bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab
initio polarizability study of polypyrrole
Authors: Toto Joseph L. , Toto Teressa Tangredi , de Melo Celso P. , Robins K. A.
Published: July 2002

The longitudinal polarizability, αzz, and second order hyperpolarizability, γzzzz, of the (C4H3N)RH2 oligopyrroles, where R=1−9, are determined at the Hartree–Fock (HF) level of theory using a 6-31G basis set. The effect of electron correlation is taken into account using Moller–Plesset perturbation theory through second order for the R=1−5 oligopyrroles. Methods for extrapolation to the infinite polymer values are explored. The convergence behavior of the correlated results as a ratio with the corresponding HF values may indicate that a scaled HF procedure could be appropriate for finding correlated estimates to the infinite polymer property values. We also propose a generalized finite field recipe which can easily be used in conjunction with popular black-box electronic structure programs for determining molecular polarizabilities through second order. The αzz and γzzzz values obtained from using this procedure on a variety of polyacetylene and polypyrrole oligomers are in excellent agreement with those derived through analytical procedures.
The gas, liquid, and solid phases of dimerizing hard spheres and hard-sphere dumbbells
Authors: Sear Richard P. , Jackson George
Published: July 2002

The complete phase diagram of a model associating molecule is determined, including the gas, liquid, and solid phases, the regions of coexistence between these three phases, and the location of the critical and triple points. The model molecule is a hard sphere with two very different attractive interactions, one a short ranged and directional attraction and the other a mean field. The first interaction only forms dimers as a molecule can only interact in this way with one other molecule. This saturable attraction mimics hydrogen and chemical bonding. The second interaction is an approximation for the dispersion forces between molecules. Thermodynamic functions for the liquid and gas phases of this model molecule are obtained from an existing theory for associating fluids but a new theory is developed for the solid phase. This is believed to be the first microscopic theory of a model associating molecule in the solid phase. In the low temperature limit no monomers are present; the system is then a fluid or solid of hard-sphere dumbbells. Simulation data are available in this limit and it is shown that in both the fluid and solid phases the theoretical predictions are close to those of simulation. The pressure equation of state for dumbbells is the most accurate theory available for the solid phase. An approximation for the free energy of a solid mixture of spheres and dumbbells is also presented.
One-electron properties of several small molecules calculated using the local density approximation within density functional theory
Authors: Duffy Patrick , Chong Delano P. , Dupuis Michel
Published: July 2002

Density functional theory (DFT) is a field enjoying a tremendous recent surge in popularity among theoretical and practical chemists alike because of its ability to more easily handle larger molecular systems than conventional ab initio methods. Until recently, however, assessment of the quality of the properties predicted (and therefore the charge density) from DFT had been limited mainly to dipole moments and their nuclear coordinate and electric field derivatives. This paper presents the calculated results for some of the one-electron properties of the eight small molecules (NH3, PH3, H2O, H2S, HF, HCl, CO, and N2). The properties chosen weight different regions of the charge density, from either very close in or at the nucleus (e.g., δ, the electron density at the nucleus) to regions farther out from the nucleus (e.g., the diamagnetic susceptibility 〈r2〉). It is found that properties which depend on an accurate knowledge of the electron density near to the nucleus are predicted poorly by the local density approximation (LDA), while others more dependent on the charge density farther out from the nucleus are predicted much more accurately, possibly due to cancellation of errors. Use of the LDA is therefore not recommended for ‘‘tight’’ properties; use of a functional employing gradient corrections would be more suitable for this purpose.
Test of a simple model of the intermolecular potential of C60 on the series of KnC60, 0<n≤6, crystals
Authors: Gamba Z. , Martinez G.
Published: July 2002

We perform a further test of a simple atom–atom model of the intermolecular potential of C60 by applying it to the alkali-metal doped crystals KnC60, 0<n≤6. The model consists in an icosahedron of 12 Lennard-Jones interaction sites and no further refinement of its LJ parameters is performed. The electrostatic interactions due to the charges transferred by K atoms are taken into account. Several statical and dynamical properties, at room temperature and zero pressure, are calculated by constant-pressure molecular dynamics methods. Comparison with available experimental data shows that the icosahedron model plus an accumulative charge at each interaction site, gives good results for n≤4, but further improvement is needed for n=6.
First row benchmark tests of the parametrized configuration interaction with parameter X (PCI-X) scheme
Authors: Siegbahn Per E. M. , Svensson Mats , Boussard Per J. E.
Published: July 2002

A recently suggested scheme termed parametrized configuration interaction with parameter X (PCI-X) for scaling the correlation energy has been applied on a benchmark test consisting of 32 first row molecules. Several different methods like Mo/ller–Plesset second-order perturbation theory (MP2), modified coupled pair functional method (MCPF), averaged coupled pair functional method (ACPF), coupled cluster singles and doubles (CCSD), and CCSD with a perturbational estimate of triple excitations [CCSD(T)] have been tested using systematically chosen basis sets ranging from double zeta (DZ) to very large atomic natural orbital (ANO) sets containing several sets of d and f functions. For each method and basis set the scaling parameter is optimized. The scaling does in all cases lead to large, sometimes dramatic, improvements of the results. Typically, using a single reference state method like MCPF the average absolute deviation compared to experiments for the benchmark goes from an unscaled value of about 20 kcal/mol down to about 2 kcal/mol. For MCPF and similar methods no improvement of the results is obtained going beyond the DZ+polarization (DZP) level. Significant improvements due to scaling occurs even at the highest level using the CCSD(T) method and the largest basis set. For medium size basis sets the present scaling is far superior to the extrapolation schemes used in the Gaussian-1 and -2 (G1 and G2) theories.
Quantum dynamics of large polyatomic systems using a classically based separable potential method
Authors: Jungwirth Pavel , Gerber R. Benny
Published: July 2002

A new method for approximate solution of the time-dependent vibrational Schrödinger equation, applicable to extended molecular systems, is presented. The new method is essentially an approximate time-dependent quantization of classical dynamics. A molecular dynamics simulation is used to obtain a separable, effective time-dependent potential for each mode, that implicitly includes also the effects of all the other modes on this degree of freedom. A time-dependent wave packet is then propagated separately for each mode, using the corresponding effective potential. The new approximation is valid for short time scale processes only, but it is easily applicable to large realistic systems. Test calculations against exact quantum and time-dependent self-consistent field (TDSCF) results are carried out for two examples; photodissociation of HI in the collinear Xe...HI cluster, and electron photodetachment from the collinear Ar...I−...Ar cluster. For illustration, the new scheme is also applied to photodetachment from large linear clusters Arn...I−...Arn (n=2–8) and the results are discussed. For the test systems, the results of the new method are virtually identical to those following from the computationally much more demanding TDSCF approach, and they are in excellent agreement with the exact results.
Accurate multireference configuration interaction calculations on the lowest 1Σ+ and 3Π electronic states of C2, CN+, BN, and BO+
Authors: Peterson Kirk A.
Published: July 2002

Using a series of correlation consistent basis sets from double to quintuple zeta in conjunction with large internally contracted multireference configuration interaction (CMRCI) wave functions, potential energy functions have been computed for the X 1Σ+g and a 3Πu states of C2 and the 1 1Σ+ and 1 3Π states of CN+, BN, and BO+. By exploiting the regular convergence behavior of the correlation consistent basis sets, complete basis set limits have been estimated that led to accurate predictions for the electronic excitation energies, dissociation energies, equilibrium bond lengths, and harmonic vibrational frequencies. The 1 1Σ+ states of CN+ and BO+ are predicted to be the electronic ground states of these species with predicted equilibrium excitation energies (Te) to the low-lying a 3Π states of 880±100 cm−1 and 5000±200 cm−1, respectively. A 3Π ground state of BN is predicted with an excitation energy to the low-lying a 1Σ+ state of just 190±100 cm−1. Identical calculations on the singlet–triplet splitting of C2 yielded a prediction of 778 cm−1 for Te, which was just 62 cm−1 above the experimental value. Accurate equilibrium bond lengths and fundamental frequencies are also predicted for BN, BO+, and the a 3Π state of CN+. Dipole moment functions have been computed by CMRCI for the ground and excited electronic states of the three heteronuclear diatomics, and these have been used to derive accurate microwave and infrared transition probabilities for these species. A dipole moment in v=0 of 5.42 D is calculated for the X 1Σ+ state of BO+, which should lead to an intense microwave spectrum. While the X 3Π ground state of BN is predicted to have a very weak infrared spectrum, this species should be observable in the microwave region since the predicted μ0 is 1.98 D. Both the microwave and infrared spectra of X 1Σ+ CN+ should be of moderate intensity.
Calculation of solvent free energies for heterogeneous electron transfer at the water–metal interface: Classical versus quantum behavior
Authors: Straus Jay B. , Calhoun August , Voth Gregory A.
Published: July 2002

A computer simulation method is developed for the study of the adiabatic heterogeneous electron transfer reactions between an ion in solution and a metal electrode. The particular system studied is the Fe2+/Fe3+ electron transfer reaction with a Pt(111) electrode. The adiabatic classical free energy curve for the reaction is computed using umbrella sampling and molecular dynamics generated by the adiabatic solution to an Anderson–Newns-like Hamiltonian. Reactive flux calculations are then performed to determine the effect of transition state recrossings on the classical adiabatic rate constant. These effects are not found to be large (κ∼0.6). The water solvent model is next quantized using Feynman path integral techniques and the quantum adiabatic free energy curve for electronic transfer is calculated. The latter calculation shows that the solvent activation free energy barrier and thermodynamic driving force for the electron transfer process can be significantly affected by the water quantization. These results suggest that classical models for water may not be adequate, or at least need to be modified, for the accurate computer simulation of many heterogeneous electron transfer reactions.
Analysis of friction kernels for n-butane isomerization in water by the generalized Langevin equation
Authors: Wan Shun Zhou , Xu Ying Wu , Wang Cun Xin , Shi Yun Yu
Published: July 2002

In this article, the isomerization reaction of n-butane is used to study the dynamic properties of n-butane in aqueous solution. It is assumed that the dihedral angle of n-butane obeys the generalized Langevin equation (GLE), in which an effective potential, a friction damping and a random torque are included. The random torque is calculated using constrained molecular dynamics (MD) simulations and the time-dependent friction kernel is determined by using the fluctuation-dissipation theorem. The results show that for the trans, gauche, and transition states of n-butane, the random torques approximately follow a Gaussian distribution. The friction kernels demonstrate a rapid initial decay and a slow sequential decay, while the half-widths of the Gaussian distributions and the initial values of the friction kernels are somewhat dependent on the conformations. The transmission coefficient for crossing the transition state is also analyzed within the framework of Kramers and Grote–Hynes theories. The result shows that the isomerization of n-butane in water is in the polarization caging regime.
Helium atom scattering study of the frustrated translation mode of CO adsorbed on the Cu(001) surface
Authors: Ellis J. , Toennies J. P. , Witte G.
Published: July 2002

The structure and vibrational modes of CO adsorbed on Cu(001) have been investigated by helium atom scattering (HAS). Below coverages of θ=0.13, CO adsorbs as isolated molecules, and the time of flight (TOF) spectra of helium atoms scattered from the surface are characterized by energy losses and gains of 3.94 ± 0.07 meV. With the aid of isotope shift measurements, this mode is unambiguously assigned to the frustrated translation mode of the adsorbed CO. In the c(2×2) structure (θ=0.5) this mode is found to show dispersion in both the 〈100〉 and 〈110〉 azimuths with frequencies ranging from 3.9 meV at the zone origin to 5.6 meV at the zone boundary. A simple force constant model with a single nearest neighbor force constant within the layer is found to be sufficient to describe the dispersion. For coverages slightly less than θ=0.5 the angular distributions of scattered helium atoms show features characteristic of vacancies in the overlayer. In the TOF spectra an additional nondispersive mode at about 4 meV is also seen and assigned to the frustrated translation of CO molecules adjacent to the vacancies in the c(2×2) overlayer. The Rayleigh mode is clearly observed in the c(2×2) structure but is found to decrease slightly in frequency while the longitudinal resonance, which is especially prominent on the clean surface, disappears completely. The results are interpreted with the aid of slab calculations.
Melting of n-butanol and n-pentanol monolayers adsorbed on graphite: Effect of molecular length on melting
Authors: Morishige K. , Sakamoto Y.
Published: July 2002

The structures and melting of n-butanol and n-pentanol monolayers adsorbed on graphite have been studied by x-ray diffraction. The structures of both monolayers change between partial and complete monolayers. The complete monolayer of butanol, as well as the partial monolayer of pentanol, take herringbone structures similar to those of butane and hexane monolayers and have zigzag chains of hydrogen bonds. Two-dimensional smectic liquid crystals are formed in melting processes of the complete monolayers of both molecules while the partial monolayers melt directly into liquids. In the mesophases their solid structures are essentially maintained. The results are compared with those of the monolayers of shorter linear-chain alcohol molecules and the effect of molecular length on the melting behavior is discussed.
Self-consistent integral-equation theory of chain-molecular liquids: Structure and thermodynamics
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

Self-consistent integral equations for the pair intramolecular and intermolecular correlation functions are derived from a general hierarchy of integral equations for chain-molecular liquids. These coupled equations are obtained by using superposition approximations for the triplet correlation functions, an approximate translational symmetry for the site–site intramolecular correlation functions and the equivalence of sites for intermolecular correlation functions. In addition to this self-consistent set of integral equations, the polymer reference interaction site model (PRISM) integral equation is also made self-consistent by coupling this intermolecular equation to the equations for the intramolecular correlation functions derived in the present theory. The intra- and intermolecular correlation functions of the self-consistent schemes considered in this work obey integral equations, and they are different from the other self-consistent schemes proposed in the literature. Self-consistent solutions for the structural properties, such as intra- and intermolecular correlation functions and structure factor, and macroscopic properties, such as chain expansion factor and thermodynamic functions of athermal polymer melts, are compared with available Monte Carlo results and other theories. For the properties examined, self-consistent solutions yield better results than the non-self-consistent calculations with ad hoc, ideal Gaussian inputs for the intramolecular correlation functions.
Orientational ordering and anisotropy in model polar clusters
Authors: Lu Dongsheng , Singer Sherwin J.
Published: July 2002

Model polar clusters of Stockmayer (Lennard-Jones + point dipole) particles exhibit spontaneous breaking of orientational symmetry in a wide range of temperatures. The form of the orientational anisotropy best accommodates the tendency of polar particles to form chains of dipoles in head-to-tail arrangement. It persists through temperatures characteristic of solid–liquid coexistence for dipole strength μ*2=μ2/εσ3=1, and to about twice the range of solid–liquid coexistence temperatures for μ*2=3. These studies extend earlier work [H. B. Lavender, K. A. Iyer, and S. J. Singer, J. Chem. Phys. 101, 7856 (1994)] in which orientational order parameters for clusters are defined and the global orientational order was observed for much smaller clusters. In this work we also study orientational order with respect to the cluster liquid–vapor interface and find significant disagreement with density functional theory predictions.
The vibrational spectra of molecular ions isolated in solid neon. XII. HCl+, (HCl)+2, ClHCl−, and O2⋅⋅HCl+
Authors: Forney Daniel , Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:HCl or a Ne:DCl sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the solid deposit includes the fundamental absorption of HCl+ or DCl+, which appears about 1% below the corresponding gas-phase band center. Another absorption, intermediate between the fundamentals of HCl and of HCl+, is contributed by an HCl-stretching fundamental of (HCl)+2. Among the important anion species present in the solid is ClHCl−, infrared absorptions of which are identified. Charge delocalization is sufficiently reduced in solid neon, compared to the heavier rare gases, that ion production from HCl does not occur at or below 10.2 eV. The electric field of the ions trapped in solid neon inhibits the rotation of HCl and leads to the appearance of a prominent HCl Q-branch absorption. In the presence of traces of oxygen, the O2⋅⋅HCl+ complex is stabilized, as evidenced by the appearance of the OO- and HCl-stretching absorptions of that species. The two O atoms are equivalent or nearly equivalent in the complex. It is suggested that photoexcitation of O2⋅⋅HCl+ leads to charge exchange, producing O+2 and HCl, with a threshold near 370 nm.
Nascent rovibrational distribution of CO(A 1Π) produced in the recombination of CO+2 with electrons
Authors: Tsuji Masaharu , Nakamura Masafumi , Nishimura Yukio , Obase Hiroshi
Published: July 2002

The dissociative electron–ion recombination processes of CO+2(X̃ 2Πg:0,0,0) has been studied by observing the CO(A 1Π–X 1Σ+) emission in the He and Ar afterglows. It was found that the CO(A:v′=0–2) states are formed in the dissociative recombination of CO+2(X̃:0,0,0) with electrons at thermal energy. The rovibrational distribution of CO(A) was N0:N1:N2=100:(T0=1000±100 K), 58±4(T1=700±50 K), and 9±2 (T2=400±100 K). The average fractions of total energy channeled into vibration and rotation of CO(A) and relative translation of the products were determined to be 〈fv〉=22%±2%, 〈fr〉=20%±2%, and 〈ft〉=58%±4%. The observed rovibrational distributions were in disagreement with statistical prior distributions, indicating that the reaction dynamics is not governed by the statistical theory. A comparison of the present results with the previous photodissociation data suggested that the CO(A:v′=0,1) states are formed through predissociation of near-resonant intermediate CO2** states coupled with a bent valence state, while the CO(A:v′=2) state is produced through predissociation of CO2** states just above the CO+2(X̃:0,0,0) state. The low CO(A:v′=2) population can be explained by the energetic constraint for thermal electrons plus CO+2(X̃:0,0,0) and/or a competition between predissociation and autoionization of CO2** states just above the CO+2(X̃:0,0,0) energy.
Diffraction mechanisms in gas-phase laser induced grating spectroscopy of vibrational overtone transitions
Authors: Booze Jon A. , Govoni Donald E. , Crim F. Fleming
Published: July 2002

We have investigated the grating formation mechanism in laser-induced grating spectroscopy by preparing transient gratings via excitation of O–H vibrational overtones in water vapor. In principle, our experiments are sensitive to three different mechanisms of diffraction: a density-based phase grating originating from local thermalization of the vibrational overtone state, a population-based phase grating caused by differences in ground and excited state polarizability volumes, and a population-based amplitude grating that appears when the probe laser is coincident with an electronic transition of the overtone-excited molecules. For water saturated air at 50–760 Torr, bulk acoustic responses originating from both collisional thermalization and from electrostriction dominate the diffraction efficiency. Measurement of the probe-wavelength dependence of the diffraction efficiency at pressures of 18 Torr and below shows that the dominant signal originates from a phase grating and that contributions from the amplitude grating are relatively unimportant at all pressures. Analysis of the temporal evolution of the diffraction efficiency suggests that the thermalization mechanism dominates at 18 Torr, despite there being only one hard-sphere collision on the time scale of the measurement. At 10 Torr, the temporal evolution of the diffraction signal begins to show evidence of a population based phase grating contribution as well. These conclusions are consistent with our measured thermalization rate constants for the vibrational overtone states and also with calculations of the effects of vibrational excitation on the molecular polarizability volume.
Geometric phase effects and wave packet dynamics on intersecting potential energy surfaces
Authors: Schön Jörg , Köppel Horst
Published: July 2002

The impact of the geometric phase on the time evolution of quantum-mechanical wave packets is studied theoretically. Two model systems of coupled electronic potential energy surfaces are compared. One of them, the well-known E×e Jahn–Teller system, comprises two conically intersecting surfaces, and the dynamics is subject to the geometric phase. The other system, describing the (E+A)×e Pseudo-Jahn–Teller effect, comprises three intersecting surfaces and the dynamics is not subject to a geometric phase. Apart from the geometric phase, the coupling to the upper surface is verified to be negligible for low-energy wave packet motion. Still, the geometric phase leads to a pronounced difference of low-energy wave packet dynamics in both systems. Most significant is the phenomenon of destructive self-interference of the two parts of the wave packet that encircle the conical intersection on opposite sides. The importance of the resulting different shape of the wave packet for a fs pump-probe spectrum is pointed out.
Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene
Authors: Lenzer Thomas , Luther Klaus , Troe Jürgen , Gilbert Robert G. , Lim Kieran F.
Published: July 2002

Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene (HFB) molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient ‘‘supercollisions’’ are also observed for all substrate–bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi-steady-state in rotational energy after only a few collisions.
Effects of chain topology on polymer dynamics: Configurational relaxation in polymer melts
Authors: Shaffer J. Scott
Published: July 2002

We report computer simulations of chain dynamics in monodisperse, linear polymer melts using a model which allows the dynamical effects of chain topology to be isolated from the individual effects of excluded volume and chain connectivity. The chain topology is controlled by either allowing or forbidding chain crossing events while maintaining the chain connectivity and excluded volume interactions between monomers. This chain crossing condition does not significantly alter the polymer structure or even the local, segment-level dynamics. For each topological condition, ‘‘crossing’’ and ‘‘noncrossing,’’ we decompose the polymer dynamics into a set of relaxation modes, calculate the autocorrelation functions for each mode, and obtain the complete distribution of internal relaxation times {τp}, where p labels the mode index. Several regimes of relaxation behavior are present, depending on the chain length N, the wavelength of the mode (N/p), and the chain topology. Noncrossing chains below a critical chain length, Nc, exhibit relaxation dynamics that are consistent with Rouse chains in the melt. Above Nc, noncrossing chains still exhibit Rouse-type dynamics in the short wavelength relaxation modes. For the modes of intermediate wavelength, however, the internal relaxation times scale with the mode wavelength according to τp∼(N/p)3 and the autocorrelation functions are strongly nonexponential. Relaxation behavior that is roughly consistent with the reptation model is finally seen for the first few relaxation modes of the noncrossing chains above Nc. In contrast to the noncrossing chains, the chains which are allowed to cross (in the presence of excluded volume and chain connectivity) exhibit Rouse-type relaxation dynamics for all long wavelength modes, even modes with characteristic wavelengths beyond the effective entanglement spacing of the noncrossing chains. Since we have controlled all factors other than chain topology which may in principle affect polymer dynamics, the results illustrate the role of chain topology in the transition from Rouse-type to reptationlike relaxation behavior in polymer melts.
The solvent spectral density and vibrational multimode approach to optical dephasing: Two-pulse photon echo response
Authors: Yang Tzyy-Schiuan , Vöhringer Peter , Arnett David C. , Scherer Norbert F.
Published: July 2002

A rigorous theoretical connection between the polarizability spectral density obtained from optical Kerr effect (OKE) measurement with the correlation function describing solvent-induced optical dephasing detected in photon echo measurements is given. The experimentally obtained spectral density has a more reasonable physical basis than model correlation function descriptions of solvent fluctuations. The experimental OKE spectrum is demonstrated to provide a natural description of solvent motions that modulate the chromophore electronic states in the case of weak induced-dipolar interactions. The chromophore optically active vibrational modes are obtained from pump-probe spectra and are employed in the calculation of echo signals. It is found that the fast decays of the two-pulse echo signals result from both solvent and solute intramolecular motions while the echo peak shifts are dominated by the solvent intermolecular modes.
Phase instabilities in charged hard-sphere mixtures. I. Binary mixtures of salt and hard spheres
Authors: Kenkare Paresh U. , Hall Carol K. , Caccamo C.
Published: July 2002

In this series of two papers we investigate phase instabilities in charged hard-sphere mixtures. Here, we derive general expressions for the thermodynamic properties of a common anion mixture and apply these to study phase separation in mixtures of salt and hard spheres. Excess thermodynamic properties due to Coulombic interactions are obtained using the analytical solutions for the mean spherical approximation closure. A detailed description of the dependence of the resulting phase diagrams on charge asymmetry of the ions, size of the neutral species, and the osmotic pressure of the mixture is presented. Binary mixtures of salt and hard spheres exhibit type III phase behavior. An increase in charge asymmetry results in an increase in the critical temperature of the mixture because enthalpic forces (ion-pairing) dominate. An increase in the size of the neutral species also results in an increase in the critical temperature of the mixture because of packing effects, which encourage phase separation. Potential applications of the model to experimental systems are discussed.
Study of HCO2 and DCO2 by negative ion photoelectron spectroscopy
Authors: Kim E. H. , Bradforth S. E. , Arnold D. W. , Metz R. B. , Neumark D. M.
Published: July 2002

Photoelectron spectra of HCO−2 and DCO−2 at 299 nm, 266 nm, and 213 nm are reported. Photodetachment accesses the 2A1, 2B2, and 2A2 states of the formlyoxyl radical, HCO2. The 2A1 state is assigned as the HCO2 ground state, although it is nearly degenerate with the 2B2 state (T0=0.027 eV), and the 2A2 state lies at T0=0.536 eV. The electron affinity of HCO2 is 3.498±0.015 eV. The spectra show partially resolved vibrational features, primarily involving progressions in the CO2 bending mode. The irregular appearance of the spectra in some regions suggests vibronic coupling between the 2A1 and 2B2 states. The possible role of the HCO2 radical as an intermediate in the OH+CO→H+CO2 reaction and in H+CO2 inelastic scattering is discussed.
Coherent ion dip spectroscopy of the ground state benzene–Ar complex: Vibration–rotation levels up to 130 cm−1 intermolecular energy
Authors: Sussmann R. , Neuhauser R. , Neusser H. J.
Published: July 2002

Coherent ion dip spectroscopy (CIS) provides high sensitivity and high resolution for the investigation of vibrational overtones in molecular electronic ground states. For a special time sequence of two coherent narrow-band Fourier transform limited nanosecond UV light pulses, with a modest delay of the pump pulse of 6.4 ns, a complete blocking of the population transfer to the upper state is achieved in the lambda-type three-level system ion dip experiment. This leads to ion dips with a depth as large as 95% and each dip represents an individual rovibronic transition. In this work, CIS is applied for the first time to a weakly bound van der Waals complex, benzene–Ar. We are able to observe six new van der Waals vibrational states up to an excess energy of 130 cm−1. The assignments are made by comparison with recent S1 excited state data of benzene–Ar and p-difluorobenzene–Ar and by analysis of the positions and intensities of the observed individual rotational lines. The frequency positions of the intermolecular vibrational states display a regular pattern up to 130 cm−1.
Solvent effects on nuclear shielding of neon
Authors: Lau E. Y. , Gerig J. T.
Published: July 2002

Computer simulations of neon dissolved in a variety of organic liquids are used to explore the origins of solvent-induced changes in the neon shielding parameter. Relying on recent theoretical calculations, it is demonstrated that short-range (van der Waals) interactions between the rare gas atom and solvent molecules are the predominant source of the shielding parameter changes. The treatment used leads to calculated solvent-induced changes for solvents as diverse as water and hexane that are in good agreement with experimental observations.
The visible excitation spectrum of jet cooled NO2: Statistical analysis of rovibronic interactions
Authors: Delon Antoine , Georges Robert , Jost Rémy
Published: July 2002

We have recorded the high resolution (150 MHz) excitation spectra of NO2 molecules cooled in a supersonic jet in two energy ranges: 16 000–19 362 cm−1 [‘‘yellow’’ range, see R. Georges, A. Delon, and R. Jost, J. Chem. Phys. 103, 1732 (1995), hereafter referred to as paper I] and 23 326–23 945 cm−1 (‘‘blue’’ range). In this paper we are interested mainly in the rovibronic properties of about 1500 rotational levels (N=1, K=0, J=1/2, and J=3/2) observed in these two ranges. Among these levels about 480 are observed via the so-called extra lines, i.e., the lines which are observable because of rovibronic couplings between bright levels (N=1, K=0, 2B2 vibronic character) and nearby dark levels. These rovibronic couplings result mainly from second order spin–orbit and orbit–rotation interactions which have been evidenced previously by Zeeman effect and anticrossing experiments [A. Delon, P. Dupré, and R. Jost, J. Chem. Phys. 99, 9482 (1993)]. By comparing the average matrix element of rovibronic interactions occuring for N=1, K=0 (J=1/2 and J=3/2), and for N=3, K=0 (J=5/2 and J=7/2) we can exclude a significant contribution from Coriolis interactions.

A model of small random matrices constructed by using the properties of the above mentioned rovibronic Hamiltonian (density of states, selection rules, and matrix elements) allowed us to reproduce the observed statistical properties of the rovibronic states: average number of extra lines per vibronic band, distribution of intensities, Fourier transform analysis, next-neighbor spacing distribution, hierarchical tree analysis, and intensity and energy correlations between J=1/2 and J=3/2 lines. All these properties confirm that the second-order spin–orbit interaction is responsible for most of the observed extra lines for low rotational levels N=1 and N=3, K=0. As a result, the average reduced rovibronic matrix element is 0.6∓0.1 cm−1 and 0.7∓0.1 cm−1 for the yellow and blue ranges, respectively. The yellow range result is in reasonable agreement with the results obtained previously under magnetic field experiments. We also derive analytical formulas applicable for the weak interaction regime and discuss the relation between the observed distribution of matrix elements and the true distribution.
The investigation of the (CO)+2 ion by dissociative ionization of argon/carbon monoxide clusters
Authors: Mähnert Joachim , Baumgärtel Helmut , Weitzel Karl-Michael
Published: July 2002

The formation of (CO)+2 ion has been investigated by the dissociative ionization of Ar(CO)2 clusters in a threshold photoelectron photoion coincidence experiment. The kinetic energy released (KER) in the reaction Ar(CO)+2→(CO)+2+Ar has been measured as a function of the internal energy of the Ar(CO)+2 ion. The comparison between the experimental KER and the statistically expected KER allows one to extrapolate back to zero KER which corresponds to the thermochemical threshold for the formation of (CO)+2. The ionization potential (IP) of (CO)2 is determined to be 12.24±0.15 eV. This leads to a binding energy of the (CO)+2 of 1.80 eV one of the strongest known for any ionized van der Waals dimer. Our data are in line with a trans planar structure of the CO dimer ion as suggested by our ab initio calculations at the CI level.
Doubly excited 1 3∑−g state of Na2: Observation and calculation
Authors: Liu Yaoming , Li Jian , Xue Meng , Chen Dieyan , Li Li , Jeung Gwang-Hi
Published: July 2002

The v=0–57 levels of the 3p+3p doubly excited 1 3∑−g state of Na2 have been observed by pulsed perturbation facilitated optical–optical double resonance (PFOODR) fluorescence excitation spectroscopy. The Tv=57=39 943.5 cm−1 of the 1 3∑−g state is 385 cm−1 above the X 2∑+g v+=0, J+=0 ionization threshold and 9 cm−1 below the Na (3p,2P3/2)+Na(3p,2P1/2) dissociation limit. No significant line broadening was observed above the X 2∑+g v+=0, J+=0 ionization threshold with our resolution. Molecular constants and the Rydberg–Klein–Rees (RKR) potential curve have been obtained from the observed data. The major constants are Te=36 519.13(17) cm−1, ωe=93.635(41) cm−1, and Be=0.118 95(90) cm−1. We have carried out an all-electron ab initio calculation for the 1 3∑−g state and transition dipole moment for the 1 3∑−g↔b 3Πu system of Na2. Molecular constants calculated from our ab initio potential curve have reasonable agreement with the experimental constants.
Theory of laser-induced desorption of ammonia from Cu(111): State-resolved dynamics, isotope effects, and selective surface photochemistry
Authors: Saalfrank Peter , Holloway Stephen , Darling George R.
Published: July 2002

A two-dimensional, two-state model is used to model the UV-laser-induced photodesorption dynamics of NH3 and ND3 from Cu(111) by solving the nuclear time-dependent Schrödinger equation. By projecting the asymptotic wave functions on the asymptotic (‘‘umbrella’’) eigenstates of NH3/ND3, we find that the molecules leave the surface vibrationally hot, in agreement with experimental data. Within individual asymptotic tunneling doublets, however, the desorbates are clearly non-Boltzmann with molecules of ‘‘gerade’’ symmetry desorbing with increased probability. Our study correlates this parity selection with details of the electronic ground state potential energy surface. An experimentally observed strong isotope effect in the desorption yields for the different isotopomers is traced back mainly to differences between the vibrational frequencies of the ‘‘umbrella’’ mode, in accord with earlier, classical models. Additionally, small tunneling and moderate zero-point contributions are observed. Finally, the possibility of bond and isotope selective photochemistry at surfaces, based on a two-photon IR+UV strategy is demonstrated.
Time-resolved diode laser spectroscopy of the ν6 band of propargyl produced by the UV photolysis of allene
Authors: Tanaka Keiichi , Harada Takeshi , Sakaguchi Kouichi , Harada Kensuke , Tanaka Takehiko
Published: July 2002

The propargyl radical (CH2C≡CH) produced by the ArF excimer laser photolysis of allene was observed by time-resolved infrared diode laser spectroscopy. More than one hundred and fifty absorption lines have been assigned to the ν6 (CH2-wagging) fundamental band of propargyl. Most of the absorption lines were observed as doublets due to the spin–rotation interaction in the 2B1 ground electronic state. The rotational and spin–rotation interaction constants derived for the ground vibrational state are, A0=9.608 47(36), B0=0.317 674(24), C0=0.307 098(24), εaa=−0.017 58(95), and εbb=−0.000 355(76) cm−1, where the figures in parentheses are 2.5 times standard deviations to be attached to the last digit. The ν6 band origin is 687.176 03(62) cm−1, consistent with the infrared spectrum observed in the argon matrix. Anomalously large vibrational changes in the A rotational constant and the ΔK centrifugal distortion constant are accounted for by the a-type Coriolis interaction between the ν6 and ν10 states, where ν10 is the CH2-rocking vibration.
Spectroscopy and intramolecular dynamics via molecular vibrogram analysis
Authors: Hirai K. , Heller E. J. , Gaspard P.
Published: July 2002

Molecular spectra have traditionally been interpreted in the frequency domain and more recently in the time domain. For simple absorption and emission spectra these are the Fourier transform of one another. In this paper we illustrate that it can be useful to examine spectra in a compromise representation which balances uncertainties in time and energy. In this paper we give the basic ideas and several examples which reveal dynamical effects hidden in either energy or time representations. The resulting ‘‘vibrogram’’ is available from experimental spectra or theoretical calculations.
Adsorption and energetics of xenon in mordenite: A Monte Carlo simulation study
Authors: Nivarthi Sriram S. , Van Tassel Paul R. , Davis H. Ted , McCormick Alon V.
Published: July 2002

Grand canonical Monte Carlo (GCMC) simulations were used to calculate energy contour maps, isodensity surfaces and adsorption isotherms of xenon in purely siliceous mordenite at 300 K using previously published potential parameters. Then, in order to understand the effect of aluminum content, Al-containing mordenites were modeled by adding the appropriate negative charge to each of the oxygen atoms in the mordenite framework and by inserting Na+ ions at fixed extra-framework positions previously identified from x-ray crystallographic data. GCMC simulations were again performed using these structure models. Both main channel and sidepocket xenon adsorption were observed in all mordenite structures. The sidepockets were found to be only singly occupied whereas the main channel allowed multiple occupancy. Sidepocket xenon adsorption became more favorable when cations (and aluminum) were introduced. In the main channel, xenon adsorbed into localized sites that were distributed in an octahedral arrangement within each unit cell with two apices in front of opposing sidepockets. These sites remained at the same location over the entire range of loading (0–16 Xe/unit cell) and cation (and aluminum) content.
Experimental and theoretical study of the B–Ne nonbonding interaction: The free-bound B 2Σ+–X 2Π electronic transition
Authors: Yang Xin , Hwang Eunsook , Dagdigian Paul J. , Yang Moonbong , Alexander Millard H.
Published: July 2002

We report a new investigation of the interaction between atomic boron, in both the ground 2p 2P and excited 3s 2S electronic states, with Ne. BNe complexes are formed in a pulsed free jet expansion and detected by laser fluorescence excitation. A broad, asymmetric feature is seen, with maximum intensity ∼270 cm−1 to the blue of the 3s 2S–2p 2PJ atomic transition. This feature corresponds to electronic excitation from the ground vibrational level of the BNe(X 2Π1/2) state into the BNe(B 2Σ+) state, which is unbound. High level ab initio configuration–interaction calculations, involving large atomic orbital bases, were carried out to describe the relevant potentials. The potential curve for the B state reveals a broad shoulder, but no well. The calculated potential curves are corrected, very slightly, by an additional scaling of the correlation energy. Spectral simulations based on these corrected curves reproduce, nearly quantitatively, the experimental spectrum.
Subpicosecond transient infrared spectroscopy of water: A theoretical description
Authors: Bratos S. , Leicknam J-Cl.
Published: July 2002

The pump–probe response of water in infrared is examined theoretically at time scales of the order of 100 fs; these times scales are characteristic of spectral diffusion. The theory is a statistical theory using correlation function description of the nonlinear optical processes involved. An analytical expression for the transient infrared signal is presented, and the corresponding time- and frequency-resolved spectra are discussed. Real time shifts of spectral bands, preceded by a redistribution of their intensities, are predicted. Physical interpretation of these findings is proposed, corroborated by the help of simple models. The intrinsic interest of this time domain is emphasized.
Vibronic structure of the cyclopentadienyl radical and its nonrigid van der Waals cluster with nitrogen
Authors: Sun S. , Bernstein E. R.
Published: July 2002

Fluorescence excitation and two color mass resolved excitation spectroscopy are employed to study the D1(2A2″)←D0(2E1″) vibronic transitions of the cyclopentadienyl radical (cpd) and its van der Waals cluster with nitrogen. The radical is created by photolysis of the cyclopentadiene dimer and cooled by expansion from a supersonic nozzle. The cpd(N2)1 cluster is generated in this cooling process. Mass resolved excitation spectra of cpd are obtained for the first 1200 cm−1 of the D1←D0 transition. The excitation spectrum of cpd(N2)1 shows a complicated structure for the origin transition. With the application of hole burning spectroscopy, we are able to assign all the cluster transitions to a single isomer. The features are assigned to a 55 cm−1 out-of-plane van der Waals mode stretch and contortional (rotational) motions of the N2 molecule with respect to the cpd radical. Empirical potential energy calculations are used to predict the properties of this cluster and yield the following results: (1) the N2 molecular axis is perpendicular to the cpd fivefold axis and parallel to the plane of the cpd ring with the two molecular centers of mass lying on the fivefold ring axis; (2) the binding energy of cpd(N2)1 is 434 cm−1; and (3) the rotational motion of the N2 molecule is essentially unhindered about the cpd fivefold axis. The molecular symmetry group D5h(MS) is applied to the nonrigid cluster, and optical selection rules exclude even↔odd transitions (Δn=0, ±2, ±4,... allowed) between the different contortional levels. Tentative assignments are given to the observed contortional features based on these considerations. The barrier to internal rotation is also small in the excited state. The results for the cpd(N2)1 van der Waals cluster are compared to those for the benzene (N2)1 and benzyl radical (N2)1 clusters.
Non-asymptotic critical behavior of the transport properties of fluids
Authors: Luettmer-Strathmann J. , Sengers J. V. , Olchowy G. A.
Published: July 2002

We extend the application of the mode-coupling theory for the dynamics of critical fluctuations in fluids into the non-asymptotic critical region. An approximate solution of the mode-coupling equations yields a set of representative equations for the thermal conductivity and the viscosity of one-component fluids which incorporates the crossover from asymptotic singular behavior near the critical point to the regular behavior of these transport properties far away from the critical point. The equations for the critical enhancements of the thermal conductivity and the viscosity depend on the background transport properties of the fluid, the equilibrium thermodynamic properties of the fluid, and one fluid-dependent wave-number cutoff qD, which is indicative of the crossover length scale. We compare our crossover model with experimental data for the thermal diffusivity, the thermal conductivity, and the viscosity of carbon dioxide and ethane.