bluuebunny/crossref_metadata_2025 huggingface dataset:
The applicability of an r-dependent integral equation parameter to the study of low-temperature Lennard-Jones fluids
Authors: Dotson Allen C.
Published: February 2003

An r-dependent parametrization of the direct correlation function is investigated as a means of (1) obtaining better values of pressure, for low-temperature Lennard–Jones fluids, than the Percus–Yevick or hypernetted chain methods provide, and (2) providing a fuller understanding of why these methods do not work well in the liquid region. Both assets and liabilities of this choice of parametrization are discussed.
Hydrodynamic interaction effects on spherical macroion diffusion
Authors: Weissman Michael B.
Published: February 2003

Hydrodynamic interactions are shown to contribute a term of order one third in volume concentration (φ) to the mutual frictional coefficient of spherical macromolecule solutions when the static structure factor is held constant. A cell-model calculation indicates a leading term in the expression for the fractional increase of the friction coefficient due to strong repulsion of (9/8)φ1/3.
Small-ion friction, hydrodynamic interaction, and charge fluctuation effects in solutions of a globular macroion
Authors: Weissman M. B. , Marque J.
Published: February 2003

The initial slope of the quasielastic light-scattering (QELS) autocorrelation function taken on solutions of charged bovine serum albumin is shown not to scale inversely with the scattering intensity as the interaction strength is varied by changing the ionic strength. This effect is shown to be nearly independent of scattering q vector over a factor of 3 variation in q2. Molecular expansion is shown not to occur. The apparent increase in the friction factor is shown to be partly due to counterion friction. A measure of the nonexponentiality of the autocorrelation functions is used together with thermodynamic parameters to calculate an upper limit for the protein charge heterogeneity.
The migration of electrons in disordered solids
Authors: Iguchi K. , Suzuki K.
Published: February 2003

The migration of electrons in disordered solids was studied by theoretical analysis and computer simulation. The transfer energy between sites was assumed to be randomly distributed while the site energy to be constant, considering the loose structure of organic solids. The theoretical analysis gives results that the root mean square displacement in initially proportional to time (coherent motion), while after a long time lapse it becomes proportional to the square root of time (diffusive motion). Also, the mean square displacement is found to be proportional to the ratio of the mean value to the width of transfer energy. When the disorder is small the computer simulation gives curves of whose characters are similar to those given theoretically. When the disorder is large, however, the simulation curves show the saturation at long time lapse, that is, the electron is localized in this case. Discussion is made on this point.
A molecular dynamic calculation of the structure of B2O3 glass
Authors: Soules Thomas F.
Published: February 2003

A molecular dynamic calculation on vitreous B2O3 reveals a structure in which boron is trigonally coordinated to oxygen. Boron atoms are at the center of approximately regular triangles. The triangles are connected at apices to form a three-dimensional network of interconnected ribbons and sheets. No tendency to form boroxol groups is found and a reinterpretation of the x-ray data of Mozzi and Warren is suggested. A comparison is made with the molecular dynamic structure of silica.
Investigation of the thermodynamic stability of the molecules YAu (g) and YIr (g) by high temperature mass spectrometry
Authors: Haque R. , Pelino M. , Gingerich K. A.
Published: February 2003

Using the high temperature mass spectrometric technique, the molecule YIr and the previously known molecule YAu have been observed in the gas phase over a Y–Ir–Au–graphite system. Thermodynamic evaluation of experimental data yielded the following thermochemical values:
The Lennard-Jones 9:3 adsorptive system. II. Prediction of the liquid density profiles by perturbation theory
Authors: Lee Lloyd L.
Published: February 2003

To predict the density profiles of liquids adsorbed onto the Lennard-Jones 9:3 (LJ 9:3) wall, we derive here two new perturbation equations (called PYP and PYP1 equations) taking a simpler adsorptive system as reference. The derivation is based on the functional derivative formalism of Lebowitz and Percus for inhomogeneous systems. The generating functional is of Percus–Yevick (PY) type. The resulting expressions are first order theories. Previous known perturbation formulas, such as the blip function equation and the enhanced PY equation, are identified as special cases (zeroth order theories) of the present equations. Connections can also be made to the Ebner–Saam–Stroud density functional equation. Numerical calculations were carried out for liquid state adsorption of Lennard-Jones 12:6 molecules and hard spheres. Two types of perturbation are examined: (i) perturbation of the density profile from the hard spheres near a Weeks–Chandler–Andersen type repulsive wall to hard spheres near a Lennard-Jones 9:3 attractive wall, and (ii) perturbation of the density profile from hard spheres near a LJ 9:3 wall to Lennard-Jones 12:6 molecules near the LJ 9:3 wall. Comparison with Monte Carlo results shows that the PYP1 approach is the most satisfactory theory among all equations tested. The blip function results are inadequate for the attractive wall, while the PYP theory gives peculiar density profiles at intermediate distances despite good agreements at shorter and longer ranges.
Experimental test of the time-dependent Ising model
Authors: Huang H. W. , Negendank William
Published: February 2003

The one dimensional Ising model and its variations have been successfully used to describe the statistical properties of polymers, such as polypeptides, DNA, etc. Recently one of us solved the hydrodynamic solution of Glauber’s time-dependent Ising model. This solution is suitable for describing kinetic processes of Ising systems. The recent data on the potassium–sodium exchange in human lymphocytes provide an ideal case for testing the theory. It was previously found that the intracellular K+ and Na+ concentrations vs extracellular K+ concentration follow the Ising isotherm. In this paper we show that the kinetic data of potassium uptake in potassium-depleted, sodium-loaded cells agree with the theoretical result.
Application of CIBS theory to the H2 molecule
Authors: McDowell Keith
Published: February 2003

The first application to a molecular system of the theory of corrections to an incomplete basis set (CIBS) is reported. The CIBS theory computer perturbative corrections to the HF energy obtained from a given incomplete basis set (IBS) and thus obtains an approximation to the HF energy in larger basis sets without the need for an SCF procedure. Computations are carried out for the H2 molecule using the differential equation format of CIBS theory.(AIP)
Exact solution of the NQR I=1 eigenvalue problem for an arbitrary asymmetry parameter and Zeeman field strength and orientation
Authors: Muha G. M.
Published: February 2003

A general, explicit, and closed-form solution is developed for the general NQR I=1 eigenvalue problem that is valid for an arbitary magnitude of the asymmetry parameter (n) and strength and orientation of the Zeeman field.(AIP)
An approximate treatment of polymer gelation in a solvent
Authors: Gonzalez A. E. , Muto S.
Published: February 2003

We present an approximate treatment of a model for gelation of long polymer chains immersed in a solvent. The polymer chains are capable of forming hydrogen bonds when they touch. Scaling results for the radius of gyration of the chains by Daoud and Jannink are used. The resulting gelation curves for different solvents, when plotted on the same temperature–concentration diagram, show the crossing which is in agreement with recent experimental results by Ruiz–Azuara et al.
Laser flash photolysis of benzene. II. Laser-induced cluster formation in gas phase
Authors: Nakashima Nobuaki , Inoue Haruo , Sumitani Minoru , Yoshihara Keitaro
Published: February 2003

Cluster formation on the nanosecond time scale was observed when benzene was excited by a KrF excimer laser at 248 nm.(AIP)
EXAFS study of dehydrated cobalt exchanged A zeolite
Authors: Morrison T. I. , Iton L. E. , Shenoy G. K. , Stucky G. D. , Suib S. L. , Reis A. H.
Published: February 2003

The authors study the structure of the transition metal exchanged zeolites by the Extended X-ray Absorption Fine Structure of the exchanged metal ion. (AIP)
Comment on ‘‘Existence of H2−, a relatively long-lived doubly charged negative atomic hydrogen ion’’
Authors: Odom R. W. , Anbar M.
Published: February 2003

In connection with the production of H2− ion, the author reports new experimental evidence which demonstrate that the fast H− ions are in fact produced by charge transfer processes rather than by autoionization of H2−. (AIP)
Model for fast, nonadiabatic collisions between alkali atoms and diatomic molecules
Authors: Hickman A. P.
Published: February 2003

Equations for collisions involving two potential surfaces are presented in the impact parameter approximation. In this approximation, a rectilinear classical trajectory is assumed for the translational motion, leading to a time-dependent Schroedinger’s equation for the remaining degrees of freedom. Model potentials are considered for collisions of alkali atoms with diatomic molecules that lead to a particularly simple form of the final equations. Using the Magnus approximation, these equations are solved for parameters chosen to model the process Cs+O2→Cs++O2−, and total cross sections for ion-pair formation are obtained as a function of energy. The results exhibit oscillations that correspond qualitatively to those seen in recent measurements. In addition, the model predicts that the oscillations will become less pronounced as the initial vibrational level of O2 is increased.
Photodetachment spectroscopy of C2− autodetaching resonances
Authors: Jones P. L. , Mead R. D. , Kohler B. E. , Rosner S. D. , Lineberger W. C.
Published: February 2003

The cross section for photodetachment of C2− is investigated in the photon energy range 14 000–20 000 cm−1 (1.75–2.5 eV). Sharp resonances due to autodetachment are observed at photon energies corresponding to transitions between high vibrational levels of the C2− X 2Σg+ state and high vibrational levels of the C2− B 2Σu+ states. The resonances are narrower than 6 GHz, and those arising from the v=5 level of the B state are about 1/10 as strong as those arising from the v=6 and higher levels. In addition, the signal from the allowed direct photodetachment of C2− X state, which would produce a smooth background cross section, is not observed, indicating that it is less than the 103 peak to background contrast ratio. A rotational analysis of the nine bands of the C2− B–X transition observed in this study, coupled with previous measurements provides a new, more precise set of spectroscopic constants for these states. These constants are then used to generate RKR potential curves for the B and X states of C2−. Based on all C2− data, the only consistent interpretation of the observations is that the electron affinity of C2 is bounded by 3.374 eV ?EA(C2)?3.408 eV. The autodetachment rate into (C2 a 3Πu+e) is deduced to be much faster than the rate into (C2 X 1Σg++e), probably as a result of strongly R dependent configuration interaction in the C2− B state. The weakness in the direct photodetachment process is attributed to poor vibrational overlap between the initial and final states.
The electronic structure of small nickel atom clusters
Authors: Basch Harold , Newton M. D. , Moskowitz J. W.
Published: February 2003

The ground state electronic structure of small nickel atom clusters (Nin, n=1–6) has been calculated using the ab initio effective core potential self-consistent field (SCF) method in a Gaussian expansion basis. The electronic configuration of the nickel atoms in the clusters is found to be very close to 3d94s1. The ground state electronic configurations for Nin generally have n unpaired 3d electrons in molecular orbitals (MO’s) spanning the same irreducible representations as the 4s atomic orbitals while the n 4s electrons fill their MO’s in accord with a simple three-dimensional Hückel model with overlap. Exceptions to this description are found in the cases of linear systems where the 3d holes prefer δ over σ symmetry and in octahedral Ni6 where a different preferred set of 3d holes is obtained. The SCF ground state wave functions correspond roughly to a model in which the 3d electrons can be viewed as weakly interacting localized 3d9 units. The clusters are bound together primarily by the 4s electrons with the 4p orbital contribution increasing in importance with cluster size and dimensionality. The binding energy per nickel atom generally increases as the size of the cluster increases, although at six atoms this quantity has not yet converged with cluster size. The density of states diagram for the occupied one electron energy levels in Ni6 is found to be very different from the corresponding types of diagrams obtained in the muffin tin (MT)–Xα method for small nickel atom clusters. This difference is examined in detail, with consideration given to the effects of relaxation energy and to the different orbital level filling criteria used in the two methods.
Study of a Lorentz-gas based on correlated walks
Authors: Fujita S. , Okamura Y. , Blaisten E. , Godoy S. V.
Published: February 2003

The dynamics of a Lorentz-gas molecule is simulated in terms of correlated walks on cubic lattices. For a finite lattice with reflecting boundary, the ergodicity is established so that the probability distribution in position and direction approaches a stationary state which is homogeneous (site-independent) and isotropic (direction-independent). Exact expressions for probability distribution in direction are obtained for simple, body-centered, and face-centered cubic lattices. They approach equilibrium exponentially with a different number of relaxation times depending on the lattice. The directional probabilities for a single-site lattice with reflecting boundary approach equilibrium in an oscillatory manner. The Boltzmann H-function for this system, however, shows a monotonic behavior, and coincides with the H-function for the same system with periodic boundary. The parameters in the model, step length and unit time, can be eliminated in the kinetic theoretical limit. In this limit, the diffusion coefficient calculated for the lattice reproduces the result by means of the Boltzmann equation within the restriction imposed on the directions of the movement.
The specific heat anomaly in triethylamine–heavy water near the critical solution point
Authors: Bloemen E. , Thoen J. , Van Dael W.
Published: February 2003

The anomaly in the specific heat Cpx has been investigated by means of a constant heating rate method. Accurate results could be obtained for ‖T-Tc‖/Tc values as small as 7×10−6. From a range shrinking procedure and nonlinear-least-squares fits to the data we obtained α=α′=0.107±0.002, which is close to the renormalization group (RG) value αRG=0.110±0.008 for the Ising model for the dimensionality d=3. For the universal amplitude ratio A′/A of the diverging part of the specific heat above and below Tc, a value of 1.75±0.03 is obtained. This is also in agreement with theoretical predictions for the Ising model. Combining the amplitude of the correlation length with the critical amplitude of the specific heat allowed us to test two-scale-factor universality. For the universal constant X=Avξ30/kB in this system, a value of (1.9±0.4)×10−2 is obtained, which is in agreement with theory and with results for other fluid systems.
Infrared spectra of matrix-isolated uranium oxides. III. Low-frequency modes
Authors: Green D. W. , Reedy G. T. , Gabelnick S. D.
Published: February 2003

Infrared absorption spectra between 50 and 350 cm−1 have been obtained of argon matrices containing UO3 and UO2. The frequencies of all three expected low-frequency vibrational modes of U16O3 have been identified and measured (±0.1 cm−1): ?3(A1)=186.2, ?5(B1)=211.6, and ?6(B2)=151.5 cm−1. The effects of 18O-substitution have been measured and interpreted to confirm these assignments. All results are consistent with a C2v, T-shaped structure for the UO3 molecule. The bending mode of U16O2 was measured at 225.2±0.5 cm−1. The measured frequencies of U16O18O and U18O2, the measured relative absorbances, and the observed mid-infrared spectrum support the assignment for UO2. In addition, broad lower frequency bands were observed that may be due to impurity-activated phonon modes of solid argon.
Coherent dynamics of multilevel systems
Authors: Amirav A. , Even U. , Jortner Joshua , Kleinman L.
Published: February 2003

In this paper we report the results of an experimental study of the coherent dynamics (CD) of the (2J+1), with J=1, 2⋅⋅⋅6, low-field, equally spaced Zeeman components of the oxygen molecule O322 (O16–O16) and O342 (O16–O18) driven by a radio frequency (rf) field in a supersonic molecular beam. A supersonic molecular beam magnetic resonance spectrometer has been employed in the ’’flop out’’ mode, where two intense inhomogeneous magnetic fields are antiparallel, so that the resonating Zeeman levels are defocused at the detector, diminishing the beam intensity. The experimental signal interrogates the total number of molecules in a given total angular momentum state J which changed their initial MJ component. The rf amplitude spectroscopy has been applied to study the dependence of the signal on the amplitude of the rf field for various Zeeman manifolds on-resonance, and to investigate the dependence of the signal on the rf frequency. An analytical solution for the coherent dynamics of a spin J system driven by an rf field, which is valid both for on-resonance and for off-resonance situations, was utilized for the analysis of the experimental data. This confrontation between experiment and theory elucidated some of the universal features of the CD of multilevel systems. Inhomogeneous dephasing of the CD effects, which is due to the velocity distribution in the beam, was quantitatively analyzed, providing a novel method for the determination of translational temperatures in supersonic beams.
The MPI spectrum of expansion-cooled ammonia: Photophysics and new assignments of electronic excited states
Authors: Glownia James H. , Riley Stephen J. , Colson Steven D. , Nieman George C.
Published: February 2003

The two- and three-photon spectrum of expansion-cooled ammonia has been recorded in the 380–500 nm region using multiphoton ionization (MPI) detection. In addition, the vacuum ultraviolet spectrum of gas phase ammonia has been measured photoelectrically in the 125–160 nm region. Features due to transitions to eleven electronic states are assigned in the 5.7–9.3 eV energy range, where only five band systems were previously assigned. By utilizing the spectral simplification provided by expansion cooling, the different selection rules for multiphoton absorption, and the differences between the MPI and VUV spectra, assignments can be made with much more certainty. Three previously assigned band systems have been reassigned, and seven additional electronic states identified. It is found that states arising from nd orbitals are responsible for the most intense features in the VUV spectrum, in contrast to previous experimental and theoretical work where they were excluded from consideration. Evidence is found for direct competition between photodissociation and photoionization of NH3 excited states. Earlier photochemical studies of ammonia are reinterpreted in light of the new spectroscopic assignments.
A holographic technique for investigating photochemical reactions
Authors: Bjorklund G. C. , Burland D. M. , Alvarez D. C.
Published: February 2003

A new experimental technique for following the progress of a photochemical reaction is described. The technique involves following the temporal growth in intensity of a hologram produced by the interference of two beams in a medium containing the species whose photochemistry is being investigated. The theory of the formation of a photochemical hologram is described and a simple expression obtained that relates the growth of the hologram to photochemical parameters of interest. The technique is used to study the photodissociation of dimethyl-s-tetrazine and hydrogen abstraction by camphorquinone in polyvinylcarbazole matrices.
Generalized Langevin theory for many-body problems in chemical dynamics: Gas-surface collisions, vibrational energy relaxation in solids, and recombination reactions in liquids
Authors: Brooks C. L. , Berkowitz M. , Adelman S. A.
Published: February 2003

Model classical trajectory simulations of gas-solid inelastic collisions, vibrational energy relaxation of a diatom in a solid, and atomic recombination reactions in a liquid are presented. These simulations are first applications of the molecular timescale generalized Langevin equation (MTGLE) theory [S. A. Adelman, Adv, Chem. Phys. 44, 143 (1980)] for condensed phase energy transfer and chemical reaction processes. The main conclusions of these model studies are as follows: (i) The MTGLE theory often provides computationally practical methods for condensed phase chemical problems. These methods are applicable with equal ease to both solid and liquid state processes. (ii) The equivalent harmonic chain heatbath modeling for reducing many-body chemical problems to effective few-body problems often converges rapidly. This rapid convergence is found for both short (subpicosecond) timescale processes (gas–solid collisions) and long (nanosecond) timescale processes (vibrational energy relaxation). (iii) The simplest harmonic chain model (heatbath replaced by a single fictitious atom) is often found to yield a qualitatively correct description of heatbath influence on both solid and liquid state processes. (iv) The MTGLE parameters ωe0 (chemical system Einstein frequency) and ω2c1 (chemical system/heatbath coupling constant) are the quantities which often determine the gross magnitude of heatbath influence on condensed phase chemical processes. For both solid and liquid state processes ωe0 determines the effective vibrational frequencies (normal modes) of the chemical system and ω2c1 determines the gross efficiency of chemical system/heatbath energy transfer. For liquid state processes ωe0 also determines the frequency of oscillations in the solvent cage, and ω2c1 also determines rigidity of the solvent cage. (ω2c1=0 means the cage is perfectly rigid).
Fluorescence of supercooled molecules as a probe for intramolecular vibrational redistribution rates
Authors: Mukamel Shaul , Smalley Richard E.
Published: February 2003

A theory is developed for the fluorescence spectra of supercooled polyatomic molecules in a jet. A single molecular mode (the ’’system’’) is assumed to be optically active and the other modes constitute a ’’bath’’ which induces line broadening and relaxation. Since the bath is finite and is initially cold, its motion is strongly correlated with that of the system. The correlated reduced dynamics of the system and bath is treated using the Mori projection operator technique. The present ’’tetradic reduced description’’ becomes useful especially in large statistical molecules where many molecular states are involved in the fluorescence. This is precisely the limit where the effective Hamiltonian formalism commonly used for the intramolecular dynamics in intermediate size molecules becomes very tedious and impractical. The fluorescence spectra in large statistical molecules may be interpreted in terms of simple rate equations and may be used to directly monitor the intramolecular vibrational redistribution of energy (IVR). We further compare the spectra predicted for statistical and intermediate size molecules with the existing experimental results in supersonic beams.
Anomalous V1←N vibrational band intensities in the low energy electron impact spectra of t
r
a
n
s-dienes
Authors: McDiarmid Ruth , Doering J. P.
Published: February 2003

A detailed examination of the extremely broad and diffuse vibrational structure of the V1←N allowed valence transition in trans-1,3-butadiene, three related trans-dienes (cis-1,3-pentadiene, trans, trans-2,4-hexadiene, and 2,3-dimethyl-1,3-butadiene), and the cis-diene 1,3-cyclohexadiene has shown that the effect observed previously by Doering, in which the lower vibrational bands of the transition are enhanced in intensity relative to the higher bands at low incident electron energies, occurs in all the trans-dienes studied. This result shows that the effect cannot be due to an underlying, forbidden electronic transition. It appears that in these trans-dienes, the configurational composition of the V1 ’’state’’ depends on the geometry of the molecule and that the configurational components of the excited state that are important in the vertical transition have different electron impact excitation cross sections than those that are important in the adiabatic transition. These results suggest that electron energy loss spectroscopy is a unique tool for probing the configurational composition of certain highly distorted excited states.
The generation and ESR investigation of the BeF radical in rare gas matrices
Authors: Knight L. B. , Wise M. B. , Childers A. G. , Davidson E. R. , Daasch W. R.
Published: February 2003

The 13 electron BeF radical has been generated and trapped in argon and krypton matrices at 12 K and studied via ESR spectroscopy. Comparisons are made between BeF and other similar radicals. The calculated dipole moment for BeF is 1.1 D. The magnetic parameters in argon are g∥=2.001, g⊥=2.001, Aiso(Be)=294(2) MHz, and Aiso(19F) =230(2) MHz. The 19F hfs is unusually isotropic for an MF radical, suggesting that rotational averaging might be occurring. Results of ab initio theoretical calculations are compared to the observed A tensors. The BeF molecule was generated by the codeposition of thermally produced Be atoms and molecular fluorine in the rare gas matrices.
Reactive scattering of rotationally excited target molecules with adiabatic theory
Authors: Choi B. H. , Poe R. T. , Sun J. C. , Tang K. T.
Published: February 2003

We present a formulation of the three-dimensional quantum mechanical reactive scattering of an atom and a rotationally excited diatomic target molecule within the framework of adiabatic distorted wave theory. This is an extension of previous work where only the rotationally ground initial state was treated while the final molecule could be in any state. The importance of the present formulation lies in the fact that the population of the rotationally excited target molecules is significant under ordinary experimental conditions. A method of obtaining exact and approximate adiabatic wave functions and energies is developed through the use of the body-fixed formulation of atom–diatomic molecule scattering. The integration in transition matrix with rotationally excited adiabatic wave function is again reduced to the three-dimensional integral by separating out the angular variables for the rigid motion of the plane for the atom–molecule system. Explicit integration formula is presented for the reactive transition matrix element. The computational feature of the present formulation is illustrated by evaluating the reactive cross section of the (D,H2) system. For 0→1 rotational transition, present calculations reproduced earlier results. For 1→1 rotational transition, for which no previous result of the adiabatic distorted wave theory is available, the present calculations yield qualitatively similar but quantitatively different angular dependence in the differential cross sections, as in those of the rotationally ground target molecule. Physical significances and further implication of the present formulation are discussed.
Solvent effects on vibrational phase relaxation and frequency shifts in isobutylene
Authors: Schindler Wolfgang , Jonas Jiri
Published: February 2003

The Raman line shapes and frequencies of the ν4 asymmetric (C=CH2) stretching mode at 1657 cm−1 and the ν9 symmetric (C–CH3) stretching mode at 805 cm−1 (both of symmetry A1) have been measured in dilute solutions of isobutylene in several polar and nonpolar solvents. The frequency shifts in the different solutions can be interpreted in terms of repulsion, dispersion, induction, and dipolar forces between molecular bonds. The induction and dipolar forces play only a minor role. The blue shift due to the repulsion forces and the red shift due to the attractive dispersion forces are nearly equal for ν9, while the dispersion forces are much more important for ν4. Analogous behavior has already been described in our earlier work dealing with pure compressed isobutylene. The frequency shift and the linewidth are connected via the first and second spectral moments, and the order of magnitude of the width can be predicted from the frequency shifts. However, quantitative calculations are more difficult for linewidths than for shifts. Qualitatively, the linewidth behavior is consistent with the result for the vibrational frequencies concerning the relative importance of the repulsive and attractive energies.
Time-resolved measurements of electron and energy transfer of rhodamine B monolayer on the surface of organic crystals
Authors: Nakashima Nobuaki , Yoshihara Keitaro , Willig Frank
Published: February 2003

Fluorescence decays of a monolayer of rhodamine B on single crystals of anthracene, phenanthrene, and naphthalene have been measured for the first time with a picosecond laser and a streak camera. The fluorescence decays were not single exponentials. Two different decay characteristics have been observed corresponding to an exo- and endo-energetic electron transfer reaction between excited dye and substrate. The short decay of 35±7 ps on an anthracene crystal is explained by the electron-transfer kinetics from anthracene to excited rhodamine B. In the cases of naphthalene and phenanthrene,the electron transfer reaction becomes endo-energetic and slower. The decays within several tens of picoseconds are analyzed in terms of two-dimensional Förster-type energy transfer to quencher sites where dyes are suggested to be in contact with each other. In connection to this phenomenon, the concentration quenching of the fluorescence of rhodamine B in aqueous solution was elucidated in terms of the Förster-type energy transfer to the nonfluorescent dimer leading to a nonexponential decay.
Thermoluminescence and emission spectra studies of 6-methylmercaptopurine and 6-methylmercaptopurine riboside x irradiated at 10 K
Authors: Cooke D. Wayne , Rhodes Joanne F. , Santi Ronald S. , Alexander Chester
Published: February 2003

In this work we report the first thermoluminescence (TL) and emission spectra studies of the sulphur-containing DNA base analog 6-methylmercaptopurine (6MeMP), and its riboside, 6-methylmercaptopurine riboside (6MeMPR),x irradiated at 10 K. The 6MeMP glow curve exhibits TL peaks at 28, 40, 68, 78, 112, and 140 K with typical emission from each peak consisting of a band possessing maxima at 500 and 530 nm. Only three prominent peaks were found in the 6MeMPR glow curve: 42, 52, and 140 K. Typical emission from each of these TL peaks consisted of a band with maxima at 455 and 485 nm. Thermal activation energies and frequency factors associated with each glow peak were extracted from the experimental data and used in formulating a model to explain the observed emission. Ultraviolet (UV)-induced glow curves, emission spectra, and photobleaching studies were also conducted in efforts to deduce the mechanisms of charge recombination in 6MeMP and 6MeMPR. The results indicate that metastable states exist in both 6MeMP and 6MeMPR which are filled by ionizing radiation. TL occurs when these trapped charges are thermally released and decay to the ground state (So) via the first excited singlet state (S1) or the first excited triplet state (T1). An energy level diagram depicting the TL emission process is presented which shows that the energy separation of S1 and T1 is approximately 0.14 eV. Further, we find that some of the TL peaks in both samples possess thermal activation energies less than 0.14 eV, suggesting that they lie between S1 and T1 in energy; these peaks are characterized by unusually small frequency factors (0.3 to 3000) s−1. Our results are consistent with a model originally proposed by Weissbluth et al. and modified by Tatake et al. to explain TL in nucleic acid bases. Finally, we suggest that the 140 K peak in 6MeMP results from the thermal destruction of an electron-adduct radical whose molecular structure and thermal properties have been previously determined by ESR studies.
Debye–Hückel theory for particles of arbitrary electrical structure
Authors: Ramshaw John D.
Published: February 2003

Classical linearized Debye–Hückel theory is formulated for a finite fluid system, of arbitrary shape, composed of rigid particles with arbitrary internal electrical structure. The multipole description is eschewed in favor of the more basic description of a particle in terms of its charge density function. This function is left arbitrary, so the particles may be charged or neutral, polar or nonpolar, etc. The theory implies that the direct correlation function c(12)=−v(12)/kT, where v(12) is the Coulomb interaction energy between the charge densities of particles 1 and 2. In the case of uncharged polar molecules, the dielectric constant may be evaluated in closed form from c(12); the result is the Langevin–Debye equation. This development removes the nonuniqueness in the original formulation of dipolar Debye–Hückel theory [J. Chem. Phys. 64, 3666 (1976)], and demonstrates that this nonuniqueness was an artifact of the multipole description rather than the mean-field approximation. Specialization to the case of simple finite dipoles shows that the nonuniqueness is associated with premature passage to the point dipole limit.
Wave vector modification of the infinite order sudden approximation
Authors: Sachs Judith Grobe , Bowman Joel M.
Published: February 2003

A simple method is proposed to modify the infinite order sudden approximation (IOS) in order to extend its region of quantitative validity. The method involves modifying the phase of the IOS scattering matrix to include a part calculated at the outgoing relative kinetic energy as well as a part calculated at the incoming kinetic energy. An immediate advantage of this modification is that the resulting S matrix is symmetric. We also present a closely related method in which the relative kinetic energies used in the calculation of the phase are determined from quasiclassical trajectory calculations. A set of trajectories is run with the initial state being the incoming state, and another set is run with the initial state being the outgoing state, and the average final relative kinetic energy of each set is obtained. One part of the S-operator phase is then calculated at each of these kinetic energies. We apply these methods to vibrationally inelastic collinear collisions of an atom and a harmonic oscillator, and calculate transition probabilities Pn1→nf for three model systems. For systems which are sudden, or nearly so, the agreement with exact quantum close-coupling calculations is substantially improved over standard IOS ones when Δn=‖nf−ni‖ is large, and the corresponding transition probability is small, i.e., less than 0.1. However, the modifications we propose will not improve the accuracy of the IOS transition probabilities for any collisional system unless the standard form of IOS already gives at least qualitative agreement with exact quantal calculations. We also suggest comparisons between some classical quantities and sudden predictions which should help in determining the validity of the sudden approximation. This is useful when exact quantal data is not available for comparison.
A 13C NMR study of the adsorbed states of CO on Rh dispersed on Al2O3
Authors: Duncan T. M. , Yates J. T. , Vaughan R. W.
Published: February 2003

The results of nuclear magnetic resonance (NMR) spectroscopy have been analyzed with respect to previous infrared studies of CO adsorbed on Rh dispersed on Al2O3 to quantify the site distribution and to describe the adsorbed state. The 13C NMR spectra account for all the 13CO adsorbed on a 2.2% Rh on Al2O3 substrate. Although the spectra from the different adsorbed states of CO overlap, the line shapes may be separated into two components based on differences in the 13C spin–lattice relaxation times. These two components have been assigned to the 13CO dicarbonyl formed on single Rh atoms and to 13CO adsorbed on Rh rafts. The component attributed to the CO adsorbed on the raft sites is further separated into linear and bridged CO state contributions based on chemical shift information, yielding a quantitative distribution of the three adsorbed states of CO on Rh. The 13CO distribution is used to estimate the molar integrated intensities of the infrared spectrum of 13CO on Rh at high coverage and to determine the degree of dispersion of Rh on the Al2O3. The 13C NMR line shapes of CO adsorbed on Rh are different from the powder pattern of Rh2Cl2(CO)4. It is suggested that the line shape of the dicarbonyl surface species is narrowed to a Lorentzian curve by reorientation at the site and the line shape of CO on the Rh rafts is modulated by exchange between sites on a single raft. The 13C relaxation time distribution provides further evidence for the existence of isolated Rh atoms on the Al2O3 surface.
Temperature reversible site structural change for SF6 isolated in an argon matrix
Authors: Swanson Basil I. , Jones Llewellyn H.
Published: February 2003

The temperature dependence of the infrared spectra of the ν3 region of SF6 in Ar was measured at 8 and 20 °K. (AIP)
Rate coefficient measurements for SO2+O=SO+O2
Authors: Slack M. , Grillo A.
Published: February 2003

The rate coefficients for the gas phase reaction SO2+O→SO+O2 have been determined between 3320 and 3760 °K from measured SO2 decay rates in shock heated mixtures of SO2-N2O-Ar. (AIP)
A condition for ignition at the stagnation point on a heated curved wall
Authors: Okamoto Masaomi , Hasegawa Eiji
Published: February 2003

The ignition conditions are established for two-dimensional stagnation flow of a combustible mixture adjacent to a convex heated wall. (AIP)
Measurement of the specific refractivities of CF4 and C2F6
Authors: Burner A. W. , Goad W. K.
Published: February 2003

The refractivities of CF4 and C2F6 have been determined with a two-beam interferometer. The values obtained fall within 1% of the values computed from the atomic refractivities of carlem and fluorine. (AIP)
Should one use complex basis functions in coordinate rotation calculations on molecules?
Authors: Simons Jack
Published: February 2003

The choice of complex basis sets in coordinate rotation method for electron-molecule scattering resonances is considered. (AIP)
Antiferromagnetic spin-1 Ising model. II. Interface structure and kinetic phase transition
Authors: Saito Y. , Müller-Krumbhaar H.
Published: February 2003

The interface structure and the kinetics of a spin-1 antiferromagnetic Ising model are investigated and applied to crystal growth. For a certain range of model parameters the system undergoes two phase transitions: crystallization (nonmagnetic to magnetic) and an order–disorder (antiferromagnetic) transition of the sublattice ordering in the solid phase. Thus there are two interfaces associated with the three different phases. The structure of each interface is described by a tanh profile in a time-dependent Ginzburg–Landau approach. Various properties of the interfaces which depend on the driving force are studied. A remarkable change in structure is observed for large interface velocities: If the interface between an ordered-solid phase and a gas phase exceeds a critical velocity, a disordered-solid phase forms in between, with a width which increases proportional to time (kinetic phase transition).
On the computation of Onsager quantum efficiency
Authors: Yip Kwok-leung , Li Leonard S. , Chen I.
Published: February 2003

A new method of computing the Onsager quantum efficiency of photogeneration is presented. This method is shown to be much faster than and as accurate as the series method used previously. Results applicable to a-Si (dielectric constant=11.5) are given as an example.
On a close correspondence between inversion methods for atom–molecule and symmetric exchange scattering
Authors: Jordan R. M. , Siska P. E.
Published: February 2003

The similarity between Gerber, Buck and Bucks1 method for obtaining atom-diatom anisotropy potential from differential cross sections for rotational transitions and the authors12 symmetric exchange systems is commented upon. (AIP).
Simultaneous infrared transitions in N2 + SF6 mixtures
Authors: Brodbeck C. , Bouanich J.-P. , Figuiere P. , Szwarc H.
Published: February 2003

A high pressure long-path cell used in the study of collision-induced simultaneous transitions in binary gas mixtures is described. An application is made to N2 + SF6 mixtures. The transition of the ν3 band of SF6 associated with the fundamental vibration of N2 has been extensively studied for low pressures of SF6 and densities of N2 up to 490 amagat. As density of N2 increases, the observed bands are markedly narrowed their absorption maxima are shifted, and the integrated intensities show a slight deviation from linearity.
15N nuclear magnetic shielding scale from gas phase studies
Authors: Jameson Cynthia J. , Jameson A. Keith , Oppusunggu D. , Wille S. , Burrell Patricia M. , Mason Joan
Published: February 2003

We have measured the 15N nuclear magnetic resonance frequencies in 15N-labeled molecules (NNO, NNO, NH3, N2, and HCN) in gas phase samples and also in CH3NO2 as neat liquid. By using the previously determined temperature dependence of samples of the these gases at various densities, we are able to reduce the measured frequencies to the zero-density limit at 300 K, and obtain shielding differences between rovibrationally averaged isolated molecules at this shielding measurements from molecular beam studies to provide an 15N absolute shielding scale based on 15NH3.
Vibrational and rotational spectroscopy of the first electronically allowed transition of α-dicarbonyls
Authors: Gurnick M. , Chaiken J. , Benson Thomas , McDonald J. D.
Published: February 2003

Laser induced fluorescence excitation spectra of the series glyoxal, methylglyoxal, biacetyl, and 2,3-pentanedione at high resolution show that except for glyoxal these molecules undergo a characteristic structural deformation during the transition. Our incomplete assignment of the methylglyoxal spectrum suggests that the methyl internal rotation and carbonyl skeletal motions (probably out of plane) are involved. The positions of the 0–0 bands shift to the blue as a methyl group is added to glyoxal (22 000 cm−1) and methylglyoxal (22 125 cm−1) but not as the side groups are extended from biacetyl (22 336 cm−1) to 2,3-pentandedione (22 340 cm−1). We cannot precisely identify the interaction between the carbonyls and methyl groups that cause these effects. As the size of the carbon skeleton increases the number of observed low frequency vibronic excitations increases and the energy at which the spectra break off decreases. These observations are consistent with the known radiationless behavior of these molecules.
State resolved rotational excitation in HD+D2 collisions. I. Angular dependence of 0→1 transitions
Authors: Buck U. , Huisken F. , Schleusener J. , Schaefer J.
Published: February 2003

In a crossed molecular beam experiment, time-of-flight distributions of HD molecules scattered from D2 have been measured at 42.2, 45.4, and 50.8 meV collision energy using the pseudorandom chopper method. In all cases rotational transitions of HD from j=0 to j=1 were resolved. At 45.4 meV complete differential cross sections have been generated from 10 ° to 120 °. They display well resolved diffraction oscillations in both the elastic 0→0 and inelastic 0→1 channels. Calculations show that these data essentially provide information on the position and the slope of the isotropic H2–H2 interaction potential near the zero point. The comparison between measurement and close coupling calculations based on the ab initio potential surface of Meyer and Schaefer shows a remarkable agreement. Only very small deviations in the positions of the diffraction oscillation extrema occur giving rise to a slight modification of the repulsive wall near the zero point. It has to be shifted by 0.1 Å to give a perfect fit to the measured data.
Kinetics and spectroscopy of near-resonant optical pumping in intense fields
Authors: Mies F. H. , Aryeh Y. Ben
Published: February 2003

The resonant Raman scattering in a three level system produces an optical pumping of molecules from the initial to the final states which are radiatively coupled to the intermediate resonant state. We consider the pumping that is produced by an intense, pulsed, monochromatic radiation field that is near-resonant with the initial resonance frequency. The Heitler–Ma damping theory is solved exactly for the case when the resonance fluorescence is weak compared to the Raman scattering, or any other processes which might cause irreversible decay of the resonant state. In particular, we examine the usefulness of dressed states to describe the intense field limit. The spectral line shape for the Raman scattering is obtained. The transient behavior following the onset of the laser pulse is examined and a careful analysis of the pumping rate and its relationship to both weak and strong field theories is presented.
Studies of reactions of importance in the stratosphere. IV. Rate constant for the reaction Cl+HOCl→HCl+ClO over the temperature range 243–365 K
Authors: Cook Jac-E L. , Ennis Christine A. , Leck Thomas J. , Birks John W.
Published: February 2003

The title reaction was studied over the temperature range 243–365 K using the discharge flow technique with mass spectrometry for detection. The rate constant was measured by detecting the loss of HOCl in the presence of a large excess of chlorine atoms. The resulting temperature dependent rate constant is given by k?0.5)×10−12exp[−(130±60)/T] cm3 molecule−1 s−1, where the stated errors include our estimate of the maximum possible systematic error. This reaction becomes a significant pathway to HCl in the stratosphere when the total ClX exceeds 10 ppbv. Upper limits for rate constants for the reactions HOCl+NO→products and HOCl+O3→products at 300 K were established to be 1.0×10−17 and 4.0×10−16 cm3 molecule−1 s−1, respectively.