bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic studies of jet-cooled CuAg
Authors: Bishea Gregory A. , Marak Ninette , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy has been applied to jet-cooled diatomic CuAg. Four band systems have been observed, three of which have been rotationally resolved and analyzed. The ground state is X 1Σ+ in symmetry, deriving from the 3d10Cu4d10Agσ2 molecular configuration. Its bond length has been determined as r0=2.3735±0.0006 Å (1σ error limits). Based on an analysis of the possible separated atom limits, three of the excited states observed (A 0+, A′ 1, and B′ 0+ ) are assigned as primarily 3d9Cu4d10Agσ2σ* in character. The observation of unusually large electronic isotope shifts in the A–X, A′–X, and B′–X band systems, similar in magnitude to those previously observed in the A–X and B–X systems of Cu2 and the s←d excitations in atomic copper, provides further confirmation that these excited states derive from the 3d9Cu4d10Agσ2σ* molecular configuration. Finally, the highest energy state observed in this work is argued to be primarily ion pair in character, and is expected to have significant contributions from both the Cu+Ag− and Cu−Ag+ ion pair states.
Static longitudinal dielectric function of model molecular fluids
Authors: Raineri Fernando O. , Resat Haluk , Friedman Harold L.
Published: July 2002

The static longitudinal dielectric function εL(k) is calculated for several polar interaction site model (ISM) fluids for comparison with related models having arbitrary short-range interactions and a set of one or more lower-order multipole moments at the centers (ΩM models). The requisite averages over the ISM fluids are calculated by the extended reference interaction site method (XRISM) using site–site hypernetted chain (HNC)-like closures modified to reproduce the correct long-range behavior of the site–site pair correlation functions. They are compared with averages over the ΩM models under the RHNC theory taken from the literature or calculated under the mean spherical approximation. We find for fluids of strong enough polarity that εL(k) is negative over a finite range of k, the low end being in agreement with recent computer simulation studies of both ISM and ΩM polar fluids. However, we confirm that the expected large-k behavior εL(k)=1 governs the ISMs, but not the ΩM models. Based on an adaptation of the color charge–color field techniques of molecular dynamics, we develop the concept of the color longitudinal dielectric function; it provides useful information about the role of the spatial extent of the molecular charge distribution on the behavior of εL(k). The ISM fluids we have analyzed include dipolar dumbbells over a wide range of bond length and polarity as well as realistic interaction site models for water and methanol. For the methanol model, we compare our εL(k) with recent computer simulation results and find substantial agreement.
Observation of several new electronic transitions of the B2 molecule
Authors: Brazier C. R. , Carrick P. G.
Published: July 2002

Several new electronic transitions of the B2 molecule have been observed in emission from a Corona excited supersonic expansion source. The rotational analyses of two bands, designated (2) 3Πu–(1) 3Πg and (1) 1Δu–b 1Δg, are presented, together with a description of three other new systems. The results are compared with recent high-level theoretical calculations.
Optical model of dissociative chemisorption: H2 on the (111), (110), and (100) faces of copper
Authors: Farrelly David , Levine R. D.
Published: July 2002

The recently developed optical model of gas-surface scattering is applied to dissociative chemisorption of a hydrogen molecule on different faces of a copper crystal. In each case, a potential of mean force is employed to describe the real part of the scattering potential. The probability of dissociative chemisorption is studied as a function of crystal face, total collision energy, parallel momentum, initial orientation of the molecule with respect to the surface, and surface temperature. The optical model is able to provide excellent qualitative agreement with experimental observations and detailed theoretical calculations.
A far wing line shape theory and its application to the foreign-broadened water continuum absorption. III
Authors: Ma Q. , Tipping R. H.
Published: July 2002

The far wing line shape theory developed previously and applied to the calculation of the continuum absorption of pure water vapor is extended to foreign-broadened continua. Explicit results are given for H2O–N2 and H2O–CO2 in the frequency range from 0 to 10 000 cm−1. The theory is based on the quasistatic and binary collision approximations and assumes for the interaction potential an isotropic Lennard-Jones part and the leading anisotropic multipolar (dipole–quadrupole) term. For H2O–N2, the positive and negative resonant frequency average line shape functions and absorption coefficients are calculated for a number of temperatures between 296 and 430 K for comparison with existing laboratory data. In general, the agreement is very good, especially at the higher temperatures for which the experimental absorption is larger and the corresponding error limits are smaller. For H2O–CO2 for which no experimental data exists, the calculations are made only at 296 K. Because of the significantly different line shapes for self vs foreign broadening, the ratio of the corresponding continuum absorption coefficients varies widely as a function of the frequency. The implications of this for absorption in the Earth’s atmosphere are discussed briefly.
Angular and vibrational effects in the sticking and scattering of H2
Authors: Darling G. R. , Holloway S.
Published: July 2002

The results of quantum mechanical simulations of H2 dissociation on metal surfaces are presented using an extension of the familiar two-dimensional ‘‘elbow’’ potential. By including corrugation parallel to the surface, it has been possible to examine the effects on the angular and energy distributions of dissociative adsorption and scattering. Additionally, trends obtained by moving the activation barrier from entrance to exit channel have been studied. To effect a closer analogy with experiment, seeding of the incident beam has been simulated by Boltzmann weighting dissociation probabilities. It is particularly important to include the experimental spread of the incident translational energy in calculations. It is found that for hydrogen dissociation on Cu and Fe, dissociative adsorption results can only be reconciled with a late barrier, while for Ni and Pd it appears to be early. For the scattered fraction, the late barrier gives rise to a significant enhancement in the diffraction of vibrationally excited molecules. This is explained in terms of the corrugation of the vibrationally adiabatic potential energy surfaces.
A density functional analysis of the restricted orientation model of liquid crystals and its implications for theories of orientational ordering
Authors: Moore Brian G. , McMullen William E.
Published: July 2002

We compare the predictions of various density functional approximation schemes as applied to the restricted-orientation, hard, rectangular-parallelepiped model of liquid crystals. These calculations illustrate some features of density functional calculations which may aid in understanding other approximate, mean-field theories of orientational ordering phase transitions and their interfaces. The isotropic–nematic coexistence curve of this model is apparently well described by the so-called third-order y expansion, and the analytical expression for the nonideal free energy derived from the simple y expansion thus provides a benchmark against which to compare the density functional approximation schemes. Using standard relations applicable to inhomogeneous fluids, we show how expansions about the bulk isotropic phase compare more favorably with the ‘‘exact’’ y-expansion results when truncated at third order than do second-order truncations. Thus this model behaves somewhat differently than other hard-particle models of fluids, notably the hard-sphere one. We also examine the possibility of expanding the free energy about the ordered phase to obtain the properties of a disordered phase. An expansion about local values of the density leads to the widely used smoothed-density approximation and a hierarchy of systematic extensions. The latter appear more stable than conventional Taylor expansions about bulk disordered phases.
Fast beam studies of NCO free radical photodissociation
Authors: Cyr D. R. , Continetti R. E. , Metz R. B. , Osborn D. L. , Neumark D. M.
Published: July 2002

The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the B̃ 2Π←X̃ 2 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the B̃ 2Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin-forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin-allowed N(2D)+CO product channel becomes energetically accessible. The spin-allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.
Fluorescence excitation spectra of jet-cooled HgBr radicals
Authors: Lipson R. H. , Jordan K. J. , Bascal H. A.
Published: July 2002

Fluorescence excitation spectra of HgBr radicals produced and rotationally cooled in a corona-excited supersonic jet discharge are presented. Both the C 2Π1/2←X 2Σ+ transition near 287.6 nm and the B 2Σ+←X 2Σ+ transition near 350.0 nm were studied. Transitions from X 2Σ+ (vX=0–2) probe very high vibrational levels in the B 2Σ+ ion-pair state (vB=34–90). Analysis of the vibrationally and isotopically resolved spectral features has yielded an unambiguous vibrational numbering for the B←X spectrum and precise B-state vibrational constants which are valid between vB=0–90.
Thermodynamic consistency in the hypernetted chain theory
Authors: Schlijper A. G. , Telo da Gama M. M. , Ferreira P. G.
Published: July 2002

An extended version of the hypernetted chain equation is presented. It allows for fluid inhomogeneity and for three-body interactions. Consistent expressions for thermodynamic functions are given. It is argued that thermodynamic consistency holds even though statistical mechanical relationships are not obeyed. Singularities in the hypernetted chain equation, related to loss of convexity of the associated free energy functional, prevent a complete description of liquid/vapor coexistence in a Lennard-Jones fluid.
Static transverse dielectric function of model molecular fluids
Authors: Raineri Fernando O. , Friedman Harold L.
Published: July 2002

It is customary in the physicochemical literature to define the static transverse dielectric function of a polar fluid according to the relation εT(k) = 1 + 4πβST(k), where β=(kBT)−1 and ST(k) is a measure of the static fluctuations of the Fourier components of the transverse part of the electric polarization field in the fluid. In this work we evaluate εT(k) for models of two classes. (1) In a DS model each molecule is a hard sphere with a point dipole at its center. It is the simplest representative of models in which the long range intermolecular interactions are generated by a set of point multipoles located at a single point in the molecule. In this case ST(k) is SM,T(k), the static correlation function of the transverse part of the dipole polarization density M̂(k). (2) In a ζDS model fluid, comprising nonideal dipolar hard spheres, the long range interactions are accounted for by two opposite partial charges on either side of the sphere’s center and separated by a distance ℓ. The ζDS model is the simplest interaction site model (ISM) of a polar fluid. For an ISM the relevant structure factor ST(k) is SPμ ,T(k), the static correlation of the transverse part of the full electric polarization density vector P̂μ(k). Here we compare εT(k) for DS and ζDS models with the help of the mean spherical approximation for the required structure functions. The ζDS–DS difference in εT(k) at large k is found to parallel the behavior of the static longitudinal dielectric function εL(k). However, εT(k), unlike εL(k), is a damped oscillatory function of k with no poles on the real k axis.
High temperature quenching cross sections for nitric oxide laser-induced fluorescence measurements
Authors: Drake Michael C. , Ratcliffe John W.
Published: July 2002

Collisional quenching cross sections for laser-induced fluorescence from NO A(v′=0) are measured for H2O, CO2, O2, NO, N2, H2, Ar, C2H6, and CO as a function of temperature from 300 to 1750 K. Room temperature cross sections vary widely (≤0.03 Å2 for N2, H2, C2H6, and Ar to ≥100 Å2 for H2O). As temperature increases, cross sections for H2O and CO2 decrease, cross sections for O2 and NO remain constant, and cross sections for N2, H2, CO, and C2H6 increase. These data provide quantitative quenching corrections for laser-induced fluorescence measurements of NO concentrations in nonreacting and reacting flows. Physical quenching models (based upon NO–Q and NO+–Q− collision complexes) are reviewed and used for experimental data correlation and extrapolation. A new quenching mechanism is proposed for weak quenchers.
Structurally determined Brownian dynamics in ordered colloidal suspensions: Self-diffusion in fluid, supercooled, and crystalline phases
Authors: Simon R. , Palberg T. , Leiderer P.
Published: July 2002

Electrostatically interacting colloidal suspensions at medium to very low salt concentrations were prepared in differently ordered phases using the method of continuous deionization. Equilibrium phase states include fluid, mono- and polycrystalline material as well as coexistence between fluid and monocrystal. A nonequilibrium supercooled fluid state is reproducibly reached by shear melting. In these phases the long time self-diffusion coefficient DL was measured by forced Rayleigh scattering, while the potential of interaction was systematically varied by changing salt concentration cs and volume fraction φ. In the equilibrium fluid DL decreases by roughly an order of magnitude as the interaction increases. This trend extends continuously into the supercooled state. In all cases crystallization is observed for DL/D0≤0.10(1). In the polycrystalline phases self-diffusion coefficients are still 1 to 2 orders of magnitude lower than in the supercooled state. Here self-diffusion increases with increasing interaction. For the monocrystalline case upper limits of DL are given. These data on the solid phases are discussed in terms of grain boundary and vacancy diffusion.
Classical atom–diatom scattering: Self-similarity, scaling laws, and renormalization
Authors: Tiyapan Ampawan , Jaffé Charles
Published: July 2002

We have examined the structure of the chattering region of the initial angle–final action plots of He scattered by I2. We demonstrate the existence of asymptotic self-similarity, determine the asymptotic scaling laws, and outline a renormalization treatment of the structures observed.
Vibrational relaxation of bulk modes perturbed by electronic state of dilute impurities
Authors: Chang Ta-Chau , Chou Shiow-Hwa , Li Hung-Wen , Lin Sheng-Hsien
Published: July 2002

Vibrational dephasing of the 1385 cm−1 vibron of host molecules (naphthalene) perturbed by electronically excited guest molecules (pentacene) was studied by the time-resolved coherent Stokes Raman spectroscopy (CSRS) in the temperature region of 6–26 K. The decay time was faster when the excitation frequency was tuned from the off-resonance to the resonance of the electronic transition of pentacene. For the resonance case, longer decay times were observed at higher temperatures (∼26 K) than at low temperatures (∼6 K). Two possible mechanisms were considered for the inverse temperature behavior. The shorter decay time in the impurity perturbed domains may be attributed to the increase of a coupling strength on the decay channel from 1385 cm−1 mode to 1365 cm−1 mode by stimulating phonon emission.
A new potential for the numerical simulations of electrolyte solutions on a hypersphere
Authors: Caillol Jean-Michel
Published: July 2002

We propose a new way of performing numerical simulations of the restricted primitive model of electrolytes—and related models—on a hypersphere. In this new approach, the system is viewed as a single component fluid of charged bihard spheres constrained to move at the surface of a four dimensional sphere. A charged bihard sphere is defined as the rigid association of two antipodal charged hard spheres of opposite signs. These objects interact via a simple analytical potential obtained by solving the Poisson–Laplace equation on the hypersphere. This new technique of simulation enables a precise determination of the chemical potential of the charged species in the canonical ensemble by a straightforward application of Widom’s insertion method. Comparisons with previous simulations demonstrate the efficiency and the reliability of the method.
Ultrafast reaction dynamics of electronically excited Ã state of ammonia clusters
Authors: Wei S. , Purnell J. , Buzza S. A. , Castleman A. W.
Published: July 2002

Femtosecond pump–probe techniques combined with a reflectron time-of-flight mass spectrometer are utilized to study the ultrafast reaction dynamics of the electronically excited Ã state of ammonia clusters. All of the detected protonated cluster ions, (NH3)nH+ n=2−6, are observed to display two distinct features with respect to preselected pump–probe time delays; a fast decay, followed by a persistent ion signal leveling off to a finite nonzero value. The fast decay is attributed to a predissociation process; while an intracluster reaction, which leads to formation of long-lived intermediates (NH3)nNH4, is responsible for the nonzero falling off regime. The results provide conclusive experimental evidence that both an absorption–ionization–dissociation mechanism and an absorption–dissociation–ionization mechanism are operative in the Ã state of ammonia clusters.
Microscopic study of inertial and viscoelastic effects in dipolar solvation dynamics
Authors: Chandra Amalendu , Bagchi Biman
Published: July 2002

In this article theoretical studies of equilibrium and dynamic solvation of a dipole in a dense dipolar solvent are presented. A microscopic expression of the solvation free energy is derived within a linear equilibrium theory of dipolar liquids. It is found that the transverse component of the solvent polarization cannot be neglected and that it makes a significant contribution to the free energy of dipolar solvation. We also present a detailed study of the dynamics of solvation of a solute dipole which includes the inertial and viscoelastic responses of the solvent and the main results are as follows. (i) The dynamics is found to be appreciably different from that of an ion because of the contribution of the transverse polar modes in the former. (ii) The size of the solute can have interesting effects. (iii) The short time inertial response of the solvent may give rise to an oscillatory decay. However, one needs to use a non-Markovian theory with the viscoelastic response of the liquid included to understand these effects and such a theory is presented here for the first time. (iv) In the overdamped and Markovian limit and in the absence of translational diffusion, the present theory gives results similar to the dynamic MSA model of Rips, Klafter, and Jortner and of Nichols and Calef.
Hydrodynamic coupling of monolayers with subphase
Authors: Sacchetti Mark , Yu Hyuk , Zografi George
Published: July 2002

The surface canal viscometry technique was used to determine the in-plane steady shear viscosity of monolayer assemblies of several amphiphilic molecules at the air/water interface. By varying the subphase viscosity by a factor of about 2.5 without affecting its surface activity, we were able to deduce the surface viscosity, which is independent of the subphase viscosity, only after correcting for the hydrodynamic coupling as prescribed by the theory of Harkins and Kirkwood. Alternatively, the subphase shear viscosity has been correctly deduced from the observed area flow rate through a canal in the limit of a vanishingly small surface viscosity where the flow rate now depends solely on the subphase viscosity through hydrodynamic coupling. By testing in these two limits, we evaluated and confirmed for the first time the validity of the hydrodynamic coupling between the monolayers and the subphase, as proposed by the theory of surface viscous flow of Harkins and Kirkwood.
The role of electronic and geometric factors in ‘‘proton tunneling:’’ A comparative study of tropolone and 9-hydroxyphenalenone by threshold photoelectron spectroscopy
Authors: Ozeki H. , Takahashi M. , Okuyama K. , Kimura K.
Published: July 2002

High resolution two-color threshold photoelectron spectra of tropolone and 9-hydroxyphenalenone (9-HPO) isolated in a free jet expansion have been measured to study the proton tunneling phenomena in the cation ground state (D0). The tunneling splitting widths of the D0 zeroth vibrational level for the both molecules lie within our experimental accuracy (2 cm−1), indicating that the proton tunneling is inhibited when compared to that obtained in their respective neutral states. By means of a comparative study of these two molecules, the tunneling inhibition has been explained in terms of a large contribution from the electronic factor which represents the changes in charge distribution of the π electrons upon ionization. The geometric factor, which is associated with the ring planarity is less important in determining the rate of tunneling in the D0 state. In the case of tropolone, we have found that the measurement of an out-of-plane skeletal vibration in various electronic states make it possible to describe the tunneling path at a higher level than a simple one-dimensional description. Moreover, the adiabatic ionization energies of 9-HPO and the deuterated 9-HPO have been determined accurately to be 65 338±5 cm−1 (8.1009±0.0006 eV) and 65 350±5 cm−1 (8.1024±0.0006 eV), respectively.
Spin-conversion relaxation in dimethyl-s-tetrazine doped n-octane: Deuteration and symmetry breaking
Authors: Orth K. , Schellenberg P. , Friedrich J.
Published: July 2002

Spectral hole burning is used to measure spin-conversion relaxation of methyl groups attached to a dye probe. The dye is dimethyl-s-tetrazine, the host is n-octane, which is, at low temperatures, a Shpol’skii-matrix. The temperature dependence of the spin-conversion relaxation follows the same pattern for the protonated and the perdeuterated methyl-rotor. However, the relaxation rates of the perdeuterated rotor are higher by almost two orders of magnitude. Partial deuteration destroys the rotor symmetry and leads to a breakdown of the correlation between nuclear spin and rotational wave function.
Energetics, structure, and compressibility of NaF determined by the periodic Hartree–Fock method
Authors: Erikson Robert L. , Eary L. Edmond , Hostetler Charles J.
Published: July 2002

Basis set development methods for molecular cluster calculations were used to develop basis sets for periodic ab initio calculations of the energy-dependent properties of ionic crystals. Two extended, Gaussian basis sets for closed-shell sodium and fluorine ions were developed to compare with two minimal basis sets in calculating the crystal structures, enthalpies of formation, and high-pressure compression behavior of the B1 and B2 phases of NaF. Accurate calculation of the energy-dependent properties to within experimental error required the use of the larger, extended basis sets. Enthalpies of formation for NaF–B1 at 0 K and 1 bar were calculated to be between −921 and −929 kJ/mol, which agrees well with an experimental value (−927±10 kJ/mol) estimated from calorimetric data. For the B1 phase, the zero-pressure bulk modulus was calculated to be between 0.485 and 0.499 Mbar, as compared to the measured value of 0.464±0.062 Mbar. The calculated molar volume at zero pressure agrees to within 0.1% of the experimental value. For the B2 phase, the zero-pressure bulk modulus was nearly the same as that for the B1 phase, ranging from 0.494 to 0.517 Mbar. However, this calculated bulk modulus for the B2 phase was considerably smaller than the experimental value of 1.03±0.19 Mbar estimated from high-pressure compression measurements. Our calculations also suggest the V02/V01 of the B2 phase is nearer to 0.871–0.895 than to the experimental estimate of 0.811. The compression curves for both phases agreed, for the most part, with the experimental data to within the experimental error. The calculated pressure for the NaF B1/B2 transition is between 0.26 and 0.29 Mbar, slightly higher than the averaged experimental value (0.23±0.03 Mbar) but within a pressure range where both phases metastably coexist along the compression curve. The calculated volume change accompanying the phase transition (−10.1%) is in good agreement with experimental data (−10.4%). The apparent discrepancies between the zero-pressure bulk modulus and the compression curves of the B2 phase are shown to be related to the range of molar volumes used in deriving the zero-pressure properties from an equation of state.
Elastic constants of the orientationally disordered phase of C2F6 by Brillouin spectroscopy
Authors: Andrews G. T. , Kiefte H. , Clouter M. J. , Zuk J.
Published: July 2002

The elastic constants and elasto-optic coefficient ratios of the orientationally disordered β phase of C2F6 have been determined using the technique of high resolution Brillouin spectroscopy. Just below the triple point, at 169 K, the values of the elastic constants (in units of kbar) are C11=26.99±0.10, C12=19.32±0.10, and C44=4.44±0.04. The temperature dependence of all three elastic constants was also determined and appears to be linear for the temperature range investigated, with ΔC11/ΔT=−0.335±0.010, ΔC12/ΔT=−0.236±0.010, and ΔC44/ΔT=0.005±0.011 kbar/K. Values of the ratios of acoustic velocities in high symmetry directions in the crystal indicate near perfect elastic isotropy and that moderately strong rotation–translation coupling is present in β-C2F6.
Chemiluminescent reaction of bismuth with F2
Authors: Cheong B. S. , Oberlander M. D. , Kampf R. P. , Parson J. M.
Published: July 2002

Chemiluminescent (CL) reaction of bismuth with F2 has been studied in a beam-gas arrangement under single-collision conditions. Time-of-flight (TOF) measurements suggest that the Bi2 dimer is the major beam reactant responsible for the observed CL. The bond dissociation energy of BiF has been estimated by observing the short wavelength limit in the B 0+—X10+ emission spectrum of BiF. The estimated lower bound of 3.7±0.1 eV is in good agreement with a recent determination of the bond dissociation energy of BiF. The vibrational distributions of the CL product states provide information on the potential energy surfaces governing the reaction.
Optical–optical double resonance spectroscopy of the 5 1Σ+g ‘‘shelf’’ state of Na2 using an ultrasensitive ionization detector
Authors: Tsai Chin-Chun , Bahns John T. , Stwalley William C.
Published: July 2002

Using optical–optical double resonance and an ultrasensitive ionization detector, we have been able to extensively study a wide range of internuclear distances for the 5 1Σ+g state of Na2. Using a combination of spectroscopic data and the ab initio calculations, we have constructed a hybrid potential which has an outer well at shelf region. For this hybrid potential, there are 54 vibrational levels in the inner well (which all have been observed in this work) and 12 vibrational levels in the outer well (of which only the last level has been observed in this work). Above the potential barrier, 78 additional vibrational levels have been observed in this work. These spectroscopic data represent about 99.8% of the potential well depth.
Perturbation density functional theory for polyatomic fluids. III. Application to hard chain molecules in slitlike pores
Authors: Kierlik E. , Rosinberg M. L.
Published: July 2002

We present the first implementation of our density-functional theory for polyatomic molecules [J. Chem. Phys. 97, 9222 (1992); 99, 3950 (1993)] to investigate the structure of nonuniform polymer melts. In this theory, derived originally from Wertheim’s perturbation theory of polymerization, the free energy of the nonuniform fluid is expressed as a functional of the full molecular distribution function and the excess contribution is calculated perturbatively over a reference fluid of monomers at the same temperature and singlet density as the real system. We compare theoretical predictions to simulations of freely jointed hard spheres confined between hard walls. We calculate the average and individual site density profiles, the bond orientation factor, and the adsorption isotherm. The theory is fairly accurate, though it tends to underestimate the depletion of chain sites near the walls at low densities and to overestimate the packing effects at high densities. It predicts very accurately, however, the density profile of chain ends.
Interatomic distances for some first row transition element dichlorides isolated in cryogenic matrices using x-ray absorption fine structure spectroscopy
Authors: Beattie Ian R. , Spicer Mark D. , Young Nigel A.
Published: July 2002

X-ray absorption fine structure (XAFS) data for several 3d transition metal dichlorides isolated in nitrogen, argon, or methane matrices have been collected and analyzed. The bond lengths obtained are in reasonable agreement with those from vapor phase electron diffraction. The results are briefly discussed with reference to Badger’s rule extended to triatomics.
Measurement of the forces between gold surfaces in water by atomic force microscopy
Authors: Biggs Simon , Mulvaney Paul
Published: July 2002

The forces between a flat gold surface and a gold-coated silica sphere have been measured in water using an atomic force microscope. A long-range attractive interaction is observed which is ascribed to the van der Waals interaction between the two surfaces. The force data agree extremely well with recent, calculated values of the Hamaker function (including retardation) for gold/water/gold. The best fit to the experimental data yields a value of 2.5±0.5×10−19 J for the unretarded Hamaker constant. In the presence of cetyltrimethyl ammonium bromide (CTAB) monolayers, electrostatic repulsion is observed at all distances for gold sphere (radius 3.3 μm) interactions with a flat gold surface. However, an attractive force is observed at very small separations for gold-coated silicon nitride tips (effective radius 0.1 μm), which is attributed to penetration of the CTAB monolayers by the sharper tip.
Solvent effect on the first hyperpolarizabilities of conjugated organic molecules
Authors: Yu Jenwei , Zerner Michael C.
Published: July 2002

The frequency dependent first hyperpolarizabilities βvec(−2ω;ω,ω) (second harmonic generation) of conjugated molecules are studied using the time-dependent Hartree–Fock theory with the inclusion of solvent effects treated with the self-consistent reaction field theory. Using the intermediate neglect of differential overlap model parametrized for spectra we obtain a plot of βvec(theory) vs βvec(expt) with a fitted slope of 1.07 when solvent effects are included and only 0.49 when the solvent effects are not included. The calculated dependence of βvec on the degree of conjugation for the (−OCH3,−NO2) series of molecules that we study is 2.1 compared to 2.2 obtained from experiment, and for the [−N(CH3)2,−NO2] series of molecules a calculated value of 1.9 is obtained compared to the 1.9 value obtained from experiment.
Ionization potentials of CaF and BaF
Authors: Jakubek Zygmunt J. , Harris Nicole A. , Field Robert W. , Gardner James A. , Murad Edmond
Published: July 2002

Although the alkaline earth–monohalide compounds, MX, have been extensively studied, their ionization potentials to MX+ are only known to within a few tenths of an eV. Using an improved electron impact ionization potential measurement to assist in arranging spectroscopic vibronic band origins into Rydberg series, we report high precision ionization potential measurements for CaF and BaF to be IP(CaF)=5.8270±0.0006 eV and IP(BaF)=4.8034±0.0004 eV. The synergism between the electron impact method and the spectroscopic method is detailed, and the minimum accuracy required of the electron impact data is assessed.
The growth of polymer crystals at the transition from extended chains to folded chains
Authors: Higgs Paul G. , Ungar Goran
Published: July 2002

We investigate models for the growth of crystals of monodisperse polymer chains. A transition from extended chain crystals to folded chain crystals occurs as the temperature is changed which is governed by the kinetics of adsorption and desorption of chains at the growth surface. Folded chains may build up on the surface of growing extended chain crystals and slow down the growth. This effect is particularly strong at the transition temperature, where the folded chains are marginally stable. Hence there is a sharp minimum in the growth velocity at this point. We compare the results of the model to experiments on crystallization of long alkanes and poly (ethylene oxide) fractions. In some cases there is an instability in the growth surface leading to patterns of crevices resembling those seen during the thickening of folded chain alkane lamellae. Sliding and rearrangement of molecules underneath the crystal surface is important to allow smooth surface growth. Sliding processes are also the dominant factor controlling the growth rate at temperatures close to the transition.
Integral equation theory of polymers: Translational invariance approximation and properties of an isolated linear polymer in solution
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

In this paper, we continue investigations on the solution methods for the generalized Percus–Yevick equations for the pair correlation functions of polymers, which were formulated in the previous papers of this series [J. Chem. Phys. 99, 4084, 4103 (1993)]. Previously, they were reduced to recursive integral equations and solved numerically. In this paper, a translational invariance approximation is used to reduce the number of integral equations to solve. In this approximation, only N integral equations out of N2 integral equations are required for a polymer consisting of N beads (monomers). The behavior of an isolated polymer is studied with three different potential models, a soft sphere, a hard sphere, and a Lennard-Jones potential. The main motivation for considering these three potential models is in testing the idea of universality commonly believed to hold for some properties of polymers. We find that the universality holds for the power law exponent for the expansion factor of polymers at high temperatures. The end-to-end distance distribution functions, intermediate distribution functions, chemical potentials, the density distributions, and various expansion factors of the polymer chain are computed from the solutions of the integral equations in the case of coiled, ideal, and collapsed states of the polymer. The expansion factors in the collapsed regime are found to obey power laws with respect to the length of the polymer and [B(T)−B(θ̄)], where B(T) is the second virial coefficient and θ̄ is a modified θ temperature. The values of these exponents approach those from the known theories of polymer collapse as the chain length becomes long and the ratio of bond length to bead radius becomes large.
Molecular clusters in liquid ethanol at room temperature
Authors: Sarkar S. , Joarder R. N.
Published: July 2002

The analysis of x-ray diffraction data of liquid ethanol at room temperature shows the existence of hexamer molecular clusters due to intermolecular hydrogen bonding. Assuming hexamer domination in the intermolecular structure, the center structure is derived, which in turn reproduces closely the available data on neutron diffraction for this system.
An extended basis set ab initio study of Li+(H2O)n, n=1–6
Authors: Feller David , Glendening Eric D. , Kendall Rick A. , Peterson Kirk A.
Published: July 2002

The structures, binding energies, and enthalpies of small molecular clusters incorporating a single lithium cation and up through six waters have been determined with extended Gaussian basis sets using Hartree–Fock and post-Hartree–Fock methods. The resulting properties are analyzed with respect to both basis set completeness and degree of correlation recovery, including core–core and core–valence effects. Although the lithium–water interaction is largely electrostatic in nature, small basis sets, lacking in polarization and near-valence diffuse functions, drastically overestimate the strength of the bond (by 20 kcal/mol or more) and underestimate the Li+...O distance by up to 0.1 Å. Their poor performance is attributable to inherent errors in describing the electric moments and polarizability of water and to large basis set superposition errors. Thus, the accuracy with which the fundamental lithium–water interaction could be modeled was primarily dependent on the quality of the Gaussian basis set and not upon the level of correlation recovery. Basis set enlargement and correlation effects both tend to reduce the strength of the Li+(H2O) bond, but produce corrections of opposite sign for the Li+...O bond length. Although correlation effects play a minor role in describing the lithium–water interaction, as the size of the cluster increases and the number of waters involved in multiple hydrogen bonds grows, correlation recovery can become significant.
Hydrogen-atom migration on a diamond (111) surface
Authors: Chang Xiao Yan , Thompson Donald L. , Raff Lionel M.
Published: July 2002

Potential barrier heights for three different hydrogen-atom migration pathways on C(111) are computed on the semiempirical hydrocarbon potential developed by Brenner [Phys. Rev. B 42, 9458 (1990)]. The barrier for diffusion to an adjacent absorption site is found to be 3.29 eV. Migration rate coefficients obtained from classical variational transition-state theory indicate that such migration processes play a minor role in diamond-film synthesis. The contribution of tunneling to the diffusion rate is shown to be negligible. The computed migration barrier to an adjacent adsorption site is in good agreement with recent results obtained by Melnik et al. using a triple-referenced, configuration interaction method.
Resonance properties from wave-packet propagation
Authors: Isele A. , Meier C. , Engel V. , Fahrer N. , Schlier Ch.
Published: July 2002

We calculate resonance energies, wave functions, and widths by propagating wave packets on a grid. A collinear model of a symmetric triatomic molecule is studied and the results are compared to those obtained from a time-independent complex scaling calculation.
Atom–molecule van der Waals complexes containing open-shell atoms. I. General theory and bending levels
Authors: Dubernet Marie-Lise , Hutson Jeremy M.
Published: July 2002

The theory needed to carry out calculations on atom–molecule van der Waals complexes containing open-shell atoms is developed. The discussion concentrates on complexes containing atoms in P states. Several possible expansions of the total wave function are described, and the matrix elements needed to construct the Hamiltonian matrix are set out. Several different angular momentum coupling cases may arise, analogous to Hund’s coupling cases in diatomic molecules. The bending energy levels of Ca–HCl, B–H2, F–H2, Cl–Cl2, and F–N2 are calculated, using simple models of the interaction potentials.
Phase diagram of a model protein derived by exhaustive enumeration of the conformations
Authors: Dinner A. , Šali A. , Karplus M. , Shakhnovich E.
Published: July 2002

An understanding of the various states available to a polypeptide chain is important for a description of the protein folding process. We use a 16-monomer chain on a two-dimensional square lattice to model a protein. This makes it possible to enumerate all self-avoiding conformations from which any equilibrium thermodynamic quantity can be calculated. By varying the external conditions of temperature and average attraction, we construct a phase diagram for the model protein. It is found to have an extended coil state, a homopolymer-like disorganized globule state, and an organized frozen globule state that corresponds to the lowest energy (native) conformation. The exact model results agree well with analytical heteropolymer theory.
Discrete variable representations of complicated kinetic energy operators
Authors: Wei Hua , Carrington Tucker
Published: July 2002

Probably the most important advantage of the discrete variable representation (DVR) is its simplicity. The DVR potential energy matrix is constructed directly from the potential function without evaluating integrals. For simple kinetic energy operators the DVR kinetic energy matrix is determined from transformation matrices and exact matrix representations of one-dimensional kinetic energy operators in the original delocalized polynomial basis set. For complicated kinetic energy operators, for which matrix elements of terms or factors with derivatives must be calculated numerically, defining a DVR is harder. A DVR may be defined from a finite basis representation (FBR) where matrix elements of terms or factors in the kinetic energy operator are computed by quadrature but implicating quadrature undermines the simplicity and convenience of the DVR. One may bypass quadrature by replacing the matrix representation of each kinetic energy operator term with a product of matrix representations. This product approximation may spoil the Hermiticity of the Hamiltonian matrix. In this paper we discuss the use of the product approximation to obtain DVRs of complicated, general kinetic energy operators and devise a product scheme which always yields an Hermitian DVR matrix. We test our ideas on several one-dimensional model Hamiltonians and apply them to the Pekeris coordinate Hamiltonian to compute vibrational energy levels of H3+. The Pekeris coordinate Hamiltonian seems to be efficient for H3+. We use Jacobi polynomial basis sets and derive exact matrix elements for (d/dx) G(x)(d/dx) , r(x)(d/dx), r(x), and (1−x)λe−xt with G(x) and r(x) rational functions. We discuss the utility of several Jacobi DVRs and introduce an improved FBR for general kinetic energy operators with more quadrature points than basis functions. We also calculate Euclidean norms of matrices to evaluate the accuracy of DVRs and FBRs.
Ab initio potential energy surface and dynamics of He–CO
Authors: Tao Fu-Ming , Drucker S. , Cohen R. C. , Klemperer William
Published: July 2002

The potential energy surface for the He–CO van der Waals interaction is calculated by the supermolecular approach using fourth-order Mo/ller–Plesset perturbation theory (MP4) with a large basis set containing bond functions. The rovibrational energies of He–CO are then calculated by the collocation method. Our ab initio surface has a single near T-shaped minimum (Rm=3.49 Å, θ=120°, Vm=−20.32 cm−1), in agreement with a recent experimental potential (R=3.394 Å, θ=121.3°, Vm=−22.91 cm−1), determined from high-resolution spectroscopic measurements, but significantly different from a previously published ab initio surface for this system. The calculated rovibrational energies are in good agreement with experiment. The explicit dependence of the intermolecular potential on the CO bond distance is also studied by MP4 calculations, and the results confirm the experimental observation that the intermolecular coordinates are approximately uncoupled from the CO bond distance.
The very fast multipole method
Authors: Petersen H. G. , Soelvason D. , Perram J. W. , Smith E. R.
Published: July 2002

The fast multipole method (FMM) has become an important alternative to traditional methods such as the Ewald method for computing the long-range interactions necessary to simulate charged or dipolar systems. In this paper, we present an improvement of this method, which we shall call the very fast multipole method (VFMM). The VFMM is shown to be a factor of about 1.2 faster than the FMM for two-dimensional systems and a factor about 2–3 times faster for three-dimensional systems without losing any accuracy for the worst case error.
Coexisting phases and criticality in NaCl by computer simulation
Authors: Guissani Yves , Guillot Bertrand
Published: July 2002

The liquid–vapor coexistence curve of NaCl is evaluated by molecular dynamics simulations in using the well-known Born–Huggins–Mayer–Fumi–Tosi (BHMFT) interionic potential. Due to the limited size of our sample (Nions=512), the calculated isotherms present van der Waals loops which can be described by an empirical equation of state whose critical parameters are Tc=3068 K, ρc=0.174 g/cm3, and Pc=105.4 bar. By comparing with the available experimental data on molten NaCl and according to corresponding states arguments, we are able to deduce from our simulation data an estimation of the critical parameters of real NaCl, namely, Tc=3300 K, ρc=0.18 g/cm3, and Pc=325 bar. The electrical conductivity of our simulated molten salt is then evaluated along the coexistence curve, in the highly compressed liquid and in the dilute gas. No evidence for an insulator-ionic conductor transition is found. Instead a continuous transition between a highly conducting fluid at high density and a low conducting gas at low temperature is pointed out. The degree of dissociation is obtained from the conductivity data and from the knowledge of the self-diffusion coefficients through a generalized Nernst–Einstein relation. The evolution of the degree of dissociation with density and temperature is corroborated by the analysis of the charge–charge distribution functions which show the occurrence of ion pairing at low density and low temperature. Finally, the mean field type behavior exhibited by our simulated molten salt is discussed in perspective with the available experimental data for other fused salts as well as with the current state of the theory concerning the criticality in ionic fluids.
Folding thermodynamics and kinetics of imprinted renaturable heteropolymers
Authors: Pande V. S. , Grosberg A. Yu. , Tanaka T.
Published: July 2002

Recently, a procedure was suggested to synthesize polymers with characteristics similar to those observed in globular proteins: renaturability and the existence of an ‘‘active site’’ capable of specifically recognizing a given target molecule. This procedure was originally studied using a computer simulation of the thermodynamics of lattice 27-mers. This analysis is extended to the thermodynamic study of longer chains (36-mers) and different types of short range interactions. We found, in the best conditions, a 50% success rate of creating renaturable heteropolymers, thus confirming the original results. Folding kinetics as examined by Monte Carlo simulation show that the imprinted sequences can reach the ground state reliably and quickly. Finally, we compare the correlations found in the imprinted sequences with those found in natural proteins. We interpret these results as the confirmation of the efficacy of the polymerization procedure.
Dipole polarizability and bond dissociation energy
Authors: Hohm Uwe
Published: July 2002

The change of thermodynamic equilibrium-state functions, such as the change in enthalpy ΔH or free enthalpy ΔG, in bond dissociation processes is well known in general. On the other hand, the systematic investigation of the change in electrical and electro-optical properties, such as the dipole polarizability α, seems to be a rather unexplored field in physical chemistry. In this Communication we report on a remarkable correlation between the change in dipole polarizability Δα and the bond dissociation energy D0298 for various dissociation processes.