bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of nitrogen atom recombination on Pyrex: Influence of the vibrationally excited N2 molecules on the loss probability of N
Authors: Lefèvre L. , Belmonte T. , Michel H.
Published: July 2002

A numerical modeling of the surface recombination of nitrogen atoms on Pyrex is developed to account for the temperature dependence of the loss probability γ on the temperature in the range 290–611.5 K. Nitrogen atom recombination is a first-order Langmuir–Hinshelwood mechanism at a temperature below 400 K where both the Langmuir–Hinshelwood and the Eley–Rideal mechanisms are taking place above. Calculated results are compared to experimental measurements obtained in an Ar–N2 Lewis–Rayleigh afterglow. Satisfactory agreement is obtained. The activation energy of the recombination processes is close to 20 kJ/mol. The weak difference between the theory and the experiment is due to the value of the model parameters and not to the influence of the vibrationally excited molecules, which is very weak. It is suggested that the rate constant for vibrational de-excitation of N2(X,v) by an adsorption–phonon mechanism is underestimated. The recombination rate of nitrogen atom on the wall of a cylindrical Pyrex tube is described by d[N]/dt|vsurfaceprocesses=−k[N], with k=γc̄N/2R. The temperature dependence of γ is provided and it is complex but a rough estimation of the rate constant is given by k=17.9 exp(−5900/RT), with T in the range 290–611.5 K.
Quantum confinement and recombination dynamics in silicon nanocrystals embedded in Si/SiO2 superlattices
Authors: Vinciguerra Vincenzo , Franzò Giorgia , Priolo Francesco , Iacona Fabio , Spinella Corrado
Published: July 2002

In this study the structural and optical properties of nanocrystalline Si/SiO2 superlattices have been investigated and discussed. Ordered planar arrays of silicon nanocrystals (Si-nc) have been formed by thermal annealing of ten period amorphous Si/SiO2 superlattices prepared by plasma enhanced chemical vapor deposition. Thermal processing of the superlattices results in well separated (by about 5 nm of SiO2) nanocrystalline Si layers, when the annealing temperature does not exceed 1200 °C. The photoluminescence (PL) properties of these layers have been studied in details. The PL peaks wavelength has been found to depend on the laser pump power; this intriguing dependence, consisting in a marked blueshift for increasing power, has been explained in terms of the longer lifetime characterizing larger Si-nc. It is also observed that these decay lifetimes exhibit a single exponential behavior over more than two orders of magnitude, in clear contrast with the typical, nonsingle exponential trends observed for Si-nc uniformly dispersed inside an insulating matrix. We attributed this peculiar behavior to the lack of interaction among nanocrystals, due to their large reciprocal distance. In agreement with the carrier quantum confinement theory, we have found that the wavelength of the PL peak can be properly tuned by changing the annealing temperature and/or the thickness of the Si layers of the superlattices, and, in turn, the Si-nc mean size. Moreover, the observed lifetimes remain very long (about 0.3 ms) even at room temperature, revealing the absence of relevant nonradiative decay processes in these samples. Furthermore, we have used the experimental PL intensities and decay times to evaluate the radiative rate as a function of the temperature; the obtained data are in good agreement with a model proposed by Calcott in the case of porous silicon. All of these data are presented, discussed, and explained within a consistent picture.
Potassium promoted C–O bond weakening on Ru(001). I. Through-metal interaction at low potassium precoverage
Authors: de Paola R. A. , Hrbek J. , Hoffmann F. M.
Published: July 2002

The interaction of carbon monoxide and submonolayer coverages of potassium on Ru(001) has been investigated with high resolution electron energy loss spectroscopy, LEED, Auger electron spectroscopy, thermal desorption, and work function measurements. The adsorption of potassium on ruthenium at submonolayer coverages (θk≤0.15) is characterized by ionic, mutually repulsive potassium atoms as evidenced by a strong work function decrease (−4.3 eV), various LEED patterns and a large desorption energy (65 kcal/mol) at low coverage. The adsorption of CO on a potassium precovered surface (θk=0.10) is nondissociative and reversible with an initial increase in the activation energy for desorption from 40 kcal/mol for clean Ru(001) to 50 kcal/mol on the potassium precovered surface. The C–O bond is anomalously weak as evidenced from vibrational spectroscopy (EELS), where C–O stretch frequencies in the range of 1400 to 1970 cm−1 are found. Observation of the first vibrational overtone indicates a strong anharmonicity of the C–O potential well, similar to that of molecularly adsorbed oxygen on Pt(111), thus indicating extremely weak C–O bonds. This is further supported by thermal desorption experiments, where complete isotopic scrambling between 12C16O and 13C18O is observed at low CO coverage. Vibrational, work function, and thermal desorption data support a through-metal interaction, where the potassium induced charge redistribution in the metal surface inreases backdonation of metal electrons into the antibonding 2π*-MO of CO, thus causing a C–O bond weakening and an increase in the M–CO bond strength. The observation of multiple states in the vibrational spectra indicate a finite range of the K–CO interaction. Both strength and range of the interaction are found to vary with the ratio of CO:K coverages. At low CO coverage, a strong, short-range interaction between K and CO is observed which causes the CO to adsorb in a side-on bonding mode. With increasing CO coverage (at constant θk) a weakening of the K–CO interaction occurs, which results in CO molecules adsorbed in metal bridge sites, with their molecular axis oriented presumably perpendicular to the surface. Structure models proposed for various potassium and CO coverages suggest a long range for this weak interaction.
Stochastic theory of resonance Raman line shapes of polyatomic molecules in condensed phases
Authors: Mukamel Shaul
Published: July 2002

A stochastic model is proposed for the calculation of resonance Raman line shapes of polyatomic molecules in condensed phases. The model assumes that the random force exerted on the molecule by the solvent causes a stochastic modulation of the electronic energy gap. Explicit expressions are derived for the absorption, the excitation spectra, and the dispersed Raman line shapes. The model interpolates all the way from homogeneous broadening (fast modulation) to inhomogeneous broadening (slow modulation). The effects of line broadening cannot be incorporated into the Kramers–Heisenberg formula, commonly used to interpret these spectra, since dephasing-induced terms appear which dominate the line shapes for large dephasing rates. A simple explanation is provided for the nature of the homogenous linewidth and for the broadening of the dispersed Raman line shapes, without having to assume an unrealistically short lifetime.
Heat aggregation kinetics of human IgG
Authors: Jo/ssang T. , Feder J. , Rosenqvist E.
Published: July 2002

Quasielastic light scattering experiments have been performed to study the aggregation kinetics of human immunoglobulins IgG. Rapid aggregation and irreversible cluster growth results when IgG solutions (2–15 mg/ml) are heated above 50 °C. The measured scattering intensity Ī and effective hydrodynamic radius R̄ can be described consistently by a Smoluchowski aggregation process. The number of clusters ni(t) containing i monomers at time t are computed. The radius of an i cluster is assumed to be Ri =R0iβ where β is the cluster exponent. This kinetic process results in the following characteristic power law behavior: R̄/R0={1+ΓR(T,C,ĉ)⋅t}αR and Ī/I0={1+ΓI(T,C,ĉ)⋅t}αI. Here R0=5.51 nm, is the monomer radius and the I0 the scattered intensity from the monomer solution at temperature T, and concentration C. A fraction ĉ∼0.48 of the monomers are heat stable and do not participate in the aggregation process. The power law behavior of R̄/R0 and I/I0 indicates scaling, and indeed a very satisfactory data collapse results for our data. The best nonlinear fit of the power law forms for R̄/R0 and Ī/I0 gives to αR =0.48±0.05, αI =1.00±0.01, and β=0.39±0.04. We also find that the heat aggregation of IgG is an activated process. Fits of the experimental data gives Gibbs free energy ΔG*=13.8±0.1 kcal/mol at 56 °C for the activated complex. The temperature dependence of the growth rates exhibits an Arrhenius behavior with an enthalpy of activation ΔH*=120±5 kcal/mol. The thermal expansion coefficient of the aggregates increases strongly with cluster size. This effect is accounted for by a weak temperature dependence of β with (dβ/dT)/β=(6.0±0.9)10−3 K−1.
Cluster size distribution in chemically controlled cluster–cluster aggregation
Authors: Family Fereydoon , Meakin Paul , Vicsek Tamàs
Published: July 2002

The dynamics of the cluster–cluster aggregation process is investigated through the time dependent cluster size distribution function using Monte Carlo simulations, scaling theory, and the Smoluchowski coagulation equation. Depending on such factors as the chemical reactivity, kinetic energy, mass, etc. of the aggregates the coagulation of two clusters may or may not take place. These effects are simulated by assuming that the probability that two clusters of sizes i and j irreversibly stick together is proportional to (ij)σ. Our results show that for constant small sticking probability cluster size distribution and its moments asymptotically scale with the same exponents as for the case when the sticking probability is unity. In the early stages, coagulation is slow and the process is chemically controlled. However, for finite sticking probability, as the aggregation process develops in time the chance that two clusters join permanently increases with the surface of the cluster and there is a crossover from chemically controlled to diffusion-limited aggregation. The effect of a finite σ is similar to the exponent γ for the mass dependent diffusion coefficient. There exists a critical value, σc, above which for a given γ the cluster size distribution changes from a bell shaped curve to a monotonically decreasing function of s, in agreement with various experiments. The simulations are found to agree with the dynamic scaling theory, but a reasonable approach to include the effect of sticking probability within the Smoluchowski equation fails to produce results that agree with the simulations.
Theta series and magic numbers for close-packed spherical clusters
Authors: Sloane N. J. A. , Teo Boon K.
Published: July 2002

In the face-centered cubic lattice each sphere is surrounded by a shell of 12 spheres, then shells of 6 and 24, the whole configuration forming a spherical cluster of 43 spheres. This paper presents a mathematical investigation into the possible numbers of spheres in such spherical clusters in the square and hexagonal lattices in two dimensions and the simple, face-centered and body-centered cubic lattices and the hexagonal close packing in three dimensions, for various choices for the center of the cluster. The theta series of these clusters are calculated and the resulting nuclearities (or magic numbers) are tabulated. Applications to cluster chemistry and physics, communication theory, and other fields are highlighted. In particular, the application of the present atom counting results to electron counting of close-packed high nuclearity metal clusters is illustrated with examples.
Fluorescence and stimulated emission S1 → S spectra of acetylene: Regular and ergodic regions
Authors: Abramson Evan , Field Robert W. , Imre Dan , Innes K. K. , Kinsey James L.
Published: July 2002

The S1 → S0 transition of acetylene has been studied by spontaneous and stimulated emission. In the spontaneous emission studies, we report new data on low-resolution dispersed fluorescence from an unrelaxed single rotational level of S1. Transitions into S0 levels from the zero-point level up to ∼28 000 cm−1 of vibrational energy are reported. Earlier investigations of the emission from relaxed acetylene in a neon matrix and of partially relaxed fluorescence from a radio frequency discharge in acetylene have been reexamined and reinterpreted in the light of the present work. The stimulated emission study employed stimulated emission pumping (SEP) for high-resolution investigations of two regions of S0 energies, one near 9550 cm−1 and another near 27 900 cm−1. Definite vibrational assignments were possible for most of the features observed in the lower energy region. This allowed the determination of two new vibrational constants, y224 and y244. The region near 27 900 cm−1 reveals a hitherto unobserved kind of vibrational behavior. A series of broad features (∼1.5 cm−1 wide) appear at intervals of ∼10 cm−1 in broadband SEP scans. At higher resolution, each of these is found to consist of many individual lines with DUMP-laser limited linewidths. This structure is discussed in terms of the Franck–Condon principle for the S1 → S0 transition and the possible relevance of the observed behavior to theories of ergodicity in quantum systems. A detailed treatment of the data, using various measures of quantum ergodicity, appears in the following paper.
On 3d bonding in the transition metal trimers: The electronic structure of equilateral triangle Ca3, Sc3, Sc+3, and Ti+3
Authors: Walch Stephen P. , Bauschlicher Charles W.
Published: July 2002

CASSCF/CCI calculations are presented for the low-lying states of Sc3 and Sc+3 and SCF/CI calculations are presented for the 1A′1 state of Ca3 arising from three ground state (4s2) Ca atoms, all for equilateral triangle geometries. The calculations use effective core potentials, developed by Hay and Wadt, which replace the Ne core but include the 3s and 3p core levels along with the valence electrons in the calculations. The bonding in Ca3 arises by 4s→4p promotion and leads to a well depth of about 0.5 eV for R(Ca–Ca)=7.5a0. For Sc3 the 4s bonding is similar to that in Ca3, but the 3d electrons are also strongly bonding leading to a 2A′2 ground state with a well depth of about 1.0 eV and R(Sc–Sc)=5.75a0. The good 3d bonding orbitals (bonding between all three atoms) are 3da″2 derived from atomic 3dπ″ and 3da1 derived from atomic 3dσ, while 3dπ′ atomic orbitals lead to 3de′ orbitals which are bonding between pairs of atoms, and the 3dδ′ and 3dδ″ derived levels are nonbonding. (Here the atomic symmetry is given with respect to an axis connecting the atom to the center of the molecule.) Based on the Sc3 calculations and preliminary calculations on Ti+3 , the bonding in V+3 and Cr+3 is also discussed.
Raman band profiles and mobility of polymethylene chains
Authors: Cho Yonghae , Kobayashi Masamichi , Tadokoro Hiroyuki
Published: July 2002

In order to elucidate quantitatively the effects of the conformational order as well as of the chain mobility on the scattering profile of the CH2 antisymmetric stretching (νa) Raman band, the temperature dependence of the half-width and the integrated intensity was investigated on single crystals of urea inclusion adduct of n-C20H42, liquid n-alkanes of n-C6H14 through n-C18H38, and linear-low-density polyethylene (LLDPE). In the case of the high-temperature (hexagonal) phase of urea-n-alkane adduct where the paraffin molecules undergo a vigorous rigid rotational motion, a significant motional broadening of the νa band was observed with a rise of temperature, keeping the integrated intensity unaltered. The trans νa band of molten n-paraffins became broader as temperature increases. These results indicate that the νa band is broadened by the conversional motion between the trans and gauche forms as well as the rigid rotation of the molecule as a whole. In the case of LLDPE, with decreasing temperature the integrated intensity of the crystalline νa band at 2883 cm−1 increased very sharply in the vicinity of the glass transition temperature, Tg. The integrated intensity of this band was concluded to be proportional to the amount of the CH2 groups which are linked to trans sequences having less mobility and are accommodated in both the crystalline and noncrystalline phases.
Collisional broadening of rotation–vibration lines for asymmetric top molecules. I. Theoretical model for both distant and close collisions
Authors: Labani B. , Bonamy J. , Robert D. , Hartmann J. M. , Taine J.
Published: July 2002

A semiclassical theory of infrared linewidths in the gas, previously developed for linear molecules, is extended to asymmetric top molecules. It includes a satisfactory treatment of the close collisions and is consequently adapted to describe the temperature dependence of the linewidths, in particular in the combustion temperature range. Numerical applications to the water vapor perturbed by nitrogen, oxygen, and argon show strong different behaviors following the nature of the perturber. For oxygen and argon perturbers, the contribution of close collisions is the predominant mechanism of line broadening. The consistency of the theoretical model is evidenced by comparison with precise measurements for H2O–N2, –O2, and Ar at 300 K. Moreover, the calculated temperature dependence of the linewidths is compared with recent experimental data for H2O–Ar between 1300 and 2300 K.
Evaluation of the scattering matrix of an arbitrary particle using the coupled dipole approximation
Authors: Singham Shermila B. , Salzman Gary C.
Published: July 2002

The coupled dipole approximation is applied to the calculation of the scattering matrix of an arbitrary particle. Both isotropic and anisotropic dipoles are used in the calculations. The forms of the matrices obtained for several types of scatterers are found to be in exact agreement with those predicted by symmetry. The method is tested quantitatively by comparison with Mie predictions for solid and coated spheres and good agreement is observed. This comparison is also used to establish appropriate magnitudes for anisotropic dipolar polarizabilities.
Hartree–Fock–Slater-LCAO implementation of the moderately large-embedded-cluster approach to chemisorption. Calculations for hydrogen on lithium (100)
Authors: Ravenek Walter , Geurts Ferdie M. M.
Published: July 2002

We discuss the implementation of Pisani’s moderately large-embedded-cluster scheme in the pseudopotential version of the Hartree–Fock–Slater-LCAO method. We confine ourselves to the chemisorption problem and use a finite representation for the substrate. As an illustration of the method, we perform calculations for the on-top adsorption of hydrogen on the unreconstructed lithium (100) surface. Embedding appears to improve the convergence of the electronic structure with cluster size. The computation time needed for cluster calculations is increased by the embedding, but it is still substantially cheaper to use the indented substrate as an embedding medium than to include it directly.
State-to-state excitation of NO(A 2Σ+, v′=0,1,2) by N2(A 3Σ+u, v′=0,1,2)
Authors: Piper Lawrence G. , Cowles Lauren M. , Rawlins Wilson T.
Published: July 2002

We have determined that the rate coefficient for quenching N2(A 3Σ+u, v′=0) by NO is (6.6±1.0)×10−11 cm3 molecule−1 s−1. Higher levels of N2(A) appear to be quenched with a similar rate coefficient. Separate studies show that the rate coefficient for the excitation of NO(A 2Σ+, v′=0−2) by N2(A 3Σ+u, v′=0) is (10±3)×10−11 cm3 molecule−1 s−1. The apparent discrepancy between the quenching and excitation rate coefficient measurements most likely results from an error in the accepted value of the lifetime N2(A). Our studies indicate that this lifetime is probably about 30% longer than currently believed. We also report rate coefficients for excitation of each of the vibrational levels 0–2 of NO(A) by each of the vibrational levels 0–2 of N2(A) relative to the rate coefficient for excitation of NO(A, v′=0) by N2(A, v′=0).
Temperature dependence of the deexcitation of He(2 3S) by atoms and molecules as studied by pulse radiolysis method
Authors: Koizumi Hitoshi , Ukai Masatoshi , Tanaka Yasushi , Shinsaka Kyoji , Hatano Yoshihiko
Published: July 2002

The temperature dependence of the rate constants for deexcitation of He(2 3S) by Ar, Kr, N2, O2, CO, NO, CO2, and C2H4 has been measured in the temperature range from 133 to 300 K by a pulse radiolysis method, and thus the collisional energy dependence of the deexcitation cross sections is obtained. The results are interpreted in terms of an electron exchange mechanism. The value of α/β, the ratio of the exponential factor of the potential width Γ(R)=A exp(−αR) to that of the interaction potential V*(R)=B exp(−βR), is obtained for each target molecule. It is found that the value of α/β decreases with the increase of the deexcitation probability.
Surface diffusion of hydrogen on Ru(001) studied using laser-induced thermal desorption
Authors: Mak C. H. , Brand J. L. , Deckert A. A. , George S. M.
Published: July 2002

The surface diffusion coefficient for hydrogen on Ru(001) at low coverage was measured using laser-induced thermal desorption techniques. In the temperature range between 260 and 330 K, the diffusion coefficients displayed Arrhenius behavior with an activation barrier Ediff=4.0±0.5 kcal and a preexponential factor D0=6.3×10−4 cm2/s. Agreement between the experimental and theoretical parameters suggests that hydrogen diffuses on the surface by moving from a threefold site to a neighboring threefold site via a twofold site. Surface contaminants such as carbon and oxygen were observed to produce dramatic effects on the hydrogen surface diffusion rate.
Coordinates for molecular dynamics: Orthogonal local systems
Authors: Aquilanti Vincenzo , Cavalli Simonetta
Published: July 2002

Systems of orthogonal coordinates for the problem of the motion of three or more particles in classical or in quantum mechanics are considered from the viewpoint of applications to intramolecular dynamics and chemical kinetics. These systems, for which the kinetic energy of relative motion is diagonal, are generated by making extensive use of the concept of kinematic rotations, which act on coordinates of different particles and describe their rearrangements. An explicit representation of these rotations by mass dependent matrices allows to relate different particle couplings in the Jacobi scheme, and to build up alternative systems (such as those based on the Radau–Smith vectors or variants thereof): this makes it possible to obtain coordinates which, while being rigorously orthogonal, may approximate closely the local ones, which are based on actual interparticle distances and are in general nonorthogonal. It is also briefly shown that by defining as variables the parameters describing the kinematic rotations it is possible to obtain nonorthogonal systems of coordinates, which are useful in the treatment of collective modes.
Dielectric friction theory of the viscosity of electrolyte solutions
Authors: Ibuki K. , Nakahara M.
Published: July 2002

The B coefficient of the Jones–Dole equation for the viscosity of electrolyte solutions is calculated theoretically in the framework of the sphere-in-continuum model. For this purpose, the Hubbard–Onsager electrohydrodynamic equation, where the coupling of the viscous and dielectric frictions are fully taken into account, is numerically solved. The resultant B coefficients are always larger than those given by the well-known Einstein theory. The present theory is compared with the early dielectric friction theory by Clark. In methanol, our results show better agreement with experiment. In water, however, the limitations of the applied continuum model are recognized.
Rate constants for electronic quenching of N2(A 3Σ+u, v=0–6) by O2, NO, CO, N2O, and C2H4
Authors: Thomas Joseph M. , Kaufman Frederick , Golde Michael F.
Published: July 2002

Rate constants for the title reactions have been measured using the discharge-flow technique, with N2(B 3Πg –A 3Σ+u) laser-excited fluorescence detection. C2H4 is an efficient quencher and exhibits little dependence of rate constant on vibrational quantum number v in N2(A). The rate constants for NO, O2, and N2O increase with v at low v, but are nearly independent of v for v≥3. CO shows a very strong dependence, with a peak in the rate constant at v=2 and a trough at v=5. These are the first data for the reactions of N2(A, v=2–6) with N2O. The other data are compared with previous measurements and discussed in terms of models of electronic-to-electronic energy transfer.
Properties of atoms in molecules: Dipole moments and transferability of properties
Authors: Bader R. F. W. , Larouche A. , Gatti C. , Carroll M. T. , MacDougall P. J. , Wiberg K. B.
Published: July 2002

This paper uses the theory of atoms in molecules to investigate the origin of molecular dipole moments. The dipole moment is given by a sum over the net charge and first moment of every atom in a molecule. The first term leads to a charge transfer contribution μc, the second to an atomic polarization contribution μa. It is shown that both terms are, in general, of equal importance in determining both the static molecular dipole moment and the moment induced by a nuclear displacement. Models which imploy only point charges and corresponding bond moments which follow rigidly the nuclear framework, i.e., models which approximate μc and ignore μa, are shown to lead to results that are incompatible with the changes that are found to occur in a molecular charge distribution during a nuclear vibration. The dipole moment is shown to be another group property that is transferable between molecules in the normal hydrocarbons, This property, along with the net charge, the energy, the correlation energy (expressed as a functional of the charge density), and the atomic volumes are transferable for methyl and methylene groups because of a corresponding transferability of the distribution of charge over the basins of the atoms in these groups up to their surfaces of zero flux in the gradient vector of the charge density. Transferability of group properties is not a result of the transferability of individual orbitals or geminals, which necessarily have tails lying outside of the group in question. Atoms have no tails and their energy, as is known from experiment, can in certain instances be transferred between systems with changes of less than 1 kcal/mol. The properties of atoms in molecules can be determined experimentally and quantum mechanics predicts these properties, just as it predicts the properties of a total system.
An examination of the 2 1A1 states of formaldehyde and ketene including analytic configuration interaction energy first derivatives for singlet excited electronic states of the same symmetry as the ground state
Authors: Allen Wesley D. , Schaefer Henry F.
Published: July 2002

In an accompanying paper by Lee, Allen, and Schaefer [J. Chem. Phys. 87, 7062 (1987)], an efficient formulation for the analytic evaluation of two-configuration self-consistent-field configuration interaction (TCSCF-CI) energy first derivatives is presented. In this paper the TCSCF-CI gradient method is adapted and applied to singlet excited electronic states of the same symmetry as the ground state. Since single-configuration self-consistent-field configuration interaction (SCF-CI) wave functions are unreliable for such electronic states due to the possibility of variational collapse, the TCSCF-CI method based on excited-state orbitals is the simplest means by which dynamical electron correlation can be incorporated effectively. Geometrical structures, excitation energies, and harmonic vibrational frequencies obtained with double-zeta plus polarization (DZP) and DZP+Rydberg (DZP+R) basis sets are reported for the 2 1A1 states of formaldehyde and ketene. The 2 1A1 state of H2CO is found to have two distinct minima with C2v symmetry: a (π→π*)1 valence minimum with re(C–O)=1.564 Å and T0=8.50 eV, and an (n→3py)1 Rydberg minimum with re(C–O)=1.216 Å and T0=7.93 eV. Some question exists on whether the (n→3py)1 state occurs experimentally at 8.11 or 7.96 eV, but in either case the agreement between theory and experiment is good. The 2 1A1 state of CH2CO is found to have only one C2v minimum with re(C–O) =1.129 Å and re(C–C)=1.403 Å. This state is predominantly Rydberg in character and can be designated (π→3px)1, but evidence is presented which suggests a moderate amount of Rydberg–valence mixing involving the (π→π*)1 ‘‘state.’’ The DZP+R CISD T0=6.94 eV prediction confirms previous assignments of the experimental band at 6.78 eV to the 2 1A1 state.
Infrared vibration–rotation selection rules for chemisorbed molecules with free internal rotation: Results for ethylidyne on Pt(111)
Authors: Malik I. J. , Brubaker M. E. , Mohsin S. B. , Trenary M.
Published: July 2002

We have studied the infrared spectrum of ethylidyne, CCH3, chemisorbed on the Pt(111) surface over the temperature range 82 to 350 K. We observe three infrared active fundamentals: the C–C stretch at 1118 cm−1, the symmetric CH3 bend at 1339 cm−1, and the symmetric C–H stretch at 2884 cm−1. The absence of three other fundamentals in our spectra confirms that the molecule has C3v symmetry on the surface with the C–C axis oriented along the surface normal as had been determined from other studies. Our IR spectra demonstrate the strict validity of the surface dipole selection rules. We also observe a weak band at 2795 cm−1 which we attribute to the first overtone of the asymmetric CH3 bend at 1410 cm−1. The intensity of the overtone is enhanced by a Fermi resonance with the symmetric C–H stretch. At 82 K the symmetric bend has an unusually narrow intrinsic width of only 1.4 cm−1. The narrowness of this band makes it a good choice for investigating the influence of free rotation about the single C–C bond on the vibrational bands. Free internal rotation in ethylidyne on supported platinum at 77 K has been reported in an NMR study. We consider the vibration–rotation selection rules for an adsorbed molecule freely rotating about the surface normal. We show that the observable vibrational bands cannot exhibit rotational fine structure. However, nonsurface dipole allowed modes observable in transmission IR studies of supported metals are much too narrow to be consistent with free rotation of the CH3 group. The NMR and IR results are consistent if the exchange of the hydrogen atoms among three equivalent sites is fast on the NMR time scale but slow on the IR time scale.
Modeling of dynamics in liquid crystals from deuterium NMR
Authors: Dong Ronald Y.
Published: July 2002

We present a superimposed rotations model that incorporates a three-site jump model to describe the trans–gauche isomerism of a hydrocarbon chain and a small step rotational diffusion model to describe the reorientation of a mesogen in an ordering potential. The model can be used to interpret the spectral densities of motion measured in liquid crystals by deuterium NMR relaxation studies. Under the simplifying assumption that rotation about each carbon–carbon bond is free, the jump model reduces to the free rotation model. Both the jump model and the free rotation model are examined using the reported J1 and J2 values in the literature for the nematogen 5CB-d15. Analytical solutions of rotational diffusion constants by numerical computation are given. The temperature dependence of these rotational diffusion constants provides the activation energies for these diffusion processes. In conjunction with the free rotation model, we found that the contribution to spin relaxation due to director fluctuations appears to show an odd–even effect along the pentyl chain of 5CB.
A unitary multiconfigurational coupled-cluster method: Theory and applications
Authors: Hoffmann Mark R. , Simons Jack
Published: July 2002

A unitary wave operator exp(G) is used to relate a multiconfigurational reference function Φ to the full, potentially exact, electronic eigenfunction Ψ=exp(G)Φ. If the reference function Φ is of a generalized complete-active-space (CAS) form, then the energy, computed as 〈Φ‖exp(−G)H exp(G)‖Φ〉 is size extensive; here H is the full N-electron Hamiltonian. The Hausdorff expansion of exp(−G)H exp(G) is truncated at second order as part of our development. The parameters which appear in the cluster operator G are determined by making this second-order energy stationary. Applications to the widely studied H2O (at the double zeta basis level) and lowest and first excited 1A1 states of BeH2 are performed in order to test this method on problems where ‘‘exact’’ results are known.
The equation of state of hard spheres and the approach to random closest packing
Authors: Song Yuhua , Stratt Richard M. , Mason E. A.
Published: July 2002

Data on the internal pressure of hard-sphere and hard-disk fluids have been available for some time from computer simulations, even at densities above the freezing density. These results for the metastable fluids suggest that the pressure diverges at the density of random closest packing. However, our examinations of these data indicates that the divergence is characterized by a fractional exponent. We show that incorporating this nonanalyticity not only enables us to construct a semiempirical equation of state which is accurate at densities well beyond that of the Carnahan–Starling equation of state, but it enables us to predict a finite entropy at random closest packing. We also show that this kind of thermodynamic singularity implies that the direct correlation function becomes infinitely long ranged with a critical exponent remarkably similar to the percolation exponent. Given the difficulties inherent in simulating hard spheres at such densities, however, we do suggest that these findings be regarded with some caution.
Adhesive hard sphere colloidal dispersions. I. Diffusion coefficient as a function of well depth
Authors: Rouw P. W. , de Kruif C. G.
Published: July 2002

When dispersed in a marginal solvent like benzene, sterically stabilized silica particles exhibit an effective attraction. This attraction is due to the interaction of solvent molecules with the stabilizing chains and is modeled by a square-well potential; the well has a depth of ε/kT=L(θ/T−1), σ≤r≤σ+Δ. L is an interaction parameter, θ the theta temperature of the chain–solvent pair, σ the particle diameter and Δ the width of the square well. An expression for the diffusivity of adhesive hard spheres is derived with the help of the general Stokes–Einstein equation. The model allows scaling to one master plot of experimentally determined diffusion coefficients for samples of six volume fractions, at 20 different temperatures and for each sample at three scattering angles. Additional results obtained with the model are discussed as well. Phase separation in the abovementioned silica dispersions occurs at a well depth of about 5 kT.
Lennard-Jones fluids in cylindrical pores: Nonlocal theory and computer simulation
Authors: Peterson Brian K. , Gubbins Keith E. , Heffelfinger Grant S. , Marini Bettolo Marconi Umberto , van Swol Frank
Published: July 2002

We present adsorption isotherms, phase diagrams, and density profiles for a Lennard-Jones fluid confined to a cylindrical pore. In particular, we concentrate on the gas–liquid transition in the pore (capillary condensation). We compare simulations for a series of radii and different temperatures with mean field density functional theory (MFT). Two forms of MFT are considered, the simple local density approximation (LDA) and Tarazona’s nonlocal or smoothed density approximation (SDA). We find that the SDA provides a quite accurate description of fluid structure in the pore and that it produces phase diagrams in good agreement with the simulation data. For larger radii and temperatures T/Tc≳0.6 the SDA shows steep rises in adsorption close to the transition. This strongly affects the shape of the coexistence curve in the T, ρ̄ plane. Here ρ̄ is defined as the average density inside the pore. This behavior is confirmed by the simulation. In contrast, LDA gives a poor representation of the fluid structure and this underlies the failure to reproduce the phase diagrams and adsorption isotherms found with SDA or simulation. For extremely small radii (R*≈1) the simulation adsorption isotherms are smooth, and for not too low a temperature they are accurately described by an approach which starts from the potential distribution theorem and uses perturbation theory for the true one-dimensional fluid.
Wavelength regulation in bacteriorhodopsin and halorhodopsin: A Pariser–Parr–Pople multireference double excitation configuration interaction study of retinal dyes
Authors: Grossjean Michael F. , Tavan Paul
Published: July 2002

A Pariser–Parr–Pople (PPP) Hamiltonian is employed to study many-electron excitations in protonated and unprotonated retinal Schiff bases. Excited states are described by a multireference double excitation configuration interaction expansion (MRD-CI) and a simplified perturbational treatment. The effects of electron correlation on the spectra of retinal dyes are analyzed and compared with experimental data. It is shown that the spectra of retinal Schiff bases are much more sensitive to the effects of protonation and charge environment than previously assumed. Based on an analysis of observations the computational results demonstrate that varying counterion distance is the essential mechanism of wavelength regulation in the retinal proteins bacteriohodopsin (BR) and halorhodopsin (HR). Spectral properties of intermediates of the photocycles of BR and HR are predicted and it is shown that available spectroscopic data are compatible with a 13,14-cis model of these cycles. Independent evidence is provided that the quantum yield of photoisomerization in BR is 0.6.
The shell structure of atoms and the Laplacian of the charge density
Authors: Shi Zheng , Boyd Russell J.
Published: July 2002

It is shown that the form of the Laplacian of the charge density provides a more complete resolution of the shell structure of atoms than the radial density function. The complete shell structure is resolved for s-block and most p-block atoms, but only the inner shells are resolved in the d-block elements. The shell structures revealed by the Laplacian of the charge density and the radial density function parallel one another where direct comparison is possible.
X-ray diffraction study of a Langmuir monolayer of C21H43OH
Authors: Barton S. W. , Thomas B. N. , Flom E. B. , Rice Stuart A. , Lin B. , Peng J. B. , Ketterson J. B. , Dutta P.
Published: July 2002

We have studied the structure of a monolayer of C21H43OH on water, in the region near close packing, by grazing incidence in-plane x-ray diffraction. For all temperatures studied the isotherms in the πa plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for π=30 dyn/cm the transition temperature is in the range 16.3<T<21.3 °C. We interpret this observation, by comparison with the properties of the lamellar crystalline n-paraffins, as a hexagonal-to-pseudohexagonal structural transition analogous to the crystal rotator II-to-rotator I transition. Our results imply that the hydrocarbon tails of the amphiphile molecules dominate the properties of the monolayer.
Mo/ller–Plesset energy derivatives
Authors: Jo/rgensen Poul , Helgaker Trygve
Published: July 2002

A Mo/ller–Plesset energy functional (Lagrangian) which is variational in all variables (the Lagrange multipliers, the orbital rotation parameters, and the orbital energies) is constructed. The variational property ensures that the responses of the orbitals and orbital energies to order n in geometrical perturbations determine the energy derivatives to order 2n+1. The Lagrange multipliers satisfy the somewhat stronger 2n+2 rule. The multipliers, orbital rotations, and orbital energy responses are determined from coupled perturbed Hartree–Fock-type equations using an exponential parametrization of the orbitals. This ensures that the orbital rotations and energy responses are treated in the same way and calculated from a single set of linear equations. Explicit expressions for energy derivatives up to third order are derived for the second-order Mo/ller–Plesset energy.
Rotary resonance recoupling of dipolar interactions in solid-state nuclear magnetic resonance spectroscopy
Authors: Oas T. G. , Griffin R. G. , Levitt M. H.
Published: July 2002

A new resonance effect in solid-state nuclear magnetic resonance (NMR) is described. The effect involves a combination of magic-angle sample rotation with irradiation of a heteronuclear spin system at the Larmor frequency of one of the spin species. If the irradiation intensity is such as to establish a match between spin nutation and sample rotation, it is shown that the heteronuclear dipolar spin interaction is selectively reintroduced into the spectrum. This allows small dipolar coupling constants to be measured in the presence of large shielding anisotropies. Applications are anticipated for determination of internuclear distances in materials lacking long-range order, such as polycrystalline materials, polymers, and surfaces.
Equation of state for the hard-sphere gas
Authors: Goldman Joseph I. , White John A.
Published: July 2002

Two equations are suggested to describe the hard-sphere system interpolating between gas and close-packed configurations. They indicate some instability and possible phase transition in the same region as that in the Adler and Wainwright molecular dynamics simulation.
Analytic energy derivatives in many-body methods. II. Second derivatives
Authors: Salter E. A. , Bartlett Rodney J.
Published: July 2002

Second derivatives of the energy correspond to second-order response properties and molecular force constants. Currently, both the theory and application of analytic second derivatives in many-body methods are limited to second-order perturbation theory. The general theory of analytic second derivatives for the coupled-cluster (CC) model is presented. The analytic expressions for the second derivative of the energy are given in terms of the response (or ‘‘relaxed’’) density, discussed in part I, and the first-derivative t amplitudes for efficient evaluation. Explicit expressions for the second derivatives of the coupled-cluster singles, doubles, and linearized triples model (CCSDT-1) are presented. Analytic derivatives for the finite-order MBPT(3) and MBPT(4) models are derived as special cases of the theory.
Absorption spectra from high vibrational levels of He2
Authors: Brooks R. L. , Hunt J. L. , Tokaryk D. W.
Published: July 2002

Absorption spectra of dense helium gas at cryogenic temperatures has been acquired while the sample was irradiated using a 6.5 MeV proton beam. By chopping the proton beam, rather than the source lamp, we were able to achieve one part in 104 spectral sensitivity. The spectra showed six new bands in 4He2 and three in 3He2. These have been identified as transitions between high-lying vibrational levels, with the strongest originating on the highest bound level of the a 3Σ+u potential. The temperature and pressure dependence of these features, as compared to low-lying molecular and atomic features, offers some insight into the reaction dynamics of this fundamental system.
Thermodynamic aspects of the vitrification of toluene, and xylene isomers, and the fragility of liquid hydrocarbons
Authors: Alba C. , Busse L. E. , List D. J. , Angell C. A.
Published: July 2002

The three xylenes, 1,2-, 1,3-, and 1,4-dimethyl benzene, provide a well-characterized family of isomeric molecules within which the melting points vary greatly while fluid-state properties such as boiling points and viscosities remain very similar. Because all pure isomers crystallize rapidly on cooling, their behavior at the low-temperature end of the liquid state, i.e., near the glass transition, has not been studied. We report here differential scanning calorimetry studies of phase relations for the three mixed isomer systems, and identify composition regions in which emulsified and also bulk mixtures can be vitrified at ordinary cooling rates. For one of the latter, 70% m-xylene+ 30% o-xylene by volume, we determine the change in heat capacity through the glass transition. On the assumption that this is the same for pure m-xylene, we assess the Kauzmann limit on liquid behavior, TK, for m-xylene to fall at 104.6 K, Tg/TK to be 1.20, and the excess entropy at Tg to be 15.6 J/mol K or 30.5% of the entropy of fusion. Equivalent data are obtained for the better-known glass former toluene, for which we find Tg/TK=1.16. Comparisons of these results are made with literature data for some saturated analogs. From these comparisons emerges the simple rule that the excess heat capacity of supercooled liquid over crystal can be described as a hyperbolic function of temperature, the one parameter of which varies linearly with the carbon number of the molecule. The plot passes through the origin, and has a greater slope for aromatic than for paraffinic molecules. This is correlated both with the greater ‘‘fragility’’ of the aromatic liquids assessed from viscosity-temperature relations normalized by the calorimetric Tg, and with a simple ‘‘liquid range’’ index Tb/Tg, where Tb is the boiling point.
Self-consistent theory of block copolymer blends: Neutral solvent
Authors: Whitmore M. D. , Noolandi J.
Published: July 2002

We present a theoretical study of the lamellar structure of diblock copolymers blended with neutral solvent. The calculations are based on numerical solutions of the equations of the mean field self-consistent theory of incompressible polymer/solvent blends, from the weak through to the strong segregation limits. For idealized model systems of symmetric copolymers and perfectly nonselective solvent, we examine the variation of the equilibrium lamellar thickness with molecular weight, Flory χ parameter, solvent quality, and copolymer volume fraction, as well as the distribution of solvent and polymer within the subdomains. We also examine a real system which has been extensively studied experimentally, namely PS-b-PI with the (nearly) nonselective solvent toluene.