bluuebunny/crossref_metadata_2025 huggingface dataset:
Near resonance interactions associated with the temperature dependence of fluorescence self-quenching of Sm3+ in POCl3:SnCl4
Authors: Chrysochoos John
Published: February 2003

Electronically excited Sm3+ ions in POCl3:SnCl4(4G5/2) interact with adjacent Sm3+ ions in their ground state (6H5/2) leading to fluorescence self-quenching (kSm3+), as well as with the solvent, POCl3, leading to fluorescence quenching by the solvent (ksolv). The interactions leading to fluorescence self quenching are temperature dependent, associated with an activation energy barrier equal to 1.3 kcal/mole. This dependence is attributed, at least in part, to conditions of near resonance established between the interacting states at higher temperatures, brought about by populating their higher energy Stark components. Values of kSm3+, corrected for such near resonance conditions, exhibit very negligible temperature dependence. An average value of (2.8±0.2) ×103 M−1 sec−1 is obtained for (kSm3+) corrected at temperatures ranging from −15 to 55°C. The quenching of the 4G5/2 state of Sm3+ by POCl3 is temperature independent. The appropriate rate constant, ksolv, has a maximum value of 32 M−1 sec−1 which corresponds to about 320 sec−1.
The particle scattering factor of branched macromolecules: computer simulation
Authors: Brostow Witold , Gruda Julian , Sochanski Jerzy S. , Turner James E.
Published: February 2003

Theoretical prediction of the particle scattering factor I (ϑ) is possible for a limited number of molecular shapes (random coils, spheres, cylinders), which can be described in terms of distances only. Given the importance of branched macromolecules, where angles also play a role, we have devised a computer procedure for calculating the particle scattering factor. Scattering was simulated for trees with alternate and with opposite branches, with straight branches and with broken branches, 2-dimensional as well as 3-dimensional, and also for systems consisting of two trees acting as a single scatterer. To elucidate various factors affecting I (ϑ), several parameters were varied, including angles between trunks and branches, angles between two linear fragments of a broken branch, and densities of linear fragments. Light scattering by generalized trees consisting of unconnected segments has been discussed. For normal trees representing real molecules, procedures enabling considerable savings in computing times have been devised.
Fluorescence spectra and kinetics of Cs2
Authors: Benedict R. P. , Drummond D. L. , Schlie L. A.
Published: February 2003

The fluorescence spectra of the A 1Σ+u–X 1Σ+g and a3πu–X 1Σ+g transitions in Cs2 have been measured as a function of temperature and Xe buffer gas density. The observed spectra were normalized to the Cs atomic emission. The Xe density dependence of the normalized fluorescence was used to determine an average formation rate constant for the A 1Σ state of 4.2×10−30 cm6/sec. Using a value of 10−29 cm6/sec for the a 3π formation rate constant, predissociation and quenching rate constants from the a 3π to the x 3Σ were determined to be 2.5×109 sec−1 and 9.8×10−11 cm3/sec. Also, the net collisional transfer rate constant from the A 1Σ to a 3π was 1.5×10−11 cm3/sec. The fluorescence spectra were also used to predict the Cs atomic excited state fraction needed to produce an inversion on the A–X transition. The experimentally inferred value was 11% compared wit a theoretical prediction of 0.25%. This result, coupled with the large loss rates, indicate that Cs2 is not an attractive laser candidate on the A–X transition.
Atom−polyatomic collisions: The role of pair correlation functions
Authors: Micha David A.
Published: February 2003

Following a brief introduction to the relation between cross sections for atom−polyatomic collisions and the atom−pair correlation functions of the isolated polyatomic, we discuss properties of the correlation functions (such as sum rules and moments) relevant to experimental interpretation. Correlation functions are obtained analytically for polyatomics with harmonic vibrational motions, in their body−fixed reference frame. Rotational motions are then described within a short time expansion which provides a physical picture of rotational energy transfer for each vibrational transition. The obtained results are valid for multiquantum vibrational transitions. Gaussian rotational distributions are found for each vibrational transition, with parameters explicitly given by the theory. Relationships among collision times and rotational– vibrational periods, and the shapes of correlation functions vs. energy transfer are briefly discussed.
Transfer of electronic excitation in collisions of metastable argon atoms with nitrogen molecules
Authors: Cutshall Euel R. , Muschlitz E. E.
Published: February 2003

Measurements have been made of the product vibrational and rotational state distributions for the process Ar*(3P2,0)+N2(X) →Ar(1S0)+N2*(C,3Πu) by spectrometric observation of the C→B fluorescence at the intersection of supersonic beams of the reactant species over the relative kinetic energy range 0.06–0.41 eV. The results are compared with the predictions of a ’’golden rule’’ model which gives the transition probabilities in the highly impulsive limit as the product of Franck–Condon and density of states factors. Below 0.17 eV, the model fits the experimentally determined v′=0/v′=1 population ratio reasonably well. At higher energies the experimental ratio rises, however, and reaches a maximum at 0.3 eV. A statistical population of rotational levels for v′=0 corresponding to rotational temperatures of 1300 °K and 1100 °K at 0.161 and 0.089 eV relative energy, respectively, is observed, while the ’’golden rule’’ predicts considerably higher rotational excitation in each case.
Theoretical studies of the valence electronic states and the 1Πu←X1Σ+g absorption spectrum of the F2 molecule
Authors: Cartwright David C. , Hay P. Jeffrey
Published: February 2003

The twelve electronic states of F2 dissociating into ground state F(2P) atoms have been investigated with ab initio polarization configuration interaction wave functions. Using a [3s2pld] contracted Gaussian basis, the theoretical spectroscopic constants (with experimental values in parenthesis) for the 1Σ+g ground state are: Re=1.427 Å (1.412 Å), De=1.85 eV(1.66 eV), ωe=946 cm−1 (924 cm−1), and ωexe=−10.6 cm−1 (−22.2 cm−1). The 3Πu state is also found to be very weakly bound (Re=1.881 Å, De=0.15 eV), while the remaining nine electronic states are strictly repulsive (aside from van der Waals minima). Molecular properties are reported for the 1Σ+g state and a detailed analysis of the 1Πu ←1Σ+g absorption is carried out. The 3Πu state appears to be the lower state in the 157 nm laser emission observed recently, which would correspond to the 23Πg→13Πu electronic transition.
Criticality in exothermic chemical reactions: Upper and lower bounds of the Frank–Kamenetzki parameter
Authors: Meinköhn Dirk
Published: February 2003

A relatively simple method is employed to derive upper and lower bounds of the critical values of the Frank–Kamenetzki parameter, which represent a critical condition for the so-called ’’thermal explosion’’. The resulting bounds are compared with the critical parameter values and their bounds which were derived by previous authors.
Nickel tetracarbonyl, Ni(CO)4. I. Molecular structure by gaseous electron diffraction. II. Refinement of quadratic force field
Authors: Hedberg Lise , Iijima Takao , Hedberg Kenneth
Published: February 2003

The molecular structure of gaseous nickel tetracarbonyl has been investigated by electron diffraction at room temperature. The analysis, based on an assumed Td molecular symmetry with corrections for the effects of vibrational motion, led to the following bond distances (rg) and amplitudes of vibration (l) with estimated uncertainties (2σ), all in angstroms: r (C=O) =1.141(2), r (Ni–C) =1.838(2), l (C=O) =0.039(2), l (Ni–C) =0.059(3), l (Ni⋅⋅⋅O) =0.061(3), l (C⋅⋅⋅C) =0.124(17), l (C⋅⋅⋅O) =0.176(11) and l (O⋅⋅⋅O) =0.244(34). The quadratic force field was refined using our structure and vibrational wave numbers from the literature in order to permit calculation of the distance corrections arising from vibrational averaging. The force constants are generally very similar to those from a previous spectroscopic study.
Electrical conduction of hemoprotein in the solid phase: Anhydrous cytochrome c3 film
Authors: Kimura Keisaku , Nakahara Yü̅suke , Yagi Tatsuhiko , Inokuchi Hiroo
Published: February 2003

Cytochrome c3 can be reduced with molecular hydrogen under the action of hydrogenase [hydrogen: ferricytochrome c3 oxidoreductase, EC 1.12.2.1] even in the solid state. The electrical conductivity of a cytochrome c3 anhydrous film containing a trace amount of hydrogenase was measured at physiological temperatures as a function of temperature and hydrogen pressure. Ferricytochrome c3 (oxidized form) and ferrocytochrome c3 (reduced form) equilibrate at a given temperature and pressure by a catalytic action of hydrogenase. Under these conditions, the conductivity of cytochrome c3 showed an unusual temperature dependence: The activation energy was positive under higher hydrogen pressure, but was negative under lower pressure. These findings are interpreted as a thermal equilibration between the ferri- and ferro- forms using the Hill equation for the reduction ratio and applying the theory of semiconduction to the electrical conductivity. The theory predicted that the activation energy of conductivity would coincide with the free energy difference between ferri- and ferrocytochrome c3, which is confirmed by measurement of the temperature dependence of the equilibrium constant. The temperature and hydrogen pressure dependences of the conductivity were reproduced well by the above theory.
Hyperpolarizabilities of substituted conjugated molecules. I. Perturbated INDO approach to monosubstituted benzene
Authors: Zyss J.
Published: February 2003

Following experimental studies on benzene substitutes, we have attempted to derive some results on their molecular hyperpolarizabilities from a semiempirical treatment of the Hartree–Fock equations extended to all valence electrons (INDO procedure). The INDO equations are modified so as to introduce a dipolar perturbation, for molecules including atoms of the third line of the Mendeleeiev classification. Hyperpolarizabilities, defined as field derivatives of the dipole moment are given finite difference expressions. Results for benzene and a number of benzene monosubstitutes are discussed.
Hyperpolarizabilities of substituted conjugated molecules. II. Substituent effects and respective σ–π contributions
Authors: Zyss J.
Published: February 2003

Introduced in a one-electron self-consistent model (D–C field perturbated INDO Hamiltonian), symmetry considerations exhibit σ and π electron contributions to second order polarizabilities of planar molecules. A family of benzene monosubstitutes is explored, their mesomeric moments and π second order hyperpolarizabilities are seen to account for the donor or acceptor nature of the substituent radical in accordance with a previous model and experiments. Influence of the INDO parametrization is studied for nitrobenzene, where an artificial variation of the oxygen affinity yields a satisfactory value for the benzene third order hyperpolarizability.
EPR relaxations of aqueous Ni2+ ion
Authors: Friedman H. L. , Holz M. , Hertz H. G.
Published: February 2003

The experimental data for proton NMR relaxation and shifts in aqueous Ni2+ solutions at 25°C have been extended, especially to high fields (8.4 T). Infinite-order perturbation theory for the EPR relaxation in the Ni2+ and a generalized set of Solomon–Bloembergen equations for the proton–electron spin–spin interaction are applied to deduce the rms value Δ and the correlation time τv for the tensor D(t) in the zero field splitting or SDS term of the electron spin Hamiltonian. Both the Brownian rotation of the Ni(H2O)62+, with correlation time τϑ, and the mean lifetime τp of protons in this complex, before exchanging with the bulk, are very important in determining the functional relation of the proton relation times and shift to the corresponding EPR quantities. The best fit of the theory to the data implies the following values for the parameters for assumed values τϑ=25 psec and τp=32 μsec: Δ=2.6 cm−1, τv=2.2 psec, TlS=T2S=2.9 psec at zero magnetic field, proton contact shift in the complex=14 ppm, proton dynamical shift in the complex at zero field=2.1 ppm. The proton dynamical shift is a second-order effect in the dipolar electron–proton spin–spin interaction. The discussion includes the comparison of some of these parameters with earlier determinations as well as a summary of additional effects that might be included in a more realistic model.
Macromolecular dimensions obtained by an efficient Monte Carlo method: The mean square end-to-end separation
Authors: Mandel Frederic
Published: February 2003

The ’’slithering snake’’ Monte Carlo technique has been used to generate a very large number of samples on the three choice square planar lattice for chains of 20, 40, 60, 100, 180, 360, and 600 links. The corresponding mean square end-to-end separations, 〈r2〉, have been calculated with greater precision than previously possible and compared with the earlier Monte Carlo calculations and with the asymptotic formula of Domb. The present Monte Carlo technique has also been used to generate end-to-end distributions for long chains. The correlation between samples for the ’’slithering snake’’ method is also discussed.
The coordination (hydration) of rare earth ions in aqueous chloride solutions from x-ray diffraction. II. LaCl3, PrCl3, and NdCl3a)
Authors: Habenschuss Anton , Spedding Frank H.
Published: February 2003

The inner sphere water coordination of the rare earth ions La3+, Pr3+, and Nd3+ in concentrated (3.4 to 3.8 m) aqueous chloride solutions have been determined from x-ray diffraction measurements. In each solution the rare earth ion exists as the [RE(H2O)9]3+ aquo complex as determined from the quantitative resolution of the radial distribution functions. The average RE3+–H2O distances are 2.580, 2.539, and 2.513 Å for LaCl3, PrCl3, and NdCl3, respectively. The average RE3+⋅⋅⋅Cl− ion pair distances are near 5.0 Å. These results, together with the octaaquo complex, [RE(H2O)8]3+, found previously for the heavy rare earth ions, Tb3+ through Lu3+, establish that the inner sphere water coordination of the rare earth ions in aqueous solutions decreases from nine to eight between Nd3+ and Tb3+.
A LEPS potential for H3 from force field data
Authors: Varandas A. J. C.
Published: February 2003

A new potential energy surface for H3 of the London–Eyring–Polanyi–Sato type has been obtained which reproduces the best available estimates for the geometry, classical barrier height, and quadratic force constants of the D∞h saddle point. Other attributes of the surface, e.g., minimum energy profile for the exchange process, spherically averaged potential V0, and leading anisotropic potential V2, are also shown to be in good agreement with the best available estimates. The simplicity of its functional form further commends it for future dynamical studies.
Theory of depolarized Rayleigh–Brillouin spectra and rotation–translation coupling in molecular liquids and solids
Authors: Wang C. H.
Published: February 2003

The dynamics of translations and rotations of molecular solids and liquids has been studied by using a microscopic theory. The effects of translation–rotation coupling on the depolarized Rayleigh–Brillouin spectrum are investigated in detail. The theory takes into account anharmonicity and thus includes both the static and the dynamic couplings. In the limit of the harmonic approximation, the present theory reduces to the earlier published result for solids. The earlier published result for ideal liquids can also be derived from the present theory. This calculation is based on linear response theory and thus provides a statistical mechanical deviation of the phenomenological theory of the depolarized Rayleigh scattering spectrum of condensed matter. The relations between the transport coefficients as well as the potential and coupling constants in the macroscopic theory and the kinetic coefficients in the microscopic theory are established. The correlation function for the lattice displacement derived from solving the macroscopic equation is shown to be in agreement with the microscopic theory. It is shown, however, that the phenomenological theory does not provide the same polarizability correlation function for solids as that obtained from microscopic theory, despite the fact that the two theories agree in the case of liquids. The effects of relaxation and resonance behavior of the depolarized Rayleigh–Brillouin spectral density are also discussed in detail.
Singlet exciton fission in pure and doped anthracene
Authors: von Burg K. , Zschokke-Gränacher I.
Published: February 2003

On the basis of the kinetic model by Merrifield for triplet exciton fusion in molecular crystals, we have derived an expression which describes the reciprocal process, i.e., the anisotropy of the prompt fluorescence in the presence of singlet exciton fission in a magnetic field. For the first time a thorough comparison of Merrifield’s theory with singlet exciton fission experiments is made. The theory is compared with experimental data for the case of homofission in pure anthracene crystals as well as for the case of heterofission in anthracene crystals doped with tetracene. The resulting kinetic parameters for the fission process have almost the same values as those for triplet fusion. The origin of a discrepancy between theory and experiment along certain crystallographic axes is discussed.
Semiclassical quantization of the low lying electronic states of H2+
Authors: Strand Michael P. , Reinhardta) William P.
Published: February 2003

The classical dynamics of an electron moving in the presence of two equally charged, fixed nuclei is presented. The manner in which the values of the three constants of the motion determine the qualitative features of the electronic trajectory is discussed. Primitive semiclassical wavefunctions and quantization conditions for the Born–Oppenheimer electronic quantum states of H2+ are derived using the canonically invariant quantization methods of Einstein, Keller, and Maslov. Because of the presence of energetic and dynamical barriers, a uniform quantization method is needed to give quantitative results for all internuclear separations. We employ a well-established uniformization which models the effective potential barrier as a parabola. Finally, the eigenparameters computed using the primitive and uniform quantization methods are compared with exact Born–Oppenheimer quantum mechanical results for the six lowest Σ states, the two lowest Π states, and the two lowest Δ states for a wide range of internuclear separations. The electronic energies computed using the uniform quantization conditions typically agree with the exact quantum results to within a fraction of a percent.
The interaction potential and the determination of some cross sections and spectra of pure O2 and O2–Ar gas mixtures
Authors: Mingelgrina) U. , Gordon Roy G.
Published: February 2003

The line shapes for microwave absorption and dispersion, depolarized Raman spectrum and the sound absorption of oxygen gas were calculated. The EPR spectrum and rotational relaxation cross sections were determined. The microwave spectrum of O2–Ar mixtures and the low pressure Raman spectrum of O2–Ar were also calculated for a range of densities at 298° K and 283° K. Good agreement was achieved with experimental results. The calculations were done using a 3- or 4-body potential, whichever was appropriate, whose parameters were determined partly from theory and partly by comparison with nonspectral data. The results showed sensitivity to variations in the potential and thus indicate the applicability of the computations for the study of intermolecular interactions, specifically their anisotropy, as well as their direct usefulness in prediction of various spectral relaxation phenomena.
Nuclear quadrupole coupling in ND4ReO4 and ND4IO4
Authors: Segel S. L. , Brown R. J. C.
Published: February 2003

The temperature and pressure dependences of the 187Re nuclear quadrupole resonance frequency in ND4ReO4 are reported, and compared with corresponding data for NH4ReO4. The temperature dependence of the deuteration shift is similar to that of the pressure coefficient of the quadrupole frequency, (∂ν/∂P)T. In both ND4ReO4 and NH4ReO4, there is a rapid change in the slope of the graph of (∂ν/∂P)T vs temperature, rather than the discontinuous change reported previously; there is no significant shift in the temperature of this feature upon deuteration. It is concluded that changes in the ammonium ion rotational motion near 200 K are the result, and not the cause, of other structural changes in the crystal. The quadrupole frequency in ND4 127IO4 has been measured at room temperature.
The ESR spectrum and linewidth measurements of atomic iodinea)
Authors: Lilenfeld H. V. , Richardson R. J.
Published: February 2003

The experimental spectrum from ground-state 2P3/2 atomic iodine is compared with a theoretical calculation using a best-fit g value equal to 1.333980±0.000006 and previously determined nuclear and electronic spin interaction constants. A rate coefficient for iodine-atom wall recombination (γ?10−3) is reported. Broadening coefficients and hyperfine interaction relaxation cross sections for the iodine-atom ground state with He, O2, and HI are determined from the line broadening of the MJ=3/2→1/2 and MJ=1/2→−1/2 transitions of I and compared with theory.
Vibrational predissociation lifetimes of the van der Waals molecule HeI2
Authors: Beswick J. A. , Delgado-Barrio G. , Jortner Joshua
Published: February 2003

In this paper we present a quantum mechanical study of the rates of vibrational predissociation of the T-shaped HeI2(B) van der Waals molecule. The van der Waals bond is characterized in terms of Morse type atom–atom interaction and by a Buckingham type atom–atom potential. The dynamics of vibrational predissociation is shown to be insensitive to the long-range part of the van der Waals potential and the Morse form is adequate for the description of this process. The close-coupling equations for nuclear motion are solved by standard numerical methods and the vibrational predissociation rates are related to the widths of the resulting resonances. The superlinear theoretical dependence of the vibrational predissociation rates on the excess vibrational energy of the molecular I2 bond is in good agreement with the experimental data. The relative contribution of intramolecular and of intermolecular terms to this superlinear dependence are elucidated, demonstrating the effects of the anharmonicity of the molecular bond on the intramolecular dynamics.
Magnetic circular dichroism of the 1S→1P transition of Mg atoms in noble gas matrices
Authors: Mowery R. L. , Miller J. C. , Krausz E. R. , Schatz P. N. , Jacobs S. M. , Andrews L.
Published: February 2003

A magnetic circular dichroism (MCD) and absorption study has been made of the 1S (3s2) →1P (3s3p) transition of Mg atoms in Ne, Ar, Kr, and Xe matrices. Triplet structure is observed in the latter three, and a detailed analysis of the MCD shows that this structure results from a splitting of the 1P excited state at a single site. An analysis of the zeroth and first MCD and absorption moments shows that the orbital angular momentum of this 1P state is partially quenched, the quenching factors being approximately 0.65, 0.75, 0.50, and 0.43 in Ne, Ar, Kr, and Xe matrices, respectively. This quenching is necessarily a consequence of out-of-state mixing (i.e., orbital mixing outside the atomic 1P excited manifold) and almost certainly reflects strong mixing of the 3p Mg orbitals with noble gas orbitals on neighboring host atoms. An analysis of higher absorption and MCD moments assuming octahedral site symmetry shows a dominant noncubic (Jahn–Teller active) mode contribution to the bandwidth in the three hosts in which the triplet structure is resolved. This suggests a Jahn–Teller explanation of the triplet structure not only for Mg but also for other atoms in noble gas matrices whose S →P transitions show the same general pattern. This pattern is characteristic of the T1×t2g Jahn–Teller case.
A zeroth order random network model of liquid water
Authors: Sceats Mark G. , Stavola M. , Rice Stuart A.
Published: February 2003

A random network model is used to generate the oxygen–oxygen pair correlation function of liquid water and amorphous solid water. The model is basically an extension of the bent hydrogen bond model proposed by Pople in 1951; it differs from the earlier work in that instead of being a parametric fit to the data, it predicts the dispersion in OO distances for each shell of neighbors, and the dispersion in OOO angles, from independent spectroscopic data. Our random network model is based on the hypothesis that there exists a separation of time scales for various classes of molecular motion in liquid water. In particular we assert that it is useful to regard libration and hindered translation as occurring in a quasistatic network of bonded molecules and diffusion and related relaxation processes as occurring much more slowly. The predicted OO pair correlation function is in good but not perfect agreement with experiment over a wide temperature range. As a further test the model is used to predict the dielectric constant of liquid water. Excellent agreement is found over the temperature range 10–300 °C. The relationship between the random network model and computer simulations of liquid water is extensively discussed.
High temperature–high pressure thermal conductivity of argon
Authors: Aggarwal Mahesh C. , Springer George S.
Published: February 2003

A conductivity column-type apparatus was built and tested for measuring the thermal conductivities of gases at high temperatures and at high pressures. Two columns of different lengths were used in order to minimize end effects. The temperature and pressure limits were established below which multicellular convection cells did not form inside the column. The thermal conductivities of argon were measured in the temperature range 400 to 750 K and in the pressure range 0.1 to 8 MPa. The data were correlated by the expression where a=0.2678×10−3, b=0.7401, A=0.2839×10−4, B =0.1650×10−4, C=−0.1065×10−6, p0=0.1 MPa, and λ is in W m−1 K−1, T in K, and P in MPa. The thermal conductivity values obtained were compared with (1) existing data, (2) with results of viscosity measurements by means of the Eucken factor, and (3) with theoretical predictions based on a model for nonattracting rigid spherical molecules.
Images of gas molecules by electron holography. I. Theory
Authors: Bartell L. S.
Published: February 2003

It is demonstrated that molecular images with an Abbe resolution limit as fine as 0.1 Å can, in principle, be reconstructed optically directly from appropriately recorded electron diffraction plates (holograms). Even when a sample of randomly oriented gas-phase molecules is chosen as a subject, reconstructed images with a well-defined physical interpretation can be made. If the molecules are selected to contain just one heavy atom—this atom to generate a strong ’’reference wave,’’ by scattering, so as to satisfy Gabor’s conditions for the production of a hologram—the reconstructed images correspond to rotational averages about the heavy atom. If no single atom dominates, the ’’image’’ is not as simple conceptually but the reconstruction is shown to display a series of rings, each of which corresponds to an interatomic distance in the molecule. The physical basis of the approach is reviewed, followed by a mathematical treatment of the electron scattered intensity and optical reconstruction.
Nonrandom isomerization of c
i
s-3-methyl-1,3-pentadiene-1,2-d2. Internal energy relaxation
Authors: Ibuki Toshio , Sugita Shoji
Published: February 2003

The unimolecular isomerization via 1,5-H atom shift of cis-3-methyl-1,3-pentadien-1,2-d2) (c-3MP-1,2-d2) was studied in the gase phase. c-3MP-1,2-d2 was produced by the cross combination of CH3CH=CCH3 and CD=CHD radicals and chemically excited at 25°C to 122 kcal/mol on the average. Nonrandomization of internal energy was observed. An effective intramolecular energy relaxation time of (3.97±0.14) ×10−11 sec was obtained by the application of RRKM theory to the isomerization reaction. The experimentally determined rate constant for the unimolecular isomerization of c-3MP-1,2-d2 with randomized internal energy was found to be 5.0×109 sec−1.
CO and H2 quantum yields in the photodecomposition of formaldehyde in air
Authors: Moortgat Geert K. , Warneck Peter
Published: February 2003

Absolute quantum yields have been measured at 25° C for the products CO and H2 resulting from the photolysis of formaldehyde admixed to air at ppm level concentrations. The dependence on wavelength and pressure was established for the region 277–355 nm. In the 290–330 nm wavelength range the quantum yields showed no pressure dependence between 380 and 800 Torr of air, at 355 nm the quantum yields increase with decreasing pressure. This effect is interpreted as being caused by collisional quenching. Data from other investigations are used to show that quenching occurs at all wavelengths greater than 335 nm, and quenching efficiencies for air, CO2, NO, and formaldehyde are estimated for formaldehyde excited at 340 and 355 nm. From a discussion of the various excited states involved in H2CO photolysis it is concluded that the CO and H2 product quenching is due to electronic quenching of vibronic levels of the S1 state. The measured quantum yields provide the first realistic data for the photochemical behavior of formaldehyde in the atmosphere. Quantum yields for H2CO loss, CO and H2 production, and radical formation are given and discussed with respect to previous data.
NMR observations of molecular motions and Zeeman–quadrupole cross relaxation in 1,2-difluorotetrachloroethanea)
Authors: Stokesb) Harold T. , Case Thomas A. , Ailion David C. , Wang C. H.
Published: February 2003

We report measurements of 19F NMR relaxation times T1, T1ρ, T1D, and T2 in the plastic crystal CFCL2–CFCL2. From the data near the melting point, we obtain the jump time for translational self-diffusion. At lower temperatures, we observe on the cold side of the T1 and T1ρ minima an unusual field dependence which is substantially less than the normal field-squared dependence. We also observe a reduction in T1 near 40 MHz due to cross relaxation between the Zeeman levels of the 19F spins and quadrupole levels of the 35Cl and 37Cl spins. We measured the cross relaxation times τIS as a function of field and found good agreement with our theoretical calculation of τIS.
Rotational distributions from photodissociations. I. Linear triatomic molecules
Authors: Morse Michael D. , Freed Karl F. , Band Yehuda B.
Published: February 2003

The generalized Franck–Codon theory of the collinear dissociation of linear triatomic molecules is presented, including a proper description of the bending vibrations in the initial bound electronic state and of the rotational motions on both the initial and the final repulsive electronic surface. The nonseparable multidimensional bound–continuum Franck–Condon integrals are reduced to a rapidly convergent series of products of one dimensional integrals. Analytical expressions are derived for rotational and orbital angular momentum distributions of the products, for scalar coupling (as in predissociations), as well as parallel and perpendicular transitions (as in direct photodissociation). This fully quantum mechanical theory makes explicit the separate and interrelated roles played by angular momentum and energy conservation. The present work is applied in a separate paper to the photodissociation of ICN, and qualitative agrement with experiment is obtained.
Rotational distributions from photodissociation. II. Results for ICN+hν→l+CN(X 2Σ+)
Authors: Morse Michael D. , Freed Karl F. , Band Yehuda B.
Published: February 2003

The quantum theory of rotational, vibrational, translational and electronic energy distributions in photodissociation is applied to the Ã continuum photodissociation of ICN. Excited state potential energy surfaces are fitted to the available experimental data, and predictions are made of the product energy distributions. General results are discussed which are applicable to other photodissociations and predissociations. In particular, the calculated rotational energy distributions display a strong sensitivity to the details of the unbound potential surface. The results of the collinear approximation for vibrational distributions in photodissociation of linear molecules are shown to be considerably modified by including rotational degrees of freedom, especially at high photon energies.
Electric fields in sandwich cells by electroabsorption and electroreflection; The Al/tetracene/nesatron cella)
Authors: Scott T. W. , Albrecht A. C.
Published: February 2003

Interfacial electric fields in photoactive Al/tetracene/indium oxide sandwich cells have been studied by electric field perturbed absorption and reflection spectroscopy. Theory shows how electroreflection relates to the electrostatic field at the very surface of tetracene, while electroabsorption depends on the intrinsic potential difference between each electrode/tetracene contact. The magnitude and orientation of each surface field and the interelectrode potential difference for the sandwich cell are reported. In addition, the spectral dependence of electroabsorption and electroreflection predicted by theory is shown to be in good agreement with experiment.
An ESR and ENDOR study of citric acid single crystals x-irradiated at 4.2 K
Authors: Finch Leon L. , Johnson Joe E. , Moulton Grace C.
Published: February 2003

Single crystals of partially deuterated citric acid were irradiated with 3 MeV x rays at 4.2 K. ESR and ENDOR data were taken at 4.2 K after warming to 100 K and recooling to 4.2 K, and also at room temperature. At low temperature, two radicals were identified from the tensors determined from the ENDOR data and fit to the ESR spectra. Radical I is the oxidized species produced by decarboxylation of the central carboxyl group, with four β-hydrogen couplings. Radical II is the reduction species produced when an electron is added to one of the end carboxyl groups. At room temperature, two stable radicals were identified from the tensors determined from the ENDOR data and fit to the ESR spectra. One of these is radical I, the decarboxylation radical, in a different molecular conformation than at low temperature, as evidenced by different values of the four hyperfine tensors. The other radical, III, shows coupling to only one α hydrogen and has the following structure:
Model of the structural phase transformation in sodium azidea)
Authors: Raich J. C. , Hüller A.
Published: February 2003

A microscopic model for the intermolecular interactions is applied to discuss the nature of the α–β structural transformation in sodium azide. NaN3 is an example of an ionic molecular crystal where changes in molecular orientations in the crystal lead to anomalous thermoelastic behavior. Sodium azide differs from other molecular crystals with elastic anomalies such as KCN in that the associated change in orientational order at the transition seems to be quite small. The model, a mean field treatment of the coupling of the rotational motions of the N3− ions to the lattice strains, is shown to lead to indirect N3−–N3− interactions mediated by the lattice. The apparent second-order nature of the transition is examined. Predictions regarding the temperature dependence of the isothermal elastic constants c44 and 1/2(c11−c12) are made.
Ligand ENDOR spectra of Cu(II) impurity complexes in α-glycine crystals
Authors: Fujimotoa) M. , McDowell C. A. , Takuib) T.
Published: February 2003

Symmetrical Cu(II) impurity complexes in α-glycine crystals have been studied with ENDOR of ligand proton and 14N nuclei at 4.2° K. It has been found that the inversion symmetry in Cu(gly)2 complexes is significant in interpreting the ENDOR spectra showing additional splittings. We propose that a pair of equivalent nuclear spins in the complex are weakly coupled via their hyperfine interactions, resulting in the observed splittings. The 14N nuclear quadrupole tensors have also exhibited the symmetry of the complex, reflecting a coupling between the identical spins. The ENDOR analysis of close protons have revealed that two amino protons and one methylene proton make a predominant contribution to the anisotropic EPR linewidths. The results indicate that the ligand amino groups are deprotonated, and the proton vacancies are localized consistent with the complex symmetry, and responsible for the electric field gradient symmetry at the 14N nuclei.
Positron annihilation in alpha-cyclodextrin: Effects of adsorbed gases
Authors: Hadley J. H. , Hsu F. H. , Yei W.
Published: February 2003

Positron annihilation lifetime measurements have been made for various adsorbed gases in the organic clathrate α-cyclodextrin. α-cyclodextrin samples were investigated under vacuum, saturated with water vapor, and under pressure of the following gases: O2, NO, N2O, CO2, He, He2, N2, and CH4. Water vapor produces a quenching of orthopositronium. Oxygen, nitric oxide, and nitrous oxide were each found to inhibit positronium formation. The interaction between Ps and these adsorbed molecules is different from that found in the gaseous state. Other gases studied had little or no effect upon the positron lifetime spectrum.
The bond additivity of Compton profiles and momentum densities for simple molecules. I. Compton profilesa)
Authors: Cade Paul E. , Henneker W.H. , Kostyla Aurora
Published: February 2003

The bond additivity of Compton profiles is theoretically explored for a sequence of linear polyatomic molecules. The Compton profiles of HCN, HNC, FCN, ClCN, NCCN, HCCCN; HCCH, FCCH, ClCCH, LiCCH; OCO, SCO, OCN−, SCN−, and NNO, using near Hartree–Fock wave functions of McLean and Yoshimine, forms the theoretical basis for this investigation. Two approaches are employed to examine bond additivity of Compton profiles. The first approach constructs the Compton profile of molecules in our repertory from others also in the list, checking for internal consistency and against the explicitly calculated result. The second approach defines bond Compton profiles and emphasizes internal consistency of bond profiles, JAB(q) calculated in various ways. The results are critically compared to previous theoretical and/or experimental bond Compton profiles.
Analytical potentials from a
b i
n
i
t
i
o computations for the interaction between biomolecules. V. The phosphate group in nucleic acids
Authors: Clementi E. , Corongiu G. , Lelj F.
Published: February 2003

Analytical atom-pair potentials are obtained from ab initio SCF–LCAO–MO computations for the – (PO4–CH2)−– group that characterizes part of the repeating unit of nucleic acids. To ensure that the – (PO4−CH2)−– group is placed in a realistic environment, as terminal group we have included part of the sugar molecule present in nucleic acids. Isoenergy contour maps have been computed for the model compound. Using a Monte Carlo program we have considered small clusters of water surrounding the phosphate group. The obtained results allow a few tentative predictions concerning the structure of water in nucleic acids.
Diatomic-molecule vibrational potentials. II. New representationsa)
Authors: Engelke Ray
Published: February 2003

Two new representations of diatomic-molecule vibrational potentials are presented. One of these includes most of the previously employed series approximations as special cases. The new representations are tested against older ones by using Peek’s ’’exact’’ numerical Born–Oppenheimer potentials for the 1sσg, 2pπu, and 3dσg states of H2+ as test problems. Accuracy comparisons are made with Dunham, Thakkar, Ogilvie–Tipping, Coulomb-subtracted Ogilvie–Tipping, and Padé representations. A central idea of the new treatment is that it is not necessary to use a global representation of the potential over the region 0?R/Re<∞. Rather one can choose separate representations suitable for the subregions R/Re?1 and R/Re?1 and then match them smoothly to each other and to the Dunham expansion at R/Re=1. Attention is focused on finding improved approximations for R/Re≳1, since this region exerts strong control on the vibrational eigenvalue spectrum and it is here, perhaps, where the older techniques are at their weakest. Known properties of the exact potential such as the R→∞ behavior and the dissociation energy can be built into the new forms a priori. If the dissociation energy is not known, the new methods allow it to be estimated with better accuracy then could be done previously. If one knows only Dunham coefficients, the Coulomb-subtracted Ogilvie–Tipping series is the superior representation. If one knows, in addition, the dissociation energy, the new representations are superior and give more accurate results on the interval R/Re?1.
A high-precision electron paramagnetic resonance study of Gd3+ in single crystals of the lanthanide ethyl sulfate nonahydrates near room temperature
Authors: Gerkin Roger E. , Rogers William J.
Published: February 2003

By methods described by Gerkin and Rogers [J. Chem. Phys. 66, 4166 (1977)], high precision values of the spin-Hamiltonian parameters b20, b40, and b60, and of the zero-field splittings, for Gd3+ dilutely substituted in single crystals of 13 lanthanide ethyl sulfate nonahydrates at room temperature (24.130° C) are presented. Since sample shape dependence is significant at the reported level of precision, the measured values were adjusted to yield corresponding values for spherical samples via the analysis of shape dependence presented by Gerkin and Rogers [J. Chem. Phys. 68, 504 (1978)]. Correlations of b20, b40, and b60, and of the zero-field splittings, with the ionic radii of the host lanthanides are presented and compared with previous results. Values of the intrinsic parameter ?2 of the superposition model analysis of b20 are obtained for Gd3+ in these salts and are displayed as a function of host ionic radius. The value −3.0 was obtained for the superposition model parameter t2. More detailed analysis of these parameters will require improved knowledge of the rare earth trivalent ion radii and of ion–ligand distances in the ethyl sulfate nonahydrate series.
Theory of electrical conduction in organic molecular crystals: Temperature-independent mobilities
Authors: Sumi H.
Published: February 2003

Almost-temperature-independent mobility (ATIM) has been observed above about 100 K for electrons in anthracene, naphthalene, and As2S3. The conduction-band structure of all these crystals is almost two- dimensional with a very narrow bandwidth in one (c′) direction, and it is in this c′ direction that the ATIM has been observed. The ATIM is ascribed to diffusive electron hopping induced by lattice distortions in the course of lattice vibrations. It is crucial to the mechanism proposed that electron motion is coherent in the two-dimensional Bloch-band states along the (a,b) plane perpendicular to the c′ direction. The steep increase of the mobility below about 100 K observed in naphthalene is ascribed to the increasing contribution of the mobility component determined by the electron transfer of the rigid lattice at low temperatures. The mobility along the (a,b) plane which shows the bandlike temperature dependence in these crystals is, on the other hand, determined only by electron transfer of the rigid lattice at all temperatures. The electron mobility in the c’ direction and that along (a,b) plane are both calculated with a unified treatment based on the linear response theory. The observed magnitude, temperature dependence, and anisotropy of the mobility are all reproduced consistently by adopting reasonable values for the energy bandwidths of electron and the electron–phonon coupling energies.
Photofragment spectroscopy and assignment of excited states of thallium iodide
Authors: Sinha Mahadeva P. , Su Robert T. , Riley Stephen J.
Published: February 2003

A molecular form of TlI was crossed by a pulsed laser and dissociation fragments detected as a function of time of flight. Spectroscopy studies were made at 266 nm.
Size dependence of delocalized treatments of the correlation problem
Authors: Malrieu J. P.
Published: February 2003

Using a perfectly localized perfectly delocalizable model problem, which reduces to N identical noninteracting electron pairs, the N dependences of various perturbative corrections for the correlation problem have been established. These theoretical results for the zero interaction case are verified to a surprising accuracy in strongly delocalized problems (π polyenes), namely the invariance of the Moller–Plesset corrections under localization of the MO’s, and its proportionality to the number of electron pairs, the Epstein–Nesbet satisfactory N dependence for localized pictures and its meaningless behavior for delocalized pictures. Using delocalized MO’s the doubly excited determinants belong to two classes with respect to their interaction with the ground state SCF determinant, N3 interacting through a N−1 matrix element, the N4 others interacting through N−2 matrix elements. These phenomena may have undesirable consequences on approximate variational CI techniques. In view of the N dependence, the use of delocalized MO’s for correlation calculations seems perfectly irrelevant.
Energy transfer in high-energy atom-oscillator collisions
Authors: Gottdiener Luis , Carmona Rafael
Published: February 2003

An expression for the energy transfer is derived for the classical one-dimensional collision of an atom with an excited harmonic oscillator with an exponential interaction. The formula does not predict independence of the average energy transfer with respect to initial oscillator excitation and approaches the correct limiting expression for high energies. Comparison is made with exact numerical results and with an existing formula.
Energy transfer in gas–surface scattering in the presence of a laser field
Authors: Lee H. W. , George Thomas F.
Published: February 2003

A classical perturbation treatment of energy transfer in low-energy gas–surface scattering taking place in the presence of a laser field is given. The laser frequency is chosen such that all transitions except surface vibrational ones can be neglected. Surface vibrations can thus be excited by both incident gas atoms and the laser radiation. It is shown that there exists an interference effect of the gas atoms and the laser radiation in transferring energy to surface vibrations. The interference can be constructive or destructive depending on the details of the experimental conditions and the system parameters.
Specific heat of bulklike NiO powder from 14 to 280 K
Authors: DuBose J. B. , Naugle D. G.
Published: February 2003

The specific heat of a bulklike NiO powder sample has been measured from 14 to 280 K. Results have been compared with previous measurements below 20 K and above 60 K as well as with predictions from inelastic neutron scattering derived phonon density of states. These measurements fill in the temperature gap in specific heat data for NiO, provide a basis for future calorimetric studies of very fine powder NiO samples, and indicate inadequacies in the inelastic neutron scattering derived phonon density of states at frequencies in the neighborhood of 3 THz and below. An anomalous contribution to the heat capacity at about 25 K is attributed to magnetic ordering of impurity Ni(OH)2 constituting about 0.5% of the sample.