bluuebunny/crossref_metadata_2025 huggingface dataset:
Wetting transitions in polymer blends: A Monte Carlo lattice simulation
Authors: Wang Jian-Sheng , Binder Kurt
Published: July 2002

Wetting transitions of binary polymer mixtures in contact with a wall were studied using lattice models by the method of Monte Carlo simulation. First-order wetting transitions were found below the critical temperature but at a moderately high volume fraction of the polymer type favored by the wall. The transitions appear to be second order at low temperatures or for short chain lengths.
Calculated Raman overtone intensities for H2 and D2
Authors: Bishop David M. , Pipin Janusz
Published: July 2002

Dynamic dipole polarizabilities pertinent to the Raman intensities of the fundamental and first two overtones of H2 and D2 have been calculated. To do this an energy-shifted sum-over-states formula has been used in conjunction with accurate electron-correlated wave functions. The results for the relevant polarizability ratios agree, in the static limit, with a previous calculation. The dynamic values are in fair agreement with the recent experimental ones for H2 but there is a strong disagreement with a previous experimental determination for D2.
Interactions energies associated with short intermolecular contacts of C–H bonds. II. A
b i
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o computational study of the C–H⋅⋅⋅H–C interactions in methane dimer
Authors: Novoa Juan J. , Whangbo Myung-Hwan , Williams Jack M.
Published: July 2002

On the basis of single-and multireference ab initio calculations on (Be)2 and (H2)2, we suggest that van der Waals systems with large highest occupied molecular orbital–lowest unoccupied molecular orbital gap can be reasonably well described by single reference MPn (n=2–4) calculations if the basis set is large enough. The binding energies of C–H⋅⋅⋅H–C contacts are then examined by performing single reference MPn calculations on (CH4)2 dimer. Our study shows that (CH4)2 is bound in all possible CH4⋅⋅⋅CH4 arrangements, and that those arrangements with more than one C–H⋅⋅⋅H–C contact lead to a greater amount of stabilization than does the arrangement with one C–H⋅⋅⋅H–C contact. The potential energy curves obtained for staggered arrangements of (CH4)2 by the MP2 calculations with the 6-311G(2d,2p) basis set are in close agreement with the experimentally deduced, isotropic potential curve.
Study of higher excited Rydberg states in ethylene
Authors: Wu C. Y. Robert , Xia T. J. , Liu G. S. , McDiarmid Ruth
Published: July 2002

The photoabsorption and photoionization spectra of ethylene have been shown to exhibit weak broad structures in the spectral region between the ionization threshold and 1060 Å. Some members of three Rydberg series that converge to the first excited electronic state Ã(2B3) of C2H+4 have been identified. They have been tentatively assigned as transitions from the ground state to the 4sσ-, 4pσ-, 3d-, and 4d-Rydberg states. The C2H4 molecule is suggested to be twisted in at least the 4sσ- and nd-Rydberg states.
The internal polarization basis set: A note of the force-adapted a
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o calculation
Authors: Liu Shu-Bin , Peng Zhou-Ren
Published: July 2002

A new version of the force-adapted ab initio basis set was proposed in this communication based on Nakatsuji’s Family basis set and our previous research. Test examples show that the internal polarization basis set, in which the polarization function is appended in the inner shells, is effective in improving the accuracy of the resultant force as well as in reducing computational efforts.
A model for adsorption of O on Mo(110): Phase transitions with nonuniversal behavior
Authors: Dünweg Burkhard , Milchev Andrey , Rikvold Per Arne
Published: July 2002

A lattice-gas model for the phase transitions of O monolayers on Mo(110) with interactions ranging up to fifth neighbors, and including a three-particle term, is studied by Bragg–Williams mean-field theory and finite-size scaling of data from Monte Carlo simulations and numerical transfer-matrix calculations. Attention is focused on the p(2×2) phase whose symmetry is that of an XY model with cubic anisotropy. The order–disorder transition lines are shown to exhibit the predicted nonuniversal behavior, and possess tricritical points. A phenomenological finite-size scaling theory for the Monte Carlo order-parameter data in the pertinent case of a third-order coupling in the Landau free energy is established. The order–order transition line between the p(2×2) and (2×1) phases is found to consist of two parts, a first-order line at low temperature, and a line at higher temperatures, along which the model has a pure Kosterlitz–Thouless phase. Good agreement is established with an experimental phase diagram based on low-energy electron diffraction (LEED) data.
Temperature dependence of the molar volume of liquid He ii
Authors: Lu Pao , Shan Yueh
Published: July 2002

We are able to reproduce the experimental volume and temperature dependence for quasiparticles of helium ii from the theoretical study reported here. We use the energy excitation spectrum together with a Hartree term as derived before [Physica Status Solidi A 159, 52 (1989)].
An analysis of 3dcorrelation effects in the bonding of atomic oxygen to Cu(100)
Authors: Illas F. , Rubio J. , Ricart J. M.
Published: July 2002

The effect of 3d correlation in the interaction of atomic O with a Cu(100) surface has been studied by using large multireference configuration interaction expansions on a Cu5O cluster model. While valence correlation arising from the five 4s1 electrons of the cluster metal atoms plus the 2s22p4 electrons of atomic oxygen is easily recovered, inclusion of the 50 electrons arising from the 3d10 shells leads to huge expansions of increasing size and to less reliable results. The study of multireference configuration interaction expansions shows that the effect of 3d correlation tends to decrease the interaction energy obtained at the valence configuration interaction level. This is in agreement with the ionic nature of the bond.
A new flexible/polarizable water model
Authors: Zhu Sheng-Bai , Singh Surjit , Robinson G. Wilse
Published: July 2002

Both geometrical flexibility and instantaneously responsive electrical polarization are incorporated into a newly developed 5-site water model that includes one oxygen atom, two partially shielded protons, and two negative charges representing lone pairs. The charges are diffusively distributed. Their values are variable in accordance with the local field. The intramolecular potential function used is the one recently developed by Dang and Pettitt [J. Phys. Chem. 91, 3349 (1987)] for a free water molecule. In order to strengthen the angular dependence of the intermolecular dimer potential, a short-range Morse-type interaction is introduced to represent specific hydrogen bonding interactions. With this model we carry out a classical constant volume molecular dynamics simulation of liquid water at mass density 0.997 g/cm3 and room temperature 298 K. Results for the liquid structure, thermodynamic properties, transport dynamics, dielectric features, and spectroscopic characteristics are presented and compared with the experimental data and other relevant computer simulations. These comparisons show a significant improvement over the 3-site flexible/polarizable model developed earlier at Texas Tech. Though about four times computationally more intensive, the new model is still simple enough to be applied to studies of liquid water in the presence of various types of local perturbations, where electrical fields and orientational effects specifically require geometric flexibility and electrical polarization.
Semiclassical calculation of self-broadening in O2, N2, and CO Raman spectra
Authors: Looney J. P. , Rosasco G. J.
Published: July 2002

Semiclassical calculations of the J and temperature dependence of the Raman Q1 and S0 self-broadened linewidths of O2, N2, and CO have been performed within the framework of Robert and Bonamy [J. Phys. (Paris) 10, 923 (1979)] with J=0–30 and T=300–1500 K. The calculation of the CO self-broadened R1 linewidths are also reported for T=100–300 K and J=0–20. Comparison of the results of these calculations with recent experiments shows good overall agreement.
Contribution to the electron distribution analysis. I. Shell structure of atoms
Authors: Kohout Miroslav , Savin Andreas , Preuss Heinzwerner
Published: July 2002

Relativistic spherically averaged numerical all-electron densities ρ were computed for the atoms Be–Ba, B–Tl, C–Pb, Cu–Au, and Zn–Hg. The Laplacian of these densities is not able to resolve the valence shell from the inner shells in case of heavy atoms, starting with the fourth row. The distribution of the local kinetic energy Ekin shows a valence maximum even for these heavy atoms, unfortunately, in a region of negative kinetic energy; i.e., nonclassically allowed. The quantity −‖∇ρ‖/ρ was also investigated. For all computed atoms, the −‖∇ρ‖/ρ diagrams are capable of describing the complete shell structure. −‖∇ρ‖/ρ is sensitive to basis set quality: poor Gaussian basis sets exhibit spurious oscillations and a premature onset of the linear decay. For the atoms B–Tl, Ba, Au, Hg, and Pb, nonrelativistic numerical calculations were performed to examine the effect of the relativity on the aforementioned quantities. Tests with pseudopotential densities reveal that for pseudopotential calculations, it is advisable to use at least two outermost shells in order to reproduce the all-electron values of ∇ 2ρ, Ekin, and −‖∇ρ‖/ρ in the valence region.
Peeling a polymer from a surface or from a line
Authors: Di Marzio Edmund A. , Guttman C. M.
Published: July 2002

We calculate the force on a long linear polymer molecule whose one end is zippered down onto a surface or onto a line and whose other end is at a perpendicular distance R from the surface or line. Random coil statistics are used for the unattached portion of the chain. The method is extended to the case when the bonds within the zippered portion are breaking and reforming. We also consider the case where the attached portion is in the form of loops and trains. Although the energy equations of state for these various systems are different from each other, the force equation of state is always given by f=((6)1/2/l)(kTΔg)1/2, where l is the bond length between monomers and Δg is the free energy change in pulling one monomer off of the surface. The force is independent of R except for small R. Applications are discussed briefly. They include (1) self-healing systems of gels and rubbers where the cross links may be hydrogen bonds; (2) adhesion; (3) the degree of crystallinity in crystal-amorphous lamellar systems; (4) the packing of DNA into the head of a bacteriophage virus and pulling apart of double stranded DNA; (5) an insight into the theory of rubber elasticity; (6) understanding the critical force for flow in thixotropic systems.
Fluorescence depolarization by electronic energy transfer in donor–acceptor pairs of like and unlike chromophores
Authors: Berberan-Santos M. N. , Valeur B.
Published: July 2002

Fluorescence depolarization by energy transfer resulting from dipole–dipole interaction (Förster type) is studied in donor–acceptor pairs of like and unlike chromophores at a fixed distance and with random and uncorrelated static orientations. For unlike chromophores, the acceptor anisotropy decay is shown to display three different extreme types of behavior. When the intrinsic decay rate of the acceptor is much faster than both the transfer rate and the donor intrinsic decay rate, the acceptor anisotropy decays from a positive value, then rises and passes through a maximum, and finally tends to a negative limiting value, yielding a zero steady-state value. The existence of a maximum is shown to be due to the peculiar relation between the orientation factor and the average angle formed by the donor and acceptor transition moments. For pairs of like chromophores, the exact anisotropy is calculated and compared with that given by an approximate treatment. It is also shown that the anisotropy of the indirectly excited partner varies with time, tending to zero, in contradiction to previous work, where it is reported to be 4% of that of the directly excited chromophore.
Self-broadening and line mixing in HCN Q branches
Authors: Pine A. S. , Looney J. P.
Published: July 2002

Q-branch spectra of the ν1+ν2 (4004 cm−1) and ν2+ν3 (2806 cm−1) combination bands and the ν1−ν2 (2599 cm−1) difference hot band of HCN have been recorded at pressures from 0.13 to 53.3 kPa (1 to 400 Torr) using a tunable difference-frequency laser. The self-broadening coefficients are the same for all three bands involving the ν2 Π bending mode and are within experimental error of those reported previously for other Σ and Π vibrational bands. The J dependence of the self-broadening coefficients exhibits a maximum near the peak of the Boltzmann population distribution, and is well described by semiclassical line-broadening theory incorporating known measured or ab initio dipole and quadrupole moments and polarizabilities, and classical trajectories using an isotropic Lennard-Jones short-range potential. Line mixing is evident in the strongly overlapped, higher pressure Q-branch profiles from the nonadditive Lorentzian superposition of the component transitions. However, line coupling is moderated by the f→e collisional cross relaxation in the l-doubled Π bending vibration. The inelastic rotational collision rates required to fit the spectral line-mixing profiles are poorly represented by empirical energy-gap fitting laws for both R→T and R→R energy transfer. An effective R→T energy-corrected-sudden (ECS) scaling law yields a satisfactory and consistent fit to the Q-branch profiles of all three bands for the pressure range studied.
Acceleration of convergence rates by high directional Monte Carlo sampling procedure
Authors: Shin Jae Kwang , Yu Jeong Yun , Jhon Mu Shik
Published: July 2002

A new Monte Carlo sampling scheme which estimates the shape of an energy hypersurface as a covariance tensor to generate variable Monte Carlo steps, is developed. The feasibility of this method is tested on 256 Lennard-Jones particles at T*=0.719, ρ*=0.85, and 216 TIP4P water molecules at T=298 K and the results are compared with those of other methods. It is found that the Lennard-Jones solid is successfully melted to liquid by the new method while it is failed by the Metropolis algorithm and the radial distribution functions correspond well to those of other established methods. The result shows that the new method has very improved features in several respects. First, the convergence property to reach the equilibrium state is better than that of other methods. Secondly, the equilibrium distribution of ensembles are not biased.
Exact integral equations for the distribution functions of liquids and liquid mixtures
Authors: Likos C. N. , Ashcroft N. W.
Published: July 2002

A known identity relating the functional density derivative of the pair distribution function g(12) of nonuniform liquids to the triplet distribution function g(123) is obtained within the grand canonical ensemble, but then generalized to the case of nonuniform liquid mixtures with either short- or long-range forces. The resulting identities are easily applied to the case of uniform one- or multicomponent liquids, and when applied to the case of a binary Coulomb mixtures, they yield the canonical sum rules relating the n-point to the (n+1)-point distribution functions. Finally, when applied to the one-component plasma (OCP), taken as a particular limiting case, the expected result that the total compressibility of the OCP must vanish in the thermodynamic limit is obtained.
Suppression of sodium nuclear magnetic resonance double-quantum coherence by chemical shift and relaxation reagents
Authors: Hutchison Robert B. , Huntley James J. A. , Jin Haoran , Shapiro Joseph I.
Published: July 2002

An investigation into the signal suppression behavior of the paramagnetic shift and relaxation reagents, Dy(P3O10)27− and Gd(P3O10)27−, with regard to their use in the nuclear magnetic resonance spectroscopic study of sodium has been performed. Measurements of T1 and T2 relaxation time constants of sodium in normal saline, Krebs–Henseleit buffer, and human blood serum, as a function of concentration of these reagents showed that, although closely coupled in the saline and K–H buffer environments, in plasma T1 and T2 become decoupled, transverse relaxation dominating in comparison to longitudinal relaxation. Linewidth measurements further suggest that relaxation in the plasma milieu is controlled primarily by inherent T2 relaxation, rather than by field inhomogeneity or diffusion effects. Quantitative single-quantum (1Q) and double-quantum (2Q) intensity measurements, biexponential T2 relaxation measurements, and parametric studies of the preparation time of the 2Q pulse sequence, were obtained in suspensions of bovine serum albumin and human erythrocytes. The observed suppression of sodium 2Q coherence by paramagnetic shift and relaxation reagents was found to exhibit a complex behavior in albumin solutions, involving the biexponential T2 decay to be expected during the preparation time of the 2Q filter pulse sequence, as well as the optimum preparation time for production of the double-quantum coherence itself. The controlling factor for both of these effects is the biexponential amplitude function in the expression for the transverse magnetization observed following application of the 2Q pulse sequence. This in turn is determined entirely by the values for the slow and fast components of biexponential relaxation in sodium, which themselves depend upon the concentration of the macromolecular binding sites for quadrupolar interaction. A similar behavior has been observed in suspensions of human erythrocytes.
Two-photon photodissociation of gaseous azulene at 325 nm
Authors: Hassoon S. , Snavely D. L. , Oref I.
Published: July 2002

The two-photon photodissociation of gaseous azulene at 325 nm is reported. Acetylene is the major photodissociation product. The quantum yields for acetylene produced by irradiation of neat azulene at its normal vapor pressure and as a function of argon bath gas pressure are reported. The quantum yield of acetylene produced from irradiated azulene at its normal vapor pressure (∼10 mTorr) is ∼(5.85±1.46)×10−6. Using a mechanism by which internal conversion precedes the absorption of the second photon, the two-photon absorption cross section is calculated.
The phase-behavior of Lennard-Jones mixtures with nonadditive hard cores: Comparison between molecular dynamic calculations and perturbation theory
Authors: Schaink H. M. , Hoheisel C.
Published: July 2002

An analytical equation of state for Lennard-Jones mixtures has recently been derived using a perturbation theory with an additive hard sphere mixture (i.e., for the collision diameter d12=(d11+d22)/2) as a reference system. Here we generalize this equation of state using a nonadditive hard sphere mixture as a reference system. Even for Lennard-Jones mixtures that obey the Lorentz–Berthelot mixing rules [σ12=(σ11+σ22)/2 and ε12 =√ε11ε22 ], we find that our generalized theory shows an improvement in the predictions of the excess Gibbs free energy and the excess volume compared to the old version of the theory. For several non-Lorentz–Berthelot mixtures the phase diagrams predicted by the equations of state with recent Gibbs-ensemble Monte Carlo and new molecular dynamics results were compared. In this comparison the van der Waals 1-fluid model as well as an effective hard sphere model were considered. In this work only the fluid–fluid phase behavior was studied. For mixtures characterized by non-Lorentz–Berthelot energy parameters the generalization of the original equation of state gives the best predictions. For a mixture characterized by a relatively large nonadditivity in the repulsion parameters the 1-fluid approximation is best. As a by-product this study yields a generalization of the MIX1 equation of state for mixtures of nonadditive hard spheres with d11≠d22.
Polarization-correlated solute–solvent states
Authors: Young Ralph H.
Published: July 2002

We introduce a new formulation of the joint electronic states of a solute and a solvent for the situation in which the solute has weakly coupled, low-lying states whose mixing must be determined. A typical solute might be an electron–donor–acceptor complex (DA) whose ionic state (D+A−) is coupled to a neutral, locally excited state, (D*A) or (DA*), of similar energy. The key feature of the treatment is that the electronic state and polarization of the solvent are allowed to correlate with those of the solute. With the present formulation, unlike earlier formulations, degeneracies are treated realistically, and different eigenstates are orthogonal. The problem of evaluating the orientational polarization of the solvent is left to future work.
Time-evolution of molecular states in electronically diabatic phenomena
Authors: Fernández Francisco M. , Micha David A.
Published: July 2002

We describe the propagation in time of quantal molecular states for electronically diabatic phenomena involving the coupling of two or more electronic states. We first show how the time propagation of a wave packet can be broken up into several stages corresponding to regions through which the wave packet moves with or without electronic couplings. This is done with a generalization of the interaction picture procedure recently shown to be quite efficient for numerical work. We next consider the propagation of total molecular states in regions with couplings by using sequences of transformations between adiabatic and diabatic electronic representations for a finite number of coupled electronic states. This requires the introduction of a general and efficient way of transforming between the two representations, that is the main concern of this contribution. The transformation is constructed using Lie-algebraic methods; it leads to simple expressions suitable for computations, and contains as a special case the coupling of two electronic states. The propagation in time through the electronically diabatic region is described using a split-operator propagator procedure suitable for electronically diabatic phenomena. We concentrate on only one molecular variable, which could be a bond distance or a bond angle, to focus on the construction of the adiabatic–diabatic transformation.
Adiabatic and quasidiabatic 2Σ+ states of BeH
Authors: Petsalakis I. D. , Theodorakopoulos G. , Nicolaides C. A.
Published: July 2002

An approach to the calculation of quasidiabatic states is presented and applied to the rather entangled spectrum of the first five BeH 2Σ+ excited states. The method of transformation from the adiabatic to the diabatic description employs a criterion of maximum nonorthonormal overlap between sequential diabatic states along the coordinate of interest, starting with the dissociation region as a reference point. The computational method is general and capable of separating out diabatic states from a set of adiabatic ones of mixed character, involving valence, Rydberg and ion-pair contributions.
Application of the three-dimensional doorway-state theory to analyses of the amide-I infrared bands of globular proteins
Authors: Torii Hajime , Tasumi Mitsuo
Published: July 2002

The three-dimensional doorway-state theory is applied to globular proteins to analyze the structure-spectrum correlation of the amide-I infrared (ir) bands in detail. The following three examples are presented. (1) The doorway states of the frequency region around 1630 cm−1 of flavodoxin and carboxypeptidase A are calculated. It is shown that the ir intensity in this frequency region largely originates from the peptide groups in the central zones of β sheets. (2) Calculations on α-lactalbumin and lysozyme clarify the vibrational motions giving rise to the difference in the amide-I band envelopes of these two proteins. (3) Contributions of the A- and E1-modelike vibrational motions of α helices to the amide-I band envelopes are analyzed. It is shown that the E1-modelike motions have large contributions to the ir intensities of the bands in the 1640–1630 cm−1 region, solving the question as to the origin of such bands observed for highly helical proteins. It is also demonstrated that the frequency splitting between the A- and E1-modelike motions depends strongly on the helix length. These three examples reveal the importance of the detailed analyses of vibrational dynamics that gives rise to characteristic amide-I band envelopes.
Photoionization energetics and dynamics of gas-phase SnCl2 by synchrotron radiation photoemission and configuration interaction calculations
Authors: Stranges S. , Adam M. Y. , Cauletti C. , de Simone M. , Furlani C. , Piancastelli M. N. , Decleva P. , Lisini A.
Published: July 2002

The photoionization from outer- and inner-valence shells of SnCl2 vapor, induced by means of molecular effusive beam technique and dispersed synchrotron radiation as an ionizing source, reveals new spectroscopic and dynamical aspects in this molecule. Spectral features related to states with main Cl 3s contribution are observed for the first time in a molecule with Cl-metal bonds. The corresponding ionization energy (IE) is 22.61 eV. A breakdown of the one-particle model is exhibited in the inner-valence spectral region. These experimental findings are compared with the theoretical predictions obtained by performing configuration interaction (C.I.) calculations for the molecular ionic states. A strongly resonant behavior in the cross section of some main lines and satellites is experimentally pointed out by tuning the excitation photon energy through resonances localized at 25.03 and 26.11 eV. CI calculations for the transition energies relative to 4d excitations have been carried out and explain the aforementioned phenomena in terms of autoionizing 4d→LUMO lowest unoccupied molecular orbital 8b1 excitations. Finally, Auger processes following the relaxation of Sn 4d hole in the free molecule SnCl2 are observed.
Theory and measurements of orientation distributions of spheroidal particles by Rayleigh–Debye–Gans light scattering
Authors: Buehler Christopher S. , Caruthers James M. , Franses Elias I.
Published: July 2002

The inverse problem of determining orientation distributions of monodisperse spheroidal particles from single and independent Rayleigh–Debye–Gans (RDG) light scattering was formulated previously [C. S. Buehler, J. M. Caruthers, and E. I. Franses, J. Chem. Phys. 92, 140 (1990)] and solved numerically only for azimuthally symmetric orientation distributions. In this paper, the problem is solved for general orientation distributions and is tested experimentally. A new instrument capable of thousands of simultaneous two-dimensional scattering measurements, over the polar (6°–30°) and the azimuthal scattering angles, is described. Measurements were done on oriented prolate polystyrene spheroidal particles, of volume 0.44 μm3 and aspect ratio 3.5, embedded in a polyvinylalcohol matrix. From the measurements and the solution of the inverse problem, reasonable orientation distribution functions were calculated. Simulations and data support the use of the technique for thoroughly probing orientations in dilute dispersions of RDG-type spheroidal particles in either liquid or solid media.
New electronic spectra of the CHFCl radical observed with resonance enhanced multiphoton ionization
Authors: Hudgens Jeffrey W. , Johnson Russell D. , Tsai Bilin P.
Published: July 2002

The structures and optical spectroscopy of the CHFCl radical and cation were studied by ab initio molecular orbital calculations and by experiment. Ab initio calculations at the MP2/ 6–311++G** theory level found that the optimum structure of the CHFCl+ (X̃ 1A′) cation is planar with r(C–H)=1.092 Å, r(C–F)=1.254 Å, r(C–Cl)=1.599 Å, ∠H–C–F=116.85°, and ∠H–C–Cl=122.14°. CHFCl (X̃ 2A′) radical is nonplanar with r(C–H)=1.083 Å, r(C–F)=1.335 Å, r(C–Cl)=1.705 Å, ∠H–C–F=113.49°, ∠H–C–Cl=116.68°, and ∠F–C–Cl=114.44°. The ab initio angle between the F–C–Cl plane and the C–H bond is Φe=38° and the inversion barrier is Binv=1190 cm−1. Using isogyric reactions to obtain empirical corrections, we calculate IPa(CHFCl)=8.37±0.05 eV. Ab initio vibrational frequencies are reported. The electronic spectrum of the CHFCl radicals was observed between 340–420 nm using one color, mass resolved, 2+1 resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectrum arises from two-photon resonances with the planar F̃(3p) [λlaser=406.7 nm, ν00=49 160(20) cm−1] and J(3d) [λlaser=361.9 nm, ν00=55 250(20) cm−1] Rydberg states. A third laser photon ionized the radicals. Both states produced the same vibrational constants: ν2 (C–H deformation)=1280(30) cm−1, ν4 (C–Cl stretch)=910(30) cm−1, ν5 (CFCl scissors)=440(30) cm−1, ν6 (OPLA)=980(30) cm−1. The REMPI spectrum exhibited v″6 = 1–6 hot bands of the CHFCl (X̃ 2A) radical. Modeling of these hot bands with a double-well potential gives the inversion barrier, Binv=1180 cm−1, and Φe=42°.
Fourier transform infrared observation of the ν2 stretching mode of the HCCCO radical in solid Ar
Authors: Jiang Qian , Graham W. R. M.
Published: July 2002

The identification of a fundamental vibration of the HCCCO radical has been made in a Fourier transform infrared (FTIR) study involving extensive isotopic measurements. An absorption at 2308.6 cm−1 has been assigned to the ν2, antisymmetric CCCO stretching mode of HCCCO which was produced by trapping in Ar the products of the vacuum ultraviolet (VUV) photolysis of a mixture of acetylene and carbon monoxide. The assignment is confirmed by measurements for a variety of D-, 18O-, and 13C-substituted isotopomers.
Growth of titanium nitride: From clusters to microcrystals
Authors: Chen Z. Y. , Castleman A. W.
Published: July 2002

Time-of-flight mass spectrometry is used to investigate (TiN)n+ clusters produced by a laser-induced plasma reactor source. The mass spectral abundance patterns indicate that the clusters have cubic structures resembling subunits of the fcc lattice of solid TiN. The primary stoichiometries observed are (TiN)n+ (n=1–126), except for TinNn−1+ (n=14, 63). The most stable structures of the clusters are cuboids, in some cases containing a completed terrace. Even at the very early stages of crystal growth, the gas-phase clusters prefer a cubic crystalline atomic arrangement with a highly symmetric structure.
The fluorescence action spectra of some saturated hydrocarbon liquids for excitation energies above and below their ionization thresholds
Authors: Ostafin Agnes E. , Lipsky Sanford
Published: July 2002

Fluorescence action spectra have been obtained for the neat liquids, cis-decalin, trans-decalin, bicyclohexyl, cyclohexane, methylcyclohexane, isobutylcyclohexane, 2,3,4-trimethylpentane, 2,3-dimethylbutane, 3-methylhexane, 3-methylpentane, n-decane, n-dodecane, and n-pentadecane at excitation energies, ε, ranging from their absorption onsets (at ca. 7 eV) to 10.3 eV. For all compounds, with the exception of cis-decalin, the fluorescence quantum yield is observed to monotonically decline with increasing ε, reaching a minimum value at an energy, εm (a few tenths of an eV above the liquid phase ionization threshold, εl) followed by a slow increase. In the case of cis-decalin, the fluorescence quantum yield remains constant over the entire range of excitation energies studied, permitting its use as a quantum counter replacing the standard sodium salicylate, at least over a spectral range from 185 to 120 nm. The recovery of the fluorescence quantum yield for ε≳εm is attributed to an increasing probability for electron ejection followed by e−+RH+ geminate recombination, to produce an excited state of RH with energy less than εl. From a simple analysis of the action spectrum, a lower bound estimate of the electron ejection probability, φ±, is obtained as a function of ε. In the case of cyclohexane, where φ± has been obtained by other techniques at ε≂10 eV, the lower bound estimate agrees with the experimental value. From this agreement, arguments are presented to make plausible the conjecture that in all these liquids, the initially produced e−+RH+ geminate ion pair first rapidly internally converts to an ion-pair state ca. 1.6 eV below εl, which then collapses to generate the fluorescent state of RH with an energy close to its absorption threshold value (ca. 7 eV).
Approximations for the inertial defects of planar molecules
Authors: Watson James K. G.
Published: July 2002

The observed zero-point inertial defect Δ0=I0cc−I0aa−I0bb is often used as a test of the planarity of a molecule, but this test is difficult to make quantitative when the information on the vibrations and vibration–rotation interactions is limited. The general formula for the inertial defect to order κ2Ie is separated into harmonic and Coriolis contributions Δ0harm and Δ0Cor, which can be expressed in various ways by use of sum rules. It is shown here that the zero-point harmonic term is given approximately by Δharm0≊Δtau0= 3K(1/ωA+1/ωB+1/ωAB−1/ωC), where K is ℏ2/2hc and the effective vibrational frequencies are given in terms of the centrifugal tensor ταβγδ by τaaaa=−16A3/ωA2, τbbbb=−16B3/ωB2, τcccc=−16C3/ωC2, and τabab=−16ABC/ωAB2, so that Δ0harm can be estimated from centrifugal distortion constants. A fit of empirical data gives Δ0≊Δ0tau[1.0292−0.0911(N−3)], where N is the number of atoms and the correction factor makes approximate allowance for Δ0Cor. Thus an observed Δ0 that is significantly negative relative to this approximation may indicate that the molecule is nonplanar or has unusually low out-of-plane frequencies.
Excitation spectra for the photoassociation of Kr–F and Xe–I collision pairs in the ultraviolet (208–258 nm)
Authors: Jones R. B. , Schloss J. H. , Eden J. G.
Published: July 2002

Photoassociation, the bound←free absorption of a photon by a colliding pair of atoms, has been studied experimentally in the Kr–F and Xe–I systems and is demonstrated to be an effective tool for deducing molecular interaction potentials and for determining the dependence of the relative electronic transition moment on internuclear separation (R) in small molecules having dissociative ground states. Specifically, the excitation (‘‘action’’) spectrum for the photoassociation of thermalized Kr(4p6 1S0)–F(2p5 2P) collision pairs, measured at 300 K in the 208–250 nm wavelength region, reveals deeply modulated Franck–Condon structure associated with transitions of Kr–F pairs from the ground state’s thermal and vibrational continua to the lowest 27 bound vibrational states (v′=0–26) of the KrF(B 2Σ) excited state. Under these conditions, photoassociation occurs in a narrow (ΔR=0.7–0.8 Å) Franck–Condon region in which the difference potential is a single valued function of R. Similar spectra have been observed for Xe(1S0)–I(2P) pairs in the 208–258 nm spectral region that arise from bound←free transitions to the lowest ∼40 vibrational levels of the XeI(B) state. Since the Kr–F and Xe–I collision pairs are photoexcited from the vibrational continuum of the ground state, the atomic pair energy distribution is thermalized and the envelope of the excitation spectrum can be interpreted directly in terms of the relative B–X transition moment, μ(R). Consequently, the behavior of μ(R) has been determined over a limited range in R and, for XeI, μ is found to fall by a factor of 2 in the ∼0.4 Å region extending from R−ReB≊0.25–0.62 Å. The XeI ground state (X 2Σ1/2+) potential in the 3.0≤R≤5.0 Å interval, derived from photoassociation spectra, is similar to that deduced from scattering experiments. Excited and ground state structural constants derived from numerical quantum simulations of the experimental spectra are presented.
Shape resonance enhancement of vibrational excitations for CO chemisorbed on Ni(111) probed via inelastic electron scattering
Authors: Ha Jeong Sook , Sibener S. J.
Published: July 2002

This paper examines how the formation of a transient negative ion during the scattering of an electron from CO chemisorbed on Ni(111) can lead to significant enhancement in the probability for vibrationally inelastic scattering. We specifically report on the incident energy dependence for transient negative ion formation for a c(4×2) overlayer. The signature for this resonance is the energy dependence of the probability for vibrationally inelastic scattering from the CO intramolecular stretch and a CO frustrated rotation, both broadly peaking in the vicinity of 18 eV. Additional support for this scattering mechanism comes from the monotonic rise in scattering intensity for both of these vibrational modes as the final (detector) angle moves towards the surface normal under otherwise fixed kinematic conditions. We also observe the presence of weak first overtone scattering for the CO intramolecular stretch under resonant scattering conditions. All of the aforementioned observations are consistent with the formation of a Σ shape resonance which is slightly lower in energy, and has a shortened lifetime, than in the gas phase. We also report dispersion measurements along the 〈112̄〉 direction for the c(4×2) structure, as well as for a saturated (√7/2×√7/2) R19.1° CO/Ni(111) overlayer which give information about intermolecular couplings in these compressed structures. Discussions are presented, including wave packet arguments, which emphasize that the presence (or absence) of vibrational excitation in a given vibrational coordinate following negative ion formation can be used to infer important details about femtosecond nuclear coordinate evolution for the system in the excited state.
Wigner trajectories and Liouville’s theorem
Authors: Sala R. , Brouard S. , Muga J. G.
Published: July 2002

It is found that, in general, Wigner trajectories satisfy Liouville’s theorem only locally, i.e., for restricted phase space and time domains. This fact limits their possible applications. Examples are provided to visualize the process of creation and destruction of Wigner trajectories. It is argued that Weyl transforms of Heisenberg operators are, however, viable alternatives to Wigner trajectories, even though they do not satisfy Liouville’s theorem either.
Ab initio study of nitroxide, HNO−
Authors: Robins K. A. , Farley John W. , Toto J. L.
Published: July 2002

The first ab initio theoretical study has been performed on the ground state of nitroxide, HNO−, in combination with a comparison study of the ground state of two chemically similar systems, peroxy radical, HO2, and the neutral nitroxyl radical, HNO. An optimum geometry and vibrational frequencies are reported for all species, and results are compared to experimental and theoretical values. A bent geometry is obtained for HNO−, with an equilibrium bond angle of 106.2°, and bond lengths of RNO=1.333 Å and RNH=1.045 Å. The calculated fundamental vibrational frequencies (in cm−1) are: ν1 (N–H stretch), 3029; ν2 (bend), 1474; and ν3 (N–O stretch), 1183. The bending frequency and the N–O stretching frequency in the ion are similar to the bending frequency and the O–O stretching frequency of HO2, while the N–H stretching frequency of HNO− is similar to the N–H stretching frequency of HNO. In addition, the GAUSSIAN 1 and GAUSSIAN 2 methods are used for the determination of an electron affinity for HNO. The calculated electron affinity of 0.23±0.10 eV is in reasonable agreement with the experimentally determined value of 0.338±0.015 eV.
Restricted applicability of Onsager’s reciprocity relations to models of interface motion
Authors: Kaplan Theodore , Aziz M. J. , Gray L. J.
Published: July 2002

Onsager’s reciprocity relations have been applied to the motion of a planar interface in a phase transformation in a two-component system, in order to provide a test of kinetic models of alloy solidification. Although applicable to such models in general, Onsager’s relations yield no information for a subclass of models in which the thermodynamic driving forces are linearly dependent to first order, as is the case for several models of alloy solidification including the continuous growth model (CGM) of Aziz and Kaplan. As a consequence, experimental tests will likely be required to distinguish between these models. If the CGM is generalized to include diffusion in the growing phase, Onsager’s relations are applicable under some circumstances.
Electronic structure of H2 and HeH+ computed directly in momentum space
Authors: Fischer Patrick , De Windt Laurent , Defranceschi Mireille , Delhalle Joseph
Published: July 2002

The possibilities of one-step iterative computations in momentum space previously developed for neutral and charged atoms are now assessed to two-electron diatomic molecular case, the helium hydride positive ion HeH+. The improvements induced in its ground state doubly occupied orbital initially approximated by different primitive Gaussian functions of increasing quality are analyzed through the calculation of test quantities such as the momentum density, 〈p−1〉, 〈p2/2〉, and 〈p4〉. Moreover the structure of the equation leading to the first iterate for HeH+ suggests that the procedure could be generalized to polyatomic systems.
The potential energy surface of Li+–CO
Authors: Grice Stephen T. , Harland Peter W. , Maclagan Robert G. A. R.
Published: July 2002

A new potential energy surface for the Li+–CO system calculated at the HF/6-311+G(2df) and MP4SDTQ/6-311+G(2df) levels of theory in a rigid rotor approximation is presented. The potential energy surface has an absolute energy minimum at θ=0°, and R=5.330 bohr of −0.716 eV. There is also a minimum at θ=180° and R=5.344 bohr of −0.475 eV, and a col at θ=87.2° and R=4.709 bohr of 0.100 eV. The potential is expanded into its angular components in a truncated Legendre expansion. The effect of nuclear polarization on the bond length of CO is investigated.
Fourier-transform infrared magneto-optical spectra of the Se2, TeSe, and Te2 molecules in rare-gas matrices at 4 K
Authors: Li S. , Van Zee R. J. , Weltner W.
Published: July 2002

The zero-field splittings [3Σ−(1 X2)−3Σ−(0 +X1)] in the ground states of the three diatomic molecules Se2, TeSe, and Te2 have been observed in infrared absorption in neon, argon, and krypton matrices at 4 K. The magnetic-dipole character of these transitions was verified by the broadening effects of applied magnetic fields of up to 4 T. The relative matrix shifts and Zeeman effects are discussed.
Intermolecular potentials calculated by an extended group function model: Theory
Authors: Ro/eggen I. , Ahmadi G. Reza , Wind P. A.
Published: July 2002

A set of computational models, denoted extended group function models, are introduced for describing intermolecular interactions of closed shell systems. The models have the following properties: They have a conceptual structure which facilitates interpretation. The models can be applied for any intersystem distances. The models are size extensive. The basis set superposition error can be eliminated at the correlation level. The models are computationally efficient. A test calculation is performed on the complex Ne⋅Li+.
Continuous wave multiquantum electron paramagnetic resonance spectroscopy. IV. Multiquantum electron–nuclear double resonance
Authors: Mchaourab Hassane S. , Christidis T. C. , Hyde James S.
Published: July 2002

We report a theoretical and experimental investigation of the interaction of a coupled electron–nuclear spin system with three electromagnetic fields: two equally intense microwave fields resonant with the electron spin, and one radio-frequency field resonant with the nuclear spin. This is an electron–nuclear double-resonance experiment where the effect of the nuclear transition is detected via changes of the electron multiple photon transitions (MQ-EPR) rather than steady-state saturation and, therefore, is called multiquantum electron–nuclear double resonance (MQ-ENDOR). The theoretical framework previously developed for the description of multiple photon phenomena in a two level system is extended to the case of a four level system. The equation of motion of the density matrix is solved in the presence of three fields, which results in five master equations relating various populations and coherences. A ten photon approximation is used to study the functional dependence on spectral parameters and determine the sensitivity of this technique to spin relaxation rates. The experimental investigation is carried out on a sample of tri-t-butyl phenoxyl radical dissolved in mineral oil. At low values of the electron saturation factor Se, the rf-swept MQ-ENDOR is a de-enhancement of the 3Q-EPR signal (i.e., the first intermodulation sidebands). As the microwave field strength increases, the MQ-ENDOR signal changes phase by 180° due to dominance of generalized saturation. Higher order MQ-EPR signals (i.e., higher order sidebands) have larger negative enhancement and tend to display smaller positive enhancement. Line splitting results if the microwave frequency difference or field strength is increased. The dependence of MQ-ENDOR displays on various spectral parameters was found to be consistent with the general trends predicted by the theory. These displays provide a convenient way to estimate electron and nuclear relaxation rates.
Heteronuclear Overhauser effect measurements in surfactant systems. IV. Direct and remote correlations within alkyl chains
Authors: Palmas P. , Tekely P. , Mutzenhardt P. , Canet D.
Published: July 2002

This paper deals with the complete interpretation of the heteronuclear Overhauser effect spectroscopy (HOESY) two dimensional nuclear magnetic resonance (NMR) experiment. This experiment aims at the determination of cross relaxation rates (having their origin in dipolar interactions) between two different nuclear species, here proton and carbon-13. Extra correlation peaks observed in the 2D map of the system under investigation (micellized sodium octanoate) are shown to arise from remote dipolar interactions and not from strong (J) coupling effects. Several experiments have been performed at different mixing times (during which the nuclear Overhauser effect builds up), allowing the observation of the whole carbon-13 magnetization evolution. Their analysis yields cross-relaxation rates, refined with respect to those derived from initial behavior. However, this requires inclusion of the effect of spin diffusion, within the proton spin system, in the data treatment. A novel approach is proposed which models the multiexponential character by means of effective proton relaxation parameters. All the results of this study are consistent with data obtained by other methods [nuclear Overhauser effect (NOE) factor determinations and frequency dependent longitudinal 13C relaxation times).
Surface mobility of C60 on SiO2
Authors: Moalem M. , Balooch M. , Hamza A. V. , Siekhaus W. J. , Olander D. R.
Published: July 2002

The interaction of a collisionless beam of thermal C60 nanoclusters with a silicon dioxide surface has been investigated with modulated molecular beam-mass spectroscopic techniques. Analysis of the amplitude and phase lag of the desorbed C60 shows the interaction mechanism to involve the elementary steps of sticking, desorption, and long-range surface diffusion. Surface diffusion coefficients determined in this measurement indicate that surface C60 nanoclusters approach two-dimensional gas-like behavior. The sticking probability of C60 clusters on SiO2 is determined to be unity. The best fit desorption rate constant kd for C60 from SiO2 is 5×1010 exp(−23 kcal/mol/RT) in agreement with previous temperature programmed desorption experiments. The efficiencies of electron impact ionization of C60 to C60+ and C60++ is measured for electron energies from 10 to 105 eV for neutrals at 875 K.