bluuebunny/crossref_metadata_2025 huggingface dataset:
Homogeneous block copolymer systems under shear flow
Authors: Onuki Akira
Published: July 2002

Effects of shear flow are investigated in melts of diblock copolymers in the homogeneous disordered phase near the microphase separation transition point. Simple approximate expressions are given for the anisotropic scattering intensity in nonlinear regimes under shear. A non-Newtonian effect and a normal stress effect are also examined, which are caused by deformations of the critical fluctuations in shear. An explicit expression is also found for the critical part of the complex shear modulus. This paper is a generalization of recent work by Fredrickson and Larson.
The interpretation of the neutron inelastic scattering and infrared absorption spectra of chromous acid using the double Morse potential model
Authors: Lawrence M. C. , Robertson G. N.
Published: July 2002

The neutron inelastic scattering and infrared absorption spectra of chromous acid are analyzed using a double Morse potential model. The proton tunneling frequency is assigned to a peak at 548 cm−1 and the ‖0〉→‖2〉 transition to the band at 2050 cm−1 in the neutron inelastic scattering spectrum. These assignments differ from those (viz. ‖0〉→‖1〉=226 cm−1, ‖1〉→‖2〉=1650 cm−1) made prior to the availability of the neutron inelastic scattering data. The proton tunneling frequency is thus assigned to a peak of considerably higher frequency. The model calculations also explain the infrared absorption spectrum of deuterated chromous acid and resolve previous discrepancies in the crystallographic data for chromous acid.
Implementation of variable time step stochastic dynamics for electronically inelastic gas–surface collisions
Authors: Garrett Bruce C. , Swaminathan P. K. , Murthy C. S. , Redmon Michael J.
Published: July 2002

A variable time step algorithm has been implemented for solving the stochastic equations of motion for gas–surface collisions. It has been tested for a simple model of electronically inelastic collisions with an insulator surface in which the phonon manifold acts as a heat bath and electronic states are localized. In addition to reproducing the accurate nuclear dynamics of the surface atoms, numerical calculations have shown the algorithm to yield accurate ensemble averages of physical observables such as electronic transition probabilities and total energy loss of the gas atom to the surface. This new algorithm offers a gain in efficiency of up to an order of magnitude compared to fixed time step integration.
On the possibility of vibrational inhibition in a bimolecular exchange reaction
Authors: Shukla C. P. , Sathyamurthy N. , Khuller I. P.
Published: July 2002

The possibility of vibrational (and translational) inhibition of a bimolecular exchange reaction H+Li2→HLi+Li arising out of kinematic considerations is pointed out.
Laser excitation spectra for matrix isolated IF: Observation of new low-lying electronic states
Authors: Nicolai Jean-Philippe , Heaven Michael C.
Published: July 2002

Laser excitation of matrix isolated IF has been used to characterize three previously unobserved electronic states. Excitation at wavelengths longer than 535 nm resulted in emission from the metastable A′ 3Π(2) state. Vibrational analysis of the A′–X system gave an approximate value of Te(A′)=13 250 cm−1. The radiative lifetime of IF (A′) was found to be about 11 ms. Excitation of IF at wavelengths shorter than 475 nm produced a long-lived (τ=5 ms) fluorescence in the 720–1500 nm region. This emission originated from a low-lying electronic state (T00≂19 040 cm−1) and terminated on the v″=9 to 22 levels of the ground state. Laser excitation spectra, recorded by scanning the laser wavelength and monitoring this emission system, showed a progression of bands from 474 to 415 nm. Franck–Condon arguments show that the absorbing state is not the emitting state. The excitation spectra provided a T00 value of 21 100 cm−1 and a vibrational constant of 500 cm−1 for the absorbing state.
Bead-spring rings with hydrodynamic interaction
Authors: Liu Tony W. , Öttinger Hans Christian
Published: July 2002

An approach to the kinetic theory of a dilute solution of bead-spring rings with hydrodynamic interaction is outlined. When the cyclic symmetry of the ring is exploited, the analysis of Hookean-spring rings is greatly simplified. Exact expressions for the time constants of a Hookean-spring ring with an arbitrary number of beads are obtained. The only effect of ring closure is to alter the spectrum of time constants. The form of the constitutive equation for a dilute solution of rings is the same as that for a dilute solution of Rouse chains. The results of Wiest et al. for Hookean-spring rings without hydrodynamic interaction, which were obtained by using a different approach, are recovered as a special case.
ESR and optical spectroscopy of matrix-isolated ethylnitrene
Authors: Ferrante Robert F. , Erickson Susan L. , Peek Brian M.
Published: July 2002

The ethylnitrene radical CH3CH2N has been produced by the interaction of gaseous ethylazide with metastable N2 or Ar generated in a microwave discharge, and has been trapped in N2 and Ar matrices at 10 K. The UV absorption spectrum in solid nitrogen consists of a series of 11 bands beginning at 335 nm with an average vibrational interval of 320 cm−1. Intensity alternations suggest that two different vibrations are excited in the transition, probably a C–C–N bend corresponding to the observed spacing and a C–N stretch with vibrational frequency near 640 cm−1. The electron spin resonance (ESR) spectrum at 8300 G indicates that ethylnitrene has a triplet ground state with zero-field splitting parameters D=1.67 cm−1, and E=0.0035 cm−1 in a nitrogen matrix. The hyperfine splittings differ slightly in the x and y lines and have the average values of 16.0 G for N and 30.6 G for the methylene H. The spectral features associated with ethylnitrene were not observed through direct photolysis of the parent ethylazide either during or after deposition. The data lend support to the conclusion that earlier assignments of the ESR spectrum of methylnitrene, CH3N, are incorrect, and may provide a lower limit to ‖D‖ for that molecule in the matrix.
Electronic structure and twisted intramolecular charge transfer in dimethylanilines
Authors: LaFemina John P. , Duke C. B. , Paton A.
Published: July 2002

The spectroscopically parameterized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission, ultraviolet absorption, and twisted intramolecular charge transfer fluorescence in N,N-dimethylaniline, p-cyano-N,N-dimethylaniline, and 3-methyl-4-cyano-N,N-dimethylaniline. Additionally, the CNDO/S3 density of valence states was used to determine a twist angle of 30° for both N,N-dimethylaniline and p-cyano-N,N-dimethylaniline via the analysis of gas phase photoelectron spectra.
Molecular cross correlation functions in inertial and noninertial frames of reference
Authors: Evans M. W.
Published: July 2002

The dynamics of interacting molecules involve the translation and simultaneous rotation of each molecule in the ensemble. This means that the frame of reference defined by the principal molecular moments of inertia, frame (1,2,3), is a noninertial frame with respect to the laboratory frame (x, y,z). Furthermore, a rotating frame of reference (1,2,3)′ can be generated from (1,2,3) a by translation for each molecule from the center of mass of the origin of (x, y,z). The equations relating velocities, accelerations, and their derivatives in one of these frames to those of another are described and then used to calculate the cross correlation functions.
Competition between binary ion molecule reactions and ternary association reactions of CH+3 with NH3 in He buffer gas in the pressure range 0.2–1.7 Torr
Authors: Saxer A. , Richter R. , Villinger H. , Futrell J. H. , Lindinger W.
Published: July 2002

The binary reaction of CH+3 with NH3 as well as their ternary association have been investigated in the regime from 0.047 to 0.2 eV, KEcm, using the Innsbruck selected ion drift tube (SIDT). In this energy regime the association rate coefficient declines from 4×10−26 to 6×10−27 cm6 s−1 and the lifetimes of the collision complex (CH+3−NH3)* range from ∼10−7 to ∼10−8 s.
Atom–surface scattering dynamics at hyperthermal energies
Authors: Amirav A. , Cardillo M. J. , Trevor P. L. , Lim Carmay , Tully J. C.
Published: July 2002

We report molecular beam scattering of hyperthermal Xe atoms over an energy range 1<Ei(eV)<10 from single crystal surfaces of GaAs(110), Ag(100), and Ge(100). The angular distributions from the corrugated surfaces show sharp backscattered rainbow maxima related to the topography of the crystal surface. In contrast the smooth surfaces yield quasispecular lobes suggestive of structure scattering. The large energy loss for all surfaces scales on average with the energy of local normal motion. A simple binary interaction model is developed which accounts for many of the phenomena observed from corrugated surfaces. With the aid of a comparison classical trajectory study, these results provide some understanding of the mechanism by and extent to which a solid can dissipate the energy of a hyperthermal collision.
Rotationally resolved isotope effect in the hot atom collisional excitation of CO2 (001) by time-dependent diode laser spectroscopy
Authors: Hewitt Scott A. , Hershberger John F. , Flynn George W. , Weston Ralph E.
Published: July 2002

High resolution diode laser spectoscopy is used to probe the rotational structure of the CO2(0001) state, produced in the inelastic scattering of hot deuterium atoms and CO2(0000). A preference for excitation of high J levels relative to low J levels of CO2(0001) by hot deuterium atoms compared to hot hydrogen atoms was observed. (AIP)
Rotational state distributions of NH(a 1Δ) from HNCO photodissociation
Authors: Spiglanin Thomas A. , Chandler David W.
Published: July 2002

We have examined the photofragmentation HNCO→NH(a 1Δ)+CO using radiation at wavelengths shorter than 230 nm. Nascent NH(a 1Δ) shows relatively little rotational excitation, accounting for less than 12% of the energy in excess of the dissociation energy. The rotational state distributions evidence less population in high rotational states than predicted by statistical theories but more than expected on the basis of a simple impulsive dissociation. A semiclassical impulsive model that describes photoproduct rotation as developing during fragmentation successfully describes the rotational state distributions of NH(a 1Δ) produced by HNCO photodissociation over a wide range of wavelengths. The success of this model in describing the NH rotational state distributions and previously measured CO rotational state distributions suggests that the excited state potential energy surface may be repulsive with minima in HNC and NCO bond angles each near 120°.
One-electron diatomics in momentum space. III. Nonvariational method for single-center expansion
Authors: Koga Toshikatsu , Matsuhashi Toshiyuki
Published: July 2002

When the Fock transformation is applied to the momentum-space Schrödinger equation for one-electron diatomics, the equation is reduced to a homogeneous Fredholm-type integral equation with a degenerate kernel. It provides a new single-center expansion formula without invoking the variational principle. The present single-center solution satisfies the zero potential energy criterion and constitutes an upper bound to the true solution. The results are illustrated for the ground and first excited states of the H+2 molecule.
Comparison of close coupling and quasiclassical trajectory calculations for rotational energy transfer in the collision of two HF molecules on a realistic potential energy surface
Authors: Schwenke David W. , Truhlar Donald G. , Coltrin Michael E.
Published: July 2002

We report rigid-rotator close coupling calculations and quasiclassical trajectory calculations for HF–HF collisions with total angular momentum zero. The results are compared to test the trajectory method.
The observation of rotational anisotropy in the second harmonic intensity from a Ag(111) electrode
Authors: Shannon Victoria L. , Koos Daniel A. , Richmond Geraldine L.
Published: July 2002

Rotational anisotropy in the second harmonic intensity is observed for a Ag(111) electrode and is compared to theory. The three and sixfold symmetric patterns for the p and s polarized SH fields reflect the ordered (111) crystal surface and demonstrate the promise of SHG as an in situ probe of surface structure at the electrode/electrolyte interface.
Raman spectroscopy and melting of nitrogen between 290 and 900 K and 2.3 and 18 GPa
Authors: Zinn A. S. , Schiferl D. , Nicol M. F.
Published: July 2002

Raman spectroscopy was used to study the melting of nitrogen from 290 to 900 K at pressures from 2.3 to 18 GPa. This work, which extends the melting by a factor of 9 over previously published results was made possible by new developments in high-temperature diamond-anvil cells. The β/δ phase boundary was also determined, and the β–δ–fluid triple point was found to be at 578±10 K and 9.9±0.5 GPa. The Raman frequencies of the vibron in fluid N2 and the ν2 vibron in δ-N2 were found to have the same pressure dependence and be independent of temperature to a good approximation. A temperature-independent pressure scale, useful to at least 900 K is approximated by P/GPa=0.4242 ν/cm−1 −987.8, where ν is the frequency of either the ν2 vibron in δ-N2 or the vibron in fluid-N2.
Influence of variable draining and excluded volume on hydrodynamic radius within Kirkwood–Riseman model: Dynamical renormalization group description to order ε2
Authors: Wang Shi-Qing , Douglas Jack F. , Freed Karl F.
Published: July 2002

The renormalization group method is employed in conjunction with the Kirkwood–Riseman and two-parameter models to study the double crossover dependence of the hydrodynamic radius of flexible polymers on both hydrodynamic and excluded volume interactions. An exact second order in ε crossover analysis is derived for the limiting cases of Gaussian and self-avoiding chains and for free-draining and nondraining chains. Our calculations show that the excluded volume and hydrodynamic interactions act competitively to affect the dynamical measures of the molecular dimensions, so that an increase in the excluded volume interaction produces a decrease in the strength of the effective hydrodynamic interaction. This coupling of the hydrodynamic and excluded volume interactions can provide an explanation of why dynamical measures of polymer dimensions are often found to scale with lower exponents than equilibrium measures, and methods are described for experimentally testing such an explanation.
Diffusional theory of termolecular recombination and association of atomic species in a gas
Authors: Flannery M. R.
Published: July 2002

A diffusional treatment of termolecular association of atomic species A and B in a low density gas is presented and applied to positive ion–negative ion recombination over the full range of masses of reactants for various classes of ion–neutral interactions. In contrast to rates given by the diffusional current, excellent results are obtained for general mass species provided a more basic expression for the association rate is introduced.
Intramolecular energy transfer and c
i
s–t
r
a
n
s isomerization in HONO
Authors: Guan Yuhua , Lynch Gillian C. , Thompson Donald L.
Published: July 2002

Classical trajectories have been used to investigate cis–trans isomerization and intramolecular energy transfer in nitrous acid. The influence of various initial normal mode excitations has been investigated. Power spectra have been used to examine the intramolecular dynamics. The rate of cis–trans conversion is strongly dependent on the site of initial excitation. The rates for cis–trans isomerization is significantly higher than those for trans–cis. The results show that the OH stretching mode is only weakly coupled to the other modes. Energy transfer out of excited states of the OH mode is slow and preferential partitioning of energy into that mode leads to a reduced rate of isomerization, while excitation of other modes gives rate enhancement. The HON bend interacts with the torsion more strongly than do the other modes.
Low-energy electron scattering from water molecules: A study of angular distributions
Authors: Gianturco F. A. , Scialla S.
Published: July 2002

We report quantal calculations of elastic differential and momentum transfer cross sections for the scattering of electrons by H2O at low and intermediate collision energies, i.e., from 2 to 20 eV. The fixed-nuclei approximation (FNA) was employed and a single-center expanded (SCE) wave function was used to represent the molecular target. The well-known divergence in the forward direction was corrected via Born closure formulas (see the text) and a parameter-free model, previously tested for methane targets, was used to describe exchange and polarization effects. The present results can be used to adequately describe angular distributions even at very small angles and can be extended beyond the largest angles that have been experimentally measured. The behavior of momentum-transfer cross sections as a function of energy, and the comparison of our results with other static-exchange (SE) calculations, which use an entirely different approach, are presented and discussed.
Outer sphere electron transfer in polar solvents. Activationless and inverted regimes
Authors: Rips Ilya , Jortner Joshua
Published: July 2002

In this paper we apply the stochastic Liouville equation for the study of outer sphere electron transfer (ET) in a polar solvent, which is characterized by a Debye dielectric relaxation. Explicit expressions, which bridge between the nonadiabatic and solvent-controlled adiabatic limits, are derived for the ET rates spanning a broad range of the energetic parameters, which include normal ET, activationless ET, and ET in the inverted region. The many-body result for the ET rate, which is accurate for symmetric ET, constitutes a useful approximate interpolation formula, which accounts well for the ET dynamics over a broad range of the energetic parameters.
Brownian dynamics simulation of rotational correlation functions of simple rigid models
Authors: Díaz F. G. , Iniesta A. , García de la Torre J.
Published: July 2002

In this paper we investigate the ability of the Brownian dynamics simulation technique of Ermak and McCammon to evaluate rotational correlation functions of rigid hydrodynamic models. Concretely, we obtain from simulated trajectories the functions 〈Pi[cos θ(t)]〉, i=1,2 where Pi is the ith Legendre polynomial and θ(t) is the angle between two orientations, separated by time t, of a given vector. This function is closely related to the decay of fluorescence anisotropy and other time-dependent electro-optical properties. The simulation results are compared with exact results for these functions. We first consider dimers in a variety of conditions regarding the rigid constraint, the hydrodynamic interaction and the relative size of the two spheres. We next study bent trimers, for which the decays are typically multiexponential. In all cases we find that the simulated results of the correlation functions and the exact ones are in rather good agreement up to a time which is determined by the finite length of the simulated trajectory.
Conformational kinetics in liquid butane by nonequilibrium molecular dynamics
Authors: Edberg Roger , Evans Denis J. , Morriss Gary P.
Published: July 2002

A new nonequilibrium molecular dynamics (NEMD) algorithm is used to calculate rate constants for trans–gauche isomerization in pure liquid butane. The new NEMD algorithm is based on macroscopic kinetic relaxation to equilibrium. The kinetic rate constant at T=292 K and ρ=0.583 g/ml is k=kF+kR =4.8×1010 s−1. The transition state theory rate constant is kTST =2.8×1011 s−1. The transmission coefficient is κ=0.17. Transition state theory overestimates the rate constant by a factor of 5.
The prediction of nuclear quadrupole moments from a
b i
n
i
t
i
o quantum chemical studies on small molecules. II. The electric field gradients at the 17O, 35Cl, and 2H nuclei in CO, NO+, OH−, H2O, CH2O, HCl, LiCl, and FCl
Authors: Cummins Peter L. , Bacskay George B. , Hush Noel S.
Published: July 2002

The electric field gradients (efg’s) at the oxygen and hydrogen nuclei in CO,NO+, OH−, H2O, and CH2O, and at the chlorine, lithium, and hydrogen nuclei in HCl, LiCl, and FCl, calculated using ab initio quantum chemical methods, are reported. Using extended Gaussian basis sets, the efg’s at the oxygen and chlorine nuclei were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. The efg’s at the hydrogen and lithium nuclei were computed as expectation values. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg’s is discussed and, where possible, corrected for. The calculated efg’s, together with the experimental nuclear quadrupole coupling constants, are used to estimate the 17O, 35Cl, and 2H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values on the basis of the present calculations are −2.64±0.03, −8.2±0.2, and 0.278 fm2, respectively, for the 17O, 35Cl, and 2H nuclear quadrupole moments.
A comprehensive interpretation of mixing effects on stationary states and dynamical behavior of the bistable ClO−2–I− reaction in a flow reactor
Authors: Boissonade J. , De Kepper P.
Published: July 2002

We propose an interpretation of mixing effects on temporal dissipative structures in CSTR in terms of micromixing. The bistability of the ClO−2 –I− reaction is extensively discussed. The micromixing process is successively represented by a mean field model and a coalescence redispersion model, together with a realistic kinetic scheme of the reaction. The latter was found more suited to the problem. The results are in good agreement with experimental observations previously reported, accounting for shifts in the transitions from thermodynamic branch to flow branch both in premixed and nonpremixed mode. It also accounts for dynamical behavior in the vicinity of the transition, including oscillatory fluctuations. It is finally suggested that micromixing processes could induce oscillations in otherwise nonoscillating conditions if the system was perfectly homogeneous.
Microscopic observation of ordered colloids in sedimentation equilibrium and the important role of the Debye-screening length. V. Binary mixtures of monodisperse polystyrene spheres in aqueous suspension
Authors: Okubo Tsuneo
Published: July 2002

A metallurgical microscope is used to directly observe the two-dimensional structures of the binary mixtures of different sizes and specific gravities of monodisperse polystyrene spheres in sedimentation equilibrium and in deionized suspensions. When the sizes of small particles are smaller than 300 nm, large (diam 1.0 μm) and dense (specific gravity=1.5) spheres are often segregated upward by the ordered structures formed by the small less dense (specific gravity=1.05) spheres and Brownian movement of the larger spheres are restricted significantly. When the sizes of the small particles in the mixtures of large dense and small less dense particles are between 300 and 500 nm, the segregation effect is prevented and various kinds of ‘‘alloy’’ structures of triangular, square, pentagonal, hexagonal, heptagonal, and octagonal patterns are observed. The interparticle distances between large–large, large–small, and small–small particles agree well with the effective sizes of spheres including the Debye-screening length. These results are consistent with the significant role of the electrical double layers under the influence of purely electrostatic repulsive forces in the effective hard-sphere model.
Synthesis of OH from reaction of O and H on the Rh(100) surface
Authors: Gurney Bruce A. , Ho W.
Published: July 2002

We report the synthesis of the OH intermediate from O and H coadsorbed on the Rh(100) surface at 90 K and heated in ultrahigh vacuum (UHV) to between 140 and 240 K; the species is stable when cooled again to 90 K. When heated to higher than ≊240 K H2O is formed and evolved from the surface, demonstrating that OH formation is an important step in H2O synthesis. Temperature programmed electron energy loss spectroscopy (TP-EELS), temperature programmed reaction spectroscopy (TPRS), and low energy electron diffraction (LEED) were employed in the study of this intermediate. The EEL spectra of the OH species is characterized by a stretch mode at 394 meV, bending mode at 114 meV, frustrated lateral translation at 82 meV, and frustrated vertical translation at 54 meV. Off-specular measurements show that the OH bending mode is entirely dipole active at an impact energy of 6eV. By monitoring the OH bend intensity normalized to the elastic intensity as the crystal temperature is linearly ramped, the kinetics of both OH synthesis and OH combination to form H2O was probed. Kinetics was obtained using the heating rate variation method (from the shift in the temperature of fastest reaction Tp with heating rate) and the coverage variation method (from the shift in Tp resulting from different reactant concentrations). An activation energy of Ef=4±1 kcal mol−1 for OH formation was obtained. We find a half-order coverage dependence indicating that OH formation occurs at the perimeters of O islands. An activation energy of Ec=24±1 kcal/mol−1 was obtained for the combination reaction. Isotopic substitution of deuterium (D) for hydrogen yielded no OD or D2O under UHV conditions, but an O covered surface heated in a D2 pressure ≥10−8 Torr formed both surface OD and evolved D2O. This difference in H and D reactivity can be explained by the combination of the observed inverse kinetic isotope effect (KIE) in D2 recombinative desorption (in which D2 desorbs faster than H2) and the observed normal KIE in OD formation (in which OD forms at a slower rate than OH).
Electronic dipole and transition moments from the relativistic CI wave function: Application to HI
Authors: Chapman Douglas A. , Balasubramanian K. , Lin S. H.
Published: July 2002

A method has been developed to allow calculation of the electronic dipole and transition moments of diatomic molecules directly from the relativistic CI wave function. In this way, the effect of spin–orbit coupling on these quantities may be rigorously ascertained. The electric dipole moment of the ground state of HI is calculated and compared to experimental data and nonrelativistic all-electron results. Transition moments between the ground state and two low-lying electronic states of HI are also calculated in the relativistic CI formalism to illustrate how the effect of spin–orbit interaction on transition moments can be studied.
Rotational perturbations between Rydberg states of Li2
Authors: Hemmerling B. , Bombach R. , Demtröder W.
Published: July 2002

With two cw single mode dye lasers selective two-step excitation of Rydberg levels of the Li2 molecule is achieved which is detected by optical double resonance polarization spectroscopy. The Doppler-free DR signals allow accurate measurements of rotational perturbations which cause level shifts and intensity anomalies due to interference phenomena. The experimental results are discussed and molecular constants for the deperturbed Rydberg states are given, which, together with the coupling matrix elements allow the accurate reproduction of the measured spectra.
Measurements of two-photon absorption cross sections for liquid benzene and methyl benzenes
Authors: Chen C. H. , McCann M. P.
Published: July 2002

Two-photon cross sections of neat benzene and methyl substituted benzenes at various wavelengths for selective excitation were measured by two-photon induced fluorescence spectra for the first time. The results show that the two-photon rate increases rapidly with the frequency of the photon.
Isotope effects in the high-resolution infrared spectrum of OC–HF
Authors: Fraser G. T. , Pine A. S.
Published: July 2002

High-resolution infrared spectra of the H–F stretching bands of natural OC–HF and enriched O13C–HF and 18OC–HF have been recorded under thermal equilibrium conditions near 195 K with a tunable difference-frequency laser. The rotational constants of the three isotopic species are consistent with the linear C–H van der Waals bond as determined by microwave spectroscopy. The isotope shifts for the band centers exhibit a curious staggering with total CO mass which we attribute to anharmonic coupling of the zero-point CO bending motion. Resolved splittings of the l doublets in a hot band originating in the CO bending vibration yield tentative assignments and rotational constants for this low frequency van der Waals mode.
The Weissenberg effect at finite rod-rotation speeds
Authors: Lodge A. S. , Schieber Jay D. , Bird R. Byron
Published: July 2002

According to a known heuristic argument, a given polymeric liquid will be expected to exhibit a positive or negative Weissenberg effect (i.e., will move up or down a rotating vertical rod immersed in it, under conditions where contact angle and centrifugal force effects are negligible) if the value of R:=1+2(σ/N1)dN2/dσ is positive or negative, where σ, N1, and N2 denote the shear stress and first and second normal stress differences in steady shear flow. The value of this heuristic treatment lies in its applicability to finite shear rate ranges for which an exact calculation is not available. The treatment includes (and generalizes) a known exact low-shear-rate limiting condition β0<0.25 for rod climbing, where β0 denotes the low-shear-rate limiting value of β:=−N2/N1. Using published experimental data for N1(σ) and N2(σ) to evaluate R, we find that positive Weissenberg effects are predicted for polyisobutylene and some polystyrene solutions (even when β=0.6), in agreement with observation, but that negative effects are predicted for other polystyrene solutions, in disagreement with observation. We cannot explain this disagreement. We also use the Curtiss–Bird kinetic theory for monodisperse polymers to evaluate R for various values of the ‘‘link tension coefficient’’ ε. Over ranges of shear rate experimentally accessible for molten polymers, we find that R>0 provided that ε>1/8. When ε=0, however, we find that R<0 for most low-to-moderate shear rates; it follows that, according to the present heuristic argument, the Doi–Edwards theory (ε=0) for monodisperse polymers predicts negative Weissenberg effects for a significant range of shear rates, in disagreement with all known measurements for molten polymers. Using the Curtiss–Bird theory for polydisperse polymers having a log-normal distribution with Mw=2Mn, we find R values greater than those found for monodisperse polymers. Inertial forces, neglected in the present treatment, could readily be included if required for application to low-viscosity liquids.
Multichannel interactions in the (1σg)2(1σu)n
sσ, n
dλ(3Σ+u,3Σ+u,3Πu,3Δu) Rydberg structures of He2
Authors: Ginter Dorothy S. , Ginter Marshall L.
Published: July 2002

We show that a minimal parameter coupled channel model based on eigenquantum defect theory can reproduce quantitatively the known Rydberg structures associated with six channels of nsσ,ndλ(3Σ+u,3Σ+u,3Πu,3Δu) v=0 ancestry in He2. Except for a few levels affected by accidental perturbations, these extensive level structures can be reproduced to within average experimental uncertainties. Previously unreported spectral analyses for transitions to the b 3Πg state from rotational levels in the nl channel segments with n=12–18 are included in this work. These spectral transitions were predicted and observed in the early stages of this investigation and were used to determine a number of new energy levels for the n=3–18 data base used in subsequent calculations. The model uses a Û matrix modified slightly from a Hund’s case (b) to case (d) transformation and energy dependent eigenquantum defects μα. Discussed in detail is a specific 14 parameter representation for ∼500 energy  levels in which 2 parameters modify  Û to include interactions between ns and nd and 12 parameters describe the variation of the μα’s with energy.
Spectra and structure of organophosphorus compounds. XXXIV. The r
s and r structures of t
r
a
n
s and g
a
u
c
h
e ethylphosphine
Authors: Groner P. , Johnson R. D. , Durig J. R.
Published: July 2002

The microwave spectra of 11 chemically different isotopic species of ethylphosphine (CH3CH2PH2) including all 13C and D single substituted species were recorded and assigned for the vibrational ground state. The b- and c-type transitions of the gauche conformers with symmetrically substituted ethyl groups and a PH2 group were found to be split due to tunneling between the equivalent forms. The spectra of these molecules were fit to a Hamiltonian containing Coriolis interaction terms. All other spectra were fit to asymmetric rotors with quartic centrifugal distortion. Complete rs structures were obtained for both the trans and gauche conformations. The r0 structures were derived from 45 and 48 rotational constants for the trans and gauche forms, respectively. Significant differences in the structures were found with the most notable one in the CCP angle which changes from 110.5° in the gauche form to 116° in the trans conformer. The methyl group in the gauche conformation is rotated away from a position symmetric with respect to the heavy atom plane. The structural differences are attributed to nonbonded repulsive interactions between the methyl and phosphino hydrogen atoms.
Inelastic collisions of CaF(A 2Π) with He and Ar: Quantum calculations and adiabatic analysis
Authors: Pouilly Brigitte , Alexander Millard H.
Published: July 2002

We present coupled-states (CS) calculations for collisions of CaF(A 2Π) with He and Ar. These calculations are based on potential energy surfaces determined by use of the Gordon–Kim electron–gas method [J. Chem. Phys. 56, 3122 (1972)]. The theoretical CS cross sections for rotationally inelastic transitions are compared with the experimental results of Dufour and co-workers [Chem. Phys. 98, 315 (1985)]. We obtain theoretical cross sections significantly greater than the experimental values. In order to discuss and interpret the results of our scattering calculations, we make use of a fully adiabatic description of the collision dynamics. This adiabatic analysis shows that the long-range part of the potential plays a key role in determining the sizes of the theoretical cross sections, and in underlying the failure of the energy sudden scaling relation to describe the calculated cross sections.
Deexcitation cross sections of He(2 1P) by atoms and molecules
Authors: Ukai Masatoshi , Nakazawa Hidenobu , Shinsaka Kyoji , Hatano Yoshihiko
Published: July 2002

The deexcitation cross sections of He(2 1P) by M (M=Kr, Xe, H2, D2, N2, CO, O2, NO, CO2, and CH4) have been obtained by a pulse radiolysis method at a mean collisional energy corresponding to room temperature. The cross sections have been explained fairly well by the Penning ionization process with a long-range dipole–dipole interaction. Bent trajectories appear to be important in the deexcitation by spherical atoms and molecules. An electron exchange process is also suggested to give an important contribution to the total decay width of the transient collision complex in highly excited autoionizing states of (HeM)*.
A multisurface DIM trajectory study of the reaction: O(1D
g)+H2(X 1Σ+g)→OH(X 2Π)+H(2S)
Authors: Kuntz P. J. , Niefer B. I. , Sloan J. J.
Published: July 2002

The diatomics-in-molecules method has been used to compute potential energy surfaces for the interactions between O(1Dg) atoms and hydrogen molecules, and a multisurface quasiclassical trajectory study of the reaction dynamics for collisions at several different reagent translational energies has been carried out using these potentials. The multisurface nature of the problem was explicitly included by using a surface-hopping computational technique, based on Landau–Zener transition probabilities, which made all of the relevant surfaces available to the trajectory. The product energy distributions predicted by three slightly different DIM models of these surfaces are computed. In all cases, the dynamics of the reactions which begin on the first excited surface are dramatically different from those of reactions which occur on the lowest surface, the only one which was considered in all previous dynamical calculations on this system.
A Monte Carlo method with different treatments for short- and long-range interactions in conformational statistics of polypeptide chains
Authors: Premilat Samüel
Published: July 2002

Separation of the conformational energy into terms associated with short-range or long-range interactions allows us to propose a new approach for using Monte Carlo calculations in statistical mechanics of molecular chains. The present method is based on a simultaneous use of statistical weight matrices calculated with short-range interactions and importance sampling applied only on the part of energy coming from long-range interactions. Distributions of end-to-end distances and probabilities of the chain units being in their different conformational states are determined on molecular models of the peptidic hormones enkephalin and β-casomorphin. Results obtained in this way are compared with values calculated using previous computational methods which are also applied on chain models including only nearest neighbor interdependent units. The relative importances of short-range and long-range interactions for the molecular conformation can thus be distinguished. Effects of residue Pro as well as of charged ends on the chain folding are well verified. It is shown that solvent effects can be introduced on long-range interactions mainly by screening the electrostatic term of the energy; extended conformations are then obtained. The proposed method lowers the computer time and by using more information on the moledular system, improves Monte Carlo calculations.
Spectral line-narrowing and saturation effects in fully resonant nondegenerate four wave mixing
Authors: Riebe Michael T. , Wright John C.
Published: July 2002

Pentacene doped benzoic acid crystals are used to study the line-narrowing capability of fully resonant four wave mixing for inhomogeneously broadened transitions. Both parametric and nonparametric nonlinear spectroscopies are used to scan the vibronic resonances of excited electronic states while maintaining resonance with a vibrational mode of the ground state and the vibrationless level of the excited state. The observed vibronic transitions correspond to modes that are coupled to the ground state mode. The spectra contain both lines that are narrowed and lines that have the full inhomogeneous width. Narrowing of the vibronic transitions is observed when site selection is performed on the vibrationless electronic transition in both parametric and nonparametric methods. Narrowing of the vibrational resonance is not observed under the same conditions showing that the vibrational and electronic inhomogeneous broadenings are not correlated. The shifts of the line positions and the changes in linewidths and relative intensities were measured across the inhomogeneous linewidth for both methods. The nonlinear signal intensity was also measured as a function of laser intensity. All of the results are consistent with a theoretical description where saturation effects are dominant.
Infrared laser spectroscopy of the a 3Σ+g–c 3Πu system of H2 and D2
Authors: Davies Paul B. , Guest Michael A. , Johnson Simon A.
Published: July 2002

Doppler limited infrared laser spectra of H2 and D2 have been recorded in an ac discharge with diode laser sources and assigned to Δv=0 and ±1 sequence bands of the a–c system. Intense absorption signals from atomic hydrogen were also detected and used to confirm that some molecular spectra occurred in stimulated emission; as was found for a known D2 laser transition. Several of the diode spectra recorded at shorter wavelengths (Δv=1 sequences) cover regions previously observed by FTIR emission experiments on the a–c system. Both fine and hyperfine structure are revealed by the present studies and a preliminary analysis shows that while the a 3Σ+g state has negligible fine structure (at this resolution), hyperfine effects are apparent and are modeled using a bβS coupling case.
Differentiation between resonance Raman scattering and single vibrational level fluorescence 5100 cm−1 above the 1B2u origin in benzene vapor by means of excitation profiles
Authors: Harmon Paul A. , Asher Sanford A.
Published: July 2002

We have measured the absolute Raman scattering cross section of the 992 cm−1 a1g ring breathing mode of benzene vapor in the 150 cm−1 region surrounding the 610150 vibronic transition to the 1B2u excited electronic state. We clearly observe emission directly from the resonant 610150 level, located ∼5100 cm−1 above the 1B2u origin. This Raman-like resonance emission is observed as an enhancement of the 992 cm−1 vibrational mode fundamental, as well as in the appearances of ν1 overtones and the progression 2ν6+nν1. Although the resonance emission from 1B2u vibronic levels has previously been denoted as ‘‘single vibrational level fluorescence’’ (SVLF), resonance fluorescence (RF), or hot luminescence (HL), we find by means of the analysis of the 992 cm−1 excitation profile that the resonance emission also contains a significant resonance Raman scattering (RRS) component. For excitation into the 610150 vibronic level of benzene vapor, the high density of background states within the molecule promotes processes such as dynamic intramolecular vibrational redistribution. These processes result in the intramolecular dephasing which determines the relative fractions of HL and RRS present in the emission from the ensemble of ‘‘isolated’’ molecules that give rise to the structured emission of vapor phase benzene. RRS derives from scattering from a superposition state while HL emission derives from an excited molecular eigenstate.
Efficiency of a combustion reaction in power production
Authors: Schell Mark , Schram Henk M. , Ross John
Published: July 2002

We analyze a thermal engine in which the source of heat is the combustion reaction CH4+2O2→CO2+2H2O. The efficiency of the thermal engine is defined as the ratio of the power output of the engine to the power input. We compare the efficiency in two modes of operation, one in which the input and output of chemical species are constants and one in which their corresponding fluxes have an imposed oscillatory component of a given amplitude and frequency. The ratio of efficiency in the oscillatory mode to the stationary flux mode can be as high as 1.3 for an amplitude of variation of the input and output fluxes plus or minus 40% of the stationary value. At low amplitudes of variation (10%), a single peak is present in a plot of the ratio of efficiency vs frequency of the fluxes. For higher amplitudes there are two peaks in that plot. One peak is associated with a response of the system that has the same period as the fluxes and the other occurs in a frequency range for which the periodic response is twice that of the perturbation. We calculate plots of the oscillatory output flux and the power output divided by that flux, which is an analog of a thermodynamic force, as a function of time, to show the importance of both resonance effects and the phase relation between the flux and the thermodynamic force in determining the ratio of efficiency.
Methods for the calculation of the self-diffusion coefficient of interacting Brownian particles
Authors: Yoshida Nobuo
Published: July 2002

Batchelor’s method of calculating the first-order concentration dependence of the self-diffusion coefficient of spherical Brownian particles is compared with our method devised previously. Both methods are based on the solution of the two-particle Smoluchowski equation as applied to a stationary state. It is shown that the difference between the methods resides only in one boundary condition which is imposed on solutions of a version of the Smoluchowski equation when two Brownian particles come into contact. Although the validity of Batchelor’s boundary condition is obvious, that of ours was not. We show how our boundary condition is justified and suggest that the two methods lead to identical results for the self-diffusion coefficient in spite of the different boundary conditions. The suggestion is confirmed for the case of hard spheres. We also derive a boundary condition, corresponding to ours mentioned above, which is applicable to solutions of a version of the time-dependent Smoluchowski equation for an equilibrium state on the basis of which the self-diffusion coefficient can also be obtained.
Photoionization of aligned molecular excited states
Authors: Appling J. R. , White M. G. , Kessler W. J. , Fernandez R. , Poliakoff E. D.
Published: July 2002

Photoelectron angular distributions of several excited states of NO have been measured in an effort to better elucidate the role of alignment in resonant multiphoton excitation processes of molecules. In contrast to previous molecular REMPI measurements on NO, (2+1) angular distributions taken for low rotational levels of the E 2Σ+ (4sσ) Rydberg state of NO exhibit complex angular behavior which is characteristic of strong spatial alignment of the optically prepared levels. Photoelectron angular distributions were also found to be strongly branch and J dependent with the lowest rotational levels of the R21+S11 branch exhibiting the full anisotropy expected for an overall three-photon process. Fluorescence anisotropies extracted from complementary two-photon fluorescence angular distribution measurements reveal small, but nonzero alignment in all rotational levels with J>1/2, in contrast to the photoelectron results. Additional photoelectron angular distributions taken for (1+1) REMPI via the A 2Σ+ (3sσ), v=0 state exhibit near ‘‘cos2θ’’ distributions characteristic of photoionization of unaligned target states. The observed photoelectron data are qualitatively interpreted on the basis of the angular momentum constraints of the excitation-induced alignment and photoionization dynamics which determine the observable moments in the angular distribution.
Nascent state distributions of Ca(4s3d 1D2), Ca(4s3d 3D
J), and Ca(4s4p 3P
J) in the collisional deactivation of Ca(4s4p 1P1) by the rare gases
Authors: Breckenridge W. H. , Merrow Clifton N.
Published: July 2002

The collisional deactivation of Ca(4s4p 1P1) by the rare gases results in essentially exclusive production of the Ca(4s3d 3DJ) states. The collisional production of the Ca(4s3d 1D2) and Ca(4s4p 3PJ) states was negligible, within experimental error, compared to production of the Ca(4s3d 3DJ) states for He, Ne, Ar, and Kr. For Xe a relative branching ratio of 0.02±0.01 was determined for Ca(4s3d 1D2) collisional production. The nascent distributions of Ca(4s3d 3D3,2,1) multiplets were nonstatistical, and varied slightly from gas to gas. The lowest-lying Ca(4s3d 3D1) multiplet was always very underpopulated. A general mechanism is proposed whereby there are avoided crossings between slightly attractive Ca⋅RG(1Π1) potentials correlating with Ca(4s4p 1P1) and repulsive Ca⋅RG(3Σ1) potentials correlating with Ca(4s3d 3DJ). Possible reasons for the preferential production of the 3DJ vs the 1D2 or 3PJ states, and the nonstatistical nascent populations of the 3DJ multiplets, are discussed.