bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical calculation of the electroabsorption spectra of polyacene crystals
Authors: Petelenz Piotr , Slawik Michal/ , Yokoi Kohei , Zgierski Marek Z.
Published: July 2002

A two-dimensional analog of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal is applied for the calculation of the electroabsorption (EA) spectra of polyacene crystals. The approach is essentially nonempirical, with most of the necessary input data estimated either from theoretical calculations or from independent experiments. Good quantitative reproduction of the experimental EA spectra is achieved, both in their absolute amplitude and intensity pattern. The large amplitude of the Frenkel exciton part of the spectra is successfully accounted for without the necessity to invoke anomalously large molecular polarizabilities. Some basic assumptions of previous analyses are shown to be invalid and future prospects of the new approach are discussed.
Zero-field-splitting and π-electron spin densities in the lowest excited triplet state of oligothiophenes
Authors: Bennati M. , Németh K. , Surján P. R. , Mehring M.
Published: July 2002

The electronic properties of thiophene oligomers (nT, n=2–8) have been investigated in the lowest excited triplet state. Theoretical calculations of the zero field splitting parameters and of the π-electron spin density have been performed and compared with previous experimental EPR results. The calculations are based on a simple π-electron (one-electron-per-site) model including electron–electron interaction at the extended Hubbard level. Optimized bond lengths result from making them self-consistent to the corresponding bond orders via Coulson’s relationship. The calculated D values decrease from D=0.0959 cm−1 for n=2 to D=0.0597 cm−1 for n=8, in agreement with EPR data. The measured as well as the calculated E values are rather small. Furthermore, we found that ZFS parameters are affected by the torsion angles between the thiophene rings. The chain length dependence of D can be rationalized comparing π-electron spin density calculations and computed bond length distortions. These clearly indicate that the triplet excitation reaches a finite extension over about four thiophene rings.
High resolution ionization-detected Raman gain spectroscopy of N2 and C6H6
Authors: Leuchs Matthias , Crew Marshall , Harrison James , Hineman Max F. , Nibler Joseph W.
Published: July 2002

Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N2 the photoionization step was via a 2+2 resonantly enhanced multiphoton ionization process using the a1Πg(v=2)←X1Σg(v=1) two-photon resonance. The detection limit is 103–104 times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1+1 ionization step permitted measurements at state densities less than 1010 cm−3 and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active ν16 fundamental.
The high-pressure range of the reaction of CH(2Π) with N2
Authors: Fulle D. , Hippler H.
Published: July 2002

The pressure dependent thermal rate constant of the reaction of CH(2Π) and N2 has been studied from 200 to 715 K at total pressures between 1 and 150 bar of helium. The CH radicals have been generated using multiphoton laser flash photolysis of CHClBr2 or CHBr3 at 248 nm and detected by saturated laser induced fluorescence (SLIF). At 200, 250, 300, 400, and 500 K falloff curves have been constructed and the high pressure limit rate constant has been determined to be k1,∞=(4.1±0.8)10−11 (T/300 K)−0.15 cm3 molecule−1 s−1. At higher temperatures thermal decomposition of the CHN2 adduct has been observed and the equilibrium constant derived by analyzing the concentration decays. By third law analysis the equilibrium constant has been evaluated with a reaction enthalpy ΔH°R (0 K)=−(97±10) kJ mol−1. Our results are compared with recent calculations of the potential energy surface (PES) and other experimental data at low pressures as well as shock tube studies. The high-pressure limiting rate constants are treated in terms of statistical reaction rate theory. A simple kinetic model has been developed to describe the measured rate constants in an extended pressure (10−3–150 bar) and temperature range (200–3500 K).
The importance of correlation effects on the bonding of atomic oxygen on Pt(111)
Authors: Illas Francesc , Rubio Jaime , Ricart Josep Manel , Pacchioni Gianfranco
Published: July 2002

The adsorption properties and the bonding nature of atomic oxygen adsorbed on the three-hollow sites of Pt9 and Pt25 cluster models of the Pt(111) surface have been studied by means of ab initio wave functions. Correlation effects have been introduced by various methods: single and multireference second-order perturbation theory, multireference configuration interaction, and density functional corrections to the Hartree–Fock energy using a variety of expressions of density functionals. While the adsorption and vibrational properties of adsorbed oxygen are sufficiently well described at the Hartree–Fock level, the adsorption energy is completely wrong as the system is unbound in the one-electron approximation while it is strongly bound at the correlated level. The use of correlation functional based on the Hartree–Fock density is not sufficient to obtain a correct estimate of the strength of the bond. We will also show that the bonding of oxygen to Pt has a partial ionic character and the effect of correlation is to enhance the importance of covalent contributions to the bonding.
Solvation effects on a model SN2 reaction in water clusters
Authors: Re Mario , Laria Daniel
Published: July 2002

We present a series of molecular dynamics experiments for the nucleophilic substitution reaction Cl−+CH3Cl→ClCH3+Cl− taking place in liquid simple point charge water nanoclusters containing 6, 16, and 32 solvent molecules at temperatures close to 200 K. A three-dimensional potential energy for the reagent interatomic interactions is employed. Equilibrium and dynamical aspects of the reactive process are investigated. Solvation effects lead to significant enhancements of the computed free energy barriers even in aggregates containing only six water molecules. The equilibrium spatial and orientational correlations describing the changes in the solvation structure along the reaction path are also presented. The reactive/product states are characterized by a fully solvated Cl− ion embedded within the cluster while the CH3Cl remains on the surface; at the transition state, the complex lies at the cluster surface adopting a linear geometry tangential to the cluster boundary. We have also monitored the time relaxation of the solvation structures as the system evolves from the transition to the stable product states. Our results show the reaction proceeds by a series of highly coordinated motions involving the different components of the reagent. Estimates for the rate constants in clusters of different sizes are also computed using the reactive flux correlation function formalism.
Exciplex vacuum ultraviolet emission spectra of KrAr: Temperature dependence and potentials
Authors: Subtil J.-L. , Jonin C. , Laporte P. , Reininger R. , Spiegelmann F. , Gürtler P.
Published: July 2002

The temperature dependence of the emissions from the 0+(3P1)and 1(3P2) Kr*Ar exciplex states in the range 85–350 K was studied using time resolved techniques, vacuum ultraviolet synchrotron radiation, and argon samples doped with minimal amounts of krypton. As the temperature is increased, the emission shifts to the blue, its width increases by almost a factor of 2, and the line shape becomes asymmetrical. The experimental line shapes have been simulated by means of Franck–Condon density calculations using the available ground state potential of Aziz and Slaman [Mol. Phys. 58, 679 (1986)] and by modeling the exciplex potentials as Morse curves. The potential parameters for the 0+ and 1 states are re=5.05±0.01 and 5.07±0.01 a0, respectively; De=1150±200 cm−1 and β=1.4±0.1 a0−1 for both states. The latter two values yield ωe=140 cm−1 and ωexe=4.3 cm−1. The energy positions of the exciplexes’s wells and their depths are compared with published results.
Generalization of the multiconfigurational time-dependent Hartree method to nonadiabatic systems
Authors: Museth Ken , Billing Gert Due
Published: July 2002

We present a generalization of the multiconfigurational time-dependent Hartree (MCTDH) scheme, originally introduced by Meyer, Manthe and Cederbaum [Chem. Phys. Lett. 165, 73 (1990)], to a general nonadiabatic system. In the course of deriving the extended working equations a new compact notation is introduced. Subsequently the equations of motion are applied to a one-dimensional two-surface model system. Calculated energy-resolved transition probabilities for the model system, treated in the MCTDH framework, are shown to be in exact agreement with direct numerically ‘‘exact’’ calculations, using a Split-operator propagation scheme. Finally a comparison is made between the convergence and the consumed CPU-time for the two methods. The two numerical formulations of the scattering problem employ, respectively, a DVR (discrete variable representations) and a FFT (fast Fourier transform) collocation scheme. We also comment on the use of negative imaginary potentials to remove artificial boundary effects in the two schemes.
Polarizable dipolar two-center Lennard-Jones fluids: Computer simulations and equation of state
Authors: Kriebel Christian , Winkelmann Jochen
Published: July 2002

Electrostatic contributions from permanent and induced dipoles have a considerable effect on the thermodynamic behavior of polar fluids. We derive a renormalized equation of state (1R-EOS) for the Helmholtz free energy of a polarizable dipolar two-center Lennard-Jones fluid, where for the first time polarizable dipolar interactions are combined with a nonspherical molecular shape. This hybrid EOS is constructed as a sum of the Helmholtz energy F2CLJ for a two-center Lennard-Jones (2CLJ) fluid and a renormalized electrostatic contribution Fel. Wertheim’s renormalization procedure for inductive interactions was applied to a recently developed equation of state for dipolar 2CLJ fluids. We demonstrate that this renormalization applies to a simulation based equation of state and we determine the influence of the polarizability on the Helmholtz energy of a polarizable dipolar 2CLJ fluid as a function of its molecular elongation. Predictions from this renormalized equation of state for thermodynamic properties of the homogeneous phase show good agreement with molecular dynamics simulation data.
Vibrational energy relaxation of adsorbate vibrations: A theoretical study of the H/Si(111) system
Authors: Ermoshin V. A. , Kazansky A. K. , Smirnov K. S. , Bougeard D.
Published: July 2002

The energy relaxation rate for the first excited stretching vibration of hydrogen atoms adsorbed on an Si(111) surface is calculated treating the motions of the H atoms quantum mechanically and computing the substrate phonon spectrum by molecular dynamics. The relaxation occurs through excitation of three bending modes and emission of a 188 cm−1 phonon.
Laser-induced fluorescence of the CH2CFO radical
Authors: Furubayashi Masashi , Bridier Isabelle , Inomata Satoshi , Washida Nobuaki , Yamashita Koichi
Published: July 2002

A new laser-induced fluorescence spectrum has been observed in the region of 307–335 nm. Since this spectrum is observed when reacting oxygen atoms with CH2CHF, or CH2CF2, or CH2CFCl and also by photolysis of CH3CFO, the fluorescing molecule is the CH2CFO (fluoroformyl methyl) radical. From an analysis of the laser-induced single vibronic level fluorescence, some of the vibrational frequencies can be assigned for the ground electronic state ν3=1724 cm−1 (C–O stretch), ν5=1211 cm−1 (C–F stretch), ν6=906 cm−1 (CH2 rock), ν7=847 cm−1 (C–C stretch), ν8=584 cm−1 (FCO bend), and ν9=416 cm−1 (CCO bend), for the excited state ν3=1790, ν5=1253, ν6=911, ν7=874, ν8=537, and ν9=421 cm−1. Ab initio calculations on the CH2CFO radical give a planar geometry with vibrational frequencies that are consistent with the observed fundamental frequencies. The vibrational frequencies show that the structure of the ground state is closer to fluoroformyl methyl (⋅CH2CFO) rather than a vinoxy-type (CH2=CFO⋅) radical. The collision-free radiative lifetimes of the excited state are 49–81 ns depending on excitation energy and vibrational modes. Strong predissociation is observed above v=1, especially in the ν3′ mode.
Accurate thermochemistry for larger molecules: Gaussian-2 theory with bond separation energies
Authors: Raghavachari Krishnan , Stefanov Boris B. , Curtiss Larry A.
Published: July 2002

Gaussian-2 (G2) theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, ΔHf0(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.
A new intermolecular potential for hydrazine clusters: Structures and spectra
Authors: Beu T. A. , Buck U. , Siebers J. G. , Wheatley R. J.
Published: July 2002

The structures of small hydrazine clusters from the dimer to the hexamer have been calculated using a standard site-site intermolecular potential and a newly developed systematic approach which is essentially based on monomer properties. Aside from the repulsive and the attractive dispersion and induction interaction special care is taken for the determination of the electrostatic interaction which is represented by a distributed multipole expansion and a penetration correction. Based on these potentials the vibrational spectra of the N-N stretching and the asymmetric NH2 wagging mode are calculated using degenerate perturbation theory. While the small shifts of the N-N stretching mode are fairly well reproduced by both potential models, large differences are predicted for the asymmetric NH2 wagging mode. Here, redshifts of –30 cm−1 are calculated for the standard and blueshifts of 100 cm−1 are obtained for the systematic potential in agreement with experiment. The analysis shows that the reason for this behavior is the careful treatment of the electrostatic term in this model.
Level-dependent damping in intermolecular vibrations: Linear spectroscopy
Authors: Farrer Richard A. , Loughnane Brian J. , Deschenes Laura A. , Fourkas John T.
Published: July 2002

A treatment of stimulated Raman intermolecular spectroscopy is presented that employs a Landau–Teller model of damping. This model incorporates a quantum-number dependence to population relaxation and pure dephasing, thereby introducing a specific temperature and frequency dependence into the damping in the intermolecular spectrum. Optical-heterodyne detected Raman-induced Kerr effect data obtained in CS2 and acetonitrile over a broad temperature range are shown to agree with the basic predictions of the model.
Dynamic solvent effects on the vibrational overtone dephasing in molecular liquids: Subquadratic quantum number dependence
Authors: Gayathri N. , Bhattacharyya S. , Bagchi B.
Published: July 2002

A theoretical study of vibrational dephasing of molecular vibrations in liquids is presented with an aim to understand the experimentally observed sub-quadratic quantum number (n) dependence of the vibrational dephasing rate, in systems like CH3I and CHCl3 and their deuterated analogues. The analysis is based on Oxtoby’s theory of vibrational dephasing but with a detailed microscopic description of the frequency dependent frictional forces on the vibrational mode. The friction on the normal coordinate in liquids is found to have a pronounced biphasic behavior with a dominant Gaussian initial component followed by a slow exponential-like relaxation. While the exponential relaxation usually assumed in Kubo’s stochastic theory leads to a quadratic n dependence of the dephasing rate, the biphasic friction is shown to give rise to the sub-quadratic n dependence. As the biphasic frictional response is expected to be a generic feature of the friction on any vibrational coordinate in dense liquids, the sub-quadratic quantum number dependence is predicted to be common to most ultrafast overtone dephasing. In addition, the calculated rates (without any adjustable parameter), are found to be in good agreement with the experimental results for the C-I stretching mode in liquid CH3I and for the C-H stretching in liquid CHCl3.
A least-action variational method for determining tunneling paths in multidimensional system
Authors: Taketsugu Tetsuya , Hirao Kimihiko
Published: July 2002

We propose a least-action variational method to determine the optimal tunneling paths in multidimensional polyatomic reactions on the basis of the minimum energy path (MEP) and the least motion path (LMP). Taking into account geometrical features of the MEP (i.e., variations of path tangent and path curvature or variations of reaction plane along the MEP), the tunneling path is determined to minimize the amount of exponential damping of the nuclear wave function in the passage through the classically forbidden region. As a demonstration, the method is applied to a polyatomic reaction, NH3+OH→NH2+H2O, in which there are highly curved regions on the MEP before and after a transition state. It is shown that the imaginary action integral calculated along the variationally determined tunneling path decreases extensively in comparison with that calculated along the MEP.
Interbranch line-mixing in CO2 (101) and (021) combination bands
Authors: Gillespie Walter D. , Meinrenken Christoph J. , Lempert Walter R. , Miles Richard B.
Published: July 2002

Absorption spectra from a mixture of 320 ppm CO2 in synthetic air (79% N2, 21% O2) were collected in the region from 3500 cm−1 to 4000 cm−1 under conditions in the range of 100–1000 atm and 295–900 K. At 295 K, both bands of the (1001), (0201) Fermi dyad show the collapse of P and R branches into a single nearly Lorentzian spectral feature as a result of interbranch line-mixing. At elevated temperatures, the presence of interbranch mixing is also clearly evident as is the presence of several hot bands. The experimental data are modeled using two methods for simulating line-mixed spectra; first, the usual line-by-line approach which relies on the binary impact approximation, and second, a simple band-averaged model proposed by Hartmann and L’Haridon [J. Chem. Phys. 103, 6467 (1995)]. The energy corrected sudden (ECS) approximation is used to generate the relaxation matrix in the first approach. Comparison with the measurement shows that the ECS method does not fit the high density data satisfactorily when adjustable parameters from the literature are used; the level of interbranch mixing must be decreased by about a factor of 2 relative to intrabranch mixing and at least 5% dephasing must be added to the ECS matrix. With these changes, the room temperature data are modeled satisfactorily, but significant discrepancies are still present in the high temperature spectra. On the other hand, the simpler band-averaged model does provide a reasonable estimate of the spectra for all temperatures when best fit values are used for mixing and broadening, but the low density data are not reproduced as well as with the ECS model. Data from high pressure absorption measurements in a 1% NO in N2 mixture as well as a 0.5% CH4 in N2 mixture are also presented without analysis, showing the effects of interbranch line-mixing in these spectra.
Statistical model for nonadiabatic decay of an exciplex strongly coupled to a dissociative continuum
Authors: Topaler Maria S. , Truhlar Donald G.
Published: July 2002

We present a statistical model for the decay of an exciplex that is strongly coupled to repulsive ground-state potential energy surface. The motion of the exciplex is assumed to be governed by the excited electronic state adiabatic potential surface, and transitions to the ground state are caused by nonadiabatic transitions at a seam of avoided crossing of two strongly coupled diabatic states underlying the adiabatic representation. The model assumes that motion in the exciplex is ergodic. The predictions of the model are tested against accurate quantum dynamics and trajectory surface hopping calculations for Na*+H2→NaH2*→Na+H2.
Vibrational contributions to the molecular first and second hyperpolarizabilities of a push–pull polyene
Authors: Kim Hyun-Soo , Cho Minhaeng , Jeon Seung-Joon
Published: July 2002

The vibrational contributions to the first and second hyperpolarizabilities of a push–pull polyene are theoretically studied by using the valence-bond charge-transfer model. The formal relationships between the vibrational and electronic contributions to the hyperpolarizabilities are derived. It is found that there exist strong correlations between the vibrational contributions and bond length alternation of a push–pull polyene. By calculating these contributions numerically, it is proven that the vibrational contributions, estimated from the IR, Raman, and hyper-Raman measurements, to the hyperpolarizabilities can also provide a measure of the nonlinear optical property.
On the molecular phase in coherent control
Authors: Lee Sungyul
Published: July 2002

Systematic analysis is carried out for molecular phase in (ω1,ω3) coherent control scheme. Two mechanisms for nonzero molecular phase are proposed. It is shown that the imaginary part of the three-photon transition operator, which is due to the contribution of the intermediate continuua, can give different phase factors for the one- and three-photon transition amplitudes, yielding very important effects of molecular phase in (ω1,ω3) coherent control. We also show that the interactions between the final continuum states can result in nonzero molecular phase, when the intermediate states are discrete. These findings are discussed in relation to Gordon and co-workers’ recent observations [Zhu et al., Science 270, 77 (1995)] on phase lag.
Electric field suppression of hopping charge recombination
Authors: Fedorenko S. G. , Burshtein A. I.
Published: July 2002

The theory of hopping reactions between charged particles in high-mobility non-polar liquids is essentially extended to account for the electric field effect on a bulk electron-ion recombination. It turns out that the rate constant of the hopping recombination decreases with the electric field contrary to what was obtained for diffusional recombination. Such a difference may serve for experimental discrimination between hopping and diffusional reaction mechanisms. Their interrelation is traced within a more general Torrey’s model of a solvated electron random walk with an arbitrary step length determined by the free electron diffusion. The coefficient of linear field dependence obtained for low fields in the contact approximation changes the sign in passing from a short mean step length (diffusional recombination) to a much larger mean step length (hopping recombination).
A theoretical study of the electronic structure and spectroscopic properties of the low-lying electronic states of the molecule SiB
Authors: Ornellas Fernando R. , Iwata Suehiro
Published: July 2002

Twenty two lowest-lying electronic states of the species SiB are described theoretically using the internally contracted multireference configuration interaction approach and natural orbitals generated from a state-averaged density matrix. Correlated consistent valence quadruple-zeta (cc-pVQZ) atomic functions are used in the expansion of the one-electron basis. Potential energy curves are presented for all the states as well as a description of the electronic structure characterizing the most relevant ones. Dissociation and excitation energies and dipole moment functions complete the electronic structure description. The solution of the radial nuclear equation allowed the determination of vibrational energies and vibrational and rotational constants. For the ground state (X 4Σ−), Re=1.918 Å and De=3.27 eV. The first excited quartet (A 4Π) is located 2.60 eV (Te) higher in energy and has a slightly shorter equilibrium distance (1.811 Å) and a smaller De, 1.00 eV; the transitions X 4Σ−−A 4Π are expected to fall in the visible (green–yellow) region. Higher-lying 4Π states show very noticeable changes in the potential function due to avoided crossings. Within about 1.5 eV from the ground state there are located five doublet states; the lowest one a 2Π, with Te=0.73 eV, Re=1.845 Å, and De=2.54 eV. For selected states, transition dipole moments, transition probabilities, and radiative lifetimes are also presented. Crossings of various states and the energetic closeness of others are expected to play non-negligible perturbative effects in the spectra. The global picture of the electronic states presented will certainly be an important aid to experimentalists in the spectroscopic investigation of this species.
Low-field hole mobility in a photorefractive polymer
Authors: Goonesekera Arosha , Ducharme Stephen , Takacs James M. , Zhang Lei
Published: July 2002

We present the time-of-flight measurements of hole mobility in a photorefractive polymer composite as a function of temperature and applied electric field. The analysis shows that the temperature dependence of the low-field mobility is in apparent disagreement with the predictions of the Gaussian disorder model and also with polaron models.
Total electron-scattering cross sections for CHF3, C2F6, C3F8, and c-C4F8
Authors: Sanabia Jason E. , Cooper Gregory D. , Tossell John A. , Moore John H.
Published: July 2002

The total electron-scattering cross section has been measured for 0–20 eV electrons incident upon CHF3, C2F6, C3F8, and cyclo-C4F8. The cross sections all have a broad maximum for electron energies near 9 eV, with more or less sharper peaks attributed to temporary-negative ion resonances at lower energies. For the linear molecules a resonance in the 3–6 eV range is associated with electron capture into the lowest unoccupied orbital. The shape of the cross section for the cyclic compound implies a stable ground-state negative ion.
Response to “Comment on ‘A study of HeI photoelectron spectroscopy on the electronic structure of the nitrate free radical NO3’ ” [J. Chem. Phys. 108, 1292 (1998)]
Authors: Wang Dianxun , Jiang Peng , Qian Ximei , Hong Gongyi
Published: July 2002

The information contents of the HeI photoelectron spectroscopy (PES) are emphasized first. The second band of the PE spectrum of the NO3 radical is attributed to electron ionization of the 1e″ orbital leading to the triplet state 3E″ of the NO3+ cation, and it is never corresponding to the excited state of the NO3 cation. A nonoverlapping between the second PES band of NO3 radical and the PES band centered on 13.02 eV for the NO2 is clearly seen from expanded PE spectrum recorded by the pyrolysis of pure N2O5. So a best PE spectrum of the NO3 radical was recorded by us until now.
Vibrational mode-selected differential scattering of NH3+ methanol (d1, d3, d4): Control of product branching by hydrogen-bonded complex formation
Authors: Fu Hungshin , Qian Jun , Green Richard J. , Anderson Scott L.
Published: July 2002

We report a study of vibrational mode effects and differential scattering in reaction of NH3+ with CD3OD, CD3OH, and CH3OD over the collision energy range from 0.1 to 5 eV. At low collision energies, abstraction of both methyl and hydroxyl D atoms is observed with roughly equal probability, even though methyl D-abstraction should be favored on both energetic and statistical grounds. Branching between the two abstraction reactions is controlled by two different hydrogen-bonded complexes. Formation of these complexes is enhanced by NH3+ umbrella bending, unaffected by the NH3+ symmetric stretch, and inhibited by collision energy. Endoergic proton transfer is mediated at low energies by a third hydrogen-bonded complex, formation of which is enhanced by both umbrella bending and the symmetric stretch. Charge transfer (CT) has a significant cross section only when the NH3+ umbrella bend excitation exceeds the endoergicity. Collision energy and symmetric stretching appear to have no effect on CT. At high collision energies all reactions become direct, with near spectator stripping dynamics. In this energy range product branching appears to be controlled by collision geometry and there are no significant vibrational effects.
Influence of hyperpolarizabilities on nonlinear Kerr effect relaxation responses
Authors: Déjardin J.-L. , Déjardin P.-M. , Kalmykov Yu. P.
Published: July 2002

The nonlinear effects arising from the first and second hyperpolarizabilities on the Kerr effect relaxation response of an ensemble of polar and anisotropically polarizable molecules in a strong dc electric field are studied by solving the infinite hierarchy of differential-recurrence relations underlying the noninertial Fokker–Planck (Smoluchowski) equation. The step-on solution is obtained by using the matrix continued fraction methods. An expression for the Kerr effect relaxation time is derived as well following the procedure of Kalmykov, Déjardin, and Coffey [Phys. Rev. E 55, 2509 (1997)] which is in full agreement with the matrix continued fraction expression. For small values of the electric field, it is shown that the first hyperpolarizability contributes to increasing the Kerr effect relaxation time while the second hyperpolarizability shifts the maximum of the relaxation time toward the lowest electric fields on increasing the value of this maximum. The dual nature of essentially two relaxation processes is also emphasized by plotting the Kerr effect relaxation spectra for various values of the first and second hyperpolarizability parameters.
Nonequilibrium unimolecular dissociation influenced by intramolecular vibrational energy redistribution
Authors: Katō Toshiko
Published: July 2002

Unimolecular dissociation rates of energized molecules influenced by the slow intramolecular vibrational energy redistribution (IVR) are formulated for no-barrier potentials of the reaction coordinate R. The master equation as to states projected on the reactive mode is presented and is solved by reducing the equation to an equivalent diffusion equation. An approximate solution for the steady state condition gives the generalized dissociation rate constant kD which is expressed as kD−1=kdiff−1+kde−1+kRRKM−1, where kdiff, kde, and kRRKM represent the internal energy diffusion rate constant, equilibrium barrier crossing rate constant by bound-continuum transitions, and the fragmentation rate constant corresponding to the flux which crosses the critical configuration R=R‡, respectively. The former two rates, which are due to IVR, are expressed by the transition kernel between states of the reactive mode, and the latter gives the RRKM rate which is valid in the rapid IVR limit. The rate limiting steps for various reactions are discussed.
Photo-induced chaos in the Briggs–Rauscher reaction
Authors: Okazaki Noriaki , Hanazaki Ichiro
Published: July 2002

Discovery of the photo-induced chaos in the Briggs–Rauscher system is reported. The chaotic oscillations were observed between the large- and the small-amplitude simple oscillatory states existent in low and high light intensity regions, respectively. Period-doubling sequence from the large-amplitude oscillations to the chaos was observed. Deterministic nature of the chaos was confirmed by the next-amplitude return map. The stretching and folding mechanism of the trajectories was revealed through the three-dimensional attractor reconstructed via the singular value decomposition method. The chemical origin of the photoinduced chaos is discussed based on the photoautocatalysis of HIO2.
Rigorous formula for the fast calculation of the electron repulsion integral over the solid harmonic Gaussian-type orbitals
Authors: Ishida Kazuhiro
Published: July 2002

A rigorous general formula for calculating the electron repulsion integral (ERI) over the solid harmonic (SH) Gaussian-type orbitals (GTOs) can be derived. A general algorithm can be obtained from this formula named as accompanying coordinate expansion (ACE) b3k3. This algorithm is capable of computing very fast SH-ERIs, especially for SH contracted GTOs. Numerical assessment is shown for the (LL|LL) class of SH-ERIs (L=2–5). It is found that the present ACE-b3k3 algorithm is the fastest among all algorithms in the literature in the floating-point-opration (FLOP) count assessment when the degree of contraction is large.
Shape distribution and correlation between size and shape of tetrahedral lattice chains in athermal and theta systems
Authors: Zifferer Gerhard
Published: July 2002

Chains embedded in the tetrahedral lattice have been produced by means of Monte Carlo simulation for athermal and for theta conditions. Nonreversal random walks (random walks without backfolding bonds) have been generated as a reference. Probability distributions of an asphericity factor δ*, of a prolatness factor S*, and of shape factors sfi* have been evaluated, the quantities being based on the orthogonal components of the squared radius of gyration taken along the principal axes of inertia. In addition, the correlation between δ* and other shape descriptors as well as between δ* and quantities characteristic of the size of configurations have been evaluated. In accordance with existing literature, the distributions H(δ*) and H(S*) are found to be very broad. The distributions H(sf1*) of the small and H(sf3*) of the large shape factor (sf1*⩽sf2*⩽sf3*, sf1*+sf2*+sf3*=1) are clearly distinct from each other, while the distribution of sf2* overlaps with that of sf1* and (slightly) with H(sf3*). Distributions of theta chains (“unperturbed” polymer) coincide fairly well with respective distributions of nonreversal random walks (which in turn are nearly identical to those of (off-lattice) random walks. As a matter of course, other shape descriptors are directly correlated with the asphericity factor δ*. Actually, for all systems evaluated, the global size of configurations is strongly correlated with δ* as well: The larger the asymmetry the larger are the dimensions of the configuration under consideration.
Hyperquantization algorithm. I. Theory for triatomic systems
Authors: Aquilanti Vincenzo , Cavalli Simonetta , De Fazio Dario
Published: July 2002

In this paper we present the theoretical concepts and methodology of the hyperquantization algorithm for the three body quantum mechanical problem. Within the framework of the hyperspherical approach to reaction dynamics, we use angular momentum algebra (or its generalization, e.g., including Hahn coefficients which are orthonormal polynomials on a set of grid points which span the interaction region) to compute matrix elements of the Hamiltonian operator parametrically in the hyperradius. The particularly advantageous aspects of the method proposed here is that no integrals are required and the construction of the kinetic energy matrix is simple and universal: salient features are the block tridiagonal structure of the Hamiltonian matrix and a number of symmetry properties. The extremely sparse structure is a further advantage for the diagonalization required to evaluate the adiabatic hyperspherical states as a function of the hyperradius. Numerical implementation is illustrated in the following paper by a specific example.
Long-time dynamics of de Gennes’ model for reptation
Authors: Barkema G. T. , Krenzlin H. M.
Published: July 2002

Diffusion of a polymer in a gel is studied within the framework of de Gennes’ model for reptation. Our results for the scaling of the diffusion coefficient D and the longest relaxation time τ are markedly different from the most recently reported results, and are in agreement with de Gennes’ reptation arguments: D∼N−2 and τ∼N3. The leading exponent of the finite-size corrections to the diffusion coefficient is consistent with the value of −2/3 that was reported for the Rubinstein model. This agreement suggests that its origin might be physical rather than an artifact of these models.
Low-energy dissociative electron attachment to CH2Br2, CHClBr2, and CCl3Br: Intermediate lifetimes and decay energetics
Authors: Parthasarathy R. , Finch C. D. , Wolfgang J. , Nordlander P. , Dunning F. B.
Published: July 2002

Dissociative electron attachment to the brominated methanes CH2Br2, CHClBr2, and CCl3Br is investigated by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity selected K(np) Rydberg atoms. The data are analyzed with the aid of a Monte Carlo collision code that models the detailed kinematics of the reactions. Measurements with CH2Br2 show that essentially all the excess energy of reaction appears in translation, indicating that the electron is captured directly into an antibonding orbital. Data for CHClBr2 show that reaction proceeds by two channels: in the first, essentially all the excess energy appears in translation; in the second, limited conversion of the excess energy to internal motions occurs, pointing to a CHClBr2−* intermediate lifetime of ∼0.1 to 1 ps. In the case of CCl3Br, electron transfer leads to the formation of both Br− and Cl− ions. Br− production is again associated with two channels: in one, essentially all the excess energy appears in translation; in the other, near statistical redistribution of the excess energy occurs prior to dissociation, pointing to a relatively long-lived (τ∼several ps) CCl3Br−* intermediate. Data for Cl− production suggest only limited energy transfer to internal motions. Possible reaction mechanisms for the various channels are discussed with the aid of ab initio calculations.
One- and two-dimensional ensemble quantum computing in spin Liouville space
Authors: Mádi Z. L. , Brüschweiler R. , Ernst R. R.
Published: July 2002

New concepts are described for nuclear magnetic resonance (NMR) implementations of spin ensemble quantum computing in one and two dimensions. Similarities and differences between ensemble and pure state quantum computing are discussed by using a Liouville space formalism based on polarization and single transition operators. The introduction of an observer spin, that is coupled to the spins carrying the quantum bits, allows a mapping of the states of a quantum computer on a set of transitions between energy levels. This is spectroscopically favorable compared to a mapping on the energy levels themselves. Two complementary parallelization schemes for quantum computing are presented: one exploits the parallel processing feature inherent in multidimensional NMR, while the other employs mixed superposition states represented by operators in Liouville space. The spin swap operation, introduced in this paper, allows a convenient extension of quantum computing to spin systems where not all spin–spin couplings are resolved. The concepts are illustrated by implementations of logic operations and identities consisting of a sequence of basic logic gate operations.
Simulation of volume polarization in reaction field theory
Authors: Chipman Daniel M.
Published: July 2002

In the reaction field theory of solvation, penetration of charge density outside the cavity nominally enclosing the solute leads to a volume polarization that contributes in addition to the commonly recognized surface polarization. In principle the exact volume polarization charge density is spread everywhere outside the cavity, but its effect can be closely and concisely simulated by a certain additional surface polarization charge density. Formal comparison is made to conductorlike screening models, and it is found that these improve on common approaches that neglect volume polarization by automatically including the simulation of volume polarization. A revised method to numerically determine this simulation is also described.
A multiconfigurational self-consistent field study of the thermal decomposition of methyl azide
Authors: Arenas Juan F. , Marcos Juan I. , Otero Juan C. , Sánchez-Gálvez Adelaida , Soto Juan
Published: July 2002

Thermal decomposition of methyl azide has been studied computationally by using the complete active space self-consistent field (CASSCF) method and Møller–Plesset theory using the CASSCF wave function as the zeroth-order wave function (CAS/MP2). The calculations have been performed in conjunction with the 6-31G* basis set. The reaction is predicted to occur in two steps via nitrene intermediate: (1) CH3N3→CH3N+N2; (2a) CH3N→H2+HCN, (2b) CH3N→H2CNH. The rate-limiting step is the N2 extrusion (1), being a competitive mechanism between a spin-forbidden path and a spin-allowed one. The calculated energy barrier height for both processes is found to be isoenergetic, ΔE=41 kcal/mol, where ΔE represents the difference between the energy at the minimum on the singlet state surface of methyl azide and the energy at the minimum energy crossing structure (ISC1) or the singlet transition state (TS1) for the spin-forbidden path and the spin-allowed one, respectively. The nitrene intermediate formed in step (1) can undergo two parallel reactions: (2a) decomposition in H2 and HCN, and (2b) isomerization to methyleneimine. H2 extrusion from the imine generated in step (2b) has been studied as well. Two high energy transition states have been found for 1,1-H2 and 1,2-H2 elimination from methyleneimine, respectively.
Electron-pair momentum densities of group 14, 15, and 16 atoms in their low-lying electronic states
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

Electron-pair intracule (relative motion) h̄(υ) and extracule (center-of-mass motion) d̄(P) densities in momentum space are studied for the 3P, 1D, and 1S terms of five group 14 atoms with p2 configurations, the 4S, 2D, and 2P terms of five group 15 atoms with p3 configurations, and the 3P, 1D, and 1S terms of five group 16 atoms with p4 configurations. Common to all fifteen atoms, the intracule densities show that a low energy term has a greater probability of finding a pair of electrons with a large relative momentum υ  than a high energy term. The Fermi hole effect in a high spin term appears naively in momentum space, and the average relative momentum 〈υ〉 is larger in a high angular momentum term. For the terms arising from the pm electronic configurations (m=2–4), the differences in the radial extracule densities are found to be almost isomorphic with the corresponding intracule ones. In a term with a high angular momentum, the average center-of-mass momentum 〈P〉 of an electron pair is always larger, and two electrons are less likely to have opposite momenta. The major origin of these differences in the electron-pair densities lies in the valence np orbitals, and the contribution of different electron-pair motions to the relative stability of terms can be explained by a rigorous relation between the kinetic energy Tnp and the second intracule 〈υ2〉np and extracule 〈P2〉np moments of the valence np subshell.
Adsorption of polydisperse polymer chains
Authors: Sear Richard P.
Published: July 2002

The adsorption of polydisperse ideal polymer chains is shown to be sensitive to the large N tail of the distribution of chains. If and only if the number of chains decays more slowly than exponentially is there an adsorption transition like that of monodisperse infinite chains. If the number decays exponentially the adsorption density diverges continuously at a temperature which is a function of the mean chain length. At low coverages, chains with repulsive monomer–monomer interactions show the same qualitative behavior.
Dynamic and transport properties of dissipative particle dynamics with energy conservation
Authors: Avalos J. Bonet , Mackie A. D.
Published: July 2002

Simulation results of the thermal conductivity ℒ of the Dissipative Particle Dynamics model with Energy Conservation (DPDE) are reported. We also present an analysis of the transport equations and the transport coefficients for DPDE based on a local equilibrium approximation. This approach is valid when the particle–particle thermal conductivity λ and the friction coefficient ζ are large. A qualitative derivation of the scaling form of the kinetic contribution of the transport of energy is derived, yielding two different forms for the kinetic contribution to the heat transport, depending on the value of λ. We find agreement between the theoretically predicted value for ℒ and the simulation results, for large λ and many particles interacting at one time. Significant differences are found for small number of interacting particles, even with large λ. For smaller values of λ, the obtained macroscopic thermal conductivity is dominated by diffusive transport, in agreement with the proposed scaling form.
Rotational spectrum and theoretical structure of the carbene HC4N
Authors: McCarthy M. C. , Apponi A. J. , Gordon V. D. , Gottlieb C. A. , Thaddeus P. , Daniel Crawford T. , Stanton John F.
Published: July 2002

Following a high-level coupled cluster calculation, the rotational spectrum of the bent HC4N singlet carbene was detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. The three rotational constants, the leading centrifugal distortion constants, and two nitrogen hyperfine coupling constants were determined to high accuracy. The rotational constants agree with those calculated ab initio to better than 0.5%. Like the isoelectronic C5H2 carbene of similar structure, HC4N was found to have fairly large centrifugal distortion and a large inertial defect. The calculated dipole moment of HC4N is 2.95 D.
Theoretical investigation of Jahn–Teller and pseudo-Jahn–Teller coupling effects on the photoelectron spectrum of allene
Authors: Mahapatra S. , Cederbaum L. S. , Köppel H.
Published: July 2002

The valence photoelectron spectrum of allene (C3H4) pertinent to the Ã 2E/B̃ 2B2 interacting electronic manifold of the corresponding cation (C3H4+) is theoretically calculated and compared with the most recent high resolution He I excited experimental recording of Baltzer et al.[Chem. Phys. 196, 551 (1995)]. A model diabatic Hamiltonian within the linear vibronic coupling scheme and ab initio calculated coupling parameters are employed in our investigations. While the resolved vibrational progressions in the photoelectron band at low energies can be attributed to the E⊗B Jahn–Teller activity within the Ã 2E electronic manifold, the diffuse structure in the photoelectron band at high energies is found to emerge from the mixing of the Ã 2E electronic manifold with the B̃2B2 electronic state via degenerate vibrational modes. The latter demonstrates the importance of the pseudo-Jahn–Teller type of interactions in the photoelectron spectrum of allene.
Infrared photodissociation spectra of the C–H stretch vibrations of C6H6+–Ar, C6H6+–N2, and C6H6+–(CH4)1–4
Authors: Dopfer Otto , Olkhov Rouslan V. , Maier John P.
Published: July 2002

Vibrational infrared photodissociation spectra of mass selected C6H6+–Ar, C6H6+–N2, and C6H6+–(CH4)1–4 ionic complexes are recorded in the spectral range of the C–H stretching vibrations. Transitions at 3095±15 cm−1 occur in all spectra and are assigned to C–H stretch fundamentals of the benzene cation in its E1g2 electronic ground state. In the case of the C6H6+–(CH4)1–4 complexes, additional transitions at 2904±7 and 3010±24 cm−1 are observed and attributed to the symmetric and antisymmetric C–H stretch vibrations of the CH4 ligands, ν1 and ν3. The deduced C–H stretching vibrations of C6H6+ in the E1g2 ground state are roughly 30 cm−1 higher than the corresponding frequencies in the A1g1 electronic ground state of the neutral species, indicating that the C–H bonds become stronger upon removal of an electron from the highest occupied e1g orbital of C6H6.