bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of spin relaxation by translational diffusion in two-dimensional systems
Authors: Korb J.-P. , Winterhalter M. , McConnell H. M.
Published: February 2003

Spin relaxation rates T−11 and T−12 are calculated for nuclear (or electron) spins diffusing on finite two-dimensional, planar, or spherical surfaces. The spin relaxation is assumed to be due to modulation of the intermolecular dipole–dipole interactions. It is shown that the mathematical divergences encountered in a number of previous theoretical treatments of this problem for infinite planar surfaces have no physical significance; these divergences are avoided by limiting the calculations to two-dimensional systems that are finite, but that are otherwise of arbitrarily large size. The theoretical relaxation rates T−11 and T−12 for finite, planar two-dimensional systems are found to have a number of unique properties that should facilitate the interpretation of magnetic resonance spectra of molecules physically adsorbed on solid surfaces. For example, the reduction in dimensionality of rapid diffusive motion yields relaxation rates typical of slow motion in three-dimensional systems. Under certain conditions the relaxation rate T−11 is strongly dependent on the size of the two-dimensional surface on which atoms or molecules diffuse. Moreover the shape of the surface (planar or spherical), which is of particular importance in the description of the two-dimensional dynamics, can profoundly alter the frequency and temperature dependences of the spin-relaxation rates. The theory appears to be directly applicable to recent experiments by J. Tabony [Prog. Nucl. Magn. Reson. Spectrosc. 14, 1 (1980)].
Rotational spectrum and structure of the complex HCNCO2
Authors: Leopold K. R. , Fraser G. T. , Klemperer W.
Published: February 2003

Radiofrequency and microwave rotational spectra of the complexes HCN CO2 and DCN CO2 have been obtained using molecular beam electric resonance spectroscopy. The spectra are characteristic of prolate asymmetric rotors. The spectroscopic constants obtained are                     These are shown to be consistent with a C2v structure in which the nitrogen of the HCN bonds to the carbon of the CO2 with a weak bond length of 3.00 Å. The HCN subunit subtends an average angle of 17.4 (2)° with the line joining the centers of mass of the two submolecules, and the average bending vibrational amplitude of the CO2 is 11(1)°. The difference between the average amplitudes of the in-plane and out-of-plane bending vibrations of the HCN is less than 4°. The stretching force constant for the weak bond is 0.049 mdyne/Å in HCNCO2, and the induced dipole moment is 0.361 D.
Reactions of HS with NO and NO2 at 298 K
Authors: Black G.
Published: February 2003

HS radicals have been generated by the photodissociation of H2S at 193 nm and their disappearance monitored by LIF. The reaction of HS with NO, like the analogous reaction of OH with NO, has been shown to involve a third body. The low pressure rate coefficients have been determined for He, Ar, and N2. The values are 2.1, 2.2, and 2.4×10−31 cm6 molecule−2 s−1, respectively, with estimated uncertainties of ±10%. The high pressure limit is (2.8±1.0)×10−11 cm3 molecule−1 s−1. The reaction with NO2 (HS+NO2→HSO+NO) is second order with a rate coefficient of (3.5±0.4)×10−11 cm3 molecule−1 s−1. An upper limit for the rate coefficient with O2 has been estimated (4×10−17 cm3 molecule−1 s−1).
A photoelectron spectroscopic study of the ground state of CF+ via the ionization process CF+(X 1Σ+)←CF(X 2Π)
Authors: Dyke J. M. , Lewis A. E. , Morris A.
Published: February 2003

The CF (X 2Π) radical has been investigated with vacuum ultraviolet photoelectron spectroscopy and ab initio molecular orbital calculations. Only one band was observed and it was assigned to the ionization CF+(X 1Σ+)←CF(X 2Π). For this band, the adiabatic and vertical ionization potentials were measured as (9.11±0.01) and (9.55±0.01) eV, respectively, and analysis of the vibrational structure in this band allowed w̄e and re for CF+(X 1Σ+) to be determined as (1840±30) cm−1 and (1.148±0.005) Å. Comparison is made with other first row diatomic fluorides that have also been studied with PES.
Symmetry reduction–vibronically induced mode mixing in the S1 state of β-methylnaphthalene
Authors: Warren Jonathan A. , Hayes John M. , Small Gerald J.
Published: February 2003

Supersonic jet excitation and single vibronic level (SVL) dispersed fluorescence spectra for α- and β-methylnaphthalene (S1 state) are presented. Marked mirror symmetry breakdown between the excitation and origin excited fluorescence spectra of the β isomer is observed. The SVL spectra reveal that mode mixing in the S1 state is too extensive to be accounted for by first order treatment of the Duschinsky effect. Vibronically induced mode mixing in the β isomer is suggested as the mechanism which leads to intramolecular vibrational energy redistribution which onsets at a significantly lower excess vibrational energy than in naphthalene. The implications of our findings for other polyatomics are discussed.
Infrared double resonance spectroscopy of V-T, R relaxation of HF(v=1): Direct measurement of the high-J populations
Authors: Haugen Harold K. , Pence William H. , Leone Stephen R.
Published: February 2003

The V-T, R relaxation of HF(v=1) by HF is studied by infrared pulse-probe transient absorption measurements using a tunable F-center laser. It is found that a substantial fraction of the relaxation occurs through the high-lying rotational levels of v=0. The results indicate that the states J=10–14 comprise ∼20%–40% of the population in the total relaxation, with the distribution increasing rapidly with decreasing J in this range. The observed signals, corrected for cascade, correspond to the fractions: J=10, ∼10%–40%; J=11, ∼4%–10%; J=12, ∼2%–6%; J=13, ∼1%–2%; and J=14, ∼0.05%–0.35%. In addition, the V-T, R rate determined here, k=1.46±0.15×10−12 cm3 molecule−1 s−1 reconfirms previous double resonance results. Also, studies involving buffer gas addition (Ar) give no indication of ‘‘bottleneck’’ effects on the high J’s. In fact, the R–R, T rates always exceed the V-T, R rate by one to two orders of magnitude. For J=11–13, the R–R, T rates range from ∼1.2×10−10 to 6×10−11 cm3 molecule−1 s−1.
First and second derivatives of two electron integrals over Cartesian Gaussians using Rys polynomials
Authors: Schlegel H. Bernhard , Binkley J. Stephen , Pople John A.
Published: February 2003

Formulas are developed for the first and second derivatives of two electron integrals over Cartesian Gaussians. Integrals and integral derivatives are evaluated by the Rys polynomial method. Higher angular momentum functions are not used to calculate the integral derivatives; instead the integral formulas are differentiated directly to produce compact and efficient expressions for the integral derivatives. The use of this algorithm in the ab initio molecular orbital programs gaussian 80 and gaussian 82 is discussed. Representative timings for some small molecules with several basis sets are presented. This method is compared with previously published algorithms and its computational merits are discussed.
Beyond linear response: Line shapes for coupled spins or oscillators via direct calculation of dissipated power
Authors: Kosloff Ronnie , Ratner Mark A.
Published: February 2003

Absorption spectra are calculated for four model systems using two different methods for the calculation: The first is the standard linear response technique, while the second involves direct calculation of the energy dissipated in the sample interacting with the applied field, thus corresponding exactly to what is in fact measured experimentally. This latter technique, which we call the direct method, agrees with a linear response (as it must) for weakly applied fields for which the first-order perturbation analysis underlying linear response is in fact valid. For cases in which the system/field interaction is stronger, the linear approximation becomes poor, and the direct method, which includes the field-induced dynamics and in itself contains no approximations, is far better. The four systems considered consist of either single or coupled pairs of harmonic oscillators or two-level systems (spins) each in a thermal bath and coupled to the field by a displacement coupling. For both linear-response and direct calculations the system/field dynamics is solved exactly and the system/bath dynamics treated using a semigroup technique. For the harmonic oscillators, for which the population in any single excited state can never become large, the nonlinear effects (those present in the direct but not in the linear response) are quite minor. For the two-level/spin/systems, however, substantial corrections to the linear-response result are found in the direct calculation. These include power (or saturation) broadening and the appearance, when the two spins are coupled strongly to each other and one is coupled strongly to the field, of a new line at the averaged frequency of the two spins. For even stronger coupling, the line shape becomes very complex (corresponding to the full 15D algebra of the two spins plus the field), exhibiting several peaks and Fano antiresonances. The direct method, which is based on work of Lebowitz, seems to offer substantial advantages for the analysis of systems in which the applied field interactions are strong: This situation, common in microwave and magnetic resonance spectroscopy, is also relevant to high-energy laser studies.
Locating transition states
Authors: Bell Stephen , Crighton James S.
Published: February 2003

A previously reported method for locating saddle points on multidimensional surfaces by a combination of aspects of the conjugate gradients method and the quasi-Newton minimization method is now described with modifications to make it more robust. A number of model potentials from the literature and some ab initio molecular potential energy surfaces are used for test and comparison purposes.
Vibrational frequencies and infrared intensities for H2CN+, protonated HCN
Authors: Lee Timothy J. , Schaefer Henry F.
Published: February 2003

The infrared spectra of polyatomic molecular ion H2CN+ is studied using CI wave functions. The vibrational frequencies and infrared intensities are calculated. (AIP)
X-ray diffraction study of CoCl2–LiCl aqueous solutions
Authors: Musinu A. , Paschina G. , Piccaluga G. , Magini M.
Published: February 2003

Three 1 M solutions of CoCl2 at Cl−/Co2+ ratios of 4, 6, and 7 were examined by x-ray diffraction. The Fourier transformed scattering data indicated inner complex formation between Co2+ and Cl− ions for the two solutions more enriched in Cl− ions. Coordination parameters of Co2+ ions were obtained by direct fitting of the structure functions.
Unified picture for spin-lattice relaxation of lipid bilayers and biomembranes
Authors: Brown Michael F.
Published: February 2003

The present study compares and interprets the 1H, 2H, and 13C spin-lattice (T1) relaxation times of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), in the liquid crystalline phase, in terms of models for the molecular dynamics of lipid bilayers. The 1H T1 times of the DPPC bilayer hydrocarbon region at two frequencies and 13C T1 data at seven frequencies, for which the relaxation is dipolar in origin, as well as the 2H T1 data at three frequencies, due to the quadrupolar interaction, can be unified and interpreted in terms of a collective model for order fluctuations. In normalizing the 13C T1 data to the 1H and 2H T1 values, a vibrationally corrected 13C–1H distance parameter of r0CH=1.14 Å has been assumed, rather than the equilibrium bond length of 1.09 Å. The analysis suggests that the behavior of the individual acyl chain segments of lipid bilayers, in the liquid crystalline phase, is similar to that of molecules in nematic fluids.
Solute–solute interactions in water. II. An analysis through the Kirkwood–Buff integrals for 14 organic solutes
Authors: Matteoli Enrico , Lepori Luciano
Published: February 2003

From accurate data of activities (a), partial molar volumes (V̄), and compressibility (K) of binary aqueous mixtures, the so-called Kirkwood–Buff integrals Gij defined by Gij=∫∞0 [ gij(r) −1] 4πr2 dr=f(a,V̄,K), (i=1,2; j=1,2), have been calculated in the whole concentration range. Fourteen water(1)-organic cosolvent(2) systems [methanol, ethanol, 1-propanol, 1-butanol, 2-methyl-2-propanol, acetonitrile, acetone, dimethylsulfoxide, tetrahydrofuran, piperidine, pyridine, 1,4-dioxane, 2-aminoethanol, 2-(dimethylamino)ethanol] have been studied at 25 °C, and two (methanol and ethanol) also at different temperatures. The Gij functions show these features in relation to the molecular structures of component 2: (1) when this component presents a large nonpolar moiety, extrema are exhibited by Gij’s at certain concentrations the more marked the larger the nonpolar portion; (2) when component two is bifunctional, Gij trend is monotonic with concentration; (3) in the temperature range 0–90 °C, G22 increases and G12 decreases with increasing T in the water-rich concentration region. By interpreting the Gij quantities as a measure of the net attraction (or repulsion) among i and j molecules and as an indication of the tendency of molecules j to cluster around a molecule i, the present results also suggested that: (a) the hydrophobic interaction (attraction) more and more increases with increasing the size of the nonpolar moiety of component two, this increase being most marked at mole fraction x2≂0.2–0.3; (b) hydrophilic groups remarkably reduce the attraction among hydrocarbon-like chains according to the sequence: OH≊NH2>NH≊O≊SO>CO >CN≊Nar; (c) the hydrophobic interaction increases with increasing temperature, in contrast to the common belief that such a phenomenon is induced by the peculiar structural features of water, and that these features fade out as temperature increases.
A perturbation analysis of the Wilemski–Fixman approximation for diffusion-controlled reactions
Authors: Weiss George H.
Published: February 2003

Some time ago, Wilemski and Fixman suggested an approximate method for calculating reaction rates for diffusion-controlled reactions. Their derivation contains a factorization assumption that makes it difficult to see how to derive higher order corrections systematically. In this paper, we assume that the reaction term can be regarded as small in a suitable sense, and develop a systematic perturbation analysis that yields the Wilemski–Fixman approximation in lowest order. This identification will be shown to imply that the Wilemski–Fixman approximation corresponds to a factorization of the resulting multiple integrals in a specific way. It will be shown that for a single localized reaction term (i.e., a delta function sink), the Wilemski–Fixman approximation leads to an exact expression for survival probability as a function of time, but the original factorization ansatz used by these authors is violated. We also develop a theory not making use of the restrictive assumption that the initial condition corresponds to an equilibrium density in the absence of reaction. Finally, we develop an exactly solvable model with a nonlocal reaction term against which the approximation can be tested.
Vibrational partition functions calculated from limited information
Authors: Isaacson Alan D. , Truhlar Donald G.
Published: February 2003

We apply several methods to estimate quantum mechanical partition functions for one-dimensional oscillators using only limited information, namely, the classical dissociation energy and the expansion of the potential through the quartic term in displacements from the minimum. The methods considered are based on the Dunham expansion or the Pitzer–Gwinn method. Various corrections are considered to try to ensure accuracy both at low temperature, where the partition function is most sensitive to the zero-point energy, and at high temperature, where it is essential to have approximate energy levels that increase in a reasonable fashion all the way to the dissociation energy. A method based on a Padé approximant modification of the Dunham expansion is found to provide a useful compromise of accuracy and simplicity for a wide variety of cases. We also show that the Pitzer–Gwinn method may be used successfully even when only limited information is available.
A model potential for the interaction of oxygen and Ag(110)
Authors: Lin J.-H. , Garrison B. J.
Published: February 2003

A model LEPS potential has been developed to describe the interaction of two and three oxygen atoms with the Ag (110) crystal face. The surface encompasses both molecular and atomic adsorption states. The oxygen–oxygen antibonding potential is assumed to be electrostatic and anisotropic in form. These modifications are necessary to account for the observed (n×1) overlayer structures where 2≤n≤7. Classical trajectory calculations have been performed in order to examine the adsorption dynamics. Adsorption into both the molecular and atomic states are observed. The probability for adsorption is the largest if the oxygen molecule is aimed towards the trough on the surface.
Treatment of intershell correlation effects in a
b i
n
i
t
i
o calculations by use of core polarization potentials. Method and application to alkali and alkaline earth atoms
Authors: Müller Wolfgang , Flesch Joachim , Meyer Wilfried
Published: February 2003

In the present approach the high reliability of ab initio techniques is combined with the easily amenable phenomenological core polarization concept for an efficient treatment of intershell correlation effects in all-electron SCF and valence CI calculations. By use of only a single adjustable atomic parameter, which is related to the radius of the core and determines the cutoff at short range, our effective core polarization potential (CPP) accounts quantitatively for dynamical intershell correlation as well as exclusion effects on the correlation energy of the core. The applications refer to alkali and alkaline earth atoms (Li to K and Be to Ca) and a detailed analysis is performed for core polarization effects on ionization energies, electron affinities, oscillator strengths, polarizabilities, van der Waals coefficients, the valence electron density, and spin densities. Very accurate results are obtained for well-known energetic properties and spin densities at the nucleus. With respect to the other applications we consider our results as the most reliable to date with an estimated uncertainty of 1%–2%.
Osmotic susceptibility and diffusion coefficient of charged bovine serum albumin
Authors: Neal Dennis G. , Purich Daniel , Cannell David S.
Published: February 2003

We report measurements of the osmotic susceptibility and mutual diffusion coefficient of solutions of charged bovine serum albumin molecules made using laser light scattering. The measurements were made as a function of solution ionic strength for various macromolecular charge states ranging from −4e to −13e. We find that the osmotic susceptibility data may be interpreted quantitatively in terms of the simple Verwey–Overbeek interaction potential, provided that adequate care is taken in computing the radial distribution function. Current theoretical treatments of the dynamics, with hydrodynamic interactions included to first order in the concentration, are found to deviate significantly (≲50%) from the diffusion coefficient data. The deviations occur under all conditions of charge and ionic strength for which the susceptibility has been reduced to less than half the ideal gas value.
Molecular beam photoionization and fragmentation of D2S, (H2S)2, (D2S)2, and H2S⋅H2O
Authors: Walters E. A. , Blais Normand C.
Published: February 2003

A molecular beam photoionization study of (H2S)2 and (D2S)2, generated in the supersonic expansion of H2S and d2S through a nozzle is reported. The ionization potentials of H2S and D2S are measured. No isotope shift is observed in the ionization potential. (AIP)
Ensemble averaged spin pair dynamics of doublet and triplet molecules
Authors: Schulten Klaus
Published: February 2003

Analytical expressions are presented which describe the spin dynamics of random ensembles of pairs of molecules, the partners being either of doublet (2D) or of triplet (3T) type. The spin dynamics are induced either by the hyperfine coupling (2D) or by the zero-field splitting (3T). The three spin transitions 1(2D1+2D2)→3(2D1+2D2), 4(3T1+2D2)→2(3T1+2D2), and 3,5(3T1+3T2)→1(3T1+3T2) are considered at low and high magnetic fields. The ensemble average is taken over all nuclear spin configurations (2D), over all molecular orientations (3T) and over possible histories which may include nuclear spin realignments by paramagnetic–diamagnetic exchange (2D) or molecular reorientations by rotational diffusion or exciton migration in random matrices (3T). The results obtained demonstrate that magnetic fields influence the processes of triplet quenching by radicals and triplet–triplet annihilation and thereby can provide information on the rotational diffusion of triplet molecules, exciton migration, and paramagnetic–diamagnetic exchange. Such effects predicted earlier by us for processes involving doublets [J. Chem. Phys. 71, 1878 (1979)] and discussed here, were recently demonstrated experimentally [F. Nolting, H. Staerk, and A. Weller, Chem. Phys. Lett. 88, 523 (1983)]. The results presented are also of general interest as the spin systems considered provide a solvable model to describe the degree of randomization in ensembles of quantum systems.
The energy storage in the Rouse model in an arbitrary flow field
Authors: Booij H. C.
Published: February 2003

The general solution of the configurational distribution function for the Rouse model of linear flexible macromolecules in solutions subjected to arbitrary flow fields obtained by D. H. King and D. F. James [J. Chem. Phys. 78, 4743 (1983)] is shown to be identical to the one derived by P. H. van Wiechen and H. C. Booij [J. Eng. Math. 5, 89 (1971)]. However, their expression for the energy storage in that model is incorrect. The relevant formula is derived and it appears that a useful approximation of the increase in Helmholz free-energy per unit volume is given by half of the trace of the stress tensor (ΔF≂ 1/2  tr P).
Electron–electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state
Authors: Brus L. E.
Published: February 2003

We model, in an elementary way, the excited electronic states of semiconductor crystallites sufficiently small (∼50 Å diam) that the electronic properties differ from those of bulk materials. In this limit the excited states and ionization processes assume a molecular-like character. However, diffraction of bonding electrons by the periodic lattice potential remains of paramount importance in the crystallite electronic structure. Schrödinger’s equation is solved at the same level of approximation as used in the analysis of bulk crystalline electron-hole states (Wannier excitons). Kinetic energy is treated by the effective mass approximation, and the potential energy is due to high frequency dielectric solvation by atomic core electrons. An approximate formula is given for the lowest excited electronic state energy. This expression is dependent upon bulk electronic properties, and contains no adjustable parameters. The optical f number for absorption and emission is also considered. The same model is applied to the problem of two conduction band electrons in a small crystallite, in order to understand how the redox potential of excess electrons depends upon crystallite size.
Statistical ensembles and molecular dynamics studies of anisotropic solids
Authors: Ray John R. , Rahman A.
Published: February 2003

We present a general discussion of the isoenthalpic–isostress molecular dynamics theories of Andersen and Parrinello–Rahman. The Parrinello–Rahman theory is shown to be applicable to the case of nonlinear elasticity if the reference state of zero strain is taken to be the state under zero stress; this brings the theory into accord with the thermodynamics of anisotropic solids for arbitrary values of the strain. For the isoenthalpic–isostress ensemble there is a microcanonical counterpart for which we present fluctuation formulas involving the constant strain specific heat, temperature coefficients of thermodynamic tension, and stiffness coefficients. The use of these various ensembles for the molecular dynamic study of polymorphic transitions in crystals is discussed.
Inherent pair correlation in simple liquids
Authors: Stillinger Frank H. , Weber Thomas A.
Published: February 2003

This paper is dedicated to the proposition that liquids possess an inherent packing structure which is determined by their collection of potential energy minima. To reveal the inherent structure in a given thermodynamic state, it is necessary to subject the dynamical system to steepest-descent ‘‘quenches’’ that remove thermal motion and distortion and leave the system in the nearest mechanically stable arrangement. Such a program has been carried out via molecular dynamics simulation on an argon-like system containing 108 atoms. Two thermodynamic states at the same reduced density (ρ*=1.0) were considered, one just above the melting temperature T*m and one at approximately 3.5 T*m . Although the pair correlation functions g(r) in these thermodynamic states differed considerably, those produced by the corresponding sets of quenches gq(r) were virtually identical. The implied inherent structure common to both states appears to be best described in terms of highly defective face-centered-cubic crystalline configurations.
Theoretical investigations of elementary processes in the chemical vapor deposition of silicon from silane. Unimolecular decomposition of SiH4
Authors: Viswanathan R. , Thompson Donald L. , Raff L. M.
Published: February 2003

The rates and mechanism for the unimolecular decomposition of SiH4 have been investigated using quasiclassical trajectory methods to follow the dynamics and Metropolis sampling procedures to average over the initial SiH4 phase space. The semiempirical potential-energy surface has been fitted to scaled SCF calculations and to a variety of experimental data. It gives the correct SiH4 equilibrium structure, reaction endothermicities, and bond energies for SiH4, SiH3, and SiH2. All hydrogen atoms are treated in an equivalent fashion. Excellent first-order decay plots are obtained for the microcanonical rates for the total SiH4 decomposition as well as for the separate decomposition channels. The low-energy pathway is found to be a three-center elimination to form SiH2+H2. The decomposition channel forming SiH3+H becomes important only at internal SiH4 energies in excess of 5.0 eV. Comparison of computed falloff curves with RRKM calculations fitted to experimental results indicates that the critical threshold energy for the three-center reaction lies in the range 2.10<E0<2.20 eV. The calculated high-pressure limiting rate coefficient is k(T,∞)=(1.08±0.02)×1013 exp(−49 600±1200/RT) s−1. The calculated distributions of relative translational energies for SiH2 and H2 reflect the fact that there is no barrier to the back reaction. Both concerted three-center eliminations and processes that resemble ‘‘half-collisions’’ of SiH3+H are found to be important decomposition pathways.
Carbon K-shell electron energy loss spectra of 1- and 2-butenes, t
r
a
n
s-1,3-butadiene, and perfluoro-2-butene. Carbon–carbon bond lengths from continuum shape resonances
Authors: Hitchcock A. P. , Beaulieu S. , Steel T. , Stöhr J. , Sette F.
Published: February 2003

Electron energy loss spectra of 1-butene, cis-2-butene, trans-2-butene, trans-1,3-butadiene, and perfluoro-2-butene in the region of carbon K-shell (C 1s) excitation and ionization have been recorded under dipole-dominated inelastic electron scattering conditions. The features observed below the C 1s I.P. in the spectra of the butenes and butadiene are assigned to promotions of C 1s electrons to unoccupied valence (π*) and Rydberg orbitals while broad features observed above the edge are assigned to σ(C–C) and σ(C–C) shape resonances. These spectra, along with carbon K-shell spectra of other hydrocarbons, are used to demonstrate that there is a quantitative relationship between carbon–carbon bond lengths and the location of σ shape resonances relative to the C 1s ionization threshold (I.P.). The C 1s spectrum of perfluoro-2-butene demonstrates dramatic potential barrier effects, namely suppression of Rydberg transitions and strong enhancement of σ(C–C) and σ(C–F) shape resonances in the region of the C 1s ionization threshold.
Complex optical potential model for electron–molecule scattering, elastic scattering, and rotational excitation of H2 at 10–100 eV
Authors: Staszewska Grażyna , Schwenke David W. , Truhlar Donald G.
Published: February 2003

We report nonempirical calculations for differential and integral elastic scattering cross sections, absorption cross sections (accounting for electronic inelasticity), and total scattering cross sections for electron–H2 scattering at 10–100 eV. The calculations are based on a complex, energy-dependent effective potential consisting of four terms: A static potential calculated from ab initio extended-basis-set Hartree–Fock (EBSHF) wave functions, a polarization potential calculated from an ab initio EBSHF adiabatic polarization potential modified by the local-kinetic-energy semiclassical polarization model to account for nonadiabatic effects, an exchange potential calculated from the EBSHF static electron density and static potential by the semiclassical exchange approximation, and an imaginary absorption potential calculated from the EBSHF static electron density and static potential by the quasifree scattering model with Pauli blocking. We obtain good agreement with all available experimental data at impact energies 10, 40, and 100 eV. Additional calculations with simplified potentials show that nonadiabatic effects on the real polarization potential and the inclusion of an imaginary absorption potential are both essential to the success of the theory except for the large-angle scattering at the highest energy, where the calculated absorption effect is too strong.
Atomic and molecular correlation energies with explicitly correlated Gaussian geminals. III. Coupled cluster treatment for He, Be, H2, and LiH
Authors: Jeziorski Bogumil , Monkhorst Hendrik J. , Szalewicz Krzysztof , Zabolitzky John G.
Published: February 2003

Čižek’s coupled-pair, many-electron theory is formulated in a first-quantized, basis set independent way. The resulting set of coupled integro-differential equations for symmetry-adapted spinless pair functions is then solved variationally using the basis set of explicitly correlated Gaussian geminals. In this way, accurate values of the correlation energies in both the linear and quadratic versions of the coupled-pair theory are obtained for He, Be, H2, and LiH. These values are expected to be saturated up to within a fraction of 1%. For Be our results are practically identical with the basis set independent coupled-pair energies obtained recently by Lindgren and Salomonson using an extensive partial-wave expansion, two-dimensional numerical integrations, and extrapolation techniques. For LiH, at the equilibrium separation of the nuclei, the correlation energy obtained using the complete coupled-pair theory amounts to −81.5 mhartrees. Since the leading (fourth-order) perturbation correction to this result is negative, this value can be viewed as a ‘‘perturbative’’ upper bound to the true nonrelativistic correlation energy. The linear coupled-pair theory gives −82.7 mhartrees for the correlation energy of LiH; this value cannot be considered as an upper bound, however. The above results are to be compared with the estimated experimental correlation energy of LiH amounting to −83.2±0.1 mhartree. A simplified theory obtained by neglecting all four-electron integrals in the quadratic part of the coupled-pair equations has been tested. For both Be and LiH the correlation energies obtained differ by only a few hundredths of a mhartree from the complete coupled-pair results.
Effects of basis set and electron correlation on the calculated properties of the ammonia dimer
Authors: Latajka Z. , Scheiner Steve
Published: February 2003

A b initio calculations are carried out for (NH3)2 with a 6-31G**(1p,2d) basis set containing diffuse polarization functions. Electron correlation is included via second-order Mo/ller–Plesset perturbation theory (MP2). At the SCF level, the equilibrium R(NN) distance is 3.54 Å and the interaction energy is −2.35 kcal/mol. Inclusion of correlation enhances the attraction substantially, increasing the energy to −4.05 kcal/mol and reducing the intermolecular separation by 0.20 Å. Comparison with previous results at the SCF level demonstrates a variety of errors including exaggerated dipole moments, underestimation of polarization energy, and sizable superposition errors with these smaller basis sets.
The magnetic behavior of clusters of the type [M3O(RCOO)6]+. An isotropic model involving dynamic distortions
Authors: Jones Donald H. , Sams John R. , Thompson Robert C.
Published: February 2003

A dynamic distortion model for the magnetic behavior of clusters which contain equilateral triangles of exchange coupled spins is derived using the assumptions of (i) isotropic coupling and (ii) a linear relationship between the coupling constants and interatomic distances. The model is shown to be consistent with experimental data for complexes of the form [M3O(RCOO)6]+, M=Fe,Cr. The spectrum of magnetic states resulting from the dynamic distortion model is found to be very similar to that of the isoceles triangle model of Kambe, but the present model is physically more satisfactory.
Kinetics of the volume melting. Nucleation and superheating of metals
Authors: Motorin V. I. , Musher S. L.
Published: February 2003

The kinetics of the bulk melting of metals and other substances, wherein atoms independently join the growing liquid phase in all points of the interface, is studied. The frequency of the formation of the liquid nuclei is obtained as a function of the macroscopic parameters which are known from experiments. It is shown that ordinary metals can be superheated up to 150 to 200 °C above the melting temperature. The result is in accordance with the experiment. The near critical nucleus growth velocity is proved to be limited by the microscopic kinetics of melting and elastic forces are insignificant. The basic results are valid in the case of crystal nuclei formation also.
On the phase space subdivision of the second virial coefficient and its consequences for kinetic theory
Authors: Rainwater J. C.
Published: February 2003

Two new methods of partitioning the second virial coefficient B into free, bound, and metastable parts, which differ from the well known partitioning of Stogryn and Hirschfelder, are presented. It is shown that the proper partitioning to use depends on the specific physical problem of interest. In particular, in the kinetic theory of moderately dense gases due to Curtiss, Snider, and co-workers, certain collision integrals reduce unambiguously to linear sums of B and its temperature derivatives for repulsive potentials, but it has not been clear to what such integrals reduce for realistic potentials. It is shown that such integrals reduce to the previously derived expressions with B replaced by one of our two new definitions of its free part. This contrasts with previous applications to real gases in which Curtiss and co-workers have used the full B and Kuznetsov has used the free part of B as defined by Stogryn and Hirschfelder. Also, original numerical calculations for the collision integrals are presented and the numerical consistency of the theory is verified.
A model for contact angle hysteresis
Authors: Joanny J. F. , de Gennes P. G.
Published: February 2003

We discuss the behavior of a liquid partially wetting a solid surface, when the contact angle at equilibrium θ0 is small, but finite. The solid is assumed to be either flat, but chemically heterogeneous (this in turn modulating the interfacial tensions), or rough. For weak heterogeneities, we expect no hysteresis, but the contact line becomes wiggly. For stronger heterogeneities, we first discuss the behavior of the contact line in the presence of a single, localized defect, and show that there may exist two stable positions for the line, obtained by a simple graphic construction. Hysteresis shows up when the strength of the defect is above a certain threshold. Extending this to a dilute system of defects, we obtain formulas for the ‘‘advancing’’ and ‘‘receding’’ contact angles θa, θr, in terms of the distribution of defect strength and defect sharpness. These formulas might be tested by controlled contamination of a solid surface.
4He n-mers and Bose–Einstein condensation in He I
I
Authors: Ghassib H. B. , Chester G. V.
Published: February 2003

A critique is presented of a recent comment by March in which he argues that the Bose–Einstein condensate may not exist in HeII. It is concluded that such a dramatic statement is not warrranted in the light of current evidence.
Functional form for representing all vibrational eigenenergies of a diatomic molecule state. IV. Application to the Br2 B(3Π+u) state
Authors: Beckel Charles L. , Kwong Robin B. , Hashemi-Attar Ali-Reza , Le Roy Robert J.
Published: February 2003

A composite expression for diatomic vibrational energy levels is here tested by application to the B state of Br2. The form is Ev=D−(vD−v)m[L/N], where Ev is the energy of vibrational level v, D the dissociation limit, vD the value of v at dissociation, and [L/N] a ratio of polynomials in (vD−v). This functional form with a variety of [L/N] is fitted to experimental data for 79,79Br2. The best fits, both with m variable and with m fixed at the value 10/3 predicted by near-dissociation theory, have an overall rms error of 0.015 cm−1. The best fits with m≡10/3 all yield realistic estimates of the coefficient C5 (of R−5, with R nuclear separation) in the long-range potential; moreover, fixing m at 10/3 offers computational and interpretational advantages. Analogous fits to Dunham polynomials in (v+1/2) are found to be distinctly less reliable than these composite ‘‘near-dissociation expansion’’ (NDE) functions with m fixed or variable. In order to test the interpolation and low-v extrapolation power of the composite functions, fits are made with 12 of the 56 Ev excluded from the data set; the NDE functions again perform as well as or better than Dunham polynomials. In addition, use of Stwalley’s mass-reduced quantum numbers in the best of the m=10/3 functions obtained for 79,79Br2 yields predicted vibrational spacings for 81,81Br2 with errors (<0.03 cm−1) only slightly larger than the experimental uncertainties.
Ion–molecule association reactions: A study of the temperature dependence of the reaction N+⋅2+N2+M → N+⋅4+M for M=N2, Ne, and He: Experiment and theory
Authors: van Koppen Petra A. M. , Jarrold Martin F. , Bowers Michael T. , Bass Lewis M. , Jennings Keith R.
Published: February 2003

The temperature dependence of the N+⋅2+N2 → N+⋅4 association has been investigated using a mass spectrometer source with coaxial electron entrance and ion exit apertures. The source resembles a miniature drift tube in that a small electric field is used to move the ions through the source. The effect of the field on the ion energy is discussed. Thermal reaction rate coefficients are obtained over the temperature range of 80 to 450 K, using He, Ne, and N2 as buffer gases. The experimental data reported here are compared with literature data as well as with the predictions of phase space theory calculations. Good agreement was found for the pure N2 system between the present results, previous experiments and theoretical calculations. Less good agreement was found between previous and present experiments when He was used as the buffer gas. In this case, the calculations tended to support the present results. The theoretical calculations also indicate a significant pressure dependence in the third-order rate coefficient which may account at least in part for the discrepancy between results.
WKB approximation for the reaction-path Hamiltonian: Application to variational transition state theory, vibrationally adiabatic excited-state barrier heights, and resonance calculations
Authors: Garrett Bruce C. , Truhlar Donald G.
Published: February 2003

We present several kinds of calculations, for reaction rates, vibrationally adiabatic barrier heights, and resonance energies and widths, in which we use the WKB approximation for vibrational energies of stretches in the reaction-path Hamiltonian. We consider both collinear and three-dimensional atom-transfer reactions. As compared to previous calculations employing the Morse approximation for vibrational energies of stretches, there is generally significant quantitative improvement in accuracy for ground-state quantities and thermal reaction rates, and there is dramatic improvement in accuracy for excited-state quantities. We also update our predictions for some three-dimensional reaction rates calculated with an accurate ab initio potential energy surface.
Compound clusters produced by the reaction of elemental vapors
Authors: Martin T. P.
Published: February 2003

Cesium clusters containing up to 15 atoms were allowed to react with a variety of gases and vapors (O2, H2O, S8, Cl2) and the products were observed in a mass spectrometer. For example, using a cluster source containing two ovens, it was possible to produce and detect CsS clusters of all conceivable compositions, from (CsS48)+ to (Cs14S)+. Certain clusters, however, stand out particularly strongly in the mass spectra. These clusters are assumed to be unusually stable. Where meaningful, the structures of the stable clusters were determined by minimizing the total binding energy assuming a pair potential.
Vibrational predissociation of the Ne–C2H4 and Ar–C2H4 van der Waals complexes
Authors: Hutson Jeremy M. , Clary D. C. , Beswick J. A.
Published: February 2003

Calculations of vibrational predissociation lifetimes and product state distributions are carried out for the van der Waals complexes Ne–C2H4 and Ar–C2H4. The technique uses a scattering wave function expanded in molecular vibrational states coupled with azimuthal basis functions describing the rotation of the C2H4 molecule about the C–C axis. A model potential based on pairwise-additive atom–atom potentials is employed. For states of the complex correlating with ethylene in its ν7 vibrationally excited state, fast vibrational predissociation to form vibrationally excited (ν10) fragments is found to be dominant. By contrast, predissociation to form ground state fragments is very slow. The shortest lifetimes calculated are 260 ps for Ne–C2H4 and 48 ps for Ar–C2H4, corresponding to linewidths of 0.02 and 0.11 cm−1, respectively.
State-resolved product detection in the overtone vibration initiated unimolecular decomposition of HOOH(6νOH)
Authors: Rizzo T. R. , Hayden C. C. , Crim F. F.
Published: February 2003

Excitation of reactant overtone vibrations in combination with laser induced fluorescence detection of unimolecular decomposition products permits measurements of reactant vibrational overtone excitation spectra and product state distributions for selectively excited hydrogen peroxide and its partially deuterated analog. The hydrogen peroxide excitation spectrum in the region of the fifth OH overtone vibration (6νOH) shows a pure local mode feature along with several others that are apparently combinations of a local mode vibration with a normal mode vibration. The product state distributions show substantial energy release into rotation with some molecules having rotational excitation approaching the limit of the total energy available. A statistical calculation incorporating conservation of total angular momentum and energy describes the observed distributions quite well for excitation of the pure local mode vibration. The products from excitation of a combination transition are in a distribution of states that differs qualitatively from a statistical outcome, possibly indicating selective energy coupling on the time scale of the unimolecular reaction.
On the evaluation of nonadiabatic coupling matrix elements using SA-MCSCF/CI wave functions and analytic gradient methods. I
Authors: Lengsfield Byron H. , Saxe Paul , Yarkony David R.
Published: February 2003

A method for the efficient evaluation of nonadiabatic coupling matrix elements of the form 〈ΨI‖∂/∂RαΨJ〉 is presented. The wave functions ΨI and ΨJ are assumed to be multiconfiguration self-consistent field (MCSCF) wave functions optimized within the state averaged approximation. The method, which can treat several states simultaneously, derives its efficiency from the direct solution of the coupled perturbed state averaged MCSCF equations and the availability of other appropriate derivative integrals. An extension of this approach to SA-MCSCF/CI wave functions is described. Here it is shown that computational efficiencies can be achieved by exploiting analogies with analytic CI gradient methods. Numerical examples for C2v approach of Mg to H2 are presented.
Measurement of atomic sodium and potassium diffusion coefficients
Authors: Silver Joel A.
Published: February 2003

The gaseous diffusion coefficients for sodium and potassium atoms have been measured as a function of temperature in a flow tube. Analysis is based on a simple relationship between diffusion and the observed alkali decay rate for the case of very reactive walls. The resulting values for the diffusion coefficient in units of cm2 s−1 at 1 atm are: The results agree well with previous measurements and the comparison with theoretical expectations is discussed.
Electrodynamics at rough metal surfaces: Photochemistry and luminescence of adsorbates near metal-island films
Authors: Garoff S. , Weitz D. A. , Alvarez M. S. , Gersten J. I.
Published: February 2003

Detailed measurements of the photochemical and photophysical properties of an adsorbate on discontinuous metal-island films are used to explore the unusual electrodynamics near rough metal surfaces. Several aspects of the properties have been measured: the magnitude the temporal decay of the fluorescence, the shape and temporal evolution of the fluorescence spectrum, and the effects on the spectrum of a photochemical hole-burning process. Dramatic increases in the fluorescent decay rate and decreases in the photochemical reaction rate as well as systematic spectral shifts of the emission of molecules experiencing the different electrodynamic environments on the island film are observed. These results reveal the strong effects of the coupling between the adsorbate and the plasma resonances localized on the islands of the film. We model our results using the electrodynamic picture which has successfully described many aspects of surface-enhanced Raman scattering and other optical processes on island films. The excellent agreement between this model and our results suggests that an important feature of the electrodynamics at these rough metal surfaces is the dipolar character of the couplings between the surface, the adsorbate, and the optical fields.