bluuebunny/crossref_metadata_2025 huggingface dataset:
Hyperfine structure of pendular states and the sign of the dipole moment of ICl A state
Authors: Durand A. , Loison J. C. , Vigué J.
Published: July 2002

Since a first study by Cummings and Klemperer [J. Chem. Phys. 60, 2035 (1974)] and further works by Friedrich and Herschbach, the dipole moments of the X and A states of ICl are considered to have opposite signs. We have performed a high resolution study of the Stark effect of the A–X transition near the 21-0 bandhead, and we have made a complete theoretical simulation of the spectrum, taking into account the hyperfine structure which is important. The comparison of the experimental spectrum with the simulation clearly prove that the dipole moments of the A and X state have the same sign. This result is in agreement with simple electronegativity arguments.
Mapping of wave-packet dynamics in a double-well potential via femtosecond pump/probe photoelectron spectroscopy
Authors: Meier Christoph , Engel Volker
Published: July 2002

The kinetic energy distribution of electrons obtained from femtosecond pump/probe ionization of Na2 is calculated. The pump laser pulse prepares a vibrational wave packet in the 1Σ+u double minimum potential of the molecule which serves as the initial state for subsequent ionization induced by absorption of another photon from the probe pulse. The time dependence of the electron kinetic energy distribution reflects details of the vibrational dynamics. In particular the splitting of the packet can be observed in the case when its mean energy equals the barrier height between the potential wells. The estimate of several molecular properties from a theoretical analysis of the electron spectra is carried through and discussed.
A model for the structure of square-well fluids
Authors: Yuste S. Bravo , Santos A.
Published: July 2002

A simple explicit expression for the Laplace transform of rg(r) for 3D square-well fluids is proposed. The model is constructed by imposing the following three basic physical requirements: (a) limr→σ+g(r)=finite, (b) limq→0S(q)=finite, and (c) limr→λσ−g(r)/limr→λσ+g(r)= exp(ε/kBT). When applied to 1D square-well fluids, the model yields the exact radial distribution function. Furthermore, in the sticky-hard-sphere limit [λ→1, ε→∞, (λ−1)exp(ε/kBT)=finite] the model reduces to Baxter’s exact solution of the Percus–Yevick equation. Comparison with Monte Carlo simulation data shows that the model is a good extension of Baxter’s solution to ‘‘thin’’ square-well fluids. For ‘‘wide’’ square-well fluids the model is still an acceptable approximation even for densities slightly above the critical density and temperatures slightly below the critical temperature.
Free energy barriers to melting in atomic clusters
Authors: Lynden-Bell R. M. , Wales D. J.
Published: July 2002

We employ an order parameter approach to investigate melting in clusters bound by the Lennard-Jones potential containing 13, 55, and 147 atoms. We find well-defined Landau free energy barriers between solidlike and liquidlike states for the two larger clusters. A barrier is also revealed in an approximate analytical calculation using only information derived from the potential energy surface. For the two smaller clusters the order parameters are calculated for a large number of local minima. This helps us to interpret the Landau free energy calculations and to comment upon the suitability of the various order parameters for the cluster melting process. Systematic quenching offers us further insight into melting events for the 55-atom cluster. Finally, we elaborate further upon the relationships between S-bends and probability distributions in different ensembles.
Reduction of degenerate four-wave mixing spectra to relative populations I. Weak-field limit
Authors: Williams Skip , Zare Richard N. , Rahn Larry A.
Published: July 2002

Diagrammatic perturbation theory combined with a spherical tensor treatment allows the degenerate four-wave mixing (DFWM) signal resulting from an isotropic molecular sample to be decomposed into a sum of three multipole moments in the weak-field (no saturation) limit. The zeroth moment gives the relative internal-state population contribution, the first moment the orientation contribution, and the second moment the alignment contribution to the DFWM spectra. This treatment makes explicit how the magnitude of the DFWM signal depends on the polarizations of the other three beams and the collisional relaxation caused by the environment. A general expression is derived for the DFWM signal for an arbitrary geometric configuration of the beams (arbitrary phase matching geometry). Under the assumption that the rates of collisional relaxation of the population, the orientation, and the alignment are the same, simple analytic expressions are found for the most commonly used experimental configurations, which should facilitate the practical analysis of DFWM spectra.
Electronic degrees of freedom and unimolecular rate constants in metal clusters
Authors: Hansen K. , Manninen M.
Published: July 2002

We calculate the modifications of the unimolecular rate constants for free metallic clusters due to thermally exited electronic degrees of freedom. The effect is essentially taken into account by substituting Helmholtz’s free energy of the electronic system for the ground state energy. The activation energy pertaining to the electronic ground states of the mother and daughter cluster is then replaced by the first difference in Helmholtz’s free energy.
Highly excited hydrogen molecules desorbed from a surface: Experimental results
Authors: Schermann C. , Pichou F. , Landau M. , C̆adez̆ I. , Hall R. I.
Published: July 2002

The rovibrational excitation of hydrogen molecules resulting from recombination of atoms on a metallic surface has been observed by means of an electronic collision method. We present the obtained H− experimental spectra reflecting the vibrational populations of the desorbed hydrogen molecules. This study brings to light the fundamental role of the nature of the metallic surface in the recombination process: excitation of higher vibrational levels (from v=5 to v=8) is observed only on those metal surfaces which strongly chemisorb atomic hydrogen. To explain these observations we propose the formation, on these surfaces, of a physisorbed H layer on the chemisorbed one.
Hyperfine measurements in the X and B electronic states of I35,37Cl: Probing the ionic character of the chemical bond
Authors: Slotterback Timothy J. , Clement Simon G. , Janda Kenneth C. , Western Colin M.
Published: July 2002

The laser induced fluorescence spectra of the B 3Π(0+)←X 1Σ+ electronic transition of I35Cl and I37Cl have been recorded with a linewidth of ∼5 MHz. This resolution allows the hyperfine splittings due to both nuclei to be measured. We report values for eQq0(I) and eQq0(Cl), for both I35Cl and I37Cl, for v′=0–2 and v″=0–2. This provides new data for both the ground and excited states. Information on the ionic character of the chemical bond of ICl in the X and B electronic states was deduced using a single electronic configuration [linear combination of atomic orbitals/molecular orbital (LCAO/MO)] model. The ionic character (fractional charge transfer) of the X state is −0.24 for v″=2, 1% less than for v″=0. Since the π and π* orbitals are completely filled for the X state, this ionic character is due entirely to the σ bonding orbital. For the B electronic state, the total ionic character was estimated from the dipole moment, and the hyperfine data were used to separately determine the ionic character of the σ type and π type molecular orbitals (−0.13 and −0.14, respectively). This is qualitatively consistent with the predictions given by the LCAO/MO model. However, the ionic character of the π type orbitals is observed to change rapidly with vibrational excitation, with the magnitude tending to zero at v′=2.
Density-functional modeling of structure and forces in thin micellar liquid films
Authors: Pollard Maria L. , Radke C. J.
Published: July 2002

Recent equilibrium force measurements on aqueous films of surfactant above the critical micelle concentration show oscillations for film thicknesses up to 50 nm. To model this phenomenon we express the micellar contribution to the disjoining pressure in terms of thickness-dependent inhomogeneous micelle number density distributions through the film. Density functional theory is used to calculate micelle density profiles, presuming the micelles to behave as charged spheres interacting with each other, and with the film interfaces, through screened-Coulomb potentials. The background electrolyte permits dilute micellar solutions to act as concentrated systems exhibiting pronounced layering in the film. For a 0.1 M sodium dodecylsulfate (SDS) film we find up to five micellar layers for a film thickness equal to ten micelle diameters (d), the layer separation scaling with the effective diameter (deff/d=1.86) which includes the micelle Debye atmosphere. The peaks are largest near the interfaces and decay toward the bulk density at the film midplane. The corresponding disjoining pressures show oscillations with the same distance scaling between the branches as in the density profiles; these values are consistent with experiment. With decreasing film thickness, the (meta-)stable disjoining pressure regions represent micellar layers in the film being forced closer together, raising the pressure until the interior layer is expelled, allowing more space between the remaining micellar layers at that thickness.

Repulsive (positive) disjoining pressures result from layer separations less than the corresponding bulk value whereas attractive (negative) regions represent more distance between layers than that in the bulk. The 0.2 M SDS disjoining pressure isotherm exhibits one additional layer than the 0.1 M case for thicknesses up to 50 nm. The pressure magnitudes of the former case are about twice that of the latter. Addition of ionic salts greatly inhibits the long-range micellar structuring. For SDS foam films, predicted disjoining pressures are much higher than measured values. Comparison with cetyltrimethyl-ammonium bromide (CTAB) micellar films in the surface forces apparatus, however, shows near quantitative agreement. The nature of the confining interfaces thus plays a key role in supporting the internal micellar structuring.
Calculation of thermodynamic properties of small Lennard-Jones clusters incorporating anharmonicity
Authors: Doye Jonathan P. K. , Wales David J.
Published: July 2002

A method for calculating thermodynamic properties of clusters from knowledge of a sample of minima on the potential energy surface using a harmonic superposition approximation is extended to incorporate anharmonicity using Morse correction terms to the density of states. Anharmonicity parameters are found for different regions of the potential energy surface by fitting to simulation results using the short-time averaged temperature as an order parameter. The resulting analytical expression for the density of states can be used to calculate many thermodynamic properties in a variety of ensembles, which accurately reproduce simulation results. This method is illustrated for 13-atom and 55-atom Lennard-Jones clusters.
An order parameter approach to coexistence in atomic clusters
Authors: Doye Jonathan P. K. , Wales David J.
Published: July 2002

We employ order parameters to investigate the dynamic equilibrium between the two lowest energy geometric isomers of LJ8, between permutational isomers of the global minimum of LJ14, and between solidlike, surface-melted, and liquidlike forms of LJ55. For LJ55 approximate analytical calculations which use only information derived from the potential energy surface can accurately model the equilibrium between the different forms. This approach provides a further illustration of how different regions of the potential energy surface influence the thermodynamics. Low-energy transition states are characterized to improve our understanding of the dynamics in the ‘‘surface-melted’’ phases. Finally, using a two state model, we consider the size dependence of the conditions necessary to produce an S-bend (Van der Waal’s loop) in the microcanonical caloric curve.
A new spectroscopic molecular mechanics force field. Parameters for proteins
Authors: Derreumaux Philippe , Vergoten Gérard
Published: July 2002

We present the development of a new spectroscopic molecular mechanics potential for proteins. SPASIBA merges the torsional, van der Waals, electrostatic, and hydrogen bond potentials of AMBER with the Urey–Bradley–Shimanouchi terms for bond lengths and bond angles. To begin, SPASIBA was consistently parametrized to structural, energies, and vibrational frequency data of model compounds representative of the 16 nonaromatic acids: n-alkanes, alcohols, acids, ethyl methyl sulfide, methyl sulfide in water, ethanethiol, dimethyl disulfide, guanidium ion, propionamide, N-methylacetamide, N-methylisobutyramide, and N-isopropylacetamide. The parameters were then transferred to N-acetyl-X methylamides (where X=Gly, L-Ala, L-Pro), the L-Leu, L-Cys, and L-Thr amino acids blocked by the carboxylate and ammonium ions, and the right-handed deca-alanine and Gly-L-Pro-Gly-Gly peptides. Results show that SPASIBA reproduces vibrational frequencies (with much higher accuracy than present molecular mechanics potentials), as well as potential energy distributions of normal modes. For the 532 fundamental frequencies considered for refining the force field, the mean frequency error is 13 cm−1. This force field, which can be incorporated in molecular dynamics simulations, is also found to provide useful insights into the conformation/vibrational spectrum relationships. This ability is illustrated on the Gly and L-Ala dipeptides, as well as the polypeptides.
The structure and chemistry of CH3 and CH radicals adsorbed on Ni(111)
Authors: Yang Q. Y. , Maynard K. J. , Johnson A. D. , Ceyer S. T.
Published: July 2002

A detailed analysis of the vibrational spectra of CH3, CH2D, and CD3 adsorbed on Ni(111) and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH4, CH3D, or CD4 is effected by molecular beam techniques. The ability to measure these spectra by high-resolution electron energy loss spectroscopy (HREELS) at higher resolution (35 cm−1) and higher sensitivity (5×106 counts/s) has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH3 radical is adsorbed with C3v symmetry on a threefold hollow site. The symmetric C–H stretch mode of CH3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH2D is a superposition of those from the decomposition of CH3 and CD3. The adsorption site of the CH species is concluded to be a threefold hollow site and the geometry of the Ni3–C–H is concluded to be pyramidal. At temperatures above 250 K, carbon–carbon bond formation between the CH species is observed to yield C2H2. Low coverages of C2H2 are shown to dehydrogenate at 400 K. High coverages of C2H2 are shown conclusively to trimerize to form adsorbed benzene in contrast to a literature report of C2H2 dissociation to adsorbed CH at these temperatures. The relative stabilities of the hydrocarbon species on Ni(111) are determined to be CH3<CH+2H <1/2C2H2+2H<1/6C6H6+H2(g).
The thermal energy dependence (10–20 eV) of electron impact induced fragmentation of C60 in molecular beams: Experiment and model calculations
Authors: Kolodney E. , Tsipinyuk B. , Budrevich A.
Published: July 2002

We have studied the dependence of electron impact induced ionization and fragmentation of C60 molecules in effusive molecular beams upon the initial thermal excitation in the temperature range of 1190–1875 K, corresponding to an average vibrational energy of 10–20 eV. This is the largest energy range of parent molecule thermal excitation ever reported for electron-impact mass-spectrometric studies. The normalized curves of electron energy (Ee) dependent ion currents of C+60 and C+58 were measured and analyzed for the temperatures (T0) of 1190, 1435, 1570, 1695, and 1875 K. Similar measurements were done for C+2n (n=26–28) fragments for T0=1190 and 1875 K. We have developed an expression for the dependence of C+58 fragment ion current i58(Ee,T0), formed via the decay process C+60→C+58+C2, on electron energy and initial temperature. Using this expression and the strong temperature dependence observed, we have proposed a simple experimental method for estimating the energy deposition function—the probability density of vibrational excitation ε by an ionizing electron of energy Ee. The effective (apparent) value of maximum deposited energy was found to be εm(Ee)=Ee−E*, where E*=30±5 eV. Possible interpretations for this surprisingly low value are discussed. Comparing the experimental i58(Ee,T0) curves with the calculated ones over the range of Ee=30–80 eV we find that for T0≤1600 K, good agreement is obtained assuming that the C60 initial internal excitation is determined by the source temperature alone.

For the higher temperature range 1600 K≤T0≤1900 K, we had to use a modified calculation taking into account radiative cooling and ensemble evaporative cooling processes along the molecular beam flight path. As a result, we have obtained an accurate simulation of the complete family of i58(Ee,T0) curves over all the temperature range measured, using a single set of independently measured physical quantities, and without any adjustable parameter. Uniqueness and sensitivity were thoroughly checked and demonstrated. The good agreement between experiment and calculation basically confirms our description of the underlying process and provides an additional support for the values of the independent physical parameters used. We have used maximum energy deposition parameter of E*=31 eV, an activation energy of E0=4.3–4.5 eV for the neutral fragmentation channel C60→C58+C2 and E1=4.0 eV for the ion fragmentation channel C+60→C+58+C2, and pre-exponential factors of A0=A1=2.5×1013 s−1. These values are very close to former ones obtained by us from analysis of time-of-flight distributions and integrated flux decay measurements of hot C60 molecular beams. Correspondence with other results reported in the literature is discussed and a two-step dissociation mechanism is proposed.
Kinetics of homopolymer collapse
Authors: Byrne Alan , Kiernan Paul , Green Daron , Dawson Kenneth A.
Published: July 2002

Langevin-equation simulation is used to study the dynamics of a simple model of homopolymer collapse at infinite dilution for polymers of lengths N=100, 200, 300, 500, and 1024 units. Lennard-Jones potentials are chosen to represent the bead–bead interactions and a harmonic potential bonds nearest-neighbor beads. The Flory coil-to-collapsed globule transition is brought about by an adjustment of the Lennard-Jones potential interactions. The average cluster size and distribution, along with the radius of gyration are used to characterize the collapse dynamics. Our results indicate that there are several kinetic mechanisms that lead to a collapsed polymer. At early times the dominant mechanisms involve cluster creation and growth by adsorption of slack polymer, whereas for later times the mechanism involves cluster coarsening. For this latter mechanism we find that the average cluster size 〈s〉 grows as 〈s〉=Atz, where z is dependent on u3, the third virial coefficient. Depending on the value of u3 we determine two values for z, first, z=0.60±0.02 for u3 not equal to zero and second z=0.66±0.03 for u3 equal to zero, both representing a departure from Lifshitz–Slyozov theory.
Molecular dynamics study of transport coefficients for hard-chain fluids
Authors: Smith Steven W. , Hall Carol K. , Freeman Benny D.
Published: July 2002

Equilibrium molecular dynamics is used to simulate fluids containing molecules modeled as chains of tangent hard spheres. A partially vectorized, efficient algorithm based on the Rapaport method has been designed that allows for very long simulation times and permits calculation of transport coefficients for short chain fluids at liquid-like densities. The self-diffusion coefficient, shear and longitudinal viscosities, and thermal conductivity are calculated for chains of length 2, 4, 8, and 16 at volume fractions ranging from 0.1 to 0.5 using a mean-square displacement approach. Results from the velocity autocorrelation functions provide information about chain motion in the bulk phase. Transport properties for the hard-sphere fluid have also been calculated for systems of 512 particles. Results for chain fluids are compared to results for hard-spheres and to the corresponding Enskog theory.
Temperature-dependent optical dephasing in sol-gel glasses: Photon echo measurements of rhodamine 101 and quinizarin doped aluminosilica xerogels
Authors: Baer Bruce J. , Chronister Eric L.
Published: July 2002

Temperature-dependent (1.2–2.1 K) photon echo measurements have been performed on quinizarin and rhodamine 101 doped into aluminosilica sol-gel glasses as well as rhodamine 101 doped poly-methyl-methacrylate (PMMA) polymer. We observe a temperature-dependent homogeneous linewidth for both quinizarin and rhodamine in the aluminosilica xerogel (ASX) matrix proportional to T2, in contrast to the T1.3±0.1 linewidth dependence typically observed for a variety of organic and inorganic glass hosts in this temperature range (including rhodamine 101 doped PMMA). The contrasting optical dephasing results for the ASX glass and PMMA suggest that the ASX glass may have unique two-level-system dynamics.
Hyperfine analysis of the mixed A 3Π1 v=28 and X 1Σ+ v=69 states of I35Cl
Authors: Slotterback Timothy J. , Clement Simon G. , Janda Kenneth C. , Western Colin M.
Published: July 2002

The A 3Π1 v=28 state of I35Cl is known from previous studies to be strongly mixed with v=69 of the X 1Σ+ state. Spectra of both these mixed states are taken at high resolution to allow accurate measurement of the hyperfine splittings due to the iodine nucleus. The hyperfine structure in both parity components of the A state is found to be perturbed, though the common rotational-electronic induced mixing should only affect the e components. Detailed analysis shows that hyperfine terms in the Hamiltonian also contribute significantly to the rovibronic state mixing. A Hamiltonian for these states is described that accounts for the hyperfine structure in both the A and X states to within the observed linewidth. The hyperfine parameters of this Hamiltonian, including the perturbation parameters, can be accounted for semi-quantitatively in terms of a separated atom model.
On the size-dependence of the static self-energy in propagator calculations
Authors: Deleuze Michaël , Scheller Markus K. , Cederbaum Lorenz S.
Published: July 2002

Various approximations are used currently to evaluate the static part Σ(∞) of the self-energy or optical potential in molecular Green’s function calculations. Since its expansion is ruled out by the linked-cluster theorem, one generally assumes a size-intensive behavior of Σ(∞) in the thermodynamic limit of an infinite system. A detailed analysis of this size-dependence property is conducted using the formulation of crystalline orbitals (CO) for stereoregular polymers. In spite of the linked-cluster theorem, this study provides evidence for a logarithmic divergence with respect to the size of a chain for some forms of Σ(∞), computed with common approximation schemes. This is the direct outcome of the long-range character of the Coulombic interaction and can be related to a violation in the number of particles within the system. A proper size-intensive behavior implies an exact cancellation of the logarithmically divergent behavior of antigraphs. The conclusions drawn from CO analysis are confirmed by numerical tests on model oligomer systems.
Decoupling of vibrational modes as a structural tool: Coverage-induced reorientation of methoxide on Mo(110)
Authors: Weldon M. K. , Uvdal P. , Friend C. M. , Serafin J. G.
Published: July 2002

The structure and reactivity of methoxide adsorbed on Mo(110) was investigated using temperature programmed reaction, x-ray photoelectron, high resolution electron energy loss and infrared reflection absorption spectroscopies. Methanol decomposes through a methoxy surface intermediate on Mo(110), with dehydrogenation and carbon–oxygen bond scission occurring at ∼400 K. The structure of the methoxy moiety is dependent on coverage, by reference to data obtained using surface infrared spectroscopy in combination with selective isotopic labeling. We demonstrate that methoxy exhibits C3v symmetry, i.e., the C–O bond is normal to the surface, at coverages below 0.17 ML. However, the C–O axis begins to tilt towards the surface at higher coverages, so that at saturation coverage (0.25 ML), two distinct methoxy species with Cs symmetry are observed with an average tilt angle of 25°±15° from the surface normal. In addition, we conclusively show that the intense features at ∼2910 cm−1 in the infrared spectrum of adsorbed methoxide are due to overtones of the methyl deformation modes, gaining intensity by Fermi resonance with the symmetric carbon–hydrogen stretching mode, in contrast to previous assignments to the in-plane asymmetric C–H stretch which, we demonstrate, occurs at above 2935 cm−1.
Dispersion energy in the coupled pair approximation with noniterative inclusion of single and triple excitations
Authors: Williams Hayes L. , Szalewicz Krzysztof , Moszynski Robert , Jeziorski Bogumil/
Published: July 2002

The second-order dispersion energy in the coupled-pair (coupled-cluster doubles) approximation has been derived. The coupled-pair amplitudes are subsequently used in a perturbation theory type expression to account for the effects of single and triple excitations. This approach selectively sums to infinite order important classes of intramonomer correlation diagrams resulting in a better theoretical description of the dispersion interaction compared to a finite-order perturbation treatment. Numerical results have been obtained for He2, Ar–H2, Ar–HF, (HF)2, (H2O)2, and He–F− in various geometries and basis sets to illustrate the performance of the nonperturbative versus perturbative treatments of the intramonomer correlation contributions to the energy of the dispersion interaction.
Spectroscopy of pendular states in optical-field-aligned species
Authors: Kim Wousik , Felker Peter M.
Published: July 2002

The results of nonlinear Raman experiments on two intermolecular bands of expansion-cooled naphthalene trimer at 0.03 cm−1 resolution are reported. The polarization and intensity dependences of the observed band contours strongly indicate that the observations pertain to transitions between pendular states that result from the interaction of a strong optical field and the permanent polarizability anisotropy of the trimer.
Dissociation pathways and binding energies of (LiH)nLi+ and (LiH)nLi+3 clusters
Authors: Antoine R. , Dugourd Ph. , Rayane D. , Broyer M.
Published: July 2002

The metastable decomposition of hydrogenated lithium cluster ions (LiH)nLi+m(m=0, 1 and 3; n≤15) is studied by using a reflectron mass spectrometer. These clusters are found to decompose by evaporation of a LiH or a Li2H2 molecule. The binding energy of these clusters are determined, using a statistical model which has been adapted to mixed clusters. Comparison with other mixed clusters suggests that (LiH)nLi+ clusters form compact cubic structure similar to pieces of a crystal lattice. For (LiH)nLi+3 clusters, the dissociation channels are more surprising, and the localization of the two excess electrons is discussed, as well as the possible existence of an energy barrier for the dissociation.
Vibronic structure of the emission spectra from single vibronic levels of the S1 manifold in naphthalene: Theoretical simulation
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

The vibrational structure of electronic spectra of naphthalene is simulated by means of a perturbative calculation of the intensities, based on the vibronic basis set. The formalism is implemented to include the contribution of Herzberg–Teller induced activity for totally symmetric modes and to describe the interference between the latter and the allowed Franck–Condon intensity. Geometries, vibrational normal modes, and vibronic coupling parameters required to model the spectra are obtained by means of ab initio and semiempirical calculations. The structure of absorption and single vibronic level fluorescence spectra is reproduced in detail and consistently for all the spectra examined. It is shown that the intensity of modes ν8a and ν5a is strongly affected by interference effects, and that Dushinsky mode mixing of totally symmetric modes plays a major role in redistributing the intensity among the vibronic bands of the spectra.
Electron transfer dynamics in MoS2 nanoclusters: Normal and inverted behavior
Authors: Parsapour F. , Kelley D. F. , Craft S. , Wilcoxon J. P.
Published: July 2002

The photophysics and electron transfer (ET) dynamics of quantum confined MoS2 nanoclusters have been studied using static and time resolved emission spectroscopy. The MoS2 nanoclusters consist of a single S–Mo–S trilayer, having diameters of ∼2.5 or 4.5 nm. Two types of electron acceptors are adsorbed on these nanoclusters: 2,2′-bipyridine (bpy) and 4,4′,5,5′-tetramethyl-2,2′-bipyridine (TMB). The ET reaction exothermicities may be varied by changing the electron acceptor or by varying the size of the MoS2 nanocluster. TMB is harder to reduce, and thus has a smaller ET driving force than bpy. The smaller nanoclusters have a higher energy conduction band, and thus have a larger ET driving force. In all cases, the ET driving force may be calculated from bulk MoS2 properties and quantum confinement theory. Both ‘‘normal’’ and ‘‘inverted’’ behaviors are observed. A reorganization energy of 0.40 eV is calculated from energy dependent ET rates.
High resolution electronic spectroscopy of 1-aminonaphthalene: S0 and S1 geometries and S1←S0 transition moment orientations
Authors: Berden Giel , Meerts W. Leo , Plusquellic David F. , Fujita Ikuo , Pratt David W.
Published: July 2002

Fluorescence excitation spectroscopy at both vibrational and rotational resolution has been used to probe the changes in energy, electronic distribution, and geometry that occur when 1-aminonaphthalene (1AN) absorbs light at ∼332 nm. The 000 band of the S1←S0 transition of 1AN is red shifted by nearly 2000 cm−1 with respect to the corresponding band of naphthalene. Additionally, it is mainly b-axis polarized, unlike the corresponding bands of naphthalene and other 1-substituted naphthalenes. Thus, 1La/1Lb state reversal occurs on 1-substitution of naphthalene with an NH2 group. The S0 state of 1AN is pyramidally distorted at the nitrogen atom. Additionally, the NH2 group is rotated by ∼20° about the C–NH2 bond. Excitation of 1AN to the zero-point vibrational level of its S1 state reduces the C–NH2 bond length by ∼0.2 Å and flattens the NH2 group along both out-of-plane coordinates. Other vibronic bands in the S1←S0 transition exhibit significantly different rotational constants, inertial defects, and transition moment orientations. An explanation for these findings is given that is based on the well-known conjugative properties of the NH2 group in chemically related systems.
Three-body interactions in fluids from molecular simulation: Vapor–liquid phase coexistence of argon
Authors: Sadus Richard J. , Prausnitz J. M.
Published: July 2002

Gibbs-ensemble molecular simulations are reported for the vapor–liquid phase coexistence of argon using the two-body Lennard-Jones potential. During the simulation, the possible effect of three-body interactions on the pressure and configurational energy of the vapor and liquid phases is estimated by performing calculations with three-body potentials. The intermolecular potentials used for the three-body calculations incorporate the influence of both three-body-dispersion and three-body-repulsion interactions. The results show that three-body repulsion makes a significant contribution to three-body interactions in the liquid phase. The effect of three-body dispersion is offset substantially by three-body repulsion.
Fifth-order nonlinear spectroscopy on the low-frequency modes of liquid CS2
Authors: Tominaga Keisuke , Yoshihara Keitaro
Published: July 2002

We have observed fifth-order optical response from liquid CS2 by six-wave mixing with five different short pulses and investigated the inhomogeneity in the spectral broadening of the low-frequency intermolecular vibrational modes. There are a couple of distinct differences in the time profile of the fifth-order signal from that of the third-order signal. The fifth- and third-order signals obtained experimentally are compared with the prediction by the theory of Tanimura and Mukamel. We simulate the third- and fifth-order signals in the two cases, the homogeneous limit case and the partially inhomogeneously broadened case with a Gaussian distribution function. The experimental results can be reproduced well in terms of two vibrational modes, one overdamped mode in the homogeneous limit and one underdamped mode with an inhomogeneous width of 2.5 ps−1.
Local weak pairs spectral and pseudospectral singles and doubles configuration interaction
Authors: Reynolds Gregg , Martinez Todd J. , Carter Emily A.
Published: July 2002

A new approximate correlation method has been developed by application of the local weak pairs approximation of Sæbo/ and Pulay to pseudospectral singles and doubles configuration interaction (SDCI) as developed by Martinez and Carter. The combination of the localization and pseudospectral approximations attacks both the problems of two-electron integral storage on disk and CI vector storage in memory that, respectively, hinder nondirect local spectral and nonlocal pseudospectral SDCI calculations individually and provides a scaling advantage over even direct local spectral SDCI calculations. The reproduction of total energies to within a kcal/mol leads to speed increases with respect to nonlocal calculations that grow larger with increasing molecular size: little or no savings for ethane and a factor of 1.1–1.6 for larger molecules studied (glyoxal, glycine, C6H2, and C8H2). The prediction of conformational energy differences with the new method appears quite promising, since energy difference predictions accurate to within a kcal/mol of the exact energy differences are obtained even when the single-point total energies are individually many kcal/mol in error. The speed increases for energy difference predictions of both local spectral and pseudospectral SDCI also grow with molecular size: from a factor of 4 in ethane and glyoxal to a factor of 6 in glycine. Additionally, when compared to the exact spectral result, the fastest local pseudospectral prediction of the conformational energy difference in glyoxal is in error by 0.2 kcal/mol and saves a factor of 10 in CPU time, indicating the prospects of combining local correlation and pseudospectral methods.
Test of the equation of state of water up to freezing at 100 and 200 °C
Authors: Grimsditch M. , Popova S. , Polian A.
Published: July 2002

The sound velocity in water at 100 and 200 °C has been determined up to the freezing pressure using Brillouin scattering. In this temperature and pressure range there is little experimental data on any physical property of water. Our results, therefore, provide a convenient test of existing equations of state. We find our results to be in good agreement with those calculated from the most reliable equation of state.
An ab initio study of cluster–surface interaction: Na clusters on the NaCl(001) surface
Authors: Häkkinen H. , Manninen M.
Published: July 2002

Structure, stability, dynamics, and the electronic structure of small sodium clusters with 2 to 12 atoms adsorbed on the NaCl(001) surface are studied using an ab initio total energy method. The interaction between the ideal surface and the metal cluster is found to be weak due to a large energy gap between the surface and the cluster single-electron states. As a result, the geometry and the electronic structure of the smallest clusters (N⩽6) is distorted only slightly if compared to the free clusters. For larger sizes it seems possible to have both 2D and 3D structure isomers with a notable interisomeric energy barrier. The cluster binding energy to the surface has a local maximum at N=6 which we interpret to be caused by electronic shell effects in 2D. The weak cluster–surface interactions are modified dramatically in the presence of surface F centers (missing chlorine atoms). The F center increases the adsorption energy of sodium adatom by 1 eV and changes the nature of adsorption from physisorption to chemisorption. Similarly, the binding of small sodium clusters to the surface is enhanced by F centers.
Free energy and entropy for inserting cavities in water: Comparison of Monte Carlo simulation and scaled particle theory results
Authors: Floris F. M. , Selmi M. , Tani A. , Tomasi J.
Published: July 2002

The process of inserting cavities in water is studied with the aim of a better description of some of the terms necessary in continuum quantum mechanical models. Free-energy changes for the formation of soft and hard spherical cavities in TIP4P water have been computed by Monte Carlo (MC) simulation with statistical perturbation theory, up to a radius of 6 Å. The free-energy change for the formation of a hard sphere, ΔGcav, is obtained combining the ΔGsol of a soft repulsive sphere with the ΔG corresponding to the process of transforming the soft sphere into a hard one. Two definitions of hard-sphere repulsive potentials have been considered, one only based on the distance of oxygens from the center of the cavity, while the other also excludes hydrogens from the same region. Differences in free energies are significant. The cubic polynomial expression ΔGcav, obtained by extrapolating the exact scaled particle theory (SPT) expression for very small excluding cavities, gives results in agreement with MC, with effective “hard-sphere” diameter for water larger than 2.77 Å. The SPT prediction is compared with other treatments based on surface tension. It is shown that a properly chosen surface and an “effective” surface tension of water lead to a good agreement with MC ΔGcav without curvature or microscopic corrections. The “effective” surface tension of water turns out to be very close to the experimental value. Some different simple ways to extend SPT expression to nonspherical cavities have been compared, for a limited number of nonspherical convex cavities modeled as n interlocking spheres, meant to mimic n-alkanes in the all-staggered conformation. Entropy changes for soft cavities have been computed with two methods, i.e., combining free energy and enthalpy computations and by finite difference methods. Discrepancies between SPT predictions and MC results are significant. The calculated probability distributions of relevant angles of first hydration shell waters are consistent with orientations where no O–H or O–lone pair vector points towards the cavity. Their variation when the cavity size increases is mostly quantitative and only the broadening of the bands observed for the largest cavities might indicate the early stage of the transition to hydration patterns peculiar to an infinite hydrophobic surface.
First-order one-electron properties in the integral-direct coupled cluster singles and doubles model
Authors: Halkier Asger , Koch Henrik , Christiansen Ove , Jo/rgensen Poul , Helgaker Trygve
Published: July 2002

An integral-direct implementation of first-order one-electron properties in the coupled cluster singles and doubles (CCSD) model is presented. The implementation increases the range of applicability of CCSD first-order one-electron property calculations significantly compared to nondirect approaches. As an application a thorough basis set investigation is performed on five diatomic molecules at the Hartree–Fock and CCSD levels for the molecular electric dipole moment, the molecular electric quadrupole moment, and the electric field gradient at the nuclei. In general, basis sets of polarized triple-zeta quality are the smallest to be recommended, and the convergence towards the basis set limit is faster at the Hartree–Fock level than at the CCSD level. Among the properties considered, the electric dipole moment is the easiest to converge. The electric dipole and especially the electric quadrupole moment require diffuse functions for high accuracy. With standard basis sets, it is not possible to calculate electric field gradients consistently within three thousandths of an atomic unit of the basis set limit—for this purpose, elaborate nonstandard basis sets are required. The electric field gradients at the nuclei in HCN and the electric dipole moment of the furan molecule are calculated at the CCSD level employing up to 417 basis functions, further demonstrating the large-scale applicability of the implementation.
Electronic structure of titanium oxide clusters: TiOy (y = 1−3) and (TiO2)n (n = 1−4)
Authors: Wu Hongbin , Wang Lai-Sheng
Published: July 2002

The electronic structure of two series of small titanium oxide clusters, TiOy (y = 1−3) and (TiO2)n (n = 1−4), is studied using anion photoelectron spectroscopy. Vibrationally resolved spectra are obtained for TiO− and TiO2−. Six low-lying electronic states for TiO are observed with five of these excited states resulting from multielectron transitions in the photodetachment processes. TiO2 is found to be closed-shell with a 2 eV highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO-LUMO-) gap. The two lowest triplet and singlet excited states of TiO2 are observed with excitation energies at 1.96 and 2.4 eV, respectively. TiO3 is found to have a very high electron affinity (EA) of 4.2 eV, compared to 1.30 and 1.59 eV for TiO and TiO2, respectively. The larger (TiO2)n clusters are all closed-shell with HOMO-LUMO gaps similar to that of TiO2 and with increasing EAs: 2.1 eV for n = 2, 2.9 eV for n = 3, 3.3 eV for n = 4. The small HOMO-LUMO gaps for the clusters compared to that of bulk TiO2 are discussed in terms of the structure and bonding of these clusters.
Modeling the concentration dependence of diffusion in zeolites. II. Kinetic Monte Carlo simulations of benzene in Na-Y
Authors: Saravanan Chandra , Auerbach Scott M.
Published: July 2002

We have performed kinetic Monte Carlo simulations of benzene diffusion in Na-Y at finite loadings for various temperatures to test the analytical theory presented in Paper I, immediately preceding this paper. Our theory and simulations assume that benzene molecules jump among SII and W sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. The theory exploits the fact that supercages are identical on average, yielding Dθ=16kθaθ2=κaθ2/6〈τ1〉[1+Keq(1→2)], where kθ is the cage-to-cage rate coefficient, Keq(1→2) is the W→SII equilibrium coefficient, 〈τ1〉 is the mean W site residence time, and κ is the transmission coefficient for cage-to-cage motion. The simulations use fundamental rate coefficients calculated at infinite dilution for consistency with the theory in Paper I. Our theory for kθ, Keq(1→2) and 〈τ1〉 agrees quantitatively with simulation for various temperatures and loadings. The simulated transmission coefficient is nearly 12 for all but the highest loadings, qualitatively validating our mean field approximation. Comparison between our theory and experimental data shows excellent qualitative agreement with tracer zero-length column data, but also shows qualitative disagreement with both pulsed field gradient NMR and frequency response data.
The development of new exchange-correlation functionals
Authors: Tozer David J. , Handy Nicholas C.
Published: July 2002

A procedure is presented for the possible systematic development of exchange-correlation functionals using ab initio electron densities and accurate total energies. For a training set of first row open- and closed-shell systems, densities are computed and are used to determine asymptotically vanishing exchange-correlation potentials. The new functional is then written as an expansion in products of the density and its gradient, and optimum expansion parameters are determined through a least squares fit involving both these potentials and accurate exchange-correlation energies. Unlike conventional functionals, the potential of the fitted functional approaches a non-zero value asymptotically, and this is achieved by introducing a self-consistently computed system-dependent shift into the fitting procedure. This shift represents the influence of the integer derivative discontinuity in the exact energy. The method has been used to determine a 21 term spin-polarized exchange-correlation functional using Brueckner Doubles or MP2 densities of 20 small systems. For those with open-shells the computed shifts are close to the hardness of the system, while for closed-shells they are considerably smaller than the hardness. These observations are consistent with theoretical requirements. A comparison of the new potential with conventional potentials highlights important differences in the inter-shell and asymptotic regions, while the values of the shifts and highest occupied self-consistent eigenvalues suggest improved asymptotic densities. The mean absolute errors in self-consistent total energies and optimized bond-lengths of systems in the training set are 0.003Eh and 0.01 Å, respectively. Comparable values are obtained for 12 first-row closed-shell systems outside the training set. Compared to conventional functionals, the new functional predicts a significantly improved classical barrier height for the hydrogen abstraction reaction H+H2→H2+H.
Nonlocal continuum solvation model with oscillating susceptibility kernels: A nonrigid cavity model
Authors: Basilevsky Mikhail V. , Parsons Drew F.
Published: July 2002

A nonlocal continuum theory of solvation is applied using an oscillating dielectric function with spatial dispersion. It is found that a convergent solution cannot be calculated using a model of a fixed solute cavity inside the solvent continuum. This is attributed to the fact that the dielectric oscillations appear as a result of coupling between polarization and density fluctuations, contradicting the concept of a fixed cavity. The theory is corrected by allowing the cavity size to vary. A cavitation energy and an interaction between the medium reaction field and the cavity size are added to the solvation free energy, and a new theory obtained by a variational treatment. The interaction term enables convergent solutions to become attainable, resulting in an oscillating electrostatic solvation energy as a function of cavity radius, the cavitation term enables these oscillations to be smoothed out, resulting in a regular, monotonic solvation free energy.
Crystal nucleation in the presence of a metastable critical point
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

Density functional theory is applied to the study of crystal nucleation in the presence of a metastable critical point. A phenomenological model for fluids with short range interactions is developed to study the influence of critical density fluctuations on the structure of the critical nucleus and the height of the barrier to nucleation. Our results show dramatic changes in the nature of crystal nucleation near the metastable critical point, with nucleation rates increasing several orders of magnitude; this behavior has important consequences for nucleation of colloids and proteins from solution. A nonmonotonic dependence of the critical cluster size on supersaturation is observed under these conditions.