bluuebunny/crossref_metadata_2025 huggingface dataset:
The dissociation energies of NF(X 3Σ−) and NCl(X 3Σ−)
Authors: Xantheas Sotiris S. , Dunning Thom H. , Mavridis Aristides
Published: July 2002

We have computed potential energy functions for the ground states (X 3Σ−) of NF and NCl using a series of correlation consistent basis sets ranging from double to sextuple zeta quality and including core-valence correlation effects in conjunction with coupled-cluster single and double excitations with perturbative treatment of triple excitations [CCSD(T)] and large internally contracted multireference configuration interaction (icMRCI) wave functions. The best estimates for the dissociation energies (De’s) are 76.6±1.3 kcal/mol for NF and 64.6±1.3 kcal/mol for NCl, respectively. Our results suggest that previous experimental estimates for the dissociation energy of NCl are in error by as much as 15 kcal/mol. The calculated spectroscopic constants for NF and NCl are in good agreement with the measured constants.
Quantum study of the Li+HF→LiF+H reaction
Authors: Aguado Alfredo , Paniagua Miguel , Lara Manuel , Roncero Octavio
Published: July 2002

In this work we present a new global fit for the potential energy surface of the LiFH system. This fit is an improvement of a recently published one [Aguado et al., J. Chem. Phys. 106, 1013 (1997)] for which more ab initio points have been calculated (from 644 to 2323). The reaction dynamics is studied using a time dependent treatment in reactant Jacobi coordinates in a body-fixed frame in which the internal coordinates are represented on a grid while Eulerian angles are described in a basis set. The centrifugal sudden approach is tested for total angular momentum J=5 and used to calculate the reaction cross section. The reaction cross section shows oscillations as a function of kinetic energy. This is a consequence of strong interference effects between reactant and product channels and is in agreement with the recent experimental data.
Laser-photolysis/time-resolved Fourier-transform absorption spectroscopy: Formation and quenching of HCl(v) in the chain reaction Cl/Cl2/H2
Authors: Eberhard Jürg , Yeh Pey-Shiun , Lee Yuan-Pern
Published: July 2002

A system to measure time-resolved Fourier-transform infrared absorption spectra of gaseous samples using a commercial step-scan spectrometer is described. To increase the signal intensity, the incident infrared light is multipassed within a White cell. Light from a photolysis laser passes through the reaction cell to initiate the reaction in the flowing gaseous sample. The variation of absorbance is obtained from the ac-coupled signal whereas phase information and a reference spectrum are from the dc-coupled signal. The system is tested by probing the temporal evolution of HCl(v) in the chain reaction of H2 and Cl2 initiated by photolysis at 355 nm. Time-resolved absorption spectra of HCl(v=0–2) were obtained with spectral resolution 0.75 cm−1 and intervals down to 5 μs. Kinetic modeling of deduced temporal profiles of HCl(v=0–2) yields rate coefficients of (1.38±0.04)×10−14 and (5.8±0.4)×10−15 cm3 molecule−1 s−1 (in which error limits represent only the uncertainty of the fit) for reactions Cl+H2→HCl(v=0)+H and Cl+H2→HCl(v=1)+H, respectively; the total rate coefficient is in agreement with previous kinetic measurements.
The geometric phase effect in chemical reactions: A quasiclassical trajectory study
Authors: Adhikari Satrajit , Billing Gert D.
Published: July 2002

The hyperspherical formulation of the vector potential arising due to the presence of a conical intersection in the adiabatic potential energy hypersurface of an A+B2 type reactive system is considered. We wish to treat the operators in the Hamiltonian (with or without vector potential) as classical variables and have seen the same direction of the relative shift in the peak position of the rotational distribution as in the quantum mechanical (QM) calculations by Kuppermann and Wu [Chem. Phys. Lett. 205, 577 (1993)]. Though the absolute peak position of the rotational distributions (with and without geometric phase) obtained by quasiclassical trajectory (QCT) calculations differ from this QM calculation, the physical effect nevertheless becomes transparent once again by our calculation which requires very little computational effort.
Tunneling currents in proteins: Nonorthogonal atomic basis sets and Mulliken population analysis
Authors: Stuchebrukhov Alexei A.
Published: July 2002

A rigorous kinetic justification is given of the expression for interatomic tunneling currents introduced in our earlier work [Stuchebrukhov, J. Chem. Phys. 105, 10819 (1996)] for the analysis of long-distance electron transfer reactions when a nonorthogonal atomic basis set is used for the description of the medium intervening between donor and acceptor. The analysis is based on the kinetic equations describing the evolution of the Mulliken populations of the atomic states in the medium during the tunneling transition of a long-distance electron transfer reaction.
Optical–optical double-resonance spectroscopy of autoionizing states of water
Authors: Glab W. L.
Published: July 2002

Double resonance excitation of vibrationally autoionizing states of the water molecule has been performed for the first time. Tunable, coherent vacuum ultraviolet light was used to excite selected rotational levels of the C̃ 1B1 state with one quantum of symmetric stretch vibrational energy (100), from which (100) Rydberg states were excited with a second dye laser. The Rydberg character of the C̃ 1B1 state leads to strong vibrational selectivity for the (100) states in the second transition; therefore, the resonances due to these states are greatly enhanced compared to the case of direct excitation from the ground state, and spectrally simplified due to the well-defined angular momentum of the intermediate state level. The spectra show unexpected peaks which might be attributable to significant 1-mixing in the C̃ 1B1 state as well as the presence of interloping linear states.
Local interactions and protein folding: A three-dimensional off-lattice approach
Authors: Irbäck Anders , Peterson Carsten , Potthast Frańk , Sommelius Ola
Published: July 2002

The thermodynamic behavior of a three-dimensional off-lattice model for protein folding is probed. The model has only two types of residues, hydrophobic and hydrophilic. In absence of local interactions, native structure formation does not occur for the temperatures considered. By including sequence independent local interactions, which qualitatively reproduce local properties of functional proteins, the dominance of a native state for many sequences is observed. As in lattice model approaches, folding takes place by gradual compactification, followed by a sequence dependent folding transition. Our results differ from lattice approaches in that bimodal energy distributions are not observed and that high folding temperatures are accompanied by relatively low temperatures for the peak of the specific heat. Also, in contrast to earlier studies using lattice models, our results convincingly demonstrate that one does not need more than two types of residues to generate sequences with good thermodynamic folding properties in three dimensions.
Entropic forces in binary hard sphere mixtures: Theory and simulation
Authors: Dickman Ronald , Attard Phil , Simonian Veronika
Published: July 2002

We perform extensive Monte Carlo simulations of binary hard-sphere mixtures (with diameter ratios of 5 and 10), to determine the entropic force between (1) a macrosphere and a hard wall, and (2) a pair of macrospheres. The microsphere background fluid (at volume fractions ranging from 0.1 to 0.34) induces an entropic force on the macrosphere(s); the latter component is at infinite dilution. We find good overall agreement, in both cases, with the predictions of a hypernetted chain-based theory for the entropic force. Our results also argue for the validity of the Derjaguin approximation relating the force between convex bodies to that between planar surfaces. The earlier Asakura-Oosawa theory, based on a simple geometric argument, is only accurate in the low-density limit.
Reaction-path dynamics with harmonic vibration frequencies in curvilinear internal coordinates: H+trans-N2H2→N2H+H 2
Authors: Chuang Yao-Yuan , Truhlar Donald G.
Published: July 2002

This paper presents a general method for treating doubly degenerate bending vibrations of bonds with 180 deg bond angles in curvilinear coordinates along a reaction path. The new treatment improves the generalized normal mode frequencies needed for rate constant calculations with variational transition state theory and for multidimensional semiclassical tunneling calculations. We apply this method to calculate harmonic vibrational frequencies and rate constants for the reaction H+trans-N2H2→N2H+H2. The new results are compared to earlier calculations employing rectilinear Cartesian coordinates in order to evaluate the sensitivity of the calculations to the coordinate system chosen.
Instantaneous perspectives on solute relaxation in fluids: The common origins of nonpolar solvation dynamics and vibrational population relaxation
Authors: Larsen Ross E. , David Edwin F. , Goodyear Grant , Stratt Richard M.
Published: July 2002

The basic idea that the instantaneous normal modes of a fluid govern its short-time dynamics has recently been used to arrive at theories for solvation dynamics and for vibrational population relaxation, theories not quite as distinct as one might have guessed for such different-looking relaxation processes. Both theories, in particular, revolve around the weighted spectra of instantaneous normal modes we call the influence spectra, with the distinctions between the different problems showing up largely in the different weightings. We show in this paper that the influence spectra reveal a surprising amount of commonality in these two processes. For the models we consider, involving an atomic solvent and relatively short-ranged intermolecular forces, the two kinds of averaged influence spectra have virtually identical shapes. Moreover, examining a single configuration of the fluid at a time reveals that both spectra are strongly inhomogeneously broadened—that is, relatively few modes contribute at any instant, despite the breadth of the configurationally averaged spectra. What is apparently responsible for this common behavior is yet a deeper similarity. If one focuses specifically on the contributing modes, it becomes clear that the reason they contribute is their ability to move one or two solvent atoms in the immediate vicinity of the solute. This observation implies that it should always be possible for us to construct a set of effective modes involving motions that would be no more elaborate than few-body vibrations but that would still allow us to predict the influence spectra. We demonstrate just such predictions in this paper, using the one or two simple binary modes which vibrate the solute against its nearest-neighbor solvent atom. Binary modes as a class account for no more than the highest 10% of the instantaneous-normal-mode frequencies, yet we find that the solute–solvent binary modes are not only responsible for all of the high frequency aspects of solvation dynamics and vibrational population relaxation, they account in a quantitative sense for the majority of both influence spectra. At least in these examples, the bulk of the mechanism by which short-time relaxation takes place is evidently no more complicated than pair motions—what the rest of the solvent decides is how and when these motions take place.
Measuring Patterson functions of inhomogeneous liquids using the nuclear dipolar field
Authors: Robyr P. , Bowtell R.
Published: July 2002

The long-range nuclear dipolar interactions in liquids are not averaged out by molecular motion and give rise to the dipolar field. In nuclear magnetic resonance experiments, this field can be used to probe the structure of heterogeneous samples. In this contribution, we demonstrate theoretically and experimentally how the signal generated by the dipolar field can provide structural information without the use of any model structure. In the limit where the dipolar field weakly perturbs the evolution of the magnetization, and where the molecular motion is not significantly restricted by the structure, the autocorrelation function, or Patterson function, of the spin density can be obtained. The signal generated by the dipolar field is measured as a function of the spatial modulation imposed on the magnetization and an integral transform of the signal amplitude yields the Patterson function. If the structure is anisotropic, a three-dimensional data set has to be acquired and Fourier transformed. If the sample is isotropic, modulation of the magnetization along a single direction is sufficient and the Patterson function can be calculated from a Hankel transform of the signal amplitude.
Quantum mechanical frequencies and matrix assignments to Al2H2
Authors: Stephens Jeffrey C. , Bolton Evan E. , Schaefer Henry F. , Andrews Lester
Published: July 2002

The three lowest lying isomers of Al2H2, the planar dibridged hydrogen D2h form, the planar vinylidene-like C2v form, and the planar monobridged hydrogen Cs form, were examined using high level ab initio methods. The highest level treatment of the energetics and harmonic vibrational frequencies, TZ2Pf CCSD(T), suggests that the dibridged and monobridged forms of Al2H2 were detected by matrix isolation infrared studies (J. Phys. Chem. 97, 10 295 1993).
Collisional population of ultra-high, ultra-long-living Rydberg states under zero-electron-kinetic-energy conditions
Authors: Bellomo Paolo , Farrelly David , Uzer T.
Published: July 2002

Zero-electron-kinetic-energy photoelectron spectroscopy (“ZEKE-PES”) is based on the pulsed field ionization of long lived Rydberg states (ZEKE states); it is generally accepted that ZEKE states have large angular momentum ℓ, which quenches electron-core interactions, but how they acquire it remains a matter of dispute. We show that {nl}→{nl′} ion-Rydberg collisions are a viable and prominent mechanism for the excitation of large-ℓ Rydberg states. We elucidate the dynamics by an exactly solvable classical model which provides a transparent and intuitive picture of the excitation of high-ℓ states. By a geometric interpretation of the dynamics we are able to predict for which values of the impact parameter and reduced velocity of the incoming ion a change of the angular momentum of the state becomes possible. We pay particular attention to the influence of the quantum defect, δl, on the {nl}→{nl′} cross section and demonstrate that, for small initial angular momenta, δl is itself a major contributor to the experimentally observed scaling of the cross section as ∼n5. This classical-quantum defect model is not only able to explain the dependence of the experimentally measured fractional population of high-ℓ states on the velocity of the incoming ion, but leads to the prediction that dipole-forbidden transitions are likely to dominate ℓ transitions induced by ion-Rydberg collisions under ZEKE-PES conditions.
Plastic crystal phases of hard dumbbells and hard spherocylinders
Authors: Vega C. , Monson P. A.
Published: July 2002

We compare the solid-fluid phase diagrams calculated from computer simulations of hard dumbbells and hard spherocylinders in the region of anisotropy where a plastic crystal phase is stable for both systems. When appropriate reduced units are used for the pressure and densities, the coexistence properties for these models are very similar. The existing evidence suggests that for both models the density change on freezing of the fluid into the plastic crystal phase becomes very small as the limit of stability of the plastic crystal phase is reached, but the transition remains first order.
Rotationally resolved multiphoton ionization photoelectron spectroscopy of the [a 1Δ]3dπ 2Φ and [a 1Δ]5pπ 2Φ Rydberg states of the SH radical
Authors: Milan J. B. , Buma W. J. , de Lange C. A. , Wang Kwanghsi , McKoy V.
Published: July 2002

Experimental and theoretical results on the rotationally resolved photoelectron spectra of the [a 1Δ]3dπ 2Φ(v=0) and the [a 1Δ]5pπ 2Φ(v=0) Rydberg states of the SH radical are presented. In particular, for the former state, ionization via a large number of members of different rotational branches is considered. The interplay between experiment and ab initio calculations, including alignment, allows for a detailed interpretation of the results. Asymmetries apparent in the rotational ionic distributions and the observed suppression of transitions involving a large change in angular momentum can be explained on the basis of the theory.
The relevance of the Laplacian of intracule and extracule density distributions for analyzing electron–electron interactions in molecules
Authors: Fradera Xavier , Duran Miquel , Mestres Jordi
Published: July 2002

A topological analysis of intracule and extracule densities and their Laplacians computed within the Hartree–Fock approximation is presented. The analysis of the density distributions reveals that among all possible electron–electron interactions in atoms and between atoms in molecules only very few are located rigorously as local maxima. In contrast, they are clearly identified as local minima in the topology of Laplacian maps. The conceptually different interpretation of intracule and extracule maps is also discussed in detail. An application example to the C2H2, C2H4, and C2H6 series of molecules is presented.
Many-body components in the integrated far-infrared absorption coefficient of diatomic molecules in spherical solvents
Authors: Roco J. M. M. , Hernández A. Calvo , Velasco S. , Medina A.
Published: July 2002

In this paper we present a quantitative study of the static cancellation effects between two- and three-body components of the total integrated absorption coefficient in the far-infrared spectra of several systems (diluted solutions of diatomic molecules in spherical solvents) with important electric multipolar induced contributions to absorption. This static cancellation decreases with an increasing order of the multipolar induced mechanism. Even more, for hexadecapole-induced dipole contributions, cancellation transforms into enhancement for all the systems considered (CO–Ar, N2–Ar, and N2–Xe at different densities and temperatures). These results are obtained first by computer simulations and second by means of the knowledge of the static structure of the fluid, that is two- and three-body static distribution functions. From both procedures results are similar.
Density profiles at liquid–vapor and liquid–liquid interfaces: An integral equation study
Authors: Iatsevitch Stanislav , Forstmann Frank
Published: July 2002

The structure of liquid–vapor and liquid–liquid interfaces in Lennard-Jones (LJ) fluids and mixtures is studied using integral equations. To obtain density distributions at interfaces between coexisting fluid phases we solve the Lovett–Mou–Buff–Wertheim equation. In this equation we approximate the direct correlation functions of the inhomogeneous fluid via interpolation between the direct correlation functions of the bulk phases. In the homogeneous bulk phases the system of the Ornstein–Zernike equation with the reference-hypernetted-chain closure is solved to obtain the direct correlation functions at coexisting densities. Density distributions and other interfacial properties are studied for a liquid–vapor interface in a pure LJ fluid, in an Ar–Kr mixture and for a liquid–liquid interface between two immiscible LJ fluids. The results are in good agreement with simulations and other theories. At low temperatures the liquid–vapor and liquid–liquid density profiles exhibit oscillating structures with periods near the diameters of the LJ spheres. Being quite weak at liquid–vapor interfaces these oscillations become very pronounced at a liquid–liquid interface between immiscible fluids.
Adiabatic and diabatic potential-energy surfaces of the CN(X 2Σ+,A 2Π)Ne complex and nonadiabatic predissociation dynamics
Authors: Yang Moonbong , Alexander Millard H.
Published: July 2002

Adiabatic and diabatic potential-energy surfaces (PES’s) for the interaction of Ne(1S0) with the CN molecule in its ground (X 2Σ+) and first excited (A 2Π) electronic states were determined using multireference configuration-interaction calculations. The three electronically adiabatic potential-energy surfaces (PES’s, 2 A′ and 1 A″) are transformed to three diabatic PES’s plus one additional PES which describes the coupling between the two diabatic PES’s of A′ symmetry which correlate asymptotically with the X 2Σ+ and A 2Π states of CN. These four diabatic PES’s are then used in the study of the predissociation of the CN(A 2Π)Ne complex, subsequent to electronic excitation of the ground-state complex [CN(X)Ne]. Two channels: Fast spin–orbit predissociation [CN(A 2Π1/2,v=3)Ne→CN(A 2Π3/2,v=3)+Ne] and slow electronic predissociation [CN(A 2Π3/2,v=3)Ne→CN(X 2Σ+,v=7)+Ne] are investigated. The quantum flux method was used to determine the positions and widths of the metastable levels of the complex. For these predissociation processes, comparison is made with the experimental investigation by Fei, Lawrence, Chen, and Heaven.
Paramagnetically induced nuclear magnetic resonance relaxation in solutions containing S⩾1 ions: A molecular-frame theoretical and physical model
Authors: Sharp Robert , Abernathy Shawn M. , Lohr Lawrence L.
Published: July 2002

The enhancement of nuclear magnetic resonance (NMR) relaxation rates produced by paramagnetic solutes is physically rather different for electron spin S=1/2 paramagnetic species than for S⩾1 species due to the presence of zero-field splitting interactions in the electron spin Hamiltonians of the latter. When the zfs energy is larger than the electronic Zeeman energy, the electron spin precessional motion is spatially quantized with respect to the molecule-fixed principal axis system (PAS) of the zfs tensor rather than along the external laboratory magnetic field. An analytical theory of the orthorhombic zfs limit has been derived in which the motion of the electron spin variables is described in the zfs-PAS and that of the nuclear spin variables in the laboratory coordinate frame. The resulting theoretical expressions are simple in form and suggest a physically transparent interpretation of the experiment. The NMR relaxation enhancement R1p results from additive contributions, R1x, R1y, and R1z, arising from the molecular-frame Cartesian components of the time-dependent electron spin magnetic moment operator μr(t). Each Cartesian component R1r depends on the dipolar power density at the nuclear Larmor frequency that is produced by the corresponding Cartesian component of μr(t). The theory displays the dependence of the relaxation enhancement on the variables of molecular structure in a very simple and physically transparent form: R1r∝r−6[1+P2(cos θr)], where r is the interspin distance and cos θr is the direction cosine of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model S=1 complex [trans-Ni(II)(acac)2(H2O)2] (acac=acetylacetonato) in dioxane solvent. The magnetic field dependence of the proton T1 of the axial water ligands has been measured over the range 0.15–1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter |E| and on the electron spin relaxation time τS,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values |E|=1.8±0.1 cm−1 and τS,z=8.0±0.3 ps.
Quantum reaction in a condensed phase: Turnover behavior from new adiabatic factorizations and corrections
Authors: Schwartz Steven D.
Published: July 2002

This paper further investigates quantum activated rate theory from the viewpoint of quantum evolution operators. It is shown that a new adiabatic separation of the quantum system-bath Hamiltonian can, in a single time step, account for quantum turnover behavior at moderate temperatures, and it is also shown how this turnover exponentially vanishes at low temperatures. It is further shown that incorporation of nonadiabatic (interaction representation form) corrections produces quantitatively accurate results at low temperatures, thus extending the applicability of the interaction representation form of nonadiabatic corrections to adiabatic evolution operators.
On the Ã and B̃ electronic states of NCO and its clusters with nonpolar solvents
Authors: Yao Jianlong , Fernandez Jose A. , Bernstein Elliot R.
Published: July 2002

The B̃←X̃ and Ã←X̃ transitions of the NCO radical and its clusters with nonpolar solvents are studied in a supersonic jet expansion by employing laser-induced fluorescence techniques. Fluorescence excitation (FE) and hole burning spectra are recorded for the NCO radical and compared to previous work. NCO is clustered with Ar, N2, CH4, and CF4 nonpolar molecules to elucidate the effect of solvation on the radical energy levels and dynamics. FE spectra are detected for NCO 1:1 clusters showing blue shifts in their spectra with respect to that of the isolated NCO radical, while their 1:n counterparts show either red or blue shifts. Potential energy surface calculations are performed to evaluate the binding energies and geometries of 1:1 clusters in the X̃, Ã, and B̃ electronic states. The relatively long decay lifetime and red shifted fluorescence wavelength range observed for B̃ state clusters suggests that they decay first through internal conversion (IC) to Ã vibronic levels, and then experience rapid intracluster vibrational redistribution (IVR) and vibrational predissociation (VP), yielding ground state solvent molecules and NCO radicals at lower Ã vibronic levels. These Ã state NCO radicals subsequently emit, generating the Ã→X̃ band.
Polarized atomic orbitals for self-consistent field electronic structure calculations
Authors: Lee Michael S. , Head-Gordon Martin
Published: July 2002

We present a new self-consistent field approach which, given a large “secondary” basis set of atomic orbitals, variationally optimizes molecular orbitals in terms of a small “primary” basis set of distorted atomic orbitals, which are simultaneously optimized. If the primary basis is taken as a minimal basis, the resulting functions are termed polarized atomic orbitals (PAO’s) because they are valence (or core) atomic orbitals which have distorted or polarized in an optimal way for their molecular environment. The PAO’s derive their flexibility from the fact that they are formed from atom-centered linear-combinations of the larger set of secondary atomic orbitals. The variational conditions satisfied by PAO’s are defined, and an iterative method for performing a PAO-SCF calculation is introduced. We compare the PAO-SCF approach against full SCF calculations for the energies, dipoles, and molecular geometries of various molecules. The PAO’s are potentially useful for studying large systems that are currently intractable with larger than minimal basis sets, as well as offering potential interpretative benefits relative to calculations in extended basis sets.
B3LYP-DFT characterization of the potential energy surface of the CH(X 2Π)+C2H2 reaction
Authors: Vereecken Luc , Pierloot Kristine , Peeters Jozef
Published: July 2002

The potential energy surface for the reaction of doublet methylidyne with acetylene has been investigated in detail using the B3LYP-DFT/6-31G** quantum chemical method. Three barrierless entrance channels lead to the formation of initially highly excited C3H3 radicals, the most stable of which is the 2-propynyl radical (propargyl). Other C3H3 isomers characterized include 1-propynyl, as well as the cyclic structures cycloprop-2-enyl and cycloprop-1-enyl which were not considered in a previous theoretical study by Walch. All identified C3H3 isomers can interconvert via transition states lying well below the entrance and exit channels. The dissociation pathways of the C3H3 radicals leading to various C3H2 isomers+H have been identified. The energetically most favorable of these exit channels was found to be the formation of singlet cyclopropenylidene+H. Other favored routes are formation of triplet prop-2-ynylidene+H and of singlet propadienylidene+H. Also identified are pathways leading to linear-C3H+H2. The transition paths of all barrierless reactions were characterized by calculating a large number of points along the reaction coordinate, allowing for a microvariational treatment of these reactions in later kinetic RRKM calculations. The kinetic and mechanistic aspects of the CH+C2H2 reaction are discussed qualitatively, based on the data obtained in this study and in the available experimental and theoretical literature.
Kinetic theory of bimolecular reactions in liquid. I. Steady-state fluorescence quenching kinetics
Authors: Yang Mino , Lee Sangyoub , Shin Kook Joe
Published: July 2002

A microscopic kinetic theory for steady-state fluorescence quenching reaction in liquid is formulated. Based on a linear reaction-Liouville equation for the distribution function in phase space, we derived a memory equation for the relaxation of singlet density function of reactants by use of Mori’s projection operator technique. The expression of the memory kernel is analyzed by the fully renormalized kinetic theory developed by Mazenko. The memory kernel includes the many-body information via a hierarchical structure of a propagator in that. This hierarchy is truncated by a disconnected approximation for the propagator governing the dynamics of an orthogonalized doublet field creating their initial correlation via a bimolecular interaction. This approximation is different from the dynamic superposition approximation for reduced distribution functions made in usual hierarchical approaches. As a result, the detailed description of reactant dynamics becomes available and the memory kernel consists of a geometric series describing the repeated ring collision (reaction) events. We obtain a self-consistent algebraic equation at the diffusion level, which is easily solved by a few iteration, for the response of concentration of reactants to a constant external perturbation. The effects of intensity of external constant perturbation are explicitly considered. The present theory yields the same result with that of the mean-field diffusion theory although the approximations and the assumptions are quite different from each other.
Reduced density matrix description of gas–solid interactions: Scattering, trapping, and desorption
Authors: Jackson Bret
Published: July 2002

A formalism is developed for modeling the interaction between a particle scattering from a surface and the thermal vibrations of the lattice, using the reduced density matrix. A short time propagation algorithm is constructed, with the dissipative terms derived directly from the full particle–bath Hamiltonian. The resulting Bloch–Redfield equation is used to study the scattering of He, Ne, and Ar from model Cu and Ru surfaces, over a wide range of energy and temperature. Trapping and desorption are also examined, and comparisons are made with experimental sticking data. The method is shown to give reasonable results and to accurately describe the relaxation to equilibrium over long times.
Theoretical study of the electronic states of Rhn+ (n=3–5)
Authors: Majumdar D. , Balasubramanian K.
Published: July 2002

Geometries and energy separations of the various low-lying electronic states of Rh3+, Rh4+, and Rh5+ clusters with different structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field method, (CASMCSCF) followed by large scale multireference singles plus doubles configuration interaction (MRSDCI) computations that included up to 1.6 million configurations for Rh3+, 1.5 million configurations for Rh4+, and 2.7 million configurations for Rh5+, were used. Four nearly degenerate electronic states were found for Rh3+. The A′6 electronic state of the pyramidal structure of Rh4+ was found to be the ground state. Although the A″6 electronic state of Rh4+ with the same geometry was found to be close in energy at the CASMCSCF level, it is 0.5 eV higher at the higher level of theory. Two nearly degenerate electronic states (A″5 and A″5) of the distorted edge capped tetrahedron structure of Rh5+ were found as candidates for the ground state of this cluster. Mulliken population analysis has been carried out on these cationic clusters to investigate their bonding properties.
Electron-electron counterbalance hole in Hartree–Fock theory
Authors: Koga Toshikatsu
Published: July 2002

The electronic extracule density E(R) and its spherical average d(R) are probability densities of finding the center-of-mass vector (rj+rk)/2 and its magnitude |rj+rk|/2 of any two electrons j and k to be R and R, respectively. Theoretical analysis of spin–orbital-pair contributions Ejk(R) and djk(R) to the extracule densities shows that within the Hartree–Fock framework, there exists an “electron-electron counterbalance hole” Ejk(0)=djk(0)=0 between two electrons in spin–orbitals j and k with the same spin and the same spatial inversion symmetry, which implies that the center-of-mass vector (or distance) of these electrons cannot be zero or these electrons cannot be exactly at the opposite positions in space with respect to the inversion center. The same is also true in momentum space. The electron-electron counterbalance hole may be considered as a complement to the familiar electron-electron coalescence or Fermi hole for two electrons with the same spin. We also point out that the electron-electron counterbalance and coalescence densities are rigorously proportional for a pair of spin–orbitals with different spins.
A novel Monte Carlo algorithm for polarizable force fields: Application to a fluctuating charge model for water
Authors: Martin Marcus G. , Chen Bin , Siepmann J. Ilja
Published: July 2002

In this Monte Carlo algorithm for polarizable force fields, the fluctuating charges are treated as special degrees of freedom subject to a secondary low-temperature thermostat in close analogy to the extended Lagrangian formalism commonly used in molecular dynamics simulations of such systems. The algorithm is applied to Berne’s SPC-FQ (simple point charge–fluctuating charge) model for water. The robustness of the algorithm with respect to the temperature of the secondary thermostat and to the fraction of fluctuating-charge moves is investigated. With the new algorithm, the cost of Monte Carlo simulations using fluctuating-charge force fields increases by less than an order of magnitude compared to simulations using the parent fixed-charge force fields.
Femtosecond dynamics of NaI ionization and dissociative ionization
Authors: Charron Eric , Suzor-Weiner Annick
Published: July 2002

The femtosecond dynamics of NaI two-body (NaI+hν→NaI++e−) and three-body (NaI+hν→Na++I+e−) break-up is investigated by quantum mechanical simulations. A first femtosecond laser pulse creates an excited molecular wave packet, and an additional ultrafast photoionization step monitors its subsequent time evolution. The theoretical results are compared with available experimental data. The branching ratio between these two competing fragmentation channels is extracted and an experimental scenario is proposed to control the outcome of this reactive half-collision.
Theoretical study of the electronic states of CS2++
Authors: Hochlaf M. , Chambaud G. , Rosmus P.
Published: July 2002

Three-dimensional potential energy functions (PEFs) have been generated for the X3Σg−, a1Δg, b1Σg+, and c1Σu− states of CS2++ using internally contracted multi-reference configuration interaction approach. The analytic forms of the PEFs have been employed in the calculations of the vibrational energy levels, the wave functions, and the Franck–Condon factors for the direct ionization process CS2→CS2+++2e. As in CS2 and CS2+, strong anharmonic resonances are found in all four states. The spectra are predicted to consist of vibrational progressions of Fermi polyads. For the 1Δg state the Renner–Teller problem has been solved and the pattern of the bending levels has been analyzed. For 14 electronic states one-dimensional collinear cuts along the charge separation path CS++S+ have been calculated and the corresponding barrier heights of the predissociating states have been determined. The experimental onset for the charge separation paths CS2++→CS++S+ at about 30 eV is found to be consistent with the calculated barrier height on the PEF of the X3Σg− state.
Quantum reactive scattering for three-body systems via optimized preconditioning, as applied to the O+HCl reaction
Authors: Poirier Bill
Published: July 2002

The evaluation of quantum scattering quantities for three-body systems is explored in conjunction with the optimal separable basis methodology, which is utilized in two different ways. First, numerical results are obtained for the zero total angular momentum case using a three-dimensional discrete variable Hamiltonian with absorbing boundary conditions and optimized preconditioning. The J≠0 results are then estimated using helicity-conserving and J-shifting approximations, after minimizing the coriolis coupling via another application of the optimal basis method. An “effective potential” interpretation of the helicity-conserving approximation is employed, which obviates the requirement of a K-varying associated Legendre basis for the angular coordinate. This treatment also leads to an improved version of the J-shifting method that automatically incorporates centrifugal distortion and other effects. Fixed-energy cumulative reaction probabilities and thermal rate constants are presented for the O+HCl→OH+Cl reactive scattering system.
Multidimensional femtosecond spectroscopies of vibrational motions in liquids: Semiclassical expansion
Authors: Chernyak V. , Mukamel S.
Published: July 2002

Fifth- (χ(5)) and seventh- (χ(7)) order electronically off-resonant Raman spectroscopies in molecular liquids are investigated using a new semiclassical expansion of the optical response which applies for weak anharmonicities and low temperatures. The leading contribution can be calculated using classical equations of motion for nuclear wave packets, even when the system itself may be highly nonclassical. Two sources of nonlinearities which generate the signals—the nonlinear dependence of the polarizability on nuclear coordinates and vibrational anharmonicities—are identified. Formal analogy between the present equations and the time-dependent Hartree–Fock equations used in electronic nonlinear spectroscopy suggests specific experimental signatures of the various nonlinearities.
Continuum percolation and depletion flocculation
Authors: Soga K. Geoffrey , Melrose John R. , Ball Robin C.
Published: July 2002

Using Brownian dynamics simulations, we investigate continuum percolation in a system of colloid particles aggregating due to depletion flocculation. For all values of aggregating potential, there is a lower threshold volume fraction of colloid particles at which a percolating cluster appears. This threshold defines a percolation “phase” boundary that crosses the phase boundary between the colloid fluid phase and colloid fluid+crystal coexistence. In the two-phase coexistence regime, the lifetime of percolated clusters increases dramatically, and therefore the percolation threshold may be related to a transient gel threshold reported from experiment. In the phase coexistence regime, there is evidence of aging due to the thermal restructuring of clusters, implying a finite lifetime for percolating structures.
Density functional theory with an approximate kinetic energy functional applied to study structure and stability of weak van der Waals complexes
Authors: Wesołowski T. A. , Ellinger Y. , Weber J.
Published: July 2002

In view of further application to the study of molecular and atomic sticking on dust particles, we investigated the capability of the “freeze-and-thaw” cycle of the Kohn–Sham equations with constrained electron density (KSCED) to describe potential energy surfaces of weak van der Waals complexes. We report the results obtained for C6H6⋯X (X=O2, N2, and CO) as test cases. In the KSCED formalism, the exchange-correlation functional is defined as in the Kohn–Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individual fragments and the explicit functional of electron density to approximate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations. Therefore, the applied implementation of KSCED requires the use of an approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn–Sham formalism. Several approximate kinetic energy functionals derived using a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 44, 768 (1991)] were considered. The functionals of this type are related to the approximate exchange energy functionals and it is possible to derive a kinetic energy functional from an exchange energy functional without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] exchange-correlation functional and the kinetic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered functionals lead to worse results. Compared to the supermolecule Kohn–Sham interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The presented KSCED results together with the previous Kohn–Sham ones [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest.
Dissociative adsorption of H2 on Cu(100): Fixed-site calculations for impact at hollow and top sites
Authors: Mowrey R. C. , Kroes G. J. , Baerends E. J.
Published: July 2002

The reaction of H2 on Cu(100) is studied using a wave-packet method to solve a four-dimensional quantum mechanical model for impact on the high-symmetry hollow and top sites. The potential energy surface (PES) is a fit to the results of density functional calculations treating a periodic overlayer of H2 on a Cu slab. The dynamics calculations include motion in the azimuthal coordinate although the PES does not depend on φ for impact on the top and hollow sites. Large dissociation probabilities (∼0.9) are found for impact at the hollow site but those for impact at the top site are lower (∼0.3). Dissociation probabilities for molecules incident with “helicoptering” motion (mj=j) are larger than those for molecules with “cartwheeling” motion (mj=0). This differs from the results of previous calculations for impact at the azimuthally corrugated bridge site which predicted comparable probabilities for the two orientations of incident molecules. The dissociation probabilities from fixed-site calculations at the different impact sites are combined to yield an averaged probability which is compared with experiment and the results of six-dimensional quantum calculations. Vibrationally inelastic scattering is predicted to occur primarily for impact at the top site.
Wavelength and temperature dependence of the absolute O(1D) production yield from the 305–329 nm photodissociation of ozone
Authors: Takahashi Kenshi , Taniguchi Nori , Matsumi Yutaka , Kawasaki Masahiro , Ashfold Michael N. R.
Published: July 2002

O ( 1 D) and O(3Pj) photofragments produced in the photodissociation of ozone in the wavelength range 305–329 nm both at 295 and 227 K have been detected directly using a technique of laser induced fluorescence (LIF) in the vacuum ultraviolet (vuv). Photofragment excitation (PHOFEX) spectra for both species have been measured by scanning the photodissociation laser wavelength whilst monitoring vuv-LIF at 115 nm [O(1D)] and 130 nm [O(3Pj)]. After applying suitable corrections for the relative detection sensitivities, suitably weighted combinations of these PHOFEX spectra were found to provide a quantitative match to the parent O3 absorption spectrum both at 295 and 227 K, thereby providing a method of determining both the wavelength and temperature dependence of the absolute O(1D) quantum yield, Φ1D(λ,T). Hot band excitation of internally excited O3 molecules and dissociation via the spin-allowed channel yielding O(1D)+O2(a 1Δg) products makes the dominant contribution to the quantum yield Φ1D(λ,T) in the wavelength range 310–320 nm. For λ>320 nm, however, both the Doppler profiles of the nascent O(1D) atoms and the temperature dependence of the O(1D) yield indicate that the O(1D) atoms must arise predominantly via the spin-forbidden channel yielding O(1D)+O2(X 3Σg−). The analysis allows a first determination of the absolute branching to this channel (Φ∼0.08) following O3 photolysis in the wavelength range 318–329 nm. The present work highlights remaining deficiencies in the latest NASA/JPL recommendations regarding the wavelength and temperature dependence of the O(1D) yield resulting from O3 photolysis at λ>320 nm.
Dynamic Stokes shift in solution: Effect of finite pump pulse duration
Authors: Georgievskii Yuri , Hsu Chao-Ping , Marcus R. A.
Published: July 2002

The time-evolution of the fluorescence spectrum of a dissolved chromophore excited by an ultrafast pump pulse is considered. The average value of the energy difference of the solute in its excited and ground states is used to describe the relaxation of the maximum of the transient fluorescence spectrum to its equilibrium value (dynamic Stokes shift, DSS). A simple formula for the normalized DSS is obtained which generalizes an earlier standard classical expression and includes the effect of a pump pulse of finite duration. As an example, dielectric dispersion data are used for a dipolar solute in water to estimate the quantum correction to the standard DSS expression. The correction is negligible when the frequency of the pump pulse is close to the maximum in the absorption spectrum, but a deviation from the standard formula can be expected for the pump pulse tuned to a far wing of the absorption band of the chromophore. An expression is given for this deviation.
Optical limiting properties and dynamics of induced absorption in C60-doped solid xerogel matrices
Authors: Schell J. , Brinkmann D. , Ohlmann D. , Hönerlage B. , Levy R. , Joucla M. , Rehspringer J. L. , Serughetti J. , Bovier C.
Published: July 2002

We present measurements of induced absorption and its dynamics in C60-doped solid xerogel matrices. The measurements are performed in a pump-test geometry using picosecond pulses generated from a frequency doubled mode locked Nd:YAG laser at 532 nm. We observe an induced absorption leading to optical limiting. We find outstanding optical limiting qualities for single shot excitation but some degradation above a certain threshold intensity for repetitive pulses. Below this degradation threshold, the material is stable and its limiting dynamics is studied. A decrease of the initial laser-induced absorption with a time constant of 150 ps is observed, down to a very long-lived constant level. We attribute this dynamics of induced absorption to a transition from excited singlet to triplet electronic states in the C60 molecule.