bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution threshold photoionization of N2O
Authors: Wiedmann R. T. , Grant E. R. , Tonkyn R. G. , White M. G.
Published: July 2002

Pulsed field ionization (PFI) has been used in conjunction with a coherent vuv source to obtain high-resolution threshold photoelectron spectra for the (000), (010), (020), and (100) vibrational states of the N2O+ cation. Simulations for the rotational profiles of each vibronic level were obtained by fitting the Buckingham–Orr–Sichel equations [A. D. Buckingham, B. J. Orr, and J. M. Sichel, Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] using accurate spectroscopic constants for the ground states of the neutral and the ion. The relative branch intensities are interpreted in terms of the partial waves of the outgoing photoelectron to which the ionic core is coupled and in terms of the angular momentum transferred to the core. The PFI technique also allows us to report an improved value for the ionization potential of N2O of 103 963±5 cm−1.
A comparative study of the electron energy loss spectrum and the surface-enhance Raman spectrum of benzene adsorbed on silver
Authors: Wolkow R. A. , Moskovits M.
Published: July 2002

EELS spectra of benzene adsorbed on cold-deposited silver films prepared at different temperatures, then cooled to 60 K before adsorbate dosing, are found to contain several normally ir-forbidden bands whose relative intensities are observed to be dose dependent, especially for surfaces prepared above 200 K. In contrast, EELS spectra of benzene adsorbed on 60 K surfaces do not change with dose and resembled the high-coverage spectra obtained with surfaces prepared at the higher temperatures. These observations are rationalized in terms of a model that assumes benzene can adsorb on silver at two types of sites, binding somewhat more strongly to one type than to the other. Dipole-forbidden modes originate from the more weakly bound adsorbate and draw their intensity, at least partly, from resonant scattering by a resonance of a transient benzene anion. These resonances appear to be suppressed in the more strongly bound benzene. EELS spectra of adsorbed pyridine, triazine, and ethylene show no indications of such resonant contributions. Impact scattering also seems to contribute unusually greatly to the intensity of several of the observed EELS bands, especially for surfaces prepared at very low temperatures, because of their roughness, causing the specular nature of dipolar scattering to be diminished and with it the special intensity enjoyed by dipole-allowed bands. SERS spectra of benzene adsorbed on cold-deposited silver and the progress of the relative intensities of bands with dose cannot be wholly reconciled with what is observed with EELS. For, example, the existence of two types of binding sites for benzene is not implied as strikingly by the SERS results. This behavior implies that adsorbed benzene molecules do not contribute equally to EELS and SERS. In particular the more strongly bound benzene molecules appear to contribute disproportionately greatly to the SERS signal to a degree that can not be ascribed entirely to the differing selection rules operating in the two surface spectroscopies.
Vibrational energy transfer of very highly vibrationally excited NO
Authors: Yang Xueming , Kim Eun H. , Wodtke Alec M.
Published: July 2002

The dependence of vibrational energy transfer on vibrational excitation has been studied using the stimulated emission pumping technique to efficiently prepare a large range of specific vibrational states of the nitric oxide molecule in its ground electronic state. Laser induced fluorescence was used to detect collisionally relaxed NO. The self-relaxation rate constants of NO(v≫1) were up to 200 times larger than that of NO(v=1). Multiquantum relaxation was found to be important at high energy and was quantified at 3.8 eV. Self-relaxation rate constants of 15N18O as well as 14N16O were measured and a large isotope effect was observed. Relaxation of NO(v″=22) with H2 was also investigated. Theoretical explanations of our experimental results were attempted and it is shown that at vibrational energy up to ≊3 eV the qualitative trends observed in these experiments such as the mass effect and the multiquantum relaxation can be explained by Schwartz–Slawsky–Herzfeld theory. A simple explanation of the anomalously high NO self-relaxation rate is given. The large acceleration of the vibrational relaxation rate above 3.0 eV is coincident with the energetic onset of high energy (NO)2 isomer complexes. More theoretical and experimental work is needed to explain the quantitative aspects of these observations.
Relativistic effects in physics and chemistry of element 105. III. Electronic structure of hahnium oxyhalides as analogs of group 5 elements oxyhalides
Authors: Pershina V. , Sepp W.-D. , Bastug T. , Fricke B. , Ionova G. V.
Published: July 2002

Electronic structures of MOCl3 and MOBr3 molecules, where M=V, Nb, Ta, Pa, and element 105, hahnium, have been calculated using the relativistic Dirac–Slater discrete-variational method. The character of bonding has been analyzed using the Mulliken population analysis of the molecular orbitals. It was shown that hahnium oxytrihalides have similar properties to oxytrihalides of Nb and Ta and that hahnium has the highest tendency to form double bond with oxygen. Some peculiarities in the electronic structure of HaOCl3 and HaOBr3 result from relativistic effects. Volatilities of the oxytrihalides in comparison with the corresponding pentahalides were considered using results of the present calculations. Higher ionic character and lower covalency as well as the presence of dipole moments in MOX3 (X=Cl, Br) molecules compared to analogous MX5 ones are the factors contributing to their lower volatilities.
Nonlinear optical properties of Langmuir–Blodgett monolayers: Local-field effects
Authors: Cnossen Gerard , Drabe Karel E. , Wiersma Douwe A.
Published: July 2002

Detailed measurements of the macroscopic second-order optical nonlinearity χ(2)(2ω,ω,ω) of Langmuir–Blodgett dye-doped monolayers are reported. The observed deviations from a linear behavior of χ(2) with increasing surface density are shown to be due to local-field effects. In order to calculate these local-field factors for disordered systems, a novel Monte Carlo type calculation is introduced. This calculation not only accounts for density variations in the monolayers but also incorporates the effect of off-diagonal elements of the (microscopic) linear susceptibility tensor. Quantitative agreement is found between the calculations and the experimental results using only the molecular hyperpolarizability as a free parameter. A method is presented to determine the tilt angle of the chromophores in Langmuir–Blodgett monolayers from the anisotropy of the linear absorption. The tilt angle determined this way is in excellent agreement with a determination by second-harmonic generation.
Efficient calculation of rovibrational eigenstates of sequentially bonded four-atom molecules
Authors: Bramley Matthew J. , Handy Nicholas C.
Published: July 2002

A new efficient and fully symmetry-adapted finite-basis variational method for calculating the rovibrational eigenstates (J≥0) of any sequentially bonded four-atom molecule is presented. The exact kinetic-energy operator T̂VR in valence coordinates is used in a scheme of successive basis-set contractions. The success of the method is demonstrated with new results for the molecules HCCH, HOOH, and HCNO, respectively linear, nonlinear, and quasilinear. The complexity of T̂VR contributes little to the computational cost, yielding a Hamiltonian matrix whose elements can all be cheaply calculated from products of arbitrarily accurate one-dimensional integrals; the dominant cost is that of matrix diagonalization. Matrices of up to 6000×6000 are used to obtain low-lying levels converged to small fractions of 1 cm−1, even for the difficult case of HOOH. The generalization to J≥0 allows the calculation of Π states for HCCH and HCNO and effective rotational constants for HCCH, all usefully converged. For the first time nine-dimensional rovibrational wave functions for four-atom systems are calculated without dynamical approximation and with basis sets well completed in all degrees of freedom. For HCCH it is straightforward to obtain hundreds of rovibrational (J=0,1,2) levels converged to better than 1.5 cm−1, opening up the possibility of the systematic refinement of the nuclear potential function V̂N.
Search of the gas–liquid transition of dipolar hard spheres
Authors: Caillol Jean-Michel
Published: July 2002

We present results of Monte Carlo simulations in the isothermal–isobaric ensemble and the Gibbs ensemble for the fluid of dipolar hard spheres. These results preclude the existence of a gas–liquid transition for a wide range of densities and temperatures.
Quantum electrodynamics of molecular nanostructures
Authors: Jenkins Jonathan K. , Mukamel Shaul
Published: July 2002

We present a microscopic account of the linear and nonlinear optical response of an assembly of molecules with nonoverlapping charge distributions and arbitrary geometry. Our approach requires only the knowledge of single-molecule wave functions. The microscopic polarization is defined by a dipole distribution for each transition; we do not make the dipole approximation and it is then unnecessary to introduce the Ewald summation technique. Equations of motion are derived which provide a quasiparticle (anharmonic oscillator) picture of the optical response. As an application, we calculate both the linear susceptibility χ(1) and the light scattering signal off a crystal in d dimensions (d=1, 2, and 3). We find that retardation does not affect χ(1), which contains a shift in the exciton frequency compared with the single molecule, but no signature of spontaneous emission. However, the scattered field is retarded and shows cooperative spontaneous emission in reduced dimensionality d=1 and 2. The present approach can be applied to ordered nanostructures as well as disordered systems such as liquids and addresses fully the effects of retardation, polaritons, and cooperativity in linear as well as nonlinear optical processes.
The monoenergetic unimolecular reaction of expansion-cooled NO2: NO product state distributions at excess energies 0–3000 cm−1
Authors: Hunter M. , Reid S. A. , Robie D. C. , Reisler H.
Published: July 2002

We report detailed vibrational, rotational, and electronic (V,R,E) distributions of nascent NO(X 2Π1/2,3/2) deriving from monoenergetic unimolecular reactions of expansion-cooled NO2. Near UV excitation above dissociation threshold (25 130.6 cm−1) prepares molecular eigenstates which are admixtures of the optically active 1 2B2 state and the ground X̃ 2A1 electronic state. The strong mixings among the vibronic states result in vibrational predissociation from states of predominantly ground state character, and the NO product state distributions (PSDs) are compared with the predictions of several statistical theories. The PSDs are combined with previously measured O(3PJ) distributions and unimolecular reaction rates, thereby providing a complete description of the decomposition of NO2 at these excess energies. All the rotational distributions show prominent fluctuations and structures, but tend on average to follow the statistical distributions predicted by phase space theory (PST). This behavior is observed in both NO(v=0) and NO(v=1) channels, although the relative population in NO(v=1) was always greater than expected by PST. The NO(v=1) fractional population is bounded by the predictions of the separate statistical ensembes (SSE) method, and recent variational Rice–Ramsperger–Kassel–Marcus (RRKM) calculations are in agreement with the experimental results. Prior distributions underestimate the degree of vibrational excitation even more than PST does, and also the relative populations of the lower NO rotational levels. The observed NO spin–orbit states are always colder than statistical. We conclude that a significant interplay between dynamical biases and statistical expectations is manifest from the onset of dissociation, and is particularly evident when the initial parent rotational state is well defined.
The quantum dynamics of hydrogen and deuterium on the Pd(111) surface: A path integral transition state theory study
Authors: Rick Steven W. , Lynch D. L. , Doll J. D.
Published: July 2002

The surface diffusion constant for hydrogen and deuterium on the palladium(111) surface is calculated using quantum mechanical transition state theory. The rate constants for diffusion into the subsurface layer are also calculated. Quantum effects are seen to be most important for the surface/subsurface transition and cause an inverse isotope effect in which the rate for deuterium is greater than the rate for hydrogen. The results of ground and excited state wave function calculations show localized hydrogenic states, despite large zero point energies, and that the preferred binding site can vary with isotope between surface and subsurface sites. In addition, estimates of the tunneling rate between the surface and subsurface are in qualitative agreement with the low temperature transition state results.
The vibrational spectra of N2 in clathrate-hydrates: A new high-pressure phase transition
Authors: van Hinsberg Mariëlle G. E. , Scheerboom Marcel I. M. , Schouten Jan A.
Published: July 2002

To investigate the interaction between the water framework and a guest molecule in a clathrate hydrate, high-pressure Raman scattering experiments on nitrogen guest molecules have been performed. The pressure dependence of the N2 vibron(s) at room temperature (295 K) has been determined and a new phase transition has been discovered at 0.84 GPa.
Molecular dynamics of adiabatic and nonadiabatic electron transfer at the metal–water interface
Authors: Rose Daniel A. , Benjamin Ilan
Published: July 2002

A molecular model for an electron transfer reaction at a solution–electrode interface is developed. The solvent diabatic free energy curves for the reaction Fe+3+e−→Fe+2 are calculated using an umbrella sampling procedure. These are used to calculate the rate of electron transfer as a function of the electrode–solution potential difference in the electronically nonadiabatic region. A model Hamiltonian for the adiabatic case is also developed and used to calculate the adiabatic free energy surface. Reactive flux correlation functions are used to determine the solvent dynamical corrections to the rate. A comparison between the molecular dynamics and the Kramers and Grote–Hynes theories is made.
Fully quantal initial-state-selected reaction probabilities (J=0) for a four-atom system: H2(v=0, 1, j=0)+OH(v=0,1, j=0)→H+H2O
Authors: Neuhauser Daniel
Published: July 2002

Exact-dynamics (six-dimensional) quantum simulations of energy-resolved initial-state-selected rearrangement reaction probabilities are presented for H2(v=0,1,j=0) +OH(v=0,1,j=0) →H+H2O, at J=0, using the time-dependent reactive-scattering formalism. A few narrow resonances appear at low reaction energies when the H2 is vibrationally excited, and are shown to be partially associated with the strong-interaction region (in addition to the asymptotic reagents channel, where the potential has an unphysical well). Vibrational excitation of the OH bond is shown to exhibit little influence on the reaction probabilities. Together with similar results due to Zhang and Zhang (J. Chem. Phys., in press), these are the first initial-state-selected simulations of exact-dynamics four-atom molecular reactions.
Surface reaction kinetics of NO on Rh{110}
Authors: Schmatloch V. , Jirka I. , Kruse N.
Published: July 2002

The reaction of NO with Rh(110) single crystal surfaces was studied by means of various surface analytical tools including static secondary ion mass spectrometry (SSIMS), temperature programmed desorption (TPD), monochromatized x-ray photoelectron spectroscopy (MXPS), and ion scattering spectroscopy (ISS). Molecular NO adsorption was found to occur below 200 K. Higher temperatures caused dissociation of adsorbed NO molecules. Complete dissociation occurred only below a critical coverage, i.e., ΘNO≤0.12. Nitrogen and oxygen molecules were the only desorption products in TPD under these conditions. For higher coverages, thermal desorption of NO occurred and was found to be associated with an activation energy Ed=130±6 kJ/mol and a preexponential νd=10(15.0±0.8) s−1 for the limit of zero NO coverage in a first order process. The dissociation of molecular NOad caused a shift of the N1s core level binding energies from 400.3 to 397.6 eV. SSIMS studies were performed in either real time or in a temperature programmed manner (TPSSIMS) and the calibrated intensities of ionic species were taken to evaluate the dissociation kinetics. Both the activation energy Edis and the preexponential k0dis for dissociation were determined from the TPSSIMS data by assuming first order kinetics in a Polanyi–Wigner ansatz. Values of Edis=15±2 kJ/mol and k0dis=10(1.9±0.5) s−1 were found. These data are compatible with the rate constant kdis=0.18 s−1 obtained in real-time measurements during the ongoing NO adsorption and dissociation at 300 K. In this case, the SSIMS data were evaluated on the basis of consecutive reaction kinetics, including site inhibition by oxygen and Kisliuk’s precursor-mediated adsorption. ISS measurements indicated that oxygen atoms can also partly diffuse into the interior of the Rh(110) crystal.
A deuterium nuclear magnetic resonance investigation of orientational order and director kinetics in aramid solutions
Authors: Fan S. M. , Luckhurst G. R. , Picken S. J.
Published: July 2002

We have studied the molecular orientational order and director kinetics of the nematic phase formed by dissolving poly-p-phenylene-terephthalamide (PPTA) in sulphuric acid using deuterium NMR spectroscopy. Measurements were made on standard polymer samples using deuterated sulphuric acid and with deuterated PPTA in normal H2SO4. The influence of temperature and polymer concentration on the quadrupolar splittings which reflect the orientational order of the solvent and the solute are reported. The biphasic gap, between the nematic and the isotropic phase, for an 8.84%(w/w) solution was found to be about 10 °C. The influence of polymer concentration on the orientational order of the polymer solute is consistent with the predictions of a modified Maier–Saupe theory. By measuring the time dependence of the spectra the kinetics of field-induced director reorientation has been studied. The reorientation process from a multidomain sample to a uniform director alignment can be adequately described using an approach in which the different domains are decoupled.
Direct reaction of gas-phase atomic hydrogen with chemisorbed hydrogen on Ru(001)
Authors: Jachimowski T. A. , Weinberg W. H.
Published: July 2002

The adsorption of gas-phase atomic hydrogen on the Ru(001) surface results in a saturation coverage of 1.42 hydrogen adatoms per primitive surface unit cell, which may be compared with a saturation coverage of one hydrogen adatom per primitive surface unit cell in the case of dissociative chemisorption of molecular hydrogen. The observed saturation fractional coverage of 1.42 results from a steady-state balance of adsorption of gas-phase atomic hydrogen and reaction of gas-phase hydrogen with chemisorbed hydrogen adatoms, which produces molecular hydrogen that desorbs from the surface at a temperature at least 150 K below the temperature of recombinative desorption of two hydrogen adatoms. The cross section of this direct reaction of hydrogen was found to be remarkably large, approximately 40% of the cross section for chemisorption of the gas-phase atomic hydrogen. The reaction was found not to depend on surface temperature nor was there an observable kinetic isotope effect.
Direct ab initio dynamics studies of proton transfer in hydrogen-bond systems
Authors: Bell Robert L. , Truong Thanh N.
Published: July 2002

We present systematic direct ab initio dynamics studies of proton transfer in hydrogen-bond systems using the tautomerization in gas phase formamidine and its monohydrated complex as model reactions. The thermal rate constants were calculated using a canonical variational transition state theory (CVT) with multidimensional semiclassical tunneling corrections within a small-curvature ground-state approximation. The reaction valleys were calculated at the second-order Mo/ller–Plesset (MP2) perturbation theory, Hartree–Fock (HF) and nonlocal Becke’s half-and-half exchange and Lee–Yang–Parr correlation (BH&H–LYP) density functional theory (DFT) levels of theory using the 6-31 G(d,p) basis set. For accurate rate constants, the potential energy along the minimum energy path was scaled to match the single-point coupled cluster calculations including single and double excitations plus correction for triple excitation [CCSD(T)] at the MP2/6-31 G(d,p) classical barrier for each reaction. In the HF rate calculations, the HF frequencies were also scaled by a factor of 0.9. We found that adding a water to assist the proton transfer significantly enhances the tautomerization rate. Tunneling contributions in both systems are quite substantial and cannot be corrected by the Wigner approximation. We found that vibrational excitation of the solvent symmetriclike stretching mode would significantly enhance the rate of proton transfer in the formamidine–water complex. We also found that nonlocal DFT methods, particular the BH&H–LYP functionals studied here, can provide accurate potential energy information for dynamical calculations. Due to the computational advantage of DFT methods, prospects for dynamical studies of large polyatomic chemical reactions are quite encouraging.
Computer simulations of cesium–water clusters: Do ion–water clusters form gas-phase clathrates?
Authors: Smith David E. , Dang Liem X.
Published: July 2002

The structure and energetics of cesium ion–water clusters have been investigated using classical molecular dynamics computer simulations and a polarizable interaction model. Recent experiments by Selinger and Castleman [J. Phys. Chem. 95, 8442 (1991)] indicate that the mass-spectral distributions for these clusters exhibit ‘‘magic number’’ oscillations at temperatures below approximately 160 K. The observed behavior of this and related charged clusters is commonly attributed to the formation of clathratelike cage structures around a central ionic species. The relationship between the structural and energetic properties of cesium ion–water clusters is reported here as a function of temperature for clusters with between 18 and 22 water molecules. The clusters exhibit solidlike dynamical behavior at kinetic temperatures below about 170 K, and liquidlike behavior at higher temperatures. A thorough analysis of energy minimized (0 K) structures indicates that the most stable clusters consist of water cages surrounding the cesium ion. These cages are related to the proposed clathratelike structures but contain additional 4- and 6-membered water rings and fewer 5-membered rings. The calculated global energy minima exhibit an energetic alternation with cluster size that is consistent with the experimentally observed mass-spectral distributions. In contrast, in the liquidlike regime there are only minor variations in calculated structural and dynamical properties as a function of cluster size. In addition, there is no statistically significant size dependence for the cluster binding energies in the liquidlike regime that might be correlated with experimental data. These results suggest that magic number stability in ion–water clusters may occur only at ‘‘low’’ energies in the solidlike cluster regime.
Photodetachment in the gaseous, liquid, and solid states of matter
Authors: Christophorou L. G. , Datskos P. G. , Faidas H.
Published: July 2002

We have made absolute cross section measurements of laser photodetachment of C6F−6 ions embedded in gaseous tetramethylsilane (TMS) and compared the results at low gas densities with measurements in nonpolar liquids and solids. The measurements indicate that the photodetachment cross section of C6F−6 in gaseous TMS is about three times larger than in liquid TMS. This is rationalized by considering the effect of the medium on both the photoabsorption and the autodetachment processes. The photodetachment cross section in both the gas and the liquid exhibits (at least) two maxima due to autodetaching negative ion states. It is argued that these are due to σ*→σ* transitions in C6F−6. The relative positions of these ‘‘superexcited’’ anionic states did not change appreciably in going from the gas to the liquid and the solid, indicating similar influences of the medium on them. As expected, the photodetachment threshold in the condensed phase is shifted to higher energies compared to the gaseous phase. This shift is consistent with recent photoelectron studies of photodetachment of C6F−6 clusters. The present study clearly shows that the photodetachment from negative ions embedded in all states of matter proceeds directly or indirectly via negative ion autodetaching states, and that for nonpolar media, the effect of the medium can be accounted for by considering the macroscopic properties of the medium described by its dielectric constant ε and refractive index n.
Resonant two photon ionization spectroscopy of the molecules V2, VNb, and Nb2
Authors: James Andrew M. , Kowalczyk Pawel , Langlois Etienne , Campbell Margot D. , Ogawa Ayano , Simard Benoit
Published: July 2002

Resonant two photon ionization (R2PI) spectroscopy was used to obtain detailed spectroscopic information on the neutral and cation ground states of the jet-cooled molecules V2, VNb, and Nb2. By recording photoionization efficiency (PIE) spectra, their adiabatic ionization potentials were determined to be 51 269(5) cm−1 (V2), 51 554(10) cm−1 (VNb), and 51 359(10) cm−1 (Nb2). In VNb, we used different ionization routes to determine that the splitting between the Ω=0 and Ω=1 spin–orbit components of the X 3Σ− ground state was 230(3) cm−1. In the case of V2 and VNb, two thresholds were observed in the PIE spectra recorded via Ω=1 intermediate states. We were thus able to assign the ground states of V+2 and VNb+ as having 4Σ− symmetry, with second-order spin–orbit splittings of 20(3) and 82(3) cm−1, respectively. A simple model was applied to calculate the locations of the 1Σ+ and 2Σ+ states which are responsible for the second-order spin–orbit splitting of the neutral and cation ground states, respectively. One-color R2PI spectroscopy was employed to determine the bond dissociation energy of VNb, the result D00=30 562(10) cm−1 being obtained. The implications of our measurements regarding the relative bond strengths of the neutral and cationic dimers are discussed.
Neutral rare-gas containing charge-transfer molecules in solid matrices. I. HXeCl, HXeBr, HXeI, and HKrCl in Kr and Xe
Authors: Pettersson Mika , Lundell Jan , Räsänen Markku
Published: July 2002

Ultraviolet-irradiation of hydrogen halide containing rare gas matrices yields the formation of linear centrosymmetric cations of type (XHX)+, (X=Ar, Kr, Xe). Annealing of the irradiated doped solids produces, along with thermoluminescence, extremely strong absorptions in the 1700–1000 cm−1 region. Based on isotopic substitution and halogen dependence of these bands, the presence of hydrogen and halogen atom(s) in these species is evident. In the present paper we show the participation of rare gas atom(s) in these new compounds. The evidence is based on studies of the thermally generated species in mixed rare gas matrices. The new species are assigned as neutral charge-transfer molecules HX+Y− (Y=halogen), and their vibrational spectra are discussed and compared with those calculated with ab initio methods. This is the first time hydrogen and a rare gas atom has been found to make a chemical bond in a neutral stable compound. The highest level ab initio calculations on the existence of compounds of type HXY corroborate the experimental observations. The mechanism responsible for the formation of these species is also discussed.
Carbon-13 hyperfine structure of the CCCCH radical
Authors: Chen Wei , Novick Stewart E. , McCarthy M. C. , Gottlieb C. A. , Thaddeus P.
Published: July 2002

The fundamental (N = 1 – 0) rotational transitions of the ground 2Σ+ electronic state of the four singly substituted 13C isotopomers of CCCCH have been measured by pulsed-jet Fourier transform microwave spectroscopy. In each isotopomer this transition is split into many well-resolved hyperfine components owing to interaction between the electron spin and the molecular rotation, the proton spin, and the 13C nuclear spin. Here, the hyperfine transition frequencies are analyzed with the higher rotational millimeter-wave frequencies described in the previous paper of McCarthy et al. to produce a precise set of rotational, centrifugal distortion, spin-rotation, and hyperfine coupling constants. In particular, the Fermi-contact interaction of the 13C nucleus has been measured at each substituted position, yielding information on the distribution of the unpaired electron spin density along the carbon chain. The Fermi-contact constants, bF(13C), of 396.8(6), 57.49(5), −9.54(2), and 18.56(4) MHz, for successive 13C substitutions starting furthest from hydrogen indicate that the electronic structure is essentially acetylenic with alternating triple and single bonds.
Continuum electrostatics in a computational universe with finite cutoff radii and periodic boundary conditions: Correction to computed free energies of ionic solvation
Authors: Wood Robert H.
Published: July 2002

Many simulations of the free energy of hydration of an ion in a polar solvent are performed in truncated periodic systems in which electrostatic forces are truncated and periodic boundary conditions are used to eliminate surface effects. Simulations allow accurate calculation of the properties of the truncated Hamiltonian because there are no long-range forces present. However, in order to compare with the real universe, it is necessary to correct for the effects of both truncated potentials and periodic boundary conditions. A method of calculating the continuum dielectric properties of a truncated periodic system is derived and applied to the case of a single ion in a polar solvent. If the continuum model is accurate at distances where the effects of the truncated potentials and periodic boundary conditions are significant, then the correction will be accurate and it will include all the effects of the truncated ion–solvent and solvent–solvent potentials, as well as the effects of the periodic boundary conditions. When the simulations of Straatsma and Berendsen [J. Chem. Phys. 89, 5876 (1988)] for the Ne(aq) to Na+(aq) transformation at 298 K and 0.1 MPa are corrected in this manner, the calculations with different cutoff radii between 0.9 and 1.2 nm and different periodic cell size give the same corrected result within experimental error. Similarly, Aqvist’s [J. Phys. Chem. 98, 8253 (1994)] calculations for a different model of the Ne(aq) to Na+(aq) transformation with periodic boundary conditions and varying cutoff schemes give self-consistent results, but these results are different from his results for the same model using the surface constrained all atom solvent method. Possible reasons for this discrepancy are discussed.
Absorption spectrum of C60 in the gas phase: Autoionization via core-excited Rydberg states
Authors: Yasumatsu Hisato , Kondow Tamotsu , Kitagawa Hiroshi , Tabayashi Kiyohiko , Shobatake Kosuke
Published: July 2002

The absolute absorption cross section of C60 in the gas phase (830–870 K) was measured as a function of the photon energy (3.5–11.4 eV) (absorption spectrum). Absorption peaks at 7.87, 8.12, 8.29, 9.2 eV and a dip at 8.45 eV observed are assigned as Feshbach resonances in the photoexcitation involving superexcited states. The superexcited states responsible for the 7.87, 8.12, and 9.2 eV peaks are assigned to be core-excited Rydberg states converging to the second, the third and the fourth ionization limits of C60 (8.89, 9.12, 10.82–11.59 eV), respectively. The 8.29 eV peak is considered to originate from vibrational excitation of a totally symmetric pentagonal pinch mode of the superexcited state responsible for the 8.12 eV peak. Further, a relative photoionization quantum yield was estimated from the absorption cross section measured and the relative photoionization cross section reported. The yield increases particularly in the vicinity of 8 eV in accordance with a high efficiency of autoionization of the superexcited states. Ionization efficiency is not high in the vicinity of the first ionization energy, probably because of rapid energy dissipation into its vibrational modes. The spectrum below the ionization energy resemble the absorption spectra of C60 in its solutions.
Is there evidence for detection of cyclic C4 in IR spectra? An accurate ab initio computed quartic force field
Authors: Martin Jan M. L. , Schwenke David W. , Lee Timothy J. , Taylor Peter R.
Published: July 2002

The quartic force field of cyclic C4 has been computed using basis sets of spdf quality and augmented coupled cluster methods. The effect of core correlation and further basis set extension has been investigated. Vibrational energy levels have been obtained using perturbation theory and two different variational approaches. A severe Fermi resonance exists between the most intense vibration, ν6, and ν3+ν5 through an exceptionally large k356=−258.2 cm−1; a large k1356 =−54.8 cm−1 causes significant higher-order anharmonicity, including a shift in ν6 of +9 cm−1. C4 appears to be an excellent test case for methods for solving the vibrational Schrödinger equation, since perturbation theory breaks down even when the above resonances are accounted for. Our best estimate for ν6, 1320±10 cm−1, may suggest its assignment to a feature detected at 1284 cm−1 in argon and 1302 cm−1 in krypton matrix, but this would imply an unusually large matrix red shift in argon.
Hartree–Fock static longitudinal (hyper)polarizability of polyyne
Authors: Toto Joseph L. , Toto Teressa Tangredi , de Melo Celso P. , Kirtman Bernard , Robins K.
Published: July 2002

The evolution in the static longitudinal polarizability and second hyperpolarizability of the C4H2 through C54H2 oligomers of polyyne is studied at the Hartree–Fock level of theory. We find that the calculated values are highly sensitive to the adopted geometry. Using improved geometries, new extrapolation procedures and careful finite field determinations we extend earlier work by others so as to provide reliable estimates for the above properties in the infinite polymer limit.
A He-atom scattering study of the frustrated translational mode of CO chemisorbed on defects on copper surfaces
Authors: Braun Jens , Graham Andrew P. , Hofmann Frank , Silvestri Walter , Toennies J. Peter , Witte Gregor
Published: July 2002

The energies of the frustrated translational vibrations parallel to the surface (T-mode) of isolated CO molecules adsorbed on various flat, regularly stepped and defective surfaces of copper have been measured using high resolution He-atom scattering. Whereas on the Cu(111) and Cu(001) surfaces CO has a T-mode energy of about 4.0 meV, at the steps of the Cu(211) and Cu(511) surfaces it is reduced to 3.0 meV. The same energy was observed for scattering parallel and perpendicular to the step edge directions. At defects, which were created by sputtering under different conditions, two additional T-mode energies of 3.2 meV and 2.5 meV were observed and are attributed to CO molecules adsorbed at step edges and at ‘‘isolated’’ defects, respectively.
A kinetic study of the interaction of gaseous H(D) atoms with D(H) adsorbed on Ni(100) surfaces
Authors: Kammler Th. , Lee J. , Küppers J.
Published: July 2002

The kinetics of reactions which occur upon subjecting D(H) covered Ni(100) surfaces with H(D) atom fluxes were investigated. At 120 K surface temperature in the H→Dad reaction HD and D2 were observed as reaction products, in the D→Had reaction HD and H2 were reaction products. As the reaction temperature was well below the hydrogen desorption temperature, a direct reaction step, like in the Eley–Rideal (ER) mechanism, is suggested to operate for HD production. However, the characteristics of the HD formation kinetics observed in the present study contradict an essential element of the ER: mechanism the rate of HD formation is not proportional to the surface coverage of the adsorbed reaction species D or H under impact of a flux of H or D atoms. Therefore, a modification of the mechanistic description of atom/surface reactions seems necessary. This modification should allow for reaction products which are completely unaccounted for in the ER picture: D2 from H→Dad and H2 from D→Had reactions. The observed strong isotope effects in the reaction efficiencies support mechanisms in which the impinging gas phase atoms get trapped in the surface potential well prior to reaction.
Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentiostatic control. I. Experimental
Authors: Strasser P. , Lübke M. , Raspel F. , Eiswirth M. , Ertl G.
Published: July 2002

The experimental characterization of the current/outer potential (I/U) behavior during the electrochemical CO oxidation on Pt(100), Pt(110) and Pt(111) is used as the first step towards a thorough investigation of the processes occurring during the electrochemical formic acid oxidation. The CO study is followed by new cyclovoltammetric results during the electrochemical formic acid oxidation on the corresponding Pt single crystals. At high concentrations of formic acid, the cyclovoltammograms revealed a splitting of the large current peak observed on the cathodic sweep into two peaks whose dependence on scan rate and reverse potential was investigated. It turned out that the presence of a sufficiently large ohmic resistance R was crucial for oscillatory instabilities. Given an appropriate resistance, all three Pt surfaces were found to exhibit current oscillations at both low and high formic acid concentrations. On Pt(100) stable mixed-mode oscillations were observed. In addition, the sensitivity of the oscillations to stirring was investigated. Whereas the period-1 oscillations were found to be independent of stirring, the mixed-mode oscillations transformed into simple oscillations with stirring. The mechanism giving rise to instability and oscillations is described.
The C–H activation reaction of methane for all transition metal atoms from the three transition rows
Authors: Wittborn A. M. Christina , Costas Miquel , Blomberg Margareta R. A. , Siegbahn Per E. M.
Published: July 2002

Parametrized density functional and ab initio based methods have been used to study the important model reaction where the C–H bond of methane is activated by transition metal atoms. All atoms from the three transition rows have been investigated. For the third transition row, spin–orbit effects are found to be quite important. These effects were obtained using two different simple schemes. In most cases good agreement between the methods and also with the sparse experimental information is obtained. Only the platinum and rhodium atoms are found to activate the C–H bond and of these only platinum can continue to form a carbene that is required for observing the reaction experimentally. The low barrier for the platinum reaction is a result of a low-energy crossing between different spin surfaces. The trends of the results are analyzed in terms of promotion energies, exchange energies, polarity of the bonds, and in terms of the relative size of the metal d orbitals. The main failure of the ab initio based computational scheme occurring for the nickel reaction is analyzed in detail.
Low-frequency-dependent effects of oscillating magnetic fields on radical pair recombination in enzyme kinetics
Authors: Eichwald C. , Walleczek J.
Published: July 2002

A model of an enzyme reaction cycle that includes the generation of a transient spin-correlated radical pair state is discussed. The recombination yield of the radical pair state is altered by external magnetic fields (radical pair mechanism). In this theoretical study, the response behavior of the enzyme to pulsed magnetic fields as well as combinations of static and sinusoidally oscillating magnetic fields is investigated by using an approach that combines enzyme kinetics with magnetic field-sensitive spin kinetics. Calculations show that the enzyme behaves like a frequency sensor that is responsive at lower field frequencies but less responsive at frequencies that are faster than the time scales inherent to the kinetic properties of the reaction cycle. There is a characteristic transition region in the frequency domain that reflects the enzyme’s relaxation behavior to time-dependent external perturbations. The transition region is characterized by using methods based on the theory of externally driven systems, including Floquet theory and the calculation of correlation functions. Model simulations suggest that time-dependent magnetic fields could be used as a tool to study the response behavior of magnetic field-sensitive enzymes.
A new spin-restricted triple excitation correction for coupled cluster theory
Authors: Crawford T. Daniel , Lee Timothy J. , Schaefer Henry F.
Published: July 2002

A new approach to the noniterative inclusion of the effects of connected triple excitations in the coupled cluster singles and doubles (CCSD) model is presented. The method is based on the recently developed Z-averaged perturbation theory, and reduces to the usual Mo/ller-Plesset-based triples correction in the absence of open shells. This new correction maintains the same invariance properties as those of the CCSD energy, and requires storage of only one-third the number of two-electron integrals as more conventional corrections. The derivation and implementation of the equations are described, and the method’s performance relative to more conventional triples corrections is assessed via benchmark calculations for a series of diatomic molecules.
Density functional and coupled-cluster study on the HNO→HON transition state
Authors: Alikhani M. E. , Dateo Christopher E. , Lee Timothy J.
Published: July 2002

The spectroscopic properties of the HNO→HON transition structure have been studied using density functional theory (DFT) and a coupled-cluster method [CCSD(T)]. The barrier height relative to the HNO minimum has been found to be 77.8 and 73.4 kcal mol−1 at the DFT and CCSD(T) levels of theory, respectively. Accordingly, the isomerization to HON should not occur to any appreciable degree even in rather severe combustion environments.
Computer simulation studies of diffusion in gels: Model structures
Authors: Netz Paulo A. , Dorfmüller Thomas
Published: July 2002

We have investigated particle diffusion through different obstacle geometries by computer simulations. The model structures used in this work — randomly placed point obstacles and cage-like structures — were chosen with the aim of represent a broad range of geometrical structures similar to gels and in order to be compared with our previous simulations of particle diffusion through polyacrylamide gels. The diffusion behavior was studied as a function of tracer size and obstacle concentration. The isomorphism between the diffusion of finite-sized tracers and the diffusion of point tracers in the presence of expanded obstacles was applied. Only hard-sphere interactions of the tracer with the immobile obstacles were considered and the theoretical description was made in terms of theory of the obstruction effect. In the case of randomly placed point obstacles an analytical expression for the dependence of the diffusion coefficient on tracer radius and obstacle concentration, applying the model of spherical cells, could be deduced. The same description was applied numerically to the other model systems. Up to moderatly high fractions of excluded volume this description was found to be successful. For very high fractions of excluded volume — higher concentrations or larger tracers — the validity of Fick’s second equation for describing diffusion breaks down and anomalous diffusion was found. The anomalous diffusion exponent diverges as the tracer size becomes comparable to the size of the pores. Analysis of the trajectory of tracers in the cases where an anomalous diffusion takes place shows a Levy-flight-like characteristic.
Dissociative chemisorption of methane on Ir(111): Evidence for direct and trapping-mediated mechanisms
Authors: Seets D. C. , Reeves C. T. , Ferguson B. A. , Wheeler M. C. , Mullins C. B.
Published: July 2002

Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with room-temperature methane gas over Ir(111) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, these calculations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to the results of a previous study of methane dissociation on Ir(110), although the reactivity of thermalized methane is approximately an order of magnitude higher on the (110) surface of iridium.
Structure of multi-component hard-sphere mixtures
Authors: Yuste Santos Bravo , Santos Andrés , López de Haro Mariano
Published: July 2002

A method to obtain (approximate) analytical expressions for the radial distribution functions and structure factors in a multi-component mixture of additive hard spheres is introduced. In this method, only contact values of the radial distribution function and the isothermal compressibility are required and thermodynamic consistency is achieved. The approach is simpler than but yields equivalent results to the Generalized Mean Spherical Approximation. Calculations are presented for a binary and a ternary mixture at high density in which the Boublı́kMansoori-Carnahan-Starling-Leland equation of state is used. The results are compared with the Percus-Yevick approximation and the most recent simulation data.
Thermodynamic stability and negative thermal expansion of hexagonal and cubic ices
Authors: Tanaka Hideki
Published: July 2002

The free energies of two low pressure ice forms are calculated over a wide range of temperatures in order to explain relative stability and their negative thermal expansivities in the low temperature regime. One-hundred proton-disordered configurations for hexagonal and cubic ices are generated. The Helmholtz free energy is approximated to a sum of the minimum potential energy, the harmonic free energy and the configurational entropy arising from the disordered nature of protons. The Gibbs free energy at a given temperature is minimized with respect to the volume of the system. This enables us to evaluate the thermal expansivity at a fixed temperature and pressure from only intermolecular interaction potentials. The negative thermal expansivity of ices in low temperature is successfully reproduced. This arises mainly from the bending motion of hydrogen bonded molecules.