bluuebunny/crossref_metadata_2025 huggingface dataset:
Light propagation and reflection by absorbing cholesteric layers
Authors: Nehring J.
Published: February 2003

The generalized Mauguin–Oseen–de Vries theory which includes absorption for light propagating along the helical axis of a cholesteric liquid crystal is investigated in detail. Expressions and examples are given that show the effect of absorption on the eigenmodes and selective reflection band. As for a nonabsorbing cholesteric, the eigenmodes in general are elliptically polarized waves when expressed in a coordinate system that rotates in step with the cholesteric structure, with the ellipses having fixed orientations relative to the local optic axis. Absorption is shown to change the ellipticity and, in some cases, the sense of the modes and to reorient the major axes of the ellipses. A Mauguin range, where nearly linearly polarized modes rotate in step with the cholesteric helix, and a selective reflection band are shown to be present even if the birefringence of the cholesteric liquid crystal vanishes. Absorption also introduces a coupling between the eigenmodes which is noticed when the energy propagation is considered. For typical values of absorption used in displays it is shown that this coupling may be neglected. An expression is derived for the reflected intensity of an absorbing layer bounded on one side by a metallic reflector.
Free-jet fluorescence excitation spectrum of t
r
a
n
s, t
r
a
n
s-1,3,5,7-octatetraene
Authors: Heimbrook Lou Ann , Kenny Jonathan E. , Kohler Bryan E. , Scott Gary W.
Published: February 2003

The fluorescence excitation spectrum from 2980 to 2650 Å of trans, trans-1,3,5,7-octatetraene in a free jet has been measured monitoring emission in the region 3250 to 3500 Å. The vibronic bands of the 1 1Bu←1 1Ag (S2←S0) transition of octatetraene narrow significantly in a seeded free jet of ∼8 ppm octatetraene in helium (FWHM∼200 cm−1 for the room temperature static gas; FWHM∼35 cm−1 in the jet). Although absolute intensities of the vibronic bands are not determined, there is significant intensity in the origin, and the vibronic development involves only a few modes. Estimated translational, rotational, and vibrational temperatures of ∼1,<45, and <170 K, respectively, were achieved in the expansion. The major source of the observed linewidth is attributed to coupling of the 1 1Bu state to the lower 1Ag states.
The methylbenzenes vis-a-vis benzene. Comparison of their spectra in the Rydberg series region
Authors: Bolovinos A. , Philis J. , Pantos E. , Tsekeris P. , Andritsopoulos G.
Published: February 2003

Absolute extinction coefficient spectra of nine gaseous methylbenzenes (toluene, o-, m-, and p-xylene, 1,2,4- and 1,3,5-trimethylbenzene, 2,3,5,6-tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene) have been recorded with a moderate resolution of 2.5 Å in the region of the Rydberg excitations below the first I.P. Term values, quantum defects, and ionization potentials have also been determined for each Rydberg series. Comparison with the benzene spectrum shows the importance of s-type Rydberg series in the spectra of these compounds.
K x-ray absorption spectra of chlorine in 3d transition-metal complexes
Authors: Sugiura Chikara , Suzuki Takanobu
Published: February 2003

The chlorine K absorption spectra from MnCl2⋅4H2O, FeCl2⋅nH2O, CoCl2⋅6H2O, NiCl2⋅6H2O, and CuCl2⋅2H2O have been investigated using a 50 cm bent-quartz crystal vacuum spectrograph. Chemical shifts are found for the first absorption maximum and discussed in relation to the d-electron affinities of the divalent metal ions. The first maxima are attributed to the transitions from the chlorine 1s level to the empty antibonding orbitals originating mainly from the metal 3d and chlorine 3p orbitals. The other absorption structures are alike through five different hydrated dichlorides and are ascribed to the core-level-to-empty-state transitions in the Cl− ion. The present spectra are compared with those of the anhydrous metal dichlorides MCl2 (M = Mn, Fe, Co, Ni, and Cu) reported previously. It is illustrated that the first peaks are varied little by changing the anhydrides into the hydrates, but the other structures are considerably altered.
The first direct detection of 99Ru and 101Ru NMR: Ru relaxation and the Ru–17O coupling constant in RuO4. Comparision with OsO4
Authors: Brevard C. , Granger P.
Published: February 2003

High resolution NMR spectra of 99Ru and 101Ru have been observed in CCl4 solution of RuO4 for the first time. The different NMR parameters (frequencies, linewidths, and T1) have been measured for both isotopes. The 17O spectra leads to 1J17O–99Ru = 23.4 Hz. The dynamical behavior of RuO4 and OsO4 is compared using the measurement of T1 and T2 on 17O, 99Ru, 101Ru, and 189Os.
High temperature, high resolution infrared spectral measurements on the HNC–HCN equilibrium system
Authors: Maki Arthur G. , Sams Robert L.
Published: February 2003

Spectral measurements have been made on HNC in equilibrium with HCN at temperatures ranging from 900 to 1250 K. From measurements of absorption intensity vs temperature it is determined that the ground state of HNC is 10.3±1.1 kcal/mol (43.1±4.6 kJ/mol) above the ground state of HCN. High resolution data are given on the following vibrational transitions for HNC near 3650 cm−1: 1000–0000, 1110–0110, 1200–0200, and 1220–0220. Measurements near 2780 cm−1 are also given for the 1000–0000 and 1110–0110 transitions for DNC. Aside from the low bending force constant, these measurements show nothing unusual about the bending potential function.
Molecular beam photoionization of (H2S)n, n = 1–7
Authors: Walters Edward A. , Blais Normand C.
Published: February 2003

The photoionization thresholds of polymers of hydrogen sulfide (H2S)n were measured for n = 1–7. They are, for increasing values of n, 10.449±0.006, 9.74±0.01, 9.63±0.01, 9.61±0.01, 9.58±0.01, 9.50±0.02, and 9.63±0.03 eV, respectively. The sequence of ionization thresholds with increasing n does not correlate well with a linear relationship to 1/n as do other nonmetal clusters reported previously. We find a binding energy for (H2S)2+ of 0.737±0.012 eV. This value diminishes for each successive cluster to n = 5. At n = 6 the incremental binding is almost as large as for trimer formation while formation of n = 7 is endoergic, that is, (H2S)7+ is unstable with respect to (H2S)6++H2S.
Further comments on the dynamics of excitation trapping
Authors: Huber D. L.
Published: February 2003

We discuss the average t matrix (ATA) and first passage time (FPT) approaches to excitation trapping. It is argued that the FPT involves unrealistic assumptions about the relative magnitudes of the donor–trap and donor–donor transfer rates. The ATA, which allows for arbitrary rates, does not have this defect. In three dimensions the ATA provides a useful description of the decay of excitation in the presence of a low concentration of traps. In lower dimensions the situation is unclear. In one dimension the ATA agrees with the asymptotic form of the exact solution for a periodic array of traps, whereas the FPT gives better agreement with exact results when the traps are randomly distributed.
An integral of the Smoluchowski equation and its application to the prescribed diffusion approximation
Authors: Parlange J-Y. , Braddock R. D. , Lockington D. , Sander G.
Published: February 2003

An integral satisfied by the solution of Smoluchowski equation is derived and illustrate its usefulness by applying it to the prescribed diffusion approximation. (AIP)
Coreless Thomas–Fermi models of atomic structure
Authors: Nyden Marc R. , Acharya Prabhat K.
Published: February 2003

We examine two atomic models which are based on Thomas–Fermi theory. In both cases we impose a shell structure by spatially partitioning the atom into zones. In the coreless Thomas–Fermi model we define two zones corresponding to the core and outer electrons. In the shell constrained Thomas–Fermi model we use one zone for each occupied shell in the atom. In both cases we obtain the full atomic densities by combining a ‖1s‖2 hydrogenic description of the core electrons with a Thomas–Fermi treatment of the outer electrons. The ionization potentials and atomic properties computed from both models are substantially more accurate than the corresponding values obtained from the original Thomas–Fermi method. The shell constrained model leads to an effective description of atomic valence shells which may be useful as a starting point for a density functional theory of molecular structure.
Structure of HIF
Authors: Bartlett Rodney J. , Kahn Luis , Purvis George D.
Published: February 2003

The triatomic, inorganic free radical of unknown structure, HIF, is characterized by ab initio correlated many-body theory. The calculations show that HIF has a bond angle of 137.5±3°, bond lengths R(HI) = 1.64±0.5 Å and R(IF) = 2.04±0.5 Å, and a ground state of 2A′ symmetry. The computed dissociation energy for the process HIF→IF+H is 25 kcal/mole, while the experimental value is 30. The bonding in HIF is very similar to a noncollinear superposition of the two diatomic molecules HI and IF. Correlated calculations for IF, HI, HF, and the electron affinity of F are also reported. The accuracy of the dissociation energy in HIF directly parallels the accuracy of the F electron affinity.
Reactions between neutrals clustered on ions
Authors: Rowe B. R. , Viggiano A. A. , Fehsenfeld F. C. , Fahey D. W. , Ferguson E. E.
Published: February 2003

The observation of a new kind of gas phase chemical reaction is reported. The reactions of O3 and N2O5 molecules with NO are greatly enhanced when the O3 and N2O5 molecules are clustered to chemically weak alkali ions. The alkali ions do not participate in chemical bonding but simply lower the activation energy for neutral reactions by the electrostatic attraction between ion and neutrals.
Generating a 3D H3 potential energy surface from the 1D surface
Authors: Agmon Noam
Published: February 2003

A 3D potential energy surface for H3 can be generated from the collinear surface by addition of the triplet repulsion between the end atoms multiplied by a ’’screening function.’’
Padé approximants and the random close packing of hard spheres and disks
Authors: Aguilera-Navarro V. C. , Fortes M. , de Llano M. , Plastino A.
Published: February 2003

It is shown that the highest order Padé approximants based on the known virial expansion in powers of the density for a classical gas of hard spheres are consistent with the empitical value of the random close packing density where the pressure diverges. (AIP)
Many-body perturbation theory and phosphorescence: Application to CH2
Authors: Phillips Philip , Davidson Ernest R.
Published: February 2003

A diagrammatic formalism is derived which facilitates the calculation of phosphorescent lifetimes. This formalism uses double perturbation theory where the spin–own–orbit interaction and the two electron fluctution potential are the perturbations. The transition amplitude between the ground state and the phosphorescent state is calculated to first order in both perturbations. The phosphorescent lifetime of the 1A1 state of methylene is evaluated, by way of illustration, and is predicted to be in the range of 40–100 s. The transition amplitude, in this case, is due primarily to the direct spin–orbit mixing of the 1A1 state with the 3B1 ground state.
Further quasicomponent distribution function analysis of liquid water. Temperature dependence of the results
Authors: Mezei Mihaly , Beveridge David L.
Published: February 2003

Metropolis Monte Carlo computer simulation results are presented for liquid water as represented by the MCY and ST2 potentials to study the temperature dependence of the calculated internal energy, heat capacity, pressure, and the molecular distribution functions and to present the hydrogen–oxygen and hydrogen–hydrogen radial distribution functions, the distribution of the near-neighbor dipole angles and pair energies, the size distribution of cavities in the liquid, and the radial distribution function for molecules with a given coordination number. A detailed comparison with available experimental data documents the degree of agreement to be expected from computations at this level of approximation and characterizes the sensitivity of results to the choice of intermolecular potential function. The simulations performed at 37 and 50 °C on the MCY water at the corresponding experimental densities reveal a small but significant shift in the direction of structure loss when compared with our previous results at 25 °C.
Elastic constants of nematic and cholesteric liquid crystals and tensor fields
Authors: Vertogen G. , Flapper S. D. P. , Dullemond C.
Published: February 2003

A simple derivation of the continuum theory of nematic and cholesteric liquid crystals is given in terms of tensor analysis. The elastic constants of a number of molecular models are calculated by conceiving these models as tensor fields.
Formulas for determining local properties in molecular-dynamics simulations: Shock waves
Authors: Hardy Robert J.
Published: February 2003

Formulas are given that relate the mass, momentum, and energy densities and the momentum and heat fluxes to the masses, positions, and velocities of the individual particles making up a system. The formulas are similar to those of Irving and Kirkwood, but have forms that are easily implemented in molecular-dynamics simulations. Even when simulating very inhomogeneous phenomena such as shock waves, the densities and fluxes exactly satisfy conservation laws for mass, momentum, and energy. Corrections to the virial formula for the pressure and to the related formulas for the stress tensor and heat flux are obtained. The relationship of the formulas given to those used by others is discussed.
Free energy of mixing, phase stability, and local composition in Lennard-Jones liquid mixtures
Authors: Nakanishi Koichiro , Okazaki Susumu , Ikari Kenji , Higuchi Tetsuji , Tanaka Hideki
Published: February 2003

By the use of umbrella-sampling technique in the Monte Carlo method proposed by Torrie and Valleau, reliable data have been established for the Helmoholtz free energy of mixing in some selected models of Lennard-Jones liquid mixtures. The variational method in perturbation theory is found to account for these Monte Carlo data. Validity of some representative semiempirical theories is discussed and it is found that none of them can reproduce the present Monte Carlo results reasonably well. Finally the phase stability of the present models is discussed in terms of both the free energy of mixing and local composition values recently obtained from our molecular dynamics calculations.
Analysis of scattering data for liquid neon: The isothermal density derivative of the pair correlation function
Authors: Haymet A. D. J. , Rice Stuart A.
Published: February 2003

From neutron scattering data for liquid neon we obtain two quantities, the isothermal density derivative of the pair correlation function and the effective pair potential, using a first-principles integral equation which contains systematic corrections beyond the superposition approximation. The calculated density derivative is compared directly with experimental results. The calculated pair potentials at three different densities reflect the statistical fluctuations in the experimental data. They are compared with a fitted potential for neon and with a pair potential obtained by inversion under the superposition approximation. The differences between the two calculations indicate the importance of triplet correlations and the inadequacy of the superposition approximation in these applications, and contribute to an understanding of long-standing doubts concerning the accuracy of effective pair potentials.
A mean free path theory of void diffusion in a porous medium with surface diffusion. II. Numerical evaluation of the effective diffusivity for arbitrary Knudsen number
Authors: Ho Fun Gau , Strieder William
Published: February 2003

A mean free path kinetic theory of gaseous void transport with simultaneous surface diffusion on the pore walls has been developed for diffusion through a porous medium. A variational upper bound expression for the effective diffusivity is applied to a model porous medium, a bed of randomly placed, freely overlapping solid spheres all of the same radius. The effects of tortuosity are rigorously considered by explicitly including the flux of diffusing material around obstructions in its path in the trial functions. Numerical calculations of the variational integrals are performed for porosities from 0.01 to 0.90 and the full range of Knudsen numbers and surface diffusivities. From asymptotic values at large Knudsen numbers, a table of effective diffusivities versus the surface diffusion coefficient is presented for pure Knudsen diffusion in the voids.
Quasipercolation: Charge transport in fluctuating systems
Authors: Schiller Robert , Vértes Ákos , Nyikos Lajos
Published: February 2003

Mobility of charge carriers in certain liquid systems is controlled by temporal fluctuations in local conductivity. Fast transport proceeds along high mobility regions similarly to traditional percolation with the difference that these regions form and fade away with fluctuations. By making use of the idea of waiting time distribution of continuous time random walk, formulas for relative mobility as a function of the expectation value of proportion of high mobility regions are suggested. The results compare reasonably well with experimental data. Under the experimental conditions given quasipercolation theory and the effective medium theory of traditional percolation do not differ too much numerically. For site percolation threshold in nonfluctuating systems the expression [(e−1)/ez]1/2 is suggested, where e is the base of natural logarithm and z is the coordination number.
Flow light scattering. III. Theoretical variation of the effect with the rate of shear and its potential practical significance
Authors: Heller Wilfried
Published: February 2003

Theoretical data are reported on flow light scattering by prolate spheroids at ϑ = ω = 90°, ω being the equatorial angle with respect to the direction of flow. The primary variables considered are (1) the length and axial ratio of the spheroids within the range of dimensions accessible to the theory of Stevenson and (2) the velocity gradient. Simple analytical procedures are outlined for the determination of spheroid dimensions from the data obtained, combined, if necessary, with additional data on flow light scattering. It is shown that flow light scattering by means of the analytical technique outlined may allow one to obtain information not only on the dimensions of spheroids at rest but also on the potential reversible or irreversible dimensional changes produced by flow.
Combination rules for intermolecular potential parameters. II. Rules based on approximations for the long-range dispersion energy and an atomic distortion model for the repulsive interactions.
Authors: Diaz Peña M. , Pando C. , Renuncio J. A. R.
Published: February 2003

Combination rules for intermolecular potential parameters based on the identification of the attractive term of the potential function with different expressions for the long-range dispersion energy and the introduction of an improved model for the combination of repulsive potentials are derived and applied to the Lennard-Jones (12–6) and Kihara potentials. The ability of the rules to predict interaction virial coefficients and unlike-pair potential parameters obtained from viscosity data is examined. Results are compared to those obtained in similar tests in part I of this study. Combination rules based on the London dispersion formula and an assumption of a geometric mean rule for the distances at which the repulsive forces are in equilibrium are shown to have advantages over previously proposed rules for the two potential functions assumed.
Kinetics of quenching of luminescent probes in micellar systems. II
Authors: Tachiya M.
Published: February 2003

In a previous paper [Chem. Phys. Lett. 33, 289 (1975)] we treated the kinetics of quenching of luminescent probes in micellar systems, assuming that the distribution of solubilized molecules among the micelles obeys Poisson statistics. In this paper we extend our treatment to a more general case where there is a limit to the number of solubilized molecules in any one micelle. Mechanisms for migration of solubilized molecules between micelles are also discussed.
Collision transfers between BH, A 1Π, v′ = 0 rotational states, induced by He, Ar, or H2 : Experiments
Authors: Nédélec O. , Dufayard J.
Published: February 2003

BH is obtained by high-frequency discharge in 2% B2H6 mixed with He, Ar, or H2. It is selectively excited by a pulsed dye laser to A 1ΙΙ, v′ = 0, J′ = 1, 4, or 7. From these rotational levels, the total collision cross sections are obtained by lifetime measurements at known pressures and temperatures. They decrease as J′ increases. They are smaller for He than for H2 or Ar. The rare gases induce only rotational transfers. H2 also induces a quenching which is much smaller than the rotational transfers. The relative intensities of the satellite lines are measured for J′ = 4 up to ΔJ′ = ±3 : the levels of the same total parity as the initial level are more populated than the levels of the opposite total parity, this propensity decreasing from He to H2 and to Ar. The polarization of the excited lines does not vary appreciably with the pressure.
Full-response pseudochannels: A new method for converging coupled-channels scattering calculations. Theory and examples
Authors: Thirumalai Devarajan , Truhlar Donald G.
Published: February 2003

We present a generalization of the matrix effective potential (MEP) formalism presented previously [D. G. Truhlar and K. Onda, Phys. Lett. A 76, 119 (1980); K. Onda and D. G. Truhlar, Phys. Rev. A 22, 86 (1980)]. In the orginal MEP we used perturbation theory to define pseudochannels that gave the correct linear response of a system to a perturbing particle in the adiabatic limit at all geometries of approach. The pseudochannel was used in conjunction with the true ground-state channel for nonadiabatic calculations of the elastic scattering and the total inelastic cross section. In the present work we use a variational approach to define pseudochannels that give the correct infinite-order (full) response of the adiabatically perturbed system. The pseudochannels are used with more than one true channel of the system; this allows the study of selected inelastic processes as well as the total inelastic cross section. The new approach is compared to the original one for vibrationally inelastic collinear collisions of He with H2 and C12. It is also compared to conventional close coupling calculations; this comparison illustrates how the method can serve as the basis of a general convergence-acceleration technique for coupled-channels calculations of inelastic cross sections.
Dipolar model for collisional energy transfer between dark and radiating excited electronic states: CaO(A′ 1Π, a 3Π) +N2O ⇄ CaO(A 1Σ+)+N2O
Authors: Alexander Millard H.
Published: February 2003

Experimental evidence indicates that collisional transfer between low-lying excited electronic states occurs readily in a wide variety of diatomic molecules, in particular the alkaline earth oxides. We present here a model for this process, based on the long-range coupling of a permanent dipole of a polar collision partner and a transition dipole between the electronic states of the molecule of interest. We specifically investigate how spin–orbit or orbit–rotation mixing of two Born–Oppenheimer states can lead to substantial transition dipoles between the eigenfunctions of the full Hamiltonian for a diatomic molecule. The collision dynamics are treated within the time-dependent Born approximation, modified to ensure statistical microreversibility. A formulation in terms of spherical tensors facilitates the application of known techniques to the collision of molecules whose eigenfunctions are linear combinations of states of nonzero electronic angular momentum. We describe the calculation of cross sections and rate constants for transfer within and between the rotational manifolds of selected near resonant vibrational levels of the (nominally) A 1Σ+, A′ 1Π, and a 3Π states of CaO, as well as for transfer between the Λ-doubling components of the A′ 1Π state. The rate constants for electronic state transfer are substantial (≳1×10−10 cm3/molecule s) over a wide range of rotational levels in the neighborhood of the various isoenergetic points, where, for a particular pair of vibrational quantum numbers, the rotational ladders cross. No evidence is seen for a continuous transfer rate, independent of rotational quantum number, even at high values of J.
Scaling of equilibrium bond lengths, chemical potential, and energy in a model of heavy tetrahedral and octahedral positive molecular ions
Authors: Lawes G. P. , March N. H.
Published: February 2003

As a starting point for the study of scaling properties of polyatomic molecules, we consider here a central field model of heavy tetrahedral and octahedral molecular ions in the Thomas–Fermi limit. For this model the main results are (i) the equilibrium bond length of the positive molecular ion has the scaling property Re = z−1/3 f(N/z, n/z), ze being the charge on the central nucleus, ne being the total charge of the outer nuclei, and N?z+n being the total number of electrons; (ii) the self-consistent potential energy V(r) scales as V(r) = z4/3g(X, N/z, n/z, x), where r = bx, the internuclear distance R = bX, with b proportional to z−13. This potential then determines the electron density and the other properties of the ground state of the molecular ion. Some numerical solutions are presented for different degrees of ionization to illustrate the method.
Unified analytical evaluation of two-center, two-electron integrals over Slater-type orbitals
Authors: Yasui Jun , Saika A.
Published: February 2003

A unified analytical expression is given for all types of two-center electron-repulsion integrals over Slater-type atomic orbitals. By means of the Neumann expansion of the inverse interelectronic distance (1/r12) in prolate spherical coordinates, Coulomb, hybrid, and exchange integrals are expressed in terms of the function W1‖ m ‖ introduced by Kotani. The present formulation allows a preliminary estimation of the number of terms in the expansion to be retained for a desired accuracy, resulting in a significant saving of computation time and working storage. The method is implemented on a computer for various type atomic orbitals from 1s to 4f with an accuracy of 12 decimal places.
An MPI search for the π→3p Rydberg states of ethylene
Authors: Gedanken A. , Kuebler N. A. , Robin M. B.
Published: February 2003

Two-photon resonances in the 56 000–64 100 cm−1 (1780–1560 Å) region of the multiphoton ionization (MPI) spectra of ethylene-h4 and -d4 have been observed and interpreted as π→3s excitations, vibronically induced by the torsional modes ν4′ and ν4\. As in the one-photon spectrum, the excitation is rich in ν2′(C = C stretch), and double and quadruple quanta of the ν4′ torsion. An earlier suggestion that π→3p transitions are responsible for anomalies in the π→3s MCD spectrum of ethylene-h4 is not supported by this work, which assigns the anomalies to one-photon π→3s absorptions vibronically induced by a single quantum of ν4′. MPI spectra of ethylene-h4 and -d4 two-photon resonant in the 61 000–71 000 cm−1 region also have been recorded and the vibrational structure analyzed. Prominent electronic origins are found at 62 905 and 66 875 cm−1 in ethylene-h4, and at 63 115 and 67 030 cm−1 in the -d4 compound. These two transitions are assigned as π→3py and π→3px, respectively, where y is in-plane short axis, and x is out-of-plane. As in the π→3s one-photon and two-photon transitions, the two-photon transitions to 3p display long progressions of ν2′ vibrations, each member of which is accompanied by the 2ν4′ and 4ν4′ torsional doublets.
The evaluation of three-dimensional rotational averages
Authors: Wagnière G.
Published: February 2003

Rotational averages of even rank up to 10 are expressed as linear combinations of invariants and are shown to be easy to evaluate by computer programming. Simple relations between averages of different rank are obtained. Averages of odd rank may be straightforwardly derived from averages of even rank.
Electronic structure of vinoxy radical CH2CHO
Authors: Dupuis M. , Wendoloski J. J. , Lester W. A.
Published: February 2003

A b initio multiconfiguration Hartree–Fock (MCHF) wave functions have been used to describe the electronic structure of vinoxy radical CH2CHO. The energy difference between the ground 2A\ state and the first excited 2A\ state is found to be 3.22 eV using a double zeta quality basis set, in good agreement with the observed transition at 3.57 eV. In addition, the calculations predict a near infrared electronic transition.
Transition probability of the 1S → 1D2 emission of atomic oxygen
Authors: Chung Sunggi , Lin Chun C. , Fischer Charlotte F. , Lee Edward T. P.
Published: February 2003

The transition probability of the 1S0 → 1D2 emission (5577 Å) of atomic oxygen has been calculated in the LS-coupling scheme. The wave functions used in this work have been calculated by the multiconfiguration Hartree–Fock method. Two sets of wave functions were generated; the larger set consists of 15 configurations for the 1S0 state and 21 for the 1D2 state, and the smaller set consists of seven (1S0) and nine (1D2) configurations. The resulting value of 1.22 s−1 agrees well with the previous calculations of recent years, and is also in essential agreement with experimental measurement.
A theoretical potential energy surface study of several states of the methoxy radical
Authors: Jackels Charles F.
Published: February 2003

Several low-lying potential energy surfaces of the methoxy radical have been studied at C3v conformations using ab initio quantum chemical techniques. A double-zeta quality basis set augmented with polarization and diffuse functions has been used throughout. Eight doublet and quartet states have been characterized as a function of C–O distance. The equilibrium conformation of the 2E ground state is found (RCO = 1.405 Å, RCH = 1.112 Å, ϑHCO = 111.3°), and the harmonic vibrational frequencies of the three a1 modes are calculated (3118, 1330, and 1017 cm−1). The ? 2A1←? 2E vertical excitation energy has been estimated to be 4.01 eV. The wave functions for the ? and ? states have been analyzed in terms of their dipole moments, population analyses, and orbital contour plots.
Discrete and quasicontinuum level fluorescence from infrared multiphoton excited SF6
Authors: Hudgens Jeffrey W. , McDonald J. D.
Published: February 2003

Infrared laser multiphoton excited SF6 shows infrared fluorescence spectra characteristic of the laser excitation process. These fluorescence experiments were conducted under dilute conditions so that the effects of collisions are essentially eliminated. After laser preparation fluorescence from molecules populating the quasicontinuum and discrete level ensembles are observed. These ensembles show distinctly different spectral characteristics which help demark energy zones which these types of levels occupy. Discrete levels which exist at lower vibrational energies show sharp fluorescence characteristic of energy localized in the laser excited mode. Quasicontinuum levels which exist at chemically interesting energies reveal that collisionless intramolecular relaxation has occured. Model calculations on thermally prepared and infrared laser prepared SF6 molecules allow the assignment of the average energy deposition into SF6 as a function of fluence, and of the population distribution of SF6 within the quasicontinuum. The model simulations of the fluorescence show that the population distribution of the quasicontinuum SF6 ensemble is nearly Boltzmann. At the lowest energies, however, a significant excess population of SF6 molecules is trapped within the discrete states.
Origin of the coloration of yellow fluorites: The O3− center structure and dynamical aspects
Authors: Bill H.
Published: February 2003

This paper demonstrates that the color of natural yellow fluorites is due to an O−3 molecule ion incorporated into the lattice. The center has several equilibrium positions. A detailed EPR and ENDOR study of this center is presented and the motional effects observed in the EPR spectrum are analyzed.
ESR investigation of matrix isolated B 16O and B 17O radicals: Comparison of nuclear hyperfine structure with a
b i
n
i
t
i
o calculations
Authors: Knight Lon B. , Wise M. B. , Davidson E. R. , McMurchie L. E.
Published: February 2003

An extensive ESR investigation of the 2Σ B16O and B17O radicals in rare gas matrices has been conducted. Several different trapping sites in neon matrices have been found and investigated as a function of temperature over the range 3–10 K both during and after deposition. Motional averaging of the nuclear hyperfine tensor seems to be occurring for two neon sites and the argon matrix. The large isotropic boron hyperfine coupling of these sites and the 17O hfs show excellent agreement with ab initio theoretical results. BO radicals trapped in a third neon site do not appear to be undergoing motional behavior. Isotropic and dipolar hyperfine parameters for this site show reasonable agreement with theoretical results. BO radicals in one neon site which is extremely sensitive to temperature effects exhibit nearly perfect ’’single-crystal-like’’ preferential orientation which has not been previously observed in rare gas matrix ESR studies. The results of this BO investigation help to correct erroneous conclusions of an earlier study. Details of the ab initio theoretical methods used to obtain the calculated magnetic parameters are presented. Newly obtained theoretical findings for the related 2Σ AlO radical are compared with previous ESR neon matrix results.
Molecular motion in anisotropic medium. I. The effect of the dipolar interaction on nuclear spin relaxation
Authors: Bernassau J. M. , Black Edward P. , Grant David M.
Published: February 2003

The diffusion equation in the presence of a restoring potential is solved and the result is applied to dipolar NMR relaxation in liquid crystal solvents. The potential imparted by the solvent is approximated by a series of rotation matrices of rank two. The solution of the diffusion equation is expressed as an infinite series of rotation matrices. It is then shown that only the terms of rank two of this series are necessary to compute the dipolar spectral densities. This theory predicts the spectral densities to be different for different z-projection numbers, even in the case of extreme narrowing.
CARS spectroscopy of molecules in supersonic free jets
Authors: Huber-Wälchli Peter , Nibler Joseph W.
Published: February 2003

CARS spectra are reported for N2, O2, H2, D2, C2H4, and various mixtures cooled by supersonic jet expansion. Analysis of the Q branch structure of the diatomic molecules permits direct measurement of rotational populations, temperatures, and densities as a function of position in the free jet zone. Good agreement is obtained with temperature results for N2 from electron beam fluorescence studies and with densities calculated from the customary isentropic expansion equations. The detection sensitivity was sufficient that, for the first time, broadband CARS spectra were obtained for a molecular beam (D2). A simple relaxation model of the expansion process yields rotational collision numbers of 10±4 And 14±4 for N2 and O2 but these values may be high because of condensation heating. The model was not successful in treating the rotational relaxation of D2 and H2 but temperature measurements show qualitatively that D2 ⋅⋅⋅ X transfer efficiency increases in the order X = D2<He<Ar<CH4.
High pressure luminescence of metalloporphyrins in liquid solution
Authors: Politis T. G. , Drickamer H. G.
Published: February 2003

High pressure luminescence measurements have been made on free-metal tetraphenyl-porphyrin (H2-TPP), as well as on Mg-TPP, Zn-TPP, and cuoctaethylporphyrin Cu-OEP), in a variety of organic liquid media. Radiative and nonradiative rates have been calculated as function of pressure for all compounds studied. It has been shown that the changes of the radiative rates are due to the change of the square of the refractive index with pressure. In the case of H2-TPP, Mg-TPP, and Zn-TPP only fluorescence was observed and the nonradiative rates are determined by viscosity. In the case of Cu-OEP only phosphorescence was detected from two states (2T1 and 4T1). The relative energies of these two excited states have been found to be pressure dependent. The radiationless rate 4T1→S0 scaled very well with the 4T1→S0 energy difference, while the intersystem crossing rate 2T1→4T1 is controlled by the 2T1→4T1 energy gap. Solvents which freeze at intermediate pressure have been also used in order to study the effects of the phase transition on the luminescence of the compounds studied.
Optical absorption spectrum of KI:Sn2+. II. Theoretical calculation of the line shape
Authors: Sivasankar V. S. , Kamishina Y. , Jacobs P. W. M.
Published: February 2003

The line shape of the optical absorption spectrum of KI:Sn2+ has been calculated at different temperatures in the range 10–300 K. The eigenvalues and eigenfunctions corresponding to the electronic states of the (a1g) (t1u) configuration of the impurity ion have been obtained within that configuration by considering a Hamiltonian which includes exchange, spin-orbit and electron-lattice interactions. The 12×12 Hamiltonian matrix was diagonalized directly, thereby dispensing with the need for a perturbation approach. Interactions of the electrons of the impurity ion with lattice vibrations of A1g, Eg, and T2g symmetry were accounted for. Integration over the interaction mode coordinates Q2, Q3,...,Q6 was carried out by the Gaussian quadrupole method and the interaction with the A1g mode achieved by the usual convolution procedure. Within the framework of the conventional theory which assumes only cubic centers and takes into account only the linear terms in the electron-lattice interaction, reasonable agreement between theory and experiment could not be obtained. Therefore, centers with tetragonal symmetry were also considered by including a term in the Hamiltonian which represents interaction with the static crystal field of appropriate symmetry due to a nearby cation vacancy. Satisfactory results were obtained only when the coexistence of cubic and tetragonal centers in the approximate ratio 3:1 was assumed. Further improvement was achieved by taking into account the quadratic terms in the electron-lattice interaction with the A1g mode. The residual discrepancy is attributed, among other factors, to the neglect of quadratic terms in the interaction with the Eg and T2g modes.
The orange arc bands of CaO. Analysis of a D,d 1,3Δ–a 3Π system
Authors: Marks Ronald F. , Schweda Hartmut S. , Gottscho Richard A. , Field Robert W.
Published: February 2003

The orange and green arc bands of CaO are shown to arise from Ca+ centered (4p,3d)-4s transitions analogous to the CaF A 2Π–X 2Σ+ and B 2Σ+–X 2Σ+ systems. The tremendous complexity of these Ca+O− systems is a consequence of the sixfold degeneracy of the O− p-hole, as compared to the nondegenerate F−(1S). The 0, 0 and 1, 1 bands of a D,d 1,3Δ–a 3Π transition are recorded by intermodulation spectroscopy and assigned (vibration, rotation, absolute parity) by nonstandard techniques. The D 1Δ state is found to lie below the isoconfigurational d 3Δ1 state and D∼d spin-orbit perturbation causes the d 3Δ state to become irregular (3Δ2≳3Δ1≳3Δ3). Two perturbation effects provide the key to the orbital interpretation of the orange and green arc bands: lambda doubling in a 3Π indicates that a π4σσ3Σ+ state lies 1500 cm−1 above a 3Π; D, d 1,3Δ∼B 1Π perturbations suggest recurrence of this 1500 cm−1 interconfigurational splitting which is the O−pσ/pπ splitting induced by the Ca+ ligand.
Photofragment infrared emission spectroscopy: Vibrational progression and potential parameters of the CH3(ν2) ‘‘umbrella’’ mode
Authors: Hermann Heinz W. , Leone Stephen R.
Published: February 2003

The out-of-plane bending vibration (ν2) of the free radical CH3 has been detected by time and wavelength-resolved infrared emission spectroscopy following dissociation of CH3I with a pulsed laser at λ = 266 nm. Three distinct Q branch features in the vibrational progression of the ν2 ’’umbrella’’ mode are observed: 3-2 (729±4 cm−1), 4-3 (772±4 cm−1), and 5-4 (811±4 cm−1). These low resolution measurements provide an approximate determination of the harmonic/quartic mixed oscillator potential parameters for the higher levels of the out-of-plane bend. The vibrational assignment is important to studies of radical excitation originating from photodissociation, which is considered in the accompanying paper.
Raman spectra of solid CH4 under high pressure. II. New phases below 9 kbar at 4.2 K
Authors: Fabre D. , Thiéry M. M. , Kobashi K.
Published: February 2003

We have studied the Raman spectra of solid CH4 at 4.2 K below 9 kbar. The vibration-libration structures of the intramolecular ν1–ν4 spectra were essentially the same as the previous data of 1.5 kbar, so that the three-site (S1, S2, and W) model of phase III proposed in paper I was utilized to analyze the observed data. It is noteworthy that one of the three bands in the ν4 spectrum exhibits a decrease in frequency with pressure. This ’’softening’’ is interpreted as a result of distortion of the local potential fields of methanes with pressure. On the other hand, the lattice Raman spectrum exhibits a marked change in the II–III phase transition at 0.5 kbar, and furthermore shows changes in the splittings of certain bands at 1.9 and 4.9 kbar. In the ν-P plots of both intramolecular and lattice vibrational bands, prominent discontinuities are observed at 1.9 and 4.9 kbar, indicative of the existence of phase II (0–0.5 kbar), phase III (0.5–1.9 kbar), and new phases IV (1.9–4.9 kbar) and V (4.9 kbar), below 9 kbar. Mode Grüneisen parameters γ have been estimated. It is found that the γ’s of phase III are much larger than those of phases IV and V. Assignments of the lattice Raman bands are made using the observed ν3 Raman spectrum, the ν-P plots and the existing far-infrared spectrum. It is concluded that the lattice Raman spectrum mainly consists of two translational bands and three librational bands arising from S1, S2, and W sites. Finally, we have estimated from the observed data the magnitude of the A-T tunneling rotational splittings of methane as a function of pressure. Results imply that the splitting at the W (weak field) sites is as large as ∼3.6 cm−1, even at P = 8 kbar. On the basis of this result, we discuss the tunneling splittings in solid CD4 phase III at normal pressure.
Calculation of Raman intensities for the torsional vibrations of ethyl halides
Authors: Bocian David F. , Schick G. Alan , Hurd J. Kathleen , Birge Robert R.
Published: February 2003

Raman intensities are calculated for the torsional vibrations of CH3CH2Cl, CH3CH2Br, CH3CH2I, CH3CHCl2, and CH3CHBr2 using an anisotropic atom-point dipole interaction model to calculate the elements of the molecular polarizability tensor. The calculated relative intensities for the members of the Δv = 2 torsional overtone progression of each of the ethyl halides are in good agreement with experiment. It is predicted that electrically anharmonic terms contribute substantially to the Raman intensities of these transitions. The Δv = 1 torsional transitions of the five molecules are predicted to be 20–30 times more intense than the overtones (although these transitions are not observed because of broadband contours and interference from other vibrational modes). Electrically anharmonic terms in the polarizability expansions also contribute substantially to the intensity of the fundamentals.
Tetraphenyl-group IV B compounds: Flexible molecules with high symmetry crystals. I. Assignment of low frequency infrared and Raman bands
Authors: Schlotter N. E. , Hudson Bruce
Published: February 2003

The vibrational spectroscopy of tetrephenylmethane and tetraphenyltin have been studied by Raman and far IR spectroscopy. The polarized spectra were recorded at both room temperature and low temperature (below 50 K) for oriented single crystals grown from solution. The optical quality and symmetry of these crystals, especially the unusually high (S4) site symmetry, provides a unique oppurtunity for the study of a large, flexible molecule in a well-defined geometry and environment. Very large extinction and polarization ratios are observed for these crystals. Using the six unique Raman and two unique IR spectra, the published x-ray data, and group theory, the observed vibrational bands can be assigned symmetry types. The eight spectral experiments highly overdetermine the assignment of the vibrations to the three (A, B, and E) symmetry classes. The results show many previously unreported low frequency bands. The polarization studies show a number of near degeneracies that cannot be resolved in unpolarized experiments. There are more bands below 300 wave numbers than can be attributed to optical phonons and combination bands and there is therefore reason to believe that there is strong mixing between the molecular and lattice degrees-of-freedom.
The rotational spectrum and molecular structure of the ethylene–HF complex
Authors: Shea J. A. , Flygare W. H.
Published: February 2003

The microwave spectrum of the ethylene–HF complex in the ground vibrational state has been assigned using a Fourier transform microwave spectrometer, employing a Fabry–Perot cavity and a pulsed supersonic nozzle as a molecular source. Ethylene–HF is nonplanar, with the axis of the HF subunit oriented perpendicular to the plane of the ethylene molecule, and bisecting the carbon–carbon double bond. A hydrogen bond is formed between the HF proton and the π-electron density of ethylene. The spectroscopic constants of ethylene–HF and ethylene–DF are:
Electric hyperfine interaction in solid CF4 at the fluorine site
Authors: Barfuss H. , Böhnlein G. , Gradl G. , Hohenstein H. , Kreische W. , Niedrig H. , Reimer A.
Published: February 2003

Using the time differential perturbed angular distribution method (TDPAD), the quadrupole coupling constant for CF4 has been measured to be νQ = 59.7(3) MHz. This value is discussed in connection with measurements of the other carbon tetrahalides and can be well understood in the framework of the Townes and Dailey theory.
Mean recombination time of diffusion controlled geminate reactions
Authors: Mozumder A.
Published: February 2003

A recently introduced method by Deutch for determining the mean passage time for diffusion controlled processes [J. Chem. Phys. 73, 4700 (1980)] has been further developed for application to the reactions of geminate pairs. First, a nonzero probability of escaping geminate recombination requires a normalized definition of mean recombination time which is done consistently. Second, only a finite dose results in a finite mean reaction time. Adopting a special outer boundary condition related to the dose, mean recombination times are calculated for ionic and neutral geminate reactions. For application in radiation- and photochemistry, fully and partially diffusion controlled conditions are applied to ionic and neutral cases, respectively.