bluuebunny/crossref_metadata_2025 huggingface dataset:
The interaction of atomic and molecular nitrogen with argon by scattering measurements
Authors: Brunetti B. , Liuti G. , Luzzatti E. , Pirani F. , Volpi G. G.
Published: February 2003

Absolute integral cross sections for collisions of N2 molecules and N atoms with Ar are measured as a function of velocity at thermal energies. For the N2–Ar case the glory structure observed in the cross section vs velocity plot does not appear to be affected by the anisotropy of the interaction. An analysis in terms of a spherically averaged potential model can be performed thus obtaining a reliable potential function. The N–Ar cross sections presented here are the first collisional study of N atoms leading to significant information on the interaction potential. Although, as shown by the magnetic behavior, the N atom beam is essentially a mixture of atoms in the two metastable 2DJ and 2PJ states, the results obtained, together with other properties of the excited atoms, indicate the presence of a low anisotropy in the N–Ar interaction. An analysis in terms of a simple and yet realistic potential model has been performed obtaining meaningful potential parameters for the N–Ar system. The results for N–Ar are tentatively inserted in a comparison of trends and regularities of the interactions of some second row elements of the periodic table with Ar.
IR-MPE/D dynamics studied by pulsed VUV laser photoionization
Authors: Feldmann D. , Laukemper J. , Welge K. H.
Published: February 2003

One-photon ionization by a pulsed VUV laser has been employed to probe infrared multiphoton excitation and dissociation (IR-MPE/D) dynamics in real time, in situ, and collision free during excitation by a CO2− laser pulse. In this first experiment acetic anhydride has been used as the parent molecule to demonstrate the feasibility and to study properties of the method. Using the 118.4 nm radiation from a VUV laser as source for photoionization, ions have been observed mass selectively with time resolution in the nanosecond range, determined by the laser pulse duration. By following production and decay rates of ions, the time dependence of the excitation and decay of parent molecules, and the evolution of neutral intermediates in primary and sequential steps could be observed, from which detailed information on excitation and dissociation pathways was deduced.
The enhancement of intense laser induced multiphoton processes by a static electric field
Authors: Thomas Gerald F.
Published: February 2003

The multiphoton absorption processes induced in model two-level systems through their interaction with intense laser and static electric fields are investigated. An analytical treatment of the degenerate system enables one to anticipate the effects of the static field on the multiphoton spectrum, including the influences of collisional damping. Using Floquet theory in the numerical treatment of a nondegenerate system confirms the enhancement of the multiphoton spectrum whose resonances will be more power broadened and will appear at higher frequencies than they would in the absence of the static field.
Relativistic effects on molecular hyperfine interactions: Application to XeF and CsO
Authors: Arratia-Perez Ramiro , Case David A.
Published: February 2003

A relativistic theory of molecular hyperfine interactions is presented, based on the use of multiple scattering theory to generate approximate solutions to the Dirac equation. This approach should be especially useful for radicals with significant orbital contributions to the hyperfine tensor. Results for XeF and CsO are in good agreement with experiment, and show that orbital interactions are important for both heavy and light atoms, and that A⊥ for fluorine in XeF is likely to be negative.
Charge partitioning by zero-flux surfaces: The acidities and basicities of simple aliphatic alcohols and amines
Authors: Stutchbury Neil C. J. , Cooper David L.
Published: February 2003

The zero-flux surface criterion for the partitioning of the total molecular electron density is investigated in order to determine the sensitivity of the populations to basis set. Electron populations obtained from this criterion are used to rationalize the acidities and basicities of simple aliphatic alcohols and amines.
Pseudoeigenvalue methods for orbital optimization. General theory and application to closed shell, open shell, and two configuration SCF wave functions
Authors: Page Michael , McIver J. W.
Published: February 2003

A general Newton–Raphson based iterative method of orbital optimization is presented. In contrast to the usual exponential transformation technique, the unitary orbital rotation matrix is specified in terms of unconstrained variables through the use of an eigenvalue equation. The method seeks improved orbitals by repeatedly constructing and diagonalizing a single symmetric matrix. The theory is applied to the closed shell, open shell, and two configuration self-consistent field (2CSCF) wave functions. In these cases, simplifying approximations greatly reduce the computational labor without seriously impeding convergence properties. Under these approximations and a particular specification of certain parameters, the closed shell case becomes identical to the traditional Roothaan method. However, an alternative specification gives a method which has superior convergence properties to the Roothaan method. The convergence properties of the general method are examined. The general criterion for the intrinsic convergence of the method and a simple test for the stability of the converged solution are given. Also, an inexpensive enhancement based on an interpolation scheme results in accelerated and forced convergence. Some aspects of the implementation of the method are discussed. Relatively minor modifications to existing closed shell computer programs allow the calculation of open shell and 2CSCF wave functions.
The structure of charged colloidal dispersions
Authors: Grimson Malcolm J.
Published: February 2003

The static structure factor S(q) for a dispersion of charged colloidal particles is obtained for large values of the wavevector q by a model of the one-component plasma (OCP) in which the direct correlation function c(r) is known analytically. The model OCP is then used as a reference system for a perturbation theory of the small q behavior of the structure factor based upon a form of the random phase approximation suitably modified to treat the inclusion of a hard core in the perturbation potential. Explicit results are obtained for the coefficients in a small q expansion of the Fourier transform of the direct correlation function c(q). The values of S(q=0) determined from the extrapolation of the experimental data are analyzed in terms of the screened Coulomb interaction from the DLVO theory of colloid stability. The calculated surface potentials are found to be smaller than the values normally associated with this type of system. The theory gives the correct qualitative form for the small q region of S(q) in dense dispersions near the melting transition. At lower particle densities, the theory predicts the existence of a shallow minimum in S(q) at small q.
Complex time dependent wave packet technique for thermal equilibrium systems: Electronic spectra
Authors: Reimers Jeffrey R. , Wilson Kent R. , Heller Eric J.
Published: February 2003

A time dependent wave packet method is presented for the rapid calculation of the properties of systems in thermal equilibrium and is applied, as an illustration, to electronic spectra. The thawed Gaussian approximation to quantum wave packet dynamics combined with evaluation of the density matrix operator by imaginary time propagation is shown to give exact electronic spectra for harmonic potentials and excellent results for both a Morse potential and for the band contours of the three transitions of the visible electronic absorption spectrum of the iodine molecule. The method, in principle, can be extended to many atoms (e.g., condensed phases) and to other properties (e.g., infrared and Raman spectra and thermodynamic variables).
Two-photon spectroscopy of the A 2Σ+–X 2Πi system of OH
Authors: Crosley David R. , Smith Gregory P.
Published: February 2003

Two-photon laser-induced fluorescence of the A 2Σ+−X 2Πi system of the OH molecule, produced in the burnt gases of a methane-air flame, has been studied. Over 100 rotational lines in the (0,0) band have been identified and the measured line strengths agree with calculations. The absolute cross section for the (0,0) band is between 2 and 8×10−32 cm4/W and the relative intensities for (0,0), (1,1), and (1,0) are 100:25:1. These are compared with theoretical calculations in order to identify the intermediate state(s) responsible for the absorption strength. Good agreement with the absolute magnitude and both ratios is obtained by assuming that nearby v, J levels of both the A and X states serve as the intermediates.
Spectral studies of cold UF6 solutions
Authors: Maier William B. , Holland Redus F. , Beattie Willard H.
Published: February 2003

Absorption spectra of UF6 dissolved in liquid xenon, krypton, nitrous oxide, and methane have been obtained between 300 and 500 nm and between 400 and 4000 cm−1. Eighteen infrared bands of UF6 were observed and were moderately (2–9 cm−1 FWHM) narrow. The band positions were found to move toward higher wave numbers ν̃ as the temperature T was increased; dν̃/dT∼0.02 cm−1/°C for the bands examined. Some anharmonicity constants were determined and were within experimental error of previous values for UF6 vapor. Integrated absorption cross sections for the ν3, ν5+ν3, ν2+ν3, and ν1+ν3 bands of UF6 were measured to be ≊1×10−16, 1.8×10−19, 5.8×10−19, and 3.9×10−19 cm2–cm−1, respectively. The solubilities of UF6 in liquid xenon, krypton, and methane have been measured.
‘‘White lines’’ in the chlorine K x-ray absorption spectra of osmium(IV), iridium(III), iridium(IV), and platinum(IV) hexachlorides
Authors: Sugiura Chikara
Published: February 2003

The chlorine K absorption spectra from K2OsCl6, K2IrCl6, K2PtCl6, (NH4)2OsCl6, (NH4)3IrCl6, (NH4)2IrCl6, and (NH4)2PtCl6 have been measured and explained in terms of molecular-orbital (MO) theory. Two conspicuous ‘‘white lines’’ have been observed at the absorption threshold of the spectra of the iridium(IV) and osmium(IV) hexachlorides in contrast to one intense white line in the spectra of the platinum(IV) and iridium(III) hexachlorides. The latter white line is ascribed to the electronic transition from the Cl 1s core level to the lowest unoccupied antibonding MO eg(σ*) for the octahedral complex ions [PtCl6]2− and [IrCl6]3− and the former white lines are attributed to the transitions to the unoccupied MO’s eg(σ*) and t2g(π*) for the octahedral complex ion [IrCl6]2− or [OsCl6]2−. The energy differences between the eg(σ*) and t2g(π*) orbitals for the complex ions [OsCl6]2− and [IrCl6]2− are estimated to be 1.8 and 1.9 eV, respectively. These white lines show the chemical shifts toward higher energies in the order of the platinum(IV) <iridium(IV) < iridium(III) < osmium(IV) hexachlorides. The other absorption structures are broad and are quite alike throughout seven hexachlorides. The chlorine K absorption spectra of the hexachlorides are little influenced by the exchange of the counterions K+ and NH4+.
Vibrational state distributions and absolute excitation efficiencies for T-V transfer collisions of NO and CO with H atoms produced by excimer laser photolysis
Authors: Wight Charles A. , Leone Stephen R.
Published: February 2003

Translation-to-vibration energy transfer from fast H atoms to NO and CO is studied by the excimer laser photolysis/infrared fluorescence method. The excited vibrational state distribution in NO resulting from collisions with H atoms at 2.3 eV initial translational energy is 0.55±0.03, 0.24±0.03, 0.07±0.03, 0.05±0.03, 0.04±0.02, 0.03±0.02, and 0.01±0.02 for v=1–7, respectively. The distribution is similar to that previously reported for H+CO collisions at the same energy [C. A. Wight and S. R. Leone, J. Chem. Phys. 78, 4875 (1983)]. However, the absolute T-V transfer efficiency for H+CO is a strong function of initial energy, increasing from 7% at 0.95 eV to 28% at 3.1 eV, whereas the efficiency for H+NO is essentially constant at 14% over the same range of initial energies. This qualitatively different behavior is not expected from simple models of T-V energy transfer, but may be attributed to differences in the attractive regions of the potential energy surfaces of the HNO and HCO transient species.
Molecular dynamics equations of motion for systems varying in shape and size
Authors: Ray John R.
Published: February 2003

A unified derivation of the molecular dynamics equations of motion for systems with changing shape and size is presented. Ambiguities in the choice of the kinetic energy of the molecular dynamics cell are also discussed. It would be useful to perform computations to compare the effects of the different expressions for the kinetic energy of the molecular dynamics cell.
Angular momentum limitation in vibrationally inelastic collisions of I2(B 3Π) with He and Xe
Authors: Dexheimer S. L. , Brunner T. A. , Pritchard David E.
Published: February 2003

We present measurements of thermally averaged level-to-level rate constants for vibrationally and rotationally inelastic collisions of I2(B 3Π) with Xe and He:I*2(vx=13, ji=41 or 91) → I*2(vf=12, jf). Comparison of these results with our previous work on the same systems for rotationally inelastic collisions with no vibrational inelasticity reveals a similarity in the collision dynamics for the vibrationally inelastic and vibrationally elastic processes. (AIP)
Defect states in polyacetylene: A photoacoustic and diffuse reflectance study
Authors: Eckhardt H. , Chance R. R.
Published: February 2003

We present a combined photoacoustic spectroscopy (PAS) and polarized diffuse reflectance (PDR) study of relatively thick freestanding cis and trans polyacetylene films in the visible-near IR regime. Both techniques are suitable for measuring low absorption coefficients in solids. The PAS signal for the cis isomer is saturated in the region of the strong interband transition. There is no absorption in the near IR except for a vibrational overtone band at 0.74 eV. Thermal isomerization leads to a very broad, featureless absorption band in the near IR consistent with a broad distribution of defect states in the bandgap of trans-polyacetylene. Polarized diffuse reflectance spectra also show a higher absorption in the near IR for the trans isomer. Upon doping with oxygen, an absorption peak grows with time with a maximum at 0.67 eV and a characteristic asymmetric line shape consistent with soliton absorption and similar to what has been observed in the absorption spectra of thin acceptor-doped (CH)x films. Our PDR results suggest an absorption coefficient of the order of 100 cm−1 for the defect absorption in undoped trans-polyacetylene.
Laser-induced fluorescence studies of ion collisional excitation in a drift field: Rotational excitation of N+2 in helium
Authors: Duncan Michael A. , Bierbaum Veronica M. , Ellison G. Barney , Leone Stephen R.
Published: February 2003

Initial results are presented for a new method of studying collisional excitation and deactivation processes of molecular ions. Translationally excited ions are prepared in the uniform electric field of a drift tube. Collisions with the inert buffer gas lead to rotational excitation (T-R). Laser-induced fluorescence (LIF) is used as a direct optical probe of the internal states of N+2 using the B 2Σ+u–X 2Σ+g transition at 391.4 nm. In this initial experiment, rotational excitation is observed for N+2 in collisions with helium at energies up to 0.054 eV (c.m.). The rotational state distribution can be described by a Boltzmann temperature corresponding to the center-of-mass collision energy, in good agreement with theory. Approximately ten collisions or less are required to obtain full equilibration of the rotational distribution. Applications of this new technique to the interpretation of ion–molecule reaction rates in drift tubes and to the study of ion-neutral energy transfer processes are discussed.
Cross sections for electron impact excitation of electronic states in UF6 at incident electron energies of 10, 20, and 40 eV
Authors: Cartwright David C. , Trajmar S. , Chutjian A. , Srivastava S.
Published: February 2003

The inelastic scattering of electrons by UF6 has been studied at incident electron energies of 10, 20, and 40 eV, and for scattering angles of 20, 60, 90, and 135 deg. Using the structure of the energy-loss spectra, and recent theoretical work as a guide, the numerous electronic states between 3.3 and 10.0 eV were combined into nine inelastic features. These nine features were then examined in the electron energy-loss spectra by using a simple model to fit all the spectra in a least-squares sense and extract absolute differential cross sections for excitation of these composite features. Absolute integral cross sections were also obtained at these three incident electron energies. Absolute cross sections for elastic scattering of electrons by the UF6 molecule are also reported at impact energies ranging from 5 to 100 eV.
Cu and Ag as one-valence-electron atoms: Pseudopotential results for Cu2, Ag2, CuH, AgH, and the corresponding cations
Authors: Stoll H. , Fuentealba P. , Dolg M. , Flad J. , Szentpály L. v. , Preuss H.
Published: February 2003

Semiempirical pseudopotentials, representing singly ionized X+ cores, have been adjusted for the Cu and Ag atoms. Polarization of the X+ cores (including core-valence correlation) is explicitly taken into account. Deviations of (unpolarized) core interactions, X+–X+ and X+–H+, from point-charge behavior are treated at the SCF level. Valence correlation is included within the framework of the spin-density functional formalism. Results are given (i) for atomic dipole polarizabilities and electron affinities, (ii) for bond lengths and dissociation and ionization energies of the diatomics XHm+ and Xm+2 (X=Cu, Ag; m=0, 1). The accuracy of our results for the dimers is 0.1 eV for binding energies and 0.05 Å for bond lengths. In contrast to the situation for the alkali dimers, Cu+2 and Ag+2 prove to be less strongly bound than the corresponding neutral dimers.
Atomic and molecular correlation energies with explicitly correlated Gaussian geminals. II. Perturbation treatment through third order for He, Be, H2, and LiH
Authors: Szalewicz Krzysztof , Jeziorski Bogumil/ , Monkhorst Hendrik J. , Zabolitzky John G.
Published: February 2003

Third-order correlation energies for He, Be, H2, and LiH were calculated using highly accurate first-order pair functions obtained recently by us [J. Chem. Phys. 78, 1420 (1983)] with a novel second-order energy functional and a large basis set of explicitly correlated Gaussian geminals. Since these strong orthogonality projection, no new types of integrals appear as compared to the second-order treatment. Consequently, the third-order calculation does not require much more time than the second-order one. For He and Be atoms, the calculated third-order energies agree very well with the most accurate results obtained using the partial-wave expansion and extrapolation techniques. We estimate that our third-order energies are accurate up to within a fraction of 1%. The sum of the calculated values of the second- and third-order energies represents 97.5%, 90.4%, 94.4%, and 95.0% of the experimental correlation energy for He, Be, H2, and LiH, respectively.
A calculation of the self-diffusion coefficient for a dilute solution of Brownian particles
Authors: Evans Glenn T. , James C. P.
Published: February 2003

The linear concentration dependence of the self-diffusion coefficient Ds is calculated for a suspension of interacting Browian particles. The Einstein expression for the mean square displacement of a tagged particle is converted into a time correlation function and evaluated using perturbation methods. The derived concentration dependence of Ds in a system of hydrodynamically interacting hard spheres is in accord with previous literature results. Furthermore, the effects of short and long range potentials on Ds are determined.
Quasifree electron mobility by the method of partial waves in liquid hydrocarbons and in fluid argon
Authors: Vertes Akos
Published: February 2003

Applicability of the fluctuation model was tested in the case of n-hexane, n-pentane, c-hexane, 2,2-dimethylbutane, 2,2,4,4-tetramethylpentane, iso-octane, and neopentane. In our model, the quasifree electrons have been assumed to be scattered by the conduction state energy fluctuations of the liquid. These fluctuations are, in turn, described as a consequence of density fluctuations. The scattering potential is supposed to be square well like and the cross section is calculated in terms of partial waves. Averages due to the density fluctuations and the electron kinetic energy distribution are determined numerically. Except for the first three materials, the calculation reproduced the experimental mobilities with reasonable values of the square well radius, which is the only fitting parameters. Further extension of the description concerning the density dependence of the low field mobility of fluid argon has been performed. The estimated fluctuation size as a function of density increases monotonically at the minimum of the mobility in accordance with the monotonic behavior of the isothermal compressibility in the same region.
Orientational order in ionic crystals containing tetrahedral ions
Authors: Klein Michael L. , McDonald Ian R. , Ozaki Yoshiaki
Published: February 2003

Molecular dynamics calculations based on atom–atom potentials and rigid nonpolarizable ions are used to study the cubic rotator phases of ammonium bromide (NH4Br). The nature of the orientational order exhibited by the ammonium ions is characterized using tetrahedral rotor functions (Mμ, μ=1,7). The quantities 〈M2μ〉 are used as static order parameters whose decay is monitored via the correlation functions 〈Mμ(t)Mμ(0)〉. In the high temperature sodium chloride structure the NH+4 ions exhibit mostly C3v order, while in the room temperature cesium halide structure Td order is dominant. As in the real crystal, cooling of the latter phase is shown to yield an antiferro arrangement of the NH+4 ions which in turn gives rise to a central peak in the zone boundary (M point) transverse acoustic phonon of appropriate symmetry. A crystal field analysis is shown to be useful in understanding the nature of the orientational order which results from a competition between the atom–atom and electrostatic interactions. The latter dominate in ammonium bromide. A similar analysis is applied to molecular dynamics calculations on the cubic high temperature rotator phases of KClO4 and Li2SO4.
Effect of concentration on the rate of chemical reactions
Authors: Ward C. A.
Published: February 2003

A method that has recently been used to examine the rate of molecular transport across a phase boundary—statistical rate theory— is extended to obtain the expression for the rate of a single step chemical reaction. The concentration dependence of the newly derived rate expression leads to the prediction of a higher reaction rate in the period immediately after the reaction has begun than any of the previous theories and gives a different physical interpretation of the ‘‘rate constant’’. When ions are reacting in solution under near-equilibrium conditions, the concentration dependence of the newly derived rate expression reduces to the conventional expression; further under these conditions, the new rate expression leads to an expression for the conventional rate constant that is equivalent to the Bro/nsted primary salt effect for the change in the rate constant with ionic strength. To examine the predicted behavior in the initial period, the predictions are compared with the measurements of ion concentration during electron exchange reactions. When these experiments were originally conducted, using radioactive labeling techniques, substantial disagreement was found between the conventional theory and the measurements. To bring them together the concept of ‘‘zero-time exchange’’ was introduced which amounts to adding a constant to the predictions of the conventional theory. The physical basis for this concept has remained undefined. When the statistical rate theory expression is compared with the data from six experiments available in the literature, it is found for five of the six that there is no measurable disagreement between the data and the theory and only small disagreement for the other case. From the point of view of statistical rate theory it is unnecessary to introduce the concept of zero-time exchange.
Theory of the electrical conductivity in the liquid–vapor interface of a simple metal
Authors: Mohanty Udayan , Rice Stuart A.
Published: February 2003

We report an analysis of the electrical conductivity in an inhomogeneous liquid metal. The particular problem addressed is the assessment of the influence of a liquid–vapor transition zone, with nonzero width and internal structure, on the components of the conductivity perpendicular to and parallel to the surface. Our analysis is developed from a generalization of the Ziman theory of conductivity of a homogeneous liquid metal. It is based on the nearly free electron-pseudopotential model of liquid metal, and is valid in the domain for which the penetration depth is large relative to the mean free path, which in turn is large relative to the nearest neighbor spacing in the liquid. We show that the parallel component of the conductivity, as a function of distance along the normal to the surface, increases continuously from the bulk value to a maximum somewhere in the liquid–vapor transition zone, then decays to zero in the vapor. The predicted anisotropy of the conductivity at the surface, σ∥≳σ⊥, is in agreement with the model conductivity profile proposed by Rice and co-workers to reconcile the optical constants of liquid Hg determined separately from reflectivity and ellipsometric data.
Toward understanding microemulsion microstructure: A small-angle x-ray scattering study
Authors: Kaler Eric W. , Bennett Karl E. , Davis H. Ted , Scriven L. E.
Published: February 2003

The microemulsion phases formed in solutions of octane, commercial surfactant, and alcohol with various brines are examined with small-angle x-ray scattering (SAXS), electrical conductivity, and viscosity techniques. Models based on monodisperse populations of swollen micelles or microemulsion ‘‘droplets’’ adequately represent the SAXS data at low volume fractions of brine. Introduction of hard-sphere interactions with the Percus–Yevick approximation allows us to model the composition dependence of the radius of gyration and isothermal compressibility up to volume fractions of brine near a percolation threshold for electrical conductivity. For brine volume fractions above the percolation threshold, a mean field attractive interaction term is needed to model the variation of isothermal compressibility; however, the same theory fails to model the composition dependence of the apparent radius of gyration. But predictions from a model for a bicontinuous microemulsion structure that is geometrically irregular yet topologically ordered and that evolves continuously into swollen (inverted) micellar solutions at low volume fraction of water (oil) are in good agreement with the SAXS and electrical conductivity data over a wide range of brine volume fractions.
Vibrational energy transfer and localization in disordered solids by picosecond CARS spectroscopy
Authors: Chronister Eric L. , Dlott Dana D.
Published: February 2003

The dephasing rate of the three longest lived vibrons of naphthalene-h8 (T1∼100 ps) is investigated by picosecond CARS in disordered mixed crystals containing naphthalene-d8. Vibrational energy transfer can either occur preferentially to one acceptor state or fairly equally to a number of acceptors. For two of the vibrons studied, the decay occurs preferentially to the lower energy d8N vibron of the same normal coordinate. The other vibron studied appears to couple fairly equally to a large number of acceptors since its corresponding d8N vibration is higher in energy and unavailable for decay. The coupling matrix elements are calculated for the specific decay routes and found to be ∼10 greater than the coupling to other acceptor states. For one of the vibrations, nonexponential decays are observed at high d8N concentrations indicating a distribution of environments and hence localization. A fit to a Voight line shape gives an estimate of the inhomogeneous distribution of frequencies experienced by this localized vibration.
ESR study of irradiated single crystals of the cocrystalline complex of cytidine: Salicylic acid
Authors: Close David M. , Sagstuen Einar
Published: February 2003

Irradiation at 77 K of single crystals of the 1:1 complex of cytidine and salicylic acid produces a phenoxyl radical formed by oxidation of the salicylic acid. Anisotropic hyperfine coupling tensors have been determined for this radical which are associated with the para and ortho hydrogens. No cytidine oxidation products (alkoxy or hydroxyalkyl radicals) were observed at 77 K. Following the decay of the phenoxyl radical at room temperature, four radicals were detected. These include the cytosine 5–yl and 6–yl radicals, formed by H addition to the cytosine ring, and an anisotropic doublet. By UV irradiation at room temperature, it is possible to convert a significant fraction of 6-yl radicals into 5-yl radicals. Hyperfine coupling and g tensors determined for the anisotropic doublet indicate that this radical is formed in the C1'-C2' region of the sugar moiety. These results indicate a shift in radiation damage away from the salicylic acid upon warming, and show that the radiation chemistry of the cocrystalline complex is different from that of the isolated bases.
The measurement of translational energy in IR MPD reactions by the transient migration technique
Authors: Rayner D. M. , Hackett P. A.
Published: February 2003

In this paper we develop mathematical and experimental refinements to the transient migration method for measuring the average translational energy of fragments produced in IR MPD. The mathematical methods presented are general for any translational energy distribution function and for the first time allow rigorous comparison of isotropic optical-time-of-flight data with results from molecular beam studies. The experimental refinements which include the use of TEM00, SLM CO2 laser pulses clipped at 80 ns pulse width by an optically triggered plasma shutter and the direct determination of the initial fragment concentration profile, remove sources of considerable error from the measurements. These approaches have been tested for four infrared photolysis schemes: (1) CF2HCl→CF2+HCl; (2) CF2Cl2→CF2+Cl2: (3) CF2Cl2→CF2Cl+Cl; and (4) CF3I→CF3+I, in which CF2 was detected by laser induced fluorescence and iodine atoms were detected by multiphoton ionization. The measured transient concentration decays are in complete accord with simulations using previously published molecular beam data for the expected translational energy distribution function. The techniques presented here are sensitive to, but do not measure, the shape of the translational energy distribution function. They measure the average expected value 〈ET〉 for an assumed distribution function. The extent and effects of this sensitivity are discussed in terms of future applications of the transient migration method.
Interpretations of the mercury halide (B 2Σ+–X 2Σ+) chemiluminescence from reactive quenching of Hg(3P2) by halogen containing molecules
Authors: Dreiling T. D. , Setser D. W.
Published: February 2003

The HgX(B 2Σ+–X 2Σ+), X=halogen, emission spectra have been recorded from Hg(3P2) atom reactions with several halogen containing molecules using a flowing afterglow reactor. The HgX* emission intensities were compared with the HgCl* emission intensity from Hg(3P2)+Cl2 to assign rate constants for HgX(B) formation. The Hg(3P2)+ diatomic halogen reactions have large kHgX* and their branching fractions for HgX(B) formation are probably unity. The polyatomic reagents generally gave much smaller kHgX*; although, a few moderately good donors, e.g., CF3I, CBr4, CCl4, and NF3 were identified. The experimental spectra were numerically simulated to assign HgX(B) vibrational energy distributions. A decrease in 〈fV(HgX)〉 was found for the Cl2, Br2, I2 series which is analogous to previous findings for 〈fV(XeX)〉 from reactive quenching of Xe(3P2). Possible reasons for the similarity are discussed. The HgX(B) vibrational energy disposal for polyatomic reagents is compared to analogous data for reactions of alkaline earth metal atoms and Xe(3P2).
The formation of OH(A 2Σ+) from H2O in a longitudinal discharge
Authors: Gerber T. , Lüthy W.
Published: February 2003

The production of OH molecules in the A 2Σ+ state by a discharge in a glass tube containing either H2O or H2O–rare gas mixtures was examined in a pressure range of about 1–10 mbar. The population density of A 2Σ+ OH molecules was determined to be in the order of 1013 cm−3. The formation of excited OH molecules is mainly due to electron impact dissociation of H2O.
The direct ultraviolet absorption spectrum of the 1Σ+g→ 1B2(1Σ+u) transition of jet-cooled CS2
Authors: Hemley R. J. , Leopold D. G. , Roebber J. L. , Vaida V.
Published: February 2003

The absorption spectrum of the linear→bent 1Σ+g→1B2 (1Σ+u) transition of jet-cooled CS2 is reported in the 47 750–53 200 cm−1 region. Adjustment of diluent gas composition enables the preparation of rotationally cold (<10 K)/vibrationally hot (≊300 K) and rotationally cold/vibrationally cold samples. The collapse of rotational envelopes on cooling facilitates the accurate determinations of frequencies and relative absorption intensities of transitions both from the zero point level and from excited vibrational levels of the ground electronic state. At 49 000–51 000 cm−1, in the region of the Franck–Condon maximum, a simple vibronic pattern emerges involving two upper state vibrational modes containing both stretching and bending contributions. In this region of the spectrum, the upper state rotational structure diverges from that of a nearly symmetric top and approaches that of the linear or quasilinear molecule. At higher energies, the vibronic structure grows in complexity, suggesting increased anharmonic mixing of these nearly degenerate modes.
Heat capacity and thermodynamic properties of p-terphenyl: Study of order–disorder transition by automated high-resolution adiabatic calorimetry
Authors: Chang Shu-Sing
Published: February 2003

The heat capacity of a sample of zone-refined, high-purity p-terphenyl has been determined from 4 to 370 K in a fully automated high-resolution vacuum adiabatic calorimeter and from 320 to 580 K in a differential scanning calorimeter. The melting point of p-terphenyl is 487.0 K and the enthalpy of fusion is 35.3 kJ/mol. A λ-type solid–solid transition occurs with a peak temperature of 193.55 K. The transition is highly reproducible without observable hysteresis, even after various thermal treatments. In the transition region which spans from 140 to 240 K, the sample reaches a state of thermal equilibrium within a period of one-half to one hour, as normally required in adiabatic calorimetry. These characteristics are desirable for the application of the equilibrium λ transitions as a calibration standard for use in dynamic calorimetry. The behavior of the λ transition at equilibrium is mapped in high temperature resolution, with small temperature increments of measurement down to 0.01 K by adiabatic calorimetry. In the temperature region between the transition and the fusion, the heat capacity of the high-temperature form of the crystal is proportional to the temperature Cp =0.94 T J/K/mol to within 1%. The heat capacity of p-terphenyl in the liquid state above its melting point of 487.0 K appears to be an extension of the heat capacity of the liquid, and the supercooled liquid, state of o-terphenyl above its glass transition temperature of about 243 K.
Perturbation theory for the Lennard-Jones diatomic fluid. I. Site-centered spherical harmonic coefficients
Authors: Nezbeda Ivo , Smith William R. , Labik Stanislav
Published: February 2003

The RAM perturbation theory is tested via calculations of spherical harmonic coefficients Gklm of the site–site pair correlation function G(12) for Lennard-Jones diatomic fluids at a range of elongations and state conditions. The accuracy ranges from good to excellent, depending upon the particular form of implementation of the theory. Although not a necessary prerequisite, the accuracy of the theory may be improved when exact values of the average site–site pair correlation function are used as input quantities in the calculations. This permits comparison with a set of alternative theories which require such input information, and which are based on the site-superposition approximation (SSA). In addition to the intrinsic advantage of possessing the ability of compute the full G(12), the accuracy of the RAM results appears always to be equal to or better than that of the SSA theories.
Solid argon revisited: Pair and many-body potentials
Authors: Barker J. A. , Bobetic M. V.
Published: February 2003

Low temperature properties of solid argon are compared with those calculated using the pair potentials of Barker, Fisher, and Watts and of Aziz and Chen, together with the best available estimates of long-range many-body interactions. The results calculated with the two potentials are very close to one another and in excellent agreement with experiment provided that the three-body interactions are included.
High pressure Raman study of vibrational relaxation in liquids of group IV tetramethyl compounds
Authors: Perry S. , Jonas J.
Published: February 2003

The vibrational relaxation in liquids composed of the group IV tetramethyl compounds C(CH3)4, Si(CH3)4,Ge(CH3)4, and Sn(CH3)4 has been studied as a function of temperature and density. The isotropic Raman lines shapes of the ν3 M-CH3 symmetric stretch, where M=C, Si, Ge, and Sn, have been measured as a function of pressure up to 4 kbar within the temperature range −15 °C–125 °C. Densities have also been determined under these experimental conditions. The isolated binary collision model for dephasing reproduces the essential features of the density and temperature dependence of the vibrational relaxation rate τ−1v . The effects of the mass of the molecule on the relaxation process are discussed.
Phase transitions in lattice solutions of semiflexible chains
Authors: Ronca G.
Published: February 2003

A mean field theory for two- and three-dimensional cubic lattice solutions of long semiflexible chains is developed using Di Marzio’s treatment of excluded volume. The configurational term of the partition function at a fixed level of macroscopic orientation is evaluated in close analogy with recent treatments of wormlike chain systems. In the presence of macroscopic orientation, the effective one-chain partition function shows typical Ising features. Mesomorphic phases of intermediate orientation are predicted to become more stable than the isotropic solution below a characteristic temperature which depends on concentration. In two dimensions, a continuous set of coexisting phases of different concentration and corresponding orientation joins the isotropic solution at the transition temperature. In three dimensions our analysis, restricted to states of uniaxial symmetry, predicts a first order transition whose order parameter decreases with concentration and approaches the limiting value 1/2 for very dilute systems. The change of the fraction of ‘‘gauche’’ bonds at the transition is modest, except in the immediate vicinity of the bulk limit, where the classical isotropic-crystalline mean field alternative is found again.
A determination of individual spectral densities in a smectic liquid crystal from angle dependent nuclear spin relaxation measurements
Authors: Barbara Thomas M. , Vold Regitze R. , Vold Robert L.
Published: February 2003

Deuterium relaxation experiments have been carried out at 38.4 MHz from 29.9 to 44.3 °C on p-butoxybenzylidene-p-n-octylaniline (40⋅8), a smectic B liquid crystal, which had been deuterated in the aniline ring and along the octyl chain. The Jeener–Broekaert pulse sequence was used to determine the spectral densities of motion J1(ω) and J2(ω), and by rotating the sample such that the optic axis was oriented at three different angles to the external magnetic field we are able to obtain information about the frequency dependence not just of J1 and J2 but also of J0. We report here the first measurements of the latter parameter at frequencies other than zero.
Carbon monoxide oxidation on the kinked Pt(321) surface
Authors: Gland John L. , McClellan Michael R. , McFeely F. Read
Published: February 2003

Carbon monoxide oxidation has been studied using high resolution electron energy loss spectroscopy, temperature programmed reaction (TPR), and titration of adsorbed atomic oxygen on a kinked Pt(321) surface characterized using Auger electron spectroscopy and low energy electron diffraction. Two previous papers detail adsorption and dissociation of oxygen and adsorption of carbon monoxide on the Pt(321) surface. Vibrational spectra taken during interrupted reaction experiments clearly indicate that atomic oxygen adsorbed on terraces reacts with CO more rapidly than atomic oxygen adsorbed near the kinked step sites. The multiple reaction limited CO2 peaks observed in the 100–400 K region can be rationalized by proposing that the activation energy for reaction of terrace and step CO with terrace and step atomic oxygen is sufficiently different to cause discernible CO2 peaks as the various species are sequentially depleted by reaction during a TPR experiment.
Properties of Sc3, Y3, and Sc13 molecules at low temperatures, as determined by ESR
Authors: Knight L. B. , Woodward R. W. , Van Zee R. J. , Weltner W.
Published: February 2003

Sc 3 and Y3 molecules have been isolated in rare gas matrices at temperatures near 4 K. ESR spectra establish that the structure of Sc3 is an equilateral triangle at 4–30 K with a 2A′1 ground state. The possibility remains that Sc3 is a fluxional bent molecule with a very low barrier to pseudorotation. The 45Sc hyperfine splitting indicates that the unpaired electron has little s character and is delocalized in 3d orbitals on the three equivalent atoms. Y3, however, is not equilateral and is most probably a bent molecule at these temperatures with the spin again distributed over the 3d atomic orbitals, but in a 2B2 ground state. La3 was not observed and is therefore judged to be a linear orbitally degenerate molecule. Under special conditions, a cluster of exceptional stability Scx, where x≥9 is formed in neon matrices. From its uniqueness and from the hyperfine structure in its ESR spectrum, it is suggested that it may be Sc13 with an icosahedral structure. This places 12 equivalent atoms at the vertices and one at the center of the icosahedron and leads to a 2Ag ground state. However, Scx as observed at 4 K could also be a dynamic Jahn–Teller molecule with only a small barrier between its distorted conformations.
Spin density distributions and g values in semiquinones
Authors: Prabhananda B. S.
Published: February 2003

From a study of several semiquinone systems, we have shown that their g values can be expressed in terms of the π-electron spin densities on the oxygens by a linear equation. This suggests the possibility of estimating the spin densities on the oxygens directly from the g values in similar systems. The spin density distribution used in establishing such a relation had been determined with the help of a new set of Q parameters for the 13C and 17O hyperfine splittings associated with the C′2CO fragment. Recognition of the existence of hydrogen bonds in solvents such as water or ethanol, by introducing an additional parameter QOOH is a new feature of this work. Q0OH=6.0 G in water or ethanol. QOOH=0 in aprotic solvents. Reliability of our Q parameters and the spin density distributions determined with their help, has been demonstrated by comparing the predicted and the observed hyperfine splittings/spin densities, not only in semiquinones, but also in 2,4,6 tri-t-butyl phenoxy radical and in benzophenone potassium ketyl. (Other authors have recommended substantially different sets of Q parameters.) It has been possible to estimate QCCC′=22.2 G, for sp3 hybridized C″ as in −C″H3, using the spin density distribution in durosemiquinone (DSQ−). Spin densities in DSQ− in ethanol were obtained with the help of our g values. Our value of QCCC″ is considerably less than the theoretical estimate of Strauss and Fraenkel, but is consistent with the predictions of Fessenden.
Spectral moments of vibrationally excited components of continuous vibronic spectra
Authors: Joens Jeffrey A. , Bair Edward J.
Published: February 2003

The spectral absorption coefficient of a continuous vibronic system is a composite of contributions which change according to the population of the vibrational levels of the lower electronic state. These components have distinctive shapes related to the shape of the vibrational wave function for a particular mode of the lower state. In addition, the component from each level of the lower electronic state is characterized by an average spectral frequency, or first spectral moment, observed as a shift from the position of the spectrum of molecules which are not vibrationally excited. The spectral moments can be calculated from sum rules. The present paper compares the spectral shifts observed in thermally excited spectra of Cl2, OCS, and O3 with those calculated using different approximations to the sum rules to examine the relative importance of: (a) the relative frequencies of the upper and lower states; (b) changes in the electronic transition moment with vibrational excitation; (c) anharmonicity; or (d) anharmonic mode coupling. It is found that each of these effects can be important, or even dominant, in individual cases.
Electric field effects on BZ chemical waves: Wave annihilation at negative fields
Authors: Schmidt S. L.
Published: February 2003

An asymptotic solution of the Field–Körös–Noyes (FKN) model of the Belousov–Zhabotinskii (BZ) reaction is coupled with electrical conduction and thermal diffusion and the parametric dependence of the wave velocity on the electric field is analyzed. Previous theoretical and experimental evidence has verified the existence of a critical field beyond which the waves are annihilated. Furthermore, theory predicts the existence of a Fisher limiting field at which the HBrO2 front of the BZ wave has the form and velocity of a Fisher wave (velocity ≥2 in dimensionless units). In this paper theoretical evidence for a second critical field is presented. The first critical field occurs at a positive field for waves moving in the positive direction; the present result predicts a wave destruction at negative fields beyond the Fisher limiting field. These results correspond to destruction by the catalyst (at positive field) and to destruction by bromide (at negative fields).