bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear induced deformation of polymers: Calculation of radii of gyration
Authors: Wang Shi-Qing
Published: July 2002

By calculating radii of gyration of a Gaussian polymer chain in simple shear flow, we study the deformed shape of an isolated macromolecule. To order ε≡4-d (d=spatial dimensionality) and to the lowest nontrivial order in the shear rate, we consider the effect of hydrodynamic interactions on the flow induced anisotropy of the radii of gyration with respect to directions specified by the flow. We predict no change in projections of polymer shape along directions perpendicular to the flow direction, in agreement with experiment. Our results also explain the negative sign and the smallness of the second normal stress.
The effects of different vibrational modes and collision energy on the reaction of acetylene cations with carbonyl sulfide
Authors: Orlando Thomas M. , Yang Baorui , Chiu Yu-hui , Anderson Scott L.
Published: July 2002

Resonance-enhanced multiphoton ionization has been used to produce beams of vibrationally state-selected acetylene cations. The ions are formed with excitation in either the symmetric C–C stretch (ν2) or in a bending mode (ν5). Reactions with OCS have been studied in a guided ion-beam mass spectrometer. Absolute cross sections for the production of OCS+ and C2HnS+ (n=1,2) are reported for collision energies ranging from 80 meV to 5 eV. Charge transfer is observed to be enhanced by bending excitation, suppressed by C–C stretching vibration, and only weakly affected by collision energy. In contrast, the C2HnS+ channels are strongly collision energy dependent, with vibrational effects that vary with collision energy. The effects of bending and stretching excitation are qualitatively similar; however, the size of the effects are different and are not what would be expected on energetic grounds. These results are contrasted to the situation for reaction of mode selectively excited C2H+2 with methane.
Role of anisotropic dispersion potential in the theory of nematic liquid crystals. II. Thermodynamic calculations
Authors: Eldredge Clark P. , Kromhout Robert A. , Linder Bruno
Published: July 2002

We reexamine the effect of the anisotropy in the intermolecular attractive (dispersion) interaction on the thermodynamic quantities characterizing the nematic–isotropic phase transition, as estimated from generalized van der Waals theory. Contrary to statements in the literature, this anisotropy significantly affects the transition temperature and the slope of the coexistence curve. The results obtained here, as in generalized van der Waals calculations in the literature, in general do not well reproduce the experimental data. Sources of these discrepancies are discussed briefly.
Second-order exchange effects in intermolecular interactions. The water dimer
Authors: Hess O. , Caffarel M. , Huiszoon C. , Claverie P.
Published: July 2002

A new method of deriving explicit formulas for the calculation of second-order exchange contributions (induction as well as dispersion) within the framework of symmetry-adapted perturbation theories is presented. It is shown how exchange contributions can be expressed as a combination of electrostatic interaction energies between suitably generalized charge distributions (overlap intermolecular charge distributions). Each of these contributions are derived within the Hartree–Fock approximation (neglect of all electron correlation effects within the noninteracting molecules) and by considering only single-electron exchange between interacting molecules. Numerical calculations for the interaction of two water molecules are presented. In the region of the equilibrium geometry, it is found that the complete second-order exchange contribution accounts for about 20% of the total intermolecular interaction energy. This contribution is essentially dominated by the exchange induction component which is found to represent approximately 1 kcal/mol (using a basis set containing 94 orbitals). To our knowledge, this is the first example of calculation of exchange induction interaction energy for a molecular system. Concerning the less important, but non-negligible, exchange dispersion component, our result is found to agree with a very recent calculation for the water dimer.
Observation of CH3Zn as a stable product in the KrF laser photodissociation of (CH3)2Zn
Authors: Jackson Robert L.
Published: July 2002

Single-photon dissociation of (CH3)2Zn has been examined at 248 nm in the gas phase. The relative yields of CH3Zn and Zn as a function of buffer gas pressure were determined by laser-induced fluorescence. With He as the buffer gas, the yield of CH3Zn increases from ∼20% at very low pressure to ∼65% at 500 Torr. The yield of Zn atoms correspondingly drops over the same pressure range. This behavior is due to the increasing probability for collisional stabilization of the primary photoproduct, vibrationally hot CH3Zn, as the buffer gas pressure increases, thereby quenching its subsequent thermal dissociation to CH3 and Zn. Information on the nascent vibrational energy distribution of the CH3Zn photoproduct is obtained by examining the probability of CH3Zn stabilization vs buffer gas pressure.
Joint product state distribution of coincidently generated photofragment pairs
Authors: Gericke K.-H. , Gläser H. G. , Maul C. , Comes F. J.
Published: July 2002

The joint product state distribution of coincident fragment pairs formed in the same elementary photodissociation process has been determined. This correlation between quantum state populations of two molecular products has been measured by high resolution Doppler spectroscopy in conjunction with level-specific detection of the ejected photofragments. One product molecule, formed in a specific quantum state, is excited by laser induced fluorescence and the wing of the corresponding Doppler profile is analyzed to determine the product state distribution of the partner fragment. In the photodissociation of jet-cooled hydrogen peroxide at 193 nm two OH partner radicals are formed with comparable angular momenta. For a specific rotation of one hydroxyl radical the product state distribution of the partner fragment is centered at about the same rotation with a width of only a few rotational quanta. The orbital angular momentum is of the order of 1 ℏ and the impact parameter is extremely small. Experimentally observed joint product state distribution and trajectory calculations on ab initio potential energy surfaces are in excellent agreement.
Multistability of steady states in competitive adsorption of two species
Authors: Córdoba A. , Lemos M. C. , Luque J. J.
Published: July 2002

Competitive adsorption of two species on a linear chain is considered. The adsorption–desorption probabilities are chosen in the Arrhenius form and it is assumed that the activation energies are partially due to interaction between nearest neighboring adatoms. Multistability can occur for an attractive interaction greater than a critical value, depending on the values of other parameters, and the least critical value corresponding to an interaction energy of ≂0.3 kcal mol−1 for room temperature. Up to seven steady states can result, three being stable. The relaxation times tend to infinitum at the critical points and the exponent α, where τJ→Jcritical∼(Jcritical−J)−α and J being a parameter gauging the strength of the interaction, has been determined for a number of cases. The analysis has been extended to two-dimensional lattices and similar results have been obtained, although Jcritical is less than that for the linear chain. Next a dimerization reaction between adatoms has been considered jointly with the adsorption–desorption process. The results obtained are again similar to the previous ones. It is noteworthy that when three stable steady states exist, depending on initial conditions, two of these states correspond to cases where one of the type of the adatoms has saturated the surface, thus displacing the other species, and practically stopping the dimerization reaction. The third state corresponds to partial coverages of the surface for both types of adatoms and the reaction rate is greater.
Infrared spectroscopic study of the rotation of chemisorbed methoxy species on an alumina surface
Authors: Beebe T. P. , Crowell J. E. , Yates J. T.
Published: July 2002

We present experimental and calculated vibration–rotation spectra as a function of temperature for the methoxy species (–OCH3 and –OCD3) chemisorbed on an alumina surface. The axis of rotation is the C–O bond axis. The model for our calculations is that of free rotation, and we describe the methods employed here in full detail. The qualitative agreement between the calculated and experimental spectra suggests that the adsorbed methoxy species is undergoing free rotational motion about the C–O bond axis.
Computer simulation study of the θ-point in three dimensions. I. Self-avoiding walks on a simple cubic lattice
Authors: Meirovitch Hagai , Lim H. A.
Published: July 2002

Using the scanning simulation method we study the tricritical behavior at the Flory θ-point of self-avoiding walks (SAWs) with nearest neighbors attractions ε (ε<0) on a simple cubic lattice (in the following paper we investigate tricritical trails on the same lattice). The tricritical temperature Tt is −ε/kBTt=0.274±0.006 (one standard deviation). The results for the radius of gyration G and the end-to-end distance R are consistent with the theoretical prediction νt=0.5 and with a logarithmic correction to scaling; the ratio G2/R2 =0.1659±0.0001 (calculated without taking into account correction to scaling) is only slightly smaller than the theoretical asymptotic value 1/6=0.1666.... The results for the partition function Z at Tt lead to γt=1.005±0.017 in accord with theory and to μt=5.058±0.014, where μt is the growth parameter; the correction to scaling in Z is found to be relatively small. For the chain length studied the divergence of the specific heat at Tt (αt≂0.3) is significantly larger than that predicted by theory, (ln N)3/11 (i.e., αt=0). Also, at Tt our data are affected by strong correction to scaling and are thus not consistent with the theoretical value of the crossover exponent φt=0.5.
A new computational approach to the chemical potential
Authors: Han K. -K. , Cushman J. H. , Diestler D. J.
Published: July 2002

A new approach to the computation of chemical potential and free energy is presented. The basic idea is to correct the underestimate obtained by the particle-removal method. Application of the new technique to several different thermodynamic states of the Lennard-Jones (12,6) fluid shows that it is capable of providing reliable estimates of the chemical potential, even at high density where standard methods encounter computational difficulties.
Freezing of simple systems using density functional theory
Authors: de Kuijper A. , Vos W. L. , Barrat J.-L. , Hansen J.-P. , Schouten J. A.
Published: July 2002

Density functional theory (DFT) has been applied to the study of the fluid–solid transition in systems with realistic potentials (soft cores and attractive forces): the purely repulsive WCA Lennard-Jones reference potential (LJT), the full Lennard-Jones potential (LJ) and the exponential-6 potential appropriate for helium and hydrogen. Three different DFT formalisms were used: the formulation of Haymet and Oxtoby (HO) and the new theories of Denton and Ashcroft (MWDA) and of Baus (MELA). The results for the melting pressure are compared with recent simulation and experimental data. The results of the HO version are always too high, the deviation increasing when going from the repulsive Lennard-Jones to the exponential-6 potential of H2. The MWDA gives too low results for the repulsive Lennard-Jones potential. At low temperatures, it fails for the full LJ potential while at high temperatures it is in good agreement. Including the attraction as a mean-field correction gives good results also for low temperatures. The MWDA results are too high for the exponential-6 potentials. The MELA fails completely for the LJT potential and the hydrogen exponential-6 potential, since it does not give a stable solid phase.
The B←X electronic spectrum of N2+–He
Authors: Bieske E. J. , Soliva A. , Welker M. A. , Maier J. P.
Published: July 2002

The electronic spectrum of N2+–He has been measured in the region corresponding to the N2+ B 2Σ+u←X 2Σ+g origin transition. The spectrum was recorded by photoexciting a mass selected beam of N2+–He ions and detecting N2+ fragments. A likely process for the fragmentation involves fluorescence to a vibrationally excited level of the ground electronic state followed by vibrational predissociation. The observed spectrum exhibits well resolved discrete structure and bears a remarkable resemblance to a cold N2+ spectrum suggesting that the potential between the N2+ ion and helium atom in both the X and B electronic states, has at most only a small barrier to internal rotation. Measurement of the shift of N2+–He transitions with respect to the corresponding N2+ lines indicates that the binding energy of the helium atom to the N2+ ion is almost the same in both the B and X electronic states.
Vibrational analysis and calculation of Franck–Condon factors for the vinoxy radical X̃(2A″) and B̃(2A″) states
Authors: Yamaguchi Makoto , Momose Takamasa , Shida Tadamasa
Published: July 2002

Vibrational analysis and calculation of Franck–Condon factors (FCF) of the vinoxy radical were performed for the two electronic states in the title. Multiconfigurational self-consistent field (MCSCF) wave functions were used to calculate the optimized geometries and the energy gradients of both states. Vibrational analysis was carried out with the harmonic approximation, and FCFs were calculated taking into account the Duschinsky rotation. The features of the experimental absorption and laser-induced fluorescence (LIF) spectra were reproduced with the calculated FCFs. Previous analyses of the experimental spectra based on an oversimplified model were critically discussed.
Electrostatic and screening effects on the dynamic aspects of polyelectrolyte solutions
Authors: Mattoussi Hedi , Karasz Frank E. , Langley Kenneth H.
Published: July 2002

We present a quasielastic light scattering (QELS) study of a synthetic polyelectrolyte compound: poly(paraphenylene vinylene) derivative dissolved in methanol. The dynamical aspects of these solutions are found to be strongly dependent on whether or not we have added an ordinary electrolyte (salt for instance). For salt-free solutions we measured two different, and a priori independent, diffusion coefficients, DC and DS, attributed, respectively, to cooperative and large scale motions. The small value of the hydrodynamical correlation length ξH, deduced from DC, reflects the large interpenetration of polyions in salt-free solutions. The chains are, consequently, highly expanded even though not rod-like in shape. This size, ξH, does not depend on the polymer concentration cp for the range scanned. The addition of even a small amount of sodium chloride progressively modifies the solution’s behavior. First, we measured only one diffusion coefficient when the salt concentration cs exceeded a certain threshold. The effective diffusion coefficient measured for these solutions depends on both the ionic strength and the polymer concentration. The type of variation of Def versus cp and cs, is compared to previous work on either polyelectrolytes and ionomers materials or on micelles. The set of data are also qualitatively compared to theoretical considerations such as the charged spheres approach and to the flexible chain model with electrostatic contributions.
Electronic and magnetic properties of a carbon atom chemisorbed on model clusters simulating the (100) surface of nickel
Authors: Fournier R. , Andzelm J. , Goursot A. , Russo N. , Salahub D. R.
Published: July 2002

Both spin-polarized and unpolarized linear combinations of Gaussian-type orbitals-model core potential-local spin density (LCGTO-MCP-LSD) calculations have been performed for clusters representing the three possible high symmetry chemisorption sites for carbon on the (100) surface of nickel. We found that the most stable chemisorption site is the fourfold hollow, in agreement with the experimental evidence. For this site, the computed equilibrium NiC distances are 1.79 and 1.77 Å at the spin-polarized and unpolarized levels, very close to the most recent experimental measurements. The calculated spin-polarized vibrational frequency perpendicular to the surface is found to be 407 cm−1 (410 cm−1 expt). The values of the binding energy are 11.5 and 11.8 eV at polarized and unpolarized levels, respectively (∼7 eV, expt); the carbon atom is strongly bound, essentially by a triple bond formed by interaction of the px, py, and pz orbitals of carbon with, primarily, the d orbitals of the four nearby surface nickel atoms. The effect of carbon chemisorption on the nickel magnetism has also been studied. The addition of the carbon atom reduces the spin magnetic moment of pure nickel by 2 or 4 μB depending on which of the two nearly degenerate nickel cluster states is taken as reference. The reduction of atomic spin magnetic moments is clearly larger on the 4 nickel atoms nearest to the carbon. The global and local (atomic) reduction in spin magnetic moments originate from some up-spin d density of states being pushed above EF, through antibonding interactions with the carbon 2p orbitals, and hence emptied.
Pure dephasing of a nonlinear vibrational mode. Non-Markovian line shape
Authors: Georgievskii Yu. I. , Stuchebrukhov A. A.
Published: July 2002

The method of thermodynamic Green’s functions is applied for calculations of spectral line shape of a vibrational transition in a molecule interacting with a thermal bath (liquid solvent or a solid state matrix) whose characteristic frequencies are much lower than the transition frequency. Line broadening arises due to cubic anharmonicity of the vibrational mode according to known mechanism of Oxtoby of dephasing in liquids. Our approach makes it possible to take into account interaction with the bath exactly, regardless of the strength of coupling, and to develop a convenient perturbation theory in nonlinearity of the mode. It is shown that at low temperatures and for strong coupling to the bath the line shape is strongly asymmetric and cannot be described in terms of a Markovian approximation usually assumed in previous studies. Actual line shape is related to the spectrum of excitations of the bath which can be studied using data on the line shape at low temperatures. Line shapes are studied both in quantum and classical case beyond the weak coupling limit and the Markovian approximation.
Calculation of rotational energy transfer rates for HD (v=1) in collisions with thermal HD
Authors: Brown Nancy J. , Longuemare Maria
Published: July 2002

State resolved rotational energy transfer rates for HD+HD collisions have been computed using quasiclassical dynamics. One of the HD molecules is given an energy corresponding to v=1 and a specific rotational state, Ji1≤6. The other HD molecule is given a rotational–vibrational energy corresponding to an eigenstate of HD but selected according to a thermal distribution at 300 K. The translational energy is selected according to the appropriate thermal distribution at 300 K. The energy transfer is dominated by translational–rotational energy exchange. The calculated rotational energy transfer rates are compared with the experiments of Chandler and Farrow. Modest agreement is achieved for Ji1≤2. Reasons for the lack of agreement are discussed.
Adsorbate migration on a solid surface: The connection between hopping dynamics and the atom-surface interaction energy
Authors: Zhang Zhenyu , Metiu Horia
Published: July 2002

The migration of an adsorbed atom at moderate temperatures is described in terms of uncorrelated jumps between lattice sites which lead to diffusion. It is widely believed that a jumping rate coefficient and therefore a diffusion coefficient can be defined only if energy exchange with the moving lattice or collisions with randomly distributed impurities give the motion of the adsorbate a random character. In this paper we examine systematically a suggestion of Haug, Wanhstrom, and Metiu, who conjectured that coupling between the adsorbate motion along the surface and its motion perpendicular to it can provide the necessary randomization and, in particular, make possible the definition of a hopping rate coefficient. We calculate the flux–flux correlation functions needed for describing the dynamics of single and double jumps by using a set of simple, but reasonably realistic, adsorbate-surface interactions. In all these calculations the lattice atoms are held fixed. We show that in spite of this, the correlation functions converge and rate constants can be defined for many of the potentials. We study in detail those features of the potential energy surface (PES) that lead to convergence and also how the shape of the PES influences the amount of recrossing (i.e., the accuracy of the transition state theory) and multiple jumping. Our results indicate that it is possible to develop a correction to the transition state theory which includes the effect of thermal fluctuations and calculates the recrossing correction by holding the lattice atoms fixed. This saves substantial computer time.
Quantum defect description of non-Rydberg molecules
Authors: Gao Hong , Greene Chris H.
Published: July 2002

Quantum defect theory, combined with the frame transformation method, is applied to analyze non-Born–Oppenheimer effects in the hydrogen molecular ion and its isotopes by calculating low-lying vibrational energy levels of the ground electronic states. The calculation, based purely on the 1sσg Born–Oppenheimer potential of H+2, shows improvement over Born–Oppenheimer results despite the fact that the hydrogen molecular ion cannot be regarded as a Rydberg molecule.
Stimulated emission pumping spectroscopy of CH3O (X̃ 2E) at the dissociation limit
Authors: Geers A. , Kappert J. , Temps F. , Wiebrecht J. W.
Published: July 2002

High-lying rotation vibration states of CH3O (X̃ 2E) radicals have been studied close to the threshold energies for the unimolecular reactions CH3O→H2CO+H and CH3O→CH2OH using the technique of stimulated emission pumping (SEP) spectroscopy. Spectra were recorded with the PUMP laser tuned to the rR21(7.5,−5) rotational lines of the Ã 2A1←X̃ 2E 000 and 310 vibrational bands and the DUMP scanned to access X̃ levels in the energy region 3000 cm−1≤Evr≤9000 cm−1. The transitions to highly excited X̃ rovibrational levels as opposed to two photon resonances to a higher electronic state was proved by the virtual identity of the SEP spectra, plotted vs term energy, with the PUMP tuned to the two Ã intermediate levels. The spectra show evidence for strong rovibronic level mixing. At Evr≊6300 cm−1, where the calculated total harmonic vibrational density of states is ρv,h≊0.43 cm−1, close to every CH3O vibrational state can be observed in the spectrum. The highest excitation energy which was reached so far (Evr≊9000 cm−1) exceeds the H–CH2O dissociation energy, ΔH0R,0K≊6800 cm−1. The observations provide the basis for a study of the unimolecular dissociation of CH3O at a fully quantum state resolved level of detail.
Photodissociation of ClNO in the S1 state: A quantum-mechanical a
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o study
Authors: Schinke Reinhard , Nonella Marco , Suter Hans Ulrich , Huber J. Robert
Published: July 2002

We investigated the photodissociation of ClNO via the S1 electronic state using a three-dimensional (3D) ab initio potential-energy surface (PES). The dissociation is found to be fast and direct. In the Franck–Condon (FC) region the slope of the potential along the dissociation path is relatively small giving rise to narrow partial absorption peaks. The total absorption spectrum therefore exhibits a broad vibrational structure which is in perfect agreement with recent measurements. The vibrational excitation of the NO fragment is small and can be qualitatively described within the adiabatic approximation. It is found to be very sensitive to the vibrational FC factor in the transition region. The rotational state distribution of NO is highly inverted with a peak around j=30. It is readily explained by the rotational reflection principle. The experimental results are satisfactorily reproduced by our calculations which underlines the overall quality of the calculated PES. Minor adjustments are necessary, however, to quantitatively reproduce the vibrational branching ratio.
Chattering collisions and their effects on gas phase rotational energy relaxation cross sections
Authors: Evans David R. , Evans Glenn T. , Hoffman David K.
Published: July 2002

Rotational energy relaxation cross sections, σR , for nitrogen in dilute atomic gases (He, Ne, Ar, and Xe) are calculated by classical trajectory simulations for a variety of simple potentials and from the Boltzmann equation for hard convex bodies in the single impulse collision approximation. The single impulse approximation, which ignores multiple impulse (chattering) collisions, is reasonably adequate to describe linear and angular momentum relaxation, but not rotational energy relaxation. For the light noble gases, the hard body derived results together with the small chattering corrections suffice to fit σR . It is also the case that chattering collisions markedly decrease σR from the single impulse approximation value for Xe–N2 in the hard-body model. However, the value of σR experimentally measured and calculated by Kistemaker and de Vries using a soft potential is considerably less than that obtained from hard-body models including chattering. Addition of a square well attractive potential of the appropriate shape to the hard core potential leads to the desired decrease in σR with well depth in accord with both the trajectory calculations and experiments of Kistemaker and de Vries. The reasons are analyzed.
Protonated disilyne, Si2H+3: Molecular structures, vibrational frequencies, and infrared intensities
Authors: Colegrove Brenda Thies , Schaefer Henry F.
Published: July 2002

The singlet potential energy surface of protonated disilyne, Si2H+3, has been investigated theoretically using the self-consistent field (SCF) and configuration interaction (CI) methods. Complete geometry optimizations and harmonic vibrational frequency analyses were performed using a double-ζ plus polarization (DZP) basis set at both the SCF and CI levels of theory. Five minima were located, with the global minimum being a highly symmetric tribridged structure. There are three minima clustered at 11–15 kcal/mol higher in energy. One of these has two bridging hydrogen bonds, another has only one and the third has none at all. A trans-bridged isomer is a higher-lying minimum. Structures corresponding to the classical and nonclassical forms of the vinyl cation, C2H+3, are shown not to be minima. Final energetic predictions are made using several extended basis sets which are at least triple-ζ plus double polarization (TZ2P) in quality. Inclusion of electron correlation is found to be very important for energetic comparisons, geometries and characterization of the stationary points. The theoretical infrared intensities may be useful in the spectroscopic identification of Si2H+3.
Three-quantum nuclear magnetic resonance spectroscopy of liquid water: Intermolecular multiple-quantum coherence generated by spin–cavity coupling
Authors: McCoy M. A. , Warren W. S.
Published: July 2002

We show experimentally and theoretically that coupling between a spin system and the NMR probe, which is responsible for radiation damping, can also act to generate multiple-quantum coherences, even in molecules such as water which ordinarily have no multiple-quantum spectrum. We discuss the implications of this observation for solvent suppression sequences and for preparing nonequilibrium spin states.
Infrared laser-induced chaos and conformational disorder in a model polymer crystal: Melting vs ablation
Authors: Sumpter Bobby G. , Voth Gregory A. , Noid Donald W. , Wunderlich Bernard
Published: July 2002

Molecular dynamics-based computer simulations are presented for the interaction of one and two infrared (IR) laser beams with a model polymer surface. When a single laser beam system is studied over a wide range of intensities, only melting of the polymer, or melting followed by bond dissociation, is observed for up to 100 picoseconds. In contrast, the two-laser simulation results exhibit a marked difference in the energy absorption behavior of the irradiated polymer which, in turn, results in multiple bond dissociations. The results for the one- and two-laser cases studied can be divided into four different classes of physical behavior: (a) the polymer remains in the solid state; (b) the polymer crystal melts; (c) the polymer ablates, but with significant melting (charring); or (d) the polymer ablates with minimal melting. Damage to the model polymer crystal from absorption of energy from either one or two lasers occurs through a mechanism that involves the competition between the absorption of energy and internal energy redistribution. The rate of energy loss from the absorption site(s) relative to the rate of absorption of energy from the radiation field determines rather the polymer melts or ablates (low absorption rates lead to melting or no change and high rates lead to ablation). A sufficiently large rate of energy absorption is only obtainable through the use of two lasers. Two lasers also significantly decrease the total laser intensity required to cause polymer crystal melting. The differences between the one- and two-laser cases are studied by adapting novel signal/subspace techniques to analyze the dynamical changes in the mode spectrum of the polymer as it melts.
An exactly soluble model for interfacial kinetics
Authors: Löwen H. , Oxtoby David W.
Published: July 2002

We study an exactly soluble model for planar interfacial kinetics for both locally conserved and nonconserved order parameters, where the Landau–Ginzburg potential is approximated by two parabolas. Long-range interactions with arbitrary interaction kernels are included. In particular, the relaxation of the initial order parameter interfacial profile to its equilibrium profile is considered. Furthermore, the steady-state solution of planar interfacial growth for a nonconserved order parameter is determined. Here, the influence of a second nonconserved order parameter is also discussed.
Spectroscopic constants and potential energy curves of 47 electronic states of InSb, InSb+, and InSb−
Authors: Balasubramanian K.
Published: July 2002

Spectroscopic constants and potential energy curves of 26 electronic states of InSb, 12 electronic states of InSb+, and 9 electronic states of InSb− are obtained using complete active space self-consistent field, first-order configuration interaction, second-order configuration interaction, and relativistic configuration interaction methods (CASSCF/FOCI/SOCI/RCI), including spin–orbit interaction. The SOCI calculations included up to 700 000 configurations. Spectroscopic constants obtained predict several allowed electronic transitions for InSb, InSb+, and InSb− which are yet to be observed. The ground states of InSb, InSb+, and InSb− are found to be X 3Σ−0+, X 4Σ−1/2, and X 2Σ+1/2 with the constants InSb X 3Σ−0+:Re =3.02 Å, ωe =121 cm−1, De =1.35 eV; X 3Σ−1 : Re =3.03 Å, ωe =136 cm−1, Te =494 cm−1; InSb+ X 4Σ−1/2 : Re =3.351 Å, ωe =63 cm−1, De =0.37 eV; and InSb− X 2Π3/2 : Re =2.695 Å, ωe =191 cm−1, De =2.5 eV. The adiabatic ionization potential and electron affinity of InSb are calculated as 6.33 and 1.41 eV, respectively. Analogous to the recently observed A 3Π–X3Σ− system of GaAs, spectral bands in the 20 200 cm−1 region are predicted for InSb. Another 3Π(II)–X3Σ− system is predicted at 15 830 cm−1. Both the 3Π states in these systems are found to be predissociated through crossing of a repulsive 5Σ− curve. The two low-lying electronic states of InSb− (2Σ+1/2, 2Π1/2) undergo relativistic avoided crossing.
Emission spectra and potentials of the 0+u and 1u states of Xe2
Authors: Messing I. , Eckstrom D. J. , Lorents D. C.
Published: July 2002

An extensive series of time-resolved measurements of Xe*2 second continuum emission spectra has been completed to reveal the spectra of the relaxed individual emitting states. Optical pumping of high-pressure Xe by a short-pulse F2 laser at 157.6 nm was used together with a gated optical multichannel analyzer detector or monochromator and boxcar integrator to time resolve the emission bands of the 0+u and 1u states. The bands are broader than expected and have center wavelengths close together. Curve fits to the data yield the parameters for the fully relaxed states: 1u:λc =171.4±0.6 nm, Δλ=12.2±0.2 nm and 0+u: λc =170.4±0.6 nm, Δλ=13.1±0.3. Temperature-dependent measurements from 246 to 413 K show a rapidly broadening band width with temperature. The results provide the first definitive measurement of the individual excimer emissions from the Xe2(0+u) and Xe2(1u) states. From these measurements and the known ground-state potential, we have derived 0+u and 1u potentials that reproduce the emission bands and temperature dependence very well.
Study of the valence electronic structure of ethyne by electron momentum spectroscopy and Green’s function methods
Authors: Weigold E. , Zhao K. , von Niessen W.
Published: July 2002

The complete valence shell electron separation energy spectra and momentum distributions are measured for ethyne by high resolution electron momentum spectroscopy at a total energy of 1500 eV. Many-body calculations of the separation energies and spectroscopic factors using Green’s function methods are carried out and compared with the data. The measured momentum distributions are compared with self-consistent-field (SCF) orbital wave functions, the SCF wave functions including d functions in the basis sets. The agreement between the measured momentum distributions and the SCF orbital momentum distributions is excellent. The inner valence 2σg orbital is found to be split severely by final state correlation effects. The agreement between the measured and calculated spectroscopic factors and separation energies is in general very good, although the measured separation energy spectra contain significant strength up to 50 eV, this strength being mainly of 2σg origin.
The extraction of dynamics from the photodetachment spectrum of ClHCl− by classical mechanical methods
Authors: Hahn Ohyeon , Gomez Llorente Jose M. , Taylor Howard S.
Published: July 2002

The main features of the photodetachment spectrum of ClHCl− are simulated using 3D classical trajectories. A classical mechanical interpretation of the quantum resonances of the ClHCl transition state region is provided. A direct correspondence between the features of the spectrum and the classical motions involved in such features is established. The demonstrated method of analysis is applicable to most complicated irregular spectra and reactions involving nuclear motions.
The electronic spectroscopy and structure of complexes of argon with 3-amino-s-tetrazine in a supersonic jet
Authors: Alfano Joseph C. , Martinez Selso J. , Levy Donald H.
Published: July 2002

The structure and spectroscopy of van der Waals clusters between argon and 3-amino-s-tetrazine (AT) has been investigated in a supersonic jet. Low resolution vibrationally resolved fluorescence excitation spectra of AT at a variety of argon gas concentrations enable us to assign the origin transitions for the clusters AT–Arn, where n=1 through 6. The frequencies of the van der Waals vibrational modes of these clusters are determined, both in the excited states from the excitation spectra, and in the ground state from single vibronic level emission spectra. High resolution spectroscopy is used to resolve rotational structure in the origin transitions of some of the clusters, and this provides structural information about the clusters. The argon atom in AT-Ar is found to be above the tetrazine ring, and the argon atoms in AT–Ar2 are found to be above and below the tetrazine ring, in all cases having a van der Waals bond length of about 3.3 Å. This work is useful in interpreting the results of our previous study on the photodissociation dynamics of van der Waals clusters between argon and AT.
The entropies and probable symmetries of the gaseous thorium and uranium tetrahalides
Authors: Hildenbrand D. L. , Lau K. H. , Brittain R. D.
Published: July 2002

The sublimation entropies of the thorium and uranium tetrahalides (except UI4 ) have been evaluated from torsion–effusion vapor pressure measurements and the results have been analyzed for compatibility with the spectroscopic and molecular constants of the gaseous molecules. New effusion pressure measurements are reported here for UCl4, UBr4, and ThI4. For UF4, UCl4, and ThF4, where thermal data are available for the solid phases, the experimental entropies of gaseous UF4 and UCl4 are incompatible with regular tetrahedral symmetries, while that of ThF4 is in close accord with a Td structure. A comparison of the entropies of sublimation shows that ThCl4 most likely has Td symmetry, while UBr4, ThBr4, and ThI4 probably do not; less-reliable total entropy data for the species are in full accord with these conclusions. The results are discussed in terms of structural information in the literature.
Vibrational states and structure of Ar3: The role of three-body forces
Authors: Horn T. R. , Gerber R. B. , Valentini J. J. , Ratner M. A.
Published: July 2002

Vibrational energies and eigenfunctions of Ar3, including some pertaining to highly excited states, are computed, and insights into their dynamical and structural properties are obtained. The method used employs the vibrational self-consistent-field (SCF) theory in hyperspherical coordinates as a first approximation. Exact results are obtained by configuration interaction, using the SCF states as an efficient basis. A focal point of the study is the effect of three-body potentials on the vibrational spectrum. Axilrod–Teller and other three-body potentials are used to examine this. It is found that the effect of three-body forces on the spectrum is substantial, and larger than effects due to uncertainties in the presently known two-body Ar–Ar potentials. This suggests that experimental spectroscopy of Ar3 may be used to determine reliable three-body forces among Ar atoms. It is also shown that the three-body double-dipole–quadrupole interaction, while less important than the Axilrod–Teller one, has a significant effect on the vibrational spectrum. Finally, a detailed analysis is made of the Ar–Ar distance distributions in the various states, of the structural distributions of Ar3, and of the properties of the wave functions. We find that the wave functions show well-ordered nodal patterns even for the highly excited large-amplitude states. Thus, these states do not correspond qualitatively to ‘‘liquid-like’’ behavior of the cluster.
High-temperature photochemical hole burning and laser-induced hole filling in dye-doped polymer systems
Authors: Furusawa Akira , Horie Kazuyuki
Published: July 2002

High-temperature photochemical hole burning (PHB) and laser-induced hole filling were studied for dye-doped polymer systems focusing on the capability of hole formation at 80 K. Hole area of a hole burnt at 4.2 K was almost conserved after cycle annealing up to 80 K irrespective of the capability of hole formation at 80 K. The extent of spectral diffusion also hardly affected this capability. Furthermore, it was not suggested to be the homogeneous width alone that affects the capability of hole formation at 80 K. Important factor for hole formation at 80 K is elucidated to be the suppression of the laser-induced hole filling at 80 K.
Spectroscopic characterizations of the 3Σ+g and 3Πg components of n
f complexes of Ne2 with n=4–6
Authors: Kim S. B. , Kane D. J. , Eden J. G. , Ginter Marshall L.
Published: July 2002

Rotationally resolved, laser-induced fluorescence spectra for the transitions nf (3Σ+g,3Πg) ←a 3Σ+u (n=4–6) in Ne2 are presented and analyzed. The rotational structures of the upper states are found to be strongly l-uncoupled. Effective rotational constants for the a 3Σ+u, nfσ 3Σ+u, and nfπ 3Π±u states with n=4–6 are reported and the structures of the nf complexes discussed.
Electric field induced change of adsorbate vibrational line strength
Authors: Lambert David K.
Published: July 2002

Electric field E changes an adsorbate’s vibrational line strength S. The dependence of S on E is described by δSE=d ln S/dE. At an electrode in an electrochemical cell the dependence is described by δSΦ=d ln S/dΦ, where Φ is electrode potential. The observed effect for CO on Pd(100) and Pt(110) electrodes in aqueous electrolytes is δSΦ=−0.135±0.086 and −0.135±0.041 V−1, respectively. The corresponding δSE for CO on Pd(100) is (−4.8±3.1)×10−9 cm/V, consistent with Bagus and Pacchioni’s ab initio calculation of −4.3×10−9 cm/V. A semiclassical explanation of δSE is given. The semiclassical δSE for CO and CN− is approximately proportional to the polarizability derivative (dα/dQ). For free CO oriented along the field direction, the semiclassical δSE=(−6.2±0.8)×10−9 cm/V. Three previous ab initio calculations for free CO gave δSE=−6.5×10−9, −4.6×10−9, and −3.6×10−9 cm/V. Experimental studies of CO on Ni(100), Pt(110), and Pd(100) indicate that (dα/dQ) of CO on these surfaces is increased relative to (dα/dQ) of free CO by factors of 6.0±5.5, 2.6±1.8, and 1.7±0.6, respectively. These values are consistent with Raman studies of CO on Ni(100) and on Pd films. Electrochemical studies of CO and CN− at polycrystalline electrodes are also discussed.
Photodissociation of adsorbed Mo(CO)6 induced by direct photoexcitation and hot electron attachment. II. Physical mechanisms
Authors: Ying Z. C. , Ho W.
Published: July 2002

Photodissociation of Mo(CO)6 adsorbed on potassium-free and potassium-preadsorbed Cu(111) and Si(111)7×7 at 85 K has been studied under ultrahigh vacuum conditions. The photodissociation yield has been measured as a function of photon power (0.5–30 mW/cm2 ), wavelength (250–800 nm), polarization (s and p), and incident angle (20°–70°). Two surface photoreaction mechanisms are considered: (i) direct electronic excitation of the adsorbate and (ii) attachment of photogenerated hot carriers to the adsorbate. The photodissociation spectra obtained on K-free Cu(111) and Si(111)7×7 exhibit the same resonant structure as the absorption spectrum of Mo(CO)6. Photodissociation of Mo(CO)6 on K-free surfaces is thus determined to be dominated by direct electronic excitation of the adsorbate, which proceeds via a single-photon process. A new photodissociation channel is opened on K-preadsorbed surfaces. The photoyield increases substantially in the UV and extends to the visible and near IR. By studying the wavelength and polarization dependences of the photoyield, it is firmly established that the new photodissociation channel is due to interaction of photogenerated hot carriers with the adsorbate. The photogenerated hot electrons tunnel through the potential barrier between the adsorbed Mo(CO)6 and substrate and attach to the Mo(CO)6 molecules. This mechanism is energetically possible in the presence of K due to a substantial up-shift in the Fermi level associated with the decrease in the work function. The negative ions formed by electron attachment are unstable and undergo dissociation.
The photodissociation of O3: A classical dynamical approach for the interpretation of the recurrences in the autocorrelation function
Authors: Farantos Stavros C. , Taylor Howard S.
Published: July 2002

Families of periodic orbits and the classical survival probability function are calculated for ozone in the excited 1B2 state. It is shown that some of the observed oscillations in the experimentally extracted and quantum mechanically calculated time autocorrelation functions can be explained by certain types of periodic orbits. Disagreements between the calculated and experimental correlation function are attributed to inaccuracies of the potential function used.
On the stability of mass action reactions in open systems
Authors: Poland Douglas
Published: July 2002

We consider the stability of chemical systems whose evolution is governed by mass action reactions. We show, with no reference to the second law of thermodynamics, that the stability of equilibrium in a closed system follows from the fact that one can express the kinetic matrix for the linearized equations near equilibrium in the form of a matrix times its transpose. This is a necessary and sufficient condition that the matrix be positive indefinite (some of the eigenvalues are necessarily zero due to the existence of conservation relations in a closed system). We then extend this approach to open systems where one finds that a component of the kinetic matrix cannot be expressed as the product of a matrix times its transpose. It is this matrix, having a simple expression in terms of stoichiometric matrices, thus expressing the basic mechanisms of the chemistry, that is potentially responsible for instabilities, oscillations, and chaos far from equilibrium. Examination of this matrix leads naturally to autocatalytic reactions as candidates for interesting behavior far from equilibrium. We conclude that all of the interesting features of mass action kinetics can be determined directly from the kinetic equations and the principle of detailed balance with no additional constraints coming uniquely from thermodynamics.
A reevaluation of the vaporization behavior of sodium oxide and the bond strengths of NaO and Na2O: Implications for the mass spectrometric analyses of alkali/oxygen systems
Authors: Steinberg Martin , Schofield Keith
Published: July 2002

There has been a long standing disagreement between our flame experiments, which predict a very stable NaO2 molecule, and Na2O(c) vaporization/mass spectrometric studies of Hildenbrand et al., which imply a weak bond strength from an inability to detect such a species. We have now reanalyzed the vaporization experiments and have identified a possible explanation for this frustrating controversy. It appears that on becoming ionized, NaO+2 fragments to Na+ and O2. As a result, mass 23 reflects p(Na)+p(NaO2). This and the changes to the thermochemical data for NaO2 modifies the earlier ion intensity/vapor pressure calibration. As a result, the previously accepted thermochemical values for NaO and Na2O need to be reduced by 18 and 11 kJ mol−1, respectively. Recommended values now become ΔHf298K (NaO)=87±4, D0(NaO)=266±4, ΔHf298K(Na2O) =−36.0±8 and D0(Na–ONa)=228±8 kJ mol−1. It also appears that the I.P.(NaO2)≤739 kJ mol−1 (7.66 eV).

The reported Clausius–Clapeyron vapor pressure curves are entirely consistent with this suggestion that the congruent vaporization of Na2O(c) is to Na and NaO2 rather than to Na and O2. A reinterpretation utilizing the previous slopes of the I(Na+) or I(O+2) signals leads to an independent measure of −90≥ΔHf298K(NaO2)≥−155 and 194≤D0(Na–O2)≤259 kJ mol−1. These are to be compared with the higher temperature flame determined values of −139±8 and 243±8 kJ mol−1, respectively. Earlier vaporization mass loss measurements of Brewer and Margrave were in approximate agreement with a vaporization to predominantly Na and O2. Analyses now show that the technique is insensitive and congruent vaporization to Na and NaO2 also fits their data. Hildenbrand and Murad measured Na+/O+2 ratios but these appeared to be a factor of 4 smaller than expected for congruency and remained unexplained. By invoking an alternate fragmentation of NaO+2 at higher energies to Na and O+2 (A.P.≊14.6 eV), with a branching ratio of ≥10%, this channel becomes the dominant source of O+2 and the observed Na+/O+2 signal ratios are consistent with congruency. These results have important implications for all mass spectrometric studies of alkali/oxygen mixtures including, for example, also carbonates and nitrates. A reappraisal for these is in order, particularly with reference to derived thermochemical values.