bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio MO studies of neutral and anionic SiCn clusters (n=2–5)
Authors: Gomei Motoki , Kishi Reiko , Nakajima Atsushi , Iwata Suehiro , Kaya Koji
Published: July 2002

The geometries and energies of SiCn and SiCn− (n=2–5) were investigated with ab initio calculations including electron correlation effects with the MP2/6-31G* method, followed by MP4 and CCSD(T) single-point calculations to determine the most stable isomers. The adiabatic electron affinities (AEA) were evaluated with the ΔMP4 method. Because of the complexity of the electronic structure of SiC2, its AEA was calculated at the CCSD(T)/aug cc-pVTZ//CCSD(T)/aug cc-pVDZ level of approximation. For the neutral SiCn clusters, the isomer having a triplet ground state has large EA, whereas the isomer having a singlet ground state has small EA. This is attributed to the bonding character between Si and C atoms in the orbital occupied by the extra electron. The calculated EA was used to assign the photoelectron spectra of SiCn− reported previously. Furthermore, the effects of Renner–Teller splitting for the linear isomers of anions are discussed.
Time-resolved fluorescence spectroscopy of high-lying electronic states of Zn-tetraphenylporphyrin
Authors: Gurzadyan G. G. , Tran-Thi T.-H. , Gustavsson T.
Published: July 2002

Depopulation of the S2 excited electronic state of the Zn-tetraphenylporphyrin (ZnTPP) was monitored by measuring the decay of S2→S0 and the rise of S1→S0 fluorescence using the up-conversion fluorescence technique with a time-resolution of 120 fs. The lifetime of the S2 electronic state, measured for ZnTPP in ethanol (τS2=2.35 ps) correlates with the risetime of S1→S0 fluorescence. This result demonstrates the depopulation of S2 to S1 via (vibrational) states with lifetimes much shorter than that of S2. The rise time of S2 fluorescence (τv=60–90 fs) was attributed to vibrational relaxation (in S2). Fluorescence anisotropy decay of the S2 state was also studied by measuring the parallel and perpendicular fluorescence components. The high initial anisotropy of r⩾0.7 is interpreted as due to the existence of a degenerate excited electronic state S2 and the corresponding fast decay time τ1=0.2 ps to the electronic dephasing of the degenerate level pair. The long component of the anisotropy decay (τ2≫10 ps) is due to rotational diffusion.
A new theoretical approach for the electrical properties of TiX2 (X=S, Se, Te) phases with density functional calculations
Authors: Doublet M.-L. , Gallego-Planas N. , Philipsen P. H. T. , Brec R. , Jobic S.
Published: July 2002

The electronic structures of layered transition metal dichalcogenides TiX2 (X=S, Se, Te) have been studied with the Amsterdam Density Functional package for periodic systems (ADF-BAND). The accuracy of this algorithm to calculate the charge transfer between the chalcogens and the metal has been tested at different levels of approximation (local-density approximation, generalized gradient corrections of Becke–Perdew and Perdew–Wang, and quasirelativistic calculations). The total and partial density of states of the three compounds, as well as the crystal orbital overlap population analysis, have been used to rationalize the electronic structure of the systems. The present results show a significant p/d-block band overlap for TiTe2, leading to a Te(5p)→Ti(3d) electron transfer and a metallic behavior. Conversely, owing to the redox competition between the metal and the chalcogens, TiS2 and TiSe2 are predicted to be a semiconductor and a semimetal respectively. These physical properties are discussed in terms of electron density displacement from the chalcogen to the titanium when going from TiS2 to TiTe2. The relativistic effects appear to have no significant influence on the global physical properties of these layered compounds, although they slightly reduce the Te(5p)→Ti(3d) electron transfer.
Rotational structure in the absorption spectra of O318 and O316 near 1 μm: A comparative study of the A23 and B23 states
Authors: Günther Jürgen , Anderson Stuart M. , Hilpert Georg , Mauersberger Konrad
Published: July 2002

Absorption spectra for O316 and O318 near 1 μ have been recorded to explore the rotational structure associated with the A23 and B23 electronic states of ozone. Rotational features within the A23←A11 1-0 band respond predictably to isotopic substitution, enabling determination of the upper state adiabatic electronic energy and asymmetric stretching frequency via isotope shift techniques. We find an adiabatic energy of 9963±4 cm−1, in excellent agreement with that determined earlier from vibronic isotope shifts observed at lower resolution, 9990±70 cm−1. We also find an asymmetric stretching frequency of 367±17 cm−1, indicating that the A23 state potential energy surface is bound against dissociation to O+O2 despite the fact that it lies above the corresponding dissociation limit. Rotational structure associated with transitions to the B23 state is detected for the first time. It responds anomalously to O18 substitution. The geometry of the upper state for O318 appears to differ markedly from that for O316 (Δθ≈−4 deg, or Δre≈+0.07A), suggesting that the B23 surface is strongly anharmonic and precluding a straightforward analysis of the isotope shifts.
Comparative Raman spectroscopy of nitromethane-h3, nitromethane-d3, and nitroethane up to 20 GPa
Authors: Courtecuisse Stephane , Cansell François , Fabre Denise , Petitet Jean Pierre
Published: July 2002

Nitromethane has been studied as a model of energetic alphatic nitro compounds. The initiation of the decomposition mechanism of nitromethane in the vicinity of its explosive decomposition pressure conditions has been studied under high static pressure at ambient temperature by Raman spectroscopy. The vibrational modes νa(NO2) and, to a lesser degree, νs(NO2) show a specific behavior at the solid (II)–solid (III) 7.5 GPa transition (at ambient temperature). A comparison of the Raman spectrum of nitromethane-h3 with new high pressure Raman spectroscopic results for nitromethane-d3 and nitroethane allows one to characterize the physico chemical behavior of these three compounds in relation to their detonic properties.
Photoelectron spectroscopy of SinH− (n=2–4) anions
Authors: Xu Cangshan , Taylor Travis R. , Burton Gordon R. , Neumark Daniel M.
Published: July 2002

Vibrationally resolved photoelectron spectra of SinH− (n=2–4) have been measured at a photodetachment wavelength of 355 nm (3.493 eV). The electron affinities of Si2H, Si3H, and Si4H are 2.31±0.01, 2.53±0.01, and 2.68±0.01 eV, respectively. Vibrational frequencies for the neutral ground states and a low-lying state of Si2H are also determined. Assignment of the electronic states and vibrational frequencies is facilitated by comparison with ab initio calculations. The calculations show that the H atom in Si4H and Si4H− is bonded to a single Si atom, in contrast to the bridged structures found for the smaller clusters. These calculations, along with photoelectron energy and angular distributions, yield a definitive assignment of the ground and nearly degenerate first excited states of Si2H.
Collision induced desorption of N2 from Ru(001)
Authors: Romm L. , Zeiri Y. , Asscher M.
Published: July 2002

The dynamics of collision-induced desorption (CID) of N2 from Ru(001) exposed to hyperthermal rare gas colliders generated in a supersonic atomic beam source have been studied. Low coverage of 0.01 ML N215 at crystal temperature of 96 K was chosen to represent a CID process of a practically isolated molecule, neglecting the effect of lateral N2–N2 interactions. The cross sections for CID of nitrogen molecules, σdes(Ei,θi), as a function of the kinetic energy and angle of incidence of Ar and Kr colliders have been measured. It was found that σdes(Ei,θi=0°) changes monotonically in the range 0–25 Å2 for beam energy in the range of 0.5–5.5 eV and is insensitive to the type of collider (Ar, Kr) as well as to the adsorbate isotope (14N2, 15N2). The threshold energy for desorption has been determined to be 0.50±0.10 eV, which is twice the binding energy of N2 to Ru(001). The cross section for CID at a fixed collider’s energy rises approximately four times as the incidence angle θi increases from 0° to 70° relative to the surface normal. Neither normal nor total energy scaling of the cross section could describe the results. The σdes(θi) scales reasonably well, however, with the tangential energy of the collider for angles above 30°. Classical molecular dynamics simulations were performed to gain better understanding of the CID process. Threshold energy and angular dependence of the cross section were reproduced very well. The predominant CID mechanism was concluded to originate from a direct rare gas–nitrogen collision, in which impulsive-bending and the motion along the surface are coupled to the adsorbate motion which leads to desorption.
Microwave measurements of rhenium quadrupole coupling in cyclopentadienyl rhenium tricarbonyl
Authors: Drouin B. J. , Cassak P. A. , Kukolich S. G.
Published: July 2002

Microwave measurements of rotational transitions for cyclopentadienyl rhenium tricarbonyl were performed in the gas phase using pulsed beam Fourier transform microwave spectroscopy. One hundred and sixty five transitions were assigned to two isotopomers of rhenium. The location of the rhenium atom near the center of mass produced significant overlap between the spectra of the two isotopomers. The data were accurately fit using a symmetric top, rigid rotor Hamiltonian that included nuclear quadrupole coupling and centrifugal distortion constants. The rotational constants obtained were B(187Re) = 724.9794(2), B(185Re) = 724.9795(2). The quadrupole coupling constants obtained were eQq(187Re) = 614.464(12) and eQq(185Re) = 649.273(14) MHz. The successful fitting of the measured spectra to a symmetric top Hamiltonian indicates that the cyclopentadienyl group retains C5v symmetry, and the Re(CO)3 group C3v symmetry in the gas phase. The measured rotational constants are in reasonable agreement with those calculated from the structural parameters obtained in the earlier x-ray work. The Re quadrupole coupling constants obtained are compared with values for other complexes.
Far-infrared spectra and two-dimensional potential energy surfaces for the out-of-plane ring vibrations of tetrahydrofuran-3-one in its S and S1(n,π*) electronic states
Authors: Lee SooNo , Meinander Niklas , Sagear Paul , Nath Deb N. , Laane Jaan
Published: July 2002

The far-infrared spectra of tetrahydrofuran-3-one show ring-bending (100–120 cm−1), ring-twisting (227–237 cm−1), difference (115–137 cm−1), and overtone (200–225 cm−1) bands. Calculations for the two-dimensional potential energy surface governing the twisting and bending motions were carried out. The minima of the potential surface correspond to twisted conformations with twist angles of approximately ±35°. The barrier to planarity is in the 1500 to 2000 cm−1 range. No saddle points corresponding to bent structures are present for the surface. A potential energy surface for the S1(n,π*) excited state, based on previous fluorescence excitation data, was also calculated. The latter has an assumed barrier to planarity of 1400 cm−1 and a barrier to pseudorotation of 880 cm−1.
High-resolution visible laser spectroscopy of the B̃ 2B1–X̃ 2A1 transition of CaNH2
Authors: Morbi Zulfikar , Zhao Chunfeng , Hepburn John W. , Bernath Peter F.
Published: July 2002

The 000B̃ 2B1–X̃ 2A1 band of CaNH2 has been recorded at high resolution with a laser ablation molecular beam spectrometer. This B̃ 2B1 state is the last of the low-lying electronic states of CaNH2 to be characterized. The pure precession model, which has previously been used to describe the spin-rotation interactions in this family of molecules was used to interpret and correctly assign the spectrum. The detailed analysis of the high resolution spectrum and energy level structure of the B̃ 2B1 state is presented.
Nonlocal continuum solvation model with exponential susceptibility kernels
Authors: Basilevsky Mikhail V. , Parsons Drew F.
Published: July 2002

An algorithm is developed for performing calculations for the nonlocal electrostatic solvation theory of an ion in a cavity, accounting for electrostatic boundary conditions. The latter implies an induced charge distribution on the cavity surface as well as an induced volume charge distribution in the medium. This approach is validated by a variational derivation which also provides a general expression for the solvation energy. The procedure, implemented for spherical ions, is tested by calculating the analytic solution for an exponential nonlocal dielectric kernel and determining the corresponding solvation energy. Parametrization is presented for a range of solvents, fitted to experimental solvation energies. © 1998 American Institute of Physics.
Giant second-order Zeeman effect in titanium (III) doped caesium gallium alum
Authors: Tregenna-Piggott Philip L. W. , O’Brien Mary C. M. , Weihe Ho/gni , Güdel Hans U.
Published: July 2002

Low temperature magnetization data obtained for titanium (III) doped caesium gallium alum show a spectacular deviation from Brillouin behavior with the magnetization highly dependent on the strength of the applied field at a given value of B/T. This arises from unprecedented higher-order contributions to the magnetization and these measurements afford the determination of the Zeeman coefficients to third order. This anomalous magnetic behavior is a manifestation of Jahn-Teller coupling giving rise to low-lying vibronic states which mix into the ground state through the magnetic field.
Transition state spectroscopy on the Li-HF system
Authors: Paniagua Miguel , Aguado Alfredo , Lara Manuel , Roncero Octavio
Published: July 2002

The transition state region of the Li-HF system is theoretically studied via infrared excitation of the ground state of the complex in the reactant valley. The absorption spectrum shows intense peaks for which LiF is produced with high efficiency (≈90%), while the reaction has a very low cross section during the collision at the same energies. The reason is that the resonances reached through optical excitation are in the vicinity of the transition state.
A pair potentials study of matrix-isolated atomic zinc. II. Intersystem crossing in rare-gas clusters and matrices
Authors: Breckenridge W. H. , Morse M. D. , McCaffrey John G.
Published: July 2002

The mechanism of 4p 1P1→4p 3PJ intersystem crossing (ISC) following excitation of the 4p 1P1 level of matrix-isolated atomic zinc is investigated using a pair potentials approach. This is achieved by extending earlier ISC calculations on the Zn⋅RG2 and Zn⋅RG3 complexes to the square planar Zn⋅RG4 and square pyramidal Zn⋅RG5 species which are the building blocks of the Zn⋅RG18 cluster used to represent the isolation of atomic zinc in the substitutional site of a solid rare-gas host. ISC predictions in these clusters are based on whether crossing of the strongly bound 1A1 states, having a 4p 1P1 atomic asymptote, occurs with the repulsive 3E states correlating with the 4p 3PJ atomic level of atomic zinc. Predictions based on 1A1/3E curve crossings for 3E states generated with the calculated ab initio points for the Zn⋅RG 3Σ(pz) states do not agree with matrix observations. Based on similar overestimation of ISC in the Zn⋅RG diatomics, less repulsive Zn⋅RG 3Σ(pz) potential curves are used resulting in excellent agreement between theory and observations in the Zn–RG matrix systems. 1A1/3E curve crossings do not occur in the Zn–Ar system which shows only singlet emission. Curve crossings are found for the Zn–Xe system which exhibits only triplet emission. The Zn–Kr system does not show a crossing of the body mode Q2, which exhibits a strong singlet emission at 258 nm while the waist mode Q3, does have a crossing, resulting in a weak singlet emission at 239 nm and a stronger triplet emission at 312 nm. The efficiency of ISC is determined from Landau–Zener estimates of the surface hopping probabilities between the 1A1 and the 3E states. Differences in the application of this theory in the gas and solid phase are highlighted, indicating that the rapid dissipation of the excited-state energy which occurs in the solid must be included to obtain agreement with observations.
The optical absorption spectrum of the octahedral RhCl63− complex: Ab initio calculations of excitation energies and the effect of spin-orbit coupling
Authors: Ribbing Carl , Gilliams Ben , Pierloot Kristine , Roos Björn O. , Karlström Gunnar
Published: July 2002

The spin-orbit states of octahedral RhCl63− in the three lattices NaCl, Cs2NaYCl6 and water solution have been investigated theoretically with multiconfigurational self-consistent field (SCF) type procedure (CASSCF), followed by a second order perturbation correlation treatment (CASPT2). The spin-orbit effects for the ground state 1A1g and the lowest excited states 3T1g, 3T2g, 1T1g and 1T2g were calculated with an effective one-electron operator and shifts of more than 1000 cm−1 were found. Different basis sets were tried: the presence of f functions on Rh and d functions on Cl is important for both excitation energies and equilibrium distances. There is agreement between computed excitation energies and the available experimental data that indicate a redshift of the 1T1g,2g← 1A1g transitions as the Rh–Cl distance increases.
N
2
on tungsten clusters: Molecular and dissociative adsorption
Authors: Holmgren Lotta , Andersson Mats , Rosén Arne
Published: July 2002

We have studied the size-dependent reactivity of W10–W60 with N2 under single-collision-like conditions by using a laser-vaporization source, a low-pressure reaction cell and a laser-ionization time-of-flight mass spectrometer. The reaction probability with the first and second N2 molecule was measured at two different cluster-source temperatures: room temperature (RT) and liquid-nitrogen temperature (LNT). For the RT clusters, a strong size dependence in the reaction probability was observed in the size range ∼10–26 atoms, with distinct local maxima at W16, W22, and W23. Upon cooling of the cluster source, the reaction probability increased significantly overall, and the relative variations with size decreased, but persisted. To get an indication of the bond strength of N2 on Wn, we heated the cluster products after reaction through irradiation with 4.02 eV photons from a XeCl excimer laser and checked for consequent desorption of adsorbate atoms or molecules. For the LNT clusters, heating with laser light caused a substantial decrease in the abundance of reaction products with nitrogen, whereas no significant change in the abundance of WnN2 was observed for the RT clusters. This indicates that a proportion of the N2 is relatively weakly bound to the LNT clusters, whereas on Wn produced at RT, only the strongly bound state/states of N2 exist. Based on comparisons with the N2-adsorption on W bulk surfaces, we conclude that the weakly and strongly bound states represent molecularly and dissociatively bound N2, respectively.
Density functional theory of polymers: A Curtin-Ashcroft type weighted density approximation
Authors: Yethiraj Arun
Published: July 2002

A density functional theory is presented that combines an exact expression for the ideal gas free energy functional with a weighted density approximation for the excess free energy functional. The weighting function required in the theory is obtained from the Curtin-Ashcroft recipe, with a bulk fluid direct correlation function from the polymer reference interaction site model integral equation theory. The theory is in quantitative agreement with computer simulations for the density profiles of freely jointed tangent sphere hard chains at a hard wall, about as accurate as the Curtin-Ashcroft theory is for hard spheres at a hard wall. For a more realistic fused-sphere chain model with fixed bond angles and bond lengths, the theory is in excellent agreement with simulations at low and intermediate densities but overestimates the magnitude of layering at high densities for short chains. The theory becomes more accurate as the chain length is increased.
Model representation of electrolyte solutions based on mass spectrographical data
Authors: Zolotoy Nathan B. , Karpov Grigory V.
Published: July 2002

A model representation of electrolyte solutions is proposed. Two assumptions are used: (a) There are k equivalent potential wells around each ion in the solution; (b) the probability of the occupation of the potential well by a solvent molecule is p<1. According to these assumptions, the probabilities wb(n) of simultaneous occupation of n wells (n=0,1,2,..,k) by solvent molecules are the terms of the binomial (p+q)k, where q=(1−p) is the probability that the potential well is empty. In consequence to the motion of the solvent molecules between the potential wells, there is an equilibrium distribution of the solution ions (cations and anions) with respect to a degree of solvation. The binomial terms wb(n) satisfactorily correlate with the corresponding terms we(n) of the experimental distributions of mass spectral line intensities of ions and ion clusters of alkaline metals evaporated from aqueous and aqueous-ethanol solutions into vacuum by means of a strong electric field. The dependence of the probability p on the solution temperature derived from the presented model is experimentally confirmed for aqueous-ethanol solution of NaI in the temperature range of 219–248 K. The enthalpy changes for the detachment of one or two ethanol molecules from Na+Etm(H2O)n ion clusters in solution (Et is ethanol, m,n=0,1,2,…) were obtained from experiments. The mechanism of the field evaporation of ions from solutions is discussed. The expression for the heat of evaporation for ion clusters is deduced.
Initial decay rate of the dynamic structure factor of polymer molecules in solution
Authors: Harnau Ludger , Winkler Roland G. , Reineker Peter
Published: July 2002

The initial decay rate of the dynamic structure factor of macromolecules in solution is briefly discussed. In particular, the theoretical calculations are compared with experimental data of polystyrene both in carbon disulfide and in benzene-d6. Our theoretical calculations agree very well with the experimental data.
The role of asymmetric molecular bonding in adsorption dynamics: Chemisorption of I2Cl6 on Si(111)
Authors: Liu Yong , Taylor Peter R. , Kummel Andrew C.
Published: July 2002

The reaction of monoenergetic I2Cl6 molecules with the Si(111)-7×7 surface is stereoselective for iodine and surface-site selective for center silicon adatoms. The adsorption starts by first forming a partial bond between an I atom of an I2Cl6 and a Si adatom on the surface so that iodine from I2Cl6 is selectively abstracted by the surface and chlorine is ejected back into the gas phase. The interaction between the asymmetric HOMO of an I2Cl6 and a partially filled Si adatom dangling bond governs the adsorption dynamics. The highly asymmetric HOMO orbital and zero dipole moment associated with I2Cl6 makes it a perfect molecular species for clarifying the role of asymmetric molecular bonding versus dipole moment in the iodine-selective abstraction of ICl by the Si(111)-7×7 surface.
Critical test of quantum cluster equilibrium theory: Formic acid at B3LYP/6-31+G* hybrid density functional level
Authors: Wendt Mark A. , Weinhold Frank , Farrar Thomas C.
Published: July 2002

The recently developed quantum cluster equilibrium (QCE) theory is used at the B3LYP/6-31+G* level of hybrid density functional theory to calculate the equilibrium cluster populations for formic acid vapor, whose quantitative cluster composition was determined by Coolidge and others through precise spectroscopic and vapor-density measurements over a wide range of temperature and pressure. Unlike previous experimental tests of QCE theory in which T,P-dependent cluster polpulations enter only indirectly (e.g., through convolutions with calculated cluster spectroscopic properties, yielding comparison with inhomogeneously braodened “average” spectral features), the present comparison is able to directly test the QCE population distributions. Nearly exact agreement between theory and experiment is found over the entire span of measurements, confirming that QCE theory satisfactorily predicts the T,P-dependent concentrations of individual cluster species, as well as the cruder population-weighted averages that enter less-sensitive comparisons with experiment.
Comparison of photon echo, hole burning, and single molecule spectroscopy data on low-temperature dynamics of organic amorphous solids
Authors: Zilker S. J. , Kador L. , Friebel J. , Vainer Yu. G. , Kol’chenko M. A. , Personov R. I.
Published: July 2002

A joint analysis of spectroscopic data obtained at liquid–helium temperatures by three line-narrowing techniques, photon echo (PE), persistent hole burning (HB), and single molecule spectroscopy (SMS), is presented. Two polymer systems, polyisobutylene (PIB) and polymethylmethacrylate (PMMA), doped with tetra-tert-butylterrylene (TBT) were studied via PE and HB techniques and the results are compared with literature data [R. Kettner et al., J. Phys. Chem. 98, 6671 (1994); B. Kozankiewicz et al., J. Chem. Phys. 101, 9377 (1994)] obtained by SMS. Both systems behave quite differently. In TBT/PIB a rather strong influence of a dispersion of the dephasing time T2 was found which plays only a minor role in TBT/PMMA. We have also measured the temperature dependence of T2 for both systems in a broad temperature range (0.4–22 K). Using these data we separated the two different contributions to the optical dephasing — due to an interaction with two-level systems and due to coupling with local low-frequency modes. The data are compared with calculations using a numerical and a semianalytical model in the presence of a large dispersion of the single molecule parameters. Furthermore, we discuss the differences of the linewidths as measured by different experimental methods.
An infrared study on CO intercalated in solid C60
Authors: Holleman Iwan , von Helden Gert , van der Avoird Ad , Meijer Gerard
Published: July 2002

The infrared (IR) absorbance spectra of CO intercalated in solid C60 have been measured as a function of temperature. The spectra show a gradual transition from a nearly free rotation of the CO molecules to a situation where their rotational motion is severely hindered. The hindering of the rotational motion of CO caused by the surrounding C60 molecules is found to be comparable to that observed for CO dissolved in a liquid. Good agreement is found between quantum mechanically calculated spectra and the measured IR spectra. The intermolecular van der Waals vibrations of a CO molecule rattling in the octahedral site of the C60 lattice have been observed at low temperatures. The IR spectra of CO intercalated in C70 are measured and compared to those observed for CO intercalated in C60.
Low-energy reactive ion scattering as a probe of surface femtochemical reaction: H+ and H− formation on ionic compound surfaces
Authors: Souda R. , Suzuki T. , Kawanowa H. , Asari E.
Published: July 2002

Capture and loss of valence electrons during low-energy (50–500 eV) proton scattering from some alkali–halide surfaces such as LiCl, NaCl, and KF have been investigated in comparison with those from the TiO2(110) and Cs-adsorbed Si(100) surfaces. The primary H+ ion survives neutralization when scattered from the highly ionized target species existing on the surface. For H− ion formation, a close atomic encounter with individual target ions is found to be important; the H− ion is formed more efficiently on the cationic site than on the anionic site despite the fact that the valence electron is spacially localized on the latter. This is because the charge state of scattered hydrogen is determined during a transient chemisorption state and amphoteric hydrogen tends to be coordinated negatively (positively) on the cationic site (the anionic site). The final charge state of scattered hydrogen is fixed at a certain bond-breaking distance (∼5.0 a.u.) from the surface where the well-defined atomic orbital of hydrogen evolves. The competing nonlocal resonance tunneling is suppressed at the ionic-compound surfaces due to the existence of a large band gap, so that hydrogen is scattered without losing the memory of such a transient chemisorption state.
Predissociation of acetylene from the Ã 1Au state studied by absorption, laser-induced fluorescence, and H-atom action spectroscopies
Authors: Suzuki Toshinori , Hashimoto Nobuhisa
Published: July 2002

The rovibrational state dependence of the fluorescence and dissociation yields of acetylene in the Ã state has been measured. The fluorescence quantum yield varies with a rotational quantum number at the V3K1 level, indicating that quenching due to electronic relaxation is present below the dissociation energy. The absolute value of the fluorescence quantum yield, determined for the first time, was only 0.13 at the V2K1 J′=2 level located 1778 cm−1 below the dissociation energy to C2H(X̃ 2Σ+)+H(2S), demonstrating that electronic relaxation is fairly efficient throughout the Ã state. From the average translational energy release calculated from the Doppler profile of H atoms, it is concluded that dissociation mainly occurs in the ã state over the barrier. However, it is also suggested that dissociation from the V4K1 and V4K2 levels occur via the ground state. The results indicate that internal conversion and intersystem crossing coexist in the decay of the Ã state acetylene.
Constraints on density functionals and density functional derivatives
Authors: Joubert Daniel
Published: July 2002

In order to improve approximations to density functionals such as the exchange–correlation functional, Exc[ρ], and the independent Fermion kinetic energy functional, Ts[ρ], it is essential to have information on the behavior of the exact functionals. Any approximation should then be made to satisfy these conditions. With this in mind, constraints on functionals and functional derivatives which reflect stability against translation, rotation and uniform scaling of the charge density for electrons moving in a fixed external potential, are derived from the minimal property of the energy functional F[ρ]=Ts[ρ]+U[ρ]+Exc[ρ] when evaluated at a v-representable density ρ(r).
Temperature-induced transition to a nonmicellar state
Authors: de Moraes J. N. B. , Figueiredo W.
Published: July 2002

We study the self-assembly of amphiphile molecules in water as a function of temperature and concentration in Monte Carlo simulations on a two-dimensional lattice, in which each water molecule occupies a single site, and each amphiphile occupies three sites. We determine the free amphiphile concentration and the aggregate-size distribution P(n) as functions of total surfactant concentration for different temperatures. The degree of micellar organization is characterized by Δ≡P(nmax)−P(nmin), where nmin and nmax>nmin are the aggregate sizes at which P exhibits its local minimum and maximum. We show that Δ vanishes continuously as we increase the temperature, and find the exponent associated with this micellar to nonmicellar transition.
Intermolecular multiple-quantum coherence transfer echoes and multiple echoes in nuclear magnetic resonance
Authors: Kimmich Rainer , Ardelean Ioan
Published: July 2002

In recent years evidence was presented that two radio frequency pulses in combination with suitably matched field gradient pulses produce cross peaks in two-dimensional NMR spectroscopy that reveal all the features of multiple-quantum/single-quantum coherence transfer signals. This experiment was termed CRAZED by Warren et al. Quite amazingly, identical signals were predicted on a purely classical basis. Thus the intriguing question arises whether this means that a classical analog to multiple-quantum coherences exists. In this study we show that this is clearly not the case. Coherence pathways of a purely single-quantum basis are excited anyway and lead independently to superimposed identical signals in parallel. Moreover, the observation of CRAZED signals per se neither disproves the adequacy of the high-temperature approximation, which was questioned in this context, nor proves a strong influence of intermolecular multiple-quantum coherences. However, the indispensable prerequisite for the description of the manifold CRAZED phenomena is that long-range dipolar interactions are taken into account either explicitly or in the continuous-limit form.
Experimental and ab initio theoretical studies of electron binding to formamide, N-methylformamide, and N,N-dimethylformamide
Authors: Desfrançois C. , Périquet V. , Carles S. , Schermann J. P. , Smith D. M. A. , Adamowicz L.
Published: July 2002

The influence of methylation upon adiabatic electron affinities of formamide (F), N- methylformamide (NMF), and N,N-dimethylformamide (DMF) is experimentally investigated by means of Rydberg electron transfer spectroscopy and calculated with the use of high-level ab initio methods. In the anions of these systems the excess electrons are captured in diffuse dipole-bound states. The methylation of formamide results in a slight increase of the dipole moment and in an increased molecular size. The two factors have opposite effects on the electron affinity. Both experimental data and theoretical results are in agreement, showing that the molecular size effect dominates and that the electron affinity noticeably decreases with the methylation.
Raman scattering in the network liquid ZnCl2 relationship to the vibrational density of states
Authors: Ribeiro M. C. C. , Wilson M. , Madden P. A.
Published: July 2002

The light scattering (Raman) spectrum of ZnCl2 has been calculated in a computer simulation directly from molecular dynamics and by using an instantaneous normal modes (INM) approach. Good agreement between the spectra is reported. The calculations use a realistic model for the fluctuating polarizability of ZnCl2, derived from earlier work on simpler ionic melts. This contains several mechanisms which couple the radiation field to the ionic motion—short-range, dipole-induced dipole and hyperpolarization. INM analysis of ZnCl2 has previously shown how the character of the underlying vibrational modes changes across the density of states. Here it is shown that the efficiency of the coupling of a given mode to the radiation field depends strongly on its character and on the polarizability mechanism, so that the Raman spectra predicted for the different mechanisms differ markedly. A consequence is that the discrete Raman bands observed at high frequency in the polarized spectrum do not coincide with the spectrum of the localized, quasi-molecular ZnCl4 units of the network. Furthermore, the “light-vibration” coupling, relating the reduced Raman spectrum to the underlying vibrational density of states, is appreciably frequency dependent and different for each mechanism.
Anion photoelectron spectroscopy of small tin clusters
Authors: Moravec Vicki D. , Klopcic Stephan A. , Jarrold Caroline Chick
Published: July 2002

The anion photoelectron spectra of small tin clusters, Snx− (x=1 through 12) are presented and compared with the anion photoelectron spectra of germanium clusters [Burton et al., J. Chem. Phys. 104, 2757 (1996)]. In general, the electron affinities of small tin clusters and germanium clusters are close. In most cases, the overall profiles of the tin cluster and analogous germanium cluster anion photoelectron spectra are remarkably similar. However, the tin cluster spectra frequently exhibit multiple electronic transitions where single broad electronic transitions appear in the germanium cluster spectra. The photoelectron spectrum of Sn3− shows more dramatic disparity: The two lowest-energy electronic transitions observed in the photoelectron spectrum of Sn3− appear to be nearly vertical, which is in sharp contrast to the vibrationally extended lowest-energy electronic transitions observed in the photoelectron spectra of Si3− and Ge3−. The differences are attributed to a reduction of Jahn–Teller effects due to competition with large spin-orbit interaction in Sn3−. The lowest-lying spectral features are assigned to the A1′+e−←E5/2 and E″+e−←E5/2 transitions, where A1′ and E″ are the spin-orbit components of the A2′3 neutral state.
Infrared spectroscopic study of rovibrational states of perdeuterated methane (CD4) trapped in parahydrogen crystal
Authors: Hoshina Hiromichi , Wakabayashi Tomonari , Momose Takamasa , Shida Tadamasa
Published: July 2002

The triply degenerate stretching (ν3) and bending (ν4) modes of CD4 in solid parahydrogen at cryogenic temperatures are studied by Fourier transform infrared spectroscopy to reveal crystal field split rovibrational structures. The observed spectra are analyzed by taking into account the crystal field potential which is constructed by the summation of the pairwise (dispersive) potential between the methane and the surrounding hydrogen molecules. By the least-squares fitting of that observed to a theoretical model the molecular constants of CD4 as well as the potential coefficient are determined as in the previous work on the CH4/p-H2 system. The potential is approximated to be proportional to the product of the polarizability of hydrogen molecule, the dipole–quadrupole polarizability of methane, and the inverse of the seventh power of the intermolecular distance between methane and hydrogen molecules. From the ratio of the potential coefficients of CH4 and CD4 the dipole–quadrupole polarizability of CD4 is found to be 0.875 times that of CH4 under the assumption that the quantum renormalization effect is negligible. A novel temperature dependence of the line shape of the ν3 and ν4 modes is discovered over the range of 4.5 to 8.0 K.
Critical properties of the oblique to homeotropic anchoring transition at nematic liquid-crystal surfaces
Authors: Braun F. N.
Published: July 2002

We revisit the oblique–homeotropic anchoring transition observed experimentally at the free surface of certain thermotropic liquid crystals. If short-range interactions are assumed, we show that the bulk critical dimension for this transition is 3, which presents a possible explanation for surprisingly mean-field-like features of the experimental data.
Solvent mediated vibrational relaxation: Superfluid helium droplet spectroscopy of HCN dimer
Authors: Nauta K. , Miller R. E.
Published: July 2002

Rotationally resolved infrared spectra are reported for the HCN dimer, grown and solvated in liquid helium droplets. This is the first study for which two different vibrational modes within the same liquid helium solvated molecule have been observed, namely those associated with the “free” and the “hydrogen-bonded” C–H stretching vibrations. Comparing the line broadening in these two bands, we conclude that the helium solvent plays an important role in the vibrational relaxation dynamics of the dimer. The rotational constants obtained from these spectra indicate that the dimer rotates more slowly in the liquid than in the gas phase.
Application of hard sphere equations of state to the Weeks–Chandler–Andersen reference system
Authors: Mulero A. , Galán C. , Cuadros F.
Published: July 2002

Six analytical expressions (four of which have been published recently) for the equation of state of a hard sphere system, together with two different expressions for the molecular diameter, have been tested in order to reproduce molecular dynamics results for pressure and potential energy of the Weeks–Chandler–Andersen reference system for the Lennard-Jones potential. The best choices for the combination of equations of state and molecular diameters in the calculation of those properties are given. It is shown how that choice may differ for different ranges of temperatures and densities, and how there is not a direct relation between the simplicity or complexity of the analytical expressions and their accuracy.
Vibrational excitation in He+(H2O)11 collisions: Quantum calculations and experiment
Authors: Brudermann J. , Buck U. , Fredj E. , Gerber R. B. , Ratner M. A.
Published: July 2002

Helium atoms are scattered from a beam of water clusters with mean size n̄ = 10 in an angular and velocity resolved collision experiment. The measured peaks are identified as elastic scattering, rotationally inelastic scattering of monomers, and vibrational excitation of the clusters. To interpret the latter processes quantum calculations are performed for He+(H2O)11 collisions using the TDSCF approximation which includes the anharmonic force field of the water clusters and energy transfer between the modes. By comparison of the calculated and experimental results, the most probable excitations correspond to energy transfer for around 7 meV and, with smaller intensities, up to 20 meV. The excitations correspond to shearing modes of the outer rings and the middle ring of the highly nonrigid cluster against each other.
Elementary mechanisms governing the dynamics of silica
Authors: Mousseau Normand , Barkema G. T. , de Leeuw Simon W.
Published: July 2002

A full understanding of glasses requires an accurate atomistic picture of the complex activated processes that constitute the low-temperature dynamics of these materials. To this end, we generate over five thousand activated events in a model silica glass, using the activation–relaxation technique; these atomistic mechanisms are analyzed and classified according to their activation energies, their topological properties and their spatial extent. We find that these are collective processes, involving ten to hundreds of atoms with a continuous range of activation energies; that diffusion and relaxation occurs through the creation, annihilation and motion of single dangling bonds; and that silicon and oxygen have essentially the same diffusivity.
N
1s
x-ray absorption study of the bonding interaction of bi-isonicotinic acid adsorbed on rutile TiO2(110)
Authors: Persson Petter , Lunell Sten , Brühwiler Paul A. , Schnadt Joachim , Södergren Sven , O’Shea James N. , Karis Olof , Siegbahn Hans , Mårtensson Nils , Bässler Margit , Patthey Luc
Published: July 2002

N  1s x-ray absorption spectra of bi-isonicotinic acid (2,2′-bipyridine–4,4′-dicarboxylic acid) on rutile TiO2(110) have been studied experimentally and quantum chemically. Differences between multilayer and monolayer spectra are explained by the adsorbate bonding to the substrate. A connection to the electronic coupling in dye-sensitized electrochemical devices is made.
Observation of a transition state resonance in the integral cross section of the F+HD reaction
Authors: Skodje Rex T. , Skouteris Dimitris , Manolopoulos David E. , Lee Shih-Huang , Dong Feng , Liu Kopin
Published: July 2002

We have studied the reaction F+HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channel F+HD→HF+D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF+D and DF+H channels were obtained in the collision energy range of 0.2–5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the transition state. Theoretical scattering calculations have been carried out using the Stark–Werner potential energy surface, which accurately reproduce the shape of the resonance feature. A detailed analysis of quantum dynamics using the spectral quantization method reveals that a quantum resonance exists near 0.52 kcal/mol, which is localized about the collinear FHD geometry. At collision energies below 1 kcal/mol, the reaction was found to proceed almost exclusively through resonant tunneling with very little contribution from direct, over the barrier, reaction. The properties of the quantum resonance, such as the position, lifetime, and partial widths were found to correlate well with the experimental results and the quantum scattering calculations.
Thermodynamics of a model for flexible polyelectrolytes in the binding mean spherical approximation
Authors: Bernard O. , Blum L.
Published: July 2002

We discuss our previous solution of the mean spherical approximation (MSA) for flexible polymers made of arbitrarily charged spheres of arbitrary diameter. Closed form analytical results for some particular cases are presented, in particular thermodynamic excess functions and an equation for the MSA screening parameter ΓT. It is found that the ideal chain approximation does not satisfy the exact Debye–Hückel limiting law since the polymer term appears with an effective charge squared that is 8.5 instead of 9 for a trimer, 14 instead of 16 for the tetramer, and 7n−16 instead of n2 for very large n. We propose a new chain approximation that takes three-body hard core exclusion into account. We show that this triplet exclusion chain approximation satisfies explicitly the Debye–Hückel limiting law for all lengths n, and also for all charge combinations.
Molecular dynamics simulations of shocked benzene
Authors: Bickham S. R. , Kress J. D. , Collins L. A.
Published: July 2002

The behavior of benzene at high temperatures and pressures is studied using nonequilibrium molecular dynamics. The interatomic forces were generated using linear-scaling tight-binding electronic structure theory on systems containing 128 and 576 molecules. The shock Hugoniot, calculated directly from the simulations without predetermining the equation of state, is compared with experiment. Piston velocities of 4 km/s or greater result in a pressure-induced polymerization. This transition is consistent with the bend in the experimental measurements of shock versus piston velocity.
A wave-packet calculation of the effect of reactant rotation and alignment on product branching in the O(1D)+HCl→ClO+H, OH+Cl reactions
Authors: Bittererova Martina , Bowman Joel M.
Published: July 2002

We report results of wave-packet calculations of the reaction probabilities for the O(1D)+HCl(v=0,j,K)→ClO+H, OH+Cl, reactions, using a recent ab initio potential energy surface [K. A. Peterson, S. Skokov, and J. M. Bowman, J. Chem. Phys. 111, 2445 (1999)]. We find a striking effect of the initial rotation and alignment of HCl on the product branching ratio.
Lattice model of living polymerization. III. Evidence for particle clustering from phase separation properties and “rounding” of the dynamical clustering transition
Authors: Dudowicz Jacek , Freed Karl F. , Douglas Jack F.
Published: July 2002

Equilibrium polymerization is studied here as a prototype for clustering transitions that commonly occur in systems of interacting particles at equilibrium. These transitions are often difficult to locate because of transition “rounding” associated with a limited extent of cluster growth, competing association or dissociation processes that initiate or inhibit clustering, and other constraints on the particle clustering dynamics. Instead of singularities in thermodynamic and transport properties, more subtle property changes signal the onset of particle clustering, explaining why clustering transitions are often overlooked or misinterpreted. We utilize a Flory–Huggins model for the equilibrium (“living”) polymerization of linear polymer chains to identify experimental signatures (features in the osmotic pressure, osmotic compressibility, and specific heat) that can be used to locate and quantify the transition “rounding” in general clustering transitions. The computation of a flattening in the concentration dependence of the osmotic pressure in the one-phase region motivates our consideration of the temperature dependence of the second virial coefficient and the variation of the theta temperature Tθ with “sticking energy” Δh as possible important indicators of particle clustering. The ratio of the critical temperature Tc for phase separation to Tθ, along with other “critical constant” ratios, such as the osmotic compressibility factor Zc, are also calculated and discussed in connection with establishing criteria for identifying particle clustering transitions and for quantifying the relative “strength” of these transitions.