bluuebunny/crossref_metadata_2025 huggingface dataset:
A stochastic model for the transport of proteins in agargel electrophoresis
Authors: Tränkle E.
Published: February 2003

The transport of proteins in agargel is described as a stochastic transition of protein molecules between the defects of the gel structure. To this end we develop a model for a regular arrangement of defect positions taking into account the defect position fluctuations only in a heuristic way. We evaluate the position, width, skewness, and excess of the protein distribution functions and compare the model predictions with experiment.
The coefficient of self-diffusion of liquid sodium from a molecular dynamics calculation
Authors: Lewis R. S. , Callaway T. O.
Published: February 2003

The coefficient of self-diffusion of liquid sodium at 127°C has been determined from data generated by a molecular dynamics study of a system of 686 atoms interacting through the potential derived by Shyu, Singwi, and Tosi. From the linear portion of the mean square displacement curve for the system, the coefficient of self-diffusion was found to be 5.87×10−5 cm2/sec.
Variable temperature 31P and 19F NMR studies of Fe(CO)3(PF3)2 and Fe(CO)2(PF3)3
Authors: Mahnke H. , Clark R. J. , Sheline R. K.
Published: February 2003

31P and 19F NMR spectra for the compounds Fe(CO)3(PF3)2 and Fe(CO)2(PF3)3 were studied in the temperature range +25 °C down to −100 °C. The four spin–spin coupling constants 1JPF, 3JPF′, 2JPP′, and 4JFF′ were evaluated as a function of temperature. Owing to the fast intramolecular exchange of axial with equatorial ligand groups in these compounds, the observed spectra are time-averaged. The magnitude of the observed time-averaged 2JPP′ coupling constant can be accounted for by the experimentally determined mole fractions for the various isomers and the trans, cis and vicinal phosphorus–phosphorus coupling constants 2JtransPP′, 2JcisPP′ and 2JvicinalPP′, respectively. An angular correlation of the phosphorus–phosphorus coupling constant is attempted. These results are compared to variable temperature ir and 13C NMR studies of these compounds. The usefulness of studying ir or Raman spectra in addition to the NMR spectra for certain nonrigid molecules which fall in the ’’very fast exchange’’ category where no temperature coalescence of NMR spectral lines can be observed, is demonstrated. The simultaneous fitting of both the time-averaged NMR spectra on the one hand and the separate ir spectra on the other is the best way to study such molecules. 151
Experimental study on the nucleation of water vapor sulfuric acid binary system
Authors: Boulaud D. , Madelaine G. , Vigla D. , Bricard J.
Published: February 2003

The formation of aerosols in a gaseous mixture of water and sulfuric acid was studied experimentally. The formation of particles having been demonstrated during previous experiments, an experimental device was developed which would make it possible to determine the rate of nucleation for different values of partial pressure of water vapor and sulfuric acid. These results were compared with the numerical values given by Mirabel and Katz on the one hand, and by Kiang and Stauffer on the other hand; it was found that the most satisfactory correlation is obtained when the value of 3.5×10−4 torr is adopted for the pressure of the sulfuric acid saturating vapor at a temperature of 20 °C.
Many-body calculations on molecules with second-row atoms: H2S and H2CS
Authors: von Niessen W. , Cederbaum L. S. , Domcke W. , Diercksen G. H. F.
Published: February 2003

The ionization potentials of H2S and H2CS are calculated by a many-body Green’s function method. Correlation energy changes in the diffuse part of the charge cloud require an extension of the polarization function basis compared to the first row atoms. When f-type functions are included all calculated ionization potentials of H2S agree with the experimental values to within 0.1 eV. The ionization potentials of H2CS agree satisfactorily with the recently determined experimental values except for the fourth band where possibly predissociation occurs. The ordering of the ionic states is the same for H2CS as for H2CO. The vibrational structure in the photoelectron spectrum of H2CS is computed.
Pressure-broadened linewidths of formaldehyde
Authors: Tejwani G. D. T. , Yeung Edward S.
Published: February 2003

Self-broadened and foreign-gas (N2 and O2) -broadened linewidths of H2CO at 300°K for a wide range of quantum numbers J and Ka, for all three types of bands, have been calculated using the Anderson–Tsao–Curnutte theory of line broadening. Due to the large dipole moment of formaldehyde, quadrupolar interactions were found to contribute negligibly to the linewidth. Because of this, in the case of H2CO–H2CO collisions, only dipole–dipole interaction has been included. For H2CO–N2 and H2CO–O2 collisions, dipole–quadrupole interaction was taken into account. Computed values for self- and N2-broadened linewidths are in good agreement with the measured values reported in the literature. Air-broadened linewidths of H2CO have also been calculated at 200°K so that the temperature dependence can be estimated.
Two-dimensional 13C NMR of highly oriented polyethylene
Authors: Opella Stanley J. , Waugh J. S.
Published: February 2003

The two-dimensional NMR procedure of separated local field spectroscopy [J. S. Waugh, Proc. Natl. Acad. Sci. USA 73, 1394 (1976)] is applied to fibers of highly oriented polyethylene. The 1H decoupled 13C chemical shift spectrum of the fiber oriented parallel to the external magnetic field is a single line, while a distinctive powder pattern results from the perpendicular orientation that is in excellent agreement with calculations. The 13C–1H dipolar splitting is 45 kHz at σ33 and nonexistent at σ22 consistent with perfectly aligned methylene groups with C–H bond length of 1.10 a.u. The splitting at σ11 is somewhat reduced from the predicted value; this finding, along with the temperature invariance of parameters, is discussed in terms of the molecular disorder and motion of the polymer.
Gravity effects on the critical equilibria of a simple mixture
Authors: Kwon O’Dae , Kim Dae M. , Kobayashi Riki
Published: February 2003

Classical thermodynamics are used to describe analytically the gravity-induced critical anomalies observable from the inhomogeneous concentration profile and also the thermodynamic stabilities of the reverse gradient for the case of a simple mixture of two miscible liquids. In particular, a derivation of the liquid–liquid coexistence curve is presented.
Knudsen forces in vacuum microbalance
Authors: Loyalka S. K.
Published: February 2003

The problem of transverse and longitudinal Knudsen forces in typical vacuum microbalance measurements is studied by the use of the BGK model and diffuse reflection at the surfaces of the inner and outer cylinders. The forces are expressed in terms of appropriate quadratures of the macroscopic velocity profiles which in turn are obtained by numerical solutions of relevant integral equations. Detailed results for several values of the Knudsen number (Kn) spanning all regimes of interest are reported and the Knudsen forces are seen to exhibit maximum in the transition regime. These results are found in good qualitative, as well as quantitative, agreement with the experimental data (on helium) of Massen et al.
Reduced Green’s functions and coupled perturbed Hartree–Fock calculations. II. Application to the static dipole polarizability of the helium isoelectronic sequence
Authors: McDowell H. Keith , Porter Richard N.
Published: February 2003

The reduced free-particle Green’s function formulation of the first-order coupled perturbed Hartree–Fock theory is applied to the calculation of the static dipole polarizability for the helium atom and for two-electron ions as a test of the formulation and to demonstrate a power-moment method of solution. The power-moment method reduces the first-order coupled perturbed Hartree–Fock (FO-CPHF) integral equation to a set of simultaneous linear equations tailored to fit the problem. The formalism facilitates a consistent treatment of three approximations to the coupled perturbed method, depending upon the potential from which the unoccupied states (appearing in the corrected Green’s function) are derived. In each case the occupied (hole) states are accurate series solutions to the zero-order Hartree–Fock equations. Results in agreement with previous calculations are reported for the helium isoelectronic sequence.
Variation of the polarizability of noble gases with density
Authors: Lallemand M. , Vidal D.
Published: February 2003

Density data for helium and neon up to 10 kbar at 25 °C are reported. The earlier experimental results for the Clausius–Mossotti function have been extended to higher densities for the whole noble gas series (i.e., up to reduced density ρ*?1). These results, combined with the machine calculation of the dipole–induced-dipole contribution with respect to density for a Lennard-Jones fluid, allow the determination of the mean individual atomic polarizability of noble gases as a function of density. In an appropriate system of units, the values of the mean polarizability fall on a single universal curve for all the reduced temperatures investigated.
Thermochemistry of gaseous UF5 and UF4
Authors: Hildenbrand D. L.
Published: February 2003

Thermochemical data for gaseous UF5 and UF4 were obtained from equilibrium and electron impact measurements made by high temperature mass spectrometry. Equilibrium measurements yielded for the gaseous reaction Ag+UF5=AgF+UF4 the enthalpy change ΔH298=16.9±1.5 kcal by second law analysis. Additionally, the sublimation of UF4(s) was studied by the torsion–effusion method, leading to ΔH298(sub) =76.2±0.5 kcal/mol. These results were combined with auxiliary thermochemical data to derive the standard heats of formation ΔHf °298(UF5, g) =−460.6±3.4 kcal/mol and ΔHf °298(UF4, g) = −377.8±0.6 kcal/mol, and the bond dissociation energies D0(F5U–F) =68.3±3.5 kcal and D0(F4U–F) =101.0±3.4 kcal. Threshold energies for formation of UF5 + and UF4 + by dissociative ionization of UF6 were determined along with the ionization potentials of UF6(14.00 eV), UF5 (11.29 eV), and UF4 (9.96 eV), leading to the BDE values D0(F5U–F) =68.0±3.2 kcal and D0(F4U–F) =102.4±3.2 kcal, in agreement with the equilibrium data. Uncertainties associated with the measurements and the interpretation of the data are discussed. The entropy of UF5(g) inferred from the equilibrium measurements was used in a third law treatment of published UF5 sublimation data, giving thermochemical results in good agreement with the above. The UF4 sublimation measurements lead to a vapor entropy that seems more compatible with a molecular configuration of lower symmetry than the obvious tetrahedral structure.
A simple model for the internal mode dependence of gas/solid-surface sticking probabilities
Authors: Doll J. D.
Published: February 2003

The role of internal molecular modes in the determination of gas/solid-surface sticking probabilities is examined. It is found that, at least for low-frequency modes, internal excitation can appreciably inhibit sticking. A simple, phenomenological model of this effect is developed.
Structure determination of liquid argon by x-ray diffraction
Authors: Kirstein B. E. , Pings C. J.
Published: February 2003

X-ray diffraction measurements and subsequent data analyses have been carried out on liquid argon at five states in the density range of 0.91–1.35 g/cc and temperature range of 127–143 °K. Duplicate measurements were made on all states. These data yielded radial distribution and direct correlation functions, which were then used to compute the pair potential using the Percus–Yevick and CHNC theories. The potential minima are in the range of −105 to −120 °K, and appear to substantiate current theoretical estimates of the effective pair potential in the presence of a weak three-body force. Numerical experiments on the data have been carried out to study the subsidiary peak phenomenon in the radial distribution function and a distortion phenomenon. The distortion phenomenon for an experiment of the type conducted partially explains why the earlier data by Mikolaj–Pings yielded argon pair potential well depths from the Percus–Yevick equation that were too shallow. The derivation of the correction procedure for distortion is presented.
Photoionization studies of the diatomic heteronuclear rare gas molecules XeKr, XeAr, and KrAr
Authors: Ng C. Y. , Tiedemann P. W. , Mahan B. H. , Lee Y. T.
Published: February 2003

The photoionization efficiency curves of the heteronuclear rare gas van der Waals molecules XeKr, XeAr, and KrAr were obtained with the molecular beam technique in the wavelength range from 790 to 1065 Å (11.64–15.69 eV). The ionization potentials were found to be 11.757±0.017 eV for XeKr, 11.985±0.017 eV for XeAr, and 13.425±0.020 eV for KrAr. From the known dissociation energies of the ground state XeKr, XeAr, and KrAr van der Waals molecules as determined by low energy molecular beam elastic scattering experiments, the binding energies for the ground state of the heteronuclear rare gas molecular ions were deduced to be 0.37±0.02 eV for XeKr+, 0.14±0.02 eV for XeAr+, and 0.59±0.02 eV for KrAr+. The photoion spectra of the heteronuclear rare gas dimers R1R2 exhibit prominent autoionization structure, which is found to correlate very well with the excited molecular Rydberg states R1*R2 and R1R2* formed by the interaction of a normal ground state rare gas atom R1 (or R2) and an excited atom R2* (or R1*), in the n′p5(2P1/2,3/2) ns (nd) configuration (where n′=3 for Ar, n′=4 for Kr, and n′=5 for Xe). The excitation of XeKr, XeAr, and KrAr is found to obey the parity selection rule Δl=±1, in agreement with previous experimental observations in the photoionization studies of Kr2 and Ar2. The Rydberg series 5p2(2P1/2) ns (nd) for Xe, and 4p5(2P1/2) ns (nd) for Kr, which is manifested as autoionization lines in the photoion spectra of the van der Waals rare gas dimers XeAr and XeKr, respectively, were red shifted with respect to the positions in the photoionization efficiency curves of Xe and Kr. This allows one to calculate the potential energy for the excited molecular Rydberg state at a distance which corresponds to the equilibrium interatomic distance of the ground state van der Waals molecule. For Xe[5p5(2P1/2) ns (nd)]+Ar(1S0) one obtains 0.06 eV and for Xe(1S0)+Kr[4p5(2P1/2) ns (nd)] 0.10 eV. These values are found to be in agreement with what one would expect from a charge induced-dipole interaction.
Group-theoretical study on the Raman tensor patterns in vibrational resonance Raman scattering: Application to the hexabromoiridate (IV) ion
Authors: Hamaguchi Hiro-o
Published: February 2003

A group-theoretical method is described which enables one to calculate the Raman tensor patterns for vibrational Raman scattering under the ’infinitely sharp’ resonance condition. A special attention is paid to the application to the molecules having degeneracy in their ground electronic states. The process of vibrational resonance Raman scattering is recognized as a succession of two vibronic transitions and its initial, intermediate, and final states are considered in vibronic space. The first-order perturbation theory is used to find the effective intermediate states for vibrational Raman scattering, a vibronic interaction Hamiltonian linear in the normal coordinates in the ground electronic state being assumed. The electric dipole transition moments are calculated group-theoretically for the transitions from the initial to intermediate and from the intermediate to final states. They are combined according to Kramers–Heisenberg dispersion formula. The method is applied to the IrBr62− ion, which has a doubly degenerate ground electronic state. The Raman tensor patterns for the three 1←0 vibrational transitions of ν1, ν2, and ν5 modes are derived and are shown to have the antisymmetric part, being consistent with the observation of the anomalous polarization in the vibrational resonance Raman scattering of this ion. The way of calculating the depolarization degrees and reversal coefficients are illustrated.
Generalized theory of semiclassical amplitudes
Authors: Heller Eric J.
Published: February 2003

The theory of quantum amplitudes in the semiclassical, h/→0 limit is considerably extended by including overlaps between Gaussian coherent states (wave packets) in the primitive ’’semiclassical algebra.’’ Traditionally, semiclassical formulations have exclusively involved overlaps between eigenstates of Hermitian operators; these are delocalized in most representations. Paradoxically, the states which are most localized and particlelike in the classical limit (wave packets) are not eigenfunctions of Hermitian observables, and have been omitted from asymptotic, formal theories of the semiclassical limit. When incorporated into a generalized theory, the Gaussian wave packet states apppear to alleviate many of the practical difficulties encountered in implementation of the Hermitian, delocalized state formulation. We present the generalized theory here, and show how it contains and reduces to the Hermitian semiclassical theory in various limits.
Temperature dependence of asymmetry parameter and mean square displacement of the iron sites in FeSO4⋅4H2O
Authors: Nascimento M. S. , Garg V. K.
Published: February 2003

Mössbauer absorption of Fe57 in synthetic monocrystals of FeSO4⋅4H2O has been studied as a function of the orientation of the crystal axis with respect to the direction of a gamma ray beam in the temperature range of 300–77 K. Analysis of the peak area ratios of the quadrupole split spectra give the EFG tensor, and hence η, and the mean square displacement <r2≳ as a function of temperature. η is in good agreement with the previously reported curve obtained by lattice sum calculations. For the MSD the polarization dependence of the absorption cross section has been taken into account in calculating the absorption spectra; and <r2≳ is 0.8×10−18 cm2 at 300 K.
Statistical mechanical theory of polymers. IV. Equation of state for the Lennard-Jones model of a single ring polymer
Authors: Ailawadi Narinder K. , Naghizadeh Jamshid
Published: February 2003

Thermodynamic functions of a single ring polymer with modified Lennard Jones potential for the binary intersegmental interactions are studied. The integral equation of the binary intersegmental correlation function g (R) is solved by a perturbation technique analogous to that of Kirkwood, Lewinson, and Alder for the case of fluids. The hard sphere model is taken as the reference solution which is expressed in terms of a parameter related to mean segment density. The binary intersegmental correlation function g (R) is computed by an iterative procedure over a wide range of density and temperatures and is used to construct the isotherms of the system. The collapse transition known from Monte Carlo computations and light scattering experiments is qualitatively reproduced.
The molecular motion in crystalline, plastic, and liquid neopentane studied by cold neutron scattering experiments
Authors: Månsson T. , Olsson L. G. , Larsson K. E.
Published: February 2003

Cold neutron scattering experiments have been performed on neopentane in the liquid state at 265 K and in the solid state at the temperatures 135, 148, 173, 213, 239, and 253 K. The measured data were interpreted by fitting computer-calculated cross sections, based on various physical models, to the experimentally obtained ones. In this procedure the translational motion was supposed to be known and the temperature dependence of the molecular rotations was particularly studied. The following was concluded: In the crystalline phase the rotations are hindered such that the molecule mainly performs small-angle librations around equilibrium orientations. In the plastic phase the rotational motions may be visualized as a rapid succession of alternating steps, described by librations and free rotations. The mean life times τ0 and τ1, of these alternating states is, however, short: between 3.5 and 0.25×10−12 s for the librations (τ0) and between 0.8 and 0.4×10−12 s for the free rotations (τ1). This implies that the free rotations involving orientational changes are strongly damped, whereas the librations are strongly damped near the melting point at 257 K and somewhat better defined near the plastic–crystalline phase transition at 140 K. Both motions are of such a short duration that the compounded motion may rather well be approximated by the Langevin model for rotation. In this model a friction coefficient is the only disposable parameter. In the liquid phase at 265 K the free rotations appear even more damped, τ1∼0.2×10−12 s, whereas the librations seem to persist slightly longer than just below the melting point τ0∼1 ×10−12 s. This implies that these molecules show a tendency of more free rotations in the solid than in the liquid for states close to the melting temperature.
Translational spectroscopy of metastable fragments produced by dissociative excitation of chlorine
Authors: Wells W. C. , Zipf E. C.
Published: February 2003

Metastable chlorine atoms were observed produced by electron-impact dissociative excitational molecular chlorine. Fragments were identified, in part, as chlorine atoms in long-lived high-Rydberg states. (AIP)
The intrinsic viscosity of dilute polymer solutions
Authors: Bixon Mordechai
Published: February 2003

A general formal expression for the intrinsic viscosity of a dilute polymer solution is derived. The derivation is based on projecting the polymer dynamics onto the subspace of bilinear products of coordinates {xiyj}. It results in an expression composed of two terms. The first includes most of the low frequency behavior and the second all of the high frequency behavior of stiff polymers. The zero frequency limit and the Gaussian model are evaluated.
The intrinsic viscosity of a suspension of nonharmonic dumbbells
Authors: Bixon Mordechai , Kaldor Uzi
Published: February 2003

The general expression for the intrinsic viscosity of dilute polymer solutions, derived in the preceding paper, is applied to a nonharmonic dumbbell model. The limit of rigid dumbbell behavior is studied. Away from the rigid limit numerical results are obtained from power series expansion, combined with Padé approximants analysis.
Population of the vibrational levels of the ground state of nitrogen by low energy electron scattering
Authors: Potter J. E. , Steph N. C. , Dwivedi P. H. , Golden D. E.
Published: February 2003

Electron scattering from a Boltzmann distribution of N2 molecules at room temperature for a range of electron energies from 0.1 to 4.0 eV is shown under proper conditions to lead to various inverted vibrational population distributions in the N2 electronic ground state. The discussion of these considerations helps explain some discrepancies between observed and calculated total cross sections. In addition, these considerations lead to a description of the role of vibrationally excited N2 in the N2–CO2 laser.
Rovibronic interactions in the photoabsorption spectrum of molecular hydrogen and deuterium: An application of multichannel quantum defect methods
Authors: Jungen Ch. , Atabek O.
Published: February 2003

Multichannel quantum defect theory (MQDT) is applied to the treatment of electron motion in molecular Rydberg states and its coupling to the vibrational and rotational motions of the nuclei. The full rovibronic Hamiltonian is taken into account and the theory formulated in terms of parameters which relate to the standard Born–Oppenheimer (BO) theory. The development is an extension of Seaton’s quantum defect theory and relies on Fano’s frame transformation method. MQDT is not restricted to the BO approximation, but includes the so-called adiabatic corrections and nonadiabatic effects (i.e., vibronic coupling and rotational l uncoupling), however strong, without requiring the explicit evaluation of the interactions between individual levels. The general formalism can be simplified for the H2 and D2 molecules in the range below or near the ionization threshold and for vibrational excitation well below the dissociation energy. The only input data required are the potential energy curve of the ion core in its ground state and the ionization potential (both of which are accurately known), plus two BO potential functions of Σ and Π symmetry, respectively. The theory has been tested in detail using recent experimental and theoretical data on the first five Rydberg absorption transitions of H2 and D2. Theoretical rovibronic levels have been obtained for the B and C states over an extended range of vibrational and rotational quantum numbers on the basis of the best ab initio potential functions currently available. They are found to agree with the experimental levels to within a few cm−1 in all cases, and represent a considerable improvement over previous calculations carried out in the BO and adiabatic approximations with the same potential curves. Conversely, MQDT has been employed to extract BO curves of comparable accuracy for the B′, D, and B″ states directly from the observed levels, without prior evaluation of molecular constants. The higher vibrational levels of the B″ and D states are subject to numerous perturbations by levels belonging to higher Rydberg states: the MQD calculation accounts for these as well, and predicts accurately all Rydberg levels with their manifold interactions, up to the ionization limit. The detailed discussion of these results is deferred, along with the extension to other molecules and to higher energies, into ranges where rotational, vibrational, or electronic preionization, predissociation, or dissociative ionization become important.
Electron spin resonance study of x-irradiated single crystals of dimethyl sulfone
Authors: Andersen Roy S.
Published: February 2003

The ESR spectra of x-irradiated single crystals of dimethyl sulfone, O2S(CH3)2 have been examined at temperatures from 77 to 353 K. Between 77–153 K the spectra are dominated by an isotropic quartet of lines of intensity ratio 1:3:3:1, centered at g=2.0026±0.0001, with a hyperfine interval of 20.9±0.2 G. This spectrum originates from a methyl radical formed by the breaking of an S–C bond. On warming, the intensity of the methyl quartet decreased markedly at 163 K, being replaced by a singlet signal after several irreversible radical conversions had taken place. This singlet, which was stable from 173 to 333 K, is characterized by g values 2.0022, 2.0061, and 2.0067±0.0001. No hyperfine structure was observed. This spectrum is attributed to the O2ṠCH3 radical. The g=2.0022 is oriented perpendicular to the –SO2 plane, as expected for an unpaired electron principally located in a 3p (π) orbital on the S atom. The other g values are symmetrically located with respect to the two S–O bonds and lie on the SO2 plane. This indicates that the remaining S–C bond, on warming, reorients itself into the SO2 plane. Observation of the 33S spectrum confirms this interpretation. Spin densities of the unpaired electron on the S atom are ρ3s=0.074 and ρ3p=0.33.
The fluorescence of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in nonpolar organic liquids for excitation energies below and above the photoionization threshold
Authors: Wu Kam-chu , Lipsky Sanford
Published: February 2003

The fluorescence from TMPD has been studied in the solvents n-hexane, methylcyclohexane, cyclohexane, isooctane, and tetramethylsilane as a function of excitation wavelength from λex =300 nm to λex?170 nm. In no solvent is there observed any apparent decline in the fluorescence yield Φf for excitation energies to ?0.7–0.9 eV above the photoionization threshold. At higher energies Φf declines slightly. However, in the presence of low concentrations of either carbon tetrachloride, chloroform, ethylbromide, or n-perfluoroheptane there is observed an abrupt increase in the quenching efficiency of the TMPD fluorescence almost precisely at an excitation energy equal to the threshold energy for photoionization εt. The quenchable species Se generated at εt is found to have the following properties: (a) the probability of its quenching follows the usual Stern–Volmer form characteristic of the quenching of a spatially coherent excited state and not of a geminate ion pair; (b) the quantum yield ηe for the production of Se in isooctane exhibits two maxima at εex=5.64eV (ηe=0.41) and εex=7.1 eV (ηe=0.66) and appears generally to be relatively insensitive to the nature of the solvent; (c) the quenching constant of Se exhibits a strong variability with the nature of the quencher and of the solvent which is qualitatively similar to the variability of the reaction rate constant of a ’’quasifree’’ electron with these quenchers in these solvents; (d) the quenching constant of Se continuously changes as εex increases above εt but the quenching probability continues to retain the Stern–Volmer form. All of these properties are shown to be consistent with the identification of Se as a diffuse but coherent excited state of TMPD and as the precursor (rather than a geminate ion pair) of the ’’escaped’’ electron observed in photoionization experiments.
Operator formalism for double quantum NMR
Authors: Vega S. , Pines A.
Published: February 2003

An operator formalism is presented which conveniently treats the interaction of a spin-1 nucleus with a weak radio frequency field. The Hamiltonian in the rotating frame is ℋ=−Δω Iz−ω1Ix+(1/3) ωQ[3 I2z−I (I+1)], where Δω is the resonance offset (Δω=ω0−ω), ω1 is the intensity of the rf field, and ωQ is the quadrupolar splitting. Nine fictitious spin−1/2 operators, Ip,i where p=x,y,z and i=1,2,3, are defined where p refers to the transition between two of the levels and i the Cartesian component. The operators, which are the generators of the group SU (3), satisfy spin-1/2 commutation relations [Ip,j, Ip,k]=i Ip,l, where j,k,l=1,2,3 or cyclic permutation. Thus each p defines a three-dimensional space termed p space. For irradiation near one of the quadrupolar satellites, for example, Δω=ωQ+δω with δω, ω1≪ωQ, it is shown that the effective Hamiltonian can be written ℋ?−δωIx,3−√2 ω1Ix,1, i.e., a fictitious spin-1/2 Hamiltonian in x space with effective magnetogyric ratio γ along the 3 (resonance offset) axis and √2 γ along the 1 (rf field) axis. For irradiation near the center we can effect double quantum transitions between m=±1. The formalism allows us to write the effective operators for these transitions. For example, if we take Δω=δω again with δω, ω1≪ωQ we find the effective double quantum (DQ) Hamiltonian H?−2 δω Iz,1−(ω21/ωQ) Iz,3. Thus the z space is referred to as the double quantum frame with effective magnetogyric ratio 2γ along the 1 (resonance offset) axis and (ω1/ωQ) γ along the 3 (rf field) axis. The limiting expressions are compared with exact calculations for arbitrary ω1 done by high speed computer. The theory is applied to various cases of irradiation including our previously reported technique of Fourier transform double quantum NMR. Various pulse sequences for preparing, storing, and maintaining the evolution of double quantum coherence are analyzed for single crystal and polycrystalline samples. Finally, the effects of rf phase on the double quantum phase are presented briefly, and the possibility of double quantum spin locking is analyzed.
Praseodymium centers in cesium bromide
Authors: Radhakrishna S. , Sivasankar V. S.
Published: February 2003

Optical absorption of praseodymium-doped cesium bromide has been studied at 300, 77, and 4.2 K. Intraconfigurational f→f transitions have been observed. An analysis of these transitions indicates that each free-ion level of the 4f2 configuration splits into (2J+1) Stark components; this implies that the symmetry of the crystal field at the impurity ion site is C2v or lower. Fluorescence emission spectrum excited by a xenon lamp is recorded at 4.2 K using a modified Cary spectrophotometer. The absorption and emission data are used together to calculate the values of the Slater integrals and the spin–orbit constant. The results are compared with those reported in the case of other host systems.
Experimental studies of mercury molecules
Authors: Drullinger R. E. , Hessel M. M. , Smith E. W.
Published: February 2003

Optically excited fluorescence spectra in pure mercury vapor have been studied over the spectral range 240–600 nm for temperatures between 400–1000 K and densities between 5×1016–2×1019 cm−3. Absorption measurements were made over the spectral range 253–334 nm, and both structured and continuum bands were observed. Several types of two photon experiments were also performed in order to probe the excited states of the mercury dimer. In addition, the mercury spectrum from mercury vapor–noble gas mixtures has also been studied for noble gas pressures up to 1 atm.
Broad band absorption of n-propylbenzene at various temperatures
Authors: McLellan C. K. , Walker S.
Published: February 2003

A study has been undertaken of the far-infrared absorption of pure n-propylbenzene in liquid, supercooled liquid, and solid phases. Broad band absorption is observed in the normal liquid. In the supercooled liquid the ’’molecular’’ mean relaxation time is very much longer than at room temperature. However, the supercooled liquid at 142 K still shows the existence of a broad band absorption near 85 cm−1 frequency very similar in appearance to that of the normal liquid at 203 K. The intensity of the lower-frequency side of the broad band does not appear to have been reduced at low temperatures. The experimental data indicate that broad band absorption is not merely a high-frequency component of the molecular relaxation process but arises from some separate type of motion. For the alkylbenzenes around room temperature, the microwave and far-infrared data could be accounted for by the overlap of a molecular relaxation process with broad band absorption.
Raman spectra and internal rotation of 1,1-dimethylcyclopropane and 1,1-dimethyl-d6-cyclopropane
Authors: Bush S. F. , Dixon P. W. , Harris W. C.
Published: February 2003

The Raman spectra of gaseous and crystalline 1,1-dimethylcyclopropane and 1,1-dimethyl-d6-cyclopropane are reported for the frequency region between 100 and 600 cm−1. Assignment of the Raman shifts at 251 and 228 cm−1 for solid 1,1-dimethylcyclopropane to the b2 and a2 torsional modes is supported by the spectral data recorded for the deuterium analog. In addition, the 2←0 transition for the b2 torsional mode is observed at 460 and 462 cm−1 in the spectra of gaseous and liquid 1,1-dimethylcyclopropane. Whereas the internal rotational barrier for solid 1,1-dimethylcyclopropane is calculated to be 3.7 kcal/mole on the basis of the low temperature frequency data, it is estimated the barrier is on the order of 3.2 kcal/mole for the gaseous state. The internal rotational parameters determined for solid and gaseous 1,1-dimethylcyclopropane are compared with barrier heights determined previously for molecules with C(sp2)–C(sp3) and C(sp3)–C(sp3) single bonds.
An analytically soluble model for the penetration of a two-dimensional barrier: Quantal and semiclassical treatment
Authors: Massmann H. , Möhring K. , Ring P.
Published: February 2003

The reflection and transmission of a wave on a two-dimensional barrier is studied quantum mechanically and semiclassically. The simple model consists of two degrees of freedom. One of them contains a barrier (of the Eckart type); the second allows only a bound motion in discrete quantum states. These two degrees of freedom are coupled. The transition probabilities for transitions between the different quantum states can, in the model considered here, be evaluated analytically, both quantum mechanically and semiclassically. Several of the so called uniform semiclassical approximations and also the initial value integral representation for the semiclassical transition amplitudes are considered and compared to the exact quantum mechanical results. Some new insight on the semiclassical treatment of the tunneling process is gained, specially concerning the proper choice of the integration path for the initial value integral representation of the transition amplitudes.
Diagrammatic perturbation theory: Many-body effects in the X1Σ+ states of first-row and second-row diatomic hydrides
Authors: Wilson Stephen , Silver David M.
Published: February 2003

Diagrammatic many-body perturbation theory is employed in a study of the X1Σ+ states oif first- and second-row diatomic hydrides at their respective equilibrium nuclear separations. All two-, three-, and four-body terms are determined through third-order in the energy within the algebraic approximation (i.e., parameterization of state functions by expansion in a finite basis). Padé approximants to the energy are constructed. From the first-order wavefunction rigorous upper bounds to the expectation value of the electronic energy are obtained from the Rayleigh quotient. Two different zero-order Hamiltonians are used, and the convergence properties of the resulting perturbation expansions are compared. In both schemes three- and four-body terms are significant, having a magnitude that is as much as 24% of the sum of the second- and third-order terms.
Product distributions for the ion–molecule reactions N++O2 and N3++O2
Authors: McCrumb J. L. , Warneck P.
Published: February 2003

The title reactions were studied using a drift tube–mass spectrometer apparatus featuring preselection of injected primary ions. Measurements of the reduced mobilities of N+ and N3+ indicate experimental conditions such that the energies of the reacting ions are close to thermal (300 K). Rate coefficients obtained for the title reactions are k3=7.5×10−10 cm3 sec−1 for the reaction N++O2 and k6=7.7×10−11 cm3/sec−1 for the reaction N3++O2, in good agreement with literature values. Several other rate coefficients for reactions of N+, N2+, and N4+ ions are also in agreement with previous data. Altogether seven reactions had to be taken into account to derive the product distributions for the title reactions. The resulting product yields are 0.45, 0.41, 0.09 for the products O2+, NO+, and O+, respectively, from the N++O2 reaction; and 0.49, 0.65 for the products NO+ and NO2+, respectively, from the N3++O2 reactions.
The optical absorption spectrum of stoichiometric neodymium nitrate hexahydrate
Authors: Caro P. , Svoronos D.-R. , Antic E. , Quarton M.
Published: February 2003

The optical absorption spectrum of single crystals of neodymium nitrate hexahydrated with the stoichiometric composition (that is, without analytical excess of nitrogen) was recorded at 4.2 K between 10 000 and 33 000 cm−1. The theoretical analysis was undertaken through a computer program which takes into account free-atom classical operators, including two- and three-body operators, as well as a crystal-field operator written in the approximate C3v symmetry of the truncated icosahedron which makes the tencorners coordination polyhedron. The Hamiltonian was applied on the basis of the 364 ‖SLJMJ〉 kets in the 4f3 configuration, that is, including J mixing. The experimental sequence of 98 Stark levels from 28 2 S+1LJ levels was reproduced with a mean square deviation of 30 cm−1 out of an 18 adjustable parameters fit (12 free-atom parameters and six crystal-field ones).
Electron spin resonance study of the radical Ph2Ṡb trapped in a single crystal matrix
Authors: Geoffroy M. , Ginet L. , Lucken E. A. C.
Published: February 2003

A radical formed by x-irradiation of a single crystal of triphenylantimony is identified as Ph2Ṡb. Its ESR spectral parameters (for 121Sb) g1=1.9804, g2=2.1044, g3=1.9959, Tx=1270 MHz, Ty=−752 MHz, Tz=−699 MHz corrected for nuclear spin–orbit interactions and decomposed into isotropic (−60.3 MHz) and anisotropic (1330.3, −691.7, −638.7 MHz) components, and the nuclear quadrupole coupling constants e2Qqx=−101 MHz, e2Qqy=−205 MHz, e2Qqz=306 MHz, have been interpreted in terms of the radicals’s electronic structure and indicate that, like the analogous Ph2Ṗ and Ph2Ȧs radicals, the unpaired electron is localized on the 5pπ valence orbital of the antimony atom and that the radical is strongly bent, ∠CSbC≃98°.
Velocity distribution of H(2s) resulting from the electron impact dissociation of H2
Authors: Hazi Andrew U. , Wiemers Karyn
Published: February 2003

We have calculated the velocity distribution of the fast, metastable hydrogen and deuterium atoms which result from the dissociation of the 1Πu(2pπu, 2sσg) autoionizing states of H2 and D2. The present model uses the previously computed potential energy curve and autoionization width of this doubly excited 1Πu state. The model treats the competition between ionization and dissociation into neutral fragments classically. In calculating the laboratory velocity distribution we take into account the thermal motion of the target molecules and the recoil due to the scattered electron. The effect of momentum transfer is important, as the calculated laboratory speed distribution depends sensitively on the angle of detection. The comparison of the experimental and the theoretical velocity distributions shows a significant discrepancy among the three measurements which have been reported to date.
Configuration interaction calculations of the satellite peaks associated with C
l
s ionization of carbon monoxide
Authors: Guest M. F. , Rodwell W. R. , Darko T. , Hillier I. H. , Kendrick J.
Published: February 2003

Large scale configuration interaction calculations of the first five 2Σ+ carbon 1s core hole states of CO are described. Configuration selection, by means of perturbation theory, from ≳30 000 bonded functions yields final transition energies in good agreement with experiment. The inadequacy of a CI expansion including only singly excited configurations from the RHF reference configuration, is shown. A small CI expansion, using selected configurations, yields calculated intensities sufficiently accurate to allow an assignment of the experimental spectrum which is not possible from the calculated transition energies alone.
Shear transformation to produce a new phase of polymeric sulfur nitride (SN)x
Authors: Baughman R. H. , Apgar P. A. , Chance R. R. , MacDiarmid A. G. , Garito A. F.
Published: February 2003

The solid-state synthesis of a new phase of (SN)x is reported. X-ray diffraction results show that mechanical shear induces a transformation of the known, monoclinic phase (space group P21/c) to a phase of orthorhombic symmetry(probable space group P212121). The unit cell parameters of the shear-induced phase of (SN)x are a=6.251, b=4.429, and c=4.807 Å. Since the approximate chain repeat dimension (b) and the chain symmetry (21) are the same before and after the transformation, the chain geometries for the two phases of (SN)x are predicted to be quite similar, if not identical. However, important differences are expected for interchain interactions that have been previously shown to be essential in understanding various properties of the monoclinic phase of (SN)x. Both phases of (SN)x are shown to be stable with respect to thermally-induced interconversion for temperatures up to 175°C. Molecular packing calculations are used to predict structural aspects of the shear-induced phase and the relationship between parent and daughter phases. The latter results are consistent with the monoclinic-to-orthorhombic transformation being martensitic in character.
’’Free’’ O–H overtone absorption of methanols in the visible region by thermal lensing spectroscopy
Authors: Swofford Robert L. , Long M. E. , Burberry M. S. , Albrecht A. C.
Published: February 2003

Absorption spectra of CH3OH, CD3OH, CH3OD, and CD3OD in the region 15 800–17 400 cm−1 are reported as seen by thermal lensing spectroscopy. Dilution studies of the protonated methanols reveal a relatively narrow (FWHM=200 cm−1) absorption peak at 16 500±10 cm−1, which is assigned to the fourth overtone (v=5) of the nonhydrogen-bonded O–H stretching vibration. The energy of this overtone is in good agreement with the predictions made by a simple one dimensional anharmonic oscillator model.
Fluorescence from S2 single vibronic levels in thiophosgene vapor: Quantum yields and quenching effects
Authors: Oka T. , Knight A. R. , Steer R. P.
Published: February 2003

The quantum yields of fluorescence from the second excited singlet state of thiophosgene vapor have been measured as a function of pressure for eight S2←S0 transitions, and parameters for the decay of the levels pumped have been calculated. The results indicate that reassignment of the origin of the S2←S0 band system to 35 112 from 36 007 cm−1 is necessary. Quantum yields of fluorescence from the lowest levels in S2 are extremely high (0.50 to 1.0) and are indicative of a very slow radiationless decay rate. Weakening of fluorescence from higher S2 vibrational levels is most likely caused by predissociation.
Semiphenomenological model for the lipid bilayer phase transition: Finite chains in three dimensions
Authors: Priest Richard G.
Published: February 2003

The exactly solvable model of the Nagle is reinterpreted and is used to form the basis of a Landau phenomenology of phospholipid bilayers with finite hydrocarbon chains. The properties of the model depend on the chain length. The results of the model calculation are in agreement with important experimental trends. It is found that infinite chain bilayers in the reinterpreted model should have transition temperatures of about 700 °K, while very short chain bilayers may have no phase transition.
Vibrational population distributions of ground state barium oxide formed in the Ba+O2 and Ba+N2O reactions
Authors: Revelli Michael A. , Wicke Brian G. , Harris David O.
Published: February 2003

The relative ground state vibrational population distributions of BaO formed in the chemical reactions Ba+O2 and Ba+N2O have been measured as a function of total pressure between 0.3 and 10 torr using the technique of dye laser induced photoluminescence. Interpretation of these results together with previously reported excited state vibrational distribution measurements by Jones and Broida for the Ba+N2O reaction suggest that a population inversion does exist between A 1Σ+(v′=1) and X 1Σ+(v″=7)BaO at ?16 torr.
Low temperature heat capacity, dielectric constant, and thermodynamic properties of solid perchloryl fluoride
Authors: Mountfield K. R. , Weir R. D.
Published: February 2003

The heat capacity of solid perchloryl fluoride, ClO3F, has been measured over the range 2.7 K<T<23 K with an estimated precision of ±1%, except below 4 K, where it becomes ±2%. Analysis of the data shows that only lattice vibrations are contributing to the heat capacity below 12.5 K and that a Schottky anomaly is not present down to 2.7 K. ΘD(T) curves have been drawn based on different sizes of vibrational unit which allows the coarse features of the lattice frequency spectrum to be determined. The dielectric constant ε of ClO3F has been measured over the range 4.2 K<T<160 K with an estimated precision of ±1%. A rise in ε is observed between 80 K and the melting point at 125.64 K. This behavior is discussed in terms of molar polarization. A correlation of residual entropy with the Raman spectrum of polycrystalline ClO3F indicates a partially ordered crystal at 0 K.
14N NQR study of molecular reorientation and phase transition in s-triazine
Authors: Zussman A. , Oron M.
Published: February 2003

14N NQR frequencies, linewidth, and spin–lattice relaxation times were measured in s-triazine as a function of temperature between 77 and 280 °K. The second order phase transition from hexagonal to monoclinic which takes place at the critical temperature Tc=198.8 °K was studied in detail. The splittings of the NQR lines in the monoclinic phase, which are proportional to the order parameter, were found to vanish with the critical exponent β=1/2 on approaching Tc. Above 210 °K the temperature dependence of the linewidth and of the spin–lattice relaxation time indicates that the s-triazine molecule undergoes molecular reorientation around its threefold axis. The activation enthalpy for this motion was found to be ΔH=14.0 kcal/mole. These relaxation phenomena were analyzed using a density matrix formalism. Explicit expressions for the decay exponents and signal function were derived.