bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-dependent wave-packet studies on the sticking of HCl to an ice surface
Authors: Wang Lichang , Clary David C.
Published: July 2002

A molecular dynamics simulation with a mixed time-dependent quantum-classical treatment is carried out for the adsorption of HCl on the (0001) surface of ice. All six coordinates of the HCl molecule are considered with two important coordinates, the vibrational coordinate r of HCl and the distance Z between the center of mass of HCl and the ice surface, treated quantally by a grid method with absorbing boundaries. The other coordinates of HCl and all coordinates of the water molecules of ice are treated classically except the intramolecular vibrational motions which are frozen. The results show that treating the two coordinates r and Z quantally is necessary in order to describe the adsorption accurately. The interaction spectra as well as other detailed dynamical information from the simulation are presented. An adsorption energy of 25 kJ/mol is obtained and compared with previous calculation and experiments. The energy transfer between HCl and the ice surface is also calculated. An effort is made to probe the effects of defects of the ice surface on the HCl scattering process.
GaAs quantum structures: Comparison between direct pseudopotential and single-band truncated-crystal calculations
Authors: Franceschetti Alberto , Zunger Alex
Published: July 2002

A single-band approach for semiconductor clusters which accounts for the nonparabolicity of the energy bands was recently used by Rama Krishna and Friesner [M.V. Rama Krishna and R.A. Friesner, Phys. Rev. Lett. 67, 629 (1991)]. We compare the results of this method (denoted here as single-band truncated-crystal, or SBTC, approximation) with a direct pseudopotential band-structure calculation for free-standing hydrogen-passivated GaAs quantum films, wires, and dots. The direct pseudopotential calculation, which includes coupling between all bands, shows that isolated GaAs quantum films, wires, and dots have an indirect band gap for thicknesses below 16, 28, and at least 30 Å (8, 14, and at least 15 ML), respectively; beyond these critical dimensions the transition becomes direct. A comparison of the SBTC approximation with the direct pseudopotential calculation shows that (i) the confinement energy of the valence-band maximum is overestimated by the SBTC method, because the zero-confinement character of this state is neglected; (ii) the confinement energy of the Γ-derived conduction state (direct band gap) is slightly overestimated by the SBTC approximation, mainly because of the assumption of infinite potential barriers at the boundaries; (iii) the confinement energy of the X-derived conduction state (indirect band gap) is severely underestimated by the SBTC method; (iv) while the SBTC approximation predicts ‘‘quantum deconfinement’’ (i.e., reduction of gap as size is reduced) for the direct gap of thin GaAs quantum wires, such effect is not present in the direct pseudopotential calculation.
The effects of covalent bonds on the localized relaxations in the glassy states of linear chain and network macromolecules
Authors: Wasylyshyn D. A. , Johari G. P.
Published: July 2002

Dipolar relaxations in the glassy states of five molecular liquids and of their partially and completely polymerized structures, both linear chain and network types, were studied by dielectric measurements for a fixed ac frequency of 1 kHz. Calorimetric measurements were made to determine the number of covalent bonds formed at different instants of polymerization, and the results were combined with the dielectric data to investigate how the height of the relaxation peak indicating localized dipole diffusion changed as the number of covalent bonds in the spontaneously polymerizing material increased. The height, or strength, of the peak due to the localized relaxation process in the molecular liquid gradually decreased and the process approached extinction as the number of covalent bonds approached its limiting value. Concurrently, a new localized glassy state relaxation process evolved at a temperature with a peak 70–90 K above that of the one observed for the molecular state and its height increased and approached a constant value as the number of covalent bonds formed approached its limiting value. These changes were quantitatively similar for the five materials, irrespective of the structure of the macromolecular product. The extinction of the sub-Tg relaxation peak observed originally for the unreacted state has features in common with structural relaxation effects.
Segmental mobilities in an athermal diblock copolymer melt far above T
g by incoherent neutron scattering
Authors: Fytas G. , Meier G. , Richter D.
Published: July 2002

We report on incoherent elastic and quasielastic neutron scattering from a symmetric poly(isoprene-1,2 butadiene) (PIP-PVE) almost athermal diblock copolymer at different wave vectors Q in the range 0.18 to 1.8 Å−1. A fixed window temperature scan between 20 K and 300 K revealed the presence of segmental dynamics above 200 K and hence line shape analysis of the incoherent quasielastic spectra Sinc(Q,ω) was performed at high temperatures far above the single glass transition Tg(=221 K) to obtain the characteristic time scales. A two step nonexponential relaxation function was found adequate to describe consistently Sinc(Q,ω) with the faster relaxation time being comparable with the segmental relaxation time in bulk PIP at similar Q. This dynamic incompatibility may arise from dynamic heterogeneities as a consequence of composition fluctuations. The observation of a bimodal relaxation distribution function in PIP–PVE with low Tg contrast at temperatures far above the single Tg emphasizes the role of the intramolecular correlations due to chain connectivity.
Theory of odd torsional transitions in the V−N resonance Raman spectrum of ethylene
Authors: Watson J. K. G. , Siebrand W. , Pawlikowski M. , Zgierski M. Z.
Published: July 2002

The V−N resonance Raman spectrum of ethylene shows a long progression in even quanta of the ground-state torsional mode ν4″(au). Bands approximately midway between the even quanta have been assigned to transitions to odd quanta of ν4″, although such transitions are forbidden according to the usual g↔/u selection rule of Raman spectroscopy. We consider the theory of the intensity of such transitions allowing for the fact that the excited state is twisted by 90° at equilibrium, using Hougen’s double group theory for the separation of the torsional and a-rotational motions. From approximate one-dimensional torsional potentials of the V and N electronic states, it is shown that qualitative agreement between observed and calculated intensities is obtained. The electronic transition moment is assumed to be proportional to cos2γ, where 2γ is the torsional angle, but the calculated relative intensities are not sensitive to the precise torsional dependence. More detailed theory will require inclusion of the CC-stretching and CH2-scissoring degrees of freedom and consideration of an avoided crossing affecting the V state.
Analysis of large-scale multi-configuration self-consistent field wave functions by expectation values of local operators
Authors: Mödl Martin , Dolg Michael , Fulde Peter , Stoll Hermann
Published: July 2002

We have developed a method to analyze multi-configuration self-consistent field wave functions for open-shell systems, which may consist of a very large number of Slater determinants, by calculating their expectation values for (local) one- and two-particle operators in a numerically efficient way. Although the proposed formalism is fully general and applies to any operators referring to a subset of arbitrary orthogonal molecular orbitals, the local operators we use here are defined in terms of localized molecular orbitals. This choice permits to perform a chemically intuitive analysis of the local spin, population and charge fluctuation, also allowing to study electron correlation effects in chemical bonding. We present some applications for ionic and covalent systems considering four small molecules, H2, N2, H2O and CH4, as well as a binary transition metal complex, [Fe2S6]6−.
Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluids
Authors: Schwarzer D. , Troe J. , Votsmeier M. , Zerezke M.
Published: July 2002

The collisional deactivation of vibrationally highly excited azulene was studied from the gas to the compressed liquid phase. Employing supercritical fluids like He, Xe, CO2, and ethane at pressures of 6–4000 bar and temperatures ≥380 K, measurements over the complete gas–liquid transition were performed. Azulene with an energy of 18 000 cm−1 was generated by laser excitation into the S1 and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by transient absorption at the red edge of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were monoexponential. At densities below 1 mol/l, collisional deactivation rates increased linearly with fluid density. Average energies 〈ΔE〉 transferred per collision agreed with data from dilute gas phase experiments. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coefficient scaled collision frequencies ZD turned over to a much weaker dependence at ZD≳0.3 ps−1. Up to collision frequencies of ZD=15 ps−1 this behavior can well be rationalized by a model employing an effective collision frequency related to the finite lifetime of collision complexes.
Potential energy surfaces and reactive dynamics of Zn(3P) with H2
Authors: Salazar Michael R. , Simons Jack
Published: July 2002

The ab initio potential energy surfaces pertinent to Zn(3P)+H2→ZnH(2Σ)+H have been calculated and are described. For thermal collisions, the dominant reactive surfaces is identified and a mechanism for the reaction on this surface, which may explain the rotational state populations in the ZnH products, is proposed. A novel dynamical technique of running classical simulations on ab initio surfaces, using a piecewise tessellation rather than a global functional fit, is briefly introduced with sample trajectories shown.
Stationary points on the lowest doublet and quartet hypersurfaces of the N3 radical: A comparison of molecular orbital and density functional approaches
Authors: Wasilewski J.
Published: July 2002

Stationary points on the doublet and quartet hypersurfaces of the N3 system are studied using the single-configurational and multi-configurational Hartree–Fock approaches and the methods which include the dynamic correlation effects on a perturbational way or through the density functional theory (DFT). A general structure of both the hypersurfaces within the 𝒞2v symmetry is explained on the basis of the Walsh diagram and studied by a simplified configuration interaction (CI) based on a small complete active space (CAS) of five valence orbitals. The minima found in this way are studied in detail by the other methods mentioned above without the symmetry constraints. The stability of minima and existence of energetic barriers are explained by the changes of the bonding — antibonding character of some valence orbitals, observed in the Walsh diagrams for both the orbital and the Kohn–Sham energies. All the methods applied indicate, that a stable ring N3 conformer should exist with the 2B1 ground state. Alternative mechanisms for a deactivation of the linear excited states of N3 are proposed.
Nonadiabatic corrections to the lowest EF 1Σg and I 1Πg− vibrational levels of the hydrogen molecule
Authors: Wolniewicz L.
Published: July 2002

EF 1Σg and I 1Πg− electronic states of molecular hydrogen are investigated and nonadiabatic ab initio energies of lower lying vibrational levels of H2, D2, and T2 are computed. The resulting term values are compared with experiment and existing theoretical results. For the I-state and the EF-state levels, with energies below −0.675 a.u., the remaining discrepancies with experiment do not exceed ±0.5 cm −1. The origin of the residual errors is briefly discussed.
Evolution of the electronic structure of cyclic polythiophene upon bipolaron doping
Authors: Kundu K. , Giri D.
Published: July 2002

The evolution of the electronic structure of cyclic polythiophene (PT) upon bipolaron doping is studied to explore the possibility of uniform charge density ground state in the metallic regime. The ground state geometry of the neutral PT, the structure of a single bipolaron, and the experimentally observed optical transitions due to it are reproduced. Since the cyclic PT doped with an odd number of bipolarons (the O configuration) creates an aromatic polyene backbone containing (4n+2)π electrons, it is driven toward the quinoid form. Consequently, we find an insulator-metal transition for dopant concentration ≥14 mol % and an ∼0.8 eV redshift in Fermi energy with respect to the neutral system at 30 mol % which agree very well with experimental findings. For an even number of bipolarons, there are two possible configurations, namely (i) the (1,1) or the T configuration and (ii) the (2,0)/(0,2) or the S configuration. The T configuration and the O configuration behave similarly and merge to a single asymptotic configuration. The energy difference per unit cell between the S and the T configurations decreases as the ring size increases. So we propose that in the sufficiently large cyclic PT the T as well as the O configurations will be the stable entities if their stability is further augmented by additional interactions.
The effective fragment model for solvation: Internal rotation in formamide
Authors: Chen Wei , Gordon Mark S.
Published: July 2002

The internal rotation of formamide with 0–5 water molecules oriented along the N–C bond has been studied by the full ab initio self-consistent field theory and using the effective fragment (EFP) method. For each case, the EFP geometries, harmonic vibrational frequencies, rotational barriers, and intrinsic reaction coordinates for the internal rotation are found to be in excellent agreement with their ab initio counterparts. The global energy minimum structures for four and five water complexes are predicted to be formamide bonded to two adjacent waters, with all water molecules in a ring. Probably due to the structural constraints, the complexes containing less than four waters have cyclic structures with the two ends of formamide connected by a sequence of water molecules. The internal rotation barrier of formamide–water complexes increases from 15.3 kcal/mol with no water to 19.0 kcal/mol with four waters and seems to saturate at four to five waters. When electron correlation corrections are added, the estimated internal rotation barrier is ∼20 kcal/mol, in very good agreement with experimental measurements.
Core-valence doubly ionized states: General aspects, examples, production mechanisms
Authors: Schulte H. D. , Cederbaum L. S. , Tarantelli F.
Published: July 2002

Electronic double vacancies with one vacancy in the valence shell and one in the core play a role in several physical processes. Such core-valence double vacancies are theoretically analyzed and related to possible experiments. The corresponding wavefunctions and energies for CO, N2, and H2CO are computed using propagator and configuration interaction methods. The numerical results are analyzed in some detail and are compared to the corresponding single valence vacancies. The analysis is performed by breaking up the binding energy of the double vacancy into the most relevant components, such as hole–hole repulsion and relaxation contributions. It is shown that the double ionization potential is essentially given by single ionization quantities. In particular, we find a kind of ‘‘Koopmans theorem’’ for those dicationic states with an outer valence hole: the double ionization potential (shifted by the core ionization energy) is approximately given by the valence orbital energy of the core ionized state. As typical for double vacancies we encounter, in addition, an interesting singlet–triplet separation problem. Intensities for the production of the dicationic states by valence ionization out of a core ionized initial state are derived. The extent of valence hole localization in the dicationic states is analyzed by a two-hole population analysis. The analysis can be used to simulate the production of core-valence vacancies via Auger decay.
Spatial correlations and solvation interaction in a two-component mixture of adhesive fluids
Authors: Jamnik A.
Published: July 2002

First, the phase behavior and the spatial correlations in the two-component mixture of adhesive fluids denoted by (1) and (2) are studied on the basis of the solution to the Percus–Yevick/Ornstein–Zernike equation. The isotherm slopes for the correlation function between unlike particles exhibit singularities at the interparticle distances which are the multiple of the molecular size of both species (1) and (2) as a consequence of the impulse character of 1-1, 2-2, and 1-2 adhesive potential of interaction. Then, the above system is treated as an adhesive solvent mixture in which the solvent mediated force between the hard solutes (3) mimicking liophobic colloids is studied. The solution of the Percus–Yevick/Ornstein–Zernike equation for a three-component mixture comprising the two-component adhesive solvent system and the hard sphere colloid is applied in the limit of vanishing solute concentration. Due to the layering of the solvent molecules, the solvation force oscillates with the periods equal to the molecular diameters of both solvent components. The force between the macroparticles in the one-component adhesive solvent [A. Jamnik, D. Bratko, and D. Henderson, J. Chem. Phys. 94, 8210 (1991)] tends to vanish at the critical condition of the model fluid. On the contrary, the solvation force in the two-component adhesive system remains finite even at the critical conditions of the solvent mixture at the specified composition.
Modeling polyethylene with the bond fluctuation model
Authors: Tries V. , Paul W. , Baschnagel J. , Binder K.
Published: July 2002

This work presents an application of recently developed ideas about how to map real polymer systems onto abstract models. In our case the abstract model is the bond fluctuation model with a Monte Carlo dynamics. We study the temperature dependence of chain dimensions and of the self-diffusion behavior in the melt from high temperatures down to 200 K. The chain conformations are equilibrated over the whole temperature range, which is possible for the abstract type of model we use. The size of the chains as measured by the characteristic ratio is within 25% of experimental data. The simulated values of the chain self-diffusion coefficient have to be matched to experimental information at one temperature to obtain a scaling for the Monte Carlo time step. The melt viscosity from the simulations as determined by applying the Rouse model is then in good qualitative agreement with experimental data over the experimentally available temperature range. The activation energy as extracted from an Arrhenius fit is different because the simulations are done at constant volume. Both experimental data and the simulation, which covers a far greater temperature range, show Arrhenius behavior for the viscosity and no indication of a finite nonzero Vogel–Fulcher temperature. For one temperature (T=509 K) various time-dependent mean-square displacements are available from atomistic molecular-dynamics simulations, and are shown to be in excellent agreement with the results from the coarse-grained model.
A study of the SO molecule with photoelectron spectroscopy using synchrotron radiation
Authors: Dyke John M. , Haggerston Darren , Morris Alan , Stranges Stefano , West John B. , Wright Timothy G. , Wright Andrew E.
Published: July 2002

The SO molecule has been studied by photoelectron spectroscopy using vacuum ultraviolet radiation from a synchrotron as the photon source. Both constant ionic state (CIS) and photoelectron spectra have been recorded. Resonances which appear in the CIS spectra recorded for selected vibrational levels of SO+ X 2Π in the photon energy region hν=11.5–15.0 eV have been assigned to excitations to Rydberg states which converge to the SO+ a 4Π and A 2Π states and autoionize to SO+ X 2Π. Also, resonances which appear in the CIS spectra recorded in the photon energy region 15.0–16.4 eV for selected vibrational levels of SO+ b 4Σ− have been assigned to excitations to Rydberg states which converge to SO+ B 2Σ− and autoionize to SO+ b 4Σ−. Photoelectron spectra recorded at selected resonant photon wavelengths show that autoionization can dramatically alter the intensities of the vibrational components in a photoelectron band and may allow extra vibrational structure to be observed. The additional information obtained from using this method to study short-lived molecules is discussed.
Observation of the “isomerization states’’ of HCP by stimulated emission pumping spectroscopy: Comparison between theory and experiment
Authors: Ishikawa Haruki , Nagao Chioko , Mikami Naohiko , Field Robert W.
Published: July 2002

Highly excited vibrational states of HCP X̃ 1∑+ were investigated by stimulated emission pumping spectroscopy. Two distinct families of vibrational states were observed in the 13400–17500 cm−1 energy region. One of them is “normal-mode-type” states, well characterized by polyads involving the bending (ω2) and CP stretching (ω3) vibrations. The other is delocalized “isomerization states” whose existence has been predicted by a recent theoretical study [J. Chem. Phys. 104, 10055 (1996)]. The present spectroscopic observations agree well with the theoretical predictions.
Optical dephasing in pentacene-doped PMMA under high pressure
Authors: Berg Otto , Chronister Eric L.
Published: July 2002

Pressure- and temperature-dependent photon echo results are obtained for pentacene doped polymethyl methacrylate (PMMA). A unique pressure effect is observed in which the optical dephasing rate increases as the pressure is increased from ambient pressure to 4 kbar, above which the optical dephasing rate is pressure independent up to 43 kbar. The present results are also compared with pressure- and temperature-dependent photon echo results for rhodamine 101 in PMMA, in which the optical dephasing rate was completely insensitive to pressure over the range 0 to 30 kbar. A negative correlation is also observed between the optical dephasing rate and the spectral hole burning efficiency. Line broadening due to pressure induced spectral diffusion may be responsible for both the increased dephasing rate and the reduced spectral hole-burning at high pressure.
Rotational analysis and assignment of the green band system of FeH to the e 6Π–a 6Δ transition
Authors: Goodridge Damian M. , Carter Robert T. , Brown John M. , Steimle Timothy C.
Published: July 2002

The technique of laser excitation spectroscopy at Doppler resolution has been used to record bands in the electronic spectrum of FeH around 532 nm. A number of lines were also recorded at sub-Doppler resolution using intermodulated fluorescence spectroscopy. Dispersed fluorescence studies were used extensively to aid the assignment of a total of 153 lines in this region of the spectrum to either the (0,0) or the (0,1) band of the e 6Π–a 6Δ transition. All the assignments from the excitation spectrum obey the selection rule ΔΩ=−1, giving subbands which involve the lowest three spin components in each state; transitions with ΔΩ=0 were seen only in dispersed fluorescence. Term values have been determined for the three components in both the ground vibrational level of the e 6Π electronic state and the first excited vibrational level of the a 6Δ electronic state (those for the v=0 level of the a 6Δ state have been determined previously). This study provides the first characterization of the spin components and the rotational levels of the e 6Π state of FeH. This state is heavily perturbed and exhibits pronounced lambda-type doubling, making it very difficult to model the energy levels using an effective Hamiltonian approach.
Theoretical studies of large water clusters: (H2O)28, (H2O)29, (H2O)30, and (H2O)31 hexakaidecahedral structures
Authors: Khan Arshad
Published: July 2002

The 28, 29, 30, and 31 mer hexakaidecahedral water clusters were studied by applying the intermediate neglect of differential overlap self-consistent field restricted Hartree–Fock method (INDO SCF RHF) after parametrization for H and O atoms. The most stable 29 and 30 mer clusters have one and two water molecules, respectively within the cavity of the distorted hexakaidecahedral cage with stabilization energy/monomer values of around 10.9 and 11.0 kcal, respectively. The 31 mer cluster with three water molecules within the cavity is less stable than the isomer with two molecules within the cavity and the third one bonded outside of the cage by around 3 kcal/mol.
Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenes
Authors: Sato Kei , Tsunashima Shigeru , Takayanagi Toshiyuki , Fujisawa Ginji , Yokoyama Atsushi
Published: July 2002

The 193 and 157 nm photodissociations of three isomers of dichloroethylene (DCE) and trichloroethylene (TCE) were investigated using a technique of photofragmentation translational spectroscopy. The photofragmentation mechanisms were constructed by analyzing the time-of-flight spectra of C2H2+, Cl+, HCl+, C2HCl+, and C2Cl2+ produced by electron impact of neutral photofragments. In the 193 nm photodissociation, both the HCl elimination and the C–Cl bond rupture were important for all the compounds examined. It was concluded that secondary dissociation of the vibrationally excited chlorinated vinyl radical produced by the C–Cl bond rupture was important even at 193 nm. In the 157 nm photodissociation, the mechanisms were similar to those at 193 nm for cis-DCE, 1,1-DCE, and TCE, while only the C–Cl bond rupture occurred for trans-DCE. This result suggests that the 157 nm photodissociation of trans-DCE proceeds via the direct photodissociation following the photoexcitation to the repulsive nσ*1 state. A minor C–H bond rupture was also found in the 157 nm photodissociations of cis-DCE and TCE. On the basis of the present mechanisms, the translational energy distributions and the branching ratios were estimated for all the possible processes.
Transition probabilities in OH A 2Σ+−X 2Πi: Bands with v′=2 and 3
Authors: Steffens Kristen L. , Luque Jorge , Jeffries Jay B. , Crosley David R.
Published: July 2002

Experimental relative vibrational band emission coefficients have been measured in the A 2Σ+−X 2Πi electronic system of the OH radical. Branching ratios for the sequences v′=2, v″=0–5 and v′=3, v″=0–6 were determined using laser-induced fluorescence in a low pressure methane–oxygen flame. We accounted for potential error contributed by rotational and vibrational energy transfer and fluorescence polarization. These transition probabilities complete a set of experimental data, which are used in a separate study to determine the optimum electronic transition moment of the OH A-X system.
A new grid-free density-functional technique: Application to the torsional energy surfaces of ethane, hydrazine, and hydrogen peroxide
Authors: Werpetinski Katrina S. , Cook Michael
Published: July 2002

We describe the implementation of a new grid-free density-functional technique for exchange-correlation potentials of ρ1/3 form (exchange-only local density-functional theory potentials). The potential is fitted to integrable functional forms by solving a set of nonlinear equations, rather than by fitting on a three-dimensional grid of points. This completely analytical method produces smooth energy surfaces and exact energy gradients. The method is found to be several times faster computationally in single-point calculations than a comparable grid-based method with a moderate number of grid points, and it is more than an order of magnitude faster for geometry optimizations. The analytical method is tested on the torsional energy surfaces of the classic isoelectronic series C2H6, N2H4, and H2O2, using the Hartree–Fock–Slater potential (α=2/3). The locations and relative energies of energy extrema, and the structural variations across the potential surfaces, are in good agreement with experimental data and the results of high-quality ab initio studies.
Electronic excitation transfer in Lennard-Jones fluid: Comparison between approaches based on molecular dynamics simulation and the many-body Smoluchowski equation
Authors: Bandyopadhyay T.
Published: July 2002

Molecular dynamics simulations were performed to study the kinetics of long-range irreversible/reversible electronic excitation transfer in a Lennard-Jones fluid where the translationaly mobile choromophores are thought to be embedded. The simulations are based on the Förster master rate equation approach which can be rederived from a stochastic Liouville formalism for excitation transfer between two identical chromophores in the weak dipole–dipole coupling regime. For energy transfer between two dissimilar partners, rate equations utilized are obtained from the first principle. The simulated kinetic results in this regime are then compared with the reaction-diffusion theoretical framework for excitation transfer. The theory is based on a many-body Smoluchowski equation for the reactant molecule reduced distribution function and makes use of a superposition approximation to truncate the hierarchy of equations. The comparison of the results show the scope and utility of the theoretical approach in the high friction limit when it is solved for the absorbing boundary condition at contact. In the low friction limit, like collisional quenching, the present reaction-diffusion formalism is found to perform poorly. When the stochastic Liouville equation in the strong dipolar coupling regime is solved combined with the molecular dynamics trajectories, the time dependent reaction probability of the donor shows oscillatory behavior and the diffusion coefficient of the medium has been found to have but little effect on this.
Diels–Alder reactions: An assessment of quantum chemical procedures
Authors: Barone Vincenzo , Arnaud Roger
Published: July 2002

Kinetic and thermodynamic parameters for the Diels–Alder reactions of butadiene with ethylene, formaldehyde, and thioformaldehyde have been computed at a variety of levels of theory with the aim of identifying the most suitable approach for general application. Especially in the presence of heteroatoms, inclusion of electron correlation significantly modifies the geometries of stationary points obtained at the Hartree–Fock level. However, energetic quantities are not very sensitive to the employed geometries provided that the structures of all the species are optimized at the same computational level. On the other hand, both extended basis sets and inclusion of most of the dynamical correlation are needed to obtain reliable energetic quantities. In particular, converged results are obtained only using a triple zeta contraction for the valence space and double sets of polarization functions on nonhydrogen atoms, whereas f functions and hydrogen polarization functions play a minor role. Among cheaper approaches hybrid density functional/Hartree–Fock methods coupled with polarized split valence basis sets provide remarkable geometries and, especially, energetic quantities.
Fullerenes coated with sulfur and phosphorous molecules
Authors: Tast F. , Malinowski N. , Heinebrodt M. , Billas I. M. L. , Martin T. P.
Published: July 2002

Clusters composed of a single fullerene molecule and several phosphorous or sulfur molecules produced in a low pressure inert gas aggregation cell are investigated by vacuum-UV photoionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra indicate that the molecules form a layer on the fullerene surface.
Exact derivation of extended thermodynamics from the Robertson statistical formalism
Authors: Nettleton R. E.
Published: July 2002

An exact derivation of nonlinear, nonequilibrium thermodynamics proposed by Robertson, effected by operating with a non-Hermitian P̂R on the Liouville equation, is applied to give an exact statistical derivation of extended thermodynamics. This includes variables that are even under time reversal and others that are their time derivatives. Entropy is given by the Jaynesian information-theoretic model. If the phenomenological coefficients are expanded in powers of thermodynamic forces, antireciprocal relations hold to all orders. Symmetric reciprocity holds exactly in linear approximation and, in the nonlinear regime, for very short times where it may apply to relaxation of fast variables, approximately. The formalism does not readily provide expressions for long-time limits and steady-state transport coefficients. One obtains exact expressions for fluxes of measurable variables as functionals of all the measurable variables. Thus addition of a hierarchy of fluxes as internal variables is incompatible with an information-theoretic entropy.
Resonance-enhanced multiphoton electron detachment spectra of C7−, C9−, and C11−
Authors: Ohara M. , Shiromaru H. , Achiba Y.
Published: July 2002

Electronic spectra of C7−, C9−, and C11− were measured in the gas phase for the first time using resonance-enhanced detachment. The band origin of the transition to the lowest excited state for each of these three anions was determined. For C7− and C9−, several other higher excited states were also detected. The 000 transitions for a series of linear chain anions from C5− to C11− were compared with those previously reported for even-numbered series. Slight differences were noted between the even- and odd-numbered series.
Nonadiabatic effects in molecular adiabatic systems: Application to linear plus quadratic E⊗e system
Authors: Baer Michael
Published: July 2002

In this publication I treat two subjects both connected to electronic nonadiabatic transitions. The first is related to the general single-state Born–Oppenheimer equation. Such an equation was recently derived by Baer and Englman [Chem. Phys. Lett. 265, 105 (1997)]; here it is derived again in a different and more instructive way. The second is related to the “linear plus quadratic E⊗e system”. For this system we derive the adiabatic–diabatic-transformation (ADT) angle [Chem. Phys. Lett. 35, 112 (1975)] and show that it is identical to the phase factor obtained by Zwanziger and Grant [J. Chem. Phys. 87, 2734 (1987)].
Spectroscopic characterization of the singly excited CaAr(4dπ 3Π), CaAr(4dδ 3Δ1) states and the doubly excited CaAr(4pπ4pπ 3Σ−) state
Authors: Kaup John G. , Breckenridge W. H.
Published: July 2002

The singly excited Ca(4s4dπ 3DJ)⋅Ar(3Π0−), and Ca(4s4dδ 3DJ)⋅Ar(3Δ1) valence states and the unusual doubly excited Ca(4pπ4pπ 3PJ)⋅Ar(3Σ−) valence state have been characterized via Resonance Enhanced Two-Photon Ionization (R2PI) spectroscopy of transitions from the long-lived Ca(4s4pπ 3PJ)⋅Ar(3Π0−) metastable state of the CaAr van der Waals molecule. Because the Ar atom can approach along the nodal axis of the quite diffuse Ca(4dδ) orbital with little repulsion, the CaAr(4s4dδ 3Δ1) state is fairly strongly bound (D0=609±50 cm−1), but less bound than the CaAr+ “core” ion (D0=775±50 cm−1). The CaAr(4s4dπ 3Π0−) state is less bound (D0=450±50 cm−1), but has a bond strength much greater than that of the lower-energy CaAr(4s4pπ 3Π0−) state (D0=60±50 cm−1). The doubly excited CaAr(4pπ4pπ 3Σ−) state is extremely strongly bound (D0=1277±50 cm−1), and this is attributed to the lack of a valence Ca(4s) electron. The Ar atom can therefore approach with minimal repulsion along the nodal axes of the two transversely aligned Ca(4pπ) electrons. The Ca(4pπ4pπ 3Σ−) state is also strongly predissociated, with a lifetime less than 10 ps.
Optimal pump-dump control: Linearization and symmetry relation
Authors: Yan YiJing , Cao Jianshu , Shen Zhenwen
Published: July 2002

A generalized eigenequation is derived for the optimal pairs of phase-unlocked pump-dump fields in the control of pure state molecular systems in the weak response regime. The associated eigenvalues are further correlated to the yields of pump-dump control, thus the globally optimal pair of phase-unlocked pump-dump fields is identified unambiguously. Presented is also a hierarchy of reduction from the general Liouville-space density matrix control formulation in the strong response regime to various linearized control equations in the weak response regime. Application to the control of a molecular ro-vibronic level in the ground electronic surface reveals a novel symmetry relation between the optimal pump field and its counterpart of the optimal dump field.
First-principles study of the structural and electronic properties of ethylene adsorption on Si(100)-(2×1) surface
Authors: Pan Wei , Zhu Tianhai , Yang Weitao
Published: July 2002

We present a first-principles density-functional study for ethylene adsorption on the Si(100)-(2×1) surface. Ethylene is di-σ bonded to the surface Si dimers with the adsorption energy of 1.81 eV. The dimer-maintained adsorption structure is found to be more stable than the dimer-cleaved one by 0.91 eV. Our calculations on the post-exposure of the ethylene-saturated surface to atomic hydrogen demonstrate the formation of Si-H bonds, the cleavage of the Si dimer bonds, and the minor changes for the chemisorbed ethylene, which is in good agreement with the recent experimental observations.
Quenched molecular reorientation and angular momentum for liquids confined to nanopores of silica glasses
Authors: Korb J.-P. , Xu Shu , Cros F. , Malier L. , Jonas J.
Published: July 2002

The results of a theoretical and experimental study of the confinement effects on molecular dynamics of nonpolar liquids in porous silica glasses prepared by the sol-gel process are discussed. The natural abundance 13C and 33S NMR spin-lattice relaxation times of liquid CS2 are reported as a function of pore radius in the range from 15 Å to 102 Å over the temperature range of 168 K to 293 K. Since spin-rotation interactions dominate the 13C relaxation at higher temperatures these experiments allowed us for the first time to follow the confinement effects on angular momentum correlation times. The low-temperature 13C T1 data and the 33S T1 data provide information about reorientational motions. In order to interpret the experimental NMR relaxation data, a theoretical model for anisotropic molecular reorientation and angular velocity, which accounts for the motional behavior of nonpolar liquids confined to nanopores, is proposed. This model predicts an increase of the reorientational and angular momentum correlation times when the pore size is decreased. Application of this theoretical model to the interpretation of the CS2 NMR relaxation data and to earlier relaxation results obtained for confined nonpolar cyclohexane-d12 liquid proved successful.
Violation of the weak noncrossing rule between totally symmetric closed-shell states in the valence-isoelectronic series O3, S3, SO2, and S2O
Authors: Ivanic Joseph , Atchity Gregory J. , Ruedenberg Klaus
Published: July 2002

The ground and next excited 1A1 potential energy surfaces of the valence-isoelectronic molecules O3, S3, SO2, and S2O have been studied using full-valence-space-multiconfiguration self-consistent-field calculations. An open and a ring minimum were found for all species. In each case, interconversion between the C2v open and closed isomers proceeds via a C2v transition state which lies on a very sharp ridge separating the basins surrounding the minima. In the immediate vicinity of these transition states, the upper state for each species is found to have a minimum, also in C2v. Furthermore, not far away on the C2v ridges, the 1 1A1 and 2 1A1 surfaces intersect conically for all molecules.
An accurate theoretical description of fluids composed of fully anisotropic molecules: Application to C2v symmetry
Authors: Alvarez M. , Lado F. , Lomba E. , Lombardero M. , Martı́n C.
Published: July 2002

We use two molecular integral equation approximations to compute the thermodynamic properties and microscopic structure of two liquids composed of planar molecules with C2v symmetry, namely SO2 and H2S. These approximations couple the exact molecular Ornstein–Zernike equation with the hypernetted chain (HNC) and reference-hypernetted chain (RHNC) closures. The theoretical results obtained for various thermodynamic states agree remarkably well with molecular dynamics calculations. In particular, the atom-atom distribution functions are very well reproduced. We find that the RHNC approximation with a hard-sphere fluid reference system offers notable improvement over HNC in the pressure calculation. We include also a self-consistent mean field calculation to incorporate the effect of polarizability on the dielectric constant of liquid SO2. Final results for this quantity are in excellent agreement with experimental values. In contrast, the model used for the electrostatic interactions in H2S leads to anomalously high permanent dipole moments, compared to experiment, and consequently to dielectric constants that are completely off the experimental data.
Energetics and dynamics of Coulomb explosion of highly charged clusters
Authors: Last Isidore , Schek Israel , Jortner Joshua
Published: July 2002

Ultrafast femtosecond Coulomb explosion of charged homogeneous (Xen) and heterogeneous doped (HIArn) small and medium sized clusters (n<60) is studied resting on the picture of a vertical high-order multiphoton ionization from the ground state nuclear configuration. The final average atomic velocity (simulated at constant charge) increases with increasing the cluster size, and at constant cluster size increases linearly with the ion charge, in accord with the predictions of an analytical model. The linear dependence of the reciprocal explosion time on the charge is also in accord with the analytical prediction. From the energetics of the Coulomb explosion (reflecting a probable initial atomic distribution of the cluster size for small clusters), a nonvertical multiphoton ionization during the Coulomb explosion cannot be inferred.
Higher-order perturbation theory for the thermodynamic properties of a solid with a truncated potential energy expansion
Authors: Shukla Ramesh C. , Lacks Daniel J.
Published: July 2002

A perturbation theory (PT) is developed in the classical limit which is based on an infinite series of diagrams composed of the loops and bubbles arising from the first- and second-order matrix elements of the PT, respectively. This theory leads to a closed form expression for the free energy, which on expansion gives an infinite power series in the temperature. Results from this theory are obtained for a Hamiltonian in which the Taylor expansion of the potential energy is truncated at the quartic term. These results are compared with results of finite summation versions of the theory up to O(λ8), with results of standard PT of O(λ2) and O(λ4), and with results of molecular dynamics (MD) simulations carried out for the same potential energy surface (i.e., the potential energy expansion truncated at the quartic term). The results show that the theory which includes all powers of temperature gives better agreement with the MD results throughout a wide temperature range than does the standard PT of O(λ2) and O(λ4).
Computer simulations of asymmetric block copolymers
Authors: Hoffmann Alexander , Sommer Jens-Uwe , Blumen Alexander
Published: July 2002

We simulate dense diblock copolymer melts using the lattice bond-fluctuation method. Letting the lengths NA and NB of the A- and B-subchains vary (with NA+NB=N) we study the dependence of the static and dynamic properties on f=NA/N. Changes in the A-B interaction parameter allow to mimic large temperature variations. Thus at low T we find, depending on f, lamellar, hexagonal or micellar structures, as evident from the appearance of Bragg-reflexes in the collective structure factor S(q); for high temperatures S(q) is well approximated by a generalized Leibler form. The single chain statics reveals non-mean-field behavior even well above the order-disorder transition (ODT). Near the ODT the copolymer chains are, as a whole, stretched whereas the blocks contract slightly; the maximal contraction occurs near the spinodal Tsp. We evaluate the mean repulsive energy felt by the monomers and its dependence on the monomer’s position along the chain. From the variance of the repulsive energy we calculate cv, the specific heat per chain; cv is continuous both near Tsp and near the ODT. Surprisingly, cv scales with ε2Nf(1−f ), where ε is the microscopic energy parameter of the simulations. As dynamical features we compute D, the diffusion coefficient of single chains and the rotational relaxation times τ of the end-to-end vector: D scales with εf(1−f ), whereas the τ-times show complex f-dependencies, facts which stress that the diffusional motion and the rotational relaxation behave differently.
Caged spin-orbit excited I*2P1/2+I*(2P1/2) atom pairs in liquids and in cryogenic matrices: Spectroscopy and dipolar quenching
Authors: Benderskii A. V. , Zadoyan R. , Apkarian V. A.
Published: July 2002

Two-photon excitation of I2 in room temperature liquid solutions and in cryogenic rare gas matrices, in the 500–600 nm range, leads to fluorescence in the near infrared. The emission spectra are assigned to the I*I*(2P1/2+2P1/2)→I*I(2P1/2+2P3/2) transitions, namely, to contact pairs of spin excited atoms which are strictly bound by the solvent cage. The spectra are analyzed to characterize cage potentials and contact distances. In rare gas solids, the caged pair relaxes radiatively, τrad=3.9, 3.1, and 2.8 μs, in Ar, Kr, and Xe. In room temperature liquids, emission from both geminate and nongeminate pairs occurs, resulting in bimodal kinetics. The fluorescence yield strongly depends on the nature of the solvent. Through a set of measurements in hydrogenated and deuterated solvents, the quenching mechanism is ascribed to dipolar energy transfer to solvent molecules: the quenching efficiency is determined by the spectral overlap between vibrational overtones of the solvent and the solute emission.