bluuebunny/crossref_metadata_2025 huggingface dataset:
The Raman excitation spectra and absorption spectrum of a metalloporphyrin in an environment of low symmetry
Authors: Shelnutt J. A.
Published: February 2003

The effects of a symmetry lowering environment on the Raman excitation spectra of a metalloporphyrin have been studied using the generalized crude Born–Oppenheimer vibronic theory. Symmetry considerations reduce the number of matrix elements introduced by the interaction of the porphyrin with its environment. In practice, matrix elements of the model are determined by fitting the Raman excitation spectra. These matrix elements contain information about the interaction of the metalloporphyrin with its environment. This procedure is facilitated if the Raman excitation spectra of the metalloporphyrin in a more symmetrical environment have been measured and analyzed, since only the additional matrix elements describing the differences in the interaction with the environment must be determined. Some of the effects of environmental interactions on the excited states of metalloporphyrins are discussed. Calculations demonstrate that Raman excitation spectra and a new tool—Raman excitation difference spectra, provide a valuable means of examining these environmental effects.
Momentum space diffusion equations for chain molecules
Authors: Evans Glenn T.
Published: February 2003

A Fokker–Planck equation is derived for the reorientational and torsional motion of a flexible butane molecule. The analysis starts with the Liouville operator for a chain molecule in a fluid and by projection operator techniques yields an equation of motion for the reduced distribution function of the Euler angles, torsion angles, and the molecule-fixed angular and torsional momenta. A diffusion equation for butane is also derived by projection operator procedures. A fictitious metric potential arises in the diffusion equation and must be deleted by an offsetting potential. The time dependence of the diffusion coefficient, to lowest order in the streaming terms, can be obtained and yields a criteria for the applicability of Brownian motion models. The second moment of the torsion angle correlation function is also obtained and demonstrates a dependence on the dihedral potential, in contrast to the second moment of rigid body orientational correlation function which depends only on kBT and the appropriate components of the inertia tensor.
Electric deflection studies of metastable acetylene
Authors: Lisy James M. , Klemperer William
Published: February 2003

Molecular beams of a metastable electronic state of acetylene are prepared by electron impact excitation. The threshold for appearance of the state is near 4.5 eV. Electric deflection measurements of C2H2, C2D2, and HC2D show the polarity of all three isotopes to be below the level of detectability. Comparison of the behavior of the metastable state with that of the ground electronic state of HC2D, where polarity is readily established by electric deflection, is made. The polarity results are shown to establish a trans geometry for the observed state.
The electronic structure of β-alumina surface with an adsorbed sodium atom
Authors: Lee Woodrow W. , Choi Sang-il
Published: February 2003

The electronic states of a sodium atom adsorbed at a site on the ac surface of β-alumina is studied by the SCF–Xα method. The system is represented by a stabilized ion cluster NaAlO3. Electrons of all atoms except 1s electrons are included in the self-consistency calculation. A singly occupied surface state is found to appear at 3.8 eV above the highest doubly occupied state. This surface state accepts the sodium 3s electron and represents an electronic state loosely bound to the sodium ion with the binding energy of 2.8 eV.
Generalized Langevin theory for many-body problems in chemical dynamics: Modelling of solid and liquid state response functions
Authors: Berkowitz M. , Brooks C. L. , Adelman S. A.
Published: February 2003

The equivalent harmonic chain heatbath modelling method [S. A. Adelman, J. Chem. Phys. 71, 4471 (1979)] for reducing problems in condensed phase chemical dynamics to effective few-body trajectory problems is tested via the calculation of typical solid and liquid state velocity response (correlation) functions χ̇ (t). Finite nearest neighbor harmonic chain models for condensed phase atomic dynamics which may be used in trajectory simulations, are constructed from moments of the spectral density of χ̇ (t) and from additional information about the small z behavior of e.g., diffusion coefficients. Model response functions are then generated numerically as specified trajectories of the chain equations of motion. The convergence of chain model response functions is investigated for the bulk elastic model (isotropic continuum elastic treatment) of a solid and for the Gaussian approximation to the Levesque–Verlet molecular dynamics results for a Lennard-Jones liquid. For both cases, the chain model heatbaths are found to give response functions which: (i) converge rapidly and systematically; (ii) exactly reproduce the chemically crucial short-time response and thus account for caging (in liquids) and initial chemical system/heatbath energy transfer; (iii) reproduce the chemically less important long time response in a qualitatively correct manner. It is thus concluded that the chain modelling techniques yield a chemically and calculationally useful reduction of condensed phase dynamics.
Proton–H2 scattering on an a
b i
n
i
t
i
o CI potential energy surface. I. Vibrational excitation at 10 eV
Authors: Schinke R. , Dupuis M. , Lester W. A.
Published: February 2003

A complete configuration interaction (CI) ground state surface for the H3+ system has been calculated using 5S and 3(Px,Py,Px) basis functions at each center. A total of 650 nuclear geometries has been considered which makes the new surface appropriate not only for scattering calculations, but also for the evaluation of the vibrational–rotational spectrum of the H3+ molecule. Significant deviations are found from the analytic Giese and Gentry potential used in many previous theoretical studies, especially for large and small nonequilibrium H–H separations which are important for vibrational excitation of the H2 molecule. Vibrational–rotational excitation cross sections have been calculated in the rotational sudden approximation where the vibrational degree of freedom is treated exactly by solving seven vibrationally coupled radial equations. The use of the new surface leads to increased vibrational excitation compared to previous calculations utilizing the same scattering approximation and to excellent agreement at 10 eV with the angle-dependent measurements of Hermann, Schmidt, and Linder.
Theory of polymer conformation based on the correlated walk model
Authors: Fujita S. , Okamura Y. , Chen J. T.
Published: February 2003

A model polymer is allowed to grow in the same direction as that of the previous unit with probability α or turn at right angles with probability β but not to reverse. The mean square end-to-end distance <R2≳ of a polymer of N units, is calculated to be 〈R2〉/a02=[(1+α)/(1−α)]N−2α (1−αN)/(1−α)], where a0 is the unit length. An exact expression for the probability that the Nth unit points in the same direction as that of the zeroth unit is also obtained. A general way of interpreting the conformation of simple polymers in terms of the present model and the connection with other theories of the polymer conformation are discussed.
Interactions and structure in aqueous NaNO3 solutions
Authors: Caminiti R. , Licheri G. , Paschina G. , Piccaluga G. , Pinna G.
Published: February 2003

Two concentrated (5 and 7 mol l−1) aqueous solutions of NaNO3 were investigated by x-ray diffraction. The features of the total radial functions calculated from experimental intensity data show that the positional correlation is not very strong in the systems investigated. However, it is possible to account for experimental data only by using models in which hexahydrated Na+ ions have definite correlations with NO3− ions, even if not through formation of isolated ion pairs. The compatibility of our observations with spectroscopic results is also discussed.
Theoretical Compton profile anisotropies in molecules and solids. VII. Zero point Compton profile anisotropies and bond polarities in alkali halide diatomic molecules
Authors: Matcha Robert L. , Pettitt Bernard M.
Published: February 2003

A correlation between bond polarities and zero point Compton profile anisotropies is shown for alkali halide molecules. The correlation derives from the fact that both quantities are determined by molecular binding and antibinding forces and are thus functionally related to nuclear charge ratios.
Generalized classical theory of intramolecular coordinate relaxation in polyatomic molecules
Authors: Vicharelli Pablo A. , McDonald F. Alan
Published: February 2003

A generalized classical theory is developed to describe intramolecular relaxation to a minimum energy configuration after coordinate distortion. The relaxation process is characterized in terms of a minimum energy direction which provides a quadratic approximation to the minimum energy path for unimolecular dissociation or intramolecular rearrangement. The present treatment includes the general case of distortion of an arbitrary number of coordinates. In addition, the intramolecular forces which bring about the relaxation are considered, and a new bonding parameter, the relaxed interaction force constant, is discussed. Results of the application of this formalism to numerous polyatomic molecules are also presented.
Reactions of fast cesium atoms with polymers of antimony pentafluoride and gold pentafluoride
Authors: Compton R. N. , Reinhardt P. W.
Published: February 2003

The observation of SbF6− and AuF6− produced in collisional ionization reactions of fast Cs atomics with polymers of SbF5 and AuF5 is reported. (AIP)
Anomalous 13CH4:12CH4 line strengths in 2ν3
Authors: Fox K. , Halsey G. W. , Daunt S. J. , Blass W. E. , Jennings D. E.
Published: February 2003

Unexpected and dramatic differences in transition intensities between 13CH4 and 12CH4 in the infra red spectra of methane are reported. (AIP)
Parametrization of ion–ion mutual neutralization rate coefficients
Authors: Miller Thomas M.
Published: February 2003

The parameterization given by Hickman for common situation where there is a large number of interaction potential curves in the encounter 2 is discussed. The extent to which the experimental data supports this parameterization is presented. (AIP)
A crossed molecular beams investigation of the reactions O(3P)+ C6H6, C6D6
Authors: Sibener Steven J. , Buss Richard J. , Casavecchia Piergiorgio , Hirooka Tomohiko , Lee Yuan T.
Published: February 2003

A crossed beams investigation of the reactions of O(3P)+C6H6, C6D6 has beeen carried out using a seeded, supersonic, atomic oxygen nozzle beam source. Angular and velocity distributions of reaction products have been used to identify the major reaction pathways. The initially formed triplet biradical, C6H6O (C6D6O), either decays by hydrogen (deuterium) elimination or becomes stabilized, most likely by nonradiative transition to the S0 manifold of ground state phenol. CO elimination was not found to be a major channel. The branching ratio between H(D) atom elimination and stabilization was found to be sensitive to both collision energy and isotopic substitution.
Dynamic Padé approximants for chemical center waves
Authors: Bose Shubha , Bose Subir , Ortoleva P.
Published: February 2003

A model of reaction and diffusion is shown to exhibit composition center waves. The analysis is based on a Padé approximant scheme carried out in a completely self-consistent way. Evidence is given to show that these patterns may exist over a domain of wave vectors (of the outer plane wave region) that may exceed that of plane waves but may have gaps of forbidden wave vectors. Furthermore multiple centers consistent with a given outer domain may exist. Chaotic centers with shock structures may also exist as attractors in systems which also have periodic center attractors under identical conditions.
Ferroelectric–ferroelastic Tb2(MoO4)3 crystal structure temperature dependence from 298 K through the transition at 436 K to the antiferroelectric–paraelastic phase at 523 K
Authors: Abrahams S. C. , Svensson C. , Bernstein J. L.
Published: February 2003

Tb2(MoO4)3 is orthorhombic below Tc=436 K with space group Pba2 and tetragonal above with space group P4̄21m. The crystal structure, previously studied at 298 K, has been determined at four higher temperatures. The integrated intensities of 2909, 1325, 1340, and 1310 symmetry-independent structure factors were measured, respectively, at 373, 438, 463, and 523 K with a CAD-4 diffractometer using graphite monochromatized MoKα radiation. The lattice constants at 373 K are a=10.3625(13), b=10.3936(18), c=10.6446(29) Å. A face-centered tetragonal cell with space group C4̄m21, for orientation identical to that of the orthorhombic cell, has lattice constants a=10.4107(4), c=10.6273(1) Å at 438 K; a=10.4126(4), c=10.6267(1) Å at 463 K; and a=10.4167(4), c=10.6257(1) Å at 523 K. The final agreement factors following structural refinement by the method of least squares are R=0.0274(373 K), 0.0145(438 K), 0.0176(463 K) and 0.0322(523 K). Atomic displacements from the positions occupied at 438 K are greater, by as much as 0.1 Å, at 298 K than those at 373 K: above Tc, the largest comparable displacement is 0.02 Å. The atomic displacements required for polarization reversal or strain reorientation at 373 K are less than those at 298 K by a maximum of slightly over 0.1 Å. Differences in the atomic paths followed under the application of an electric or stress field from those in a thermal field are highly significant, ranging in length to more than 0.2 Å for two oxygen atoms. Two of the three independent MoO42− tetrahedra vibrate highly, the third somewhat less, anisotropically both above and below Tc. The three independent tetrahedra rotate gradually from the positions occupied at Tc, through angles less than 8°, as the crystal is cooled to 298 K.
Microwave rotational spectrum and internal rotation in g
a
u
c
h
e ethyl alcohol
Authors: Kakar Ramesh K. , Quade C. Richard
Published: February 2003

The microwave torsional–rotational spectra of several species of gauche ethyl alcohol have been assigned and analyzed. The species studied are CH3CH2OH, CH3CH2OD, CH2DCH2OH, and CH2DCH2OD, both methyl symmetric and asymmetric forms of the last two species. Rotational coefficients have been determined for all species. For the normal species, the components of the dipole moment have been determined as ‖μa‖=1.264±0.010 D, ‖μb‖=0.104±0.008 D, and ‖μc‖=1.101±0.016 D. The tunneling frequency between the two gauche substrates is measured to be Δ=97 734.3 MHz and the gauche–trans energy difference is measured to be 41.2±5.0 cm−1 for the normal species. The first three coefficients in the Fourier expansion of the hydroxyl internal rotation potential energy are determined to be V1=57.0 cm−1, V2=0.8 cm−1, and V3=395.0 cm−1. The barrier to methyl internal rotation for gauche CH3CH3OH is The barrier to methyl internal rotation for gauche CH3CH3OH is determined to be V3=1331 cm−1. Analysis of the interaction between methyl and hydroxyl internal rotations for CH2DCH2OH and CH2DCH2OD determine, for −OH: Vs1s2=8±2 cm−1; and for −OD: Vs1s2=4±1 cm−1 for the coefficient of sinα1 sinα2 in the effective internal rotation potential energy for these low symmetry species.
Is a two-state model approach applicable to liquid water?
Authors: Endo Harumi
Published: February 2003

It has been suggested that a two-state model theory is applicable to many physical properties of liquid water. On the basis of various proposed two-state water models, we found the calculated adiabatic relaxational compressibility of liquid water to exceed the measured value. By use of the empirical coefficient obtained by a two-state model approach to the isobar expansivity, it is found that the calculated value of the adiabatic compressibility at infinite frequency has a negative value for the various two-state models proposed. In the calculation of heat capacity (CV,∞) at constant volume and infinite frequency, the result also contradicts physical observations such that Cv,∞<0. Therefore, it seems that these model theories are inapplicable to liquid water.
Magnetic field dependence of nuclear spin polarization as a criterion for the triplet-Overhauser mechanism
Authors: Hutton Richard S. , Roth Heinz D. , Schilling Marcia L. Manion
Published: February 2003

The magnetic field dependence of nuclear spin polarization effects is evaluated as a mechanistic criterion for the triplet-Overhauser mechanism. The relation between CIDNP intensity and magnetic field strength is generally complex since at least three of the four steps of this mechanism are magnetic field dependent. The effects observed during the photoreaction of trifluoroacetophenone with dimethoxybenzene in acetonitrile show a high field maximum, which is compatible with the triplet-Overhauser mechanism, and two low field maxima, which can be assigned to S–T± mixing in a radical ion pair. These assignments were confirmed by the quencher concentration dependence at the fields that give rise to maximum effects.
The effect of internal energy on the addition of hydroxyl radicals and deuterium atoms to aromatic hydrocarbons
Authors: Sloane Thompson M. , Brudzynski Richard J.
Published: February 2003

This paper reports a study of the reactions of hydroxyl radicals and deuterium atoms with toluene and 1,3,5-trimethylbenzene under single collision conditions. The internal (vibrational and rotational) energy of the hydrocarbon was varied while the distribution of relative kinetic energies of collision was kept constant. The amount of OH–aromatic hydrocarbon complexes which reached the mass spectrometer detector decreased as the vibrational and rotational temperature of the hydrocarbon was increased. The amount of hydrogen atom displacement product and methyl radical displacement product decreased slightly. If the dynamics of the methyl radical displacement reaction are similar to the analogous F+ aromatic hydrocarbon reaction (arguments are presented which indicate that this may well be the case), then our results show that there is no significant increase in the branching ratio of the methyl radical displacement path with increased hydrocarbon temperature. This means that the complexes which reached the detector at low hydrocarbon temperature but which decomposed before reaching the detector at higher hydrocarbon temperatures decomposed back to reactants rather than to products. The amount of D–aromatic hydrocarbon complexes reaching the detector increased as the hydrocarbon temperature was increased. This result probably means that there is a barrier to complex formation and that vibrational and/or rotational energy of the hydrocarbon is effective at overcoming this barrier.
Sublevel phosphorescence spectra of anthraquinone. II. Molecular distortion in pentane solvent
Authors: Murao Toshiro , Azumi Tohru
Published: February 2003

The sublevel phosphorescence spectra of anthraquinone originating from single site in pentane matrix are observed. Different from the spectra in hexane and in octane, the 0–0 band appears strongly for all the three sublevel phosphorescence spectra. Radiative rate constants at the 0–0 band are of the order kxr ≳ kzr ≫ kyr (x axis perpendicular to the molecular plane and z axis is along the C=O bond). This result is interpreted as due to the distortion of the anthraquinone molecule along its b3u normal coordinate.
Behavior of hydrogen-bonded ferroelectrics KH2AsO4 and KD2AsO4 at high temperatures
Authors: Huong Pham V. , Hilczer B.
Published: February 2003

The Raman spectra of KH2AsO4 and KD2AsO4 ferroelectrics are recorded at various temperatures between 300 and 463 K. Firm evidence of a high temperature phase transition is found for KH2AsO4 at 413 K and for KD2AsO4 at 453 K. The transition is shown to be of first order and nondestructive. The role of the proton dynamics in the mechanism of this transition is also evidenced.
The ultraviolet absorption spectrum of liquid water
Authors: Quickenden T. I. , Irvin J. A.
Published: February 2003

The absorptivities of carefully purified water have been measured at 1 nm intervals in the wavelength range 196 to 320 nm. The measured absorptivities fell monotonically from a value of (1.26±0.03) m−1 at 196 nm, to a value of (0.0100±0.0006) m−1 at 320 nm. The water was purified by ion–exchange, followed by four subsequent distillations, the first being carried out under mild oxidizing conditions, and the last being performed in all-silica apparatus, the water vapor being heated to a temperature of 870 K in the presence of oxygen to remove residual organic impurities. The absorptivities were measured using a differential pathlength method and a correction for the effect of double reflections in the absorption cells was derived and applied to the data. The measured absorptivities were much lower than all previously reported values in the 200–300 nm region, but gave considerably better agreement with theory and aligned well with the edge of a set of recent vacuum UV measurements on high purity water. It appears that previously reported absorptivities of liquid water in the 200–300 nm region are too large by up to two orders of magnitude, due to the presence of oxygen and of persistent absorbing impurities. The small absorption peak reported around 275 nm by Larzul et al. [C. R. Acad. Sci. Paris 261, 4701 (1965)] and which has often been cited to corroborate electron impact evidence for the existence of a low-lying 3B1 state of the water molecule, was not observed, despite adequate sensitivity and stability. The broad and unstructured absorption observed across the 200–300 nm region was only slightly in excess of that attributable to the aggregate of Rayleigh scattering and the tail of the well known ? 1A1→1B1 absorption of water.
Temperature dependence of the diamagnetism of water
Authors: Philo J. S. , Fairbank W. M.
Published: February 2003

We have measured the magnetic susceptibility of liquid water over the 20 to 60 °C range. By using a new instrument based on superconducting technology, we obtain a precision nearly two orders of magnitude higher than previous studies. Notably absent from these data is any evidence for the thermal anomalies reported in several earlier studies. The data vary smoothly with temperature to within ±0.0001% of the diamagnetism, and show no abrupt changes in slope. These data will be useful for magnetic studies of molecules in aqueous solution, for calibration purposes, and for correcting NMR chemical shifts. With continued improvements in theoretical calculations, it should be possible to use magnetic susceptibility data to obtain useful information on molecular interactions in the liquid state.
Zero-field ODMR studies of the T1 states of bromine-containing 1,4-dihalobenzenes
Authors: Shinohara Hisanori , Hirota Noboru
Published: February 2003

The zero-field optically detected magnetic resonance (ODMR) spectra of bromine-containing para-dihalobenzenes have been obtained monitoring the low temperture (1.3 K) phosphorescence from both x-trap emissions and emissions from different hosts. From the ODMR the various decay rate constants and the spin sublevel populations are determined. The zero-field splittings of parabromochlorobenzene and paradibromobenzene are consistently explained in terms of the second order spin–orbit mixing with 3πσ*(3σπ*) and 3nπ* states, indicating the presence of these states near the T1 states. The radiative and radiationless decays from the y and z sublevels also involve the mixing with 3πσ*(3σπ*) and 3nπ* states, which are also supported by the phosphorescence sublevel spectra. The decay from the x sublevel is likely due to the distortion from planarity. Finally, some indirect evidence for the presence of the 3πσ*(3σπ*) and 3nπ* states (lying relatively close to the T1 states) is presented by the phosphorescence excitation spectra.
Thermodynamics in finite time: A chemically driven engine
Authors: Ondrechen Mary Jo , Berry R. Stephen , Andresen Bjarne
Published: February 2003

The methods of finite time thermodynamics are applied to processes whose relaxation parameters are chemical rate coefficients within the working fluid. The direct optimization formalism used previously for heat engines with friction and finite heat transfer rates—termed the tricycle method—is extended to heat engines driven by exothermic reactions. The model is a flow reactor coupled by a heat exchanger to an engine. Conditions are established for the achievement of maximum power from such a system. Emphasis is on how the chemical kinetics control the finite-time thermodynamic extrema; first order, first order reversible, and second order reaction kinetics are analyzed. For the types of reactions considered here, there is always a finite positive flow rate in the reactor that yields maximum engine power. Maximum fuel efficiency is always attained in these systems at the uninteresting limit of zero flow rate.
Collision induced absorption spectra for gas phase chemical reactions in a high power IR laser field
Authors: Orel Ann E. , Miller William H.
Published: February 2003

A previously derived classical model for describing the interaction of molecular systems with high power IR lasers has been used to obtain the collision induced absorption spectrum of a state-selected, monoenergetic reactive collision system. The spectrum can be written as the Fourier transform of a particular dipole correlation function, and both the correlation function and the absorption spectrum are considered. Examples treated are a one-dimensional barrier problem, reactive and nonreactive collisions of H+H2, and a modified H+H2 potential surface that leads to a collision intermediate.
X-ray photoelectron spectroscopy of Co(II), Ni(II), and Cu(II) acetylacetonate vapors
Authors: Perera J. S. H. Q. , Frost D. C. , McDowell C. A.
Published: February 2003

Gas phase x-ray photoelectron spectra of some transition metal acetylacetonates, M(AcAc)2, (M=Co,Ni,Cu) are reported. The metal 2p, 3s, and 3p core levels and the O 1s and C 1s binding energies have been obtained. Satellites were observed to higher binding energies from the metal 2p, 3s, and 3p levels and the O 1s levels. From a comparison of the solid phase and the present gas phase results the effects of changes in symmetry upon satellite structure were studied without changing the central metal atom or the ligand. The results suggest that the satellites seen in these transition metal 3s level spectra arise from shakeup mechanisms rather than from multiplet splittings.
A self-consistent study of ion adsorption and discrete charge effects in the electrical double layer
Authors: Chan Derek Y. C. , Mitchell D. John , Ninham Barry W.
Published: February 2003

A self-consistent treatment is given for ion size effects, specific ionic adsorption, and discrete surface charge effects in the electrical double layer at a planar surface. Ionic interactions are considered in the Mean Spherical Approximation and specific adsorption is modelled by the sticky potential of Baxter. Analytic results include the surface charge–surface potential relationship, the adsorption isotherm, and a comparison with the surface dissociation model of Ninham and Parsegian.
Matrix isolation infrared and mass spectrometric studies of the vapors formed by the thermal decomposition of Te6O11Cl2
Authors: Westphal G. H. , Rosenberger F. , Cunningham P. R. , Ames L. L.
Published: February 2003

The gaseous products of the thermal decomposition of Te6O11Cl2 (s) were studied by mass spectrometry and matrix isolation infrared spectroscopy. The temperature range investigated was 340 to 900 °C. The majority vapor species was found to be TeOCl2 together with minor amounts of TeCl4, TeCl2, and Cl2. The frequencies of the normal modes of vibration of TeOCl2 were measured and a normal coordinate analysis performed. The standard thermodynamic functions of TeOCl2(g) were also calculated.
Laser induced phonons: A probe of intermolecular interactions in molecular solids
Authors: Nelson Keith A. , Fayer M. D.
Published: February 2003

A new type of density dependent spectroscopy is developed and applied to the study of acoustics phonons and excited state–phonon interactions in molecular crystals. It is shown that intermolecular separations in solids can be varied in a controlled manner along a selected crystalline direction and that the consequences of the molecular displacements can be observed optically in time-resolved transient grating, absorption, and fluorescence experiments. The technique involves the generation of specific acoustic waves (phonon modes of definite wave vector) by transient grating optical excitation of the sample. The resulting time-dependent density changes cause time-varying spectral shifts, the magnitude of which are dependent on the detailed nature of the anisotropic intermolecular interactions in the crystal, e.g., anisotropic van der Waals interactions in molecular crystals can be directly investigated. First a formalism is presented which permits the determination of the nature of the phonons produced (longitudinal, quasilongitudinal, or quasitransverse) by transient grating excitation in anisotropic crystals. Next, the effect of the transient grating generated phonons on experimental observables (absorption, fluorescence, and transient grating) due to anisotropic spectral shifts is obtained. Model calculations illustrate the results. Finally experimental data for the perylene pure crystal excimer system and the pentacene in p-terphenyl mixed system are presented. In the perylene system, the various types of phonons predicted by theory are observed and phonon dispersions are obtained for several crystal directions. In pentacene in p-terphenyl, the theoretically predicted phonon-induced spectral shifts are clearly observed.
The chemisorption of acetylene and ethylene on Rh(111): A low energy electron diffraction (LEED), high resolution electron energy loss (ELS), and thermal desorption mass spectrometry (TDS) study
Authors: Dubois L. H. , Castner D. G. , Somorjai G. A.
Published: February 2003

We report the results of a detailed investigation of the chemisorption and reactivity of acetylene (C2H2) and ethylene (C2H4) on the Rh(111) single crystal surface. Below 270 K ELS measurements indicate that acetylene chemisorbs on Rh(111) with its C–C bond oriented parallel to the surface forming an approximately sp2 hybridized species. LEED investigations show that both C2H2 and C2H4 form metastable (2×2) surface structures on Rh(111) below 270 K. An irreversible order–order transformation occurs between 270 and 300 K to a stable c(4×2) hydrocarbon overlayer. The stable species formed from both molecules are identical. Hydrogen addition to chemisorbed acetylene is necessary to complete this conversion. The geometry of the adsorbed ethylene species does not change during this transformation although the overlayer structure does. This stable hydrocarbon species is identical to the hydrocarbon species formed from the chemisorption of either C2H4 or C2H4 and hydrogen on Pt(111) above 300 K. The addition of H2 to chemisorbed C2D2 or C2D4 results in H–D exchange, but no change in the adsorbate geometry is detected by ELS. Decomposition of these molecules occurs on the Rh(111) surface above ∼420 K.
State-to-state cross sections for elastic and inelastic electron scattering by N2 at 20–35 eV, including resonant enhancement of vibrational excitation
Authors: Onda Kunizo , Truhlar Donald G.
Published: February 2003

We have calculated integral and differential cross sections for elastic scattering and rotational, vibrational, and rotational-vibrational excitation at 20, 25, and 30 eV. We have also calculated partial cross sections and eigenphase sums at these energies and at 35 eV. We present a detailed study of the resonance enchancement of the vibrational excitation cross section in this energy region.
Observation of molecular motion in a scattering experiment
Authors: Kleyn A. W. , Khromov V. N. , Los J.
Published: February 2003

Differential cross section experiments for neutral scattering in K, Cs+O2 collisions have been performed in the energy range 25–100 eV. The structure observed for the differential cross section is caused by the rapid oscillation of the intermediate O−2 ion.
The chemisorption of hydrogen on the (110) surface of iridium
Authors: Ibbotson D. E. , Wittrig T. S. , Weinberg W. H.
Published: February 2003

The chemisorption of hydrogen on Ir(110) was studied under ultra-high vacuum conditions with thermal desorption mass spectrometry, LEED, contact potential difference measurements and ultraviolet photoelectron spectroscopy. The Ir(110)-(1×2) reconstructed surface is stable in hydrogen at pressures from 10−9 to 10−5 Torr and surface temperatures from 130 to 1000 K, the conditions investigated. No streaks or spots were observed in the LEED pattern due to the presence of hydrogen. Absolute coverage measurements indicate the saturation density at 130 K on Ir(110) is (2.2±0.2)×1015 atoms cm−2. Thermal desorption measurements indicate that hydrogen obeys second order desorption kinetics and exhibits two features, β1 and β2 states, with intensities 2:1, respectively, which exchange isotopically with one another. However, β2 hydrogen obeys first order adsorption kinetics with an initial sticking probability S0 equal to unity, while β1 hydrogen has an S0 equal to 7×10−3 and obeys second order kinetics. Rate parameters for hydrogen desorption from Ir(110) were calculated as a function of fractional surface coverage. A sympathetic increase in the rate parameters up to at least half of saturation is observed for the β2 state of hydrogen, where Ed and νd assume the values 23 kcal mol and 1.5×10−2 cm2 s−1, respectively. The increase is attributed to adatom–adatom attractive interactions of hydrogen in the β2 state. For the β1 state, the energy of desorption, in kcal mol−1, is given by Ed=17−10ϑ from ϑ equal to 0.4–0.7, and the pre-exponential factor, νd, varies weakly as a function of coverage with an average value of (2÷×2)×10−7 s−1. The contact potential difference (CPD) increases linearly with coverage up to 0.20 eV and reaches a maximum at 0.30 eV for the β2 state of hydrogen for T?300 K. A continuous decrease in the CPD occurs over the β1 state up to saturation at 140 K and becomes lower than the clean surface contact potential for total fractional coverages above 0.9. Probable binding sites for the β1 and β2 states are inferred from the absolute coverage measurements and the CPD measurements. Ultraviolet (HeI,hv=21.2 eV) photoelectron spectra of hydrogen show a broad H(1s)level centered approximately at 6.1 eV below the Fermi level. A strong decrease in the d-band emission occurs near the Fermi level after adsorption of the β2 state which suggests preferred binding locations for this state of hydrogen on the Ir(110)-(1×2)surface.
Thermochemical studies of the BF2 radical
Authors: Lau K. H. , Hildenbrand D. L.
Published: February 2003

The BF2 radical was generated under equilibrium conditions by the reaction of both SF6 (g) and BF3 (g) with B (s) in an effusion cell at 1600 to 1800 K, and was identified and thermochemically characterized by mass spectrometry. Enthalpy data derived from the analysis of several gaseous equilibria in the B–F and B–Ca–F systems led to concordant results on BF2, and to the standard enthalpy of formation ΔfH°298(BF2)=−120.0±4 kcal/mol, about 20 kcal/mol less negative (less stable) than the previously accepted value. From electron impact threshold measurements, the ionization potential of BF2 was evaluated as 8.84±0.10 eV. The results show the FB–F bond to be 70 kcal weaker than the B–F bond, and 59 kcal weaker than F2B–F in the BF3 bond dissociation sequence. Implications of the results on the chemical bonding in the B–F system and comparison with a theoretical treatment of BF2 are discussed briefly.
Millimeter wave measurements of the rotational spectra of ClF, BrF, BrCl, ICl, and IBr
Authors: Willis Robert E. , Clark William W.
Published: February 2003

The rotational spectra of all twelve stable isotopic species of ClF, BrF, BrCl, ICl, and IBr were observed and measured in the millimeter wave region by means of a sensitive microwave spectrometer. Transitions were detected over a wide range of frequencies for molecules in both the ground vibrational state and several excited states. The rotational spectrum of each molecule was split by the nuclear quadrupole interaction. Altogether, 250 new lines were measured. These correspond to 136 pure rotational transitions. Values of the Dunham coefficients Y01, Y11, Y21, Y31, Y02, Y12, and Y03 were obtained from a computer analysis of the measured frequencies. From these coefficients a number of equilibrium constants were derived to significantly greater accuracy than in previous work. In particular, the equilibrium distance, re, was found to two or three more significant figures.
Studies of the binary reactions of H3O+⋅(H2O)0,1,2 ions and their deuterated analogues with D2O, H2O, and NH3
Authors: Smith D. , Adams N. G. , Henchman M. J.
Published: February 2003

The rate coefficients and product ion distributions for the binary reactions of H3O+⋅(H2O)0,1,2 and D3O+⋅(D2O)0,1,2 ions with D2O and H2O, respectively, and with NH3 have been studied at 300 K using a selected ion flow tube (SIFT) apparatus. The ions were created in a flowing afterglow ion source and after mass filtering were injected at low energy into the SIFT. All the reactions proceeded at or near the gas kinetic limit. In the D2O and H2O thermoneutral isotopic exchange reactions, the distribution of H and D amongst the product ions and neutrals was seen to be purely statistical. This implies that these reactions proceed via the formation of an intermediate long-lived association ion in which total randomization of the H and D atoms takes place prior to unimolecular decomposition. No appreciable isotopic exchange occurred in the exothermic NH3 reactions which apparently proceeded via the simpler mechanisms of D+ (or H+) or D3O+ (or H3O+) transfer. The differing mechanisms for the H2O and D2O reactions compared to the NH3 reactions are rationalized in terms of the thermicities of the reactions and the lifetimes of the respective intermediate ions.
The effects of isolated vacancy defects on the thermodynamic properties of harmonics crystals: Cell cluster method
Authors: Garza Cesar M. , Huckaby Dale A.
Published: February 2003

A general derivation of the cell cluster theory for harmonic systems is given. The effects of a vacancy defect on the harmonic contribution to the high temperature Helmholtz free energy, the zero point energy, and the constant volume heat capacity of a face-centered cubic crystal with first-neighbor interactions were calculated using the cell cluster method. Similar calculations were also performed for the triangular lattice.
Simple formulas for dichroism analysis. Orientation of solutes in stretched polymer matrices
Authors: Nordén B.
Published: February 2003

The multiplicative property of linear dichroism relative to axes of ’’statistical’’ rotation has allowed a simple, general formulation of the reduced dichroism of solutes oriented in uniaxially stretched polymer matrices: LDr=3 Sp ΣiSiiOi, i=x, y, z; where Sp=1/2ℱ0π/2 (3 cos2ϑ−1) df (ϑ) is a factor characterizing the orientation of the polymer. Sp was derived as a function of stretch for the case when f(ϑ) is the distribution of independent directions p (not necessarily attached to polymer chains) obtained in an isotropic-substrate sphere when this is deformed into a spheroid at constant volume. Sii define two independent local orientation parameters which depend on the solute–polymer interaction; Oi are optical factors given by the moment directions of the absorbing transitions. The model was tested for a few solutes, with known moment directions, in stretched films of polyvinyl alcohol (PVA) and polyethylene (PE). The agreement between predicted and observed LDr, using the model distribution, was excellent for two elongated dyes in PVA but only qualitative in PE. A significant variation of Sii with the degree of stretch indicates that the solute orientation is not due only to interaction with a single neighbor chain, a behavior which is consistent with a repulsive interaction.
Vibrational relaxation in jet-cooled alkyl benzenes. II. Fluorescence spectra
Authors: Hopkins J. B. , Powers D. E. , Mukamel S. , Smalley R. E.
Published: February 2003

Dispersed fluorescence spectra of a series of laser-excited n-alkylbenzenes cooled in a supersonic free jet have been obtained under effectively collision-free conditions. The series includes all members of the n-alkylbenzene family from methyl- through n-hexylbenzene. Pulsed laser excitation was cleanly made into the 000, 6b01, 1210, and 18a10 vibronic bands of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. Resonance fluorescence from the O00 excitation experiments permitted clear assignment of the six ring modes (6a, 6b, 1, 12, 18a, 9a) which in combinations and short progressions dominate the fluorescence spectra of these molecules. Except for 6a and 1, the frequency of these ground state vibrations is found to be constant throughout the alkylbenzene series to better than 1 part in 100. Fluorescence from the 6b10, 1210, and 18a10 excitation experiments with alkylbenzenes of successively longer chains showed clearly the onset and increasing importance of intramolecular vibrational relaxation (IVR) occurring in the 1B2(ππ*) state prior to emission. By comparing the relative intensity of the resonance and relaxed fluorescence, a lower limit to the rate of the IVR process has been measured as a function of the type of system mode excited (6b1, 121, or 18a1) and as a function of the size and nature of the bath (the alkyl chain in its various conformations). As expected this IVR rate increases with increasing alkyl chain length. For 6b10 excitation (vibrational energy, Ev=530 cm−1) the IVR process is first observable in n-(t)-butylbenzene where the IVR rate, kvr, is ⩾5>106 sec−1. This rate increases to ⩾4×107 sec−1 and ≳109 sec−1 in 6b10 excited n-(t)-pentyl- and n-(t)-hexylbenzene, respectively. For 1210 excitation (Ev=933 cm−1) kvr⩾108 sec−1 for n-(t)-propylbenzene and kvr×109 sec−1 for all higher n-(t)-alkylbenzenes. Vibrational relaxation is found to be substantially slower for alkylbenzenes in the gauche conformation: kvr<6×105 sec−1 for 6b10-excited n-(g)-pentylbenzene. With increasing alkyl chain length the spectrum of the relaxed fluorescence becomes sharper and closer in frequency and pattern to the resonance fluorescence of the 000-excited molecule. This indicates an increasing involvement in the IVR process of bath modes in the alkyl chain which are far removed from the benzene ring.
High pressure study of slow phosphorescence in La2O2S:Eu.
Authors: Webster G. A. , Drickamer H. G.
Published: February 2003

The effect of pressure has been measured on the decay rate of the slow phosphorescence in La2O2S:Eu. Although the trap structure is complex, the decay curves could be approximated as the sum of two exponentials for purposes of discussion. Activation energies for detrapping were extracted from the decay times. For the ’’shallow traps,’’ the activation energy increases linearly with pressure at a rate of 13 cm−1/kbar. For the ’’deep traps’ the change in activation energy with pressure is ∼4 cm−1/kbar at low pressure and increases to about 14 cm−1/kbar by 40 kilobar. One can conclude that the ’’shallower trap’’ (thermally released) storage increases with pressure, as does the depth of the shallowest traps studied. The fraction of the holes captured by the shallowest traps also increase with pressure.
High-resolution one and two photon excitation spectra of t
r
a
n
s, t
r
a
n
s-1,3,5,7-octatetraene
Authors: Granville Mark F. , Holtom Gary R. , Kohler Bryan E.
Published: February 2003

One and two photon excitation spectra for the lowest energy singlet transition (2 1Ag←1 1Ag) of trans, trans 1,3,5,7-octatetraene in n-octane at 4.2 K(0–0 at 28 56l cm−1) have been measured and analyzed. The one photon spectrum is found to be vibronically induced while the two photon spectrum is electronically allowed. The contribution of the higher lying 1 1Bu←11Ag transition (0–0 at approximately 32 100 cm−1) to the observed two photon excitation spectrum is found to be negligible allowing vibrational overtones of the 2 1Ag←1 1Ag system to be observed. On the bases of frequency, intensity, and one or two photon allowedness, 128 measured lines are assigned as combinations of 4 antisymmetric and 11 symmetric normal modes.
Collision-induced energy absorption and vibrational excitation by intense laser radiation in CH3F
Authors: Forber R. A. , McNair R. E. , Fulghum S. F. , Feld M. S. , Feldman B. J.
Published: February 2003

Vibrational energy absorption and transfer in diatomic and polyatomic molecules from intense resonant laser pulses is studied in the regime where energy flow is determined by vibrational–vibrational (V–V) collisions. Simple theoretical models for single mode (diatomic) and two mode (polyatomic) oscillators, based on coupled rate equations with the addition of a laser pumping term, are presented. By invoking quasiequilibrium (temperature) assumptions on the vibrational level populations (confirmed with computer simulations) simple expressions are derived for the absorbed energy in the single mode case; and for two modes similar formulas based on a two-temperature model are derived and discussed in relation to the one mode model. For many cases of interest, CH3F included, the influence of a second mode on the total energy absorption is insignificant, with most of the energy flowing into the mode being pumped at nearly the same rate as for a single mode oscillator. The experimental work studies the absorption of the P(32) line of CO2 by 13CH3F, and the subsequent energy transfer among the several modes of the molecule. Approximately 1/4 J over 2 to 3 μsec delivers several quanta per molecule to the vibrational degrees of dreedom. Fitting the theoretical model to the measured absorption produces a V–V rate constant of γVV =1.0±0.5 μsec−1 Torr−1 for the ν3 mode. A determination of the energy partitioning among the several modes, by means of a technique for measuring the absolute energy stored in each, shows that roughly 50% of the absorbed energy is stored in the ν3 vibration with the remainder distributed among the other modes. Detailed knowledge of the partitioning allows confirmation of a specific energy flow path for CH3F.
Electron spin resonance studies of magnetic soliton defects in polyacetylene
Authors: Weinberger B. R. , Ehrenfreund E. , Pron A. , Heeger A. J. , MacDiarmid A. G.
Published: February 2003

We present electron spin resonance studies of the magnetic defect in undoped polyacetylene. Temperature dependences of the absorption spectra of (CD)x and (CH)x over the range 2–295 K have been obtained. The properties of the magnetic defect in fully isomerized trans samples and in cis-rich samples are compared. The data are consistent with a description in terms of a mobile neutral magnetic defect in the trans polymer and a stationary one in the cis-rich polymer. The spin resonance linewidths are determined by unresolved hyperfine splittings resulting from interaction of the electron spin with many nuclear spins within the envelope of the spatially extended magnetic defect. The results of analyses of the detailed line shapes and linewidths are discussed in terms of the bond alternation domain wall (soliton) model of the magnetic defect.