bluuebunny/crossref_metadata_2025 huggingface dataset:
The effects of impurity scattering and transport topology on trapping in quasi-one-dimensional systems: Application to excitons in molecular crystals
Authors: Wieting R. D. , Fayer M. D. , Dlott D. D.
Published: February 2003

A model is developed which permits a detailed examination of the effects of impurity scattering and deviations from a strictly one-dimensional transport topology on the trapping rate and on the mean-square displacement of mobile species in systems which are nearly one dimensional in their transport characteristics, i.e., quasi-one-dimensional. The model is applied specifically to Frenkel excitons in molecular crystals, but may be readily adapted to other types of systems. Both coherent (wavelike) and incoherent (diffusive) microscopic modes of exciton transport are considered. In the strictly one-dimensional limit following pulsed optical excitation, a time-dependent trapping rate function is obtained as opposed to the commonly employed trapping rate constant. It is demonstrated that the coherent and incoherent trapping rate functions have identical dependencies on time and on impurity concentration and the macroscopic rate of transport can be calculated. To treat deviations from strictly one-dimensional transport topology, it is necessary to consider anisotropic walks on the system’s ’’superlattice,’’ i.e., the array of molecular chain segments which are bounded by scattering impurities. Montroll’s Green function formalism is employed to obtain solutions to various walk topologies which have not previously been reported in the literature. In the quasi-one-dimensional case, a time-dependent trapping rate function is also obtained unless the walk on the superlattice is nearly isotropic. The various trapping rate functions are then employed to provide explicit expressions for the time-dependent trap population which is proportional to the intensity of time resolved optical emission, a widely used experimental observable. Finally, the exciton mean-square displacement for a quasi-one-dimensional system with scattering impurities is shown to remain basically one dimensional even in situations where exciton trapping behaves in a manner best described in terms of a three-dimensional topology.
NMR study of adsorbed water. II. Molecular motions in the monolayer hydrate of halloysite
Authors: Cruz M. I. , Letellier M. , Fripiat J. J.
Published: February 2003

In the electrically neutral interlayer space of endellite, the water monolayer does not exhibit a preferential orientation such as that observed in phyllosilicates with water hydrating the interlayer cations (see, for example, the first paper in this series). The influence of the surface is to provoke an anisotropy of the molecular motions which can be separated into a rapid 180° flipping motion about the C2 axis and a slow tumbling about an axis perpendicular to C2. The activation energy of the former motion is 4.6 kcal mole−1, whereas that of the latter is 3.6 kcal mole−1. The proton and deuteron spin–spin and spin–lattice relaxation times can be accounted for by this simple model. Two other types of experimental results complement this model. The observed infrared OH frequencies suggest weak hydrogen bonds between water molecules and surface lattice hydroxyls, whereas strong hydrogen bonds lock water molecules to each other. The molar heat capacity of adsorbed water parallels that of ice up to the temperature (130 °K) where a torsional motion about the C2 axis is transformed into the 180° flipping motion.
Low frequency vibrational spectra, methyl torsional potential functions, and internal rotational potential of methyl vinyl ether and methyl-d3 vinyl ether
Authors: Durig J. R. , Compton D. A. C.
Published: February 2003

The vibrational spectra (infrared and Raman) of gaseous methyl vinyl ether and methyl-d3 vinyl ether have been recorded below 500 cm−1. A large amount of torsional data are reported and the observed bands are assigned on the basis of an equilibrium between an s-cis conformer and a high energy conformer which is shown to be the gauche form. For methyl-d3 vinyl ether, the torsions are well separated in frequency and they are not greatly coupled, whereas for the ’’light’’ molecule, the torsional spectra are quite complicated, presumably due to coupling and/or a perturbation between the methyl and asymmetric torsional modes. Both series of transitions for the methyl and methoxy torsional modes were assigned for the methyl-d3 vinyl ether molecule and the assignments were confirmed by the observation of a large number of double jumps for both torsions. The potential function governing the asymmetric torsional mode was found to be V1=−413±6 cm−1 (1.18 kcal/mole), V2=1434±21 cm−1 (4.10 kcal/mole), V3=1177±10 cm−1 (3.36 kcal/mole), and V6=−96±3 cm−1 (0.27 kcal/mole) with the s-cis conformer being more stable than the gauche conformer (torsional angle 114°) by approximately 402 cm−1 (1.15 kcal/mole). The barriers to s-cis–gauche, gauche–gauche, and gauche-s-cis interconversion were found to be 2215 cm−1 (6.33 kcal/mole), 290 cm−1 (0.83 kcal/mole), and 1774 cm−1 (5.07 kcal/mole), respectively. The barriers to methyl rotation for the s-cis and gauche conformers were found to be slightly higher than 1610 cm−1 (4.60 kcal/mole) and 514 cm−1 (1.47 kcal/mole), respectively. Possible reasons for this large variation are discussed. Tentative assignments of the torsional modes for the light compound are also given and discussed.
Transitions in the electronic absorption spectra of c
i
s- and t
r
a
n
s-difluoroethylene
Authors: McDiarmid Ruth
Published: February 2003

The absorption spectra of cis- and trans-difluoroethylene and their perdeuterated isomers were measured between 1200 and 1650 Å. The spectrum of each molecule is analyzed into several Rydberg series to the lowest ionization potential, and the 3s-Rydberg transition to the second ionization potential of the trans-molecule. Interactions are observed between the 3d and 4s Rydberg transitions in cis-difluoroethylene and between the 3s transition to the second ionization potential and the entire s-Rydberg series in trans-difluoroethylene. From these interactions the d-Rydberg series in each molecule is deduced to be a dδ series.
Laser-excited fluorescence of calcium dimer in inert gas matrices
Authors: Miller John C. , Andrews Lester
Published: February 2003

A strong, red, laser-excited fluorescence has been observed for Ca2 in inert gas matrices. Band origins were located using calcium-44 substitution and the spectroscopic constants were determined for the ground state van der Waals molecule in several different sites in solid argon and one site in solid xenon. Spectroscopic constants for the Ca2 excited state in solid neon (ω′e=118±2, ωex′e=0.3±0.1, and T0–0=15 150±20 cm−1) provide an estimate for the yet to be determined gas-phase constants for the upper state of this red transition. The observation of both unrelaxed and relaxed emission from Ca2 depending on the site and matrix is discussed in terms of guest–host interaction.
Transition probabilities at threshold for the photoionization of molecular nitrogen
Authors: Peatman Wm. B. , Gotchev B. , Gürtler P. , Koch E. E. , Saile V.
Published: February 2003

The relative transition probabilities at threshold for the photoemission of molecular nitrogen to vibrational levels of the X (2Σ+g), A (2Πu), and B (2Σ+u) states of the N+2 ion have been determined with high resolution threshold photoelectron spectroscopy using synchrotron radiation. These threshold cross sections are found to be strongly influenced by ionizing Rydberg states which are not necessarily apparent in the absorption or photoion spectrum of nitrogen. This interaction leads in the X (2Σ+g) state of N+2 to rotationlike structure in the observed vibrational transitions and in the A (2Πu) state to a smooth but non-Franck–Condon distribution of vibrational intensities. In addition to the cross section data, the molecular parameters for the X, A, and B states of N2 have been determined.
Light scattering of disordered ordinary and metastable phases of KCN as a function of pressure
Authors: Wong P. T. T.
Published: February 2003

The pressure-induced phase transition between KCN I and KCN II has been studied by the light scattering method. Under hydrostatic conditions, the transition is reversible and is completed over a range of 3 kbar pressure. The orientation of the CN− ion in the metastable KCN II is disordered as in KCN I. The pressure dependences of the frequency, and the integrated intensity of the CN stretching mode and of the integrated intensity of the broad scattering band in the frequency region near the exciting laser line, have been measured for both phases. The mechanism giving rise to the strong scattering near the exciting line is discussed.
Photoionization of molecular hydrogen
Authors: ONeil Stephen V. , Reinhardt William P.
Published: February 2003

A theoretical formulation of diatomic photoionization is given in terms of an entirely body-frame Hund’s case b description. Within this formalism, the electronic wavefunctions for both the bound initial state and the free final state of photoionization are described with conventional configuration interaction wavefunctions expanded in a Gaussian basis. The Stieltjes imaging procedure is used to recover the continuum oscillator strength from the discrete eigenspectrum. The continuum oscillator strength is combined with variationally determined vibrational wavefunctions to yield model photoelectron spectra, specific v–v ionization cross sections, a photoion kinetic energy spectrum, dissociative ionization spectra, and a total photoionization spectrum. For a series of photon energies near threshold, cross sections from these calculations are compared with previous theoretical results and with recent experimental measurements.
Relaxation processes and chemical kinetics
Authors: Skinner James L. , Wolynes Peter G.
Published: February 2003

The role of relaxation processes in determining the rates of activated events has long been a point of discussion in chemical physics. In this paper, we re-examine this issue. We idealize the problem as the classical motion of a particle in a one-dimensional potential coupled to a heat bath. This situation is described by a kinetic equation with a ’’collision operator’’ gℓc. An expansion in powers of the damping constant g is developed. This expansion is not limited to the case of high activation barriers. We compare results for various choices of the collision operator and provide a new derivation of Slater’s new rate theory. A Padé approximant approach unifies our low g results with those in the high g, i.e., diffusive, regime.
Low field hyperfine structure in the EPR spectra of Mn2+ containing glasses
Authors: Schreurs J. W. H.
Published: February 2003

Hyperfine structure observed at g′≃9.4 and g′≃4.3 in the EPR spectrum of Mn2+ in several glasses is analyzed on the basis of the theory of Griscom and Griscom [J. Chem. Phys. 47, 2711 (1967)] and Aasa [J. Chem. Phys. 52, 3919 (1970)]. The complex hyperfine structure at g′=4.3 is found to be a superposition of three sextets, but at g′=9.4 only a single set of six hyperfine lines is observed.
The radius and limiting conductivity of a spherical ion in a polar solvent
Authors: Reff Israel
Published: February 2003

The limiting conductivity of a spherical ion in a polar solvent is calculated. The Brownian motion of the ion is determined by viscous forces, the electric field of the permanent dipole moment of solvent molecules and by the induced polarization of the solvent. The ionic radius is calculated from experimentally observed conductivities.
Electron density and density matrix approximations using potential energy error bounds
Authors: Pakkanen Tapani A. , Whitten J. L.
Published: February 2003

The possibility of simplifying expansions of the total electron density, ρ (1), and the first order density matrix, γ (1,1′), derived from a given total wavefunction is considered based on the minimization of positive definite error bound integrals, ε=〈ρ (1)−ρ′ (1) ‖ν12‖ρ (2)−ρ′ (2) 〉 and εγ=〈γ (1,2)−γ′ (1,2) ‖ν12‖γ (1,2) −γ′ (1,2) 〉. Expansions, ρ′ and γ′, are in terms of a subset of the full basis and are designed for iterative use in energy variational calculations. In all of a variety of cases investigated, simple one-center expansions are found to be very accurate for the ρ expansion which determines the total coulombic energy of the system. However, correspondingly simple, one-center γ (1,2), expansions produce significant errors in the exchange, self-energy when the electron distribution is highly delocalized requiring that important two-center distributions be included for accurate calculations. Applications to localized electron distributions and to problems in which a lower level of accuracy is acceptable are discussed.
Laser magnetic resonance spectroscopy of SO in the ? 1Δ state with a CO2 laser as a source
Authors: Yamada Chikashi , Kawaguchi Kentarou , Hirota Eizi
Published: February 2003

Four vibration–rotation transitions of 32S16O in the 1Δ state were observed by laser magnetic resonance spectroscopy with a C18O2 laser in the 9 μm region as a source. Additional measurements were also made on two transitions of 34S16O. The band origin, the rotational constant, and the g factor in the v=1 state were determined by fixing the molecular constants in the ground vibrational state to those obtained either from microwave spectra or from reanalyzed electron paramagnetic resonance spectra. The second-order effect on the molecular g factor was discussed. The equilibrium bond length was determined to be 1.48886±0.00026 Å. The harmonic frequency and the vibrational anharmonicity constant were calculated from the band origins of the two isotopic species to be ωe=1115.349±0.081 cm−1 and ωexe=6.978±0.041 cm−1, respectively. The errors are all 2.5 times the standard deviations.
Electron spin–lattice relaxation times of melanin
Authors: Sarna T. , Hyde James S.
Published: February 2003

Saturation recovery measurements of the electron spin–lattice relaxation times of the free radicals of melanin are reported. ’’Natural’’ melanin gently extracted from the choroid of bovine eyes was used. Over a wide range of experimental conditions, the relaxation time remained nearly unchanged; A characteristic value is 15 μsec. Progressive saturation measurements also were made and results varied widely among samples in the series, which presumably is a consequence of variations in the transverse relaxation time.
Vibrational deactivation of CO(v=1) by oxygen atoms
Authors: Lewittes Mark E. , Davis Christopher C. , McFarlane Ross A.
Published: February 2003

Collisional relaxation of vibrationally excited carbon monoxide (v=1) by oxygen atoms has been measured from 265 to 389 °K using the laser induced fluorescence technique. Atom concentrations were measured with an ESR spectrometer. The measured rates, which vary from 7.5(+9.4−7.5) 102 sec−1 torr−1 at 265 °K to 4.2±2.0 103 sec−1 torr−1 at 389 °K show good agreement with existing high temperature shock tube results on a Landau–Teller plot. These rates indicate that atomic oxygen may be the dominant quenching species in many carbon monoxide chemical lasers.
Photolytic separation of D from H in cryogenic solutions of formaldehyde
Authors: Maier II William B. , Freund Samuel M. , Holland Redus F. , Beattie Willard H.
Published: February 2003

Isotopic enrichment of hydrogen has been demonstrated in cryogenic solutions. Formaldehyde is photolyzed in liquid xenon and found to decompose with low quantum yield. Identified photolytic products include CO and CO2. Absorption spectra of formaldehyde dissolved in liquid xenon are presented. The ratios (1.28–2) of the photodestruction rates of isotopically labeled formaldehyde are similar to the ratios (1.2–3) of their respective absorption cross sections. The photochemistry is briefly discussed.
Characterization of alkoxy free radicals
Authors: Box Harold C. , Budzinski Edwin E. , Potienko George
Published: February 2003

Alkoxy radicals have been produced by x irradiation in a variety of compounds containing hydroxy groups. The relationship between the g tensor and its correlated hyperfine coupling tensors are explored as an additional means for characterizing alkoxy radicals. A simple model of the spin density distribution in an alkoxy radical is able to account for the main features of the anisotropic coupling. The model gives 0.75 spin density in a p orbital of the oxygen atom and a small positive density along the C–H bond axis.
Kinetics of the deactivation of I(52P1/2) by Br2. I. Quenching of excited iodine atoms
Authors: Wiesenfeld J. R. , Wolk G. L.
Published: February 2003

The collisional deactivation of electronically excited iodine atoms, I(52P1/2), upon collision with Br2 has been monitored using time-resolved resonance absorption spectroscopy in the vacuum ultraviolet. The rate constant (at 295° K) characterizing the deactivation process was determined to be 5.6±0.2×10−11 cm3 molecule−1 sec−1. Furthermore, the physical quenching of I(52P1/2) leading to formation of ground state iodine atoms, (I(52P3/2), was shown to play a monor role in the overall deactivation of the excited atom. This demonstrates the significance of the reaction I(52P1/2)+Br2→IBr+Br(42P1/2,3/2) in the removal of the excited atoms upon collision with Br2.
Kinetics of the deactivation of I(52P1/2) by Br2. II. Production of electronically excited bromine atoms, Br(42P1/2), by chemical reaction
Authors: Wiesenfeld J. R. , Wolk G. L.
Published: February 2003

The production of bromine atoms in the ground, 42P3/2, and electronically excited, 42P1/2, states following the reactive deactivation of I(52P1/2) by Br2 was monitored by time-resolved resonance absorption spectroscopy. Observation of the absorption transient, corresponding to attenuation of resonance emission by atoms in the reaction vessel, demonstrates that the predominant products of these reactive collisions is Br(42P1/2), approximately 80% of the atoms being formed in the electronically excited spin-orbit state. The deactivation of I(52P1/2) by halogens is discussed in terms of a simple molecular orbital model which suggests that electronically adiabatic production of Br(42P1/2) proceeds following side-on attack of the iodine atom on the Br2 molecule.
Radiation damage mechanisms in single crystals of creatine monohydrate
Authors: Wells James W. , Ko Ching-Lung
Published: February 2003

ENDOR spectroscopy is utilized to define the temperature dependent sequence of molecular fragmentation processes occuring in x-irradiated single crystals of creatine monohydrate. Two conformations of the primary reduction product =OOĊ–C(H2) –N(CH)3–C(NH2)2+ are found to undergo a series of subtle changes before deamination. The resultant radical −OOC–ĊH2 then induces hydrogen abstraction to form a final room temperature product −OOC–ĊH–N(CH3) –C(NH2)+. An unknown initial oxidation species is found to decarboxylate forming the radical H2Ċ–N(CH3) –C(NH2)2+ which, although similar to the deamination product, exists at room temperature. The stability of this species is attributed to a delocalization of spin indicated by calculation and measurement.
Weak isotope effect in the condensed phase vibrational relaxation of a nonhydride molecule: NO(a 4Π)
Authors: Goodman J. , Brus L. E.
Published: February 2003

The 1933 Å ArF excimer laser has been used to directly time resolve the vibrational relaxation of 14N16O, 15N16O, and 14N18O into the v=0 B 2Π1/2 and v=0 a 4Π5/2 states in solid Ar host. A weak ’’conventional’’ istope effect is observed, with the smaller vibrational quanta tending to relax faster. However, the rates do not strictly correlate with either the isotopic reduced mass or the asymmetry of vibration. The host lattice phonons may directly accept energy during these relaxation processes. Intersystem crossing between v=0 B 2Π and v=8 a 4Π is shown to involve the individual spin orbit components of a 4Π.
Absorption of infrared radiation by a large polyatomic molecule: CO2 laser irradiation of disulfur decafluoride
Authors: Lyman John L. , Leary Kevin M.
Published: February 2003

Absorption cross sections, reaction probability, isotopic selectivity, and reaction product distributions have been measured for disulfur decafluoride (S2F10) irradiated with CO2 laser radiation. The absorption measurements covered a five-decade energy fluence range at ten laser frequencies in the region of the 938 cm−1 S2F10 absorption feature. S2F10 absorbs laser radiation much more easily than SF6 at similar infrared band position and energy fluence.
Accurate H3 dynamics on an accurate H3 potential surface
Authors: Walker Robert B. , Stechel Ellen B. , Light John C.
Published: February 2003

Converged quantum close coupling calculations for the hydrogen exchange reaction on the chemically accurate potential surface of Siegbahn and Liu, using the fit of Truhlar and Horowitz, are reported. Comparisons are made with the Porter–Karplus potential energy surface.
Simulation of n-butane using a skeletal alkane model
Authors: Weber Thomas A.
Published: February 2003

A simulation of a fluid n-butane system has been carried out using a straight-chain skeletal model. The calculations reported here involve fluid densities from 288.80 to 721.99 kg/m3 at several distinct temperatures. In the course of this investigation the linear self-diffusion constant, the rate of torsional gauche/trans relaxation and rotational tumbling of the fluid have been studied. This model shows a high degree of cooperativity between the molecular vibration and rotation, and the bulk fluid motion.
Dissociative photoionization of t-butyllithium
Authors: McLean W. , Murray P. T. , Baer T. , Jarnagin R. C.
Published: February 2003

Tertiary-butyllithium, Li4(t-bu)4, was examined by photoionization spectroscopy and optical absorption in the wavelength range 2000–950 Å. An absorption edge and ionization onset were found together at 6.2±0.25 eV. The ion Li4(t-bu)3+ and neutral isobutylene were found as photolysis products between the onset and 8.1±0.25 eV. Additional ionic fragments, Li4(t-bu)2+, Li4(t-bu)+, and Li2(t-bu)+ appeared at 8.1±0.25 eV. Hydrocarbon ions appeared above 11 eV.
The photodissociation lifetime of the van der Waals molecule I2He
Authors: Johnson Kenneth E. , Wharton Lennard , Levy Donald H.
Published: February 2003

The photodissociation lifetime of the van der Waals molecule I2He has been measured as a function of v, the iodine stretching vibrational quantum number. Lifetimes were deduced from the measured width of the R-branch head of the fluorescence excitation spectrum obtained in a supersonic free jet. The linewidth (HWHM) varied from 0.0120 cm−1 for v′=12 to 0.0700 cm−1 for v′=26, corresponding to lifetimes of 221 psec (v′=12) and 38 psec (v′=26). The inferred lifetimes were compared to calculated lifetimes provided by a recent theory of vibrational predissociation.
Molecular rotation and reorientation: Microscopic and hydrodynamic contributions
Authors: Hynes James T. , Kapral Raymond , Weinberg Michael
Published: February 2003

The relative roles of microscopic and hydrodynamic contributions to molecular rotation and reorientation are examined within the framework of the microscopic boundary layer theory recently proposed by the authors. The theory is applied to rough spheres, for which computer simulation data are available, and to experimental results on spherical top molecules. Attention is focused on rotational diffusion constants, the kappa parameter introduced by Kivelson et al., and orientational relaxation times. It is shown that, while collective effects are present and often nonnegligible, the motion of small molecules is dominated by its microscopic aspects. Experimental trends which can incorrectly suggest dominance by hydrodynamic contributions are discussed in some detail. Finally, the transition to the regime where collective effects are dominant is considered.
Crossed beam measurements of rotational quantum transitions in low-energy H+–H2 scattering
Authors: Schmidt H. , Hermann V. , Linder F.
Published: February 2003

Pure rotational excitation j=1→3, 1→5, 1→7 and vibrational excitation v=0→1, 0→2 have been measured in collisions of H+ with H2 at Ec.m.=4.67 and 6 eV. Transition probabilities for these processes are presented. Rotational excitation is the dominant inelastic process at Ec.m.=4.67 eV compared to vibrational excitation. At Ec.m.=6 eV the energy transfer to vibration and rotation is of comparable magnitude. The general characteristics of the scattering processes can be described by the semiclassical theory of Giese and Gentry. A quantitative comparison between experiment and the results of the calculations of Schinke and McGuire allows a detailed test of approximations made in their quantum mechanical treatment.
PNO-CI and CEPA studies of electron correlation effects. VI. Electron affinities of the first-row and second-row diatomic hydrides and the spectroscopic constants of their negative ions
Authors: Rosmus Pavel , Meyer Wilfried
Published: February 2003

Near equilibrium potential curves for the ground states of the first-row and second-row diatomic hydride negative ions have been calculated using PNO-CI and CEPA wavefunctions. The spectroscopic constants for the bound AH− ground states are given and the Franck–Condon factors for the processes AH−→AH are derived. The calculated intensity ratios support fully the assignments of the photoelectron spectra but the shifts in the re values which have been deduced from the observed intensity ratios are shown to have the wrong sign in most cases. The calculated electron affinities show systematic errors ranging from−0.15 eV to −0.35 eV. The yet unobserved BH− and and AlH− are predicted to be stable with estimated affinities of about 0.15 eV.
Vibrational relaxation of small molecules in the liquid phase: Liquid nitrogen doped with H2 or D2
Authors: Manzanares I. Carlos , Ewing George E.
Published: February 2003

Experimental results, using a pulsed laser technique to study the vibrational relaxation of nitrogen in solutions with H2 or D2 show that the system N2–H2 relaxes 100 times faster than N2–D2. The rate constants are kN2–H2= (1.5±0.2) ×105 and kN2–D2 = (1.8±0.4) ×103 s−1 (mol fraction)−1. From the experimental rate constants, transition probabilities 〈P〉N2–H2=9.0×10−9 and 〈P〉N2–D2=1.5×10−10 were calculated using a simple liquid model. This model treats the liquid as a dense gas and assumes that isolated binary collisions are causing the vibrational relaxation. Theoretical calculations of transition probabilities based on the first order distorted wave approximation were used to estimate the importance of V–T, V–T,R, or V–V channels to the relaxation of vibrationally excited nitrogen in the presence of H2 or D2. These calculations favor the V–T channel over the V–T,R channel for both N2–H2 and N2–D2 systems at 77 K. Relaxation of N2 by H2 is more effective than by D2 because collisions of the lighter isotopes occur with higher velocities. Relaxation by the V–V channel is unimportant in these systems. Our conclusions are consistent with those of others who have shown that the V–T channel is primarily responsible for vibrational relaxation for similar systems, CO–H2 and CO–H2 in the gas phase at low temperatures.
Kinetic theory of chemical reactions in liquids. II. Spatial nonequilibrium effects for a reversible reaction
Authors: Pagitsas Michael , Kapral Raymond
Published: February 2003

The effects of particle diffusion on the rate coefficients of a bimolecular isomerization reaction occurring in a dense fluid medium are investigated by kinetic theory. Emphasis is placed on the spatial nonequilibrium effects which occur as a result of the slow diffusive motion in a liquid, and the structure of the chemical relaxation time and rate coefficients when both forward and reverse rate processes are treated consistently. Since the kinetic theory calculations are microscopic and do not rely on a diffusion equation approach, a more accurate representation of the short time evolution in such reacting systems is obtained. Competition effects which occur at higher densities of the reacting species are also studied.
An application of the generalized Fokker–Planck equation to the dynamics of dilute polymer solutions
Authors: Adler Ronald S.
Published: February 2003

The nonlinear formalism developed by Zwanzig and Mori is utilized to derive a kinetic equation for the distribution of monomer phase space coordinates and a coarse-grained momentum density. Several simplifying approximations are then introduced into the exact kinetic equation. The resulting approximate description is shown to be closely related to the starting equations of the Freed–Edwards theory. The former differs, however, due to the presence of a non-Stoke’s frictional term which accounts for dissipation of monomer momentum fluctuations relative to the local velocity field of the solvent. Two applications of the approximate description are considered. A derivation of an equation for the two-time configuration space distribution function ψ (y, y′, t) is presented, where y denotes the collection of monomer position vectors. It is demonstrated that ψ (y,y′,t ) satisfies an equation similar to the Kirkwood–Riseman equation. Nonlinear couplings of the polymer distribution function to monomer momenta and the momentum density of the solvent lead to a diffusion tensor in which hydrodynamic interactions are characterized by a coarse-grained Oseen tensor. The correlation function formulation of the intrinsic viscosity proposed by Stockmayer et al. is extended to finite wavevectors and polymer concentrations. The specific viscosity is identified as the sum of two terms involving the mechanical contribution to the polymer momentum flux tensor and the diffusion current of chain segments.
Raman spectroscopic evidence for conformational deformation in the high pressure phase of polytetrafluoroethylene
Authors: Rabolt J. F. , Piermarini G. , Block S.
Published: February 2003

An investigation of polytetrafluoroethylene (PTFE), nC20F42, and a random copolymer of tetrafluoroethylene and hexafluoropropylene (TFE–HFP) under pressure has been carried out using Raman spectroscopy in conjunction with a diamond anvil cell. Both nC20F42 and the TFE–HFP copolymer were found to undergo a phase transition in the 7–9 kbar range similar to PTFE as evidenced by the observation of the 625 cm−1 Raman band characteristic of phase III. With increasing pressure in the 10–52 kbar range, a continual variation in intensity of the bands at 285 and 395 cm−1 was observed and found to differ for the three fluorocarbon materials studied. By correlating a change in the I285/I395 ratio with a change in sample crystallinity (as determined by x- ray), it has been determined that an increase in transplanar content with pressure is responsible for the observed band intensity changes in the high pressure phase of PTFE.
Selective deuteration and the anomalous temperature dependence of proton spin–lattice relaxation in polycrystalline p-t
e
r
t-butyltoluene
Authors: Banks Grace , Wei I. Y. , Johnson Charles S.
Published: February 2003

Proton spin–lattice relaxation times T1 are reported for p-t-butyltoluene (I); the derivatives having the toluene methyl group relaced with CH2D (II), CHD2 (III), and CD3, (IV); and the derivative having all of the ring positions deuterated (V). The measurements were made on polycrystalline samples in the temperature range 4–260 K at both 17.1 and 36 MHz. Raman spectra are also reported for these compounds at 77 K. All of the compounds show T1 minima between 100 and 200 K which are associated with motions of the protons in the t-butyl group and, except for compound V, with protons attached to the ring. Compounds I, II, and V exhibit two T1 minima below 10 K which are presumably associated with motions of the protons in the toluene methyl group. The Raman peaks do not permit the form of the potential function for internal rotation of this methyl group to be established. It is argued that a potential having both three and sixfold contributions is consistent with the low temperature T1 data. For compound II, which contains CH2D, the 17.1 MHz experiment gives frequency dependent T1 minima at 13.5 and 52.6 K which show apparent activation energies of 80±20 and 640±10 cal/mol, respectively. If these energies are associated with the torsional splitting E1−E0 and the activation energy Vmax−E0, respectively, the required ratio of V6 to V3 contributions in the potential function is 0.71.
The fluorescence excitation spectrum of chromyl chloride (CrO2Cl2) cooled in a free jet expansion
Authors: Blazy Joseph A. , Levy Donald H.
Published: February 2003

The fluorescence excitation spectrum of CrO2Cl2 in a free jet has been studied using a single mode tunable CW dye laser. Spectra were recorded for the A4 (origin), A5, and D5 bands. Sequence congestion has been entirely eliminated and detailed rotational structure of the bands observed for the first time. Assignment of the A4 origin band indicates the excited state to be B1 in vibronic symmetry under the C2v point group. The best least squares fit rotational constants are: A″=0.1073±0.0002 (cm−1); A′=0.0910±0.0003 (cm−1), B″=0.0620±0.0004 (cm−1); B′=0.0659+0.0003 (cm−1); C″=0.0521±0.0010 (cm−1); C′=0.0524±0.0010 (cm−1). Assignment of the D5 band indicates it is based on the same electronic transition but involves a symmetric vibration in the excited state. Possible geometries for the ground and excited states are also reported.
Relativistic electronics structure of PbS and PbSe molecules
Authors: Yang Cary Y. , Rabii Sohrab
Published: February 2003

The electronic orbitals of PbS and PbSe are calculated using a scattered-wave technique for the solution of the one-electron Dirac equation for a polyatomic muffin-tin potential. The resulting orbital transition energies are in good agreement with experimental absorption and emission data.
Pyroelectric 3La(IO3)3⋅HIO3⋅7H2O. Crystal structure of the transition metal iodates. V.
Authors: Abrahams S. C. , Bernstein J. L.
Published: February 2003

3La(IO3)3⋅HIO3⋅7H2O is pyroelectric and piezoelectric at room temperature and crystallizes in the orthorhombic space group Aba2 with four formulas in the unit cell. The lattice constants at 298 °K are a=13.18008(10), b=21.72076(10), c=12.15514(18) Å (for λ CuKα1=1.540562 Å). The integrated intensities of most reflections within a hemisphere of reciprocal space with radius (sinϑ)/λ=1.02 Å−1 were measured on two different crystals, each with a different diffractometer, resulting in 2742 symmetry-independent Fmeas for one set and 4038 Fmeas for the second set of structure factors. The crystal structure was solved by use of Patterson and Fourier methods, and refined by the method of least squares. The final agreement factor R=0.063 for the first set and 0.040 for the second, for models in which all atoms but hydrogen undergo anisotropic thermal vibrations. Normal probability plot analysis is used to compare the two sets of data and parameters. The absolute configuration of the crystal has been determined. Evidence for lattice damage caused by x irradiation is presented. The iodate ions form an oxygen-sharing network, with one independent La3+ occupying distorted bicapped, the other distorted equatorially-girdled, trigonal prismatic interstices. Each of the five independent IO−3 ions form triangular pyramids with three short I–O bonds of average length 1.803 Å: Three iodine atoms are associated with two next-nearest oxygen neighbors at 2.80 Å, one has three next-nearest neighbors also at 2.80 Å, and the fifth has four next-nearest neighbors at 2.91 Å, on average. An extensive O⋅⋅⋅O hydrogen bond network, with shortest distance of 2.619 Å, connects water molecules and iodate ions. Evidence for H3O+ formation by the iodic acid proton, and for the positions of the hydrogen atoms associated with the four independent water–oxygen atoms, is discussed.
Stochastic classical trajectory approach to relaxation phenomena. II. Vibrational relaxation of impurity molecules in Debye solids
Authors: Nitzan Abraham , Shugard Mary , Tully John C.
Published: February 2003

The stochastic classical trajectory approach is extended through introduction of a systematic class of phonon mode densities. Convenient algorithms are presented for generating the required random forces and damping integrals corresponding to mode spectra which approach, as closely as desired, the Debye spectrum. Extension to realistic mode densities involving irregular and discontinuous features is discussed. Application to vibrational relaxation of impurities in solids demonstrates that rates can depend sensitively on the structure of the phonon density of states, particularly at low temperatures.
Small angle neutron scattering by n-alkane chains
Authors: Yoon D. Y. , Flory P. J.
Published: February 2003

The angular dependence of neutron scattering by n-C16H34 and n-C36H74 in a medium having the approximate composition CD2 has been calculated from the positions of the actual scattering loci, the hydrogen atoms, on the basis of the rotational isomeric model for n-alkane chains. The theoretical calculations satisfactorily represent the experimental results of Dettenmaier for n-C36H74 and n-C16H34 both in the melt and in solution over the scattering range 0<μ?0.4 Å−1, where μ=4π/λ sin(ϑ/2). Thus, it is demonstrated that the configurations of short as well as long sections of n-alkane chains are not perceptibly perturbed by intermolecular interactions in the melt.
Single-particle motions in liquid water
Authors: Chu Shu-Yuan , Sposito Garrison
Published: February 2003

A physical picture of the single-particle motions in liquid water is developed as a generalization of the relaxing cage concept that has been employed to describe the single-particle motions in classical monatomic liquids. In the generalized picture, the solid-like cage that surrounds a given water molecule in the liquid at short times is assumed to exhibit several normal modes whose frequencies are essentially the same as those at resonance positions in the frequency spectrum of ice Ih. The coupling of these modes to all of the other normal modes in ice via a Hamiltonian that destroys the symmetry of the lattice is considered to be a principal cause of the decay of the resonance modes that produces the differences between the frequency spectra for ice and liquid water. A perturbation–theoretic method is applied to solve the classical equation of motion for the center-of-mass velocity of an H20 molecule in the liquid, as a formal representation of the physical picture developed. This approach produces an expression for the velocity autocorrelation function that is identical with the well known three-pole approximation if the cage surrounding a molecule in the liquid exhibits only one normal mode. In the general case of several cage normal modes, the model in its simplest form produces an expression for the velocity autocorrelation function that is a weighted sum of three-pole approximation terms, one for each cage normal mode. A numerical calculation using 18 observed resonance frequencies in the ice Ih density of states results in a frequency spectrum and velocity autocorrelation function that agree well with molecular dynamics estimates.
The infrared spectroscopy of chemisorbed molecules; a dynamical theory of the line shape
Authors: Metiu Horia , Palke William E.
Published: February 2003

A one dimensional model is used to examine the infrared line shape for the vibration of an atom chemisorbed on a solid. The calculation assumes that the mechanism of broadening is the coupling between the vibration of the atom and those of the lattice. It is shown that this mechanism cannot explain the large linewidths observed experimentally in case the solid is a metal. We believe that in this situation the broadening of the line is caused by the coupling between the vibrational motion of the chemisorbed molecule and the electronic degrees of freedom of the metal.
Thermally stable negative ions of polar molecules
Authors: Garrett W. R.
Published: February 2003

A simple molecular pseudopotential method is utilized to outline the strengths of permanent and induced moments of polar molecules which are sufficient to insure the existence of stable negative ions with specified electron affinities in the range of thermal collisional energies.
Measurements of the nitric oxide ion vibrational absorption coefficient and vibrational transfer to N2*
Authors: Bien Fritz
Published: February 2003

The NO+(v) absorption coefficient and quenching of NO+ vibration in the v=1 and v=2 states by N2 have been experimentally determined. Integrated absorption coefficients corrected for a thermal equilibrium distribution are: S1u(1–2 P 5) =430 cm−2 atm−1 and S1u(2–3 R2) =653 cm−2 atm−1. Quenching of NO+(v) by N2 at 297 °K was found to give rates of k=2.5×10−12 cm−3 sec−1 for NO+(v=2) and k=3.0 ×10−12 cm3 sec−1 for NO+(v=1). An estimate of 3 ×10−13 cm3 sec−1 was found for quenching of NO+(v=2) by NO at 297 °K.
Vibration relaxation time measurements via CRIIF (coherent Raman induced infrared fluorescence)
Authors: Devir A. D. , Bauer S. H.
Published: February 2003

The population of an excited, totally symmetric state can be enhanced via a raman transition by exposing the sample simultaneously to a pumping frequency (ωL) and a Stokes frequency (ωS), such that (ωL−ωS) precisely matches the desired transition. Subsequent transfer of excitation to dipolar states, either inter- or intra- (v,v), can be detected from risetime and decay of infrared fluorescence. Thus, energy transfer probabilities from directly pumped, preselected states, whether these are totally symmetric or not, can be measured. An experimental configuration to test this concept was set up using an oscillator–amplifier single pulse ruby laser both for ωL and for pumping an infrared dye to generate ωS. Representative results were obtain for the D2/HCl system. The basic requirements for power, linewidth matching, and spacial superposition were analyzed. We conclude that our experiments demonstrated the feasibility of using CRIIF to measure v→v transfer rates from directly pumped totally symmetric states. There is a possibility that, for homoatomic molecules, sufficient populations of excited vibrational states could be generated to test their enhanced chemical reactivities.
Rate constants for rotational excitation in NH3–He collisions
Authors: Davis Stephen L. , Boggs James E.
Published: February 2003

The semiclassical method (classical translational motion and quantum mechanical rotational motion) is used to calculate cross sections and rate constants for (J?3, K) → (J?4, K) rotational excitations in NH3–He collisions on Green’s uniform electron gas (Gordon–Kim) potential surface. The assumptions involved in the use of the rigid rotor approximation for the inversion motion are examined in some detail. Agreement between semiclassical and accurate close-coupling cross sections is good, comparable to the agreement between the coupled-states and the close-coupling cross sections. A ’’semiclassical coupled-states’’ approximation was found to give reliable cross sections except at low energies. Microwave double resonance ΔI/I values were calculated from the rate constants and compared with experiment. Good agreement was found for the Δk=3 transitions, but poor agreement for the Δk=0 transitions. Possible explanations for the discrepancies are discussed.
Negative ion properties of fluoranil, chloranil, and bromanil: Electron affinities
Authors: Cooper C. D. , Frey W. F. , Compton R. N.
Published: February 2003

Bound and excited negative ion states of p-fluoranil (FA), p-chloranil (CA), and p-bromanil (BA) are studied from experiments involving collisions of electrons, Na, K, and Cs beams with FA, CA, AND BA. Reaction thresholds for producing FA−, CA−, and BA− are determined as a function of the temperature of the target molecules. The electron affinities corresponding to 0 °K were measured to be 2.92±0.2 eV for FA, 2.76±0.2 eV for CA, and 2.44±0.2 eV for BA. FA, CA, and BA attach ∼ 0 eV electrons and FA attaches 0.5 eV electrons to form metastable parent negative ions with lifetimes in the millisecond range. These lifetimes are a function of the electron energy. Dissociative and nondissociative resonances observed via electron attachment to p-benzoquinone (PBQ) and the ’’anils’’ are correlated with the optically determined 2B3g′, 2B1u, and 2Au states of PBQ− and the anion radicals of several substituted quinones.
General method for calculating the escape probability in diffusion-controlled reactions
Authors: Tachiya M.
Published: February 2003

We present a general method for calculating the escape probability in diffusion-controlled reactions. This method is applied to several cases of practical interest, including the escape probability from a pair of reactive sinks.
Crossed molecular beam studies on the interaction potential for F(2P)+Xe(1S)
Authors: Becker C. H. , Casavecchia P. , Lee Y. T.
Published: February 2003

For the evaluation of the ground state XeF interaction potential, angular distributions of F atoms scattered off Xe were measured in crossed molecular beam experiments at collision energies of 2.11, 10.5, and 13.9 kcal/mole. F atoms, produced by thermal dissociation of F2 at 700 °C in a supersonic expansion using rare gas carriers, contain ∼78% F (2P3/2) ground state and ∼22% F (2P1/2) spin–orbit excited state. Consequently, three electronic states X1/2, I3/2 emerging from the 2P3/2 +1S0 asymptote, and II 1/2 from the 2P1/2+1S0 asymptote are involved in the scattering. A simple elastic approximation, neglecting interstate coupling, is used for the calculation of differential scattering cross sections in the evaluation of interaction potentials. Experimental results are found corroborating the spectroscopically derived potential of Tellinghuisen et al. (ε=3.359 kcal/mole, rm=2.293Å) for VX1/2(r), and the conclusion that VI3/2(r) and VII1/2(r) are in close resemblance to the ground state Ne+Xe interaction potential.
Magnetization modes and evolution matrices for some simple spin systems in anisotropic media
Authors: Werbelow Lawrence G. , Grant David M. , Black Edward P. , Courtieu Jacques M.
Published: February 2003

The time evolution of complete sets of coupled magnetization mode variables for the AX2, AX3, and X3 spin one-half systems are presented. The chosen modes prove to be the natural kinetic variables for free and partially constrained nuclear magnetic relaxation studies in both spacially isotropic and spacially anisotropic environments. The relationship between spacial anisotropy, the concomitant failure of time reversal symmetry, and the appearence of complex valued angular correlation time constants is also discussed. It is rationalized that the absence of time reversal symmetry necessitates the introduction of certain zero frequency phase coherence variables which linearly couple into the magnetization (population) mode variables. Thus, relaxation studies may indeed distinguishbetween degenerate irreducible multiplicities.
Time dependent formulation of polyatomic photofragmentation: Application to H3+
Authors: Kulander Kenneth C. , Heller Eric J.
Published: February 2003

We present a new approach to photofragmentation which through its explicit time dependence avoids the difficulties normally associated with calculation of Franck–Condon processes. No scattering eigenfunctions or Franck–Condon overlaps are required, yet the formulae are exact within the same assumptions as the usual time independent theory. Semiclassical implementation of the method is described in connection with a successful application to H+3, which involves, in our model, a multiple continuum breakup final state on one of two excited state surfaces treated. Full arrangement and energy partitioning information are given on both surfaces in a calculation whose main numerical effort involves running five classical trajectories.