bluuebunny/crossref_metadata_2025 huggingface dataset:
Long polyene and polydiacetylene oligomers: Pariser–Parr–Pople investigation of the geometric and electronic structures in the first 1B
u excited state
Authors: Brédas J. L. , Dory M. , André J. M.
Published: July 2002

We present Pariser–Parr–Pople (PPP) calculations on long polyene and polydiacetylene oligomers containing up to 32 carbon atoms. Working at both PPP restricted Hartree–Fock and PPP configuration interaction with single excitations levels, we study the excitation towards the 1 1B+u state. We report, as a function of chain length, the evolution of the transition energies and the modifications in geometry and atomic charges along the backbones. Comparison is made between the two series of oligomers. In the longer molecules, localization of the excitation is found, in agreement with the excitonic character of the transition to the 1 1B+u state.
Conductivity mechanisms in phthalocyanine-based ‘‘molecular metals’’: Calculation of the temperature-dependent resistivity
Authors: Hale Paul D. , Ratner Mark A.
Published: July 2002

The temperature dependence of the conductivity σ in the low-dimensional material, NiPcI (Pc=phthalocyanine), is investigated using a transport theory for noninteracting electrons in a tight-binding band scattered by one- and two-phonon processes. The lattice motions found to be the dominant sources of resistivity along the chain are the longitudinal stretch and the interplanar twist (libron), with the former being a first-order scattering process (σ1p ∝1/T) and the latter a second-order mechanism (σ2l ∝1/T2). The resulting conductivity has a room-temperature value of ∼400 Ω−1 cm−1 which increases to almost 4000 Ω−1 cm−1 at 50 K, with an overall temperature dependence of σ∼T−1.4, in good agreement with experiment. It is found that at low temperature, one-phonon scattering is most important, while at room temperature one-phonon and two-libron scattering contribute nearly equally to the resistivity. The accuracy of the bandwidths and of the electron/phonon coupling constants, calculated using first-principles electronic structure methods, shows the value of these techniques for studying conduction in ‘‘molecular metals’’, while the good agreement of our calculated resistivity plots with those measured experimentally both supports the straightforward narrow-band picture used for the conduction process, and provides valuable insights into the design of optimal molecular metals of this class.
Time and spectrally resolved luminescence and relaxation processes in solid neon
Authors: Coletti F. , Debever J. M. , Zimmerer G.
Published: July 2002

The spectral distribution and the time evolution of the luminescence of atomic (a-STE) and molecular type self-trapped excitons (m-STE) of solid neon is measured under primary excitation with a continuous or pulsed electron beam. In high resolution spectra, several new luminescence bands are established. In addition to the luminescence of excitons localized at different trapping sites, the emission of desorbing neon atoms show up, indicating a close connection between self-trapping of excitons at the surface and desorption. The spectral and temporal behavior of m-STE emission displays the step-by-step vibrational relaxation of the Ne*2 centers. The decay curves of high vibrational levels of the m-STE are the first direct measurement of the relaxation rates. Good agreement with recently published values obtained with a different method is found.
Chemical reaction network sensitivity analysis
Authors: Larter Raima , Clarke Bruce L.
Published: July 2002

Traditional sensitivity analysis of a chemical reaction system’s periodic trajectory shows how the oscillation depends on the reaction rate constants. We develop sensitivity analysis within the framework of a stoichiometric network analysis, which is a theory that relates the dynamics of complex chemical reaction systems to feedback loops in the reaction network. The combined theory shows how the properties of a chemical oscillation are related to parameters that reflect the topology of the reaction network rather than to the rate constant parameters. The theory is applied to a Brusselator limit cycle as an example.
The effect of dipole moment on diffusion controlled bimolecular reaction rates
Authors: Northrup Scott H. , Smith Jeffrey D. , Boles Jeffrey O. , Reynolds John C. L.
Published: July 2002

Calculation of diffusion controlled bimolecular reaction rates for complicated macromolecular systems is made possible by a computer simulation approach based on the Brownian dynamics (BD) trajectory method. This is applied to the calculation of the diffusion controlled reaction rate between two charged spherical molecules, one of which represents a protein which possesses a strong dipole moment and asymmetric chemical reactivity. The role of the position of the dipole moment relative to reactive surfaces having various extents in media of varying ionic strength is assessed. No dipolar electrostatic effect on the diffusional rate constant of an isotropically reactive dipolar protein is observed. When the dipolar protein has a small axially symmetric reactive surface (e.g., of 10° extent), the dipole exerts a fivefold enhancement on the rate when the dipole vector is along the reactive patch axis, and retards the rate by a factor of 10 when oppositely disposed. A significant electrostatic effect persists through the physiological ionic strength regime. The positioning of dipolar charges relative to the protein surface at fixed dipole strength has an important effect on the electrostatic influence of the dipole. The variation of the protein dipole relative to its reactive surface can in fact be achieved in practice by chemical modification or site-directed mutagenesis.
X-ray absorption near-edge structure of complex compounds (NH4)3RhCl6, K3RuCl6, and Ru(NH3)6Cl3
Authors: Sugiura Chikara , Kitamura Michihide , Muramatsu Shinji
Published: July 2002

X-ray absorption spectra of chlorine K edge, ruthenium LIII edge, and rhodium LIII edge from (NH4)3[RhCl6], K3[RuCl6], and [Ru(NH3)6]Cl3 have been measured with a Johann-type 50 cm bent crystal vacuum spectrograph. The white lines due to the transitions to the incompletely filled antibonding molecular orbital (MO) 2t2g(π*) or nonbonding orbital t2g and the empty antibonding MO 3eg(σ*) for the octahedral complexes have been observed in the Cl K edge, Ru LIII edge, and Rh LIII edge regions. It is found that the two absorption peaks at the Ru LIII edge reflect the ligand-field effect. An interpretation for experimental results of the Cl K edge and the Ru LIII edge absorption spectra in K3RuCl6 on the basis of the MO scheme leads us to the conclusion that the white lines at the Cl K edge suffer from the effect of core hole.
Room temperature rate constant for the reaction of Na with Cl2
Authors: Silver Joel A.
Published: July 2002

The room temperature rate constant for the reaction of atomic sodium with Cl2 is measured to be (6.7±0.9)×10−10 cm3 molecule−1 s−1. This experiment is performed in a fast flow reactor where Na is detected using laser-induced fluorescence. The results of this measurement are compared with simple collision theory.
Cross sections for production of atomic thallium states in photodissociation of thallium halides
Authors: Scott D. A. , Piper J. A.
Published: July 2002

The cross sections for production of 62P1/2 and 62P3/2 ground and metastable atomic thallium states via UV dissociation of the thallium halides TlCl, TlBr, and TlI have been measured at wavelengths 193, 222, 249, 308, and 353 nm using time-resolved atomic resonance absorption spectroscopy. The measured cross sections, which were independent of halide densities up to densities ∼1017 cm−3, were found to represent only 10% to 36% of the total UV absorption cross section at the appropriate wavelength.
C–N bond formation in the gas phase reaction of C+ with NH3
Authors: Curtis R. A. , Farrar J. M.
Published: July 2002

We present a study of the reaction of C+ with NH3 at collision energies near 2 eV. The reaction proceeds through a long lived intermediate which is hypothesized on the basis of ab initio calculations to be the nonclassical cation HCNH+2 produced by insertion of C+ into the N–H bond. Mass 28 production appears to proceed through a substantial exit channel barrier, suggesting that N–H bond cleavage in the intermediate complex yields HCNH+, hypothesized to be important in interstellar synthesis of large molecules. The recoil distribution is significantly broader than the predictions of phase space theory. Mass 27 production yields a recoil distribution which is also broader than predicted by a statistical theory and which appears to be a superposition of two channels releasing different amounts of kinetic energy. We hypothesize that the intermediate complex decays by (1,1) geminal elimination of H2 to yield HCN+, while HNC+ is also produced by (1,2) vicinal elimination from the same intermediate.
The reactions of NO2/N2O4 with Ag: A surface enhanced Raman scattering and ellipsometric measurement
Authors: Dorain Paul B. , Boggio Joseph E.
Published: July 2002

This paper, to our knowledge, reports the first detailed kinetic study of surface chemical reactions on Ag metal powder using surface enhanced Raman scattering (SERS). A pulse technique was used to admit NO2/N2O4 gas to the Ag powder surface in the temperature range of 20–80 °C. The initial chemical step is the formation of AgNO2 which subsequently decomposes autocatalytically into AgNO3, NO, and Ag. It is the product Ag metal atoms which aggregate, forming the Ag microstructures necessary for the SERS enhancement mechanisms to be operative. Ellipsometric and scanning electron microscope measurements confirm the existence of the Ag microstructures which are embedded in the AgNO3 surface film. Subsequent pulses of NO2/N2O4 rapidly react with the Ag microstructures which suggests that the process may be used to study in situ heterogeneous catalysis of other gases on Ag metal surfaces at ambient and elevated temperatures.
Dynamic perturbation theory of vibrational energy transfer in a nonrigid molecular system
Authors: Tachibana Akitomo , Nagaoka Masataka , Yamabe Tokio
Published: July 2002

A theory for van der Waals molecules (AB---C) is presented, which is based on the concept that a van der Waals molecule can be considered as a nonrigid molecule because it is loosely bound in comparison with proper chemical bonds. A perturbation method is performed for the three-body internal Hamiltonian which has been obtained for the case in which the angular momentum of the whole molecular system is equal to zero (J=0). A new idea, the dynamic perturbation mechanism of vibrational energy transfer, is presented which originates in the coupling of the kinetic energy of the chemical and van der Waals bonds. This is applied to the vibrational predissociation process of the van der Waals molecule and linewidths and lifetimes are calculated by estimating the dynamic coupling. The decay rates calculated by this new mechanism are quite large and are comparable to those values estimated by the usual treatment of the perturbation potential.
Study of the visible emission induced by IR multiple-photon excitation of OsO4
Authors: Kompa K. L. , Lamprecht H. , Schröder H. , Puretzky A. A. , Tyakht V. V.
Published: July 2002

The visible luminescence induced in the OsO4 molecule by the action of CO2-laser radiation was studied in a wide range of laser fluences (2–1000 J/cm2). A novel approach consisting in separating successive dissociation and fluence was developed experimentally to study the visible emission induced by multiple-photon excitation (MPE) in an IR-laser field. Three different luminescence stages with definite fluence thresholds were found in MPE of the OsO4 molecule. The luminescence spectra and the velocities of the luminescent species were measured at each stage. A theoretical model is proposed which explains the observed three-stage emission in OsO4 by the following sequence of processes taking place during the IR-laser pulse: inverse electronic relaxation (IER)–multiple-photon dissociation (MPD) of the parent molecule (OsO4–first stage)—IER–MPD of the primary fragment (OsO3-second stage)—IER–MPD of the secondary fragment (OsO2-third stage).
On the relationship between principal fluctuations and stability coefficients in multicomponent systems
Authors: Debenedetti Pablo G.
Published: July 2002

There is a univocal correspondence between each of the (n+2)! stability coefficients of an n-component system and mean squared fluctuations of Gibbs-space variables (entropy, volume, molecule number) in which the subsystem under study is defined through a single extensive Gibbs-space variable (principal fluctuations). The relationship between principal fluctuations and stability coefficients [Eq. (53)] is derived in this paper starting from the usual expressions involving explicit calculation of the inverse of an (n+1)×(n+1) matrix. A general equation between higher order energy variations in fluctuating subsystems [Eq. (22)] is also derived. The relationship between principal fluctuations and stability coefficients yields, as particular cases, expressions which had been used previously for the limiting case of single-component systems, or extended to solutions through heuristic arguments.
A model for photoinjection into polar fluids
Authors: Bennett G. T. , Thompson J. C.
Published: July 2002

The presence of fluctuating dipoles in a polar liquid has been suggested to cause an intrinsic tail of localized states extending below the conduction band edge at −V0. If such preexisting states are the final states for photoinjection into such a liquid, then they cannot be described by the conventional treatment which assumes free-particle states. In this elementary model, the effect of both free and localized final states on the Fowler theory for photoemission is determined. Well above threshold Fowler’s 1/2-power law is found. The near-threshold behavior is governed by the exponential tail in the final density of states. The results compare well with recent results on photoinjection into H2O and NH3. We find V0 to lie near −0.80 eV in NH3.
Study of eigenvalue singularities from perturbation series: Application to two-electron atoms
Authors: Arteca Gustavo A. , Fernández Francisco M. , Castro Eduardo A.
Published: July 2002

A simple method for studying real singularities from power series expansions is developed. Singularity positions, critical exponents, and critical amplitudes are calculated for a number of quantum-mechanical eigenvalue problems. Special attention is devoted to the Z−1 perturbation series for two-electron atoms. Present results for the states (1s2) 1S, (2p2) 3P, and (1s2s) 3S reveal inaccuracies in previous calculations. In particular it is suggested that the singularity for the 3S state occurs at Z=1 and that the energy lies in the limit of the continuous spectrum of H−.
Nature and location of excited charge-transfer states in porphyrin dimers: Development of preliminary design characteristics for biomimetic solar energy conversion systems
Authors: Petke J. D. , Maggiora G. M.
Published: July 2002

The ground and excited electronic states of a magnesium porphyrin–porphyrin (MgP–P) dimer in a variety of geometries have been investigated by ab initio configuration interaction calculations. The lowest four excited singlet states were found to be locally excited monomer-like (π, π*) states in each geometry, except for dimers in which the macrocycles were constrained to lie within the van der Waals separation. The lowest MgP+–P− radical pair state CT1 was found much higher in energy than the lowest (π, π*) state; CT1←S0 transition energies varied approximately linearly with macrocyclic center-to-center distance, but were only weakly affected by relative macrocyclic orientation. Coordination of the Mg atom by chloride ion was shown to differentially stabilize CT1 relative to (π, π*) states, to the extent that CT1 is positioned near or below the lowest (π, π*) state. Moreover, estimates of the differential stabilization of CT1 by a polarizable medium suggest that combined point-charge and medium effects are sufficient to stabilize CT1 below the lowest (π, π*) state in a variety of geometries of the complex.
An a
b i
n
i
t
i
o calculation of the near-equilibrium potential energy surface and vibrational frequencies of H2Br+ and its isotopomers
Authors: Botschwina P. , Zilch A. , Rosmus P. , Werner H.-J. , Reinsch E.-A.
Published: July 2002

The near-equilibrium potential energy surface of H2Br+ was calculated by SCEP-CEPA using a basis set of 82 contracted GTOs. The equilibrium geometry is predicted to be re=1.441 Å and αe=92.1°. Vibrational frequencies were calculated variationally and the fundamentals of H279Br+ (in cm−1) are ν1=2403, ν2=1073, and ν3=2398. An unusual isotopic effect, due to vibrational anharmonicity, is discussed. The calculated proton affinity of HBr of 587 kJ mol−1 is in excellent agreement with experimental values.
Angular momentum polarization produced by molecule–surface collisions in a Knudsen flow
Authors: Hermans L. J. F. , van der Tol J. J. G. M. , Beenakker J. J. M.
Published: July 2002

In a flowing Knudsen gas, molecular angular momentum polarization can be produced by tangential forces resulting from nonspherical molecule–surface interaction. This is investigated by studying the change in the flow due to molecular precession around an external magnetic field. Various types of polarization are observed in CH4, N2, and HD flowing through a Au-plated channel. The results suggest a strong correlation between type of polarization produced and molecular structure. These data call for model calculations which include surface corrugation.
The effect of initial S1 vibrational mode on the photodissociation H2CO → H2+CO
Authors: Apel Eric C. , Lee Edward K. C.
Published: July 2002

SRL lifetimes of the jet-cooled S1 H2CO molecules with low J′ and K′ values have been studied for the excitation of v′4=1 and 3, v5=1, and v′6=1. The v4 excitation leads to considerably faster S1 u S0 radiationless decay than the v′5 and v6 excitation, showing the vibrational mode specificity and correlation predicted by the molecular dissociation theory.
Molecular dynamics in intense fields. IV. Beyond the dipole approximation
Authors: Bandrauk André D. , Kalman O. F. , Nguyen Dang Tung T.
Published: July 2002

The dressed molecular Hamiltonian for molecules in strong electromagnetic fields is derived in the Bloch–Nordsieck (BN) and electric field (EF) representations beyond the dipole approximation. Both representations, which are related by simple unitary transformations generate photon coherent states beyond the dipole approximation. It is shown that applying the dipole approximation to the complete molecule–field equations leaves recoil corrections of order (v/c) which are field dependent. This implies that in the BN representation all Coulomb potentials will be modified at high intensities. In particular, electron–electron repulsions are subject to field dependent photon recoil corrections, which for very intense fields necessitate relativistic corrections as well.
Nonlinear force and tensorial mobility in a kinetic theory for polymer liquids
Authors: Honerkamp J. , Öttinger H. C.
Published: July 2002

A stochastic differential equation for the global length and orientation of a flexible macromolecule is studied. A non-Hookean internal force law, a tensorial mobility, and an approximate form of the hydrodynamic interaction are taken into consideration. An equation for the equal-time correlation function is obtained in mean-field approximation, the constitutive equation is derived and the material functions are calculated for steady elongational and shear flows.
AgCl–Ag cluster enhanced optical second harmonic generation from an electrode surface
Authors: Marshall C. D. , Korenowski G. M.
Published: July 2002

Evidence is given for the presence of AgCl–Ag cluster complexes on a silver electrode surface. The complexes are formed during electrochemical cycling of the electrode in a KCl electrolyte. These cluster complexes provide a resonance enhancement for surface optical second harmonic generation from the electrode during electrochemical reduction of adsorbed AgCl oxidation product. The complexes possess a strong electronic resonance in the visible wavelength region and may be related to species in the silver halide photographic process. Recent surface enhanced Raman scattering studies of this surface have demonstrated a resonance enhancement from similar cluster structures. Previous second harmonic studies on this system have not identified any resonance enhancement from surface complexes. The studies of this paper address the question of resonance enhancement from surface sites or clusters and their contribution to second harmonic generation from these interfaces. In addition, multiphoton induced luminescence studies of the surface clusters are presented. These studies indicate that the silver clusters associated with the AgCl–Ag cluster complex formation are six atoms in size or less. Second harmonic generation and cluster luminescence are also used to follow and elucidate molecular level events at the electrode surface during oxidation and reduction in the electrochemical cycling experiments.
Trapping site geometry of N2H+4 radical ion in x-irradiated single crystals of N2H5HC2O4: An ENDOR study
Authors: Sagstuen E. , Awadelkarim O. , Lund A. , Masiakowski J.
Published: July 2002

The radical ion N2H+4 has been produced by x irradiating single crystals of hydrazinium hydrogen oxalate (N2H5HC2O4) and studied by ESR and ENDOR spectroscopy. The structure of the trapping site has been elucidated by observing 1H ENDOR transitions not only from the radical but also from distant protons. One of the latter originates from the proton lost by N2H+5 upon radical formation. Evidence that proton transfer occurs through a hydrogen bond is presented.
Polarized and depolarized Raman spectra of liquid carbon disulfide in the pressure range 0–10 kbar. I. Vibration frequencies, C–S bond length, and Fermi resonance
Authors: Ikawa S. , Whalley Edward
Published: July 2002

The effect of pressure on the polarized and depolarized Raman spectra of liquid carbon disulphide, i.e., the peak frequencies, bandwidths, and relative intensities of both the allowed ν1 and 2ν2 bands and the interaction-induced ν2 and ν3 bands, have been measured at 22 °C up to 10 kbar. This paper discusses the effect of pressure on the frequencies and on the relative isotropic intensity of the ν1 and 2ν2 bands. The frequency of the ν1 band increases linearly with pressure, within the experimental uncertainty, at the rate 0.16±0.01 cm−1 kbar−1, and the frequencies of the ν2, ν3, and 2ν2 bands decrease nonlinearly. The frequency shifts are described by second-order perturbation theory with the molecular anharmonicity and the intermolecular interaction as perturbations. The leading terms of the shifts consist of the same derivative of the interaction potential, multiplied by different anharmonicity constants, and the shifts of the ν1 and 2ν2 bands suggests that the C–S bond length decreases at the rate 2×10−4 Å kbar−1. The relative isotropic intensity of the 2ν2 and ν1 bands increases with pressure at the rate 0.050 kbar−1, whereas the anisotropic 2ν2 intensity relative to the isotropic ν1 intensity is independent of pressure to the experimental precision of ∼0.005. The effect of pressure on the second derivative of the isotropic and anisotropic parts of the polarizability with respect to the bend coordinate was estimated as 1.1×10−43 C m2 V−1 kbar−1 and ∼0, respectively, from these values.
On the use of action-angle variables for direct solution of classical nonreactive 3D (Di) atom–diatom scattering problems
Authors: Smith Neil
Published: July 2002

Application of action-angle variables for the physical description and direct numerical integration of the exact Hamilton’s classical equations of motion for 3D nonreactive atom–diatom collision systems is discussed in detail. The Hamiltonian, action-angle equations of motion, and necessary coordinate transformations are presented for both rigid and vibrating rotor models of the diatom. Generalization to the vibrating-rotor model of the diatom is done via the introduction of internuclear position and momentum variables (r,pr), and subsequent canonical transformation (r,pr) →(V,ψV) to vibrational action-angle variables. We present a quantitatively realistic interatomic diatom potential model U(r) which allows exact solution of diatom energy ‘‘eigenvalues’’ E(V,J), and which facilitates analytical inversion of the transformation equations to obtain virtually exact expressions for r(ψV;V,J) without resort to any dynamical approximations. Use of rotational and especially vibrational action-angle variables (J,ψJ) and (V,ψV) to describe internal diatom motion [in place of their traditional Cartesian counterparts (r,pr)] allows large savings in computation time, a minimization in the number of necessary dynamical variables, exact treatment of vibrational–rotational coupling, and a trivial assignment of final ‘‘quantum’’ numbers (vf, jf ) which is useful for classical trajectory calculations of vib-rotationally inelastic cross sections.
Atom–diatomic molecular reactive scattering: Investigation of the hyperangular integration
Authors: Wolniewicz L. , Hinze Juergen
Published: July 2002

Different formulations of the quantum-mechanical Hamiltonian of the triatomic hydrogen system in hyperspherical coordinates are investigated. Numerical solutions of the adiabatic hyperangular states have been computed for several hyperspherical radii using cubic spline finite elements. It is found that the fully symmetric hyperspherical coordinates of Johnson are most appropriate for a coupled channel calculation of reactive scattering.
A method for the deconvolution of incompletely resolved CARS spectra in chemical dynamics experiments
Authors: Anda Andrew A. , Phillips David L. , Valentini James J.
Published: July 2002

We describe a method for deconvoluting incompletely resolved CARS spectra to obtain quantum state population distributions. No particular form for the rotational and vibrational state distribution is assumed, the population of each quantum state is treated as an independent quantity. This method of analysis differs from previously developed approaches for the deconvolution of CARS spectra, all of which assume that the population distribution is Boltzmann, and thus are limited to the analysis of CARS spectra taken under conditions of thermal equilibrium. The method of analysis reported here has been developed to deconvolute CARS spectra of photofragments and chemical reaction products obtained in chemical dynamics experiments under nonequilibrium conditions. The deconvolution procedure has been incorporated into a computer code. The application of that code to the deconvolution of CARS spectra obtained for samples at thermal equilibrium and not at thermal equilibrium is reported. The method is accurate and computationally efficient.
On the multiple-scattering-cluster model of the transition metal atoms isolated in rare gas matrices
Authors: Braga M. , Taft C. A.
Published: July 2002

Spin-polarized multiple-scattering Xα calculations have been performed for Mn Ar12 and Mn Ar6 clusters in order to describe the substitutional and interstitial Mn atom trapped in a crystalline Ar matrix. The calculated spin densities are used to interpret the experimental ESR hyperfine parameters for both the neutral Mn atom and its positive ion. Our results for the isolated neutral atom are in good agreement with the experiment, however, they are completely different in the case of the Mn+ ion. Some explanations for this discrepancy are considered and a critical discussion of the capabilities and limitations of the multiple-scattering-cluster model applied to impurities in van der Waals solids is also given.
The UV-visible emission spectra of oxygen and nitrogen in solid hydrogen and deuterium hosts
Authors: Brooks Robert L.
Published: July 2002

Spectra in the wavelength range of 300–600 nm have been acquired during proton-beam irradiation of samples of solid hydrogen and deuterium at 4.2 K. Emission spectra from the transitions A′ 3Δu→X 3Σ−g of O2, 4S–2D of N i, and 1D–1S of O i have been measured and attributed to an air impurity in the sample. Comparison is made of the vibrational constants derived from the molecular bands, the spectral shifts of all features, and the lifetime of the 2p3 2D multiplet of N i obtained in the solid hydrogens with those obtained by matrix isolation studies using N2 and rare gas hosts.
Experimental studies of the vapor phase nucleation of refractory compounds. III. The condensation of silver
Authors: Nuth Joseph A. , Donnelly Kimberly A. , Donn Bertram , Lilleleht Lembit U.
Published: July 2002

Measurements are reported of the critical supersaturation necessary to initiate the vapor phase nucleation of silver over the ambient temperature range 500 K≤T≤950 K at total pressures of 250 and 500 Torr of hydrogen. The results are compared with the predictions of both classical nucleation theory and the theory of a physical spinodal developed by Rasmussen, neither of which adequately describe our observations. The nucleation of silver appears to be dependent on the total pressure of the system in a manner which suggests that three-body reactions are necessary in order to form the initial silver clusters (e.g., dimers) from the monoatomic vapor.
Surface-enhanced Raman scattering from p-nitrobenzoic acid
Authors: Roth P. G. , Venkatachalam R. S. , Boerio F. J.
Published: July 2002

The surface-enhanced Raman scattering (SERS) from thin films formed by p-nitrobenzoic acid (PNBA) adsorbed onto silver island films has been investigated. SERS spectra obtained using low laser powers were very similar to the normal Raman spectra of the sodium salt of PNBA and were characterized by strong bands near 1600 and 1355 cm−1 and by weaker bands near 1395, 1115, and 875 cm−1. The band near 1395 cm−1 was assigned to the symmetric stretching mode of carboxylate groups, indicating that PNBA was adsorbed as a metal salt. When PNBA films were irradiated at high laser powers, a rapid reaction occurred. The bands near 1355 and 1115 cm−1 gradually decreased in intensity and a strong band near 1460 cm−1 and a strong doublet near 1150 cm−1 gradually appeared. The band near 1460 cm−1 and the doublet near 1150 cm−1 were attributed to azodibenzoate formed by the reductive coupling of PNBA molecules at the silver surface during laser irradiation. When adsorbed PNBA films were irradiated at low laser powers, the reaction still occurred but at a much lower rate. Reduction of PNBA was probably thermally induced but a photochemical mechanism may also be possible.
Classical differential cross sections in surface scattering
Authors: Horn T. C. M. , Kleyn A. W. , Gislason E. A.
Published: July 2002

A formula for the differential cross section for classical scattering of structureless particles from a solid surface is derived. In this process energy can be exchanged, so that a 3×3 Jacobian is needed for the description. Using this derived differential cross section, the definition of rainbows is reconsidered. For elastic scattering from a hard wall it is then proven that a rainbow occurs if and only if a catastrophe occurs. Examples of rainbows are given corresponding to several kinds of roots of the Jacobian, including ones where no individual elements of the Jacobian are zero. Finally, an example of an analytic expression for a cross section for inelastic scattering is presented.
Quasielastic hot-electron transport in solid N2 films
Authors: Keszei E. , Jay-Gerin J.-P. , Perluzzo G. , Sanche L.
Published: July 2002

Low-energy (0–20 eV) electron transmission experiments on thin (0–6.5 nm) solid molecular nitrogen films deposited at 17 K on a platinum substrate are reported. A theoretical model, based upon surface scattering with various angular distributions and quasielastic isotropic bulk scattering, is described to analyze the results in the energy range 2.5–7 eV relative to vacuum. Electronic conduction-band densities of states and electron scattering mean free paths are extracted by fitting the model to experimental data.
Consistency check for radial distribution functions of water
Authors: Lie George C.
Published: July 2002

From a relation between the thermal compressibility and radial distribution function, it is shown that the accuracy of the generally accepted Narten and Levy’s experimental radial distribution functions (RDFs) for water is very low, compared with newer experimental results. The most recent experimental RDFs obtained by Soper and Phillips not only pass the consistency check derived, but also have the best overall agreement with the simulated results of Lie and Clementi.
Electronic structures of H2O⋅BF3 and related n–v addition compounds: A combined EELS–UPS study in vapor phase
Authors: Durrant M. C. , Hegde M. S. , Rao C. N. R.
Published: July 2002

The detailed electronic structure of the n–v addition compound H2O⋅BF3 has been investigated for the first time by a combined use of electron energy loss spectroscopy (EELS) and UV photoelectron spectroscopy (UPS) augmented by MO calculations. The calculated molecular orbital energies of H2O⋅BF3 agree well with the UPS results and have been used to assign the electronic transitions obtained from EELS and to construct an orbital correlation diagram.
A molecular representation of Al3++H charge transfer reactions
Authors: Kirby K. , Heil T. G.
Published: July 2002

The charge transfer reaction Al3++H→Al2++H+ has been studied within a molecular representation appropriate for slow to intermediate velocity collisions. Ab initio electronic potential energy curves were computed for the lowest four 2Σ+ states of AlH3+, which dissociate to the limits Al3++H and Al2+(2S,2P0,2D)+H+. The configuration interaction wave functions were constructed to represent the four states in an unbiased fashion yielding asymptotic energy splittings within 1% of experimental values. Radial and dipole coupling matrix elements were obtained as a function of internuclear separation. Two formal criteria are proposed to evaluate the accuracy of the radial coupling elements which, in this case, were obtained through the Hellman–Feynman theorem. The nuclear scattering equations were solved in the semiclassical straight line trajectory approximation for the range of barycentric collision energies 0.01–1.0 keV/amu.
On the possibility of using differential cross section measurements for the electronic excitation of adsorbates by an electron beam, to determine the adsorbate orientation
Authors: Nagano Seido , Luo Zi-Ping , Metiu Horia , Huo Winifred M. , Lima Marco A. P. , McKoy Vincent
Published: July 2002

We show, by detailed electron–molecule scattering calculations, that the angular dependence of electron energy loss spectra in which an adsorbate is electronically excited can be used to identify the orientation of the molecule with respect to the surface and the nature of the final states. The calculations are exploratory and were carried out for an H2 molecule. The transition amplitude for electron–molecule scattering is calculated by using the Schwinger variational principle with two open channels. The effects of the surface were introduced through a semiquantitative model which treats the surface as a partly reflecting, flat mirror.
On the equivalence of the normal and local mode representations
Authors: Kellman Michael E.
Published: July 2002

The local modes Hamiltonian is rewritten exactly in terms of coupled anharmonic normal modes. Reexpressed in this way, the Hamiltonian contains leading-order 2:2 Darling–Dennison and 1:2 resonances, and terms corresponding to higher-order resonances in the highly anharmonic regime.
Crystal structures and phase transition of the cyanospinel K2Hg(CN)4
Authors: Gerlach P. N. , Powell B. M.
Published: July 2002

The crystal structures of the high temperature (paraelastic) and low temperature (ferroelastic) phases of the cyanospinel K2Hg(CN)4 have been determined by neutron powder diffraction. At 298 K the crystal has the spinel structure and the cyanide coordination is ordered in the sequence N–C–Hg–C–N. Below the phase transition (111 K) the structure is trigonal with space group R3̄c. At the transition Hg(CN)4 tetrahedra adjacent along [111] rotate in antiphase as rigid units by ≊7° and the K ion shifts by ≊0.1Å. The transition has an ordering wave vector q0≡{1/2 1/2 1/2}. Comparison is made with the structural characteristics of other cyanospinels and it is suggested that the transitions cannot be understood in terms of steric interactions but must be a dynamic effect.
Equality of two different formulas for the surface tension: A direct proof
Authors: Waldor Michael H. , Wolf Dietrich E.
Published: July 2002

We prove the equivalence of the ‘‘Triezenberg–Zwanzig’’ and the ‘‘Kirkwood–Buff’’ formulas for the surface tension of a liquid–vapor interface without referring to the dynamical properties as previous proofs did.
Recurrence relations for the expansion of Slater-type orbitals about displaced centers
Authors: Rico J. Fernández , López R.
Published: July 2002

We show here that the coefficients for the expansion of any Slater-type function over a displaced center can be obtained by three simple (and numerically stable) recurrence relations starting at the expansion coefficients of the 1s or 2s orbitals. We present also compact (and very fast to compute) formulas for these initial quantities.
Spectra and structure of small ring molecules. XLVII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid fluoride
Authors: Durig J. R. , Badawi H. M. , Bist H. D. , Little T. S.
Published: July 2002

The microwave spectrum of cyclobutylcarboxylic acid fluoride, c-C4H7CFO, has been recorded from 18.0 to 40.0 GHz. The a-type R-branch transitions have been observed and assigned for the ground and two vibrationally excited states of the asymmetric torsional mode as well as for the first vibrationally excited state of the ring puckering fundamental. The rotational constants were determined from the frequency fit to a rigid rotor model for the ground vibrational state to be: A=5467.35±0.03, B=1884.76±0.01, and C=1558.64±0.01 MHz. These constants are shown to be consistent with an equatorial-gauche conformation (i.e., the CFO group is in the equatorial position relative to the ring and the C=O bond is eclipsing, or nearly so, one of the C–C bonds of the ring). From relative intensity measurements the frequency for the asymmetric torsion for this conformer is estimated to be 72±10 cm−1. From the Stark effect the dipole moment components were determined to be: ‖μa‖ =2.97±0.02, ‖μb‖ =0.84±0.01, ‖μc‖ =0.35±0.01, and ‖μt‖ =3.11±0.01 D. The central line of an additional conformer has been identified with a B+C value of 3482 MHz which is consistent with the expected value for the equatorial-trans conformer. The infrared (3500 to 30 cm−1) and Raman spectra (3200 to 30 cm−1) have also been recorded for the gaseous and solid states of cyclobutylcarboxylic acid fluoride.

Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. From the relative intensities of the Raman lines of the gas at 918 cm−1 (equatorial-trans) and 926 cm−1 (equatorial-gauche) as a function of temperature, the enthalpy difference was found to be 352±100 cm−1 (1.01 kcal/mol) with the equatorial-gauche being more stable. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. These results are compared to similar quantities for some related molecules.
Vibrational study of m
e
t
a-dinitrotetramethylbenzene single crystal. Infrared reflectance and transmission and polarized Raman spectra
Authors: Cataliotti Rosario Sergio , Paliani Giulio , Sorriso Salvatore
Published: July 2002

Polarized IR and Raman spectra have been measured for meta-dinitrotetramethylbenzene single crystals. Using different scattering and polarization geometries in the Raman effect and polarized reflectance and transmission infrared spectra, all the polarizability tensor matrix elements and electric dipole moment variations have been determined for the monoclinic crystal of this molecule. Lifting of degeneration, at the Brillouin zone center, of the longitudinal and transverse components of the infrared phonons have been detected and attributed to long range electrostatic interactions of the molecules in the unit cell.
Effective-range theory in two dimensions
Authors: Adhikari S. K. , Gibson W. G. , Lim T. K.
Published: July 2002

A parametrization of low-energy two-dimensional quantum scattering is given which is in the tradition of familiar three-dimensional effective-range theory. A scattering length differing from previous treatments is introduced whose behavior reflects the repulsive or attractive nature of the interaction, and whether or not bound states are present. However its definition is subject to an unknown length parameter. An effective-range parameter is also defined and together with the scattering length explains the nonexistence of the Efimov effect in two dimensions. It is also shown that the Ramsauer–Townsend effect exists in 2D. Application to the case of a square-well potential reveals direct relationships between the unknown length parameter and the effective range to the actual range of the potential.
The vibrational relaxation of DCl(v=1) in liquid xenon and HCl(v=1) in liquid krypton
Authors: Krueger Herman , Knudtson J. Thomas , Vlahoyannis Yannis P. , Weitz Eric
Published: July 2002

The relaxation rate constants for DCl(v=1) in liquid xenon at 211 K and HCl(v=1) in liquid krypton at 190 K are reported. Both the DCl–DCl and HCl–HCl rate constants are similar to the corresponding gas phase rate constants at the same temperature, and the relaxation appears amenable to an isolated binary collision (IBC) description. As in the gas phase, the rate constant for DCl–DCl deactivation in liquid xenon is found to be smaller than that for HCl–HCl relaxation in liquid xenon, despite the smaller energy defect for the DCl relaxation process. This suggests that rotational degrees of freedom are also involved in the liquid phase vibrational relaxation of these molecules. The observed relaxation rates for DCl(v=1) by liquid xenon and HCl(v=1) by liquid krypton are found to be within an order of magnitude of values estimated from various forms of the IBC model. More careful comparisons await values of the appropriate gas phase relaxation rate constants near 200 K. Finally, certain features of the mechanism for production of vibrationally excited HCl and DCl are considered.
Chemiluminescent reactions of second-row atomic ions. I. Al++H2→AlH+(A 2Π, B 2Σ+)+H
Authors: Müller B. , Ottinger Ch.
Published: July 2002

Light emission from the impact of Al+(1S,3P) ions on H2 molecules at 5.2 to 13.8 eVCM was spectrally analyzed. It was found to consist of two band systems. Of the known A 2Π–X 2Σ+ transition, numerous new bands were observed, allowing a first experimental determination of vibrational constants. In addition, a predicted B 2Σ+–X 2Σ+ system was observed for the first time. Relative emission cross sections for the two systems and for the 1S and 3P reactant species were measured. They parallel closely the trends in the isovalent system B++H2. One important difference is, however, the existence of a large (∼3.5 eV) activation barrier in the dominant Al++H2 reaction channel. This is explained in terms of adiabatic potential hypersurfaces.
Resonance ionization photoelectron spectroscopy of lanthanide elements
Authors: Donohue D. L. , Young J. P. , Smith D. H.
Published: July 2002

The photoelectron spectra of Sm, Nd, and Pr have been measured following laser-induced resonance ionization. The results obtained shed light on unexplained ionization processes observed by many workers in the field of resonance ionization mass spectrometry. Electron energy spectra can provide information about the initial energy level in a multiple-photon sequence leading to ionization. Also present is information about the low-lying energy levels of the atomic ions produced. A number of identified and unknown processes are presented for these three elements along with the results of power dependence measurements.
Incipient triple points in films on lamellar halides
Authors: Niskanen K. J.
Published: July 2002

I point out that the results of a recent vapor pressure isotherm study of noble gas monolayers physisorbed on several lamellar halides by Millot, Larher, and Tessier can be explained in terms of an incipient triple point in some of the films and the usual liquid–gas critical point in others, whereas a critical point in all of the films is not in agreement with model calculations. Lamellar halides would thus provide a system in which the crossover, presumably via a critical endpoint, between the two cases can be studied experimentally.
Molecular orientation on metal surfaces by electrostatic interactions: The adsorption of cyclopentene on a stepped (221) silver surface
Authors: Alvey Mark D. , Kolasinski Kurt W. , Yates John T. , Head-Gordon Martin
Published: July 2002

Step defects at metal surfaces are known to cause a local dipole at the surface, thereby creating an enhanced surface electric field in their vicinity. Using the electron stimulated desorption ion angular distribution (ESDIAD) technique, cyclopentene is observed to be ordered by the interaction of its permanent dipole moment with the electrostatic field at the steps on the Ag (221) surface. This ordering is not seen for cyclopentene on Ag (111). The experimental results agree with estimates of the interactional energy of the dipole with the electric field at the steps, and the direction of orientation is consistent with calculations of the sign of the cyclopentene dipole moment.
Bistable limit cycle oscillations in chemical systems. II. Mechanisms for noise-induced transitions
Authors: Celarier Edward , Kapral Raymond
Published: July 2002

External noise coupled to Rössler’s model of a nonlinear chemical system is considered, and noise-induced transitions between coexisting three-looped limit cycles are studied. Two mechanisms account for these transitions. These are analogs of mechanisms which have been identified for the noise-induced transitions of a stochastic, discrete-time dynamical system. The effects of changing the statistical properties of the external noise process are also investigated. The dynamical response of the system has a straightforward interpretation in terms of the structure of the basins of attraction. The correlation time of the noise process is one important determinant of the system’s response to the fluctuations. In particular, if the correlation time is short, compared to the characteristic time for the oscillations of the system, the analogy with the discrete-time mechanisms for transitions breaks down.
Velocity modulation laser spectroscopy of negative ions: The infrared spectrum of hydrosulfide (SH−)
Authors: Gruebele Martin , Polak Mark , Saykally Richard J.
Published: July 2002

Twenty-five transitions in the v=0→1 and v=1→2 bands of 32SH−, as well as in the fundamentals of 33SH− and 34SH− have been measured with a diode laser by the velocity modulation technique. The data set was fit to an effective Hamiltonian through sextic terms to determine equilibrium molecular parameters. Comparison with the neutral SH radical shows a strong similarity in the anion and neutral molecular constants, as in the case of OH−. The agreement of experiment with ab initio calculations is found to be quite good. The rotational and vibrational temperatures of the species were estimated to be 360±50 and 960±200 K, respectively.