bluuebunny/crossref_metadata_2025 huggingface dataset:
The coupling between librational motions and conformational transitions in chain molecules. II. The rotor chain represented by the master equation for site distributions
Authors: Moro Giorgio J.
Published: July 2002

The model system of the rotor chain is considered in order to analyze the coupling between conformational transitions and the librational motions, i.e., the small amplitude displacements within the energy wells of the stable states. The master equation for site (conformer) distributions (MESD) on the torsional angle displacements is the suitable formalism which allows one to describe at the same time both types of motions. While in the previous work the MESD was proposed on a phenomenological ground, the formal derivation of the MESD is now provided by means of a projection procedure. The ingredients of the MESD, in particular the coordinate-dependent rate kernel, are determined unambiguously in this way. A reduced equation without site indeces is recovered by applying the MESD to the rotor chain with equivalent potential minima. A new procedure based on the Gaussian approximation for the site distribution is introduced to solve the reduced equation. This allows one to calculate rather easily correlation functions for different observables. A good agreement is found in the comparison with the available Brownian molecular dynamics simulations on the same system. The behavior of the orientational correlation functions is examined in detail in order to disentangle in the overall relaxation the contributions of the conformational dynamics from the effects of the librational motions.
A quantum random-walk model for tunneling diffusion in a 1D lattice. A quantum correction to Fick’s law
Authors: Godoy Salvador , Fujita Shigeji
Published: July 2002

With the help of quantum-scattering-theory methods and the approximation of stationary phase, a one-dimensional coherent random-walk model which describes both tunneling and scattering above the potential diffusion of particles in a periodic one-dimensional lattice is proposed. The walk describes for each lattice cell, the time evolution of modulating amplitudes of two opposite-moving Gaussian wave packets as they are scattered by the potential barriers. Since we have a coherent process, interference contributions in the probabilities bring about strong departures from classical results. In the near-equilibrium limit, Fick’s law and its associated Landauer diffusion coefficient are obtained as the incoherent contribution to the quantum current density along with a novel coherent contribution which depends on the lattice properties as [(1−R)/R]1/2.
Geometries, singlet-triplet separations, dipole moments, ionization potentials, and vibrational frequencies in methylene (CH2) and halocarbenes (CHF, CF2, CCl2, CBr2, and CI2)
Authors: Russo Nino , Sicilia Emilia , Toscano Marirosa
Published: July 2002

The geometrical structure, harmonic vibrational frequencies, ionization potentials, and singlet-triplet gaps of simple substituted halocarbenes (CHF, CF2, CCl2, CBr2, and CI2) have been investigated by using the linear combination of Gaussian-type-orbital local-spin-density method. Optimized geometries, as well as vibrational frequencies, are in good agreement with available experimental data. The obtained values of singlet-triplet splittings (ΔEST) computed taking into account the nonlocal corrections are very close to experimental and previous theoretical investigations employing extended configuration interaction contributions. Many of the calculated properties obtained here have not yet been determined both experimentally and theoretically.
Direct electronic energy transfer in the presence of static site-energy disorder–dipolar couplinga)
Authors: Mollay Bernhard , Kauffmann Harald F.
Published: July 2002

The single-step energy transfer between randomly distributed donors and acceptors has been analyzed in the presence of static site-energy disorder. Exact expressions for the donor survival probability have been formulated with jump-frequencies that depend on both spatial and energy-coordinates. By using the factorization approximation and the continuum limit the procedure yields, for multipolar interaction, approximate, closed-form solutions of the Kohlrausch–Williams–Watts (KWW) functional form with a generalized energy-function λε ≳ 1, which influences the time-scale of the KWW-decay but does not affect the exponent α. For dipolar coupling and 3D transfer (α=1/2), both the energy-specific f(t;ε) and the energy-averaged donor relaxation 〈 f 〉 (t) have been Laplace inverted to yield the distributions of transition frequencies φ1/2(ν;ε) and Φ1/2(ν), respectively. The analysis of λε containing the energy-dependence of transition frequencies and the energetic spread of sites has been performed on the premises of a balance-equation for uphill processes and a Gaussian density-of-states function for the site-energy fluctuation. This allows the time and frequency-domain analogs of donor relaxation to be discussed as a function of the initial energy of excitation ε, the energetic width of fluctuating sites σ, and the energy gap δε̄ between the mean values of donor and acceptor distribution. The functional dependences of energy-specific responses, i.e., the characteristic deceleration of the KWW-profiles and the log frequency-shift of the corresponding frequency spectra as well as the pronounced deviation that may occur for broad-band excitation have been investigated in detail. Finally, the circumstances under which such relaxations are leading to the ordinary KWW-law (λε = 1) have been discussed by considering the exact limiting procedures.
On the problem of a suitable definition of the cluster in embedded-cluster treatments of defects in crystals
Authors: Pisani C. , Orlando R. , Corà F.
Published: July 2002

The perturbed-cluster embedding scheme [J. Chem. Phys. 92, 7448 (1990)] adopts a self-consistent procedure where, at each cycle, a molecular cluster solution is calculated, and subsequently corrected for realizing its coupling to the crystalline environment. In the present work, it is shown that a proper design of the cluster is of critical importance for the scheme to work satisfactorily. Indications are given for defining clusters that are at the same time ‘‘self-satisfied,’’ and optimally fitted to the environment, so that correction terms are minimized. The procedure is tested in the case of defects in crystalline silicon.
The average number of kinks of a short polyelectrolyte chain: A Monte Carlo study
Authors: Brender Chava , Danino Meir
Published: July 2002

An interesting connection between the mean straight length 〈ls〉 and the defined average number of kinks 〈nkink〉 in a polyelectrolyte chain configuration is found and described. 〈nkink〉 is found to be the dominant parameter throughout the entire range of the Bjerrum length λ and very sensitive to salt concentration. Identification of λm where (nkink values for various chain lengths coincide, as a percolation threshold is suggested. Considering the kink as a bond, it is found to be much more significant than the contacts described in a previous article as a new type of bonds. It is shown that the family of the physical statistical bonds includes not only contacts but also kinks. The kink fraction in a chain and the effective screening of 〈nkink〉 are defined and described for various values of λ. Through the use of these new parameters, pronounced size effects appear clearly. A connection between polymer systems and molecular cluster systems is proposed.
The phase transition temperature at 145 K in cadmium calcium acetate hexahydrate as a function of Cu2+ probe concentration
Authors: Shields H. , Kleman T. G. , De D. K.
Published: July 2002

The temperature of the structural phase transition in cadmium calcium acetate hexahydrate as a function of Cu2+ concentration has been studied with electron paramagnetic resonance (EPR). The phase transition temperature was found to vary from 144 to 124 K as the amount of copper varied from 0.07% to 1.3% of the sample weight. An extrapolation of this data predicts a structural phase transition at 145 K in pure cadmium calcium acetate hexahydrate. Good fits of the data to second-order phase transition theory was obtained with critical exponents close to 0.30.
Optical emission generated by collisions of 5 eV O(3P) atoms with surface-adsorbed hydrazine
Authors: Orient O. J. , Martus K. E. , Chutjian A. , Murad E.
Published: July 2002

Optical emission has been observed corresponding to vibrational bands in the NH (A 3Π→X 3Σ−) electronic transition during collisions of 5 eV, ground-state oxygen O(3P) atoms with MgF2 and Ni surfaces continuously exposed to a beam of hydrazine (N2H4). The NH emission intensity is observed to be about five times greater for MgF2 than for Ni. No dependence on temperature was observed for either surface in the range 240–340 K, implying that the NH-producing intermediate species is tightly bound. The half-lifetime for desorption of hydrazine from each surface was measured. This was found to be 120 min for the MgF2 surface at 240 K, and ≤20 min for Ni. After exposure the surface composition was measured using x-ray photoelectron spectroscopy (XPS) on the exposed and unexposed areas of both targets.
Direct energy transfer from excited organic adsorbates to intrinsic defect sites in silicalite
Authors: Leheny A. R. , Turro N. J. , Drake J. M.
Published: July 2002

The decay of the excited singlet state of an aromatic molecule such as naphthalene adsorbed to the surface of silica is nonexponential. This nonexponential decay is shown to result from a multipolar interaction between the excited adsorbate and the intrinsic defects of silica. We present experimental evidence for direct energy transfer between the donor molecules, naphthalene and 2-methoxynaphthalene, and acceptor defects sites of a crystalline silica zeloite (silicalite). The principal condition for direct energy transfer, spectral overlap between the optical absorption of the defects and the fluorescence emission of the adsorbate is demonstrated for this system, and the nonexponential relaxation of the donor is shown to be described by a Forster-type stretched exponential. Based on the spectral overlap and energy transfer rate determined, the intrinsic defect density of silicalite is calculated to be 3±2×1020 cm−3.
Atom confinement in helicoidal cavities
Authors: Gonzales-Platas J. , Breton J. , Girardet C.
Published: July 2002

The interaction between an atom and a neutral helicoidal cavity is calculated for varying parameters—helix pitch and radius, matter density on the helix, number of helices schematizing the cavity,... . General conditions for the applicability of the continuum approximation, for the occurrence of geometrical confinement leading to atom supermobility in the helicoidal system, and for the influence of confinement on the adsorption energy are given when the interactions are described by a Lennard-Jones potential. It is shown that the atom remains confined along the symmetry axis of the cavity when the helix/atom radius ratio is smaller than or approximately equal to 1.2. In that case, the potential is analytic, linear with respect to the helix number, and flat for sufficiently large matter densities on the helices.
Theoretical modeling of photodissociation dynamics of CH3I on LiF(001)
Authors: Huang Zhi-Hong , Guo Hua
Published: July 2002

A new method is developed for the simulation of atoms and molecules interacting with ionic surfaces. This approach, based on a quasi-two-dimensional Ewald sum and a two-dimensional Fourier transformation, is capable of evaluating the long-range Coulomb interactions for a semi-infinite ionic solid. We have applied this method to investigate the photodissociation dynamics of CH3I on a LiF(001) surface. All the degrees of freedom of the adsorbed molecule are considered and the excited state dissociation potentials of CH3I are described by analytical functions derived from a recent ab initio calculation. The substrate (LiF) is represented by 6×6×3 movable atoms surrounded by static ions at their equilibrium positions. The adsorbate/substrate interaction is modeled as a sum of Coulomb and Lennard-Jones pairwise potentials. A phenomenological term is introduced to account for the adsorbate/adsorbate interaction. The equilibrium configurations of the system at a given temperature are obtained by a Monte Carlo method, which shows that there exist two stable configurations with the CH3I molecular axis perpendicular to the surface, either methyl up or down. The dissociation dynamics of the adsorbate is studied with a classical molecular dynamics method and the angular, kinetic energy, and rovibrational distributions of the fragments are calculated. When the molecule is adsorbed with the methyl up, the methyl fragment dissociates into the vacuum promptly with kinetic energy and internal state distributions similar to those in the gas phase. If the molecule is adsorbed with the methyl down, however, the methyl fragment could collide with iodine after rebounding from the surface, transferring a significant amount of kinetic energy to the iodine. A much broader and more energetic kinetic energy distribution of the iodine fragments is observed under such circumstances. The energy transfer is most effective when the parent molecule orients parallel to the surface normal and decreases as the angle deviates from this direction. We also observed a substantial increase in the rotational angular momentum of the methyl fragment and a cooler vibrational distribution for the umbrella mode as a result of the collision.
Computer simulation study of the validity of scaling and renormalization group theories for two-dimensional star polymers
Authors: Bishop Marvin , Clarke Julian H. R. , Freire Juan J.
Published: July 2002

Monte Carlo simulations employing the pivot algorithm have been performed for off-lattice polymer systems in order to investigate the behavior of two-dimensional linear and star polymers. Theoretical predictions for the overall radius of gyration and the end-to-end distance were found to be in reasonable agreement with the simulation data for a small number of arms. However, the disagreement with theory in the case of the radius of gyration of individual arms for larger numbers of arms was well outside the statistical error of our data.
Umbrella-sampling realization of ‘‘Widom’’ chemical potential estimation
Authors: Ding Kejian , Valleau John P.
Published: July 2002

The ‘‘Widom’’ or ‘‘test-particle’’ technique for the computational estimation of excess chemical potentials of model fluids has been very useful at low and moderate densities. However, as usually implemented, it becomes inefficient and eventually unreliable on going to high densities, due to inadequate sampling. This article describes a straightforward solution to this problem by the use of ‘‘umbrella sampling’’ to ensure that all parts of phase space relevant to the estimation are included. The method is illustrated by application to the Lennard-Jones system at kT/ε=1.2 and at various densities up to ρσ3=0.9.
Dissociation of doubly charged CH2=CD2 and CH2=CF2 in the region of valence shell photoexcitation
Authors: Ibuki Toshio , Imamura Takashi , Koyano Inosuke , Masuoka Toshio , Brion C. E.
Published: July 2002

Dissociative double photoionization of 1,1-dideuteroethene (CH2=CD2) and 1,1-difluoroethene (CH2=CF2) has been studied using the photoion–photoion coincidence (PIPICO) technique and monochromated synchrotron radiation over the photon energy range of 37–85 eV. PIPICO branching ratios and threshold energies for dissociative double photoionization show two dominant exit channels of the doubly charged ions (i) central C=C bond cleavage to form CXm++CYn+ fragment ions at a threshold below 37 eV photon energy. The decomposition into H++C2HD2+, D++C2H2D+, and F++C2H2F+ are also appreciable, and (ii) the fragmentation into H+(D+ or F+)+CXm+ takes place at hν≳37 eV. Comparison of the present PIPICO branching ratios with the recent theoretical studies of the electronic states of the outer valence doubly charged ethene ion [Liegener, Chem. Phys. 92, 97 (1985) and Ohrendorf et al., J. Chem. Phys. 91, 1734 (1989)] indicates that double ionization involving release of one πC=C electron results in central C=C bond breakage, while that of two electrons of the σCH, σCC, and C 2s orbitals of ethene forms two smaller fragment ions of the H++CXm+type.
A parametric method 3 (PM3) study of trans-stilbene
Authors: Galvão D. S. , Soos Z. G. , Ramasesha S. , Etemad S.
Published: July 2002

We report a comparative modified neglect of diatomic overlap (MNDO), Austin method one (AM1), and parametric method 3 (PM3) study of trans-stilbene (tS) in its ground, excited (singlet and triplet), and ionic (positive and negative polarons and bipolarons) states. We have also calculated the barrier for ring rotation about the backbone single bond. Our results show that PM3 geometries are superior to MNDO and AM1, at least for tS. PM3 predicts, in contrast with MNDO, AM1 and even ab initio 3-21G, a coplanar structure for tS, in accordance with recent experimental data. Singlet and triplet energies obtained from heats of formation are in surprisingly good agreement with experimental data.
Interpretation of the hyperfine coupling constants of the boron trimer in rare-gas matrices
Authors: Fernández Berta , Jo/rgensen Poul , Simons Jack
Published: July 2002

By carrying out a systematic basis set and electron correlation investigation, we have determined accurately the isotropic and anisotropic parts of the hyperfine coupling tensor of the B3 molecule using the multiconfiguration self-consistent-field restricted–unrestricted method. The anisotropy of the computed tensor is significantly larger than the experimental uncertainty. This indicates that the B3 molecules must be freely tumbling in neon, argon, and krypton matrices at 4 K. The spin polarization of the 1s orbital on each B atom is found to be very small in the B3 molecule. This implies that the isotropic hyperfine coupling is dominated by valence-orbital contributions rather than by 1s-orbital contributions.
Generation of an exact three-dimensional quadrupole electric field and superposition of a homogeneous electric field within a common closed boundary with application to mass spectrometry
Authors: Wang Y. , Wanczek K. P.
Published: July 2002

The exact three-dimensional rotationally symmetric quadrupole field can be generated by closed boundaries with continuously varied potential, especially with linearly variable potential, in the ideal case by simple cone-shaped boundaries with linearly variable potential. An example for the application of the field is the storage of charged particles inside the closed boundaries. Within the same cone-shaped boundaries, a homogeneous ideal field in the direction of the symmetry axis can be generated and superimposed without interaction with the quadrupole field. This field can be employed for excitation of the kinetic energy of the stored charged particles. Potential distribution of both fields within one cone-shaped boundary with linearly variable potential are simulated separately. A new electrode structure for ion trap is suggested. The signals induced by the coherently moving ions on the electrodes can be analyzed by Fourier transformation.
Molecular beam pump/probe microwave-optical double resonance using a laser ablation source
Authors: Fletcher D. A. , Jung K. Y. , Scurlock C. T. , Steimle T. C.
Published: July 2002

The first successful pump/probe microwave-optical double resonance experiment using a laser ablation/reaction scheme for molecular beam production has been performed. Pure rotational transitions at frequencies up to 52 GHz have been recorded for the transient refractory compounds YF, YO, and SrOH at a resolution of <30 kHz [full-width at half- maximum (FWHM)]. The observed three lowest pure rotational transition frequencies of YF (X 1Σ+) were analyzed to produce an improved set of rotational constants, B=8683.6156(11) MHz and D=0.007 521(74) MHz. The three lowest pure rotational transitions of SrOH (X 2Σ+) were analyzed to give the spectroscopic parameters (in MHz), B=7470.8180(4), D=0.006 25(3), γ=72.706(1), γD=−0.0021(2); bF (H)=1.713(2) and c (H)=1.673(5). The proton magnetic hyperfine interactions were interpreted in terms of a molecular orbital description for the X 2Σ+ state.
Visible absorption spectroscopy of the B 2A1–X 2B2 transition of fluoroformyloxyl radical, FC(O)O
Authors: Maricq M. Matti , Szente Joseph J. , Li Z. , Francisco Joseph S.
Published: July 2002

We report the visible absorption spectrum of the fluoroformyloxyl radical. The spectrum is assigned to the B 2A1–X 2B2 transition in FC(O)O. The origin of the excitation spectrum is observed at 13 150 cm−1 (760 nm). Extensive progressions in the OCO bending mode of the B state, ν′3 = 610 cm−1, dominate the spectrum. Three other progressions are observed: the OCO wag mode, ν′6 = 840 cm−1; the CF stretching mode, ν′2 = 1110 cm−1; and the asymmetric CO stretching mode, ν′4 = 2820 cm−1. Two series of hot bands also appear in the spectrum. From these bands the frequency of the OCO bending mode, ν″3 = 520 cm−1, is assigned for ground state FC(O)O. The experimental spectrum is compared to the results of ab initio MO calculations. These include second-order Mo/ller–Plesset perturbation theory (UMP2) and quadratic configuration interactions theory with single and double excitations (QCISD) for both the X 2B2 and B 2A1 states. The origin of the B 2A1–X 2B2 transition and the predicted vibrational frequencies are found to be in reasonable agreement with experiment.
Gaussian-2 theory using reduced Mo/ller–Plesset orders
Authors: Curtiss Larry A. , Raghavachari Krishnan , Pople John A.
Published: July 2002

Two variations of Gaussian-2 (G2) theory are presented. In the first, referred to as G2 (MP2) theory, the basis-set-extension energy corrections are obtained at the 2nd order Mo/ller–Plesset (MP2) level and in the second, referred to as G2(MP3) theory, they are obtained at the MP3 level. The methods are tested out on the set of 125 systems used for validation of G2 theory [J. Chem Phys. 94, 7221 (1991)]. The average absolute deviation of the G2(MP2) and G2(MP3) theories from experiment are 1.58 and 1.52 kcal/mol, respectively, compared to 1.21 kcal/mol for G2 theory. The new methods provide significant savings in computational time and disk storage.
Local ordering in asymmetric chain fluids
Authors: Kuespert Daniel R. , Donohue Marc D.
Published: July 2002

A thermodynamic study of local ordering in amphiphilic chain fluids based on the sphere-and-bond formalism of Kuespert and Donohue [Langmuir 9, 431 (1993)] is presented using the Flory equation for entropy of mixing and the mean-field approximation for fluid energy. After introducing general equations, a model of an amphiphile chain dissolved in a monomeric solvent is derived and its bulk and microphase equilibria are investigated.
Spectroscopic cluster size effects
Authors: Jortner Joshua , Ben-Horin Narda
Published: July 2002

In this paper we examine the spectral moments of the absorption line shapes of aromatic- molecule⋅(rare gas)n heteroclusters to provide analytic expressions for the spectral shifts (δν), which are determined by the first moment, and for the homogeneous linewidths (Γ), which are determined by the central second moment (Δ) of the line shape. δν originates from the cumulative contributions of dispersive pair interactions, while Δ and Γ manifest the short-range dynamic nuclear fluctuations. Our analysis elucidates some of the general features of these spectroscopic observables, e.g., their dependence on the cluster structure and their size dependence, as well as some of their specific characteristics, e.g., their temperature dependence. We predict a weak temperature dependence of δν (=a+bT with b≪a/T) and a strong temperature dependence of Δ(∝T1/2), which is borne out by molecular dynamics (MD) simulations. We have derived cluster size equations (CSE) relating the spectroscopic observables of the finite cluster with those of the infinite bulk system. The excluded volume corrections for δν(n)∝n−1 and for Δ(n)∝n−3 are different, in accordance with the nonuniversability principle for CSEs. The predictions of the CSEs concur with experimental and MD simulation data for δν(n) and with MD simulation data for Δ(n). Finally, we have addressed dimensionality scaling of spectroscopic cluster properties, applicable for the situation of a common dimensionality (D≠3) for the cluster and for the infinite bulk. This analysis may be useful for the spectroscopic interrogation of doped clusters of low dimensionality, e.g., wire clusters (D=1) and planar clusters (D=2), as well as fractal clusters, e.g., clusters of porous materials.
Energy transfer in the 31,214151 Fermi-resonant states of acetylene. I. Rotational energy transfer
Authors: Frost Michael J.
Published: July 2002

An infrared–ultraviolet double resonance technique is used to probe the state-to-state rotational energy transfer dynamics of self-relaxation in acetylene. The output of an optical parametric oscillator at ∼3 μm is used to excite C2H2 to a rotational level within one of its Fermi-resonant 31,214151 states. By fixing this wavelength and scanning the frequency-doubled output of a tunable dye laser, laser induced fluorescence signals arising from collisional population of rotational levels within both dyads are observed and state-to-state rate constants for rotational relaxation are obtained. Rotational relaxation to J levels within the pumped (upper energy) Fermi-dyad accounts for 74% of the total rate of loss of the population of the J=12 level, whereas relaxation to J levels in its partner accounts for only 16%. A further 7% of the absolute rotational relaxation rate is accounted for by vibrational relaxation out of the mixed levels, leaving only 3%–4% of the total relaxation to be accounted for.
Temperature dependence of the quenching of O(1S) by simple inorganic molecules
Authors: Capetanakis F. P. , Sondermann F. , Höser S. , Stuhl F.
Published: July 2002

The quenching of metastable O(1S) atoms by H2 , O2 , CO, CO2 , N2O, and SF6 was studied in the temperature range 210–470 K. Very low concentrations of O(1S) atoms were generated in the pulsed vacuum ultraviolet (VUV) Lα photolyses of N2O and CO2 . The atoms were detected by their forbidden emission at around 557.7 nm, the intensity of which was enhanced by the formation of exciplexes in the presence of Ar. The quenching by H2 and SF6 is very inefficient and occurs without a significant activation energy. The rate constants for the quenching by CO and CO2 can be represented by activation energies near 10 kJ mol−1 but very different frequency factors. The collisions of O(1S) with O2 and N2O lead to different products which likely are formed with different activation energies. It is proposed that generally quenching of O(1S) proceeds by diabatic processes.
Direct laser-induced emission detection of the S1 and T1 states of germanium dichloride: Pyrolysis jet spectroscopy and ab initio studies
Authors: Karolczak J. , Zhuo Q. , Clouthier Dennis J. , Davis W. M. , Goddard J. D.
Published: July 2002

Spectra of jet-cooled germanium dichloride were obtained by pyrolysis of trichlorogermane in the throat of a supersonic jet. Laser-induced emission excitation spectra were recorded for the weak 450–400 nm and strong 320–300 nm band systems, both of which were vibrationally analyzed. Ab initio predictions of the excited state geometries, vibrational frequencies, and excitation energies were made to aid in assigning the spectra. The strong ultraviolet band system is assigned as Ã 1B1–X̃ 1A1 with upper state vibrational frequencies of ν1=354 cm−1 and ν2=104 cm−1. It is the direct analog of the 580–440 nm band system of dichlorocarbene. The weaker band system in the visible is shown to be the ã 3B1–X̃ 1A1 transition, with upper state vibrational frequencies of ν1=393 cm−1 and ν2=118 cm−1. This is the first report of direct laser-induced phosphorescence detection of the excited triplet state of any of the carbene or heavier carbene analogs.
Charge transfer and collision-induced dissociation reactions of CO++ with the rare gases at Elab=49 eV
Authors: Rogers Steven A. , Price Stephen D. , Leone Stephen R.
Published: July 2002

Multiple product channels are observed for the reaction of 13CO++ with each of the rare gases (Rg) at Elab=49±1 eV. A beam of 13CO++ is produced by electron impact ionization and is mass selected using a quadrupole mass spectrometer. The ion beam is focused into a collision region and the reaction products are monitored using time-of-flight mass spectrometry. Relative yields for the production of 13C+, O+, and 13CO+ are measured directly. Absolute charge transfer reaction cross sections for collisions of 13CO++ with He, Ne, Ar, and Kr are estimated by comparing the Rg+ production with that for the charge transfer reactions of doubly charged rare gas ions with neutral rare gas atoms. The cross sections are found to range from 0.9−0.9+1.5 Å2 for collisions of 13CO++ with He to 37.5±19.6 Å2 for collisions with Kr. The reaction of 13CO++ with He proceeds almost exclusively into the collision-induced dissociation channel. The branching fraction for collision-induced dissociation is smaller for reactions with Ne and almost disappears for Ar, Kr, and Xe. As the relative importance of the collision-induced dissociation process decreases, branching into the charge transfer channel increases. The charge transfer reactions of 13CO++ with Ar, Kr, and Xe are shown to populate excited, dissociative electronic states of 13CO+ selectively. These effects are modeled successfully using Landau–Zener theory in conjunction with reaction window theory.
Control of the adiabatic electronic state in ab initio molecular dynamics
Authors: Fois Ettore S. , Penman James I. , Madden Paul A.
Published: July 2002

The problem of keeping the electronic state at its adiabatic value in the course of ab initio molecular dynamics runs with the Car–Parrinello method is discussed. Attention is focused on the difficulties which arise when chemical bonds are broken and formed in the course of the atomic motion. A solution to the problems is the coupling of independent Nosé–Hoover thermostats to the up and down spin orbitals. The choice of suitable parameters to control the thermostats is discussed. The success of the method is illustrated in studies of the dissociation/recombination of Na2 and the isomerization of the Na3 molecule by a process of pseudorotation.
Equation of state for the soft-sphere fluid from a direct summation of the virial series
Authors: Maeso M. J. , Solana J. R.
Published: July 2002

An equation of state for the inverse-twelfth-power soft-sphere fluid is obtained by direct summation of the virial series. To do so, a generalization of the Carnahan–Starling method for obtaining the equation of state of the hard-sphere fluid is used. The equation of state obtained in this way reproduces accurately the simulation data for both the stable and metastable fluid regions. Agreement remains good up to the neighborhood of the glass transition where the equation of state predicts that the soft-sphere fluid becomes unstable.
Structure of benzene and phenol chemisorbed on Ni{110}
Authors: Bu H. , Bertrand P. , Rabalais J. W.
Published: July 2002

The chemisorption of benzene and phenol on a clean Ni{110}–(1×1) surface and an oxygen predosed Ni{110}–(3×1)–O surface near room temperature has been investigated by time-of-flight scattering and recoiling spectrometry accompanied by shadow cone calculations. The Ne scattering and H, C, and O recoiling fluxes exhibited strong angular anisotropies as a function of beam incident (α) and crystal azimuthal (δ) angles. These anisotropies are due to C and O atoms shadowing their neighboring atoms within the benzene molecules and resulting phenoxide species, demonstrating that scattering and recoiling spectrometry is capable of providing structural information on polyatomic molecular systems. The results show that both benzene and phenoxide are chemisorbed as molecules which have very good short-range order despite the absence of long-range order observable by low energy electron diffraction. Both benzene and phenoxide are oriented nearly parallel to the surface, with a maximum inclination angle of 15°. The C atoms in the para positions of benzene and the C–O bond in phenoxide are oriented along the <001≳ azimuth. The C–H bond is bent out of the plane of the hexagonal ring so that the H atoms are above the C atom plane. Chemisorption on the oxygen predosed surface results in a reaction in which a H atom is abstracted from both benzene and phenol with the formation of surface hydroxide groups; the molecules remain well ordered on this surface also.
Spectroscopic characterization of the lowest Π and Σ bending states of ArHCN
Authors: Drucker S. , Cooksy A. L. , Klemperer W.
Published: July 2002

The lowest excited bending states, Σ1 and Π1, of the ArHCN complex have been measured by millimeter-wave electric resonance optothermal spectroscopy. The principal molecular constants determined for the Σ1 state are ν0=164 890.790(12) MHz; B=1958.8571(37) MHz; D=−0.075 23(29) MHz; eqaaQ=0.825(27) MHz; and μa=−0.521(30) D. For the Π1 state, the constants are ν0=181 984.4126(47) MHz; B=2031.3624(17) MHz; D=0.153 35(16) MHz; eqaaQ=0.904(11) MHz; and μa=0.273 02(63) D. The leading Σ1–Π1 coupling constants are the Coriolis coefficient β0=1016.998(13) MHz and the transition dipole moment μb=2.2535(57) D. The rotational constants for the two bending states indicate that the average separation between the argon and the HCN center of mass contracts by roughly 0.5 Å compared to the linear ground state. This is consistent with the nearly T-shaped average geometry for each state established by analysis of the dipole moments and quadrupole coupling constants. Agreement between this work and prior theory confirms attribution of the anomalous distortion and isotope effects in the ground state to extreme angular–radial coupling. The relative sign of the dipole moments for the Σ1 and Π1 states is resolved in this work, allowing an unambiguous interpretation of the angular information. Assuming Laguerre angular distributions, we obtain that the Σ1 state wave function has a maximum at an angle of 108° with a halfwidth of 49°, and that the Π1 state maximum is at 80° with a halfwidth of 37°. This estimate for the Σ1 state angular distribution indicates that although the state is not antilinear (ArNCH), as was expected, it does approach this configuration. The Π1 state is nearly a free rotor eigenstate, showing that the angular part of the potential surface near 90° is extremely flat. The combined data from the ground, Σ1, and Π1 states reflect virtually the entire angular coordinate along the radial minimum of the potential, and should provide a reliable benchmark for ab initio potential energy surfaces for ArHCN near the bottom of the well. We compare the data to predictions from available models.
Intramolecular delocalization of Π electrons in polythiophenes with fixed conformation: A spectroscopic study
Authors: Zerbi Giuseppe , Radaelli Roberta , Veronelli Maurizio , Brenna Elisabetta , Sannicolò Franco , Zotti Gianni
Published: July 2002

The main feature of β,β′-bridge–α,α′-bithienyls is that their conformation is controlled by the bridge length. This class of molecules could be useful for the understanding of the intra and intermolecular processes which determine the conductivity of the corresponding polymers. In this work we have recorded and studied (i) the electronic spectra of the monomers, (ii) the vibrational spectra (IR, Raman λexc=514 nm, FT–Raman λexc=1064 nm) of the monomers and polymers in the pristine state, (iii) the IR spectra of the doped polymers, and (iv) the IR spectra (probe) of the polymers photoinduced by visible light (pump). The interpretation of the spectra is made in terms of the ‘‘effective conjugation coordinate’’ theory. Poly(cyclopentabithiophene) P(T+1) has the largest π-electron delocalization with a wide distribution of conjugation lengths. In poly(cycloheptabithiophene) P(T+3) π electrons are highly localized, while an intermediate situation is observed in poly(dihydrobenzobithiophene) P(T+2). The various aspects of intra and intermolecular hopping of charge carriers are discussed.
Teaching lasers to control molecules in the presence of laboratory field uncertainty and measurement imprecision
Authors: Gross Peter , Neuhauser Daniel , Rabitz Herschel
Published: July 2002

An iterative optimization algorithm for designing laser fields to control molecular motion which utilizes laboratory input (test fields) and output (resulting product yields) information is proposed. Laboratory uncertainties such as laser field noise and limited precision in the product yield measurements are included in the simulations of the experiments. Two simulated examples of implementation of the algorithm are presented: selective electronic excitation in a model four-state system and maximizing dissociation yield of the hydrogen fluoride molecule. Both examples demonstrate that, even with the inclusion of laboratory uncertainties, the experimental learning-based algorithm is a potentially feasible method of controlling molecular motion and possibly manipulating chemical reactions.
Rotational transfer, an angular momentum model
Authors: McCaffery Anthony J. , Alwahabi Zeyad T. , Osborne Mark A. , Williams Colin J.
Published: July 2002

We have re-examined critical experiments on collision induced rotational transfer (RT) and conclude that the probability of RT is controlled by the factors that control the probability of angular momentum (AM) change. The probability of energy change seems less important in this respect. In the light of this we suggest a model for RT in which the probability of AM change is calculated directly and present a formalism for this purpose. We demonstrate that such a calculation leads to an exponential-like fall of RT probabilities with transferred AM, a consequence of the radial dependence of the repulsive part of the intermolecular potential. Thus in this AM model, the exponential gap law has a simple physical origin. The AM model we describe may be used as the basis of an inversion routine through which it is possible to convert RT data into a probability density of the repulsive anisotropy. Through this model therefore it is possible to relate experimental RT data directly to the forces that are responsible for rotational transfer. The hard ellipse model is used in this work to relate calculated anisotropies to a form that includes an isotropic component. The result is a representation of the intermolecular potential through which new insights into the RT process are gained.
Dynamics of relaxation at strongly interacting polymer–solid interfaces: Effects of chain architecture
Authors: Adriani Paul M. , Chakraborty Arup K.
Published: July 2002

The relaxation dynamics of a polymer chain strongly adsorbed to a solid surface are simulated via a kinetic Ising model that includes chain connectivity constraints (steric hindrance, rotational strain, and configurational entropy). The two polymer architectures examined consist of one or two chemisorbing functional groups per segment. In both architectures, the chemisorbed polymer chain is trapped in nonequilibrium conformational states at low temperatures, but relaxes to equilibrium at higher temperatures with stretched exponential (KWW) relaxation kinetics. The average relaxation time for the two pendant group architecture has a strongly non-Arrhenius temperature dependence that obeys the Vogel–Fulcher law. In contrast, average relaxation times for the one pendant group architecture cannot be described by the Vogel–Fulcher law.
Rovibrational product distributions of O2+ from the reaction of O+(4S) with CO2
Authors: Walter C. W. , Cosby P. C. , Peterson J. R.
Published: July 2002

The nascent rovibrational population of O2+ produced from the reaction of near-thermal O+(4S)+CO2 is measured by observing the translational energy spectrum of its dissociative charge transfer products following neutralization in Cs vapor at keV energies. It is found that of the 1.19 eV exothermicity in the formation reaction, the product O2+ is formed on the average with 0.48 eV of internal energy partitioned nearly equally between vibrational excitation (0.27±0.03 eV) and rotational excitation (0.22±0.03 eV). Populations measured for O2+(v=0−5) are: 0.38, 0.30, 0.18, 0.09, 0.05, 0.01, respectively. The O2+ rotational temperature decreases with increasing vibrational excitation, from Trot=3100 K in v=0 to Trot=450 K in v=5. The present populations show some disagreement with recent flow tube measurements. Details of the technique and a determination of the O2+ rovibrational population following electron-impact ionization of O2 are also presented. The O+(4S)+CO2 reaction and its products are important in the ionospheres of Mars and Venus and in CO2 releases in the Earth’s ionosphere.
A new band system of nitrogen: Observation of the N2(G 3Δg→W 3Δu) transition
Authors: Bachmann R. , Ottinger Ch. , Vilesov A. F.
Published: July 2002

The first experimental observation of the N2(G 3Δg→W 3Δu) transition is reported on here. The emission forms part of the spectrum of the so-called N2 beam afterglow, a spontaneous luminescence emitted by a molecular beam of N2 issuing from an intense d.c. discharge. Using a high performance charge-coupled device (CCD) optical multichannel detector, 18 bands of a new band system were observed with 2 Å full width at half-maximum (FWHM) resolution in the 350–650 nm region. Three well-resolved v″ progressions were analyzed. From a comparison with the known vibrational spacings in the N2(G) and N2(W) states, they could be assigned unambiguously to the N2(G→W) transition. This observation allows the energy of the N2(G) state to be determined as Te=89 505 cm−1 or 11.10 eV, thereby also fixing the location of the previously observed H(3Φu) state at Te=107 328 cm−1 or 13.31 eV.
Rotational spectrum of a quantum rotor adsorbed on a rough and disordered surface: Para-H2 and ortho-H2 on amorphous ice
Authors: Buch V. , Silva S. C. , Devlin J. P.
Published: July 2002

Experimental Raman spectra are presented for the J=0→J=2 transition of para-H2 and for the J=1→J=3 transition of ortho-H2 adsorbed on the amorphous ice surface. Analysis of the spectra is based on a path integral Monte Carlo simulation of the H2 adsorbate-surface system. Rotational line broadening is due to the removal of the (2J+1)-fold degeneracy of the J manifolds of the ‖M〉 states. The line broadening pattern is traced to the characteristics of the anisotropic gas-surface interaction. The theoretical model, which does not contain any adjustable parameters, provides a simple quantitative interpretation of the spectra.
Comment on: Computation of the energy V of an excess electron in dense helium and argon
Authors: Lopez-Castillo J.-M. , Frongillo Y. , Plenkiewicz B. , Jay-Gerin J.-P.
Published: July 2002

In this Comment, we correct an unfortunate statement made by Boltjes, de Graaf, and de Leeuw [J. Chem. Phys. 98, 592 (1993)] in regard to a previous work of us [J. Chem. Phys. 96, 9092 (1992)] on the calculation of the conduction-band energy minimum V0 of an excess electron in fluid argon using path-integral molecular-dynamics simulations.
Spin–orbit and rotational autoionization in HCl and DCl
Authors: Zhu Y.-F. , Grant E. R. , Lefebvre-Brion H.
Published: July 2002

The rotationally resolved spin–orbit autoionization spectrum of HCl has been obtained in double resonance via levels of lowest angular momentum in v=0 of the 4pσ D 1Π state. A self-consistent assignment is offered for the portion of the spectrum ranging over principal quantum numbers from 13 to 18. Positions established in HCl point to series of similar quantum defect converging to the rotational ground state of DCl+ 2Π1/2. HCl features are further confirmed by comparison with earlier double-resonance spectra obtained via the F 1Δ state, and with vuv autoionization spectra observed in single-photon transitions from the neutral ground state. Multichannel quantum defect theory calculations have been carried out, which introduce 28 parameters corresponding to the quantum defects, transition moments, and s–d l-mixing coefficients for Rydberg states of HCl in Hund’s case (a). As output they predict positions and intensities for transitions involving all 53 channels accessible in Hund’s case (e) for l ranging from 0 to 2 and J from 1 to 2. The same parameters for both isotopes yield reasonable correspondence between experiment and theory for all the major resonances in HCl and DCl from principal quantum number 13 to 18 (about 70 resonances), with an average ΔE less than 3 cm−1.
Phase separation by surface induced nucleation
Authors: Nilsson Svante , Linse Per
Published: July 2002

The effect of a surface on the phase stability of solvent–solvent and solvent–polymer mixtures is analyzed within a lattice model. The location of the surface nucleation curve, which describes the limit of the metastability due to the presence of a surface, depends in a sensitive way both on the chain length of the polymer and on the interactions with the surface. The metastable phase region, as defined by the binodal and spinodal curves, can be reduced by more than a factor of two due to the presence of a planar surface.
The photodissociation dynamics of dichloroethenes at 214 and 220 nm
Authors: Sato Kei , Shihira Yoshitaka , Tsunashima Shigeru , Umemoto Hironobu , Takayanagi Toshiyuki , Furukawa Katsutoshi , Ohno Shin-ichi
Published: July 2002

The nascent rotational distributions of HCl (v=0, 1, and 2) generated in the photodissociation of three isomers of dichloroethenes (DCE) at 214 and 220 nm were measured under molecular beam conditions. HCl molecules were probed by a (2+1) resonantly enhanced multiphoton ionization technique combined with time-of-flight mass spectrometry. The rotational distributions of vibrationally excited HCl (v=1 and 2) molecules were Boltzmann-type, while those of HCl (v=0) could not be represented by a Boltzmann distribution and consisted of two components. These results suggest that there are more than two processes in the photodissociation of DCE. Cl(2P3/2) and Cl*(2P1/2) could also be detected when DCE were photodissociated. The branching ratios of Cl*(2P1/2) to Cl(2P3/2) obtained in the present work were much larger than those obtained at 193 nm.
Theoretical study of the visible and ultraviolet spectra of chromyl chloride (CrO2Cl2)
Authors: Yasuda Koji , Nakatsuji Hiroshi
Published: July 2002

The excitation spectrum of chromyl chloride (CrO2Cl2) is studied theoretically by the symmetry adapted cluster-configuration interaction (SAC-CI) method. The calculated spectrum agrees well with the observed spectrum. In this report, we describe the assignments of the visible and ultraviolet spectra up to 8.0 eV and discuss the nature of the excitations.
Rotational spectrum of the trimethylamine-hydrogen iodide dimer: An ion pair (CH3)3NH+⋅⋅⋅I− in the gas phase
Authors: Legon A. C. , Rego C. A.
Published: July 2002

The ground-state rotational spectra of the two isotopomers [(CH3)3N,HI] and [(CH3)3N,DI] of a dimer formed between trimethylamine and hydrogen iodine have been observed in the gas phase by the pulsed-nozzle, F–T microwave technique. The spectroscopic constants B0, DJ, DJK, χ(14N) and χ(I) have been determined for both isotopomers and have the values 894.0954(2) MHz, 0.010 17(18) kHz, 5.880(10) kHz, −2.451(8) MHz, and −341.204(14) MHz, respectively for [(CH3)3N,HI]. It is concluded, by comparing the iodine nuclear quadrupole coupling constant χ(I) and the intermolecular stretching force constant kσ with those of model hydrogen-bonded (H3P...HI) and ion-pair (Na+...I−) analogs, that (CH3)3NH+...I− is the appropriate description for the gas-phase dimer. An experimentally defined fractional ionic character indicates that the extent of proton transfer increases smoothly along the series [(CH3)3N,HX], where X=F, Cl, Br, and I. Simple energetic considerations are shown to be in agreement with this conclusion.