bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of the reverse reaction on the experimental determination of energy transfer rate constants
Authors: Huo Winifred M. , Dillon Michael A.
Published: February 2003

The effect of the reverse reaction in the experimental determination of energy transfer rate constants is re-emphasized. It is shown that the pressure independence of the measured rate constant, frequently used experimentally as an indicator of the absence of reverse reactions, may have more complicated sources like the presence of a third channel of dissipating the excited species. Thus, pressure independence alone is not a reliable indicator of the absence of reverse reactions. The quenching of Ar 3P1 by H2 is used as an illustrative example with the energy transfer rate constant and the rate constant of the reverse reaction estimated using the Born approximation [W. M. Huo, J. Chem. Phys. (preceding paper)].
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o geometry and vibrational frequencies for lithium peroxide
Authors: Yates J. H. , Pitzer R. M.
Published: February 2003

The minimum energy geometry of lithium peroxide (Li2O2) was found to be of D2h symmetry. The O–O bond distance was computed to be very nearly the same as in hydrogen peroxide, and the O–Li–O angle to be 51.3°. The vibrational frequencies were calculated by distorting the molecule according to the desired internal coordinate. The frequencies and force constants are compared with available experimental data. The ionic nature of the bonding is supported by a Mulliken population analysis.
On stochastic reductions in molecular collision theory: Projection operator formalism; application to classical and quantum forced oscillator model
Authors: Schatz George C. , McLafferty Frank J. , Ross John
Published: February 2003

A method is developed for treating complex molecular collision processes through the application of stochastic reduction formalisms. We begin by describing a projection operator method for decomposing a complicated collision system into two (or more) subsystems, each of which is assumed to be weakly correlated (not necessarily weakly interacting) with the others. Approximations to this correlation are then introduced, and this results in a set of coupled equations for the reduced density operators (or classical phase space distributions) associated with each subsystem. We then examine the classical mechanical application of this theory to the forced oscillator model of V–T energy transfer. Arguments of multiple time scales are used to uncouple the stochastically reduced equations of motion, and thus we may evaluate the memory kernel analytically. This leads to a single diffusion equation for the time evolution of the action in the oscillator during the collision. Comparison with the corresponding exact results indicates excellent agreement of low order moments of the classical distributions of action in the limit of small energy transfer (i.e., ΔE/h/ω<1). Of particular note is the fact that our stochastic theory predicts an average energy transfer (first moment) in exact agreement with the exact result independent of magnitude of the energy transfer. In a related application of our general stochastic formalism, we consider the quantum mechanical forced oscillator model. This problem is treated in two different ways: (a) through the use of reduced density matrices (which leads to master equations), and (b) through the Wigner equivalent formalism (which is formally analogous to the classical treatment). The resulting transition probabilities obtained from these two equivalent applications are shown to be identical. Comparison of stochastic and exact quantum results indicates quantitative agreement of the probabilities for ΔE/h/ω?0.1, and average agreement of the probabilities for larger values of the energy transfer.
Nuclear recoil chemical activation studies. Vibrational energy transfer from cyclobutane-t
Authors: Nogar N. S. , Spicer Leonard D.
Published: February 2003

A method is demonstrated for the determination of intermolecular energy transfer efficiencies in systems chemically activated by nuclear recoil reaction. Relative vibrational energy transfer efficiencies are determined for highly excited cyclobutane-t formed with ∼5 eV of internal energy in the hot tritium for hydrogen replacement reaction on cyclobutane in the presence of several inert bath gases. The pressure dependence of the hot yields is used to probe the overall reaction mechanism and results indicate that a sizable fraction of the hot reaction product does not undergo competitive unimolecular decomposition. The general systematics of these side reactions are discussed. From the composition dependence of the unimolecular reaction at a constant pressure of 800 torr, the relative energy transfer efficiencies for the respective bath gases are found to be c-C4H8, 1.0; CF4, 1.05; N2, 0.32; He, 0.12; Ne, 0.24; Ar, 0.25; Kr, 0.31; Xe, 0.39. Simple collision models for the cyclobutane–noble gas cases suggest V–T transfer occurs most efficiently through delocalized interactions. Furthermore, angular momentum considerations indicate low impact parameter collisions are most effective in forming transition modes through which statistical redistribution of energy can occur.
Hard-core trigonal planar lattice gas on a triangular lattice
Authors: Stanford David J. , Huckaby Dale A.
Published: February 2003

Using the grand canonical ensemble, series approximations to the thermodynamic properties of a triangular lattice gas of hard-core, trigonal planar molecules have been obtained. An analysis of these series indicates an order–disorder phase transition, probably of the second-order type, occurs at a number density ρ=0.6ρO, a pressure P=1.3ρOkT, and an activity z=2.4, where ρO is the number density at closest packing.
Dispersed fluorescence observations of the CO(A 1Π→X 1Σ+) transitions from photodissociation of CO2
Authors: Phillips E. , Lee L. C. , Judge D. L.
Published: February 2003

Spectra from 1500 to 1800 Å have been obtained showing CO(A 1Π→X 1Σ+) fluorescence arising from photodissociation of CO2. A line emission source provided the incident photons at 15 selected wavelengths from 449 to 955 Å. Analysis of the spectra confirms that the CO(A 1Π→X 1Σ+) band system dominates the vuv fluorescence at incident photon wavelengths from 700 to 923 Å. Vibrational populations of the CO(A 1Π) electronic state for v=0, 1, and 2 are seen to follow Poisson distributions. The onset of dissociation to CO(A 1Π)+O(1D) at 801 Å, the threshold for that process, is apparent from the relative increase in the A 1Π (v=0) population. At the minimum energy required to produce CO(A 1Π)+O(1S), no increase in the v=0 population is observed. Absolute partial cross sections for production of the CO(A→X) bands at incident photon wavelengths of 901.2 and 923.2 Å are reported. The data also show the electronic transition moment to decrease linearly with the r centroid to ∼1.25 Å, consistent with previously established results; however, at larger values of the r centroid the moment increases. The fluorescence observed using incident photons at still shorter wavelengths, 449 Å<λ<500 Å, consists primarily of atomic carbon lines, with an upper limit for the cross section for production of CO(A 1Π) of ∼0.07 Mb.
Laser-assisted excitation transfer as a single-atom multiphoton process
Authors: Milonni P. W.
Published: February 2003

An approach to the calculation of cross sections for laser-assisted excitation transfer processes is outlined for the dipole–dipole case. The internal atomic variables are treated in the Heisenberg picture, while the simplest form of semiclassical collision theory is used for the atomic trajectories. A simple physical picture in the spirit of the Weizsäcker–Williams method emerges: The laser-assisted excitation transfer may be treated approximately as a single-atom multiphoton process in an applied field.
Measurement of energy distributions of secondary electrons ejected from water vapor by fast electrons
Authors: Vroom D. A. , Palmer R. L.
Published: February 2003

Energy distributions of the secondary electrons ejected from water vapor ionized by fast electrons have been measured. The energies of the primary electron beam ranged from 1 to 10 keV and ejected electron energies up to 300 eV were considered. Measurement of the secondary electron spectra has been made for electrons ejected over the total sphere and at angles of 30°, 60°, 90°, 120°, and 150° with respect to the primary electron beam.
Variational calculations of vibrational properties of ozone
Authors: Carney G. D. , Langhoff S. R. , Curtiss L. A.
Published: February 2003

A variational method is used to obtain vibrational–rotational properties for ozone from an experimental quartic force field. Band positions, average structures, matrix elements for calculating infrared intensities, and effective rotational constants are reported for 16O3 and its 18O isotopic species. Also, the degree to which the vibrational energies and properties are converged is investigated as a function of the basis set parameters and basis set size, and of the method of obtaining the Hamiltonian matrix elements. A convenient procedure for assigning the vibrational states is developed for variational vibrational wavefunction expansions.
Theory and applications of an atoms in molecules approach to the Xα-SCF method
Authors: Tylicki J. , Hood R. J. , Gallup G. A. , Eckhardt C. J.
Published: February 2003

We have studied the Xα-SCF problem with an atoms in molecules approach. LCAO-molecular orbitals are used and the molecular charge and exchange densities are built up from atomic contributions. We have applied our method to CH3F with subsequent comparison to ab initio calculations. The Xα-SCF dipole moment of CH3F is 1.76 D compared with an experimental value of 1.79 D. We also give calculations of TCNQ and TTF with comparisons to recent Xα calculations using the overlapping sphere modification of a muffin-tin potential. Quadrupole moments for TCNQ and TTF have also been determined.
The determination of hydrogen coordinates in lanthanum nicotinate dihydrate crystals by Nd+3-proton double resonance
Authors: Hutchison Clyde A. , McKay David B.
Published: February 2003

The EPR (electron paramagnetic resonance) and proton ENDOR (electron nuclear double resonance) spectra of single crystals of lanthanum nicotinate dihydrate with Nd+3 dilutely substituted for La+3 have been obtained and described by appropriate spin Hamiltonian operators. The ENDOR signals of protons lying within 5 Å of the Nd+3 have been resolved and assigned to specific protons. The physical interpretation of the measured anisotropic rare earth ion–proton hyperfine interactions has been given. The spatial coordinates of the protons in the principal axis system of the electron Zeeman interaction matrix ge have been obtained with standard deviations ranging from 0.002 to 0.015 Å, with an average of 0.006 Å. The average standard deviation for the coordinates of the 4 H2O protons nearest to the Nd+3 ion, three of which form hydrogen bonds with nicotinate nitrogen atoms, is 0.004 Å. The average standard deviation for their distances from the Nd+3 is 0.003 Å. It is clear that by means of suggested improvements of the reported techniques it should be possible to measure proton coordinates to 0.001 Å. The methods developed show promise for applications to problems concerning the structure of organic molecules, in particular organo–metallic complexes and large molecules of biological interest, such as proteins, to which paramagnetic ions may be attached.
On a multiple reflection time domain method in dielectric spectroscopy: Application to the study of some normal primary alcohols
Authors: Bottreau André Marie , Dutuit Yves , Moreau Jacques
Published: February 2003

The authors propose a time domain method of total reflected signals on a dielectric sample in a coaxial line. This method enables the determination of the complex constant of propagation of the medium to be studied and, consequently, its complex permittivity. Contrary to other similar methods employed up to now, this method needs neither iterative calculation nor nomograms. The validity of this multiple reflection approach is illustrated by the study of several primary alcohols. The values of the permittivities and relaxation frequencies thus acquired are in good agreement with those obtained elsewhere.
Laser excitation of rare gas metastables and the resulting fluorescence distribution
Authors: Bondybey V. E. , Miller Terry A.
Published: February 2003

Laser excitation spectra have been obtained from metastable states of the rare gas atoms, He, Ne, Ar, Kr, and Xe. The relative population of the atomic metastable states have been obtained several centimeters downstream from a dc discharge source. Dispersion of the laser-induced fluorescence indicates that relaxation of the excited atomic levels is very extensive at the 1–10 torr pressures used.
Stress–strain behavior in polymer networks containing nonlocalized junctions
Authors: Graessley William W. , Pearson Dale S.
Published: February 2003

Two models of entanglement interaction in polymer networks are examined. One is based on Edwards’ principle of topological classification, the other on interstrand links which are free to slip along the strand contours. Relationships between stress and strain in simple elongation are developed for networks which contain such elements as well as permanent crosslinks. In each case departures from neo-Hookean behavior are found. The apparent modulus decreases with strain in uniaxial tension and remains approximately constant in uniaxial compression as observed in real networks. If the network is expanded, such as would occur through addition of a swelling solvent, the departures from neo-Hookean behavior become smaller, again as observed experimentally. It thus appears that chain entanglements, portrayed here by two different nonlocalized junction models, may account for many of the differences in behavior between real networks and predictions of the phantom network theories.
Ion–molecule reactions in SiH4–D2 mixtures
Authors: Allen W. N. , Cheng T. M. H. , Lampe F. W.
Published: February 2003

The ion–molecule reactions characteristic of SiH4–D2 mixtures have been elucidated and the reaction cross sections measured as a function of relative kinetic energy. D2+ reacts with SiH4 to yield SiH3+, SiH2+, SiH+, and Si+ by exothermic processes that proceed with a total cross section near the Langevin orbiting value. D3+ reacts with SiH4 to form only SiH3+ with a cross section also near the Langevin value. The absence of SiH2D+ suggests direct H− transfer from SiH4 rather than the intermediate formation of SiH4D+. All reactions of Si-containing ions in their ground states with D2 are endothermic. Energy thresholds for the deuterium atom pickup reactions of Si+ and SiH2+ yield thermochemical results in satisfactory agreement with values obtained by other methods. SiH3+ reacts with D2 to form SiH2D+ solely, a result indicating that only HD is eliminated from an SiH3D2+ intermediate and showing the nonequivalence of D and H in the silanium ion.
Resonance Raman spectra of metalloporphyrins. Effects of Jahn–Teller instability and nuclear distortion on excitation profiles of Stokes fundamentals
Authors: Shelnutt J. A. , Cheung L. D. , Chang R. C. C. , Yu Nai-Teng , Felton R. H.
Published: February 2003

Excitation profiles of depolarized modes (blg, b2g) in Ni etioporphyrin I exhibit marked 0–0 resonance Raman intensity upon Q-band excitation. By contrast, inversely polarized Raman scattering (a2g) is stronger with 0–1 excitation. Within the crude Born–Oppenheimer approximation, a theoretical model is presented which explains the 0–0 enhancements of depolarized modes as arising from interference of intermanifold (Q –B) and intramanifold (Q –Q) vibronic coupling. Comparison of computed and observed excitation profiles shows that a weak Jahn–Teller distortion is present in the Q state. The vibronic model predicts behavior of Raman intensity in the B (Soret) electronic state. Evidence of increased vibronic coupling of both 400 and 1365 cm−1 polarized vibrations (a1g) is found in chromium tetraphenylporphyrin with the appearance of a supernumerary peak at 1400 cm−1 in the excitation profile of the 400 cm−1 vibration. Excitation profiles of a2g modes in the chromium complex exhibit an apparent decrease in excited state vibrational frequencies due to increased coupling. Estimates of vibronic coupling strengths for the Q state are given for the nickel and chromium porphyrins.
Intermolecular potential of anisotropic systems: LiF–Ar and LiF–Kr
Authors: Reed Kent A. , Wharton Lennard
Published: February 2003

High resolution differential cross sections for LiF scattering from Ar and Kr were measured in the thermal velocity range with varying LiF rotational temperature. An extremely narrow velocity distribution in the LiF beam, which reduced the equivalent temperature of the experiment to typically below 1 K, made it possible to resolve the high frequency oscillations of the differential cross section. Rainbow oscillations were identified and measured. Effects of experimental convolution were discussed and treated. Average potential well depths and radii were determined. It was inferred from the breadth of the primary rainbows that the potential well is not pathological in possessing extremely deep or shallow depths (ε) for certain orientations: standard deviation of ε is about 50%. The persistency of the high frequency oscillations with angle is evidence of a quite spherical locus of rm, although the locus is not necessarily concentric with the center of mass. The high frequency oscillations and in part the rainbow oscillations are damped by rotational transitions. The differential cross section is largely independent of the rotational state distribution. Calculations using an optical model for the scattering and assumed angular dependent potentials permitted a quantitative test of a theoretical long range potential model and also permitted a fit of an empirical angular dependent model at the radius of the onset of these transitions. The quantitative test indicated that neglected repulsive terms are important in the theory. The fitted empirical model was consistent with the measured dispersion in ε. The radius at which rotational transitions become significant has been determined and lies outside rm, approximately at the rainbow radius, but inside the radius probed by total scattering cross section measurements. The potential well parameter rm=3.8 Å, ε=4.1×10−14 erg and rm=3.7 Å, ε=5.3×10−14 erg were obtained for LiF–Ar and LiF–Kr, respectively.
Alkali halide surfaces: Measurements and modeling of adsorbate motions and reactions at surface defects
Authors: Bjorklund Robert B. , Lester Joseph E. , Spears Kenneth G.
Published: February 2003

Results are presented for a mass spectrometric study of reactions between alkali halide molecular beams and vaporizing single crystals of different alkali halides. Shuttering the beam yields a temperature dependent decay of reaction products from their steady state surface fluxes. We determined Arrhenius pre-exponentials and activation energies from the decay rates of unreacted beam diatomic (monomer), dimer products, and beam–crystal ion exchange products (monomers). The kinetic curves were primarily composed of a slow decay with a smaller amount of a faster decay. The pre-exponentials and activation energies exhibit a close correlation (lnAαEa) for energies ranging from 2–57 kcal/mole. This compensation effect has allowed our direct measurement of several processes in a narrow range of first order decay constants (0.02–4.0 sec−1). Analysis of the decay curves and extensive modeling of the crystal surface defects identify the measured activation barriers with specific surface processes. We have assigned dimer product and unreacted beam monomer activation energies to barriers against motion from ledge to surface desorption sites. Beam–crystal ion exchange reactions occur at kink defects and the rate determining step for monomer product formation is the dissociation of a dimer intermediate at the kink. Some decay measurements showed evidence for bulk diffusion of diatomic and atomic species.
Alkali halide surfaces: Adsorbate binding energies and structures at surface defects
Authors: Bjorklund Robert B. , Spears Kenneth G.
Published: February 2003

We describe an electrostatic method of calculating equilibrium configurations of alkali halide diatomic molecules (monomers) and tetratomic dimers adsorbed on rigid alkali halide crystal surfaces. Results are presented for adsorbate binding energies and geometries on the smooth surface, at single-step ledge defects, and at kink (corner) defects. The results are consistent with our recent experimental measurements and we explore several features of the adsorbate–substrate interaction. Cyclic dimers are more weakly bonded than monomers on the smooth surface, but binding strengths for both are nearly equivalent at ledges. Linear dimers are stabilized at the defect binding sites. For all species, the kink position is the strongest adsorption site. Adsorbate structures at the different sites are dependent on the relative magnitudes of adsorbate–adsorbate and adsorbate–substrate interactions.
Alkali halide surfaces: Potential energy surfaces for diffusion and dimer dissociations
Authors: Bjorklund Robert B. , Spears Kenneth G.
Published: February 2003

We report results from electrostatic calculations of activation barriers for surface motions of alkali halides adsorbed on alkali halide (100) crystal faces. The results include energy surfaces for adsorbate diffusion along single-step ledges and dimer motions at idealized kink defects. Activation barriers for diffusion and desorption along the crystal surface and ledges are nearly equal, which implies that the pre-exponential factors for both rates are significant in determining diffusion distances for adsorbates. Dimer motions at kink defects either interconvert several possible stable isomers or lead to diatomic dissociation products. The kink’s role in such movements is to distort the different dimer configurations and to strongly bond one diatomic pair during dissociation. There is a direct relationship between the structure of the adsorbed dimer and the diatomic products formed from specific dimer motions.
Calculation of Raman scattering parameters for methane and halomethanes from an atom dipole interaction model
Authors: Applequist Jon , Quicksall Carl O.
Published: February 2003

Derivatives of molecular polarizability tensors with respect to vibrational normal coordinates are calculated using an atom dipole interaction model for CH4, CD4, CF4, CCl4, CHF3, CH3Cl, CHCl3, CF3Cl, CFCl3, CH3Br, CF3Br, and CH2Cl2. Isotropic atom polarizabilities obtained from previous studies of molecular polarizability are used. Derivatives of atom polarizabilities with respect to bond lengths are adjusted to give a best fit to the mean polarizability derivative ?′ for 29 totally symmetric modes as found from published gas phase Raman scattering intensities and depolarization ratios. A satisfactory fit to ?′ is obtained, but the calculated anisotropies γ′ correlate only roughly with the experimental values. The signs of the calculated ?′ values are consistent with the view that the mean molecular polarizability increases with increasing bond length for all normal modes in which bond stretching motions predominate.
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o calculations of oscillator and rotatory strengths in the random-phase approximation: Twisted mono-olefins
Authors: Bouman Thomas D. , Hansen Aage E.
Published: February 2003

A b initio (STO-nG) computations of ordinary and rotatory intensities of low-lying electronic transitions are presented for twisted ethylene and twisted trans-2-butene in the random-phase approximation (RPA). The intensities are computed in both dipole length and dipole velocity forms, as well as the mixed form for the oscillator strength, and the convergence of these formally equivalent results is examined in the RPA and several other methods for constructing the electronic excitation: the virtual orbital, or single-transition, approximation (STA), the monoexcited configuration-interaction, or Tamm–Dancoff, approximation (TDA), and one version of the higher RPA (HRPA). We show that the RPA has consistent advantages over the TDA for calculation of CD as well as ordinary intensities. Our computations confirm that a localized, ethylenic chromophore is indeed adequate to account for the low-lying CD spectrum in mono-olefins. Further, even with minimal valence-shell basis sets, our RPA rotatory strengths agree essentially completely in both sign and magnitude with the experimental results of Mason and Schnepp on trans-cyclooctene.
Rotational energy effect upon the branching fraction for reactive decay of the CsF+K collision complex
Authors: Stolte S. , Proctor A. E. , Pope W. M. , Bernstein R. B.
Published: February 2003

The influence of the rotational energy of the CsF molecule upon its reactivity with K has been measured by the crossed molecular beam technique over a range of relative translational energy Ētr from 3–6 kcal mol−1. The experiments involve the endoergic (1.8 kcal mol−1) reaction CsF+K⇄[CsFK]→Cs+KF, for which the reactive branching fraction FR had been previously measured in this laboratory using velocity-selected but otherwise ’’thermal’’ (Boltzmann) beams. In the present investigation, beams of low rotational states of CsF have been prepared by means of a quadrupole state selector focusing field placed beyond the usual slotted-disk velocity selector. This arrangement provides so-called ’’low-J’’ beams of CsF, with known speed distributions, which are then crossed with a well-characterized K beam. The measurements consist of angular distributions of reactive (i.e., Cs) and nonreactive (i.e., CsF) scattering for the ’’low-J’’ beam compared to the ’’thermal’’ reactant beam at essentially the same Ētr. The reactive yield and thus the reactive branching fraction FR is found to be significantly smaller for the ’’low-J’’ beam relative to the ’’thermal’’ beam of CsF. An analysis of the state selection and focusing properties of the quadrupole lens sytem has been carried out which accounts satisfactorily for the observed velocity dependence of the CsF focused beam intensity. This analysis yields a calculated rotational state distribution P (J), from which the average rotational energy Ērot of the reactant CsF molecules is obtained at each of the Ētr values studied. Ērot ranges from 0.13 to 0.04 kcal mol−1 for Ētr in the range 3–6 kcal mol−1, whereas Ērot for the ’’thermal’’ CsF beam is 2.52 kcal mol−1. (Thus the effect of applying the rod voltage to the quadrupoles is to ’’switch off’’ some 2.4 kcal mol−1 of CsF rotational energy.) This analysis also predicts the observed enhancement of the total scattered signal due to the focusing, which implies that the cross section for complex formation is the same (within ≈±10%) for the low-J and the thermal reagent molecules. By combining the data on the Ētr-dependence of FR for the rotationally cold CsF beams with the previous results for thermal beams it is possible to compare the relative importance of rotational versus translational energy at a given total energy. It is found that rotational energy of the CsF is significantly less effective than relative translational energy in promoting reactive decay of the complex. The theoretical implications of this result are discussed.
Spin diffusion effects in partially spin–lattice relaxed NMR spectra of solid block copolymers
Authors: Lind A. C.
Published: February 2003

Room-temperature proton NMR measurements of 65 wt % poly(dimethylsiloxane) (DMS) and 35 wt % bisphenol-A polycarbonate (BPAC) block copolymers with number average DMS block lengths ranging from 20 to 100 monomer units have shown effects which can be explained in terms of a gradient of net spin population along the DMS blocks during recovery from a nonselective 180° spin inverting pulse. This gradient occurs because the spin–lattice relaxation of the BPAC component is faster than that of the DMS component and spin diffusion takes place along the DMS blocks to the BPAC, causing spins at the centers of the DMS blocks to relax slower than those at the ends. High-resolution solid NMR spectra were obtained with a multiple-pulse technique, which permitted independent measurements of the DMS and BPAC relaxations. Unusual line shapes in partially relaxed broadline spectra were interpreted as being caused by enhanced motional narrowing of the protons at the centers of the DMS blocks in addition to the gradient of net spin population along the DMS blocks. The results of a spin diffusion model, which was developed to aid interpretation of the experimental results, were in quantiative agreement with the data. This model fits the data only if the diffusion process is limited by diffusion along the length of the DMS blocks and not by a single diffusion barrier between the DMS and BPAC blocks.
Charge transfer at photoconductor–liquid interfaces
Authors: Hartmann G. C. , Noolandi J.
Published: February 2003

The electrical properties of a photoconductor–liquid interface are investigated by a study of transient photostimulated charge transfer from phthalocyanine into an insulating hydrocarbon liquid. The dependence of charge transfer on electric field, light intensity, and concentration of impurities added to the liquid was measured. A theoretical rate constant model is developed for the various processes which contribute to the transient current. Comparing this to the measurements, we find evidence for surface traps at the interface. A model for the desorption rate of ions from the interface is used to obtain an estimate of the adsorption energy of the ion at the interface. Finally, a discussion of probable charge transfer mechanisms is presented.
Dimer-formation rate coefficients from measurements of terminal dimer concentrations in free-jet expansions
Authors: Knuth Eldon L.
Published: February 2003

Dimer formations in free jets of rare gases are considered as a simple example of recombination processes. In order to extract values of the local recombination-rate coefficient kf from measurements of the terminal dimer mole fraction xA2, a sudden-freeze model is used. Of the several kinetic models considered, only Bunker’s model of stabilization of virtual orbiting pairs by collisions with third bodies yields realistic values of the collision effectiveness. Procedures are developed for (1) predicting the terminal value of xA2 for given source conditions and known kf, (2) deducing kf for given source conditions and measured xA2, and (3) selecting appropriate source conditions for specified xA2 and known kf. Data for Kr, Ar, and Ne from six experimental studies and one computer study are correlated by a steric factor of 0.4 and an activation energy of 0.6 times the potential-well depth ε. The simplified relation xA2?0.5[noσ3(ε/kTo)7/5 (d*/σ)2/5]5/3, where no is the source number density, To is the source temperature, d* is the source-orifice diameter, σ is the zero-potential interaction radius, and k is Boltzmann’s constant, predicts terminal dimer mole fractions to an approximation suitable for technical applications. It is suggested that a critical value of xA2, namely, about 0.01, is associated with the initiation of significant trimer formation.
Dissociation energies of CaBr, SrBr, BaBr, and BaCl from mass spectrometric studies of gaseous equilibria
Authors: Hildenbrand D. L.
Published: February 2003

High temperature gaseous equilibria involving CaBr, SrBr, BaBr, and BaCl were studied by mass spectrometry, and the derived heats of reaction were used to evaluate the dissociation energies of the monohalides. The derived values measured by reference to D°0(AlBr) and D°0(AlCl), are D°0(CaBr) =73.4±2.2 kcal/mol; D°0(SrBr) =78.7±2.2 kcal/mol; D°0(BaBr) =85.5±2.2 kcal/mol; and D°0(BaCl) =103.3±2.0 kcal/mol. These values are in good agreement with results of recent flame equilibrium determination, and with semitheoretical values calculated from both the Rittner polarizable-ion model and ionicity-corrected Birge–Sponer extrapolations. On the contrary, values of D°0 derived from chemiluminescent metal oxidation reactions are rather higher than those obtained by any of the other methods.
Mechanisms of intersystem crossing in aromatic hydrocarbons
Authors: Fujimura Y. , Shimakura N. , Nakajima T.
Published: February 2003

A rate expression for intersystem crossings S-T in aromatic hydrocarbons is formulated by means of a Green’s function technique. The rate constant involves not only diagonal terms of spin–orbit coupling and nonadiabatically induced spin–orbit coupling but also cross terms between them that are usually neglected. It is shown that promoting modes for intersystem crossings can be classified into two groups according to the perturbations causing intersystem crossings. One group is governed by vibronic spin–orbit coupling matrix elements and the other by vibronic coupling matrix elements which involve the nuclear momentum operator. An approximate calculation of vibronic spin–orbit coupling matrix elements is carried out for the S1(1B2u) -T intersystem crossing in benzene. The results indicate that C–H out-of-plane vibration ν10 acts as a dominant promoting mode for both S1-T1 direct and S1-T2-T1 indirect mechanisms.
Electron paramagnetic resonance of divalent nickel in the low temperature phases of magnesium fluosilicate and magnesium fluotitanate
Authors: Rubins R. S. , Kwee Kian-Kwet
Published: February 2003

The EPR spectra of Ni2+ in single crystals of MgSiF6⋅6H2O and MgTiF6⋅6H2O have been studied at and above 77 °K in the frequency ranges 12–18 and 23–25 GHz. Phase transitions occur on warming the crystals above room temperature at (302±1) °K in MgSiF6⋅6H2O and at (318±1) °K in MgTiF6⋅6H2O, and on cooling at (301±1) °K for both crystals. The EPR spectra at 77 °K were fitted to the spin Hamiltonian ℋ=gβH⋅S+DSz2 +E (Sx2−Sy2) with g=2.250±0.005, ‖D‖= (2.16±0.01) cm−1 and E= (0.210±0.001) cm−1 for MgSiF6⋅6H2O and g=2.245±0.005, ‖D‖= (2.3±0.1) cm−1 and E= (0.149±0.001) cm−1 for MgTiF6⋅6H2O. In MgSiF6⋅6H2O, the spectrum in the perpendicular plane showed a sixfold rotational symmetry and consisted of two sets of three lines related by a rotation of (22±1) ° about the c axis. In MgTiF6⋅6H2O, a single three line spectrum with a sixfold rotational symmetry was observed in the perpendicular plane. In both crystals, the parameter E increased appreciably with temperature. In MgSiF6⋅6H2O, the temperature gradient ∂E/∂T was approximately 3.3×10−4 cm−1/°K in the range 77 to 240 °K and then decreased with increasing temperature above 240 °K. In MgTiF6⋅6H2O, ∂E/∂T was approximately 4.7×10−4 cm−1/°K in the range 120 to 290 °K and then decreased with decreasing temperature below 120 °K.
Theoretical studies of highly expanded free jets: Influence of quantum effects and a realistic intermolecular potential
Authors: Toennies J. Peter , Winkelmann Klaus
Published: February 2003

An earlier approximate theory proposed by Knuth and Fisher and Miller and Andres for solving the Boltzmann equation to calculate speed ratios and temperatures in free jet expansions has been extended to take account of quantum effects and realistic potentials in the collision integrals. The calculations show that quantum effects lead to a substantial increase in terminal speed ratios in helium but not in heavier gases. For He the speed ratios depend sensitively on the shape of the potential. Despite the approximate nature of the theory the predictions are in reasonable agreement with recent experiments. The theory predicts that the presently highest achieved speed ratio in He of 225 can be increased to 700 if condensation does not occur. At this speed ratio the quasiequilibrium gas phase temperature is roughly 10−3 K.
The binding energy between NO and NO+
Authors: Ng C. Y. , Tiedemann P. W. , Mahan B. H. , Lee Y. T.
Published: February 2003

The photoionization efficiency curve of the nitric oxide dimer (NO)2 was obtained in the wavelength range from 1460 to 1210 Å (8.492–10.246 eV). The ionization potential of (NO)2 was found to be 8.746±0.015 eV. From this, the ionization potential of NO and the dissociation energy of (NO)2 (0.069±0.004 eV) obtained by Billingsley and Callear, the dissociation energy of NO(2Π, v′′=0) –NO+(1Σ+, v′=0) is deduced to be 0.59±0.02 eV. From the comparison of the structure of the photoion yield curve of NO and (NO)2 it is concluded that NO(2Π, v′′=0) –NO+(1Σ+, v′) is bound when NO+(X1Σ+) is in the v′=0 or 1 vibrational state. This conclusion is in agreement with the dissociation energy of (NO)+2 obtained.
The ultraviolet–visible spectra of diatomic, triatomic, and higher nickel clusters
Authors: Moskovits M. , Hulse J. E.
Published: February 2003

The uv–visible spectra of Ni2 and Ni3 trapped in argon matrices are reported. Three bands were observed for the former, two with ν00 at 377 nm and 529 nm respectively and ω0 approximately 330 cm−1 and one continuous band centered at 410 nm. Two bands of Ni3 were identified, a continuous one at approximately 420 nm and one with ν00 at 480 nm and ω0 of 200 cm−1. The spectra of higher, uncharacterized clusters were also observed. These showed one intense, broad band centered at approximately 335 nm. Nickel colloid formed upon prolonged warmup of the matrix and its spectrum compares well with that calculated from Maxwell-Garnett theory. The HOMO → LUMO transition is found to increase in energy as one progresses from Ni2 to bulk nickel. The observed spectral transitions are compared with reported values computed by various approximate molecular orbital calculations, and several discrepancies are found between the predicted and observed energy differences.
The pressure dependence of the dipolar correlation function of CO2 in N2 at moderate densities
Authors: Laulicht I. , Brender C.
Published: February 2003

The dipolar correlation function of CO2 in N2 has been measured by the band shape analysis of the infrared stretching mode at various pressures in the 20–140 bar range. The depth of the minimum in the correlation function is found to be a convenient experimental parameter. It is linear with pressure and converges to the classical free rotor value at very low pressures. The correlation time is linear with pressure in the 20–140 bar range, but its intercept at zero pressures is very different from the classical free rotor value. The mean torque has been calculated from the experimental second and fourth spectral moments. It is found to be a nonlinear function of pressure, which converges to zero at zero pressure. The second moments are lower than 2kT/I by 30–20%. Results are compared to previous experiments.
Deuterium relaxation of chloroform dissolved in a nematic liquid crystal
Authors: Vold Regitze R. , Vold Robert L.
Published: February 2003

The deuterium relaxation of CDCl3 dissolved in a thermotropic nematic liquid crystal has been studied at 13.0°C and analyzed in the framework of Redfield theory. From the results of a selective Carr–Purcell experiment, a nonselective and two selective inversion - recovery measurements, spectral densities of motion at ω=0, ω=ω0, and ω=2ω0 have been determined, and the results are discussed in relation to present theories for relaxation of solutes in ordered media.
The conformation of cyclooctane: An experimental determination by NMR in an oriented solvent
Authors: Meiboom S. , Hewitt R. C. , Luz Z.
Published: February 2003

The proton NMR spectrum of partially deuterated cyclooctane dissolved in a nematic solvent has been studied in detail. The direct coupling constants between proton pairs and between carbon-13 and a proton have been measured and assigned. Of the many conformations that have been suggested for cyclooctane, the boat-chair is the only one for which the calculated coupling constants will reproduce the observed ones, and it is concluded that this is the dominant conformation. It has been shown by Anet and Basus that another conformation, probably the crown, is present to an amount of about 6% at room temperature. The present results seem to confirm this, but the method is not very sensitive for small admixtures of other conformations, and the actual fraction present, though clearly small, could not be determined. The carbon bond angles were found to deviate very appreciably from tetrahedral. The average C–C–C angle was determined as 118° and the H–C–H angle as 103°, with an estimated uncertainty of ±2°.
Chemi-ionization reactions of alkali dimers with halogen molecules
Authors: Reck Gene P. , Mathur B. P. , Rothe Erhard W.
Published: February 2003

A crossed beam apparatus is used to characterize ionic products of thermal energy collisions of alkali dimers M2 with homonuclear halogens X2 and with IBr and ICl. All possible alkali–halogen combinations were studied, except Rb2 with ICl and IBr. When energetically possible the ionic products are mainly M+ and X−. When this path is closed two alternative paths are observed. These yield M2X+ and X−, or M+ and MX2 −. The results are discussed in the framework of a mechanism proposed by Lin, Whitehead, and Grice. The binding energy of the triatomic ions is computed with the use of an ionic model.
The velocity autocorrelation function and the diffusion coefficient for a dilute hard sphere gas
Authors: Eder Otto J.
Published: February 2003

The velocity autocorrelation function and the diffusion coefficient for a dilute hard sphere gas are calculated without solving the Boltzmann equation directly. The basic assumption used is the hypothesis of molecular chaos. The diffusion coefficient obtained is compared with other solutions of this problem.
Al+O3 chemiluminescence: Perturbations and vibrational population anomalies in the B 2Σ+ state of AlO
Authors: Lindsay D. M. , Gole J. L.
Published: February 2003

Ozone reacts with aluminum entrained in an argon buffer gas to yield the B 2Σ+–X 2Σ+ spectrum of AlO. The measured B 2Σ+ vibrational populations for this reaction follow a markedly non-Boltzmann distribution, exhibiting local maxima at vibrational levels v′=6, 8, 12, and 14. This behavior is attributed to an initial chemical reaction Al+O3→AlO(A 2Π) +O2 followed by the collision induced rearrangement AlO(A 2Π) +Ar→AlO(B 2Σ+)+Ar. The spin–orbit interaction in AlO connects ro-vibronic levels of the A 2Π and B 2Σ+ states. Consequently, collisional energy transfer is particularly efficient for the most strongly perturbed levels of the B 2Σ+ state. For Al+N2O, an approximate Boltzmann distribution in the AlO B 2Σ+ state population indicates that this reaction proceeds by a differing mechanism. Consistent with the proposed mechanism for Al+O3 is the appearance of ’’extra’’ band heads representing normally Franck–Condon forbidden A–X transitions which become allowed because of a small admixture of B 2Σ+ character. The relative intensities of the extra and ’’main’’ transitions are strongly dependent upon the argon buffer gas pressure. A quantitative description of this dependence gives an estimate for the amount of mixing between A 2Π and B 2Σ+ and for the rate of energy transfer between these two states. The measured rate constant for the most strongly perturbed level is comparable in magnitude to the rate constant expected for hard sphere collisions between argon and AlO. The present results allow some understanding of the continuum emission observed in the upper atmosphere release of aluminized grenades. They also may be pertinent to the construction of a visible chemical laser.
The free energy of a vacancy
Authors: Cook Robert , Negri David
Published: February 2003

The configurational free energy change ΔA/kT associated with the introduction of a vacancy defect into a two-dimensional triangular lattice with nearest neighbor interactions is calculated by lattice dynamics in the classical high temperature limit and found to be −1.680±0.002.
Angular correlations in linear copolymers from the compositional dependence of their dipole moments. IV. Local environmental effects in undiluted poly (4-chlorostyrene, styrene) copolymers
Authors: Tréhu Y. M. , Work R. N.
Published: February 2003

Assumptions which are basic to the copolymer method of evaluating local, intrachain angular correlations in undiluted polymers have been examined experimentally through measurements of effective dipole moments, made at temperatures above the glass transition, of poly(4-chlorostyrene,styrene) copolymers, both undiluted and diluted with polystyrene. Changes in the environment in the vicinities of specified sequences of structural units that accompany the changes in copolymer composition contribute not more than about 3% to the conformational parameter g-1, where g is the Kirkwood correlation factor, specialized to a one-dimensional system, for poly(4-chlorostyrene). This suggests that the copolymer method is capable of providing meaningful information relating to the conformations of polymer molecules in the undiluted state.
X-ray and electron scattering from diatomic molecules in the first Born approximation
Authors: Epstein Joel , Stewart Robert F.
Published: February 2003

The total and elastic x-ray scattering intensities from the diatomic molecules CO, N2, and O2 have been calculated within the Waller–Hartree theory. Near Hartree–Fock quality wavefunctions with extensive basis sets at experimental Re values have been employed. The calculated intensities reflect the full accuracy of the published wavefunctions. The electron–nuclear interference terms have also been computed and tabulated so that studies of high energy electron scattering intensities can be made. An independent atom model has been constructed from atomic wavefunctions, which served as a starting basis for calculations of the molecular wavefunctions. It is found in the scattering range for which 4π sinϑ/λ?10 Å−1, neither the total nor the elastic x-ray intensities can be sensibly neglected compared to the electron–nuclear interference terms for the three diatomic molecules studied in this work.
Theory of differential phase fluorometry: Detection of anisotropic molecular rotations
Authors: Weber Gregorio
Published: February 2003

A general description of transformations in the excited state is employed to derive the general equations for the differential delay and the modulation ratio of the fluorescence owing to different pairs of excited species present. These equations yield directly the differential delay and the modulation ratio of the polarized components of the fluorescence from a rotating spherical molecule. Similar equations for a rotating irregular molecule are then derived from the sine and cosine transforms of the impulse response of the polarized components of the fluorescence emission. It is shown that for excitation at a wavelength at which the limiting polarization is high, namely, 1/2 to 3/11, the maximum differential tangent observed for anisotropic rotations is uniformly lower than the value for the sphere. The magnitude of this tangent defect permits an estimate of the minimum standard deviation of the principal rotational rates of the ellipsoid of inertia that describes the rotations of the molecule.