bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave packet studies of nonadiabatic effects
Authors: Rama Krishna M. V.
Published: July 2002

Variational and wave packet perturbation theory methods are implemented to study the effects on photodissociation and scattering cross sections due to nonadiabatic coupling between two electronic potential energy surfaces. The simplest variational method we implemented can handle large coupling strengths between surfaces, but its restriction to one Gaussian wave packet per surface is found to be too severe in some instances. In such situations the wave packet perturbation theory, which coherently superimposes multiple Gaussians on each surface, overcomes the difficulties encountered by the variational method and is seen to give quite accurate photodissociation and Landau–Zener cross sections over a wide energy range, so long as the coupling between surfaces is reasonably small. In this paper we give details of the appropriate computational schemes to adopt for calculating photodissociation and scattering cross sections from propagating wave packets. In addition, we discuss the strengths and weaknesses of Gaussian wave packet methods for studying nonadiabatic effects.
The temperature and concentration dependencies of diffusion coefficients of some helium–hydrocarbon and helium–fluorocarbon systems
Authors: Dunlop Peter J. , Bignell C. M.
Published: July 2002

Binary diffusion coefficients are reported as functions of concentration and temperature for helium–hydrocarbon and helium–fluorocarbon systems. An empirical correlation is made between the temperature-dependent effective cross sections for diffusion and the number of electrons in the heavy species interacting with the light helium atoms.
Monte Carlo simulations of hydrophobic polyelectrolytes. Evidence for a structural transition in response to increasing chain ionization
Authors: Hooper Herbert H. , Beltran Sagrario , Sassi Alexander P. , Blanch Harvey W. , Prausnitz John M.
Published: July 2002

Monte Carlo simulation has been used to study the configurational properties of a lattice-model isolated polyelectrolyte with attractive segment–segment interaction potentials. This model provides a simple representation of a hydrophobic polyelectrolyte. Configurational properties were investigated as a function of chain ionization, Debye screening length, and segment–segment potential. For chains with highly attractive segment–segment potentials (i.e., hydrophobic chains), large, global changes in polymer dimensions were observed with increasing ionization. The transformation from a collapsed chain at low ionization to an expanded chain at high ionization becomes increasingly sharp (i.e., occurs over a smaller range of ionization) with increasing chain hydrophobicity. The ionization-induced structural transitions for this model hydrophobic polyelectrolyte are analogous to pH-induced transitions seen in real polyelectrolytes and gels. These studies suggest a simple explanation for such transitions based on competing hydrophilic and hydrophobic interactions.
About the first solvation shell of protonated hydrates: H3O+(H2O)6
Authors: Rahmouni A. , Kochanski E. , Wiest R. , Wormer P. E. S. , Langlet J.
Published: July 2002

A b initio SCF and semiempirical calculations have been performed on some geometrical configurations of the system H3O+(H2O)6, with either three or four water molecules in the first solvation shell. The dispersion energy has been evaluated from perturbation theory. It is shown that the pair approximation overestimates the stability of the second category of structures. However, the present work confirms that geometries with either three or four water molecules in the first solvation shell are close in energy. Comparison is done with results obtained from several semiempirical expressions and parameters available in the literature.
An approach to the ‘‘channel three’’ phenomenon of benzene
Authors: Sobolewski A. L.
Published: July 2002

The nonradiative decay of benzene excited within the S1 manifold is studied by assuming an open vibrational potential for the S2 state that leads to spontaneous breaking of hexagonal symmetry along a path for valence isomerization. The S2 state crosses the S1 electronic configuration (at the channel three threshold) as well as the ground state along the reaction path. The rate constants of nonradiative decay of the S1 state (in fact the S1uS2 internal conversion) and the S2 state (S2uS0 IC) are both quantum mechanically calculated by invoking microcanonical averaging over all vibrational levels accessible at a given excess of vibrational energy. The results obtained account for a number of features observed in spectroscopy of benzene at the channel three threshold.
Cluster and barrier effects in the temperature and pressure dependence of the photoisomerization of t
r
a
n
s-stilbene
Authors: Schroeder J. , Schwarzer D. , Troe J. , Voß F.
Published: July 2002

The pressure and temperature dependence of the photoisomerization rate coefficient of trans-stilbene in the S1 state have been measured in the solvents C2H6, C3H8, C4H10, Xe, Co2, SF6, and CHF3. At constant temperature, the pressure dependences up to 6 kbar can be well represented by the Kramers–Smoluchowski model. The comparison of results in different solvents clearly indicates the importance of reactant–solvent cluster formation modifying the height and imaginary frequency of the barrier. The change of the temperature dependence with pressure points towards a multidimensional barrier of nonseparable character. Multidimensional barrier effects manifest themselves most clearly via the temperature dependence of the rate coefficient in the Kramers–Smoluchowski limit.
Electronic decay of core hole excited states in molecular oxygen
Authors: Lapiano-Smith D. , Lee K. , Ma C.-I. , Wu K. T. , Hanson D. M.
Published: July 2002

Electron energy spectra characterizing the relaxation of molecular oxygen from different core hole excited states are reported. By using monochromatic synchrotron radiation, spectra are obtained with excitation at the π* and σ* resonances and at various energies above the ionization threshold. The spectra obtained with excitation above threshold are very similar and result from Auger decay. The spectra obtained at the σ* resonance result from spectator autoionization, and those for the π* resonance result from both spectator and participator autoionization. The σ* resonance is broadened by spectral congestion, and the autoionization spectra indicate the presence of three components. Configurations and states produced by the electronic decay are identified, and a correlation between spectator screening and term values is documented.
Photoinduced processes and resonant third-order nonlinearity in poly (3-dodecylthiophene) studied by femtosecond time resolved degenerate four wave mixing
Authors: Pang Yang , Prasad Paras N.
Published: July 2002

We have investigated the dynamics of resonant third-order optical nonlinearity of chemically prepared poly(3-dodecylthiophene) by the degenerate four wave mixing technique using 60 fs pulses at 620 nm. The measured effective value of χ(3) is 5.5×10−11 esu, sixfold smaller than that obtained with 400 fs pulses, emphasizing the pulse width dependence of effective χ(3) when the relaxation time of the photogenerated excitation responsible for the optical nonlinearity is comparable to the pulse width. Within the resolution of the optical pulse, the rise time of the nonlinear response is instantaneous and the dominant decay occurs within 200 fs, revealing that the short time, nonlinear response is derived from the initially photogenerated excitons. A detailed analysis of the total decay behavior is consistent with the polaron dynamics of the conformational deformation model proposed by Su, Schrieffer, and Heeger for a conjugated linear polymer with bond alternation.
Excitation energies with multireference many-body perturbation theory
Authors: Meissner Leszek , Kucharski Stanislaw A. , Bartlett Rodney J.
Published: July 2002

Excitation energy calculations with multireference many-body perturbation theory (MRMBPT) are theoretically and numerically studied. An extension of the Hose–Kaldor (HK) scheme is presented, which removes disconnected terms and, hence, the size-extensivity error for higher-order MRMBPT approximations. The excitation problem requires the use of an incomplete model space for which connectivity of the effective Hamiltonian Heff, is incompatible with intermediate normalization. In our formulation a proper choice of the model space as an introduction of a ‘‘quasiintermediate’’ normalization leads to the connected structure of Heff. This guarantees size extensivity of the method which generally could not be achieved with the earlier Hose–Kaldor (HK) framework based upon intermediate normalization. Special attention is paid to the case when the Hartree–Fock (HF) approximation is used in the zeroth-order step. In this specific case the HK formalism applied to a subspace of the model space spanned by singly excited determinants gives the same result through third order which means that in the HF case disconnected contributions to Heff disappear and size extensivity is preserved to that order. MRMBPT(3) results for N2 and CO are presented to offer illustrative comparisons with the recently proposed EOM-CCSD and Fock space MRCCSD results.
Molecular-dynamics simulations of the photodissociation of ICl adsorbed on a MgO(001) surface
Authors: McCarthy M. I. , Gerber R. B.
Published: July 2002

Molecular-dynamics (MD) simulations are used to study the vibrational properties of ICl adsorbed on an MgO(001) surface, and the photodissociation dynamics of the molecule after excitation to a 1Π electronic state. The electronic ground-state simulations show that ICl lies nearly parallel to the surface and occupies a single orientational site at surface temperatures below 150 K. Above 350 K the molecule hops between two orientational sites on the surface, and at 500 K full rotational diffusion of the adsorbate in the surface plane occurs. The multiplicity of sites and the onset of rotational diffusion at high T were found to greatly affect the dissociation dynamics and its temperature dependence. The photodissociation simulations show that only a fraction of the Cl atoms and some of the I atoms (which have a much higher binding energy) leave the surface following photolysis (at these energies). The fraction of Cl atoms leaving the surface subsequent to photodissociation at 50 K is ∼0.5, and it decreases as T is raised to 150 K. The trajectories show that Cl atoms leave the surface preferentially for initial ICl orientations in which the Cl end ‘‘points down.’’ This orientation ensures that the escaping atom rapidly collides with the surface atoms. Momentum transfer due to surface local roughness is crucial for the Cl to acquire ‘‘escape velocity’’ normal to the surface. The angular intensity distribution of the Cl atoms is sensitive to surface corrugation, and the energy distribution of the photofragments strongly reflects the Cl/surface collision stage of the process. It is concluded that photodissociation experiments can provide information both on surface local structure and on photofragment/surface interaction and energy transfer.
Multiphoton spectroscopy of Rydberg states of tetrachloroethylene
Authors: Williams Bradley A. , Cool Terrill A. , Rohlfing Celeste M.
Published: July 2002

Spectroscopic studies of two-photon resonant vibronic bands of the (π,np) and (π,nf) Rydberg series of jet-cooled C2Cl4 are reported. Polarization ratio measurements for each transition and comparisons of experimental and calculated chlorine isotope shifts permitted the unambiguous assignment of vibronic band structure. Ab initio self-consistent field calculations of the harmonic frequencies for the optimized ground state geometry of C2Cl4 were performed for each of the seven isotopic chlorine compositions to assist interpretation of the isotope shift measurements. The 4p-Rydberg bands are quite weak and strongly perturbed, suggestive of localized Rydberg–valence state configuration mixing. The nf-Rydberg series provides a measurement of the first ionization potential for C2Cl4 of 75 216±5 cm−1 in good agreement with previous measurements. Mass analysis of ion fragmentation patterns reveals a multiphoton ladder switching excitation mechanism.
Fourier transform spectroscopy of C4H (butadiynyl) in Ar at 10 K: C–H and C≡C stretching modes
Authors: Shen L. N. , Doyle T. J. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared (FTIR) isotopic study of the C4H (butadiynyl) radical produced by trapping the products from the vacuum ultraviolet photolysis of diacetylene (C4H2) and 1,3-butadiene (C4H6) in solid argon at 10 K has been carried out in the midinfrared from 400–3900 cm−1. A variety of carbon-13 and deuterium substituted C4H2 and C4H6 parent molecules were used to produce various isotopomers of C4H. On the basis of their isotopic behavior two absorptions at 3307.4 and 2083.9 cm−1 have been assigned to the ν1, C–H stretching and ν2, C 3/4 C stretching modes. The previous identification of the other C 3/4 C stretching mode, ν3=2060.6 cm−1, which was based on the effects of deuteration, has been confirmed and the corresponding frequencies for various 13C-substituted isotopomers have been measured. The following frequencies have been obtained for ν1, ν2, and ν3 for C4H isotopomers: 3307.2, 2078.9, and 2023.8 cm−1 for H–C 3/4 C–C 3/4 13C; 3307.2, 2069.9, and 2032.5 cm−1 for H–C 3/4 C–13C 3/4 C; 3307.2, 2066.8, and 2054.0 cm−1 for H–C 3/4 13C–C 3/4 C; 3293.3, 2048.9, and 2057.4 cm−1 for H–13C 3/4 C–C 3/4 C; 3292.5, 2050.6, and 2024.6 cm−1 for H–13C 3/4 C–C 3/4 13C; 3292.5, 2050.6, and 2030.0 cm−1 for H–13C 3/4 C–13C 3/4 C; 3306.3, 2063.3, and 2019.5 cm−1 for H–C 3/4 13C–C 3/4 13C; 3306.3, 2050.6, and 2018.0 cm−1 for H–C 3/4 13C–13C 3/4 C; 3290.1, 2007.9, and 1981.6 cm−1 for H–13C 3/4 13C–13C 3/4 13C; and 2579.3, 2056.5, and 2049.7 cm−1 for D–C 3/4 C–C 3/4 C.
Electronic transmission coefficient for outer-sphere electron transfer reactions in solution: A Landau–Zener formalism
Authors: Khan Shahed U. M. , Zhou Zheng Yu
Published: July 2002

The Landau–Zener formulation and literature values of electronic transition matrix were utilized to compute the theoretical values of electronic transmission coefficient of several outer-sphere electron transfer reactions in solution. The slopes of the energy surfaces that are needed for the Landau–Zener equation were obtained using both intermediate neglect of differential overlaps (INDO/2) molecular orbital and classical improved average dipole orientation (IADO) methods. Theoretical results of electronic transmission coefficient of electron transfer reaction obtained using values of slopes from INDO/2-MO (molecular orbital) as well as IADO methods are found in close agreement with the quasiexperimental values of electronic transmission coefficient obtained from experimental data of rate constant. These theoretical values of electronic transmission coefficient, as well as those from experimental values of rate constant are found to be less than unity. These results indicate that outer-sphere electron transfer reactions in solution involving aquo and amine complexes of the transition metal ions studied in this work are nonadiabatic in nature. Theoretical results of electronic transmission coefficient obtained using values of slopes from the classical improved average dipole orientation (IADO) method is found in close agreement with those obtained from the quantum chemical INDO/2-MO method and also with those from experimental values of rate constant and hence justifies the validity of the use of the former.
High precision study of chemical relaxation in the system N2O4=2NO2 by photoacoustic resonance spectroscopy
Authors: Fiedler M. , Hess P.
Published: July 2002

The chemical relaxation of the N2O4 dissociation was studied using Ar+ laser excited acoustic resonances of a cylindrical cavity. The resonance profiles of the first radial mode were measured as a function of pressure and temperature by a computer controlled setup. The N2O4 dissociation rate constant and the mean rotational and vibrational relaxation time of the N2O4–NO2 mixture were obtained for temperatures between 273 and 317 K by fitting a detailed theoretical model of the resonator to the measured values for frequency dispersion, resonance broadening, and strength of the photoacoustic signal. Excellent agreement between theory and experiment was obtained for the frequency dispersion and the photoacoustic signal. An empirical correction function had to be introduced to account for discrepancies between the theoretically predicted and measured values of the resonance broadening which was in the percent region. A value of 6.1×10−15 cm3/molecule s was determined for the rate constant of N2O4 dissociation in the low pressure limit at 298 K. The activation energy of the N2O4 dissociation for 20 mbar pressure was determined to be EA=36 kJ/mol. A comparison of the measured low-pressure limit of the unimolecular rate constant with theoretical predictions of RRKM theory yielded values for the mean collision efficiency of N2O4 and NO2 between 0.95 and 0.5 in the temperature range 273–317 K.
Intersystem crossing in collisions of aligned Ca(4s5p 1P)+He: A half collision analysis using multichannel quantum defect theory
Authors: Dubs Richard L. , Julienne Paul S. , Mies Frederick H.
Published: July 2002

A half collision analysis of alignment effects on intersystem crossing in the collisions of Ca(4s5p 1P) with He has been performed using generalized multichannel quantum defect theory (MCQDT). The theory provides a rigorous analytical representation of the numerically exact closed-coupled scattering wave functions. The half collision analysis results in a factorization of the full quantum collision problem into a number of simpler quantum mechanical problems which reflect different regions of development during the collision. A WKB-assisted, frame transformation approximation to the incoming half collision matrix is tested numerically and is found to be useful in projecting out information on the ‘‘locking radius’’ concept.
The effect of the composition of the oil phase on the microstructure and dielectric behavior of water-in-oil microemulsions
Authors: Robertus C. , Joosten J. G. H. , Levine Y. K.
Published: July 2002

A comparison is made between the clustering process occurring the sodium-di-2-ethylhexyl-sulfosuccinate microemulsion in the four organic solvents: n-decane, iso-octane, n-heptane, and n-hexane. The clustering process is studied by small angle x-ray scattering (SAXS) and dielectric relaxation spectroscopy. The structural parameters obtained from the analysis of the SAXS experiments using the sticky hard sphere droplet model are used to calculate the dielectric permittivity and are compared with results from the dielectric relaxation experiments. It is found that an increase in the attraction between microemulsion droplets when the chain-length of the solvent molecule is increased.
Kinetic behavior of diffusion systems in the presence of non-Markovian boundaries
Authors: Dayan Ido , Weiss George H.
Published: July 2002

Previously, the authors have calculated a number of properties of diffusion in the presence of partially reacting boundaries, in which the probability that a particle impinging on the boundary will react is not a constant. This is reflected in the appearance of a boundary condition nonlocal in time. Here a number of consequences for kinetic behavior of such a nonlocal boundary condition are derived. The circumstances under which results derived from the Smoluchowski model or its generalization are asymptotically valid are discussed, as well as conditions that lead to deviations from the predictions of such a theory.
Bimolecular annihilation reactions with immobile reactants: Unequal reactant concentrations
Authors: Schnörer H. , Kuzovkov V. , Blumen A.
Published: July 2002

We study the bimolecular annihilation reaction A+B→0 with immobile reactants interacting via exchange. We extend our previous analyses to the case of unequal concentrations of A and B particles. A comparison between the direct simulation of the reaction process and an analytical approach, which is based on Kirkwood’s superposition approximation, shows reasonable agreement, although deviations are larger than in the case of equal A and B concentrations. Interestingly, the asymptotic decay forms for the particle concentrations (when formulated in terms of a suitable reaction length) parallel those encountered in diffusion-limited reactions. Here the diffusion length lD=(Dt)1/2 gets replaced by the reaction radius ξ=ln t. We discuss implications of this finding for a qualitative (but quite accurate) picture of the temporal evolution behavior.
Dative circular dichroism: An overlap-dependent perturbation analysis for magnetic dipole allowed transitions
Authors: Schipper Pieter E.
Published: July 2002

The conventional independent systems/perturbation (ISP) treatment for the circular dichroism of magnetic dipole allowed transitions of an achiral chromophore (A) in a chiral environment (C) is extended to include analytically the effect of weak overlap between the A and C systems. The derivation exploits the recently developed simplified group function (SGF) approach, which resolves the interaction between two weakly overlapping groups into separable and additive overlap-independent long range contributions (excitonic effects on which the ISP analysis is based) and overlap-dependent dative terms. A theoretical justification for the group (chromophore) descriptions implicit in the SGF analysis is presented through the concept of equivalent representations. The results provide clear criteria for when the ISP results must be augmented with the dative contributions, and also provide a clear rationale for why the ISP/dative approach may be expected to be of relatively broad validity.
Analysis of a model for the ripple phase of lipid bilayers
Authors: McCullough W. Scott , Perk J. H. H. , Scott H. L.
Published: July 2002

We report two-dimensional Monte Carlo, mean-field, and exact one-dimensional studies of a statistical-mechanical model for the ripple (Pβ′) phase of hydrated phosphatidylcholine lipid bilayers. The model is a p-state chiral clock model coupled to an Ising model through the chiral field. The microscopic parameters of the model are fixed by independently obtained values for pairwise molecular interactions. The one-dimensional model possesses a ‘‘floating fluid phase’’ characterized by exponentially decaying, spatially modulated correlations. Solutions of the mean-field equations for the model include modulated phases but these are found to be metastable states, and the mean-field phase diagram is dominated by phases characteristic of the p-state chiral clock model. Monte Carlo simulations also reveal clock model phases. However, when unphysical effects due to mod p counting in the Hamiltonian are eliminated, the Monte Carlo simulations reveal a modulated phase, intermediate in temperature between a high-temperature disordered phase and a low-temperature ordered phase which is identified with the chain tilt (Lβ′) phase.
A study of molecular supersonic flow using the generalized Boltzmann equation
Authors: Randeniya L. K. , Smith M. A.
Published: July 2002

We report a model based on the generalized Boltzmann equation to predict the properties of a free-jet expansion of a single-component gas, with particular emphasis on polyatomic flow. Contrary to many other studies on this subject, we do not assume a constant beam speed. A new set of boundary conditions is obtained for the solutions to the moment equations which describe the evolution of the flow properties. The variation of the different temperature components, speed, and density as a function of distance is presented assuming an anisotropic Maxwellian distribution function. An expression for the bulk rotational relaxation time is also derived from first principles. Comparison is made between the new model and currently available models.
The transition temperatures and dynamics for the argon–xenon N=7 mixed cluster series
Authors: Robertson Daniel H. , Brown Franklin B.
Published: July 2002

Results of molecular-dynamics calculations with quenching for the N=7 Ar–Xe mixed cluster series are reported. The phase-transition temperatures have been determined and are discussed. The microscopic transitions between catchment regions on the potential-energy surface are examined as well as the total amount of time spent in each of these catchment regions. The mixed cluster results are compared to the homogeneous results. The heterogeneous dynamics are found to more closely reproduce the homogeneous results if the heterogeneous catchment regions are classified according to their associated homogeneous parent minimum.
Two dimensional 31P nuclear magnetic resonance coherence transfer experiments under magic-angle sample spinning
Authors: Kubo Atsushi , Root Andrew , McDowell Charles A.
Published: July 2002

In this paper, we report on two dimensional (2D) 31P cross polarization (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments on the coupled two-spin systems, sodium pyrophosphate decahydrate, Na4P2O7, 1OH2O (SP) and tetraphenyldiphosphine-1-oxide, (C6H5)2PP(O) (C6H5)2 (TPPO), including antiecho (COSY), double-quantum NMR, and zero-quantum NMR experiments. These experiments are generalizations of the absolute mode 2D Fourier transform antiecho COSY performed under MAS condition by Kentgens, de Boer, and Veeman [J. Chem. Phys. 87, 6859 (1987)]. The 2D sideband intensities for these experiments on polycrystalline samples are shown theoretically to be real. There are two mechanisms of coherence transfer; homonuclear J coupling and dipolar coupling. Theory shows that the zero-quantum signal for the coupled two spins can not be observed by using a (CPx−τ−(π)x−τ−(π/2)x−t1−(π/2)−t2 pulse sequence, when the coherence transfer is due to J coupling. When, however, the coherence transfer is induced by the flip-flop term of the dipolar coupling Hamiltonian, the zero-quantum signal can be observed by that pulse sequence. The preparation time dependences of the double-quantum and the zero-quantum sideband patterns, are expected, when the coherence transfer is induced by dipolar coupling. The zero-quantum signal was very weak for TPPO, while it was strong for SP. The apparent preparation time dependence of the zero-quantum sideband pattern was observed for SP. These results suggest that the coherence transfer is mainly due to J coupling in TPPO, where the two 31P nuclei have different isotropic chemical shifts. While, on the other hand, the dipolar coupling is more important in SP, where the two 31P nuclei have the same isotropic chemical shifts but different orientations of the chemical shift tensors. The 2D sideband intensities of the antiecho COSY spectrum of TPPO were calculated, and the relative orientation of the two chemical shift tensors was determined.
X-ray diffraction study of the structure of a monolayer methanol film adsorbed on graphite
Authors: Morishige K. , Kawamura K. , Kose A.
Published: July 2002

The x-ray diffraction measurements of methanol adsorbed on graphite have been taken over the temperature range 32–150 K and the coverage range 0.02–0.17 molecule Å−2. The diffraction data are analyzed to yield the configuration of the molecule on the surface by means of the least-squares fitting of the whole pattern. The data show clearly that zigzag chains of alternating weak and strong hydrogen bonds, as evidenced by the long and short 0⋅⋅⋅0 separations, are formed in the crystalline monolayer of methanol on graphite. The crystalline solid film near the complete monolayer melts around 142 K in comparison to bulk methanol which melts at 175.4 K.
Infrared laser spectroscopy of H2 and D2 Rydberg states. I. Application of the polarization model
Authors: Davies P. B. , Guest M. A. , Stickland R. J.
Published: July 2002

Multistate perturbation theory has been used to assign high resolution Rydberg spectra of H2 and D2 in the infrared. The theoretical model treats interactions between the H+2 ion core and the spatially extended case (d) Rydberg electron in terms of the multipole moments and polarizabilities of H+2. The long range interaction model of Sturrus, Hessels, Arcuni, and Lundeen [Phys. Rev. A 38, 135 (1988)] was refined to include terms up to r−8 in the Rydberg electron radial matrix element. Implementation of the multipole moment–polarization matrix diagonalization model was carried out at three levels: Rotation only (‘‘pure precession’’), vibrationally extended, and full multichannel. At each level the calculations were made to order r−4 (low order), r−6 ( (1)/(2) V6) and r−8 (high order). The most generally successful model was found to be the (1)/(2) V6 vibrationally extended model.
Electronic structure of Rh, RhH, and Rh2 as derived from a
b i
n
i
t
i
o (configuration interaction) calculations
Authors: Illas F. , Rubio J. , Cañellas J. , Ricart J. M.
Published: July 2002

The electronic structure of Rh, RhH, and Rh2 in their ground and low-lying electronic states has been studied using nonempirical relativistic pseudopotentials and moderately large basis sets including f-type functions. The energy splitting 4F–2D, ionization potential and electron affinity of Rh have been obtained and compared with experimental results. A rather good agreement being found in all cases. For RhH the 3Δ, 3Π, 3Φ, 3Σ− and 1Σ+ electronic states were studied. The ground state being 3Δ in agreement with previous theoretical studies. The calculated De for RhH was found to be in a nice agreement with the experimental value. For Rh2 eight electronic states symmetrically dissociating into the ground state of the isolated atoms have been selected. Moreover a search for lowest electronic states has also been performed among those electronic states dominated by electronic configurations which (at re ) do not correlate with the ground state of the isolated atoms. The lowest electronic state was a 5Δu, the De corresponding to this state is 1.500 eV which is only 62% of the experimental value. The possibility of a metal–metal multiple bond in Rh2 has also been considered by studying the lowest 1Σ+ electronic state. This state was found to lie higher than all the quintuplets considered in this work.
The bending vibrational levels in C2D2 (X̃ 1Σ+g)
Authors: Huet T. R. , Herman M. , Johns J. W. C.
Published: July 2002

Improved sets of data have been recorded and analyzed in order to study the bending rovibrational energy levels of C2D2, in the regions of the ν5–ν4 difference band (FIR) and of the ν5 fundamental band (IR). A matrix model is setup to deal with a Darling–Dennison coupling between sets of bending levels as well as with the usual l-type resonances. A simultaneous fit of the new data together with earlier microwave measurements produces an extended set of very accurate parameters, including for the first time in C2D2 s45 and qkt.
Theoretical prediction of ultrahigh vibrational excitation using infrared picosecond pulse trains: Coherent absorption of several photons each of a different frequency
Authors: Dibble Bridget Gourley , Shirts Randall B.
Published: July 2002

Using an IR picosecond pulse train, ultrahigh vibrational excitation in a Morse oscillator is achieved. Following a suggestion by Paramonov and Savva, pulse-train parameters can be chosen such that within 100 pulses (about 20 ps) there is significant excitation into the eighth excited vibrational state. We improve on their work by (1) using physically realistic matrix elements and potential parameters; (2) using Floquet theory to more efficiently perform the numerical calculation; (3) discussing the molecular excitation sensitivity to pulse-train parameters: pulse spacing, pulse width, central laser frequency, and peak laser fluence; and (4) showing that the vibrational excitation is a coherent multicolor, multiphoton process.
Why doesn’t PVF2 form a 31 helix?
Authors: Zhang Renshi , Taylor P. L.
Published: July 2002

The possibility of the existence of a 31 helix as a stable phase for poly(vinylidene fluoride) is investigated on the basis of intrachain and interchain interactions. The stable phase for PVF2 at low temperatures is well known to be the α phase, with a conformation of tgtg′, even though a calculation of the intrachain energy of a hypothetical helix leads to a value lower than that of the α phase. In this paper we discuss how the interchain energy acts to make the α phase the more stable conformation. Ingredients in this calculation include the compactness of chain packing and the results of imposing the condition of commensurability on the chain structure.
Avoiding truncation errors in Hartree–Fock theories of periodic systems
Authors: Raynor S.
Published: July 2002

A simple ab initio procedure involving Madelung-like correction terms to the integrals is derived for the energy and Fock matrix elements in 1D, 2D, and 3D Hartree–Fock models of infinitely periodic systems. Applications to ionic and nonionic linear systems show that convergence is greatly enhanced with this approach. In addition, an efficient scheme for the truncation of the lattice sums is discussed and shown to provide maximum convergence with minimum investment in multicenter integral evaluation.
Laser induced clustering in thiophenol
Authors: Kollia Z. , Zafiropulos V. , Fotakis C. , Stockdale J. A. D.
Published: July 2002

Results are reported of laser−induced clustering in mixtures of thioph vapor with nitrogen. The formation of micron size was observed. (AIP)
Inversion of gas–surface scattering data for potential determination using functional sensitivity analysis. I. A case study for the He–Xe/C(0001) potential
Authors: Ho Tak-San , Rabitz Herschel
Published: July 2002

A general iterative inversion procedure based on functional sensitivity analysis is presented for determining the gas–surface interaction potential from low energy elastic scattering data. Formally, Tikhonov regularization, singular function analysis, and a recently developed exact transformation technique are implemented to render the inversion stable and efficient. Specifically, the simulation of helium scattering from a rigid periodic xenon monolayer on the graphite (0001) face is considered. It is found that the functional sensitivity densities of the diffraction intensities with respect to the He–Xe/C(0001) potential contain profound information, thus are invaluable in guiding the inversion of scattering data to yield the potential. Although, unequivocal determination of the full three-dimensional potential from the inevitably incomplete experimental data may be difficult, we demonstrate that simulated input data consisting of a finite number of polar scan specular intensities can be used to accurately recover the underlying He–Xe/C(0001) potential. The recovered potential has been obtained without imposing any explicit functional form on the potential per se. The resulting procedure is quite promising for treating real laboratory data.
External magnetic field effects on the β band emission of NO
Authors: Sumitani M. , Abe H. , Nagakura S.
Published: July 2002

Magnetic field effects on the β band chemiluminescence of NO were studied for three cases in which the excited state (B 2Π) was produced through different reaction processes. The chemiluminescence from the v′=0 level was quenched for the three cases by a magnetic field. The magnetic field effect upon that from the v′=2 level was dependent on the reaction processes. No effect was observed for that from the v′=4 level. These findings can be explained in terms of the magnetically induced interaction between the B 2Π state and the a 4Π or b 4Σ state.
A Born–Oppenheimer-type separation for the treatment of the dynamics of heavy + light-heavy systems
Authors: Kubach C. , Vien G. Nguyen , Richard-Viard M.
Published: July 2002

The dynamics of heavy + light-heavy systems are investigated using a Born–Oppenheimer-type separation between the light and the heavy nuclear motions. The method is illustrated using the IH+I system with the extended LEPS hypersurface A of Manz and Romëlt. Hydrogenic wave functions are obtained for clamped iodine nuclei and provide potential energy curves and couplings that govern the I–I motion. Two objectives are pursued, namely, the study of the H-atom exchange process in the IH (v=0)+I collision at thermal energy and the study of the dissociation of the I2H quasimolecule that follows the photodetachment of the IHI− anion. The proposed approach allows for an understanding of the origin of bound states and resonances involved in these processes. Simple calculations of total cross sections for the H exchange reaction show that they exhibit a strong dependence upon the rotational state of the reactant IH molecule. The widths of quasibound states of I2H are calculated and the rotational levels of the IH molecule that result from the corresponding dissociation are determined. The present results confirm an analysis of experimental data by Weaver et al. Detailed comparisons with the best to date calculations of Schatz show the power of the proposed method which promises to be very useful for full quantal 3D calculations of the dynamics of heavy+light-heavy systems.
The Hall mobility of excess electrons in n-pentane–neopentane mixtures
Authors: Itoh Kengo , Holroyd Richard , Nishikawa Masaru
Published: July 2002

Measurements of the Hall and drift mobilities are reported for mixtures of n-pentane and neopentane up to 30% n-pentane and for the temperature range 20–150 °C. Under these conditions the Hall mobility is approximately equal to the drift mobility. The results suggest that the electron remains quasifree. Changes in the mobility are attributed to enhanced scattering of quasifree electrons.
Surface photochemistry of divalent metal alkyls on SiO2
Authors: Shaw Ping S. , Sanchez Esaul , O’Neill James A. , Wu Zhen , Osgood R. M.
Published: July 2002

Metal-alkyl molecules adsorbed on chemically prepared silicon surfaces have been studied using infrared total-internal reflection and ultraviolet transmission spectroscopies, temperature-programmed desorption spectroscopy (TPD), and mass spectroscopy of laser induced desorption (LID) products. For chemisorbed species, the surface hydroxyl groups have been shown to be the prime adsorption sites. In addition, we determined the identity of the chemisorbed species and their fragments under ultraviolet photodissociation. Surface photodissociation was observed to be strongly wavelength dependent at 193 and 248 nm.
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o study of the electronic magnetic circular dichroism spectrum in acetylene: The B̃←X̃ and 1 1B2←X̃ transitions
Authors: Jensen James O. , Adams George F. , Chabalowski Cary F.
Published: July 2002

This study utilizes a newly implemented method based on first-order perturbation theory for calculating the electronic magnetic circular dichroism (MCD) spectra in molecules. As an initial application, the MCD band maxima have been calculated for the B̃(1 1Bu)←X̃(1 1Ag) and 1 1B2←X̃(1 1A1) electronic transitions in the trans and cis bent conformations (respectively) of acetylene. The band intensity is assumed to come entirely from the ℬ0 term in the MCD equations of Stephens, which explicitly includes a first-order perturbation correction to the two electronic states involved in the transition. The wave functions are determined using ab initio quantum chemical techniques including state averaged CASSCF and multireference CI. There has been speculation that the 1 1B2←X̃ band system might overlap the B̃←X̃, and be part of the reason for the diffuse nature of the spectrum in the 185–170 nm region. This study considers this claim. The current calculations predict MCD band maxima for the 0–0 and 1–0 vibrational bands in the B̃←X̃ to be Δεmax0=−3.48 and −5.82, respectively, while experiment gives −0.8 and −1.6. This is 103 times larger than the largest band maximum [Δεmax0(3–0)=−0.0037] calculated for the 1 1B2←X̃ transition. This study also finds the absorption oscillator strengths for the B̃←X̃ to be a factor of 60 larger than that found in the 1 1B2←X̃. Thus while these results do predict both the B̃←X̃ and 1 1B2←X̃ transitions to lie in the same spectral region, they do not support the hypothesis that the 1 1B2←X̃ is a major contributor to either the absorption or MCD intensity in the 185–170 nm region.
Hard-cube analysis of the steric effect in molecule–surface scattering
Authors: Ionov Stanislav I. , Bernstein Richard B.
Published: July 2002

A modified hard-cube model is employed to account for the recent experimental results of the Bernstein group on steric effects in the scattering of polyatomic molecules on a graphite (0001) surface. In the low surface temperature (Ts) limit simple analytical formulas are obtained for Ts dependences of the probability of direct scattering, the average postcollisional translational energy and the average tangent of the scattering angle. The model accounts for the surface temperature dependences of the scattering probability and the angular distributions of direct scattering over the whole surface temperature range. It also gives the experimentally observed functional dependence of the steric effect in scattering probability vs the surface temperature, but fails to give quantitative agreement.
Quantum chemistry without diagonalization: An extension of the Lanczos method to molecules and solids
Authors: Riedinger Roland , Benard Marc
Published: July 2002

We implement here the recursion method and its extensions to the case of nonorthogonal bases (Riedinger et al., 1989) for determining the electronic structure of molecules of intermediate size. The recursion method, which is a variant of the Lanczos method, has been introduced by Heine, Haydock, and Kelly in solid state physics during the 1970s. It provides an invaluable tool for studying the electronic structure of solids and amorphs in direct space, without explicit diagonalization, in the case of orthogonal bases. Applications with nonorthogonal bases are made on the decavanadate ion (V10O28)6−, described within the extended Hückel model. Our extension of the recursion method may even be applied to molecules of large size or to infinite systems, where the usual methods of quantum chemistry fail. It applies to crystals as well as to glasses and molecules, for which no translational symmetry exists.
Theoretical study of the radiative properties of the triplet states of the NH radical: Transition dipole moments, radiative lifetimes, photodissociation cross sections
Authors: Kirby Kate P. , Goldfield Evelyn M.
Published: July 2002

Ab initio transition dipole moments between the X 3Σ− and the A 3Π states of NH and the 2 3Σ− and 2 3Π dissociative states have been computed. These transition dipole moments have been used to compute photodissociation cross sections and interstellar photodissociation rates for NH. Photodissociation rates for NH in the interstellar radiation field range from 1.9 to 4.7×10−10 s−1 depending on the field used. Direct dissociation via the 2 3Σ− and 2 3Π states is found to be the only important pathway for photodestruction of NH in diffuse interstellar clouds. A large photodissociation cross section is found for the A 3Π–2 3Σ− transition and a method for photolyzing NH in the laboratory is suggested. Einstein A coefficients and radiative lifetimes have also been computed for the A–X transition and are compared with recent theoretical and experimental work.
Thermal diffusion factors for equimolar He–Ar from 80 to 640 K and equimolar He–Xe from 140 to 270 K
Authors: Hurly J. J. , Taylor W. L. , Menke D. A.
Published: July 2002

New experimental measurements of the thermal diffusion factor are presented for the He–Ar and He–Xe system which serve to better define the temperature dependence of αT, particularly at low temperatures. The present results and all previously published data were used to obtain correlation functions accurate to within a few percent. Parameters for a Hartree–Fock dispersion (HFD)-type potential with individually damped dispersion terms were derived allowing accurate theoretical calculations to be made for transport properties, scattering cross sections, and the second virial coefficient.
Influence of molecular reorientational processes on the circular intensity difference scattering by optically active media in dc electric field
Authors: Woźniak S. , Kielich S.
Published: July 2002

The influence of reorientational processes on circular intensity difference scattering (CIDS) in a static electric field E0 is analyzed for systems of optically active dipolar molecules with the point group symmetries 3, 4, 6, and ∞ and of optically active nondipolar molecules with the symmetries 32, 422, and 622 for three experimental configurations: (i) at E0 perpendicular to the plane of scattering; (ii) E0 parallel to the incident light beam; and (iii) E0 parallel to the wave scattered in the direction of observation. The effect is described in terms of molecular reorientation functions and is dependent on the experimental field configuration, the electro-optical properties of the molecules, and the field strength E0. Numerical calculations of molecular reorientation functions are performed for arbitrary degrees of electric saturation. The effects are accessible to observation in solutions of macromolecules by advanced laser and optoelectronic techniques.
Ion pairing effects in poly(propylene glycol)–salt complexes as a function of molecular weight and temperature: A Raman scattering study using NaCF3SO3 and LiClO4
Authors: Schantz S. , Torell L. M. , Stevens J. R.
Published: July 2002

The symmetric stretch modes for the anions in poly(propylene glycol) (PPG) complexed with NaCF3SO3 and LiClO4 at an ether oxygen of alkali metal cation ratio of 16:1 have been studied as a function of the molecular weight of PPG and temperature using Raman spectroscopy. The splitting of these modes has been analyzed in terms of ‘‘free’’ anions and ion pairs. We have observed that the number of ion pairs increases with increasing molecular weight and increasing temperature. The effect is greater in the PPG–NaCF3SO3 complex than in the PPG–LiClO4 complex. We have found a simple exponential relationship for the number of free anions in a master-plot representation using a shift factor T* which increases with increasing molecular weight. These results are discussed in terms of the entropy and energetics of polymer–salt systems.
Proton transfer in symmetric and weakly asymmetric H bonds
Authors: Vodná Ľubica
Published: July 2002

Microscopic theory of proton transfer in a symmetric and weakly asymmetric H-bond complex within some larger molecular system is formulated. The potential of the proton motion is assumed to be the double well one and a two-level approximation is used. The proton motion in the double well potential is taken to be strongly coupled to the end atom vibration of the H bond, which is subsequently coupled to other degrees of freedom forming a heat bath. The solution of the problem by means of Liouville equation and projection operator technique allows us to show (at least on this model) the connection between the rate constant formulation and the microscopic theory.
Conserved quantities and the effect of external input on the decay of density fluctuations in steady state reaction–diffusion systems
Authors: Molski Andrzej
Published: July 2002

Using the fluctuation dissipation theory developed by Keizer a study is made of the long-time asymptotics of the particle number correlation function, Gij(r,t)=〈δρi(r,t)δρj(0,0)〉 (i,j=A,B), for steady states of diffusion mediated reactions with external random input. For the reaction A+B→P we find a power decay, Gij(r,t)∝t−ν, with the exponent ν whose value depends on the type of input and the dimensionality of the system d. In the case of an uncorrelated input ν=1/2 in three spatial dimensions. When particles are added locally in pairs ν=d/2. For the reaction 2A→P the correlation function decays exponentially fast. These results are discussed in terms of the existence of a quantity which is conserved by the reaction and the stochastic properties of the input process.