bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface segregation in alloys: LEED, Auger, and gas adsorption study of segregation of Au to the (111) surface of Ni
Authors: Burton J. J. , Helms C. R. , Polizzotti R. S.
Published: February 2003

The (111) surface of Ni single crystals containing less than 1% Au was studied using Auger spectroscopy, LEED, and gas adsorption. The heat of segregation of gold to the Ni(111) surface, obtained from equilibrium measurements of the surface composition as a function of temperature, was found to be 12±2 kcal/mole. This result is not temperature, was found to be 12±2 kcal/mole. This result is not adequately described by existing theories of surface segregation. We also observed a variety of other complex phenomena. For gold surface concentrations above about 50%, an ordered overlayer of gold formed on the nickel substrate. Exposure of a partially gold covered surface to oxygen causes the disappearance of the gold. Small amounts of gold on the surface effectively poison it for adsorption of CO.
Experimental determination of the temperature–concentration diagram of flexible polymer solutions by neutron scattering
Authors: Cotton J. P. , Nierlich M. , Boué F. , Daoud M. , Farnoux B. , Jannink G. , Duplessix R. , Picot C.
Published: February 2003

Using the neutron scattering technique, we have measured the temperature dependences of the characteristic correlation lengths in semidilute polymer solutions in the vicinity of the theta (Θ) point. From the analysis of the data in the Guinier range, with a sample containing a few labeled chains, we find that the radius of gyration Rg has the dependence Rg∼τ0.125±0.005, where τ is the reduced temperature (T−Θ)/Θ. The scattering experiment from unlabeled samples, in the intermediate range of the momentum transfer, shows a decrease of the screening length ξ, ξ∼τ−0.25±0.01. These two dependences are in agreement with a recent calculation [M. Daoud and G. Jannink, J. Phys. (Paris) (to be published)] based on de Gennes’s theory that the theta temperature is in fact a tricritical point. A scaling law in molecular weight and concentration is tested below Θ. A temperature concentration diagram is drawn with the above mentioned experimental results.
The Ne3 nonadditive potential energy surface
Authors: Novaro O. , Nieves F.
Published: February 2003

The Ne3 potential energy surface is calculated in the SCF–MO–LCAO approximation in order to estimate the nonadditive contributions at short range separations. Several geometrical configurations are presented comprising diverse isosceles triangles and linear symmetrical and asymmetrical Ne3 geometries.
Centrifugal distortion in ArHCl
Authors: Novick Stewart E. , Janda Kenneth C. , Holmgren Stephen L. , Waldman Marvin , Klemperer William
Published: February 2003

Measurements of centrifugal distortion constants for four isotopic species of the weakly bound, wide amplitude bender ArHCl are reported. Their relationship to an effective radial potential and the associated vibrational wavefunction is discussed. Although the stretching vibrational frequency and the resulting force constant do not have a straight forward relationship to the nonharmonic, angle averaged radial potential, an interpretation of the centrifugal distortion constant in terms of the standard deviation—or width—of the radial wavefunction is proposed. It is shown that a harmonic approximation for the standard deviation of the wavefunction accurately reflects a realistic wavefunction for a complicated potential surface. The derived constants for the four isotopic species are as follows: D0(kHz) ω (cm−1) k (mdyn/Å) σ= (〈R2〉−〈R〉2)1/2(Å) ArH35Cl 20.0(4) 32.4 0.0117 0.166 ArH37Cl 19.0(4) 31.9 0.0114 0.165 ArD35Cl 17.1(4) 34.4 0.0134 0.160 ArD37Cl 16.4(4) 33.7 0.0132 0.159.
Diatomics-in-molecules potential energy surfaces. III. Non-Hermitian formulation
Authors: Tully John C. , Truesdale Carlton M.
Published: February 2003

A reformulation of the diatomics-in-molecules method is presented. Because energies are obtained directly from the non-Hermitian matrix S−1H, overlap is included exactly in the present formulation. The simplicity and potential accuracy of the method are demonstrated by application to the H3 molecule, with inclusion of ionic configurations.
Interpretation of ortho–para hydrogen conversion
Authors: Truhlar Donald G.
Published: February 2003

The interpretation of statistics of the nuclear spin states in the ortho–para hydrogen conversion by hydrogen atoms is considered. It is shown that the observable rate coefficient (which is the sum of the forward and reverse rate coefficients for ortho→para conversion) equals the distinguishable-atom rate coefficient in the classical limit. In addition to spin-1/2 nuclei, we also consider spin-1 nuclei and spin-0 nuclei.
General theory of circularly polarized emission and magnetic circularly polarized emission from molecular systems
Authors: Riehl J. P. , Richardson F. S.
Published: February 2003

A theory of circularly polarized emission (CPE) based on the formalism and methods of quantum electrodynamics is developed for chiral luminescent molecular systems and for achiral luminescent molecular systems placed in a magnetic field. Differential transition probabilities for the (spontaneous) emission of left- and right-circularly polarized light are calculated and these quantities are related to the observables of CPE spectroscopy. The influence of photoselection and reorientational relaxation (due to rotary Brownian motion) upon the CPE observables (with and without an applied magnetic field) is considered. Expressions for emission anisotropy factors are derived and these factors are related to the molecular transition integrals which determine the CPE and total emission intensities. Results are obtained for two excitation–emission geometries—180° (head on) and 90° (right angle).
Self-reversal in the exciton phosphorescence of 1,4-dibromonaphthalene
Authors: Macfarlane R. M. , Konzelmann U. , Burland D. M.
Published: February 2003

Self-reversed line shapes have been observed in the low temperature phosphorescence spectrum of 1,4-dibromonaphthalene crystals when the crystals are illuminated by a collimated laser beam. Self-reversal is a consequence of reabsorption of the phosphorescence emission. The precise shape of the self-reversed lines is a very sensitive measure of small changes in the unreabsorbed emission spectrum with respect to the absorption spectrum and energy shifts as small as 1% of the linewidth can be measured. For dibromonaphthalene we attribute these changes to macroscopic variations in strain within the crystal. Numerical calculations of the line shapes based on a simple but realistic model are compared with the experimental line shapes.
The thermodynamics of ionization of gaseous oxides; the first ionization potentials of the lanthanide metals and monoxides
Authors: Ackermann R. J. , Rauh E. G. , Thorn R. J.
Published: February 2003

The first ionization potentials of the gaseous lanthanide metals and monoxides have been determined by electron impact from the appearance potentials of ionization efficiency curves. A method of simultaneous and intercomparative measurements with known standards was used and the results for the lanthanide metals are in excellent agreement with values obtained previously from spectroscopic and surface ionization studies. In the early part of the lanthanide sequence the ionization potentials of LnO(g) are less than those of Ln(g), whereas the converse is true in the latter part. The differences in the ionization potentials of LnO(g) and Ln(g) are simply related to the differences in the dissociation energies of LnO(g) and LnO+(g). Values of D0(LnO+) are derived. The nature of the chemical bonding in LnO(g) and LnO+(g) is examined for the lanthanide sequence by means of an electrostatic point-charge model. The assumption of monotonic variation of the interatomic distance and the electrostatic repulsion parameter and the nonmonotonic variation of the polarization energy derived from the known f-to-d electronic transitions are all mutually consistent and describe rather closely the variation in bonding in LnO(g); a similar scheme without the f-to-d transition energies is shown to describe the variation in the bonding of LnO+(g).
Open and closed channel resonances in the collinear inelastic scattering of He by H+2
Authors: Chapman Frank M. , Hayes Edward F.
Published: February 2003

Collinear quantum mechanical calculations are reported for the inelastic scattering of He by H+2 at energies below the reaction threshold. The inelastic transition probability curves show pronounced quantum oscillations as a function of energy due to both open and closed channel resonances. An analysis of these resonances indicates that the open channel resonance is associated with tunneling through a small barrier and the closed channel resonance is a Feshbach or compound state resonance corresponding to vibrational excitation of H+2 to a closed vibrational state. The compound state resonances are similar to those found earlier to be important in the reaction He+H+2→HeH++H.
Theory of vibrational relaxation and infrared absorption in condensed media
Authors: Lin S. H.
Published: February 2003

In this paper, we develop a theoretical model for vibrational relaxation of vibrons in condensed media based on the breakdown of the adiabatic separation of vibron (high frequency) and phonon (low frequency) motion. The conditions under which the adiabatic approximation of the vibron–phonon system is valid are examined. The present model is also applied to the ir absorption in condensed media; the band shape function and the dipole correlation function are obtained. The relation between the vibrational relaxation and the ir absorption is discussed.
Low energy scattering of 6Li+ from H2O
Authors: Siskind B. , Alexander M. H. , Coplan M. A.
Published: February 2003

Collisions of 6Li+ with H2O at laboratory energies of 10.0 and 17.5 eV have been studied. Energy distributions for the scattered ions at laboratory angles to 20° were measured. From the distributions, contour plots of intensity per unit velocity space were constructed. The plots show inelastic collisions to occur with high probabilities. The degree of inelasticity increases with increasing scattering angle and varies from a few tenths of an eV to a few eV. Differential cross sections summed over the elastic channel as well as all inelastic channels were also calculated from the experimental data. They are compared with theoretical cross sections based on the ab initio potential surface of Clementi and Popkie. Good agreement is found.
Mechanisms of formation of diatomic and triatomic hydrogen negative ions
Authors: Schnitzer Rafael , Anbar Michael
Published: February 2003

The rates of production of diatomic and triatomic negative hydrogen ions were studied in a mass spectrometer as a function of pressure and of isotopic composition. A hollow cathode duoplasmatron was used as the ion source. At 1 torr D2 the relative ion currents are D−:D−2:D−3=1:10−6:10−7. H/D isotope effects in the formation of partially or totally deuterated analogs of H−2 and H−3 were observed and provided additional insight on the mode of formation of these molecular ions.
Heat capacity of 242PuO2 from 12 to 350°K and of 244PuO2 from 4 to 25°K. Entropy, enthalpy, and Gibbs energy of formation of PuO2 at 298.15°K
Authors: Flotow Howard E. , Osborne Darrell W. , Fried Sherman M. , Malm John G.
Published: February 2003

Heat capacity measurements have been made on a 22.5 g sample of 242PuO2 from 12 to 350°K and on a 3.6 g sample of 244PuO2 from 4 to 25°K. The heat capacity curve has the normal sigmoid shape. At 298.15°K the heat capacity C°p, entropy S°, enthalpy H°−H°0, and tempered Gibbs energy (G°−H°0)/T are, respectively, (66.25±0.26) J °K−1⋅mole−1, (66.13±0.26)J °K−1⋅mole−1, (10 784±43) J mole−1, and −(29.96±0.12) J °K−1⋅mole−1. This value of the entropy is significantly lower than the one previously derived from heat capacity measurements on 239PuO2. At 298.15°K the standard entropy of formation and the standard Gibbs energy of formation are −(195.2±0.4) J °K−1⋅mole−1 and −(998.0±0.7) kJ mole−1, respectively.
Laser photoluminescence of calcium molecules
Authors: Sakurai Katsumi , Broida H. P.
Published: February 2003

Calcium diatomic molecules were produced in a heat-pipe oven at temperatures from 1000 to 1200 K. Photoluminescence of Ca2 has been studied with various laser sources and with white light. Discrete, sharp line spectra are attributed to known bound states of Ca2. A broad continuum emission with an oscillatory intensity distribution is associated with emission to the repulsive part of the lower state. A weak continuum emission to another repulsive state also has been observed. The observed strong emission from the atomic resonance lines at 422.7 and 657.3 nm probably is caused by dissociation of excited Ca2 molecules. The laser-excited photoluminescence experiments suggest that the ground state of Ca2 is repulsive and that the lowest observed bound state of Ca2 correlates with the 3P atomic state.
Crossed-beam chemiluminescent studies of alkaline earth atoms with ClO2
Authors: Engelke F. , Sander R. K. , Zare R. N.
Published: February 2003

Chemiluminescence resulting from the reaction M+ClO2, M=Mg, Ca, Sr, or Ba, has been observed in crossed molecular beams. Two special features are found: (1) selective formation of MO* in the A′ 1Π state in preference to the isoenergetic A 1Σ+ state; and (2) the formation of MCl* in various electronic states. Although the MCl* product results from attack of the central Cl atom in ClO2, its cross section is comparable to that of MO*. The cross section for chemiluminescence is a small fraction of the total reaction cross section, which is larger than gas kinetic. From the chemiluminescent spectra the relative electronic, vibrational, and rotational population distributions are derived and the following lower bounds are placed on the MO dissociation energies: D00(CaO) ?109.7±3.5 kcal/mole; D00(SrO) ?107.7±3.5 kcal/mole; and D00(BaO) ?133.6±3.5 kcal/mole. The reaction dynamics are discussed in terms of an electron transfer from M to the half-filled antibonding orbital of ClO2.
Long range vibrational energy transfer from ND and NH(A 3Π) to CO and N2 in solid Ar
Authors: Goodman J. , Brus L. E.
Published: February 2003

Three exothermic, yet near resonant (ΔE=13–126 cm−1) vibrational energy transfer processes from ND (A 3Π) to 12CO and 13CO in solid Ar have been investigated. The transfer quantum yield and the nonexponential time-resolved donor population both follow Förster (dipole–dipole) kinetics as a function of acceptor concentration. The rates exhibit a strong energy gap (transfer exothermicity) law of the form Kα exp(−ΔE/28 cm−1). Reverse transfer of population from 13CO to ND (A 3Π) is not observed, thus indicating a fast sequential exactly resonant transfer of quanta among CO molecules. CO increases the ND X 3Σ→A 3Π inhomogeneous linewidth; however, the transfer kinetics are independent of excitation wavelength within the inhomogeneous linewidth. Transfer from NH(A 3Π) to 12CO is anomalously fast in view of the large ΔE=728 cm−1. Theory is used to relate the energy transfer rates to the vibrational relaxation rates of the isolated donor and acceptor. The hydride dissipates the exothermicity into lattice phonons more efficiently than the deuteride. Limited data suggest a weak energy gap law for the hydride of the form K α exp(−ΔE/310 cm−1).
Energy-dependent cross sections for quenching of Li(2p 2P) by several gases
Authors: Lin Shen-Maw , Weston Ralph E.
Published: February 2003

The lifetime of Li(2p 2P), in the presence of H2, D2, N2, CO, I2, CO2, C2H4, C2H6, and C2F6 as quenching gases, has been directly determined by the single-photon time-correlation method. Li(2P) atoms were produced with varying amounts of translational energy by photodissociation of LiI with light of wavelenghts from 2000 to 2300 Å. Large quenching cross sections, comparable to those observed for heavier alkali metal atoms, were found. The results are discussed in terms of an orbiting trajectory model and a ’’harpooning’’ model.
Magnetic field and concentration dependence of CIDNP in some quinone photolyses: Further evidence for an Overhauser mechanism
Authors: Adrian F. J. , Vyas H. M. , Wan J. K. S.
Published: February 2003

A study has been made of the magnetic field dependence of chemically induced nuclear spin polarization (CIDNP) of fluorine nuclei in the photolysis of tetrafluoro-1,4-benzoquinone and tetrafluoro-1,4-hydroquinone in benzene. Also studied was the benzoquinone concentration dependence of proton CIDNP in the photolysis of benzoquinone in CDCl3. These studies are compared with the theory of an Overhauser mechanism of CIDNP in which the photoexcited triplet mechanism produces electron-spin-polarized semiquinone radicals whose polarization is transferred to the nuclear spins by electron–nuclear cross relaxation. The results support the conclusion, previously reached for the tetrafluoro-1,4-benzoquinone case on the basis of other evidence, that this Overhauser mechanism contributes to CIDNP in these systems.
Sufficiency conditions for the validity of the j
z-conserving coupled states approximation
Authors: Kouri Donald J. , Heil Timothy G. , Shimoni Yair
Published: February 2003

The jz-conserving coupled states (jzCCS) approximation is examined by beginning with the exact body frame Lippmann–Schwinger (BFLS) equation derived recently by Kouri, Heil, and Shimoni. It is shown how one may derive the jzCCS equation in such a way as to enable sufficiency conditions to be obtained for the validity of the jzCCS approximation. In addition, the method of derivation leads to a more general jzCCS approximation that includes states of definite parity. The ordinary jzCCS amplitude density equations result when one assumes that the even and odd parity amplitude densities are equal. The behavior of the jzCCs method for fixed total angular momentum J and varying energy E and for fixed E and varying J is discussed. In addition, the method enables us to show clearly, for fixed E and J, the roles played by the various regions of internuclear scattering distance. The resulting sufficiency conditions may be stated as follows: (a) When the turning point Rt is larger than the impact parameter X0, the jzCCS method will be accurate. (b) When the turning point is smaller than the impact parameter but the difference is not too large, then the accuracy deteriorates as X0−Rt increases. The jzCCS method depends on X0−Rt through the constants Z(i)J(jλ‖j0λ0), i=1,2, defined by Σλ′∫Rtx0dR R2 𝒥̃λλ′Jj(kjR) ζ+j(jλ′‖j0λ0‖R) and Σλ′∫xRtdR R2 ℋ̃λλ′Jj(kjR) ζ+J(jλ′‖j0λ0‖R), and if these constants are sufficiently small, then the jzCCS method will be accurate. Conversely, if Z(i)J(jλ‖j0λ0) are large, or the potential is sufficently long ranged [even though the Z(i)J(jλ‖j0λ0) are small] so as to build up significant λ transitions, then the jzCCS method may break down. However, breakdown is initiated in the short range (Rt⩽R⩽X0) region. (c) The jzCCS method works least well for transitions j→j′ where both j, j′ are large since then, the impact parameter X0 (approximately equal to (J+J+1/2)/kj) is very large and X0−Rt can be quite large. (d) Transitions are poorly treated by the jzCCS close to their threshold because X0≃ (J+j+1/2)/kj will be very large. We emphasize that our criterion (a) is a sufficient condition only. Also, even though condiitons (b) – (d) apply good results may still be obtained owing to fortuitous cancellation of errors.
One-photon intrinsic photogeneration in anthracene crystals
Authors: Lyons L. E. , Milne K. A.
Published: February 2003

Steady state one-photon intrinsic photogeneration in anthracene crystals has been measured using 1M Na2SO3 aqueous solutions as electrodes, as a function of photon energy and electric field (up to 6×107 V m−1). Anomalies in previous measurements with aqueous electrodes have been resolved. The values of absolute photoconduction yield now agree for aqueous, blocking, and metal electrodes, and for pulsed and steady state measurements. The Onsager theory of geminate recombination explains the shape of the current-field dependence better than does a theory of Knights and Davis. Interpretation of the current-field curves in terms of Onsager’s theory yields values of r0 (the initial separation of the charges in the ion pair) as a function of photon energy and dielectric constant, assuming a delta function distribution for r0. Two ion pair states appear to be formed: at hν=3.9 to 4.2 eV, r0A=1.8 nm (for εr=3.8); at 4.5 to 5.0 eV, r0B = 2.5 nm. For hν=4.2 to 4.5 eV, r0 undergoes a smooth transition. The maximum value (0.1) of φ1, the one-photon ion pair production efficiency, occurs at 4.2±0.15 eV, near the maximum in η, the two-photon ion pair yield. The agreement of peaks in φ1 and η with a peak at 4.1 eV in σ2F, the two-photon absorption cross section, is discussed. From the energy dependence of φ1 and the magnetic field effect on prompt fluorescence, it is concluded that, in the process of singlet exciton fission into two triplet excitons, the lowest ion pair state is not the fissionable singlet. Only a very small [0.07(±0.03) %], direction independent, magnetic field effect on the intrinsic photocurrent was observed (at a photon energy of 4.5 eV).
An eigenphase study of the resonances in the rotational excitation of H2 by Li+ immediately above the j=2 threshold
Authors: Dickinson A. S. , Kouri D. J. , Wells C. A. , Lester W. A.
Published: February 2003

The resonance structure in the cross section for rotational excitation of H2 by Li+ in a small energy range above the j=2 threshold has been examined. The usual close-coupling formalism has been employed with the Lester hypersurface. The observed resonances are believed to be shape resonances strongly perturbed by the anisotropic potential, which makes characterization of the resonances extremely difficult. The use of the eigenphases has been helpful in the study of these resonances.
Collisional deactivation of Br(4 2P1/2) by hydrogen isotopes
Authors: Wiesenfeld J. R. , Wolk G. A.
Published: February 2003

The quenching of electronically excited bromine atoms Br(4 2P1/2) upon collision with H2, HD, and D2 has been studied at 300 °K. The temporal profile of the Br(4 2P1/2) concentration was monitored by observation of the time-resolved attenuation of bromine resonance radiation at 153.2 nm. The probabilities of deactivation by the hydrogen isotopes are presented and compared to theoretical treatments based either upon long-range coupling via mulitpolar interactions or explicit treatment of the nonadiabatic crossing probabilities computed using semiempirical potential surfaces. These results are then examined with a view toward assessing the importance of near-resonant electronic–vibrational energy transfer in such an elementary system.
Spin coupling of two identical complex ions. II. Comparison with the experimental data: fine structure and g values
Authors: Amoretti G. , Varacca V.
Published: February 2003

The theory outlined in a preceding paper is applied to the actual calculation of g values and fine structure splittings for three different compounds, using two types of wavefunctions. The computational detail and approximations are discussed. The final comparison with the experimental data shows that our theory allows a very good agreement between experimental and calculated results to be reached. Moreover, it shows the possibility of evaluating a large number of parameters for the compounds under study.
EPR spectroscopic investigations of metal halides doped with transition metal ions. II. Iridium (III) -doped AgCl and AgBr
Authors: Eachus R. S. , Graves R. E.
Published: February 2003

The paramagnetic species (IrCl6)4− and (IrBr6)4− have been identified as products in the light-induced decomposition of iridium (III) -doped AgCl and AgBr single crystals. Evidence is presented to show that secondary electron and hole traps such as Ni2+ and Fe2+ grossly affect the rate of filling and emptying of the more abundant Ir3+ sites. Kinetic data obtained from the thermally induced decay of the Ir2+ traps locate these levels 0.5±0.01 eV and 0.44±0.01 eV below the conduction bands in silver chloride and silver bromide, respectively.
Three-body association reactions of H+ and H3+ ions in hydrogen from 135 to 300 K
Authors: Johnsen Rainer , Huang Chou-Mou , Biondi Manfred A.
Published: February 2003

Rate coefficients for the reactions H++2H2?h3++H2 [Reaction (1)] and H3++2H2?H5++H2 [Reaction (2)] have been determined at several temperatures in a drift-tube mass spectrometer apparatus. Reaction (1) was found to have forward rate coefficients of (3.0±0.3) and (4.3±0.4) ×10−29 cm6/s at a temperature of 300 K and at temperatures between 135 and 160 K, respectively. Reaction (2) was found to have forward rate coefficients of (12±2), (9.5±3), (6.2±2.0), and (4.6±1.5) ×10−30 cm6/s at 156, 195, 203, and 210 K, respectively. Equilibrium constants for Reaction (2) were determined in the temperature range 176–320 K. The temperature dependence of the equilibrium constant indicates a dissociation energy of (0.35±0.03) eV for breakup of H5+ into H3+ and H2.
Summation of the Morse pairwise potential in gas–surface interaction calculations
Authors: Goodman Frank O.
Published: February 2003

The work of Cabrera and Goodman [J. Chem. Phys. 56, 4899 (1972)] is extended to the case of the Morse pairwise potential. A closed-form result is obtained for the Fourier components of the gas–surface interaction potential which results from summation of the Morse potential over all lattice sites. Particular attention is paid to the (0,0) component, that is, the surface-averaged potential, for which illustrations of the work are given for the system He–W and He–LiF.
Molecular dynamics studies of the vitreous state: Simple ionic systems and silica
Authors: Woodcock L. V. , Angell C. A. , Cheeseman P.
Published: February 2003

Some applications of molecular dynamics calculations to the vitreous state have been examined for simple ionic MX and MX2 type glasses. The MX system which corresponds to a hypothetical vitreous and supercooled amorphous KCl is found to undergo an isobaric transition in the region of 0.3 Tf and an isothermal transition at around 35 kbar (T=Tf). The transition is evidenced by a discontinuity in second order thermodynamic properties and may be associated with a virtual disappearance of translational diffusion. By employing a variety of irreversible stress histories, the vitreous state in this case is shown to be insensitive to the thermodynamic stress history up to a maximum computationally permissible relaxation time of around 10−10 sec. Simulations are also reported for some ionic liquids of MX2 stoichiometry which, in contrast with KCl, have glass-forming ability (BeF2, ZnCl2, and SiO2). Although the relaxation times involved in laboratory glass formation far exceed the long-time limit of computer ’’experiment,’’ the configurationally arrested states which are produced by simulation exhibit many characteristic features of the experimental glasses. Changes of heat capacity consequent on configurational arrest in the case of KCl are similar in magnitude to those observed at the experimental glass transition of simple ionic glasses. The radial distribution for simulated SiO2 ’’glass’’ is very similar to that for vitreous silica, and a simulated compression–vitrification–expansion cycle for this system produces a ’’densified’’ glass, as observed in laboratory experiments. A rattle-and-correlated-jump mechanism appears suitable for the description of diffusion in this liquid. Limitations and further applications of MD calculations pertaining to the vitreous state are discussed.
Lattice vibrations of crystalline benzene. II—Librational modes
Authors: Sataty Y. A. , Ron A.
Published: February 2003

All infrared and Raman active lattice modes of crystalline benzene were calculated using a six-center hexagonal potential function. The interction was expressed in terms of one parameter, d, the side of the hexagon. The agreement is good for the translational modes and reasonable for the librational ones.
Heat-pipe-oven reactor studies. III. Chemiluminescence from ternary flame systems
Authors: Luria Menachem , Eckstrom Donald J. , Benson Sidney W.
Published: February 2003

We present a summary of results obtained in a quantitative study of the chemiluminescence from several ternary flame systems (volatile polyhalide, alkali metal, and oxidizer) carried out in a heat-pipe-oven reactor (HPOR). The polyhalides studied were: CCl4, GeCl4, SiCl4, SnCl4, PCl3, BCl3, BI3, AsCl3, SCl2, and NF3. Sodium vapor was mostly used for the reduction, and N2O for the oxidation. The emission characteristics of these ternary flames can be classified in three groups: (1) Flames that produced emission from electronically-excited diatomic species. We observed radiation from upper electronic states of SnO, PO, BO, NO, and probably GeO, as well as from CN and C2. These flames also produced radiation from excited alkali atoms; (2) flames that produced strong emission from the various electronic levels of the alkali atoms up to the ionization limit, but no radiation from oxides. This group included SCl2, NF3, and CCl4, reacting with alkali vapors in the presence of N2O; (3) weak flames that produced emission only from the lowest resonance level of Na. This group included SiCl4, AsCl3, and BI3. The photon yields in the first group were on the order of 10−2 to 10−4; in the second group, up to 3%; and in the third group, less than 10−6. The photon yields were strongly pressure-dependent, with the highest yields obtained at the lowest pressure (∼50 mtorr in most cases), which suggests very large quenching cross sections. Possible excitation mechanisms for the atomic and molecular emissions are discussed.
Chemi-ionization in atom–diatom collisions
Authors: Preston Richard K. , Cohen James S.
Published: February 2003

Total cross sections and branching ratios are reported for the ionization processes resulting from collisions between He atoms and H2 molecules.(AIP)
Note on absorptions due to H2, HD, and D2 in argon matrices
Authors: Smith George R. , Warren Jonathan A. , Guillory William A.
Published: February 2003

The infrared spectra of matrix isolated D2, HD, and H2 are reported. The differences in the spectra of D2, HD, and H2 are discussed in terms of symmetry of a molecule. (AIP).
Dynamics of adsorption: Model calculations for several interaction potentials
Authors: McCreery Jane Hylton , Wolken George
Published: February 2003

A model potential to describe the interaction of a diatomic molecule with a rigid solid surface has been derived previously. Using this interaction potential, classical trajectory calculations have been carried out to study the dynamics of adsorption on a solid surface. Several forms of the interaction potential were employed so as to observe the effect of different types of potential surfaces on adsorption dynamics, and both H2 and HD were used in these model calculations in order to study the isotope effects on adsorption. Sticking probabilities for the different cases are reported and the validity of the rigid surface model is discussed.
Laser-fluorescence study of the reactions of alkaline earth atoms with BrCN: Spectroscopic observation of the alkaline earth monocyanides
Authors: Pasternack Louise , Dagdigian Paul J.
Published: February 2003

The reactions of M+BrCN, where M=Ca, Sr, and Ba, have been studied by laser-induced fluorescence. The dominant reaction pathway is the formation of the alkaline earth monocyanides (MCN); for the Sr and Ba reactions weak features due to products of the other possible pathway, MBr and CN, are also observed. This study represents the first spectroscopic observation of the MCN molecules. The dominant features of their excitation spectra are broad ’’quasicontinua’’ several hundred angstroms wide which occur at the same wavelengths as the spectra of the homologous alkaline earth monohalides (MX). Thus, the electronic structure of the MCN molecules is qualitatively similar to that of MX, which has been adequately explained by an ionic model, as discussed previously by Dagdigian, Cruse, and Zare. Radiative lifetimes of the MCN electronic states have been determined by direct observation of the fluorescence decay after pulsed laser excitation. The lifetimes are: CaCN: B state 40.8±1.5 nsec, C state 165±38 nsec; SrCN: B state 51.2±6.2 nsec, C state 104.4±6.3 nsec; and BaCN: C state 229±13 nsec, where the quoted uncertainties represent three standard deviations. The MCN and MX B state lifetimes are similar in magnitude, but the MCN C state lifetimes are considerably longer than the homologous MX C 2Π lifetimes. One possible interpretation is that the monocyanide excited C states, unlike those of the halides, are not simply described by one electron excitations into nonbonding metal atom orbitals but also include some excited state CN− character.
Time and space dependent energy distributions of atoms from surfaces
Authors: Modak Ashok T. , Pagni Patrick J.
Published: February 2003

Time and space dependent energy distributions for atoms effluxing from a surface are obtained analytically using the harmonic soft-cube model. The effluxing atoms consist of a desorbed fraction and a reflected fraction. The tangential energy distribution of the desorbed fraction is assumed to have equilibrated with the surface, while that of the reflected fraction is assumed unchanged. The normal energy distribution of the desorbed fraction depends on the adsorbed atom energy distribution obtained here from the governing master equation without the steady state approximation. The normal energy distribution of the reflected fraction is obtained from the soft-cube model for the collision dynamics. As expected, due to the gas–surface potential well, the desorbed fraction of the flux from the surface has a noncosine distribution. The reflected fraction has a lobular distribution about the specular angle. Time dependent scattering patterns are obtained as a function of the interacting atomic species, the incident energy distribution, and the gas and surface temperatures. Comparison with experimental desorption and adsorption rates and with scattering patterns from clean surfaces is possible.
Quantization for lattice-gas models of water
Authors: Herrick David R. , Stillinger Frank H.
Published: February 2003

Several recent studies have shown that some of the unusual properties of liquid water can be reproduced with a classical lattice-gas model, wherein the host lattice is body-centered cubic. In this paper we quantize both rotational and translational motion in those models, using suitable hopping operators. Several alternative forms are possible for the rotational kinetic energy; we provide comparisons for each with the experimental spectrum. Variational calculations have been performed for (H2O)2, (D2O)2, and (T2O)2 ground states on the lattice to estimate hydrogen bond destabilization by zero-point motion. Finally, expressions have been developed for thermodynamic-property and distribution-function quantum corrections that should be useful in classical lattice−gas simulations of water via computer.
Intermolecular interactions in polymorphic p-dichlorobenzene crystals: The α, β, and γ phases at 100 °K
Authors: Wheeler George L. , Colson Steven D.
Published: February 2003

The structures of the supercooled α and β phases of p-dichlorobenzene are determined at 100 °K and a detailed comparison made with the γ-phase structure previously reported at that temperature [Acta. Crystallogr. B 31, 911 (1975)]. The effects of temperature are investigated by a redetermination of the β-phase structure at room temperature and by measurement of the thermal expansion coefficients for the α- and β-phase crystals. The average atomic thermal displacements are found to be in the expected thermodynamic order. For chlorine we find Ū (β) =0.184 Å, Ū (α) =0.169 Å, and Ū (γ) =0.157 Å at 100 °K. Rigid body analysis of our data gives good agreement between calculated and observed librational lattice frequencies. New limits are placed on the density changes associated with the phase transitions [ΔV (α→β) =0.1±0.4% and ΔV (γ→α) =0.5±0.4%]. The effects of crystal field on the molecular geometry are limited to a slight out-of-plane displacement (0.045 Å) of the chlorine atoms in the γ phase. The possibility of such distortions in the high temperature α and β phases is masked by slight positional disorder (∼0.02 Å). The chlorine–chlorine interactions are found to be attractive and anisotropic. This indicates the necessity of including anisotropic terms in semiempirical atom–atom potential functions. All presently available functions, including Coulomb–Coulomb terms, are found to be inadequate for this reason.
A temperature dependent kinetics study of the reaction of OH with CH3Cl, CH2Cl2, CHCl3, and CH3Br
Authors: Davis D. D. , Machado G. , Conaway B. , Oh Y. , Watson R.
Published: February 2003

Reported in this study are temperature dependent rate data for the reaction of OH with the partially halogenated methane species CH3Cl, CH2Cl2, CHCl3, and CH3Br. The nominal temperature range covered was 245–375 K. The appropriate Arrhenius expressions are kA= (1.84±0.18) ×10−12 exp[−(2181±70/RT)], kB= (4.27±0.63) ×10−12 exp[−(2174±161/RT)], kC= (4.69±0.71) ×10−12 exp[(2254±214/RT)], kD= (7.93±0.79) ×10−13 exp[−(1766±116/RT)]. Units are cm3 molecule−1⋅s−1. No simple correlations between Eact and C–H bond strengths were found. The impact of these halogenated species on stratospheric ozone is also discussed.
The use of electron paramagnetic resonance matrix spin flip satellite lines as a tool in the structural determination of paramagnetic centers in disordered solids
Authors: Bales Barney L. , Lesin Eric Steven
Published: February 2003

It is shown that measurement of the first moment of one proton spin flip satellite with respect to the EPR main line, when combined with other methods, leads to a determination of the effective number of near neighbor protons and an effective distance to these protons in disordered solids. The method is applied to existing data for hydrogen atoms trapped in irradiated acids and results in neff(H2SO4) =3.1, reff(H2SO4) =2.06 Å and neff(H3PO4) =3.4, reff(H3PO4) =2.30 Å. In the case of one near neighbor proton, it is shown that the satellite line is always narrower than the main line. For Gaussian shaped lines, an explicit expression is given for the main and satellite linewidth difference in terms of the hyperfine coupling parameters.
The electronic structure of molecules by a many-body approach. I. Ionization potentials and one-electron properties of benzene
Authors: von Niessen W. , Cederbaum L. S. , Kraemer W. P.
Published: February 2003

The ionization potentials of benzene are studied by an ab initio many-body approach which includes the effects of electron correlation and reorganization beyond the one-particle approximation. The calculations confirm the assignment of the photoelectron spectrum experimentally proposed by Jonsson and Lindholm: 1e1g(π), 2e2g, 1a2u(π), 2e1u, 1b2u, 1b1u, 2a1g, 1e2g in order of increasing binding energy. To definitely establish the ordering of the ionization potentials in the second band, which has been very controversial, the corresponding vibrational structure has been calculated. A number of one-electron properties are calculated in the one-particle approximation and compared to experimental work and other theoretical calculations.
Orientational order and enthalpic measurements on binary mixtures at the N–S
A transition: Comparison with McMillan’s models
Authors: Achard M. F. , Hardouin F. , Sigaud G. , Gasparoux H.
Published: February 2003

We have studied binary mixtures which present a nematic to smectic A transition leading continuously from a first order transition to a second order transition. The evolution of the order parameter, close to the N–SA transition is observed by using diamagnetic susceptibility measurements. In addition, the transitional enthalpies were determined. A correlation between those different data is shown. We discuss the results with respect to the rigid molecule model proposed by McMillan to describe the N–SA transition.
New formulation of the pseudopotential theory for atoms with two valence electrons
Authors: Szasz Levente , Brown Lawrence
Published: February 2003

The equation for the correlated pair function of two valence electrons of an atom is transformed into a pseudopotential equation in such a way that the pseudopotential equation has exactly the same energy eigenvalue spectrum as the original equation. The new equation is much simpler mathematically than the original, since its solutions do not have to be orthogonalized to the core functions. In the first application of the new equation the model Hamiltonian approach of Hellman will be shown to be an excellent approximation to the exact equation. It is concluded that the model potential approach can be expected to yield results which will be practically as accurate as the results of the ab initio calculations. The new transformation is also given for the correlated pair equation of two core electrons.
Ion model and equilibrium configuration of the gaseous alkaline-earth dihalides
Authors: Guido M. , Gigli G.
Published: February 2003

The polarized ion model has been applied to calculate binding energies, equilibrium configurations, and force constants of the gaseous alkaline-earth dihalides. The capability of the model to predict bent structures has been proved and good agreement between experimental and calculated bond angles as well as binding energies has been achieved.
Dielectric friction and the viscosity of ionic solutions
Authors: Clark Gary J.
Published: February 2003

The methods employed by Zwanzig for determining the contribution of dielectric relaxation to ionic mobilities in electrolyte solutions can also be applied to viscosities. The results show that the Jones–Dole B coefficients should depend upon solvent properties in much the same manner as ionic friction coefficients; there is an inverse dependence upon ionic radius. B coefficients calculated from this treatment are of comparable magnitude to experimentally measured values.
High energy-resolution studies of electron impact optical excitation functions. II. The first positive system of N2
Authors: Estler Ron C. , Doering John P.
Published: February 2003

The relative emission cross sections for the (4,1), (4,2), and (5,2) bands of the N2 first positive system were measured in the threshold region using a high energy-resolution electron spectrometer coupled to an optical detection system. The maxima in the relative emission cross section for the (4,1) and (4,2) bands occurred at 10.68±0.08 eV and the maximum for the (5,2) band occurred at 11.00±0.08 eV. Structure was observed in the excitation functions which arose from the presence of channels for the population of the N2 B 3Πg state other than direct excitation by electron impact at certain energies. The contribution from C 3Πu→B 3Πg cascade is the major source of additional B 3Πg state population. The contribution from this process reaches a maximum at an incident electron energy near 14 eV. The E 3Σ+g state is observed to contribute slightly to the B state population near 12 eV through cascade and/or collisional deactivation. A third secondary feature is observed in the optical excitation functions close to threshold. Measurement of direct electron excitation functions for the v=4 and v=5 vibrational levels of the B 3Πg state by electron scattering techniques as well as delayed coincidence optical experiments on the first positive (4,1) band emission confirm the presence of a secondary process in this region. Radiative cascade from the N2 A 3Σ+u and/or W 3Δu states is the most reasonable explanation of this effect.
Striations in chemical reactions
Authors: Huberman B. A.
Published: February 2003

We present a theory of symmetry breaking in chemical reactions that takes into account both the kinetic aspects and the phase coexistence of the reacting species. Using a free energy functional that contains contributions from a concentration gradient in the reactants, simple criteria are derived for the conditions that lead to broken symmetries. Analytic expressions are obtained for the wavelength of the spatial structures and their characteristic time constants in terms of thermodynamic quantities. In the one dimensional case we also derive an expression for the interphase profile. We show that for a simple autocatalytic reaction it is possible to observe periodic patterns of chemical composition if the system is sufficiently far from phase coexistence equilibrium at a given time. Reactions with both positive and negative rate constants are analyzed in detail, and the role of nonlinearities in the time evolution of the system is discussed.
Laser-induced fluorescence determination of internal-state distribution of OH produced by H+NO2 in crossed molecular beams
Authors: Silver Joel A. , Dimpfl William L. , Brophy John H. , Kinsey James L.
Published: February 2003

The relative populations of rotational states in the v=0 and v=1 vibrational states of OH produced in the reaction of H+NO2→OH+NO in crossed molecular beams were measured by laser-induced fluorescence. The excited vibrational state v=1 was found to be produced at 1.3±0.3 times the rate of v=0. High degrees of rotational excitation were also observed. The rotational-state distributions were analyzed in information theoretic terms. A linear surprisal was found, which yielded a rotational surprisal parameter λROT=−2.69±0.57 for v=0.
Polyhalide photofragment spectra. I. Two-photon two-step photodissociation of methylene iodide
Authors: Kroger Peter M. , Demou Peter C. , Riley Stephen J.
Published: February 2003

Photofragment spectroscopy of CH2I2 at a wavelength of 266 nm shows evidence for a two-photon, two-step dissociation process. The first photon absorption causes transition to a repulsive excited state of B1 symmetry that breaks up rapidly to yield highly internally excited CH2I radicals and an I atom. On average, between 80% and 90% of the effective available energy for this process appears in CH2I rovibrational excitation. Once formed, a sizable fraction of the CH2I fragments may absorb a second photon, dissociating to CH2 radicals and another I atom. Energy distribution analysis indicates that the CH2 fragments are both translationally and internally excited, although quantitative estimation of their rovibrational excitation is complicated by the possibility of electronic excitation of one or more fragments in the two-step process. An approximate analysis of the CH2 fragment recoil angular distribution allows a crude resolution of the CH2I fragment internal excitation into its rotational and vibrational components. In the absence of electronic excitation, 40% of the CH2I internal energy is found to be rotational and 60% vibrational, qualitatively consistent with the picture of impulsive recoil of a semirigid radical. Kinetic analysis of the dependence of CH2I and CH2 fragment production on photolyzing light intensity permits extraction of the cross section for second photon absorption. From this, it is estimated that the extinction coefficient for CH2I radical absorption at 266 nm is 30 l mole−1⋅cm−1.