bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular and applied modulation effects in electron electron double resonance. IV. Stationary ELDOR of very slowly tumbling spin labels
Authors: Hyde James S. , Smigel Murray D. , Dalton Larry R. , Dalton Lauraine A.
Published: February 2003

The investigation of very slowly tumbling spin labels by stationary electron electron double resonance (ELDOR) is discussed. When a Zeeman modulation frequency of 270 Hz was employed, spectra which were independent of modulation frequency but not modulation amplitude resulted. Under such conditions, the ELDOR technique permits characterization of rotational processes with correlation times from 10−7 to 10−3 sec even though normal electron spin resonance (ESR) spectra are insensitive to motion in these regions, appearing to be superimposable with the ESR powder pattern of a static random collection of molecules. Quantitative analysis of stationary ELDOR spectra is accomplished employing a density matrix treatment that explicitly includes the interaction of the spins with the applied electromagnetic radiation and Zeeman modulation fields. The effect of molecular motion inducing random modulation of the anisotropic hyperfine and electron Zeeman interactions can be calculated employing either an orthogonal eigenfunction expansion method or a transition rate matrix method. The present communication reports investigation of the rotational diffusion of the spin label probes 2,2,6,6−tetramethyl−4−piperidinol−1−oxyl and 17β−hydroxy−4′,4′−dimethylspiro− [5α−androstane−3,2′−oxazolidin]−3′−oxyl in sec−butylbenzene. Experimental spectra are compared with computer simulations of spectra carried out for isotropic Brownian (limit of infinitesimal step size) and jump diffusion (arbitrary step size) models. Best fit is obtained for finite step sizes, i.e., 0.15 rad, and significant deviation of microscopic from macroscopic viscosity is observed at the highest viscosities.
The activated, dissociative chemisorption of methane on tungsten
Authors: Winters Harold F.
Published: February 2003

The dissociative chemisorption of methane on tungsten has been investigated for temperatures between 600 and 2600°K and it is shown that at least two processes are operative. The apparent activation energies are 10.2 kcal/mole (1200°K < T < 2600°K) and 2.7 kcal/mole (600°K < T < 1000°K). The sticking probability varies from 3×10−3 at 2600°K to ∼10−4 at 600°K. The sticking probabilities are independent of pressure for the clean surface but are shown to decrease as the surface concentration of carbon or nitrogen increases. Above ∼1900°K the sticking probability was independent of the number of molecules previously reacted, indicating that carbon was diffusing into the bulk of being desorbed. A large kinetic isotope effect with rather unique features is observed. Tentative explanations of these phenomena are suggested.
Proton and deuteron radiolysis of argon matrix samples of O2 and Cl2. Infrared spectra of charged species
Authors: Andrews Lester , Ault Bruce S. , Grzybowski Joseph M. , Allen Ralph O.
Published: February 2003

Argon matrix samples containing O2 and Cl2 have been bombarded with 2.0 keV protons and deuterons during condensation at 15°K. In the oxygen experiments, bands due to O3 and HO2 appeared. Of most interest is isolated O−3 at 804.3 cm−1. Deuteron radiolysis produced increased yields of the 644 cm−1 band which is confirmed as a DAr+n species. Proton and deuteron radiolysis of chlorine−containing samples produced HCl2x and DCl2x in high yields. Owing to the strong electron scavenging capability of chlorine, these species are probably anions.
Arrival time distribution in high pressure mass spectrometry. VI. Formation and reaction of negative ions in carbon dioxide
Authors: Mitchum R. K. , Freeman J. P. , Meisels G. G.
Published: February 2003

The formation and reactions of O− in CO2 have been investigated using a pulsed chemical ionization mass spectrometer. Inhibition of negative ion formation by SF6 is interpreted as evidence that the attachment of electrons to atomic oxygen is a major source of O−. The termolecular rate constant for the association of O− with CO2 was found to be independent of the nature of the precursor of O− and was at thermal energies 1.5×10−27 cc2 sec−1 independent of the identity of the third body. At increasing drift fields, this rate constant was found to decrease slightly; this is interpreted by a reduction in the lifetime of the intermediate complex with ion energy and by deviations from ion−molecule collision rates from those calculated on the basis of point charge and molecule.
Pulse radiolysis studies of time dependent spectral shifts of the solvated electron in ethanol and deuterated ethanol glasses at 76 K
Authors: Klassen N. V. , Gillis H. A. , Teather G. G. , Kevan Larry
Published: February 2003

The initial (?200 nsec) spectra of the trapped electron are similar in C2H5OH, C2H5OD, C2D5OH, and C2D5OD glasses at 76 K with λmax ∼1300 nm. The kinetics of the spectral shifts to the stable values of λmax near 540 nm are similar for C2H5OH and C2H5OD and resemble the kinetics of a series of first order processes or of tunneling reactions. Structure seems to be present in the partially relaxed spectra and may result from specific geoemetrical orientations of first solvation shell molecules.
Mössbauer effect and x-ray studies of the phase transition in iron hexammine salts
Authors: Asch L. , Shenoy G. K. , Friedt J. M. , Adloff J. P. , Kleinberger R.
Published: February 2003

The Mössbauer effect of 57Fe has been studied between 1.6 and 300 K in iron hexammine salts Fe(NH3)6X2, where X = Cl, Br, I, ClO4, BF4. A phase transition has been inferred from the sudden splitting of the single line (observed at room temperature) into a quadrupole doublet below a critical temperature, Tc. A static distortion of the lattice is confirmed by x−ray determination of a monoclinic structure of Fe(NH3)6Cl2 in the low temperature phase. The strength of the Mössbauer resonant absorption was determined as a function of temperature. The measurements show an abrupt decrease at Tc in passing from the low to the high temperature phase. The small value of the latter is attributed to the scattering of acoustic phonons by the NH3 liberons. The temperature dependences of the quadrupole interactions below Tc cannot be described by conventional theories and suggest temperature dependent crystal fields. These may arise through tilting oscillations of the NH3 groups in the low temperature phase. This behavior in the low symmetry phase, together with the general features of the transition itself (hysteresis, coexistence of the two forms without line broadening) leads to the conclusion that the transition is of second order but turns to first order before reaching the second order critical point.
The Ursell-function structure of the memory function
Authors: Hwang L. P. , Anderson C. F. , Friedman H. L.
Published: February 2003

The integrodifferential equation which defines a memory function f (t) in terms of the time−correlation function g (t) generates ordered Ursell functions having the cluster property. Thus f (t) has an expansion f(2) (t) + f(3) (t) + ... in which f(n) (t) is an n−1 fold time integral whose integrand vanishes when the interval between any two successive times is much greater than a certain correlation time of the system. An interaction representation is used in deriving these results and it is found that f(n) (t) depends, in a certain sense, upon the nth power of the interaction term H1 of the Hamiltonian. The integrand of f(n) (t) is closely related to the ’’cumulant averaged’’ Liouville operators introduced by Kubo and Tomita and developed further by Kubo, Freed, and van Kampen. Thus, in the Markoffian limit f(2) (t) is simply related to the Redfield relaxation matrix. However, the peculiar time−ordering problems of the cumulant expansion theory do not appear here. Except in very simple cases, all of these results depend upon identifying g (t) as a correlation matrix (Kubo) and the f (t) is the corresponding memory matrix. As a first application the memory function for the EPR relaxation of aqueous Ni++ is calculated in terms of the spin parameters. It is assumed that fluctuating zero−field splitting causes the relaxation, but it is not assumed that the EPR relaxation obeys Bloch’s equations.
Molecular polarizability and vibrational Raman tensor elements of the H+2 molecule
Authors: Schulman Jerome M. , Detrano Robert
Published: February 2003

Molecular polarizability and Raman matrix elements for the bound vibrational states of H+2 are calculated. The increase in the magnitude of these matrix elements with vibrational quantum number and the ’’structure’’ of the contributions from the intermediate vibronic continuum functions are explored.
Non-single-particle excitations in finite Fermi systems
Authors: Cederbaum L. S.
Published: February 2003

In order to describe properly non−single−particle excitations in finite Fermi systems in terms of Green’s functions, it is necessary to build upon an approximation of the self−energy part which has the appropriate analytic properties. A distribution function Γ enables us to construct approximations of the self−energy part with suitable properties in a straightforward way. The formalism is applied to the case where the three lowest orders of the expansion of the self−energy part are given as a starting point, leading to a simple secular equation for the poles of this function. Excitations accompanying ionization in C3O2 are calculated as examples of non−single−particle excitations and are compared with experiment.
Luminescence of NaOH, KBr:OH−, and KBr:Tl+ under slow electron impact
Authors: Huang Timothy , Hamill William H.
Published: February 2003

Under low-energy electron impact the principal luminescence band of thin-film NaOH at 420 nm resembles that of KBr:OH− at 410 nm, both at ∼ 300 °K. The lowest energy luminescence excitation peaks occur at 4.5 and 6.5 eV for NaOH and at 4.9 and 6.0 eV for KBr:OH−, all below the first optical excitons at 7.7 and 6.7 eV, respectively. The excitations of the 320 and 360 nm bands of KBr:Tl+ are very similar to those for KBr:OH−. Electron characteristic energy loss spectra for the three systems correlate with luminescence excitation spectra. Host sensitization of 3.0 eV emission of the OH− activator cannot involve the self-trapped exciton of KBr which emits at 2.3 eV. Neither can the emissions of thallium-doped KBr at 3.4 and 3.8 eV be so accounted for. Direct excitation of the activators by electron impact is very improbable and host sensitization is assumed. In doped KBr, unrelaxed excitons at ∼4.9 eV may undergo cubic relaxation and diffuse by hopping to the activator site where they are trapped and emit (360 nm, KBr:Tl+) or transfer (320 nm, KBr:Tl+ and 420 nm, KBr:OH−).
Mean configurational relaxation rates in finite length polypeptides
Authors: Jernigan R. L. , Ferretti J. A.
Published: February 2003

Expressions for the mean relaxation rate <r≳ and the time correlation function <ϑ (0) ϑ (t) ≳ of the helix fraction are derived for polypeptides in the helix–random coil transition region. The resulting equations are based upon a general description of the time rate of change of the configurational probabilities in the form of a set of coupled differential equations. Quantities are expressed in terms of the Zimm–Bragg equilibrium statistics and the neighbor dependent transition rates for an individual peptide unit. Both <r≳ and <ϑ (0) ϑ (t) ≳ exhibit strong chain length dependent behavior throughout the transition region. The quantities 1/<r≳ show maxima at some ϑ?0.5 within the range of the transition. As chain length increases, these curves sharpen and their maxima shift toward ϑ = 0.5, asymptotically. Results for <r≳ are identical to those calculated by an alternative method which follows from a theory developed by Schwarz. Effects of finite chain length can persist to greater than 1000 peptide units.
Chemiluminescence and photoluminescence of diatomic iron oxide
Authors: West J. B. , Broida H. P.
Published: February 2003

Band emission from the Infrared, Orange, and Blue systems of FeO is observed in chemiluminescent flames resulting from the reaction of Fe with O3, N2O, NO2, and discharged O2. Flames are produced at a relatively low temperature (700 K) in flowing inert gas at pressures of 10−3 atm. Bands not previously reported are observed and ascribed to FeO. Reaction of Fe with O2 produces continuumlike emission of unidentified origin; however, laser photoluminescence studies confirm ground state FeO is present. Photon yields for the Orange systems of FeO formed in reactions with O3 and N2O are found to be on the order of 2% and 0.2%, respectively. Photoluminescence of FeO is excited with a krypton ion laser and a cw, tunable dye laser. Ground state vibrational constants of ω″e = 875.8 cm−1 and ω″ex″e = 4.6 cm−1 are determined from the laser excited photoluminescence spectra. Using the tunable dye laser, a rotational constant for the upper state of the B 0,0 band (λ = 579 nm) is found to be B′0 = 0.497 cm−1. Radiative lifetimes of the Orange system upper electronic levels are measured using a pulsed, tunable dye laser excitation source. Lifetimes are found to be on the order of 450±100 nsec.
Collisions between systems of Coulomb particles. I. Small-angle scattering for time-dependent fields
Authors: Gryziński Michal/
Published: February 2003

Collisions between stationary systems of charged particles, i.e., systems in a dynamic equilibrium, are investigated in a so-called small-angle approximation. The interaction of unperturbed systems, described in terms of time-varying electric multipoles (with the dependance on time given by Fourier analysis) is discussed in detail. It is found that the exponential cutoff for the interaction is a characteristic feature of systems with fields periodically varying in time. Relations important for development of the low energy atomic collision theory are derived.
ESR study of the kinetics of the reactions of O(3P) atoms with CS2 and OCS
Authors: Wei Chiu-Nan , Timmons Richard B.
Published: February 2003

Fast flow studies of the reaction rates of O(3P) atoms with CS2 and OCS, using the ESR detection method, were studied. In the reaction with CS2, studied over the temperature range of 218–293 °K, linear least squares treatment of the data obtained leads to an Arrhenius rate constant expression of (1.66±0.23) ×1013 exp(−1280±70/RT) cm3 mole−1⋅sec−1 for the reaction O(3P) + CS2 → CS + SO. The reaction between O(3P) and OCS was studied over a temperature range of 239–404 °K and an Arrhenius expression of (1.2±0.2) ×1013 exp(−4270±70/RT) cm3 mole−1⋅sec−1 was obtained. The Arrhenius parameters obtained in this work are in reasonably good agreement with some of the existing literature results on these reactions. Although these results lead to rather small adjustments in the Arrhenius parameters recommended, the new values are significant with respect to a comparison of the ratio of Arrhenius factors for these two reactions. This point is discussed explicitly at the end of this paper.
Photogeneration of free carriers near 4000 Å at 77 °K in anthracene crystals
Authors: Fort A. , Coret A.
Published: February 2003

A new mechanism of free charge creation in the singlet absorption region at low temperature in anthracene single crystals is suggested. At 77 °K an important photocurrent appears in both a and b polarized light, preceding the usual conduction due to free singlet exciton annihilation. This photoconduction is attributed to the interaction of charged defects with trapped excitons.
Electronic structure of luminescence center of ZnS:Ag phosphors
Authors: Uehara Yasuo
Published: February 2003

The fluorescence, excitation, and reflection spectra of ZnS:Ag phosphors at room temperature and at 77°K were measured in detail. A number of excitation bands characteristic of Ag+ center were found in the spectral region from about 3400 to 5000 Å. The ligand-field analysis of the excitation spectra was performed, based on the localized center model of luminiscence. Good agreement between the calculated and experimental energies was obtained. Thus the excitation and absorption spectra of ZnS:Ag phosphors are unambiguously correlated to the electron transitions between the ground state A1(1S0) with 4d10 configuration and the excited states T2(1D2, 3D2 and 3D3) in zincblende, and A1 and E (1D2, 3D1, 3D2 and 3D3) in wurtzite, having 4d95s configuration. The well-resolved blue, bluish green, and green emission bands characteristic of Ag+ center were observed and ascribed to the reverse process of the absorption transitions. Furthermore, the excitation lines and bands in the uv region below about 3400 Å were analyzed in terms of excitons or phonon-coupled excitons.
Shell model investigation of the Raman scattering in the ordered and disordered phases of NH4Br
Authors: Geisel T. , Keller J.
Published: February 2003

We describe a formalism for the Raman scattering from orientationally disordered solids, in particular for the ammonium halides. The electronic polarizability is treated in a shell model. Applying the theory to the ordered phase of NH4Br we analyze the extreme scattering anisotropy of the 56 cm−1 band observed by Wang and Wright. This allows us to determine the temperature dependence of the order parameter and to deduce a potential for the orientation dependent interaction of the Br shell with neighboring H atoms. With the use of this potential predictions can be made for all the polarizability coefficients which enter numerical calculations of the Raman scattering intensity in the disordered phase. These calculations are in good agreement with experiment and show that one does not have to assume short range order among the NH4+ ions to explain the appearance of sharp bands in the Raman spectra of the disordered phase.
A neutron diffraction study of the diaquohydronium ion in yttrium oxalate trihydrate
Authors: Brunton G. D. , Johnson C. K.
Published: February 2003

Five structures of yttrium oxalate trihydrate and its deuterated isomorphs have been determined from least-squares refinement of neutron diffraction data. Three structures with different H–D isotopic ratios were determined from 25 °C data and two structures with the same isotopic ratios were determined from data collected at 60 and −155 °C. The cell dimensions for the structures are as follows: YHOX 25 °C, a0 = 8.6962(6) Å, c0 = 12.824(3) Å YDOX 25 °C, a0 = 8.6989(7) Å, c0 = 12.818(3) Å YHDOX 25 °C, a0 = 8.704(2) Å, c0 = 12.832(2) Å YHDOX 60 °C, a0 = 8.708(2) Å, c0 = 6.435(3) Å YHDOX −155 °C, a0 = 8.677(4) Å, c0 = 12.751(7) Å. In each of the structures a proton or deuteron is on a twofold symmetry axis and nearly centered between two H2O molecules forming an H5O2 ion. The O–H–O bond is symmetrical but not linear; the z parameters for H and O are slightly different. The O–H–O distance in the diaquo ion is 2.442(2) Å, and the O–D–O distance is 2.454(2) Å. The four other protons in the diaquo ion (paired by symmetry) form nonlinear hydrogen bonds with O atoms in the oxalate molecules [2.643(2) and 2.680(2) Å]. The respective O–H–O angles are 178.2(3)  ° and 167.9(3)  °. There is a nonequilibrium temperature dependence of the partitioning of hydrogen and deuterium among the available proton positions. The centered proton in the H5O2 is the preferred site for hydrogen and the H/D ratio increases with decreasing temperature. Between 155 and 25 °C the structure of YH(C2O4)2⋅3H2O becomes progressively disordered with respect to the H5O2+ ion. At approximately 53 °C, there is a dimorphic transition in the structure and both the independent water molecule and the diaquo ion become disordered and the space group changes from P42/n to P4/n; c0 is halved. The progressive disorder is superimposed upon the P4/n disorder.
Proton resonance and metamagnetism in Mn(COOCH3)2⋅4H2O
Authors: Spence R. D.
Published: February 2003

Using proton resonance we have verified the existence of a metamagnetic transition in manganous acetate tetrahydrate which had been previously proposed to explain low field magnetization data. The transition was found to occur when a field of 6 Oe was applied along the a axis. The observation of the transition was facilitated by the existence of one narrow line in the proton resonance spectrum which appears to originate from one of the acetate groups. Fields greater than 6 Oe along the a axis produce a mixed antiferromagnetic and saturated paramagnetic state which gradually turns into a pure saturated paramagnetic state as the field is increased. Two-dimensional magnetic ordering appears to exist above the Néel temperature.
Quasiclassical trajectory study of the molecular beam kinetics of the deuterium atom–hydrogen halide exchange reactions
Authors: Raff L. M. , Suzukawa Henry H. , Thompson Donald L.
Published: February 2003

Unadjusted quasiclassical trajectory computations have been carried out to simulate the molecular beam scattering of thermal D atom beams at 2800 °K crossed with beams of HCl and HI at 250 °K. Total reaction cross sections, energy partitioning distributions, and differential scattering cross sections have been computed for the exchange reactions D+HCl → DCl+H and D+HI → DI+H while total reaction cross sections are reported for the corresponding abstractions, i.e., D+HCl → HD+Cl and D+HI → HD+I. For the exchange reactions, the computed reaction cross sections are within the range estimated from the crossed beam experiments. The calculated average energy partitioned into relative translational motion of products is in near quantitative agreement with the beam results, and the predicted differential scattering cross sections appear to be in qualitative accord with the beam experiments. The over-all agreement between theory and experiment indicates that previously computed values for the thermal rate coefficients for the exchange reactions are of the right order and that a systematic error exists in the interpretation of photolysis data in the hydrogen–hydrogen halide systems.
Breakdown of selection rules in the Raman spectra of KDP
Authors: Srivastava M. K. , Wang C. H.
Published: February 2003

Raman spectra of A1 and B1 modes of KH2PO4 single crystal are studied experimentally by varying the orientation of the crystal axes relative to the incident and scattered light directions. It has been found that the scattering intensities depend on the k vector of the scattering phonons, thus invalidating the selection rules in the first order Raman scattering.
Angular distributions, kinetic energy distributions, and excitation functions of fast metastable oxygen fragments following electron impact on CO2
Authors: Misakian M. , Mumma M. J. , Faris J. F.
Published: February 2003

Dissociative excitation of CO2 by electron impact has been studied using the methods of translational spectroscopy and an angular distribution analysis. Earlier time-of-flight studies revealed two overlapping spectra, the slower of which has been attributed to metastable CO(a 3π) fragments. The fast peak is the focus of the present study. Threshold energy, angular distribution, and improved time-of-flight measurements indicate that the fast peak actually consists of five overlapping features. The slowest of the five features (1) is found to consist of the metastable O(5S) produced by predissociation of a Σu+ state of CO2 into O(5S) + CO(a 3π). Oxygen Rydberg fragments originating directly from a different Σu+ state are believed to make up the next fastest feature (2). Mechanisms for producing the three remaining features are discussed.
Interpretation of c
i
s-hexatriene absorption spectrum. The Duschinsky effect
Authors: Petelenz Piotr , Petelenz Barbara
Published: February 2003

The differences between the uv absorption spectra of trans- and cis-hexatriene are interpreted in terms of the coupling between the ground state normal modes in the electronic excited state, i.e., the Duschinsky effect. The model Hamiltonian is derived and its eigenvalues and eigenvectors are found by means of the standard perturbation theory. The spectra of both molecules are simulated at the computer. A good agreement with experiment is found. Some approximations involved are discussed and the possibilities of further experimental verification are pointed out.
Frozen core approximation, a pseudopotential method tested on six states of NaH
Authors: Sachs Edward S. , Hinze Juergen , Sabelli Nora H.
Published: February 2003

A pseudopotential formulation, more appropriately called the frozen core approximation (FCA), is presented in detail. This FCA is tested by performing MCSCF calculations on the six low lying states, X1Σ+, A1Σ+, a3Σ+, c3Σ+, B1Π, and b3Π of NaH. The results obtained are compared with those of an analogous previous MCSCF calculation on these states without the use of FCA, i.e., with all orbitals optimized. It is found that energies are obtained rather accurately with FCA; however, calculated molecular properties are affected more strongly and the shapes of the potential curves appear to be distorted somewhat by FCA. It is argued that no pseudo- or model-potential calculation can give errors less than FCA unless the potential is made valence shell dependent. This, however, would be analogous to a full calculation, and the savings due to a pseudopotential approximation would be lost.
Graph theory and molecular orbitals. XII. Acyclic polyenes
Authors: Gutman I. , Ru??ić B. , Trinajstić N. , Wilcox C. F.
Published: February 2003

A graph-theoretical study of acyclic polyenes is carried out with an emphasis on the influence of branching on several molecular properties. A definition of branching is given and several branching indices are analyzed. The case of polyenes without a Kekulé structure is discussed briefly. The main conclusions are: (a) thermodynamic stability of conjugated polyenes decreases with branching, but (b) reactivity, in general, increases with branching.
Molecular and applied modulation effects in electron electron double resonance. V. Passage effects in high resolution frequency and field swept ELDOR
Authors: Percival Paul W. , Hyde James S. , Dalton Lauraine A. , Dalton Larry R.
Published: February 2003

The observation of electron electron double resonance (ELDOR) linewidths for nitroxide radicals which are significantly less than the hyperfine envelope widths is reported. For a 1.5×10−3M solution of 2,2,6,6-tetramethyl-4-piperidinol-1-oxyl (TANOL) in sec-butylbenzene (SBB) between the temperatures of −40 °C and −70 °C, the ELDOR linewidths correspond to single spin packet widths determined by the effective electron spin–spin relaxation rates. An analysis of the precise dependence of frequency swept absorption and dispersion ELDOR signals upon microwave radiation field intensities and upon the details of the applied Zeeman modulation and accompanying phase-sensitive detection is also presented. Quantitative prediction of molecular and applied modulation effects is accomplished employing a modified density matrix treatment. Best fit parameters for TANOL in SBB at −63 °C include we(0) = 1.75×105 Hz, Te1e(0) = 1.3×10−6 sec, Te2e(0) = 2.7×10−7 sec, wn(14N) = 2.6×105 Hz, wn(all 1H) = 0 Hz, τ2 = 2×10−10 sec, ? = 2.00587, g∥ = gzz = 2.00210, g⊥ = (1/2)(gxx + gyy) = 2.00775, ? (14N) = 43.235 MHz, A∥(14N) = Azz = 92.65 MHz, A⊥(14N) = (1/2)(Axx + Ayy) = 18.53 MHz, ? (1H,axial CH3) = 1.28 MHz, ? (1H,axial CH2) = 1.57 MHz, ? (1H,equatorial CH2) = 0.84 MHz, ? (1H,all other) = 0. The theoretical and experimental results afford crucial insight into the design and performance of ELDOR experiments for optimum sensitivity in the determination of spectroscopic parameters and molecular relaxation rates.
Comparison between model potential and Hartree–Fock results for diamond
Authors: Euwema R. N. , Greene R. L.
Published: February 2003

Two Hartree–Fock calculations were performed on diamond. One included the 1s core, while the other used the model potential proposed by V. Bonifacic and S. Huzinaga [J. Chem. Phys. 60, 2779 (1974)] in which the 1s core is excluded. The binding energies agree to 0.015 Ry/atom, the x-ray structure factors and Compton profiles to well within 0.5 percent, and the rms deviation for the valence and low conduction eigenvalues is 0.012 Ry. Some of this disagreement may be due to slightly different Gaussian contractions, integral approximations, etc.
Theoretical studies on the isolated spin cluster complex, tetrameric cobalt (II) acetylacetonate Co4(C5H7O2)8: Effects of competing superexchange interactions
Authors: Bonner Jill C. , Kobayashi Hanako , Tsujikawa Ikuji , Nakamura Yukio , Friedberg Simeon A.
Published: February 2003

According to x-ray studies, Co4(C5H7O2)8 is an assembly of linear, tetrameric Co2+ ion clusters, which are effectively magnetically isolated. An examination of superexchange paths suggests that this tetramer has two different kinds of nearest-neighbor exchange constants plus a next-nearest-neighbor exchange constant. The only bulk experimental data available so far are the powder susceptibilities from room temperature down to about 0.35 K. This paper demonstrates that this information is apparently sufficient to infer that all three exchange constants are antiferromagnetic in type and have values in the region of Jnn1 ∼ 5 K, Jnn2 ∼ 10 K, and Jnnn3 ∼ 4 K. The interactions therefore compete; and they compete so as to give a paramagnetic character to the low temperature behavior. This is in interesting contrast to the behavior of a sister compound, trimeric nickel (II) acetylacetonate, where the paramagnetic character results from a dominant nearest-neighbor ferromagnetic exchange constant. The exchange constants for the cobalt tetramer are unusual in the light of current ideas concerning superexchange, and additional, confirmatory experiments would be desirable. Accordingly, specific, exact, theoretical predictions are made for the specific heat and single crystal magnetization for comparison with future experiments.
Photofragment spectrum of ICN
Authors: Ling Joyce H. , Wilson Kent R.
Published: February 2003

We have used photofragment spectroscopy to study the near ultraviolet photodissociation of ICN at 37 550 cm−1 (266.2 nm) by crossing an ICN molecular beam with a pulsed fourth harmonic neodymium laser. A mass spectrometer is used to measure the densities of both the recoiling CN and I fragments as a function of recoil direction with respect to the polarization direction of the laser light and as a function of flight time. From these measurements, the fragment center-of-mass distributions with respect to angle, total translational energy, and total internal energy are calculated. Two peaks are observed in the internal energy distribution, both with angular distributions peaking parallel to the electric vector of the light, indicating that the dissociation process or processes must be predominantly derived from parallel transitions. This conclusion appears to rule out both of the previous spectral assignments for this region. The angular distributions show that the ICN excited state lifetimes are short compared with rotation. The internal energy distribution is such that the fragments are produced with population inversion. The lower internal energy peak, corresponding to 40% of the fragments, has insufficient energy to electronically excite either fragment, and can be matched by the assumed production of a mixture of v = 2, Trot = 1100°K and v = 3, Trot = 400°K CN fragments. This match is not unique, and while appreciable v = 4 and higher contributions may be ruled out, contributions from v = 0 and 1 with correspondingly higher rotational energies are possible. The higher internal energy peak, corresponding to 60% of the dissociation, can be matched by the assumed production of ground state I and ? 2Πi electronically excited CN fragments with no vibrational excitation and little or no rotational excitation. Other possibilities for this peak which cannot be ruled out by these experiments are the formation of both CN and I fragments in their ground electronic states with a very narrow combination of CN vibrational (maximum v = 4) and rotational excitation, or alternatively the production of I* 2P1/2 with no vibrational but appreciable rotational excitation of the CN. The latter possibility would, however, contradict experimental conclusions by Donovan et al. Possible models for the two peaks seen in the dissociation process include (i) the excitation of two separate ICN upper states, and (ii) the interaction between two upper states, which may be viewed as the initial exclusive production of excited ? 2Πi CN fragments followed by electronic quenching of a portion of them by a ’’half-collision’’ with the recoiling I atom, leaving the quenched CN fragments vibrationally and rotationally excited.
Relative efficiencies of O(1S) production from photodissociation of Co2 between 1080 and 1160 Å
Authors: Koyano I. , Wauchop T. S. , Welge K. H.
Published: February 2003

Relative O(1S) emission intensities following the phtotdissociation of CO2 have been measured between 1080 and 1160 Å as a function of incident photon energy at 2–3 Å wavelength intervals with 3.2 Å bandpass. Most of the structure in the absorption spectrum of CO2 was clearly folowed by the O(1S) intensities in the range 1098–1160 Å, whereas the structure was not followed below 1098 Å. Relative quantum efficiencies for the photodissociative production of O(1S) from CO2 were highest in the range 1095–1135 Å and dropped on both sides of this range.
Photochemical studies of platinum(II) -doped AgBr by EPR spectroscopy
Authors: Eachus R. S. , Graves R. E.
Published: February 2003

Platinum(II) ions, incorporated into AgBr by melt–growth techniques, occupy substitutional positions in this material and directly trap photoproduced electrons. The reduced platinum center, (PtBr6)5−, which is thermally stable at or below 200 °K, has been studied by electron paramagnetic resonance spectroscopy. This center is tetragonally contracted, with the odd electron distributed between the metal ‖5dz2≳ and ‖5s≳ levels and bromide ligand ?-antibonding orbitals. The photoproduction of the (PtBr6)5− center can be achieved with white-light exposures, but it is most favorable at 510 and 650 nm.
Structural effects on the ultrasonic properties in the normal–supercooled liquid transition region of diphenyl ether and o-terphenyl
Authors: D’Arrigo Giovanni
Published: February 2003

Ultrasonic absorption and velocity were measured in diphenyl ether and o-terphenyl over a temperature range extending from the liquid to the supercooled region. Experimental results indicate a different degree of association in the two liquids with different volume viscosity mechanisms. While a coexistence of structural and thermal relaxation processes appears to operate in the polar diphenyl, only structural effects can be used to interpret the observed behavior in the terphenyl. Calculated ratios of structural to shear viscosity (?2 for diphenyl and ≈2.7 for terphenyl) agree with previous determinations in analogous systems. In both liquids, at a few degrees above the freezing point, the sound velocity shows a positive departure from the linear behavior observed at higher temperature. This effect is attributed to the onset of some structural change occurring in the molecular aggregates present in this region.
Approximate solutions in multiple reflection time domain spectroscopy
Authors: Claasen T. A. C. M. , van Gemert M. J. C.
Published: February 2003

Simple and accurate relations are derived to approximate the transcendental equation of the reflection coefficient in multiple reflection time domain spectroscopy. From these relations the dielectric permittivity can be found analytically; the results are shown to be significantly more accurate than approximations derived previously for all possible permittivity situations.
Adiabiatic a
b i
n
i
t
i
o potential curves for the B′ 1Σ+u state of H2
Authors: Ford A. L. , Browne J. C. , Shipsey Edward J. , DeVries Paul
Published: February 2003

There is reported here an accurate ab initio calculation of the fixed nuclei potential curve for the B′ 1Σ+u state of H2 together with approximate values of the adiabatic corrections to the potential curve for this state. The state is observed to have two minima in its potential with the very small maxima occurring at approximately, R=5.5a0. Dipole transition moments and oscillator strengths for X 1Σ+g ↔ B 1Σ+u transitions are given for a wide range of internuclear separations.
Multiple scattering theory. I. Off-shell t matrix elements and momentum space wavefunctions for local two body potentials
Authors: Brumer P. , Shapiro M.
Published: February 2003

An efficient method for computing off-shell t matrix elements and bound state momentum space wavefunctions for any local two body potential is presented. Applications are made to several sample atom–atom interactions.
Optical study of LaAlO3:Eu at temperatures approaching the rhombohedric → cubic transition
Authors: Faucher M. , Caro P.
Published: February 2003

The LaAlO3:Eu fluorescence spectrum was investigated at 77, 300, and 500 °K, that is, approaching the rhombohedric→cubic transition. A progressive modification of spectra is noted, owing to a shift in point symmetry at the europium site (D3 → Oh). Assignment of irreducible representations of the levels is aided by the observation of degeneracies. A calculation of the crystal-field parameters Bkq was tried, based on the model of point charges and coordinated with the structural data for LaAlO3 (R 3̄ c). The calculated values are very different from the experimentally measured parameters; however, these latter can be approached fairly well if the calculated parameters B2q, B4q, and B6q are multiplied by factors of 0.26, 1.93, and 7.5, respectively. Diagonalizing the interaction matrix of the 4f6 configuration thus accounts for the experimental splitting of the 7F levels at 77, 300, and 500 °K. A priori calculation of the Bkq parameters thus permits an approximation of the x parameter which characterizes the deformation of perovskite with respect to the ideal cubic form.
Collision-induced emission from S(1S) by He, Ar, N2, H2, Kr, and Xe
Authors: Black G. , Sharpless R. L. , Slanger T. G.
Published: February 2003

Collision-induced emission from S(1S) has been studied for collisions with He, Ar, N2, H2, Kr, and Xe. The S(1S) was made by OCS photodissociation at 1610 Å. The emitted intensity increased linearly with the added gas pressure. This behavior can be described by rate coefficients for induced emission on the 1S→1D transition of (5.6±0.9) ×10−20 cm3 molecule−1⋅sec−1 for helium, (4.2±0.3) ×10−18 for argon, (3.3±0.2) ×10−18 for nitrogen, (1.73±0.15) ×10−18 for hydrogen, (1.5±0.1) ×10−17 for krypton, and (1.1±0.05) ×10−16 for xenon. Xenon was also found to enhance the 1S→3P emission intensity with a rate coefficient of (5.5±1.0) ×10−19 cm3 molecule−1⋅sec−1. Induced emission is a major (and possibly exclusive) path for deactivation of S(1S) by all these gases except hydrogen.
Pseudopotential transformation of correlated-pair equations
Authors: Szasz Levente , Brown Lawrence
Published: February 2003

A pseudopotential transformation for correlated-pair equations is derived that yields solutions that are pseudowavefunctions, i.e., they do not have to be orthogonal to the core functions. The approximate solutions for the transformation will be much simpler to compute, but they do not involve a loss of accuracy. (AIP)
Infrared absorption and reflection studies of organic radical salt: K+TCNQ−
Authors: Iqbal Zafar , Christoe C. W. , Dawson D. K.
Published: February 2003

The bulk reflectivity and absorption (using alkali halide matrices and thin film dispersions) of polycrystalline K+TCNQ− in the wavelength range between 30 and 4000 cm−1 have been examined in detail. The sharp reflectivity peaks have been analyzed using a classical oscillator fitting procedure to give the peak and oscillator parameters. Most of the observed infrared peaks have been assigned to the fundamental modes of TCNQ− and lattice vibrations. The temperature dependence of the CN stretching peaks in particular have been studied in the 300–433 °K temperature range. The reversible and gradual disappearance of one set of CN stretching peaks in K+TCNQ− has been tentatively linked to a possible cell halving phase transition at Tc∼379 °K. The results are compared with the spectrum of Rb+TCNQ− which is isomorphous with the postulated high temperature phase of K+TCNQ−.
Long-lived K
a=0, 2B1 states of NO2: A direct measurement using a tunable dye laser
Authors: Haas Y. , Houston P. L. , Clark J. H. , Moore C. B. , Rosen H. , Robrish P.
Published: February 2003

Narrow-band (1 GHz) excitation coupled with selection of specific emission bands has been used to measure the lifetimes of some Ka=0, 2B1 states of NO2. In contrast with previous experiments and expectations of short lifetimes based on analysis of possible perturbations in the electronic manifold, these states exhibit long collisionless lifetimes of 33±4 μsec. Their quenching cross sections are found to be larger than gas kinetic, and much larger than that for the unresolved fluorescence.
Contraction effects in ideal networks of flexible chains
Authors: Ronca G. , Allegra G.
Published: February 2003

The mean-square distances between various points of regular Gaussian networks are calculated using a new electrical analog and solving exactly a set of difference equations both in two and in three variables through an appropriate Fourier lattice transformation. The results show that the mean-square radius of gyration of the network is of the same order of magnitude as that of a single chain for networks having a three-dimensional connectivity, while it increases logarithmically with the total number of chains for networks having a two-dimensional connectivity.
NMR study of sodium ion motion in single crystal beta-alumina
Authors: Chung Intaik , Story Harold S. , Roth Walter L.
Published: February 2003

The motion of sodium ions in single crystals of beta-alumina has been studied via its effects on NMR linewidth and electric quadrupole interaction. A single line is seen between −160 and +220 °C at Larmor frequencies from 4 to 19 mHz, indicating rapid ionic motion. The average coupling constant remains almost constant over the temperaure range. The average asymmetry parameter is nonzero and decreases with temperature, reflecting the change in off-axis site population. An activation energy of about 0.17 eV has been estimated from the linewidth changes in general agreement with that found from diffusion and conductivity measurements.
Cluster-type calculations of electronic structures of crystals by the method of linear combinations of atomic orbitals
Authors: Menzel W. Paul , Mednick Kenneth , Lin Chun C. , Dorman C. Franklin
Published: February 2003

First-principle cluster-type calculations of the electronic energy structures for the silicon and lithium fluoride crystals by the method of linear combinations of atomic orbitals (LCAO) are presented. The Hamiltonian is that of the infinite crystal (with a Slater-type approximation for electron exchange) and the basis functions are linear combinations of localized functions centered at the atomic sites within a cluster. By means of the Gaussian technique all the multicenter integrals associated with the Hamiltonian and overlap matrix elements are readily evaluated. For the localized functions in the basis set, we use (i) atomiclike orbitals, expanded in terms of the Gaussian-type orbitals, corresponding to the core and valence states of the free atoms and (ii) single Gaussians with various exponents. With exclusively atomiclike basis orbitals we show that one can reproduce the valence bands of the infinite crystals reasonably well using a cluster of eight or ten shells. However, to obtain representative conduction bands, particurly energy band gaps, we find it necessary to supplement the atomiclike orbitals by a few single Gaussians. In these cluster calculations, no energy levels are found in the crystal band gap region because the Hamiltonian is that of an infinite crystal with no physical surfaces present.
The ultraviolet vapor absorption spectrum of 2-furaldehyde: Electronic assignments and vibrational analysis
Authors: Zwarich R. , Rabinowitz I.
Published: February 2003

The vapor absorption spectrum of 2-furaldehyde has been photographed in the 20 000–50 000 cm−1 region. The spectrum is unusual in that it displays two separate electronic transitions with sharp structure, the origin band in both cases having a 5 cm−1 bandwidth. The 26 000–34 000 cm−1 absorption region is assigned as an Ã 1A″(nπ*) ←X̃ 1A′ transition and the 36 000–41 000 cm−1 region as a B̃ 1A′ (ππ*) ←X̃ 1A′ transition with its transition moment direction tentatively thought to be nearly parallel to the ring–CHO bond. The assignments are in accord with a CNDO configurational interaction description by Del Bene and Jaffe [J. Chem. Phys. 48, 4050 (1956)]. A detailed vibrational analysis of the Ã–X̃ system based upon identification of the polarization of the bands by their rotational contours and a recent assignment of the fundamental vibrations in the ground state leads to the following conclusions. The intensity in the electronically allowed part of the absorption spectrum is dominant. There is considerable vibronic intensification in the Ã←X̃ transition of 2-furaldehyde with the activity of the out-of-plane ring modes being comparable to the out-of-plane aldehyde modes. The Herzberg–Teller activity of the ring modes indicates that the n-electron density is delocalized into the ring. A comparison of the rotational contours of the vibronic bands in the Ã–X̃ system with those of the B̃–X̃ system suggests that the B̃–X̃ system is not the source of intensity for the Ã–X̃ system. Nearly all of the absorption bands in the B̃–X̃ system have rotational contours similar to the origin, indicating that intensity borrowing is not important. Most of the spectrum can be analyzed in terms of the combinations and progressions formed by two excited state modes: ν19(180 cm−1), the in-plane wagging motion of the aldehyde against the furan ring, and ν9(1271 cm−1), the ring stretching mode.
Excitation functions for the reactions of Ar+ with CH4, CD4, and CH2D2
Authors: Wyatt J. R. , Strattan L. W. , Chivalak S. , Hierl P. M.
Published: February 2003

Integral reaction cross sections as a function of initial translational energy (0.4–30 eV c.m.) are reported for isotopic variants of the exoergic ion-molecule reaction Ar++CH4 → ArH++CH3. The excitation functions, which maximize at about 5 eV and decrease at lower collision energies, appear to possess translational energy thresholds at about 0.1 eV. At the higher energies there is a large isotope effect favoring abstraction of H over D. The observed threshold behavior, unusual for exoergic reactions of positive ions, is discussed in terms of the formation of an ArCH4+ intermediate complex at low collision energies.