bluuebunny/crossref_metadata_2025 huggingface dataset:
High resolution study of the ν1 vibration of CH3 by coherent Raman photofragment spectroscopy
Authors: Triggs Nancy E. , Zahedi Mansour , Nibler Joseph W. , DeBarber Peter , Valentini James J.
Published: July 2002

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational–rotational Q-branch structure of the ν1 symmetric stretch of methyl radicals produced by 266 nm laser photolysis of methyl iodide. Spectra were recorded in both flow cells and free jet expansions at instrumental resolutions ranging from 0.25 to 0.005 cm−1. Due to the high recoil velocity of the CH3 fragment, Doppler and collisional broadening of the transitions is appreciable. Even at the highest resolution such broadening of the transitions leads to interference effects among the closely spaced Raman transitions that influence both the line positions and intensities in the observed CARS spectra. The molecular parameters (cm−1) obtained from the analysis are ν1=3004.42(4), αB1=0.0851(8), αC1=0.0475(7), DN1−DN0=−0.000 046(8), DNK1−DNK0=0.000 083(20), and, with assumptions, DK1−DK0=−0.000 039. These results and infrared data in the literature yield a CH bond length of 1.08378(5) Å for the (1000) state and, with some assumptions, an equilibrium bond length Re of 1.076 Å for this prototypic case of sp2 bonding.
Infrared (0.9–1.65 μm) emission from thallium iodide photoexcited with ultraviolet excimer laser radiation
Authors: Herrera S. D. , Schlie L. A. , Black G.
Published: July 2002

The infrared emission spectra from 0.9 to 1.65 μm produced when thallium iodide vapor is photodissociated by excimer laser wavelengths of 193, 248, 308, and 351 nm are reported. In this spectral region, only atomic thallium transitions were observed. No emission from metastable atomic iodine at 1.315 μm (5 2P1/2→5 2P3/2) or thallium at 1.283 μm (6 2P3/2→6 2P1/2) was ever detected in contrast to strong 1.315 μm signals obtained by ultraviolet (UV) excimer laser dissociation of C3F7I. The failure to detect metastable atomic emission from TlI photodissociation is explained by the fast quenching of the metastable states during superelastic collision with electrons produced by photoionization of the TlI. Consequently, a pulsed iodine 1.315 μm laser based on photodissociation of thallium iodide does not seem practical.
Classical dynamics study of HONO using constrained trajectories
Authors: Qin Yue , Thompson Donald L.
Published: July 2002

Constrained classical trajectory calculations were carried out to investigate intramolecular vibrational energy redistribution (IVR), cis–trans isomerization, and rotation–vibration interactions in HONO. Relaxation of the OH bond stretching mode, initially excited to the second overtone state, was monitored by using a local-mode approximation. In order to determine the roles of the various modes in the IVR and cis–trans isomerization, the dihedral, HON bending, or ONO bending angle was dynamically constrained by incorporating Lagrangian multipliers into Hamilton’s equations of motion. The results show that the in-plane HON bending and out-of-plane torsional motions play important roles in the IVR. A strong interaction between the torsional and HON bending modes is a major factor in causing cis-HONO to isomerize at significantly greater rates than trans-HONO. Molecular rotation significantly enhances IVR, however, it has little effect when the torsional coordinate is frozen at its equilibrium value.
The application of negative imaginary arrangement decoupling potentials to reactive scattering: Conversion of a reactive scattering problem into a bound-type problem
Authors: Last Isidore , Neuhauser Daniel , Baer Michael
Published: July 2002

In this work is presented a time-independent treatment of a reactive system employing negative imaginary decoupling potentials. We discuss two aspects: (a) we show how with the help of these potentials a reactive scattering problem is converted into a bound-type problem, and (b) we show that a reactive treatment can be carried out entirely in the products arrangement channel without the use of the reagents arrangement channel. By doing that we are able to obtain exact reactive state-to-state S matrix elements or transition probabilities.
Simple theory of diffuse structure in continuous uv spectra of polyatomic molecules. II. Photodissociation of bent symmetric triatomics
Authors: Braunstein M. , Pack R. T
Published: July 2002

We present two simple models for calculating photoabsorption spectra of bent symmetric triatomic molecules, one designed for light-heavy-light systems with an inactive bending mode and a more general one with no assumptions about atomic mass ratios and where bending motion may be active. Both models are extensions of an earlier model [R. T Pack, J. Chem. Phys. 65, 4765 (1976)] for the photodissociation of collinear symmetric triatomic molecules which showed that excitation to a single dissociative electronic state can give absorption spectra with rich structure due to bound degrees of freedom perpendicular to the dissociation coordinate. Similarly, the present models predict structure in the absorption spectra which is easily understood in terms of the vibrational frequencies and bond lengths, and the steepness and height of the excited state barrier. As a test case, we use the models to calculate the photoabsorption spectra of the X 1A1→A 1B1 transition in water which is known to involve a single dissociative excited state surface. We find that these models, which only require knowledge of the ground-state potential-energy surface near the minimum and the excited state potential-energy surface in the barrier region, are quite successful and may serve as simple tools to investigate photoabsorption spectra of other molecules.
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9Xe nuclear magnetic resonance studies of xenon in zeolite CaA
Authors: Jameson Cynthia J. , Jameson A. Keith , Gerald Rex , de Dios Angel C.
Published: July 2002

The average 129Xe nuclear magnetic resonance (NMR) chemical shift for xenon atoms in alpha cages of zeolite CaA is observed in a single peak dependent on xenon loading (〈n〉=0.5–8.9 Xe atoms/alpha cage) and temperature (240–360 K). The general increase of the shift with increasing average number of xenon atoms per alpha cage is shown to be due largely to the changing distribution of occupancies with increasing 〈n〉, coupled with increasing increments in the chemical shifts of Xen with increasing n. Except at the highest loadings, the results obtained for xenon in CaA are predicted nicely on the basis of δav(T)=(1/〈n〉)Σnnδn(T)Pn (〈n〉,T), where the fractions Pn of alpha cages containing n Xe atoms are imported from the Pn measured in xenon in zeolite NaA. The high loading data in CaA are interpreted in terms of contributions to the average 129Xe chemical shifts associated with xenon atoms in the window positions.
Microwave spectrum, dipole moment, structure, and internal rotation of the cyclopropane-sulfur dioxide van der Waals complex
Authors: Andrews Anne M. , Hillig Kurt W. , Kuczkowski Robert L.
Published: July 2002

The rotational spectrum of the cyclopropane-sulfur dioxide complex was observed by Fourier transform microwave spectroscopy. The spectrum exhibited a- and c-dipole selection rules with the c-dipole transitions split into doublets of unequal intensity separated by about 150 kHz. The structure has Cs symmetry with the sulfur and carbon atoms all lying in the ac plane; the oxygen and hydrogen atoms straddle the plane. The sulfur dioxide plane is nearly parallel to a C–C bond edge. The distance from the center of mass of the SO2 to the C–C bond center is 3.295 Å. The dipole moment of the complex is 1.681(1) D, with components μa=0.815(1) D and μc=1.470(1) D. The splittings in the spectrum arise from an internal rotation of the cyclopropane subunit about its local C2 axis which lies nearly along the line connecting the centers of mass.
High resolution x-ray absorption spectroscopy of linear hydrocarbons adsorbed on noble metal surfaces
Authors: Rabus H. , Arvanitis D. , Domke M. , Baberschke K.
Published: July 2002

Recent progress in the performance of soft x-ray monochromators has revealed new features in soft x-ray molecular photoabsorption spectra. We present here a comprehensive high resolution x-ray absorption study on the linear hydrocarbons C2H2n and C2D2n, adsorbed on Ag(100) and Cu(100) at 50 K and in the solid and the gas phase. A line shape analysis is given, for the bound as well as for the continuum resonances and the absorption thresholds, by means of models easily accessible to the experimentalist, leading to a quantitative understanding of the observed spectra, including the vibrational envelopes. Changes in intramolecular bond lengths upon core-hole excitation can be determined and tracked as a function of deuteration and chemisorption. The effects of the substrate bonding on the vibrational envelopes are analyzed.
Chemisorbed CO site interconversion on Ni(111) induced by the electric field of physisorbed second layers
Authors: Xu Zhi , Yates John T. , Wang L. C. , Kreuzer H. J.
Published: July 2002

A new and general phenomenon related to the electrical polarization of a condensed layer has been observed: A polar overlayer physisorbed on top of a chemisorbed CO layer on Ni(111) causes a CO site interconversion from terminally bound CO to bridged bound CO species in the CO underlayer. It has been found that the site interconversion observed is independent of the molecular identity of the physisorbed overlayer, but is closely related to the work-function decrease induced by the polarized overlayer. This phenomenon is explained in terms of the interaction of CO with a very strong electric field, which is of the order of 0.3 V/Å, produced by the polarized overlayers. The electric field causes a rehybridization of underlayer chemisorbed CO species producing a redistribution of CO molecules to different adsorption sites on Ni(111).
Applying the variational principle to a spin-boson Hamiltonian
Authors: Amann Anton
Published: July 2002

Ground states of a spin-boson Hamiltonian, describing one two-level system (a spin) coupled to infinitely many harmonic oscillators (bosons), are studied. This spin-boson Hamiltonian is a prototype for the description of a ‘‘small’’ system (e.g., a molecule) coupled to its environment. The respective ground-state vector(s) are approximated by a linear combination of two coherent-state vectors corresponding to the two levels of the spin. Interest concentrates mainly on phase-transition phenomena (generation of superselection rules) in case the parameters of the Hamiltonian (level splitting of the spin, frequencies of the field modes, and coupling constants) exhibit an infrared singularity. The resulting phase diagrams are shown to satisfy reasonably well the rigorous bounds derived by Spohn, and in particular distinguish between the superohmic, ohmic, and subohmic regime in the sense of Leggett. Nevertheless, the approximation method used is simple enough so that everything can be explicitly calculated. Former results by Pfeifer as well as results by Emery and Luther, Zwerger and Harris and Silbey are extended and discussed.
The heat capacities of V3P and V3Si
Authors: White B. E. , Cotts E. J. , Myers C. E. , McCreary J. C. , Kematick R. J.
Published: July 2002

The heat capacities of V3P and V3Si were measured in the temperature range from 150–350 K. At 298.15 K, we find Cp(V3P)=22.3±.2 J/g atom K and Cp(V3Si)=22.6±0.2 J/g atom K. At 150 K the heat capacity of V3P is 6% smaller than that of V3Si. Our data provide support for a value of 25.6 J/g atom for the standard entropy of V3Si at 298.15 K, and result in an estimated value of 23.5 J/g atom for the standard entropy of V3P at 298.15 K.
Use of the fundamental equation to derive expressions for the transformed Gibbs energy, entropy, and enthalpy of a gaseous reaction system at a specified partial pressure of a reactant
Authors: Alberty Robert A. , Oppenheim Irwin
Published: July 2002

When the chemical potentials or partial pressures of some reactants are externally fixed, a Legendre transform can be used to define a new thermodynamic potential that provides the criterion of equilibrium and leads to a new fundamental equation that provides the means for computing the equilibrium composition and deriving equations for the transformed thermodynamic properties of the equilibrium system. This method is applied to a simple system and to the polymerization of the alkylbenzenes in the presence of ethylene, which involves an infinite number of isomer groups.
Charge-transfer excitons and χ(2) of molecular monolayers
Authors: Dubovsky Oleg , Mukamel Shaul
Published: July 2002

The second-order nonlinear susceptibility χ(2) is calculated for a monolayer of donor–acceptor substituted molecules, using equations of motion which map the problem onto the dynamics of coupled anharmonic (exciton) oscillators. New exciton resonances, which may change in character from Frenkel through charge transfer to Wannier excitons, are predicted as the intermolecular charge-transfer coupling is turned on.
Formation of the superhalogen ion SF−7 via electron attachment to SF6 clusters
Authors: Rauth T. , Grill V. , Foltin M. , Scheier P. , Märk T. D.
Published: July 2002

Electron attachment to SF6 and SF6 clusters has been investigated in a molecular beam/electron impact ion source/mass spectrometer system. In accordance with recent theoretical predictions we were able to produce the superhalogen ion SF7− by electron attachment to SF6 clusters. SF7− has not been previously observed; its relative attachment cross section curve shows features more similar to that of F2− from SF6 than that of F− from SF6.
Immiscibility induced chain stretching, local segregation, and formation of locally ordered domains in diblock copolymers
Authors: Tang Hai , Freed Karl F.
Published: July 2002

An analogy between diblock copolymer melts and incompatible homopolymer blends is used to introduce a model of chain stretching and local segregation in diblock copolymers. This model describes the observed non-Gaussian behavior of diblock copolymers in the disordered state and is used in a scaling analysis that exhibits the nonuniversal nature of the high scaling molecular weight power law for the collective correlation length in the intermediate segregation regime.
A diagram technique for nonorthogonal electron group functions. I. Right coset decomposition of symmetric group
Authors: Kantorovich L. N. , Zapol B. P.
Published: July 2002

For an N-electron system partitioned into a set of electron groups, an expanded arrow diagram (EAD) technique is proposed. It is useful when the wave function of the system is approximated by an antisymmetrized product of mutually nonorthogonal group functions. The technique provides a graphic representation of the right (left) coset decomposition of the symmetric group SN with respect to its subgroup S0 generated by all the intragroup permutations. The connection between the EADs and arrow diagrams of Matsen, Klein et al., and Wilson is stated and used to prove the validity of the arrow diagram representation of the double coset decomposition of SN with respect to S0, as well as to obtain the decomposition coefficients explicitly. The technique is applicable to systems composed of any finite or infinite number of electron groups each containing an arbitrary number of electrons.
Diagram technique for nonorthogonal electron group functions. II. Reduced density matrices and total energy
Authors: Kantorovich L. N. , Zapol B. P.
Published: July 2002

In part I, both the arrow diagram (AD) and expanded AD decompositions of the antisymmetrization operator A for an N-electron system with wave function represented by the product of mutually nonorthogonal group functions have been considered. Based on them, new diagrams for decompositions of normalization and overlap integrals, reduced density matrices, as well as for total electronic energy of the system are proposed and discussed in detail in the present part. The rules for evaluation of the contribution of each diagram in the form of an analytical expression are obtained. Both the strong and p-orthogonality approximations are discussed.
Theoretical study of the dissociation of HF+ in the B 2Σ+ state by quantum-mechanical methods
Authors: Cacelli Ivo
Published: July 2002

The dissociative pathway of the HF+ molecular ion, initially in its B 2Σ+ electronic state, is studied by a full quantum-mechanical technique. The vibronic time-dependent wave function is expanded on the twelve lowest 2Σ+ electronic states computed by a limited configuration-interaction calculation. The nuclear functions are projected on a monodimensional grid and the time-evolution operator is represented by its (1,1) Padè approximant. The calculations are performed both in the adiabatic and in a semidiabatic representation of the electronic states. The decay pattern shows strong vibronic interactions often involving several states at a time. The results are compared with those obtained in a previous calculation, where the nuclear motion was treated by a classical trajectories approach.
Monte Carlo simulation of a surface reaction model with local interaction
Authors: Luque J. J. , Jiménez-Morales F. , Lemos M. C.
Published: July 2002

Influence of the interaction between nearest-neighbor adatoms in a reaction of catalyzed oxidation of carbon monoxide has been studied by Monte Carlo simulation. The transition probabilities are chosen in the Arrhenius form, and the activation energy is divided into two additive terms, corresponding to the action of the substrate and to the interaction between nearest adatoms, respectively. When the interaction makes desorption easier or hinders adsorption the behavior is similar: Three steady state regimes or phases were observed; in the first phase, the surface is poisoned by oxygen; in the second phase there is a reactive steady state in which carbon dioxide is continuously produced, and in the third phase, the surface is poisoned by carbon monoxide. The transition from the O-poisoned phase to the reactive phase is continuous, or second order, and the transition from the reaction to the CO-poisoned phase is first order. The same occurs when the interaction is not considered. The interaction makes the second zone wider, where carbon dioxide is being produced. When the interaction makes easier adsorption or hinders desorption only two phases are observed: In the first phase the surface is poisoned by oxygen and in the second phase it is poisoned by carbon monoxide and the transition from one to another is first order.
Interactions of the B3 cluster with H atoms and H2 molecules
Authors: Hernandez Ramon , Simons Jack
Published: July 2002

A b initio electronic structure calculations on several σ- and three-center bonded B3H and B3H2 structures that correlate with the ground and first excited states of B3 (plus H or H2 ) have been carried out using correlation-consistent polarized valence double-zeta basis sets and complete-active-space self-consistent-field treatments of electron correlation. Geometries, electronic energies, and local harmonic vibrational frequencies were determined for two locally stable structures of B3H and three stable and one metastable structures of B3H2 as well as for transition states connecting such structures. Reaction energies for several processes and a barrier for the B3+H2→B3H2 reaction have also been calculated. A picture of the B3 reactivity is given in terms of two coupled potential energy surfaces and their avoided crossings. The relevance of our findings to future experimental work and similarities with the chemistry of boranes and related reactive species are emphasized.
Thermal rate constants in collinear atom transfer reactions by optimizing the position of the reactants/products dividing surface
Authors: Rom Naomi , Moiseyev Nimrod , Lefebvre Roland
Published: July 2002

On the basis of Miller–Schwartz–Tromp [J. Chem. Phys. 79, 4889 (1983)] formula for the flux autocorrelation function, and its derivation in hyperspherical coordinates by Park and Light [J. Chem. Phys. 94, 2946 (1991)], we study the sensitivity of the thermal rate constant, k(T), for a collinear transfer reaction calculated within the framework of the finite basis set approximation, to the position of the dividing surface between reactants and products, s0=ρα0. Illustrating 2D numerical examples of calculating k(T) for symmetric, H3, and nonsymmetric, Mu–H2 and Mu–D2, potential surfaces by optimizing the value of α0 are given. In all cases (symmetric and nonsymmetric potential surfaces) the optimal dividing surface has to be chosen such that dk(T)/dα0=0. For symmetric potential surfaces tan(2α0)=(mBM/mAmc)1/2, whereas, for highly nonsymmetric potential surfaces, the optimal dividing surface has to be chosen numerically. It is shown that with very limited basis set this numerical stationary solution may be associated with an inflection point rather than a minimum, and therefore k(T) is not necessarily an upper bound to the true thermal rate.
Dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) as studied by light scattering and neutron scattering
Authors: Floudas G. , Higgins J. S. , Fytas G.
Published: July 2002

Dynamic light scattering (depolarized Rayleigh and polarized Rayleigh–Brillouin) and quasielastic neutron scattering are employed to study the dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) (Tg=184 K). The depolarized Rayleigh scattering measurements were made in the temperature range from 303 to 433 K, the polarized Rayleigh–Brillouin measurements in the range from 263 to 433 K, and the quasielastic neutron-scattering measurements in the range from 37 to 312 K and in the Q range from 0.33 to 1.84 Å−1. The orientation times for DOP, obtained from a single Lorentzian fit to the experimental depolarized spectra at high T, are in good agreement with recent dielectric data for the primary (α) relaxation. However, at lower T, the viscosity increases more strongly than the orientation times and the Stokes–Einstein–Debye equation which can adequately describe the dynamics in the high-T range is insufficient at temperatures close to Tg. The relaxation time obtained from the Rayleigh–Brillouin experiment is about 1 order of magnitude faster than the orientation times. In the neutron-scattering experiment we find a strong decrease of the elastic intensity and a corresponding increase of the quasielastic intensity around Tg.

The data analysis with respect to the dynamics (from a two Lorentzian fit) revealed the existence of three processes affecting the high-frequency range: (i) a ‘‘fast’’ (τ2∼10 ps) Q-independent motion with weak T dependence (E2=1.54 kcal/mol), (ii) a ‘‘slow’’ Q-dependent motion, and (iii) a flat background increasing with T and Q. The fast process is discussed in terms of a very localized motion of the phenyl group (β relaxation) and, as such, as a precursor of the the primary (α) relaxation. The relaxation time of this process (τ2) was found to compare nicely with the time τmax from the Rayleigh–Brillouin (RB) experiment suggesting that the latter is caused by fast localized motions. The slow process is discussed in terms of the jump-diffusion model. The activation energy associated with the jump-diffusion times is 6.1 kcal/mol and it is associated with large-scale diffusional motion of the DOP molecule. The relaxation times obtained from this process are compared with the relaxation times obtained from the depolarized and dielectric techniques for the primary relaxation. Finally, the background can be identified with fast local motions and/or low-frequency excitations relaxing outside the energy window of our experiment.
Equilibrium properties of transition-metal ion–argon clusters via simulated annealing
Authors: Asher Robert L. , Micha David A. , Brucat Philip J.
Published: July 2002

The geometrical structures of M+(Ar)n ions, with n=1–14, have been studied by the minimization of a many-body potential surface with a simulated annealing procedure. The minimization method is justified for finite systems through the use of an information theory approach. It is carried out for eight potential-energy surfaces constructed with two- and three-body terms parametrized from experimental data and ab initio results. The potentials should be representative of clusters of argon atoms with first-row transition-metal monocations of varying size. The calculated geometries for M+=Co+ and V+ possess radial shells with small (ca. 4–8) first-shell coordination number. The inclusion of an ion-induced-dipole–ion-induced-dipole interaction between argon atoms raises the energy and generally lowers the symmetry of the cluster by promoting incomplete shell closure. Rotational constants as well as electric dipole and quadrupole moments are quoted for the Co+(Ar)n and V+(Ar)n predicted structures.
Molecular dynamics simulation of Xe diffusion on the Si(100)-2×1 surface
Authors: Utrera Luis , Ramírez Rafael
Published: July 2002

The diffusion of Xe atoms physisorbed on the Si(100)-2×1 surface at submonolayer coverage has been studied by molecular dynamics simulation. Diffusion coefficients were calculated at temperatures between 40 and 150 K. The surface diffusion of adatoms appears as a thermally activated process with activation energy of 2.99 kJ/mol. The microscopic mechanism responsible for the surface diffusion of the adsorbed atoms was investigated comparing the simulation results with the Chudley–Elliot and random-walk models. An analysis of the particle trajectories shows that the diffusive paths of the adatoms can be described as a non-Markovian chain, due to jump correlations induced by adatom–adatom interactions. An extension of the Chudley–Elliot model allowed us to determine jump-distance distributions from the intermediate self-scattering function Fs(k,t). A Lorentzian distribution describes very well the motion of Xe atoms in terms of jump diffusion.
On the ℏ→0 limit of the Schrödinger equation
Authors: Ling Song
Published: July 2002

Contrary to the previous belief, it is found that the time dependent Schrödinger equation is not always reduced to the Hamilton–Jacobi equation in the ℏ→0 limit. An explicit example is given.
Branching ratios for infrared vibrational emission from NO(X 2Π,v′=2–13)
Authors: Rawlins W. T. , Fraser M. E. , Miller S. M. , Blumberg W. A. M.
Published: July 2002

The ratios of overtone and fundamental vibrational Einstein coefficients for NO(X 2Π) have been measured by spectrally resolved infrared chemiluminescence near 2.7–3.3 μm and 5.2–6.8 μm. The reactions of N(2D,2P) with O2, in the presence of a small background of He in a cryogenic low-pressure reactor, generated vibrationally excited, rotationally cold (60 K) NO(v), whose emission spectra were recorded with high spectral resolution. Least-squares spectral fitting analysis of the observed overtone and fundamental spectra gave vibrational band intensities, whose ratios at each emitting vibrational level v′ yielded the (Δv=2)/(Δv=1) Einstein coefficient ratios for v′=2–13. The results provide comparisons to previous theoretical and experimental data, and reflect the behavior of the dipole moment function for NO(X 2Π). The measured ratios indicate an overtone Einstein coefficient A2,0=0.94±0.11 s−1 for an assumed fundamental value A1,0=13.4 s−1.
Critical slowing down of nuclear growth along the liquid/liquid coexistence curve
Authors: Ikier Ch. , Klein H. , Woermann D.
Published: July 2002

The growth rate of a second liquid phase is studied in demixing process of n-butoxyethanol/water mixtures. Following the liquid/liquid coexistence curve from far-off critical compositions to the critical point, the growth rate of the droplets slows down markedly. The values of the mutual diffusion coefficient D extracted from the growth rate data are found to scale approximately with a power law of the form D ∼‖ (T − Tc)/Tc‖0.79.
Monte Carlo study of the shear modulus at the surface of a Lennard-Jones crystal
Authors: Eerden J. P. v.d. , Knops H. J. F. , Roos A.
Published: July 2002

In this paper, we give a microscopic definition of local elastic constants. We apply this to the numerical evaluation of the shear modulus of an interface which is sharp as compared with the interaction range. The algorithm is applied to a study of the (001) face of a face-centered-cubic (fcc) Lennard-Jones crystal. The vanishing of the shear modulus gives an estimate of the melting temperature of the first layer which is well below the bulk triple point. Some theoretical aspects of surface melting are briefly discussed.
Influence of the electronic asymmetry in NH (1Δ) state Λ doublets on the photodissociation dynamics of HN3 and DN3
Authors: Gericke Karl-Heinz , Lock Michael , Fasold Renate , Comes Franz Josef
Published: July 2002

The influence of the electronic asymmetry in the 1Δ(A′) and 1Δ(A″) Λ doublets of NR (R=H,D) on the photodissociation dynamics of hydrazoic acid (RN3) has been investigated. Hydrazoic acid was prepared in its first excited electronic state, Ã1A″. A variety of scalar (internal state and translational energy distribution) and vectorial (angular distribution, rotational alignment, correlation between translational and rotational motion) properties of the ejected NH or ND fragment were analyzed by Λ-doublet-specific Doppler profile measurements. While the population of the 1Δ(A′) and 1Δ(A″) states are equal, the vector correlations for both Λ sublevels are different. NR(A″) products are preferentially ejected in the original plane formed by the parent, and the recoil of NR fragments in the symmetric Δ(A′) state is preferentially perpendicular to that plane. The vector correlation between the translational and the rotational motion of the fragment also indicates a strong nonplanar dissociation geometry for NR products in the Δ(A′) state. About 50% of the ND(A′) product rotation is generated by a torsional motion, while 80% of the ND(A″) fragments are formed with J being aligned perpendicular to the recoil direction (MJ=0).
Jet spectroscopy and excited state dynamics of SiH2 and SiD2
Authors: Fukushima Masaru , Mayama Shinya , Obi Kinichi
Published: July 2002

Silylene radicals, SiH2 and SiD2, are generated in a supersonic free jet by ArF laser (193 nm) photolysis of phenylsilane and phenylsilane-α-d3, respectively. LIF excitation and dispersed fluorescence spectra are measured for the ν2 vibronic bands of the Ã 1B1−X̃ 1A1 transition. The heterogeneous predissociation to Si (3P)+H2 is proposed from the anomalous rotational structure in the excitation spectra; the rotational lines of the r(1) subbranch (K′a=0←K■a=1) have stronger intensity than those of the r(0) subbranch (K′a=1←K■a=0), though the latter is expected to be stronger due to the low temperature Boltzmann distribution in the jet. The time-resolved excitation spectra demonstrate shorter lifetime of K′a=1 rovibronic levels in the Ã 1B1 state.

The heterogeneous predissociation is interpreted with the second order perturbation: Ã 1B1 –(a-type Coriolis)→X̃ 1A1 -(spin–orbit)→ã 3B1→Si(3P)+H2. It is demonstrated experimentally that there is a potential barrier associated with the dissociation path of ã 3B1→Si(3P)+H2, the height of which is estimated to be 1540–2160 cm−1 from the bottom of the Ã 1B1 state. The electronic transition moment of the Ã 1B1–X̃ 1A1 transition is estimated to be ‖μe‖2=0.26e2a20 from the Einstein equation for spontaneous emission using measured fluorescence lifetimes for single rovibronic levels with Ka=0 and calculated Franck–Condon factors. The onset of a second predissociation channel, Ã 1B1→Si(1D)+H2, at the (0,7,0) vibronic level of SiH2 Ã 1B1 is manifested as a sharp decrease in the observed fluorescence lifetime for the v′2=7, J′=0 level relative to that predicted for a pure radiative lifetime.
High resolution Fourier transform stimulated emission and molecular beam hole-burning spectroscopy with picosecond excitation sources: Theoretical and experimental results
Authors: Hartland Gregory V. , Joireman Paul W. , Connell Leslie L. , Felker Peter M.
Published: July 2002

Theoretical and experimental results relating to the picosecond laser implementation of Fourier transform stimulated emission spectroscopy (FT-SES) and Fourier transform hole-burning spectroscopy (FT-HBS) in molecular beams are presented. It is shown that the resolution in the schemes is only limited by the length of the interferogram taken, and not by factors such as the excitation pulsewidths, excitation bandwidths, or the delay between the pump and probe laser pulses. In addition, the factors which determine the homogeneous and Doppler broadening in FT-SES and FT-HBS spectra are examined. It is found that ground-state damping factors determine the homogeneous broadening in FT-SES and excited-state damping factors determine such broadening in FT-HBS. Doppler broadening in both of the methods is shown to be that associated with two-photon resonant vibrational transition frequencies rather than one-photon resonant vibronic transition frequencies. The characteristics of the Fourier transform methods are compared to those of their frequency-domain counterparts.
The dynamics of succinonitrile in the plastic phase by subpicosecond time-resolved optical Kerr effect
Authors: Foggi P. , Righini R. , Torre R. , Angeloni L. , Califano S.
Published: July 2002

The relaxation kinetics of succinonitrile in the plastic crystalline phase has been investigated between 250 and 320 K by transient optical Kerr effect with femtosecond pulses. Three different noninstantaneous contributions to the signal time profile have been identified. The fastest one is a subpicosecond decay, attributed to the relaxation of the damped librational and torsional vibrations of the molecules. The intermediate decay time, ranging from 4 ps at 323 K to 30 ps at 250 K, is interpreted as due to rotations of the trans molecules about the shortest inertia axis which bring the molecule from one cube diagonal to another. The slowest decay time ranges from 28 ps at 323 K to 190 ps at 250 K and agrees very well with previous measurements with different experimental techniques. This decay is interpreted as due mostly to rotational motions of the gauche molecules jumping from one equilibrium position to another in the unit cell. The activation energy for the two processes is 3.7 and 4.3 kcal/mol, respectively, for the intermediate and slow kinetics.
Electronic spectra of N+2–(He)n (n=1, 2, 3)
Authors: Bieske E. J. , Soliva A. M. , Friedmann A. , Maier J. P.
Published: July 2002

Ionic clusters of nitrogen and helium have been investigated by recording their electronic spectra in the near UV. Structured bands belonging to 14N+2 –He, 14N+2–(He)2, 14N+2–(He)3, and 15N+2 –He have been measured between 390 and 392 nm close to the N+2 B 2Σ+u←X 2Σ+g band origin. Spectra were obtained by exciting mass selected cluster cations with tunable laser radiation and recording the photodissociation cross section as a function of wavelength. The data support the hypothesis that the He– – –N+2 interaction potential has only a small barrier to internal rotation in both the X and B electronic states. A lower estimate for the dissociation energy of the N+2 –He cluster of 101 cm−1 is inferred.
Experimental study of the reactions of N2(A 3Σ+u) with CH3CN and HCN: The effect of vibrational energy in N2(A)
Authors: Tao Wen , Golde Michael F. , Ho Grace H.
Published: July 2002

The rate constants and product channels have been determined for the reactions of N2 (A 3Σ+u, v=0–3) with CH3CN and HCN. The discharge–flow technique was employed, with LIF (laser induced fluorescence) detection of N2(A,v) and CN(X,v′) and resonance fluorescence detection of H atoms. For N2(A,v)+HCN, the rate constant increases from (5.8±0.9)×10−12 cm3 s−1 for v=0 to (3.1±0.5)×10−11 cm3 s−1 for v=3, and for N2(A,v)+CH3CN, the rate constant increases from (1.3±0.2)×10−11 (v=0) to (6.0±0.9)×10−11 cm3 s−1 (v=4). A large dependence of the product channels on v is also observed. While cleavage of the C–H bond is the principal channel for reaction of N2 (A, v=0) with both reagents, dissociation of CH3CN to CH3+CN is the major channel for reaction of N2 (A, v>2). The data are discussed in terms of initial attack of N2(A) at the CN bond of the reagent, followed by vibrational energy redistribution within the molecule.
Cross relaxation of polymer dispersed liquid crystal droplets
Authors: Cross Collin W. , Fung Bing M.
Published: July 2002

A study of proton relaxation for polymer dispersed liquid crystal samples was performed using the techniques of magic-angle spinning and magnetization transfer. By the use of these procedures it is possible to determine the cross relaxation rate between the polymer and the liquid crystal directly. The cross relaxation rate is dependent on the curing temperature, which affects the size of the liquid crystal droplets. It is also dependent on the curing time, which affects the droplet size as well. The cross relaxation rates for the aromatic and aliphatic protons of the liquid crystal are very similar, indicating that the liquid crystal molecules probably orient themselves lengthwise on the polymer surface. Differential scanning calorimetry analysis was also performed in order to study the effect of curing time upon the thermal properties of the polymer.
Reactions of barium atoms with triatomic oxidants. I. Ba+NO2
Authors: Davis H. Floyd , Suits Arthur G. , Lee Yuan T.
Published: July 2002

Angular and velocity distributions of the neutral products resulting from the reaction Ba+NO2 were measured using the crossed molecular beams method. Despite a large reaction exoergicity (ΔH=−61 kcal/mol), formation of the dominant ground state BaO(1∑)+NO products results primarily from decay of long-lived Ba+NO−2 collision complexes, even at incident collision energies as high as 59 kcal/mol or with electronic excitation of the Ba atom. A large fraction of the reaction exoergicity is channeled into product translational energy. This rather unusual behavior results from a large exit potential-energy barrier for decay of the initially formed singly ionic Ba+NO−2 intermediate to ground state doubly ionic Ba2+O2−. A secondary source of forward scattered, internally excited BaO results from a direct reaction without the involvement of long-lived intermediates. An additional minor channel, formation of BaNO+O is observed from ground state Ba+NO2 at high collision energies by a direct reaction mechanism. Unlike the dominant BaO+NO channel, which involves harpooning at the first ionic-covalent curve crossing, formation of BaNO from reaction of ground state Ba likely results from the small range of collision geometries which are able to avoid long range electron transfer. The BaNO+O channel was enhanced substantially by electronic excitation of the incident barium atom. However, BaNO from reactions of electronically excited Ba primarily resulted from decay of collision complexes, rather than from a direct mechanism.
Mean-field treatment of local binding processes
Authors: Di Cera Enrico
Published: July 2002

A mean-field treatment of ligand binding to a biological macromolecule containing multiple binding sites correctly describes the essential thermodynamic features involved in the effects observed locally at the level of individual sites. Equilibrium and steady state phenomena can be analyzed within a simple framework that is particularly useful in practical applications. Analysis of the complex electron binding properties of cytochrome aa3 reveals the origin of the negative electron binding capacity observed experimentally for cytochrome a.
Collision energy-resolved Penning ionization electron spectra of unsaturated hydrocarbons with He*(2 3S) metastable atoms
Authors: Takami Tomohide , Ohno Koichi
Published: July 2002

Collision energy dependencies of the partial ionization cross sections of ethylene, propene, butadiene, and benzene upon collision with He*(2 3S) metastable atoms were measured. Band intensities of the π bands decrease with the increase of the collision energy. This tendency indicates that interaction potentials are attractive when He*(2 3S) approaches the π orbitals of unsaturated hydrocarbons. The intensity of a strong extra band at an electron energy of 3.7 eV for benzene shows a similar collision-energy dependence with those of π bands. This extra band is assigned to the ionization from the π(1e1g) orbitals associated with the excitation from the π(1e1g) orbitals to the π*(1e2u) orbitals. The reason why the extra band of the Penning ionization electron spectrum of benzene is more enhanced than that of the ultraviolet photoelectron spectrum can be ascribed to the existence of the He*(2 3S) atom at the time the ionization occurs. The shapes of the σ and π bands change with the collision energy, which reflect the nature of the interaction potential.
Fermi resonance states in the natural isotopically mixed CS2 crystal
Authors: Ranson P. , Ouillon R. , Perrin B. , Lemaistre J.-P.
Published: July 2002

High-resolution Raman spectroscopy (HRRS) is used to analyze the anharmonic interactions between the modes ν1 and ν2 in the CS2 crystal and its natural isotopic aggregates. Statistics of aggregates (mainly formed with molecules containing 34S and 33S isotopes) are calculated. Using the exciton theory formalism, a unified description of the Fermi resonance going from the molecule to the crystal is provided and used to calculate the various eigenmodes of aggregates. HRRS allows an accurate analysis, in the ν1 frequency domain, of the complex band structure due to the various configurations of isotopic aggregates (monomers, dimers, trimers,...). In the crystal, the weak Fermi resonance interaction (ν1, 2ν2 ) leads to the appearance of bound states ν+ and ν−. HRRS allows one to clearly separate the two Raman active Davydov components (Ag and B3g ) in both the ν− and ν+ bands. Their thermal evolution is also given and compared to other incoherent as well as coherent Raman data.
Reversible and irreversible formulation of unimolecular reactions
Authors: Aguda Baltazar D. , Pritchard Huw O.
Published: July 2002

A comparison is made between the reversible and irreversible formulations for a series of model isomerization reactions, and the conditions under which the differences are important are demonstrated.
Modified extended Hückel band calculations on conjugated polymers
Authors: Hong Sung Y. , Marynick Dennis S.
Published: July 2002

In order to more accurately predict band gaps, corresponding to π–π* transitions of one-dimensional conducting polymers, the formula for the off-diagonal elements, Hαβij in the extended Hückel (EH) band calculation method was modified according to the form Hαβij=K1(Hααii +Hββjj)exp(−K2Rαβ) Sαβij. Parametrizations for the constants K1 and K2 were performed so as to yield reasonable band gaps for the pure hydrocarbon polymers trans-polyacetylene, poly(para-phenylene), and poly(phenylene vinylene). Since there is a large difference in bond alternations along polymeric chains between ab initio and modified neglect of diatomic overlap optimized geometries, especially for heterocyclic polymers, the valence orbital exponents of oxygen, nitrogen, and sulfur were separately adjusted, depending on the chosen geometry, to reproduce the band gaps of polyfuran, polypyrrole, and polythiophene. It is found that geometrical relaxations in the presence of heteroatoms strongly affect the C1–C4 interactions as well as bond alternations, which in turn affect the band gap. Modified EH band calculations were performed for various polymers. The predicted band gaps had average errors of ca. 10% (less than 0.3 eV) compared to the experimental values, and the method produced band structures consistent with electron-energy-loss spectroscopic observations.
Ferromagnetic interaction in poly(m-aniline): Electron spin resonance and magnetic susceptibility
Authors: Yoshizawa Kazunari , Tanaka Kazuyoshi , Yamabe Tokio , Yamauchi Jun
Published: July 2002

Ferromagnetic interaction among the spins in poly(m-aniline) has been suggested by electron spin resonance and magnetic susceptibility measured at low temperatures. From the g-value analysis, these spins are found to be located on nitrogens although they are in some degree delocalized on phenyl rings. Doping of this polymer with iodine leads to an increase in the spin concentration up to 1020 spins⋅(g-polymer)−1. Saturation magnetization of 0.30 emu⋅G⋅g−1 and spin clusters of S=1 are observed in the iodine-doped polymer from the temperature and field dependence of the magnetization. Through-bond interaction, depending on the topology of a polymer skeleton, is considered to operate between rather distant spins through the spin polarization with antiferromagnetic array of the phenyl rings.
Adsorption state selectivity of ultraviolet–laser-stimulated desorption of NO chemisorbed on Pt(001) at 80 K studied by (1+1)-resonance-enhanced multiphoton ionization
Authors: Mase Kazuhiko , Fukutani Katsuyuki , Murata Yoshitada
Published: July 2002

Photostimulated desorption of NO chemisorbed on Pt(001) at 80 K has been studied by the (1+1)-resonance-enhanced multiphoton ionization technique. A linearly polarized ArF excimer laser (λ=193 nm, 6.41 eV) is used as the pump laser. The NO desorption yield increases drastically when the amount of NO exposure exceeds ∼1.8 L. This result shows that the amount of NO species with a large cross section for photostimulated desorption increases drastically at higher NO coverages.
A unitary group formulation of the complete active space configuration interaction method. II. An approach based on the subgroup chain U(n=n+n1+n2) = U(n)×U(n1 +in2) = U(n)×U(n1)×U(n2)
Authors: Burton P. J. , Gould M. D.
Published: July 2002

In this paper, we examine some aspects of complete active space configuration interaction (CASCI) calculations where excitations from the core are taken into account. We show that it is appropriate to work in a basis for the irreducible representations (irreps) of the orbital unitary group symmetry adapted to the subgroup U(n0)×U(n1)×U(n2) (where n0, n1, and n2 are the number of core, active, and external orbitals, respectively, and n=n0+n1+n2). We demonstrate that this approach has the following advantages; (i) it allows the efficient isolation of single, double, and higher excitations from the reference space; (ii) it demonstrates explicitly the internal–external factorization of generator matrix elements; (iii) it allows the determination of the matrix elements of arbitrary generator products by simple matrix multiplications.
Polarization effects on resonance enhanced two-photon dissociation of H2 by the X→(E
F+G
K+I) transition via intermediate B and C states
Authors: Banerjee Sima , Chakrabarti M. K. , Bhattacharyya S. S. , Saha Samir
Published: July 2002

The generalized resonance enhanced two-photon dissociation (RETPD) cross sections of H2 are calculated for crossed and circular (same- and opposite-sense) polarizations of the two-photon fields using the reduced density operator formalism for the transitions X 1Σ+g(vg=0, Jg=0–4) →(ω1,ε̂1)VUVB 1Σ+u (C 1Πu)[vi=0, 3, Ji =0(1)–3]→(ω2,ε̂2)UV/visible [(EF+GK)1Σ+g+I 1Πg), Ef,Jf]. The wavelength (λ2) of the second photon is so chosen that ionization from the resonant B(C) state is energetically forbidden. The effects of polarizations on the wavelength and quantum number dependences of RETPD profiles have been studied. For vi=3 of both B and C states, a strong polarization dependence of RETPD cross sections is observed. The same- and opposite-sense circular polarizations have about the same RETPD profiles for the B state except at the shape resonance positions due to the final I state quasibound level, while the profiles are quite different for the C state.