bluuebunny/crossref_metadata_2025 huggingface dataset:
Triplet direct correlation functions in the hard-sphere fluid
Authors: Curtin W. A.
Published: July 2002

The weighted density approximation (WDA) to the Helmholtz free energy of an inhomogeneous liquid is used as a generating functional for the triplet direct correlation function c(3) (k,k′) of the hard-sphere fluid. The results are compared to a recent Monte Carlo determination of c(3) for hard spheres and good agreement is found. The WDA results are slightly inferior to those obtained by the free energy model of Rosenfeld, although the WDA predicts the existence of a hard sphere face-centered-cubic (fcc) crystal phase at high densities while the model of Rosenfeld does not.
Molecular dynamics simulation of the vibrational properties of a disordered N2O crystal. I. Lattice frequencies
Authors: Cardini Gianni , Schettino Vincenzo
Published: July 2002

The consequences of head to tail disorder in crystalline N2O have been studied by means of molecular dynamics simulations. Results are presented for the infrared and Raman spectrum of the lattice vibrations. The effect of the simulation box size and of the degree of head-to-tail disorder is considered. Single particle and collective mode properties are analyzed. The origin of line broadening associated with the head-to-tail disorder is discussed.
Random heteropolymers in layered fluids
Authors: Garel T. , Orland H. , Shakhnovich E. I.
Published: July 2002

We study a simple model of a random heteropolymer, without excluded volume, in layered fluid systems of the form (i) ABA or ABC, where A, B, C denote three immiscible fluids, (ii) AB+hard wall, (iii) ABABA... . The quenched randomness is contained in the affinities {ξj; j=1,2,...,N} of the N links of the chain with the different fluids: these affinities are taken as random independent variables. Case (i) may be of interest for the localization of proteins in membranes, and (ii) pertains to the adsorption onto a wall. Using a replica-symmetric theory with a Hartree variational procedure, we study the behavior of the chain as a function of temperature. Various transitions [localization in case (i), adsorption in (ii)] are then exhibited. In the periodic situation (iii), the quenched and annealed randomness lead to the same results.
Radiative relaxation rate determination by competitive photodissociation of n-butylbenzene ions
Authors: Uechi Guy T. , Dunbar Robert C.
Published: July 2002

The radiative relaxation rate of n-butylbenzene ions was measured using competitive photodissociation as a thermometric tool. Cooling curves showing the loss of internal energy of the ions with time were obtained at three pressures. An infrared radiative cooling rate of 0.5 s−1 was obtained from the data. This value is comparable to other cooling studies done in polyatomic ions having internal temperatures of the order of 200 K above room temperature. The radiative cooling of polyatomic molecules is compared with that of diatomics. Observations are in accord with theoretical expectations in showing that for polyatomics the infrared radiative cooling rate constants are slow and strongly dependent on internal temperature, in contrast to diatomics where they are fast and temperature independent.
Production of vibrationally excited O+2 in the reaction of O+ with CO2
Authors: Viggiano A. A. , Morris Robert A. , Dale F. , Paulson John F.
Published: July 2002

The production of vibrationally excited O+2 in the reaction of O+ with CO2 has been studied as a function of temperature. The measurements were made in a variable temperature-selected ion flow drift tube by the monitor ion technique. The principal vibrational level produced is found to be O+2(v=1), with approximately 45%–51% of the O+2 produced in this state. The remainder is O+2(v=0), representing 23%–43%, and O+2(v≥2), representing 6%–32%. More vibrational excitation is observed at higher temperature, but this temperature dependence may be obscured due to vibrational quenching. The various vibrational levels of O+2 observed here were found to react with Xe at different rates, with O+2(v=1) having the largest rate constant. Rate constants for several reactions pertinent to this study are also reported.
Observation of the U1←0(1) transition of solid deuterium
Authors: Chan Man-Chor , Oka Takeshi
Published: July 2002

Infrared absorption in the 3 μm region of normal and ortho enriched solid deuterium samples at 4.2 K has been restudied with improved signal-to-noise ratio using a Bomem FTIR spectrometer. The zero-phonon U1←0(1) transition of deuterium was observed for the first time in the spectrum of normal D2. In addition, we also observed the phonon branches UR(0) [of the U1←0(0) transition] and UR(1) [of the U1←0(1) transition], which were not seen in previous work. The analysis of the spectra and the effect of p-D2 molecules on the intensities and linewidths of the transitions are discussed. The hexadecapole moment of the U1←0(0) transition was estimated to be 0.121 ea40 from the experimental integrated intensity.
Raman overtone intensities measured for H2
Authors: Shelton D. P.
Published: July 2002

The Raman spectra of the vibrational fundamental, first overtone and second overtone transitions of the H2 molecule were recorded using visible and ultraviolet argon–ion laser excitation. The ratios of transition polarizability matrix elements, α01,21/α01,11 and α01,31/α01,11, were determined from the measured intensities of the Q(1) Raman lines v,J=0,1→v′,1 for v′=1,2,3. The experimentally determined value of the Raman first overtone matrix element is in good agreement with the value from the best ab initio calculation.
Effect of polydispersity and second virial coefficient on light scattering by randomly cut random coils
Authors: Reed Christopher E. , Reed Wayne F.
Published: July 2002

The light scattering by an initially monodisperse population of Gaussian random coils in a theta solvent is described by the well known Debye function: P(θ)=D[u(θ)] =(2/u2)(e−u−1+u), where u=u0≡〈S2〉q2. S is the radius of gyration, and q=(4πñ/λ)sin(θ/2) is the magnitude of the scattering vector, λ being the vacuum wavelength of the incident light, ñ the index of refraction, and θ the scattering observation angle. If the molecules undergo random scission, P(θ)=D(u0+r), where r is the average number of scissions per original molecule. In a good solvent, one should include the effect of the second virial coefficient A2 on the light scattering. In the single contact approximation this can be done by using Kc/R=1/MP(θ)+2A2c for an originally monodisperse solution. K is a constant, R the absolute Rayleigh scattering ratio, and c the concentration. The above equation is generalized to originally polydisperse solutions and branched random coils without loops. We discuss its mathematical limits and range of validity, and how to apply it to experimental situations. It is speculated that it may work fairly well when the penetration function ψ is less than about 0.1. We also discuss possible methods of extending its range of validity.
What can gas phase reactions tell us about solution reactions?
Authors: Li Y. S. , Wilson Kent R.
Published: July 2002

Much of the behavior of simple gas phase reaction dynamics can be understood in terms of simple pictures based on the shapes of the underlying potential energy surfaces and the masses of the reagent atoms. Our aim here is to investigate to what extent such gas phase models can be used to understand properties of solution reactions. In particular, we examine for a solution reaction the validity of the Evans–Polanyi rule that an early potential barrier favors translational excitation of the reactants and vibrational excitation of the products, with the converse holding true for a late barrier. The test is performed by using molecular dynamics simulations for an asymmetric linear transition state A+BC→AB+C atom exchange reaction in argon solvent. We calculate for both gas and solution reactions the partitioning among translational, rotational, and vibrational energy during the reaction process. We find that for a short time period (−65 to 65 fs where t=0 is at the barrier top), in which the forces from the intrinsic gas phase potential dominate, the Evans–Polanyi rule can be carried over into the solution reaction. The gas phase vibrational energy distributions persist in solution over a much longer period. In particular, this calculation illustrates for an early barrier linear transition state potential in solution that a solvent induced fluctuation in the reagent translational energy is considerably more effective than a fluctuation in vibrational energy in prompting reaction. The resulting reaction products are formed highly vibrationally excited. For the reverse late barrier reaction, a solvent induced fluctuation in vibrational energy is needed for reaction and the resulting products are initially highly translationally excited. We expect that on the proper time scale, many other gas phase reaction dynamics rules will also carry over to solution reactions, particularly in cases in which the reagent–solvent interaction is weak.
Applications of the generator coordinate approximation to diatomic systems. III. Curve-crossing problems
Authors: Broeckhove J. , Claessens M. , Lathouwers L. , Van Leuven P. , Deumens E. , Öhrn Y.
Published: July 2002

The generator coordinate approximation, previously applied to vibration–rotation levels near potential-energy minima, is now worked out for curve-crossing situations. We define the weak and strong adiabatic coupling limits. For weak adiabatic coupling both the adiabatic and generator coordinate approximations become exact. In the strong adiabatic coupling limit the adiabatic approximation breaks down, whereas the generator coordinate approximation again reproduces the exact solutions. These theoretical results are confirmed by calculations for a Hamiltonian modeled to the EF,GK 1Σ+g curve crossing in the electronic spectrum of the hydrogen molecule.
Molecular beam optical Stark spectroscopy of YF
Authors: Shirley Jeffrey , Scurlock Chris , Steimle Timothy , Simard Benoit , Vasseur Michael , Hackett P. A.
Published: July 2002

The molecular-beam-optical Stark spectrum of the B1Π(v=0)−X 1Σ+(v=0) band system of YF has been recorded and analyzed. The permanent electric dipole moment μ and the magnetic hyperfine parameter a for the B1Π state were experimentally determined to be 2.96(4) D and 146.8(3) MHz and the experimentally determined value for μ(X 1Σ+) is 1.82(8) D. The sign of the magnetic hyperfine parameter indicates that the major contribution to the B1Π state is from a⋅⋅⋅πδ configuration. The determined μ(X 2Σ+) value is compared with theoretical predictions and the ratio μ(X 1Σ+)/μ(B 1Π) is rationalized in terms of plausible molecular orbital descriptions for the two electronic states.
Electron impact excitation of coronene
Authors: Khakoo M. A. , Ratliff J. M. , Trajmar S.
Published: July 2002

A preliminary study of the electron-impact excitation of thermally evaporated coronene at 550° C was carried out using electron-energy-loss spectroscopy. Measurements of the energy-loss spectra of coronene at high (100 eV) and low (5–20 eV) impact energies are presented. One of the high-energy spectra was converted to an apparent generalized oscillator strength spectrum and compared to the photoabsorption spectrum of coronene. Observations concerning vibrational excitation of coronene by electron impact are also presented and discussed.
Spin–rotation and hyperfine structure in the X 2Σ+ state of yttrium monosulfide by molecular-beam laser-radio-frequency double resonance
Authors: Azuma Y. , Childs W. J.
Published: July 2002

The molecular-beam laser-radio-frequency double-resonance method has been used to measure the spin–rotation and magnetic hyperfine structure of yttrium monosulfide (YS) in its X 2Σ+ electronic ground state. The spin–rotation constant γ is found to be positive, unlike that of YO. The Fermi contact and dipolar hyperfine interactions (due to the spin I=1/2 of 89Y) are found to be rather close to the corresponding quantities in YO. The contact hfs constant b in the excited B 2Σ+ state of YS was determined by combining the directly measured X 2Σ+ splitting information with B–X optical hfs observations.
Optimal phase modulation in stored waveform inverse Fourier transform excitation for Fourier transform mass spectrometry. II. Magnitude spectrum smoothing
Authors: Guan Shenheng
Published: July 2002

A smoothing method for generating optimal SWIFT (stored waveform inverse Fourier transform) excitation waveforms used in Fourier transform mass spectrometry (FT-ICR or FTMS) was previously proposed to substitute time-domain waveform apodization procedures. This work gives a detailed analysis of the simple smoothing procedure. The effect of the smoothing procedure on magnitude spectral edges can be easily expressed in analytical format so that the frequency resolution of excitation can be easily analyzed. The relation between the time domain apodization and the smoothing of magnitude spectra in the frequency domain is established. This provides a convenient method to estimate the time duration required for accommodation of excitation power leakage from the power distribution limits. A method for generating nonconstant frequency resolution excitation waveforms is proposed.
Determination of the electric dipole moment of HN+2
Authors: Havenith M. , Zwart E. , Leo Meerts W. , ter Meulen J. J.
Published: July 2002

The electric dipole moment of the linear molecular ion HN+2 was determined by measuring the isotope shifts of the rotational g factors of different isotopic species. We studied the Zeeman effect of the R(6) rotational transition at 650 GHz. In a magnetic field of 5.4 T the rotational transition split into two components, separated by 2.2–2.5 MHz. The gR factors were determined for H14N14N+, H14N15N+, and H15N14N+. The dipole moment for H14N14N+ was determined as (3.4±0.2) D, which is in excellent agreement with the theoretical value. The transitions were observed by direct absorption spectroscopy with a tunable FIR sideband spectrometer. The ions were generated in a modified anomalous discharge placed in a superconducting magnet.
The crystalline site symmetry and its effect on the vibrational spectra of a weakly hindered molecule. V. Crystallization effects in NH4B(C6H5)4
Authors: Roberts Malcolm P.
Published: July 2002

Changes in the vibrational spectra for molecules that nearly freely rotate or perform rotational tunneling at low temperatures are explained through the systematic study of the combined symmetry of the molecule, the site, and the molecule’s nuclear spins. The changes observed for different crystallizations in the low temperature vibrational spectra of the NH+4 ion dilute in KTPB and in NH4B(C6D5)4, the NH3D+ ion in ATPB and NH4ClO4, and of solid CH4 II are examined and explained as arising from slight perturbations of the crystalline site symmetry.
Micellar radical pair decay
Authors: Bittl Robert , Schulten Klaus , Turro Nicholas J.
Published: July 2002

The magnetic-field dependence of hyperfine coupling-induced radical pair processes in micelles is studied theoretically. We show that experimental observations on reaction dynamics of triplet benzylic radical pairs can be explained by a hyperfine interaction mechanism when the exchange interaction between the unpaired spins in the radical pair confined to a micelle is taken into account. The theoretical description applied involves a stochastic Liouville equation. The time-dependent solutions of the stochastic Liouville equation qualitatively reproduce the observed magnetic-field and isotope dependence of micellar radical pair decay.
Laser-induced photofragmentation of triethylaluminum: Modeling H-atom production
Authors: Brum Jeffrey L. , Deshmukh Subhash , Koplitz Brent
Published: July 2002

A rate-equation approach is presented that models H-atom formation during the pulsed laser photolysis of a triethyl metal compound, the specific case being triethylaluminum excited at 193 nm. An excimer laser initiates the chemistry under collisionless conditions, and H atoms are produced that are detected using two-photon (121.6+364.7 nm) ionization. Experimentally, the H-atom intensity is monitored as a function of photolysis laser power. Mechanistically, the primary photodissociation step is postulated to involve cleavage of the metal–carbon bond, thereby producing an ethyl radical. This species can then either: (1) form C2H4 and H directly; or (2) absorb an additional photon and produce an H-atom photofragment. The rate equations and their solutions allow one to calculate how H-atom production should vary as a function of photolysis laser power, and the interplay between the two H-atom production channels is calculated for various absorption cross sections and dissociation rates. A comparison with experimental power dependence data suggests that an overall one-photon dissociation process predominates under the experimental conditions actually used.
Diffusion-controlled reactions in a polydisperse medium of reactive sinks
Authors: Given James A. , Blawzdziewicz Jerzy , Stell George
Published: July 2002

We study diffusion-controlled reactions in which a reactive solute diffuses through a medium containing static, reactive, spherical traps of many different sizes. We focus on the cases of impenetrable, i.e., nonoverlapping traps, and of randomly overlapping traps. Bounds for the trapping rate are derived using trial functions of the kind developed by Doi, and by Weissberg and Prager. It is shown that the bounds for trapping rate are relatively insensitive to dispersivity of trap size when they are plotted against the proper scaling variable. The trap volume fraction is the proper scaling variable for randomly overlapping traps. The mean density of traps ρ〈a〉 is shown to be the proper scaling variable for nonoverlapping traps. It is shown that the low-density limits of both the Doi and the Weissberg–Prager bounds fail to reproduce the radius-averaged single-trap solution of Smoluchowski. We give a generalization of these classes of bounds that has the proper behavior. In evaluating trapping bounds for impenetrable traps, the material correlation functions are evaluated for the first time for a nontrivial polydisperse system.
Long-range, nonadiabatic effects in statistical adiabatic channel models: Dynamic orientation of diatomic fragments formed in the decomposition of long-lived triatomic complexes
Authors: Dashevskaya E. I. , Nikitin E. E. , Troe J.
Published: July 2002

Using the example of the decomposition of a long-lived triatomic complex into an atom and a diatom, it is shown that nonadiabatic interactions between adiabatic channel states at large interfragment distances strongly reduce the dynamic rotational orientation of the diatom. The degree of orientation is expressed in terms of a number of open adiabatic channels and ‘‘slipping factors.’’ These factors are estimated for an anisotropic potential following an inverse power law.
Orbital evolution and promotion effects in the photoionization dynamics of 2Σ− Rydberg states of OH
Authors: Stephens J. A. , McKoy V.
Published: July 2002

In this paper, we discuss the photoionization dynamics of the D 2Σ−(1π23pσ) and 3 2Σ−(1π24sσ) Rydberg states of OH, emphasizing the critical role that Rydberg orbital evolution plays at intermediate to larger internuclear distances in determining vibrational and rotational molecular ion distributions. The orbital evolution process is discussed in terms of diabatic and adiabatic molecular states, united atom–separated atom correlation rules, and quantum defect functions. Vibrationally resolved photoelectron spectra and angular distributions for resonance enhanced multiphoton ionization (REMPI) of OH via the D 2Σ−(1π25σ) and 3 2Σ−(1π26σ) Rydberg states are considered as examples. The results and conclusions are relevant to vibrationally and rotationally resolved REMPI studies of all first-row molecular hydrides, due to the similarity of their electronic structure and correspondence to their associated united atom.
Non-Franck–Condon effects in photoionization of the 3 3Π Rydberg state of NH
Authors: Wang Kwanghsi , Stephens J. A. , McKoy V.
Published: July 2002

Strong non-Franck–Condon behavior is predicted to occur in the vibrationally resolved photoionization spectra of NH for (3+1) resonance enhanced multiphoton ionization processes via the 3 3Π Rydberg state. The non-Franck–Condon effects are interpreted on the basis of rapid orbital evolution, Cooper minima, and internuclear distance dependence of the dipole transition moment and cross sections. A Cooper minimum occurs in the 5σ→kπ channel at small internuclear distances, where NH resembles its united atom, oxygen. The iterative Schwinger variational method and multiplet-specific ion potentials are employed in the calculation of the photoelectron continuum wave functions. Cross sections and asymmetry parameters for photoionization of the NH ground state leading to the X 2Π, a 4Σ−, and A 2Σ− ions are also reported.
Simulation of coherent energy transfer in a hydrogen bonded amide chain by Fermi resonance
Authors: Clarke David L. , Collins Michael A.
Published: July 2002

Classical simulations are used to examine vibrational energy redistribution in a chain of hydrogen bonded N-methylacetamide molecules following excitation of a NH stretching mode or Amide-I mode at one end of the chain. The dynamics take place on a complex potential energy surface constructed from ab initio calculations and empirical surfaces, refined with reference to observed vibrational frequencies. Simulations are used to characterize the dynamical effects of variation of several important potential parameters, and to determine the existence of coherent energy transfer by Fermi resonance mechanisms. Results are discussed with reference to simple models.
Nonequilibrium thermodynamic analysis of Michaelis–Menten kinetics
Authors: Mou Chung-Yuan , Liang Chung-Hsien
Published: July 2002

We developed a method of analysis of nonequilibrium steady states of chemical reaction system. The analysis is applied to systems of Michaelis–Menten type reactions. We focus our attention on the consequence of the quasi-steady-state approximation to the rate of entropy production. Under separation of time scale, the description of the complete kinetic system can be contracted to slow variables. We find that the resulting rate of entropy production can be written in a new and revealing way consisting of steady-state contribution, intermediate species-specific affinity production, and channel-mixing entropy terms. The species-specific term in entropy production is in the canonical form so that a thermodynamiclike description of far-from-equilibrium steady state becomes possible. Under the quasi-steady-state approximation, we find the rate of entropy production is first order. This applies also to more complicated models. Finally, we discuss the implications in stochastic fluctuations of the free energy of the system. The Michaelis–Menten type of reactions considered in this paper do not show the possibility in gains of power from fluctuating external environment.
The interaction between macroparticles in molecular fluids
Authors: Attard Phil , Wei Dongqing , Patey G. N. , Torrie G. M.
Published: July 2002

Potentials of mean force for pairs of charged and neutral macroparticles immersed in simple dipolar hard sphere and waterlike solvents are obtained using the reference hypernetted-chain theory. In general, the results depend rather strongly upon both the solvent and the size of the macroparticles. For smaller neutral macroparticles, the potential of mean force in both solvents is essentially a short-ranged hydrophobic interaction with very little structure. For larger neutral particles in water, short-ranged structural effects likely due to solvent ordering near the flatter surfaces are evident. For charged macroparticles, the potential of mean force is found to be essentially a superposition of three distinct contributions: a short-ranged structural contribution which clearly depends upon the granular nature of the solvent; a repulsive ‘‘cavity’’ term which decays asymptotically as r−4; and of course there is the long-ranged Coulombic interaction. At intermediate separations, this repulsion is the principal correction to Coulomb’s law and it is shown to be much more important in molecular solvents than predicted by comparable dielectric continuum calculations. Furthermore, it is shown that this contribution can be represented with reasonable accuracy by simple analytical approximations.
Infrared vibrational studies of CO adsorption on ultrathin Cu films on a Rh(100) surface
Authors: He J.-W. , Kuhn W. K. , Leung L.-W. H. , Goodman D. W.
Published: July 2002

Adsorption of CO on ultrathin Cu films supported on a Rh(100) surface has been studied using infrared reflection absorption spectroscopy (IRAS). At low Cu coverage (<0.3 ML), CO on Cu shows a higher vibrational frequency than the gas phase CO stretching frequency (2143 cm−1). This blue shift is interpreted as arising from CO adsorption on Cu atoms and/or two-dimensional atom clusters with a small positive charge, i.e., Cuδ+n, formed via charge transfer at the Cu–Rh(100) interface. As the Cu coverage increases, the CO vibrational frequency for Cu–CO red shifts. On an annealed Cu overlayer (≳0.3 ML), the IR spectra show a sharp peak at 2095 cm−1 which is interpreted to correspond to CO adsorbed on well-ordered pseudomorphic Cu domains. CO adsorption on Rh(100) was found to be blocked by 0.6 ML Cu. For a high coverage of Cu (∼7 ML) annealed to 850 K, a peak at 2076 cm−1 is observed and identified as corresponding to CO adsorption on three-dimensional Cu clusters.
The sites of Gd3+ in the luminescent matrix La1−xGdxMgAl11O19: Single crystal structure determination and site-selective excitation of Gd3+
Authors: Salem Y. , Linares C. , Jacquier B. , Saine M. C. , Gasperin M. , Lejus A. M. , Vivien D.
Published: July 2002

Single crystals of La1−xGdxMgAl11O19 for x=0.02 to 1 have been grown from the melt by the Verneuil (flame fusion) method. The localization of Gd3+ ions in the matrix has been obtained using x-ray diffraction and Gd3+ fluorescence techniques giving the average and the local structure of the material. The resolution of the crystal structure, of La0.4Gd0.6MgAl11O19 homolog, indicates that this compound is of the distorted magnetoplumbite (MP) type (hexagonal P63/ mmc). Lanthanide ions lie in the mirror plane in two kinds of sites: the (2d) regular MP one (D3h symmetry) occupied by La3+ ions, the distorted (12j) one (Cs symmetry) partially filled up and containing only Gd3+. Some oxygen ions of the Ln coordination polyhedron may be missing leading to a lowering of the true symmetry of the sites. Site selective excitation of the fluorescence of Gd3+ and emission spectra have been carried out on crystals with different x values. It indicates that Gd3+ ions are distributed mainly among two sites, A and B. Crystal field analysis of the splitting of the 6P terms of Gd3+ determined on the excitation spectra show that site A is close to the ideal D3h symmetry while site B is a strongly distorted site. This leads to the identification A=(2d), B=(12j). The occupancy is larger for site B than for site A in agreement with the refinement of the structure. Selective laser excitation into the A site induces emission of the B sites as a result of energy transfer which is demonstrated by the fluorescence decay. No evidence of energy migration is found for gadolinium content up to 100%. Fluorescence spectroscopy and crystal structure determination appear complementary to obtain a detailed description of the sites of Gd3+ in La1−xGdxMgAl11O19.
Nuclear spin relaxation in paramagnetic solutions. Effects of large zero-field splitting in the electron spin Hamiltonian
Authors: Sharp Robert R.
Published: July 2002

Expressions are derived describing nuclear spin relaxation in paramagnetic salt solutions under conditions where the electron spin Hamiltonian is dominated by a uniaxial quadratic zero-field splitting (zfs) interaction. In this situation, the electron spin vector is quantized along molecular axes rather than along the external magnetic field. By expressing the time dependence of the electron spin operators, written in the molecular coordinate frame, in the Heisenberg representation and then transforming these expressions to the laboratory coordinate system, simple closed form expressions for the paramagnetic nuclear relaxation increment have been derived. Electron–nuclear dipole–dipole and scalar relaxation mechanisms are considered. The resulting expressions parallel those of Solomon–Bloembergen–Morgan theory, but are valid in the zfs limit rather than the Zeeman limit. Nuclear relaxation rates in the zfs and Zeeman limits exhibit characteristic qualitative differences, some of which have been noted in earlier studies. Of particular note is the fact that the scalar contribution to T−11p is much larger in the zfs than in the Zeeman limit. In most circumstances, T−11p=T−12p in the zfs limit, while in the Zeeman limit, scalar relaxation usually contributes significantly only to T−12p. A vector model of this phenomenon is suggested. The results are valid for arbitrary values of the electron spin quantum number but they assume that electron spin relaxation is in the Redfield limit, i.e., that the correlation times of the coupling between electron spin and the lattice be short on the time scale of electron spin relaxation. This condition is probably satisfied widely when the static zfs is large.
The size distribution of rigid segments in a one-dimensional equilibrium fluid
Authors: Burgos E. , Bonadeo H.
Published: July 2002

The possibility of obtaining information on the size distribution of a rigid segment 1D fluid from its experimentally determined radial distribution function, subject to random noise, was investigated. It was found that a direct determination of the size distributions is impossible since the solutions are unstable even for very low levels of noise. Using the moments of the radial distribution function, however, it is possible to obtain reliable data on the density and packing fraction of the fluid and on the average size and dispersion of the size distribution, and good information on its asymmetry parameter and kurtosis, which may allow a distinction between different shapes.
Transport properties of polydisperse fluids. II. Effect of solute polydispersity on the bulk and shear viscosities of a solute–solvent mixture of hard spheres in the Enskog approximation
Authors: Xu Jiasai , Stell George
Published: July 2002

The bulk and shear viscosity expressions for a binary mixture of hard spheres in the Enskog transport theory are generalized to the case in which one of the species (the solute) becomes polydisperse. The effect of polydispersity is expressed in terms of correction factors to results for a binary mixture with a monodisperse solute. These correction factors are evaluated for both a log-normal size distribution of solute particles with the solute mass-size relation of power-law form and a log-normal mass distribution of solute particles with fixed solute-particle size. The results reported here can be easily generalized to a multicomponent mixture involving several polydisperse species and several monodisperse species.
Orientational analysis of pelargonic acid at liquid–solid interfaces, drops, and ultrathin liquid films, by polarized Raman spectroscopy
Authors: Yahiaoui Bechir , Masson Michèle , Harrand Monique
Published: July 2002

Polarized Raman spectra of pelargonic acid (PA), CH3–(CH2)7–CO2H, at liquid–solid interfaces were recorded, using the excitation of the surface plasmon of silver. In this device, the exciting electric field decreases exponentially (or quasi-exponentially) into the sample. Therefore, the molecules in the vicinity of the solid surface are preferentially illuminated. This study was performed on two types of systems: 1°—One interface: a drop of PA is deposited on a solid surface [silver, silica, cetyltrimethylammonium bromide (CTAB), or Langmuir–Blodgett (LB) layers of barium stearate]; 2°—two interfaces: liquid ultrathin films of PA are placed between silica or barium stearate LB layers; the thickness of the film (250 to 5 nm) being measured by an interferential method. The shape of the 2800–3000 cm−1 band, which is very sensitive to the chain conformation, was studied. Since in the convenient polarization, the spectra of the drops were different from the spectrum of the isotropic liquid (classical cell), we can assume that the PA molecules have an anisotropic distribution in the vicinity of a solid surface and that the molecules are, therefore, more or less uniaxially oriented.

The orientation is dependent on the nature and the smoothness of the solid surface, as is the distance over which the orientation spreads. (1°) If the distance of orientation is short relative to the penetration depth (≂100 nm) of the exciting wave, most of the illuminated molecules belong to an isotropic distribution: there is, therefore, little difference between the spectra of the drop and of the isotropic liquid, but, as in the ultrathin films only the oriented molecules are illuminated, spectra of films and liquid are very different. (2°) If the orientation distance is long (PA on barium stearate LB layers), the greater part of the illuminated molecules are oriented: the spectrum of the drop is therefore very different from the spectrum of the bulk (isotropic liquid); and the spectra of ultrathin films are similar to the spectrum of the drop. The differences observed in the band shapes are due to an increasing intensity of the peaks assigned to the symmetric vibrations of the CH2 and CH3 groups. These modifications can be explained by the comparison of the calculated scattering activities of uniaxially oriented molecules and random ones. The causes of the orientation and conformation change of the hydrocarbon chain have been discussed.
Studies of graphite intercalation compounds using time-differential perturbed angular distributions
Authors: Martin P. W. , Bichard J. W. , Budtz-Jo/rgensen C.
Published: July 2002

The technique of time-differential perturbed angular distributions (TDPAD) was used to investigate the structure of graphite samples intercalated by AsF5 vapor. Following population of the 198 keV state (τ=125 ns, Q=0.072b) of 19F by the 19F(p, p′γ) reaction, measurements of the γ-ray angular distribution were performed to give information on quadrupole interactions. Theoretical fits reveal the presence of two static electric field gradient (efg) interactions, the principal one corresponding to the formation of CF bonds, with quadrupole frequency νQ=57.4±0.8MH z. Although information on graphite intercalation chemistry was not forthcoming from the measurements, it is shown that the strong texture exhibited in the perturbation spectra can be explained in terms of the angular dependence of the efg, suggesting a novel method for the characterization of graphite intercalation compounds.
Observation of collisional ionization of chromium atoms in high Rydberg states and the N4 scaling relation
Authors: Rooney Daniel T. , Chaiken J.
Published: July 2002

We have used mass selected multiphoton dissociation/excitation/collisional ionization (MPD/MPE/CI) spectroscopy to study benzene chromium tricarbonyl (BCT) in an effusive molecular beam under collisionless and collisional conditions. Under collisionless conditions, our results are entirely consistent with, but not identical to, our earlier nonmass selected published experiments under bulk gas conditions. The spectroscopic results confirm that we can produce chromium atoms in a variety of well defined Rydberg states (7S3[Ns3d5], N=30–43). Following the state selective preparation of the chromium Rydberg atoms, by MPD of BCT followed by MPE of the ground state chromium atoms, they can be allowed to collide with ground electronic state benzene chromium tricarbonyl molecules producing ground state chromium ions and other products. We observe the scaling of the cross section for this collisional ionization process as a function of the Rydberg electron principal quantum number. The cross sections are a strong function of the Rydberg electron principal quantum number N, even when N is very large. Scaling as N4, the cross sections suggest that dipolar interactions with molecular colliders can be quite efficient at inducing collisional ionization of Rydberg atoms.
Rotationally and vibrationally inelastic scattering of glyoxal at E=80 meV
Authors: Kroes G.-J. , Rettschnick R. P. H. , Dateo C. E. , Clary D. C.
Published: July 2002

Results of quantum calculations on rotationally and vibrationally inelastic scattering of 1Au(S1) trans-glyoxal by H2,He, and Ar(E=80 meV) are presented. For scattering of 00 glyoxal and 72 glyoxal by H2, a quantitative comparison with a recent crossed beam experiment is made. The calculated cross sections for rotationally inelastic scattering are in good agreement with the experimental values, and the trend that σ(72, k=0→72, k′) falls off faster with k′ than σ(00, k=0→00, k′) is reproduced. Our calculations do not reproduce the experimental finding that, for the initial 72 level, the cross sections for the Δv7=−1 and Δv7=+1 transitions are nearly identical. The calculations on 72 glyoxal + He and Ar show important differences with 72 glyoxal + H2 in the competition of rotational excitation with vibrational excitation. Model calculations also presented here yield some interesting trends in the k′ dependence of the cross sections σ(00, k=0→x1, k′) on the vibrational mode x excited.
Observation of highly excited bending levels in NCO(X̃ 2Π)
Authors: Patel-Misra Dipti , Sauder Deborah G. , Dagdigian Paul J.
Published: July 2002

New NCO Ã 2Σ+–X̃ 2Π vibronic bands in the (0,v2,0)–(v1,v2,v3) sequences, with (v1,v3)=(0,0), (1,0), (0,1), (1,1), and (2,0), are reported. These bands have been observed in laser fluorescence excitation experiments of NCO produced in the CN+O2 reaction, where CN was prepared by 193 nm photolysis of cyanogen. The measured bandhead wave numbers were used to derive term values for a number of NCO(X̃ 2Π) vibronic levels, which were then fit to a model Hamiltonian.
Application of the reduced-equations-of-motion formalism to the scattering of molecular NO from a LiF (001) surface at nonzero temperature
Authors: Kay Ronald D. , Jones Patrick L. , Coltrin Michael E. , Riley Merle E.
Published: July 2002

The reduced-equations-of-motion (REOM) model for gas–solid energy transfer in molecule–surface collisions [D. J. Diestler and M. E. Riley, J. Chem. Phys. 89, 4137 (1989)] is extended to include effects of nonzero surface temperature by combining the REOM damping theory with a stochastic partial velocity reset (PVR) algorithm [M. E. Riley, M. E. Coltrin, and D. J. Diestler, J. Chem. Phys. 88, 5934 (1988)]. The REOM/PVR procedure, which involves integration of the gas–molecule EOM above the frozen solid with stochastic resetting of the molecule’s velocities, is tested by comparing results for NO+LiF(001) scattering with those of a previous stochastic-trajectory study of this system [R. R. Lucchese and J. C. Tully, J. Chem. Phys. 80, 3451 (1984)]. The REOM/PVR results reproduce the trends in the stochastic-trajectory results very well for translational and rotational energy transfer as a function of the various system parameters. However, it is found that the coupling of the vibrational mode, whose frequency is greater than that of the Debye frequency of the solid, is not treated accurately by the REOM theory, which is based on adiabatic approximation to the solid’s response function.
The adsorbate state specific photochemistry of dioxygen on Pd(111)
Authors: Wolf M. , Hasselbrink E. , White J. M. , Ertl G.
Published: July 2002

The ultraviolet-photochemistry of molecularly adsorbed oxygen on Pd(111) has been studied using pulsed laser light with 6.4 eV photon energy. Three processes occur upon irradiation: desorption of molecular oxygen, conversion between adsorption states, and dissociation to form adsorbed atomic oxygen. By using time-of-flight spectroscopy to detect the desorbing molecular oxygen and post-irradiation thermal desorption spectroscopy (TDS) to characterize the adsorbate state, a detailed picture of the photochemical processes is obtained. The data indicate that the O2 molecules desorbing with low translational energies from the saturated surface as well as the conversion of adsorbed molecules between binding states are induced by the photoinduced build-up of atomic oxygen on the surface. Analysis of a proposed reaction model reproduces the observed data and yields detailed rates. Polarization analysis indicates that the photochemical processes are initiated by electronic excitations of the substrate.
Quasihomogeneous line broadening of a hydrogen-bonded polymer, investigated by picosecond infrared holeburning
Authors: Graener H. , Lösch T. , Laubereau A.
Published: July 2002

Transient spectral holes with lifetimes of several picoseconds are investigated in the infrared absorption band of the OH stretching vibration of the terpolymer polyvinyl butyral. A large variation of the quasihomogeneous linewidth from 7–45 cm−1 is determined from the measured holewidth in the temperature range 80–330 K and interpreted in terms of anharmonic coupling to a low frequency mode ν2. The result ν2=135±8 cm−1 provides strong evidence for indirect dephasing via the OH⋅⋅⋅O bridge stretching vibration. The inhomogeneous broadening of the total band is verified by the picosecond infrared holes. The inhomogeneity is related to an approximately static distribution of hydrogen bridges generated by local disorder and displays a different temperature behavior.
Selective multiple quantum 1H nuclear magnetic resonance in chemically exchanging systems: Applications to liquid crystalline solutions
Authors: Gamliel D. , Luz Z. , Maliniak A. , Poupko R. , Vega A. J.
Published: July 2002

Multiple quantum (MQ) proton NMR spectroscopy is applied to chemically rearranging systems. These systems include liquid crystalline solutions of s-trioxane, which undergoes ring inversion and cyclooctatetraene (COT), which undergoes a bond shift rearrangement. The MQ spectra are recorded by a modified version of the selective detection method of Wokaun and Ernst. In this modification the phases of both the rf pulses and of the detector are cycled in concert, yielding a more efficient and potentially more accurate method for recording the MQ transitions. Experimental fourth and sixth order MQ spectra are presented for respectively the s-trioxane and COT solutions over wide temperature ranges. The results are compared with simulated spectra calculated using exact as well as approximate expressions for the dynamic line shapes. Potential uses and advantages as well as limitations of MQ spectroscopy for studying dynamic systems are discussed.
Elementary processes in the scattering of NO molecules from a diamond surface
Authors: Matsuo Y. , Vach H. , Châtelet M. , Häger J. , Roth C. , Flytzanis C. , Walther H.
Published: July 2002

We report on the scattering of supersonic NO molecules from a diamond (111) surface. Using state selective laser techniques combined with mass selective quadrupole measurements, we observe two novel features in the molecule–surface interaction: first, a scattering behavior which is dependent on the direction of the incoming molecules; and second, surface residence times that are unusually long, but temperature independent. The former observation can be traced back to the topography of the unreconstructed diamond (111) surface, the latter one can be understood when a temporary function of a metastable surface–molecule complex is assumed.
Microwave and far infrared spectra, r structure, barriers to internal rotation, and a
b i
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i
t
i
o calculations for 2-fluoropropane
Authors: Guirgis G. A. , Nanaie H. , Durig J. R.
Published: July 2002

The microwave spectra of five isotopic species of 2-fluoropropane, (CH3)CH2DCFH, (CH3)2CFD, (CH3)CD3CFH, (CD3)2CFD, and (CH3)213CFH, have been recorded from 12.4 to 39.7 GHz. The b- and c-type R-branch transitions have been observed and assigned for the ground state. Utilizing the rotational constants for these five isotopic species along with those reported earlier for the normal species the following r0 structural parameters have been determined: r(C–C) =1.522±0.007 Å, r(C–F)=1.398±0.013 Å, CCC =113.37±0.79°, and CCF=108.19±0.41°. All of the carbon–hydrogen parameters have also been determined from the rotational constants except for the r(C–Hsec) which was obtained from its frequency in the infrared spectrum. The far infrared spectra of 2-fluoropropane-d0, -d3 and -d7 in the gas phase were recorded with a resolution of 0.10 cm−1. Both torsional fundamentals along with several hot transitions were assigned for the three isotopic species. The barrier to internal rotation of the methyl rotors has been determined with the two-coupled rotor model to be 1149±69 cm−1 (3.29±0.12 kcal/mole). Both potential coupling terms, V33 and V′33, have been determined for the -d0, -d3 and -d7 isotopic species. The complete equilibrium geometry has been determined from ab initio Hartree–Fock gradient calculations employing both the 3-21G and 6-31G* basis sets. These results are compared to the corresponding quantities for some similar molecules.
Competing mechanisms and products’ properties for the Be+HF reaction
Authors: Alvariño José Maria , Hernández Maria Luz , Margarido Joaquin , Laganà Antonio
Published: July 2002

Reactive properties of the Be+HF reaction have been investigated theoretically using a new fit of the ab initio potential energy values. The dependency of the reactive dynamics upon both the partition of the initial energy among the various diatomic degrees of freedom and the features of the potential energy surface is analyzed by discussing the structure of detailed products’ properties and the evolution of the reactive cross section with collision energy. Dynamical properties of competing reaction paths are also investigated by carrying out a detailed examination of selected trajectory plots.
Cluster diffusivity: Structure, correlation, and scaling
Authors: Kang H. C. , Thiel P. A. , Evans J. W.
Published: July 2002

We have investigated the diffusion of clusters on a triangular lattice using Monte Carlo simulations. A cluster is defined as a two-dimensional collection of particles which are connected to each other, either directly or indirectly through other particles in the cluster, by nearest-neighbor bonds. Each particle is allowed to hop, with probability αδb/2/(α−δb/2+αδb/2), to a vacant nearest-neighbor site with the constraint that the hop does not break the cluster. The change in the number of bonds is given by δb. The equilibrium clusters are correlated animals with structure controlled by the parameter α. We show that the diffusion coefficient of a cluster can be decomposed into two factors. One is a measure of the weighted length of the ‘‘active’’ perimeter and the other is a measure of the correlation between pairs of steps taken by the cluster during its walk. The perimeter measure is asymptotically proportional to cluster size N, as anticipated for ramified animals, but it crosses over to N1/2 dependence for smaller compact clusters with α>1. Our focus is on the accurate determination of the size and structure dependence of the correlation factor, which is more sensitive to statistical fluctuations. As a result, we describe the scaling of the cluster diffusion coefficient with cluster size.