bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation functions in the capillary wave model of the liquid–vapor interface
Authors: Bedeaux Dick , Weeks John D.
Published: July 2002

We study the capillary wave model of the liquid–vapor interface and derive expressions valid in arbitrary dimensions for the density profile and the density–density pair correlation function. The latter is inverted to give an explicit expression for the direct correlation function. We examine the effects of long wavelength density fluctuations on these functions in dimensions d<3 and discuss the applicability of the model to a real liquid–vapor system.
An EXAFS study of the structure of the metal–support interface in highly dispersed Rh/Al2O3 catalysts
Authors: van Zon J. B. A. D. , Koningsberger D. C. , van’t Blik H. F. J. , Sayers D. E.
Published: July 2002

Four highly dispersed and fully reduced rhodium on alumina catalysts with different particle sizes in the range 6–12 Å were investigated with the EXAFS technique in order to derive information about the structure of the metal–support interface. This information can only be obtained when the signal-to-noise ratio of the experimental EXAFS data is high enough and accurate reference compounds and a modified way of data analysis are used. With the aid of phase and amplitude corrected Fourier transforms it was possible to detect a small additional signal which could be ascribed to a Rh–O bond. Since the catalysts were fully reduced and since the intensity of the small signal increased with decreasing particle size, the oxygen neighbor was assigned to be originated from the metal–support interface. From the intensity of the Rh–O bond it was estimated that, on the average, each interfacial rhodium atom is surrounded by 2–3 oxygen ions of the support. The detected Rh–O bond has a coordination distance of 2.7 Å which is about 0.6 Å larger than the first coordination distance in Rh2O3 (2.05 Å). The coordination distance of 2.7 Å can be explained by assuming an interaction between metallic rhodium (atomic radius 1.34 Å) and ionic oxygen belonging to the support (ionic radius 1.4 Å). This would possibly imply an ion-induced dipole bonding between the metal particle and the support.
Two-photon excitation spectroscopy and excited state decay kinetics of isolated diphenylbutadiene
Authors: Horwitz James S. , Kohler Bryan E. , Spiglanin Thomas A.
Published: July 2002

We report the two-photon excitation spectrum and decay kinetics of the two-photon prepared states for diphenylbutadiene in a helium free jet. The spectrum is that of a symmetry allowed two-photon transition and is assigned using 17 harmonic fundamentals. A reexamination of the previously reported one-photon excitation spectrum allows the assignment of several additional u symmetry fundamentals of diphenylbutadiene’s excited 1Ag state. Curves of intensity vs time for light detected at two times the wave number frequency of the laser minus a Stoke’s shift of between 1000 and 9000 cm−1 show two components. One, due to two-photon resonance enhanced hyper-Raman scattering, reproduces the temporal profile of the exciting laser. The other, due to fluorescence from the excited 1Ag state, decays exponentially. The fluorescence lifetime decreases with increasing excitation frequency from 100 ns for 0–0 band excitation to 13 ns for excitation at 1235 cm−1 above the origin, consistent with the onset of a nonradiative decay channel previously observed for diphenylbutadiene in one-photon studies. Above this energy only hyper-Raman scattering is detected.
Fluorescence lifetimes and Zeeman quantum beats of single rotational levels in 3B2 carbon disulfide
Authors: Loge Gary W. , Tiee J. J. , Wampler F. B.
Published: July 2002

The lowest observed triplet state of carbon disulfide, the R 3B2 state, was directly excited using a pulsed, narrow bandwidth dye laser (0.05 cm−1). The first reported collision-free lifetimes of this state of CS2 were obtained, and the changes in lifetime for single rotational and vibrational levels were observed. The effect of rotation and vibration on the lifetimes indicates that the R 3B2 state has strong spin-orbital coupling with the higher lying V 1B2 state, which gives the R 3B2 state such short lifetimes (2–5 μs) for a triplet state. A magnetic field was observed to have no effect on the lifetimes, which is consistent with this strong spin-orbital coupling scheme. Zeeman quantum beats were observed on the fluoresence decay curves in a magnetic field. Beat frequencies were obtained as a function of magnetic field strength to obtain permanent magnetic dipole moments for the R 3B2 state vibrational levels. The magnetic dipole moments were found to be independent of rotational angular momentum (N dependence) indicating a strong spin-orbital coupling, consistent with the lifetime data. The collisional depolarization rate was qualitatively observed to be competitive with and slightly faster than electronic quenching.
The stabilities of the hydrogen fluoride trimer and tetramer
Authors: Liu Shi-Yi , Michael Daniel W. , Dykstra Clifford E. , Lisy James M.
Published: July 2002

A b initio calculations have been carried out to determine the dissociation pathways for the hydrogen fluoride trimer and tetramer that are accessible with single photon excitation of an HF stretching vibration. This requires evaluation of the stabilities of the small HF clusters. Electron correlation effects are found to contribute 1.0 kcal to the stability of the trimer, compared to 0.5 kcal for the dimer. The equilibrium stabilities with respect to dissociation into monomers are 4.6 kcal for the dimer, 14 kcal for the trimer, and 25 kcal for the tetramer. With zero-point vibrational effects, single photon vibrational excitation is sufficient for dissociation of the trimer into three monomers. The orientational structural parameters of the trimer and tetramer, like those of the dimer are found to be well explained by dipole and quadrupole moment interactions.
An HTP kinetics study of the reaction between ground-state H atoms and NH3 from 500 to 1140 K
Authors: Marshall Paul , Fontijn Arthur
Published: July 2002

The H+NH3 reaction has been investigated using the high-temperature photochemistry (HTP) technique. H(1 2S) atoms were generated by flash photolysis of NH3 and monitored by time-resolved atomic resonance fluorescence with pulse counting. The rate coefficient for 660≤T≤1140 K is given by k(T)=(5.7±2.8)×10−10 exp[(−8650±410)K/T] cm3 molecule−1 s−1, where the uncertainties represent one standard deviation based on a propagation of error treatment including systematic errors. The Arrhenius plot reveals curvature between 500 and 660 K which follows the results of quantum mechanical tunneling calculations based on transition state theory and an Eckart potential.
Convergence of Magnus and Magnus-like expansions in the Schrödinger representation
Authors: Salzman W. R.
Published: July 2002

General and greatly simplified methods are presented for calculating terms in the exponent in Magnus and Magnus-like expansions to first order in a ‘‘small’’perturbation and to infinite order in the overall Hamiltonian in the Schrödinger representation. These techniques are applied to four simple but important models and it is shown that in each case the Magnus exponent diverges for some range of the parameters of the model. This result casts serious doubt on the utility of the Magnus expansion in the Schrödinger representation. Several of the problems are also treated in the interaction representation giving results which converge throughout the useful range of parameters. There is no evidence that a similar question exists in this representation.
Statistical mechanics of solutions of semiflexible chains: A path integral formulation
Authors: Bhattacharjee Somendra M. , Muthukumar M.
Published: July 2002

A path integral formulation has been used to study the behavior of a single chain in the solution of many semiflexible chains. The Flory–Huggins approximation on lattices suggests that the chains undergo a phase transition from a high temperature isotropic phase to an anisotropic phase where the chains are rod-like. Using a self-consistent approximation, we, however, find that, at any stiffness, long chains (in the limit of the chain length going to infinity) remain in the disordered state.
The laser induced fluorescence spectrum and radiative lifetime of PO2
Authors: Hamilton Peter A.
Published: July 2002

The ultraviolet spectrum of PO2 first observed in absorption by Verma and McCarthy has been studied by laser induced fluorescence for the first time. The spectra are similar in many respects to those observed in the visible system of NO2 and no predissociation is observed. The loss of rotational structure is attributed instead to mixing with one or more near continuous ‘‘background’’ states, with the amount of mixing apparently related to excitation of the bending vibration. The radiative decays are observed to be nonexponential but are accurately described by a double exponential form. This gives collision free radiative lifetimes of about 0.5 and 4.5 μs for the structured and continuous states, respectively, with the effective lifetime of the structured state varying strongly with the amount of mixing. Collisional quenching rate constants are roughly constant at (6±1)×10−10 and (4±1)×10−11 cm3 molecule−1 s−1 for the two states, with the very rapid quenching rate of the structured state probably being for nonradiative transfer to the background continuum. From observations of the wavelength dependence of the fluorescence the ground state ν2 and ν1 frequencies are found to be about 387 and 1117 cm−1, respectively. The emission is very extensive and strongly red shifted and lends further evidence that these states of PO2 are responsible for the chemiluminescence observed in phosphorus/oxygen reactions.
Structure and properties of polyaniline as modeled by single-crystal oligomers
Authors: Shacklette L. W. , Wolf J. F. , Gould S. , Baughman R. H.
Published: July 2002

A single-crystal charge–transfer complex of a phenyl-end-capped tetramer of polyaniline has been synthesized and studied along with a similar dimer of polyaniline. Structural, optical, and electrochemical studies of these oligomers in various oxidation states provide detailed information which has been used to model the structure of polyaniline and its evolution during electronic doping. These studies of the polymer and its oligomers suggest that the emeraldine salt form of the polymer (50% doping per nitrogen) is a preferred low-energy structure. The preference for this structure leads to phase segregation in doped compositions having average doping levels less than 50%.
Concentration fluctuation of stiff polymers. II. Dynamical structure factor of rod-like polymers in the isotropic phase
Authors: Doi Masao , Shimada Takayuki , Okano Koji
Published: July 2002

The dynamical structure factor of the solution of rod-like polymers is calculated by using a newly proposed kinetic equation. The initial decay rate of the structure factor is obtained as a power series of the scattering vector k. It is found that (i) the coefficient of k2, which gives the cooperative diffusion constant, increases with concentration due to the excluded volume of rod and (ii) the coefficient of k4 changes sign from positive to negative. These results agree qualitatively with experiments.
High resolution absorption spectrum of the molecular eigenstates of pyrazine
Authors: van Herpen W. M. , Uijt de Haag P. A. M. , Meerts W. Leo
Published: July 2002

A method is demonstrated to measure high resolution absorption spectra, in the near ultraviolet region, with a bolometer in a molecular beam. The high resolution absorption spectrum of the P, Q, and R branches of the 1B3u (0–0) electronic transition of pyrazine was recorded. The observed residual Doppler linewidth is reduced to 50 MHz in the near ultraviolet region. Simultaneously with the absorption spectrum, the laser induced fluorescence spectrum was observed. A comparison is made between excitation and absorption intensities of the molecular eigenstate spectrum. It is concluded that substantial differences exist between both spectra. The intensities are not proportional, which is most clearly visible in the Q branch. This confirms earlier time resolved measurements and low resolution absorption spectra. However, the quantum yield of various, closely spaced molecular eigenstates is in the same order of magnitude. This observation justifies the approximation of absorption intensities by excitation intensities in some work.
Order parameter and packing studies in nematic and smectic A phases by x-ray diffraction
Authors: Haase W. , Fan Z. X. , Müller H. J.
Published: July 2002

X-ray diffraction measurements have been made on the nematic phase of T15 (4-cyano-4′-n-pentyl-p-terphenyl), as well as on the nematic and smectic A phases of RNO2 [4′-nitrophenyl-4-(4′-n-hexyloxy-benzoyloxy)-cinnamate]. The x-ray equipment used allows comparably high resolution measurements of liquid crystals. The inner reflexes show that both compounds possess a bilayer structure supposed as overlapping of polar cyano or nitro group of one molecule and the phenyl ring of the neighboring molecule. The molecular orientational distribution function f (β) has been calculated on the basis of intensity simulations of the outer reflexes. The orientational order parameter P2(cos β), P4(cos β), 〈P2〉 and 〈P4〉 have been determined from f (β). A strong decrease of 〈P2〉 and 〈P4〉 is observed at the smectic A–nematic phase transition temperature of RNO2.
Infrared and Raman spectra of t
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a
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s-1,3,5,7-octatetraene and normal-coordinate analysis based on a
b i
n
i
t
i
o molecular orbital calculations
Authors: Yoshida H. , Tasumi M.
Published: July 2002

The infrared and Raman spectra of trans,trans-1,3,5,7-octatetraene have been observed. It has been found that the spectral data reported in 1959 for this compound contain bands due to either impurities or geometrical isomers other than the trans,trans form. Band assignments are performed by using the results of ab initio molecular orbital (MO) calculations with the 6-31G basis set. Important features of the vibrational modes and vibrational force field are discussed in some detail.
Adiabaticity and nonadiabaticity in bimolecular outer-sphere charge transfer reactions
Authors: Beratan David N. , Onuchic José Nelson
Published: July 2002

A model for outer-sphere bimolecular electron transfer reactions is presented which is based on a rate expression correct in the adiabatic, nonadiabatic, and intermediate dynamical regimes for an overdamped solvent coordinate. A complete discussion of the time-scale separations needed to define a bimolecular rate for these electron transfer reactions is given. Bimolecular reaction rates are predicted to display distinct solvent and electronic coupling dependencies, defined by a few experimentally variable parameters, which we now identify. Also, a significant intermediate dynamical regime is predicted to exist in which the rate is sensitive to both electronic coupling and nuclear relaxation. Evidence that both extreme dynamical regimes have been accessed experimentally is recalled, and strategies are presented for fully mapping out the dependence of rate on the electronic coupling and relaxation times.
The benzene ground state potential surface. IV. Discrimination between multiple E1u force field solutions through infrared intensities
Authors: Goodman Lionel , Ozkabak Ali G. , Wiberg Kenneth B.
Published: July 2002

Accurate values for integrated intensities of the infrared active 13C6H6 fundamentals, ν18, ν19, ν20, and ν11 (Wilson notation) have been measured and redetermined for ν18 and ν19 in C6H6 and C6D6. The 13C6H6 intensities are I18=6.52±0.15, I19=12.60±0.20, I20=55.6±1, and I11=74.6±3 km/mol. Unlike C6H6 and C6D6, interfering transitions in 13C6H6 are minor and these intensities can be used as a critical test for theoretical predictions of atomic polar tensors. The ν18 intensities in C6H6 and C6D6 (7.48±0.15 and 7.09±0.14 km/mol, respectively) and the ν19 intensity in C6D6 (2.51±0.12 km/mol) are measured to be substantially lower than the literature values. The qualitative intensity pattern of benzene in-plane fundamentals uniquely discriminate among the eight possible real E1u force field solutions obtained from frequency information alone. Isotopically invariant dipole moment derivatives, ∂μ/∂S18a, ∂μ/∂S19a, and ∂μ/∂S20a are 0.494±0.005, 0.395±0.016, and 0.770±0.008 D/Å, respectively, obtained from the 13C6H6 experimental intensities and the complete experimental force field of Part II. Using these quantities and the L−1 matrix (Table III), dipole moment gradients for C6H6 become ∂μ/∂Q018a =+0.298, ∂μ/∂Q019a =+0.371, and ∂μ/∂Q020a =+0.814 D/Å. Mode decomposition matrices expressing normal modes of benzene in terms of isotopically labeled molecule modes have been used to definitively determine the C6H6 dipole gradient signs. The signs are in agreement with theoretical calculations. The D6 isotopic labeling effect on C6H6 ν18 intensity provides a sensitive test of E1u force field quality and reveals the inadequacy of present theoretical force field approaches. Ab initio atomic polar tensors have been obtained both at the HF level, using several basis sets up to the 6-311+G(d,p) and at the

MP2 level up to the 6-31+G(d) basis set. The dipole derivative for the CC stretch is highly sensitive to both basis set (particularly diffuse functions) and correlation effects. Qualitative CH and CC stretching dipole derivative and intensity predictions by the MP2/6-31+G(d) calculation are encouraging (i.e., within 15% of the experimental values). However, the same calculation yields 20% and 45% errors for the CH bending dipole derivative and fundamental intensity, respectively.
Enhanced energy transfer between donor and acceptor molecules near a long wire or fiber
Authors: Hua X. M. , Gersten J. I.
Published: July 2002

A theory is developed for the energy transfer between donor and acceptor molecules near a long metallic wire or dielectric fiber. The theory is valid for arbitrary wave lengths. Interference effects are noted and discussed.
Spectroscopic characterization of the ã 3Π−, b̃ 3Π2, and E 3Σ+ states of ZnAr
Authors: Bennett Robert R. , Breckenridge W. H.
Published: July 2002

The a 3Π0− , b 3Π2, and E 3Σ+ states of ZnAr have been characterized spectroscopically. The ZnAr 3ΠΩ van der Waals molecules were synthesized in a supersonic jet from metastable 3PJ Zn atoms produced by the laser vaporization of a Zn rod. High resolution rotational spectra of 11 vibrational bands were analyzed to obtain the spectroscopic constants. The ZnAr 3ΠΩ states were found to be intermediate between Hund’s cases (a) and (c), but closer to case (a). Spin–orbit calculations predict the location of the unseen 3Π0+,1 bound and 3Σ+0−,1 repulsive states. The Zn(4s5s)Ar(3Σ+) state is even more strongly bound than the analogous Hg(6s7s)Ar(3Σ+) state, and it is postulated that the Ar atom is inside the outer maximum of the Rydberg Zn(5s) orbital.
On the nature of dense CO adlayers
Authors: Persson B. N. J. , Tüshaus M. , Bradshaw A. M.
Published: July 2002

We have performed Monte Carlo simulations in order to study the ordered structures formed by CO on Pt{111} at high coverage. The results are compared with LEED and infrared (IR) spectra. The calculations are based on a recently constructed potential energy surface for CO on Pt{111} and a CO–CO interaction potential deduced from the variation of the CO binding energy with coverage. Ordered adsorbate structures are obtained at θ=0.5, 0.6, 0.67, and 0.71 in the simulations. The so-called compression structures (θ>0.5) are stabilized by the energy lowering which results when CO molecules at the high density domain walls move away from the on-top sites because of the unbalanced repulsive CO–CO interactions. If this relaxation channel is blocked, disordered adsorbate structures occur. We present the resulting (θ, T) phase diagram and discuss its qualitative properties. The LEED data show ordered structures at θ=0.5, 0.6, and 0.71, but, in contrast to previous results, or perhaps to the interpretation thereof, not at θ=0.67. The IR data show that the compression structures still consist of CO molecules adsorbed on distinct surface sites. Finally, we discuss the changes in adsorbate structures which would result from variations in the CO–substrate potential energy surface and, in the light of these results, briefly look at the Cu{111}–CO, Ni{111}–CO, and Pd{111}–CO chemisorption systems.
Quenching of A 2Σ+ OH at 300 K by several colliders
Authors: Wysong Ingrid J. , Jeffries Jay B. , Crosley David R.
Published: July 2002

Collisional quenching of the v′=0 level of the A 2Σ+ state of the OH molecule has been studied for a variety of collision partners. The pressure dependence of time-resolved, laser-induced fluorescence furnishes the quenching cross sections σQ. OH radicals are produced in a microwave discharge or by photolysis of HNO3 at 193 nm, always in sufficient Ar bath to produce a thermal rotational population at 300K in the laser-excited A 2Σ+ state. For Kr and Xe, the σQ are 8 and 27 Å 2, respectively; comparison with a prior study suggests a decrease in σQ with increasing rotational level and/or increasing temperature. σQ (O2)=18 Å 2,and σQ (H2O)=80 Å 2; cross sections were measured for selected freons and butanes also important in tropospheric laser measurements of OH.
Theoretical investigation of model polymers for eumelanins. II. Isolated defects
Authors: Galvão D. S. , Caldas M. J.
Published: July 2002

We investigate here defects in initially ordered polymers of 5,6-indolequinone, in one or other redox form. The defects studied include aggregation of the carboxyl radical into one skeleton monomer, the aggregation of a host monomer in a lateral misplaced position, and faults in the polymerization sequencing. Our study is carried out, as in the first paper I, through Hückel π-electron theory, and results are compared to the perfect structures in I. Our results indicate that the end-type defect suggested as an electron capture center in I is not deactivated by these other structural defects, and that new capture centers might be introduced that could also be responsible for the acceptor behavior of melanins.
A critical examination of data on the dissociative adsorption and associative desorption of hydrogen at copper surfaces
Authors: Michelsen H. A. , Auerbach D. J.
Published: July 2002

We examine experimental data on the activated dissociative adsorption and associative recombination of hydrogen at copper surfaces with respect to the role played by molecular vibrational states. We use models describing the variation of the adsorption probability with the vibrational state, kinetic energy, and angle of the incident molecules, establishing the parameters of the models by a nonlinear least squares fit to adsorption data. Using the principle of detailed balance, we apply these models to the corresponding data on associative desorption thus comparing adsorption data with desorption angular, velocity, and internal state distributions. The most consistent picture resulting from this analysis is that the adsorption has significant contributions from both H2(v=0) and H2(v=1) and that these components have markedly different translational thresholds. Within the framework of this picture we are able to resolve the apparent contradiction between the strong angular dependence of the kinetic energy required for adsorption and the lack of angular dependence of the mean kinetic energy of desorption. We also partially resolve the apparent discrepancy in interpretation of the role of H2(v=1) in recent adsorption experiments.
Binary nucleation in acid–water systems. I. Methanesulfonic acid–water
Authors: Wyslouzil B. E. , Seinfeld J. H. , Flagan R. C. , Okuyama K.
Published: July 2002

Experimental measurements of binary nucleation between methanesulfonic acid and water vapor were carried out for relative acidities (Ra), 0.05<Ra<0.65, and relative humidities (Rh), 0.06<Rh<0.65, using a continuous flow mixing-type device. The number concentration of particles leaving the nucleation and growth tube was measured as a function of the initial relative humidity and the relative acidity in the temperature range from 20 to 30 °C. Particle size distributions were also measured and found to vary with the amount of water and acid present. The system was simulated to predict the total number of particles and the total mass of acid in the aerosol phase using a simple integral model and classical binary nucleation theory allowing for the formation of acid–water hydrates in the gas phase. At low particle concentrations, condensation rates did not significantly change the saturation levels and the nucleation rates were estimated from the total number concentration data as functions of Ra, Rh, and temperature. The values of experimental and theoretical nucleation rates differed significantly, with Jexpt/Jtheor changing as a function of temperature from 10−8 to 10−4 as temperature varied from 20 to 30 °C. This work represents the first systematic experimental study of the temperature dependence of binary nucleation.
Desorption kinetics of hydrogen from the Si(111)7×7 surface
Authors: Reider G. A. , Höfer U. , Heinz T. F.
Published: July 2002

Thermal desorption of hydrogen from the Si(111)7×7 surface was investigated using optical second-harmonic generation to monitor the hydrogen coverage from 0.2 monolayer to below 0.01 monolayer. The results of isothermal desorption measurements are found to be compatible neither with simple first nor second-order kinetic behavior. It is suggested that different binding sites available for Si–H monohydride states on the Si(111)7×7 surface give rise to the apparent intermediate reaction order.
A Monte Carlo study of surface diffusion coefficients in the presence of adsorbate–adsorbate interactions. I. Repulsive interactions
Authors: Uebing C. , Gomer R.
Published: July 2002

Chemical, jump, and tracer diffusion coefficients, D, Dj, and D*, respectively, for the case of nearest-neighbor repulsive interactions have been determined by Monte Carlo simulations on a fixed square lattice. D was determined by the fluctuation autocorrelation function method and from the Kubo–Green equation, using values of 〈(δN)2〉/〈N〉 determined from isotherms. It was found that the two methods give essential agreement over the entire coverage and temperature range examined, 0.73≤T/Tc≤7.33, 0.1≤θ≤0.8. The changes in activation energies for D, D*, and Dj as function of T and θ were determined and can be understood in terms of the phase diagram of the system.
Mode-specific subpicosecond photodissociation dynamics of the methyl iodide B state
Authors: Wang P. G. , Ziegler L. D.
Published: July 2002

Vibronically specific, subrotational period lifetimes of the predissociated B state (201–190 nm) of CH3I and CD3I are determined by spontaneous resonance Raman scattering measurements of depolarization ratios and rotationally resolved Raman excitation profiles (REPs). The lifetime of the electronic origin, methyl rocking (ν6), and methyl umbrella (ν2) band are the same in CH3I (0.5 ps) and CD3I (1.2 ps). However, the predissociation rate decreases by a factor of ∼3 when one quantum of ν3, the methyl–iodine stretch, is excited on the B-state surface. The only vibrational mode which is found to accelerate the B-state predissociation rate is the a1 C–H (C–D) stretch. The lifetime of the ν1 vibronic level decreases by nearly an order of magnitude for CH3I (60 fs) and by a factor of 2 for CD3I (0.6 ps) relative to the corresponding electronic origin level. In addition, K-resolved REP analysis reveals predissociation rates to be K independent. These mode-specific predissociation rates are ascribed to the shape of the multidimensional reaction coordinate in the region of the curve crossing between the bound (B state) and unbound surfaces.
Ultraviolet photoelectron spectra of coinage metal clusters
Authors: Taylor K. J. , Pettiette-Hall C. L. , Cheshnovsky O. , Smalley R. E.
Published: July 2002

Ultraviolet photoelectron spectra (UPS) were recorded for mass-selected negative clusters of copper (1–411 atoms), silver (1–60 atoms), and gold (1–233 atoms), using photodetachment lasers at 6.4 and 7.9 eV photon energy. The results provide a direct estimate of the vertical electron affinity (EA) of these clusters and information on the evolution of the d bands of copper and gold as a function of cluster size. The large even/odd alternation of EA in small clusters of these metals in earlier work is found to largely disappear as the cluster size exceeds 40 atoms. The ellipsoidal shell model is shown to be consistent with the observed EA behavior of all three metals, the predicted spherical shell closing at cluster 58 being evident for silver and gold. The UPS data show a smooth evolution of the d band toward that of the bulk metal.
Collective dynamics of liquid carbon tetrachloride studied by inelastic neutron scattering and computer simulation
Authors: García-Hernández M. , Martínez J. L. , Bermejo F. J. , Chahid A. , Enciso E.
Published: July 2002

An inelastic neutron scattering (INS) study of carbon tetrachloride (CCl4) at two different temperatures within its liquid range is reported. The analysis of the spectra reveals the presence of a collective inelastic response throughout the explored Q-range (0.3 Å−1≤Q≤2.2 Å−1). The wave vector dependence of the excitation energies (‘‘dispersion relations’’) is investigated by means of the analysis of the measured spectral intensities in terms of a phenomenological (viscoelastic) model. In order to help with the analysis of the experimental intensities a molecular dynamics study has been carried out. The analysis of the calculated dynamical factors is also performed within the same approximation and the results are tested by comparison with those stemming from the experimental observations.
Optical response of concentrated colloids of coinage metals in the near-ultraviolet, visible, and infrared regions
Authors: Farbman I. , Levi O. , Efrima S.
Published: July 2002

The optical response of two-phase composite materials (cermet topography with coinage metals) at high filling factors under a wide range of particles sizes, frequencies, and experimental conditions is calculated using a model that combines multipolar interactions and particle size effects. The validity of the model is established for a real system of a silver colloid and the simulations are discussed in terms of the microscopic structural parameters and the interaction between the particles. The limitations of the Maxwell–Garnett model are explored and the transition between a dense system which primarily reflects light and a dilute composite which mostly transmits light is followed with the same model.
Luminescence properties of Ni2+ in CsCdBr3
Authors: de Viry D. , Tercier N. , Denis J. P. , Blanzat B. , Pellé F.
Published: July 2002

Spectroscopic properties of Ni2+ ions substituted in the unidimensional compound CsCdBr3 were investigated from 9 to 300 K. The structured features of the absorption spectrum have been interpreted on the assumption of a trigonal distortion of the Ni2+ site. Four emissions were observed and assigned to the 1T2 →3T2,3T1,1E and to the 3T2 →3A2 transitions in order of decreasing energy. Energy-level calculations were performed in the usual cubic model and the results compared to the values obtained in a trigonal-field framework. The lifetimes have been measured and fitted by the Mott law and using the Struck and Fonger model.
Bent valence excited states of CO2
Authors: Spielfiedel A. , Feautrier N. , Cossart-Magos C. , Chambaud G. , Rosmus P. , Werner H.-J. , Botschwina P.
Published: July 2002

The conical intersection regions on the potential energy functions of the valence excited 1,3Σu−, 1,3Δu, 3Σu+, and 1,3Πg states of CO2 have been investigated by ab initio calculations. Using large scale multireference configuration interaction the ordering of the lowest valence excited states of CO2 has been calculated to be 3B2, 3A2, 1A2 followed by 1B2. All these states have bent equilibrium structures and lie energetically below their dissociation asymptotes. The near equilibrium parts of the potential energy functions have been mapped in three dimensions by multiconfiguration self-consistent field calculations. The 1,3B2 and 1,3A2 states differ in their equilibrium angles (118° and 127°, respectively), and have much longer equilibrium distances (around 1.26 Å) than the electronic ground state. Anomalously low values of ca. 800 cm−1 have been calculated for the wave number of the antisymmetric stretching vibrations of the 1A2, 3B2, and 1B2 states. The crossings between the bent valence excited states in the geometry region 90°≤αOCO≤150° and 1.1 Å≤RCO≤1.4 Å have been located.
On the classical limit of quantum thermodynamics in finite time
Authors: Geva Eitan , Kosloff Ronnie
Published: July 2002

The finite time performance of quantum heat engines has been examined with emphasis on the classical, high temperature, limit. Two basic engine models were studied, differing by their consistency of working fluid: the harmonic engine, consisting of noninteracting harmonic oscillators, and the spin-j engine, consisting of noninteracting spin-j subsystems. The two models represent two distinct types of engines, with bounded vs unbounded Hamiltonians, and with creation and annihilation operators of the Fermionic vs the Bosonic type. The analysis is based on the time derivatives of the first and second laws of thermodynamics. Explicit relations linking quantum observables to thermodynamic quantities are utilized. The dynamics of the engines was modeled by the semigroup approach. The engines were optimized with respect to various target functions: power, entropy production, and efficiency, while subject to finite cycle duration. The main strategy of optimization was based on the Euler–Lagrange equation, and is similar to that previously applied by Salamon and Nitzan for the investigation of Newtonian engines [J. Chem. Phys. 74, 3546 (1981)]. The optimal cycles obtained at the classical limit are not of the Curzon–Ahlborn type, i.e., the internal temperature along the thermal branches is not constant. This result is in conflict with Newtonian thermodynamics, where the optimal cycles are of the Curzon–Ahlborn type. Nonetheless, some of the main features of Newtonian thermodynamics, such as the Curzon–Ahlborn efficiency at maximum power, are reproduced at the classical limit. This makes it possible to establish a ‘‘thermodynamic correspondence principle.’’ This principle asserts that the phenomenological Newtonian thermodynamic approach provides an asymptote of a theory based upon the more fundamental semigroup quantum approach. It is argued that the asymptotic nature of Newtonian thermodynamics is twofold since its validity is restricted by two demands: that of high temperatures and that of proximity to equilibrium.
Hard sphere cavity in a liquid-density functional approach
Authors: Samborski A. , Stecki J. , Poniewierski A.
Published: July 2002

The system of particles, interacting via a reasonably realistic intermolecular potential, in contact with a hard spherical wall is studied. The weighted density approximation is used to determine the density distribution and the liquid–wall surface tension. The effect of the wall curvature on the density at contact and the surface tension is investigated. The predictions of the model calculations are compared with the results of the computer simulations and a qualitative agreement is found. It is suggested that the asymptotic expansion of the surface tension and the density at contact in powers of the curvature may not be valid for cavities of microscopic size.
Quantum yields for OH production from 193 and 248 nm photolysis of HNO3 and H2O2
Authors: Schiffman A. , Nelson D. D. , Nesbitt D. J.
Published: July 2002

The absolute quantum yields (Φ) for OH production from 193 and 248 nm photolysis of HNO3 and H2O2 are measured at room temperature using flash kinetic spectroscopy in a flow tube. The OH radicals are produced by excimer laser photolysis and probed via direct absorption of high resolution, tunable IR laser light. The resulting quantum yields are found to be ΦHNO3193=0.47±0.06, ΦH2O2193=1.22±0.13, ΦHNO3248=0.75 ±0.10, and ΦH2O2248= 1.58±0.23. These results indicate quantum yields for both precursors at both wavelengths which are less than the maximum possible values of 1 for HNO3 and 2 for H2O2. The present measurements are discussed in light of contrasting results suggested from other work.
Quasidegenerate perturbation theory with multiconfigurational self-consistent-field reference functions
Authors: Nakano Haruyuki
Published: July 2002

A quasidegenerate perturbation theory based on multiconfigurational self-consistent-field (MCSCF) reference functions is derived. The perturbation theory derived here is for multistate, where several MCSCF functions obtained by the state-averaged MCSCF method are used as the reference and an effective Hamiltonian is constructed by perturbation calculation. The energies of states interested in are obtained simultaneously by diagonalization of the effective Hamiltonian. An explicit formula of the effective Hamiltonian through second order is derived as well as general formalism, and is applied to calculate potential curves of the system H2, Be–H2, CO, NO, BN, and LiF. The results agree well with those of full configuration interaction or multireference single and double excitation configuration interaction methods for both the ground and the excited states.
Raman scattering and microwave dielectric studies of the structural phase transition in the quasi-one-dimensional ferromagnet (CH3)4NNiBr3
Authors: Musfeldt J. L. , Poirier M. , Jandl S. , Renard J.-P.
Published: July 2002

We report polarized Raman spectra and 9.6 GHz dielectric constant measurements of the quasi-one-dimensional ferromagnet (CH3)4NNiBr3 (TMNB) at temperatures above and below the 134 K structural phase transition (Tc). The dielectric response is typical of materials for which a structural phase transition occurs, rising upon approach to Tc and decreasing sharply at Tc; ε1 falls gradually in the low temperature phase. There is a small hysteresis in the dielectric constant at Tc, indicative of the first order nature of the transition. The Raman spectra display striking changes through the structural phase transition as well. At room temperature, we observe the weak signature of four of the five Raman active BX3 modes. Below Tc, we observe splitting in several degenerate vibrational modes, one of which is a BX3 chain mode. This suggests that the phase transition is related to a symmetry breaking structural distortion in which the NiBr3 chains are intimately involved. The behavior of TMNB through Tc is in contrast to that of many other hexagonal magnetic materials, most notably TMNC, where the phase transition occurs with a doubling of the unit cell and an ordering of the tetramethylammonium counterions. We suggest that this difference is related to a combination of electrostatic and lattice expansion effects. Small frequency shifts of the phonons and a variation of the slope in the dielectric constant also provide evidence for a second phase transition near 95 K, which we relate to complete counterion ordering as in the case of TMCC.
Separation of quadratic and linear external field effects in high J quantum beats
Authors: Auzinsh M. , Ferber R. , Stolyarov A. V.
Published: July 2002

We discuss quantum beats in electronically excited molecular states with high rotational angular momenta J appearing in time resolved fluorescence in conditions of quadratic and linear energy shift dependence on magnetic quantum number M and external field strength. Density matrix formalism is used to obtain in explicit form the expressions for time dependent fluorescence intensity after δ-function pulsed excitation. In case of pure quadratic Stark effect, which is typical for 1Σ state diatomics, excited state quantum beats for J≫1 exhibit a regular, or ‘‘grill’’ structure, consisting of narrow equidistant ‘‘principal’’ peaks with equal relative amplitudes on the exponential decay background. At linear polarized excitation the time intervals between the adjacent peaks are 2π/ω20, ω20 being the splitting frequency between coherently excited M-sublevels with M=2 and M′=0. If an admixture of linear contribution is present in field induced level shifts, the grill structure is superimposed by a single frequency harmonic modulation. A special geometry was found in which the quadratic beats are fully absent and the modulated grill pattern is brought into existence only by the influence of linear term. Such a case takes place when the light polarization vector in fluorescence is directed at 45° angle with respect to the exciting light polarization vector and yields the most sensitive way to separate quadratic and linear contribution. We considered the examples when the first order term appears by a combined action of electric and magnetic field, as well as due to the e–f level electric field induced mixing, with the parameters typical for the NaK molecule.
Rotational and vibrational relaxation of the ν1/2ν2 Fermi dyad in CO2 gas from Raman-infrared double resonance experiments
Authors: Roche C. , Millot G. , Chaux R. , Saint-Loup R.
Published: July 2002

Time-resolved Raman-infrared double resonance experiments have been conducted on CO2 gas in order to determine collisional relaxation rates of energy levels. These total depopulation constants are measured in a three-level double resonance scheme. A pulsed Raman excitation populates the studied state over a brief time interval (7 ns). The ensuing collisional depopulation of this level is monitored by a continuous probe CO2 laser whose transition originates in the same level. The transient absorption on the probe laser gives the time dependent population behavior. The high selectivity of the lasers allows the study of vibrational and rotational levels. The relaxation rates, of the (1000) vibrational state and, for the first time, of the J=14–34 rotational levels of the (0200) vibrational state, have been measured. The derived broadening coefficients have been compared to those calculated by a semiclassical model.
Product branching fractions in the reactions of NH(a 1Δ) and NH(X 3Σ−) with NO
Authors: Okada Satoru , Tezaki Atsumu , Miyoshi Akira , Matsui Hiroyuki
Published: July 2002

Reactions of the imidogen radical NH with NO have been investigated in its electronically excited a 1Δ and ground X 3Σ− states, where UV- and VUV-LIF were used to monitor radical and atomic species in the 193 nm photolysis of HNCO/NO mixtures at 295±3 K in 20 Torr He. The rate constants for the reactions of NH(a 1Δ; v=0,1)+NO→products (1) and NH(X 3Σ−; v=0,1)+NO→products (2) were determined. No apparent effect on the rate constants of the vibrational excitation of NH was observed in either reaction. H and OH were observed as products in both reactions, whereas physical quenching of NH(a 1Δ) to NH(X 3Σ−) is dominant in Reaction (1). In addition, O(3P) was found to be produced in Reaction (1) for the first time. Product branching fractions in these reactions were determined using proper reference reactions, along with addition of Xe to control the relative contributions of Reactions (1) and (2). The branching fractions in Reaction (1) were: 0.57 for quenching (1a), 0.20 for H+N2O (1b), 0.14 for OH+N2 (1c), 0.08 for the sum of O+N2H (1d), and O+N2+H (1e), and in Reaction (2): 0.65 for H+N2O (2a), 0.30 for OH+N2 (2b). These results were found to be consistent with former theoretical predictions that Reactions (1) and (2) produced on the same doublet surfaces. It was elucidated that the observed delay of OH(v=0) production was due to the vibrational relaxation of OH(v≥1) produced in the reactions.
Automated multidimensional coherent optical spectroscopy with multiple phase-related femtosecond pulses
Authors: Wefers Marc M. , Kawashima Hitoshi , Nelson Keith A.
Published: July 2002

Multidimensional spectroscopy with femtosecond phase-related pulse sequences, in which the relative delays and optical phases of pulses are scanned during spectroscopic observation, is demonstrated on electronic transitions in atomic potassium vapor. The pulse sequences were constructed using emerging femtosecond pulse-shaping techniques that allow for automated generation of complex ultrafast waveforms. Sequences of two and three pulses with specified optical phase relationships and interpulse delays were used to prepare specified coherent states.
The vibrational energy levels in acetylene 12C2H2: Towards a regular pattern at higher energies
Authors: Temsamani Mohammed Abbouti , Herman Michel
Published: July 2002

We have gathered 122 known experimental data concerning the vibrational energy levels of acetylene, C2H2, up to 12 000 cm−1. We have fitted the data simultaneously to a model based on the clustering of the levels induced by a set of eight anharmonic resonances. That set is carefully built from the literature information on various isotopomers of acetylene. The model takes full advantage of the anharmonic pattern and systematically block diagonalizes the entire vibrational energy matrix. The 122 input energies are reproduced within a standard deviation of 0.74 cm−1 with 35 fitted parameters. Decisive support to the model is brought by using the resulting vibrational parameters and matrix eigenvectors to reproduce energies, principal rotational parameters, and relative intensity features. The cluster picture is discussed, in terms of the 7 normal modes of vibration in acetylene, according to the theoretical model developed by Kellman [M. E. Kellman and G. Chen, J. Chem. Phys. 95, 8671 (1991)].
Ab initio calculation of rovibronic transition spectra of CaH
Authors: Leininger Thierry , Jeung Gwang-Hi
Published: July 2002

The ground (1 2Σ+,X) and excited electronic states, 1 2Π (A), 2 2Σ+ (B,B′), 1 2Δ, 2 2Π (E), 3 2Σ+ (D), 4 2Σ+ (C), 5 2Σ+ (K), 3 2Π (L), and 2 2Δ, of CaH are studied by ab initio calculation, with extensive configuration interactions. Calculated spectroscopic constants, in particular, the excitation energies, are more accurate than in previous calculations and agree well with the currently available experimental data. This work confirms the existence of a single vibrational level, v′=4, bound to the second well of the B state calculated from the experimental data by Martin [J. Chem. Phys. 88, 1797 (1988)]. Calculated oscillator strengths for the B–X and D–X show irregular band structures because of multiple avoided crossings of the potential energy curves. The avoided crossing between the D and C states should be taken into account to explain the apparent perturbations observed in the experimental spectra. The wave functions, dipole moments, and transition dipole moments of these states are discussed in detail.
Coupled channel bound states calculations for alkali dimers using the Fourier grid method
Authors: Dulieu Olivier , Julienne Paul S.
Published: July 2002

The Fourier grid Hamiltonian method is shown to be a powerful method to provide an accurate determination of the bound state spectra of coupled electronic states in alkali dimers. Using accurate ab initio potentials, the perturbations in the spectra of the coupled states A 1Σ+u and b 3Πu in Na2 are reproduced in excellent agreement with spectroscopic studies. A few predictions are also presented for the heavier species Cs2, for which a complete study, both experimental and theoretical, is still needed.