bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of the Fe+ cation with heavy noble gas atoms
Authors: Heinemann Christoph , Schwarz Joseph , Koch Wolfram , Schwarz Helmut
Published: July 2002

The diatomic iron–noble gas complexes FeAr+, FeKr+, FeXe+, and FeRn+ have been theoretically investigated by means of quantum-chemical calculations including an extensive treatment of electron correlation. Potential energy curves and spectroscopic constants for the lowest 4Δ and 6Δ states are derived from an open-shell coupled-cluster approach and the relative energies of all seven low-lying electronic states are evaluated by the multireference configuration interaction method. While in FeAr+ the lowest quartet and sextet states are found to be energetically almost degenerate, the heavier Fe+–noble gas molecules are predicted to exhibit 4Φ ground states. From a qualitative point of view bonding in these species is shown to be electrostatic in origin with intrinsically higher interaction energies for the quartet as compared to the sextet states. For calibration purposes, also an accurate calculation of the 4F(4s03d7)–6D(4s13d6) energy difference in the atomic Fe+ cation is provided.
Zero-kinetic-energy pulsed-field ionization spectroscopy of the a 1Δ state of SH+ (SD+)
Authors: Milan J. B. , Buma W. J. , de Lange C. A.
Published: July 2002

The results of a zero-kinetic-energy pulsed-field ionization study on the a 1Δ (v+=0) excited ionic state of SH+ (SD+) obtained via two-photon excitation of the [a 1Δ]3dπ 2Φ (v′=0) Rydberg state and subsequent one-photon pulsed-field ionization are reported. Accurate ionization energies as well as rotational constants are obtained. A detailed comparison between the rotational branching ratios resulting from the pulsed-field ionization process and those of a direct ionization process is made. The results elucidate the dynamics of the high-n Rydberg states involved in pulsed-field ionization of SH (SD).
Photodissociation of HOCl: A model for the prediction of the OH Λ-doublet and Cl spin–orbit product state distributions
Authors: Offer Alison R. , Balint-Kurti Gabriel G.
Published: July 2002

HOCl is a closed shell system in its ground electronic state. In common with many molecules it is photodissociated to give open shell fragments. In this paper the photodissociation process HOCl(X 1A′)+hν→HOCl(2 1A′)→OH+Cl is investigated. A model is proposed to predict the OH product spin–orbit and Λ-doublet state distributions as well as the distribution of the Cl atom spin–orbit states. The model considers both the electronic structure of the system and the nuclear motion. Predictions are made not only of the individual fragment state distributions and of their dependence on total initial angular momentum and photon frequency, but also of the degree of correlation between the production of the different states of the two open shell fragments. The computed results agree with the available experiments, in particular the current theory is the first to agree with experimental findings in predicting a smoothly varying OH rotational state distribution. It is demonstrated that the model reproduces the frequency dependence of the absorption line shape as computed using more exact procedures. In the simple treatment presented here rotational transitions during the dissociation process are ignored, as is the detailed coupling of the electronic states in the asymptotic region. Some of the remaining disagreements between the current predictions and the experimental results are ascribed to these approximations in the model. It is hoped that the predictions of the correlated product state distributions will stimulate coincidence experiments on the photodissociation process.
Multicenter molecular integrals of spherical Gaussian functions by Fourier transform convolution theorem
Authors: Moharerrzadeh Mohammad , Chiu Lue-Yung Chow
Published: July 2002

The two-electron four-center integral of the homogeneous solid spherical harmonic Gaussian-type functions (GTF’s), r2n+lYlm(r̂)exp(−αr2), has been evaluated analytically by decomposing it into a linear combination of two-center integrals through coincidence of centers. The two-electron two-center integrals are integrated analytically through the Fourier transformation convolution theorem. A compact integration formula is obtained for a general two-electron irregular solid spherical harmonic operator [4π/(2L+1)]1/2YLM (r̂12)/r(L+1)12. This formula is applied to evaluate two-center integrals of the Coulomb repulsion, the spin–other–orbit interaction and the spin–spin interaction by letting L=0, 1, and 2, respectively. The integration results are in terms of the spherical Laguerre GTF’s, Ln′l′+1/2(σR2)Rl′Yl′m′ (̂)exp(−σR2), of the relative nuclear coordinate plus one error-type F-function term. One-electron multicenter integrals have also been evaluated through Fourier transformation convolution theorem. These are the integrals of two and three center overlap, three-center multipole moments rLYLM(r̂) (e.g., L=1 and 2 for dipole and quadrupole moment, respectively), and three-center irregular solid spherical harmonics YLM(r̂)/r(L+1) (e.g., L=0, 1, and 2 for nuclear attraction, spin–orbit interaction, and electron spin–nuclear–spin interaction, respectively). All of the integral results are given explicitly in terms of the relative nuclear coordinates and the Gaussian exponential parameters. Explicit expressions of derivatives can be easily obtained to generate force constants, to optimize geometries and to optimize Gaussian exponential parameters.
Lattice-gas model of NO decomposition on transition metals
Authors: Zhdanov V. P. , Kasemo B.
Published: July 2002

Decomposition of an adsorbed NO molecule is possible provided that it has an empty nearest-neighbor site for the oxygen (or nitrogen) dissociation product. For this reason, the coverage dependence of the rate constant of this step is usually described by the equation kr(θ)=(1−θ)kr(0) containing the fraction of empty sites. Employing the lattice-gas model, we show that this dependence can be dramatically modified by repulsive lateral adsorbate–adsorbate interactions. If the lateral interaction in the ground state (intact NO molecules) is negligible, the interaction in the activated state may suppress the reaction with increasing coverage. In this case, the decomposition rate constant can often be represented as kr(θ)≊(1−θ)nkr(0), where n≳1. This expression is, however, not applicable near saturation coverage. If the lateral interaction in the ground state is non-negligible, the rate constant of NO decomposition first increases with increasing coverage and then rapidly drops as θ→1. For relatively strong adsorbate–adsorbate interactions (≥2 kcal/mol), the region where the rate constant decreases with increasing coverage is rather narrow (θ≥0.8).
Electronic states of Al and Al2 using quantum Monte Carlo with an effective core potential
Authors: Greeff C. W. , Lester W. A. , Hammond B. L.
Published: July 2002

The diffusion Monte Carlo method is applied in conjunction with an ab initio effective core potential to compute energies of some neutral and charged states of Al and Al2. The computed ionization potentials, electron affinities and dissociation energies differ from measured values by at most a few hundredths of eV. The computed dissociation energy of Al2 agrees with the most extensive CI calculations. It appears that our dissociation energy for Al−2 is the most accurate to date. The quality of the results indicates that the use of the pseudopotential is not an important limitation on the accuracy of these calculations. Variational wavefunctions with Boys-Handy correlation functions are found to give more than 70% of the correlation energy with 8 optimized parameters. These optimized trial functions are used together with numerical integration to localize the pseudopotential.
The generalized hyperfine sublevel coherence transfer experiment in one and two dimensions
Authors: Hubrich Michael , Jeschke Gunnar , Schweiger Arthur
Published: July 2002

The generalized hyperfine sublevel coherence transfer pulsed ESR experiment is described that consists of a nuclear coherence generator, a first evolution period, a nonselective microwave π pulse, a second evolution period, and a nuclear coherence detector. Several incrementation schemes to perform one-dimensional experiments are discussed, namely, a four-pulse electron spin echo envelope modulation scheme for measuring combination frequencies, two types of nuclear coherence transfer echo experiments (DEFENCE) to record dead time free electron spin echo envelope modulation spectra of disordered systems, and a new hyperfine spectroscopy experiment for the direct measurement of hyperfine frequencies. The one-dimensional experiments can be combined to four two-dimensional schemes. In addition to hyperfine sublevel correlation spectroscopy (HYSCORE) where two nuclear frequency dimensions are correlated, three new two-dimensional experiments are introduced that correlate the hyperfine with the nuclear frequency, the combination frequency with the nuclear frequency, and the combination frequency with the hyperfine frequency. With a properly selected two-dimensional incrementation scheme the digital resolution and signal-to-noise ratio of the spectra can be improved and artifacts can be suppressed. The predicted features of the one- and two-dimensional experiments are verified experimentally for both ordered and disordered systems using echo and coherent Raman beat detection.
Solvation and solvent effects on the short-time photodissociation dynamics of CH2I2 from resonance Raman spectroscopy
Authors: Kwok Wai Ming , Phillips David Lee
Published: July 2002

Resonance Raman spectra of CH2I2 have been obtained at excitation wavelengths of 369, 355, and 342 nm in cyclohexane solution and in methanol solution at excitation wavelengths of 355 and 342 nm. Resonance Raman spectra were also measured for CH2I2 in the vapor phase with an excitation wavelength of 355 nm. The resonance Raman spectra of CH2I2 exhibit most of their intensity in fundamentals, overtones, and combination bands of modes nominally assigned as the I–C–I symmetric stretch, the I–C–I bend, and the I–C–I antisymmetric stretch vibrations. The absorption spectra and resonance Raman intensities of the gas phase and methanol solution phase diiodomethane spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode coefficients from normal coordinate calculations were used to convert the motion of the wave packet on the excited electronic state surface from dimensionless normal coordinates into internal coordinates of the molecule. The short-time photodissociation dynamics of diiodomethane in the vapor phase shows that the two C–I bonds are lengthening by the same amount, the I–C–I angle becomes smaller, the H–C–I angles become larger, and the H–C–H angle becomes smaller.

The two C–I bonds appear essentially equivalent in the Franck–Condon region of the gas phase photodissociation which implies that the molecule chooses which C–I bond is broken after the wave packet has left the Franck–Condon region of the potential energy surface. Comparison of the gas phase resonance Raman spectrum with solution phase spectra obtained in cyclohexane and methanol solvents reveals that the short-time photodissociation dynamics are noticeably changed by solvation with a large solvent-induced symmetry breaking observed. In the Franck–Condon region of the solution phase diiodomethane photodissociation in methanol solvent the two C–I bond become larger by differing amounts, the I–C–I angle becomes smaller, the H–C–H angle becomes smaller, and the H–C–I angles differ from the corresponding gas phase values. During the initial stages of the solution phase photodissociation (at least in methanol and cyclohexane solvents) the two C–I bonds are not the same and this suggests that the molecule chooses which C–I bond will be broken soon after photoexcitation.
Potential energy surface for interactions between N2 and He: Ab initio calculations, analytic fits, and second virial coefficients
Authors: Hu Ching-Han , Thakkar Ajit J.
Published: July 2002

An ab initio potential energy surface (PES) for the interaction of rigid N2 with He is calculated by supermolecular fourth-order Mo/ller–Plesset perturbation theory. The computations involve full counterpoise corrections and large basis sets including bond functions. The 61 ab initio points on the PES are fitted to a 21-parameter algebraic form with an average absolute error of 0.39% and a maximum error less than 1.2%. The characteristics of the fitted PES are compared with those of previous surfaces. Unlike the older surfaces, our PES has the anisotropy thought to be required for a proper description of experimental data. Pressure second virial coefficients are calculated from our surface and compared with experimental values.
Time-dependent spectral calculation of bound and resonance energies of HO2
Authors: Dai Jiqiong , Zhang John Z. H.
Published: July 2002

We report detailed numerical studies for bound and resonance states of HO2 using a time-dependent (TD) wave packet approach. The energies of bound and resonance states are calculated from an energy spectrum which is obtained by time→energy Fourier transform of the autocorrelation function. Numerous bound and resonance (both inelastic and reactive) states are identified and their energies are obtained, all from a single wave packet calculation. We employed a Gaussian function to generate the Gaussian spectrum which is defined by convoluting the autocorrelation function with a Gaussian weighting function in the time→energy transform. As shown in a previous paper, the Gaussian spectrum is positive definite and each spectral peak corresponds to a true eigenenergy which makes the picking of eigenenergies straightforward. The wave functions of highly excited bound states and essentially all resonance states show complicated nodal patterns and their spectroscopic assignments seem to be impractical. Our calculated energy spectrum above the reaction threshold shows similar structures as the energy dependence of the reaction probabilities obtained from a previous reactive scattering wave packet calculation. The DMBE IV potential energy surface (PES) of Varandas and co-workers is used in the present calculation.
Raman spectroscopy and theoretical modeling of HCl vibrational frequency shifts in high pressure argon
Authors: Devendorf George S. , Ben-Amotz Dor , de Souza Luís E. S.
Published: July 2002

Raman vibrational frequencies of HCl in argon were measured at pressures up to 110 MPa. The mean frequency of the asymmetric Q-branch is shown to accurately measure vibrational shifts through a density region where line shape changes due to motional narrowing render the peak maximum an inaccurate measure of pressure induced frequency shifts. A semiclassical, analytical expression utilizing Hutson’s HCl–Ar pair-potentials is used to determine the derivative of the HCl vibrational frequency with respect to Ar density in the limit of zero density. The predictions are in reasonable agreement with experimental results, although the experimental frequency shifts are about 20% smaller (less redshifted) than theoretical predictions, which may represent the influence of multibody interactions. Experimental HCl Raman Q-branch and S-branch linewidths and peak shifts are compared qualitatively with previous R-branch (IR absorption) results. Separation of the vibrational (Q-branch) and rotational parts of the frequency shift suggest that the rotational contribution is positive (blueshifted) for all J values and approaches zero with increasing J.
State-resolved differential scattering in open-shell collisions: Cl(2P3/2)+HCl from high-resolution infrared-laser Dopplerimetry
Authors: Zhao Zhong-Quan , Chapman William B. , Nesbitt David J.
Published: July 2002

Time- and frequency-resolved IR-laser absorption methods are used to probe state-resolved collisional energy transfer in open-shell collisions of Cl(2P3/2) radicals with HCl(J) in the near single-collision regime. Translationally ‘‘hot’’ Cl(2P3/2) radicals are formed by excimer laser photolysis of Cl2, which then collide with a room-temperature distribution of HCl peaked at J≂3. Final-state distributions of the HCl are monitored via transient absorption detection of a cw IR probe laser by the collisionally populated states (J=4, 5, 6,...,12). In previous work [J. Chem. Phys. 102, 7046 (1995)], these transient signals are used to extract absolute integral collisional cross sections for state-resolved rotational energy transfer into final J states. In the present study, high-resolution IR Dopplerimetry with the single-mode probe laser is used to measure translational distributions of the collisionally populated HCl(J) as a function of final J state. Analysis of these translational distributions leads to state-resolved differential scattering cross sections for rotational energy transfer, which exhibit a strong propensity for forward scattering into all levels observed (J≤12). These results are compared with quasiclassical trajectory calculations (QCT) on a recently modified potential energy surface of Schatz and Gordon. The theoretical analysis is in good agreement with experiment, with the angular distribution dominated by forward scattering for most of the final HCl rotational states. However, for the very highest J states collisionally populated, the QCT calculations predict a shift from predominantly forward to more isotropic scattering that is not evident in the experimental results.
A perturbation theory study of adlayer CO on NaCl(100)
Authors: Meredith Andrew W. , Stone Anthony J.
Published: July 2002

We have constructed a potential energy function to describe the interaction of CO molecules with each other and with the (100) surface of NaCl. It uses distributed multipoles to describe the electrostatic interactions, distributed polarizabilities to describe induction, anisotropic dispersion coefficients taken from work by Rijks and Wormer, and anisotropic atom–atom repulsion terms derived from ab initio intermolecular perturbation theory. We have also investigated the structure and charge distribution of the NaCl surface. We find that an isolated CO molecule on the NaCl(100) surface adopts a position with the CO axis perpendicular to the surface and the C atom over a surface Na+ ion, but in monolayers of CO the molecules are tilted by about 28°. There are two 1×1 and two 2×1 structures, all with very similar energies; a further 1×1 structure with the molecules perpendicular to the surface is an index-2 stationary point with energy only 32 cm−1 above the lowest-energy structure. The results are consistent with the experimental infrared spectra and with the measured enthalpy of adsorption.
Reaction dynamics of the four-centered elimination CH2OH+→CHO++H2: Measurement of kinetic energy release distribution and classical trajectory calculation
Authors: Lee Tae Geol , Park Seung C. , Kim Myung Soo
Published: July 2002

Mass-analyzed ion kinetic energy (MIKE) spectrum of CHO+ generated in the unimolecular dissociation of CH2OH+ was measured. Kinetic energy release distribution (KERD) was evaluated by analyzing the spectrum according to the algorithm developed previously. The average kinetic energy release evaluated from the distribution was extraordinarily large, 1.63 eV, corresponding to 75% of the reverse barrier of the reaction. A global analytical potential energy surface was constructed such that the experimental energetics was represented and that various features in the ab initio potential energy surface were closely reproduced. Classical trajectory calculation was carried out with the global analytical potential energy surface to investigate the causes for the extraordinarily large kinetic energy release. Based on the detailed dynamical calculations, it was found that the strained bending forces at the transition state and strengthening of the CO bond from double to triple bond character were mainly responsible for such a significant kinetic energy release. In addition, the dissociation products H2 and CHO+ ion were found to be rotationally excited in the trajectory calculations. This was attributed to the asymmetry of the transition state and the release of asymmetric bending forces. Also, the bending vibrational modes of CHO+ and the H2 stretching mode, which are coupled with the bending coordinates, were found to be moderately excited.
Computation of electron affinities of O and F atoms, and energy profile of F–H2 reaction by density functional theory and ab initio methods
Authors: Jursic Branko S.
Published: July 2002

The validity of hybrid and nonlocal DFT methods are tested on examples of systems which are difficult to model by way of quantum chemistry techniques. The electron affinities for the oxygen and fluorine atoms were calculated. The exothermicity, the barrier for the fluorine atom reaction with the hydrogen molecule, and the energy of the H–F bond and its distance were computed with DFT methods, as well as, with ROHF, MPn, and QCISD(T) ab initio methods. The computations were performed by using various basis sets, with 6-311++G(3df,3pd) as the largest. The obtained results are compared with the experimental values. The results of the Becke3LYP hybrid method is in qualitative agreement with experimental results and in the majority of the cases reassembles the high cost QCISD(T) calculation results. Considering the modest computational cost for DFT methods, Becke3LYP/6-311+G(2d,2p) is suggested as the standard theory model for computation, and Becke3LYP/6-311++G(3df,3pd) as the model for generating highly accurate results. They should be applicable to relatively sizable chemical systems.
The 1550–1460 Å region of CS2
Authors: Baker Jacob , Couris Stelios
Published: July 2002

The 1550–1460 Å region of CS2 has been investigated by both (1+1′)+1 and (3+1) resonance enhanced multiphoton ionization and the spectra obtained compared to previous one photon and electron impact studies. The results of this study indicate that the main feature in this region is due to an optically allowed transition to either a 1Πu valence state or a 1Σ+u Rydberg-valence type state and that this state is strongly predissociated. There is also evidence of gerade valence states in this region but there is no evidence of gerade Rydberg states.
Theoretical study of the intramolecular isotope effect in the reaction of F+HD
Authors: Song Ju-Beom , Gislason Eric A.
Published: July 2002

Quasiclassical trajectory calculations on the Muckerman V potential energy surface were carried out for the reaction F+HD(υ=0, J)→FH+D or FD+H. The reactive cross sections for the two product channels as well as the isotope effect were calculated for J=0 over the relative collision energy range 0.8–900 kcal/mol and also for 0≤J≤10 at a collision energy of 2 kcal/mol. The results are compared to those for F+H2→FH+H. The goal of this work was to understand the remarkable isotope effect behavior in both sets of calculations. It proved useful to decompose the reactive cross sections QR(FH) and QR(FD) into the product of three factors; the cross sections for F hitting the H end and the D end of HD, Qhit(FHD) and Qhit(FDH); the probabilities of reacting to give either FH or FD once the H end or D end is hit, PR(FHD) and PR(FDH); and the corrections for knockout collisions at both ends of the molecule, CKO(FHD) and CKO(FDH). (A knockout collision is one where the F atom initially hits the H end but reacts to form FD or vice versa.) All three factors make important contributions to the isotope effect. At low collision energies for J=0 FD is favored over FH because F hits the D end of HD more often and because the overall reactivity is higher at the D end. Both effects become less important at higher energies, but above 30 kcal/mol knockout reactions strongly favor the production of FD. The isotope effect as a function of J is quite remarkable. Although FD is favored for J=0 at E=2 kcal/mol, by J=3 no trajectories hit the D end, and no DF is formed. But at J=7 DF product reappears, coinciding with the onset of knockout reactions and an increase in reactivity at both ends of HD. We attribute all of these effects to the ability of the system at high J to rotate through the barrier separating the reactant valleys at each end of HD. The results obtained in this paper are explained by reference to special features of the potential energy surface and to the displacement of the HD center of mass towards the D atom.
Kinetic energy matrices in a basis of equidensity orbitals
Authors: Harriman John E.
Published: July 2002

The matrix of the kinetic energy operator can be divided into two components, one of which is equivalent to the matrix of a function so that it is effectively local. This decomposition is basis-set dependent and is particularly simple when equidensity orbitals are used. It is shown that for a one-dimensional problem the norm of the local component is √5/3 times the norm of the whole kinetic energy matrix, independent of the density used to define the orbitals. In the three-dimensional case this ratio depends on the density but reasonably simple expressions are obtained.
Rotational diffusion and internal motions of circular DNA. I. Polarized photon correlation spectroscopy
Authors: Chirico Giuseppe , Baldini Giancarlo
Published: July 2002

Dilute solutions of DNA plasmid pUC18 (2687 base pairs) have been studied by polarized photon correlation spectroscopy. The autocorrelation functions of the scattered light have been measured and fitted to a sum of exponentials. When a two exponential fit of the autocorrelation functions is performed, the slower relaxation rate corresponds to the expected translational diffusion coefficient of the plasmid while the faster one shows a linear behavior below K2≂5×1014 m−2, being superlinear at larger K2 values. The translational diffusion coefficient at T=25° is Dt=5.4±0.25×10−12 m2/s while the extrapolation of the fast relaxation rate to K2=0 yields the tumbling rotational coefficient R⊥=1.5±0.2 kHz. Monte Carlo simulations of the plasmid conformation have been performed with different sets of dynamical parameters (bending and torsional rigidities, Debye length), and the hydrodynamic coefficients have been computed according to known algorithms. The translational diffusion coefficient determined experimentally shows good agreement with the results of Monte Carlo simulations, while the simulated rotational tumbling coefficient is slightly larger than that found experimentally.
An amorphous monolayer: Infrared spectroscopic and theoretical studies of SO2 on NaCl (100)
Authors: Berg Otto , Ewing George E. , Meredith Andrew W. , Stone Anthony J.
Published: July 2002

At temperatures between 100 and 110 K, exposing the (100) face of NaCl to unsaturated SO2 gas yields a stable adlayer. Infrared spectra of adsorbed SO2 contain complex resonances near the origins of the molecular symmetric and asymmetric stretching vibrations. On photometric grounds the absolute coverage of the surface is found to be one molecule per exposed Na+Cl− ion pair. The spectra of this monolayer consist of several sharp lines overlapping one broad feature for each molecular mode. By comparison to vibrational excitons in simpler systems, the coexistence of crystalline and amorphous adlayer structures is strongly indicated. This partial ordering is pressure dependent, and develops spontaneously on a time scale of minutes. The disordered component, in contrast, is never in equilibrium with the gas phase. Computational simulations have detailed the microscopic basis of this behavior. Accurate ab initio models of the SO2 molecule and NaCl(100) surface were used in a Monte Carlo simulation of the experimental conditions. At both half and full coverage, an amorphous two-dimensional condensate developed. This is minimally consistent with the polarized infrared photometry. Seemingly equilibrated Monte Carlo runs retained some memory of the initial molecular configuration, again consistent with hysteresis observed in the spectroscopic experiments. No structural order developed in the simulations, but the energetic state of affairs was clarified: the potential driving adsorption and condensation is deep in comparison to available thermal energy, but relatively insensitive to molecular orientation.
Application of the quasi-Gaussian entropy theory to the calculation of thermodynamic properties of water and methane in the liquid and gas phase
Authors: Apol M. E. F. , Amadei A. , Berendsen H. J. C.
Published: July 2002

In this article we investigate the applicability of the statistical Gamma state as following from the quasi-Gaussian entropy theory, where all thermodynamic properties at every temperature are obtained from the knowledge of the potential energy distribution at one temperature. We compared for a typically polar system (water) and an apolar one (methane) the experimental heat capacity and entropy data with the predictions of the theory at various densities, ranging from the almost ideal gas to typical liquids. Interestingly, the behavior of water and methane is quite similar. Low-density gases and fluid-liquid systems can be described as weakly perturbed Gamma states. For intermediate densities a more complex statistical state arises. In order to describe more accurately the fluid-liquid regime, we propose in this paper a confined Gamma state, based on the division of phase-space into two different regions: one of them described by an exact Gamma state and another very unstable one. We conclude that typical fluid-liquids can be described very well by this new Gamma state approximation. We also try to give a physical interpretation of the two parts of phase space that arise from the model. The high accuracy of the theory over a large temperature range makes the approach very suitable for the prediction of thermodynamical properties at, for example, supercritical conditions.
Energetics of charge transfer reactions in solvents of dipolar and higher order multipolar character. I. Theory
Authors: Perng Baw-Ching , Newton Marshall D. , Raineri Fernando O. , Friedman Harold L.
Published: July 2002

We extend a recent molecular theory of solvation dynamics to accommodate static solvent effects on the energetics of charge transfer (CT) processes. Our theory is based on a simple renormalized linear response development which incorporates nonlinear aspects of equilibrium solvation. It can accommodate polarizable solvent molecules as well as the limiting case represented by electronically rigid interaction site model (ISM) solvent molecules. We focus on the diabatic free energy profiles governing CT processes in solute donor–acceptor systems of chemical interest. By studying CT in ISM solution models we naturally cover both the short range and long range solute-solvent interactions, thereby enabling applications to CT in solvents of higher multipolar as well as dipolar character. We derive expressions for the key energetic parameters of a CT process; the solvent reorganization energy, the solvent contribution to the change in thermodynamic free energy, and the optical absorption and fluorescence frequencies.
Quantum phase space theory for the calculation of v⋅j vector correlations
Authors: North Simon W. , Hall Gregory E.
Published: July 2002

The quantum state-counting phase space theory commonly used to describe ‘‘barrierless’’ dissociation is recast in a helicity basis to calculate photofragment v⋅j correlations. Counting pairs of fragment states with specific angular momentum projection numbers on the relative velocity provides a simple connection between angular momentum conservation and the v⋅j correlation, which is not so evident in the conventional basis for phase space state counts. The upper bound on the orbital angular momentum, l, imposed by the centrifugal barrier cannot be included simply in the helicity basis, where l is not a good quantum number. Two approaches for an exact calculation of the v⋅j correlation including the centrifugal barrier are described to address this point, although the simpler helicity state count with no centrifugal barrier correction is remarkably good in many cases. An application to the photodissociation of NCCN is consistent with recent classical phase space calculations of Klippenstein and Cline. The experimentally observed vector correlation exceeds the phase space theory prediction. We take this as evidence of incomplete mixing of the K states of the linear parent molecule at the transition state, corresponding to an evolution of the body-fixed projection number K into the total helicity of the fragment pair state. The average over a thermal distribution of parent angular momentum in the special case of a linear molecule does not significantly reduce the v⋅j correlation below that computed for total J=0. Predictions of the v⋅j correlations for the unimolecular dissociation of NCNO and CH2CO are also provided.
A molecular theory of frequency and wave-vector-dependent dynamic response functions of electrolyte solutions
Authors: Chandra Amalendu , Jana Debasish , Bhattacharjee Subir
Published: July 2002

A molecular theory of frequency and wave-vector-dependent dynamic response functions of electrolyte solutions is presented. The theory is based on linear response theory and a set of molecular hydrodynamic equations. The theory properly includes the static and dynamic interionic correlations through ionic structure factors and van Hove functions. Analytical expressions for frequency and wave-vector-dependent charge, ion moment, and ion current susceptibilities are derived and evaluated. It is found that the dynamic response at finite wave vectors can be very different from that at zero wave vector (or at long wavelength). An important application of the present theory is also discussed where we have studied the dynamics of ion atmosphere relaxation around a newly created solute charge.
The sulfur reaction in small ionized carbonyl sulfide clusters
Authors: Frazão J. M. A. , Lourenço J. M. C. , Áurea Cunha M. , Laranjeira M. F. , Los J. , Moutinho A. M. C.
Published: July 2002

Ionic species obtained by electron impact on (OCS)n clusters produced by expansion of OCS mixed with argon at two different (OCS/argon) concentrations are studied as a function of stagnation pressure and electron energy. Ionization efficiency curves of homogeneous cluster ions (OCS)+n, homogeneous sulfur cluster ions S+m and inhomogeneous ions (OCS)nS+m are analyzed. Several reactions paths, ending in S+m, have been identified. The strong correlation between reaction path and cluster size is discussed. Finally, by minimization of the potential function created by dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions between OCS molecules, we propose the geometrical configurations of neutral (OCS)n (n=2–5) clusters.
The effect of the nature of the interaction potential on cluster reaction rates
Authors: Venkatesh R. , Marlow W. H. , Lucchese R. R. , Schulte J.
Published: July 2002

The effect of two different interaction potentials, a two-body and a many-body potential, on thermal cluster reaction rates was studied for 2–13 atom nickel clusters using the classical trajectory method. The reaction rates were computed for cluster–monomer and cluster–cluster collisions at T=1200 K, using the bulk and dimer parametrized Lennard-Jones (LJ) potentials and were compared with the rates previously obtained for these collisional events by using a more realistic many-body tight-binding second moment approximation (TB-SMA) potential. For cluster–monomer collisions, close agreement exists between the reaction cross section results for dimer fitted LJ (LJD) potential and TB-SMA potential suggesting that the cluster–monomer collisions may be dominated by pairwise interactions. The bulk fitted LJ potential (LJB) underestimates the sticking cross section results of the other two potentials for most cluster sizes. This discrepancy however appears to be due to the relatively smaller cluster binding energies obtained for this potential as a result of which a larger cross section for dissociation is observed. For cluster–cluster collisions, for most cluster sizes, no agreement exists between the reaction cross section results for the three potentials. The discrepancy between the cross section results for the LJ potentials and the TB-SMA potential appears to lie in the difference in the scaling of cluster energy with cluster coordination for these two types of potentials (i.e., linear for LJ vs square root dependence for TB-SMA). Some characteristics of the cross section results of both LJB and LJD potentials correlate with the relative cluster stability pattern for the LJ clusters. For TB-SMA case, no such correlation exists, which however is consistent with the smooth and featureless size distributions observed experimentally for nickel and other transition metals. The cut-off used in the TB-SMA potential appears to lead to a significant underestimation of the total reaction cross section for N=13, in the case of the cluster–cluster collisions. The results of this study indicate that the rate calculations may be sensitive to both the nature and parametrization of the simulation potential depending on the temperature range considered and cluster growth process simulated.
Up–down–up–... multiphoton excitation of two-level systems
Authors: van Opbergen M. , Dam N. , Linskens A. F. , Reuss J. , Sartakov B.
Published: July 2002

Multiphoton up–down–up–...(UDU) excitation in a two-level system was realized under adiabatic conditions. Spectra of C2H4 excitation were investigated and UDU peaks were observed. The dependence of peak amplitudes on the powers and the frequencies of exciting fields were investigated experimentally as well as by computer simulation based on the rotating wave approximation (RWA). Agreement between experimental and theoretical results was observed.
Quantum beat study of the nuclear hyperfine structure of OD and Ar⋅OD in their A 2Σ+ electronic states
Authors: Carter Robert T. , Povey Ian M. , Bitto H. , Huber J. Robert
Published: July 2002

The nuclear hyperfine structure of OD and Ar⋅OD in their A 2Σ+ electronic states has been studied by quantum beat spectroscopy. The very cold transient species were produced in a supersonic expansion using a pulsed discharge nozzle. Coherent excitation of hyperfine (hf) states, arising from one fine structure (OD) or rotational (Ar⋅OD) level, created quantum beats on the fluorescence decay. The beat frequencies, which correspond to energy separations between hf levels, could be measured to ±75 kHz. The splitting of the hf levels into their Zeeman components was investigated in a weak magnetic field. A fit of the zero field and Zeeman data yielded the relevant constants for the nuclear magnetic and electric quadrupole hyperfine interactions as well as the pertinent g-factors in each species. In the case of OD, the hf parameters agree well with those reported previously but are more accurately defined. For Ar⋅OD the previously unknown hyperfine and spin-rotation parameters of the A 2Σ+ state were determined. A comparison of the hf parameters in the two systems allowed assessment of the effect of van der Waals complex formation on the electron distribution. Thus complexation is found to reduce the unpaired electron density on the deuteron by 7% which is indicative of significant chemical bonding between the Ar atom and the OD moiety in the A 2Σ+ state of Ar⋅OD. For both systems, the g-factors gS and gl obtained suggest an admixture of other, possibly quartet, electronic states into the A 2Σ+ state.
A comparative study of CO2–Ar potential surfaces
Authors: ter Horst Marc A. , Jameson Cynthia J.
Published: July 2002

Twelve potential energy surfaces that have been proposed for the CO2–Ar interaction have been considered in detail. The anisotropies of these surfaces are compared and their ability to predict the interaction second virial coefficient as a function of temperature has been examined. Intermolecular bending and stretching quadratic force constants predicted by each and the mean square torque calculated for each are compared with the experimental values. Quantum diffusion Monte Carlo simulations provide the average rotational constants and geometry for the ground vibrational state as well as the dissociation energy in each case. These are compared with the experimental values. Classical trajectory calculations were carried out to obtain 45 types of thermal average cross sections for six of these surfaces. Various thermophysical properties such as mixture viscosity, mixture thermal conductivity, and diffusion coefficient, calculated from these cross sections and the NMR relaxation cross sections, are compared with experimental data. It is found that the spectroscopic constants define the depth and shape of the well at the global minimum, whereas the NMR cross sections and mean square torque probe the anisotropy in a broader sense. The thermophysical properties (viscosity, diffusion coefficient, and thermal conductivity) are not strongly discriminating between the surfaces, whereas the temperature dependence of the second virial coefficient detects the weaknesses in the low and upper repulsive walls of those surfaces that were modified specifically to improve greatly the shape of the well so as to reproduce the spectroscopic constants.
The potential energy surface of He–HCN determined by fitting to high-resolution spectroscopic data
Authors: Atkins Keith M. , Hutson Jeremy M.
Published: July 2002

Two potential energy surfaces for He–HCN are determined by least-squares fitting of parameterised functional forms to data from high-resolution microwave and millimeter-wave spectroscopy [Drucker et al., J. Phys. Chem. 99, 2646 (1995)]. The two potentials both have significantly deeper wells than suggested by the ab initio supermolecule calculations of Drucker et al. Both potentials have linear or near-linear equilibrium geometries, He–H–C–N, but the shapes of the well depth functions away from the linear geometry are significantly different. The existing experimental data are thus not sufficient to probe this potential feature in detail. Predictions of spectroscopic properties that would allow the new potentials to be tested and refined are given.
Cage exit probability versus excess energy in the photodissociation of matrix-isolated HCl
Authors: Gödderz K. H. , Schwentner N. , Chergui M.
Published: July 2002

Dissociation efficiencies for excitation of the repulsive A 1Π state of HCl were recorded in Xe, Kr, and Ar matrices for photon energies between 5 and 10 eV from the content of dissociation products and quantum efficiencies were derived with the absorption spectra. Influence of temperature and preparation conditions was investigated. The quantum efficiency rises monotonically in Xe from an excess energy of 1.4 eV above the gas phase dissociation energy on, saturates around 2.4 eV and remains then essentially constant up to 4 eV. In Ar and Kr, it saturates around 2 eV and in Ar an absolute efficiency of about 0.18 is determined at 3.7 eV. Results of molecular dynamics calculations and a statistical model agree qualitatively but the observed saturation at low excess energies is not well described and the absence of a temperature effect in Ar needs further consideration.
The coalescence range of the α and β processes in the glass-forming liquid bis-phenol-C-dimethylether (BCDE)
Authors: Alvarez F. , Hoffman A. , Alegría A. , Colmenero J.
Published: July 2002

In this paper, we introduce a method of analysis of the dielectric relaxation data, which results to be very suitable for studying the coalescence of the α and β processes in the time domain. We show that, at least in the case of the glass-forming liquid BCDE (bis-phenol-C-dimethylether), the coalescence process can be well described by assuming that both, the α and β processes, behave as statistically independent processes. This simple assumption allows to describe the coalescence range of the two processes by extrapolating the low temperature behavior (where both processes are well separated), contrary to more classical approaches in which one needs to introduce a temperature at which the mechanism of the β process changes.
Electron spin resonance matrix isolation and ab initio theoretical investigations of 69,71GaH2, 69,71GaD2, H69,71GaCH3, and D69,71GaCD3
Authors: Knight Lon B. , Banisaukas John J. , Babb Robert , Davidson Ernest R.
Published: July 2002

First time electron spin resonance studies are reported for various isotopomers of GaH2 and HGaCH3. The radicals were generated in neon matrices at 4 K by the ultraviolet photoexcitation of Ga which undergoes insertion reactions with H2 and CH4. Ab initio calculations with a large uncontracted basis and configuration interaction with all single excitations from the spin-restricted Hartree–Fock configuration gave good agreement with the experimental results and supported the free atom comparison method interpretation of the hyperfine interactions. A comparison with similar radicals is presented, including BH2, AlH2, HAlCH3, HAlOH, and AlH+.
Quantum activated rates—an evolution operator approach
Authors: Schwartz Steven D.
Published: July 2002

This article presents a derivation of the rate of reaction in the quantum activated rate problem. In this problem, one studies the rate of a chemical reaction when the reaction is placed in a dissipative bath. Our derivation defines the rate in terms of the flux autocorrelation function and proceeds via the recently developed interaction representation for nonadiabatic corrections to adiabatic evolution operators. This methodology is an infinite order resummation of nonadiabatic corrections to evolution operators. The approach produces an analytic expression which yields accurate results over a range of temperatures, viscosities and system parameters through the Kramers turnover region.
Nonequilibrium phenomena in spectral diffusion physics of organic glasses
Authors: Fritsch K. , Friedrich J. , Kharlamov B. M.
Published: July 2002

We performed two different types of spectral diffusion experiments on persistent spectral holes. In all cases, we measured the holewidth as a function of time. The two experiments differed in their initial conditions: In the type 1 experiment (the ‘‘aging experiment’’), the sample was cooled from room temperature to the final temperatures which were 100 and 800 mK, respectively. Holes were burnt at various time intervals after the final temperature was reached. In the type 2 experiment (the ‘‘cycling experiment’’), the sample was allowed to relax for a period of about 10 days. Then, a hole was burnt and subjected to a temperature cycle. In all cases, the time dependence of the holewidths was strongly nonlogarithmic. The temperature cycled hole showed a narrowing regime which prevailed for the whole observation period of roughly one week. We will show that the deviation from the logarithmic time dependences is a nonequilibrium phenomenon. All features observed could be modelled within the standard tunneling model.
Towards the atomistic simulation of phase coexistence in nonequilibrium systems
Authors: Baranyai András , Cummings Peter T.
Published: July 2002

A theoretical approach is presented which represents the first attempt known to the authors to develop molecular dynamics algorithms capable of modeling phase coexistence between two nonequilibrium steady state phases confined in a closed (E,V,N) system. We deal exclusively with shearing liquids because of their importance in rheology. In the present paper, as in the equilibrium Gibbs ensemble Monte Carlo technique for systems at equilibrium, the coexisting phases have no physical contact but their dynamics are coupled in order to reach mechanical, thermal, and composition balance between bulk regions of the two phases. The thermal balance is maintained by requiring zero net heat flow across a hypothetical boundary. This can be achieved by starting from equilibrium and gradually increasing the strength of the external field (the shear rate) in a quasistatic process. For particle interchanges we invoke the Evans–Baranyai variational principle which is at the very least a good approximation for similar simulated steady state systems far from equilibrium. Results of several model calculations are presented. The limitations and the implications of the methods are discussed.