bluuebunny/crossref_metadata_2025 huggingface dataset:
Nuclear Magnetic Relaxation in Hydrogen—Rare Gas Mixtures
Authors: Foster Kenneth R. , Rugheimer John H.
Published: February 2004

The proton spin lattice relaxation time (T1) has been measured in binary mixtures of hydrogen and rare gases over the temperature range of 165–265°K and at densities of roughly 20 amagats and rare gas fractions of 0%-60%. From this data the contribution to the proton T1 from H2—rare gas collisions was found by extrapolating the measurements to zero concentration of hydrogen. The data have been fit by coupled channel molecular scattering calculations, assuming infrequent transitions of the hydrogen molecule between rotational levels, and using a Lennard-Jones (12, 6) potential for the isotropic and angular-dependent parts of the intermolecular potential. The calculated thermally averaged reorientation cross sections can be represented in a way (suggested by the Bloom—Oppenheim theory) which makes clear the sensitivity of the various parameters of the potential to the data. The calculations indicate that the apparent anisotropy in the attractive part of the H2—rare gas interaction is approximately twice as great as that predicted by molecular polarizability measurements; this is probably a consequence of choosing an oversimplified model for the potential. The Bloom—Oppenheim theory predicts much larger reorientation cross sections than does quantum scattering theory and yields values for the molecular anisotropy which are in approximate agreement with the polarizability data.
Infrared Spectra and Intermolecular Potentials of Matrix-Isolated Nitromethane and Nitromethane-d3
Authors: Verderame F. D. , Lannon J. A. , Harris L. E. , Thomas W. G. , Lucia E. A.
Published: February 2004

The infrared spectra of protonated and deuterated nitromethane isolated in a nitrogen matrix have been recorded from 4000 to 250 cm−1. Peaks have been assigned to monomer and dimer species by varying the concentration of nitromethane in the matrix. The frequencies of the monomer have been used in a normal coordinate analysis and the force constants evaluated agree well with those of the gas phase. The Cartesian displacements of the atoms for each normal mode are reported. These displacements showed that for the ``NO2 antisymmetric vibration'' there was considerably more motion of the hydrogen atoms. Lennard-Jones-type potentials were used to calculate dimer configurations with a minimum intermolecular potential energy, taking into account both dispersion and electrostatic forces. The charges on the atoms needed for the calculation of electrostatic forces were calculated from molecular orbital theory using both the MWH method and the INDO method. The position of minimum potential energy is a function of distance between a pair of nitromethane molecules and the angular rotation of two molecules with respect to each other. With the configuration of minimum potential energy and the Cartesian displacements in hand, the shifts and splittings due to intermolecular coupling could be evaluated by applying two-site, molecular exciton theory. The calculated shifts and splittings agree well with those observed in the spectra giving credibility to our calculations. The results will make it possible to estimate the moments of inertia of the dimer and force constants for the pair necessary for assessing partition functions for dimer formation under all conditions of temperature and pressure.
Raman Scattering from Nematic Liquid-Crystalline Azoxybenzenes
Authors: Amer Nabil M. , Shen Y. R.
Published: February 2004

We have investigated the Raman spectra of four nematic members of the homologous series 4,4′-bis (alkoxy) azoxybenzene in the solid, nematic, and liquid phases for a spectral range of 10–1900 cm−1. These spectra show great similarity, as one would expect from the similarity of their molecular structure. A few Raman modes, particularly the intermolecular ones, exhibit quasidiscontinuous changes at the phase transitions. These changes are qualitatively interpreted as the result of modification of the intermolecular interaction induced by the phase transitions.
EPR Study of O3 − on Magnesium Oxide
Authors: Wong Ning-Bew , Lunsford Jack H.
Published: February 2004

Molecular oxygen reacts with O− on the surface of magnesium oxide to form the ozonide ion, O3−. The EPR spectrum of this ion was used to verify its formation. Oxygen-17 was introduced into the molecule either via the O− ion or the O2 molecule. The spectrum is characterized by g1=2.0172, g2=2.0100, and g3=2.0014, with | a3 a |=26 G, | a3 b |=82 G, and | a3 c |=65 G. Here, a3 a is the hyperfine splitting for the oxygen atom which was originally O−. The splitting along the other principal axes was less than 5 G. A model is suggested in which a terminal oxygen of the bent molecule is in the plane of the surface, and the plane of the molecule is perpendicular to the surface. The ozonide ion slowly dissociated at 25° to form O2−. The oxygen-17 isotope distribution indicates that the superoxide ion was largely derived from molecular oxygen, rather than the O− ion.
Broken Path Model of Reactive Collisions: Application to Collinear (H, H2)
Authors: Middleton Paulette B. , Wyatt Robert E.
Published: February 2004

Within the context of the collinear H + H2 reaction, an approximate technique for reactive scattering on realistic potential surfaces is presented. Essentially, the broken path model provides a novel framework for the derivation of propagation matrices. The model is constructed by segmenting the potential surface into rectangular or pie-shaped regions. In each region, the potential is approximated by two simple analytical functions associated with translational and vibrational motions. Local Cartesian coordinates are defined in each region. Since adjacent local coordinate axes are offset and at angles to each other, the asymptotic regions are connected by a ``broken path.'' The potential approximations and coordinate system allow us to solve a set of uncoupled Schrödinger equations for translation of the wavefunction across a box. Coupling occurs at the boundaries where matching of the wavefunctions and normal gradients is required. Propagation matrices are defined for each box; they include both translation and connection into the next region. A set of propagation matrices transforms the wavefunction from the initial to final asymptotic regions. Calculations are presented to show the effects of small variations in the model, the influence of closed channels, and nonadiabaticity. We find that sensitivity to variation in region size and potential approximations increases as the total energy increases. However, deviations are reduced if we employ all channels required for convergence. The importance of closed channels and nonadiabatic effects are seen by comparing results from adiabatic (one channel studies) and nonadiabatic calculations. In addition, if reaction path curvature, which contributes to nonadiabaticity, is neglected, the accuracy of the results is reduced. For all our calculations we use the exact reaction probability as a test of our accuracy.
Dissociation of HBr and Br2 in Shock Waves
Authors: Westberg Karl , Greene E. F.
Published: February 2004

The rate of dissociation of HBr has been measured at temperatures in the range 1500–2700°K in shock waves by recording the light emitted when two Br atoms combine to form Br2(1IIu). The experiments show that the activation energy for HBr dissociation cannot be substantially different from the HBr bond energy. In particular, the experimental data are consistent with the assumption that the rate coefficient of the reaction HBr+M→ H+Br+M, M=Ar or HBr, is 1.2× 1019 T−2 exp(−DHBr/RT)M−1· sec−1, where DHBr is the bond dissociation energy of HBr (88 kcal/mole). The rate coefficients for the dissociation of Br2 by Ar, He, and Br were determined by shock heating mixtures of Br2 and Ar or He. The results are in agreement with those reported by other workers. The temperature and concentration dependence of the light emitted behind bromine—argon shocks have been examined, and the results are consistent with the premise that over a 1500–3000°K temperature range, the emission at λ595 nm is due to the reaction 2Br(2P3/2)⇌ Br2(1Πu)→ Br2(1Σg+)+hν.
Quantum Yield of Solvated Electrons from Ferrocyanide in Various Water—Alcohol Mixtures
Authors: Shirom Miriam , Tomkiewicz Micha
Published: February 2004

The ultraviolet photoejection of electrons from ferrocyanide, in various water—alcohol mixtures, was studied by using N2O as a scavenger. The quantum yields of solvated electrons were obtained. The results can not be interpreted in terms of a single parameter of the solvent such as dielectric constant, relaxation time, or diffusion coefficient. Mozumder's theory was chosen as a possible way of interpretation. The agreement between the calculated and experimental results is good, provided enough care is taken in choosing the relaxation times. The thermalization length of the ejected electrons was found to be 30 Å, independent of the particular water—alcohol mixture.
Structural Influences on Adsorption Energy. II. CO on Ni(100)
Authors: Tracy J. C.
Published: February 2004

The adsorption of CO on Ni(100) has been studied using LEED, Auger, and work function measurements and was found to be completely reversible. The isosteric heat of adsorption was approximately 1.30 eV/molecule independent of coverage up to 0.61 monolayers. Three surface structures were observed: (1) a disordered phase at high temperature and/or low coverage, (2) a c(2× 2) phase at coverages up to and including 0.5 monolayers at sufficiently low temperature, and (3) a compressible hexagonal phase between 0.61 and 0.69 monolayers. Evidence is presented which supports an attractive CO–CO interaction at coverages of less than 0.5 monolayers. A number of two-dimensional phase transitions were observed and suggest an interesting T—θ phase diagram with a eutecticlike triple point. The importance of scrupulous cleaning procedures in allowing all possible adsorbed phases to be observed has again been demonstrated.
Structural Influences on Adsorption Energy. III. CO on Cu(100)
Authors: Tracy J. C.
Published: February 2004

CO adsorption on Cu(100) has been found to be completely reversible over the temperature range 77–300°K and pco=10−8−10−4 torr using LEED and work function measurements. The isosteric heat has been determined as a function of coverage, and strong correlations have been found with surface structural changes. An in-registry→out-of-registry transition was observed at 0.5 monolayer suggesting once again that localization on high coordination substrate sites may not be so important as commonly believed.
Photoelectron Spectroscopy of High-Temperature Vapors. I. TlCl, TlBr, and TlI
Authors: Berkowitz J.
Published: February 2004

A cylindrical-mirror electron energy analyzer has been constructed for use with molecular beams of unstable or nonpermanent gases. The apparatus has been used to study the He(I) photoelectron spectra of TlCl, TlBr, and TlI. For TlCl and TlBr, three peaks are observed. A surprising result of the analysis is that the broad peak, presumably due to removal of an electron from a bonding orbital, corresponds to the ``lone-pair'' pII orbitals of the halogen. Although removal of such an electron in a system such as HX or CH3X gives rise to sharp peaks, indicating that this orbital is nonbonding in the covalent sense, its removal from an ionic molecule destroys the ionic bond, making the corresponding peak broad. The TlI spectrum is more involved and suggests that the covalent contribution to bonding is competitive with the ionic contribution.
Theory of Electron Transfer Reactions in Simple Fluids
Authors: Schmidt P. P.
Published: February 2004

The purpose of this paper is to explore with as much detail and accuracy as possible the feasibility of formulating a theory of solution electron transfer reactions utilizing nonequilibrium statistical mechanics and the existing theory of simple fluids. Within the confines of a clearly stated, reasonably flexible set of approximations this has been accomplished. The theoretical analysis utilizes Yamamoto's generalization to chemical reactions of the Kubo nonequilibrium statistical mechanical response theory. We examine systems composed of electroactive solutes dissolved in an electrolytic solvent. The paper has two parts. The first part contains the treatment leading from a general Hamiltonian operator for a system of electron donors and acceptors dissolved in a solvent (with supporting electrolyte present) to an expression for the rate constant. The Hamiltonian operator adopted contains no physical solvent model assumptions such as, for example, that represented by the dielectric continuum approximation. The second part of the paper is devoted to the examination of a model, but representative, system: the exchange reaction H+H+=H++H. This is done to establish a correspondence between the theory and typical chemical interpretation, viz., the identification of activation energies and frequency factors in an Arrhenius type rate constant expression. While the H; H+ exchange has no physical reality, it is conceptually simple, and, apart from the hydrogenic mass, does correspond to systems such as Fe+2; Fe+3. The main reason for choosing this system is the clear simplicity of the Coulomb matrix elements and the hard sphere character of the reactants. Thus, this system typifies outer sphere transfer couples. In addition, the dielectric continuum model is used in the example calculations to compare this theory with that of Levich and Dogonadze. As a result of the study of the model system, we demonstrate a dependence on transport properties of the system. In particular, the electron transfer kinetics depend on the transport of reactants to a transfer state configuration. This transport differs from the familiar solution diffusion in that the simultaneous movement of the reactants and the distortion of the environment to a configuration favorable to electron transfer must occur. The general treatment of electron transfer reactions presented in this paper should prove valuable in a number of related areas, especially in assessing the validity of the use of the dielectric continuum approximation and the Born—Oppenheimer separation. Moreover, the approach provides the best route to the theoretical examination of more complicated electron transfer systems as typified by the inner sphere examples.
14N Nuclear Quadrupole Resonance in Compounds Containing N–N Bonds. II. Hydrazides
Authors: Sauer Enrique G. , Bray P. J.
Published: February 2004

14N nuclear quadrupole resonance studies at 77°K were performed in 14 hydrazides. The results indicate extensive delocalization of the charge in the pi orbital of the substituted nitrogen atom; the charge lost to the acyl oxygen is of the order of 0.30 electrons. This loss of pi electrons is partially balanced by an increased polarization of the sigma bonds. In symmetrical hydrazides this amounts to an excess charge density of order of 0.10 electrons. In hydrazides retaining an −NH2 group, the charge in the N–N bond is polarized towards the substituted nitrogen atom amounting to 0.07 electrons transferred to the substituted nitrogen atom.
Dynamics of Sodium and Potassium Bifluoride: Infrared, Raman, and Neutron Studies
Authors: Rush J. J. , Schroeder L. W. , Melveger A. J.
Published: February 2004

The crystal and molecular dynamics of NaHF2 and KHF2 have been studied by infrared and Raman spectroscopy and by neutron inelastic scattering. Infrared absorption spectra have been measured in the region of the bending (v2) and asymmetric stretching (v3) vibrations of NaHF2 and KHF2 containing 2%, 11%, and ≈ 80% DF2−. The spectra show a rather striking reduction in the widths of the DF2− absorption peaks as well as shifts in peak frequencies as the percent DF2− is lowered toward a ``defect'' concentration. These results indicate that the considerable width of the infrared absorption bands assigned to the v2 and v3 modes in pure NaHF2 and KHF2 is associated with the coupling of near-neighbor HF2− oscillators having similar frequencies or energy states. The Raman spectrum for NaHF2 shows peaks at 630.5 and 145 cm−1 which can be unambiguously assigned, respectively, to the HF2− symmetric stretching mode (A1g) and the Eg librational lattice mode. The KHF2 Raman spectrum is similar to a previous result, with some differences in the frequencies of the lattice-mode peaks. The combined neutron and far-infrared spectra on NaHF2 provide information on the acoustic and optical translational lattice modes. An approximate vibrational frequency distribution is derived from the neutron spectrum and compared with previous spectroscopic and theoretical results on NaN3. A detailed assignment of the internal and lattice modes for both sodium and potassium bifluoride is presented.
Calculation of Coefficients in the Power Series Expansion of the Long-Range Dispersion Force between Atoms
Authors: Starkschall George , Gordon Roy G.
Published: February 2004

By applying an Unsöld approximation to the expression for the second-order perturbation energy between two atoms and assuming no overlap of charge distributions, we have derived an expression for C2n, the coefficient of the R−2n term in the attractive potential, in terms of C6 and ground-state averages of even powers of the electron—nucleus distance in the isolated atom. Values of C8 and C10 are calculated for H, He, Ne, Ar, and Kr, and estimates are made as to the accuracy of the calculations. Several comments are made on the convergence of the power series expression for the second-order energy.
Spectra and Kinetics of the Hydroperoxyl Free Radical in the Gas Phase
Authors: Paukert Thomas T. , Johnston Harold S.
Published: February 2004

The absorption spectrum of the hydroperoxyl radical (HO2) has been obtained by the molecular-modulation technique. The radical was formed by the photolysis of hydrogen peroxide at 2537 Å, by the photolysis of ozone in the presence of hydrogen peroxide at 2537 Å, and by the photolysis of Cl2 in the presence of hydrogen peroxide at 3500 Å. The vibrational frequencies of HO2 have been observed to be 1095, 1390, and 3410 cm−1. Details of the vibrational spectrum are consistent with the molecular geometry: H–O bond distance 0.96 Å, O–O bond distance 1.3 Å, and H–O–O angle approximately 108°. The absorption spectrum of HO2 in the ultraviolet has a maximum at 2100 Å. Kinetic analysis of the modulated absorption signals shows that the HO2 radical decays by a process second order in HO2 concentration. The rate constant for the disproportionation reaction HO2+HO2→ H2O2+O2 was found to be 3.6± 0.5× 10−12 cm3/molecule· sec in agreement with a reported value of 3× 10−12 cm3/molecule· sec. The absorption cross section of HO2 at 1420 cm−1, the absorption maximum of the 1395 cm−1 band, is approximately 5× 10−20 cm2/molecule. The absorption cross section at 2100 Å is 4.5× 10−18 cm2/molecule.
Mixture-Model Approach to the Theory of Classical Fluids
Authors: Ben-Naim A.
Published: February 2004

A family of distribution functions is defined for a classical fluid. Interpretation of these distribution functions as mole fractions of quasicomponents suggests that a one-component fluid may rigorously be viewed as a mixture of any number of components. It is suggested that a detailed study of these ``quasicomponent distribution functions'' may be useful in gaining insight into the way an extensive thermodynamic quantity is ``spread'' over the various ``species.'' Some bearings on the existing theories of liquid water are discussed, and it is concluded that the so-called ``continuous'' and ``mixture-model'' approaches to the theory of liquid water are equivalent from the formal point of view.
Scattering of Helium, Neon, Argon, Krypton, Xenon, and Deuterium from a Tungsten (110) Surface Characterized by LEED
Authors: Weinberg W. H. , Merrill R. P.
Published: February 2004

Rare gas (He, Ne, Ar, Kr, and Xe) scattering distributions have been measured from a tungsten (110) surface at an incidence angle of 45°. The gas temperature was 295°K, while the surface temperature varied between 375 and 1300°K. The positions of the peak maxima and the broadening of pseudospecular scattering of He and Ne correlate with the FRI models, whereas the Ar–Xe results are anomalous in that these aspects of the scattering do not correlate with either the FRI or soft-cube models because Ar, Kr, and Xe are dominated by trapping at the surface. An estimate of the fraction of the rare gas atoms initially trapped at the surface has been made. Deuterium scattering distributions have been measured and are characterized by a relatively large diffuse fraction which is attributed to an internal excitation of the diatomic molecule rather than to trapping at the metal surface. Comparisons of these data with scattering from the (111) face of fcc crystals suggests three phenomenologically distinguishable scattering regimes, quasielastic, inelastic, and trapping dominated scattering.
Electronic Absorption Spectra of 3,3′-Dimethyldiazirine
Authors: Robertson L. C. , Merritt J. A.
Published: February 2004

The high-resolution electronic absorption spectra of 3,3′ -dimethyldiazirine are presented. Vibrational analyses are given for two electronic systems, the allowed B12 (n π*) ←1A1 system with its 0–0 band at 27 923 cm−1 and the spin forbidden B32 (n π*) ←1A1 system with its origin band at 23 747 cm−1. In addition, evidence is presented for another electronic system 126 cm−1 to the red of the singlet—singlet system. The N=N stretching vibration is the most prominent feature observed in all the electronic systems. A corresponding analysis is also presented for 3,3′ -dimethyl-d6-diazirine.
Hindered Molecular Rotation in Liquids
Authors: O'Reilly D. E.
Published: February 2004

Expressions for the angular velocity autocorrelation function of a classical spherical top rigid rotator and the NMR correlation time τ2 are derived. A general relationship of the product of the spin—rotational correlation time (τ1) and τ2 is presented which is proportional to the mean square angular displacement in a diffusive rotational step. The present expression for τ2 is compared with that based on the quasilattice random flight (QLRF) model. The most characteristic difference between the two expressions for τ2 is that present expression depends on the square root of the moment of inertia while the QLRF relationship depends on the square root of molecular mass. In favorable situations this difference can be used to ascertain the mechanism of molecular reorientation such as for NH3 and ND3. Root mean square angular displacement in a diffusive step are computed from observed values of τ2 and compared with the results of infrared measurements. A value for the time τ1 for liquid water at 34°C is calculated from the results of recent computer experiments of Rahman and Stillinger and the product τ1τ2 is evaluated. This result indicates that the molecular rotation in liquid water is not small step rotational diffusion.
High-Resolution Inelastic Neutron Scattering from Some Aromatic Molecular Polycrystals for Study of Crystal Excitations and Anharmonic Effects
Authors: Reynolds P. A. , Kjems J. K. , White J. W.
Published: February 2004

Inelastic neutron scattering spectra have been obtained at high resolution from a number of aromatic polycrystals, mainly halogenated benzenes, in the range 4 cm−1-1600 cm−1, at room temperature and below. Since the spectra observed reflect only nuclear motion (in contrast to optical spectroscopy), analysis reveals direct information on the harmonicity. In para-dichlorobenzene the angular dependence of the intensity of incoherent scattering, both elastic and inelastic, the relation between the total inelastic and total elastic intensities, and the temperature dependence of these spectra (and those for other substances) are all consistent with a weakly anharmonic model of the dynamics (phonons which interact with one another to give a temperature dependent lifetime and frequency). Assuming that multiphonon effects are negligible, this weakly anharmonic model is valid in p–C6H4Cl2 up to room temperature, 30°K below the melting point. At low temperatures (100°K) the phonon lifetimes are long enough to allow the observed critical points to be interpreted using quasiharmonic models. At low temperatures many critical points are observed at this high resolution (e.g., 19 for p–C6H4Cl2). In the ``intermolecular'' region in p–C6H4Cl2 and 1, 2, 4, 5–C6H2Cl4 at 200 cm−1 and above no dispersion in the modes is observed. In the ``intermolecular'' region, less than 150 cm−1, spectra for p–C6H4Cl2, p–C6H4Br2 and C10H8 were observed where absolute intensity and main features are well reproduced by Pawley's rigid-molecule model of lattice motion, but less well than Brillouin Zone center optical data is reproduced by this model.
Millimeter Wave Rotational Spectrum of HSSH and DSSD. III. Q Branches of DSSD
Authors: Winnewisser Gisbert , Helminger Paul
Published: February 2004

The Q branches of the K=2← 1 and K=3← 2 rotational subbands of the ground state of D32S32SD have been measured in the frequency range 200–350 GHz. The locations of the subband centers have been determined to be at ν0(1)=209 741.9± 0.1 MHz and ν0(2)=349 544.5± 0.1 MHz for the K=2← 1 and K=3← 2 rotational subbands, respectively. The most prominent and unusual feature encountered in the spectra of the rQ1 and rQ2 branches is that the observed K doubling of the K=3← 2 rotational lines is larger than the K doubling of the K=2← 1 Q-branch transitions of the same J value. This anomalous K doubling of the Q branches is shown to be caused by the superposition of two contributing effects, the asymmetry splitting and centrifugal-distortion splitting. This concept has been established in the preceding paper. For the K=1 levels both contributions to the K doubling are equally important, while for the K=2 levels the K doubling is entirely due to the centrifugal-distortion splitting. The analysis of a total of some 350 measured and assigned absorption lines indicates clearly that together with the three rotational constants, six quartic distortion constants are sufficient to explain the spectrum. A discussion of the spectra of rQKa branches for Ka≥ 3 is given, although these transitions have not been observed.
Millimeter Wave Rotational Spectrum of HSSH and DSSD. IV. P, Q, and R Branches of HSSH
Authors: Winnewisser Gisbert , Helminger Paul
Published: February 2004

The earlier microwave measurements on the ground-state rotational spectrum of HSSH have been extended into the submillimeter wave region to a wavelength of 0.7 mm, or 420 GHz, for the observation of the Q branch of the K=2← 1 rotational subband. In the lower-frequency range 50–200 GHz, various Q-, R-, and P-branch transitions have also been assigned. From these measurements the ground-state rotational constants are (in megahertz): A=146 858.170± 0.032, B=6 970.430± 0.003, C=6 967.689± 0.003, which yield an asymmetry parameter bp=−0.98× 10−5 or κ =−0.999960. The most remarkable feature of the perpendicular-type spectrum is the anomalous splitting of the K=2 energy levels. It is found that the splitting of the K=2 levels is not only one order of magnitude larger than that expected from the small asymmetry of the molecule, but also that the sequence of these levels is reversed as compared to their ``normal'' order, determined by the asymmetry splitting. These anomalies are explained as in paper II of this series by the additional K-type doubling (centrifugal-distortion splitting) caused by the centrifugal-distortion operator (Ĵ+4+Ĵ−4) in the rotational Hamiltonian. Unlike DSSD, the centrifugal-distortion splitting in HSSH is nearly masked by the larger asymmetry splitting.
Polarized Fluorescence and Absorption Spectra of Tetracene in p-Terphenyl
Authors: Kruse Norman J. , Small Gerald J.
Published: February 2004

Polarized absorption and fluorescence spectra of the 5000 Å B12u⇌1A1g transitions of tetracene (naphthacene) dissolved in p-terphenyl have been measured at 4.2°K. The vibronic band-widths are sufficiently sharp to permit resolution of a 3 cm−1 site splitting and to enable assignments to be made of the 12 ground and excited state a1g and several b3g fundamentals between 300 and 1600 cm−1. The phonon structures in absorption and fluorescence are characteristic of strong and weak electron—phonon coupling, respectively. A dependence of the structure in the tetracene fluorescence spectrum on the wavelength of the exciting light has been observed. It is shown that this wavelength dependence is the result of unusual crystal site relaxation processes which occur through electron—phonon coupling mechanisms.
Transmission Properties of a LiF–O2 Filter for Radiation Resulting from Electron Impact on H2: Cross Sections for Lyman-α Radiation
Authors: Carrière J. D. , de Heer F. J.
Published: February 2004

In many experiments Lyman-α radiation has been detected by means of the iodine-vapor-filled ultraviolet photon counter of Brackmann, Fite, and Hagen in combination with a LiF–O2 filter. In the case of electrons incident on molecular hydrogen, Fite and Brackmann have used the detector for determination of the cross section for ``countable ultraviolet radiation'' (cuv) being chiefly composed of Lyman-α radiation and an unknown fraction of molecular radiation. In the present investigation the transmission properties of the LiF–O2 filter are investigated. At 100 eV impact energy it is found that 26% of the transmitted radiation is molecular radiation. Considering this fraction, it is possible to evaluate the cross sections for Lyman-α radiation from Fite and Brackmann's cuv cross sections. These cross sections agree very well (within about 6%) with those of de Heer and Carrière, obtained by a different procedure.
Classical Dynamics of the Reaction of Fluorine Atoms with Hydrogen Molecules. II. Dependence on the Potential Energy Surface
Authors: Muckerman James T.
Published: February 2004

The sensitivity of classical trajectory calculations of the reactions of fluorine atoms with H2 and HD to the particular choice among several ``acceptable'' potential energy surfaces is investigated. The hypothesis that the percent of the total energy available to the reaction products that is in the form of vibration energy can be predicted by the ``energy release'' on the potential energy surface is tested using this set of related H2F surfaces. The present results suggest that the main conclusions of a previous study of the dynamics of the reactions of F atoms with H2, HD, and D2 are features of the classical dynamics on any reasonable H2F potential energy surface of the generalized LEPS type.
Study of Relaxation in Free Jets Using Temperature Dependence of n-Butane Mass Spectra
Authors: Milne Thomas A. , Beachey Jacob E. , Greene Frank T.
Published: February 2004

Molecular beam studies of the temperature dependence of n-butane fragmentation patterns were carried out under several conditions. Under effusive-flow molecular-beam formation, the temperature dependence of fragmentation to several ions from n-butane agrees well with conventional studies. Under free-jet expansion molecular beam formation, the fragmentation of n-butane to several ions indicates substantial vibrational relaxation of the neutral molecule. Dilution of n-butane with argon at room temperature gives rise to parent/fragment ion ratios that suggest that the dimer or higher clusters are contributing substantially to ion peaks normally attributed to the monomer. The unusual ions (C4H10)n+ are readily generated. Implications of the large observed vibrational cooling and its method of detection are discussed.
Studies in Molecular Dynamics. X. Corrections to the Augmented van der Waals Theory for the Square Well Fluid
Authors: Alder B. J. , Young D. A. , Mark M. A.
Published: February 2004

The validity of the augmented van der Waals theory of fluids is demonstrated by a rigorous evaluation of the next three terms in the reciprocal temperature expansion of the Helmholtz free energy for the square-well fluid. Each term in the dense fluid is shown to be an order of magnitude smaller than the preceding one except the fourth, which is comparable to the third. Analysis of molecular dynamics data leads to a peak in the heat capacity at the coexistence curve which is not reproduced by the finite series. This peak in turn leads to realistic values of the critical indices α and α′ while the other indices β, γ, and δ have their classical van der Waals values.
Theoretical Determination of Vibrational Transition Probabilities in Diatomic Molecules
Authors: Alexander Millard H.
Published: February 2004

Vibrational transition probabilities in diatomic molecules are given by the square of off-diagonal matrix elements of the molecular dipole-moment function M (R). Taken separately, neither ab initio molecular calculations nor molecular beam studies of the vibrational dependence of the electric dipole moment provide enough information about M (R) to determine uniquely these off-diagonal matrix elements. We develop a method to combine the available experimental and theoretical information about M (R) so as to yield a maximum of precision in the transition amplitudes. The method invokes the linear programming algorithm. As a numerical application we present and compare with experiment results for various vibration-rotation transitions in the LiH and KF molecules.
Recombination of Ions in a Coulomb Field in the Presence of a Scavenger
Authors: Magee John L. , Tayler A. B.
Published: February 2004

The method of ``matched perturbation solutions'' is used in an analytical treatment of the recombination of a pair of ions in the presence of a scavenger. The perturbation series contains terms in integral powers of c8 1/2 and the solution presented here is carried out explicitly to the term in cs. This solution gives the effect of scavenger on recombination rigorously for small scavenger concentrations. It agrees with prescribed diffusion in the c8 1/2 term.
Mean Spherical Model Integral Equation for Charged Hard Spheres. II. Results
Authors: Waisman Eduardo , Lebowitz Joel L.
Published: February 2004

We continue our investigation of the solution of the mean spherical model integral equation for systems of charged hard spheres and charged hard sheets (in one dimension). The general method of solution was presented in Paper I of this series. This paper contains explicit expressions for the structure functions and thermodynamic properties of a variety of such systems in one and three dimensions. The results all have a very simple form and are in good agreement with various machine computations. When the charges on the particles vanish our results coincide with those obtained from the Percus—Yevick equation for hard spheres while in the limit of zero hard core diameters the results go over into those obtained from the linearized Debye—Hückel theory.
In—Cavity Laser Raman Spectroscopy of Vapors at Elevated Temperatures. As4 and As4O6
Authors: Brumbach Stephen B. , Rosenblatt Gerd M.
Published: February 2004

The 632.8-nm radiation inside the cavity of an He–Ne laser is used to excite vibrational Raman spectra of As4 vapor at ∼ 610°C and As4O6 vapor at ∼ 440°C. As4 peaks are observed and assigned as follows: v1(a1), 344 cm−1; v2(e), 210 cm−1; and v3(f2), 255 cm−1. As4O6 vapor peaks are seen at 185, 253, 381, 409, 496, and 555 cm−1. An outside-cavity room-temperature spectrum of powdered As4O6 shows bands at 85, 189, 269, 371, 470, 560, and 782 cm−1. At a sample temperature of −178°C, the 85-cm−1 peak shifts to higher frequency by 4.4 cm−1. The molecular f2 fundamentals of As4O6 vapor are reassigned at 253, 346, 496, and 782 cm−1 based upon the disappearance of the 85-cm−1 low-frequency band upon going from solid to vapor, the temperature dependence of the 85-cm−1 band of the solid, comparison with assignments for P4O6, and comparison of computed absolute entropies of As4O6 vapor with entropies obtained from vapor pressure measurements, as well as literature single-crystal polarization measurements and infrared absorptions.
Monte Carlo Studies of the Relaxation of Vector End-to-End Length in Random-Coil Polymer Chains
Authors: Kranbuehl David E. , Verdier Peter H.
Published: February 2004

The effects of excluded volume interactions upon the dynamical behavior of random-coil polymer chains are studied by obtaining autocorrelation functions for vector end-to-end length of lattice-model chains of 9, 15, 33, and 63 beads by a Monte Carlo simulation technique. It is found that relaxation of the vector end-to-end length requires from 4 to 7 times as long as relaxation of its square, in contrast to the predictions of simple models without excluded volume effects.
Nuclear Magnetic Resonance Study of Hydrogen Diffusion in Yttrium Trihydride
Authors: Weaver H. T.
Published: February 2004

Nuclear magnetic resonance measurements of spin-lattice relaxation time and resonance line shapes have been carried out as a function of temperature in the range 77–715°K for 1H contained in yttrium hydride. The composition ranged from YH2.61 to YH2.94 and included three samples in the hydrogen-deficient trihydride phase. Within limited temperature ranges samples with higher hydrogen concentrations displayed complex line shapes and nonexponential magnetization recovery. These phenomena were associated with the formation of regions of YH3 within the hydrogen-deficient structure.
Effects of Variations of the Potential-Energy Surface on the Attributes of Simple Exchange Reactions: Classical Calculations
Authors: Fong K. P. , Diestler D. J.
Published: February 2004

The results of numerically exact classical-trajectory calculations for the collinear collision of H+H2 on a series of Wall-Porter potential-energy surfaces are presented and compared with previously reported accurate quantum-mechanical results. The effects of variations of (1) the total energy, (2) the barrier height, (3) the curvature of the reaction path, and (4) the slope and ``width'' of the potential barrier along the reaction path on various attributes of the reaction are determined by systematically changing selected parameters of the surface. Attributes of primary interest are (1) the reaction probability (as a function of energy), (2) the effective threshold for reaction, and (3) the degree of vibrational excitation of the product, as measured by the fraction of the total energy which ends up as vibrational energy of the product. It is observed that the classical results agree better with the quantum-mechanical for some surfaces than for others. Also, although the detailed structure of the reaction probability curves is not well predicted in the classical approximation, the classical and quantum-mechanical results agree on the ``average,'' indeed, incredibly well for some surfaces.
On the Measurement of Transverse Relaxation Rates in Complex Spin Systems
Authors: Vold Robert L.
Published: February 2004

Selective measurement of transverse relaxation rates of each line in a complex NMR spectrum can be accomplished by subjecting the spin system to a nonselective Carr-Purcell sequence of rf pulses, Fourier transforming the free precession following the last echo maximum, and repeating the experiment for several different durations of the echo train. Effects of homonuclear spin coupling on such measurements are exmined in detail. It is shown that such coupling can produce severe distortion of the apparent time decay of partially relaxed spectral lines. Criteria for avoiding the distortion are established, and a general scheme for interpreting selectively measured transverse relaxation rates in the framework of Redfield theory is proposed.
Vacuum-Ultraviolet (147 nm) Photolysis of Carbon Suboxide in the Presence of Methane
Authors: Tschuikow-Roux E. , Inel Y. , Kodama S. , Kirk A. W.
Published: February 2004

The vacuum-ultraviolet photolysis of C3O2 in the presence of CH4 and Ar has been investigated at 25°C using the 147 nm Xe resonance line. The principal gaseous reaction products include CO, C2H2, C2H4, and some C2H6. The yield of acetylene decreases with increasing methane or inert gas pressure while the opposite trend is observed in the case of ethylene. Diagnostic experiments with added H2 and CO have shown that excess H2 effectively suppresses C2H4 formation, while excess CO virtually eliminates the production of C2H2. The results are interpreted in terms of the participation of 1D and 3P carbon atoms and are consistent with recent flash photolysis-kinetic spectroscopy work on the C3O2/CH4 system. It is proposed that C2H2 formation can be attributed entirely to the reactionC(1D)+CH4→ C2H2+H2,whereas C2H4 results from reaction of methane with ground state C2O(3Σ) radicals, producted in a secondary process C(3P)+CO+M→ C2O(3Σ)+M,C2O(3Σ)+CH4→ C2H4+CO.Both Ar and CO deactivate C(1D) to C(3P), however, relative to its rate of attack on methane, the deactivation of C(1D) by argon is an inefficient process.
Atomic and Ionic Radii of Superheavy Elements
Authors: Fricke B. , Waber J. T.
Published: February 2004

Atomic and ionic radii are presented for the elements E104–E120 and E156–E172. It is shown that a number of effects correlated with the large relativistic contraction of orbitals with low angular momentum leads to smaller atoms for higher atomic numbers. It is expected that Cs is the largest atom in nature.
NMR Determination of Some Deuterium Quadrupole Coupling Constants in Nematic Solutions
Authors: Millett Francis Spencer , Dailey Benjamin P.
Published: February 2004

The analysis of the NMR spectra of suitable nematic liquid-crystal solutions appears to offer a convenient and desirable method for the determination of small nuclear quadrupole coupling constants for molecules in the liquid state. Nuclear quadrupole coupling constants have been determined for deuterium in the C–H bond of a number of small molecules. Assuming that the field gradient has axial symmetry, values of e2qQ/h along the bond axis are as follows: CH3CD3167± 18 kHzC3D6184± 20C2D2198± 7CH3C2D199.4± 2.0DCN199.0± 3.0. The results are discussed in terms of various theoretical calculations of the field gradient at the deuteron, and a compilation of some of the accurate experimentally determined deuterium coupling constants is presented.
Neutron Diffraction Study of Iron(II) Chloride Tetrahydrate, FeCl2· 4H2O
Authors: Verbist Jacques J. , Hamilton Walter C. , Koetzle Thomas F. , Lehmann Mogens S.
Published: February 2004

A three-dimensional neutron diffraction study has revealed the positions of hydrogen atoms in a 55.9% deuterated crystal of iron(II) chloride tetrahydrate (FeCl2· 4H2O). The space group is P21/c with Z=2 and unit cell parameters: a=5.885(3), b=7.180(6), c=8.514(4) Å, and β=111.09(2)°. The conventional R factor is 0.027, assuming a partially disordered model and anisotropic extinction. Hydrogen atom positions corresponding to a minimum of calculated electrostatic energy are compared with those from the crystal structure. Previous attempts to determine hydrogen positions in FeCl2· 4H2O from NMR spectra are analyzed in the light of the results obtained in this study.
Third-Order Constitutive Equations and Transport in Rarefied Gases
Authors: Cha C. Y. , McCoy B. J.
Published: February 2004

A description of transport of either heat, momentum, or mass in the transition regime is based upon third-order constitutive equations for the fluxes. The one-dimensional steady-state equation which governs the dependent variable P* (either dimensionless temperature, velocity, or concentration) as a function of distance x* is the linearized fourth-order differential equationN2(d4P*/dx*4) + (d2P*/dx*2)=0,where N is proportional to the Knudsen number. A solution is proposed based on reasonable boundary conditions for parallel plate geometry. Expressions are derived for the profile of P*, the slip or jump of P* at a wall, the effective transport coefficient, and the flux. The expressions have the proper limits for the continuum and free molecule regimes, and compare well with other theories for the transition regime and with subsonic experimental data.
Cooperative Optical Transitions in the Oxygen Dimer: Excimer States, Selection Rules, and Oscillator Strengths
Authors: Zarur George Louis , Chiu Ying-Nan
Published: February 2004

An alternative and approximate method is used to compute the dipole intensity of the cooperative optical transition in an oxygen molecular dimer, O4. This approximation attributes the interaction of the two oxygen molecules to the overlap of their valence electrons and constructs the composite molecular states of the dimer by permuting all four πg valence electrons (two from each molecule) in a Slater determinant. The overall symmetry of the possible composite, dimeric and exciton states is determined for different collision geometry belonging to D∞h, C2vx, C2hy, C2vz, D2h, C3yz, C2, C3xz, and S4 point groups. The polarization of the transition is determined from these overall symmetries of the dimer. The transition matrix elements of the dimer are weighted by overlap integrals and are expressed in terms of the angles between the two molecules. This is in contrast to the conventional method which gives transition matrix elements weighted by electrostatic matrix element divided by an energy factor. The latter conventional method mixes in allowed states by intermolecular electrostatic interaction plus electron exchange. The present computation provides an expedient way of giving the relative contribution to intensity of the various orientations and geometries of the collision complex. The present formalism may be used in the cooperative optical transitions of the dimers of other homopolar diatomic molecules. The group theoretical analysis of the excimer states will be useful in the study of molecular crystals of linear molecules in various lattice geometries, in polarization studies and in the nonadiabatic vibronic energy transfer and chemical reactions between two linear molecules.
Pulse Radiolysis in Lanthanide Aqueous Solutions. I. Formation Spectrum and Chemical Properties of Divalent Europium, Ytterbium, and Samarium Ions
Authors: Faraggi M. , Tendler Y.
Published: February 2004

The reactions of the hydrated electrons with Eu(III), Yb(III), and Sm(III) have been investigated. Absorption spectra of transient species produced by the pulse of electrons have been obtained and assigned to the Laporte allowed transition between 4fn and 4fn−1 5d levels of the divalent ions. The excitation energy values Efd for these ions in aqueous solution are in good agreement with those found or calculated in crystal. The specific rates of reactions of Eu(II), Yb(II), and Sm(II) with OH radicals, O2, N2O, H2O2, NO2−, NO3−, BrO3−, IO3−, Cu2+ and the electron transfer reaction of Sm(II) with Yb(III) and with Eu(III) have been determined. It has been found that the order of reactivity is Sm(II) > Yb(II) > Eu(II); this order is correlated with the electrode potential E0 of divalent ions.
Pulse Radiolysis Studies in Lanthanide Aqueous Solutions. II. Formation, Spectrum, and Some Chemical Properties of Praseodymium(IV) in Aqueous Solution
Authors: Faraggi M. , Feder A.
Published: February 2004

Pulse radiolysis of Pr(III) gives a transient absorption spectrum with a maximum at 290. This is due to the formation of Pr(IV) through the reaction of OH radicals and Pr(III). The rate of the reaction has been determined by four different methods and the rate constant was found to be 1–4 × 106M−1 sec−1. The variation of the optical density of Pr(IV) ions with pH lead to the conclusion that the form of Pr(IV) is Pr(OH)3+ and Pr4+ in neutral and acid solutions, respectively. The equilibrium constant of hydrolysis, Pr4+ + H2O ⇄ Pr(OH)3++ H+, was found to be 7 × 10−4. The spectrum obtained is assigned to the Laporte-allowed transition between 4f and 5d. The specific reaction rate constants of Pr(IV) with water, HSO4−, NO2−, H2O2, and Br− have been determined, as well as those of Pr(III) with eaq− and H atoms.