bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman spectroscopic study of a phase transition in (TMA)2UCl6 and (TMA)2SnCl6
Authors: von der Ohe W.
Published: February 2003

A first order phase transition in (TMA)2UCl6 and (TMA)2SnCl6 with transition temperatures Tc1=116 and 142 K, respectively, upon decreasing and Tc2=131 and 156 K, respectively, upon increasing the temperature, is studied by Raman spectroscopy. The phase transition in (TMA)2SnCl6 is reported here for the first time. The Raman spectra of both crystals in both phases are incompatible with O5h, the space group indicated by x-ray investigations. In (TMA)2UCl6 the high temperature space group appears to be D112d, the low temperature structure has either C54h symmetry or a two-formula unit primitive volume. In (TMA)2SnCl6 one phase has a distortion containing a component of A2u symmetry species, the other phase has a two-formula unit primitive volume. The structural changes involve at least two independent distortions, one to be removed, the other one to be introduced. In neither crystal is the space group of one phase a subgroup of the space group of the other phase. The phase transitions are of first order on account of the symmetry change alone. They are of strongly first order, evidenced by the constancy of the peak frequencies above and below Tc, particularly of the frequency separations between neighboring bands, and by abrupt changes of the frequencies at Tc. The only band with a temperature dependent peak frequency, in the high temperature phase of (TMA)2SnCl6, is likely to be the soft mode of another phase transition of this crystal, occuring above its decomposition temperature. In the phase transitions under study vibrational bands appear and disappear by their intensity increasing from or decreasing to zero. Some bands which only exist in the high or low temperature phase, respectively, coexist over a temperature range of several degrees. In (TMA)2UCl6 the intensity change of two electronic Raman bands indicate a volume change at Tc of several percent. Preliminary scans of (TMA)2ZrCl6 at 90 and 300 K show that this crystal also undergoes a similar phase change. In the Appendix the equal cell subgroups of O5h are related to the symmetry species of the distortions having the form of a zone center vibrational eigenvector of the original structure.
Reaction rates and energy distributions among reaction products for the H+Cl2 and Cl+H2 reactions
Authors: Wilkins Roger L.
Published: February 2003

Rate coefficients have been calculated for the bimolecular exchange reactions of H with Cl2 and Cl with H2. Three-dimensional classical trajectories of the collision dynamics of these reactions have been calculated using London–Eyring–Polanyi–Sato (LEPS) potential energy surfaces. The results of this trajectory study are as follows. For the H+Cl2 reaction at room temperature, the relative rate coefficients for formation of HCl into specific vibrational states are: k (v′=1) =0.01, k (v′=2) =0.65, [k (v′=3) =1.0], k (v′=4) =0.10, and k (v′=5) ?0. The symbol v′ indicates the product HCl vibrational state. The relative rate coefficient k (v′=2)/k (v′=3) has a slight temperature dependence, and the other relative rate coefficients k (v′≠2)/k (v′=3) are temperature dependent. The mean fraction (?v′=Ev′/ Etotal and ?J′=EJ′/Etotal) of the total energy entering vibration plus rotation is ?v′+?J′= (0.46+0.05) =0.51. For the Cl+H2 (v=0) reaction, ?v′+?J′= (0.37+0.16) =0.53; and for Cl+H2(v=1), ?v′+?J′ = (0.57+0.11) =0.68. Both Cl+H2 reactions exhibit efficient conversion of reaction energy into internal excitation of the product HCl molecule. A smaller fraction of reaction energy becomes relative translational energy in the Cl+H2(v=1) reaction than in the Cl+H2(v=0) reaction. The reaction Cl+H2 is greatly enhanced by increased vibrational energy in the reagent molecule H2. The Cl+H2(v=1) reaction generates an inverted population of HCl(v′=1) molecules that can, under proper conditions, produce lasing from the v′=1 to v=0 transition. Temperature-dependent rate coefficients are provided for many reactions that have not been measured experimentally.
Time-dependent wavefunctions for two independent perturbations
Authors: Leipert Thomas K. , Grant David M.
Published: February 2003

A method of investigating the time development of two independent perturbations on a nuclear or electronic spin ensemble is presented. Without introducing unduly restrictive assumptions, a set of linear dependent differential equations is obtained, suitable for further numerical analysis. An explicit and general wavefunction Ψ(1)(t) is derived for the presence of one time-dependent perturbation. Explicit expressions for Ψ(2)(t), the wavefunction of the spin ensemble under the influence of two time-dependent perturbations, have been determined for various specific experimental conditions. The coefficients of expansion of Ψ(2)(t) are presented in a form directly applicable for the determination of density matrices in an interaction representation. Qualitative conclusions based on these expressions are in agreement with reported experimental observations.
Vibrational energy transfer in polyatomic molecule collisions. I. High order corrections to the transition probabilities of vibrational energy transfer
Authors: Jolicard G. , Galatry L.
Published: February 2003

An expansion technique of the evolution operator using the MAGNUS formalism enabled us to study the transition probability at one, two, three, and four quanta of vibration between the oscillators of two polyatomic molecules during a collision. In this paper the collisions are supposed to be not energetic enough to cause a series of multiple transitions, and the theory allowed us to study the validity of the first approximation of Born. At the same time the present work proposes a preliminary approach to the influence of the cubic and quartic potential constants and of the vibrational resonance on the vibrational transitions induced by collisions between polyatomic molecules. The theory is then applied to the collision CO2–He.
Multiconfiguration studies of some low-lying bound states of VH
Authors: Henderson George A. , Das G. , Wahl Arnold C.
Published: February 2003

Multiconfiguration calculations using a hybrid of multiconfiguration self-consistent field (MCSCF) and configuration interaction (CI) methods in a basis of Slater-type atomic orbitals (STO’s) of nominal accuracy are carried out for some low-lying bound states of VH. All the lowest states of the symmetries, 3Σ−, 5Σ−, 3Π, 5Π, 3Δ, 5Δ, 3Φ, and 5Φ, that dissociate to the ground states of the constituent atoms are bound and so are some of the excited states that have higher-energy atomic dissociation products. The ground state is the lowest 5Δ state for which the spectroscopic constants are computed to be Re=3.33 bohr, De=1.87 eV, and ωe=1585 cm−1. Potential curves and the computed spectroscopic constants are presented for the lowest states of all the above symmetries.
Vibrational relaxation of small molecules in the liquid phase: Liquid nitrogen doped with O2, CO, and CH4
Authors: Calaway Wallis F. , Ewing George E.
Published: February 2003

The vibrational relaxation time of liquid nitrogen doped with CH4, CO, and O2 at 77 °K has been measured by pulsed laser and spectroscopic techniques. Rate constants for V→V energy transfer from N2 to the dopants and V→T relaxation times have been obtained. These rates are consistent with a simple model which essentially treats the liquid as a high density gas where isolated binary collisions cause vibrational relaxation.
Rotational correlation times for ethanol by deuteron magnetic spin–lattice relaxation
Authors: Chen Teng ko , Beyerlein A. L. , Savitsky G. B.
Published: February 2003

The deuterium T1 relaxation times of perdeuterated ethanol in CCl4, benzene, and water solutions have been measured. The results are compared with theoretical determinations of the rotational correlation time. (AIP)
27Al–13C coupling constants in trimethylaluminum dimer and its derivatives
Authors: Yamamoto Osamu
Published: February 2003

The 27Al–13C coupling constants in dimers of trimethylaluminum, dimethylaluminum chloride and bromide, and diethylaluminum chloride, and in the trimethylaluminum–diethylether complex were obtained by line shape analyses of the unresolved broadened signals of the proton-decoupled 13C NMR spectra in a wide temperature range. There is a remarkable parallelism between 27Al–13C coupling constants in trimethylaluminum dimer and its derivatives and the 11B–1H coupling constants in diborane derivatives. This comes from the similarity in the electronic structure of the two types of electron deficient compounds, and can be accounted for in the framework of the Fermi contact mechanism for the spin coupling.
CO2 laser irradiation of ethyl iodide: A laser driven thermal explosion
Authors: Bellows James C. , Fong Francis K.
Published: February 2003

The CO2 laser irradiation of ethyl iodide in the pressure range 7–80 torr has been thoroughly investigated. The results can best be described as a laser driven thermal explosion (LDTE). The LDTE is characterized by a high temperature zone in which the laser energy is converted to heat. If the temperature of this zone is high enough, reaction begins. Higher absorptivity of the products leads to thermal instability and explosion. Temperatures in the system have been measured and adequately account for the entire reaction. A computer simulation based on purely thermal reaction confirms the conclusion that the reaction is entirely thermal. The importance of the high temperature reaction zone in infrared irradiation experiments is discussed.
Colliding beam measurements of ICR collision processes between alkali ion–atom pairs
Authors: Riggin M. , Bloom M.
Published: February 2003

A colliding beam ion cyclotron resonance experiment has been developed and used to study rate constants for symmetric (resonant) and asymmetric charge exchange between potassium ion–atom pairs and between sodium ions and potassium atoms in the range of ionic energies from 0.1–2.0 eV. The results for symmetric charge exchange are in agreement with theory, but the rate constant for the exothermic process Na++K→Na+K+ at near thermal energies appears to be much larger than predicted. The changes in the ion cyclotron double resonance (ICDR) signals of ions A+, when ions B+ were heated using ICR methods, were found to be sensitive in magnitude and sign to the difference between the static field strength and the A+ ICR resonant field, as well as the intensity of the rf electric field used to heat the B+ ions. These results have been interpreted in terms of a previously published theory which takes into account explicitly the spatial distribution of ions in the ICR cell. Semiquantitative information is obtained on the energy dependence of asymmetric charge transfer between Na+ and K and resonant charge transfer between 39K+ and 41K at very low energies.
A Raman spectroscopic study of the structural aspects of K2MgCl4 and Cs2MgCl4 as solid single crystals and molten salts
Authors: Brooker M. H.
Published: February 2003

Polarized Raman spectra have been obtained for oriented single crystals of K2MgCl4 and Cs2MgCl4 at 77 and 298 K. The data is in excellent agreement with factor group analyses based on the space groups I4/mmm (D174h) and Pnma (D162h) for the K2MgCl4 and Cs2MgCl4 crystals. In K2MgCl4 the magnesium is surrounded by six chloride ions in a distorted octahedral arrangement with a network structure such that neighboring octahedra share corners. In Cs2MgCl4 a discrete tetrahedral MgCl42− species is present. The 35Cl–37Cl isotopic splitting of the symmetric stetching mode of the tetrahedral MgCl42− species has been resolved at 77 K and is similar to that observed for CCl4. Raman spectra for the high temperature solids and molten salts suggest that the coordination number of magnesium changes from six in solid K2MgCl4 to four in the melt, whereas Cs2MgCl4 melts with retention of the MgCl42− tetrahedral complex. Additional evidence is presented to support previous reports that the MgCl42− tetrahedral species is the principal complex ion in the melts, although a fraction of the magnesium appears to be present in a polynuclear complex, perhaps Mg2Cl62−.
Molecular orbital model for antimony and bismuth luminescent centers in antimonates M(II) Sb2O6 (M=Ca,Sr,Ba)
Authors: Pedrini C.
Published: February 2003

A detailed semiempirical molecular orbital calculation is presented for computing the energy levels of various luminescent centers in the antimonates. In order to interpret the excitation spectrum of the nondoped compounds, the MO calculations are carried out on two clusters: (SbO6)7− and (SbO6)9−. In the first one, the central metal is Sb5+ with a point symmetry D3; in the second one the metallic ion is Sb3+, occupying the M(II) site with a point symmetry D3d. The results for (SbO6)7− do not agree at all with experiment; therefore, the possibility of a charge transfer process from the ligands to the Sb5+ ion is eliminated. However, good agreement with experiment is found for the (SbO6)9− calculations, supporting the idea that the electronic transitions may be assigned to centers Sb3+, produced by an oxido-reduction process, Sb5+?Sb3+, during the preparation of the compounds and placed at M(II) sites. The results concerning the pseudomolecule (BiO6)9− are in good agreement with the experiment in the case of antimonates doped by bismuth, permitting one to assign the observed excitation bands of bismuth to the low-lying excited triplet states.
Solvent effects on chain fluctuations in highly concentrated polyisobutylene solutions. A new description derived from high resolution NMR spectroscopy
Authors: Cohen-Addad J. P. , Roby C.
Published: February 2003

The purpose of the present paper is twofold. First it is shown that a standard analysis of high resolution NMR spectra can be defined to derive a physical parameter Δ, reflecting chain dynamics in polymer systems. This property was illustrated by observing temperature and solvent concentration effects on high resolution NMR spectra of protons linked to high molecular weight polyisobutylene chains. Three types of solvents were used, giving analogous results: CDCl3, CS2, and C7D8. Observations were made on broad concentration and temperature ranges. Attention was particularly focused on highly concentrated systems which are not easily observed from viscoelastic experiments. Then the description of solvent concentration and temperature effects on NMR data is investigated. It is shown with great accuracy that the parameter Δ−1 varies according to a linear function of the ratio c=ns/nm, where ns and nm are the solvent molecule and monomeric unit number, respectively: Δ−1=Δ−10 +δ−1c (δ−1 is a constant characterizing the polymer–solvent system). This linear representation is shown to apply also to the description of concentration effects on diffusion parameters previously measured on other polyisobutylene–solvent systems. This linear representation is contrasted to the conventional Williams–Landel–Ferry (WLF) representation used to describe free volume effects usually considered as well-reflected by diffusion parameters. Also, temperature effects on NMR data were not found to be described according to the WLF form but according to the following representation: the product TΔ−1 is a linear function of the absolute temperature T. It is concluded that chain fluctuations observed from NMR lines cannot be characterized by parameters such as the steady-state viscosities derived from viscoelastic measurements.
Vibrational energy transfer in HCl–CO mixtures
Authors: Seery Daniel J.
Published: February 2003

The rate of energy transfer from the first vibrational level of HCl to CO has been determined. Measurements have been made for the vibrational relaxation process for CO in mixtures with HCl by monitoring the 4.7 μ emission following shock heating. The time-resolved emission intensity profile is then reconstructed by a model calculation to determine the best rates for the reactions: HCl(v=1)+CO(v=0) →HCl(v=0)+CO(v=1) and HCl(v=0)+CO→HCl(v=1)+CO. The model calculations were carried out using a computer program which numerically integrates the system of gas dynamic, reaction kinetics, and state equations and calculates the vibrational population–time profiles for HCl and CO for an assumed reaction mechanism at the desired temperatures, densitites, and gas velocities. The rate constants for the HCl–CO V–V transfer and the HCl–CO V–T vibrational relaxation determined in this study show good agreement with comparable data in the literature.
Molecular ions and electron transport in rare gas lattices: C−2 formation mechanism and X 2Σ+g↔B 2Σ+u spectroscopy
Authors: Brus L. E. , Bondybey V. E.
Published: February 2003

Tunable laser excitation of the C−2 B 2Σ+u →X 2Σ+g emission spectrum in rare gas lattices uncovers a strong Personov effect—narrowing in emission of an inhomogeneously broadened absorption spectrum. Single site emission spectra show moderate coupling with delocalized lattice phonons, and the zero phonon ’’quasiline’’ broadens and shifts red as a function of increasing temperature. The solid phase spectroscopic constants suggest that C−2 is a substitutional guest in Ar, Kr, and Xe, while displacing two or more atoms in Ne. The formation of C−2 in 1216 Å photolyzed, acetylene doped rare gas solids can be understood in terms of single photon ionization schemes in dielectric media. A long range (10–20 Å) electron tunneling from the C−2 excited B state in Ne to a nearby cation is observed with rate constant ?103 sec−1. A general classification of ion pairs in dielectric media as either ’’coulomb’’ or ’’chemically bonded’’ is proposed.
A stochastic theory of cluster growth in homogeneous nucleation
Authors: Kitahara Kazuo , Metiu Horia , Ross John
Published: February 2003

A critical cluster in homogeneous nucleation can be defined in two ways. Thermodynamically, the size of the critical cluster corresponds to a maximum of the free energy of the system; kinetically the critical cluster is defined such that all supercritical clusters grow and all others decay. We describe the evolution of a cluster by a postulated stochastic master equation in which the transition probabilities are given by a formula proposed by Langer. We derive from these assumptions, by a path integral method, an equation for the most probable evolution of the size of the cluster and show thereby that under these circumstances the two definitions are equivalent.
Nonrandomization of energy in unimolecular reaction. The effects of high pressures on the apparent rate constant.
Authors: Oref I.
Published: February 2003

This paper proposes a general mechanism which explains the effects of high pressures on the effective unimolecular rate coefficient 〈ka〉. By assuming steady state concentrations of the energized moiety and molecule an expression for 〈ka〉 is obtained. Analysis of the expression for 〈ka〉 shows that nonrandom effects of energy distribution can be obtained only when the unimolecular rate coefficient of the energized molecule is large enough so as to provide an independent channel for decomposition. A comparison is made with work reported in the literature.
Magnetic susceptibility of 243AmO2
Authors: Karraker D. G.
Published: February 2003

The magnetic susceptibitlity of 243AmO2 from 2.2 to 97 °K. The sample showed an antiferromagnetic transition at 8.5±0.5 °K and to regions of temperature-dependent paramagnetism above 15 °K. (AIP)
Electron impact excitation of the gamma bands of nitric oxide
Authors: Imami Mahmood , Borst Walter L.
Published: February 2003

The excitation of several pronounced γ bands of NO by electron impact was studied from threshold to 1000 eV. A high optical efficiency was achieved by using an ellipsoidal collision chamber with mirrored interior surfaces. The interaction volume viewed was located at one of the foci of the ellipsoid. A 0.25 m double monochromator and single photon counting techniques were employed to monitor molecular band intensities. The optical sensitivity between 2000 and 4000 Å was obtained with the molecular branching ratio method by monitoring 1NG CO+, 1NG N+2, 2PG N2 band intensities and suitable progressions in the γ system of NO. Second order effects were avoided by low NO densities (∼10−1 torr) in the collision chamber. No ionization gauges were used during measurement, thus eliminating the possible dissociation of NO and subsequent formation of N2. Absolute cross sections for γ bands were obtained by normalizing to the 2PG (0,0) band of N2. For this purpose, a known mixture of NO and N2 was prepared in a gas handling manifold using a manometer. For the unambiguously identifiable (0,1), (0,2), (1,0), (1,5), and (2,7) γ bands of NO, the cross sections peaked near 18 eV and had values of 2.2, 1.8, 6.3, 1.3, and 0.57×10−19 cm2, respectively, with a possible error of 25%. The relative peak cross sections agree with calculated band intensities within 20%.
NMR study of motion in hydrazine sulfate
Authors: Harrell J. W. , Peterson E. M.
Published: February 2003

Proton rotating frame relaxation time (T1ρ) measurements have been made on hydrozine sulfate from 137 to 495 °K, and single crystal deuteron resonance measurements have been made from 297 to 501°K. The T1ρ measurements exhibit minima at 167 and 315 °K and decrease very rapidly above 473 °K. The deuteron resonance spectrum is constant up to 483 °K and narrows into a single central line above 488 °K. The measurements below the phase transition at 481 °K are analyzed in terms of NH3 reorientation and 180 °K flips of the N2H6++ ions. Above the phase transition the measurements are analyzed in terms of translational diffusion of the N2H6++ ions.
Effects related to pair correlation of π electrons
Authors: Kresin V. Z. , Litovchenko V. A. , Panasenko A. G.
Published: February 2003

A good deal has been written about correlation effects of electrons in molecules. We consider a somewhat special type of correlation, viz., the pair correlation of π electrons in large molecules. Various attraction mechanisms which lead to this correlation are discussed. We study the behavior of the large π electron systems in external electric and magnetic fields (i.e., anomalous diamagnetism, polarizability, analog to the Kirkwood formula). It has been shown that in a strong magnetic field pair correlation leads to an additional reduction of the magnitude of the 0–0′ transition. We also consider collective excitations and multiplet structure of the π electron spectrum.
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o computation of force constants. The second and third period hydrides
Authors: Schlegel H. Bernhard , Wolfe Saul , Bernardi Fernando
Published: February 2003

The force method has been employed to calculate the force constants of HF, H2O, NH3, CH4, HCl, H2S, PH3, and SiH4. The computations were performed using wavefunctions generated by both the STO-3G and 4-31G basis sets that are standard in the Gaussian 70 program system. The 4-31G basis set provides reliable harmonic force constants, but those computed with the STO-3G basis set are poor. Cubic force constants computed at the 4-31G level are fair to very good. Cubic stretching force constants are reproduced especially well, regardless of the basis set. Quartic stretching force constants are good, but quartic bending constants are in error. A component analysis has been developed and relations have been found that may be useful for qualitative and quantitative discussions.
Crossed molecular beam study of F+CH3I
Authors: Farrar J. M. , Lee Y. T.
Published: February 2003

The reaction F+CH3I→IF+CH3 has been studied at two relative collision energies of 2.6 and 14.1 kcal mole−1 using the crossed molecular beam technique. Angular and velocity distributions of the reactively scattered IF products have been measured. At the lower collision energy the distributions are consistent with reaction occurring through a complex for which the lifetime is many rotational periods. At the higher collision energy, the complex lives only a fraction of a rotational period. The energy distributions measured for these systems are shown to be fairly consistent with RRKM and phase space statistical models; in order to make a more reliable statistical calculation on this system, the stability of the intermediate complex, CH3IF, has been determined in independent measurements of the threshold for the reaction F2+CH3I→CH3IF+F.
Light-induced fluorescence of short-lived species produced by a pulse of radiation: The benzophenone ketyl radical
Authors: Hodgson B. W. , Keene J. P. , Land E. J. , Swallow A. J.
Published: February 2003

The optical absorption spectra, kinetic behavior and other properties of numerous short-lived species have been studied by flash photolysis and pulse radiolysis. By a simple modification of the technique we have now been able to measure the emission produced when a short-lived species, in this case the benzophenone ketyl radical, is itself excited by absorption of light, a property which is normally observable only when transient species are stabilized in a rigid medium. (AIP)
Dependence of free ion yield on electrical field strength in a dielectric liquid irradiated by 60Co γ rays
Authors: Casanovas J. , Grob R. , Blanc D. , Brunet G. , Mathieu J.
Published: February 2003

A corrected curve of free ion yield versus electric field is given for n-hexane as well as new curves for 2-methylpentane, 2,2,4-trimethylpentane, and carbon tetrachloride. Comparison with the Onsager theory of geminate ion recombination does not give reasonable agreement over the entire range of field strengths for any of the functions used.(AIP)
Low energy dynamics for reactive collisions of C+ with D2
Authors: Herbst E. , Champion R. L. , Doverspike L. D.
Published: February 2003

The reaction C++D2→CD++D has been studied with an ion-beam, gas-target scattering apparatus over the energy range 0.64–1.58 eV. The results indicate that no long-lived complex is formed in the reaction, however the product excitation and the backscattering indicate a strong three-body interaction. (AIP)
Matrix elements of the tight-binding method for lattices with D144h symmetry (part 2)
Authors: Jouanin C. , Albert J. P. , Gout C. , Daude N.
Published: February 2003

We have evaluated the matrix elements of the Hamiltonian in the LCAO method for all the crystals with D144h symmetry.
The electronic structure of nitrogen dioxide. I. Multiconfiguration self-consistent-field calculation of the low-lying electronic states
Authors: Gillispie Gregory D. , Khan Ahsan U. , Wahl A. C. , Hosteny R. P. , Krauss M.
Published: February 2003

Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO2 has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self-consistent-field (MC–SCF) wavefunctions have been computed for the low-lying X̃ 2A1, Ã 2B2, B̃ 2B1, C̃ 2A2, 4B2, 4A2, and 2Σ+g electronic states of NO2. The minima of the Ã 2B2, B̃ 2B1, and C̃ 2A2 states have all been found to be within 2 eV of the minimum of the X̃ 2A1 ground state; for these states, C2v potential surfaces have been constructed for purposes of a spectral interpretation. The 4B2, 4A2, and 2Σ+g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO2 ab initio calculations in three important areas: (1) The double-zeta-plus-polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained:     The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 Å for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the 2Σ+g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 Å and 1.42 Å and the inversion barrier is approximately 800 cm−1.
Proton spin–lattice relaxation in smectic TBBA
Authors: Blinc R. , Luzar M. , Vilfan M. , Burgar M.
Published: February 2003

The angular, temperature, and frequency dependences of the proton spin–lattice relaxation rate T−11 in the smectic A phase of TBBA have been determined. An analysis is made of the possible relaxation mechanisms in smectic A systems. The results seem to show that the angular dependence of T−11 is produced by the modulation of the intramolecular dipolar interactions due to the coupling between self-diffusion and local ’’director’’ fluctuations. The low temperature smectic phases of TBBA have been as well investigated. The decrease in the self-diffusion constant due to two-dimensional translational ordering at the smectic C→smectic H transition is shown to produce a dramatic increase in T1 and decrease in T1D. The rotation of the chain segments abruptly slows down on going from smectic H to smectic VI and discontinuously freezes out on going from smectic VI to smectic VII.
Dissociation of methane by electron impact
Authors: Winters Harold F.
Published: February 2003

The total dissociation cross section for methane is reported for electron energies between 10 and 500 eV. A maximum cross section of 4×10−16 cm2 occurs at 85 eV. Dissociation into ionic and neutral fragments have about equal probability for energies between 50 and 500 eV. At lower energies most of the fragments are uncharged, ground state, or long lived excited state molecules. The experimental technique is discussed in detail and criteria established for determining gases for which similar methods can be applied.
Laser-pulsed phosphorescence studies of the distance dependence for triplet–triplet energy transfer
Authors: Strambini Giovanni B. , Galley William C.
Published: February 2003

The distance dependence for the rate of triplet–triplet energy transfer between aromatic chromophores is established from time-dependent phosphorescence measurements. The phosphorescence decay of carbazole which serves as the energy donor is followed as a function of time in the presence of increasing concentrations of naphthalene (acceptor) in a rigid glass of 2:1 (v/v) ethanol–diethyl ether at 77 °K. A 10 nsec pulse at 337 nm from a N2 laser is used to selectively excite the donor. The nonexponential phosphorescence decays of the donor in the presence of acceptor can be interpreted in terms of an exponential (Dexter) variation of the rate of triplet–triplet energy transfer with intermolecular separation. Interpretation of the distance dependence for transfer in terms of donor–acceptor wavefunction overlap emphasizes the extension of molecular wavefunctions from the nuclei resulting in transfer over distances of greater than 10 Å. Transfer over these distances with the efficiencies observed can be understood in terms of a decrease in the effective nuclear charge of the carbon atoms of the donor and acceptor to a value of +1.0.
Relativistic and correlation effects in the 21.2-eV photoemission spectrum of atomic lead
Authors: Süzer S. , Banna M. S. , Shirley D. A.
Published: February 2003

Photoelectron spectra were obtained for atomic lead in the 700–800 °C range, using HeI radiation (21.2 eV) and a special, high-temperature cell. Relativistic effects were obvious, most notably in the relative intensity of 1.000:0.071 in the peaks arising from Pb+(s2p; 2P1/2):Pb+(s2p; 2P3/2) states. This is a consequence of the Pb(s2p2; 3P0) ground state being in fact mostly s2p1/22. Correlation effects were also apparent. Several final states inaccessible from an s2p2 initial-state configuration were reached; this was explained by admixtures of the configurations sp2d and s2d2 into the initial state. All transitions that would be allowed by the presence of these peaks were actually found. These ’’direct’’ observations of configuration interaction give an operational meaning to this concept that is not available to most spectroscopic methods. Energy-loss peaks were observed at higher pressure and attributed to excitation of optical levels in neutral Pb by photoelectrons.
Ion mobility measurements in dielectric liquids
Authors: Winokur Peter S. , Roush Marvin L. , Silverman Joseph
Published: February 2003

Ion mobility measurements were made on n-hexane and a 0.22M solution of biphenyl in n-hexane using a time-of-flight technique. A collimated beam of x rays from a flash x-ray generator was used to produce a layer of ions near one electrode of a parallel-plate conductivity cell. Part of the charge layer was swept under the influence of an electric field to the opposite electrode, and the motion of the charge was observed by means of a current-versus-time measurement. Mobilities have been determined by fitting theoretical current–time curves to experimental ones. The theoretical current–time curves were obtained by solving numerically the differential equations governing carrier motion during a mobility measurement. Diffusion, bimolecular recombination, motion under an applied electric field, and induced bulk liquid motion were included in the theoretical model. The measured spatial dose distribution, radiation yield data, applied voltage, and electrode separation were input constants to the theoretical calculation, while values for the anion mobility, cation mobility, and induced liquid acceleration were variable parameters. The theoretical model yields the spatial distribution of charge carriers and its time dependence during the charge transport. It also describes the electric field between the electrodes as a function of space and time and demonstrates the effects of space charge. The current–time curves generated from the model are in good agreement with those obtained experimentally. The theoretical model that includes the effects of induced bulk liquid motion yields values of mobility that are significantly lower than those determined experimentally by use of inflection or ’’half-steady-state’’ points. Measurements made on purified n-hexane showed that a radiation pulse produces more than one slow carrier of each sign. When biphenyl was added to the sample, one negative and two positive species were observed. Our theoretical fits to experimental curves obtained from the irradiation of a 0.22M solution of biphenyl in n-hexane yields a mobility value for the negative species of 3.6×10−4 cm2 V−1⋅sec−1. This negative species is believed to be the biphenylide anion.
Absorption and dispersion of the field induced dielectric increment in caprolactam–cyclohexane solutions
Authors: Hellemans L. , De Maeyer L.
Published: February 2003

The nonlinear dielectric effect in solutions of ε-caprolactam in cyclohexane is studied by a new differential technique over the frequency range of 2 to 100 MHz. The frequency dependent real and imaginary parts of the field induced permittivity increment are obtained. Cole–Cole plots show a concentration dependent dielectric relaxation, to which a single Debye term is fitted. The relation of relaxation times with concentration is not consistent with a simple dimerization mechanism. Static permittivity measurements indicate the presence of polar associates. An expression is derived including those contributions to the observed nonlinear effect, which are frequency independent within the range studied. Calibration measurements on diethyl ether–cyclohexane solutions are reported and the effect measured in pure cyclohexane is discussed.
Profiles of hydrogen stretching ir bands of molecules with hydrogen bonds: A stochastic theory. I. Weak and medium strength hydrogen bonds
Authors: Bratos S.
Published: February 2003

A theory is proposed to describe the shape, in inert solutions, of hydrogen stretching ir bands of complexes with hydrogen bonds. The theory uses stochastic arguments and is related to general theories of ir band profiles of liquids. The cases of weak and medium strength hydrogen bond are treated separately. In the former case the band profile is, essentially, a broad asymmetrically distorted Gaussian produced by an anharmonic coupling between the high frequency AH stretching mode and the perturbed low frequency AH⋅⋅⋅B stretching mode. In the latter case the profile is, essentially, a very broad Gaussian intersected by a number of flat or deep, broad or narrow Evans-type transmission windows. Here, the band shaping mechanisms are the anharmonic coupling between the AH stretching and different low frequency external motions and the Fermi resonance between states involving the AH stretching and some other internal modes. Spectral manifestations of hydrogen bonded liquids represent the most extreme case of vibrational broadening and the presence of the Evans holes is its characteristic feature.
General TRIPLE resonance on free radicals in solution. Determination of relative signs of isotropic hyperfine coupling constants
Authors: Biehl R. , Plato M. , Möbius K.
Published: February 2003

For the first time a general type of electron–nuclear–nuclear TRIPLE resonance has been performed on radicals in liquid solution. In this method two inequivalent nuclei are irradiated simultaneously at their respective NMR frequencies, the resonance being detected by an enhancement of a saturated ESR line. This general TRIPLE experiment, the analog of which is known already in the solid state (DOUBLE ENDOR), gives direct information about relative signs of hyperfine coupling constants. Several important aspects of the experimental setup are described. The experimental results are discussed on the basis of a simple relaxation model.
Multimode contributions to the anomalous shear viscosity
Authors: Garisto Frank , Kapral Raymond
Published: February 2003

The results of a mode–mode coupling calculation of the anomalous shear viscosity are presented. By explicitly including trilinear variables in the generalized Langevin equation expressions for the anomalous wave vector and frequency dependent shear viscosity are obtained which incorporate both vertex and frequency corrections. Explicit calculations of the anomalous viscosity of xenon have been carried out. The vertex corrections are found to be important and give rise to an additional anomalous contribution which, close to the critical point, is about 20% of the contribution considered in previous calculations. The inclusion of this effect leads to a much closer agreement with the experimental results. In addition, results are presented for the wave vector and frequency dependence of the shear viscosity which are relevant for Rayleigh linewidth calculations.
Mechanism for quenching of singlet state p-xylene vapor by nitric oxide
Authors: Burke John McKenzie , Schuh Merlyn D. , Sporborg Hamilton M.
Published: February 2003

Using a flash sensitized biacetyl emission technique it has been found that the principle mechanism by which nitric oxide quenches singlet state p-xylene vapor is enhanced intersystem crossing, not internal conversion. The kinetics analysis also produced an estimate for the quantum yield of unquenched S1→T intersystem crossing of 0.6.
Crossed molecular beam study of chemiluminescent reactions of Group IIIb atoms with O2
Authors: Manos D. M. , Parson J. M.
Published: February 2003

Crossed-molecular beams have been used to study reaction of O2 with La to form electronically excited LaO (A 2Π, B 2Σ+, and C 2Π), with Y to form YO (A 2Π), and with Sc to form ScO (A 2Π). No other emitters are observed under single-collision conditions. Relative rate constants for forming product vibrational and electronic states, extracted from wavelength scans, are found to be close to predictions made by prior statistical distributions obtained with no dynamical constraints. The collision energy dependences of the rate constants for forming specific vibrational states, determined by varying the O2 beam velocity, also are close to the prior predictions over a range of about 2–8 kcal/mole.
Radiative and nonradiative transitions in the first excited singlet state of simple linear aldehydes
Authors: Hansen D. Alan , Lee Edward K. C.
Published: February 2003

Radiative and nonradiative rates of asymmetric linear aldehydes (C2∼C6) have been measured in the gas phase. Observe radiative lifetimes are compared to calculated radiative lifetimes obtained from the Strickler–Berg (SB) expression. The calculated values are about the same as the observed values in large aldehydes, but the former is about six times greater than the latter in acetaldehyde. The radiative rates are about four orders of magnitude smaller than the nonradiative rates. The two most important nonradiative processes are S1→T1 intersystem crossing and type II processes, the latter playing an increasingly important role at high energy in the aldehydes with γ-hydrogens. The contrasting radiative behavior of the asymmetric aldehydes from that of the symmetrical ketones is intriguing and suggests important differences in their excited state equilibrium geometries and transition moments.
Electron paramagnetic resonance study of hydrogen and fluorine atom addition to SO2
Authors: McDowell C. A. , Herring F. G. , Tait J. C.
Published: February 2003

The electron paramagnetic resonance spectrum of the species formed in the near uv photolysis of mixtures of argon or krypton containing 1% HI and SO2, 1% CF3OF and SO2, or F2SO2 has been investigated at 4.2 K. The species formed when an HI/SO2 mixture is photolyzed has been identified as the symmetric σ radical HSO2. The assignment of the principal components of the g and hyperfine tensors was aided by our observation of preferential orientation of the trapped radical. The species formed when a CF3OF/SO2 mixture is photolyzed in the far uv has been identified as the symmetric σ radical FSO2. The g and A tensors have been determined from an exact solution of the spin Hamiltonian assuming a noncollinear tensor axis system.
A comparison of experimental determinations of electron affinities of pi charge transfer complex acceptors
Authors: Chen E. C. M. , Wentworth W. E.
Published: February 2003

The absolute electron affinities of pi charge transfer complex acceptors have been examined and the ’’best’’ values have been chosen. All of the results obtained by the magnetron method, including the estimates for hexafluorobenzene and tetracyanoethylene, were accepted. However, the magnetron results for anthraquinone and benzoquinone are not in agreement with charge transfer complex and half-wave reduction potential data. The half-wave reduction potential data and the charge transfer complex data for all of the other acceptors for which absolute electron affinities are available were found to be consistent with the usual correlation equations and their associated assumptions. The parameters from these correlations have been used to calculate the absolute electron affinities for about 150 acceptors.
Potential surface distortion and orbital reorganization upon change of electronic state. Formaldehyde
Authors: Özkan İlker , Chu San Yan , Goodman Lionel
Published: February 2003

Near Hartree–Fock calculations on the 1A1 ground, 2B1, 2B2 cationic, 2B1 anionic, and 1,3A2 optical excited states of formaldehyde are reported. Reorganization effects are analyzed at the group symmetry and orbital levels. The results are as follows: Energy—large reorganization effects are found on excitation to all states. Comparison of equivalent excitations to the isoelectronic molecule ethylene shows that the reorganization effects are significantly more severe in the polar H2CO. Orbitals—the b2 and π orbitals are strongly sensitive to the electronic state, leading to poor frozen orbital predictions of excited state properties (e.g., dipole moments). The frontier 2b2 orbital contains strongly bonding and antibonding regions depending on the electronic state; consequently it is better to label it as frontier sigma (σf) rather than as nonbonding (n). Dipole moments—the large dipole moments in the σfπ* states can be largely understood from the centroids of σf and 1b1(π) shifting towards the oxygen in σfπ*. Potential surfaces—only the B1 anionic and A2 optical states are predicted to have nonplanar potential minima. Fermi correlation alone cannot explain either the smaller out-of-plane angle or larger inversion barrier height of the 1A2 vs 3A2 states.
Electronic symmetries, dipole moments, and polarizabilities of molecules in Rydberg states determined from electric-field effects on absorption spectra
Authors: Scott John D. , Russell B. R.
Published: February 2003

The effects of intense electric fields (∼ 2×105 V cm−1) on the gas-phase, ultraviolet absorption spectra of acetone and dimethylsulfide are investigated for the purpose of determining the changes in the electric dipole moments and polarizabilities and the transition polarizations corresponding to three bands assigned to Rydberg transitions. The bands investigated are the 1954 Å band of acetone assigned to the 3s←n (1B2←1A1) Rydberg transition and the 1959 and 2284 Å bands of dimethylsulfide assigned to the 4p←n (1A1←1A1) and 4s←n (1B1←1A1) Rydberg transitions, respectively. Interpretation of the results revealed that, for the transition in acetone and the 4p←n and 4s←n transitions in dimethylsulfide, the dipole moments are reduced from the ground-state values by 0.48±0.02 D, 0.47±0.03 D, and 0.42±0.04 D, respectively, and the mean polarizabilities are increased by 17.3±1.5×10−24 cm3, 11.7±1.3×10−24 cm3, and 9.3±2.0×10−24 cm3, respectively. The transition polarizations were all found to be in agreement with their particular Rydberg assignments. Derivations of the mathematical equations necessary for the interpretation of the experimental data are presented.
The elastic and inelastic scattering of diatomic molecules by atoms: CsI+Ar, CsCl+Ar, CsI+Xe
Authors: Armstrong W. D. , Conley R. J. , Creaser R. P. , Greene E. F. , Hall R. B.
Published: February 2003

Measurements of differential cross sections for the elastic and inelastic scattering of CsI by Ar, of CsI by Xe, and of CsCl by Ar are reported at three relative translational energies E in the range 4–17 kJ mol−1. Both excitation and de-excitation of the molecules, which have internal energies approximately those of a gas at 844 K, occur with cross sections approaching those of elastic scattering for center-of-mass angles of 40° or greater. No rainbow structure is apparent; presumably either E is below the threshold for the transition from orbiting to rainbow scattering, or E is so high that any rainbow structure is obscured by the scattering near the primary beam, or more probably the molecular asymmetry and dipole moment cause complicated trajectories. The absence of structure corresponding to purely vibrational transitions, even when this would be resolved by the apparatus, suggests that a substantial fraction of the inelastic events involve changes in rotational energy.
Effects of pressure and temperature on the optical absorption spectrum of lead azide
Authors: Downs D. S. , Christoe C. W.
Published: February 2003

Optical absorption spectra of lead azide (PbN6) have been obtained at room temperature for pressures up to 22 kbar and at 1 bar for temperatures down to 77 K. The shift of the 3.35 eV absorption peak is discussed in terms of various exciton models.
Dissociative ionization of molecules by electron impact. I. Apparatus and kinetic energy distributions of D+ ions from D2
Authors: Stockdale J. A. D. , Anderson V. E. , Carter A. E. , Deleanu Liliana
Published: February 2003

A crossed beam apparatus for the study of electron impact dissociative ionization of molecules is described. D+ time-of-flight and energy spectra are presented for impact of electrons on D2, from energies near dissociative threshold to 300 eV. In addition to the group of D+ ions peaking near 8 eV kinetic energy from dissociative ionization via the 2Σ+u state of D+2, groups of ions of lower kinetic energy peaking near 5, 2, and possibly near 1 eV are observed. These ions appear to be due to excitation of states of D2 or D+2 lying near to or above the 2Σ+u state of D+2.