bluuebunny/crossref_metadata_2025 huggingface dataset:
Validity of the Ruff-MKW-boiling point method: Vapor pressure of liquid cesium chloride, viscosity coefficient of gaseous cesium chloride, and gaseous interdiffusion coefficient for cesium chloride in argon and helium
Authors: Wahlbeck P. G. , Myers D. L. , Hendrixson T. L. , Pierson F. D. , Pyles S. A.
Published: February 2003

The Ruff-MKW-boiling point method is used to determine equilibrium vapor pressures, viscosity coefficients of the sample vapor, and the gaseous interdiffusion coefficients of the sample vapor in an inert gas. The experimental data are mass flow rates from a sample container which has a capillary exit, in the presence of an inert gas at a measured pressure, under isothermal conditions. The sample studied in these experiments was CsCl(l). The measured vapor pressures are in very good agreement with the equation previously given by Kelley. From the kinetic theory of gases, the experimental viscosity coefficients gave rise to a molecular diameter of 5.69 Å. The gaseous interdiffusion coefficients are compared to and are in agreement with the kinetic theory of gases.
Treatment of dipole orientation in the double layer at electrodes: Analysis of conditions leading to polarization catastrophes
Authors: Marshall S. L. , Conway B. E.
Published: February 2003

Molecular polarization contributions to the potential profile and differential capacity of electrode-solution interfaces are compared, on the basis of the primitive model for the double layer, according to three simple representations of the orientation of solvent dipoles at the electrode: the Watts–Tobin/BDM two-state model, the Fawcett three-state model, and a Debye–Langevin-type continuum-of-orientations model. The so-called Cooper–Harrison catastrophe, in which the induced polarization qdip of the inner layer is calculated to exceed the surface charge density qM of the electrode, is shown to arise for all models considered, and is therefore not, as supposed previously, a specific artifact of the two-state model with respect to which it was first noted. This polarization catastrophe is shown to result from an oversimplified but frequently employed representation of the polarizing field as E1=4πqM. When the field assumed to act on the molecules in the polarization expressions is represented as E1=Δφ1/d (where Δφ1 is the potential difference across the inner layer and d is its thickness), or as E1=4π(qM−qdip), i.e., taking into account the reactive polarization of the dipole layer, there result implicit equations for qdip and Δφ1 which provide self-consistent values of these quantities as a function of qM or of electrode/solution potential difference V. Anomalous behavior is then shown not to arise. These equations are shown to be formally consistent with the conditions of continuity of potential and dielectric displacement, applied to the interfaces assumed to be present in the given model of the double layer. The calculation procedures which are described may be readily extended to other models which have been proposed to describe the polarization of the compact double layer.
Fe* production from electron impact on Fe(CO)5
Authors: Hale Brian C. , Winn John S.
Published: February 2003

A crossed beam experiment is reported. A strong effusive beam of Fe(CO)5 was crossed with an electron beam at right angles. Atomic iron fluorescence was observed in the 200−600 nm range. (AIP).
Fluorescence line narrowing in percolating systems
Authors: Huber D. L.
Published: February 2003

We calculate the decay of the sharp line fluorescence in an inhomogeneously broadened system excited by monochromatic light. The optically active ions are distributed at random on a lattice. It is assumed there is transfer of optical excitation only between nearest neighbors. Particular emphasis is placed on the behavior near the critical percolation concentration. The time dependence of the decay is established in various regimes.
Measurement of the (quasi)-self-diffusion coefficient of solutions of Brownian macroparticles
Authors: Phillies George D. J.
Published: February 2003

D s of a dilute suspension of 0.15 μ polystyrene spheres moving through concentrated suspensions of 0.038 μ polystyrene spheres was determined with light scattering spectroscopy. Solutions from which small ions had been rigorously excluded were compared with suspensions containing such ions. If the concentration dependence of Ds is Ds=D0 (1+λφ), φ being the volume fraction of 0.038 μ spheres, removal of the small ions makes λ more negative. This behavior of λ agrees qualitatively with a dynamic friction model for Ds, but is inconsistent even as to sign with calculations based on the generalized Smoluchowski equations, as recently summarized by Evans and James [J. Chem. Phys. 79, 5553 (1983)].
Universality of the coexistence curves in a polymer solution
Authors: Izumi Yoshinobu , Miyake Yasuhiro
Published: February 2003

On the basis of a geometrical consideration on the surface and of the homogeneous properties for coexistence curves, we deduce a scaling form τ/τc =F(φ/φc) for the family of coexistence curves in a polymer solution, where τ is the reduced temperature, τc the critical reduced temperature, φ the concentration, and φc the critical concentration. It is shown that the scaling form provides a reasonable representation of existing experimental results. The relevance to some current theoretical predictions of this equation is discussed.
Influence of crystal fields on the quasimetallic reflection spectra of crystals: Optical spectra of polymorphs of a squarylium dye
Authors: Tristani-Kendra M. , Eckhardt C. J.
Published: February 2003

The reflection and Kramers–Kronig absorption spectra have been obtained from the monoclinic and triclinic polymorphs of a squarylium dye, 2,4-bis(4-diethylamino-2-hydroxy phenyl) cyclobutadienediylium-1,3-diolate. The extremely different optical responses were found to arise from two molecular singlet transitions of essentially long axis polarization. Successful application of a four oscillator molecular exciton–polariton theory required the use of point charge densities rather than point dipoles in an extension of the theory which employed both interactions between the two singlets as well as a frequency dependent lattice damping. An intermolecular charge transfer transition is also assigned.
Study of low-lying electronic states of Ni atoms in solid inert gas matrices
Authors: Cellucci Thomas A. , Nixon Eugene R.
Published: February 2003

Laser-induced fluorscence studies of low-lying energy states of Ni atoms isolated in Ne, Ar, and Kr matrices show that the atoms in Ne matrices are gas-like in nature, with a 3F4(3d84s2) ground state and levels only slightly perturbed by the matrix environment. The Ar and Kr matrices contain not only these gas-like atoms but also Ni atoms in a second kind of environment. The ground state of these atoms is 3D3(3d94s1), which we estimate lies about 200 cm−1 below 3F4.
The microwave spectra of propyne in the 17–70 GHz region for the ground V1=1 and V9=1 vibrational states
Authors: Ware J. M. , Roberts J. A.
Published: February 2003

The rotational spectra of the ground V10=1 and V9=1 vibrational states of propyne have been determined for the frequency range of 17–70 GHz. The ground and V10=1 measurements have been shown to be in excellent agreement with constants derived from higher J values by other authors. This was the first time that a sufficient number of lines in the V9=1 state had been measured to make a reasonable assignment. It was found that the data could be fit to Amat’s theory for Vt=1 states in C3V molecules only if an additional term of 0.653 (J+1) was added to the frequencies predicted for the K=±1, l=∓1 lines. Our assignments are in disagreement with those of Jaseja based only on the J=2→3 transitions measured by Gordy. Our data completes the low J rotational spectra for the ground V10=1 and V9=1 states of CH3CCH.
Mechanism of thermal electron attachment to O2: Isotope effect studies with 18O2 in rare gases and some hydrocarbons
Authors: Shimamori Hiroshi , Hotta Hiroshi
Published: February 2003

Thermal electron attachment to 18O2 has been studied at room temperature for mixtures with rare gases (He, Ne, Ar, Kr, and Xe) and hydrocarbons (CH4, C2H6, C3H8, and neo-C5H12) as third bodies (M), and the results are compared with those for the corresponding 16O2 systems. The obtained three-body attachment rate constants (k18M ) for rare gases and CH4 are nearly equal to, or even less than those for the 16O2 systems (k16M ), which strongly suggests that the attachment to van der Waals molecules (or the vdW-M mechanism) predominates in these systems. On the other hand, for C2H6, C3H8, and neo-C5H12 k18M ’s are about twice greater than k16M ’s, thus indicating the importance of the Bloch–Bradbury (or the B–B) mechanism. All the isotope-effect data imply that the well-known feature of the marked dependence of the three-body rate constants upon nature of third bodies originates mainly from the B–B mechanism, and also that the vdW-M mechanism becomes important only when the B–B rate constant is very small, as in the case of rare gases, N2, and CH4.
Comparison of theories for the translational and rotational diffusion coefficients of rod-like macromolecules. Application to short DNA fragments
Authors: Tirado M. Mercedes , Martínez Carmen López , de la Torre José García
Published: February 2003

Two theories relating the translational and rotational diffusion coefficients Dt and Dr of a rod-like macromolecule to its length and diameter, proposed by Broersma [J. Chem. Phys. 74, 6989 (1981)], and Tirado and García de la Torre [J. Chem. Phys. 71, 2581 (1979); 73, 1986 (1980)] are shown to predict different values of the coefficients for a particle of given dimensions. Next, we use the two theories to analyze existing experimental data of sedimentation coefficients s and translational and rotational diffusion coefficients of short DNA fragments, and obtain values of the hydrated diameter of DNA d which is treated as an adjustable parameter. The results are compared with the expected value, d≂26Å. This comparison favors clearly the Tirado–Garcia de la Torre theory in the case of Dt and s. For Dr, and using a rise per base pair r=3.4 Å, this theory gives best agreement for all the data examined, while when r=3.3 Å, the agreement depends on the source of data.
Equivalent potentials for equations of state for fluids of nonspherical molecules
Authors: Williams G. O. , Lebowitz J. L. , Percus J. K.
Published: February 2003

We analyze the extent to which the equation of state and other thermodynamic properties of systems of hard nonspherical molecules can be obtained from a density independent hard sphere reference system. We conclude that the median and Barker–Henderson prescriptions effectively reproduce all data now available. We discuss the motivation for these two formulations in detail.
Size effects in the structure and dynamics of CO2 clusters
Authors: Torchet G. , Bouchier H. , Farges J. , de Feraudy M. F. , Raoult B.
Published: February 2003

Clusters are formed in a free jet expansion and studied by electron diffraction, in the range 102–105 CO2 molecules per cluster. When clusters contain more than about 103 molecules, crystallographic methods provide mean structural and dynamical parameters while a comparison between diffraction patterns and calculated diffraction functions of a simple homogeneous model is used in the case of smaller clusters. CO2 clusters are found to present the bulk cubic structure even when they contain less than some 102 molecules. When decreasing the mean cluster size down to some 103 molecules, we observe a linear decrease in the lattice parameter and an increase in the rms atomic displacements, this increase being larger for O atoms than for C atoms due to the molecular librational motions. The calculated diffraction functions make it possible to study quantitatively the correlated motions of nearest neighbor molecules and are in very good agreement with the experimental curves in the range 102–103 molecules per cluster.
Dynamics of O(1D) reactions with bifunctional substrates: HCN
Authors: Carpenter Barry K. , Goldstein Neil , Kam Anthony , Wiesenfeld John R.
Published: February 2003

The products of the reactive deactivation of O(1D) by HCN are monitored using laser fluorescence spectroscopy. The dominant channel for reaction leads to formation of NH(a 1Δ)+CO(X 1Σ+); less than 10% of the reactive encounters yield OH(X 2π)+CN(X 2Σ+). The rotational distribution of NH(a 1Δ) is significantly colder than prior prediction probably as the result of a kinematic effect, while that of CN(X 2Σ+) is slightly cooler, but both are generally well described by a Boltzmann temperature. There appears to be an inversion of the v″=1/v″=0 population ratio in NH(a 1Δ), although the present data confirm that previously calculated Franck–Condon factors for the c 1π←a 1Δ transition do not accurately reflect the observed intensities. The dominant reactive pathway involves insertion of O(1D) into the C–N bond to form an oxazirine intermediate, with subsequent migration of the hydrogen from carbon to nitrogen leading to formation of the observed products. This interaction proceeds initially on the 1A′ ground state surface, but analysis of orbital symmetry conservation reveals that a nonadiabatic transition to the low-lying 1A″ surface is required prior to hydrogen migration.
T-V and V–V energy transfer in collinear collision of two diatomic molecules. An application of label variable classical mechanics
Authors: Shi Shenghua
Published: February 2003

The label variable classical mechanics method has been applied to the calculation of vibrational transition probabilities for T-V and V–V energy transfer in the collinear collision of two identical molecules where quantum resonance plays a crucial role. The results show that this very simple practical approach can handle the phase problem surprisingly well.
BKLT equation for reactive scattering. II. Extensions and application to collinear nonsymmetric systems
Authors: AbuSalbi N. , Kouri D. J. , Shima Y. , Baer M.
Published: February 2003

The BKLT integral equations were recently introduced with the aim of applying them in numerical studies of exchange collisions. So far only symmetric systems were treated. In this work we extend the treatment to nonsymmetric collinear systems (D+H2 and F+H2) and compare the results with those obtained from a more established method. In addition we also introduced a third set of equations and tested its relevance.
Evaluation of complex higher-order semiclassical phase integrals
Authors: Luppi Jussi , Pajunen Petri
Published: February 2003

A method for evaluating complex semiclassical first- and higher-order phase integrals is derived and tested. The method is an extension and generalization of an earlier procedure devised for evaluating real phase integrals. The semiclassical approach is tested by calculating first- and higher-order complex phase shifts. It is shown that the first-order results often are too inaccurate, but the accuracy in third order typically is sufficient and that the higher-order terms readily give reliable estimates of the accuracy achieved.
(Heisenberg) exchange and electrostatic interactions between O2 molecules: An a
b i
n
i
t
i
o study
Authors: Wormer Paul E. S. , van der Avoird Ad
Published: February 2003

The electrostatic and exchange interactions between two ground state 3Σ−g O2 molecules have been calculated ab initio by means of first order exchange perturbation theory. The nonorthogonality problem has been handled in a second-quantized hole-particle formalism by a generalization of Wick’s theorem. The splitting between the spin states, S=0, 1, and 2, of the O2–O2 dimer is accurately represented by the Heisenberg Hamiltonian. By means of a spherical expansion for the orientational dependence and exponential functions for the distance dependence of the expansion coefficients, complete analytic potential surfaces have been evaluated, both for the spin-independent term in the Heisenberg Hamiltonian ∼(ΔE) and for the exchange coupling parameter J. The strong anisotropy and distance dependence of J indicate that magnon–libron and magnon–phonon coupling in solid O2 are likely to be strong. A simple four-electron model containing the O2 open shells only reproduces the structure dependence of J qualitatively, but not quantitatively.
Far IR continua caused by large polarizabilities of intramolecular NLi+ ⋅⋅⋅N ⇄ N ⋅⋅⋅ Li+N bonds due to cation motion
Authors: Brzezinski Bogumil , Zundel Georg
Published: February 2003

Aromatic N bases, compounds 1–4 and aliphatic N bases, compounds 5 and 6 were studied by IR spectroscopy pure and as 1:1 mixtures with LiClO4. In the pure LiClO4 solutions the Li+-ion motion band caused by vibrations of Li+ in a cage of acetonitrile molecules is found at about 400 cm−1. In the case of the solutions of the aromatic N bases 1-3 with LiClO4, continua are observed instead of the ion motion band, beginning at about 450 cm−1 and extending toward smaller wave numbers. They indicate the presence of intramolecular NLi+⋅⋅⋅N ⇄N⋅⋅⋅Li+N bonds in which the Li+ ion may fluctuate. These Li+ bonds are homoconjugated, i.e., structurally symmetrical. The continua demonstrate that these bonds show large polarizability, so-called Li+ polarizability, due to the Li+ motion. Caused by their large polarizabilities they interact strongly with their environments. With the aliphatic N bases complexes with the Li+ ions are formed, but instead of the continua an ion motion band at 400 cm−1 occurs. This result shows that N⋅⋅⋅Li+⋅⋅⋅N bonds with single minimum are present. The different behavior of the aromatic and the aliphatic compounds is explained since the affinity for the Li+ ions is much higher with the aromatic compared with the aliphatic N bases.
On the thermal broadening of zero-phonon impurity lines in absorption and fluorescence spectra
Authors: Hsu D. , Skinner J. L.
Published: February 2003

We examine the problem of the absorption zero-phonon line shape for dilute impurities in crystals. We consider the usual two level electronic model, where both the ground and excited state Born–Oppenheimer surfaces are harmonic in the phonon coordinates. The difference between the two surfaces (the electron–phonon interaction) has terms which are both linear and quadratic in the phonon coordinates. In contrast to the usual perturbative theories, we calculate the zero–phonon line broadening and shift to all orders in the electron–phonon interaction. We find that only the quadratic term is responsible for line broadening, and that at T=0 K this contribution vanishes. Our results are presented as integrals, which can be performed analytically or numerically, involving the weighted phonon density of states. We also show that within the model, the zero-phonon lines in the absorption and fluorescence spectra coincide exactly for all temperatures. Our results resolve the theoretical controversy produced by the two previous attempts to solve the line shape problem for strongly coupled electron–phonon systems. The work by Osad’ko is shown to be correct.
Two-photon spectroscopy of ions in crystals: Cu+ and Ag+ in the alkali halides
Authors: Payne Stephen A. , Goldberg Andrea B. , McClure Donald S.
Published: February 2003

The two-photon (TP) spectra of the Cu+ and Ag+ impurities in some alkali chloride hosts have been obtained; here the ion is situated in a nearly octahedral chloride environment. Since the d→s transitions observed in the near UV are electronically allowed in TP spectroscopy (δ=3 gm), a simple theory of the polarization studies, in conjunction with the DV-Xα computer code led to a definitive characterization of the 1Eg, 1T2g crystal field splitting of the d9s excited state. We also obtained evidence of the Jahn–Teller effect in the 1Eg(d9s) state as well as a good understanding of the mixing of the 1Eg, 1T2g states resulting from the dynamic and static displacements of Cu+. The absolute value of the two-photon cross section was measured and compared to a calculated value.
Two-dimensional nuclear magnetic resonance studies on a single crystal of l-alanine. Separation of the local dipolar fields; and 2D exchange spectroscopy of the 14N relaxation processes
Authors: Naito A. , Barker P. B. , McDowell C. A.
Published: February 2003

Two types of 2D NMR techniques, namely, separated local field 2D NMR (SLF 2D NMR), and 2D exchange NMR spectroscopy were applied to a single crystal of l-alanine at room temperature. In the SLF 2D NMR experiments, we found that the 13C–1H dipolar field at the Cα carbon nucleus could be separated not only from the chemical shift interaction, but also from the 13Cα–14N dipolar field. The angular variation of the 13Cα–1H dipolar splitting was measured when the static magnetic field was rotated about three orthogonal axes (a, b, and c axes). The 13Cα-1H dipolar coupling tensor was determined and the Cα–H bond length was evaluated to be 1.073 Å. In the 2D exchange NMR experiment for Cα carbon nucleus, the off-diagonal cross peaks due to the single quantum and the double quantum transitions for the spin-lattice relaxation processes of the adjacent 14N nucleus were observed. The single quantum transition rate constant was evaluated to be 0.8 s−1, and the double quantum transition rate constant was estimated to be much smaller. Inspection of the experimental results of the 2D exchange NMR, together with the theory, indicates that, (1) the double quantum cross peaks which appeared when a long mixing time (τm=1.0 s) was used, is brought about by two consecutive single quantum processes, and (2) the main spin-lattice relaxation process of the NH+3 nitrogen nucleus is the fluctuation of 14N–1H dipolar interaction rather than the fluctuation of 14N quadrupole interaction.
Quantum dynamics of the three-dimensional F+H2 reaction. I. Energy partitioning and entropy analysis in the collision complex
Authors: McNutt Joe F. , Wyatt Robert E. , Redmon Michael J.
Published: February 2003

Analysis is presented of the quantum dynamics of the three-dimensional F+H2→FH+H reaction for total angular momentum J=0. First, the method (coordinates, Hamiltonian, basis sets, close-coupling method, and boundary conditions) of solving the Schrödinger equation is reviewed, with emphasis on numerical construction of the scattering wave function in the region of the collision complex. Then, four types of analysis of the collision complex are presented: (1) translational wave functions for the dynamically significant channels, (2) vibration-rotation energy partitioning, (3) vibration-rotation entropies, (4) variation with position along the reaction coordinate of the total scattering wave function density. Emphasis is placed upon variations in these quantities as the system passes through a quantum resonance (near total energy 0.36 eV). In paper II of this series, the total scattering wave function density and flux are analyzed in the region of the collision complex.
Temperature dependence of the structure factor S(Q) of liquid-like ordered colloidal dispersions
Authors: Härtl Wolfram , Versmold Heiner
Published: February 2003

A liquid-like ordered polystyrene latex suspension of particle diameter σ=109 nm and concentration ρ=1.62×1018 particles/m3 was investigated via its light scattering structure factor S(Q) in the temperature range from 21 to 70 °C. Contrary to previous investigations we did not find a rapid collapse of the liquid-like structure at higher temperature. Comparison of the experimental S(Q) with structure factors derived in the rescaled mean spherical approximation (RMSA) of Hansen and Hayter indicates that the experimental S(Q) at 70 °C are slightly less structured than predicted by the model. With the assumption of an extra ionic strength at 70 °C of I=1.0×10−6 mol/ℓ due to impurities the experimental S(Q) can well be rationalized in terms of the RMSA liquid model.
Kinetic energy and angular dependence of activated dissociative adsorption of N2 on W(110): Observed insensitivity to incidence angle
Authors: Auerbach D. J. , Pfnür H. E. , Rettner C. T. , Schlaegel J. E. , Lee J. , Madix R. J.
Published: February 2003

Measurements are reported of the dependence of the dissociative adsorption probability on the incidence angle of nitrogen beam. The supersonic molecular beam of N2 is incident on a W(110) surface mounted in a UHV chamber. The measurements were controlled by a feedback system interfaced to a computer. (AIP)
Picosecond excitation and selective intramolecular rates in supersonic molecular beams. I. SVL fluorescence spectra and lifetimes of anthracene and deuterated anthracenes
Authors: Lambert W. R. , Felker P. M. , Zewail A. H.
Published: February 2003

Fluorescence spectra and decay rate results for jet-cooled anthracene and some deuterated anthracenes excited to single vibronic levels in their S1 manifolds are presented. Consistent with our quantum beat results (see accompanying paper), fluorescence spectra exhibit the manifestations of negligible, then limited, and finally, extensive intramolecular vibrational energy redistribution (IVR), as the vibrational energy in S1 is increased. The decay rate results indicate that the primary decay channel of the anthracenes for S1 vibrational energies of 0 to 5600 cm−1 is a radiationless transition involving a small electronic energy gap. These decay rate results also display manifestations of negligible IVR at low energies and extensive IVR at higher energies. The energy at which the decay rates (time-resolved spectra) indicate extensive IVR is about 1800 cm−1.
The spectroscopy of dimethyl-s-tetrazine cooled in a supersonic free jet
Authors: Haynam Christopher A. , Brumbaugh Donald V. , Levy Donald H.
Published: February 2003

The fluorescence excitation and dispersed emission spectra of dimethyl tetrazine have been observed in a supersonic free jet. Measurements of vibrational structure in the low resolution fluorescence excitation spectrum and in the single vibronic level emission spectrum have allowed us to confirm and extend vibrational assignments made in the static gas and solid matrix. The spectrum is dominated by progressions in the mode ν6a with strong progressions in ν1 as well. Excited vibrational levels of ν6a in the upper electronic state appear to be perturbed. The progression in ν1 is considerably more intense in dimethyl tetrazine than it is in tetrazine. The vibrational modes ν2, ν8a, ν16a, and ν16b have been assigned. Of these, the assignments of the first three agree with previous work, while the assignment of ν16b is at a much lower frequency. Rotational structure has been resolved at the origin band, and torsionless m=0 transitions have been assigned. Analysis of the m=0 levels indicates that the ring bond lengths and internal angles and their changes upon electronic excitation are similar to those of s-tetrazine and methyl-s-tetrazine.
The spectroscopy, photophysics, and photochemistry of the dimer of dimethyl tetrazine
Authors: Haynam Christopher A. , Brumbaugh Donald V. , Levy Donald H.
Published: February 2003

The dimer of dimethyl tetrazine (DMT)2 has been prepared in a supersonic free jet and its structure and dynamics have been studied by laser induced fluorescence spectroscopy. The binding energy of the excited π*←n electronic state of the dimer was significantly greater than the binding energy of the ground electronic state, and we have called the DMT dimer a mild excimer. The geometry of the DMT dimer was determined from the analysis of the rotational structure in the spectrum of the perdeutero isotopic species and was found to have the two DMT rings stacked and parallel with one ring slipped with respect to the other. At our lowest temperature the perdeutero species was found to have nonrotating methyl groups while the h12 isotopic species had freely rotating methyl groups at all temperatures. The structure of the DMT dimer was consistent with the observation of two exposed aromatic faces as determined by the observation of two van der Waals molecules formed by binding one or two helium atoms to the dimer. When excited vibrational levels of the excited electronic state were pumped, the emission spectrum was dominated by emission from levels reached by intramolecular vibrational relaxation of the initially excited level. The rate of intramolecular vibrational relaxation out of the 6a1 level was found to be 17±6×109 s−1 as determined from the line broadening observed in the fluorescence excitation spectrum. Emission from electronically excited monomers produced by photodissociation of the DMT dimer was observed, but relaxation was always the dominant process and appeared to be much faster than photodissociation.
The decoupled direct method for calculating sensitivity coefficients in chemical kinetics
Authors: Dunker Alan M.
Published: February 2003

A version of the direct method for calculating first-order sensitivity coefficients is extended to nonlinear, time-dependent models defined by stiff differential equations. In this approach the auxiliary equations for the sensitivity coefficients are solved separately from the model equations. Accuracy and stability are maintained by using exactly the same time steps and numerical approximations in calculating the sensitivities as are used in calculating the model solution. The decoupling procedure also greatly increases the efficiency of the method by taking advantage of the fact that the auxiliary equations for different sensitivity coefficients are quite similar. The decoupled direct method is applied to stiff chemical mechanisms for the oxidation of hydrocarbons in the atmosphere, the pyrolysis of ethane, and the oxidation of formaldehyde in the presence of carbon monoxide. Sensitivity coefficients are also calculated for the three mechanisms by a method employing Green’s function and by actually varying the input parameters. Based on these results, the decoupled direct method has advantages in simplicity, stability, accuracy, efficiency, storage requirements, and program size over other methods, including those using Green’s function. Specifically, the decoupled direct method is as much as a factor of 6 more efficient than a recent version of the Green’s function method. Extensions of the decoupled direct method are also discussed.
Induction and dispersion forces in the electron gas theory of interacting closed shell systems
Authors: Harris Robert A.
Published: February 2003

In this paper we exactly incorporate induction forces and partially include dispersion forces into the electron gas theory of interacting closed shell systems.
The interaction of methanol with Ru(001)
Authors: Hrbek Jan , DePaola Robert A. , Hoffmann Friedrich M.
Published: February 2003

The adsorption, desorption, and decomposition of methanol on a clean Ru(001) surface at 85 K has been examined with electron energy loss spectroscopy, multiple mass thermal desorption spectroscopy, low energy electron diffraction, and a work function probe. Methanol adsorbs readily on Ru(001) and is found to decompose at submonolayer coverages even at low temperature (85 K). Two decomposition pathways are observed: oxygen–hydrogen bond breaking (CH3OH→CH3O–M+H–M) and carbon–oxygen bond breaking (CH3OH→H2O+C–M+2H–M). The methoxy species either recombines with hydrogen and desorbs as methanol between 220 and 250 K via second order reaction kinetics (n=1.85; E*D≂14 kcal/mol; ν(2)=10−2 cm−2 s−1); or further decomposes to form carbon monoxide and hydrogen. The conversion of the methoxy species into carbon monoxide begins at 220 K and is completed at 300 K. The methoxy conversion is accompanied by the gradual formation of a p(2×2) LEED pattern which disappears after CO desorption. The second reaction channel, i.e., C–O bond breaking, results in the formation of water, which desorbs at 210 K, and in surface carbon, which was detected with oxygen titration. The results are discussed and compared to methanol decomposition on palladium and nickel and demonstrate the unique ability of Ruthenium for both C–O bond cleavage and formation of hydrogenatable methoxy species.
Vibrational excitation, harpooning, and sticking in molecule–surface collisions
Authors: Gadzuk J. W. , No/rskov J. K.
Published: February 2003

The problem of vibrational excitation of a diatomic molecule scattering from a metal surface is considered for encounters in which the molecular electron affinity level crosses the surface Fermi level, thus allowing for electron transfer back and forth between metal and molecule during the scattering process. The problem is formulated within a diabatic representation in terms of a Landau–Zener–Tully–Preston curve hopping at the location where charge transfer or harpooning occurs, following related theory for other surface charge transfer processes. Account is taken of both the time dependence of the affinity level position and width due to the translational motion of the molecule. Vibrational excitation probability distributions for scattered molecules are calculated. Under certain circumstances, these are obtained in analytic form using a semiclassical wave packet dynamics model. A novel mechanism in which the energy redistribution from translational to internal vibrational modes gives rise to sticking of the undissociated molecule on the surface is presented.
Determination of reorientational correlation functions in ordered fluids: IR absorption spectroscopy
Authors: Dozov I. , Kirov N. , Fontana M. P.
Published: February 2003

Fourier band analysis is applied to obtain the reorientational correlation functions from IR spectra of ordered nematic liquid crystals. Applying the equations derived in this work on vibrations not symmetric about the long molecular axis the rotational diffusion coefficient Drot of 4, n-octyl 4′-ethoxytolane molecules in nematic phase was determined and its temperature dependence was investigated. It is shown that Drot can also be obtained using the differences in the half-widths of a band recorded with E∥n and E⊥n. This was done for both 4, n-pentyl 4′-methoxytolane and 4, n-octyl 4′-ethoxytolane. The molecular dynamics in the investigated aligned nematic phase is also discussed.
Computer simulation of Raman scattering from molecular fluids
Authors: Ladanyi Branka M. , Levinger Nancy E.
Published: February 2003

We present the results of molecular dynamics (MD) simulations of the nonvibrational contributions to Raman spectra of dense fluids of linear molecules. We include orientational and collision induced (CI) contributions to depolarized scattering time correlation and the CI contribution to the isotropic scattering time correlation. The intermolecular potential is represented by a two site Lennard-Jones model. The bond length and potential parameters are chosen to correspond to those of O2, and CO2. We consider two pair polarizability models, one based on the dipole–induced dipole (DID) interactions between molecular centers, the other on the DID interactions between Lennard-Jones sites. For both models, we study the variations of effective polarizabilities, integrated intensities, spectral moments, and time correlations with density and temperature. We find that the orientational-CI cross correlations are the main interaction-induced contribution to depolarized spectra. Effective polarizabilities, as well as the intensity and the second moment, M2(2), of the depolarized spectrum are all found to have a similar dependence on the fluid structure, which results in the linear density dependence of these quantities. This finding is confirmed by experimental results for M2(2) of fluids structurally similar to the ones considered here. We demonstrate how Raman MD results may be used to evaluate the Rayleigh effective anisotropic polarizability in the ‘‘product’’ approximation. In the case of CO2, significantly different density dependence of the CI part of the LS spectra is predicted by the two DID models.
Temperature dependence of the three-body association of Cl−, NO−2, and NO−3 with SO2
Authors: Böhringer H. , Fahey D. W. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

The temperature dependence of the three body association reactions X−+SO2+He →X−SO2+He with X−=Cl−, NO−2, and NO−3 were studied at temperatures from 150 to 375 K in a flowing afterglow apparatus. The measured rate constants are given by the expressions k1=3.9×10−29 (300/T)2.1(±0.4) cm6 s−1, k2=2.4×10−28 (300/T)2.65(±0.2) cm6 s−1, k3=1.65×10−28 (300/T)3.75(±0.2) cm6 s−1. The increase of the rate constants with decreasing temperature are similar to those observed previously for positive ion association reactions with reactants of comparable complexity. The increase in the temperature coefficient with molecular complexity reflects the role of vibrational motion in the intermediate complex lifetime.
A direct interaction model for chemiluminescent reactions
Authors: Prisant M. G. , Rettner C. T. , Zare R. N.
Published: February 2003

A fully general direct interaction with product repulsion (DIPR) model is developed to aid in the interpretation of product population and alignment data from beam-gas chemiluminescence reactions of the type: A+BC→AB*+C, where AB* is an electronically excited diatomic product. In this model an electron jump occurs at relatively large A–BC reagent separation which initiates a strong repulsive interaction in BC. This is followed by an attractive interaction between A and B, which is less rapid than the BC repulsion. Product repulsion is taken to be distributed as in photodissociation (the DIP extension of the DIPR model), and the attractive and repulsive energy releases are considered to be separable. The electronic energy of the BC product is subtracted from the total available energy to yield an effective exothermicity which is set equal to the sum of the attractive and repulsive energies. Given reaction exothermicity, reagent and product molecular constants, and repulsive interaction parameters, this model yields the product alignment, vibrational distribution, and rotational distribution for each possible product electronic state. Application to the Ca(1S0)+F2→CaF(B 2∑+)+F reaction shows good agreement with experimental results. It is suggested for Ca+F2 that the CaF* alignment originates from a collinear orientational preference for reaction of the reagents.
Theoretical study of substituent effects on CH stretching frequencies
Authors: Raghavachari Krishnan
Published: February 2003

Substituent effects on CH stretching frequencies are explored by computing isolated CH frequencies of several two- and three-heavy-atom systems containing CH bonds. Effects of polarization functions (6-31G* basis set) and electron correlation (second-order Mo/ller–Plesset perturbation theory MP2) are investigated in all cases. The effects of d functions are mostly small, though significant contributions are seen in some cases. Electron correlation effects are extremely important in describing the CH frequency shifts resulting from substitution of electronegative groups. Frequency shifts calculated at the MP2/6-31G* level are in very good agreement with the corresponding experimental values. Frequency splittings resulting from chemical interaction of equivalent CH bonds and the associated deuterium substitution effects are computed in some cases. Good agreement is obtained in cases where experimental results are available.
A path integral Monte Carlo study of liquid neon and the quantum effective pair potential
Authors: Thirumalai D. , Hall Randall W. , Berne B. J.
Published: February 2003

The path integral Monte Carlo (PIMC) method is used to simulate liquid neon at T=40 K. It is shown that quantum effects are not negligible and that when the quantum effective pair potential is used in a classical molecular dynamics simulation the results obtained for the radial distribution function agrees with that predicted by a full path integral Monte Carlo simulation. The validity of this procedure is further shown by comparing the results obtained by this method to experimental measurements of liquid neon at T=35 K.
Comparison of core-hole excitation spectra of organic donor/acceptor molecules in the vapor and condensed phases: p-Nitroaniline, 2-amino-6-nitronaphthalene, and 1-amino-4-nitronaphthalene
Authors: Freund H.-J. , Slaughter A. R. , Ballina S. M. , Banna M. S. , Bigelow R. W. , Dick B. , Lex J. , Deger H. M.
Published: February 2003

Vapor phase N1s and O1s core-hole spectra of the organic donor/acceptor molecules 2-amino-6-nitronaphthalene (2,6-ANN) and 1-amino-4-nitronaphthalene (1,4-ANN) are presented and compared to condensed phase results. 2,6-ANN and 1,4-ANN are found to exhibit broad single peak N1s(NO2) signals in the gas phase which become split into resolvable components in the solid. The spectral differences noted on going from the vapor phase to the solid parallel previous results on p-nitroaniline (PNA). Unlike the situation encountered in PNA (and 2,6-ANN), however, the O1s(NO2) spectrum of 1,4-ANN also indicates a moderately increased splitting as well as an apparent decrease in shake-up intensity on going from the vapor to the condensed phase. Spectral differences between the two phases are addressed within the CNDO/S(CI) equivalent-core approximation including up to doubly excited ‘‘singlet-coupled’’ configurations as a means of elucidating the apparent core-hole induced intermolecular coupling. Comparison computations are presented on PNA, 2,6-ANN, and 1,4-ANN. X-ray crystallographic measurements on 2,6-ANN indicate an intermolecular donor/acceptor pairing scheme as in PNA. Dimer models are therefore used to computationally address solid state interactions. The N1s(NO2) spectral differences observed in PNA, 2,6-ANN, and 1,4-ANN on going from the vapor to the condensed phase are attributed to strong core-hole induced intermolecular orbital mixing. Explicit inclusion of doubly excited configurations in both the monomer and dimer (solid) computations is essential to achieve theoretical assignments of the N1s(NO2) and O1s(NO2) spectra consistent with experiment. The results are analyzed in terms of dominant single and doubly excited contributions.
A comparison of experimentally measured and calculated 11B quadrupolar coupling constants in the carboranes
Authors: Leffler Amos J.
Published: February 2003

The 11B quadrupolar coupling constants for ortho, meta, and para B10C2H12 have been experimentally determined by an NMR method. Theoretical values for the coupling constants and asymmetry parameters were calculated using the Xα-SCF program. Agreement between experimental and theoretical values is poor and the possible causes are discussed.
Crossed beam study of reaction of van der Waals molecule O+(NO)2 →NO*2+NO: Angle-resolved chemiluminescence detection
Authors: Honma K. , Kajimoto O.
Published: February 2003

Bimolecular reaction of NO dimer with O atom was studied by use of a crossed beam apparatus. NO dimer was generated by the supersonic expansion of NO seeded in He. Oxygen atom beam, generated by the microwave discharge, was crossed with (NO)2 beam and the chemiluminescence from the product NO*2 was measured as a function of the scattering angle. The angular distribution is anisotropic and peaking at θlab=0, with respect to (NO)2 incidence. This fact suggests sideways or backward scattering of O atom in (NO)2+O reaction.
Propensity rules in rotationally inelastic polar molecule collisions involving 2Σ+ molecules: CaCl(X 2Σ+)–CH3Cl
Authors: Bullman S. J. , Dagdigian Paul J.
Published: February 2003

Integral state−resolved cross sections are presented for the scattering of CaCl with CH3Cl target. This particular system was chosen so that the potential could be represented as the long−range dipole−dipole interaction. Fluorescence was observed by using a single mode dye laser as excitation source. Rotationally inelastic cross sections were determined from measurement of line intensities as a function of scattering gas pressure. (AIP)
Basis set limit geometries for ammonia at the SCF and MP2 levels of theory
Authors: DeFrees D. J. , McLean A. D.
Published: February 2003

A systematic approach to the Hartree–Fock limit geometry of ammonia was used to determine the limiting bond length, r(NH)=0.998±0.001 Å, and the limiting bond angle (HNH) =108.1±0.1°. Multiple sets of polarization functions, d functions on hydrogen, and f functions on nitrogen are all needed to converge the bond angle to within ±0.1° of the limiting value. Geometry changes due to MP2 correlation mostly become smaller as the basis set increases in size. The MP2 structures are not as well converged to a basis set limit as the SCF structures: r(NH)=1.007±0.002 Å and (HNH)=107.5±0.4°.
Comment on ‘‘A theoretical study of coherence effects in charge transfer collisions: Application to Na+Li+’’
Authors: Hertel I. V. , Schmidt H.
Published: February 2003

The theortical study of Orel and Kulander, regarding charge exchange collisions of excited alkali atoms is commented upon. It is noted that theoretical results can be better correlated to experimental ones if reported in terms of amplitudes and phase factors, or in terms of collisional density matrix elements. (AIP)
Absolute partial electron impact ionization cross sections of Xe from threshold up to 180 eV
Authors: Stephan K. , Märk T. D.
Published: February 2003

Partial electron ionization cross section ratios and functions of Xe were determined in the low energy regime (≤180 eV) using a refined mass spectrometric technique. The experimental results are compared with previous measurements and calculations.
Fold and cusp catastrophe structures in rotational rainbow scattering
Authors: Wolf F. , Korsch H. J.
Published: February 2003

The rainbow structure of rotationally inelastic differential state-to-state cross sections for anisotropic interaction potentials with an attractive well is investigated within the semiclassical infinite order sudden approximation. The typical structural features of the rainbow catastrophes are closely related to the topology of the potential surface. The observed cusped rainbow singularities can be tamed by two-dimensional Airy and Pearcey uniformization techniques.
Rotational and electronic relaxation in pulsed supersonic beams of NO seeded in He and Ar
Authors: Zacharias H. , Loy M. M. T. , Roland P. A. , Sudbo A. S.
Published: February 2003

Resonantly enhanced two-photon ionization has been used to obtain detailed information about the rotational and electronic relaxation in pulsed supersonic expansions of nitric oxide seeded in helium and argon. Due to the sensitivity of the method we were able to measure rotational distributions up to high quantum numbers J″>24.5. Measurements were made for stagnation pressures ( p0) and nozzle diameters (d) in the range 0.44≤p0d≤22 [Torr cm] for NO/Ar beams, and 0.88≤p0d≤36 [Torr cm] for NO/He beams. In general non-Boltzmann rotational population distributions were observed. Furthermore, we found that for NO/He beams, the two electronic substates 2Π1/2 and 2Π3/2 of NO were not in local equilibrium. These observations can be understood in terms of a simple model using state-to-state collision cross sections combined with the empirical hydrodynamic equations to describe the isentropic expansions.
Nonorthonormal CI for molecular excited states. I. The sudden polarization effect in 90° twisted ethylene
Authors: Petsalakis I. D. , Theodorakopoulos G. , Nicolaides C. A. , Buenker R. J. , Peyerimhoff S. D.
Published: February 2003

The accurate and efficient calculation of properties of excited states, especially those involved in near-degeneracies and valence-Rydberg mixings, may depend crucially on the zeroth order orbitals employed for the description of a few important configurations. When self-consistent field orbitals specific for the states of interest are employed, one becomes involved with nonorthonormal basis sets, which circumstance has conceptual as well as computational implications. In this work, we have developed a nonorthonormal configuration interaction (NONCI) method and have applied it to the calculation of the ‘‘sudden polarization’’ effect in the zwitterionic excited states of ethylene, in the spirit of a state-specific theory. A very small NONCI, (6×6), yields similar results to those from previous large CI calculations. In particular, the sudden rise of the dipole moment as a function of the pyramidalization angle of the CH2 group of the 90° twisted Z state is predicted while it is shown that the inclusion of NON at 0° angle (D2d symmetry) is sufficient to recover the correct electronic charge distribution (zero dipole moment).