bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized Langevin theory for inhomogeneous fluids: The transverse current–current correlation function
Authors: Grant Martin , Desai Rashmi C.
Published: February 2003

We consider a one component system of coexisting liquid and vapor phases with a diffuse planar interface and investigate the transport properties along the interface. We start with a formally exact, coupled set of generalized Langevin equations for the fluctuations in the number density, momentum density, energy density, stress tensor, and heat flux in an inhomogeneous fluid. We use these equations to calculate the transverse current–current correlation function RT for the liquid–vapor system. In contrast to homogeneous fluids, RT depends on the nonlocal viscoelastic properties (even in the long wavelength limit). These viscoelastic properties are related to ’’shearing’’ and ’’stretching’’ of the interface. The calculation of RT indicates a natural way of introducing nonlocal elastic moduli and nonlocal frequency dependent viscosities for inhomogeneous systems.
Abstraction vs exchange in the reaction of D+HCl
Authors: Miller John C. , Gordon Robert J.
Published: February 2003

The gas phase reaction of D with HCl was studied at room temperature by the method of flash photolysis, with resonance fluorescence detection of S, H, and Cl atoms. The D and Cl atoms were observed to decay exponentially, whereas no H atom signal from the exchange reaction was detectable. From the calibrated sen;sitiavity of the H atom detector, an upper limit of 0.002 was set for the ratio of rate constants for the exchange vs abstraction channels.
An a
b i
n
i
t
i
o determination of the rate constant for H2+C2H→H+C2H2
Authors: Harding Lawrence B. , Schatz George C. , Chiles Richard A.
Published: February 2003

The first ab initio determination of the rate constants for the reaction of ethynyl radical C2H with molecular hydrogen is presented. The potential energy surface was determined in the seagent and Saddle point regions using configuration interaction methods and the rate constant was evaluated using transition state theory. (AIP)
A space-fixed sudden approximation for total differential cross sections
Authors: Siska P. E.
Published: February 2003

A high-order space-fixed sudden approximation based on the theory of Cross is presented and shown to give improved agreement with exact close coupling results for low-energy He–CO atom-rigid rotor scattering, when compared with the infinite-order-sudden (IOS) approximation. (AIP)
A simple model for the influence of fluctuations on explosive reactions
Authors: Rigos A. A. , Deutch J. M.
Published: February 2003

A simple stochastic model is approximately treated by use of a mean passage time argument to examine the influence of fluctuations on explosive behavior. (AIP)
Effect of reaction path curvature and dimensionality on the accuracy of classical transition state theory
Authors: Miller William H.
Published: February 2003

Using a simple dynamical approximation based on the reaction path Hamiltonian of Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)], it is shown how reaction path curvature and dimensionality affect the accuracy of classical transition state theory. Specifically, an explicit expression is obtained for the critical energy up to which classical transition state theory is exact, and this critical energy is seen to vary inversely with the curvature of the reaction path. The model also shows that for energies above this critical energy transition, state theory deviates less from the correct result the larger the dimensionality (i.e.,the number of degrees of freedom) of the system. Finally, for the canonical (i.e.,thermally averaged) rate constant, as opposed to the microcanonical case, the fractional error in transition state theory (i.e.,the transmission coefficient) is found in certain cases to be independent of dimensionality.
Transfer and quenching rate constants for XeF(B) and XeF(C) state in low vibrational levels
Authors: Brashears H. C. , Setser D. W.
Published: February 2003

The relative XeF(B–X) and XeF(C–A) emission intensities from the steady-state vacuum ultraviolet photolysis of XeF2 have been used to measure the B–C transfer and quenching rate constants of XeF molecules in low vibrational levels. The rare gases N2, CF4, SF6, F2, NF3, CF3H, CF3Cl, HF, CO2, and XeF2 were investigated as buffer gases at room temperature. The transfer rate constants are much larger than the quenching rate constants for He, Ne, Ar, Kr, N2, CF4, and SF6. For Xe, NF3, CHF3, and CClF3 transfer is only 2–4 times faster than quenching and for F2, HF, and CO2 quenching is faster than B–C state transfer. Quenching for XeF(D) was studied for rare gases and for N2. No convincing evidence was found for three-body quenching by the rare gases and their quenching of the XeF(B, C) and XeF(D) states are reported as two-body processes for pressures below ∼5 atm. The XeF(D) quenching rate constants are of the same magnitude as the B–C state transfer rate constants. The photochemical and collisional (metastable rare gas atom) dissociative excitation of XeF2 and KrF2 are summarized in the Appendix.
Reactive scattering and electron detachment in collisions of halogen negative ions with isotopic hydrogen molecules
Authors: Huq M. S. , Fraedrich D. S. , Doverspike L. D. , Champion R. L. , Esaulov V. A.
Published: February 2003

Total cross sections for reactive scattering and collisional electron detachment have been measured for collisions of F− and Cl− with H2, D2, and HD. The collision energy extends from below the energetic thresholds for the processes studied up to a laboratory energy of about 300 eV. Reactive scattering is found to be the dominant inelastic channel for the case of the F− projectile. Isotope effects are observed in all cross sections. Electron detachment of F− is found to occur by two distinct mechanisms. A striking difference in the reactive and detachment cross sections is observed when Cl− is substituted for F− in that the electron detachment cross section is generally larger than that for reactive scattering. As in the F− case, isotope effects are also present for the Cl− projectile. The isotope effects observed for electron detachment of Cl− suggest that detachment cannot be described by any unique mechanism.
Mechanism and kinetics of excited-state relaxation in internally hydrogen-bonded molecules: 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole in solution
Authors: Huston Alan L. , Scott Gary W. , Gupta Amitava
Published: February 2003

We report the ground state absorption recovery kinetics (at 355 nm) and the fluorescence spectra and kinetics of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole (I) following excitation at 355 nm in several solvents at room temperature. Measured lifetimes range from 14 to 75 ps. Fluorescence lifetimes are shorter than the corresponding ground state recovery times, indicating the intervention of intermediate forms during the excited state relaxation process. The measured fluorescence quantum yield is lower than one predicted by the usual calculated radiative rate for the near UV, strongly absorbing singlet state of this molecule. Decay rates are slower in ethanol than in nonhydrogen bonding solvents indicating that external interference with the intramolecular hydrogen bond in (I) slows the relaxation rate. The room temperature decay rates are not strongly affected by the solvent viscosity. Deuteration of the molecule produces only a slightly more rapid ground state recovery rate than in the protonated species. A model involving excited state proton transfer is presented for the decay mechanism, rationalizing the known experimental data.
Dynamical constraints on the transfer of angular momentum in rotationally inelastic collisions of I2(B 3Π) with He and Xe
Authors: Dexheimer S. L. , Durand M. , Brunner T. A. , Pritchard David E.
Published: February 2003

We have measured thermally averaged level-to-level rate constants for rotationally inelastic collisions of I2(B 3Π) with Xe and He using the method of laser induced fluorescence. We present the values of 149 rate constants with average errors less than 10% for initial rotational levels ranging from 41 to 113 in the v′ = 13 vibrational level. For I*2–He, we report the first experimental evidence for a limitation of maximum angular momentum transfer on level-to-level rate constants and conclude that dynamical, rather than energetic constraints govern the collisional process. We find that the I*2–Xe rate constants follow a power law in both energy and angular momentum transfer, a behavior observed in most previous experimental work on rotationally inelastic collisions.
Rigid bender model for the out-of-plane vibrations of C
s and C2v tetra-atomic molecules
Authors: Clouthier D. J. , Goddard J. D.
Published: February 2003

A formula for the evaluation of the mass-weighted coordinate Q for out-of-plane bending which is applicable to Cs or C2v tetra-atomic molecules has been derived. This formula is tested numerically against earlier, more approximate methods of determining Q. The effects of approximations for Q on the vibrational energy levels of a double minimum potential and on the determination of the equilibrium out-of-plane angle are examined. The present formula is exact within the rigid bender constraints and is more widely applicable than previously published methods.
Dynamics of proton migration in free base porphines
Authors: Sarai Akinori
Published: February 2003

We investigated the proton tautomerization in free base porphines by considering the dynamical coupling between the proton migration and the skeletal motion. We estimated the conformation change of the skeleton due to the tautomerization. The estimated geometry of the skeleton in the linear proton configuration indicated that the large skeletal deformation from D4h symmetry is mainly caused by the change in the valence structure due to the proton configuration change. The strong vibrational coupling indicates that the deformation motion of skeleton will destroy the coherence of the proton migration. Thus, the proton migration should be considered as an incoherent hopping process. A quantum mechanical model which incorporates the effect of skeletal vibrations was described, and the temperature dependence of the proton migration was discussed in detail. The result was in agreement with the previous suggestions [A. Sarai, Chem. Phys. Lett. 83, 50 (1981)] that a considerable part of the activation energy is contributed by the skeletal vibrations, and that the proton migration should include the bending motion as well as the stretching of N–H. The quantum mechanical study also indicated that the synchronous proton migration, in which the central two protons migrate simultaneously, is a dominant process at low temperatures, but the asynchronous process, in which the protons migrate in a successive way, can make considerable contribution to the proton migration at high temperatures.
The motion of charged colloidal particles in electric fields
Authors: Teubner Max
Published: February 2003

General formulas are derived for the force and torque on a charged rigid macromolecule in electric fields. The theory is based on the Smoluchowski mean field approximation and yields the motion in terms of the electric potential and certain geometric quantities of the particle. If convection can be neglected in the ion conservation equations, the theory makes any further hydrodynamic considerations unnecessary. The formulas of Smoluchowski, Hückel, and Henry for electrophoretic mobility are obtained as simple consequences. Smoluchowski’s formula is generalized to nonconstant surface potentials. The torque on an uncharged rod in moderately strong field is considered.
Thermodynamics of liquid cesium–cesium halide mixtures at high temperatures
Authors: Yokokawa Harumi , Kleppa O. J.
Published: February 2003

The molar enthalpies of mixing of cesium with cesium fluoride, chloride, and iodide have been measured by transposed temperature drop calorimetry at 925 K. For these systems, the enthalpies of mixing can be expressed empirically by the relation ΔHM = x(1−x) (a+bx) where x is the mole fraction of cesium metal. The parameters a and b were found to have the following values: In order to obtain entropies of mixing, the enthalpies were compared with Gibbs energies derived from available cryoscopic data. The enthalpies and excess entropies were found to exhibit very similar trends : (1) The enthalpy and excess entropy both show the strongest asymmetry in Cs–CsF, i.e., in the system which has the smallest halide ion; (2) The limiting value of the partial properties of cesium in cesium halide increases with decreasing size of the halide ion; (3) The limiting value of the partial properties of cesium floride in cesium is very low compared to the other cesium halides. From the thermodynamic properties the concentration fluctuation properties have been derived. A detailed analysis has been made of the entropies of mixing in terms of the ionic configuration. This analysis indicates a close relation of the ionic configurational entropy to the shielding properties due to electrons and ions.
Statistical rate theory of interfacial transport. III. Predicted rate of nondissociative adsorption
Authors: Ward C. A. , Findlay R. D.
Published: February 2003

The statistical rate theory approach is used to derive the expression for the rate of adsorption of a nondissociating gas on a solid surface. By comparing the predicted adsorption with the available experimental results, one of the central assumptions in the statistical rate theory can be examined. On the basis of these experimental results, with which the theoretical predictions are found to be in close agreement, the assumption would appear to be valid. By being in close agreement with the experimental results throughout the experimental period, the adsorption rate expression derived from the statistical rate theory is an improvement over the presently available theories.
Statistical rate theory of interfacial transport. IV. Predicted rate of dissociative adsorption
Authors: Findlay R. D. , Ward C. A.
Published: February 2003

The statistical rate theory approach is used to derive the expression for the rate at which a diatomic gas dissociatively adsorbs on a solid surface. By comparing the predictions with available experimental evidence it is possible to examine one of the central assumptions in the statistical rate theory. The predictions were found to be in close agreement with two independent sets of experimental results; thus giving support to the basic assumption of the statistical rate theory. The close agreement extended throughout the experimental period; thereby making this approach an improvement over the presently available theories.
The origin of H+ in electron stimulated desorption of condensed CH3OH
Authors: Stockbauer Roger , Bertel Erminald , Madey Theodore E.
Published: February 2003

It is observed that low energy electron excitation of CH3OH results in H+ ejection due to C-H bond clearage. (AIP)
Force constants for the ethylene molecule developed from the Green’s function technique
Authors: Crowder C. D.
Published: February 2003

The Green’s function reported by wolfram and Asgharian has been used to obtain force constants for the ethylene molecule in Cartesian coordinates. The internal coordinate force constants are also discussed. (AIP)
Selective summation in generalized perturbed Hartree–Fock theory
Authors: McDowell Keith
Published: February 2003

An exact relationship between an arbitrary but well defined set of spin orbitals and the Hartree–Fock spin orbitals is derived using the techniques of diagrammatic many-body perturbation theory. The resulting nonlinear integral equation is appropriately linearized and the exact Hartree–Fock energy is developed as an expansion in the linearized approximation. The expansion is found to be in agreement with previous work which was based on the selective summation of third-order topologies to infinite order. In addition, a general form of coupled perturbed Hartree–Fock is obtained which uses the arbitrary choice of spin orbitals as the reference set.
Stability of doubly charged alkali halide clusters
Authors: Martin T. P.
Published: February 2003

The total energy surfaces of doubly ionized alkali halide clusters have been investigated assuming purely ionic, two-body interactions. All clusters having the form NanCln−2++ are found to be at least metastable. However, for n<10, binding energy can be gained (after overcoming a barrier) on losing a Na+ ion.
Free volumes and liquidlike clusters in soft-core dense liquids and glasses
Authors: Hiwatari Yasuaki
Published: February 2003

With the particle configurations generated by molecular dynamics calculations for the soft-core system, free volumes of each particle are calculated in dense, supercooled liquids and glasses. These free volumes, the size distributions are used to divide the particles into two classes, the solidlike and liquidlike particles. The fraction p of the liquidlike particles and the connected liquidlike clusters with neighboring liquidlike particles are examined. Average cluster size and liquidlike neighbors are obtained as a function of p. It is shown that liquid–glass transition is associated with the decrease of p with increasing reduced density and the transition, is described by a type of percolation transition, which occurs at about pc?0.2 (critical concentration).
New measurements of the ion content of evaporation-humidified air
Authors: Carlon Hugh R.
Published: February 2003

New measurements by electrical conductivity of the ion content of untreated room air humidified by evaporating liquid water in a closed container are presented. Large populations of the singly charged hydrated ions can be maintained in the container for an hour or more after the liquid water evaporation source is removed. The new results suggest that evaporation produces large populations of neutral molecular clusters of many water molecules, and that the neutral clusters dissociate to a small extent into ions that can be monitored by electrical conductivity using cells of a new design to monitor the temperature and humidity dependencies of the neutral cluster populations. Data are presented showing ion population function versus saturation ratio (fractional relative humidity) and time at near-ambient temperatures. Ion population is very dependent on the saturation ratio and time of contact of the vapor with liquid water. This strong dependence impacts accepted electrostatic and insulator theories. Data also are presented from experiments in which ion content was measured in fully saturated moist air at temperatures to 50 °C, where the air was humidified by steam and ion populations in excess of 109/cm3 were observed.
On the number of distinct sites visited in 2D lattices
Authors: Henyey Frank S. , Seshadri V.
Published: February 2003

We present analytic results for the asymptotic behavior of Sn, the number of distinct sites visited in an n-step random walk on two-dimensional lattices using a combination of contour integration and saddle point techniques. We obtain results that agree very well with exact results (n?500) and numerical simulation (n∼105).
Photoemission and electron mean free paths in liquid formamide in the vacuum UV
Authors: Birkhoff R. D. , Heller J. M. , Painter L. R. , Ashley J. C. , Hubbell H. H.
Published: February 2003

Our apparatus has been modified to permit measurements of absolute photoemission as well as reflectance of liquids in the vacuum UV. Photon fluxes are determined by filling the tandem ion chambers with argon. An electrometer is connected to the sample cup, which must be properly biased, to read electron currents leaving the cup. After argon ion drift into the cup is taken into account, the yields of photoelectrons per incident photon and per absorbed photon are obtained. These yield curves for liquid formamide (HCONH2) are similar to each other over the energy range studied (16–25 eV). They are nearly constant between 16 and 18 eV at 13.5% and decline toward higher energies. The surface and volume plasmon energies for formamide are near 19 and approximately 27 eV, respectively. We suggest that photoelectron excitation of surface plasmons may enhance the photoemission yield, while excitation of volume plasmons may reduce it. Values of electron mean free path derived from the yields and a simple three-step model for photoemission are found to be about two to three times larger than predicted by theory.
Laser optogalvanic study of HCO Ã state predissociation
Authors: Vasudev R. , Zare R. N.
Published: February 2003

Widths of lines in the (0, 90, 0)–(0, 01, 0) band of the HCO ? 2A\–? 2A′ transition are recorded using a single-mode dye laser, revealing a strong dependence on the rotational quantum number N. The HCO radical is produced in a radio frequency discharge in acetaldehyde (CH3CHO) vapor and the ?–? transition in the 570–630 nm region is detected by laser-induced changes in the discharge impedance. The variation of widths with N is interpreted in terms of Coriolis interaction and K-type resonance of the (0, 90, 0)Σ(K′ = 0) level with nearby strongly predissociated Π(K′ = 1) and Δ(K′ = 2) levels, respectively, of the upper state.
Rapid convergence of V–V energy transfer calculated using adiabatic basis functions. I. An accurate two-state model for low-energy resonant V–V energy transfer. II.
Authors: Thirumalai Devarajan , Truhlar Donald G.
Published: February 2003

We report a series of calculations designed to show the efficiency of adiabatic basis sets for V–V energy transfer in diatom–diatom collisions. We do find that these basis sets are very efficient for both symmetric and nonsymmetric cases. For resonant V–V energy transfer at low energies we obtain quantitative accuracy with a two-state model. The only dynamical calculations required for this model are two phase shifts.
Photodissociation and photodesorption line shapes
Authors: Casassa Michael P. , Celii Francis G. , Janda Kenneth C.
Published: February 2003

Photodissociation line shapes are derived based on two-level-plus-decay models. The simplest model involves only dissociative decay of the upper level and applies to laser-induced vibrational predissociation of van der Waals molecules. A more general model which includes quenching processes that compete with dissociation is presented in a form applicable to laser-induced desorption. Both models yield insightful line shape expressions with explicit dependence on laser power and frequency, irradiation time, absorption strength, and phenomenological rate constants. Implications of the line shape formulas for several recent experiments as well as feasibility of studying more complex systems are discussed.
First passage times and the kinetics of unimolecular dissociation
Authors: Carmeli Benny , Nitzan Abraham
Published: February 2003

Approximate solutions for multistep master equations describing the time evolution of product formation in multiphoton or thermal unimolecular reactions are investigated. In particular, a method based on fitting the first few moments of the passage time distribution associated with the given stochastic process to proposed simple expressions for the product yield function is studied. It is shown that reasonable agreement with the exact numerical solution of the corresponding master equation is obtained with a two parameter fit (using two passage time moments) and an excellent agreement is obtained with a three parameter fit (using three passage time moments). In no case studied does a need arise for more than a three-moment description and the quality of available experimental results makes the simpler two-moment description sufficient in most cases. Analytical solutions for the first and second passage time moments are obtained for simple discrete and continuous master equation models. Expressions for the incubation time and the reaction rate are obtained in terms of these solutions. The validity of discretizing a continuous master equation (which is an important simplifying step in evaluating the time evolution associated with multiphoton dissociations in the presence of collisions, or with thermal unimolecular reactions involving large molecules) is studied using both the approximate two-moment solutions and exact numerical solutions. It is concluded that a proper discretization of a continuous master equation may be carried out provided ε≪kBT, where ε is the discretization energy step, kB the Boltzmann constant, and T the effective (density of states weighted) temperature. A larger discretization step can be used if only the incubation time is required. Using the approximately discretized master equation, we next calculate the effect of collisions on the incubation time and the rate of multiphoton dissociation using a model constructed to correspond to the unimolecular dissociation of tetramethyldioxethane. Incubation times are found to be less sensitive to collisions then the reaction rates. Finally, we investigate the applicability of the passage time moments method to describe the time evolution of product formation in a system whose dynamics is determined by a quantum mechanical Liouville equation. Again the two-moment description provides a reasonable and the three-moment approximation a good approximation to the exact solution. The three-moment approximations, however, cannot be used when the pressure (i.e., the dephasing rate) is too low.
Laser absorption measurements of OH, NH, and NH2 in NH3/O2 flames: Determination of an oscillator strength for NH2
Authors: Chou M. S. , Dean A. M. , Stern D.
Published: February 2003

We have used laser diagnostics to probe atmospheric pressure ammonia–oxygen flames. Absorption from a tunable dye laser was used to measure concentration profiles of OH, NH, and NH2 radicals at fuel equivalence ratios φ of 1.28, 1.50, and 1.81. The absolute concentrations of OH and NH and the product of the NH2 concentration and the NH2 oscillator strength were derived as a function of height above the flat flame burner. These measurements indicate that the reaction NH2+OH?NH+H2O is equilibrated near the flame front, as well as in the post flame region. It was possible to use these measurements to derive an oscillator strength fi = (2.04±0.44)×10−4 for the PQ1,7 line in the (0,9,0)←(0,0,0) vibrational band of the A 2A1←X 2B1 transition of NH2. Rotational temperatures of OH were obtained from absorption measurements on a variety of rotational lines. These temperatures suggest the possibility of excess rotational energy in OH in the flame front regions of the φ = 1.28 and φ = 1.50 flames.
Nascent metastable products in the reactions Ba+NO2 and Ba+N2O
Authors: Hsu Yen Chu , Pruett J. Gary
Published: February 2003

The chemiluminescent reactions Ba+N2O and Ba+NO2 are studied using laser-induced fluorescence under molecular beam and low pressure flow conditions. The studies indicate the nascent electronic branching between chemiluminescent and metastable levels for the two reactions. In both reactions the predominant product channel is the ground electronic state. For the more exothermic Ba+N2O reaction, the exothermicity is mainly deposited into vibrational and rotational excitation of ground electronic state levels which exceeds the energies of the first few excited electronic states of BaO. Excited electronic states which are populated are also vibrationally and rotationally hot. For the Ba+NO2 reaction the predominant ground state products are vibrationally cold, but rotationally hot. Excited states produced are both vibrationally and rotationally cold. Selective production of particular metastable electronic states is seen.
Vibrational relaxation of CH3F in inert gas matrices
Authors: Young Linda , Moore C. Bradley
Published: February 2003

The deactivation mechanism of the CH stretching fundamentals and bending overtones in matrix isolated CH3F has been determined for dilute samples as a function of host and temperature using infrared laser excited vibrational fluorescence. The decay mechanism is the same for all hosts; depopulation of any of the levels near 3000 cm−1 occurs via rapid (<5 ns) V→V transfer to the CF stretching overtone level 2 ν3. Subsequent stepwise relaxation occurs from 2ν3 to ν3 and from ν3 to ground. Decay rates of 2ν3 and ν3 have been determined through temporal resolution of 1050 and 2100 cm−1 fluorescence, respectively. These rates show a dramatic dependence on host lattice, an increase of two orders of magnitude in going from Xe to Ar matrices. Lifetimes depend only weakly on temperature.
Statistical energy partitioning in dissociation to several products
Authors: Baer Tomas , DePristo Andrew E. , Hermans J. J.
Published: February 2003

Energy partitioning in dissociation of a highly energized molecules or ion to several fragments is discussed in the framework of the statistical theory. Linear momentum conservation is incorporated and the microcanonical distribution function is replaced by the canonical one. This yields the momentum and internal state probability distribution functions of the fragments which in turn provides a simple relation for the average kinetic energy of each fragment for the case of a simultaneous dissociation to several products. The resulting average kinetic energies are compared with those of a sequential dissociation to the same final products. Significant differences are noted which makes it possible to distinguish experimentally between sequential and simultaneous dissociation.
Interrupted coarse-grained theory of unimolecular relaxation and stimulated recurrences in photoexcitation of a quasicontinuum
Authors: Yeh J. J. , Bowden C. M. , Eberly J. H.
Published: February 2003

We present an approximate analytic theory of photoexcitation of a quasicontinuum. The theory accounts for unimolecular dipole dephasing relaxation and partial dipole recorrelations. It is based on systematic repeated interruption of coarse-grained dynamical evolution of the system’s wave function. We predict sign reversals and discontinuities in transition rates, and other quasiperiodic phenomena. The theory gives fair agreement with computer simulations and reproduces features of two known exactly soluble models.
Laser intensity dependence of mulitphoton excitation vs collisional relaxation in CF2HCl and CF2CFCl
Authors: Stephenson John C. , Blazy Joseph A. , Li Chang-lin , King David S.
Published: February 2003

CO2 laser pulses for which the intensity vs time profile is rectangular (10 or 50 ns duration) were used in the multiphoton excitation of CF2HCl and CF2CFCl dilute in high pressure (400 Torr) argon. Energy deposition was measured by optoacoustic detection and CF2 product yield by laser-excited fluorescence. Even at low yield (e.g., 10−3) more than 100 photons were absorbed per CF2CFCl molecule, while for an identical yield, CF2HCl absorbed 200 times less energy. For the same laser fluence, the higher intensity 10 ns pulses gave more yield from CF2CFCl (factors up to 500 were observed) than the less intense 50 ns pulses; for CF2HCl, the two intensities give the same yield. For both molecules, the two intensities gave the same optoacoustic signal for a given fluence. These results are related to the dependence on reactant energy of the competing ratio of collisional deactivation to laser excitation.
Photochemical dynamics of 79Br2 in Xe matrices
Authors: Beeken Paul , Mandich Mary , Flynn George
Published: February 2003

The photochemical dynamics of Br2 trapped in a Xe matrix have been investigated. Following excitation into either the B 3Π(0+u) or 1Π(1u) states, emission occurs in the far red and infrared region. Analysis of the emission allows the emitting states to be characterized as B 3Π(0+u) v′ = 0→X 1Σ+g, and (tentatively) the A′ 3Π(2u)→X1Σ+g. The spectrum of the B→X emission was recorded and exhibited unusually broad vibronic emission lines when compared with Ar spectra. The B→X emission lifetime was measured to be 3.6±0.5 μs and is in excellent agreement with the expected value for the radiative lifetime. The risetime of the B signal was found to be limited by the detection system (∼15 ns). The tentatively identified A′→X emission has a lifetime of 4±1 ms. The A3Π(1u)→X 1Σ+g emission resolved clearly in Ar and Kr matrices is altogether absent from Br2 isolated in Xe. Experiements were performed to measure the relative intensities of the B/A′ emissions in Xe and compare them with earlier results from Ar matrices. The Xe B/A′ ratio was found to be nearly 100 times greater in Xe then in Ar. The observations were found to be independent of temperature from 12 to 60 K and independent of concentrations from 1:500 to 1:10 000, Br2 in Xe.
Proton magnetic resonance study of classical reorientations and tunneling in LiNH4SO4
Authors: Reynhardt E. C. , Watton A. , Petch H. E.
Published: February 2003

Nuclear magnetic resonance studies have been made of the protons and lithiums in LiNH4SO4 at temperatures below 285 K. The proton line shape at 4.2 K has shown that tunneling rotation of the NH+4 ion around one of its C2 axes is favored over the others. Additional tunneling around threefold axes is also present. The ground torsional level structure of the NH+4 ion has been determined. The classical reorientations of the two nonequivalent NH+4 groups can be described as simultaneous two and threefold reorientations with estimated activation energies ranging from 1.0 to 1.6 kcal/mol. Slight differences in activation energies have been observed during cooling and heating cycles below 133 K. The 7Li relaxation time is determined to a large extent by the reorientations of the NH+4 ions. Evidence has been found that the phase transition at 133 K is accompanied by a slight rearrangement of the Li–O–S skeleton.
Determination of the rate parameters and products for the reaction of hydroxyl radicals with nitric acid
Authors: Jourdain J. L. , Poulet G. , Le Bras G.
Published: February 2003

The discharge flow-EPR technique has been used to measure the rate constant and the product distribution for the reaction of OH with HNO3, which is important for stratospheric modeling. The rate constant is found to be (7.3±2)×10−15 exp[876±85)/T] cm3 molecule−1 s−1 in the range 251–403 K. Besides, the in situ titration of NO3 by NO has shown that the reaction of OH with HNO3 only produces NO3 and H2O at room temperature.
Frequency dependence study of the proton spin relaxation in the re-entrant nematic phase
Authors: Dong Ronald Y.
Published: February 2003

Proton T1 measurements are reported for two 60CB/80CB mixtures as functions of temperature and Larmor frequency. The frequency and temperature dependences of T1 support that different relaxation mechanism is responsible for spin relaxation in the nematic and in the re-entrant nematic phase. While the molecular self-diffusion is effective in the nematic phase, the orientational order director fluctuations play a dominant role in the re-entrant nematic phase.
X-ray absorption studies of Cu(en)2+2 exchanged Y zeolite
Authors: Morrison T. I. , Shenoy G. K. , Iton L. E. , Stucky Galen D. , Suib Steven L.
Published: February 2003

X-ray absorption studies have been performed on Cu(en)2+2 exchanged Y zeolite. The copper EXAFS shows that there is little change in the equatorial copper-ligand bond lengths, but the XANES is indicative of axial distortions. Both the EXAFS and XANES can be interpreted to show that the complex remains intact upon exchange and that it is structurally and electronically similar to the bis(ethylenediamine) copper (II) complex in solutions.
Beam maser spectroscopy on CH3CCD and CD3CCH
Authors: Cogley C. D. , Tack L. M. , Kukolich S. G.
Published: February 2003

The J = 1→0 transition frequencies in CH3CCD and CD3CCH were measured using a beam maser spectrometer. Hyperfine structure from deuterium quadrupole coupling was well resolved. The measured deuterium quadrupole coupling tensor components along the symmetry axis are eQqaa = 228±2 kHz for CH3CCD and eQqaa = −55.0±0.5 kHz for CD3CCH. The coupling strength along the bond direction for CD3CCH is eQqzz = 174±6 kHz. These results are compared with the other deuterium quadrupole coupling values. Measured rotational constants are B = 7 778 170±1 kHz for CH3CCD and B = 7 355 767±1 kHz for CD3CCH.
Topological rubber elasticity theory. I. Basic theory
Authors: Iwata Kazuyoshi
Published: February 2003

This theory is based on an assumption that topological states of a network are determined by a set T0 of the Gauss linking coefficient (GLC) defined for all the possible pairs of loops in the network. Since elements of T0 are not linearly independent, a complete basic subset T1 of T0 is chosen as a set of independent topological invariants of the system. An equilibrium distribution function Φ(r, ϑ), in regard to r, a set of the all coordinates of junction points, and θ a set of the all GLC for strand pairs in the network, are computed first using the topological moment method presented in a previous paper [K. Iwata and T. Kimura, J. Chem. Phys. 74, 2039 (1981)], and it is transformed into Ψ(r, τ), a distribution function in regard to r and τ, a value of T1. In the calculation of free energies, it is assumed that (1) the network is composed of identical cells, each of which contains sufficiently large number of strands and junction points; (2) the network is periodic in regard to the cells; and (3) relative displacement of weight centers of the cells under macroscopic deformation is exactly affinelike. The free energy of the system under macroscopic deformation F̃(λ, r0) is defined by F(λ, τ) = kT ln∫Ψ(r, τ)dr, F̃(λ, r0) = ΣτF(λ, τ)Ψ(τ,‖r0),where λ is a set of parameters for macroscopic deformation, r0 is the position of the junction points at the time of network formation, and Ψ(τ‖r0) is a distribution function of τ under the condition that r is fixed at r0; the integration in regard to r is performed with restrictions (1), (2), and (3) stated above so that F̃(λ, r0) becomes a function of λ. The free energy F̃(λ, r0) consists of F̃e, an elastic free energy of the network, and ΔFm, an excess free energy of mixing. F̃e is composed of three terms F̃0, F̃1, and F̃2. The zeroth term F̃0 is an elastic free energy of a network in which the all junction points deform affinley and θ is fixed at its initial value θ0 at the time of network formation. The first term F̃1 is a correction due to nonaffine displacement of the junction points. The second term F̃2 is a correction due to fluctuation of θ. The zeroth term F̃0 is further divided into F0,ph, a term coming from the entropic force acting between ends of the strands and F̃0,top, a term coming from the topological interaction among the strands. It will be shown in the following paper (part II), where the present theory will be applied to SCL networks, that the principal term of the elastic free energy is F̃0,top.
Topological rubber elasticity theory. II. SCL networks
Authors: Iwata Kazuyoshi
Published: February 2003

The theory presented in part I [Iwata, J. Chem. Phys. 76, 6363 (1982)] is applied to networks having a simple-cubic-lattice (SCL) regular connection pattern, for which the projection matrix Γ* is computed easily. Derivatives of elastic free energies in regard to parameter λ for macroscopic deformation ∂F̃e/∂λ are computed numerically for isotropic deformations (swelling or deswelling) and for simple deformations (extension or contraction under swelling by α times). The initial arrangement of junction points r0 is assumed to be exactly SCL, and δ = d0/νb is chosen as one of parameters in the calculation, where d0 is an end-to-end distance of the strands at the time of network formation, ν is a degree of polymerization in regard to the strands, and b is a statistical length per monomer. A repeating cell is chosen as a cube composed of 3×3×3 ( = 27) junction points and 3×27 ( = 81) strands. The following are found in this work. (1) Among four terms ∂F0,ph/∂λ, ∂F̃0,top/∂λ, ∂F̃1/∂λ, and ∂F̃2/∂λ of the derivative of the elastic free energy, the principal term is ∂F̃0,top/∂λ, which comes from the topological interaction among the strands; the phantom network term ∂F0,ph/∂λ is only a small correction to the net stress. (2) In isotropic deformations, the elastic free energy takes a minimum at λ0, a little below λ = 1; for compression below λ0, a strong postitive inner pressure, which comes from the topological repulsive forces among the strands, arises. (3) In simple deformations, the Mooney–Rivlin term appears for unswollen systems and it disappears as swelling of the network proceeds. Experimental plans are proposed which will reveal the existence of the topological repulsive interactions in the networks.
Kinetic model for desorption of paired adatoms
Authors: Luque J. J. , Córdoba A.
Published: February 2003

By employing an extension of the Glauber–Ising model, the kinetics of the desorption of paired adatoms from a linear chain is studied. Three competitive mechanisms for changing a state of the system are allowed, namely desorption from or readsorption onto the chain or migration of the adatoms from site to site on the chain. The transition probabilities depend on an external field, on the interaction between nearest neighbors, and on three proper rate constants characterizing desorption, readsorption, and migration processes. An infinite set of coupled kinetic equations is obtained from the master equation. In order to compute the adsorption degree ϑ and the fraction of adjacent adatom pairs η, a ’’closure’’ approximation is taken. The steady-state solution is in agreement with the exact result of the equilibrium statistical mechanics. We find that relaxation far from equilibrium is, in general, not describable by a single exponential. The relaxation is slower when mobility does not exist than when adatoms are mobile, though the system behavior is similar in both cases. Equilibrium state is reached most quickly when lateral interaction does not exist and the final adsorption degree is 0.5. Attractive lateral interaction yields relaxation slower than when interaction is null, while repulsive interaction favors an initial relaxation very quickly, and the system reaches an adsorption degree lower than the final one, passes through a minimum and tends to equilibrium state slowly.
Metal cluster oxidation: Chemiluminescence from the reaction of sodium polymers (Nan, n≥3) with halogen atoms (X = Cl, Br, I)
Authors: Crumley W. H. , Gole J. L. , Dixon D. A.
Published: February 2003

Chemiluminescent emission is studied for gas phase Na polymers oxidezed by Cl, Br, and I. The clusters are produced by ’’free jet’’ supersonic expansion. (AIP)
Highly excited HCN: The inapplicability of classical dynamics
Authors: Lehmann K. K. , Scherer G. J. , Klemperer W.
Published: February 2003

A realistic potential surface is used to calculate HCN trajectories. (AIP)
Hypochromic effect in the bending mode of water at low temperature in Zn(ClO4)2⋅6H2O
Authors: Patel M. B. , Bist H. D.
Published: February 2003

The medium intensity infrared (IR) active bending mode of water near 164 cm−1 in zinc perchlorate hexahydrate disappears almost completely at low temperatures.
Vibrational threshold equal to the barrier height for an endothermic reaction: Li+FH→LiF+H on an a
b i
n
i
t
i
o potential-energy surface
Authors: NoorBatcha I. , Sathyamurthy N.
Published: February 2003

Vibrational threshold equal to the barrier height for an endothermic reaction: Li+FH→LiF+H on an ab initio potential energy surface is considered. (AIP)
On the bonding and structure in dihydroperoxyl (HO2)2 and H2O4
Authors: Diem Michael , Tso Tai-Ly , Lee Edward K. C.
Published: February 2003

Ultraviolet photolysis of H2CO and C2H2O2 in an O2 matrix at 12–20 K gives (HO2)2. The infrared absorption spectrum of (HO2)2 shows internal hydrogen bonding. A head-to-tail, six-membered cyclic dimer structure is proposed. On warming to 30 K, (HO2)2 disappears and H2O2 is formed.
Variational theorem for the generalized diffusion matrix
Authors: Fixman Marshall
Published: February 2003

A variational theorem is developed for a generalized diffusion matrix that describes the motion of one or more objects in a continuous solvent. The solvent motion obeys the linearized Navier–Stokes equation. The objects interact with the solvent through frictional forces. The magnitude of the frictional force density may vary from point to point in the object, and is controlled by a spatially varying friction tensor. If the friction tensors have non-negative eigenvalues at all spatial points, the variational principle becomes a minimization principle. The approach is illustrated for the two sphere problem in the nondraining limit, for which several exact calculations are available for comparison.
Equation of state and electrical conductivity of water and ammonia shocked to the 100 GPa (1 Mbar) pressure range
Authors: Mitchell A. C. , Nellis W. J.
Published: February 2003

Dynamic equation-of-state data for liquid H2O and NH3 were measured in the shock pressure range 30–230 GPa (0.3–2.3 Mbar) using a two-stage light-gas gun. Electrical conductivities of water were also measured in the shock pressure range 28–59 GPa (280–590 kbar). The experimental techniques to measure the electrical conductivity in a 50 ns time interval and to cool the target holders to liquid ammonia temperatures (230 K) are described. The H2O data are discussed in terms of the statistical mechanics model of Ree. At temperatures above 3000 K significant molecular ionization occurs.
The temperature of shock-compressed water
Authors: Lyzenga G. A. , Ahrens Thomas J. , Nellis W. J. , Mitchell A. C.
Published: February 2003

Temperatures from 3300–5200 K were measured in liquid H2O shocked to 50–80 GPa (500–800 kbar). A six-channel, time-resolved optical pyrometer was used to perform the measurements. Good agreement with the data is obtained by calculating the temperature with a volume-dependent Grüneisen parameter derived from double-shock data and a heat capacity at constant volume of 8.7 R per mol of H2O.
The adsorption and reaction of methanol on Pd(100). I. Chemisorption and condensation
Authors: Christmann K. , Demuth J. E.
Published: February 2003

The adsorption, desorption, and condensation of methanol on a clean Pd(100) surface has been studied in UHV at 77 K using high-resolution electron energy loss spectroscopy, thermal desorption spectroscopy, UV photoemission, work function measurements, and low energy electron diffraction. CH3OH adsorbed readily with a high initial sticking probability which remains nearly constant up to monolayer coverages (∼6×1014/cm2) indicating precursor state adsorption kinetics. The first 20% of this methanol spontaneously decomposes, but can largely recombine to desorb as methanol. The nature of this species and other decomposition products are discussed in part II. The remaining methanol chemisorbs nondissociatively having a bonding energy of 10.8 kcal/mol; no long-range order can be observed. The first physisorbed and later condensed layers are readily distinguishable and have desorption energies of 9.0 and 7.2 kcal/mode, respectively. The adsorption of methanol is accompanied by a continuous work function decrease to a final value of −1.60 eV after the deposition of ∼3 layers. A layer-by-layer growth mechanism is observed with physisorbed methanol showing first order kinetics and the absence of hydrogen bonding. This physisorbed layer still alters the surface dipole layer and shows some signs of additional interactions with the surface not present in the subsequent condensed layers. The subsequent condensed layers exhibit fractional order kinetics, which is interpreted to reflect hydrogen bonded ’’chains’’ of methanol molecules similar to that occurring in the melt of crystalline methanol.