bluuebunny/crossref_metadata_2025 huggingface dataset:
On the anisotropic energy transfer to random acceptors
Authors: Blumen Alexander
Published: February 2003

We present exact expressions for the ensemble averaged decay of the excitation of a donor molecule, due to the energy transfer via anisotropic dipolar interactions to acceptors distributed randomly on a lattice. We compare in two special cases the approximate, Förster-type expressions, valid for low acceptor concentrations and long times, with the exact results: in the case of a completely random angular distribution and in case of a preferential orientation. The connection to the decay law for isotropic interactions [Blumen and Manz, J. Chem. Phys. 71, 4694 (1979)] is also presented. Whereas the approximate expressions show similar qualitative behavior in all cases, their regions of validity (compared to the exact results) differ considerably; the difference is especially large for the decay due to acceptors which are randomly distributed on the lattice but are orientationally ordered.
Time resolved study of rotational energy transfer in A 2Σ+(ν′ = 0) state of OH in a flame by laser induced fluorescence
Authors: Stepowski D. , Cottereau M. J.
Published: February 2003

Time resolved study of rotational transfer in the A 2Σ+(ν′ = 0) state of OH in a flame is performed by analyzing the fluorescence spectra scanned at different delays after the short exciting laser pulse. The conditions under which a three level model apply are discussed. Then comparison between this kinetic model and experiment provides direct total transfer rate out of the J′ = 7 1/2 excited level: The specific transfer cross sections derived are 70 Å2 for N2 and 18 Å2 for Ar. The fluorescence spectra show a strong propensity for spin conservation upon rotational transfer. As the delay increases, the rotational population distribution among the upper state tends to a Boltzmann distribution.
Multiphoton ionization mass spectroscopy of deuterated analogs of acetaldehyde: Evidence for deuterium scrambling
Authors: Fisanick G. J. , Eichelberger T. S.
Published: February 2003

Fragmentation patterns as a function of laser flux are obtained for CD3CHO using a quadrupole mass spectrometer for excitation at the origin of the two-photon resonant n→3s Rydberg transition at 55 138 cm−1. Extensive fragmentation is found with product ion branching ratios showing a strong dependence on laser flux. Fragments such as CDO+ appear, indicating that bond rearrangement occurs. In the case of the CD+3, CD2H+ ion pair, the CD2H+ scrambled product dominates, although in no case does the scrambling reach the statistical limit obtained by assuming that all hydrogen species are equivalent. The individual ion signal data can be fit with a rate equation model. Isotopic selectivity for mixtures of CD3CDO/CH3CHO is demonstrated; no partially deuterated products are found, indicating that the scrambling in CD3CHO is not collisionally induced.
Multiple photon excited SF6 interaction with silicon surfaces
Authors: Chuang T. J.
Published: February 2003

Infrared laser induced SF6–silicon interactions have been studied and the surface reaction yields have been determined as a function of the laser frequency, the laser intensity, and the gas pressure in both perpendicular and parallel beam incidences on the solid surfaces. The results clearly show that vibrationally excited SF6 molecules promoted by CO2 laser pulses are very reactive to silicon, particularly when the solid is simultaneously exposed to the intense ir radiation. The laser excitation of the Si substrate alone cannot cause the heterogeneous reaction to occur. The present gas–solid system thus provides an example which clearly establishes the direct correlation between surface reactivity and vibrational activation. Additional experimental measurements also demonstrate that the thermal fluorine atoms generated by SF6 multiple photon dissociation at high laser intensities can react with silicon to form volatile product. The study thus provides further insight into the silicon–fluorine reaction dynamics.
The laser induced fluorescence spectrum of trapped BrCN cations
Authors: Grieman Fred J. , Mahan Bruce H. , O’Keefe Anthony
Published: February 2003

Laser induced fluorescence spectra of mass selected BrCN+ confined in a three dimensional quadrupole trap are presented. Using this technique, the B̃ 2Π–X̃ 2Π system of BrCN+ is investigated. Approximate values for the C–Br stretching frequencies (ν′1=441±18 cm−1, ν1″=509±16 cm−1) and the difference of the spin–orbit splitting constants (A″–A′=224±43 cm−1) are determined. However, the spectrum is complicated by irregular vibrational and spin–orbit splitting spacing, yielding a large uncertainty in these values. The existence of sequence bands and a Fermi resonance interaction provides possible explanations for these irregularities.
Raman intensities of sulphur α-S8
Authors: Domingo C. , Montero S.
Published: February 2003

On the basis of the available force fields for the S8 ring the vibrational Raman intensities of an α-S8 single crystal and of a randomly oriented polycrystalline sample are interpreted in the frame of the bond polarizability model. The whole set of Raman tensors of the S8-molecular ring and of the single crystal including relative signs are reported, as well as the polarizability parameters for the S–S bond. These magnitudes and the related Raman scattering coefficients are finally reported in an absolute scale by means of the optical refractivities of the crystal.
Electron spin resonance of Gd3+ in trifluorides of La, Ce, Pr, and Nd
Authors: Misra Sushil K. , Mikolajczak Pawel , Korczak S.
Published: February 2003

An EPR study of Gd3+-doped single crystals of CeF3, LaF3, PrF3, and NdF3 has been made in order to study the systematics of spectra in these homologous isostructural hosts. The method of preparation of samples is presented. The measurements are carried out at all three—room, liquid nitrogen, and liquid helium—temperatures. Thus, the absolute signs of all the spin Hamiltonian parameters are determined from intensities of lines at liquid helium temperatures. The parameters are evaluated from a simultaneous fitting of all lines observed in the zx and xy planes in a rigorous least-squares fitting procedure, which utilizes numerical diagonalization of the spin Hamiltonian matrix. As far as the systematics of the parameters are concerned, it is found that the parameter b02 varies linearly with the atomic radius of the host rare-earth ion, while the parameter b22 does not show any such behavior. It it hoped that the experimental results presented in this paper would be helpful in further theoretical understanding of the interaction of the Gd3+ ion with its environment due to simplicity of the rare-earth trifluoride crystal structure, in that in these crystals there are no water molecules present.
The effective diffusion coefficient detected by dynamic light scattering
Authors: Russel W. B. , Glendinning A. B.
Published: February 2003

The wave number dependence of the diffusion coefficient measured by dynamic light scattering is predicted in the dilute limit through a relatively simple, but rigorous, equilibrium analysis of the diffusion process. For hard sphere interactions with ak≲0.2 the gradient diffusion coefficient should be detected, but for ak≳4 the process resembles self-diffusion.
The gas dynamics of a pulsed supersonic nozzle molecular source as observed with a Fabry–Perot cavity microwave spectrometer
Authors: Campbell E. J. , Buxton L. W. , Balle T. J. , Keenan M. R. , Flygare W. H.
Published: February 2003

The gas dynamics of a pulsed supersonic nozzle molecular source are investigated by using a pulsed Fabry–Perot cavity microwave spectrometer to obtain free induction decay signals from rotational two-level systems in the gas expansion. An equation is derived giving the time domain emission signal line shape as an integral over the active molecular distribution in the beam. The Doppler splitting phenomenon is discussed in detail. Experimental line shapes are deconvoluted to give molecular velocities, dephasing times, and density distributions. We find that the density distribution of active molecules from the the pulsed nozzle varies rapidly in time, starting with a depletion on the nozzle axis at short times after the nozzle is opened, and changing to on-axis concentration at longer times. Results obtained with the gas nozzle axis oriented at angles ranging from 0 ° to 90 ° with respect to the direction of propagation of the microwaves are reported.
Phonon spectra of methane physisorbed on graphite
Authors: Maki Kazuo , Klein Michael L.
Published: February 2003

We have investigated certain aspects of the dynamics of methane monolayers physisorbed on graphite. In particular, phonon frequencies have been calculated by applying the self-consistent harmonic approximation to a two-dimensional array of methane molecules arranged as stable tripods on a triangular lattice. The methane–graphite interaction was not considered explicitly, and only three degrees of freedom per molecule were taken into account: two translations parallel to the surface and rotation about the axis normal to the surface. A new intermolecular potential proposed by Righini and present authors was employed. It was found that the translation–rotation coupling plays an important role in determining the anharmonic frequency shift. The results are in good agreement with a recent neutron incoherent inelastic scattering experiment in which the momentum transfer is parallel to the graphite surface. The two main peaks observed around 9 and 5 meV are assigned to librational and translational modes, respectively.
On deviations in Ruedenberg energy formula for molecules
Authors: Sen K. D.
Published: February 2003

The deviation Δ=E−3/2εkνkεk in the Ruedenberg2 energy formula is discussed in terms of atomic parameters. The formula Δ∼εa(Eza−21ee) is found to give good approximation to Δ. Ea and Veea represent the core energy and interelectronic energy of the constitutent atoms in the molecule.(AIP)
The fluorescence excitation and dispersed fluorescence spectra of the van der Waals molecules I2N2 and I2N2He
Authors: Johnson Kenneth E. , Levy Donald H.
Published: February 2003

van der Waals molecules I2N2 and I2N2He have been observed in gas mixtures expanded through a supersonic nozzle. Fluorescence excitation and dispersed fluorescence spectra of these molecules is reported. (AIP).
Measurement of the depolarization ratio of Rayleigh scattering at absorption bands
Authors: Anglister J. , Steinberg I. Z.
Published: February 2003

Measurements of the depolarization ratio ρv of light scattered by the pigments lycopene and β-carotene at the red part of their absorption bands yielded values which are very close to the theoretical value 1/3 of a fully anisotropic molecular polarizability, i.e., that due to an electric dipole moment. Measurements of ρv at the blue edge of the visible absorption band of pinacyanol chloride yielded a value of 0.75 at 472.2 nm, which is the maximum value that a depolarization ratio can assume, and is attained if the average molecular polarizability is zero. This is possible only if the diagonalized polarizability tensor has at least one negative element to counterbalance the positive ones. A negative refractive index at the blue edge of the absorption band is thus experimentally demonstrated.
The statistical mechanics of the electrical double layer: Stress tensor and contact conditions
Authors: Carnie Steven L. , Chan Derek Y. C.
Published: February 2003

The aim of this paper is twofold: (i) to clarify the definition of the mechanical (hydrostatic) stress tensor and the Maxwell (electrostatic) stress tensor in nonuniform electrolytes or Coulombic systems, and (ii) to derive contact conditions for an electrolyte at a charged hard wall to include (a) ion size effects, (b) imaging effects at the wall, and (c) effects due to the molecular nature of the solvent (the civilized model). The last result is valid for a general nonlinear dielectric whose molecules can have any electrostatic multipole interactions.
The influence of quantization on the onset of chaos in Hamiltonian systems: The Kolmogorov entropy interpretation
Authors: Kosloff Ronnie , Rice Stuart A.
Published: February 2003

An extension of the concept of Kolmogorov entropy to quantum mechanical systems is given. Using the Kolmogorov entropy as a common basis for discussion, the onset of chaotic motion in classical mechanical and quantum mechanical systems is compared. It is found that if the spectrum of the system is discrete, the Kolmogorov entropy is zero, and therefore that a bounded quantum mechanical system cannot have chaotic motion like that observed for a corresponding classical mechanical system. This analysis leads to the prediction that if the distributions of states are similar for two bounded systems which in the classical limit have, respectively, quasiperiodic and chaotic motion, wave packets for the two systems decay at similar rates as found by Brumer and Shapiro. The relationship between the defined Kolmogorov entropy, previous interpretations of ’’KAMlike’’ onset of chaos in quantum mechanical systems, and the role played by preparation and observation of a system in influencing the intramolecular dynamics, are discussed.
Nonequilibrium time dependent theory of hot atom reactions. III. Comparison with Estrup–Wolfgang theory
Authors: Shizgal B.
Published: February 2003

Time dependent solutions of the Boltzmann equation for a hot atom speed distribution function are obtained with a new discrete ordinate method of solution. The hot yields that are calculated for a model system agree to within approximately 20% with the Estrup–Wolfgang theory provided the reactivity is not too large. Although no steady state sets in before reaction occurs, the justification for the use of a time independent collision density function is given in terms of a time averaged distribution function.
Photodissociation in intense laser fields: Predissociation analogy
Authors: Bandrauk André D. , Sink Michael L.
Published: February 2003

Using the dressed molecule picture, it is shown that photodissociation in intense laser fields can be treated as a predissociation induced by the radiative coupling betweem the initial bound state and final dissociative state. Ar2+ is used as an example as it has a large electronic transition moment typical of molecular ions. It is found that at intensities of 1014 W/m2, one can no longer speak of Franck–Condon factors and that one must introduce into the description new adiabatic states created by the intense fields. It is further suggested how these new laser induced electronic states might be detected experimentally.
A quasiclassical trajectory study of reagent vibrational excitation effects in the OH+H2→H2O+H reaction
Authors: Schatz George C.
Published: February 2003

Quasiclassical trajectories have been used to determine reaction rate constant enhancements and product state energy partitioning in the OH(v)+H2(v′)→H2O+H reaction for (v,v′)= (0,0), (1,0), (0,1), and (1,1). An analytical fit to the accurate ab initio potential surface of Walch and Dunning was used in the Monte Carlo calculations. Final H2O vibrational states were assigned using the histogram method to bin the good action variables governing H2O vibrational motions. These actions were calculated by using second order classical perturbation theory to solve the vibrational Hamilton–Jacobi equation. The resulting integral reaction cross sections and thermal rate constants, indicate that OH vibrational excitation leads to a very small enhancement (only a factor of 1.28) in the thermal rate constant at 300 K. H2 excitation, on the other hand, causes a large reduction in the reaction activation energy (from 0.18 eV to 0.03 eV) and a large enhancement in the rate constant (a factor of 393 at 300 K). These results, as well as the ground reagent state thermal rate constants are in good agreement with experiment. We also find that simultaneous H2 and OH vibrational excitation gives a result which is just the superposition of the separate excitation results. Reagent rotational excitation causes a decrease in the reaction cross section, with the rate of decrease larger when H2 is excited than OH. An examination of product state energy partitioning indicates that nearly all of the additional energy coming from reagent vibrational excitation ends up as product vibration, with all of it going to the H2O stretch modes when OH is initially excited, and a more random distribution of the three H2O modes when H2 is excited. This mode specificity of energy flow contrasts with the nonspecific vibrational mode distributions obtained for the reaction from the ground state reagents.
Electronic structure and dissociation curves for the ground states of Tl2 and Tl2+ from relativistic effective potential calculations
Authors: Christiansen Phillip A. , Pitzer Kenneth S.
Published: February 2003

The dissociation curves for the ground states of Tl2 and Tl2+ were computed using a generalization of the molecular relativistic ω–ω coupling formalism of Lee, Ermler, and Pitzer. Relativistic effects, as represented by the Dirac equation, were introduced using effective potentials generated from atomic Dirac–nFock wave functions using a generalization of the improved effective potential formulation of Christiansen, Lee, and Pitzer. Our calculations show that the ground state of Tl2+ is 1/2g with computed De and Re values of 0.58 eV and 3.84 Å. For Tl2 we find that the ground state is 0u− but the 0g+ and the 1u states are only slightly higher in energy; the potential curves for these states are repulsive to about 3.5 Å and then essentially flat beyond that radius. While corrections for correlation will increase De somewhat, Tl2 is only weakly bound in any of these states which dissociate to normal atoms. The cause is undoubtedly related to the large spin-orbit splitting between the 6p1/2 and 6p3/2 thallium spinors.
Potential model for the interaction of two like S state atoms involving spin symmetry
Authors: Feltgen R.
Published: February 2003

One of the problematic parts of the interaction of two atoms is the dispersion interaction resulting from interatomic electron correlation. Based on available variational and perturbational ab initio results for two like S state atoms with only s valence electrons, a model formula for the dispersion interaction is constructed in a way that different spin states of the system are considered. If the exchange dispersion component can be treated as a correction, then the formula contains two free parameters, one characterizing the orbital contraction and the other the exchange contribution. To this formula the Hartree–Fock energy with an intra-atomic correlation correction is added, thus giving a reliable two-parameter model for the full interaction. The two parameters can be fitted to experimental data of a system under consideration to get the potential.
A theoretical study of selected singlet and triplet states of the CO molecule
Authors: Cooper David M. , Langhoff Stephen R.
Published: February 2003

Potential energy curves have been obtained for the X 1Σ+, B 1Σ+, C 1Σ+, (4)1Σ+, I 1Σ−, A 1Π, and a 3Π electronic states of CO using self-consistent-field plus configuration-interaction techniques and a large Slater basis augmented with diffuse functions. The calculated spectroscopic constants Te, re, ωe, and ωexe are in excellent agreement with those from experiment, especially for the low-lying valence states. Calculated dipole moment functions for the X 1Σ+, A 1Π, and a 3Π states also agree well with those from previous theoretical calculations. In particular, our a 3Π dipole moment function provides further support that the two previous theoretical calculations are preferable to the dipole moment function measured by Wicke and Klemperer. Calculated electronic transition moments for the A 1Π–X 1Σ+ (CO fourth positive system), B 1Σ+–X 1Σ+, C 1Σ+–X 1Σ+ and (4)1Σ+–X 1Σ+ band systems all vary significantly with internuclear distance. Our theoretical radiative lifetimes for the lowest vibrational level of the A 1Π, B 1Σ+, and C 1Σ+ states are 6.2–9.9, 11.2, and 2.1 nsec, respectively. These theoretical results are in generally good agreement with the experimental measurements.
Intermolecular potentials for ammonia based on SCF–MO calculations
Authors: Hinchliffe A. , Bounds D. G. , Klein M. L. , McDonald I. R. , Righini R.
Published: February 2003

The results of SCF molecular orbital calculations on the ammonia dimer have been used in part to parameterize a set of atom–atom potentials. When combined with a charge distribution which reproduces the experimental dipole and quadrupole moments of the monomer, and with independent estimates of the dispersion energy, the resulting intermolecular potential yields a fair description of certain properties of the condensed phases of ammonia. Liquid ammonia is predicted to have a weakly associated character.
A comparison between hydrogenic and Thomas–Fermi expectation values
Authors: Katriel Jacob , Nyden Marc R.
Published: February 2003

The Z dependence of various expectation values is studied for pure and screened hydrogenic atomic models. These results are compared to the corresponding Thomas–Fermi and Hartree–Fock values, providing insight into the transiton from the regular, exponentially decaying densities of real atoms to the Z→∞ Thomas–Fermi limit.
Collisional energy transfer from excited polyatomic molecules produced by infrared multiple photon absorption
Authors: Baldwin Alan C. , van den Bergh Hubert
Published: February 2003

A simple energy-grained master equation is used to model data on the multiphoton dissociation of CF2HCl as a function of laser fluence and argon bath–gas pressure. Information on energy transfer from vibrationally excited CF2HCl is obtained from an analysis of the competition between laser pumping and collisional energy removal using suitable parametrizations of the laser intensity and the collision process.
Variational transition state theory and vibrationally adiabatic transmission coefficients for kinetic isotope effects in the Cl–H–H reaction system
Authors: Garrett Bruce C. , Truhlar Donald G. , Magnuson Alan W.
Published: February 2003

Variational transition state theory and semiclassical adiabatic ground-state transmission coefficients are applied to calculate the kinetic isotope effects for D and T substitution in the reaction Cl+H2 at 245–445 K. The calculated isotope effects differ significantly from those calculated previously using conventional transition state theory and semiempirical potential energy surfaces. We use variational transition state theory and conventional transition state theory with the Wigner tunneling correction to adjust three new semiempirical surfaces to the experimental data. No one set of calculations is completely successful. The potential energy surfaces that are most successful at predicting the HD/DH intramolecular kinetic isotope effect have the earliest saddle points (the saddle points are collinear with R‡Cl–H=2.64–2.78a0, R‡H–H=1.88–1.72a0). For each surface studied except one, the canonical variational transition states are located past the saddle point for some of the isotopic reactions and earlier than the saddle point for others. The exceptional surface is the one of Valencich and co-workers; that surface has an early saddle point, but the variational transition states are always earlier than the saddle point.
Elastic constants of nematic liquid crystalline Schiff’s bases
Authors: Leenhouts F. , Dekker A. J.
Published: February 2003

The three elastic constants of a number of structurally related nematic liquid crystalline Schiff’s bases have been measured as a function of temperature. The results are new in the sense that they exhibit a clear distinction in behavior between compounds with a rigid molecular structure and compounds with a molecular structure consisting of a rigid core and a flexible alkyl chain. In particular, it has been found that the ratio of the bend and splay elastic constants K3/K1 increases for compounds with a rigid molecular structure with increasing length to width ratio. This trend is the reverse of that observed wihin homologous series, for which K3/K1 has been found to decrease with increasing length of the alkyl chain. New is the experimental finding that the twist elastic constant K2 of the homologs studied is independent of the length of the alkyl chain. Finally, the experiments show that the average elastic constant K̄1+K2+K3)/3 is proportional to the smectic–isotropic transition temperature. The results can only be partly understood within the framework of existing theoretical models.
Topological distribution functions and the second virial coefficients of ring polymers
Authors: Iwata Kazuyoshi , Kimura Tsunehisa
Published: February 2003

A new method (a topological moment method) is presented for treating statistical problems of ring polymer chains of which the topological state (Gauss linking coefficient) is prescribed. It is shown that the topological interaction among ring polymers formally resembles the excluded volume effect among linear polymers. A ’’topological parameter γ’’ which corresponds to the binary cluster integral β in the excluded volume problem is newly introduced for representing the strength of the topological interaction among submolecules. The parameter γ is given as a function of bond length, bond angle, internal-rotational potential, and of interaction potentials among chain elements. Analytical expressions for the distribution function of the intermolecular distance and the second virial coefficient are derived and they are computed numerically.
Splittings of the librational ground state of CH3D physisorbed on the surface of graphite
Authors: Maki Kazuo
Published: February 2003

Energy levels within the librational ground state manifold of CH3D in a trigonal field are calculated for a monolayer system physisorbed on the surface of graphite. They are classified into two groups: The upper two levels correspond to the stable tripod orientation with the CD bond pointing upward from the surface, and the lower five levels correspond to those with the CD bond pointing toward the surface. The inelastic neutron scattering cross sections are calculated. The transitions between the two groups are found to be negligible. The spectrum for a powdered sample of graphite (Vulcan III) is well reproduced, and that for a partially oriented sample (Papyex) is predicted with momentum transfer both parallel and perpendicular to the surface. Heat capacity anomalies are calculated and found to be sensitive to the spacing between the two groups of levels.
Molecular dynamics study of a single polymer chain in solution
Authors: Bruns W. , Bansal R.
Published: February 2003

We have carried out a computer simulation of the dynamics of a model polymer chain in solution. The polymer is represented by a nonamer linked through bonds which are constrained to be fixed (bead–rod model). All particles are assumed to interact via a Lennard-Jones potential and the simulations have been performed at a temperature and density which corresponds to the triple point of L-J fluids. Equilibrium properties of the chain such as end to end distance, radius of gyration, structure function, and polymer asphericity are computed. The effect of the attractive term in the L-J potential on the end to end distance and radius of gyration is discussed. The self-diffusion coefficient and time-dependent autocorrelation functions of various dynamical variables have also been calculated. All these results have been compared with those obtained for an isolated chain and wherever possible the effect of the solvent on the chain structure is commented. A qualitative comparison of present results with those of other authors is also made.
Triplet–triplet energy-pooling processes in magnesium vapor
Authors: Breckenridge W. H. , Nikolai W. L. , Stewart J.
Published: February 2003

Experimental observations and computer simulation results are reported for the annihilation of the triplet states Mg(3s3p3PJ) excited by pulsed laser radiation.(AIP)
Green’s function Monte Carlo calculation for the ground state of helium trimers
Authors: Cabral F. , Kalos M. H.
Published: February 2003

The ground state energy of weakly bound boson trimers interacting via Lennard-Jones (12,6) pair potentials is calculated using a Monte Carlo Green’s Function Method. Threshold coupling constants for self binding are obtained by extrapolation to zero binding.
Laser photolysis of O3/H2 mixtures: The yield of the H+O3→HO2+O reaction
Authors: Force A. P. , Wiesenfeld J. R.
Published: February 2003

The collisional deactivation of O(2 1D2) by H2 and D2 was found to proceed with a rate constant 1.0±0.1×10−10 cm3 molecule−1 sec−1. The hydrogen atoms produced by O(2 1D2) +H2→OH+H were observed to react with the O3 source molecule with the rate coefficient 3×10−11 cm3 molecule −1 sec−1. Oxygen atoms were seen to be formed with a similar rate and approximately 40% of the reactive encounters of H with O3 were estimated to result in the production of oxygen atoms instead of the expected OH‡ and O2. Possible mechanisms involving collisionally induced dissociation of O3 by vibrationally excited hydroxyl radicals do not appear to account for the observed oxygen atom kinetics in this system, thus suggesting that H+O3→HO2+O may indeed be a significant pathway in the reaction of hydrogen atoms with ozone.
Molecular ion formation in decaying plasmas produced in pure argon and krypton
Authors: Grössl M. , Langenwalter M. , Helm H. , Märk T. D.
Published: February 2003

The formation of molecular ions Ar2+ and Kr2+ in decaying plasmas of the respective rare gases is studied at pressures between 0.2 and 3 Torr at room temperature. Measurements of the decay of He2+ in He are made to test the experimental technique used. The reaction rate coefficient for the process Ar+(2P3/2)+2 Ar→Ar2++Ar is found to be (2.35±0.2)×10−31 cm6 s−1. The corresponding reaction rate coefficient in krypton is found to be (2.2±0.2)×10−31 cm6 s−1. Using a recently proposed reaction model [Helm and Varney (1978)] for the excited fine structure state (2P1/2), the pressure dependence of the reaction rate coefficient describing the destruction Ar+(2P1/2)+2Ar→products is calculated. The pressure dependence and magnitude of the coefficient found are in agreement with the experimental findings of Liu and Conway (1975) for the above process. The process could not be studied experimentally in our system due to the low density of Ar+(2P1/2) in our discharge. The low density is inferred indirectly and the process responsible for the fast removal of the excited ion species in the argon plasma is regarded to be the superelastic collision process with slow plasma electrons Ar+(2P1/2)+e→Ar+(2P3/2)+e+0.18 eV for which a rate coefficient ≳10−7 cm3 s−1 is estimated. At late times in the afterglow a significant source of ionization is observed in argon. This source is consistent with ionizing collisions of metastable argon atoms. Its importance increased with pressure since the diffusive loss of metastable atoms becomes insignificant as the pressure is raised.
Temperature dependence of the reaction rate for H+CF3Br
Authors: Silver D. M. , deHaas N.
Published: February 2003

Using the flash photolysis–resonance fluorescence technique, the rate of the reaction H+CF3Br was measured over the temperature range 225–377 K. The rate constants are found to obey k=(1.53±0.19)×1013 cm3 mol−1 s−1 exp[(−4341±70) cal mol−1/RT] over the measured range.
Magnetic properties of HeH+
Authors: Bishop David M. , Cheung Lap M.
Published: February 2003

A calculation of the magnetic-susceptibility components and rotational g value of HeH+ is reported. Rovibronic wave functions, with explicit inclusion of the interelectronic coordinate, are used.
Localized excitations in pyrazine and p-benzoquinone. A valence bond model
Authors: Martin Richard L.
Published: February 2003

A valence bond (exciton) model involving the broken symmetry solutions to the Hartree–Fock equations provides reasonably accurate excitation energies for the 1,3(n→π*) excited states and the 2n ionic states of pyrazine and para-benzoquinone. An approach to the nonorthogonal CI is outlined which dramatically simplifies the calculation of the matrix elements.
A study of cationic disorder in nitrosyl tetrafluoroborate, by comparison of the heat capacity of this salt with that of potassium tetrafluoroborate from 9 to 304 K
Authors: Callanan J. E. , Granville N. W. , Green N. H. , Staveley L. A. K. , Weir R. D. , White M. A.
Published: February 2003

The heat capacity Cp of the salt nitrosyl tetrafluoroborate, NOBF4, has been measured from 10 to 304 K, and that of potassium tetrafluoroborate from 9 to 295 K. The heat capacity–temperature curve of the nitrosyl compound has a λ-type anomaly with Cp reaching a maximum at ∼67 K, and with an associated entropy gain estimated at 0.54R. Using Cp of the potassium salt as a reference, it appears that just above the λ point Cp of the nitrosyl salt once more starts to rise anomalously. The two-dimensional librational or rotational movement of the nitrosyl ion is probably that of a restricted rotator in a somewhat unsymmetrical potential field, and admittedly it is difficult to estimate reliably the contribution to Cp from this mode of motion. Nevertheless, the total heat capacity of the nitrosyl salt for at least 200 K is such that it seems certain that it embodies a component reflecting the gradual gain by the nitrosyl ion of orientational disorder. We estimate that the total configurational entropy acquired on reaching 300 K is approximately 2.3R, or about R ln10, and since Cp at this temperature is still anomalously high, this entropy term must rise even higher with a further increases in temperature.
Electrostatic potentials and fields in infinite point charge lattices
Authors: Heyes D. M.
Published: February 2003

The Coulomb potentials and fields in infinite point charge lattices are represented by series expansions in real and reciprocal space using a method similar to that devised by Bertaut. A systematic investigation is made into the relative convergence characteristics of series derived by varying the charge spreading function which is required in the theory. A number of formulas which are already in use are compared with new expressions derived by this method.
X-ray study of phase transition in [Co3(η-C5H5)3S2]
Authors: Kamijo Nagao , Terauchi Hikaru
Published: February 2003

The temperature dependences of the lattice constants and the x-ray scattering intensity of the superstructure reflections are measured in [Co3(η-C5H5)3S2] crystal. The results are interpreted in terms of electron–phonon interaction.
Microwave spectrum, spin–rotation, and hyperfine interaction constants, dipole moment, molecular structure, and harmonic force constants of the FSO radical
Authors: Endo Yasuki , Saito Shuji , Hirota Eizi
Published: February 2003

The FSO radical was identified by its microwave spectrum, in the reaction of carbonyl sulfide with microwave discharge products of an oxygen/carbon tetrafluoride mixture. Spectra of the normal and 34S species in the ground vibrational state and of the normal species in the first excited bending state were observed in the frequency region from 30 to 80 GHz. The rotational constants, centrifugal distortion constants, spin–rotation coupling constants including their centrifugal distortion terms, and magnetic hyperfine coupling constants of the fluorine atom were determined from analyses of the observed spectra. The off-diagonal components of the spin–rotation and hyperfine coupling εab+εba and Tab were obtained with good accuracy. These molecular constants lead to the conclusion that FSO is a π radical and its electronic ground state is 2A″.The following structure parameters were calculated from the rotational constants of the normal and 34S species: r(S–F)=1.602(3) Å, r(S–O)=1.452(3) Å, and &FSO=108.32(6)°. The dipole moment was determined from Stark effect measurements to be μa=0.374(12) D, μb=1.624(9) D, and μtotal=1.666(13) D. The harmonic force field and three vibrational frequencies were estimated from the observed centrifugal distortion constants and inertia defect.
Kinetics of the formation of Xe(6p[1/2]) and Xe(6p[3/2]2) by vacuum-ultraviolet irradiation
Authors: Di Stefano G. , Lenzi M. , Margani A. , Xuan C. Nguyen
Published: February 2003

The (6p[1/2]0) and (6p[3/2]2) states of Xe have been produced by vacuum-ultraviolet irradiation and selectively detected by their emissions at 828.01 and at 823.16 nm due to the transitions (6p[1/2]0→6s[3/2]1) and (6p[3/2]2→6s[3/2]2), respectively. Both states are not radiatively coupled to the ground (1S0) level according to the electric dipole selection rules. A study of the intensity of the emission lines and of the relative quantum yields as a function of pressure allows a kinetic mechanism to be outlined consistent with the present data. Experiments have been carried out at nine selected wavelengths corresponding to peculiar regions in the Xe vacuum-ultraviolet absorption spectrum and the main result is the observation of a marked difference in the behavior of the system as far as the formation of the two relevant Xe states is concerned. The (6p[1/2]0) level shows a pressure dependence which is very often higher than the (6p[3/2]2) level and very close to the pressure dependence of the absorption of the incident radiation.
A model for parametric analysis of the intensities in the infrared spectra
Authors: Galabov B.
Published: February 2003

A model for parametric analysis of IR intensities based on the use of molecular polar parameters related with vibrational distortions of separate bonds is described. The parameters represent derivatives of the total dipole moment with respect to coordinates describing the changes in the length and orientation of each bond in a molecule. The mathematical procedure is given using matrix representation. The analysis results in determination of bond polar vectors obtained from the intensities of stretching modes, and bond polar (3×3) matrices calculated from the intensities of deformation modes. The rotational contributions to the intensities in the case of polar molecules are explicitly considered. An example of application is presented in detail.
Vibrational relaxation of O3 by H2O and D2O
Authors: Vlahoyannis Yannis , Gordon Robert J.
Published: February 2003

The vibrational relaxation of laser excited O3(001) by H2O and D2O has been studied over the temperature range 295–473 °K. The CO2 laser-induced chemiluminescent reaction of NO+O3(001) was utilized to monitor the decay rate of vibrationally excited ozone. The relaxation probability for both water isotopes has a strong inverse temperature dependence, with D2O being four times more efficient than H2O. The results are discussed in terms of a metastable complex model for energy transfer.
A kinetic theory of dense fluids. Test-particle kinetic equation
Authors: Jhon Myung S. , Dahler John S.
Published: February 2003

Mori’s form of the generalized Langevin equation is applied to a system of particles whose pair interactions are sums of short-ranged, rigid-core repulsions and longer-ranged continuous interactions. It is assumed that the memory function decays exponentially from its initial value at a rate characterized by a single, wave number dependent relaxation time. The test–particle kinetic equation resulting from this one key assumption is strongly reminiscent of that associated with the well-known Rice–Allnatt theory. However, in addition to Fokker–Planck and Enskig operators similar to those of the Rice–Allnatt theory, it also includes coupling between rigid-core scattering events and the diffusive, Brownian motions of the particles as well as contributions associated with three-particle rigid-core collisions. A systematic method of solution is developed and applied to a simplified form of this kinetic equation. Solutions generated by this procedure involve matrix elements of the linearized Enskog operator computed in the basis of generalized Hermite polynomials which are eigenfunctions of the Fokker–Planck operator associated with the long-ranged, continuous portion of the pair potential. The approximate solutions obtained are used to compute the density and velocity autocorrelation functions and the coefficient of self-diffusion as well.
Statistical thermodynamics of micellar solutions
Authors: Stillinger F. H. , Ben-Naim A.
Published: February 2003

In order to study the behavior of surfactant solutions near their critical micelle concentrations (CMC’s), the osmotic pressure and monomer concentration functions have been examined in the complex plane of total surfactant concentration y. By considering some simple models in detail it has been established that a locus of branch-point singularities for both functions intersects the positive real y axis at the CMC’s. The proximity of the closest branch-point pair straddling the real axis is related on the one hand to mean micelle size, and on the other hand to the sharpness of the critical micelle phenomenon. A perturbation procedure has been constructed which can help to locate the physically important branch points near the positive real axis for the general ideal solution case.
A theory for ion binding and phase equilibria in charged lipid membranes. II. Competitive and cooperative binding
Authors: Copeland Bruce R. , Andersen Hans C.
Published: February 2003

Statistical mechanics is used to treat the competitive binding of protons and divalent cations to charged, acidic phospholipid membranes. Two models for competitive binding are described. The first model includes only simple competitive binding, in which protons and divalent cations compete for identical sites on individual lipids. The second model allows for cooperative binding of pairs of lipids by single divalent cations (cooperative bridging) in addition to simple competitive binding. The laws of mass action, an equation for the equilibrium area of a membrane, and a formula for the lipid chemical potential are obtained for each model. These results are used to develop equations for the membrane phase transition temperature. Calculated curves of transition temperature versus logarithm of bulk divalent cation concentration are presented and discussed for several different cases. It is demonstrated that large increases in transition temperature can be obtained at high divalent cation concentrations if there are different kinds of divalent cation binding in the solid and fluid membrane phases.
Molecular dynamics study of solid β-N2
Authors: Klein Michael L. , Lévesque Dominique , Weis Jean-Jacques
Published: February 2003

A computer simulation study of orientationally disordered solid β-N2 is reported for the state condition V=26.1 cm3/mol, T=47 K. We utilized an hexagonal system of 288 molecules interacting via a Raich–Gillis intermolecular potential. Particular attention is given to the dynamical structure factor S(Q,ω) and its dependence on the momentum transfer h/Q. In this regard the present model agrees considerably better with experimental observations than previous work based upon a simple atom–atom potential with no electrostatic quadrupole–quadrupole interaction.
Theory of scattering of atoms by one- and two-dimensional periodic hard corrugated surfaces with attractive potentials
Authors: Scribani Luigi , Goodman Frank O.
Published: February 2003

An exact theory of elastic scattering of atoms by a two-dimensional periodic hard corrugated surface with an attractive potential is presented. The theory is essentially a modification of a combination of the RR′ method of Garcia and Cabrera [Phys. Rev. B 18, 576 (1978)] and the method of Garcia, Goodman, Celli, and Hill [Phys. Rev. B 19, 1808 (1979)]. The working of the theory is illustrated by comparing its results with previous exact no-well calculations made by the RR′ and RR′Z methods, and by applying a simple square-well model of the H–NaF(001) system to some diffraction and bound-state-resonance data of the Erlangen group.
The angular dependence of scattered electron spectra of neon and argon in the region of quasidiscrete autoionizing states
Authors: Dillon M. A. , Spence David
Published: February 2003

Using 200 eV incident electrons, the energy-loss spectra is measured for e-Ar and e-Ne collisions. It is demonstrated that information on quasi-discrete autoionizing states can be obtained via the Born approximation.(AIP)
Kinetics of the reaction of CH3O2 with NO
Authors: Ravishankara A. R. , Eisele F. L. , Kreutter N. M. , Wine P. H.
Published: February 2003

The kinetics of the reaction CH3O2+NOk1→products were studied over the temperature range 240–339 K. CH3O2 radicals were prepared by 355 nm laser photolysis of (1) azomethane in the presence of O2, or (2) Cl2 in the presence of CH4 and O2 The temporal profile of NO2, a product of Reaction (1), was followed by monitoring the laser induced fluorescence excited by the 488 nm output of a cw argon ion laser. k1 was found to be nearly temperature independent with an average value of (8.1±1.6)×10−12 cm3 molecule−1 s−1. At 298 K, k1 was independent of the pressure and the identity of the diluent gas. The yield of NO2 from Reaction (1) was directly measured to be near unity (1.06±0.24) at 298 K.
Peak power dependence of rovibronic populations below the quasicontinuum during IR multiphoton absorption
Authors: Brenner D. M.
Published: February 2003

Rovibronic levels of the ground state of thiophosgene, perturbed by IR multiphoton absorption, are observed under collisionless conditions by laser-induced fluorescence. Evidence is presented that at peak powers of the CO2 laser≳3 MW/cm2 an energy distribution is produced in which the vibrational populations of levels v=0–4,6,8 of the mode ν4 are depleted or not populated, even though the pumped mode is 2ν4. Only at lower peak powers (longer pulse duration) are population increases observed. Under the latter conditions, it appears that 3ν4 is populated to a smaller extent than [ν1+ν3] to which it is coupled by strong Coriolis interaction. At room temperature no evidence for rotational ’’hole burning’’ effects during IR pumping are observed in the vibrationless level. The data suggest, in fact, that all rotational levels interact with the laser field, even when thiophosgene is rotationally cooled.
Time-resolved infrared spectral photography: Study of laser-initiated explosions in HN3
Authors: Avouris Ph. , Bethune D. S. , Lankard J. R. , Ors J. A. , Sorokin P. P.
Published: February 2003

Recent improvements in the technique of time-resolved infrared spectral photography (TRISP) are described. With this technique CO2 laser-induced thermal explosions of gas phase HN3/DN3 mixtures were investigated. HCl gas added to the mixtures was utilized to probe the transient temperature of the reaction. An induction period and a phase of rapid energy release were clearly identified and characterized both with respect to temperature and utilization of reactant material.
Concerning the chemical potential of few-electron systems
Authors: Katriel Jacob , Parr Robert G. , Nyden Marc R.
Published: February 2003

We examine the implications of a theoretical framework which assume a continuous dependence of atomic (molecular) energies on the number of electrons. Although this function E(Z,N) has discontinuous slopes in the limit as Z approaches infinity with N finite, it is shown that one can formulate a representation of E(Z,N) which is analytic for finite Z and has the appropriate limiting behavior. Assuming analyticity, for one-electron systems there is derived a formula for the chemical potential μ(1)=E(1)+(∂C/∂N)N=1, where C is the electron repulsion energy. From numerical data it is argued that (∂C/∂N)N=1 is nonzero for one-electron systems and that C is negative for N<1. The formula for the chemical potential is used in an empirical fit of atomic energies and to clarify a recently proposed definition of atoms in molecules.