bluuebunny/crossref_metadata_2025 huggingface dataset:
Moment theory of electron thermalization in gases
Authors: Knierim K. D. , Waldman M. , Mason E. A.
Published: February 2003

A time-dependent moment method for solving the Boltzmann equation, not restricted to elastic collisions, is applied to the description of the thermalization of a beam of electrons in a gas. An exact solution is also obtained for diffusion and velocity relaxation by elastic collisions at very short times, before appreciable energy is lost. These results are compared with a recent approximate theory of Mozumder for electron thermalization, using two model systems: the Maxwell model (constant collisions frequency), and the rigid-sphere interaction (constant collision cross section). All results are exact for the Maxwell model, but for rigid-sphere interactions the errors in the velocity and energy relaxation times from Mozumder’s method are approximately 25%. Many real systems are therefore probably described satisfactorily by the approximate theory, unless perhaps the cross sections have a peculiar energy dependence or inelastic collisions are important. If more accurate results are needed, the present moment method gives a systematic procedure for the calculation of higher-order approximations. Although the specific examples treated here consider only elastic collisions, the moment method applies to the case of inelastic collisions as well.
A new Monte Carlo method for direct estimation of cluster partition functions. Application to micellar aggregates
Authors: Pratt Lawrence R.
Published: February 2003

A new Monte Carlo method for direct and unbiased estimation of cluster partition functions is established. The method is a generalization of the technique of inversely restricted sampling used in the study of linear polymer statistics. The essential generalization is a correct weighting of the various branching events possible in the cluster growth algorithm. The method applies directly both to ordinary continuous space clusters as well as to simpler lattice clusters. As an example, the method is applied to the calculation of cluster partition functions for the hard core chain molecule aggregates associated with a simple model micellar aggregate investigated previously.
Statistical mechanics of helical wormlike chains. XIV. Radius and shape of branched chains
Authors: Fujii Motoharu , Nagasaka Kyosuke , Shimada Jiro , Yamakawa Hiromi
Published: February 2003

Analytical expressions for the mean-square radius of gyration chains, such that the initial orientations (Euler angles) of branches at branch points are fixed with respect to other branches (of the star) or to the main chain (of the comb), corresponding to real branched chains. It is shown that for regular stars, the factor g, as defined as the ratio of 〈S2〉 of the branched chain to that of the linear chain of the same total contour length L, is independent of the relative initial orientations of branches for the reduced contour length Lb of each branch greater than about 100. In particular, for regular stars of rather strong helical nature, the behavior of g is shown to be similar to that of 〈S2〉/L for the corresponding linear chains; it first increases to its maximum with increasing L and then decreases monotonically to its coil limiting value. This behavior does not correlate with the value of the characteristic ratio C∞ of the corresponding real linear chain. A comparison of the present model is made with the rotational isomeric state model with respect to g for regular stars, taking as examples polymethylene, polyoxymethylene, and poly-DL-alanine. The agreement (and the disagreement) between the two models is discussed rather in detail. The moments of inertia tensor (its mean principal values) are also evaluated for regular stars approximately by replacing its three invariants by their averages as in the case of linear chains. It is then shown that the molecular shape depends more strongly on the relative initial orientations of branches at small L than does g, and is independent of them for Lb≳10, and that at a given L in the latter range as well as in the coil limit, the spherically symmetric shape is attained gradually with increasing branching.
High resolution fluorescence excitation and quantum beat spectroscopy of the first electronically allowed transition of perdeuteromethylglyoxal
Authors: Chaiken J. , McDonald J. D.
Published: February 2003

Laser induced fluorescence excitation spectra of supersonic nozzle beam cooled methylglyoxal-d4 reveal that a deformation occurs during the radiative electronic transition that destroys the planarity of the carbonyls. Consistent with the expectation that deuteration results in an increased number of low frequency vibrations, the number of observed vibronic lines increased and the energy of the breakoff of sharp vibronic structure above the O–O band decreased compared to the protonated molecule. As for the protonated molecule, the observed vibronic structure can be almost completely assigned on the basis of two fundamentals and a repeating pattern of lines that involves the methyl internal rotation and the cis–trans internal rotation of the carbonyls. Systematic anharmonicities involving the pattern and only one of the two fundamentals suggests which molecular motions correspond to the observed fundamentals. Quantum beat and Zeeman effect experiments unambiguously show the triplet states that are responsible for the radiationless dynamics that are known to occur in these molecules. All quantum beat data were analyzable using our previously published perturbation theory method and yield triplet densities which are intermediate compared to methylglyoxal-h4 and biacetyl-h6 and in good agreement with an estimate obtained by direct state coupling. The density of effectively coupled triplet states increases with the energy of the initially prepared singlet state and the intramolecular coupling is 2–15 MHz independent of the amount of vibrational-rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry and conservation of total energy, total angular momentum, and nuclear spin. Zeeman experiments indicate extensive coupling of all molecular angular momenta which initially decouple at only ∼10 G. In this low field regime complicated splittings are observed which correspond roughly to g values of about 67% that of a free electron. At larger fields of about 50–60 G, the microsecond fluorescence is nearly completely quenched, and although we cannot completely explain this effect at present, we suspect both intra- and intermolecular processes are possible.
Influence of coordinating water on the chlorine K absorption spectra of hydrated metal dichlorides: MgCl2⋅6H2O and SrCl2⋅6H2O
Authors: Sugiura Chikara
Published: February 2003

The chlorine K absorption spectra from MgCl2⋅6H2O and SrCl2⋅6H2O have been measured with a 50 cm bent crystal spectrograph. Results are evaluated by comparing the spectra with the chlorine K absorption spectra of MgCl2 and SrCl2 reported previously. A significant change has been found between the spectra of the hydrated and anhydrous dichlorides: The absorption fine structures of the former are remarkably smoothed out. It is illustrated that the change is due to the water molecules. A tentative explanation is also made for the first absorption peak of the spectra of MgCl2 and SrCl2.
Spectroscopy and nonradiative relaxtion of propynal
Authors: Kelley D. F. , English J. H. , Bondybey V. E. , Rentzepis P. M.
Published: February 2003

The spectroscopy and relaxation dynamics of propynal in condensed media were studied by wavelength and time resolved laser-induced fluorescence techniques. Ã 1A′ fluorescence quantum yields are low, with intersystem crossing into the ã 3A′ state being the dominant relaxation pathway. The ã 3A′ state phosphorescence seems to occur in solid neon with near unity quantum yield. The very detailed spectroscopic information obtained from the emission and excitation spectra suggests a remarkably weak medium perturbation in solid neon.
Director dynamics and NMR applications of nematic liquid crystals spinning at various angles from the magnetic field
Authors: Courtieu Jacques , Alderman D. W. , Grant David M. , Bayles J. P.
Published: February 2003

The dynamics of the director of a nematic liquid crystal spinning in a magnetic field are studied. A simple average potential theory which gives a qualitative understanding of the director behavior is presented along with an exact quantitative treatment of situations in which the director is stationary in the laboratory frame. NMR spectra of an A2 spin system, 19F in CF2CBr2, are used to verify the theory. For nematic liquid crystals in which the magnetic susceptibility anisotropy Δχ = (χ∥−χ⊥) is positive, the director aligns along the spinning axis when the sample is spun at a rate much greater than that characteristic of director reorientation and the angle between the spinning axis and the magnetic field is less than the magic angle ϑm = 54.74°. Past the magic angle the director distributes in the plane perpendicular to the spinning axis. For liquid crystals in which Δχ is negative, alignment parallel to the spinning axis occurs for angles greater than ϑm and distribution in the plane occurs for angles less than ϑm. By spinning very close to the magic angle the averaged dipolar coupling between the nuclei of a molecule dissolved in the nematic phase can be reduced by up to two orders of magnitude. This reduction is used to convert the ABC pattern of 19F in CF2CFBr to a first order spectrum. Such simplification removes one of the most severe limitations of the chemical application of NMR in liquid crystal solvents. Also obtained is a 13C spectrum of a liquid crystal with the proton dipolar couplings almost totally eliminated by spinning alone.
The quenching of Na(3 2P) by H2: Interactions and dynamics
Authors: Truhlar Donald G. , Duff James W. , Blais Normand C. , Tully John C. , Garrett Bruce C.
Published: February 2003

An eight-configuration diatomics-in-molecules formalism with symmetrically orthogonalized diabatic input is used to calculate the three lowest potential energy surfaces and nonadiabatic couplings for Na+H2 collisions. The singlet diatomic input for NaH and the singlet and triplet diatomic input for H−2 are chosen to reproduce the accurate adiabatic curves for those cases, and the diatomic input for H2 is based on theoretical interpretations of the resonance states. Trajectory calculations are carried out using the calculated 2 2A′ potential surface and nonadiabatic couplings, and the choice of H−2 input is found to sensitively affect the H2 vibrational excitation in the resonance-line quenching process. One H−2 curve, based on the work of Wadehra and Bardsley, yields an amount of vibrational excitation similar to that observed experimentally. The resulting set of potential energy surfaces provides a reasonable microscopic interpretation of the chemical dynamics of the quenching process Na(3p2P)+H2(v = 0, low j) →Na(3s2S)+H2(v′,j′).
A study of long-range anisotropic potentials for CsF, CsCl, and KCl interacting with the rare gases based on measured state-to-state rotationally inelastic cross sections. II. Comparison with calculations for model potentials
Authors: Meyer Gerhard , Toennies J. Peter
Published: February 2003

In a previous paper measurements of state-to-state rotational excitation cross sections for small angle scattering for the systems CsCl and KCl colliding with rare gases were reported. In the present paper the latest available theoretical models for calculating the long-range potentials are used in the framework of the Rittner model to obtain an accurate anisotropic potential including terms up to R−9 and anisotropies up to P5 (cos γ) (γ is the angle between the internuclear distance R and the bond axis) for CsF, CsCl, and KCl interacting with all the rare gases. With these potentials, inelastic cross sections are calculated in the IOS approximation and averaged over the apparatus geometry for direct comparison with the experiemntal data. The comparison shows good agreement within about 30% for CsF and CsCl, where classical induction terms dominate with significantly smaller theoretical cross sections for KCl. Since for KCl the anisotropic dispersion terms are expected to be dominant, the results suggest either that the small amount of covalent bonding has a severe effect on the potential or that the available theoretical methods do not describe the anisotropic long-range potential.
Model studies of the kinetics of collisional population transfer between dark and radiating excited electronic states: CaO(A′ 1Π)+N2O⇄CaO(A 1Σ+)+N2O
Authors: Alexander Millard H. , Osmolovsky Michael G.
Published: February 2003

In a previous article [J. Chem. Phys. 76, 429 (1982)] we presented a model for collisional energy transfer between dark (A′ 1Π, a 3Π) and radiating (A 1Σ+) excited electronic states of the alkaline earth oxides. The inelastic transitions result from coupling between the electric dipole of the collision partner and a transition dipole in the alkaline earth oxide, which arises from the non-Born–Oppenheimer coupling between the rovibronic manifolds of two different electronic states. Here we use the rate constants reported in the previous article to investigate population flow from the nominally v = 6 manifold of the A′ 1Π state of CaO into the nominally v = 0 manifold of the A 1Σ+ state, induced by collisions with N2O. The master equation is solved in the steady state limit. The resulting populations are then used to simulate the pressure dependence of the (0–0) band of the CaO A 1Σ+→X 1Σ+ spectrum, and investigate the variation of the predicted spectral features with respect to changing the conditions which characterize both the rate of formation of the excited states as well as the rate of translational loss out of the zone of observation. At low to moderate target gas pressure the major effect of collisions is A′→A population transfer in the region of the largest coupling between the two rotational manifolds. The concomitant intensity increase and the spectral variation in the A state emission are qualitatively similar to features seen in experimental spectra of Irvin and Dagdigian, which we present here. The pressure dependent changes in the emission spectra are extremely sensitive to the assumed rate of translational loss, which must be taken into account in any comparison between experiments carried out under flame and molecular beam conditions. Finally, we demonstrate that although it is possible to fit with a simple two-level kinetic model the observed pressure dependence of the total A state emission, summed over rotational lines, the resulting kinetic parameters which characterize this fit may bear little relation to those which characterize the master equation for the coupling between the underlying rotational manifolds.
Equations of state for rigid disks and spheres
Authors: Devore Jerald A. , Schneider Erika
Published: February 2003

The equations of state for rigid disks and spheres are discussed. Alder and Hoovers’17 approximants are found to yild compressibility factors close to those of Woodcock1 and Baram and Luban2. (AIP)
Comment on ‘‘Dispersion induced dipole moments’’
Authors: Crowell A. D.
Published: February 2003

An earlier result arguing that multipole dispersion interactions between a pair of spherically symmetric atoms would lead to permanent dipoles in each atom directed along the line of centers if the multipole expansion were carried through the dipole quadrupole terms is reinvestigated following a suggestion by Mahanty. The original calculation employed a variational approximation in which the trial function was linear in the terms of a multipole interaction potential and which included only one-dimensional dipole quadrupole interactions along the line of centers. When the three-dimensional dipole quadrupole terms are employed, using an otherwise similar linear trial function, it is found that spherical symmetry causes the dipole moment of each species to vanish in contrast to the previous result. (AIP)
Viscothermal theory of sound wave propagation in chemically reacting mixtures of nondiffusive fluids
Authors: Margulies Timothy S. , Schwarz W. H.
Published: February 2003

Measurements of the absorption and dispersion of forced acoustic waves propagating in fluids in equilibrium have been used to determine thermostatic properties and the kinetics of reactions. Here is presented a theoretical treatment of this problem using the linearized balance equations of mass, linear momentum, and energy. The reacting fluid mixture is considered to be linearly viscous (Newtonian), Fourier heat-conducting, and nondiffusive. General results are obtained in the form of a biquadratic characteristic equation for the complex propagation variable χ = −(α+iω/c), where α is the attenuation coefficient, c is the phase speed of the progressive wave, and ω = 2πf is the angular frequency. This represents a modified form of the Kirchhoff–Langevin equation which includes the effects of viscosity and thermal conduction and is applicable to a system of coupled reactions. The reactions were treated using ’’normal modes’’ for a set of extent-of-reaction variables and the equations were necessarily symmetrized so that results for the reacting, but inviscid and nonconducting case, could be expressed in simple forms and compared to previous work. For the single reaction case, the comparison is exact; however, for the multiple reaction case, the forms derived here differ somewhat from those used by other investigators and should be noted.
An electron energy loss study of the triplet states of SO2
Authors: Avouris Ph. , Demuth J. E. , Schmeisser D. , Colson S. D.
Published: February 2003

Electron energy loss studies are reported for solid SO2 in the 0–14 eV range. Electron beam energy is varied so as to observe triplet excitations. (AIP)
Inequivalent C–H oscillators of gaseous alkanes and alkenes in laser photoacoustic overtone spectroscopy
Authors: Wong James S. , Moore C. Bradley
Published: February 2003

The overtone spectra of the C–H stretching vibrations of several gaseous alkanes and alkenes were observed using laser photoacoustic spectroscopy. Resolvable peaks are seen for each chemically or sterically equivalent C–H bond and are assigned using the local mode model. The fifth overtone transition energies decrease linearly with increasing C–H bond lengths. Spectral shifts corresponding to 0.001 Å bond length differences are observed. Linear correlations of bond length and anharmonicity with fundamental C–H stretching frequency allow a Morse potential for CH bonds to be defined in terms of a single parameter. The integrated cross section per C–H oscillator for the fifth overtone spectra varied mostly within a factor of 2 about the average value of (1.08±0.28)×10−23 cm2 cm−1.
Dynamic light scattering studies of collagen
Authors: Hwang J. S. , Cummins H. Z.
Published: February 2003

Monodisperse rat skin collagen solutions obtained through purification by density gradient separation have been studied by dynamic light scattering. Autocorrelation functions were measured at 31.8° and 90° scattering angles. Calculations of the translational diffusion coefficient DT and the rotational diffusion coefficient DR from the measured correlation functions of scattered light have been carried out in this study. A new approach to simulatanously determining the translational and rotational diffusion coefficients was used to estimate both the length of the macromolecules and the viscosity of the solutions. Our result for L is 2715±100 Å.
Photon correlation spectroscopy of polystyrene near the glass transition under high pressure
Authors: Patterson G. D. , Stevens J. R. , Carroll P. J.
Published: February 2003

Photon correlation spectroscopy is employed to study the slowly relaxing density and anisotropy fluctuations in bulk atactic polystyrene as a function of pressure at 146 °C from 1 bar to 1.33 kbar. The light scattering relaxation function observed as a function of pressure is identical in shape to that observed previously as a function of temperature. The average relaxation time changes by nearly 4 decades per kbar as the polystyrene sample approaches the glass transition along the isothermal path. The present results represent the first step in the study of light scattering under high pressure near the glass transition in polymers.
Nonresonant excitation of plasmon surface polaritons on silver gratings in air and in electrolyte
Authors: Kunz R. E. , Gordon J. G. , Philpott M. R.
Published: February 2003

The emission of elastically scattered light from radiative plasmon surface polariton (PSP) modes of a silver grating has been observed when the grating is struck by a laser beam at an angle that does not excite PSP modes directly. This nonresonant angular excitation occurs through the involvement of surface roughness, which provides additional momentum matching sources. Two types of experiments are described, the first for gratings in air and the second for a grating electrode immersed under potentiostatic control in an aqueous electrolyte. Roughness scattering increased greatly upon immersion and a new peak not associated with PSP modes was observed in the p→p and s→s scattering channels.
Coexistence curve near the binary mixture critical solution point: Noncritical fluctuations and Fisher renormalization
Authors: Vause C. , Sak J.
Published: February 2003

It is shown, within the framework of the fluctuation theory based on a microscopic Landau–Ginzburg–Wilson Hamiltonian for a two-component fluid, that under the constraint of fixed total liquid density the temperature versus composition phase diagram possesses a diameter that satisfies the rectilinear law in the asymptotic region near the critical solution point. The width of the coexistence region as well as the leading corrections to scaling have Fisher-renormalized exponents. These results also apply to mixtures with both upper and lower critical solution points.
Theory of spin-lattice relaxation in lipid bilayers and biological membranes. 2H and 14N quadrupolar relaxation
Authors: Brown Michael F.
Published: February 2003

Based on a previous, more approximate treatment [M. F. Brown, J. Magn. Reson. 35, 203 (1979)], expressions are derived for the quadrupolar spin-lattice (T1) relaxation rates of 2H and 14N in lipid bilayers. Results are presented for the most general, anisotropic rotational diffusion model describing the segmental or molecular reorientation in lipid bilayers, and the analysis is extended to include relatively slow fluctuations of the local director with respect to the macroscopic bilayer normal. Numerically computed values of T1 for the diffusion model suggest that, even for extremes of ordering and motional anisotropy, such a model cannot by itself quantitatively account for the observed 2H T1 values of multilamellar dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), in the liquid crystalline state, as a function of temperature and frequency. The contribution from relatively low frequency motions is modeled in terms either (i) a simple noncollective model in which the slow motions are described in terms of a single effective correlation time, or (ii) a collective model in which the relatively slow reorientation is described by a distribution of correlation times, corresponding to collective fluctuations of the instantaneous director. The experimentally observed dependence of the 2H T1 relaxation rates on the acyl chain segmental order parameter SCD and the resonance frequency ω0 are most consistent with a collective model for slow molecular reorientations in lipid bilayers. The 2H T1 data for the saturated DPPC bilayer, in the liquid crystalline state, can be quantitatively described by a relaxation law of the form T−11 = Aτf+BS2CD ω−1/20 as observed for simpler nematic and smectic liquid crystals. The first (A) term is suggested to correspond to trans–gauche isomerizations of the lipid acyl chains, while the (B) term describes collective bilayer modes which predominantly influence the frequency dependence of the relaxation. In contrast to earlier conclusions [M. F. Brown et al., J. Chem. Phys. 70, 5045 (1979)], the dominant contribution to the 2H T1 relaxation rates of the saturated DPPC bilayer may arise from collective order fluctuations rather than fast local motions. The value of τf∼10−11 s obtained by extrapolating T−11 to infinite frequency or zero ordering is consistent with the correlation times calculated from 2H or 13C T1 data for n-alkanes of equivalent chain lengths, suggesting that the microviscosity of the bilayer hydrocarbon region is not appreciably different from that of paraffinic liquids.
ESR studies of the anomalous phase transition in crystalline galvinoxyl radical
Authors: Mukai Kazuo , Ueda Kyozo , Ishizu Kazuhiko , Deguchi Yasuo
Published: February 2003

The ESR spectra of powder samples of pure galvinoxyl and 1% galvinoxyl diluted in phenol matrix is measured in the temperature range 77–293 K.(AIP)
Scaling relations and analytic solution for the Thomas–Fermi metal–dielectric surface
Authors: Dahl D. , Halley J. W.
Published: February 2003

We give an explicit solution for a jellium metal–dielectric interface in the Thomas–Fermi approximation. The solution obeys a scaling relation which permits solutions for any ε to be obtained from the solution with ε = 1. The resutls are useful as a guide to the study of several metal-insulator and electrochemical problems. (AIP)
Nucleation and colloidal growth in concentration gradients (Liesegang rings)
Authors: Venzl Gerd , Ross John
Published: February 2003

We analyze nucleation and colloidal growth in the presence of concentration gradients of electrolytic reactants by means of a postulated set of reaction–diffusion equations, which contain a sink (or reaction) term to account for nucleation and subsequent growth. These equations are more realistic than previous ones used in formulations of the Ostwald hypothesis of Liesegang ring formation, in which that formation is supposed to be a prenucleation process, but in limits contain the previous formulations. We calculate length and time scales of our equations and estimate values of the physical quantities involved. Numerical solutions of our equations predict nucleation to occur as a spatially continuous process, in agreement with experiments. In previous formulations spatial discontinuties in nucleation were obtained by unwarranted approximations or ad hoc assumptions (boundary conditions).
Comments on pattern formation in precipitation processes
Authors: Venzl Gerd , Ross John
Published: February 2003

We investigate the instability theory of spatial pattern formation in precipitating systems which focuses on post-nucleation phenomena in contrast to the supersaturation (prenucleation) theory of Ostwald. A scale analysis shows that very slow particle growth kinetics are necessary to match the experimental observations; this result rules out diffusion limited growth of colloidal particles. We further show that colloidal particle diffusion is quantitatively unimportant for the process of structure formation and does not provide a macroscopic length scale for diffusion limited growth kinetics.
Energy unresolved differential cross section for electron scattering by H2
Authors: Kol/os Wl/odzimierz , Monkhorst Hendrik J. , Szalewicz Krzysztof
Published: February 2003

Elastic, inelastic, and total energy unresolved differential cross sections for high energy electron scattering by the hydrogen molecule have been computed in the first Born approximation. An accurate wave function in the form of a 36-term expansion in correlated Gaussian functions was used for the electronic ground state of H2. The computed quantities have been averaged over the zero point vibrations and over thermal populations of the rotational states. The results are compared with the data derived from experiments.
Heat conductivity in a moderately dense hard sphere gas
Authors: Eder Otto J. , Lackner Thomas
Published: February 2003

The three contributions to the heat conductivity in a moderately dense hard sphere gas are calculated using the Green–Kubo expressions for the kinetic, the cross, and the potential part of the time-correlation function. The starting point is the calculation of the relevant correlation functions within the ’’successive uncorrelated binary collision’’ model. It is shown that the cross-correlation function is proportional to the kinetic correlation function and that the potential correlation function contains two contributions, one proportional to the kinetic correlation function and the other proportional to a Delta function. This latter part is due to the instantaneous nature of the momentum and energy transfer in a hard sphere collision. The time integrals over these correlation functions provide the three contributions to the heat conductivity coefficient.
Statistical mechanics of an adsorbed quantum gas
Authors: Guo Zhi-Chun , Bruch L. W.
Published: February 2003

The one- and two-body clusters for a low-density adsorbed quantum gas are calculated, including effects of the periodic structure of the substrate by a perturbation expansion in powers of the Fourier amplitudes of the adatom-substrate potential. The formalism is applied to helium adsorbed on basal plane graphite and quantitative comparisons are made with experimental data. Effects of substrate-mediated interactions and of adatom vibrations perpendicular to the adsorbing surface are included. Other possible applications of the formalism are examined.
Mass spectrometric equilibrium study of the molecule Hg2
Authors: Hilpert K.
Published: February 2003

The vaporization of Hg/l was studied by high temperature mass spectrometry with a Knudsen cell in the temperature range 300–386 K. The partial pressure of the molecule Hg2 was determined as: log(p/Pa) = −5859 K/T+11.19. The following enthalpies of vaporization and dissociation were obtained for Hg2 (g):ΔvapH°298 = 114±2 kJ mol−1 and D°298 = 8±2 kJ mol−1.
Diffusion coefficients of interacting Brownian particles
Authors: Ohtsuki Toshiya , Okano Koji
Published: February 2003

Starting from a N-particle diffusion equation, the effective particle (self-) and concentration diffusion coefficients of interacting Brownian particles are studied theoretically. These two diffusion coefficients defined by distinct ways are different for interacting systems. By taking into account both potential and hydrodynamic interactions, the expression for the frequency-dependent particle diffusion coefficient exact to linear order in number density is derived. Detailed calculations of both diffusion coefficients are performed for model systems of hard spheres with a repulsive or an attractive long-range potential. The effects of potential and hydrodynamic interactions on these two diffusion coefficients are clarified. The velocity autocorrelation function is also calculated and non-Markovian behaviors are discussed.
Vapor pressure and thermodynamics of Bk-249 metal
Authors: Ward John W. , Kleinschmidt Phillip D. , Haire Richard G.
Published: February 2003

The vapor pressure of 249Bk metal has been measured over the range 1100–1500 K, using combined target and mass spectrometric Knudsen effusion techniques. Vapor pressure data are combined with an appropriate estimate for the solid heat capacity and the estimated crystal entropy S°298, to produce enthalpy, entropy, and free energy functions from 298 to 2000 K. The vapor pressures are described by the equations: Log10 P(atm) = −(15 718 ±253)/T+578±0.21 [solid, 1107–1319 K], Log10 P(atm) = −(14 902 ±244)/T+5.14±0.17 [liquid, 1345–1528 K] from which we calculate Δ H°298 (2nd law) = 73 960±1150 cal/mol [solid], Δ H°298 (2nd law) = 73 980±1120 cal/mol [liquid], and Δ H°298 (3rd law) = 74 220±180 cal/mol [solid and liquid].
OH− and OD− threshold photodetachment
Authors: Schulz P. A. , Mead Roy D. , Jones P. L. , Lineberger W. C.
Published: February 2003

Threshold photodetachment cross sections for OH− and OD− at photon energies close to their electron binding energies have been obtained using a crossed tunable laser-negative ion beam apparatus at a resolution of 25 μeV. The data reveal many clear features associated with photodetachment of single rotational levels of the negative ion, producing single rotational levels of the neutral and a near zero energy electron. From the frequencies of the observed thresholds, the spectroscopic constants of OH− and OD− are found. The rotational constants of the vibrational ground state are B0 = 18.7409(45) cm−1 and D0 = 2.052(45)×10−3 cm−1 for OH− and B0 = 9.9852(48) cm−1 and D0 = 0.553(33)×10−3 cm−1 for OD−. The electron affinities of OH and OD are measured to be 14 741.03(17) cm−1 and 14 723.92(30) cm−1, respectively. The observed bound–free transitions satisfy selection rules only slightly different from the selection rules observed in bound–bound spectroscopy. The relative intensities of the transitions show the transition from Hund’s case (a) to case (b) as the rotation in OH increases. The intensities are in agreement with a theory based on an intermediate complex which dissociates into jj-coupled constituents. The shape of the cross section near threshold, a probe of the long range electron–OH interaction, is found to depend on the final rotational state of the OH neutral. This dependence is studied in detail. Analogous studies on photodetachment of OD− confirm our deductions.
Observation of a Rydberg series of CO in the two-electron excited region
Authors: Wu C. Y. R.
Published: February 2003

Using synchrotron radiation as a continuum background, the absorption spectrum of CO in the 380–470 Å was taken by using a double ion chamber. A Rydberg series in the 450 Å (27.5 eV) region has been observed to converge to the F 2Π, a two-electron excited state of CO+. The series limit determined at 28.11 eV is in excellent agreement with the vertical ionization potential of the F 2Π state measured by the high resolution photoelectron spectroscopy. A few dissociation processes leading to the productions of atomic ions and excited atomic fragments are also discussed.
Raman study of vibrational dephasing in haloalkanes
Authors: Maleknia S. , Ho S. , Schwartz M.
Published: February 2003

The Raman spectra of the carbon–halogen stretching mode was measured as a function of temperature in the five liquids: ethyl bromide, isopropyl chloride and bromide, and t-butyl chloride and bromide. The derived isotropic (vibrational relaxation) correlation times showed a very clear trend. The magnitude of τiso was quite similar in both chloroalkanes (in the range from 1.2 to 1.5 ps), whereas, the values in all three bromoalkanes were virtually superposable and markedly longer (1.8–2.1 ps) than in the chloro compounds. These results, together with those from earlier studies on methyl iodide, provide evidence that vibrational dephasing efficiencies (τ−1iso) of the C–X mode in haloalkanes vary in the order Cl≳Br≳I. It was further observed that, in contrast to the rather unusual temperature dependence of vibrational relaxation in methyl iodide, τiso either remained approximately constant or decreased with temperature in the compounds studied here. Modulation times calculated from the Kubo expression were found to be significantly longer (approximately 1 ps) in the isopropyl halides than in the t-butyl compounds, where τm’s were of the same magnitude as Enskog collision times. Vibrational dephasing times calculated from the Fischer–Laubereau isolated binary collision model were of the same order of magnitude but longer than the experimental correlation times.
Dynamic frequency shifts in pseudorotating Na3
Authors: Lindsay D. M. , Thompson G. A.
Published: February 2003

ESR spectra assigned to pseudorotating Na3 molecules have been obtained by clustering atomic sodium in an argon matrix. Both a pseudorotating spectrum and the previously reported ’’stationary’’ spectrum are observed. As in the analogous potassium case, the pseudorotating spectrum is attributed to a rapid exchange between the three equivalent potential wells of a Jahn–Teller distorted trimer. The ESR spectra show an extreme alternating linewidth effect. Even at T≳30 K, only those four hyperfine (hf) components which do not broaden are observed. The relaxation matrix formalism is applied to pseudorotating Na3 and derived expressions give a good fit to the measured line positions. These show 5–10G ’’dynamic frequency shifts,’’ much larger in magnitude than normally encountered in exchanging radicals. The measured g value and hf constants for pseudorotating Na3 are in close correspondence with those predicted from the stationary trimer parameters. Together the two spectra determine the sign of the hf coupling in Na3.
Temperature effects on energy pooling in laser CO2 multiphoton dissociation
Authors: Oref I.
Published: February 2003

Temperature effects on collision induced energy pooling is discussed. The probability that a molecule will have energy above a threshold value E0 is calculated for 300 and 1000 K, different initial distributions, and E0 = 30 and E0 = 60 kcal/mol. Some of the distributions are truncated at E0. It is found that energy pooling does take place and that the fraction of molecules transported above E0 depends on the initial energy distribution, temperature, and number of photons absorbed as well as the number of degree of freedom.
Mean activity coefficient for polyelectrolyte in mixed aqueous solutions of polyelectrolyte and simple electrolyte
Authors: Iwasa K.
Published: February 2003

The mean activity coefficient for polyelectrolyte in a mixed aqueous solution of polyelectrolyte and simple electrolyte is derived based on the condensation formalism previously reported [Biophys. Chem. 9, 397 (1979)]. Consistency is examined between the limiting laws, namely, for osmotic coefficient, for simple electrolyte activity coefficients, and for polyelectrolyte activity coefficient.
The diffusion thermoeffect in ternary nonelectrolyte liquid mixtures
Authors: Platt Gerald , Vongvanich Tevin , Rowley Richard L.
Published: February 2003

Expressions for that portion of the heat flux which is due to composition gradients in multicomponent nonelectrolyte liquid mixtures have been formulated in terms of heats of transport. The resultant thermal energy equation in combination with component mass balance equations constitute coupled partial differential equations descriptive of the diffusion thermoeffect in liquid mixtures. These equations were solved with a double perturbation technique subject to initial and boundary conditions consistent with experimental design conditions considered appropriate for successful measurement of multicomponent heats of transport. Based on the adiabatic type cell, the expected temperature response, dependent on cell variables, experimental conditions, and system thermophysical properties, has been modeled to provide design information for successful and optimum heats of transport measurements. In ternary systems, the two independent heats of transport can be experimentally determined from temperature response measurements only if two experiments are performed at the same mean composition but with different initial composition gradients. Experimental heats of transport in ternary systems are obtainable based upon the design principles of this work.
X-ray diffraction study and models of liquid ethane at 105 and 181 K
Authors: Sandler S. I. , Lombardo M. G. , Wong D. S.-H. , Habenschuss A. , Narten A. H.
Published: February 2003

The structure functions for liquid ethane at (1) T = 105 K and a molecular density ρ = 0.012 70 Å−3, and (2) T = 181 K and ρ = 0.011 10 Å−3 have been derived from new x-ray diffraction data. Treating the –CH3 group as a single scattering site, carbon–carbon (methyl–methyl) intermolecular pair distribution functions have been obtained from the structure function data by Fourier inversion. Using Monte Carlo simulation, no two-site exponential-6 interaction model, with sites at the observed C–C bond length, could be found which reproduce the carbon–carbon distribution function. By considering a different class of interactions models, a model was found which reproduced the distribution function derived from experiment and accurately predicts the second virial coefficient of ethane.
A molecular beam study of the NO interaction with Pt(111)
Authors: Serri J. A. , Cardillo M. J. , Becker G. E.
Published: February 2003

Using angle and time resolved molecular beam techniques, an investigation of the low coverage adsorption-desorption kinetics of NO on Pt(111) is made. The experiments are carried out over a crystal temperature range of 300 K<Ts<900 K. For Ts≳500 K, the sticking probability s≳0.9. NO adsorbs molecularly with little dissociation (<5%). The desorption rate is found to be strongly dependent on the incident beam flux and trace amounts of chemisorbed oxygen (<1%) on the surface. These findings suggest that steps play the dominant role in low coverage kinetics for a nominally flat crystal. We present a model which incorporates the effect of steps, explains the nonlinearity of the desorption kinetics, and reconciles the disagreement between these results and previous molecular beam studies.
Excluded-volume theory of polymer–protein interactions based on polymer chain statistics
Authors: Hermans Jan
Published: February 2003

A theory is presented of mutual steric exclusion of protein molecules and randomly coiled polymer molecules, in which the former are presented as impenetrable rigid particles and the latter as flexible segmented chains obeying Gaussian statistics. The limiting case of small polymer chains and large protein molecules is treated first, by approximating the protein surface as planar; this gives an excluded volume equal to the sum of protein volume V and a term in Vh0/R, i.e., protein volume times the ratio of rms end-to-end distance of the polymer and radius of the protein. Several cases of steric exclusion are treated next, using a quite general approach, by which excluded volume can be expressed as V/〈m〉, the ratio of protein volume and the average number of elements that are simultaneously excluded by, or that simultaneously exclude, one protein molecule. Expressions for 〈m〉 are successively obtained for thin rods arbitrarily finely divided into segments, for uniformly randomly distributed segments, for segments linked into very long polymers, and for segments linked into finite polymers. In all these cases the protein is represented as a sphere; in addition an expression for the excluded volume of very long polymers and cylindrical protein molecules is obtained. Excluded volume of spherical proteins and very long polymers is found to be simply proportional to the radius of the protein and the square of the rms end-to-end distance of the polymer, while the larger excluded volume of proteins and finite chains is conveniently expressed as the product of the result for very long chains and a series expansion in powers of R/h0. Comparison with results of Monte Carlo simulation shows all predictions of theory to be excellent, with two exceptions. As a result of the use of Gaussian statistics for polymers of any chain length, the theory does not adequately describe the case of stiff polymer chains and small protein molecules. As a result of a second, at present unavoidable, approximation in the calculation of 〈m〉 for polymers of finite chain length, theory systematically predicts excluded volumes that are too low by a factor that varies from 1 to 2. As a result, this theory makes useful predictions for short polymers, as long as h0/R does not exceed 1 and for long polymers, as long as R/h0 does not exceed 0.5. Comparison with experimental data is found to be good; the prediction of proportionality of excluded volumes of globular proteins and high molecular weight polymers with protein radius agrees with recent observations with poly(ethylene oxide) and predicted excluded volumes of serum albumin and this polymer at three different polymer molecular weights agree within 10% with recently observed values.
Frequency distribution function for benzene
Authors: Batheja Pinky , Garg Suresh , Kothari L. S.
Published: February 2003

The frequency distribution functions of benzene are calculated using the measured value of the specific heat at constant volume (1,2). (AIP)
Lateral diffusion in fluid systems
Authors: MacCarthy John E. , Kozak John J.
Published: February 2003

It is shown that an extension of the free−volume model introduced by Cohen and Turnbull (see Refs. 6−8), can serve as a starting point in the development of a microscopic theory of lateral diffusion in membranes and monolayers. (AIP)
Comment on ‘‘Absorption and dispersion in the O2 microwave spectrum at atmospheric pressures’’
Authors: Rosenkranz P. W.
Published: February 2003

The spectral line coupling coefficients Wjk given by Smith1 are discussed further. It is pointed out that the value of Σj Wjk implicitly determines the high-frequency asymptote of the absorption spectrum. (AIP)
Electron diffraction studies of hot molecules. II. ‘‘Anharmonic shrinkage effects’’ in SF6, CF4, and SiF4
Authors: Goates Steven R. , Bartell Lawrence S.
Published: February 2003

Least squares refinements of diffraction intensities for SF6, CF4, and SiF4 yielded nonbonded internuclear distances that lagged increasingly behind values expected from the bond lengths, the higher the temperature. The observation that observed nonbonded shrinkages greatly exceed calculated shrinkages cannot be attributed to a deficiency in the theoretical expressions customarily invoked. It is explained, instead, by anharmonic effects in bending modes that give the illusion of anomalous shrinkage where it does not, in fact, exist. These effects, which skew the nonbonded distribution functions and displace the true mean distances from the peak maxima, contain heretofore unexploited information about anharmonic potential constants.
Evidence for the primary decomposition of propylene oxide to singlet methylene
Authors: Chou Jim-Son J. , Adams Thomas E. , Grant Edward R.
Published: February 2003

Using laser-induced fluorescence as a probe for 1CH2, we have determined that infrared multiphoton dissociation of propylene oxide yields singlet methylene as a primary product. Experiments with specifically labeled 1-d2 propylene oxide confirm, by showing only 1CD2, that the carbene is ejected exclusively from the terminal position of the heterocycle and that fragmentation is faster than any competing hydrogen migration process.
Interstitial integrals in the multiple-scattering model
Authors: Swanson John R. , Dill Dan
Published: February 2003

We present an efficient method for the evaluation of integrals involving multiple-scattering wave functions over the interstitial region. Transformation of the multicenter interstitial wave functions to a single center representation followed by a geometric projection reduces the integrals to products of analytic angular integrals and numerical radial integrals. The projection function, which has the value 1 in the interstitial region and 0 elsewhere, has a closed-form partial-wave expansion. The method is tested by comparing its results with exact normalization and dipole integrals; the differences are 2% at worst and typically less than 1%. By providing an efficient means of calculating Coulomb integrals, the method allows treatment of electron correlations using a multiple scattering basis set.