bluuebunny/crossref_metadata_2025 huggingface dataset:
Dipolar relaxations in solid (CF4)1−x(CClF3)x
Authors: Böhmer Roland
Published: July 2002

(CF4)1−x(CClF3)x shows a eutectic phase diagram with a eutectic temperature TE =69 K and a eutectic concentration xE =0.54. In the mixed crystal regime x<0.015, dielectric audio frequency spectroscopy revealed the dynamics of weakly coupled CClF3 molecules performing thermally activated reorientations. An analysis of the data was performed in terms of a relaxator model including a critical comparison of different distributions of relaxation times. The static susceptibilities of the mixed crystals suggested a two-phase model of CF4, where rigid latice domains coexist with paraelastic regions. Several pure CClF3 samples were investigated using different thermal treatments. The dielectric dispersion of this compound is described by the reverse Havriliak–Negami expression and possible explanations for the origin of the relaxation spectra are discussed. The behavior of the CClF3 rich mixed crystals is considered briefly.
Simplified trajectory method for modeling gas–surface scattering: The NO/Pt(111) system
Authors: Jacobs D. C. , Zare R. N.
Published: July 2002

A model is presented to describe the dynamical processes of trapping/desorption as well as direct and indirect inelastic scattering on single-crystal surfaces. Newton’s equations of motion are integrated for a system consisting of a rigid rotor interacting with a slab of 19 surface atoms. The surface atom which is closest to the center of mass of the molecule is permitted to translate only along the surface normal. In turn, this mobile surface atom is harmonically coupled to a microcanonical heat bath consisting of three subsurface atoms. This method is much less computationally intensive than the typical generalized Langevin equation (GLE) approach. Direct comparison is made between the predictions of this model and experiment for the NO/Pt(111) system. In the case of trapping/desorption, the model accurately describes the observed dependence of rotational alignment on rotational quantum number. For the inelastic scattering regime, the model successfully reproduces the degree of rotational excitation and qualitatively accounts for the observed rotational alignment. In addition, the model predicts correlations between final state velocity and final state rotational angular momentum (both direction and magnitude), as well as the effect of molecular orientation and surface impact parameter on the overall trapping probability.
Threshold level laser photoablation of crystalline silver: Ejected ion translational energy distributions
Authors: Helvajian H. , Welle R.
Published: July 2002

We have conducted an experiment which measures, for a single laser shot, the ejected mass and nascent velocity distributions of ionic species ablated at laser fluences near the threshold for ion production. Our results show that for a crystalline silver target, the laser-ablated ion products are ejected with fixed kinetic energy equal to 9±1 eV (3 eV FWHM). The kinetic energy of the ejecta (Ag+,Ag+2, adsorbed Fe+) do not vary with wavelength (351 and 248 nm), nor with the ion product mass, and within limits are independent of the incident laser intensity. We do see a wavelength dependence in the threshold for ion production and in the dimer/monomer (Ag+/Ag+2) ion ratio. A number of possible mechanisms are presented to explain the data, although none is without some objection. Among these, the process whereby desorption is induced by electronic transitions (DIET processes) has some merit in explaining our data.
Breakdown of Onsager’s conjecture on distance dependent polarization relaxation in solvation dynamics
Authors: Chandra Amalendu , Bagchi Biman
Published: July 2002

Onsager’s conjecture that the polarization relaxation of solvent molecules closer to a newly created ion occurs slower than those which are far away is examined by using a molecular theory. We find that although Onsager’s conjecture is correct when only the rotational diffusion mechanism of polarization relaxation is considered, it breaks down completely for a significant translational contribution to polarization relaxation. The consequences of this result in solvation dynamics of ions and dipoles are discussed.
Intramultiplet energy transfer in the collisions of 3p 4D nitrogen atoms with nitrogen molecules
Authors: Jeffries Jay B. , Copeland Richard A. , Crosley David R.
Published: July 2002

Two-photon excitation at 211 nm combined with wavelength-resolved detection near 870 nm is used to study intramultiplet energy transfer in the 2s22p23p 4D0 state of nitrogen atoms resulting from collisions with nitrogen molecules. The thermally averaged cross sections at 300 K vary from 36±10 Å2 for 4D01/2→4D03/2 to 3±3 Å2 for 4D07/2→4D01/2. The cross sections for ΔJ=1 are larger than for ΔJ=2 which in turn are larger than for ΔJ=3. When 3p4D07/2 is excited with linearly polarized light, the fluorescence polarization is nonzero and independent of N2 pressure. Thus, the cross section for transfer between magnetic sublevels is smaller than those for transfer out of that fine-structure level.
Confirmation of anomalous pressure dependence of linewidths of the electron paramagnetic resonance spectrum of molecular oxygen
Authors: Utter R. G. , Gardiner W. C.
Published: July 2002

Intrinsic linewidths of the MJ=−1 to MJ=0 electron paramagnetic resonance (EPR) transitions of O2 for total rotational quantum numbers N=3, 5, and 7 were redetermined by the second harmonic zero crossing method using direct calibration of modulation depth and analytic accounting for the effect of harmonic distortion of the magnetic field modulation waveform. The collisional linewidths were found to be 4.38±0.05, 4.26±0.07, and 4.22±0.10 MHz/Torr, respectively, for N=3, 5, and 7, corresponding to relaxation/reorientation cross sections of 0.679, 0.661, and 0.655 nm2. Anomalous negative zero-pressure linewidths detected in an earlier study were also given by linear regression analysis of the present data at pressures over 0.5 Torr. Extending the pressure range down to 0.05 Torr using a voltage controlled oscillator (VCO) to mimic tracking gaussmeter signals showed that the linewidth does appear to extrapolate finally to a physically meaningful positive value at zero pressure. No satisfying interpretation of the measured intrinsic widths being approximately 200 kHz too small to yield physically meaningful linearly extrapolated zero-pressure widths was found.
Exact solution of boundary value problems involving linear processes of growth and transport in addition polymerization
Authors: Rabeony H. M. , Reiss H.
Published: July 2002

Exact mathematical descriptions are obtained for the following process. Polymers are initiated and then propagated by a chain addition reaction in a convectionless medium in a cylindrical vessel. An external field (e.g., gravity) may be present, and the polymers may diffuse to the vessel walls where they are irreversibly lost. Both chain transfer and termination are absent, but the propagation constants, diffusion coefficients, and mobilities in the field depend arbitrarily on polymer size. Exact expressions are derived for the concentrations of polymers of each size, as functions of time and position. The results are important to the study of gas phase polymerization, but may have other applications.
Phase transitions of lipid bilayers. II. Mean field theory
Authors: Jones Francis P. , Tevlin Patrick , Trainor L. E. H.
Published: July 2002

We use the mean field theory of Pink et al. with the ab initio calculations of the density of states of Tevlin et al. to calculate various features of the main phase transition of a lipid bilayer using fewer fitting parameters than in previous models. Good agreement with experiment is obtained for the calculated transition temperature, Gibbs free energy, enthalpy, and average number of gauche rotations per acyl chain. The average chain area is poorly predicted; we expect this to be corrected once cooperative packing of the acyl chains is taken into account. The nematic order along the chain is also calculated and found to agree qualitatively with experiment.
Optical excitation spectra of adsorbates on dielectric surfaces
Authors: Mavroyannis Constantine
Published: July 2002

Optical absorption spectra of adsorbates on dielectric surfaces have been considered at low coverage. Radiative and nonradiative processes have been taken into account. Expressions for the spectral functions have been derived describing the spectra of the symmetric and antisymmetric modes, respectively. At low coverage and when the damping of the surface electron–hole pairs is much greater than the effective radiative damping, the relative intensities of the symmetric and antisymmetric modes take positive and negative values describing the physical processes of absorption (attenuation) and stimulated emission (amplification), respectively. Thus, peaks of the symmetric and antisymmetric modes arising from two nearby excited states of the adsorbed atom may cancel each other out, provided that the frequency profiles of the peaks in question overlap. Numerical calculations have been made for the spectra of rare-gas atoms He, Ne, Ar, Kr, and Xe which are physisorbed on graphite. The resulting excited geometrical configurations arising from the cancellation process between the spectra of the symmetric and antisymmetric modes have been computed as a function of the distance R from the atom to the dielectric surface, and they are graphically presented. The disappearance or the persistence of the excited geometrical configurations may help to determine the distance R at which the atom is physisorbed on the dielectric surface under investigation.
Structural investigation of a nonconjugated conducting polymer by solid state 13C nuclear magnetic resonance spectroscopy
Authors: Cholli A. L. , Thakur M.
Published: July 2002

The charge-transfer mechanism in 1,4 cis-polyisoprene is studied using solid state 13C NMR spectroscopy as a function of doping level of iodine. The CP/MAS NMR data show that the concentration of double bonds decreases as a consequence of doping. The overall results are consistent with previous observations and indicate that cation radicals –C+‖–C⋅‖– are formed upon doping and charge transfer. The double bonds are reformed as the doped samples are compensated with ammonia indicating reversibility of the doping. The measurements also indicate that the chain mobility is restricted in the doped regions compared to the undoped regions.
Conformational stability, barriers to internal rotation, vibrational assignment and a
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o calculations of 2-fluoropropenoyl fluoride
Authors: Durig J. R. , Wang Ai-Ying , Little T. S. , Brletic P. A. , Bucenell J. R.
Published: July 2002

The far infrared spectrum of gaseous 2-fluoropropenoyl fluoride CH2CFCFO, has been recorded at a resolution of 0.10 cm−1 in the 350–35 cm−1 region. The asymmetric torsional fundamental of the more stable s-trans (two double bonds oriented trans to one another) and the high energy s-cis conformations have been observed at 84.1 and 68.5 cm−1, respectively, each with excited states falling to lower frequencies. From these data, the asymmetric torsional potential function governing internal rotation about the C–C bond has been determined. The potential coefficients are V1=−201±4, V2=2106±16, V3=382±6, V4=−41±5, and V5=−74±3 cm−1. The s-trans to s-cis and s-cis to s-trans barriers have been determined to be 2232 and 2124 cm−1, respectively, with an enthalpy difference between the conformations of 108±26 cm−1 (309±74 cal/mol). From variable temperature studies of the Raman spectrum, the conformational enthalpy difference has been determined to be 187±39 cm−1 (535±112 cal/mol) and 370±82 cm−1 (1058±234 cal/mol) for the gas and liquid, respectively. A complete assignment of the vibrational fundamentals observed from the infrared (3500–50 cm−1) spectra of the gas and solid and the Raman (3200–10 cm−1) spectra of all three physical states is proposed. All of these data are compared to the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing both the 3-21G and 6-31G* basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.
Force constants of solvent polarization fluctuations: Softening at intermediate wave vectors
Authors: Chandra Amalendu , Bagchi Biman
Published: July 2002

A microscopic calculation of the wave vector (k) dependent force constant [K(k)] of solvent polarization fluctuations in a dipolar liquid is presented. We find that the force constant of the pure liquid undergoes a pronounced softening at intermediate wave vectors (kσ≂2π, where σ is the solvent molecular diameter). An analysis of the modification of the solvent polarization force constant because of the presence of a polar solute molecule is also presented. In agreement with the calculation of Carter and Hynes, we find that this modificaton is a nonlinear effect. In the present density-functional theory, the contribution of the solute enters through the space- and orientation-dependent triplet direct correlation function. We use the convolution approximation of the triplet direct correlation function to derive an analytic expression for the solute-induced contribution to both free energy and force constant. A preliminary numerical calculation shows that the long-wavelength (k→0) force constant increases because of the presence of the solute, which is in agreement with Carter and Hynes and also with Kakitani and Mataga. The significance of these results in charge transfer processes in a dipolar liquid is discussed.
Magnetic circular dichroism spectroscopy and magnetization studies of iron atoms isolated in xenon matrices
Authors: Graham Robin G. , Grinter Roger
Published: July 2002

Absorption and magnetic circular dichroism (MCD) spectra of iron atoms isolated in xenon matrices have been measured and their magnetization properties studied. The high signal intensity and optical quality of the matrices has allowed the assignment of many more transitions than hitherto. The magnetization measurements reveal considerable guest host interaction, though there is little evidence of this in the spectra. Detailed analysis of the magnetization data shows that they can be interpreted quite well in terms of an essentially axial crystal field splitting of the 5 D4 ground state which places the two ‖MJ‖=4 levels about 100 cm−1 below the ‖MJ‖=3. By extending the theoretical treatment to include a small mixing of the two ‖MJ‖=4 levels it is possible to improve the fit of the higher temperature data. These results demonstrate the value of MCD measurements, particularly where a system is electron paramagnetic resonance (EPR) silent.
Observation of ‘‘magic numbers’’ in the population distributions of the (NH3)n−1NH+2 and (NH3)nH+2 cluster ions: Implications for cluster ion structures
Authors: Peifer William R. , Coolbaugh M. Todd , Garvey James F.
Published: July 2002

We have measured the relative yields of the cluster ions (NH3)n−1NH+2 and (NH3)nH+2 produced by electron impact ionization of a supersonic molecular beam of neutral ammonia clusters, as functions of cluster ion size, neutral beam stagnation temperature, and electron impact energy. Our observation of a magic number at n=7 in the distribution of (NH3)n−1NH+2 cluster ions is interpreted in terms of an intracluster bimolecular association reaction between a nascent NH+2 daughter ion and an adjacent NH3 solvent molecule, giving rise to a protonated hydrazine product ion. Our observation of magic numbers at n=5 and n=8 in the (NH3)nH+2 cluster ion distribution is consistent with the production of the N2H+8 Rydberg radical cation via an intracluster ion–molecule reaction. Our experimental results are consistent with recent theoretical predictions concerning the structure and stability of N2H+8 ions.
An atom–atom encounter model of energy transfer from polyatomic molecules
Authors: Sceats Mark G.
Published: July 2002

The atom–atom encounter model for collisional energy transfer by monatomic colliders previously developed for diatomics [M.G. Sceats, Chem. Phys. Lett. 151, 281 (1988)] is extended to polyatomic molecules. For small molecules, such as triatomics, the density of states is sufficiently low at high energies that the result can be enumerated for each state. For large polyatomics a statistical approach based on either microcanonical or effective canonical distributions can be used when independent degrees of freedom are assumed. An effective anharmonicity at high energy is incorporated by inclusion of diagonal terms for these independent degrees of freedom. The primary feature of the model is that it accounts for kinematic effects, such as momentum transfer efficiency and the adiabatic nature of collisions, which are not accounted for in ergodic theories of energy transfer.
Alternate solution to a discrete car parking problem
Authors: Texter John
Published: July 2002

A few years ago, P. J. Flory [J. Am. Chem. Soc. 61, 1518 (1939)] solved a statistical problem analogous to determining the random-irreversible saturation formation of a 1×2 overlayer on a linear n lattice, by deriving an expression for the fraction of empty lattice points. Methods are presented here for both the enumeration and construction of saturation configurations. This alternate approach is derived from the theory of partitions and incorporates shift and sum-rule operators for the calculation of configurational probabilities. The fraction of filled lattice points is evaluated, and the configurational probabilities provide distribution functions for saturation configurations having different saturation levels (for fixed n). These distribution functions can be used to calculate sequence selective properties in addition to average saturation levels. Applications are illustrated in the analysis of the randomness of dehydrochlorination of polyvinylchloride and the distribution of UV-induced cyclobutyl-thymine dimers in Tetrahymena pyriformis DNA.
A new technique for grand canonical Monte Carlo simulation: Application in a hard-disk system
Authors: Caliri A. , da Silva M. A. A. , Mokross B. J.
Published: July 2002

We present a new Monte Carlo approach which is capable to reduce significantly the two-phase coexistence region by constraining the system to remain fairly homogeneous in the transition region. This is achieved by introducing the chemical potential as an extra intensive thermodynamic variable in the canonical average. The technique developed was applied in a system containing 224 hard disks. The approach can, on the other hand, be applied in a general way to other systems.
Use of high observing power in electron spin resonance saturation-recovery experiments in spin-labeled membranes
Authors: Yin Jun-Jie , Hyde James S.
Published: July 2002

Biomolecular collision rates of 14N-containing nitroxide radical labeled stearic acid with similar 15N-containing species in dimyristoylphosphatidylcholine (DMPC) liposomes have been determined using the saturation-recovery electron spin resonance (ESR) method. It is shown that high microwave observing powers can be used to obtain these rates, thereby increasing the signal-to-noise ratio by about ten times relative to methods previously used. The data are analyzed using the rate-equation approach with inclusion of the observing transition probability. A number of solutions to these equations for other experimental situations and for pulse and continuous wave electron–electron double resonance (ELDOR) are also presented. The bimolecular collision rate of labels at the C16 position was found to be 20% greater than for labels at the C12 position, suggesting that the interaction distance differs at these two positions. Even though the high observing power does not affect the ability to extract bimolecular collision rates, the effective spin-lattice relaxation rates are altered. Plots of these latter rates versus power are linear and the slope can be used to determine the absolute microwave field at the sample for a given input power.
Ab initio determination of mode coupling in HSSH: The torsional splitting in the first excited S–S stretching state
Authors: Herbst Eric , Winnewisser G. , Yamada K. M. T. , DeFrees D. J. , McLean A. D.
Published: July 2002

A mechanism for the enhanced splitting detected in the millimeter-wave rotational spectra of the first excited S–S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S–S stretching state and excited torsional states, has been investigated in part by the use of ab initio theory. Based on an ab initio potential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second-order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.
Theory of amphiphilic liquid crystals: Multiple phase transitions in a model micellar system
Authors: Taylor Mark P. , Berger Alan E. , Herzfeld Judith
Published: July 2002

A variety of amphiphilic molecules reversibly and spontaneously self-assemble in aqueous solution forming a polydisperse population of asymmetric aggregates. In binary water/surfactant systems, aggregates may be either oblate or prolate and form corresponding orientationally ordered phases. Such binary systems are modeled using a lattice statistics calculation of the configurational entropy of a polydisperse collection of hard rods and plates, combined with a phenomenological description of aggregate assembly. Using parameter values relevant to micellar surfactant systems, a series of phase diagrams and associated particle size distributions have been computed. Depending upon the competition between rod and plate growth, the concentration dependent transition sequences I→A, I→P, and I→A→P (where I≡isotropic micellar, A≡axial nematic, and P≡planar nematic) are seen. The A→P transition occurs when rod and plate growth are equally favorable and is discussed in detail. Qualitative comparisons are drawn between the calculated phase behavior and experimental results. No stable biaxial nematic phases are predicted for binary systems in the absence of positional order.
Determination of the geometry of deuterated tryptamine by rotationally resolved electronic spectroscopy
Authors: Wu Yenchune R. , Levy Donald H.
Published: July 2002

The low and high resolution fluorescence excitation spectra of d3-tryptamine have been observed in the environment of a cold, supersonic molecular beam. As in the case of h3-tryptamine, six bands due to the origins of different conformers have been found in the low resolution spectrum of deuterated tryptamine. A previous paper reported that conformers labeled A and B and conformers labeled D and E of tryptamine have identical rotational structures. However, for deuterated tryptamine those conformers have distinguishable rotational structures. Analysis of the rotational structure in the high resolution electronic spectra of five of the bands was used to determine the geometries of the different conformers. Conformers A, B, and F have a gauche conformation with respect to the rotation about the Cα–Cβ bond while conformers D and E have an eclipsed conformation. The geometries of conformers A and B and conformers D and E differ only in the orientation of the amino group. In these structures the angles of the internal rotation of the amino group are 180°, 60°, 180°, 60°, and −60° for conformers A,B, and D–F, respectively. Feature C consists of two overlapped conformers, and it is suggested that these conformers have the amino group trans to the indole ring.
Thermoinduced and photoinduced reactions of NO on Si(111)7×7. II. Effects of potassium coadsorption
Authors: Ying Z. C. , Ho W.
Published: July 2002

The adsorption, thermoreactions, and photoreactions of NO coadsorbed with potassium on Si(111)7×7 at 90 K have been investigated using work-function measurements, high-resolution electron energy loss spectroscopy, and mass spectrometry. A minimum in the work function of Si(111)7×7 at 90 K vs the potassium exposure is observed and it is suggested that higher K exposures passed the work-function minimum result in the formation of K multilayers, which upon thermal heating desorb at 315 K. Submonolayers of potassium introduce a new adsorption configuration of NO on Si(111)7×7 at 90 K. This new NO species exhibits a weak intramolecular bonding and competes with NO adsorbed in other configurations. During thermal heating, this NO species gradually dissociates, thus contributing to N2 recombinative desorption, and at relatively high K coverages leads to desorption of N2 and N2O at 555 K via N2O synthesis from NO. The surface after thermal heating to >555 K is depleted of molecular species and covered with atomic N and O. Preferential surface oxidation occurs in the presence of potassium. Under photon irradiation (300–900 nm), desorption of N2, NO, and N2O is observed. The photodesorption intensities decrease monotonically as the K coverage increases. The experiments provide further evidence that the dominant contribution to the three photodesorbed species comes from molecularly adsorbed NO and that N2O is synthesized under photon irradiation.
Self-diffusion on the Lennard-Jones fcc(111) surface: Effects of temperature on dynamical corrections
Authors: Cohen J. M. , Voter A. F.
Published: July 2002

Surface self-diffusion constants have been calculated for the single component Lennard-Jones fcc(111) system using the dynamical corrections formalism for transition state theory (TST). At high temperatures, these results are found to be in agreement with previous molecular dynamics calculations. Over the extended temperature range in which this method is valid, deviations from Arrhenius behavior are observed. At lower temperatures, a noticeable contribution to the diffusion constant stems from trajectories in which the adatom recrosses the TST boundary, often due to a direction-reversing collision with the substrate atom on the far side of the binding site. This produces a dip in the dynamical correction factor centered around a reduced temperature of T=0.038. At higher temperatures, the expected multiple-jump effects are observed.
A new determination of the dissociation energy of acetylene
Authors: Green Peter G. , Kinsey James L. , Field Robert W.
Published: July 2002

A new upper bound for the dissociation energy of acetylene, D00(HCC–H) =529.89(±0.01) kJ/mol, has been determined by Stark anticrossing spectroscopy. The zero-pressure extrapolated (unimolecular) decay rates of levels of S1 (Ã 1Au) v′3=2 and 3 (quanta of the trans-bending vibration) increase upon application of an electric field of 113 kV/cm. We attribute this increase in molecular decay rate to predissociation rather than any other Stark-induced nonradiative or radiative phenomenon. The lowest level (v′3 =2, J′=2, K′=1) from which we have observed an increase in decay rate (i.e., predissociation) has an internal energy of 44 295.65 cm−1 relative to v=0, J=0 of S0 (X̃ 1∑+g). This corresponds to a value about 24 kJ/mol lower then the consensus upper bound result from four different recent experimental determinations (including one of ours) as well as current ab initio results. The new value agrees, however, with earlier work and with recent modeling studies of acetylene pyrolysis kinetics.
Phonon wings in inelastic neutron scattering spectroscopy: The harmonic approximation
Authors: Tomkinson J. , Kearley G. J.
Published: July 2002

The inelastic neutron scattering (INS) spectra of some ammonium halides and ammonium nitrate are analyzed in terms of the internal vibrations of the NH+4 ion convoluted with multiphonon lattice modes up to the eighth term. Due to the low mass of this ion, most of the intensity of the internal-mode region of the INS spectra is in the ‘‘phonon wings,’’ differences between the salts in this spectral region being mainly due to differences in their lattice-mode spectra. Refinement of the lattice Debye–Waller factor is included in the profile refinement of interatomic force constants.
On the primary process in the plasma-chemical and photochemical vapor deposition from silane. III. Mechanism of the radiative species Si*(1P ) formation
Authors: Tsuda Minoru , Oikawa Setsuko , Sato Kazufumi
Published: July 2002

The mechanism of the radiative species Si*(1P0) formation from SiH4 was investigated with ab initio molecular orbital–configuration interaction calculations on two reaction routes: (a) the simplest one-step reaction mechanism SiH4→Si+2H2 and (b) the second simplest two-step reaction mechanism SiH4→SiH2+H2→Si+2H2. The conclusions obtained are as follows: (i) the one-step mechanism (a) operates effectively in the generation of the radiative species Si*(1P0), (ii) no effective path was found for the formation of Si*(1P0) in the two-step mechanism, (iii) the radiative species Si*(1P0) originates from the fifth lowest excited singlet state 11T1 of SiH4 at the Td structure in the one-step mechanism (a), whereas the nonradiative species Si(1D) is produced spontaneously from the fourth lowest excited singlet state 11E in both reaction mechanisms of (a) and (b), (iv) the nonradiative species SiH2(11B1) is spontaneously generated from the lowest excited singlet state 11T2 of SiH4, (v) the electronic state of the radiative Si*(1P0) has Rydberg character and the emission of Si*(1P0) at 288.2 nm is approximately expressed by 3p4s(1P0)→3p2(1D) where 4s is a Rydberg atomic orbital (AO). It was also concluded that the contributions of 3d AOs are important for making a description of the quantitative energy separations among the lower lying singlet states, 1D, 1S, and 1P0 of Si atoms.
Cluster motion on metals: Ir on Ir(111)
Authors: Wang S. C. , Ehrlich Gert
Published: July 2002

Diffusivities have for the first time been quantitatively measured for two-dimensional clusters of iridium atoms on a metal surface, Ir(111). As cluster size increases from two to seven atoms, the diffusivity generally diminishes, but tetramers are an exception. It is shown that their high mobility comes about because of weak interatomic binding, and a low activation energy for diffusion, rather than from a change in the mechanism of motion, which involves displacements of individual adatoms independent of cluster size.
Characteristic modes of oscillatory chemical reactions
Authors: So/rensen P. Graae , Hynne F. , Nielsen K.
Published: July 2002

We show how eigenvectors of the Jacobi matrix J and its transpose JT at a supercritical Hopf bifurcation can be used for quantitative characterization and analysis of models of oscillatory chemical reactions. Eigenvectors of J determine the oscillations and the principal transients and can be expressed in terms of concentrations or reaction currents. Important reactions of a system can be easily identified this way. A pair of complex conjugate eigenvectors of JT associated with the bifurcation determines all independent quenchings of the oscillations. It can be used for quantitative comparison with experiment and in systematic search for better models. We combine the two sets of eigenvectors in a quantitative analysis of the effect of using a phase space of reduced dimension as an approximation to the full phase space. The analysis is illustrated by an explicit reduction of a five-dimensional Oregonator based model of the Belousov–Zhabotinskii reaction to a four-dimensional phase space involving a ‘‘quasi-species.’’ The reduced phase space is tangent to a slow manifold at the bifurcating stationary point. Reconstruction of amplitudes and phases of the oscillations from incomplete quenching data is explained in terms of the reduced phase space.
Photoisomerization of diphenylbutadiene in low-viscosity nonpolar solvents: Experimental manifestations of multidimensional Kramers behavior and cluster effects
Authors: Gehrke Ch. , Schroeder J. , Schwarzer D. , Troe J. , Voß F.
Published: July 2002

The photoisomerization of diphenylbutadiene was studied by picosecond absorption spectroscopy over wide pressure and temperature ranges in liquid and supercritical alkanes, CO2, SF6, and He. The reaction shows typical features of a thermal unimolecular reaction on the S1 potential energy surface. The rate can be expressed by a combination of standard unimolecular rate theory and Kramers–Smoluchowski theory. However, multidimensional behavior manifests itself in the transition to the gas phase low pressure range as well as to the high density Kramers–Smoluchowski range: in the former case, the low pressure limit of a unimolecular reaction of the polyatomic molecule is approached; in the latter case, the effective imaginary barrier frequency shows a marked apparent temperature dependence. The experiments also suggest contributions of reactant–solvent cluster interactions, which modify the barrier height even in nonpolar solvents.
Mie scattering from thin anisotropic spherical shells
Authors: Lange Benjamin , Aragón Sergio R.
Published: July 2002

Exact simple closed form solutions for the electromagnetic scattering problem for a thin spherical shell composed of radially oriented optically anisotropic scattering elements have been obtained. The solution is valid for arbitrary shell size and index of refraction, but is limited to the case of small shell thickness compared to the shell radius. Connections with the Rayleigh–Debye approximation are clearly established and the polarizability of the shell is given in terms of the dielectric constants. It is found that the Rayleigh–Debye approximation correctly computes the effect of the anisotropy in this case. These results are useful in the interpretation of light scattering experiments from phospholipid vesicle dispersions.
The derivation of the rotational potential function from atom–atom potentials. V. Hindered rotation of ammonia molecules in solid ammonia
Authors: Smith David
Published: July 2002

The rotational potential functions of ND3 and NH3 are calculated from atom–atom potentials. The site symmetry of the ammonia molecules in solid ammonia is C3. The hindered rotational energy levels of the molecules in ND3 and NH3 are computed by diagonalizing the Hamiltonian matrices. The librational frequencies and the activation energy of the ammonia molecules determined from the hindered rotational energy levels are compared to the observed results. The heat capacities of both NH3 and ND3 have been analyzed using the computed hindred rotational energy levels. The difference in the low temperature heat capacities of ND3 and NH3 is interpreted in terms of the hindered rotational energy levels of the molecules.
Variational determination of optimal orbitals for electron scattering
Authors: Hinze Juergen , Hamacher Peter
Published: July 2002

The Kohn variational principle for the (negative) logarithmic derivative can be used to solve the Schrödinger equation variationally for given energies in the continuum. The MCSCF formulation of the problem permits the optimization of the configuration expansion coefficients as well as the orbitals, out of which the configuration state functions are constructed. We elucidate the correspondence between the variational determination of the orbitals and the Configuration-Interaction approach.
Optically heterodyned polarization spectroscopy. Measurement of the orientational correlation function
Authors: Alavi D. S. , Hartman R. S. , Waldeck D. H.
Published: July 2002

Polarization spectroscopy has been developed as a useful method for the investigation of molecular reorientation in both liquid phase solutions and in the gas phase. This technique has the advantage of measuring a single particle orientational correlation function directly but the disadvantage of averaging over rotation in all electronic states. Described and characterized herein is a variant of this technique, optically heterodyned polarization spectroscopy, which is able to disentangle various contributions to the signal and determine the rotational relaxation of the solute molecule in different electronic states independently. This work also demonstrates the measurement of the normalized value of the orientational correlation function at time zero, r(0), without extensive normalization of laser parameters. Lastly, various technical advantages of the optically heterodyned method are discussed.
The rotational spectrum of the CH radical in its a 4Σ− state, studied by far-infrared laser magnetic resonance
Authors: Nelis Thomas , Brown John M. , Evenson Kenneth M.
Published: July 2002

The CH radical has been detected in its a 4Σ− state by the technique of laser magnetic resonance at far-infrared wavelengths. Spectra relating to different spin components of the first three rotational transitions have been recorded. The molecule was generated either by the reaction of F atoms with CH4, with a trace of added oxygen or by the reaction of O atoms with C2H2. The observed resonances have been analyzed and fitted to determine the parameters of an effective Hamiltonian for a molecule in a 4Σ state. The principal quantities determined are the rotational constant B0=451 138.434(94) MHz and the spin–spin parameter λ0=2785.83(18) MHz. Proton hyperfine parameters have also been determined.
Lattice dynamics and vibrational spectra of polypyrrole
Authors: Tian B. , Zerbi G.
Published: July 2002

The interpretation of the infrared and Raman spectra of pristine polypyrrole as 1-d lattice is presented as the result of lattice dynamical calculations. Geometries and vibrational force fields are derived from semiempirical quantum chemical calculations and/or from the optical spectra of oligomers. This paper lays the background information which are used in the second paper, which immediately follows, where the spectra of the conducting polymer are extensively discussed.
Smectic–smectic phase transitions in binary liquid-crystalline mixtures of DB5–T8
Authors: Fontes Ernest , Lee Wah Keat , Heiney Paul A. , Nounesis George , Garland Carl W. , Riera Antoni , McCauley John P. , Smith Amos B.
Published: July 2002

We have used high-resolution x-ray diffraction and ac calorimetry to study the liquid–crystalline smectic-A-smectic-A phase transitions in binary mixtures of polar mesogenic compounds DB5 [benzoic acid, (4-cyano)-4-[(4-pentylphenoxy) carbonyl] phenyl ester] and T8 [(E)-benzoic acid, 4-octyloxy-4-[2-(4-cyanophenyl) ethenyl] phenyl ester]. A detailed synthesis for each of these compounds is presented. The smectic A1 phase in this system shows two collinear diffuse x-ray maxima at incommensurate wave vectors, indicating a tendency to form the predicted Ai1 phase with two strongly coupled density modulations having incommensurate wavelengths. The Ad peak position, while both temperature and concentration dependent, is remarkably constant along the N–Ad phase boundary. In the vicinity of 16 mol % T8, both the x-ray and heat-capacity results are reproducibly history dependent. The simultaneous observation of sharp A2 and Ad peaks most likely results from phase coexistence, but a condensed incommensurate Ai2 phase cannot be ruled out.
Rate constants for the reaction, H+D2→HD+D, over the temperature range, 724–2061 K, by the flash photolysis-shock tube technique
Authors: Michael J. V.
Published: July 2002

Thermal rate constants measured by the flash photolysis-shock tube (FP-ST) technique are reported for the reaction, H+D2→HD+D, over the temperature range, 724–2061 K. H-atom concentration has been monitored by atomic resonance absorption spectroscopy (aras). The results can be represented by the Arrhenius expression: k1=(3.95±0.35)×10−10 exp(−5919±95 K/T) cm3 molecule−1 s−1, to within ±25% over the temperature range. These results are then combined with lower temperature direct determinations, and a three parameter expression is derived which expresses the rate behavior between 256–2061 K: k1=1.69×10−17T1.10 exp(−3527 K/T) cm3 molecule−1 s−1. The experimental results are then compared to theoretical calculations that utilize ab initio potential energy surfaces that are presumably the most exact that have ever been determined. Thus, the theoretical to experimental comparison constitutes a stringent test of the ab initio surfaces and the dynamical calculations in which they are used. The conclusion from this comparison is that transition state theory supplies a high quality prediction for the rate behavior, being within ±30% of the experimental data over the entire temperature range.
Linear thermodynamic analysis of the reversible Selkov model: An interpretation of the Chatelier-like principle for local concentration fluctuations near thermodynamic equilibrium
Authors: Dutt Arun K.
Published: July 2002

Thermodynamic analysis of the reversible Selkov model (a simple kinetic model describing glycolytic oscillations) has been done by an entropy production technique of Prigogine and it is shown that only the autocatalytic step can destabilize the steady state in this model. It is derived that at thermodynamic equilibrium, the product δS δP is always a positive quantity which appears to be a Chatelier-like principle for local concentration fluctuation applicable to the autocatalytic step S ⇄ P of this model.
A direct method of studying reaction rates by equilibrium molecular dynamics: Application to the kinetics of isomerization in liquid n-butane
Authors: Brown David , Clarke Julian H. R.
Published: July 2002

In this article we describe a straightforward and general method for studying chemical reaction kinetics in equilibrium systems using molecular dynamics computer simulation. We have applied it to the problem of determining the rate constants for the trans–gauche isomerization processes in liquid n-butane. The method relies upon the fact that if a dynamic equilibrium exists in the system then it must apply to any chosen subset of the system. The power of the method is demonstrated by its high precision and in the discovery that a mechanism for direct gauche to gauche interconversions through the trans well is required to explain the data. The connection between the method applied here and other approaches to the determination of rate constants is discussed. The correlation function for isomerization dynamics is usually formulated as a collective property of the system. Its use leads to poor statistics when applied to simulation data. It is shown that the relaxation functions are equivalent to single particle correlation functions which fully define the isomerization kinetics. An additional observation from our 1.8 ns simulation is that the trans–gauche equilibrium in model liquid n-butane is shifted by ∼1% to excess gauche in comparison to the ideal gas at the same temperature. This is a smaller solvent effect than has been reported in the past.
Hydrodynamic description of suspensions of Brownian particles
Authors: Alarcon Waess O. , Garcia-Colin L. S.
Published: July 2002

Using the formalism of extended nonequilibrium thermodynamics we propose a set of variables which are relevant in describing the space- and time-dependent properties of suspensions consisting essentially of Brownian particles. The time evolution equations for the nonconventional variables are derived, which together with the appropriate time evolution equations for the subset of the conventional variables, form a complete set of equations for the full set of dynamical variables. These equations are used to derive the form of the dynamical structure factor under the assumption that hydrodynamic interactions are negligible. The results are compared and discussed in the light of those obtained by other methods.
Spectral hole burning and holography. II. Diffraction properties of two spectrally adjacent holograms
Authors: Renn Alois , Meixner Alfred J. , Wild Urs P.
Published: July 2002

The first paper in this series presented a detailed analysis of the diffraction efficiency of a single holographic grating obtained by spectral hole burning. This paper covers the influence of phase changes which occur during the burning of a single hologram, and the interaction of spectrally adjacent two laser induced gratings. A theoretical description of the holographic efficiencies which takes into account the influence of the relative phases between the two gratings. Experimental results are presented and discussed in terms of the theory.
Symmetry adapted Fourier solution of the time-dependent Schrödinger equation
Authors: Shi Yun , Tannor David J.
Published: July 2002

The application of the time-dependent Schrödinger equation/Fourier method to triatomic systems in hyperspherical coordinates is described. In particular, we consider the high symmetry situation of three identical particles, and focus on the reduced dimensional dynamics in the hyperspherical coordinate χ, which contains all the symmetry of the general problem. The periodic structure of the wave functions in the χ coordinate leads to a discrete spectrum in k space. The additional finite symmetry of the wave functions in this coordinate gives rise to selection rules, analogous to rotational selection rules for symmetric top molecules, which ensure that only one out of six of these integer k values has nonzero intensity. It is possible to reduce the range of the wave function (and the calculation) by a factor of 6 by selecting only one replica of the coordinate space wave function. The FFT automatically increases the spacing in k space by a factor of 6, eliminating the lines in k space with nonzero amplitude. For each different symmetry of wave function in coordinate space it is necessary to shift the compacted grid in k space by a different amount, in accord with k space selection rules.
Vibrational energy as a hydrodynamic variable
Authors: Visscher P. B. , James John C.
Published: July 2002

Study of the relaxation of vibrational energy in a polyatomic fluid is usually based on a simple definition of the vibrational energy as the sum of the energies of the internal degrees of freedom of the molecules. Extensive computer simulations have shown that this is not in fact the best variable to use in a hydrodynamic theory because it is not the most slowly relaxing variable. We propose an alternative definition (omitting very hot vibrators) which relaxes much more slowly, and is therefore more useful on a hydrodynamic time scale.
Energy relaxation in dense fluid mixtures
Authors: Khalil-Yahyavi B. , Châtelet M.
Published: July 2002

A theoretical model based on isolated binary interactions is generalized by using a realistic isotrope interaction potential for energy relaxation in dense fluid mixtures. We have applied with success this model, the soft-spheres model, to experimental vibrational energy relaxation time τ data of N2(v=1) in highly compressed gas mixtures of D2–N2 at room temperature as a function of gas density and D2 concentration. We show the limitations of the hard-spheres model. The predictions of the ‘‘soft-spheres’’ model is also compared to experimental data of energy relaxation of O2 in N2 or Ar liquid systems as a function of mixture ratio.
Reactive scattering and electron detachment in O− collisions with H2. Nascent product energy distributions
Authors: Esaulov V. A. , Champion R. L. , Grouard J. P. , Hall R. I. , Montmagnon J. L. , Penent F.
Published: July 2002

Results of the measurements of the energy distributions of secondary electrons and H− ions produced in low relative energy (1 to 10 eV) collisions of O− with H2 are presented. The electron distribution for the associative detachment reaction is found to be narrow and peaks at almost zero energies, indicating the formation of a highly vibrorotationally excited molecule. The H− energy distributions are structured and indicate the formation of vibrationally excited OH. For low collision energies this excitation is small. The H− energy distributions are found to resemble closely the ones obtained in dissociative attachment in electron–H2O scattering.
Reply to the Comment on: ‘‘Lifetime of the A 2Σ+, v′=0 state of HS’’
Authors: Loge Gary W. , Tiee J. J.
Published: July 2002

The results of measurements of the lifetime of the excited states of SH, using Hanle effect and by measuring linewidths in LIF−molecular beam is explained.(AIP)