bluuebunny/crossref_metadata_2025 huggingface dataset:
Doublet→quartet and doublet→doublet electronic transitions in NO2 by electron impact
Authors: Rianda R. , Frueholz R. P. , Kuppermann A.
Published: February 2003

The electron-impact energy-loss spectrum of nitrogen dioxide (NO2) has been measured at impact energies of 25, 50, and 75 eV, and scattering angles varying from 5° to 80°. A previously unreported spin-forbidden doublet→quartet transition was observed at 4.49 eV, in excellent agreement with theoretical calculations. Doublet→doublet transitions were observed at 2.95, 5.81, 7.48, 8.64, 9.69, 10.52, 10.68, 10.94, and 11.20 eV, in agreement with previous experimental and theoretical work. In addition, numerous doublet→doublet transitions to superexcited states were observed.
Motion of wave packets in regular and chaotic systems
Authors: Moiseyev Nimrod , Peres Asher
Published: February 2003

The quantum energy spectrum of a system which is classically integrable consists of families of nearly equidistant levels. There is no such regularity for a classically chaotic system. As a consequence, a small wave packet, initially centered in a regular region of phase space, will slowly disperse while following the (almost) periodic classical trajectory. A wave packet placed in a chaotic region will disperse much more rapidly. These predictions are illustrated by calculating the evolution of two wave packets with the same mean energy in the Hénon–Heiles model.
The heat of dissociation of H5+ derived from measurements of ion mobilities
Authors: Elford M. T.
Published: February 2003

The mobility of hydrogen ions in hydrogen at 240, 274, and 295 K has been measured over the E/N range 0.5–10 Td and at gas number densities in the range 8×1016–5×1018 cm−3 using a variable length drift tube of the four gauze type. The absolute error and statistical scatter in the mobility values are estimated to be <±0.5% and <±0.15%, respectively. The equilibrium constant for the reactions H3++H2+H2⇄H5++H2 was obtained by fitting to the variation of the measured mobility with gas number density at low E/N values. Checks were made to ensure the reactions were in equilibrium in the drift space. The derived equilibrium constants and the heat of dissociation 250 meV were found to agree with those of Elford and Milloy and to disagree with those of previous workers. Possible causes for the disagreement are discussed. The zero field reduced mobility of a number of impurity ions in H2 at 293 K was measured. The values were as follows (in cm2 s−1 V−1), CH5+ (11.8±0.1), H3O+ (10.5±0.1), N2H+ (12.1±0.1), and H5O2+ (11.0±0.1).
Quantum studies of inelastic collisions of NO(X 2Π) with Ar
Authors: Orlikowski Tadeusz , Alexander Millard H.
Published: February 2003

Coupled-states (CS) and close-coupling (CC) calculations for collisions of NO(X 2Π) with Ar are reported, based on the potential surface of Nielson, Parker, and Pack [J. Chem. Phys. 66, 1396 (1977)]. The CS approximation is shown to be accurate and was used to generate converged cross sections at total energies of 0.035, 0.0555, and 0.0708 eV. Comparison with CS cross sections, obtained by neglecting the spin and electronic orbital angular momentum of the molecule, indicate that this neglect can introduce substantial error. At low J, the calculated cross sections are unaffected by inclusion of the spin-rotation coupling terms in the molecular Hamiltonian. Both for transitions within the Ω=1/2 spin-orbit manifold and for Ω=1/2→3/2 transitions we find good qualitative agreement with the relative J→J′ cross sections, summed over the Λ doublets, determined experimentally by Joswig and Andresen [J. Chem. Phys. 77, 2204 (1982), and to be published]. In particular the Ω=1/2→3/2 cross sections are about an order of magnitude smaller than the cross sections for transitions within the Ω=1/2 manifold and also show an increase with increasing ΔJ, contrary to what is usually seen in rotationally inelastic collisions. For transitions within the Ω=1/2 and Ω=3/2 manifolds, the sudden limit propensity rules and scaling relations derived earlier [J. Chem. Phys. 76, 5974 (1982)] are validated, and a new propensity rule established. By contrast, the sudden limit propensity rules and scaling relations appear to be of poor accuracy for transitions between the two spin-orbit manifolds.
On the time required to reach fully developed flow in pulsed supersonic free jets
Authors: Saenger Katherine L. , Fenn John B.
Published: February 2003

A new and simple model is presented for estimating the minimum pulse duration (‘‘valve open time’’) required for a pulsed nozzle to produce a supersonic beam comparably ‘‘cold’’ to that obtained from a continuous source. The model is based on the notion that cooling will be complete if the leading edge of the beamlet reaches the terminal temperature prescribed by the steady state flow equations before the sound wave created at the back of the pulse by the nozzle’s closing catches up with the leading edge. The times predicted from this model for reasonable stagnation conditions are all ≲10 μs.
Variational transition state theory calculations for an atom–radical reaction with no saddle point: O+OH
Authors: Rai Sachchida N. , Truhlar Donald G.
Published: February 2003

We have extended the general polyatomic canonical variational theory formalism of Isaacson and one of the authors to improved canonical and microcanonical variational theory. We have calculated the rate constants for the reaction in the title over the temperature range 200–2500 K using all three variational theories and the Melius–Blint ab initio potential energy surface. The results are compared to canonical variational calculations based on the reaction-path interpolation scheme of Quack and Troe, to the trajectory calculations of Miller, and to experiment. We find that the microcanonical variational transition states have a strong energy dependence and the generalized free energy of activation curves have two maxima. Quantization effects appear to be important at the lower temperatures, and recrossing effects may be important at higher temperatures.
Intersystem crossings of intermediate-case molecules
Authors: Takahashi K.
Published: February 2003

Collision-induced intramolecular nonradiative transitions through vibronic coupling are calculated by the resolvent method for medium-size molecules (intermediate case). The time-dependent perturbation theory is treated by an inverse Laplace transformation of the resolvent (i ‖ [E−H]−1 ‖ i). The poles and residues of the resolvent are obtained by the Baker–Sasakawa (BS), Brillouin–Wigner (BW), and Rayleigh–Schrödinger (RS) perturbation expansion methods, with the BS method giving the best values. A collision-induced stochastic perturbation method is introduced for obtaining the imaginary part of the eigenvalues, i.e., the line breadth of vibronic levels which indicates the occurrence of vibrational or rotational relaxations in the final electronic state. The calculated value of the 1A″→3A″ intersystem crossing rate of propynal shows good agreement with the experimental value. The calculated damping pattern of the excited state shows oscillatory behavior. The present theory affords an explanation for intramolecular multifrequency quantum beats.
Calculation of NMR chemical shifts. IV. Hydronium and hydroxyl ions
Authors: Fukui H. , Miura K. , Tada F.
Published: February 2003

Magnetic shielding constants are calculated for the hydrogens and the oxygens in H2O, H3O+, and OH−. Calculated shieldings of protons show that the proton resonance absorption in hydronium ion would be displaced downfield about 9 ppm from the gaseous water proton absorption as a standard and that the hydroxyl proton resonance would be shifted about 12 ppm to high field from the same standard. On the other hand, calculated oxygen shieldings show that the 17O NMR absorption in H3O+ would be in a higher field than that of the water oxygen signal and that the oxygen resonance of OH− would be in a lower field. The calculated results for the oxygen shielding changes are opposite to the theoretical predictions for the protons. These theoretical shieldings are compared with the experimentally determined shifts.
Exact-exchange extension of the local-spin-density approximation in atoms: Calculation of total energies and electron affinities
Authors: Baroni Stefano , Tuncel Eda
Published: February 2003

Total energies and electron affinities of selected atoms are calculated in a local-spin-density scheme where the exchange is treated exactly. A much improved overall description of the electronic structure is obtained with respect to the usual Hartree–Fock and local-spin-density approximations. The joint use of the exact expression for the exchange and of the local approximation for the correlation gives much better total energies and allows to avoid the failures of the above two schemes in the calculation of negative-ion binding energies. The accuracy for total energies of first and second row atoms is better than 2 eV, while the typical error for electron affinities is 0.2 eV.
Generalized quantum Liouville equation: Its solution by wave packet dynamics
Authors: Coalson Rob D. , Karplus Martin
Published: February 2003

A generalized quantum Liouville equation of the form iℏ(∂ρ̂/∂t)=Ĥuρ̂−ρ̂Ĥl for the density operator  ρ̂(t) is introduced; the quantum mechanical Hamiltonians Ĥu and Ĥl are, in general, different operators. Such an equation is of interest for a variety of problems involving systems at finite temperatures, including the calculation of vibrational and electronic spectra of molecules that are initially distributed over a range of eigenstates. It is shown that the generalized Liouville equation can be solved by exploiting its equivalence to a time-dependent Schrödinger equation in the coordinate space representation. In particular, this equivalence makes it possible to utilize techniques of Schrödinger wave packet propagation to compute the time evolution of the desired operator. Application of Gaussian wave packet dynamics and its extensions is considered and shown to be justified when the coordinate representation of  ρ̂(t) remains essentially Gaussian throughout the course of the relevant dynamics. As an illustration of the method, the finite temperature electronic absorption of a model molecule with two degrees of freedom is calculated. Other applications of the method are discussed.
An energy-density equation for isoelectronic changes in atoms as revealed from the Z dependence of the electrostatic potential at the nucleus for isoelectronic series of atoms
Authors: Teruya Hirohide , Anno Tosinobu
Published: February 2003

It is shown that a density functional ∫[ρ(r,Z)/ r]dr for an atom can be established from the data on the electrostatic potential at the nucleus V0,A for series of ions isoelectronic to that atom. The actual form has been obtained for the 2–10 electron atoms in their ground configurations within the Hartree–Fock level of approximation. For the He isoelectronic series, a similar attempt has been made by using the V0,A data in which the electron correlation is taken into account. It has been demonstrated that the resulting formula reproduces the difference in the total energy between two members of an isoelectronic series very well through the Hellmann–Feynman theorem, even when the difference in the atomic number is large. Relevance of the present results to the Z expansion theory of the total electronic energy of atoms is also discussed.
Pressure and temperature dependence of the permittivity and density of 1,1-dimethoxy-2-propanone
Authors: Vij J. K.
Published: February 2003

The static permittivity and density of 1,1-dimethoxy-2-propanone have been measured in the temperature range −30 to 100 °C and at atmospheric pressure to 3.5 kbar. The dipole moment increases at atmospheric pressure from 2.75 D at −30 °C to 2.89 D at 99 °C and increases by 0.9% in the pressure range 0.001–3.5 kbar, when the density increases by about 20%. The results are interpreted in terms of the barrier to the free rotation of the methoxy groups and a reduction in the free energy difference with pressure between the conformers with a large and a small dipole moment.
Axial–biaxial phase transition in discotic liquid crystals, studied by deuterium NMR
Authors: Goldfarb D. , Belsky I. , Luz Z. , Zimmermann H.
Published: February 2003

Truxenehexaalkanoates (TxHAn) exhibit a variety of discotic mesophases including the sequence ND⇄Drd⇄Dho. The ordering characteristics of these phases were studied by deuterium NMR of deuterated solutes, in particular C6D6. The results indicate that during the phase transition the minor susceptibility axis of the Drd phase transforms into the director axis of the axial phases. It is suggested that in the Drd phase, in alternating columns the mesogen molecules are tilted with respect to the columns axes in opposite directions.
Resonance Raman excitation profiles of bacteriorhodopsin
Authors: Myers Anne B. , Harris Robert A. , Mathies Richard A.
Published: February 2003

We have measured resonance Raman excitation profiles for 12 vibrational modes of light-adapted bacteriorhodopsin using 27 excitation wavelengths ranging from 472 to 752 nm. The excitation profiles are quite similar and have approximately the shape and width of the absorption spectrum. Calculations using a 29-mode harmonic sum over states with large amounts of inhomogeneous broadening (Gaussian standard deviation 750 cm−1) yield good fits to the absorption and excitation profile band shapes and to the absolute absorption and Raman cross sections. These calculations also give nearly correct intensities for most of the observed overtone and combination bands. The absolute Raman cross sections allow us to differentiate between the homogeneous and inhomogeneous contributions to the spectral breadth. Our upper limit of 250 cm−1 for the homogeneous linewidth, as well as the absence of strong low-frequency Raman lines, implies that the lack of resolution in the bacteriorhodopsin absorption is due primarily to inhomogeneous effects. Finally, the permissible contribution of torsional vibrations to the spectral diffuseness and the consistency of these results with an excited state isomerization are explored using wave packet propagation on a linear excited state torsional potential.
The effect of composition on the noncoincidence of the isotropic and anisotropic Raman frequencies and its intrepretation by means of a simple dielectric model
Authors: Giorgini Maria Grazia , Fini Giancarlo , Mirone Paolo
Published: February 2003

The difference between the anisotropic and the isotropic frequencies of the C=O stretching bands of acetone, acetophenone, methylbenzoate, and dimethylformamide was measured in several mixtures of known dielectric constant. Its change with concentration can be explained by making the assumption that the interaction energy of the dissolved dipoles is described by the dielectric model of Onsager–Fröhlich over the whole composition range. The same model accounts for the change of the above difference in neat liquids with temperature.
Transient absorption and luminescence of rare gas crystals
Authors: Dössel O. , Nahme H. , Haensel R. , Schwentner N.
Published: February 2003

In time resolved luminescence spectra, taken within the temperature range of 11–80 K, the two lowest excited states of rare gas crystals are observed. One of them, the long lived 3Σ+u state is identified as the initial state for transient absorption. The transient absorption spectra (11–80 K) indicate strong similarities between self-trapped excitons in the crystals and free excimers, but only in the energy region up to 1.5 eV above the lowest excited state 3Σ+u. Higher energy levels and the continuum states of the self-trapped exciton are strongly influenced by solid state effects.
Theoretical study of the far infrared absorption spectrum of dense nitrogen
Authors: Guillot B. , Birnbaum G.
Published: February 2003

The far infrared absorption spectrum of dense nitrogen is calculated by assuming that the induction mechanism is quadrupolar and that the intermolecular potential is predominantly isotropic. The profile is then described as the convolution of a translational spectrum and a rotational spectrum. The latter expresses the free rotations of the molecules, whereas the former, calculated from the Zwanzig–Mori theory of Brownian motion, is characterized by two translational modes, a diffusive low frequency mode and an oscillatory high frequency mode. The translational spectrum is found to be responsible for the density dependence of the absorption spectrum, contributing significantly to the profile at high densities and for all frequencies. The parameters of the theory are analytically given and the influence on the profile of a small anisotropy in the intermolecular potential is estimated by a perturbation approach.
A quasiclassical trajectory analysis of the ionization-dissociation behavior of the van der Waals molecule ArH2
Authors: Hashim A. , Hutchinson John S. , Weiner E. R.
Published: February 2003

The ionic species ArH+2 may be formed in an ion–molecule collision or by ionization of the van der Waals molecule ArH2. The ion is unstable in either case, but its internal energy distribution and, hence, its dissociation behavior depends on the method of formation. A trajectory calculation is described which simulates the ionization-dissociation of ArH2, initially formed in a supersonic expansion. Cross sections for the possible dissociation channels and product energy distributions are obtained. The vibrational and rotational energy of H2 in the ArH2 precursor are determining factors in the dissociation behavior.
Selective vibrational excitation of the ethylene–fluorine reaction in a nitrogen matrix. II
Authors: Frei Heinz
Published: February 2003

The product branching between 1,2-difluoroethane and vinyl fluoride (plus HF) of the selective vibrationally stimulated reaction of molecular fluorine with C2H4 has been studied in a nitrogen matrix at 12 K and found to be the same for five different vibrational transitions of C2H4 between 1896 and 4209 cm−1. The HF/DF branching ratio of the reaction of F2 with CH2CD2, trans-CHDCHD, and cis-CHDCHD was determined to be 1.1, independent of precursor C2H2D2 isomer and particular mode which excited the reaction. These results, as well as the analysis of the mixtures of partially deuterated vinyl fluoride molecules produced by each C2H2D2 isomer indicate that the product branching occurs by αβ elimination of HF(DF) from a vibrationally excited, electronic ground state 1,2-difluoroethane intermediate. Selective vibrational excitation of fluorine reactions in isotopically mixed matrices t-CHDCHD/C2H4/F2/N2 and CH2CD2/C2H4/F2/N2, and in matrices C2H2/C2H4/F2/N2 revealed a high degree of isotopic and molecular selectivity. The extent to which intermolecular energy transfer occurred is qualitatively explained in terms of dipole coupled vibrational energy transfer. A study of the loss of absorbance of the C2H4 ⋅ F2 pairs in case of ν9 as a function of both the laser irradiation frequency within the absorption profile, and the ethylene concentration showed that the C2H4 ⋅ F2 absorption is inhomogeneously broadened. Substantial depletion of reactive pairs which did not absorb laser light is interpreted in terms of Forster transfer.
A CARS investigation of HBr and H2 collisions with hot H atoms produced by ArF laser photolysis of HBr
Authors: Quick C. R. , Moore David S.
Published: February 2003

Coherent anti-Stokes Raman spectroscopy is used to examine the rotational and vibrational state distributions in HBr and H2 molecules excited by collisions with translationally hot H atoms produced by ArF laser photolysis of HBr. CARS detection of Br atoms and of rovibrationally excited H2 produced by the H+HBr reaction is demonstrated for the first time.
Dynamics of liquid state chemical reactions: Photodissociation dynamics and geminate recombination of molecular iodine in liquid solution
Authors: Balk M. W. , Brooks C. L. , Adelman S. A.
Published: February 2003

The dynamics of photodissociation, cage breakout, and subsequent recombination of molecular iodine in model Lennard-Jones solvents designed to simulate liquid carbon tetrachloride and ethane is studied via classical stochastic trajectory simulations based on the molecular timescale generalized Langevin equation (MTGLE) of motion for liquid state chemical reaction dynamics [S. A. Adelman, J. Chem. Phys. 73, 3145 (1980)]. Also presented for comparison purposes are parallel trajectory studies based on a matrix Langevin equation characterized by friction coefficients which depend on the I2 internuclear separation R. These studies show that dynamical solvent effects arising from molecular timescale correlated solute/solvation shell motions play a basic role in the I2 photodynamics. The nature and magnitude of these dynamical solvent effects are governed by the MTGLE force constants, especially, the basic force constants ω2e0(R) and ω2c1(R). Dynamical solvent effects in particular: (i) can lead to the production of quasibound pairs of iodine atoms even if the static or mean I2 effective interaction is repulsive. The fundamental vibrational frequency of the quasibound pairs is governed by ω2e0(R) which determines the modification of the static effective interaction due to instantaneous elastic cage restoring forces exerted on the I2 molecule; (ii) largely determine the efficiency of cage breakout (and hence the nongeminate yield) which occurs during the initial stages of the photolysis process. This breakout efficiency depends on a competition between ω2e0(R) [large ω2e0(R) retards breakout] and ω2c1(R) [large ω2e0(R)promotes breakout]; (iii) determine, via the strength of solute-shell coupling as measured by ω2c1(R), the rate and qualitative character of the energy relaxation of highly vibrationally excited nascent I2 product. Moreover, the interplay between the efficiencies of the initial cage breakout step and the final vibrational stabilization step of the photolysis process determines the observable picosecond timescale transient electronic absorption spectrum. Because of this interplay, systems with very different underlying dynamics can exhibit similar transient absorption spectra (on the timescale of typical experiments). This is illustrated by the similarities in the absorption spectra of I2 in liquid CCl4 (high geminate yield and relatively inefficient vibrational stabilization) and I2 in liquid C2H6 (low geminate yield and relatively efficient vibrational stabilization). Finally, simplified methods to handle the complicating effects of nonlinear solvent response to very fast initial solute motion and of nonadiabatic motion involving several I2 electronic states are described.
Tetramethyldithiadiselenafulvalene (TMDTDSF): Properties of its hexafluorophosphate salt and alloys with tetramethyltetraselenafulvalene
Authors: Wudl F. , Walsh W. M. , Hauser J. J. , DiSalvo F. J. , Nalewajek D. , Rupp L. W. , Waszczak J. V. , Felder R. J. , Poehler T. O. , Chaikin P. M. , Lacoe R.
Published: February 2003

Preparation and some properties of tetramethyldithiadiselenafulvalene (TMDTDSF) are described. It forms the expected (donor)2 X salts. Properties of the PF−6 salt are described as well as properties of alloys of the title compound with tetramethyltetraselenafulvalene (TMTSF). (TMDTDSF)2PF6 is a relatively poor conductor even though it is isomorphous with the high-pressure superconductor (TMTSF)2PF6. The profound difference in properties between these two solids is ascribed to disorder in the arrangement of sulfur and selenium atoms along the donor stacks in (TMDTDSF)2PF6. Alloying very small amounts of TMDTDSF into (TMTSF)2PF6 causes the metal-to-insulator transition temperature (TMI) to move up smoothly from 11 K for pure (TMTSF)2PF6 to ∼200 K for pure (TMDTDSF)2PF6 due to increasing disorder as TMDTDSF is incorporated.
Response function theory and its application to polymer dynamics
Authors: Jhon M. S. , Dahler J. S.
Published: February 2003

A response theory is constructed for systems with deterministic, nonrandom dynamics which are not governed by Hamiltonian mechanics. This theory is used to compute the response of a polymeric solution (modeled as elastic dumbbells suspended in a Newtonian solvent) to a flow field. This externally imposed flow displaces the system from its initial state of equilibrium and causes each function of state to respond in a characteristic way. The response function of greatest interest is the fluid stress tensor. We present a correlation function expression for this tensor and derive Kubo-like formulas for a number of material functions, namely, the shear and extensional viscosities and the primary and secondary normal stress coefficients associated with laminar flows. Finally, an expression is obtained for the extra stress due to turbulent flow.
Frequency spectrum and dynamic correlations of concentrated polymer liquids
Authors: Ronca G.
Published: February 2003

Elastic interactions between polymer chains in concentrated liquids are incorporated into the equation of motion by adding a memory term whose kernel is related to the relaxation modulus of the system. Below a characteristic chain length mechanical and dynamical properties are predicted to approach Rouse-like behavior. For long chain systems, an approximate self-consistency condition is obtained for the dynamic viscosity in the form of a nonlinear integral equation. In this limit, the mechanical spectrum is shown to split into a molecular weight independent branch separated from terminal relaxations by a wide plateau region whose elastic modulus is calculated as a function of the same parameters commonly used for the Rouse characterization of low molecular weight polymers. Concentration dependences of plateau modulus, entanglement length, and transitional relaxation time are correctly predicted. Zero shear viscosity and terminal relaxation time are shown to scale with the same power of the molecular weight. Heuristic considerations on the viscosity exponent suggest a value of three. The calculated expression of the single chain scattering function in the transition region shows a continuous change from Rouse-like behavior to flat response with decreasing wave vector. Application to multichain scattering by concentrated solutions predicts long-time exponential behavior only above a critical scattering wavelength.
A photon correlation spectroscopy study of the dynamic behavior of poly(α-methylstyrene) in moderately concentrated solution
Authors: Selser J. C.
Published: February 2003

The dynamic behavior of a series of moderately concentrated solutions of a high molecular weight poly(α-methylstyrene) PAMS in toluene was investigated using photon correlation spectroscopy. The dependence of the shape of the scattered light temporal autocorrelation function on polymer concentration and scattering wave vector was carefully determined and the results did not agree with those predicted by scaling theories. Instead, the best description of the PCS results is provided by a new theory due to Ronca, which correctly predicts the general features of the observed dependence of the correlation function shape upon scattering wave vector and polymer concentration. While in the Ronca theory, entanglement behavior and its effect on the correlation function are treated expressly, current scaling theories do not consider entanglements in treating the dynamic behavior of moderately concentrated solutions. Considering the representative character of the solutions studied here, it is concluded that a successful description of the dynamic behavior of such systems requires consideration of entanglement behavior.
A phenomenological study of heterogeneous chemical reactions of mercuric chloride on heated stainless steel surfacesa)
Authors: Wang R.-G. , Dillon M. A. , Spence David
Published: February 2003

We have made the first measurements of the products of surface chemical reactions of mercuric chloride on heated stainless steel using the technique of electron-energy-loss spectroscopy as an analytic tool. We find that HgCl2 reacts with hydrogen in the stainless steel to form hydrochloric acid HCl. Under our experimental conditions HCl becomes the cominant species observed in our spectra at surface temperatures greater than about 150 °C. The possible consequences of our observation on the efficiency of mercuric halide lasers is discussed. (AIP)
ESR studies of Ba(ClO4)2⋅3H2O and Ba(ClO4)2⋅3D2O single crystals γ irradiated at 77 K
Authors: Sastry D. L. , Rama Rao K. V. S.
Published: February 2003

The paramagnetic defect centers observed in Ba(ClO4)2⋅3H2O and Ba(ClO4)2⋅3D2O single crystals γ irradiated at 77 K are identified as ClO3 and ClO42−. ClO42− occurs at three chemically inequivalent conformations. Two of them are metastable and occur at three magnetically inequivalent sites each. The third one is relatively more stable and occurs at only one site with axial symmetry. It is found that the ESR spectra of these centers could be well understood in terms of the (ClO3−, O−) model proposed for these centers in KClO4 by Byberg. The temperature up to which the metastable configurations of these radicals are stable has increased to 135 K in the hydrated crystals and to 183 K in the deuterated crystals when compared to a value of 100 K in KClO4. The observation of the ClO3 indicates that the radiation damage mechanisms in this lattice are essentially different from those in KClO4. These observations are attributed to the effect of waters of hydration in the present lattice. The orientation and the proton superhyperfine interaction of the (ClO3−, O−) radicals has also been discussed.
Polarization propagator approach to the dynamic nuclear electric shielding in LiH molecule
Authors: Lazzeretti P. , Rossi E. , Zanasi R.
Published: February 2003

The frequency dependence of the dipole electric polarizability and electric shielding tensors of hydrogen and lithium nuclei in the LiH molecule has been studied via some approximations to the polarization propagator method, i.e., STA, TDA, and RPA, allowing for dipole length, velocity, acceleration, and mixed formalisms. The RPA nuclear electric shieldings are in a fairly good agreement with the corresponding experimental data, a discrepancy being observed for the parallel component at the Li nucleus.
EXAFS study of bromomanganate ions in molten salts
Authors: Crozier E. D. , Alberding N. , Sundheim B. R.
Published: February 2003

Extended x-ray absorption fine structure spectra measurements were made on molten (Bu4N)MnBr3 to determine its coordination geometry which has remained in doubt because of its disproportionated structure in the solid form. Comparison with results on solid and molten (Bu4N)2MnBr4 imply a Br to Mn coordination in (Bu4N)MnBr3 of 3.14 ± 0.36 at an average distance of 2.46 ± 0.03 Å. It is concluded that the nominal MnBr3 moiety is, in fact, trigonal—a species previously unknown in liquids. The EXAFS of (Bu4N)2MnBr4 give a Br–Mn bond length of 2.50±0.02 Å when small corrections for an asymmetric pair distribution are included. The variation of the disorder parameter with temperature is in accord with measured asymmetric stretch frequencies of the MnBr= anion. No abrupt change in the disorder parameter of bond length occurs at the melting point. The near edge absorption spectra of both compounds are unexpectedly similar considering the difference in coordination numbers.
A new type of cluster expansion and time-dependent adiabatic perturbation theory on finite and infinite lattices
Authors: Briels W. J.
Published: February 2003

A new type of cluster expansion is proposed, which is used to calculate the properties of the lower part of the spectrum of a general lattice Hamiltonian. The method does not involve power series in the number of lattice sites. Surface effects are easily incorporated into the theory in a natural way. Application to the 1-d Ising model in a transverse magnetic field demonstrates that good results can be obtained with moderate computational effort.
Structure and intermolecular excitations of β-N2 at 400 MPa and 55 K
Authors: Powell B. M. , Dolling G. , Nieman H. F.
Published: February 2003

Elastic and inelastic neutron scattering measurements have been made on a single crystal of β-N2 at 55 K and a pressure of 400 MPa. The integrated intensities of 34 Bragg reflections were measured and analyzed in terms of symmetry-adapted harmonics. None of the coefficients of the higher-order harmonics could be determined and, within experimental error, the N2 molecule is randomly oriented. No well-defined librational modes were observed and even translational acoustic phonons are broad. The line shapes of translational acoustic phonons propagating along the hexagonal axis are analyzed in terms of both Gaussian and Lorentzian line shapes and in terms of the Michel–Naudts theory of rotation–translation interactions. In terms of the latter theory, β-N2 is in the slow relaxation regime and has a strong rotation–translation interaction. The temperature dependence observed for four of these line shapes suggests that the librational excitations are no longer overdamped at 46 K. Values for three of the elastic constants were determined and are (in units of 109 N m−2): C11=4.43±0.14, C33=5.92±0.14, and C44=0.61±0.02.
Surface spinodals and extended wetting in fluids and polymer solutions
Authors: Nakanishi Hisao , Pincus P.
Published: February 2003

A novel concept of ‘‘surface’’ spinodal decomposition is introduced in the context of wetting and prewetting first order transitions for simple fluids in contact with a bounding wall. Qualitative and semiquantitative discussions are made within the framework of a phenomenological Landau theoretical model for semi-infinite systems with a (short-ranged) wall potential. A concept of ‘‘extended wetting’’ is also proposed, whereby even for T<TW (the wetting temperature) a macroscopically thick ‘‘wetting’’ layer can form near the wall upon rapidly quenching the solution into the coexistence region. These ideas are carried over to polymer solutions and blends near their consolute points where the characteristic time scales should be more manageable.
Ab initio energy calculations for the intramolecular rotational potential of polyacetylene
Authors: Rao B. K. , Darsey J. A. , Kestner N. R.
Published: February 2003

Using ab initio methods, calculations have been performed on the rotational states of polyacetylene. Using a 4–31G basis set a gauche state is found involving a 40° rotation from the cis state. It is 0.6 kcal/mol lower in energy than the cis state. The trans state is the lowest energy state with an energy 2.60 kcal/mol lower than the gauche. Trans–cisoid to trans isomerization is proposed to occur by a rotation about the single C–C bonds in the backbone with a rotational barrier of 2.61 kcal/mol. The lack of sensitivity which arises when using a STO-3G poor basis set is also demonstrated.
A general method for approximating the electronic correlation energy in molecules and solids
Authors: Colle Renato , Salvetti Oriano
Published: February 2003

A detailed analysis of a method proposed earlier for calculating a good approximation to the electronic correlation energy is presented here. The principal advantages of this method compared to the usual CI techniques are discussed.
Electrohydrodynamic domain patterns in freely suspended layers of nematic liquid crystals with negative dielectric anisotropy
Authors: Faetti S. , Fronzoni L. , Rolla P. A.
Published: February 2003

The electrohydrodynamic domain patterns observed in freely suspended layers of nematic liquid crystals with negative dielectric anisotropy are investigated. Two different domain patterns, in low and high frequency electric fields, occur. Both the director lines and the hydrodynamic flow lines associated with these patterns are investigated in detail. The threshold voltage of the instability is measured as a function of thickness and distance between the electrodes. The experimental results agree with a two-dimensional theoretical approach based on the Carr–Helfrich’s model.
Quasiresonant excitation transfer in molecular aggregates
Authors: Parson R. P. , Kopelman R.
Published: February 2003

Excitation transfer in finite molecular aggregates is analyzed in the context of the Haken–Strobl model. Explicit solutions are presented for a trimer and a rectangular tetramer. Special emphasis is placed upon population transfer among subunits (monomers, dimers) and its relationship to energy transfer, and upon the problems associated with coherence of this transfer process. These aggregates serve as models for the problem of excitation transfer in disordered media, where partial coherence resulting from short-range interactions has been largely ignored. Our most intriguing result is the greatly diminished effectiveness of the longer-ranged transfer in the presence of short-range clusters. Under some conditions the ensuing energetic mismatches may well dominate the overall energy transport and render invalid the usual description in terms of hopping among individual sites. An application to triplet energy transport in isotopic mixed naphthalene crystals is given; it is seen that the reduced efficiency of non-nearest-neighbor transfer processes reinforces the two-dimensional characteristics of the energy transport.
Densities and partial molar volumes of supercooled aqueous solutions
Authors: Sorensen C. M.
Published: February 2003

Density measurements of aqueous solutions of ethanol, t-butanol, and hydrazine in the temperature range from 50 °C down to nucleation are presented. From these data, solute partial molar volumes and temperatures of maximum density are determined. The partial molar volumes of the alcohol solutions at low concentration become small below −30 °C, and the characteristic minimum in the partial molar volume vs concentration curve vanishes. The hydrazine solutions display similar behavior except no minimum is observed. It is also found that the temperature of maximum density washes out at finite concentrations and solutions with higher concentration do not seem to nucleate but may be forming glasses. These results are discussed in terms of a liquid clathrate structural model of water which becomes multiply connected to fill all space as the water is supercooled.
Fine-structure spectrum of O2–rare gas van der Waals molecules
Authors: van der Avoird Ad
Published: February 2003

A simple analytically solvable model is presented for the rotation-vibration-spin states in O2–rare gas atom complexes, which predicts the form of the fine-structure spectrum. It is shown that this spectrum looks basically different from the pure O2 spectrum, due to the partial quenching of the O2 angular momentum by the anisotropic O2-rare gas interaction potential. The model contains two properties of the O2–X complex as parameters: the ground state average of the anisotropic potential 〈V2(R)〉0 and the end-over-end rotational constant 〈R−2〉0, where R is the O2–X distance. Adjusting these parameters yields quantitative agreement with the spectra that have recently been obtained from two different empirical potentials via accurate dynamical calculations. The model can be used for interpreting the experimental fine-structure spectrum of O2Ar which is currently being measured.
Single crystal ESR studies of a nitroxide spin label. I. Determination of the G and A tensors
Authors: Tabak M. , Alonso A. , Nascimento O. R.
Published: February 2003

Experimental results are reported for the G tensor and the nitrogen hyperfine A tensor of a stable nitroxide radical 4-hydroxi-2,2,6,6- tetramethyl-piperidine-1-oxyl (TANOL), introduced as impurity in a matrix of known structure, 4-hydroxi-2,2,6,6-tetramethyl-piperidine, gxx=2.0099, gyy=2.0061, gzz=2.0024, Axx=5.3 G, Ayy=7.0 G, Azz=35.0 G. Due to the perturbation in to the crystal structure introduced by TANOL it was not possible to determine the orientation of the tensors relative to the molecular bonds but only relative the crystal axes. The presence of hydrogen bonds as well as the crystal packing both strongly influence the orientation of the G and A tensor. The value of the nitrogen hyperfine tensor was determined for the first time for this nitroxide and correlates well with the hydrogen bonding of the NO fragment.
Detection of the ν2 bands of CD2 and CH2 by infrared diode laser spectroscopy
Authors: McKellar A. R. W. , Yamada Chikashi , Hirota Eizi
Published: February 2003

Five transitions in the ν2 fundamental band of the CD2 radical have been detected in the 707 to 718 cm−1 region using a tunable diode laser spectrometer and Zeeman modulation. The CD2 transitions were of the form N0,N←N1,N−1 with N=3 to 7. Two transitions of CH2 were also observed around 892 cm−1. Analysis of the CD2 results yields the ν2 band origin as well as the rotational constant 1/2(B+C) and the spin–spin interaction parameter D for the excited state. The data also provide the basis for a refined determination of the structure and potential parameters of methylene, and the experience gained here in the generation of long absorption paths of the molecule should prove useful in future experiments.
Effect of the relative velocity on the collision dipole in a pair of atoms
Authors: Humbert J. , Galatry L.
Published: February 2003

It is shown that the dispersive collision induced dipole in a pair of atoms is dependent on the relative velocity of the partners. Two formulations taking into account this effect are successively presented. In the first one, the origin of the induced dipole lies in the energy coupling between the fluctuating instantaneous dipole of an atom and the reaction field due to the presence of the neighbor; the effect of the relative velocity is introduced through a Fourier expansion of the time dependent internuclear distance. The second method is based on the consideration of the secular equation for approximate instantaneous normal modes of the whole system. In the two cases, the vanishing velocity limit allows to find again a known result. For a nonzero relative velocity, a limited expansion is obtained and the two lowest order terms are given. Order of magnitude of the effects are estimated in a model of harmonic oscillators.
Single collision ion–molecule reactions at thermal energy: Rotational and vibrational distributions from N++CO→N+CO+
Authors: Guyer Dean R. , Hüwel Lutz , Leone Stephen R.
Published: February 2003

A new apparatus is developed and used to obtain nascent vibrational and rotational distributions in the ground electronic state of CO+ formed from the charge transfer reaction N+(3P)+CO (X 1Σ+)→N(4S)+CO+ (X 2Σ+,v,J)+0.52 eV, at approximately thermal energies. The device utilizes a flow tube for the production of thermal N+ ions in a helium buffer and a large diameter sampling orifice which delivers the ions via a mild free jet expansion into a low pressure chamber. The expansion is crossed by a stream of reactant CO molecules and the CO+ product states are probed by laser-induced fluorescence. Although the energy available is sufficient to populate CO+ vibrational states up to v″=2, the major vibrational channel in the CO+ product is v″=0. The relative vibrational distribution is found to be: Nv=0≳0.81 (observed under single collision conditions), Nv=1<0.15 (not observed), and Nv=2≊0.04 (observed only under nonsingle collision conditions). The rotational distribution in the v″=0 state is characterized closely by a Boltzmann distribution with a temperature of 410±50 K. This represents a fractional energy disposal into rotation of only 2%. Nearly all of the reaction exothermicity is therefore released into translational recoil. These results are considered in terms of simple dynamical models of the charge transfer process.
Delayed fragmentation of acetonitrile upon short-pulse UV irradiation: Possible role of long-lived superexcited intermediates
Authors: Craig B. B. , Faust W. L. , Weiss R. G.
Published: February 2003

We have studied fragment emission in the spectral range 200–900 nm, following short-pulse UV excitation (25 ps, 266 nm, 0.2–10 mJ) of acetonitrile. The dominant components of emission are the CN B 2Σ+→X 2Σ+ violet system and the C2 d 3Πg→a 3Πu Swan system. Emissive CH and NH are also observed. Each of these diatomics is produced with a pressure-dependent rate. An instrumental risetime of 0.6 ns enabled study of the rise and fall of violet and of Swan emission over the pressure range 0.27–10 Torr. The formation kinetics suggest that there are long-lived intermediates to the violet and Swan emissive populations. These precursors are not observed in emission. They have zero-pressure lifetimes of 20 to 30 ns; in collisions with the parent they are removed with large bimolecular rate constants (≳10−9 cm3 s−1). With the use of isotopically labeled acetonitrile, we have established that major fractions of CN B 2Σ+ and of C2 d 3Πg are formed intramolecularly. Further, two channels of intramolecular CN B 2Σ+ production are recognized. They share a common risetime, which may indicate a common precursor. The preferred channel retains the CN association of the parent. In the other channel, the carbon atom originates in the methyl group of the parent, suggesting rearrangement within an intermediate. We believe that the precursors are superexcited or highly excited species C2HnN** (0≤n≤3).
The equation of state of molecular hydrogen at very high densitya)
Authors: Ross M. , Ree F. H. , Young D. A.
Published: February 2003

In the preceding paper new shock-wave results were reported on liquid hydrogen and deuterium up to a pressure of 76 GPa (760 kbar). In the present paper an effective pair potential is determined from these results and used in calculations of fluid and solid isotherms, melting curves, and the metallic transition pressure. The agreement with available data is good and the metallic transition is predicted to be near 300 GPa (3 Mbar).
Crystal structures of solid bromoform
Authors: Myers R. , Torrie B. H. , Powell B. M.
Published: February 2003

The crystal structures of the three solid phases of deuterated bromoform have been determined using the neutron powder profile refinement technique. At 273 K, there are two molecules per unit cell in a hexagonal structure with space group P3̄. The molecules are statistically disordered to give a pseudosymmetry with space group P63/m. Rapid cooling of the sample freezes out the dynamic disorder and results in a metastable structure with trigonal space group P3̄, in which a given molecule is surrounded by molecules of the opposite orientation with C–D bonds in the c direction. This structure can be viewed as being made up of molecular bilayers parallel to the basal plane with the deuterium atoms in the interior of a bilayer. The transformation from this metastable trigonal phase to the stable phase at low temperatures involves mainly a translational motion of one bilayer with respect to the neighboring bilayers. Assuming threefold molecular symmetry, this structure has been determined at 14 and 220 K. It is found to be triclinic (space group P1̄) with two molecules per unit cell.
Orientational ordering, site structure, and dynamics for octahedral molecules in low temperature matrices: SF6 and SeF6 in rare gas solids
Authors: Jones Llewellyn H. , Swanson Basil I.
Published: February 2003

From polarization studies of high resolution IR spectra of SF6 and SeF6 trapped in noble gas solids we show that much of the structure observed for the stretching mode represents site symmetry split components for low symmetry trapping sites, the triply-degenerate ν3 mode being split into a doubly- and singly-degenerate mode. Most of the sites showing polarization are orientationally ordered with the singly-degenerate component perpendicular to the substrate. We attribute the driving force for ordering to guest–host interaction potentials which result in registry between the molecules and the (111) growth plane during deposition. The observed orientational ordering combined with high temperature annealing studies has allowed the identification of the symmetry of certain trapping sites and further analysis of vibrational dephasing dynamics. Several sites with the same nominal symmetry and structure can be tracked through the matrices discussed herein. The implications of the ordering of impurity structure in a host lattice formed by vapor deposition are discussed.