bluuebunny/crossref_metadata_2025 huggingface dataset:
ESR studies of damage processes in X-irradiated high purity a-SiO2:OH and characterization of the formyl radical defect
Authors: Griscom D. L. , Stapelbroek M. , Friebele E. J.
Published: February 2003

A range of high purity type III synthetic silicas (Suprasil 1, Spectrosil, Dynasil 1000) has been subjected to 100 keV X irradiation at 77 K and the induced ESR spectra have been recorded at 100 K before and after successive pulse anneals to higher temperatures. Absolute spin concentrations are determined as functions of radiation dose, anneal temperature and time, prior irradiation history, and sample supplier. Defects monitored comprise E′ centers, oxygen-associated hole centers (OHCs), atomic hydrogen, and a previously unidentified defect in a-SiO2 characterized by a 13.3 mT doublet centered on g=2.0. The 13.3. mT doublet is ascribed here to formyl radicals HCO produced by the reaction of radiolytic hydrogen atoms with minute amounts (≲0.1 ppm) of dissolved carbon monoxide present in Suprasil 1 and Dynasil 1000 but not Spectrosil. This identification is based on production kinetics and the similarity of the spin Hamiltonian parameters measured at 30 K to those previously reported for the formyl radical in solid carbon monoxide. Motional effects apparent in the HCO spectra at ≳100 K are interpreted with the aid of computer line shape simulations, and inferences are drawn concerning the interaction of the HCO molecule with the glass network. The isochronal anneal data are discussed in some detail and an influence of radiolytic molecular hydrogen on the color-center bleaching kinetics is postulated.
Two-photon absorption photothermal spectroscopy using a synchronously pumped picosecond dye laser. Thermal lensing spectra of naphthalene and diphenylbutadiene
Authors: Fang Howard L. , Gustafson T. L. , Swofford Robert L.
Published: February 2003

We demonstrate the use of a synchronously pumped picosecond dye laser as the excitation source for two-photon photothermal (thermal lensing) spectroscopy. The two-photon absorption spectrum of naphthalene in CCl4 is recorded in the range 31 450–35 200 cm−1, and the measurement is in agreement with the previously reported two-photon excitation spectrum. Similar agreement is found for the spectrum of diphenylbutadiene in the range 30 000–35 200 cm−1. It is demonstrated how the technique can provide accurate values for two-photon absorption cross sections by comparison of the thermal lens signals from one- and two-photon absorption. The two-photon absorption measurement in naphthalene indicates a symmetry-forbidden, vibronically induced transition, while the measurement in diphenylbutadiene indicates a fully symmetry-allowed two-photon transition. Both cross section measurements are in agreement with previously reported results.
The He i photoelectron spectra of CO2, CS2, and OCS: Vibronic coupling
Authors: Kovač Branka
Published: February 2003

High resolution He i photoelectron (PE) spectra of CO2, CS2, and OCS have been recorded and re-examined. Resolved spin-orbit splittings in the first doubly degenerate electronic states of CO+2, CS+2, and OCS+ are 160, 440, and 350 cm−1, respectively, and are in good agreement with the results obtained from emission spectra analysis. The fine structure analyses of the four lower systems reveal that nontotally symmetric modes are active upon ionization in addition to the totally symmetric ones. This indicates that the Born–Oppenheimer approximation is not valid in these cases, i.e., that nonvanishing interactions between and within electronic states take place. One quantum of bending mode couples X̃ (2Πig) with B̃ (2Σ+u), Ã (2Πiu) with C̃ (2 Σ+g) in cases of CO+2 and CS+2, and 2Πi with 2Σ+ states in OCS+. The Ã (2Πi) and B̃(2Σ+) states of OCS+ deserve special attention since considerable mixing of states is expected in the overlapping region.
Spin lattice relaxation of coupled spin 1–spin 1/2 system in anisotropic phases. Molecular dynamics of CHDCl2 in nematic phases
Authors: Voigt J. , Jacobsen J. P.
Published: February 2003

The general theory of the spin dynamics of the spin 1–spin 1/2 system is described. The normal mode equations for the spin lattice relaxation is established. The elements in the relaxation matrix are evaluated under the assumption that only intramolecular dipole–dipole interaction, quadrupole interaction, and intermolecular proton dipole–dipole interaction are important. The contributions from the interference term between the intramolecular dipole–dipole and the quadrupole relaxation has also been included. 1H and 2H spin lattice relaxation measurements have been performed on methylenechloride-d1 (CHDCl2) in two different nematic phases, Mercks phase V and ZLI 1167. Evidence for the existence of the interference effect between the dipole–dipole and the quadrupole relaxation has been found in both phases. The relaxation behavior is different in the two phases. The differences have been explained in terms of the ‘‘director fluctuation’’ theory.
Inner shell excitation of matrix-isolated silver
Authors: Hormes J. , Happel G.
Published: February 2003

The absorption spectra of silver atoms isolated in Ar, Kr, and Xe matrices have been measured in the wavelength region below 190 nm. From our spectra, we can assign all multiplets belonging to the 4d105s → 4d95s5p transition and, in argon, even the strongest members of the 4d105s → 4d95s6p transition. Matrix shifts and crystal field splittings are discussed in comparison to the results for the resonance transition 5s → 5p. The influence of the matrix on the various atomic levels is discussed in a simple picture assuming that the average radius of the wave function for the excited electron is the dominant parameter for the interaction.
Application of the semiclassical perturbation (SCP) approximation to diffraction and rotationally inelastic scattering of atoms and molecules from surfaces
Authors: Hubbard Lynn M. , Miller W. H.
Published: February 2003

The semiclassical perturbation (SCP) approximation of Miller and Smith [Phys. Rev. A 17, 17 (1978)] is applied to the scattering of atoms and molecules from crystal surfaces. Specifically, diffraction of He from LiF, and diffraction and rotationally inelastic scattering of H2 from LiF are treated, and the SCP model is seen to agree well with earlier coupled-channel and quantum sudden calculations. These tests of the SCP model are all for ‘‘soft’’ interaction potentials, e.g., of the Lennard-Jones Devonshire variety, but it is also shown that the model behaves correctly in the limit of an impulsive hard-wall potential function. The SCP picture thus appears to have a wide range of validity for describing the dynamics of gas-surface collisions.
13C and 14C kinetic isotope effects in the catalytic oxidation of CO over ZnO
Authors: Kobal I. , Senegačnik M. , Kobal H.
Published: February 2003

13C and 14C kinetic effects in the reaction CO+1/2 O2→CO2 over ZnO catalyst were experimentally determined. The k12/k13 and k12/k14 ratios were found to be temperature independent in the temperature range studied (200–500 °C) and amounted to 1.0101±0.0010 and 1.0204±0.0019, respectively. Interpretation of the experimental values, following Bigeleisen’s formalism, reveals that (CO2)‡ with an interband angles of (90±10)° and planar (CO3)‡ with two interband angles in the range of (120±10)° may be considered as activated complexes of the rate determining and isotope effect fractionation governing step of the reaction mechanism.
The effect of frequency dependent friction on isomerization dynamics in solution
Authors: Bagchi Biman , Oxtoby David W.
Published: February 2003

We investigate the effect of the frequency dependence of friction on the rate of photochemical isomerization in solution. Recent experiments have shown a significantly smaller effect of viscosity on rates at high viscosities than that predicted by Kramers’ theory. We show that this partial saturation of viscosity effects arises from the fact that the effective friction in the barrier region is much smaller than its zero frequency value at high viscosities. The fractional values of the exponent α (where kiso=A (ηs0)−α and ηs0 is the zero frequency shear viscosity) observed in recent experiments naturally arise when the frequency dependence of the friction is taken into account. The physical significance of these results is discussed.
The Enskog theory for multicomponent mixtures. I. Linear transport theory
Authors: López de Haro M. , Cohen E. G. D. , Kincaid J. M.
Published: February 2003

The Enskog theory for dense multicomponent fluid mixtures is developed. Two versions are considered—the standard theory and the revised theory. Explicit expressions for all the transport coefficients (shear and bulk viscosity, thermal conductivity, mutual and thermal diffusion coefficients) in terms of the sizes, masses, and concentrations of the constituents of the mixture are given in the third Enskog approximation. Applications will be discussed in subsequent papers.
Concentration dependent friction coefficients of polymer molecules in dilute solutions. I
Authors: Muthukumar M.
Published: February 2003

A cluster expansion theory is developed for the self- and cooperative-friction coefficients of polymer chains in the steady-state limit as a virial series in concentration. The derived equations are general and are valid for both good and theta solutions. The self- and cooperative-friction coefficients are shown to be the same in a theta solvent. As a test of the theory, the leading concentration dependence of the translational and rotational friction coefficients are calculated for θ solutions. Our results for the coefficient of concentration dependence of the translational friction coefficient is (ks)θ=0.75 [η] (where [η] is the intrinsic viscosity) in excellent agreement with the experimental data of Mulderije. The details of the explicit calculations are given in the following paper.
Concentration dependent friction coefficients of polymer molecules in dilute solutions. II
Authors: Muthukumar M. , DeMeuse Mark
Published: February 2003

Using a multiple scattering cluster expansion approach, an explicit calculation of the leading terms of the concentration dependent translational and rotational frictional coefficients is presented. Within the preaveraging approximation, a value of 0.75 [η] is obtained for the leading factor for the translational friction coefficient where [η] is the intrinsic viscosity. This is in excellent agreement with the recent experimental results of Mulderije. Similarly, a value of 0.43 [η] is obtained for the same term for the rotational friction coefficient.
The measurement of the lifetime of a 3Πu state of Na2
Authors: Shimizu Fujio , Shimizu Kazuko
Published: February 2003

Lifetime of the a 3Πu0 state of Na2 in the collisionless regime was measured by the time resolved laser induced fluorescence of the a 3Πu0–X 1Σ+u transitions. For levels which have a close coincidence with the A 1Σ+u level, the observed lifetime was explained by the radiative decay induced by the mixing with the perturbing A 1Σ+u level. For other levels, it was found to be 3.0–4.5 μs.
On the auxiliary functions Ḡq−N
N′ for calculation of multicenter integrals over Slater-type orbitals
Authors: Guseinov I. I.
Published: February 2003

Simple recurrence reations are presented for the auxiliary functions Ḡ9−1N1. The multicenter integrals over stater type orbitals STO’s can be calculated by using the auxiliary functions Ḡ9−1N1. (AIP)
Comment on ‘‘Dirac states for unit position and momentum: Phase consistency of their angular momentum representations’’
Authors: Hahne G. E.
Published: February 2003

The argument presented makes two points about Snider’s [J. Chem. Phys. 76, 3543 (1982)] transformation between ‘‘unit position’’ states ‖r̂〉 and ‘‘unit momentum’’ states ‖p〉 for the motion of a structureless particle in three space: Snider’s transformation (1) implicitly defines an operator p̂ that has a classical limit a unit vector that is perpendicular to both r̂ and l=r×p, while on a classical scattering trajectory for r→∞, the physical unit vector p/p≊±r̂, and (2) attempts, inconsistent with the kinematics of states and observables of the system, to pass from r̂ to p̂ amplitudes of a state vector without any reference to the scalar r or p dependence of the wave functions.
Electronic excited state transport among molecules distributed randomly in a finite volume
Authors: Ediger M. D. , Fayer M. D.
Published: February 2003

A theoretical study of electronic excited state transport among molecules randomly distributed in a finite volume is carried out. Two special cases of the general transport and trapping problem are treated. A truncated series expansion in powers in the chromophore density is used as an approximation for one component systems (i.e., donor–donor transport only). In two component systems of donors and traps, the Förster limit, in which transfer can occur only from donors to traps due to low donor concentrations, is solved exactly for a finite spherical volume. In both cases, the results presented demonstrate that time-dependent observables can be significantly altered in finite volume systems relative to infinite volume systems. These calculations have implications for the interpretation of experiments performed on real finite volume systems, e.g., energy transport among the chromophores of an isolated polymer chain.
Diffusion of optical excitation at finite temperatures
Authors: Huber D. L.
Published: February 2003

We study diffusion of optical excitation as a function of the ratio of the temperature to the inhomogeneous linewidth Δ. When T≪Δ the diffusion constant associated with one-phonon-assisted dipolar transfer over a Gaussian line is suppressed by the factor exp[−aΔ2/T2]. The effect is attributed to a decrease in the effective density of ions participating in the transfer.
The multistate kinetics of nucleation in the presence of an aerosola)b)
Authors: McGraw Robert , Marlow William H.
Published: February 2003

The classical Becker–Döring homogeneous nucleation theory is rigorously extended to include monomer and cluster removal mechanisms capable of coupling to the usual Becker–Döring equations for evaporation and growth. Exact expressions are obtained in a compact continued fraction form for the steady state cluster distribution and nucleation rate in the presence of an aerosol. The unmodified classical theory is recovered in the limit of a particle free vapor. Recently proposed models for new particle formation in the presence of an aerosol and nucleation in a nonuniform vapor are shown to be contained within the present formulation. Condensation curves are calculated for a series of model condensable vapor species covering a wide volatility range in the presence of an aerosol. For those vapors having low equilibrium vapor pressures (less than 10−5 Torr at the highest aerosol concentrations considered) significant departures from predictions based on the unmodified classical theory are found.
A stochastic model for reversible pressure denaturation of chymotrypsinogen
Authors: Gardiner C. W.
Published: February 2003

The relaxation rate characteristic of pressure denaturation of chymotrypsinogen is explained by a stochastic model in which both volume and shape changes are taken into account. All features of the data are explained qualitatively, though quantitative checks are not possible without more data.
On the temperature derivative of the surface tension at a critical end point
Authors: Robert M. , Tavan P.
Published: February 2003

It is shown that, according to the van der Waals theory of fluid interfaces, the surface tension of the interface between a This result holds for any number of phases and independently varying densities and is not restricted to classical values of the critical exponents.
A Mössbauer spectroscopic study of the six-coordinate high-spin ferrous compound (meso-tetraphenylporphinato) bis(tetrahydrofuran) iron(II)
Authors: Boso Brian , Lang George , Reed Christopher A.
Published: February 2003

Mössbauer spectra of a polycrystalline form of the six-coordinate high-spin ferrous compound (meso-tetraphenylporphinato) bis(tetrahydrofuran) iron (II) have been recorded over a range of temperatures (4.2–195 K) and magnetic fields (0–6.0 T). Analysis of the spectra using a phenomenological model of the internal magnetic field and using an S=2 spin Hamiltonian, where applicable, yield the sign of Vzz negative, η=0.4, D=6.0 cm−1, E/D=0.1, and Ã*/g*N βN =(−7.2, −7.2, and −24.3 T). These results suggest that the iron experiences an octahedral crystal field, trigonally distorted in the (1, 1, 1) direction, producing a prolate orbital dz2 as the ground state. Crystal field calculations confirm this interpretation by reproducing the spin Hamiltonian parameters listed above. The calculation predicts an orbital doublet 1667 cm−1 above the ground state. Comparisons with deoxyheme proteins and their synthetic analogs suggest some common gross features of the orbital state and structure-related trends in the character of the ground quintet.
Diffusion controlled processes among stationary reactive sinks: Effective medium approach
Authors: Cukier R. I. , Freed Karl F.
Published: February 2003

We formulate an effective medium theory for diffusion controlled processes among randomly distributed, stationary, reactive sinks. An implicit expression for the exact ‘‘self-energy’’ of the combined reactive density field and sinks is obtained in terms of the exact medium propagator. The lowest order truncation of this expression can be solved by numerical iteration for the self-energy. The rate and diffusion coefficients as a function of sink concentration are extracted from this self-energy. We also consider the next order truncation which explicitly introduces the pair correlation function of the sinks.
Microwave absorption studies of interface phenomena at ZnO electrodes
Authors: Bogomolni Roberto A. , Tributsch Helmut , Petermann Günter , Klein Melvin P.
Published: February 2003

In order to explore the feasibility of microwave absorption techniques as a tool for the study of interfaces and electrode reactions we have measured the electrochemical and photochemical properties of ZnO in contact with an aqueous electrolyte by conventional electrochemical procedures and simultaneously by electrodeless detection of microwave absorption at 9.5 GHz in a resonant cavity. Pronounced potential dependent microwave absorption signals were found which proved to be influenced by illumination and electrochemical surface reactions. The comparison of microwave absorption with the electrochemically measured electrode behavior indicates, that microwave absorption data can provide interesting complementary information on the nature and reactivity of charge carriers and dipoles in the electrode/electrolyte interface. The proposed method is so sensitive that signals arising from fractions of monolayers will be detectable. A comprehensive interpretation of the results is not yet attempted. Further experimental studies on simple electrode systems are needed to clarify the mechanism. Because of its nondestructive nature, the microwave method could develop into a valuable new technique in surface chemistry.
On the use of the Ewald summation in computer simulation
Authors: Adams D. J.
Published: February 2003

Monte Carlo calculations for the restricted primitive model at a point in the molten salt region have been made using a variety of summation methods. The empirical conclusion is that the Ewald summation is an excellent method for this work. It is shown that it does not produce significantly anisotropic pair distributions. It is shown that unlike some of the other methods considered it does not have a systematic bias towards lower energy for some configurations. It is argued that the shape of its effective pair potential does not lead to distortion of the melt structure. However, an approximate form of the Ewald summation is shown to produce significant distortions of the melt structure for the case examined.
On the motion of an ion near a conducting wall: Correlation effects
Authors: Mazo Robert M.
Published: February 2003

When a charged particle moves near a conducting wall, the induced current exerts a retarding force on the particle. The magnitude of this retarding force for a single ion has previously been computed. Here, the effects of ionic spatial correlations in an electrolyte solution are examined. We find that, at large distances from the wall, there is partial screening of the retarding force. At small distances, where the entire effect is largest, the previously considered single ion contribution is dominant.
Quantitative theory of solubility of supercritical fluids
Authors: Gitterman Moshe , Procaccia Itamar
Published: February 2003

The phenomenon of enhanced solubility of solids in supercritical fluids is analyzed. The entire region of pressures is considered on the basis of the thermodynamic approach, independently of specific models. General aspects of the phenomenon have been discovered by considering the slopes of solubility curves. These slopes are shown to diverge near the two existing critical points, namely the lower and upper critical end points. In the case of dilute solutions, a curve of the locus of maxima of slopes of solubility curves is found in the reduced pressure-reduced temperature plane. All available experimental data are unified under this approach and are found to agree with theoretical analysis. The results obtained can be used to optimize supercritical fluid extraction.
14N and 35Cl double resonance study of the phase transition in the intercalated bilayer compound C10H21NH3Cl
Authors: Seliger J. , Žagar V. , Blinc R. , Arend H. , Chapuis G.
Published: February 2003

The pure nuclear quadrupole (NQR) spectra of 14N, 35Cl, and 37Cl have been measured in C10H21NH3Cl as a function of temperature. The chlorine NQR frequency νQ (35Cl)=1215 kHz at 47 °C is determined by the N–H––Cl hydrogen bonds and is the lowest reported so far. The 14N quadrupole coupling data e2qQ/h=760 kHz. η=0 shows that the C–N–H––Cl backbone is practically rigid at this temperature. Above 53 °C the effects of the phase transition from an intercalated to a nonintercalated structure show up in the N–H––Cl backbone as a sharp drop of the NQR frequencies to ∼60% of their rigid lattice values. A model accounting for this effect has been proposed.
The reactive quantum Boltzmann equations: A derivation from an arrangement channel space representation and BBGKY hierarchy
Authors: Evans J. W. , Hoffman D. K. , Kouri D. J.
Published: February 2003

A rigorous derivation of the reactive quantum Boltzmann equations is presented for systems where breakup and recombination are excluded. The use of an arrangement channel space representation allows an exact decomposition of the N particle density matrix into components for different chemical compositions and an exact definition of reduced species density matrices (as opposed, e.g., to standard projection operator techniques). This necessitates the use of the combinatorially complex arrangement channel BBGKY hierarchy which, however, avoids the need for the usual heuristic specification of collision terms. Another advantage is that scattering equations generated for the reactive and nonreactive many body T matrices appearing in the Boltzmann equations have ‘‘well-behaved’’ kernels (unlike the corresponding Lippmann–Schwinger equations). From the derived equations we readily obtain, e.g., reaction-diffusion equations and nonequilibrium expressions for the chemical reaction rates.
Intramolecular relaxation in N=2 Hamiltonian systems: The role of the K entropy
Authors: Hamilton Ian , Brumer Paul
Published: February 2003

Autocorrelation functions and averages (over distributions) of dynamical variables, plus coarse grained entropy techniques are used to explore relaxation in two-degree-of-freedom (i.e., N=2) oscillator systems. The K entropy is shown to provide relaxation times for initial distributions modeling energy deposited in zero order modes as well as delay times for initially localized distributions.
Enthalpy of formation and experimental lattice energy of sodium 7,7,8,8-tetracyanoquinodimethanide
Authors: Metzger Robert Melville , Arafat El Sayed , Kuo Chiun S.
Published: February 2003

The enthalpy of formation of NaTCNQ(c) was determined by rotating-bomb semimicro combustion calorimetry to be ΔH°f,298.15 =348.40±6.68 kJ mol−1. The ionic salt is more stable than its ionic gaseous constituents [Na+ (g), TCNQ− (g)] by 756.43±15.15 kJ/mol; the Madelung energy is only 67.2% of the experimental crystal binding energy (assuming full charge transfer ρ=1.0).
On the rate of absorption of Brownian particles by a black sphere: The connection between the Fokker–Planck equation and the diffusion equation
Authors: Naqvi K. Razi , Waldenstro/m S. , Mork K. J.
Published: February 2003

It is shown that, if one takes the first two moments of the Fokker–Planck equation, one obtains, in the lowest order, the telegraph equation irrespective of whether one uses a full-range or a half-range decomposition of the distribution function; it is pointed out that the exact boundary condition, according to which the distribution function for emerging particles vanishes at the surface of an absorbing (black) sphere of radius R, may be replaced, in either case, by Marshak’s boundary condition j+(R, t) =0, where j+ is the outward radial current. Drawing on Wilemski’s work, the second moment equation is finally replaced by Fick’s law, obtaining thereby the diffusion equation and the radiation boundary condition.
Third body efficiencies for collision-induced dissociation of diatomics. Rate coefficients for H+H2→3H
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

Fixed-energy cross sections, thermal-translation rate constants, energies of activation, opacity functions, and probabilities of reaction as functions of initial relative orientation angle are presented for the reaction H+H2(v, j)→3H on the lowest-energy potential surface for selected initial vibrational-rotational states (v, j) of H2. In agreement with the experimental result for steady dissociation of a thermal mixture of initial states, the collision-induced dissociation process for these state-selected processes is found to be considerably enhanced for H as collision partner as compared to Ar as collision partner. We find that the cross sections for collision-induced dissociation are strongly increasing functions of initial internal energy and initial vibrational quantum number. The general trend at 4500 K is that the rate constants increase more rapidly than the equilibrium populations decreases as we increase v. The activation energies for complete dissociation are larger than the fixed-state binding energies by 0.2–1.1 eV.
Determination of cesium ion–rare gas potentials from total cross section measurements
Authors: Rajan Malini S. , Gislason Eric A.
Published: February 2003

Total cross sections have been measured for Cs+ ions scattered by He, Ne, Ar, Kr, and Xe in the range EΘR=5–1000 eV deg. Here, E is the laboratory energy of the Cs+ beam and ΘR is the resolution angle of the apparatus. The cross sections have been inverted to obtain accurate estimates of the potential V(R) over a wide range of R including the attractive well region. The results are compared with other theoretical and experimental work on these systems. The agreement with other experiments is good, except for the Cs+–He system, which is the most difficult diatomic to study because of its shallow potential well.
Rate constant for the reaction of O(3P) with HBr from 221 to 455 K
Authors: Nava D. F. , Bosco S. R. , Stief L. J.
Published: February 2003

Absolute rate constants for the reaction O(3P)+HBr→OH+Br have been measured over the temperature range 221–455 K using the flash photolysis technique combined with resonance fluorescence detection of O(3P) atoms. For each of the five temperatures at which the reaction was studied, the rate constant was shown to be independent of variations in [HBr], total pressure (Ar), and flash intensity (i.e., initial [O]). A linear least-squares treatment of the data in Arrhenius form yields the expression k7=(6.73±1.93)×10−12 exp(−1537±81/T) cm3 molecule−1 s−1 for 221≤T≤455 K, where the indicated errors are at the one standard deviation level. There is a suggestion of curvature in the Arrhenius plot below 298 K. These results are compared with two previous studies of the temperature dependence which covered a narrower range of temperatures and used O+NO chemiluminescence to monitor O(3P). Consistency with an estimate of k7 at 773 K is evaluated and rate constants for atomic bromine reactions derived from a study of the HBr inhibited thermal H2/O2 reaction at this temperature are examined in light of recent direct measurements. Comparison is also made of the activation energy measured here for the O+HBr reaction with recent calculations employing a modified bond energy–bond order method.
Universal even-tempered basis sets for negative molecular ions
Authors: Cooper David L. , Wilson Stephen
Published: February 2003

Universal even-tempered basis sets of exponential-type functions for negative molecular ions are developed. The near linear dependence which may arise in molecular basis sets containing diffuse functions is discussed. Applications to the ground states of the CH− and the OH− anions are presented.
The concept of quasispin and its use for the study of physical and chemical properties of alternant conjugated hydrocarbons
Authors: Čížek Jiří , Pauncz Ruben , Vrscay Edward R.
Published: February 2003

The concept of quasispin is introduced in an elementary way in order to study the physical and chemical properties of alternant conjugated hydrocarbons in the Pariser–Parr–Pople and Hubbard models. The physical interpretation of quasispin is presented. Further, it is shown how the number of states of a given spin, quasispin, and ionization level can be enumerated. Finally, methods for the construction of simultaneous eigenfunctions of spin and quasispin are discussed in full detail for the case of hexatriene.
An electron spin echo study of the photoexcited triplet state of tetracene in p-terphenyl crystals at room temperature
Authors: Yu Hsiang-Lin , Lin Tien-Sung , Sloop David J.
Published: February 2003

Room temperature magnetic resonance studies of the photoexcited triplet state of tetracene in p-terphenyl crystals have been carried out by the electron spin echo technique. The studies yield the following structural and dynamical information: (1) zero-field splittings; D/hc=±0.0551(1) and E/hc=∓0.0042(1) cm−1; (2) spin densities: ρ1(α)=0.055, ρ2(β)=0.016, ρ5(γ)=0.217, ρ13=−0.025, and ρ15=−0.027; and (3) relative triplet population rates: px≊py≳pz.
Infrared–microwave double resonance spectroscopy of the SiF4 ν3 fundamental using a tunable diode laser
Authors: Takami Michio , Kuze Hiroaki
Published: February 2003

Pure rotational spectrum of silicon tetrafluoride SiF4 is investigated in the ν3 vibrational state by infrared–microwave double resonance. Frequency tunable semiconductor diode lasers are used as an infrared pumping radiation source. 106 radiofrequency transitions among the tetrahedral fine structure splittings and 60 microwave transitions among the various Coriolis sublevels are measured with an accuracy of a few tens of kilohertz. Seven scalar and four tensor constants of the excited state are determined accurately. A part of the badly congested infrared Q-branch spectrum is analyzed on the basis of assignment by double resonance. Three ground-state constants B0, D0, and D044 are determined.
Electron-beam pumped kinetic study of ArF* (B) state
Authors: Chang R. S. F.
Published: February 2003

ArF (B–X) emission at 193 nm has been studied as a function of pressure in Ar/F2, Ar/NF3, Ne/Ar/F2, and Ne/Ar/NF3 mixtures under long pulse electron-beam excitation. Fluorescence yield, formation yield, as well as apparent quenching rate constants of the ArF* (B) state by Ne, Ar, F2, and NF3 have been measured. While the ArF* formation yield is about the same as per neon or argon excitation produced, highest fluorescence yield is obtained from a Ne/Ar/F2 mixture. Using Ne as the main diluent gas prevents the rapid conversion of ArF* to Ar2F* and Ne itself is a slower quencher of ArF* than Ar.
The singlet π* ← n spectrum of jet-cooled acetaldehyde
Authors: Noble Marcus , Apel Eric C. , Lee Edward K. C.
Published: February 2003

The supersonic jet fluorescence excitation spectrum of the singlet π* ← n transition of CH3CHO has been recorded between 29 700 and 30 800 cm−1. The lower frequency vibrational peaks have been analyzed in terms of two upper electronic state (S1) vibrations; the out-of-plane bend (ν′14) and the CH3 torsion (ν′15). A threefold potential with a barrier height of V′3=653 cm−1 gave an accurate fit of the observed ν′15 levels of the S1 state. The allowed transition to the electronic origin has been observed. The value of T0=29 771 cm−1 is considerably higher than recent estimates .
Laser induced fluorescence spectra of fluorophenol cations in a Ne matrix
Authors: Bondybey V. E. , English J. H. , Miller Terry A. , Shiley R. H.
Published: February 2003

Laser induced fluorescence and/or absorption spectra of the cations of 2,3,5,6-tetrafluorophenol, 2,3,5,6-tetrafluorothiophenol, and 3,5-difluorophenol have been obtained in a Ne matrix. The spectra of C6HF4OH+ are much better resolved than in the gas phase. The gas phase congestion is likely caused by the near degeneracy of the X̃ and Ã electronic states whose separation is now measured at 207 cm−1. The spectrum of C6H3F2OH+ represents a deperturbed example of the Jahn–Teller distorted sym-C6F3H3+ ion. C6H3F2SH+ shows only a broad featureless absorption.
ENDOR analysis of H-addition radicals and their conversion in irradiated crystals of tryptamine hydrochloride
Authors: Theisen Håkon , Sagstuen Einar
Published: February 2003

In a previous study it was shown that the dominant radiation-induced room-temperature radical in single crystals of tryptamine⋅HC1 is formed by a net H addition to C7 of the indole ring. In the present work, it is demonstrated that this species is the precursor for two other indole H-addition radicals. By storage at room temperature, the C7 adduct slowly converts into the C4 adduct. The formation of the C4 adduct could easily be followed by ENDOR. The ENDOR spectra are characterized by hyperfine coupling to six protons bonded to the indole moiety. Upon annealing the crystal above 70 °C, the C7 adduct is more rapidly converted into the C4 adduct, but then the annealing also induces a simultaneous conversion into the C2 adduct. This species also is characterized by six proton hyperfine interactions in the strong-coupling region. The C7 add→C2 add process could be reversed by exposure to UV light at room temperature, whereas the C7 add→C4 add process is irreversible upon this procedure. Semiempirical molecular orbital calculations support the interpretations made. The mechanisms of the different processes is discussed in light of the crystal and molecular structure of the compound, and related to similar processes in other compounds, e.g., the purine and pyrimidine bases.
An APS-XPS study of vanadium pentoxide
Authors: Curelaru I. M. , Suoninen E. , Minni E.
Published: February 2003

We have measured the APS and XPS spectra of V2O5(V6O13). The time variation of the APS spectrum indicates that the chemical composition of the sample gradually changes during electron bombardment. A comparison between our APS and XPS spectra, and the literature data on XES, XAS, and the energy level diagram of molecular orbitals obtained by a cluster model calculation, illustrates the local breakdown of the one-particle orbital model of the solid.
An EPR study of a radical anion in x-ray irradiated trifluoroacetamide single crystals at 3 K
Authors: Samskog P.-O. , Kispert L. D.
Published: February 2003

The radical anion CF3ĊONH2− was identified by EPR spectroscopy in a single crystal of trifluoroacetamide x-ray irradiated at 3 K. The primary radical gave anisotropic hyperfine couplings to a fluorine of 139, 78, and 63 G, with g values 2.0061, 2.0047, and 2.0029. The maximum fluorine coupling is directed close to one of the C–F bonds, indicating a delocalization of spin density into that bond. The radical anion decomposes thermally by defluorination, giving ĊF2CONH2. By bleaching the anion with visible light a ĊF3 fragment is formed. A complete reaction mechanism both for the primary oxidation and reduction products consistent with the observed species is proposed.
Charge transfer complexes of molecular oxygen in NaClO3 and KClO3 studied by resonance Raman and infrared spectroscopya)
Authors: Bjerre Nis , Bates J. B.
Published: February 2003

The complex point defects [ClO−,O2] and [ClO−2,O2], formed by photolysis of crystalline NaClO3 and KClO3 at 15 or 77 K, were studied by resonance Raman and infrared spectroscopy. The vibrational frequencies correspond closely to those of the individual species whereas a variety of other properties is peculiar to the complexes. Thus, they have optical absorptions in the visible spectral region, the vibrations of both components are resonance Raman active under excitation with visible light, and the vibration of O2 is active in the infrared. A model which accounts for these properties in terms of charge transfer between the two components of the complex is described. Estimates based on the intensities of the infrared and optical absorptions suggest that 0.01–0.03 electron is transferred from ClO− to O2 in the ground state of the complex. The effects of the librational motion of the O2 molecules and the photoinduced reorientations of some of the defects are described.
Effects of overmodulation on saturation transfer EPR signals
Authors: Robinson Bruce H.
Published: February 2003

Recent calculations have been published which simulate the effects of anisotropic Brownian motion on saturation transfer EPR spectra, in which the effects of over modulation were not explicitly included because of very long computation times. Overmodulation is the only spectrometer parameter needed to be incorporated into this theory for complete agreement with experimental spectra. Here we suggest various approximations for the effects of modulation which will avoid long computation time. We derive an equation for the spin-lattice relaxation time T1e and the spin–spin relaxation time T2e which takes into account the effects of the Zeeman modulation amplitude. Spectra calculated using these modified relaxation times are overlaid with experimental spectra of fully perdeuterated 15N nitroxyl spin labels for several different modulation amplitudes, with no other adjustable parameters. The conclusion is that modified relaxation times adequately account for overmodulation effects of STEPR signals in the case of isotropically tumbling systems. The physical basis for this and related approximations is discussed.
Critical point shifts in films
Authors: Nakanishi Hisao , Fisher Michael E.
Published: February 2003

Critical behavior in thin films is discussed with attention to the example of phase separation in binary fluid mixtures between parallel plates. The analyses focus on the dependence of the shift in critical temperature ΔTc, critical field, etc., on the film thickness D, and on the nature of the walls as modeled by a surface field or chemical potential h1 which acts near the walls and leads to preferential adsorption of one of the bulk phases. Mean field theory for an Ising/lattice-gas model is utilized and the resulting asymptotic scaling functions for the shifts ΔTc etc. are computed within Landau theory by analytic and numerical methods. Series analyses for simple cubic lattice Ising model films with h1=0 are used to estimate universal features of three-dimensional systems: specifically, if ξ(ΔT) is the bulk correlation length, determined, say, via scattering experiments, at ΔT=T−T∞c≳0 then the shift ratio D/ξ(‖ΔTc‖) is about 2.89 for h1=0 but 4.61 for h1→∞, compared with mean field values π and 5.0699. Crossover effects for small nonzero h1 may lead to 10%–15% errors in estimating the exponent for the decay of ΔTc with D. The relation of the theory to recent experiments and the connection with wetting phenomena are discussed briefly.
Influence of Gaussian fluctuations on a model kinetic system exhibiting explosive behavior
Authors: Chandler Elaine , Deutch J. M.
Published: February 2003

We examine the influence of fluctuations on a model kinetic equation which exhibits explosive behavior. The particular model kinetic equation ẋ=kx2−αx has fixed points at x=0 and x=x1=(α/k). For constant coefficients k and α, the trajectory x(t) diverges in a finite time if the initial value x0≳x1, and x(t) approaches zero if x0<x1; fluctuations in the coefficients may change this deterministic behavior. We consider three different models incorporating Gaussian fluctuations. In case A, the rate coefficient k(t) has a fluctuating part; in this case it is possible to determine exactly the entire probability distribution p(x,t‖x0) as well as N(t) the probability that the system has not exploded (for a given x0) at time t. In case B, the rate coefficients k(t) and α(t) contain identical fluctuating parts; we show that this type of fluctuating behavior has no influence on the transition between stable and unstable behavior. In case C, the rate coefficient α(t) contains a fluctuating part. An approximation solution leads to an effective kinetic equation which is identical to the deterministic equation except that α is replaced by αeff=α(1−αq), where q is a measure of the strength of the fluctuation in α(t). This model kinetic equation is helpful for illustrating how fluctuations may influence kinetic systems that contain explosive character, including the problem of passage over a barrier in the overdamped limit.
A Monte Carlo simulation study of the two-dimensional melting mechanism
Authors: Allen Michael P. , Frenkel Daan , Gignac William , McTague John P.
Published: February 2003

We report here a Monte Carlo study of the thermodynamic and structural properties of a two-dimensional system of 2500 particles interacting by a repulsive inverse sixth power potential. Particular effort was made in the melting region, both to identify the defect structures and to ascertain the thermodynamic changes associated with the transition. Significant precursor fluctuations occurred on both the solid and fluid sides, and the unbinding of dislocation pairs was observed on melting. More complex defect patterns, including grain boundary loops, were also generated near melting. The Kosterlitz–Thouless, Halperin–Nelson, and Young dimensionless elastic constant approached the universal value 16π in the solid, and the orientational correlation range tended to diverge in the fluid near freezing. However, we were unable to distinguish between a very weak (ΔV/V≂0.7%) first order and a continuous transition, since both spatial structures and temporal relaxations became exceedingly long.
Criticality in the Yvon–Born–Green and similar integral equations
Authors: Fisher Michael E. , Fishman Shmuel
Published: February 2003

The critical behavior implied by the Yvon–Born–Green integral equation for the pair correlation function of a fluid, and by a class of similar integral equations, is considered. By using the inverse correlation length κ, which vanishes at a true critical point, as a small parameter the integral equations are reduced, via a moment expansion, to a nonlinear differential equation which describes the critical behavior of the correlation function outside the (finite) range of the potential. Analysis then shows that for spatial dimensionalities d≳4 any critical behavior is, asymptotically, precisely of classical, Ornstein–Zernike scaling form. However, for d≤4 either there is no true criticality, in the sense that both the correlation length and the compressibility KT always remain finite, or the behavior becomes anomalous in that the net correlation function h(r)≡g(r)−1 must, in general, become negative for intermediate and large distances r as κ→0. In the latter case, asymptotic and universal scaling behavior occurs, with a critical point decay exponent η=4−d, and KT→+∞ for d≳d0≂2.2, although KT→−∞ for 2≤d<d0. For d<2 the compressibility always remains finite.
Laser-pulsed plasma chemistry: Laser-initiated plasma oxidation of niobium
Authors: Marks R. F. , Pollak R. A. , Avouris Ph. , Lin C. T. , Théfaine Y. J.
Published: February 2003

We report the first observation of the chemical modification of a solid surface exposed to an ambient gas plasma initiated by the interaction of laser radiation with the same surface. A new technique, which we designate laser-pulsed plasma chemistry (LPPC), is proposed for activating heterogeneous chemical reactions at solid surfaces in a gaseous ambient by means of a plasma initiated by laser radiation. Results for niobium metal in one atmosphere oxygen demonstrate single-pulse, self-limiting oxide growth induced by a pulsed CO2 laser. X-ray photoelectron spectroscopy (XPS or ESCA) was used to monitor surface chemical composition changes and thickness control of thin (1 to 5 nm) reaction product layers. The dependence of single-pulse oxide growth upon laser fluence is observed to be monotonic for oxide thicknesses up to 5 nm. Composition of the oxide Nb2O5−δ, formed by such an optically driven plasma, is similar to that formed by low-temperature oxidation processes such as rf plasma oxidation; however, the valence defect δ of the LPPC oxide is a least two to five times lower. Interdiffusion at the oxide/metal interface becomes important at higher irradiances and is activated by direct optical coupling with the solid or by plasma-mediated thermal coupling. Under ultrahigh vacuum, CO2 laser irradiances greater than 0.9 J cm−2 per pulse thin the surface oxide.
Scattering from disordered surfaces in the sudden approximation
Authors: Gersten J. I. , Gerber R. B. , Dacol D. K. , Rabitz H.
Published: February 2003

The angular intensity distribution of atomic beams scattered from partially disordered solid surfaces is investigated. Quantitative relations are established between correlation functions that describe the structural disorder, and several features of the scattered intensity distribution. To simplify the collision dynamics, the study is carried out in the framework of the sudden approximation, which assumes that momentum changes perpendicular to the surface are large compared with momentum transfer due to surface corrugation. Simple relations are obtained between the structural correlation functions and the scattering at near-specular angles, at extreme off-specular angles, and for directions close to that at which the surface scattering rainbow appears for the corresponding ordered lattice system. Detailed expressions for the case of step disorder are presented. The results suggest that molecular beam scattering should be a powerful probe of structural properties of disordered surfaces.
The diamagnetic-susceptibility anisotropy of O2(3Σ) from the temperature dependence of the Cotton–Mouton effect
Authors: Kling H. , Dreier E. , Hüttner W.
Published: February 2003

The magnetic-field induced birefringence of oxygen has been measured between 200 and 400 K, and at liquid-nitrogen temperature. The experimental results can be understood if the rotational dependence of the coupling between electronic spin and molecular frame is properly accounted for. In order to do so a quantum mechanical expression for the Cotton–Mouton constant, applicable to any molecule, has been derived. Data evaluation yields ξ‖‖−ξ⊥=−17.7(26)×10−30 cm3 for the susceptibility anisotropy, and η̄=−6.5(34)×10−42 cm3/G2 for the hyperpolarizability anisotropy of the ground-state oxygen molecule. Corresponding parameters for the nitrogen molecule are also reported.