bluuebunny/crossref_metadata_2025 huggingface dataset:
XPS and UPS investigation of PF6 doped and undoped poly 3-methyl thiophene
Authors: Nelson A. J. , Glenis S. , Frank A. J.
Published: July 2002

XPS and UPS techniques have been used to characterize electrochemically grown conductive films using two different solvent systems of poly (3-methyl thiophene) in the PF6 doped and undoped states. Core-level and valence-level spectra have yielded information on the nature of the polymeric cation and its associated PF6 anion as well as structural disorder effects in these polymers. Results have revealed that addition of the PF6 anion causes structural disorder within the polymer and that the PF6 anion interacts with the highest occupied nonbonding lone pair orbital. Solvent system effects are also evident.
Medium effects on the molecular electronic structure. I. The formulation of a theory for the estimation of a molecular electronic structure surrounded by an anisotropic medium
Authors: Hoshi Hajime , Sakurai Minoru , Inoue Yoshio , Chûjô Riichirô
Published: July 2002

A theory has been developed for estimation of the electronic structure of molecules embedded in an anisotropic (inhomogeneous) medium. It is assumed that the medium surrounding a solute molecule is composed of more than two polarizable dielectrics with different dielectric constants and that the dielectrics contact each other through arbitrary shaped boundaries. Then, the charge distribution in the solute molecule interacts with the dielectrics through a reaction field. The boundary element method is introduced to obtain a numerical solution to the Poisson equation with necessary boundary conditions. The expressions for the total energy of the molecule and the Helmholtz free energy of the system are given under the Hartree–Fock–Roothaan approximation. The Fock matrix is derived by taking into account nonlinearity of the Schrödinger equation in wave function. The present theory is generally applicable to the problem of medium effects when the medium surrounding a molecule is inhomogeneous and could be approximated by a multidielectric system.
Dicationic states of hydrocarbons and a statistical approach to their Auger spectra
Authors: Ohrendorf E. M.-L. , Tarantelli F. , Cederbaum L. S.
Published: July 2002

The valence doubly ionized states of acetylene and ethane are computed and discussed, complementing previous investigations on other hydrocarbons via the Green’s function method. The vertical double ionization potentials are used for the interpretation of the Auger spectra of the molecules. The analysis is performed by employing a simple statistical approach to the Auger rates where the states are weighted by their two-hole components. For all hydrocarbons studied the resulting theoretical spectra exhibit maxima in good agreement with the experimental Auger peaks and reproduce the essential features of the experimental spectra. Strong final-state correlation effects are found for acetylene, ethylene, and benzene. They lead to a breakdown of the molecular orbital picture of double ionization which seems to be a typical phenomenon for unsaturated molecules. This phenomenon manifests itself in the Auger spectrum by the appearance of a high density of dicationic states which cannot be described in the independent-particle approximation. By contrast, the Auger spectrum of ethane is found to be dominated by the appearance of main states accompanied by weak satellite states at high energy. There is a correspondence between many-body effects in the Auger spectrum and the chemical bond. General aspects of correlation effects in dicationic states are discussed and related to the case of singly ionized states.
An a
b i
n
i
t
i
o study of the magnetic hyperfine properties of F−2(2Σ+u)
Authors: Feller David
Published: July 2002

Gaussian basis sets derived from the new Dunning ‘‘correlation consistent’’ sets have been used in conjunction with multireference single- and double-excitation configuration interaction (CI) wave functions to determine the magnetic hyperfine properties of the F−2 radical anion and the electron affinity of F2. The best computed values, using an extension to the polarized, valence quadruple zeta basis (cc-pVQZ) are: Aiso(19F)=772 MHz (expt=785 MHz), Adip=890 MHz (expt=901 MHz), and EA=2.61 eV (expt=3.08±0.10 eV). Estimates which crudely account for basis set incompleteness and CI truncation lead to the following, slightly different, values: Aiso (19F)=802 MHz, Adip =898 MHz, and EA=2.94 eV. The sensitivity of Aiso to changes in the F–F bond length results in an uncertainty on the order of ±20 MHz in the isotropic hyperfine property. Such close agreement between theory and experiment suggests that any rare gas matrix effect in the recent electron spin resonance (ESR) measurements on F−2 should be small, i.e., the matrix results should agree well with any future gas phase values. Correlation effects beyond the standard singles and doubles level are estimated to contribute approximately 50 MHz (∼30%) of the 155 MHz correlation correction to Aiso.
Conformation of weakly charged polyelectrolytes at a solid–liquid interface
Authors: Varoqui R. , Johner A. , Elaissari A.
Published: July 2002

The adsorption characteristics of polyelectrolyte chains on a plane surface are derived. The approach uses the mean-field arguments first developed by de Gennes [Rep. Prog. Phys. 32, 187 (1969)] and Richmond and Jones and Richmond [J. Chem. Soc. Faraday Trans. 73, 1062 (1977)] for the evaluation of the configuration probability of a neutral chain at an interface. Electrostatic interactions among monomers are treated through the linearized Poisson–Boltzmann equation. The treatment is restricted to dilute solutions and to weakly charged polyelectrolytes. Simply analytical expressions are derived for the concentration profile and the adsorption isotherm which are expressed in terms of two characteristic lengths L1 which characterizes the range of surface forces and L2, the Debye–Hückel length, which characterizes the range of electrical forces. Once these variables are fixed, it appears that the adsorption cannot exceed a critical value Γm which is tuned by the temperature.
Scaling properties of induction times in heterogeneous nucleation
Authors: Shneidman Vitaly A. , Weinberg Michael C.
Published: July 2002

In the asymptotic limit of a high nucleation barrier we obtain the heterogeneous-to-homogeneous induction time ratio as a function of the contact angle. The result is in excellent agreement with the data of numerical simulations by Greer et al. [J. Cryst. Growth 99, 38 (1990)]. An expression to describe individual (nonreduced) induction times is also obtained.
Transition rates and Judd–Ofelt intensity parameters of tripositive americium in a fluorozirconate glass
Authors: Brundage R. T. , Svatos Michelle M. , Grinbergs Raitis
Published: July 2002

We have applied the Judd–Ofelt theory of f–f transition rates to the absorption and fluorescence of americium in a fluorozirconate glass. The intensity parameters calculated from infrared absorption transitions are inconsistent with the absorption and emission rates observed for the 5L6′ state at 19 960 cm−1, but are in reasonable agreement with the measured fluorescence lifetime of the 5D1′ state at 17 240 cm−1.
A laser spectroscopic study of gas-phase TiH
Authors: Steimle T. C. , Shirley J. E. , Simard B. , Vasseur M. , Hackett P.
Published: July 2002

The results of the first laser spectroscopic study of gas-phase titanium monohydride, TiH, are described. Survey laser induced fluorescence (LIF) scans over an extensive region of the visible spectrum revealed no new band systems that could be attributed to TiH. The known band system at 535 nm was studied by Doppler limited LIF, sub-Doppler LIF, and sub-Doppler/Stark LIF techniques. The 535 nm optical features are consistent with an assignment to the X 4Φ(Ω=3/2)–4Γ(Ω=5/2) subband. The permanent electric dipole moments of 2.998 and 2.455 D were determined for the excited and ground states, respectively.
Spatial pattern formation in the oscillatory NO+CO reaction on a Pt(100) surface and its vicinal orientations
Authors: Veser G. , Imbihl R.
Published: July 2002

Spatial pattern formation in the NO+CO reaction on a cylindrical Pt single crystal surface (axis parallel [001] direction) has been investigated using photoemission electron microscopy (PEEM) as an in-situ method to image the lateral adsorbate distribution during the reaction with a resolution of ≊1 μm. The experiments were conducted in the 10−6 Torr range, between 380 and 430 K, under conditions where the (100) orientation exhibits oscillatory behavior. Of the different orientations of the [001] zone which are present on the surface only the orientational range between (100) and (310) was found to be very active in NO dissociation and hence in the surface reaction. A sharp phase boundary meandering between (210) and (410) parallel to the [001] direction separates the active from the inactive zone on the cylinder surface. In the active zone between (100) and (310) one finds propagating reaction fronts and complex spatiotemporal patterns. The velocity of the reaction fronts is strongly anisotropic with the maximum occurring in the [001] direction, parallel to the steps on the cylinder surface, while the minimum velocity is observed perpendicular to the step edges. At constant temperature the spatial patterns display turbulent behavior. Due to averaging, these patterns are associated with a stationary reaction rate. Upon a small temperature change of ≊1–3 K the turbulent pattern orders into a target pattern with parallel reaction fronts. Macroscopically, this transition corresponds to the excitation of kinetic oscillations which then decay after a number of cycles as the pattern becomes turbulent again. The occurrence of damped oscillations in the NO+CO reaction on Pt(100) can thus be explained as being due to the absence of an efficient synchronization mode.
Rate constants for the N2O reaction system: Thermal decomposition of N2O; N+NO→N2+O; and implications for O+N2→NO+N
Authors: Michael J. V. , Lim K. P.
Published: July 2002

Experiments on the thermal decomposition of N2O have been carried out in reflected shock waves using the atomic resonance absorption spectrometric (ARAS) technique for monitoring the product O atoms. The results provide a calibration for the ARAS system as well as values for the decomposition rate constant, k1, between 1540 and 2500 K. The present results can be represented as k1=5.25×10−10 exp(−27 921 K/T) cm3 molecule−1 s−1. Additionally, rate constants for the reaction, (2) N+NO→N2+O, have been measured by the laser photolysis-shock tube (LP-ST) technique between 1251 and 3152 K. NO serves as both the photolytic source of N(4S) atoms and the reactant molecule. N atoms are monitored by the ARAS method. The results do not show significant temperature dependence and can be represented by k2=(3.7±0.8)×10−11 cm3 molecule−1 s−1 over the experimental temperature range. Values for the reaction, (−2) O+N2→NO+N, can be derived from these results. The results are compared with earlier studies and are then theoretically discussed in terms of transition states taken from an ab initio electronic structure calculation and also from a dispersion force model. Neither model accurately predicts the observed behavior, and this suggests that further theoretical work with trajectory calculations be attempted to assess the importance of recrossing.
Studies of molecular conformations by ultraviolet laser spectroscopy and quantum chemical calculations: Inversion potentials in ground and excited singlet states of jet-cooled 1,4-dihydronaphthalene
Authors: Chakraborty Tapas , Del Bene Janet E. , Lim Edward C.
Published: July 2002

Studies of inversion potentials have been carried out for gas-phase 1,4-dihydronaphthalene (DHN) using ultraviolet laser spectroscopy and ab initio quantum chemical calculations. The analyses of the experimental fluorescence excitation and dispersed fluorescence spectra in a supersonic free jet indicate that the equilibrium conformation of DHN is nonplanar in both the ground and lowest excited singlet states. However, the barrier to inversion is smaller than the energy of the zero-point vibrational level, so that DHN behaves as a quasiplanar molecule. The ground-state structure of DHN computed at the correlated MP2/6-31G(d) level of theory is also nonplanar, with a dihedral angle of 148° and an inversion barrier of 0.5 kcal/mol. Comparisons are made for the series 1,4-cyclohexadiene (1,4-dihydrobenzene), DHN, and 9,10-dihydroanthracene.
Correlation effects on Raman scattering from low-energy vibrational modes in glasses. II. Experimental results
Authors: Achibat T. , Boukenter A. , Duval E.
Published: July 2002

New experimental results of low-frequency Raman scattering from different glasses are presented. They are compared to the vibration density of states obtained from inelastic neutron scattering to deduce the light-vibration coupling coefficient C(ω) as a function of the vibration frequency. It was found that C(ω)∝ω2 at very low frequency ω<20 cm−1 in inorganic glasses (SiO2,B2O3) and that C(ω)∝ω at low frequency in polymer glasses and in inorganic glasses for ω≳20 cm−1. Our experimental results for glasses are interpreted by assuming no correlation at very low-frequency and only radial correlation at low-frequency. A full correlation would exist in silica-aerogels in which the effect of random fluctuations would be negligible.
Total energy calculation for CdxSn1−xTe
Authors: Freytag B. , Rössler U. , Karch K. , Grosch G. H. , Range K.-J.
Published: July 2002

In the mixed crystal system CdxSn1−xTe, the transition from a ten-electron (x=0) to an eight-electron system (x=1) is realized, which is accompanied by a change in the crystal structure from rocksalt (SnTe) to zincblende (CdTe). The structure and lattice constants of solid solutions, grown (in part under high pressure) for nearly the whole mixing range, are known. We use norm-conserving pseudopotentials in connection with the local density and virtual crystal approximation for ab initio calculations of the stable crystal structure and the lattice constant in dependence on the composition x of the system. Our calculated results confirm Vegard’s rule and give a crossover from rocksalt to zincblende at about x=0.9 in accordance with experimental data. The crossover is accompanied by a change in the binding character, which can be seen in a change of the electronic charge density.
A reexamination of the vibrational–vibrational energy transfer from N2(v) to NO+
Authors: Viggiano A. A. , Morris Robert A. , Paulson John F. , Brown Eileen R. , Sutton Emmett A.
Published: July 2002

We have reexamined whether N2(v=1) transfers its vibrational quantum to NO+(v=0). In light of recent results that show that CH3I does not charge transfer rapidly with NO+(v=1), we show that previous measurements could not have detected vibrational–vibrational (V–V) energy transfer from N2(v=1) to NO+(v=0). We have made measurements to examine this process by using C2H5I as the monitor for NO+(v=1). Our results show that NO+(v≳0) is indeed produced from reaction of NO+(v=0) with N2(v) but these results cannot be used to distinguish between resonant V–V energy transfer from N2(v=1) to NO+(v=0) and V–V, T energy transfer from higher levels of N2(v).
Statistical mechanics of line liquids
Authors: Bhattacharya I. , Mohanty U.
Published: July 2002

We study the vortex liquid phase structure factor by a synthesis of liquid state physics for point particles with a generalization of Ornstein–Zernike-type description to line liquids. The decay of the in-plane Fourier components k‖ of the structure factor S(k‖,z) along the z-axis due to line wandering are obtained at high flux densities. We find that S(k‖,z=0) vanishes linearly as k‖→0 signifying that the system is a line in three dimensions.
Dynamics of surface enrichment. Phenomenology and numerical results above the bulk critical temperature
Authors: Puri Sanjay , Frisch H. L.
Published: July 2002

We present a phenomenological model for the dynamics of binary alloys in contact with a surface which has a preferential attraction for one of the components of the alloy. The bulk system is described by the usual Cahn–Hilliard equation whereas the surface is mimicked by two boundary conditions. We also present numerical results from this model for the case when the system temperature is above the bulk critical temperature. Specific predictions are made which should be verifiable experimentally.
Solvent induced polarizabilities and hyperpolarizabilities of para-nitroaniline studied by reaction field linear response theory
Authors: Mikkelsen Kurt V. , Luo Yi , Ågren Hans , Jo/rgensen Poul
Published: July 2002

Polarizabilities and hyperpolarizabilities and their frequency dispersions are calculated for para-nitroaniline immersed in solution using a reaction field response model. Single- and multiconfigurational wave functions are self-consistently optimized in the presence of the reaction field of the solvent and used as reference states for response calculations of the dipole spectra, polarizabilities, and hyperpolarizabilities at zero and finite frequencies. The hyperpolarizabilities are obtained either by finite field generated analytical first polarizability calculations or using a two-state model where all quantities are calculated from the reaction field response method. The effect of the polarity of the solvent on the quantities entering the two-state model (the ground and charge transfer state dipole and transition moments and transition energy) are investigated in some detail. All are found to contribute to the steep dependence of the hyperpolarizability as a function of the polarity of the solvent. The relative variation of the hyperpolarizability with the polarity of the solvent is well reproduced at correlated level calculations.
Collisional alignment of CO2 rotational angular momentum states in a supersonic expansion
Authors: Weida Miles J. , Nesbitt David J.
Published: July 2002

The rotational alignment of CO2 seeded in a supersonic expansion is measured using a general, direct absorption method based on fast (75 kHz) polarization modulation and phase sensitive detection with of a narrow band tunable IR laser. The anisotropic distribution of ‖MJ‖ states is created by a directed velocity slip between the carrier gas and a nonspherical seed gas. Strong alignment signals are observed in a pinhole expansion that depend systematically on the carrier gas, stagnation pressure, and J state. In a slit expansion, however, no alignment is detected for comparable conditions. The observed effects are quite significant and occur at rather modest expansion conditions (e.g., 2.5% CO2 in He at 1000 Torr). As much as 60% enhancement of n⊥/n∥, i.e., the ratio of molecules with J directed perpendicular vs parallel to the expansion axis is obtained. The data indicate that the observed alignment is correlated with the magnitude of the velocity slip, and that elastic collisions where ΔJ=0 and ΔMJ≠0 are primarily responsible for creating and sustaining the alignment.
Using the noninteracting cluster theory to predict the properties of real vapor
Authors: Saltz David
Published: July 2002

We examine the nonideal behavior of real vapor in the context of the theory of noninteracting molecular clusters. The vapor is treated as a perfect mixture of clusters, which in equilibrium attain a distribution in size determined by formation energies ΔGi, where ΔGi is the energy required to form a cluster of i molecules from i molecules in bulk saturated liquid. A theory for the ΔGi gives an equation of state that captures the nonideal behavior of the vapor; conversely, equation of state data provide a validation of the theory. In this paper, we compare the predictions of this equation of state to experimental data. Utilizing the ΔGi proposed by Dillmann and Meier and based on Fisher’s droplet model, we compute the vapor compressibility along the saturation curve for several nonpolar substances and obtain excellent agreement with experiment. We also compute the third virial coefficient for these substances and observe correct qualitative behavior; in the case of benzene and n-octane, for which some data are available, we find rough agreement with experiment. The conventional kinetic theory of homogeneous nucleation, which is based on the assumption of noninteracting clusters, demonstrates that the cluster series equation of state can be continued past the saturation point to describe metastable vapor, a claim that no nonvirial equation of state can make a priori. Furthermore, the noninteracting cluster theory readily accommodates results of more detailed calculations of molecular clusters (e.g., results of Monte Carlo or molecular dynamics studies). These considerations and the success of the simple Fisher–Dillmann–Meier model in predicting the behavior of nonideal vapor suggest possible avenues of investigation in equation of state research.
Electronic structure of the copper(II) ion doped in cubic KZnF3
Authors: Dubicki Lucjan , Riley Mark J. , Krausz Elmars R.
Published: July 2002

The absorption and magnetic circular dichroism spectra of 1%–3% copper(II) doped into the cubic perovskite host KZnF3 are measured over the temperature range 1.8–300 K. Sharp magnetic dipole allowed transitions 2Eg(Γ8)→2T2g(Γ7,Γ8) are observed together with accompanying vibrational fine structure. The spectra are interpreted on the basis of a tetragonally elongated CuF4−6 ground state geometry which arises from strong Jahn–Teller coupling. This results in a statistical distribution of three equivalent elongations which can undergo reorientation on the electron paramagnetic resonance time scale. The Jahn–Teller coupling within the 2T2g multiplet is partially quenched by spin–orbit coupling and the lowest Kramers doublet Γ7 has an octahedral geometry, while the higher lying Γ8 state has a small tetragonal distortion. This system presents an excellent opportunity to study the spectroscopy of the copper(II) ion in a cubic environment.
Calculation of the differential two-photon ionization cross section of H2S
Authors: Cacelli I. , Carravetta V. , Rizzo A. , Moccia R.
Published: July 2002

Differential cross section of nonresonant two-photon ionization of the outer valence shell of H2S is computed using ab initio methods for several choices of photon polarization and experimental setup. The theoretical model employed is based on the second-order perturbation theory in the dipole and vertical transition approximations and the calculations are performed adopting a large basis set of L2 integrable functions. The excitations to intermediate states are described in the random phase approximation, while the final states are described in the static-exchange approximation adopting for the continuum orbitals a K-matrix technique that allows a proper description of the angular distribution of the emitted electrons.
How do molecules move near T
g? Molecular rotation of six probes in o-terphenyl across 14 decades in time
Authors: Cicerone Marcus T. , Blackburn F. R. , Ediger M. D.
Published: July 2002

Time resolved optical spectroscopy was used to observe molecular rotation over more than 14 decades in time for six probes in o-terphenyl (OTP). In contrast to previous studies, probe rotation times are found to depend significantly upon probe size in the deeply supercooled regime. Systematic deviations from the temperature dependence of the Debye–Stokes–Einstein equation are observed, however, these deviations are relatively small. These observations are inconsistent with some models of cooperative molecular motion near Tg which invoke rigid aggregates or locally liquidlike regions. The width of the relaxation spectrum (characterized by the KWW β parameter) systematically decreases with increasing probe size. Near Tg, the largest probe (rubrene) rotates with nearly a single exponential correlation function. Based on the observed trend in β, it is estimated that OTP is homogeneous on length scales greater than 2.5 nm at Tg.
Transition state for the dissociation of tetrahedral N4
Authors: Dunn Kevin M. , Morokuma Keiji
Published: July 2002

The Woodward–Hoffmann forbidden dissociation of tetrahedral N4 to 2N2 has been explored at several levels of ab initio molecular orbital theory. In Cs symmetry, both reactants and products belong to the A′ representation but have different orbital occupancy at the restricted Hartree–Fock (RHF) level. The minimum in the seam of crossing between the reactant and product RHF potential energy surfaces provides a qualitative estimate of the transition state. With a multiconfigurational wave function, this becomes an avoided crossing with a true transition state. The barrier to dissociation at our best level of theory is 63 kcal/mol.
Dynamics of hydrogen-bonded liquids confined to mesopores: A dielectric and neutron spectroscopy study
Authors: Mel’nichenko Yu. B. , Schüller J. , Richert R. , Ewen B. , Loong C.-K.
Published: July 2002

In this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 Å pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen-bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low- and high-frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature Tg by up to ΔTg≊+47 K; and (ii) relatively ‘‘free’’ molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in Tg for the α process with ΔTg≊+5 K is maximal for the monomer liquid and gradually diminishes with increasing molecular weight or decreasing intermolecular hydrogen bonding. The inelastic neutron spectrum of confined propylene glycol shows the boson peak as expected in bulk strong and intermediate glass formers in the vicinity of Tg. This effect can be attributed to the finite-size induced crossover from long wave vibrations characteristic of a continuous medium to localized vibrations in a confined geometry.
Exchange–Coulomb model potential energy surface for the N2–Ar interaction
Authors: Dham Ashok K. , McCourt Frederick R. W. , Meath William J.
Published: July 2002

An exchange–Coulomb model potential energy surface has been developed for the N2–Ar interaction. This potential energy surface is based upon recent results for the Heitler–London interaction energy, long-range dispersion energies, the temperature dependencies of interaction second virial, binary diffusion, and mixture shear viscosity coefficients, microwave spectra of the van der Waals complex, and collision broadening of the depolarized Rayleigh light scattering spectrum. The adjustable parameters in the final potential surface have been determined by fitting the temperature dependence of the virial coefficient (within experimental uncertainties) and the frequencies of three representative lines of the microwave spectrum of the 14N2–Ar van der Waals complex. A fine tuning of these parameters was achieved by considering the temperature dependence of the binary diffusion and mixture shear viscosity coefficients. Calculated frequencies for all observed microwave transitions of the various N2–Ar isotopomers lie within 0.1% of the experimental values. Excellent agreement is obtained with experimental values of binary diffusion and mixture shear viscosity data for all temperatures and compositions. Agreement with the bulk gas relaxation phenomena, while not outstanding, is acceptable, given the accuracy with which these phenomena are currently determinable. Detailed comparisons are made with predictions of the two best literature N2–Ar potential energy surfaces. This new potential surface gives the best overall agreement with experiment for N2–Ar mixtures.
On the accuracy of valence correlation energies in pseudopotential calculations
Authors: Dolg M.
Published: July 2002

Total valence correlation energies of X5+, X+, X, X−, and X2 (X=F,Cl,Br,I) have been calculated with the coupled-cluster method for the all-electron and the pseudopotential case using large uncontracted basis sets containing up to g-type functions. The overestimation of valence correlation energies due to the modified nodal structure of the pseudo-orbitals is discussed. The errors are found to be less than 10% in all cases of chemical interest and reduce to 2 mH or less in correlation contributions to ionization potentials, electron affinities or binding energies.
A new class of collective excitations: Exciton strings
Authors: Mazumdar S. , Guo F. , Meissner K. , Fluegel B. , Peyghambarian N.
Published: July 2002

Optical excitation in a strongly neutral quasi-one-dimensional mixed-stack charge-transfer solid results in an exciton state, in which the electron and the hole are bound by electrostatic Coulomb interactions that are large compared to the one-electron hopping. We present a joint theoretical–experimental demonstration of a new class of collective excitations, multiexcitons or exciton strings, consisting of a string of several (more than two) bound excitons, in a prototype neutral charge-transfer solid. The stability of the multiexciton states arise from the combined effects of one dimensionality and strong Coulomb interactions. Theoretically, we show that in narrow band one-dimensional semiconductors with long range Coulomb interactions, the occurrence of stable 2-exciton string (biexciton) necessarily implies stable higher multiexcitons. Experimentally, evidence for the multiexciton strings is demonstrated by femtosecond pump–probe spectroscopy of anthracene pyromellitic acid dianhydride. Excellent qualitative agreement is found between the calculated and the measured differential transmission spectra. Photoinduced absorptions to the 2-exciton string at low pump intensity and to the 3-exciton string at high pump intensity are observed, in agreement with the theory of excited state absorption. The 2-exciton string is confirmed also by a direct two-photon absorption measurement. The binding energies of the 2-exciton and the 3-exciton strings are obtained from the experimental data. The larger binding energy of the 3-exciton is in agreement with theory.
Characterization of ground and excited electronic state deprotonation energies of systems containing double bonds using natural bond orbital analysis
Authors: Badenhoop Jay K. , Scheiner Steve
Published: July 2002

Natural bond orbital analysis is applied to the ground and excited states of a set of neutral, cationic, and anionic doubly bonded species HnC=XHn (X=C, N, O) isoelectronic with ethylene. The character of the excitation is correlated with calculated charge shifts and geometry changes upon relaxation. For these planar molecules, depopulation of the π bond or population of the π* antibond causes an out-of-plane twist or pyramidalization upon relaxation correlated to the amount of charge shift. These nonplanar distortions generally lower the energy more than changes in bond lengths and angles. Population of a σXH* antibond by more than ∼0.4 e often leads to dissociation of that proton. The character and symmetry of the transition are used to match excited states in the protonated and deprotonated species so as to extract an excited state deprotonation energy. The vertical deprotonation energy of the π→π* state tends to be higher than that of the ground state due to greater electronic destabilization of the deprotonated species, while Rydberg excited states take less energy to deprotonate. Adiabatic deprotonation energies can be greater or less than that of the ground state depending on whether the protonated or deprotonated species is more stabilized by geometry relaxation.
Vibrational spectroscopy of small water complexes embedded in large liquid helium clusters
Authors: Fröchtenicht Ralf , Kaloudis Michael , Koch Martin , Huisken Friedrich
Published: July 2002

Infrared molecular beam depletion spectroscopy has been employed to study the vibrational spectroscopy of water molecules and small water polymers [(H2O)n, n=2,3,4] embedded in large liquid helium clusters (HeN, 100≤〈N〉≤5000). The spectral region between 3500 and 3800 cm−1 was covered with an injection-seeded optical parametric oscillator. The following vibrational bands could be located and investigated: for the monomer the ν3 asymmetric stretch, for the dimer the asymmetric stretch of the proton acceptor molecule and the free and bonded O–H stretches of the donor molecule, for the trimer both free and bonded O–H stretches, and for the tetramer the free O–H stretch. By comparison with the data on free gas phase water complexes, it was found that the helium host clusters induce only minor perturbations in form of small frequency redshifts and that they constitute an ideal nano-matrix. Furthermore, it was found that both the water molecule and the dimer rotate almost freely in the host cluster and that the internal-rotationlike tunneling motion of the water dimer is not quenched. Due to the extremely low internal temperatures, a splitting of the trimer band associated with the O–H ring vibration could be resolved for the first time. This splitting indicates that the trimer structure is asymmetric, as has been predicted by theoretical calculations.
Asymmetry and scaling in polymer contraction kinetics
Authors: Poland Douglas
Published: July 2002

We continue our investigation of a model lattice polymer where only local conformational changes (cooperative trans ↔ cis transitions) are allowed requiring the diffusion of randomness into the interior from the ends of the chain for the process of contraction of an initially extended (all-trans) chain. Simulations of the kinetics show that in the early stages of the dynamics there is a marked tendency for the cis states entering the chain to exist largely on either odd or even numbered sites in the chain thus generating a sublattice asymmetry or order in the molecule, a pattern that is largely lost at equilibrium. A similar soluble continuum model suggests that the net amount of cis in the chain should increase with time approximately as a simple exponential with the relaxation time proportional to the square of the chain length. This suggests that if a given dynamic function is plotted versus the scaled time (time divided by the chain length squared) one will obtain a general function independent of chain length. Simulations of the kinetics of the net amount of cis in the system support this scaling precisely. The difference in the number of cis states on the odd and even sublattices approaches a limiting scaled form, but develops a maximum as the chain length increases. The kinetics of the decrease in the end-to-end distance on contraction slowly approaches a limiting form in the scaled time, developing a t−1/2 tail as the chain length goes to infinity. This later behavior is similar to the critical slowing down in the mean-field approximation to relaxation in the three-dimensional Ising model near the critical point.
Time- and product-resolved photodissociations of bromotoluene radical cations
Authors: Kim Byungjoo , Koo Shin Seung
Published: July 2002

Photodissociations of o-, m-, and p-bromotoluene radical cations have been studied in the wavelength range 575–475 nm using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. The parent ions were prepared by charge-transfer reactions of bromotoluenes with toluene-d8 radical cations produced by two-photon ionization of toluene-d8 at 266 nm. Bromotoluene radical cations dissociate to C7H7+ by loss of Br. The dissociation rates were measured by time-resolved photodissociation spectroscopy. Structures of C7H7+ from one-photon dissociation were identified by their bimolecular reactivities with toluene-d8. The C7H7+ products from all three isomers were identified as the benzyl cation. No unreactive tropylium ions were detected within experimental limits. The rate constants measured in this work were combined with the previous photoelectron-photoion-coincidence results to refine activation parameters for the Rice–Ramsperger–Kassel–Marcus rate-energy curves, k(E), for the low barrier rearrangement process. The activation barriers are estimated to be 1.66, 1.80, and 1.78 eV for the o-, m-, and p-bromotoluene radical cations, respectively, whereas the entropy changes for the activation, ΔS‡(1000 K), are −9.6, −7.2, and −5.6 eu., respectively. The mechanism of the rearrangement process is presented to account for the predominant formation of the benzyl cation.
The electronic transition moment function for the Bu+(3Π)↔X 1Σg+ transition in I2
Authors: Tellinghuisen Joel
Published: July 2002

A new method is devised for extracting diatomic electronic transition strengths, as functions of internuclear distance R, from total radiative decay rates as a function of the vibrational level υ in the excited electronic state. The method involves least-squares inversion and utilizes an efficient and reliable sum rule for total radiative decay. When applied to the available data for the B–X transition in I2, the method indicates that the transition strength peaks at 1.85±0.1D2 near R=3.3 Å. This result is in qualitative agreement with most prior studies, from which, however, the level of quantitative consistency must be rated as poor.
Predicting the viscosity of alkanes using nonequilibrium molecular dynamics: Evaluation of intermolecular potential models
Authors: Allen William , Rowley Richard L.
Published: July 2002

Nonequilibrium molecular dynamics (NEMD) viscosity simulations of branched and linear alkanes at liquid densities were performed using both united-atom (UA) and all-atom (AA) intermolecular potential models in order to study the relative efficacy of the models in predicting fluid viscosity. Both models were used in conjunction with fixed bond lengths and bond angles, but different torsional potentials were investigated. The commonly used Ryckaert–Bellemans intermolecular potential model, which accurately predicts viscosities for short straight-chain alkanes, produced values for branched and long-chain alkanes that were significantly below experimental values. Likewise, a more complex UA model that uses transferrable site potentials and is commonly used to simulate thermodynamic properties also under predicted viscosities for branched and long-chain molecules. The UA models were also found to be density dependent, substantially under predicting viscosity at high liquid densities for all model fluids tested. Predicted viscosities using AA intermolecular potential models were generally substantially too large compared to experiment when using model parameters from the literature, even though thermodynamic properties were adequately predicted. However, evidence suggests that accurately modeling the hydrogen interactions and the rotation potential of methyl groups is essential for accurate viscosity simulations. Therefore, a new set of parameters for the hydrogen interactions was regressed using viscosity simulations of 2-methylpropane and n-pentane. Like the UA model, the AA model with the new parameters is still somewhat density dependent, but gives reasonably accurate predictions of viscosity for most fluids.
Rovibrational energy surfaces of triatomic water-like molecules
Authors: Makarewicz Jan
Published: July 2002

The formation of fourfold energy clusters in high J-multiplets of triatomic waterlike molecules is analyzed theoretically. The semiclassical method is used to determine the rovibrational energy surfaces and the effective adiabatic rotational Hamiltonians for the ground and excited bending states in these molecules. The adiabatic separation of the rotational and the internal molecular motion used in this paper enables significant insight into the mechanism of the strong rotation–vibration interaction in rapidly rotating molecules. The critical values of the angular momentum Jc, at which the bifurcation responsible for the clusterization effect occurs, are calculated. The results from the previous classical studies [see I. N. Kozin and I. M. Pavlichenkov, J. Chem. Phys. 104, 4105 (1996)] and quantum mechanical calculations [O. L. Polyansky, P. Jensen, and J. Tennyson, J. Chem. Phys. 101, 7651 (1994)] on the structure of the J-multiplects in the H2O molecule are revised and the formation of the fourfold energy clusters can now be considered well understood.
Chain relations of reduced distribution functions and their associated correlation functions
Authors: Alavi Saman , Wei G. W. , Snider R. F.
Published: July 2002

For a closed system, the integration (trace in the quantum case) over one particle of a reduced distribution function is related to the reduced distribution function of one lower order. The particular details of this “chain” relation depend sensitively on the detailed manner in which the reduced distribution functions are defined, specifically their normalization. Correlation functions are defined in terms of reduced distribution functions, which fixes the normalization of the correlation functions and, provided they exist, their associated chain relations. Chain relations for the correlation functions are shown to exist for normalizations of generic type but not for normalizations of specific type. The normalization requirement is shown, in general, to prevent the direct association of correlation functions with physical clusters, which is commonly assumed in the literature. These relations are illustrated for an ideal gas of monomers and dimers. The effect of taking the thermodynamic limit on the chain relations for this system is discussed. This illustrates how the thermodynamic limit generally destroys the chain relations.
Charge density wave transition and instability in interchain coupled organic ferromagnets with next-nearest-neighbor hopping interaction
Authors: Wang W. Z. , Yao K. L. , Lin H. Q.
Published: July 2002

Considering interchain interaction and the intrachain next-nearest-neighbor hopping interaction, within the self-consistent-field Hartree–Fock approximation, we deal with two neighboring organic ferromagnetic chains. We take into account the π-electrons, the Hubbard repulsion, and antiferromagnetic correlation between π-electrons and side radical electrons. It is shown that there appear to be two kinds of charge density waves successively when the next-nearest-neighbor hopping interaction is greater than a critical value, which decreases with the interchain coupling. The first charge density wave is accompanied by a bond order wave and the second one has no lattice distortion. The spin density wave along the main chain is modulated by the charge density wave. It is also found that the interchain coupling and the next-nearest-neighbor hopping integral destabilizes the ferromagnetic ground states.
Structure and dynamics of intermediate benzene–argon clusters: (C6H6)Arn, n=13–40
Authors: Easter David C. , Bailey Lino , Mellot James , Tirres Michael , Weiss Todd
Published: July 2002

Size-resolved benzene–argon clusters, (C6H6)Arn, n=13–40, were generated by supersonic expansion and studied by ultraviolet resonant two-photon ionization spectroscopy through benzene’s B2u←A1g601 transition. The size-specific sequence of cluster spectra reveals six features that can be isolated, allowing for an independent analysis of each feature’s evolution. In the n=13–40 range, each cluster spectrum is well described by the sum of between two (n=13) and five (n=17–24,26–29) Gaussian features. Only two spectral features (D,E) span the 28-cluster sequence, while each of the other four features appears over a limited consecutive cluster size range (A:n=14–39; B:n=17–29; C:n=14–24; F:n=26–40). The evolution of the spectral shift, width, and relative amplitude is traced for each feature. The observations are rationalized through a model that has been previously developed for the discussion of benzene–argon cluster data. We report for the first time a “high shift” spectral feature, observed at a spectral shift of −80 cm−1, which approaches the bulk shift more closely than anything previously reported.
Vacuum ultraviolet photodissociation dynamics of acetonitrile
Authors: Moriyama Masashi , Tsutsui Yoshiya , Honma Kenji
Published: July 2002

Vacuum ultraviolet photodissociation dynamics of CH3CN and CD3CN has been studied at wavelength near the Lyman-α transition (121.6 nm). Two products, CN(B 2Σ) and H or D, were observed. The CN(B 2Σ–X 2Σ) chemiluminescence spectrum was analyzed to determine vibrational and rotational distributions of CN(B). The vibrational distribution was Boltzmann-like and agreed with a statistical distribution. Although the rotation of the v=0–3 states were also characterized by the Boltzmann distribution, the average rotational energy was higher than the statistical energy disposal. The line shape of the Lyman-α laser-induced fluorescence of H and D showed a Gaussian Doppler profile, which was ascribed to the statistical translational energy distribution of H and D. Isotope effects were determined for both products. While no isotope effect was observed for energy partition into the vib-rotation of CN(B) and the translation of H and D, relative yields of two products showed an isotope effect, i.e., CN(B) formation was favored for CD3CN over CH3CN and the reverse was true for H(D) atom formation. These results suggest a dissociation mechanism where fast internal conversion occurs after the primary photoexcitation and the vibrationally hot molecule leads to the H(D) atom elimination which competes with C–C bond dissociation.
Many-body effects in the trapping problem with a field
Authors: Makhnovskii Yurii A. , Maslova Marina E. , Berezhkovskii Alexander M.
Published: July 2002

We study how an external bias field influences the Brownian particle survival in a medium with traps. The emphasis is on the many-body aspect of the problem. A general path integral approach is used to present the particle survival probability P(t) in the form providing a uniform description of the process over the whole range of time and for any value of the field strength. It is shown that at low fields the many-body effects determine the long-time behavior of P(t). At high fields, they manifest themselves only as a small correction to the rate constant predicted by the single-body theory. Particular attention is given to the one-dimensional case where an exact solution can be obtained. A major observation is that the difference between the exact and the mean-field expressions for P(t) (considered as a measure of the magnitude of the many-body effects) behaves nonmonotonically with the field. This suggests that an optimal choice of the field may facilitate an experimental observation of the many-body effects in the trapping kinetics.
Role of vibrational softening in fast dynamics of an amorphous polyimide far below Tg
Authors: Hansen J. , Kanaya T. , Nishida K. , Kaji K. , Tanaka K. , Yamaguchi A.
Published: July 2002

We report quasielastic neutron scattering results on an amorphous polyimide (AURUM) in a temperature range from 10 to 300 K below the glass transition temperature Tg(=250 °C). As temperature increases from 10 K, anharmonic excess scattering intensity appears at around 200 K or about 300 K below Tg. This excess scattering is very similar to the so-called fast process in picosecond order observed for glass-forming polymers as well as organic and inorganic low molecular weight glass-forming materials. We first analyzed the spectra in a simple way assuming validity of the Bose scaling for the vibrational density of states and a Lorentzian for the additional relaxational process, and found that the excess scattering intensity appears in the frequency range below the Boson peak and the characteristic time is independent of temperature which are common features for the fast processes reported previously. The second analysis using the recent vibration-relaxation (VR) model, which is more realistic, indicated that softening of the vibrational modes may play an important role for the change of spectra from inelastic-like to quasielastic-like. Nevertheless, relaxational process is still necessary to describe the excess scattering although the temperature range examined is very far below Tg.