supercollisions. Analysis of the trajectories of supercollisions in the bank shows that the condition for obtaining supercollisions are dynamic in nature. The atom approaches the molecule perpendicularly and it is in phase with a highly excited out-of-plane motion and/or is hit by a fast rotating molecule. This also agrees very well with the previous work quoted above. It is found that collisions, including supercollisions, are short lived. ∼60% of all inelastically scattered collisions last less than 140 fs and the rest last less than 500 fs. The number of long lived complex forming collisions is negligible.
On the fundamentals of the gradient theory of van der Waals
Authors: Cornelisse P. M. W. , Peters C. J. , de Swaan Arons J.
Published: July 2002

In this study a simple relation has been derived for the influence parameter in the gradient theory of van der Waals in terms of simple accessible quantities like temperature, the equilibrium densities, and the equilibrium isothermal compressibilities. Application of this relation leads to a substantially better agreement between interfacial tensions computed from the gradient theory and tensions obtained from experiment and simulation. The basis for this novel relation is an expression that connects the influence parameter to the second moment of the direct correlation function of pure fluids at states within the binodal region. The direct correlation functions for this study have been obtained from solving of the Ornstein–Zernike equation and the Percus–Yevick (PY) or hypernetted chain (HNC) closure relations for a Lennard–Jones (LJ) fluid. Special attention was paid to the behavior of solutions in the vicinity of the nonsolution region. It was shown for the PY closure that at the low density side the isothermal compressibility remains finite at the boundary of the nonsolution region. Along the isotherms and isochors the isothermal compressibility terminates at this boundary in so-called square root branch points. The isothermal compressibility diverges on the high density side although the correct location of the spinodal locus could not be found because of numerical inaccuracies. Diverging compressibilities are never encountered as the solution boundary is reached using the HNC closure. In all cases the isothermal compressibility terminates in square root branch points along the isotherms and isochors. In addition, computations show that at this boundary, the second moment of the direct correlation function seems to diverge for both the PY and the HNC closure. Comparison of the tensions obtained from the gradient theory with those obtained from a partial summation of the gradient expansion shows that at low temperatures the former results are ∼50% higher. Comparison of the results obtained from the latter model with experiments and simulations shows good agreement.
Droplet microemulsions at the emulsification boundary: The influence of the surfactant structure on the elastic constants of the amphiphillic film
Authors: Gradzielski M. , Langevin D. , Sottmann T. , Strey R.
Published: July 2002

The droplet structure of nonionic oil/water (O/W) microemulsions has been investigated by small-angle neutron scattering (SANS). The sum 2κ+κ̄ of the elastic moduli that can be deduced from the experimental polydispersity compares well with the values independently deduced from interfacial tension measurements, thereby showing that our determination of the elastic constants appears to be reliable. In this study, nonionic surfactants of different chain length and with different head groups were investigated (polyethylene glycol monoalkylether CiEj and alkyldimethlyamine oxides). The interpretation of the experiment data shows that the elasticity of the amphiphilic film increases with the thickness of the amphiphilic film. However, it does not depend significantly on the type of the head group but is primarily determined by the length of the alkyl chains of the surfactant. In mixtures of surfactants of different chain lengths, the sum 2κ+κ̄ is found to vary linearly with the molar composition of the mixture.
Diffusion of noninteracting and interacting lattice gases on heterogeneous surfaces: The bivariate random barrier model
Authors: Uebing C. , Pereyra V. , Zgrablich G.
Published: July 2002

The Monte Carlo technique is used to model surface diffusion on energetically heterogeneous surfaces. Simulations are performed for the bivariate barrier model with a concentration cH of high barriers distributed at random. The variation of the chemical and tracer diffusion coefficients with particles coverage and with cH is discussed both for interacting and noninteracting particles. Two different regimes are found for cH below and above the critical concentration for bond percolation. Above percolation the diffusion coefficients are found to vary as cH−1/T, a result which is not predicted by effective medium approximation (EMA).
Two-photon Zeeman spectroscopy of terbium (III) elpasolites
Authors: Berry A. J. , Denning R. G. , Morrison I. D.
Published: July 2002

Polarised two-photon excitation (TPE) spectroscopy is a powerful technique for probing the energy levels of f-block elements at high symmetry sites because most pure electronic transitions are allowed by the selection rules. TPE in a magnetic field provides additional information, through the resolution of Zeeman components, evaluation of g-values, and the appearance of intensity in forbidden transitions. Here we present the selection rules and polarisation dependence for two-photon absorption at cubic sites, in a magnetic field. Polarised TPE spectra for the elpasolites Cs2NaTbX6 (X=F, Cl) are in excellent agreement with theory. The sensitivity of the second order Zeeman effect to the orientation of a cubic crystal relative to the magnetic field is illustrated, together with successful modelling of the Zeeman interaction for a number of multiplets.
Electronic relaxation and ion desorption processes of solid Si(CH3)2Cl2 following Si 2p core-level excitation
Authors: Chen J. M. , Lu K. T. , Liu R. G. , Lay J. W. , Liu Y. C.
Published: July 2002

High-resolution Si L23-edge x-ray absorption near-edge structure (XANES) total electron yield (TEY) spectrum and photon stimulated ion desorption (PSID) spectra of condensed Si(CH3)2Cl2 have been measured in the energy range of 102–115 eV using synchrotron radiation. Excitation from Si 2p to a Si–C antibonding orbital enhances the CH3+ formation, while excitation to the Si–Cl antibonding orbital gives rise to a pronounced Cl+ production. This indicates that the character of the bound terminating orbital has a significant influence on the fragmentation processes. The selective enhancement of H+ yield at a peak of ∼106.0 eV is interpreted in terms of the excited electron in the bound orbital with strong C–H antibonding character. Applying resonant photoemission spectroscopy, the spectator Auger process was found to be the dominant decay channel for the resonantly excited Si(2p) core hole of condensed Si(CH3)2Cl2. The close resemblance of the TEY and PSID spectra of solid Si(CH3)2Cl2 was attributable to spectator Auger decay of core excited states and subsequent Coulomb repulsion of multivalence hole final states as proposed by the Knotek and Fiebelman mechanism.
Cooperativity and stability in a Langevin model of proteinlike folding
Authors: Berriz Gabriel F. , Gutin Alexander M. , Shakhnovich Eugene I.
Published: July 2002

We present two simplified models of protein dynamics based on Langevin’s equation of motion in a viscous medium. We explore the effect of the potential energy function’s symmetry on the kinetics and thermodynamics of simulated folding. We find that an isotropic potential energy function produces, at best, a modest degree of cooperativity. In contrast, a suitable anisotropic potential energy function delivers strong cooperativity.
Theoretical study of the dissociation reaction HFCO→HF+CO: New ab initio potential function and classical trajectory analysis
Authors: Yamamoto Takeshi , Kato Shigeki
Published: July 2002

A total of 3261 ab initio energies calculated at the RHF/MP2 level were used to obtain an analytical representation of the potential energy surface (PES) for the title reaction considering all the vibrational degrees of freedom. The analytical potential is constructed by switching three local representations of the PES utilizing a distancelike function, and it reproduces well the ab initio energies up to 20 kcal/mol above the dissociation threshold with the root-mean-square (rms) deviation equal to 1.5 kcal/mol. Two types of classical trajectory studies, i.e., power spectra calculations and product-state distribution analysis, were performed to assess the reliability of the present potential function. The results were found to be in good agreement with the available experimental ones.
Method for predicting IVR-limited unimolecular reaction rate coefficients
Authors: Shalashilin Dmitrii V. , Thompson Donald L.
Published: July 2002

We present a method based on diffusion theory (i.e., a classical master equation) for calculating unimolecular reaction rates at high energies where reaction is limited by the IVR (intramolecular vibrational energy redistribution) rate. The method, which we refer to as intramolecular dynamics diffusion theory (IDDT), uses short-time (a few fs) classical trajectory results to determine the characteristic times for the evolution of an initial microcanonical distribution, or, more specifically, the rate of IVR between the reaction coordinate and the “bath” modes of the molecule. The IDDT method accurately predicts the rate of Si–Si bond fission in Si2H6 in the nonstatistical, IVR-controlled regime, as demonstrated by comparisons with the results of a standard classical trajectory simulation. The method requires much less computer time than do the standard classical trajectory calculations. The method can be used to obtain results from the dynamical regime down to the statistical regime (near threshold), where rates can be calculated by Monte Carlo variational transition-state theory (MCVTST). Thus, the combined procedures can be used to calculate unimolecular reaction rates in large molecules for realistic potential energy surfaces over the entire energy range. The main approximation is the assumption of classical mechanics.
The dynamics of field-induced director reorientation for a nematic phase comprised of flexible molecules
Authors: Ciampi E. , Emsley J. W. , Luckhurst G. R. , Timimi B. A. , Kothe G. , Tittelbach M.
Published: July 2002

The effect of rotating the nematic phase of 4-n-pentyl-4′-cyanobiphenyl (5CB) at a frequency ωr about an axis perpendicular to an applied magnetic field has been studied using deuterium NMR spectroscopy. The alignment of the directors produced by the competing magnetic and rotational torques was monitored by the effect produced on Δνi, the deuterium quadrupolar splittings at all five alkyl chain and one biphenyl core sites in 5CB. The director orientation was found to be independent of the site monitored. The change in the director orientation produced by stopping the rotation of a sample was followed by recording deuterium spectra during the reorientation process. The ratios of the quadrupolar splittings, Δνi(t)/Δνi(t=0), are found to be site independent, which shows that measuring the director reorientation rates by this method will produce the same result independently of which molecular site is monitored. This contrasts with recent reports of experiments which found that monitoring director reorientational dynamics via a biphenyl core observable gave a different result than when the process was followed by measuring one corresponding to a site in the chain. The possible reasons for the observation of a site dependence of director reorientational dynamics are discussed.
The Bardeen–Cooper–Schrieffer approach to electron correlation in the density matrix formalism
Authors: Piris M. , Montero L. A. , Cruz N.
Published: July 2002

A generalization of the reduced density matrix theory when the number of particles is not fixed is given here. The first and second order reduced density matrices are defined with respect to a Bardeen–Cooper–Schrieffer ground state for molecules with an even number of electrons. This formalism is extended to molecules with an odd number of electrons. Valatin’s generalized Hartree-Fock method is applied to the functional of energy. A new one-particle operator, V̂q, is thus obtained. This operator has two new terms. It takes into account all the correlation effects mainly through a new non-local potential L̂q — the Exchange and Time Inversion Potential. The operator V̂q thus correlates the motion of two particles with anti-parallel spins and determines a new set of spin-orbitals. The new set of equations is established and solved by using the MO-LCAO approximation. The method was tested for several molecules with good results. The calculated vertical ionization potentials compare satisfactorily with the experimental data.
Theoretical study of hydrogen bonding in liquid and gaseous N-methylformamide
Authors: Ludwig R. , Weinhold F. , Farrar T. C.
Published: July 2002

Temperature-dependent quadrupole coupling constants (qcc), χQ, and asymmetry parameters, ηQ, of neat liquid and gaseous N-methylformamide were calculated theoretically. These calculations are based on standard ab initio self-consistent-field (SCP) methods at the 6-31G* level for the six most abundant clusters which were found via the quantum cluster equilibrium (QCE) model of liquids. The cluster sizes vary from one to six molecules and include linear and cyclic structures. Using QCE theory the equilibrium populations of the different clusters were calculated for temperatures between 250 and 600 K. These calculations indicate that at low temperatures six membered rings and linear tetrameters are the dominant species. At higher temperatures the cluster populations change and these two clusters are increasingly replaced by linear trimers and dimers. In the gas phase the expected monomers as well as a substantial fraction of dimers are found. The temperature dependencies of the calculated qcc values for the nitrogen and the amide deuterium nuclei are in good agreement with results from NMR relaxation time experiments. The changes of geometry with temperature agree with experimentally measured shifts from the gas to the liquid phase.
Nonlinear effects in dipole solvation. I. Thermodynamics
Authors: Matyushov Dmitry V. , Ladanyi Branka M.
Published: July 2002

The method of Padé truncation of perturbation expansions for thermodynamic potentials of molecular liquids is extended to the calculation of the solvation chemical potential of an infinitely dilute dipolar solute in a dipolar liquid. The Padé form is constructed to include nonlinear solvation effects of dipolar saturation at large and the linear response quadratic solute dipole dependence at small solute dipoles. The theory can accommodate polarizable solvents. The limiting case of electronically rigid solvent molecules is tested on the nonlinear reference hypernetted chain (RHNC) approximation for dipolar liquids. At high solvent polarities the Padé solvation chemical potential exceeds that of the RHNC. For both treatments, the nonlinear solvation contribution is found to pass through a maximum as a function of solvent polarity indicating that orientational saturation created by the solute breaks down with increasing solvent-solvent dipolar coupling. The Padé form of the chemical potential provides an analytical solution applicable to spectroscopic and electron transfer calculations involving solvation of fictitious complex-valued dipoles.
Nonlinear effects in dipole solvation. II. Optical spectra and electron transfer activation
Authors: Matyushov Dmitry V. , Ladanyi Branka M.
Published: July 2002

We present a theoretical analysis of the effect of nonlinear dipole solvation on steady-state optical spectra and intramolecular electron transfer (ET) reactions. The solvation nonlinearity is attributed to saturation of a dipolar liquid produced by the solute dipole. The treatment explores the perturbation expansion over the solute-solvent dipolar interaction truncated in the form of a Padé approximant. The optical line shape and the free energies along the ET reaction coordinate are related to the chemical potential of solvation of a fictitious solute with a complex-valued dipole moment. Due to solvent dipolar saturation the spectrum of dipolar fluctuations is confined by a band of the width 2Elim. Solvation nonlinearity was found to manifest itself for optical transitions with high dipole moments in the initial state, most often encountered for emission lines. In this case, the spectral line approaches the saturation boundary Elim bringing about “line squeezing” and decrease of the line shift compared to the linear response prediction. In the nonlinear region, the line shift dependence on the solute dipole variation Δm switches from the quadratic linear response form ∝Δm2 to a linear trend ∝|Δm|. The bandwidth may pass through a maximum as a function of |Δm| in the saturation region. Nonlinear solvation results thus in a narrowing of spectral lines. For a transition with solute dipole enhancement, the bandwidth in emission Δe is therefore lower that in absorption Δa: Δe<Δa. As a result, the plot of βΔa,e2, β=1/kBT against the Stokes shift ℏΔst demonstrates the upward deviation of βΔa2 and downward deviation of βΔe2 from the linear response equality βΔa,e2=ℏΔωst. We also explored the nonlinearity effect on charge separation/charge recombination activation thermodynamics. The solvent reorganization energy was found to be higher for charge separation (λ1) than for charge recombination (λ2). Both are smaller than the linear response result. For the reorganization energies, the discrepancy between λ1 and λ2 is relatively small, whereas their temperature derivatives deviate significantly from each other. The theory predictions are tested on spectroscopic computer simulations and experiment. Generally good quantitative agreement is achieved.
Resonances in the photodepletion spectrum of the Ba…FCH3 weakly bound complex
Authors: Skowronek S. , Pereira R. , González Ureña A.
Published: July 2002

Fano type resonances were observed in the photodepletion action spectrum of the Ba…FCH3 complex. They can be interpreted as a result of a significant coupling between the bound excited potential of the complex and the (open channel) ionic potential leading to BaF*+CH3 reaction products.
Kinetics and dynamics in the photodissociation of the allyl radical
Authors: Deyerl Hans-Jürgen , Gilbert Thomas , Fischer Ingo , Chen Peter
Published: July 2002

The direct observation of the products, kinetics and translational energy release from the photodissociation of the allyl radical, C3H5, upon excitation in the near-uv is reported. A statistical analysis of the data shows that they are in agreement with allene formation being the dominant H-loss reaction channel.
Conceptual and technical improvement of the reverse Monte Carlo algorithm
Authors: Tóth Gergely , Baranyai András
Published: July 2002

We performed Metropolis Monte Carlo and a comprehensive set of reverse Monte Carlo simulations using the same Lennard-Jones liquid to analyze the algorithmic correctness of the latter method. We found that the present reverse Monte Carlo technique is biased because it samples relatively disordered arrangements more often than ordered ones, which leads to a poor acceptance ratio and questionable relevance to the structure of the real system. On the basis of our results, we propose changes in the algorithm. To remove the deficiency we applied a combined acceptance criterion, which in addition to the very restrictive difference minimization procedure, uses an entropy related factor to select from the possible configurations. Further improvement can be achieved if the simulations are performed on the structure factor and the radial distribution function simultaneously. If the radial distribution function is the target function, incorporation of the r-dependent measurement error in the acceptance criterion is essential to obtain results similar in quality to the structure function techniques. The modifications enhance the performance of the reverse Monte Carlo method without increasing the computational effort.
A theory for electron transfer across the electrode/electrolyte interface involving more than one redox ion
Authors: Boroda Yuri G. , Calhoun August , Voth Gregory A.
Published: July 2002

A theory for electron transfer between a metal electrode and several redox species in a polar electrolyte is developed within the context of the Anderson–Newns Hamiltonian. Analytical expressions for the adiabatic ground state solvent free energy surfaces are derived, and it is shown how these expressions can be evaluated self-consistently. The shape of the adiabatic potential surfaces, and thus the thermodynamic and equilibrium rate characteristics of the electron transfer process, are shown to strongly depend on several key parameters. The effect of an external electric field on the free energy surface is also analyzed.
Initial state resolved electronic spectroscopy of HNCO: Stimulated Raman preparation of initial states and laser induced fluorescence detection of photofragments
Authors: Brown Steven S. , Berghout H. Laine , Crim F. Fleming
Published: July 2002

Stimulated Raman excitation (SRE) efficiently prepares excited vibrational levels in the ground electronic state of isocyanic acid, HNCO. Photofragment yield spectroscopy measures the electronic absorption spectrum out of initially selected states by monitoring laser induced fluorescence (LIF) of either NCO (X 2Π) or NH (a 1Δ) photofragments. Near threshold, the N–H bond fission is predissociative, and there is well-resolved rotational and vibrational structure in the NCO yield spectra that allows assignment of Ka rotational quantum numbers to previously unidentified vibrational and rotational levels in the ν1 N–H stretch and ν3 N–C–O symmetric stretch fundamentals in the ground electronic state of HNCO. The widths of NCO yield resonances depend on the initial vibrational state, illustrating one way in which initial vibrational state selection influences dissociation dynamics. Initial excitation of unperturbed ν1 (N–H stretch) states leads to diffuse NCO yield spectra compared to excitation of mixed vibrational levels. The higher energy dissociation channel that produces NH (a 1Δ) has coarser structure near its threshold, consistent with a more rapid dissociation, but the resonance widths still depend on the initially selected vibrational state.
Absorption study of an aggregated porphyrin under high pressure
Authors: Chan I. Y. , Hallock A. J.
Published: July 2002

The N,N’-diacid of meso-tetrakis(4-sulfonylphenyl)porphin forms both J- and H-aggregates. Both the B- and Q-transitions exhibit a J-band. Under typical experimental conditions, the monomer and H-bands are also observed. We have measured the pressure dependence of the various spectral bands up to 40 kbar. All data show linear dependence of spectral red shift with pressure. The monomer Soret band has the highest pressure sensitivity. The pressure slope of the B-band is 3.6 times greater than that for the Q-band for the J-aggregates. A simple excitonic theory predicted a ratio of nearly 11. It appears that there is a substantial solvent shift contribution to these pressure derivatives.
Anisotropy-induced polarization effects in degenerate four-wave mixing spectroscopy: A new scheme for probing molecular orientation and alignment
Authors: Müller Thomas , Wasserman Thierry A. W. , Vaccaro Patrick H. , Johnson Bruce R.
Published: July 2002

Recent theoretical predictions of anisotropy-induced polarization components in the signal wave generated from gas-phase degenerate four-wave mixing spectroscopy are verified experimentally by probing optically pumped ensembles of jet-cooled molecules that are characterized by well-defined orientation and/or alignment parameters. Aside from corroborating recent multipole moment analyses, detailed simulation of recorded data sets demonstrates the feasibility of performing a new class of resonant four-wave mixing measurements designed to extract angular momentum anisotropy information from target species in a manner that is both background-free and insensitive to isotropic contributions.
Guided ion beam studies of the reactions of Vn+ (n=2–17) with O2: Bond energies and dissociation pathways
Authors: Xu J. , Rodgers M. T. , Griffin J. B. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the reactions of Vn+ (n=2–17) with oxygen is studied using a guided ion beam mass spectrometer. In all but the smallest clusters, the primary reaction process at low energies is the formation of a vanadium cluster dioxide ion which then loses one or two vanadium atoms or a vanadium oxide diatom (VO). Vanadium atom loss is the preferred reaction pathway for large clusters (n⩾5), whereas loss of VO is more favorable for the smallest reactant clusters (n⩽4). As the collision energy is increased, these primary products dissociate further by loss of additional vanadium atoms. Bond dissociation energies of the vanadium cluster oxides are determined by analysis of the kinetic energy dependence of several different products. The effect of oxygen atoms on the stabilities of vanadium cluster ions is discussed and compared with bulk phase thermochemistry.
Theoretical and electron spin resonance studies of the H⋯H, H⋯D, and D⋯D spin-pair radicals in rare gas matrices: A case of extreme singlet–triplet mixing
Authors: Knight Lon B. , Rice William E. , Moore Louie , Davidson Ernest R. , Dailey Robert S.
Published: July 2002

The H⋯H, H⋯D, and D⋯D spin-pair radicals have been thoroughly investigated in neon, argon, krypton, and xenon matrices near 4 K by electron spin resonance (ESR). A theoretical model has been developed that treats these spin-pairs as weakly interacting atoms. The model includes the effects of Σ/3Σ1 mixing in the analysis of the observed ESR spectral results and yields a consistent set of magnetic parameters for these three isotopomers in all four rare gas hosts. The consideration of H atoms interacting with other H atoms over a distribution of internuclear distances in the rare gas lattice is included in the theoretical and experimental analyses. Application of the model to earlier ESR results for H⋯CH3 reveals a value for its Heisenberg exchange interaction (J) which is found to be considerably larger than that for the H⋯H spin-pair. The effects of methane and neon on the J value are calculated for these spin-pairs. The H⋯H case is unusual in that the nuclear hyperfine interaction (A) is considerably larger than D (the anisotropic dipole–dipole magnetic interaction between electrons) which is much larger than J. The H⋯H spin-pairs exhibit internuclear distances greater than 7 Å and have the following magnetic parameters (MHz) based upon this model of “weakly interacting atoms;” giso=2.0016, Aiso=1426, D=−200, and J=6. Since a distribution of distances is involved, other spin-pairs would be separated by even greater distances in the matrix and thus have smaller absolute values of D and J.
Structural transformation in the collapse transition of the single flexible homopolymer model
Authors: Hu Wenbing
Published: July 2002

The structural transformation in the coil-globule transition of a single flexible lattice chain has been investigated using dynamic Monte Carlo simulations. The results based upon ensemble averaging illustrated that for the homopolymers with limited chain length, an intermediate state with a dense-core and molten-shell structure reversibly occurs in the transition region. It was attributed to a special microphase separation behavior in an isolated coil, performing with densifying the dense core and contracting the thin shell. The continuous appearance of the size transition and its tendency to discontinuity at the theta temperature with the chain length approaching infinity were illustrated by the coexistence curves of the monomers with limited chain length. A possible explanation and its implications to the general mechanism of protein folding are also discussed.
A comparative high-resolution study of predissociation linewidths in the Schumann-Runge bands of O2
Authors: Dooley P. M. , Lewis B. R. , Gibson S. T. , Baldwin K. G. H. , Cosby P. C. , Price J. L. , Copeland R. A. , Slanger T. G. , Thorne A. P. , Murray J. E. , Yoshino K.
Published: July 2002

Results are presented of a comparative study in which three distinct high-resolution experimental techniques (vacuum-ultraviolet laser spectroscopy, laser-induced fluorescence spectroscopy and vacuum-ultraviolet Fourier-transform spectroscopy) were used to study predissociation in the Schumann-Runge bands of O2 B 3Σu−(v′)←X 3Σg−(v″) with v′=13 and 14. Our measurements are the first to be performed at high resolution for these levels and represent a significant advance on previous knowledge, characterizing completely the fine-structure and rotation dependencies of the B 3Σu−(v=13 and 14)-state predissociation for the first time. The measured fine-structurespecific linewidths will result in significant improvements in the parameterization of models describing predissociation of the B-state and will have an impact on the development of realistic photochemical models of the terrestrial atmosphere. Good agreement was found between linewidths measured using vacuum-ultraviolet laser spectroscopy and laser-induced fluorescence spectroscopy, but unexpected difficulties arose in determining quantitative linewidths using vacuum-ultraviolet Fourier-transform spectroscopy. For each experimental technique, the instrumental resolution had to be carefully controlled and monitored in order to ensure reliable interpretation of the measured spectra.
Correlated Monte Carlo electron-pair density for the atoms helium to neon
Authors: Sarsa A. , Gálvez F. J. , Buendı́a E.
Published: July 2002

The Monte Carlo method to obtain the electron-pair density for the atoms helium to neon has been applied. The wave functions of Schmidt and Moskowitz [J. Chem. Phys. 93, 4172 (1990)] to take into account the dynamic correlation among the electrons have been used. For the atoms Be, B and C we have considered the nondynamic correlation due to the near degeneracy 2s−2p by means of a configuration interaction wave function and for Li and Be we have also varied the central part of the wave function. A study of the differences between the correlated and the Hartree–Fock results has been carried out. Finally we have also calculated the interelectronic moments, 〈r12n〉, and the value of the electron pair density at the coalescence point for all the atoms considered.
Infrared spectroscopy of overtones and combination bands
Authors: Jakob P. , Persson B. N. J.
Published: July 2002

We present a detailed discussion on infrared spectroscopy of vibrational combination bands and overtones of adsorbate systems. For the case that the (dynamical) lateral coupling between the adsorbates is dominated by dipole coupling, we present general results for the absorption spectra which can be used to analyze experimental data and deduce the bond anharmonicity δω. The theoretical results are used to analyze experimental line shape data for the combination band of the C–O and the Ru–CO stretch vibrational modes of CO adsorbed on Ru(001), as well as the overtone of the C–O stretch vibration for the same adsorbate system. It is found that for strong lateral coupling (and weak anharmonicities) asymmetric line shapes are common; strong anharmonicities lead to the formation of localized two-phonon bound states besides a continuum of delocalized two-phonon states. However, even then the extraction of anharmonic parameters can be severely impeded by dynamic line shifts of the localized overtone band through virtual transfers of vibrational quanta to neighboring oscillators. We further consider the mechanisms which contribute to the broadening of overtone excitations and show that the energy relaxation rate for the overtone is twice as large as for the fundamental, while the (pure) dephasing rate is approximately four times higher than that of the fundamental excitation.
Crystal structure and low-temperature methyl-group dynamics of cobalt and nickel acetates
Authors: Nicolaï B. , Kearley G. J. , Johnson M. R. , Fillaux F. , Suard E.
Published: July 2002

The crystal structures of cobalt and nickel acetate tetrahydrate have been determined at room-temperature and liquid-helium temperature by neutron powder diffraction of the fully deuterated salts. Molecular mechanics and ab initio methods based on these structural results have then been used to calculate the rotational potentials experienced by the methyl groups. We have also used inelastic neutron scattering to measure the rotational potential via the rotational tunneling spectrum of the methyl groups, and this has enabled us to compare different methods for the calculation of partial charges in these ionic compounds. Good agreement between the observables and calculations has been obtained for both compounds when ab initio methods are used to recalculate partial charges at every step of the methyl rotation.
Relationship between static vibrational and electronic hyperpolarizabilities of π-conjugated push–pull molecules within the two-state valence-bond charge-transfer model
Authors: Bishop David M. , Champagne Benoı̂t , Kirtman Bernard
Published: July 2002

The two-state valence-bond charge-transfer (VB-CT) model for π-conjugated push–pull molecules is analyzed to yield several parameter-independent relations between vibrational and electronic hyperpolarizabilities. Ab initio computational tests show that these relations are not satisfied. As a byproduct of the ab initio calculations, we have found cases where the static vibrational first hyperpolarizability is much larger than the static electronic first hyperpolarizability. Our results also shed light on a controversy regarding normal coordinate polarizability derivatives.
Optimal representation for semiclassical surface hopping methods
Authors: Herman Michael F.
Published: July 2002

A semiclassical surface hopping expansion of the propagator is developed for a general representation of the “fast” variable quantum states. The representation can be the adiabatic or diabatic representation or any representation between these two. A particular representation is defined, which is optimal in the sense that it minimizes the integrated interstate coupling. The coupling is integrated over a suitable classical trajectory in this definition. Calculations for a simple one-dimensional curve crossing model problem show that the use of this optimal representation can significantly reduce the importance of multihop terms in the expansion. An approximation to this optimal representation is proposed, which is much simpler to implement numerically. Calculations for the model curve crossing problem demonstrate that this approximate optimal representation provides integrated couplings that are very close to those obtained for the optimal representation. These results suggest that this approximate optimal representation provides a computationally attractive representation for use with semiclassical surface hopping methods, when studying problems with curve crossings.
Proposal and numerical test of a simple diabatization scheme
Authors: Thiel A. , Köppel H.
Published: July 2002

The theory of constructing diabatic electronic wave functions for strongly coupled potential energy surfaces is briefly reviewed. The use of a particularly simple diabatization scheme is advocated which removes only the leading terms of the nonadiabatic, or derivative, coupling elements. It is tested numerically for a well-established E⊗e Jahn–Teller problem and shown to lead to very good (often excellent) agreement with the exact results.
Compton profile study of bonding in BeO
Authors: Joshi K. B. , Jain Rajesh , Pandya R. K. , Ahuja B. L. , Sharma B. K.
Published: July 2002

The isotropic Compton profile of BeO has been measured using a γ-ray Compton spectrometer which employs a 5 Ci annular Am241 source. The measurement is compared with the theoretical HF-LCAO Compton profile published by Lichanot et al. The measurement has also been compared with our calculations based on the RFA model employing different ionic wavefunctions. It is seen that the experimental Compton profile is in very good agreement with the HF-LCAO calculations. To determine the nature of bonding, equal-valence-electron-density Compton profile of BeO has been compared with that of the isovalent compound MgO. It is observed that the bonding in BeO is partially covalent and less ionic than MgO in agreement with the conclusions of the HF-LCAO calculation.
Quasielastic helium atom scattering measurements of microscopic diffusional dynamics of H and D on the Pt(111) surface
Authors: Graham Andrew P. , Menzel Alexander , Toennies J. Peter
Published: July 2002

The microscopic diffusional dynamics of H and D on Pt(111) have been studied over length scales between 3 and 14 Å by quasielastic helium atom scattering. Data taken over a wide range of parallel wave vectors (0.3 Å−1⩽|ΔK|⩽3.1 Å−1) in the surface temperature range 140 K⩽Ts⩽250 K and at coverages 0.05 ML⩽ΘH⩽0.66 ML, provide evidence for an isotropic single jump mechanism with an activation energy of 68±5 meV and a pre-exponential factor of D0=1.1±0.5×10−3 cm2 s−1 at low coverages. The diffusion coefficient is 2 to 3 orders of magnitude higher than found in an earlier laser induced desorption experiment, whereas the dependence on coverage is similar.
Structure of molten MCl3 systems from a polarizable ion simulation model
Authors: Hutchinson Francis , Rowley Adrian J. , Walters Malcolm K. , Wilson Mark , Madden Paul A. , Wasse Jonathan C. , Salmon Philip S.
Published: July 2002

Computer simulations of a range of molten salts of stoichiometry MX3 using a polarizable, formal charge ionic interaction model are described. The systems studied — LaCl3, TbCl3, and YCl3 — span a range of cation sizes and the interaction model is a “generic” one, in the sense that the cation size is the only parameter in the interaction potential which distinguishes one system from another. The liquid structures predicted from the simulations are compared with recently obtained neutron diffraction data. Excellent agreement is found, except that the first coordination shell seems to be too tightly bound in the computer simulations. The cation in LaCl3 is found to be 7–8 fold coordinate, and the coordination number drops to 6 for the smallest cation considered (Y3+), so that the coordination number in these systems does not change substantially on melting, in contrast to earlier reports. The polarization effects promote a significant degree of edge-sharing between these coordination polyhedra relative to predictions of a simple ionic pair potential. Associated with these changes is a shift to smaller scattering vectors of the prepeak in the structure factor and an increase in the fluidity.
Geometry optimization in generalized natural internal coordinates
Authors: von Arnim Malte , Ahlrichs Reinhart
Published: July 2002

The automatic generation of natural internal coordinates is extended to all cases except multiply fused ring systems, i.e., “cages.” This is achieved by a relatively simple code; most of the geometrical cases can now be treated by a single subroutine. To handle molecules containing cages we combine the natural internal coordinates with the delocalized coordinates of Baker, Kessi, and Delley. We have also modified the delocalized coordinates to separate modes with strongly different force constants. The new methods show very good convergence properties in geometry optimizations. Our test set of molecules covers a wide range of atoms and of different bonding situations. For such molecules good internal coordinates are particularly important because no reliable initial force constant matrix can be obtained to accelerate convergence.
High resolution infrared absorption spectra of methane molecules isolated in solid parahydrogen matrices
Authors: Tam Simon , Fajardo Mario E. , Katsuki Hiroyuki , Hoshina Hiromichi , Wakabayashi Tomonari , Momose Takamasa
Published: July 2002

We present high resolution (∼0.01 cm−1) infrared absorption spectra of the ν4 band of methane doped parahydrogen (CH4/pH2) solids produced by two different techniques: gas condensation in an enclosed cell at T≈8 K, and rapid vapor deposition onto a T≈2 K substrate in vacuum. The spectrum of the rapid vapor deposited solid contains a novel progression of single peaks with ≈5 cm−1 spacing, superimposed over the known spectrum of CH4 molecules trapped in sites of D3h symmetry in hexagonal close-packed (hcp) solid pH2. New theoretical calculations of the rovibrational transitions of a tetrahedral molecule in an external field of Oh symmetry permit the assignment of this new progression to CH4 molecules trapped in crystalline face centered cubic (fcc) regions of the pH2 solid. Annealing of the rapid vapor deposited samples to T≈5 K decreases the intensities of the CH4/pH2(fcc) absorptions, and results in intensity changes for parallel and perpendicularly polarized CH4/pH2(hcp) transitions. We discuss these phenomena, and the narrow (0.01–0.04 cm−1 full width at half-maximum) absorption linewidths, in terms of the microscopic structure of the pH2 hosts.
Ab initio theoretical studies on photodissociation of HNCO upon S1(1A″)←S(1A′) excitation: The role of internal conversion and intersystem crossing
Authors: Kaledin Alexey L. , Cui Qiang , Heaven Michael C. , Morokuma Keiji
Published: July 2002

Photodissociation of isocyanic acid, HNCO, was studied with high-level ab initio methods. Geometry optimizations of stationary points and surface crossing seams were performed with the complete active space self-consistent-field (CASSCF) method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory (CASPT2). The three product channels that participate in the photodissociation process are [1] HN(X 3Σ−)+CO at 86.0 (calculated 79.6) kcal/mol, [2] H+NCO(X 2Π) at 109.7 (108.7) kcal/mol, and [3] HN(a 1Δ)+CO at 122.2 (120.8) kcal/mol. The four electronic states, S0, S1, T1, and T2, that interconnect these channels were studied in detail. S1 exhibits dissociation barriers to both, channel [2] and [3], whose respective reverse heights are 11.3 and 1.2 kcal/mol, in good agreement with experiment as well as previous theoretical works. The two triplets, T1 and T2, show barriers of similar heights for HN bond fission, while S0 has no barriers to either channel. Various key isomerization transition states as well as numerous minima on the seam of surface crossings (MSX’s) were also found. At photoexcitation energies near channel [3] threshold, products to channel [3] are likely to be formed via S1→[3] (if enough energy in excitation) and S1→S0→[3]. Channel [2] can be formed via S1→S0→[2]; (HN-mode quanta)+S1→T1→[2]; S1→T2→[2]; S1→T2→T1→[2], and channel [1] via S1→S0→T1→[1], S1→T1→[1] and S1→T2→T1→[1]. At higher photoexcitation energies the S1→[3] pathway is expected to be dominant while S1→[2], with the higher activation energy, is expected to drop rapidly. Also addressed are such important issues as the impact of a vibrationally excited HN mode on a channel [2] yield, and the band origin of the S1←S0 excitation spectrum.