bluuebunny/crossref_metadata_2025 huggingface dataset:
Pressure effects on the Raman spectra of phospholipid membranes: Pressure induced phase transitions and structural changes in 1,2-dimyristoyl 3-s
n-phosphatidylcholine water dispersions
Authors: Wong P. T. T. , Murphy W. F. , Mantsch H. H.
Published: February 2003

In a study of the pressure dependence of the Raman spectrum of an aqueous dispersion of dimyristoyl phosophatidylcholine from ambient pressure up to 5 kbar, three phase transitions have been observed at 150 bar and at 1 and 2.6 kbar. From the behavior of the spectrum, a correlation may be made between these phase transitions and temperature-induced changes observed at 24, 14, and −60 °C, respectively. The major cause of the pressure-induced frequency shift of all the Raman bands is the pressure-enhanced interchain anharmonic interaction rather than the pressure-induced compression of intramolecular bonds. It is further shown that the large angle reorientational fluctuations about the long axes of the acyl chains decrease with increasing pressure.
The energy dependence of the forward and backward rate coefficients for clustering reactions of Li+ ions with Ar, Kr, and Xe
Authors: Takebe M. , Satoh Y. , Iinuma K. , Seto K.
Published: February 2003

The energy dependence of the clustering reactions of Li+ ions with Ar, Kr, and Xe has been measured in the pressure range 2–4 Torr using a drift tube without mass spectrometer. The energy (center of mass) of the Li+ ions was varied over the range 41–67 meV in Ar, 43–129 meV in Kr, and 48–207 meV in Xe. The energy of the cluster ions was varied over the range 40–44 meV in Ar, 40–48 meV in Kr, and 40–43 meV in Xe. All the gases were maintained at room temperture. It is found that the forward rate coefficients are inversely proportional to the ion energy in all cases. The backward rate coefficients increase rapidly with increasing cluster ion energy. Arrhenius plots of the backward rate coefficients gives the activation energy as 0.34 eV in Ar, 0.45 eV in Kr, and 0.49 eV in Xe.
‖m
l‖ changing in collisions between xenon Rydberg atoms and CO2
Authors: Slusher M. P. , Higgs C. , Smith K. A. , Dunning F. B. , Stebbings R. F.
Published: February 2003

Thermal energy collisions between xenon Rydberg atoms in n = 31, l≳3, ‖ml‖≳3 states, and CO2 are reported. The rate constant is derived for those collisions which result in a change in ‖ml‖. The value so obtained is similar to that obtained earlier for Xe(nf)–CO2 collisions resulting in a change in l. It is argued that these observations imply that collisions result in simultaneous changes in l and ‖ml‖.
Quenching and vibrational energy transfer in Li2(B 1Πu)
Authors: Dubreuil B. , Chaleard C.
Published: February 2003

Radiative lifetimes, quenching, and excitation transfer in the v′ = 0, 1, 2, 3, 4 vibrational levels of Li2(B 1Πu) are studied by means of time resolved laser spectroscopy in a heat pipe type oven. Radiative lifetimes in the range 8.35–8.6 ns compare well with recent theoretical results. Quenching and excitation transfers towards neighboring vibrational levels are mainly due to collision with Li atoms with rate coefficients in the range 1.9–5.6 10−9 s−1 cm3. These values are two orders of magnitude larger than the rate coefficients for quenching and exitation transfers induced by neon atoms. These results can be used in the rate equations of the optically pumped Li2 laser.
The f–f radiationless transitions in lanthanide complexes
Authors: Strek W.
Published: February 2003

A general theory of the f–f radiationless transitions in lanthanide complexes is developed within the framework of the dynamic coupling model. The two different mechanisms are discussed: the static mechanism linked with the metal bond involving vibrations as the promoting modes and the dynamic mechanism linked with the internal ligand vibrations as the promoting modes. It is found that the electronic factors for the f–f radiationless transitions may be calculated as a sum of three products of the unit tensor operators Uλ and the parameters η̄λ independent of the nature of the electronic states involved in the transition. An approximate expression for the rate constant is derived and possible application to the quantitative analysis of the f–f radiationless transitions is discussed.
Collisional excitation of CO2 by N2, O2, and Ar
Authors: Ryali S. B. , Fenn J. B. , Kolb C. E. , Silver J. A.
Published: February 2003

We have measured cross sections for the T-V excitation of the asymmetric mode of ground state CO2 molecules in collision with N2, O2, and Ar molecules at relative velocities in the range from 2 to 5 km/s. Excitation was brought about by interacting two slightly collimated molecular beams in the field of view of an InSb photovoltaic detector which monitored resulting radiation in the 4.3 μm band. The excitation cross sections for all three combinations of partners were of similar magnitude and had a strong dependence upon collision velocity. In the case of N2–CO2, for which we have the most data, the measured cross sections range from about 4 × 10−19 cm2 at 2 km/s to about 1 × 10−17 cm2 at 5 km/s.
Translational energies of fragment ions in the multiphoton ionization of benzene
Authors: Carney Thomas E. , Baer Tomas
Published: February 2003

The average kinetic energies of the frament ions C+, C+2, C3H+3, and C4H+3 formed in the multiphoton ionization of benzene have been measured from the fragment ion time of flight distributions. The results indicate that the distributions of kinetic energies are characteristic of thermal ones and that these four fragments are formed with initial average translational energies of 0.26, 0.19, 0.13, and 0.13 eV, respectively. These energies are considerably lower than those expected on the basis of the statistical theory calculations, which were calculated for a number of different dissociation mechanisms. It is shown that the fragment ion kinetic energies can be a sensitive probe for evaluating reaction mechanisms.
Rotationally inelastic collisions between a diatomic molecule in a2Π electronic state and a structureless target
Authors: Alexander Millard H.
Published: February 2003

We present the full close-coupling formulation of the collision between a diatomic molecule in a 2Π state in the Hund’s case (a) limit and a structureless target. Due to the possibility of transitions between spin-orbit and/or Λ-doubling states the scattering displays an additional degree of complexity not seen in rotationally inelastic collisions of 1Σ+ molecules. The well-known coupled states and infinite-order-sudden (IOS) approximation techniques can be applied in a straightforward manner. The factorization and scaling relations between the various cross sections, which are valid in the energy sudden limit, are explored. For transitions within either spin-orbit manifold (Ω = 1/2, Ω = 3/2), these scaling relations allow both matrices of cross sections, for parity conserving the parity violating transitions, to be expressed in terms of the cross sections for parity conserving transitions out of the Ω = 1/2, J = 1/2 level. Under conditions in which either a Born or sudden formulation of the collision dynamics is appropriate, we show that at large values of the total angular momentum, transitions which conserve the parity index of the molecular wave functions will be strongly favored, a propensity rule which has been seen in previous experimental studies of rotational relaxation in 2Π molecules. A scaling relation is also derived for the cross sections for transitions between the two spin-orbit manifolds. For these processes, however, the propensity toward conservation of the parity index only occurs in the case of transitions which are elastic in the total angular momentum. The IOS formulation of the collision dynamics is then extended to a Hund’s case (b) representation of the molecular wave function. Although a complete factorization is no longer possible, one can still show that at large J there will exist a strong propensity toward conservation of the alignment between S (the spin-angular momentum of the molecule) and N (the vector sum of the nuclear rotational angular momentum R and the z-component Λ of the electronic orbital angular momentum). In the case (b) limit a propensity toward conservation of the parity index will still exist, but less strongly than in the case (a) limit.
On the nature of the bonding in Cu2
Authors: Bauschlicher Charles W. , Walch Stephen P. , Siegbahn Per E. M.
Published: February 2003

The ground state of Cu2 is found to arise from the 2S(4s13d10) +2S(4s13d10) limit and to involve a 4s–4s sigma bond pair. The dominant bond pair correlations are left–right and angular, with the former lengthening the bond and the latter contracting the bond, so that at the two-electron MCSCF level the Re is slightly longer (0.02a0) than at the SCF level. Correlation of the 3d electrons shortens the bond by 0.19a0, leading to a final bond length of 4.35a0, which is 0.15a0 longer than experiment. This error is of the same magnitude as twice the relativistic contraction of the 4s orbital of the 2S state of the Cu atom (0.13a0) and most of the remaining error in Re is thought to be due to this relativistic contraction.
Electronic structure and chemical bonding in metallic clusters of binary and ternary transition metal chalcogenides. I. SCF MS Xα study including relativistic effect of PbMo6S8
Authors: Le Beuze L. , Makhyoun M. A. , Lissillour R. , Chermette H.
Published: February 2003

Cluster calculations of the electronic structure and charge distribution of PbMo6S8 have been performed using the SCF Xα scattered-wave method. These results compare fairly well with those obtained with solid state methods and ionization energies are in agreement with XPS experimental spectra. Moreover, the chemical bonding, particulary the interactions between the Mo6 metallic clusters with its chalcogenide neighboring, is discussed by comparing the results obtained for the Mo6 and (Mo6S8)2− units. These main features are also illustrated by using both the partial wave population analysis and contour map of some bonding molecular orbitals of the valence band. The importance of relativistic effects on such metal 4d compounds is discussed.
Triplet exciton transfer and fusion in isotopically mixed molecular crystals
Authors: Brown R. , Lemaistre J.-P. , Megel J. , Pee Ph. , Dupuy F. , Kottis Ph.
Published: February 2003

We present experimental results on triplet energy transfer from traps to super-traps and on triplet exciton fusion in substitutionally disordered naphthalene crystals, monitored by the phosphorescence and by the delayed fluorescence of the traps and of the super-traps. The concentration dependence of these emissions at fixed temperatures has been discussed abundantly by other authors. In this work, we present new results on their dependence on temperature and on the source of excitation, which have, respectively, been little or not at all reported on before. Comparison of experiments done with selective and with nonselective excitation of the traps shows that the results depend on the method of preparation of the trap excitons. The existence of threshold temperatures for these emissions is interpreted as evidence of phonon assisted processes, for which mechanisms are proposed. A significant drop in transport efficiency at high temperatures is observed for trap concentrations from 4% to 8%. This drop with trap concentration is discussed in relation to the random distribution of traps and to the likely importance of clusters of ’’conducting’’ sites for energy transport in random systems.
Triplet state Stark spectroscopy above the phosphorescence origin of 2,4,5-trimethylbenzaldehyde
Authors: Winkler I. C. , Hanson D. M.
Published: February 2003

The electric field effect on several of the bands in the phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde, isolated in a durene single crystal at 4.2 K, was measured. While the weak origin of the first triplet state at 4140 Å was sufficiently narrow (∼1 cm−1) to resolve the Stark splitting, the intense bands in the region previously assigned to the second triplet state near 4060 Å were broad (∼10 cm−1). A selective laser excitation technique was developed to determine the Stark splitting of these bands. It was found that the most intense peaks in the 4060 Å region have the same dipole moment difference vectors, within the measurement uncertainties, as the T1 origin. This result is unexpected since the first triplet state is known to be ππ* in character, while the second is thought to be nπ*. An explanation of these observations in terms of coupling of the T2 (nπ*) state to a set of nearby T1(ππ*) vibronic states is proposed.
Theory of ESR parallel-edge lines of slowly tumbling molecules
Authors: Kivelson Daniel , Lee Sook
Published: February 2003

For each hyperfine component we have examined the sharp feature, called the ’’parallel-edge line,’’ which is observed at the limit of the ESR rigid powder and slow-tumbling spectra associated with those molecules in a liquid which have their symmetry axes oriented nearly parallel to that of the applied field. The parallel-edge linewidths are well determined in the second derivative ESR spectra. The motional broadening and shift of the parallel-edge lines which arise from the rotational modulation of the anisotropic molecular magnetic parameters can be separated from motionally narrowed inhomogeneous effects by taking the difference of parallel-edge linewidths and shifts of two hyperfine components. These differences in linewidths and shifts of two parallel-edge hyperfine lines are predicted to vary as τ2−1/2, where τ2 is the rotational correlation time. An analysis of the parallel-edge lines enables us to estimate slow tumbling times from ESR spectra without carrying out full-scale line shape simulations, which require an exact knowledge of motionally narrowed inhomogeneous linewidths.
Importance of initial and final states as intermediate states in two-photon spectroscopy of polar molecules
Authors: Dick Bernhard , Hohlneicher Georg
Published: February 2003

Two-photon transitions may proceed via a mechanism not involving virtually excited intermediate states. For some examples, it is shown numerically that this mechanism may account for the largest contribution to the two-photon cross section in polar molecules, if the dipole moment strongly alters with excitation. Possible consequences for the two-photon detection of ’’forbidden’’ states in polar polyene systems are discussed.
Raman study of the temperature and pressure dependence of vibrational relaxation in propyne
Authors: Zerda T. W. , Perry S. , Jonas J.
Published: February 2003

The Raman isotropic band shapes of the ν3 (C≡C stretch) vibration of propyne complexed with acetone have been measured as a function of temperature and pressure. The experimentally determined correlation functions 〈ω(0)ω(t)〉 have been calculated. It is shown that these functions have a long-time tail which is negative. This effect is explained in terms of a slowly relaxing rotational motion which causes the environment of the scattering propyne molecule to change on a time scale comparable to the vibrational relaxation time. The short-time portion of the 〈ω(0)ω(t)〉 function is shown to be dominated by collisional effects. The isotropic band shapes of the ν3 vibration have also been measured for solutions of propyne in inert solvents (tetramethylsilane and carbon disulfide). The analysis of the band shapes suggests that, in these cases, the band broadening is in the ’’motionally narrowed’’ limit.
Photoacoustic study of a UV initiated solid-state polymerization
Authors: Prock A. , Shand M. L. , Chance R. R.
Published: February 2003

We report a detailed photoacoustic study of a highly exothermic solid-state reaction, the photopolymerization of diacetylene crystals. Both the amplitude and the phase shift of the photoacoustic signal are monitored during the course of the polymerization at various UV excitation wavelengths and various chopping frequencies. A one-dimensional heat-diffusion model employing impedance boundary conditions is shown to fit the experimental data quite adequately. We conclude from the theoretical interpretation of the results that the quantum yield for photopolymerization is very high (∼100) and that the polymer acoustically couples much more effectively to the gas than does the monomer.
Na2(A 1Σ+u→X 1Σ+g) fluorescence accompanied by a continuous spectrum
Authors: Katô Hajime , Matsui Tooru , Noda Chifuru
Published: February 2003

A suitably adjusted single-mode line of the krypton ion laser at 5682 Å has been found to excite Na2 to the single level A 1Σ+u(v′ = 34, J′ = 50). Then, a series of resonance fluorescence to different vibrational levels of the ground state X 1Σ+g have been observed up to the point of convergence. A continuous spectrum with banded diffraction structure has been contiguous to it and extended toward the small wave number region. The spectrum has been quantitatively accounted for by quantal calculations.
Resonant nonlinear optical mixing using the phase conjugate configuration: Spectroscopic studies
Authors: Souma H. , Heilweil E. J. , Hochstrasser R. M.
Published: February 2003

We introduce the phase conjugate configuration for the study of ultrashort relaxation processes of the excited states of molecules having complex electronic spectra. This three beam configuration allowed us to distinguish between the optical and thermal portions of the four-wave nonlinear mixing in the frequency domain, and the typical characteristic features of the third-order nonlinear susceptibilities for all of the interesting difference frequency regions. The dispersive characteristics of the third-order nonlinear susceptibility, χ(3)(ωi = 2ω1−ω2) were interpreted by using a three-level model including spectral cross relaxation with which we discuss the various relaxation pathways and rates associated with the excited electronic states of cresyl violet and rhodamine 6G in ethanol.
Statistics of polymer chains embedded on a diamond lattice. II. Results for arbitrary t
r
a
n
s and g
a
u
c
h
e weightings
Authors: Schroll W. K. , Walker A. B. , Thorpe M. F.
Published: February 2003

A polymer of finite length is embedded on a diamond lattice where the angle between adjacent monomers is cos−1(−1/3) = 109°. We set up a transfer matrix formulation and show how the characteristic function Cn(k) can be expressed in terms of the eigenvalues and eigenvectors of the transfer matrix. Results are presented for chains of various lengths and for different trans and gauche weightings. The results are particularly interesting and simple in the stiff chain limit, where the chains are shown to obey a scaling relation.
Radiative rate of the 6 2D3/2 state of atomic bismuth
Authors: Patel D. , Pritt A. T. , Coombe R. D.
Published: February 2003

Experimental measurements of a high resolution absorption spectrum of the 6 2S3/2–6 2D3/2 transition in bismuth and the radiative lifetime of the 6 2D3/2 state are reported.
Mobilities and collision cross sections of Cl− ⋅ nH2O ions in argon, krypton, and xenon
Authors: Jówko Antoni , Armstrong David A.
Published: February 2003

Clustered chloride ions of the formula Cl− ⋅ nH2O, with n predominantly 3 or 4, have been made in argon, krypton, and xenon, and their mobilities measured by a drift technique as a function of inert gas pressure over the range 25 to 1000 Torr. Mobilities at 296 K and inert gas densities of 2.69×1019 molecule cm−3 were: 2.03, 1.26, and 0.81 cm2 V−1 s−1 in argon, krypton, and xenon, respectively. Comparison with theory shows that a significant inverse sixth power attractive potential is required to explain the magnitudes of the collision cross sections.
A hard-disk system in a narrow box. I. Thermodynamic properties
Authors: Wojciechowski K. W. , Pierański P. , Mal/ecki J.
Published: February 2003

The partition function and thermodynamic properties of a system of hard discs (of diameter σ) in a periodic box of width D⩽√3̄σ was calculated analytically.
Quantum stochasticity and unimolecular decay
Authors: Shapiro M. , Child M. S.
Published: February 2003

The origin of quantum stochasticity is studied via a simple model of decaying coupled anharmonic oscillators. It is shown that the decay rates of individual eigenstates, as well as mixed states, are never stochastic, even in regions where classical stochasticity exists. However, decay rates of (coherent) wave packets do show a transitional behavior, similar to the classical one, as the energy is raised. The transition is shown to arise from a competition between two rates, the dissociation rate, depending on the wave function amplitudes, and the dephasing rate, depending on the energy spectrum, and to occur when the dephasing becomes faster than the dissociation. It is suggested that only properties which depend simultaneously on the wave function-amplitude and the energy spectrum can display this transitional behavior. It is also shown that strong coupling is not a necessary condition for stochasticity.
Cluster theory for concentration dependence of shear viscosity for suspensions of interacting spheres. II. Calculation of Huggins coefficient
Authors: Muthukumar M. , Freed Karl F.
Published: February 2003

Explicit calculations are presented for the Huggins coefficient for the viscosity of suspensions of hard spheres using the formal results of part I. The calculations are presented to make contact with previous calculations of the Huggins coefficient for hard spheres, and to develop the mathematical methods necessary to evaluate the integrals appearing in our cluster theory for the higher order terms in powers of concentration of the viscosity of suspensions. Our hard sphere Huggins coefficient calculations agree with previous works.
Electrostatic contribution to the persistence length of a polyelectrolyte
Authors: Le Bret Marc
Published: February 2003

The electrostatic contribution to the persistence length is computed through the integration of the average electrostatic potential at the surface of a toroidal polyion over its charge parameter, when specific interaction between the mobile ions of the salt and the fixed charges of the polyion are ignored. When the radius of curvature of the polyion is very large, we assumed that the electrostatic potential may be developed in series as a function of the curvature. With this assumption, it may be computed from one-dimensional integrations only. The assumption is then numerically verified by performing a painstaking two-dimensional integration for finite values of the radius of curvature. Values for the electrostatic contribution to the persistence length are given in two limit cases: in the presence of an excess of added salt and in the total absence of added salt. The role of the dielectric constant and of the mobility of the ’’fixed’’ charges of the polyion is also determined.
Low temperature specific heat of metal-rich alkali metal oxides
Authors: Gmelin E. , Simon A. , Brämer W. , Villar R.
Published: February 2003

The specific heats of the metallic oxides Cs11O3, Cs7O ( =̂ Cs10Cs11O3) and Rb7Cs11O3 have been measured in the temperature range of 0.50⩽T⩽100 K. The results are in agreement with a bond model of ionic clusters of the composition Cs11O3 which are weakly coupled to one another and to the matrix of additional alkali metal atoms via metallic bonding. The Debye temperatures are extremely low [θ0(Cs11O3) = 41 K, θ0 (Cs7O) = 45 K, and θ0 (Rb7Cs11O3) = 47 K]. Anomalies in the lattice specific heat at low temperatures (T⩽5 K) are interpreted as due to low frequency vibrations of the heavy Cs11O3 clusters (molecular weight m=1510 g mol−1), corresponding to a characteristic Debye temperature of θD≃4 K.
A molecular theory of the solid–liquid interface. II. Study of bcc crystal–melt interfaces
Authors: Oxtoby David W. , Haymet A. D. J.
Published: February 2003

We have extended and applied our previously developed order parameter theory for the solid–liquid interface [J. Chem. Phys. 74, 2559 (1981)]. We show how the differential equations for interfacial structure from our earlier work may be reformulated in terms of a variational principle and demonstrate that the quantity minimized gives the surface free energy. We point out a useful analogy between the order parameter equations and the equations of classical dynamics. We carry out explicit calculations of the interfacial structure of the bcc 100 and 111 crystal–melt interfaces and obtain estimates of the solid–liquid interfacial free energy for sodium and potassium; the free energies differ by less than 3% for the two different faces of each crystal. Our calculations suggest the existence of a ’’structured liquid’’ region in the interface, characterized by a liquidlike average density, but solidlike lattice ordering. The interfacial transition zone is quite broad, 16–20 Å in width and comprising 10–15 layers. We compare our results to computer simulation data and discuss their implications for microscopic mechanisms of crystal nucleation and growth from the melt.
Generalized van der Waals theories and the asymptotic form of the density profile of a liquid–vapor interface
Authors: Barker J. A. , Henderson J. R.
Published: February 2003

Generalized van der Waals theories for the free energy and density profile of an inhomogeneous fluid are used to obtain an expression for the tails of the density profile of a liquid–vapor interface of an argon-like fluid. We find that the density profile decays asymptotically into the bulk regions as the inverse cubed power of the distance from the center of the interface. We estimate the effect to be too small to be quantitatively significant.
First order corrections to the correlation functions of charged hard spheres
Authors: Ciccariello S. , Gazzillo D.
Published: February 2003

Radial distribution functions of the most typical symmetrical and unsymmetrical restricted primitive models have been evaluated by taking into account the first order corrections in their renormalized cluster expansions. The linear approximation turns out satisfactory at small densities, while the exponential one is very poor in the same cases where HNC fails. (AIP)
An XPS study of the UV reduction and photochromism of MoO3 and WO3
Authors: Fleisch T. H. , Mains G. J.
Published: February 2003

Crystalline powder pellets of MoO3 and WO3 were irradiated with 253.7 nm radiation at room temperature in vacuum, in the presence of H2 and in the presence of Hg vapor and H2. Electronic changes induced in the surface of the solids were studied by x-ray photoelectron spectroscopy. In vacuo both MoO3 and WO3 turned blue upon irradiation and the XPS spectra supported the reversible formation of +5 oxidation states which reverted upon exposure to air. When MoO3 was irradiated in the presence of H2, a hydrogen/molybdenum bronze containing Mo in the +6, +5, and +4 oxidation states and hydroxyl groups were irreversibly formed. Exposure to UV radiation in the presence of Hg vapor and H2 enhanced the formation of Mo+4 and hydroxyl groups. Irradiation of WO3 in the presence of H2 and Hg vapor/H2 mixtures produced no further reduction of tungsten compared to irradiation alone. The conclusions based upon core electron emissions from molybdenum, tungsten, and oxygen were supported by changes in the valence band spectra.
New method of characterizing slow-tumbling motions for nitroxide spin labels/probes from second-derivative ESR powder spectra
Authors: Lee Sook , Ames D. P. , Brown I. M.
Published: February 2003

A simple, direct method is proposed for characterizing the extent of slow-tumbling molecular motions of nitroxide spin labels or spin probes in amorphous substances. This method essentially consists of determining accurate values for the ’’single-crystal,’’ first-derivative peak-to-peak linewidths of nitroxides from their second-derivative hyperfine power spectra and hence employing the dependence of the linewidths on the nitrogen nuclear-spin-magnetic quantum number M resulting from molecular motion.
G-sum rule separation of overlapping vibrational bands
Authors: Brown Deborah Illman , Bruns Roy E.
Published: February 2003

The G intensity sum rule is used to determine a separation ratio of 1.52:10 for the ν2, ν5 overlapped band of CHF3 and to resolve the sign ambiguities in the ∂p/∂Qi values of both CHF3 and CDF3. The method can be applied to molecules for which one isotopic species contains overlapping bands corresponding to normal modes of different symmetry whereas another species is free from such separation problems. This technique should prove useful as a check on computer simulation band separations and in cases where experimental separation is not possible.
Franck–Condon approach to collisional dephasing of spectral lines in liquids
Authors: Grimbert Danielle , Mukamel Shaul
Published: February 2003

We consider the broadening of spectral lines in liquids at low temperatures. To lowest order in density, the relevant microscopic information is given by the spectral function J(ω) which is related to the single perturber spectrum. The line shape may then be calculated using J(ω). We have calculated J(ω) both classically and quantum mechanically for a collinear model and show quantitatively how different they are for typical experiments in cryogenic liquids. The absorption line shape I(Δ) is then evaluated using J(ω) and we discuss how sensitive it is to the details of J(ω). This gives us an estimate of the microscopic information content of collisionally broadened line shapes.
Photoionization of argon clusters
Authors: Dehmer P. M. , Pratt S. T.
Published: February 2003

Argon clusters were produced in a free supersonic molecular beam expansion of pure argon at room temperature and the photoionization efficiency curves of the trimer through hexamer were measured in the wavelength regions from threshold to 700 Å. A study of the Ar+3 photoionization efficiency curve as a function of nozzle stagnation pressure shows that fragmentation of heavier clusters can dominate the spectrum, even near threshold, and even when the nozzle conditions are such that the Ar+4 intensity is only a small fraction of the Ar+3 intensity. The Ar+3 photoionization efficiency curve, obtained using nozzle stagnation conditions such that no heavier ions were detected, exhibits several broad peaks near threshold which show similarities to bands of the dimer. At high nozzle stagnation pressures, the photoionization efficiency curves for Ar+3 to Ar+6 are nearly identical due to the effects of fragmentation. These spectra exhibit two very broad features which are similar to features observed in the solid. The threshold regions for all the positive ions show extremely gradual onsets, making it difficult to determine the appearance potentials accurately. The appearance potentials for Ar+2 and Ar+3 are 855.0±1.5 and 865.0±1.5 Å, respectively, yielding a value of 0.18±0.05 eV for the dissociation energy of Ar+3. The appearance potentials for the heavier clusters Ar+4 through Ar+6 are all approximately 870±2 Å.
Nuclear magnetic resonance spectroscopy in cholesteric liquid crystals. I. Orientational order parameter measurements
Authors: Shivaprakash N. C. , Prasad J. Shashidhara
Published: February 2003

Orientational order parameters for cholesteric liquid crystalline compounds—cholesteryl benzoate, methyl carbonate, ethyl carbonate, and laurate—have been estimated by studying the dipole–dipole splittings in the wide line NMR absorption spectra. The temperature dependences of the S factor agree closely with those of nematic liquid crystals.
Hyperthermal K–TeF6 molecular beam scattering
Authors: Wagner A. F. , Young C. E. , Pobo L. G. , Wexler S.
Published: February 2003

Angular distributions of K+ product ions from collisions of a beam of hyperthermal K atoms with a cross beam of thermal K–TeF6 molecules were determined at 13.7 and 23.7 eV (lab). The angular yields of K atom products from the same system were too low to permit measurement of angular distributions. From the integrated yields, the K+ ion/K atom branching ratio was determined to be greater than 103. In addition to the extremely large branching ratio, the differential cross sections exhibited several other unusual characteristics: (a) the lack of small angle scattering, corresponding to virtual absence of covalent scattering, (b) two peaks in the differential cross section with an outer rainbow feature at very large scattering angles (∼275  eV deg). The observations are unexpected from previous experimental and theoretical studies of electron transfer reactions and from the electronic and structural properties of TeF6 and TeF6−. A simplified dynamics model based on formation of electronically excited TeF6− in the initial electron transfer, followed by inner crossings leading to formation of electronically and vibrationally unexcited TeF6− or dissociation to TeF5− and other ionic products, has been developed which accounts for the experimental results. The model suggests that the observed two peaks in the differential cross section are due to the production of TeF6− (inner peak) or TeF5− and other ionic dissociation products (outer peak). The model also suggests that the observed branching ratio requires a vertical electron affinity of ⩽1.9 eV, much lower than its adiabatic electron affinity of 3.3 eV.
Radiative lifetimes and collisional energy transfer rate constants in Ar of the Ar(3p55p) and Ar(3p55p′) states
Authors: Inoue G. , Setser D. W. , Sadeghi N.
Published: February 2003

The radiative lifetimes and collisional deactivation rate constants of Ar[3p5(2P01/2)5p] and Ar[3p5(2P03/2)5p] levels with argon atoms have been measured by time resolved laser induced fluorescence combined with a pulsed discharge for preparation of populations in the Ar(4s) precursor levels. The radiative lifetimes of Ar(5p) levels are 100–140 ns, and their two-body deactivation rate constants are in the range of (1.2–5.6)×10−10 cm3 atom−1 s−1. About half of the two-body energy transfer is intramultiplet relaxation. Only about 20% of the Ar(5p) radiative decay is via 5p–4s transitions; the reminder is via transitions to the 5s and 3d states. The radiative cascading through the 5s and 3d states populates the 4p states with conservation of the inner core configuration. From the time dependence of the emission intensities from the 4p levels, the radiative lifetimes of the 5s and 3d states were estimated to be ∼50 ns.
Effect of ionic strength on the kinetics of ionic and micellar reactions in aqueous solution
Authors: Dung Mei Hsu , Kozak John J.
Published: February 2003

The effect of electrostatic forces on the rate of reaction between ions in aqueous solutions of intermediate ionic strength is studied in this paper. We consider the kinetics of reactions involving simple ionic species (1–1 and 2–2 electrolyte systems) as well as kinetic processes mediated by the presence of micellar ions (or other charged organizates). In the regime of ionic strength considered, dielectric saturation of the solvent in the vicinity of the reacting ions must be taken into account and this is done by introducing several models to describe the recovery of the solvent from saturation to its continuum dielectric behavior. To explore the effects of ion size, charge number, and ionic strength on the overall rate constant for the process considered, we couple the traditional theory of ionic reactions in aqueous solution with calculations of the electrostatic potential obtained via solution of the nonlinear Poisson–Boltzmann equation. The great flexibility of the nonlinear Poisson–Boltzmann theory allows us to explore quantitatively the influence of each of these effects, and our simulations show that the short-range properties of the electrostatic potential affect primarily kinetically controlled processes (to varying degrees, depending on the ionic system considered) whereas the down-range properties of the potential play a (somewhat) greater role in influencing diffusion-controlled processes. A detailed examination is made of ionic strength effects over a broad range of ionic concentrations. In the regime of low ionic strength, the limiting slope and intercept of the curve describing the dependence of log kD on I1/2/(1+I1/2) may differ considerably from the usual Debye–Hückel limiting relations, depending on the particular model chosen to describe local saturation effects. The results in the regime of high ionic strength for all models considered converge to the Smoluchowski limit, and thus differ significantly from the traditional Debye–Hückel results. As a specific application of our theory, we consider the reaction between pyrene solubilized in cetyltrimethylammonium bromide (CTAB) and e−aq. We estimate the chemical rate kR and then calculate the dependence of the overall rate on the ionic strength; the results obtained are found to be in excellent agreement with experiment.
Theoretical investigation of rotational rainbow structures in X–Na2 collisions using CI potential surfaces. III. Rigid-rotor X = Ne scattering
Authors: Schinke Reinhard , Müller Wolfgang , Meyer Wilfried
Published: February 2003

A thorough investigation of rotational rainbow structures in differential cross sections for the prototypical Ne–Na2 system is presented. The scattering calculations are performed using an accurate CI potential energy surface, which includes electron correlation effects for the bond orbital of Na2 and the L-shell orbitals of Ne together with the dispersion attraction between the two subsystems using the method of self-consistent electron pairs (SCEP). The surface is dominantly repulsive and highly anisotropic. A very shallow van der Waals minimum of about 0.3 meV is obtained at large internuclear distances. Coupled states and infinite-order-sudden differential cross sections are compared for a wide range of collision energies which allows for a critical test of the energy-sudden condition. The applicability of the centrifugal sudden approximation for Ne–Na2 is also discussed. In particular, we investigate the dependence of the rotational rainbow structures on collision energy and initial rotational state. The positions of the primary rotational rainbows obtained with the CI and the corresponding Hartree–Fock surface are compared to experimental results. We find satisfactory agreement for the CI surface but considerable deviations if the Hartree–Fock surface is used. Finally, we compare the scattering results for Ne–Na2 with those for He–Na2 and conclude that at low energies (E≲100 meV) the drastic differences are mainly due to the different masses rather than the potential energy surfaces.
Ni–Cl bonding in concentrated Ni(II) aqueous solutions at high Cl− /Ni2+ ratios. An x-ray diffraction investigation
Authors: Magini M. , Paschina G. , Piccaluga G.
Published: February 2003

Three concentrated Ni(II) solutions at Cl−/Ni2+ ratios of 4 and 5 have been examined by the x-ray diffraction technique. The Fourier transformed scattering data straightforth indicate inner complex formation between Ni2+ and Cl− ions. Octahedral Ni(H2O)6−nCl2−nn (n = 0, 1, 2) complexes are the species which are consistent with the distribution curves and the structure functions. The average numbers of bonded chloride ions per nickel atom have been determined in each solution and the reliability of the complexation numbers as well as of the structural parameters critically examined. The existence of an intermediate-range order in NiCl2 solutions is discussed within the frame of the current literature and the present findings.
Quantum theory of polarization in liquids: Exact solution of the mean spherical and related approximations
Authors: Thompson Mark J. , Schweizer Kenneth S. , Chandler David
Published: February 2003

The mean spherical approximation and related integral equations theories (such as the linearized hypernetted chain equation) are studied for a fluid composed of atoms or spherical molecules with quantum mechanical fluctuating internal dipoles. We derive the solutions of these equations for the case in which the intramolecular restoring force for a fluctuating dipole is harmonic (i.e., a quantum Drude model). In the limit of low oscillator frequencies, the solutions reduce to those deduced by Pratt on the basis of classical theory. We discuss the frequency dependence of the fluid renormalizations of atomic polarizabilities, and show that for the zero frequency applications discussed by Pratt, the classical theory is correct. We find, however, that the finite frequency quantum effects play a dominant role for many experimentally relevant properties. Generalizations of the quantum theory to include features such as charge overlap and hyperpolarizabilities are also discussed. The relationship between low order quantum mechanical perturbation theory and the integral equation theories is described.
The radiative lifetime of the C 3Πu state of N2
Authors: Dumont Michel N. , Remy François
Published: February 2003

New measurements of the radiative lifetime of the C 3Πu state of N2 have been made using the single photon delayed-coincidence technique. No significant variation of the lifetime with v′ having been found the proposed value is τ = (39.2±3.9)ns. This is in good agreement with published values that we consider as the more reliable ones following the criteria discussed in this paper.
Dynamics of liquid state chemical reactions: R-dependent correlation functions for I.+I. in CCl4
Authors: Brooks C. L. , Adelman S. A.
Published: February 2003

Reagent configuration dependent solvent correlation functions, which are required for implementation of the MTGLE approach [S. A. Adelman, J. Chem. Phys. 73, 3145 (1980)] to problems in liquid state chemical reaction dynamics, are calculated via the molecular dynamics (MD) method for a Lennard-Jones system designed to simulate two interacting iodine atoms dissolved in liquid carbon tetrachloride. For this case, the correlation functions depend only on the iodine–iodine internuclear separation R. The R-dependence of the correlation functions, and related computed quantities of the MTGLE parameters, is found to be governed mainly by excluded volume effects arising from ’’squeeze-out’’ of solvent molecules from the region between the iodine atoms. Additional calculations designed to test approximations in the theory and simplifications, like use of small box-sizes and the superposition approximation, are also presented.
Catastrophe theory and molecular charge densities: Determination of w in the universal unfolding for triatomic molecules
Authors: Melnick S. J. , Whitehead M. A.
Published: February 2003

The parameter w in the universal unfolding f(y, z) = y2z−1/3z3+wy2+uy+vz has been defined for triatomic systems, using the molecular point group symmetry and the atomic ground state electronic structure. The treatment of larger molecular systems is related to this definition of w.
Ab initiostudies of structural features not easily amenable to experiment. 23. Molecular structures and conformational analysis of the dipeptideN-acetyl-N′-methyl glycyl amide and the significance of local geometries for peptide structures
Authors: Schäfer Lothar , Van Alsenoy C. , Scarsdale J. N.
Published: February 2003

The molecular structures of four conformations of N-acetyl–N′-methyl glycyl amide were refined by geometrically unconstrained ab initio gradient relaxation on the 4–21G level. The most stable form I contains a seven-membered ring closed by hydrogen bonding. A second local minimum II is less than 1 kcal/mol above I and represents the fully extended form with a five-membered hydrogen bonded ring. The two other minima refined, III and IV, are open forms which are 4–5 kcal/mol less stable than I. The refined geometries make it possible to estimate the significance of local geometries, in contrast to standard geometry, in the various conformations. It is found that bond distances in different conformations can vary by up to 0.02 Å, and important backbone bond angles can vary by up to 7°. Except for the symmetrical form II, small deviations from amide planarity (H–N–C = 0 angles of 3–10°) are the rule, even though the equilibrium structure of the unperturbed amide group in 4–21G space is planar. It can be concluded that local geometry relaxations at different points of the potential energy surface of a peptide system can amount to several Kcal/mol per residue and should be an important aspect of protein conformational analysis.
Hindered rotational energy levels of linear ions in trigonal and tetragonal crystalline fields
Authors: Smith David
Published: February 2003

The hindered rotational energy levels of linear ions in crystalline fields of trigonal and tetragonal symmetry have been computed. For a trigonal crystalline field, D3d symmetry, and a tetragonal crystalline field, D2h symmetry, the J = 2 and J = 4 terms have been included in the potential function. The theory of a linear ion in a trigonal field is applied to the hindered rotation of the hydrosulfide ion in the alkali metal hydrosulfides. The librational frequencies of the alkali metal azides KHF2 and TlN3 are analyzed using the model of a linear ion in a tetragonal crystalline field.
The statistical thermodynamics of microemulsions. II. The interfacial region
Authors: Talmon Y. , Prager S.
Published: February 2003

We show that the Voronoi model of microemulsion phase equilibria developed in an earlier paper can be extended to provide a description of the interfacial region. For two-phase equilibria at near-critical surfactant concentrations, we have calculated the influence parameters which control the interface thickness, and carried through a van der Waals (mean field) analysis to obtain interfacial composition profiles showing an excess surfactant concentration. Interfacial tensions calculated using oil/water domain sizes available from light scattering measurements give values similar to the minima reported under optimal conditions of salinity and co-surfactant concentration.
Investigations of model polymers: Dynamics of melts and statics of a long chain in a dilute melt of shorter chains
Authors: Bishop Marvin , Ceperley David , Frisch H. L. , Kalos M. H.
Published: February 2003

We report additional results on a simple model of polymers, namely the diffusion in concentrated polymer systems and the static properties of one long chain in a dilute melt of shorter chains. It is found, for the polymer sizes and time scales amenable to our computer calculations, that there is as yet no evidence for a ’’reptation’’ regime in a melt. There is some indication of reptation in the case of a single chain moving through fixed obstacles. No statistically significant effect of the change, from excluded volume behavior of the long chain to ideal behavior as the shorter chains grow, is observed.
Determination of contraction coefficients for replicated Gaussian primitives
Authors: Graf P. , Mehler E. L. , McLean A. D. , Chandler G. S.
Published: February 2003

Replication of primitive Gaussian basis functions is used to minimize energy losses in contracted Gaussian basis set. A method for obtaining near optimum contrction coefficients for the replicated functions and requiring least squares solutions of a set of linear equations is described. Repeated self-consistent field calculations are thereby avoided. An application of the method to production of a contracted basis set for phosphorus is outlined.
Electron-coupled spin–spin coupling tensor for 31P in tetraethyldiphosphine disulfide
Authors: Tutunjian Pierre N. , Waugh J. S.
Published: February 2003

The traceless part of the 31P–31P spin–spin coupling tensor was determined from a single crystal NMR study of tetraethyldiphosphine disulfide (TEPS). The electron-coupled part of this tensor, obtained by subtracting the known dipolar part, admits of two assignments. These have different anisotropies (δI = +1491 Hz, δII = +5861 Hz). Both show approximate axial symmetry (ηI = 0.1, ηII = 0.02) about the P–P bond, supporting the intuitive notion that J coupling is primarily determined by the local sigma-bonding electrons.