bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid–liquid coexistence in hard-core Yukawa systems
Authors: Renkin M. , Hafner J.
Published: July 2002

The modern density functional theory of freezing has been applied to hard-core liquids with repulsive and attractive Yukawa tails. The freezing properties are calculated as a function of the strength and the decay length of the Yukawa interaction. The calculated phase diagrams are put into a critical perspective with the phase diagrams of hard-core, adhesive hard-core, inverse-power, and Lennard-Jones systems.
On the correct electronic ground state of Tc(g)
Authors: Rard Joseph A. , Rand Malcolm H. , Thornback John R. , Wanner Hans
Published: July 2002

The electronic ground state of Tc(g) is 6S5/2, which arises from a 4d55s2 valence electron configuration. However, there are several treatises and review articles in which the ground state is incorrectly given as S9/2 with a valence electron configuration of 4d65s1. The origin of this incorrect assignment was traced to the misinterpretation of a paper on the hyperfine splitting of the optical spectrum of technetium, and to confusion between nuclear and electronic spins.
Forbidden singlet–singlet transitions in collisional processes
Authors: Lee A. R. , Enos C. S. , Brenton A. G.
Published: July 2002

In an atomic collision in which total spin angular momentum is conserved, retention of singlet status by either collision partner is strictly forbidden when the other collision partner undergoes a change in multiplicity. The absence of such forbidden transitions appears to be the only stringent test of spin conservation. This is strikingly manifested in the translational energy spectra of boron ions, B+, scattered off CO molecules, with suppression of the interaction channel B+(1S)+CO(X 1Σ+)→B+(1S)+CO (a 3Πrf). For projectile ions in nonsinglet ground states, the singlet–triplet X 1Σ+→a 3Πr excitation of CO is an unusually prominent feature.
Trapping and flow among random arrays of oriented spheroidal inclusions
Authors: Miller C. A. , Kim I. C. , Torquato S.
Published: July 2002

The effective trapping rate k associated with diffusion-controlled reactions among random distributions of spatially correlated and uncorrelated, oriented spheroidal traps of aspect ratio ε is determined from Brownian motion simulations. Data for k are obtained for prolate cases (ε=2, 5, and 10), oblate cases (ε=0.1, 0.2, and 0.5), and spheres (ε=1) over a wide range of trap volume fractions (φ2) and satisfy recently obtained rigorous lower bounds on k for this statistically anisotropic model. The results for the trapping rate for correlated traps always bounds from above corresponding results for uncorrelated traps. Generally, the trapping rate k, for fixed φ2, increases with decreasing aspect ratio ε, showing a precipitous rise in k as the spheroids become disklike. Using a recent theorem due to Torquato [Phys. Rev. Lett. 64, 2644 (1990)], data for the trapping rate k can be employed to infer information about the fluid permeability tensor K associated with slow viscous flow through porous media composed of the same arrays of oriented spheroidal particles.
Spectral moment method versus least-squares Franck–Condon analysis for vibrationally resolved absorption spectra
Authors: Lee Soo-Y. , Lee S. C.
Published: July 2002

An accurate, back-of-the-envelope spectral moment method is described to obtain molecular parameters of excited state molecules and ions from vibrationally resolved absorption spectra. Since it is not an iterative procedure, the spectral moment method is clearly much faster than the least-squares Franck–Condon analysis for obtaining molecular parameters, and the procedure described here can even be implemented on a hand-held calculator. Some of the drawbacks of Franck–Condon analysis are overcome by the spectral moment method. Comparison of molecular parameters for a diverse range of excited state molecules and ions obtained by the spectral moment method versus Franck–Condon analysis or rotational spectroscopy reveal the high accuracy of the procedure here. Simulations of vibrationally resolved absorption spectra show that the spectral moment method with the Morse potential approximation gives an envelope that matches very well the observed spectra.
Intensities and asymmetry features of electronic Raman spectra of TmPO4. Third-order one-particle parametrization scheme
Authors: Smentek-Mielczarek Lidia
Published: July 2002

The intensities and polarization characteristics of the transitions from the ground state to the crystal field levels of the 3H6 multiplet of TmPO4 are analyzed and compared with experiment. Emphasis is directed toward the asymmetry features of the Raman spectra. The discussion is based on the results of ab initio calculations performed within the third-order electron correlation theory formulated in terms of perturbed functions. The various contributions to the scattering tensor have been evaluated for the complete radial basis sets (including continuum states), and they represent the impact of the singly excited configurations 4fN−1l′, for l′=d,f,g.
Electron correlation properties of doubly excited states. Berylliumlike vs heliumlike systems
Authors: Martín F. , Bachau H. , Galan P. , Riera A. , Yáñez M.
Published: July 2002

We report a detailed study of electron correlation properties of the (3,n) and 1s2(3,n) resonant doubly excited states of He- and Be-like systems. In both cases, angular correlations are almost constant along the isoelectronic series, whereas radial correlations vary with nuclear charge. It is shown that angular configuration mixings in the Z=∞ limit could be used to classify 1s2(3,n) doubly excited states for high and intermediate values of Z. Calculation of their energies as continuous functions of the nuclear charge (Z diagrams) shows the existence of many avoided crossings at low values of Z, so that no systematic classification can be easily obtained for many doubly excited states in that region.
A self-consistent mean-field calculation of a homopolymer deoxyribose nucleic acid strand separation: Bond breaking in a macromolecule
Authors: Chen Y. Z. , Prohofsky E. W.
Published: July 2002

We develop a self-consistent calculation of the probability of base pair opening from room temperature through the critical behavior at melting for poly(dA)⋅poly(dT). The method uses as input only force constants and interaction potential parameters refined at room temperature plus one parameter describing the open state.
Generalized Langevin equations for molecular dynamics in solution
Authors: Xiang Tian-xiang , Liu Fang , Grant David M.
Published: July 2002

This paper rigorously derives several classes of generalized Langevin equations (GLE) which describe the motion of an interacting many-body system in a heat bath. The fundamental assumption in deriving these GLEs is that the Liouville operator representing the system–heat bath interaction LSR commutes with the global Liouville operator L. An important second dissipation–fluctuation theorem is put forward relating the memory kernel of the friction force to the correlation function between the potential force of the system and the random force from the heat bath and to the auto- and cross-correlation functions of the random forces. Unlike most of the previous treatments in which the system, the heat bath and the system–heat bath interaction are greatly simplified, the properties of the system, the heat bath and the system–heat bath interaction remain general in this study. In particular, the interaction Hamiltonian may be arbitrarily nonlinear, so that the present theory is applicable to any physical system. Finally, a unified treatment for the GLEs, originally proposed by Ciccotti and Ryckaert for the systems in which there is no interaction among particles, is extended to many-body systems with internal interactions and in a heat bath.
Wetting transitions at a solid–fluid interface
Authors: Dhawan S. , Reimel M. E. , Scriven L. E. , Davis H. T.
Published: July 2002

Nonlocal density functional free energy theories are used to investigate the nature of the wetting transition of a Lennard-Jones fluid near a Lennard-Jones wall. In particular, the wetting behavior is investigated for argon films on a solid carbon dioxide substrate for which the Lennard-Jones parameters are known. In accordance with previous predictions of local density functional theories, the transition is found to be of first order. The locus of the prewetting line of the Ar–CO2 system is calculated. Density profiles obtained from the Tarazona and generalized hard rod models are both in qualitative agreement with Monte Carlo simulations of Finn and Monson, although Tarazona’s model performs better quantitatively.
Partial structure factors in colloidal silica mixtures determined with small-angle neutron scattering contrast variation
Authors: Duits M. H. G. , May R. P. , Vrij A. , de Kruif C. G.
Published: July 2002

Small-angle neutron scattering experiments at various contrasts were performed on concentrated mixtures of colloidal spheres differing in size. Both colloidal components consisted of fairly monodisperse silica cores coated with a layer of octadecyl chains. Cyclohexane was used as dispersing medium; variation of the contrast was achieved by using mixtures of 1H -cyclohexane and 2H -cyclohexane. Scattered intensities were measured at three volume fractions up to 0.4, at equal partial volume fractions. The different contrast dependence of the scattering amplitudes of both colloids allowed us to calculate partial structure factors. This was done using a method which has not been reported previously. Describing the intraparticle structures with layered-sphere models, and using a decoupling approximation, three partials were obtained from a system of linear equations. The scattering curves at the various contrasts constitute a consistent data set, at all volume fractions. Although the separate components interact like hard spheres, the partial structure factors in the mixture reflect marked deviations from hard-sphere behavior. Their nature can be qualitatively explained with an attractive interaction between unequal particles. This is confirmed with simple model calculations.
Accurate a
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o calculations of the quadrupole moment of acetylene. A combined study of basis set, correlation, and vibrational effects
Authors: Lindh R. , Liu B.
Published: July 2002

The quadrupole moment of acetylene has been studied at the multiconfiguration self-consistent field (MCSCF) and multireference single and double configuration interaction (MRSDCI) level of theory. At the MCSCF level the π-CI complete active space SCF (CASSCF) and the valence-CI CASSCF were employed. The subsequent MRSDCI calculations were continued until the reference space included all configuration state functions (CSFs) of the MCSCF wave function with a coefficient larger than 0.01 [MRSDCI(0.01)]. The higher level basis sets in this study were all based on van Duijneveldt’s C(13s 8p) and H(6s) and extensions of that basis set. The study shows in a consistent way that both the one- and n-particle spaces are saturated at the highest level of theory. The study has revealed that in addition to the well known increase of the quadrupole moment due to the inclusion of polarizing functions in the basis (typically 0.20 a.u.), the inclusion of electronic correlation in the model wave function as well as vibrational corrections will decrease the quadrupole moment significantly more, −0.66, −0.49, and −0.36 a.u., for the correlation correction and zero-point correction for HCCH and DCCD, respectively. The most accurate computations predict the quadrupole moment of HCCH, including zero-point correction, to be 4.29±0.12 a.u., which discriminates the experimental estimates of 4.03±0.30, 4.28±0.30, and 4.57±0.30 a.u. (the first being the favored value). The quadrupole moment of DCCD is computed to 4.42±0.10 a.u. In the study it was observed that in contradiction to previous experiences the use of the model equilibrium geometries rather than the experimental geometry gives a smoother convergence as the level of theory is increased. The effects of basis set quality and electron correlation on the quadrupole moment are studied in detail. These effects are analyzed with reference to the redistribution of the electronic charge.
Fractal basin boundaries for the forced autocatalator
Authors: Liu Junli , Scott S. K.
Published: July 2002

The evolution of the basins of attraction of coexisting stable (periodic) states in a forced bistable system is considered. One attractor undergoes a period-doubling cascade to chaos. The basin of attraction also becomes fractal (d = 2.07) at sufficiently high forcing amplitude, leading to the loss of integrity of this state−but this is not coincident with the onset of chaos.
Third harmonic generation of extreme ultraviolet radiation in a nitrogen continuous free jet zone of silence
Authors: Faucher O. , Aguillon F. , Campargue R.
Published: July 2002

The interpretation of third harmonic generation, as obtained by crossing and focusing a pulsed laser beam on the axis of a continuous supersonic jet of nitrogen, is discussed and treated quantitatively. The geometry of the laser modes, as well as the inhomogeneity of the free jet zone of silence surrounded by the shock wave structure and the background gas, are taken into account. In this way, the variation of the phase matching factor is calculated in terms of the axial distance from the nozzle to the laser focus. This makes it possible to obtain the free jet characteristics from careful measurements of two-photon resonant, or nonresonant, frequency conversion rates. Thus the variations along the jet axis of the number density and the rotational temperature (decreasing down to 6 K), are obtained in good agreement with theoretical predictions.
Thermodynamic properties of fluorine monolayers deposited on graphite
Authors: Kumar S. , Etters R. D.
Published: July 2002

The properties of F2 monolayers on graphite are calculated using both a constant pressure and constant density Monte Carlo procedure with deformable periodic boundary conditions. A wide range of thermodynamic quantities and order parameters are determined over the temperature range 0≤T≤35 K. The ground state is found to be an incommensurate, centered rectangular structure with molecules oriented in the plane of the substrate in a herringbone arrangement. At T=19 K, a phase change occurs into a triangular structure, accompanied by an orientational transition. Details of the phases and the transition are given.
Phonon excitations in methane solid monolayers
Authors: Michael J. R. , Hakim T. M.
Published: July 2002

The dynamic form factor S(q,ω) for in-plane phonons in solid methane monolayers is evaluated at different temperatures in the self-consistent phonon theory. The multiphonon components and interference contributions are not important in the momentum region considered and the significant broadening observed in the one-phonon term can be contributed to anharmonic interactions in the monolayer.
The photon-induced reactions of chemisorbed CH3Br on Pt{111}
Authors: Radhakrishnan G. , Stenzel W. , Hemmen R. , Conrad H. , Bradshaw A. M.
Published: July 2002

The photochemistry of chemisorbed CH3Br on Pt{111} has been investigated using high resolution electron energy loss spectroscopy (HREELS) and thermal desorption. The primary photon-induced reaction involves the cleavage of the C–Br bond, giving rise to chemisorbed CH3 and Br, both of which can be identified in HREELS. From the angular dependence of the loss peaks, the symmetry of the CH3 surface complex is shown to be C3v. HBr can also be identified in subsequent thermal desorption. Experiments performed directly with HBr on Pt{111} indicate that molecular HBr adsorbs dissociatively on this surface. This result, in combination with observations of the C–H vibrational mode as a function of temperature, shows that the production of HBr arises from a secondary surface reaction between Br and CHx fragments. Based on the wavelength dependence of the fragmentation cross section and the photoemission spectrum of adsorbed CH3Br the primary photon-induced reaction to a charge transfer excitation is ascribed.
Observation of three-level and collisionally induced four-level infrared–infrared double-resonance effects in 12CH3F
Authors: Shin Uhyon , Song Quan , Schwendeman R. H.
Published: July 2002

A series of three-level and collisionally induced four-level infrared–infrared double resonance effects have been recorded and analyzed for 12CH3F. In each case a transition in the ν3 band was pumped by a fixed-frequency near-resonant CO2 laser while a transition in either the fundamental ν3 band or the 2ν3←ν3 band was probed by a continuously tunable infrared-microwave sideband laser. The recorded line shapes were analyzed either by the usual three-level double-resonance equations or by a four-level line shape equation based on a sum of Keilson–Storer collision kernels. The Δk=3n selection rule for collisionally induced rotational transitions has been confirmed and the four-level double resonances for Δk=0 show sharp transferred spikes. In addition, all the four-level double resonances include a Gaussian contribution attributed to V–V energy transfer that does not follow apparent selection rules. The widths of the spikes increase as the absolute value of the difference in the J values of the pump and probe increases. The line shapes of the spikes have been used to estimate values of the root mean square change in velocity upon collision.
Fundamental stretching frequencies in 1H13C14N– –1H19F: J-dependent predissociative excited state lifetimes in ν3 (C≡N) stretching vibration
Authors: Quinones A. , Ram Ram S. , Bevan J. W.
Published: July 2002

Static gas phase rovibrational analyses have been used to determine the four fundamental stretching band origin frequencies (in cm−1) of 1H13C14N– – –1H19F: ν1=3716.100(3), ν2=3292.1029(3), ν3=2088.9209(7), ν4=167.407(5). Estimated lifetimes in the ν1 and ν3 vibrationally predissociating states are determined to be τ(ν1)=0.052(7) ns; and τ(ν3)=1.1(3) ns, J′≤16; τ(ν3)=0.32(5) ns, J′≥17. The availability of a higher HF rotational energy channel is proposed as a possible mechanism for explaining this sudden change in excited state lifetime.
Planar traveling waves in the oscillatory oxidation of CO over polycrystalline catalysts
Authors: Levine Herbert , Zou Xiaoqin
Published: July 2002

We have used the oxide model proposed by Sales, Turner, and Maple to calculate spatial patterns in the oscillatory oxidation of CO over polycrystalline Pt, Pd, and Ir catalysts. Specifically, by introducing CO diffusion, we have studied the dispersion of one-dimensional traveling waves and their linear stability. These results could be tested by experiments which resolve the spatial structure of the catalytic oscillations.
Vibrational coherence effects in the pump–probe studies of photochemical predissociation
Authors: Engel Volker , Metiu Horia
Published: July 2002

We examine how the results of pump–probe experiments with short pulses are affected by the coherence of the excitation process, in the particular case when the pump causes the predissociation of the molecule. To document and understand this process we perform exact quantum mechanical calculations and examine simple models. The exact calculations are performed on a model of CH3ONO photodissociation. We examine the effect of coherence when the probe excites the transition complex and when it excites the NO molecule produced by photodissociation. In both cases the vibrational coherence leads to oscillations in the dependence of the two pulse laser-induced fluorescence (LIF) signal on the delay time between the pulses. In the first case the effect is due to the coherent excitation of the predissociation resonances. In the second the important factor is the coherence of the NO vibrational state produced by photodissociation. The physical factors affecting this process are discussed in detail. One of the interesting findings is that the relative translational energy distribution affects in an important way the manner in which vibrational coherence modifies the two pulse LIF signal.
Dielectric phase transition and symmetry change of constituent molecules in proton–deuteron mixed crystals of squaric acid
Authors: Moritomo Y. , Katsufuji T. , Tokura Y.
Published: July 2002

We have investigated effects of random substitution of proton sites with deuterons on the temperature- and pressure-induced phase transitions in squaric acid crystal, which consists of ferroelectrically ordered molecular sheets of [C4O4]2−(SQ2−) skeletons and intervening protons or deuterons. In the isotopically mixed crystals (H1−xDx )2SQ, the critical temperature Tc for the dielectric phase transition was found to increase linearly with the deuterium concentration x, while the critical pressure Pc increases nonlinearly with x at 300 K. Around the pressure ∼20 kbar above Pc, the constituent SQ2− skeletons in the paraelectric phase show a symmetry change from the asymmetric (C1h ) to centrosymmetric (C4h ) form, which is likely caused by the deformation of the correlated proton (deuteron) potential.
Coupled electromagnetic modes between a corrugated surface and a thin probe tip
Authors: Girard C. , Bouju X.
Published: July 2002

A self-consistent formalism is presented in order to determine the dispersion equation of the coupled electromagnetic modes between a dielectric probe tip of arbitrary shape and a rough surface. A microscopic picture of the matter is used for describing the dielectric response of the tip, and the coupling with the substrate is introduced from a dynamical matrix expressed in terms of propagators. Retardation effects may be included without formal difficulty through an appropriate response function describing the surface of the solid (local or nonlocal). An advantage of such a calculation lies in the possibility of simulating dielectric tips of arbitrary shape without introducing boundary conditions at the surface of the probe. Connection with atomic force microscopy and near field detection by local probe will be discussed.
Comparison of mechanistic model with experimental observation: The Ar(2p2)→Ar(1s4) emission signal in the pulse radiolysis of argon
Authors: Richmann Michael K. , Firestone Richard F.
Published: July 2002

The temporal behavior of atomic and molecular ions and of several classes of neutral states lying above the Ar(1s) manifold has been mathematically modeled in a successful attempt to reproduce observed Ar(2p2)–Ar(1s4) emission signals at 296 K with Ar pressures in the 100–200 Torr region. In addition, traces of SF6 were added to remove the effects of the slow electron thermalization associative ionization cycle characteristic of pure Ar. The proposed model mechanism necessarily includes additional reaction steps required by the addition of SF6. Good agreement was obtained between observed and calculated emission profiles only after the introduction of Ar+2 as an important thermalizer of fast electrons in pure Ar. Previously published rate constant values for the various elementary steps assumed to comprise a simplest probable mechanism were found to be generally acceptable.
The simultaneous three-body dissociation of CF2I2
Authors: Wannenmacher Elizabeth A. J. , Felder Peter , Huber J. Robert
Published: July 2002

The photodissociation dynamics of difluorodiiodomethane CF2I2 following 248 nm excitation were studied using the time-of-flight crossed laser-molecular beam technique. There is clear evidence that CF2I2 undergoes exclusively a simultaneous three-body dissociation. Two different reaction channels are observed: the dominant one (86%) yields CF2+I(2P1/2)+I(2P3/2) while the less efficient one (14%) produces the same fragments but in the ground state CF2+I(2P3/2)+I(2P3/2). The angle I–C–I between the recoil velocity vectors of the two departing I atoms was determined to be 120 °. The measured anisotropy parameters of βCF2=−0.8 for the CF2 fragments and βI=+1.1 for the I atoms (in both reaction channels) imply that the excited state symmetry of CF2I*2 is B1 (molecular symmetry C2v) and also indicate that the excited state lifetime is significantly shorter than a rotational period. Furthermore, the dissociation energy for the rupture of both C–I bonds was determined to be D0≤53 kcal/mol. Based on this dissociation energy a heat of formation for CF2I2 of ΔH0f, 0 K=−46 kcal/mol was calculated.
Theory of anharmonically modified Coriolis coupling in the S1 state of benzene and relation to experiment
Authors: Helman Adam , Marcus R. A.
Published: July 2002

Avoided crossings between quasidegenerate rovibrational states in the Doppler-free two-photon excitation of the 141 mode in the S1 excited state of benzene are treated theoretically. Two sets of avoided crossings in plots of spectral line frequency vs J at a given K and ΔK have been reported experimentally between an initially prepared ‘‘light’’ state (141 in zeroth order) and dark states, namely, one which in zeroth order is a 51101161 state, the other being in zeroth order a 62111 and/or possibly a 31161 state, implicated earlier by Neusser et al. The identification of these states makes the phenomenon an excellent candidate for treatment of the avoided crossing via a Van Vleck transformation, no other basis set states being needed for the diagonalization in order to extract the important features. Two successive transformations are used for handling direct coupling and coupling via virtual states. The dominant calculated contribution to the coupling is, jointly, Coriolis plus cubic–cubic anharmonic interactions between vibrational modes.

Playing less of a role are Coriolis terms in which the inverse moment of inertia tensor is expanded up to quadratic terms in the coordinates. There results a 5×5 (for coupling to 51101161 ) and a 3×3 (for coupling to 62111 or 31161 ) matrix of the transformed Hamiltonian, each of which can also be described, if desired, to a very good approximation by a 2×2 matrix. The coupling element V0 and the difference of the rotational constants for the light and dark states (ΔB) are obtained from the plots of line position vs J(J+1) obtained. For the 141 to 51101161 and for the 141 to 62111 couplings the theoretical results are in reasonable agreement with the experimental results, no adjustable parameters being employed. For a coupling of 141 to 31161 the calculated V0 would be much too high compared with experiment (a factor of 10), the coupling involving the exchange of only three instead of four vibrational quanta. A situation in which the 141 state is coupled to the 62111 state to yield an avoided crossing and off-resonantly coupled to the 31161 state would be consistent with some experimental results and not affect the reasonable agreement of the slope difference and splitting for the avoided crossing plots.
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o variational calculations of the vibrational properties of Li+3, Li2Na+, LiNa+2, and KLiNa+
Authors: Searles D. J. , von Nagy-Felsobuki E. I.
Published: July 2002

A rovibrational Hamiltonian has been derived in terms of rectilinear displacement coordinates which is based on the Watson Hamiltonian. Moreover, it is a generalization of the Carney and Porter analysis for D3h triatomic systems [J. Chem. Phys. 65, 3547 (1976)] and Carney et al. analysis for C2v triatomic systems [J. Chem. Phys. 66, 3724 (1977)]. It is therefore the most general form of the Watson Hamiltonian which is applicable to a bent triatomic system. Ab initio variational calculations using this Hamiltonian are presented for vibrational properties of Li+3, Li2Na+, LiNa+2, and KLiNa+.
Scattering from uniform, cylindrically symmetric particles in liquid crystal phases
Authors: Hamley I. W.
Published: July 2002

Theory is presented for the analysis of the intraparticle component of diffraction patterns from uniaxial fluid phases of cylindrically symmetric particles via a series expansion method. Diffraction from micelles, for example, can be modeled as the scattering from uniform particles, which are considered here. Knowledge of the particle self-correlation function enables the intraparticle component of the scattering to be calculated. The series expansion approach allows direct information on the orientational ordering of the particles to be extracted in terms of Legendre polynomial order parameters. Provided that high rank order parameters can be extracted from the scattering data, the full orientational distribution function can be obtained. However, at small Q, over a Q range which decreases with increasing particle anisotropy, it is shown that high rank order parameters cannot be measured so that the full distribution function is not obtainable. Scattering coefficients for monodisperse uniform cylinders, spherocylinders and ellipsoids are calculated and compared to each other.
The static scattering function and optical birefringence of a deformed, ideal polymer chain
Authors: Altenberger Andrzej R. , Rosa Epaminondas , Dahler John S.
Published: July 2002

An exact formula is derived for the van Hove static scattering function of an ideal chain under traction. Results obtained from this formula are applied to the description of affinely deformed elastomers. Calculations are performed for two models of polymeric chains with the aim of establishing connections between the models and experimentally observable characteristics of macroscopic samples.
Observation of the visible absorption spectrum of H2O+
Authors: Das Biman , Farley John W.
Published: July 2002

The Ã 2Ã1–X̃ 2B̃1 system of H2O+ has been observed, using laser absorption spectroscopy in a velocity-modulated discharge. A total of 78 transitions between 14 794 and 15 475 cm−1 have been observed with an uncertainty (1 SD) of 0.02 cm−1, including 76 transitions in the (0,7,0)–(0,0,0) band and 2 in the (0,8,0)–(0,0,0) band. This species is important for cometary astronomy, and intriguing for molecular physics because of its prominent Renner–Teller interaction. Careful measurements were made of the relative intensities of the absorption lines, which were measured to an accuracy of 13% (1 SD). This is the first observation of the Ã–X̃ transition in absorption; all previous data were obtained in emission with conventional grating spectroscopy. The transition frequencies of our new data are in good agreement with previous work, and have improved accuracy. The new data have definite rejection of the interfering lines from excited neutral H2 that plagued previous work. Compared with previous work, the new data have the first quantitative measurement of intensities. The ratio of the Franck–Condon factors I8/I7=0.99±0.43 has been measured for the first time, where Iv=FCF[(0,v′,0)–(0,0,0)].
Persistent infrared spectral hole burning of NO−2 ions in potassium halide crystals. I. Principle and satellite hole generation
Authors: Ambrose W. P. , Sethna J. P. , Sievers A. J.
Published: July 2002

New features are resolved within the internal vibrational mode spectra of NO−2 defects in KCl, KBr, and KI crystals at low temperatures using high-resolution Fourier transform spectroscopy and persistent infrared spectral hole (PIRSH) burning separately and together. With interferometry it has been discovered that the vibrational linewidths of the different modes range over a factor of 300—from 0.01 cm−1 to ∼3 cm−1 and, with PIRSH burning, it has been demonstrated that the narrowest lines are inhomogeneously broadened while the broadest ones are homogeneously broadened. PIRSH’s have been found in some internal modes and combination bands of the NO−2 molecule when pumped with low-intensity single-mode lead salt diode lasers; however, detectable persistent holes are not produced in all of the modes because of a competition between hole production and relaxation by tunneling at low temperatures. This competition results in a hole burning intensity, below which hole relaxation overwhelms hole production and only small holes may be produced.

The most unusually shaped absorption features are the V-shaped notches in the reorientational tunneling fine structure at the NO−2 bending mode frequency in KCl and KBr. Of all the internal modes that do show pronounced PIRSH burning, these V-notched absorption bands exhibit the most striking behavior. Multiple satellite PIRSH’s are detected at frequencies away from the single-mode laser burn frequency with a broadband probe beam produced by a high-resolution Fourier transform interferometer. An explanation for these satellite holes is derived from temperature, plastic deformation, and uniaxial stress dependence measurements on the KCl:NO−2 absorption spectrum. We find that the inhomogeneous broadening of the KCl:NO−2 ν2 reorientational tunneling fine structure is dominated by degenerate rotor level splitting produced by random crystal strains. Degenerate perturbation theory of the rotor level splitting in the strain field is found to match very closely the V-shaped inhomogeneous distribution of levels associated with the KCl:NO−2 reorientational tunneling fine structure. The general conclusion is that whenever strain splitting of a doubly degenerate level dominates the inhomogeneous broadening, then the absorption spectrum displays zero strength in the distribution at zero splitting and a linear increase in absorption coefficient away from this frequency generating the observed V-shaped notch in the absorption profile.
The electrochemical properties of poly(2,5-thienylene vinylene) synthesized via water-soluble precursor polymer
Authors: Onoda Mitsuyoshi , Morita Shigenori , Iwasa Toshinori , Nakayama Hiroshi , Yoshino Katsumi
Published: July 2002

The electrochemical, optical, and magnetic properties in poly(2,5-thienylene vinylene), PTV, in comparison with those of polythiophene, PT, during electrochemical p-type doping have been investigated by cyclic voltammetry, optical absorption spectrum, and electron spin resonance (ESR) measurements. The evolution of localized states is evidenced by the spectral change associating with doping. In the lightly doped state only two absorption peaks originating from the transitions between two gap states and the valence band appear within the gap region at about 0.6 and 1.2 eV. The spin susceptibility increased by about 1 order of magnitude from 3.7×10−7 to 2.6×10−6 emu/mol upon doping up to a dopant concentration of about 2.8 mol %. With further increasing dopants concentration, the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was inconsistent with the polaron density assumed to be formed by dopants of 2.8 mol %. Namely, even at lightly doped state, the newly developed species turn into p-type spinless bipolarons. The band gap of PTV was evaluated to be about 1.8 eV and smaller than that of PT by about 0.3 eV. This result can be interpreted in terms of a conformation change in the molecular structure, such as a change in the torsion angle between neighboring thiophene rings. The electronic band structure of PTV was determined. The bipolaron states in PTV appears in the band gap: about 0.6 eV below and above the conduction and the valence band, respectively. The bottom of the conduction band of PTV is in almost the same position as that of PT and successful n-type doping in PTV is expected.
Energies and widths of the lowest 1Σ+g states of H2
Authors: Isaacson Alan D.
Published: July 2002

Positions and widths for the lowest 1Σ+g doubly excited autoionizing states of H2 at several internuclear separations have been obtained by the calculation of Siegert eigenvalues. This approach involves the direct computation of the complex resonance energy in a basis set of both real and complex Slater orbitals. When the complex orbitals are centered between the two atoms, the numerical results are in fair agreement with previous theoretical treatments, although the present width does not rise quite as much for larger internuclear separations. In addition, the sensitivity of the present results on the basis set is studied and improvements to the basis set which should provide converged positions and widths for these resonances are discussed.
Photodissociation of triatomic molecules: Formulation of the three-dimensional problem
Authors: Neto J. J. Soares , Linderberg Jan
Published: July 2002

A variational approach for calculating the cross section of the photodissociation process of triatomic molecules is put forth as a generalization of a formulation used previously for fully three-dimensional calculations of transition probabilities for the reaction H2+H→H+H2 and the rovibrational spectrum of H+3. It is based upon the generator coordinate method and the hyperspherical coordinates and the evaluation of the scattering wave function employs the R-matrix theory.
Surface charging process in colloidal suspensions: A new statistical approach
Authors: Spalla Olivier , Belloni Luc
Published: July 2002

We introduce a new statistical-mechanical model of the surface charge process in colloidal suspensions. The chemical adsorption of ions on the surface is described with a narrow and deep well in the pair potential between the spherical colloids and the ions. This extends the Cummings and Stell model of bulk chemical reactions. In this simple case of association of spherically symmetric atoms into diatomic molecules, we verify that the model fulfils the law of mass action involving the activities of the reactant and product particles. In the case of surface reactions, we solve numerically the hypernetted-chain integral equation for the equilibrium properties of the system. From the pair distribution functions we deduce the implicit structural charge of the colloid as well as the electrostatic condensation of the counter ions, the ion–ion correlations inside the chemical shell and the colloid–colloid effective repulsion.
Normal mode analysis of the conformational fluctuation of polymethylene chains in their nearly extended states
Authors: Yamamoto Takashi
Published: July 2002

The conformational motion of a nearly stretched polymethylene molecule is studied by a normal mode analysis of the dihedral angle fluctuations. It is assumed that the conformation can be described by the dihedral angles only, the bond length, and the bond angle being fixed. The simplified model Hamiltonian consists of the intramolecular energy due to the dihedral angle distortions of the C–C bonds and the effective intermolecular energy which favors the stretched chain conformations. The Hamiltonian is expanded, around the transplanar state, in a quadratic form of the dihedral angles {τi}, and the normal modes of the dihedral angle fluctuations are obtained through diagonalization of the Hamiltonian. The normal modes are found to be classified into the in-phase and the out-of-phase modes; in the former modes the adjacent bonds make concerted rotations in the same direction resulting in the twisted chains, while in the latter modes they make counterrotations which give rise to the large out-of-plane bending of the chain. The power spectra of the dihedral angle fluctuations are calculated. They are found to be composed of the in-phase and the out-of-phase components; the spectra qualitatively reproduce those obtained in our previous computer simulation. A new variable, the curvature of the chain contour, is devised and is used throughout the discussion; it is found to be very useful in the description of the conformational fluctuations.
Classical collision complexes in the D+H2(v=0, j=0)→HD(v′, j′)+H reaction
Authors: Aoiz F. J. , Herrero V. J. , Sáez Rábanos V.
Published: July 2002

Detailed quasiclassical trajectory calculations of the D+H2(v=0, j=0)→HD(v′=0, j′)+H have been carried out in the range of collision energy from 0.35 to 1.25 eV. The calculated v′j′ state resolved differential cross sections and opacity functions show analogous structures to the ones obtained by accurate quantum mechanical results, that is, a peak along a line in the E−θ (or E−J) plane, that was attributed to broad resonances. Analysis of present results in terms of the duration of the collision indicates that those trajectories pertaining to these peaks proceed through the formation of short lived collision complexes with lifetimes of 15–35 fs.
Sequential clustering reactions of SiD+ with SiD4: Rapid growth to kinetic dead-end structures
Authors: Mandich M. L. , Reents W. D.
Published: July 2002

Sequential clustering reactions of SiD+ with SiD4 are monitored in the trapped ion cell of a Fourier transform mass spectrometer. At thermal energies, SiD+ initially clusters by rapid addition of silylene accompanied by elimination of D2. This growth sequence halts after reaching the dead-end structure Si4D+7, which grows further only by a slow termolecular process to form Si5D+11. Nonthermal cluster growth reactions are also observed which generally result in elimination of additional D2 molecules as compared to the thermal reactions. Thus the nonthermal product ions are more silicon rich than the thermal product ions. Some of the resulting nonthermal product ions react further with SiD4, but quickly form dead-end structures which cease to react. Both the forward and back reaction probabilities and products have been determined experimentally for each step of the growth sequence. These are used in combination with phase space theory to model the transition state energies involved in the microscopic pathways that have been elucidated by Raghavachari using ab initio electronic structure theory. The excellent quantitative agreement for these energies, to within 0.12 eV, between the experimentally derived values and those calculated by Raghavachari supports the growth pathway found by ab initio calculations. This pathway also shows why further growth of Si4D+7 can only occur by inefficient bimolecular attachment of SiD4. These experimental results strongly indicate that the sequential growth of SiD+ in reactions with SiD4 will not lead to large hydrogenated silicon particles even under the conditions of higher temperatures, pressures, and ion energies found in silane plasmas.
Hidden symmetry and explicit spheroidal eigenfunctions of the hydrogen atom
Authors: Sung Stella M. , Herschbach Dudley R.
Published: July 2002

The Schrödinger equation for a hydrogenic atom is separable in prolate spheroidal coordinates, as a consequence of the ‘‘hidden symmetry’’ stemming from the fixed spatial orientation of the classical Kepler orbits. One focus is at the nucleus and the other a distance R away along the major axis of the elliptic orbit. The separation constant α is not an elementary function of Z or R or quantum numbers. However, for given principal quantum number n and angular momentum projection m, the allowed values of α and corresponding eigenfunctions in spheroidal coordinates are readily obtained from a secular equation of order n−m. We evaluate α(n,m;ZR) and the coefficients gl(α) that specify the spheroidal eigenfunctions as hybrids of the familiar ‖nlm〉 hydrogen-atom states with fixed n and m but different l values. Explicit formulas and plots are given for α and gl and for the probability distributions derived from the hybrid wave functions, ∑lgl(α)‖nlm〉, for all states up through n=4. In the limit R→∞ these hybrids become the solutions in parabolic coordinates, determined simply by geometrical Clebsch–Gordan coefficients that account for conservation of angular momentum and the hidden symmetry. We also briefly discuss some applications of the spheroidal eigenfunctions, particularly to exact analytic solutions of two-center molecular orbitals for special values of R and the nuclear charge ratio Za/Zb.
Electric-field-enhanced laser-induced plasma spectroscopy of jet-cooled metal-based ion–molecule complexes
Authors: He K. X. , Hammond T. D. , Winstead C. B. , Gole J. L. , Dixon D. A.
Published: July 2002

Electric-field-enhanced laser-induced plasma spectroscopy is a relatively simple and general approach to the mapping of electronic states and internal mode structure in small metal-based molecular complexes. In an exemplary study, weakly bound excited-state ion–molecule complexes of aluminum have been formed and monitored. Emission spectra associated with Al+CO(Al+OC) and Al+H2 complexation show clear vibrational structure tentatively associated with the Al+ -molecule stretch and/or triatomic bending mode. These features are correlated precisely with transitions among several excited states of the Al+ ion which extend throughout the visible and ultraviolet. Evidence is also obtained for Al+N2 complexation associated with select Al+ excited states. All complex emissions which can be readily ascribed involve singlet-(Al+) –singlet-(CO,H2,N2) interactions. Resulting singly charged electronically excited molecular-ion complexes are likely formed in orbiting collisions of the Al+ ion. Observed vibrational-level separations are consistent with quantum-chemical calculations on the ground electronic 1Σ states of Al+CO, Al+OC, Al+H2, and Al+N2. Broadening and vibrational structure associated with an Al++–N2 interaction are apparent, as excited-state complex formation in this system dominates that for the Al++–CO interaction. These results are considered in view of an anticipated Coulomb explosion in multiply charged ions. Extensions to the vacuum ultraviolet for the study of weakly bound ground-state Al+ complexes and extrapolations to other metal-based systems are suggested.
Dissociation of diatomic gases
Authors: Hansen C. Frederick
Published: July 2002

The Landau–Zener theory of reactive cross sections has been applied to diatomic molecules dissociating from a ladder of rotational and vibrational states. Although the preexponential factor of the Arrhenius rate expression is shown to be a complex function of the dimensionless activation energy E*/kT, the average over all states in the ladder is well represented by a single factor that varies about as T−n, where the coefficient n is the order of unity. This relation agrees very well with experimental data for dissociation of O2 and N2, for example. While the reactive cross section theory does not agree with experimental rate data when applied only to dissociation from ground state molecules, it gives good agreement when applied to a ladder of rotation–vibration states, since the bulk of the dissociations occur from states within about kT of the dissociation limit. The results validate previous empirical assignment of a single preexponential factor in the Arrhenius expression and justify the extrapolation of the expression well beyond the range of data. The theory is then used to calculate the effect of vibrational nonequilibrium on dissociation rate. For Morse oscillators the results are about the same as for harmonic oscillators, and the dissociation from a ladder of equilibrium rotational and nonequilibrium vibrational states is close to an analytic approximation provided by Hammerling, Kivel, and Teare for harmonic oscillators all dissociating from the ground rotational state.
α- and β-relaxation and the criteria for rotational diffusion
Authors: Kivelson Daniel
Published: July 2002

A simple model for α- and β-rotational relaxation (detected through the behavior of the lth order spherical harmonics) in supercooled liquids predicts that the relaxation frequency of the latter is proportional to l(l+1) and that of the former (confirmed or at least suggested experimentally) is independent of l. This has been taken to mean that the β relaxation takes place by means of small angular jumps (diffusion) and the α relaxation by means of long angular jumps. However, because the amplitude of the α relaxation decreases rapidly with increasing l, despite the fact that the α-relaxation frequency is independent of l, the motion may be diffusional in accord with the observations of Spiess et al. This is discussed in terms of a simple model.