bluuebunny/crossref_metadata_2025 huggingface dataset:
The use of symmetrized valence and relative motion coordinates for crystal potentials
Authors: McMurry H. L. , Hansen Flemming Y.
Published: February 2003

Symmetrized valence coordinates are linear combinations of conventional valence coordinates which display the symmetry of a set of atoms bound by the valence bonds. Relative motion coordinates are relative translations, or relative rotations, of two or more strongly bonded groups of atoms among which relatively weak forces act. They are useful for expressing interactions between molecules in molecular crystals and should be chosen, also, to reflect the symmetry of the interacting groups. Since coordinates defined by these procedures possess elements of symmetry in common with the bonding electron distributions, the force constants in the potential should be more amenable to calculation in terms of energy changes in the electronic ground state which accompany displacements of the atoms from equilibrium. It is easier to determine force constants for fitting experimental data because interaction constants coupling coordinates of unlike symmetry with regard to the crystal point group are necessarily zero. They may be small, also, for coordinates which belong to different representations of the local symmetry when this is not the same as for the crystal. Procedures are given for defining the coordinates, and for assuring that the potential energy is invariant to crystal translations and rotations. The secular equation is derived by expressing the kinetic and potential energies in terms of components of mass adjusted basis vectors which are chosen so that high and low frequency modes can be separated approximately. The necessity to remove redundancies among the coordinates in the potential is avoided.
Multiphoton ionization mass spectroscopy of acetaldehyde
Authors: Fisanick G. J. , Eichelberger T. S. , Heath Barbara A. , Robin M. B.
Published: February 2003

Details of the positive ion fragmentation patterns of acetaldehyde produced by collisionless multiphoton ionization are presented. Fragments are generated in the ion collection region of a quadrupole mass spectrometer and analyzed with unit mass resolution. Daughter ions are found corresponding to the absorption of up to five photons. All fragment ions show the same excitation spectrum, which closely resembles that obtained with total ion collection in both the mass spectrometer and a conventional parallel plate ionization cell. Individual fragment ion production depends strongly on laser power, going approximately as Ip, where p=1.17±0.07 for CH3CHO+, up to p=3.54±0.09 for CH+3. Total ion production varies as p=2.19±0.08. Considerable deviation from a simple power law dependence can be found for some fragments, however. A rate equation approach is used to explain the experimental results.
Evaluation of kinetic theory collision integrals using the generalized phase shift approach
Authors: Thomas James A. , Curtiss C. F.
Published: February 2003

The collision integrals arising in the treatment of binary gas mixtures in applied magnetic fields are shown to be straightforward generalizations of those partially evaluated by L. W. Hunter and R. F. Snider, and utilized by Hunter in his treatment of the effects of a magnetic field on the shear viscosity and thermal conductivity of single component diatomic gases. These collision integrals are further developed by use of the generalized phase shift formalism, which permits the evaluation of several summations and angle integrations and leads to considerable simplification of the expressions.
Emission of the Hα and Hβ lines excited by electron impact on H2
Authors: Ortiz M. , Campos J.
Published: February 2003

The delayed coincidence method has been used to obtain the intensity decay curves of the two first lines of the Balmer’s series of atomic hydrogen. The excited hydrogen atoms have been produced by electron impact on H2 followed by dissociation. The measurements were made at pressures in the range 1–25 mTorr and with electron energies in the range 15–100 eV. From this experiment, the lifetimes of the 3s, 3d, 4s, 4p, and 4d excited levels have been deduced. The obtained values are in agreement with the theoretical data. The relative contributions of the nl states to the total Hα and Hβ lines have been found.
Collisional ionization of Cs and K by O2
Authors: Stockdale J. A. D. , Warmack R. J.
Published: February 2003

Collisional ionization of Cs and K atoms by O2 has been studied in a crossed beam geometry from threshold to ∼100 eV alkali laboratory system energy. The energy spectra of forward directed positive (Cs+ and K+) and negative (O− and O−2) ions were measured by electrostatic energy analysis. The dominant O−2 negative ion is associated with forward scattering of the alkali positive ion in the center-of-mass system but O− production appears associated with backward scattering of the alkali ion.
Calculated spectroscopic constants for the X 3Σ−, 1Δ, 1Σ+, 3Π, and 1Π states of CCO. The heat of formation of CCO
Authors: Walch Stephen P.
Published: February 2003

Geometries, excitation energies, and vibrational frequencies have been calculated for the X 3Σ−, 1Δ, 1Σ+, 3Π and 1Π states of CCO using POL–CI wave functions based on GVB wave functions with a polarized valence double zeta basis set. The results are compared to known spectroscopic constants for the X 3Σ− and 3Π states of CCO. The calculated Te’s (experimental values in parenthesis) are 0.95 eV for 1Δ, 1.58 eV for 1Σ+, 1.49 eV (1.44) for 3Π and 3.49 eV for 1Π. The calculated vibrational frequencies are ν1=1838 cm−1 (1978), ν2=367 cm−1 (379.4), ν3=1086 cm−1 (1074) for X 3Σ− and ν1=1841 cm−1 (2045.7), ν2=580 cm−1 (607.8), ν3=1097 cm−1 (∼1270) for 3Π. The Do (C–CO) is calculated to be 30.6 kcal/mole with an estimated error of ∼23 kcal/mole (based on comparable calculations for ketene) leading to an estimated Do (C–CO) of 54 kcal/mole and a ΔH°fo of 89 kcal/mole.
Absolute rate constant for the reaction of O(3P) with CH3SSCH3 from 270 to 329 K
Authors: Lee Jai H. , Tang I. N.
Published: February 2003

Rate constants for the oxidation reactions of organic sulfur compounds have become increasingly important due to the potential role of these compounds in the formation of tropospheric and stratospheric sulfate aerosols. The present study deals with the reaction of the ground state oxygen atom with dimethyl disulfide (DMDS). The discharge fast flow–resonance fluorescence technique was used to monitor the decay of O(3P) concentration in the presence of excess CH3SSCH3 under pseudo-first-order conditions. Over a temperature range from 270 to 329 K, and a pressure range from 0.52 to 2.60 Torr, no variation of the rate constant with temperature and pressure was observed. The results are best represented by the temperature and pressure independent value of (2.12±0.22)×10−10 cm3 molecule−1 s−1, which is about one-half the theoretical collision frequency. The zero activation energy for this reaction is discussed in terms of the temperature dependence of other sulfur compound–oxygen atom reactions reported in the literature.
Vibrational relaxation in jet-cooled p
a
r
a-alkylanilines
Authors: Powers D. E. , Hopkins J. B. , Smalley R. E.
Published: February 2003

Absorption and fluorescence spectra have been obtained for the 1B2(ππ*)←1A1 transition of a series of para-alkylanilines cooled in a supersonic free jet. The series consists of all p-n-alkylanilines through n-butyl- as well as p-isopropylaniline and aniline itself. For those members of the series having an alkyl chain length of three or greater, two distinct molecular conformations are found to be populated in the jet with roughly equal likelihood. Spectra of both the extended chain conformation [p-n-(t)-alkylanilines] and the coiled chain conformation [p-n-(g)-alkylanilines] show that two optically active aniline vibrations are largely unaffected by changes in the length and nature of the n-alkyl chain. One of these vibrations (the inversion motion of the –NH2 group) would be expected to be particularly weakly coupled to the torsions and bends of p-alkyl chain. Nonetheless, fluorescence spectra obtained when this vibration is excited in the 1B2 state show that intramolecular vibrational relaxation occurs into the chain modes within a nanosecond.
On laser-induced harpooning reactions
Authors: Weiner John
Published: February 2003

Switching of chemical reactivity by a nonresonant laser field in simple gas-phase collisions of the type A+BC→AB+C is discussed in terms of a 2nd-order optical/collisional perturbation. A simple formula relating laser-induced harpooning cross sections to the laser power density is derived and applied to Hg/Cl2 collisions. Calculation shows that a gas-kinetic cross section for the reaction (Hg+Cl2+h/ω→HgCl*+Cl) may be achieved in the presence of an ArF laser at a power density of 109 W/cm2.
Critical heat capacity of NH4Br and NH4BrxCl1−x single crystals
Authors: Lushington K. J. , Garland C. W.
Published: February 2003

A high-resolution ac calorimeter has been used to measure the heat capacity of NH4Br, NH4Br0.5lCl0.49, NH4Br0.26Cl0.74 and NH4Br0.04Cl0.96 single crystals over the range of 200–300 K. The behavior of Cp near the order–disorder transition in these crystals and in NH4Cl and ND4Cl is analyzed to obtain the critical exponent α′ in both the ’’ferro’’-ordered and ’’antiferro’’-ordered phases. In the ferro regime (NH4Cl, ND4Cl, and NH4Br0.04Cl0.96), the transition is weakly first order but quasitricritical behavior (α′?0.5) is observed. In the antiferro regime, the data for NH4Br and NH4Br0.26Cl0.74 (but not NH4Br0.51Cl0.49) can be well represented using α′=0.20 but may also be consistent with a crossover from Gaussian to cubic (d=3, n=3) critical behavior. The effects of inhomogeneous internal strains on Cp and the order parameter are discussed.
Temperature dependence of the far IR spectrum of α-N2
Authors: Medina F. D.
Published: February 2003

The temperature dependence of the far IR spectrum of α-N2 has been studied for a sample at constant volume. The two modes observed near 50 and 72 cm−1 have frequencies which increase with temperature. The integrated intensity of the mode near 50 cm−1 remains constant with temperature, within experimental uncertainty. The Grüneisen parameter of this mode is estimated to be ∼3.
The interaction of two spherical colloidal particles in electrolyte solution. An application of the hypernetted-chain approximation
Authors: Patey G. N.
Published: February 2003

The hypernetted-chain approximation is solved numerically for a pair of identical spherical colloidal particles immersed in an electrolyte solution. We consider a simple model system where the ions are hard spheres with embedded point charges and the colloidal particles are much larger hard spheres bearing a uniform surface charge distribution. The surface charge density is considered to be constant independent of the particle separation. We obtain the potential of mean force acting between the colloid particles and the structure of the double layer about a single particle. It is found that under some conditions the colloid-colloid interaction is attractive at small separations even in the absence of van der Waals forces. This result should be of interest in the theory of colloidal stability.
Molecular hydrogen exchange: A study of HD(v=5)+HD(v =0)→H2+D2
Authors: Herman Irving P.
Published: February 2003

A likely bimolecular pathway for the reaction HD(v=5)+HD(v =0)→H2+D2 at room temperature is investigated by cw intercavity dye laser excitation. Followed by measurement of the D2 photoproduct. (AIP)
On the optimum trajectory in semiclassical calculations of inelastic collisions
Authors: Gordon Robert J.
Published: February 2003

Semi-classical formulation of molecular scattering theory is discussed. It is argued that the arithmetic mean of the initial and final state velocities va should be used in the calculations. (AIP)
Molecular beam experiments on the sticking and accommodation of molecular hydrogen on a low-temperature substrate
Authors: Govers T. R. , Mattera L. , Scoles G.
Published: February 2003

Molecular beams, microcalorimetry, and mass spectrometry are used to study the physisorption kinetics of H2 and D2 molecules on a low temperature substrate. The energy transferred to the surface of a liquid-helium cooled bolometer by an incident room temperature beam of H2 and D2, and the number density of the scattered molecules are measured. The sticking coefficient S and the accommodation coefficient α are obtained as a function of N, the surface coverage by H2 or D2. For both isotopes, S and α depend strongly on N. S(H2) changes from 0.07±0.05 to 0.80±0.02 and α(H2) from 0.31±0.07 to 0.97±0.01 as N increases from ?0 to 3×1015 molecule cm−2. In the same coverage range, S(D2) changes from 0.25±0.05 to 0.92±0.04 and α(D2) from 0.3±0.1 to 0.975±0.025.
The Kirkwood instability in a mean field context
Authors: Klein W. , Grewe N.
Published: February 2003

We demonstrate for systems with pair potentials of the form V(r)=γdφ(γr) that the Kirkwood instability analysis of the first equation in the BBGKY hierarchy is correct in the limit γ→0. The cancellation of singular behavior, which occurs for models with short range potentials when higher order equations in the BBGKY hierarchy are considered, does not occur in this mean field (γ→0) limit.
Study of the corrosion of aluminum by CCl4 using inelastic electron tunneling spectroscopy
Authors: Ellialtiog̃lu R. M. , White H. W. , Godwin L. M. , Wolfram T.
Published: February 2003

Inelastic electron tunneling spectroscopy (IETS) has been used to determine the molecular species which occur on an aluminum oxide surface during metallic corrosion by carbon tetrachloride. IETS spectra were obtained for CCl4 adsorbed on aluminum oxide and the observed vibration modes were assigned by comparison with infrared and Raman frequencies. Modes were observed which could be associated with the molecular species CCl4, AlCl, AlCl2, AlCl3, ⋅CCl3, C2Cl6, and the complex CCl+3[AlCl4]−. These species are those in the reaction sequence proposed by Stern and Uhlig for the corrosion of aluminum by CCl4. The results do not provide information on whether the reaction is chemical or electrochemical in nature. The fact that no modes were observed which would reflect oxygen–carbon bonding, and the presence of a large number of aluminum–chlorine modes suggest that the corrosion mechanism is by reaction of the solvent with exposed aluminum atoms.
Initial vibrational state dependence of resonant excitation and ‘‘dissociative attachment’’ in electron–N2 scattering
Authors: Huetz A. , Gresteau F. , Hall R. I. , Mazeau J.
Published: February 2003

A beam of vibrationally excited ground state nitrogen molecules has been generated using a microwave discharge. The effect of this vibrational energy on the different decay processes of the A 2Πu state of N−2 formed by electron impact in the 7–13 eV region has been studied using an electron-impact spectrometer. Firstly, the ’’boomerang’’ oscillations occuring in the excitation cross section of the A 3Σ+u levels increase in amplitude when the initial level is v=1. This effect, as well as the measured cross sections are well described by calculations using the local complex potential model. Secondly, the cross section ratio for the ’’dissociative attachment’’, process yielding unstable N−(3P) from the v=0 and 1 levels has been evaluated indicating that the vibrational energy has almost no effect. The calculations further indicate that this ratio only rises to 4 when going from v=0 to 4 which is in sharp contrast to H2 where, in the 4 eV region the same amount of vibrational energy increases the dissociative attachment cross section by a factor of 360.
Determination of frequency shifts by Raman difference spectroscopy
Authors: Laane Jaan , Kiefer Wolfgang
Published: February 2003

The characteristics of the Raman difference spectra which result when one Raman signal arising from a particular chemical species is subtracted from a second Raman signal due to the same species in a different system, have been examined for both Lorentzian and Gaussian band profiles. The mathematical relationships between δ (frequency separation in the difference spectra), d (intensity of difference spectra), Δ (frequency separation between the two Raman spectra), I0 (Raman band height), and Γ (Raman bandwidth) have been derived. The relationship between the ratios d/I0 and Δ/Γ is most useful for the determination of very accurate values of Δ, the frequency shift. These can often be determined to within ±0.01 cm−1.
GaSe Faraday rotation near the absorption edge
Authors: Villaverde A. Balbin , Donatti D. A.
Published: February 2003

We measured the optical dispersion of the interband Faraday rotation of GaSe near the absorption edge at the temperature of 298 °K. From our data it is possible to confirm that a direct allowed electronic transition takes place between the valence and the conduction bands in GaSe.
Approximating the master equation by Fokker–Planck-type equations for single-variable chemical systems
Authors: Gillespie Daniel T.
Published: February 2003

The stochastic evolution of a well stirred chemically reacting system containing a single time-varying species X is accurately described by the master equation, in which the total number x of X molecules is an integer variable. We investigate here the legitimacy of approximating the master equation by a Fokker–Planck type partial differential equation in which x is treated as a real variable. Taking the position that any partial differential equation may be regarded as a legitimate approximation to the master equation if and only if it reduces to the master equation when subjected to a proper discretization procedure, we deduce the following: For the special case in which the various chemical reactions can alter the X molecule population by no more than one molecule at a time, a second order (two term) Fokker–Planck equation suffices. However, for the more general case in which reactions are allowed that alter the X molecule population by two molecules at a time, it is necessary to use at least a fourth order (four term) Fokker–Planck equation. In all cases it is assumed that x is large compared to unity; however, no assumptions are made about the way in which the transition probability rates and the molecular population fluctuations scale with the size of the system.
The photolytic cage effect of iodine in the gas phase
Authors: Zellweger J. M. , van den Bergh H.
Published: February 2003

The laser flash photolysis of iodine has been studied over a wide pressure range in several bath gases. At low pressures the observed quantum yield for photolysis at 6943 Å is independent of the gas density, whereas at higher pressures the quantum yield decreases with increasing gas density. This phenomenon is discussed in terms of the cage effect, and the measured yields are compared with model calculations. The experiments also provide information on the second order rate constants for iodine atom combination.
Surface diffusion and spin polarization: Two dimensional CIDEP
Authors: Monchick L.
Published: February 2003

An asymptotic Green’s function method that proved successful for spin polarization–diffusion processes in bulk solution is extended to similar processes on surfaces. An approximate solution indicates that the dependence on the exchange interaction is similar in two and three dimensional processes, the dependence on time and magnetic interactions, dissimilar. The latter difference is attributed to a fundamentally different feature of two dimensional diffusion-controlled processes: two particles undergoing random walk motion that have collided once are certain to collide again.
EXAFS as a probe of local coordination environments of transition metal ions in polycrystalline zeolite catalysts. Co(i
i) in hydrated zeolite –A and –Y
Authors: Morrison T. I. , Reis A. H. , Gebert E. , Iton L. E. , Stucky G. D. , Suib S. L.
Published: February 2003

Synthetic zeolites exchanged with transition metal or rare earth ions are of immense importance in catalysis, both commercially and in basic research; however, difficulty in growing single crystals and problems of long range disorder make crystallographic studies formidable tasks. In this study, extended x-ray absorption fine structure (EXAFS) spectroscopy has been successfully applied to characterize the local coordination of Co(ii) ions in polycrystalline zeolite –A and –Y, thereby establishing the utility of this technique for structural study of intrazeolitic complexes. The Co(ii) ions in Co18.2Na20.7–Y⋅xH2O are found to be ’’solutionlike’’, six-coordinate with Co–O bond distances of 2.06(1) Å, but in Co4.3Na3.4–A⋅xH2O and Co5.5Na1.0–A⋅xH2O, they are five coordinate with Co–O distances of 2.07(1) Å. The latter result is compatible with several published spectroscopic measurements, but is in conflict with a previous single crystal x-ray diffraction study; this discrepancy has been critically analyzed. The experimental data were collected for ∼17 h per sample on an ’’in house’’ EXAFS facility employing a standard 1 kW Mo x-ray tube and focussing optics. Hence, EXAFS appears to be a powerful tool for investigating short range order even in materials as complex as zeolites.
Lifetimes and collisional quenching cross sections of single rotational states in bromine
Authors: Luypaert R. , De Vlieger G. , Craen J. Van
Published: February 2003

Lifetimes and self-quenching cross sections of individual rotational levels in the B 3Π0+u state of 79Br2 and 81Br2 were measured by observing the fluorescence decay following excitation by acousto-optically chopped, single mode Ar+ and Kr+ lasers. The results, which depend strongly on v′ and J′, were interpreted in terms of gyroscopic predissociation. This type of predissociation, exhibiting maxima in the neighborhood of v′=33 and v′≲18, appears to be much stronger in bromine than in iodine.
l-changing collisions of Xe(n
f) Rydberg atoms with SF6
Authors: Kellert F. G. , Higgs C. , Smith K. A. , Hildebrandt G. F. , Dunning F. B. , Stebbings R. F.
Published: February 2003

The observation of an l-changing effect in collisions of metastable Xe atoms and Sf6 is reported. The rate constants for l-changing collisions were found to be large and therefore considered important in studies of Rydberg collision processes involving SF6. (AIP)
An approximation of the Coulomb force for molecular dynamic calculations
Authors: Soules Thomas F.
Published: February 2003

An approximate expression is given for the long range attractive Coulomb froce. This expression can conveniently be used in molecular dynamics calculations and avoids ambiguities of other approximations. (AIP)
Comment on ‘‘Flexible vs rigid constraints in statistical mechanics’’
Authors: Pechukas Philip
Published: February 2003

The differences between rigid and flexible constraints on molecular motion are commented upon. The consequences of molecular dynamics calculations are discussed. (AIP)
Systematic study of Politzer’s energy relation for atoms
Authors: Teruya Hirohide , Anno Tosinobu
Published: February 2003

The parameter k appearing in Politzer’s energy relation for atoms E=kVNe, which connects the total electronic energy E of a given atom with its nuclear–electronic interaction energy VNe, has been determined systematically using the near Hartree–Fock data given by Clementi for a wide variety of atoms and ions. It turns out that k for an isoelectronic series of atoms approaches 0.5, which is appropriate to a hydrogenlike atom. It has been found that the general pattern of k vs Z curves can be explained very well by Slater’s simple model for electronic structure of atoms with the idea of a screening effect due to inner electrons upon outer electrons.
The electronic structure of ferrous heme with imidazole as the fifth ligand. I. SCF–LCAO–ASMO and CI calculation on the low-lying multiplets and optically allowed excited states
Authors: Seno Yasunobu , Kameda Noboru , Otsuka Jinya
Published: February 2003

A semiempirical molecular orbital calculation is performed on the porphin–Fe–imidazole system. As basic atomic orbitals, lone pair σ orbitals of the N atoms coordinating to Fe are employed in addition to 3d ortibals of Fe and π orbitals of N and C atoms. Molecular orbitals are obtained by the usual SCF procedure for the closed shell configuration. Then the calculation of the open shell states is carried out by a careful configuration interaction method. The ground state is identified with one component of 5E. The energy level scheme suggested by magnetic measurements and by ligand field theory for low-lying quintet, singlet, and triplet states is well reproduced. Corresponding to a broad absorption band in the visible region and weak bands in the near IR, x- and y-polarized electronic transitions, two d→d transitions and six d→π (porphin) transitions as well as π→π transitions, are calculated to be in a range of excitation energy 9–20 kK. Two z-polarized transitions, d→4dgx (porphin) and d→π′ (imidazole), also become fairly low in excitation energy, 2 and 11 kK, respectively.
N2–N2 interaction potential from a
b i
n
i
t
i
o calculations, with application to the structure of (N2)2
Authors: Berns Rut M. , van der Avoird Ad
Published: February 2003

The short range electrostatic and (first order) exchange contributions to the N2–N2 interaction energy have been calculated ab initio as a function of the N2 orientations and the distance (139 geometries). Using a numerical integration procedure, the results have been represented analytically in the form of a spherical expansion. At R=0.3 nm this expansion is accurate to better than 0.5% if we include the first 18 independent terms, to 2% if we truncate after LA=LB=4, and to 16% if we truncate after LA=LB=2. In combination with the long range multipole expansion results (electrostatic R−5, R−7, R−9 terms, dispersion R−6, R−8, R−10 terms) calculated by Mulder et al., this yields an anisotropic N2–N2 interaction potential in the region of the van der Waals minimum, which can be fairly well represented also by a site–site model. The potential is in good agreement with the available experimental data for the gas phase and for the ordered (α and γ) crystal phases of solid N2. The structure of the van der Waals molecule (N2)2 is discussed; its energy is lowest for the crossed structure: ΔEm=1.5 kJ/mol, Rm=0.35 nm (for the isotropic potential the well characteristics are ΔEm=0.75 kJ/mol and Rm=0.417 nm). The (staggered) parallel and the T-shaped structures are slightly higher in energy. The internal N2 rotation barriers vary from 0.2 kJ/mol (17 cm−1) to values comparable with the dissociation energy.
Low energy electron impact excitation and radiative decay of the d3Δ state of CO
Authors: Paske W. C. , Twist J. R. , Garrett A. W. , Golden D. E.
Published: February 2003

The lifetimes of the d3Δ(v′=4,5) levels of CO have been measured using a delayed coincidence technique. The lifetime for the 3Δ1 subbands were found to be 9% shorter than the 3Δ2 and 3Δ3 subbands while none of the lifetimes showed the J dependence previously reported. The lifetimes of the 3Δ2,3 subbands of the v′=4 and v′=5 levels were found to be 4.67±0.33 μs and 4.02±0.32 μs while the lifetimes of the 3Δ1 subbands of the v′=4 and v′5 levels were found to be 4.27±0.31 μs and 3.69±0.34 μs, respectively. Long lived cascades with lifetimes of 16.1±0.9 μs and 13.3±1.1 μs were found to feed the v′=4 and v′=5 levels, respectively. Quenching cross sections and the (d→X):(d→a) branching ratio are given for v′=4,5 and discrepancies with previous works are discussed.
Interactions of ion beams with surfaces: Dynamics of the reaction of N2+ with rhenium
Authors: Hu Hsin-Kuei , Fukuda Yasuo , Baldwin David A. , Murray P. Terrence , Rabalais J. Wayne
Published: February 2003

Reactions of N2+ ion beams with the surface of polycrystalline rhenium foil over the range 150–3 000 eV have been studied by the techniques of x-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS). The reactions produce a nitride layer of the type ReNx, 0.21⩽x⩽0.40, with x varying as a function of ion kinetic energy and depth into the surface. The nitride layer extends from the surface down to the penetration depth of the atoms, which varies from ∼15 Å at 0.3 keV to ∼76 Å at 2.5 keV ion energy. The product nitride and chemisorbed nitrogen on Re are distinctly different, exhibiting N1s binding energies of 397.9 and 396.8 eV, respectively, and TDS maxima near 425 and 600 °C, respectively. The value of x in ReNx increases linearly with the nitrogen ion flux and reaches a steady state condition at a dose of 9.5 ×1016 ions/cm2 which is determined by the opposing rate of nitride formation and the sputtering rate by impinging N2+ ions. An expression describing the rate of nitration as a function of the reaction cross section σr and the sputtering cross section σs is derived. The simplified expressions for the limiting cases of initial rates (t→0) and the steady state (t→∞) product concentration, which are used to interpret the experimental results, yield σr≃3×10−18 cm2 for the N2++Re(metal)→ReNx reaction at an ion impact energy of 500 eV.
Determination of the Madelung potential of ionic crystals with a polar surface by the Ewald method
Authors: Lee Woodrow W. , Choi Sang-il
Published: February 2003

Application of the Ewald method to the evaluation of the Madelung potential of a crystal slab possessing two-dimensional periodicity has been studied. The method involves a simple differential equation to be solved. When each crystallographic plane parallel to the surface contains equal amounts of opposite charges, the unique boundary condition leads to a unique solution and the correct Madelung potential in the surface region and the bulk. If the planes are not charge neutral, the boundary condition is ambiguous. A slab consisting of a dipoleless repeating unit layer satisfies a unique boundary condition. The method is applied to NaCl, CsCl, and β-alumina. A simple surface modification is shown to produce the slab bulk self-potential equal to the infinite crystal bulk self-potential.
Mössbauer effect of 129I in n–σ and π–σ charge-transfer complexes of iodine in the frozen solution
Authors: Sakai H. , Maeda Y. , Ichiba S. , Negita H.
Published: February 2003

The Mössbauer effect of 129I was applied to investigate charge-transfer complexes of the n–σ and π–σ types in the carbon disulfide frozen solution at 16 K. In the case of n–σ type complexes such as triethylamine–iodine and pyridine–iodine, two chemical species of iodine, which correspond to the bridging and terminal iodines, were observed in the Mössbauer spectra. It is elucidated that a charge-transfer bond is described as a mixture of a four-electron three-center bond and a covalent bond involving dσpσ hybrid orbitals in the bridging iodine. On the other hand, for the π–σ type complexes such as benzene–iodine and methylated benzene–iodine the Mössbauer spectra show only one chemical species of iodine. The acceptor orbital of iodine is thought to be described by an spd hybrid orbital. That is, a small amount of s and d characters contributes to the formation of the charge-transfer bond. From the Mössbauer parameters the charge densities and the charge distributions of iodine in these complexes were obtained.
Effect of reagent translation on the dynamics of the exothermic reaction Ba+HF
Authors: Gupta Arunava , Perry David S. , Zare Richard N.
Published: February 2003

The Ba+HF (v=0)→BaF+H reaction has been studied as a function of relative collision energy (3–13 kcal/mole) using a crossed beam geometry in which a seeded HF beam intersects a thermal Ba beam. The vibrational and rotational distributions of the BaF product are determined from computer simulations of its excitation spectrum. The reaction cross section is found to have a low threshold (∼1 kcal/mole). With increasing collision energy the cross section increases to a maximum in the range of 6–8 kcal/mole. An upper bound of 15 Å2 is placed on the absolute value of the reaction cross section. The fraction f ′ of energy appearing in translation, rotation, or vibration of the products is roughly constant over the range of collision energies studied with nearly half going into product translation and the remainder being divided nearly equally between product rotation and vibration. However, while <f ′rot≳ increases slowly with collision energy, <f ′vib≳ first rises then falls and <f ′trans≳ first falls then rises, the crossover occuring at collision energies for which the reaction cross section reaches its maximum. The fractional energy disposal in the different product modes is qualitatively consistent with the calculations of phase space theory, but the detailed behavior cannot be matched. However, the product rotational distribution observed for each vibrational level agrees closely with the predictions of this model, which is a consequence of the kinematic constraint for this mass combination. The Ba+HF reaction as a function of collision energy shows both similarities and differences with the corresponding studies for Ba+HCl and Ba+HBr. Possible explanations for this contrasting behavior are presented.
Polarizability calculations with the SCF method. III. Ethylene, butadiene, and hexatriene
Authors: Zamani-Khamiri O. , McIntyre E. F. , Hameka H. F.
Published: February 2003

We evaluate the nonlinear third-order electric susceptibilities of ethylene, trans-butadiene, and trans-hexatriene by combining the Pariser–Parr–Pople method and the extended Hückel method. It follows from the nature of the calculations that the theoretical results are quite sensitive to approximations since the final results are obtained from various positive and negative contributions. If we allow for a possible 10% error in each contribution, then our result and the corresponding error margins are γ=−0.03±0.03 for ethylene, γ=1.35±0.75 for trans-butadiene, and γ=9.92 ±5 for trans-hexatriene, all values are expressed in terms of 10−36 esu. We feel that these values are consistent with experimental results obtained by Ward and Elliott.
A model for the formation and stabilization of charged water clathrates
Authors: Holland Paul M. , Castleman A. W.
Published: February 2003

A model for the formation and stabilization of charged water clathrates is presented which accounts for observed anomalies in H+(H2O)n ion distributions. These anomalies are observed in both ion cluster and neutral expansions and are consistent with the sizes expected for clathrate ions. That the same sizes are observed in both ion cluster and neutral expansions strongly suggests that a rapid ionic process is responsible for their formation. The proposed model is based on the high mobility and bonding effects of the ’’excess’’ proton in water. Computer simulations suggest that ’’excess’’ proton movement in a water clathrate would be suitable for stabilizing the clathrate structure as well as giving it access to a large number of nearly degenerate proton configurations. The formation of clathrates in charged water clusters of proper size can be ascribed to the following: rapid ’’excess’’ proton movement, a strong preference of the H3O+ for a 3-coordinate bonding structure (which is compatible with hydrogen bonding), and finally, relatively slow processes leading to thermal disorder.
MPI spectroscopy of expansion-cooled ammonia: Structure and dynamics at 50 K
Authors: Glownia James H. , Riley Stephen J. , Colson Steven D. , Nieman George C.
Published: February 2003

The multiphoton ionization MPI spectrum of ammonia cooled by a supersonic expansion is recorded in the 360–500 nm region. Rotational structure is greatly simplified over that observed at room temperature, facilitating quantitative comparisons with theoretical predictions of rotational intensities for three-photon absorption using either linear or circular polarized light. The previous assignment of the new three-photon resonant ?′ state is supported, and the location of its origin confirmed. Small systematic deviations are found between the calculated and observed line strengths. These differences can be understood in terms of two phenomena not accounted for in the static gas line strength theory. There is the possibility of a non-Boltzmann distribution of rotational states following the expansion cooling. Also, the effects of the competition between rotational predissociation and photoionization of the excited ? state can result in a degradation of transitions involving higher upper state rotational levels. The observed deviations suggest that both of these processes may be important in the MPI spectrum of expansion cooled ammonia.
Monomer–eximer dynamics in spread monolayers. I. Lateral diffusion of pyrene dodecanoic acid at the air–water interface
Authors: Loughran Thomas , Hatlee Michael D. , Patterson L. K. , Kozak John J.
Published: February 2003

In this paper, we present a combined experimental and theoretical study of monomer–eximer dynamics in spread monolayers. The system studied here is a spread monolayer of 12-(1-pyrene) dodecanoic acid (PDA) and oleic acid at the air–water interface of a neutral aqueous solution. The microscopic kinetic behavior of the PDA probe in the monolayer was monitored through the eximer–monomer, steady-state photoexcitation of the pyrene moiety. The steady-state ratio of eximer to monomer intensities, I2/I1, was found to depend linearly on the mole fraction of PDA in oleic acid over the concentration range studied. These data were interpreted theoretically by constructing a reaction-dynamic model to describe the lateral diffusion and subsequent interaction of the pyrene species. Using analytical results from the theory of random walks on lattices with traps, and from data from Monte Carlo simulations, an estimate is obtained for the two-dimensional diffusion coefficient of the pyrene probe in the monolayer; this estimate is 1.7×10−6 cm2/sec.
Raman spectra of liquid n-alkanes. II. Longitudinal acoustic modes and the g
a
u
c
h
e–t
r
a
n
s energy difference
Authors: Scherer James R. , Snyder Robert G.
Published: February 2003

Low-frequency bands in the Raman spectra of the liquid n-alkanes, n=9–20, have been identified with extended and certain nearly extended rotameric forms of these molecules. The bands are associated with the intense longitudinal-acoustic-mode (LAM-1) band that appears in the Raman spectra of crystalline n-alkanes. For liquids, the intensities of these bands are highly temperature dependent. This dependence was measured for the n=11–14 n-alkanes and was used to determine an average value for ΔE, the energy difference between the trans and gauche states of the CC bond. The value determined for ΔE, 508±50 cal/mole, is very near the value commonly assumed in statistical calculations based on the rotational isomeric model. No evidence for local order was found even at temperatures just above the melting point of n-C13H28 and n-C14H30.
Manganese hexacyanomanganate: Magnetic interactions via cyanide in a mixed valence Prussian blue type compound
Authors: Klenze R. , Kanellakopulos B. , Trageser G. , Eysel H. H.
Published: February 2003

A Prussian blue type compound of the stoichiometric composition Mn(CN)3⋅xH2O (x=0.57) is formed as the final product of hydrolysis of Na3[Mn(CN)6] in perchloric acid. IR and visible spectra as well as structural and magnetic data show that it is a mixed valence compound consisting of sixfold N-coordinated Mn(II) and sixfold C-coordinated Mn(IV) in a cubic face-centered lattice. The compound exhibits ferrimagnetism with a Curie temperature of 48.7 K. A molecular field treatment assuming two collinear spin sublattices provides an adequate description of the observed magnetic behavior. The calculation of exchange integrals, which was based on a Heisenberg model, reveals that the magnetic ordering is almost exclusively induced by an antiferromagnetic interaction between Mn(II) and Mn(IV) ions. This interaction is described in terms of a super exchange involving cyanide as bridging ligand.
Photofragmentation infrared emission studies of vibrationally excited free radicals CH3 and CH2I
Authors: Baughcum Steven L. , Leone Stephen R.
Published: February 2003

Time and wavelength-resolved infrared fluorescence techniques are used to study the photofragmentation dynamics of CH2I2 and CH3I at the excimer laser wavelengths of 248 and 308 nm. Emission is detected from vibrationally excited CH2I and CH3 radicals as well as from the excited iodine atoms [I*(2P1/2→2P3/2)] produced in the photolysis. A complete infrared fluorescence spectrum of the highly excited CH2I radical is obtained as a function of time after the 248 nm dissociating laser pulse, providing both spectroscopic and vibrational deactivation data for the radical. Significant CH2I emission is observed at all wavelengths, indicating that excitation occurs into a very high density of states, nearing the vibrational quasicontinuum. Stronger emission features are observed in the region of the C–H stretching vibrations, the CH2 bending motion, and a combination band of these two modes. Deactivation rates for various spectral features of the highly excited CH2I radical with CH2I2 and argon are presented, along with a discussion concerning the relaxation of the highly excited radical spectrum. In the photolysis of CH3I at 248 nm, infrared fluorescence is observed directly from the out-of-plane bend of the CH3 radical. In addition, at 248 nm, the quantum yield (ΦI*) for I*(2P1/2) production is measured to be 0.81±0.03 for CH3I and 0.46±0.04 for CH2I2, while dissociation of CH2I2 at 308 nm yields ΦI*=0.25±0.02. The I* quenching rates with CH3I and CH2I2 are measured to be (2.5±0.4)10−13 and (3.6±0.3)10−13 cm3 molecule−1 sec−1, respectively.
Computational kinetics and sensitivity analysis of hydrogen–oxygen combustion
Authors: Dougherty Eugene P. , Rabitz Herschel
Published: February 2003

Kinetic modeling calculations on the H2–O2 system have been carried out with an extensive reaction set to probe the vicinity of the three explosion limits. Sensitivity analysis is used throughout this investigation to study system behavior, in particular, to elucidate mechanistic details. The concentrations and sensitivity profiles are discussed in light of the appropriate experimental results and existing theories of hydrogen combustion. The results indicate the present model to be useful over a wide pressure–temperature range. The reaction set is also used to probe the sensitivities for an experimental study designed to measure the rate constant of an important elementary reaction, H+O2+M→HO2+M, involved in this system. The versatility of the reaction set is also demonstrated by a study of a related chemical reaction, the decomposition of hydrogen peroxide. Finally, prospects for utilizing the methods and results of this study to examine other complex kinetic schemes are discussed.
V–V exchange involving H2O molecules: A calculation of the N2*–H2O deactivation rate constant
Authors: Nagel J. , Rogovin D.
Published: February 2003

A calculation of the N2*+H2O→N2+H2O* deactivation rate constant is presented which is in fair agreement with measurement. The process is found to be well described by quasiresonant VR energy exchange induced by hard core forces. The large rates are due to the vast number of VR modes H2O has available to absorb energy. Long ranged and hydrogen bond forces are considered but found to be relatively unimportant for VV exchange. The calculated rate constant was dominated by interactions in which the molecules collided with certain specific initial orientations.
Rate constant and transmission coefficient in the diffusion theory of reaction rates
Authors: Mangel Marc
Published: February 2003

The diffusion theory of reaction rates, originally described by H. A. Kramers, is extended and new results are derived. It is shown that valuable information can be gained by using backward diffusion equations (equations in which the initial values are independent variables), in addition to the Fokker–Planck equation. Three theoretical formulations of the rate constant are described. The first formulation uses transition state theory. The second formulation uses a modification of Kramers’s theory. In the third formulation, the rate constant is defined to be the reciprocal of the mean time to cross a given energy barrier. The three formulations of the rate constant are compared with each other and with Monte Carlo experiments. By using the backward equation, it is possible to calculate the transmission coefficient. The theoretical results are compared with Monte Carlo experiments.
Influence of argon bombardment on the reactivity of (110) platinum with oxygen
Authors: Miranda R. , Ibañez J. , Rojo J. M. , Salmeron M.
Published: February 2003

The influence of 500 eV Ar+ bombardments on the oxygen adsorption by an initially well annealed (1×2) reconstructed Pt (110) surface has been studied. It is found that the bombarded surface is more reactive (vs oxygen) than the initial one. The effects of bombardment are extremely rapid. Ion doses as small as 0.05 Ar+/ surface atom reduce the superlattice spot intensity by 50%. The oxygen surface concentration increases by 100% after an ion dose of only 0.01 Ar+/surface atom. Oxygen adsorption on a heavily bombarded surface, showing a (1×1) LEED pattern, gives rise to a structure with a (1×2) symmetry. Subsequent removal of the oxygen by interaction with the residual gas restores the (1×1) pattern. Although the influence of the surface defects cannot be dismissed, local rearrangement of the surface after ion bombardment seems a more likely explanation of these effects.
Chemical potential of a solute at infinite dilution in a hard disc fluid by Monte Carlo simulation
Authors: Zollweg John A.
Published: February 2003

Methods are described for finding the chemical potential of a hard disc solute of any radius at infinite dilution in a hard disc fluid or solid. A new method is also described for finding the pair distribution function near contact from the same data that are used to calculate the chemical potential for small solute discs. The methods have been applied to a system of 1024 hard discs at densities in the dense fluid and solid region. The results for the equation of state are compared with other simulation results and a Padé approximant to the virial series. The chemical potential results are compared with scaled particle theories and with the integrated equation of state.
Electron localization in dense nitrogen vapor
Authors: Wada Toshinori , Freeman Gordon R.
Published: February 2003

The density normalized mobility μn of thermal electrons in saturated nitrogen vapor decreases by two orders of magnitude between n=5.7×1019 and the critical density 6.7×1021 molecule/cm3. This large effect is attributed to temporary anion formation involving resonance rotational states. The process is much more extensive than the quasilocalization of electrons in dense xenon and hydrocarbon vapors. The process is approximately first order in nitrogen at n<2×1021 molecule/cm3, but the order increases progressively with density at n≳2×1021 molecule/cm3. The apparent mean lifetime δτ of the anions at 127 K is 4×10−13 s at n<2×1021 molecule/cm3, but increases at higher densities and reaches 6×10−12 s at n=6.7×1021 molecule/cm3. The value of δτ in the low density gas decreases with increasing temperature.
Sound velocity of supercooled water down to −33 °C using acoustic levitation
Authors: Trinh E. , Apfel R. E.
Published: February 2003

The sound velocity in supercooled water has been measured at 54 kHz down to −33 °C by means of an acoustic levitation method. The accuracy of the results is estimated to be within ±1.3%. Comparison with published data obtained at 5 GHz and 925 MHz indicates an anomalous sound velocity dispersion.