bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical exponent for the viscosity of four binary liquids
Authors: Berg Robert F. , Moldover Michael R.
Published: July 2002

We have measured the viscosity of four binary mixtures near their consolute points: (1) methanol+cyclohexane, (2) isobutyric acid+water, (3) nitroethane+3-methylpentane, and (4) 2-butoxyethanol+water. The viscosity data are consistent with the power-law divergence: η∼‖T−Tc‖−y, with an apparent viscosity exponent in the range 0.0404<y<0.0444. Recent theoretical estimates for y are near 0.032, which is outside the experimental range. The value of y is independent of whether the critical point is an upper or a lower consolute point and of whether the approach toward Tc is at constant pressure or at constant volume. Our torsion oscillator viscometer is unique in its simultaneous low frequency (∼1 Hz) and low shear rate (∼0.1 s−1), allowing its use close to the critical point before encountering non-Newtonian fluid behavior associated with critical slowing down. Nevertheless, we find quantitative evidence for viscoelasticity near the critical point.
Adiabatic and diabatic potential energy surfaces for collisions of CN(X 2Σ+, A 2Π) with He
Authors: Werner Hans-Joachim , Follmeg Bernd , Alexander Millard H.
Published: July 2002

The interaction potential energy surfaces for CN(X 2∑+, A 2∏)+He have been computed from ab initio MCSCF and MCSCF-CI wave functions using an extensive basis set. In the presence of the He atom the two degenerate components of the CN 2∏ state split into wave functions of A′ and A″ symmetry, and the symmetry of the 2∑+ state reduces to A′. The two adiabatic potentials for the A′ states are transformed to a diabatic basis, which yields a fourth potential energy surface V1, describing the collision-induced electrostatic coupling between the two A′ states. The degree of mixing of the two diabatic A′ states has been determined by integration of the relevant nonadiabtic coupling matrix elements and, in a simpler method, from the coefficients of the MCSCF configurations. Both procedures yield virtually identical results. The nonadiabatic coupling matrix elements are strongly peaked near the CN bond distance at which the X 2∑+ and A 2∏ states cross in the isolated molecule. The diabatic coupling potential V1, however, is only weakly dependent on the CN bond distance, and decreases exponentially with the CN–He separation. Near the classical turning points for room temperature collisions the magnitude of V1 is approximately 50 cm−1. The V1 potential shows a bimodal character as a function of the collision angle θ. These results are discussed in connection with recent experiments of Dagdigian and co-workers.
Bonding and stabilities of small silicon clusters: A theoretical study of Si7–Si1
Authors: Raghavachari Krishnan , Rohlfing Celeste McMichael
Published: July 2002

A b initio calculations have been performed to study the structures and energies of intermediate-sized silicon clusters (Sin, n=7–10). All geometries have been optimized at the Hartree–Fock (HF) level of theory with the polarized 6-31G* basis set. The harmonic vibrational frequencies have been evaluated at the HF/6-31G* level of theory. Electron correlation effects have been included by means of fourth order Mo/ller–Plesset perturbation theory. The most stable structure for Si7 is a pentagonal bipyramid and the lowest energy calculated structures for Si8–Si10 correspond to capped octahedral or prismatic geometrical arrangements. The evolution of the cluster geometries with increasing size is discussed. Clusters containing four, six, seven, and ten atoms have been identified as ‘‘magic numbers’’ for small silicon clusters, both theoretically and experimentally. The hybridization and bonding in small silicon clusters is discussed. Our results are used to interpret the recent photoelectron spectra of negative silicon cluster ions.
Approximately diabatic states from block diagonalization of the electronic Hamiltonian
Authors: Pacher T. , Cederbaum L. S. , Köppel H.
Published: July 2002

In many cases of interest there is only a small number of electronic states which interact with each other through the nuclear motion and are well-separated energetically from the other states. The aim of this work is to find a nearly diabatic (quasidiabatic) representation for the coupled states in which their coupling becomes small. Block diagonalization of the electronic Hamiltonian can accomplish this goal. It is achieved by a unitary transformation T which is uniquely determined by a ‘‘least action principle’’ which demands that T ‘‘does not do anything but block diagonalization.’’ By the help of the transformation we arrive at a decoupling of the relevant states from the rest and have to deal with a small matrix Hamiltonian. We have investigated in detail this block diagonalization procedure and succeeded in calculating the matrix elements of the total Hamiltonian in the new basis in closed form, in a nonperturbative way. Qualitative criteria are given to decide in which cases the method can successfully be applied to obtain quasidiabatic states. Particular attention is paid to the derivative couplings within the block of the interacting states. We have calculated that part of these couplings present in the adiabatic basis which will be transformed away and also the residual couplings which remain in the quasidiabatic basis after the transformation. The general results are exemplified on two model examples, describing a Σ–Π coupling in a linear molecule and a pseudo-Jahn–Teller effect, respectively. In addition, we briefly discuss the applicability of the proposed method in ab initio calculations on the example of the ethylene dication.
Gaussian-1 theory: A general procedure for prediction of molecular energies
Authors: Pople John A. , Head-Gordon Martin , Fox Douglas J. , Raghavachari Krishnan , Curtiss Larry A.
Published: July 2002

A general procedure is developed for the computation of the total energies of molecules at their equilibrium geometries. Ab initio molecular orbital theory is used to calculate electronic energies by a composite method, utilizing large basis sets (including diffuse-sp, double-d and f-polarization functions) and treating electron correlation by Mo/ller–Plesset perturbation theory and by quadratic configuration interaction. The theory is also used to compute zero-point vibrational energy corrections. Total atomization energies for a set of 31 molecules are found to agree with experimental thermochemical data to an accuracy greater than 2 kcal mol−1 in most cases. Similar agreement is achieved for ionization energies, electron and proton affinities. Residual errors are assessed for the total energies of neutral atoms.
Ionization energies of OH−3 isomers
Authors: Ortiz J. V.
Published: July 2002

Structures for hydride–water, hydroxide–H2 and double-Rydberg isomers of OH−3 are optimized at the MBPT(2)/6-311++G(d,p) level. While the first two isomers have nearly equal total energies, the double-Rydberg isomer is 1.68 eV less stable. Vertical ionization energies of the isomers are calculated with electron propagator theory and a 6-311++G(2d,2p) basis augmented with extra diffuse functions. The result for the hydride–water complex, 1.50 eV, is in excellent agreement with a recent photoelectron experiment, while the value for the hydroxide–H2 structure, 2.27 eV, is substantially different. Calculations are performed on a double-Rydberg anion with a pyramidal, C3v structure, yielding a vertical ionization energy of 0.43 eV. Corresponding neutral structures are optimized and provide adiabatic ionization energies. Harmonic vibrational frequencies are calculated for the anionic structures.
Dissociation of methanol and ethanol activated by a chemical reaction or by light
Authors: Satyapal Sunita , Park Jeunghee , Bersohn Richard , Katz Benjamin
Published: July 2002

When energized sufficiently either vibrationally or electronically, ROH (where R is methyl or ethyl) can dissociate to form H atoms and RO radicals. We have determined the translational energy release (〈ETr 〉=0.82Eavl ) and angular distribution (β=−0.60±0.03) from the laser induced fluorescence spectra of H atoms produced in the 193 nm photodissociation of CD3OH. We have also determined that the quantum yield for producing H from CD3OH is 0.86±0.10. In contrast, the reaction of O(1D)+CH4 which produces vibrationally excited CH3OH, has a quantum yield for producing H atoms of roughly 0.25 with only 22% of the available energy released as translation. We conclude that although the total available energy is the same in both cases, the dissociation of photoexcited methanol is prompt whereas the dissociation of chemically activated methanol shows some degree of internal vibrational equilibration.
Chemical probes of metal cluster ionization potentials
Authors: Parks E. K. , Klots T. D. , Riley S. J.
Published: July 2002

A procedure is described for the determination of metal cluster ionization potentials (IPs) using available excimer laser lines that gives error limits substantially smaller than traditional bracketing experiments. It is based on the observation that the adsorption of ammonia on cluster surfaces lowers cluster IPs, and that the IP lowering is linear in the number of adsorbed NH3 molecules. By determining the minimum number of NH3 molecules needed for ionization by the various excimer lasers, an approximation to the dependence of IP on coverage can be deduced. Extrapolation of this dependence to zero coverage gives the bare cluster IPs. Results are presented for clusters of iron, cobalt, and nickel having from 4 to 100 atoms. The effect of molecular adsorption on cluster IPs is analyzed theoretically, and the comparison with experimental results used to estimate the effective dipole moment of NH3 molecules adsorbed on these clusters. Comparison of the bare cluster IPs with the simple spherical drop model suggests that for transition metal clusters the Fermi level can be a significant function of cluster size.
The effects of phase transitions, surface diffusion, and defects on surface catalyzed reactions: Fluctuations and oscillations
Authors: Vlachos D. G. , Schmidt L. D. , Aris R.
Published: July 2002

Kinetic oscillations in catalytic reactions are often attributed either to attractive absorbate–absorbate interactions between adatoms or to required vacant sites for chemical reaction and are usually described by ordinary differential equations. These mechanisms are examined as possible causes for oscillations in surface reactions using Monte Carlo simulations. Self-sustained oscillations driven by a phase transition on the catalyst surface are found. It is shown that spatial inhomogeneity of the surface controls the existence of oscillations which therefore cannot be modeled using ordinary differential equations. The problem of synchronization of numerous oscillators on the surface is investigated. In particular, the influence of adatom migration and surface defects on Hopf bifurcation is examined. It is demonstrated that sites on surface with strong binding energy result in more chaotic oscillations whose amplitude decreases. The influence of concentration and distribution of defects on the kinetic oscillations is discussed.
Crossed molecular beams study of the reaction D+H2→DH+H at collision energies of 0.53 and 1.01 eV
Authors: Continetti R. E. , Balko B. A. , Lee Y. T.
Published: July 2002

This paper reports the first product differential cross section (DCS) measurements for the D+H2→DH+H reaction as a function of laboratory (LAB) scattering angle with sufficient resolution to resolve product DH vibrational states. Using a D-atom beam produced by the photodissociation of DI at 248 nm, product velocity and angular distributions were measured at 12 LAB angles at a nominal collision energy of 0.53 eV and at 22 LAB angles at a nominal collision energy of 1.01 eV with a crossed molecular beams apparatus. After correction of the raw product time-of-flight (TOF) spectra for modulated background, a comparison with recent exact quantum mechanical scattering calculations was made using a Monte Carlo simulation of the experimental conditions. The simulation showed that although the theoretical predictions agree qualitatively with the measurements, some significant discrepancies exist. Using the Monte Carlo simulation, a best-fit set of DH(v,j) DCS’s which showed good agreement with the measurements was found. At the detailed level of the state-to-state DCS, significant differences were observed between theory and experiment for rotationally excited DH(v, j) products. The discrepancies observed suggest that some regions of the current ab initio H3 potential energy surfaces, particularly the bending potential at high energies, may need further examination.
Following steepest descent reaction paths. The use of higher energy derivatives with a
b i
n
i
t
i
o electronic structure methods
Authors: Page Michael , Doubleday Charles , McIver James W.
Published: July 2002

New methods of determining the steepest descent reaction path in dynamical calculations based on the reaction path Hamiltonian are investigated. These methods use first and second energy derivatives and are correct to third order in the Taylor series expansion of the path in the arc length. The reaction H+H2O→H2+OH using small basis set three electron-three active orbital CASSCF wave functions with first and second analytically calculated energy derivatives is used as a test platform. Reaction paths using a number of different step sizes are computed and their accuracy determined by evaluating the deviations of the computed path curvature from the exact curvature and the RMS deviation of the computed transverse frequencies from the exact values, criteria more sensitive and more relevant to dynamical studies than are geometrical parameters. The new methods are compared to one another in computational efficiency and accuracy. For this example reaction and level of theory it is found that, in addition to being more accurate at any step size, the new methods are less expensive at any accuracy than those that utilize only first energy derivatives.
Examination of the 2A′2 and 2E″ states of NO3 by ultraviolet photoelectron spectroscopy of NO−3
Authors: Weaver A. , Arnold D. W. , Bradforth S. E. , Neumark D. M.
Published: July 2002

The photoelectron spectrum of the NO−3 anion has been obtained at 266 and at 213 nm. The 266 nm spectrum probes the 2A′2 ground state of NO3. The 213 nm spectrum represents the first observation of the 2E″ lowest-lying excited state of NO3. The 2A2 band shows vibrational progressions in the ν1 symmetric stretch and the ν4 degenerate in-plane bend of NO3. Our analysis of this band indicates that the NO3 ground state has a D3h equilibrium geometry and is vibronically coupled to the 2E′ second excited state via the ν4 mode. We also obtain the electron affinity of NO3, 3.937±0.014 eV, and the heat of formation of NO3 at 298 K, 0.777±0.027 eV (17.9±0.6 kcal/mol). The 2E″ state of NO3 lies 0.868±0.014 eV above the ground state. The 2E″ band shows complex and extensive vibrational structure. Several possible assignments of this structure are discussed.
Surface structure and thermal motion of n-alkane thiols self-assembled on Au(111) studied by low energy helium diffraction
Authors: Camillone Nicholas , Chidsey Christopher E. D. , Liu Gang-yu , Putvinski T. M. , Scoles Giacinto
Published: July 2002

Low energy helium diffraction has been used to study the packing and thermal motion of the terminal CH3 groups of monolayers of n-alkane thiols self-assembled on Au(111)/mica films and a Au(111) single crystal surface. At low temperatures (<100 K), the terminal CH3 groups are arranged in domains containing a hexagonal lattice with a lattice constant of 5.01 Å. As the length of the carbon chain is shortened, an abrupt decrease in the diffraction peak intensities is observed for CH3(CH2)9SH/Au(111)/mica, and no diffraction is observed for CH3(CH2)5SH/Au(111)/mica. This is indicative of a sudden decrease in surface order at around ten carbon atoms per chain. A semi-quantitative estimation of the average domain size of each monolayer surface shows a maximum of 46 Å at intermediate chain length [CH3(CH2)13SH/Au(111)/mica], decreasing to 26 Å at longer [CH3(CH2)21SH/Au(111)/mica] and 41 Å at shorter [CH3(CH2)9SH/Au(111)/mica] chain lengths. No phase transitions could be detected at the surfaces of these monolayers from 35 K to 100 K, but as expected for a soft material, the thermal motion of the n-alkane thiol molecules increases with increasing surface temperature and reduces the diffraction intensities to zero at around 100 K. The relative mean square displacements of the surface CH3 groups along the directions perpendicular and parallel to the surface have been calculated from the temperature dependence of the diffraction peak intensities using the standard Debye–Waller formalism. The measured values are in good agreement with the results from a recent molecular dynamics simulation. [J. Hautman and M. Klein, J. Chem. Phys. 93, 7483 (1990).]
The small shear rate response of electrorheological suspensions. I. Simulation in the point–dipole limit
Authors: Klingenberg D. J. , van Swol Frank , Zukoski C. F.
Published: July 2002

The electrorheological (ER) response is defined as the rapid and reversible change in the rheological properties of nonaqueous suspensions due to the application of large electric fields [∼O(1 kV/mm)]. Orders of magnitude increases in suspension viscosities at small shear rates are commonly observed, and are believed to be due to the field-induced formation of fibrous structures. A molecular dynamics-like simulation technique is developed to investigate the ER response at small shear rates. The suspensions are modeled as monodisperse suspensions of hard, dielectric spheres contained in a Newtonian fluid between parallel plate electrodes, and subjected to electrostatic and hydrodynamic forces. The results predict a dynamic yield stress with a concentration dependence that agrees well with experimental results. The magnitudes of the simulated stresses are smaller than the experimental values, a result of approximations in the model. This issue is addressed in the second paper of this series.
Charge transport in disordered molecular solids
Authors: Borsenberger P. M. , Pautmeier L. , Bässler H.
Published: July 2002

Hole mobilities have been measured in vapor deposited films of 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) and TAPC-doped bisphenol-A-polycarbonate (BPPC). Over an extended range of temperatures, the mobilities decrease with increasing field at low fields. At high fields, a log μ∝E1/2 relationship is observed with a slope that approaches zero at high temperatures. The results are described within the framework of the disorder transport formalism. By comparison of the experimental results with Monte Carlo simulations, we show that the observed behavior is a signature of the simultaneous presence of diagonal and off-diagonal disorder. Agreement between simulation results and experiment is excellent. Generalizing these results provides a framework for determining the magnitude of the relevant diagonal and off-diagonal disorder parameters from an analysis of mobility measurements.
Dynamics of the oxidation of CO on Pt(111) by an atomic oxygen beam
Authors: Mullins C. B. , Rettner C. T. , Auerbach D. J.
Published: July 2002

We have determined the angular and velocity distributions of the CO2 product of the interaction of gas-phase O atoms with CO adsorbed on Pt(111) at 90 K. The desorption intensity is found to be strongly peaked along the surface normal, falling approximately as cos12 θf with increasing final angle, θf. The velocity distributions are found to be highly non-Boltzmann with mean energies of ∼0.6 eV, decreasing slightly with increasing θf.
Room temperature Raman scattering studies of external and internal modes of solid nitrogen at pressures 8≤P≤54 GPa
Authors: Schneider H. , Häfner W. , Wokaun A. , Olijnyk H.
Published: July 2002

Raman spectra of solid nitrogen were studied from 8 to 54 GPa at room temperature. Between 8 and 16 GPa three out of four possible lattice modes of the δ–N2 phase were observed. Above 16 GPa up to eight lattice modes can be distinguished, seven of which correspond to lattice modes observed at 15 K in the ε–N2 phase. Branching of existing lattice and vibronic modes above 20 GPa and above 40 GPa indicate additional slight structural modifications. The observed pressure shift of vibrons of the 14N–15N isotopic species appears to favor factor group effects instead of bond weakening to be responsible for the downward shift of one vibronic mode at higher pressures.
Equilibrium and nonequilibrium solvation and solute electronic structure. III. Quantum theory
Authors: Kim Hyung J. , Hynes James T.
Published: July 2002

The electronic structure of a solute in a polar and polarizable solvent depends on the nonequilibrium (or equilibrium) state of the solvent. Here we present a theory for this phenomenon, at the level of a dielectric continuum description of the solvent, characterized by an orientational polarization Por and an electronic polarization Pel. The entire range of electronic coupling between solute electronic states is considered. The present theory supersedes, in important respects, our earlier work [H. J. Kim and J. T. Hynes, J. Phys. Chem. 94, 2737 (1990); J. Chem. Phys. 93, 5194 (1990); 93, 5211 (1990)] by including a full quantization of Pel; this is a feature recently shown in a model study for weak electronic coupling [J. N. Gehlen, D. Chandler, H. J. Kim, and J. T. Hynes, J. Phys. Chem. (to be published)] to be necessary for, e.g., a correct description of electron transfer activation free energies and transition states. The quantization of Pel is effected via a coherent state formulation, coupled with a multiconfiguration self-consistent representation of the solute-Pel wave function. Nonequilibrium free energies and solute electronic structure are found and depend explicitly on the comparative time scales of a transferring electron in the solute and of Pel. The corresponding equilibrium relations are also found.

For activated electron transfer, the current theory recovers (and generalizes) the conventional expression for the activation free energy, in both the electronically nonadiabatic and adiabatic regimes, for small electronic coupling. For larger electronic coupling—but still within the regime of activated electron transfer—the theory predicts an increased activation free energy compared to conventional results, associated with the (partial) solvation by the electronic polarization of delocalized solute electronic structure of the transition state. This trend is the same as that previously reported by us, but is smaller in magnitude due to the finite time scale of the transferring electron. The same is true for even stronger electronic coupling, characteristic of delocalized complexes, SN1, SN2, and proton transfers. Our previous predictions of novel spectroscopic aspects for delocalized complexes are confirmed.
Energy transfer studies on N2(X 1Σ+g,v) and N2(B 3Πg)
Authors: Piper Lawrence G.
Published: July 2002

We have developed a novel discharge–flow technique for studying the quenching of N2(X 1Σ+g,v″≥5) and N2(B 3Πg) by a variety of molecules. The technique involves adding small number densities of N2(A 3Σ+u) to a flow of N2(X,v) producing, thereby, N2(B). By comparing N2(B) fluorescence intensities generated when a quencher is added to the N2(X,v) flow 2–3 ms before N2(A) addition with intensities observed after the N2(X,v) and quencher have been mixed for times of 10–30 ms, we can separate the effects of N2(B) fluorescence quenching from effects of N2(X,v) quenching. Our results indicate that CH4, CO2, CO, O2, and N2O quench N2(B) at rates approaching gas kinetic while H2, N2, SF6, and CF4 are about ten times slower. Rate coefficients for N2(X,v″≥5) quenching by H2 and N2 are on the order of 10−15 cm3 molecule−1 s−1, those for CO2, CH4, and CF4 roughly an order of magnitude faster, and CO and N2O yet another order of magnitude faster.
Wave packet correlation function formulation of scattering theory: The quantum analog of classical S-matrix theory
Authors: Tannor David J. , Weeks David E.
Published: July 2002

A novel time-dependent quantum mechanical formulation of scattering theory is developed which is well suited for the calculation of individual S-matrix elements. Wave packets corresponding to well-defined reactant and product channel quantum numbers are propagated in to the interaction region using Mo/ller operators, the former forward in time and the latter backwards in time. The S-matrix element Sβα(E) is then simply related to the Fourier transform at energy E of the time-dependent correlation function between the two wave packets in the interaction region. The symmetric treatment of reactants and products allows the entrance and exit channel dynamics to be performed highly efficiently using different coordinate systems and different interaction representations. As a result, the formulation is expected to provide an improved route to the calculation of S-matrix elements using quantum mechanical, as well as semiclassical propagation methods. The new formulation combines elements of classical S-matrix theory, the wave packet correlation formulation of spectroscopy, and quantum transition state theory, and should be a good starting point for a variety of new approximations to and interpretations of individual S-matrix elements.
Femtosecond experiments and absolute rate calculations on intervalence electron transfer of mixed-valence compounds
Authors: Tominaga Keisuke , Kliner Dahv A. V. , Johnson Alan E. , Levinger Nancy E. , Barbara Paul F.
Published: July 2002

New experimental results and theoretical calculations are reported for the optical electron transfer (ET) and subsequent reverse radiationless ET for the molecules (NH3)5RuIIINCRuII(CN)−5 and (NH3)5FeIIINCRuII(CN)−5. A procedure is presented for extracting many of the key parameters in ET theory from a combined analysis of resonance Raman data and the optical absorption ET band shape. Using these parameters, ET rates have been calculated using contemporary ET models. The experimental and theoretical rates agree within the uncertainty of the theoretical predictions, which results from an uncertainty in some of the parameters. The results demonstrate that inertial solute/solvent interactions and intramolecular sources of fast energy fluctuations play an important role in ultrafast ET kinetics for these compounds.
Electric field effects in the near-threshold photoionization spectrum of nitric oxide
Authors: Pratt S. T.
Published: July 2002

Zero-kinetic-energy–photoelectron spectroscopy (ZEKE-PES) and time-of-flight mass spectrometry are used to investigate the near-threshold, two-color photoionization of NO via the A 2Σ+, v=0 level. Pulsed-field ionization of Rydberg states within 20 cm−1 of the ionization threshold is shown to proceed via a diabatic mechanism. Particular emphasis is given to the effect of predissociation on the signal produced by delayed field ionization. A small (∼3 V/cm) dc electric field is shown to produce a dramatic decrease in the field ionization signal in both the NO+-ion and ZEKE-electron channels. This decrease is thought to be due to an increase in the predissociation rate caused by the dc electric field. The implications of these experiments for mass-analyzed threshold ionization are discussed.
Vibrational relaxation in simple fluids: Comparison of theory and simulation
Authors: Tuckerman M. , Berne B. J.
Published: July 2002

General theoretical expressions for the dephasing and energy relaxation times of a stiff oscillator in simple fluids are derived from the GLE and a critical discussion of the dynamic processes in these systems is given. In addition new methodological aspects of stochastic and full molecular dynamics simulations are discussed. The new reversible integrator based on the Trotter factorization of the classical propagator is used to directly simulate the vibrational energy and phase relaxation of a stiff classical oscillator dissolved in a Lennard-Jones bath. We compare the ‘‘real’’ relaxation from full MD simulations with that predicted by Kubo theory and by the generalized Langevin equation (GLE) with memory friction determined from the full molecular dynamics. It is found that the GLE gives very good agreement with MD for the vibrational energy relaxation, even for nonlinear oscillators far from equilibrium. The dephasing relaxation is also well approximated by the GLE.
Time-dependent photodissociation of methyl iodide with five active modes
Authors: Hammerich Audrey Dell , Manthe Uwe , Kosloff Ronnie , Meyer Hans-Dieter , Cederbaum Lorenz S.
Published: July 2002

Advances in the time propagation of multidimensional wave packets are exploited to present the A-band photodissociation dynamics of methyl iodide for five active vibrational modes on the three relevant excited ab initio potential surfaces. The five modes considered represent all of the experimentally observed dynamical activity. The only modes neglected are the asymmetric C–H stretch and the asymmetric deformation of the methyl group. The kinetic energy operator corresponding to these five degrees of freedom is derived. The fully quantum mechanical calculation was implemented upon grids using 2880 distinct time-dependent configurations, determined by the multiconfigurational time-dependent Hartree algorithm, for each electronic state. All of the currently known experimental results regarding the umbrella vibration, symmetric C–H stretching vibration, perpendicular rotation, and parallel rotation of the photodissociated methyl radical fragment are well reproduced. The full wavelength dependence of all of these quantities is determined. The wavelength dependence of the energy deposited into translational, vibrational, and rotational motion is also given. The time evolution of the modes is presented in the context of correlated motion and its effect upon the dissociative process. Many of the details of the dynamics inherent to the conically intersecting nature of the excited surfaces is delineated. In particular it is shown that the Jahn–Teller distortion of the 1Q1 state is irrelevant in contributing to the perpendicular character of resonance Raman depolarization ratios. Results are compared and contrasted to previous calculations employing the collinear pseudotriatomic model with optimized empirical surfaces or the bent pseudotriatomic model with the same ab initio surfaces.
Ultra-low temperature kinetics of neutral–neutral reactions: New experimental and theoretical results for OH+HBr between 295 and 23 K
Authors: Sims I. R. , Smith I. W. M. , Clary D. C. , Bocherel P. , Rowe B. R.
Published: July 2002

The first determination of the rate of reaction of OH radicals with HBr at temperatures below 249 K is reported. Rate constants measured from 295 to 23 K increase monotonically with decrease in temperature and are faster than has previously been thought at the temperatures present in the mid and low stratosphere. The observed negative temperature dependence is well predicted by a simple formula deduced from quantum scattering calculations employing the rotating bond approximation.
Coupled-cluster theory employing approximate integrals: An approach to avoid the input/output and storage bottlenecks
Authors: Rendell Alistair P. , Lee Timothy J.
Published: July 2002

By representing orbital products in an expansion basis, certain classes of two-electron integrals are approximated for use in CCSD(T) calculations (singles and doubles coupled-cluster plus a perturbational estimate of the effects of connected triple excitations). This leads to a very large reduction in disk storage and input/output requirements, with usually only a modest increase in computational effort. The new procedure will allow very large CCSD(T) calculations to be undertaken, limited only by available processor time. Using the molecular basis as the expansion basis, explicit numerical comparisons of equilibrium geometries, harmonic frequencies, and energy differences indicate that the error due to the use of approximate integrals is less than the error associated with truncation of the molecular basis set.
A photoluminescence study of poly(phenylene vinylene) derivatives: The effect of intrinsic persistence length
Authors: Gettinger C. L. , Heeger A. J. , Drake J. M. , Pine D. J.
Published: July 2002

We report the results of light scattering, absorption, excitation, and emission spectroscopy of three polyphenylene vinylene (PPV) derivatives; poly[2-methoxy, 5-(2′-ethyl-hexyloxy-p-phenylene- vinylene] (MEH-PPV), poly[2-butoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV), and poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV) in solution with p-xylene. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the polyphenylene vinylene backbone and that this change in stiffness has dramatic effects on the photoluminescence of polyphenylene vinylene. We have determined the luminescence quantum efficiencies of the polyphenylene vinylene derivatives relative to a known standard, Rhodamine 6G, and find that the photoluminescence can be greatly enhanced by increasing the intrinsic stiffness of the polymer backbone. The stiffest polymer, poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV), has a quantum efficiency of 0.66±0.05. The quantum efficiency decreases to 0.22±0.05 for poly[2-butoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV) and 0.20±0.05 for poly[2-methoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene] (MEH-PPV), the most coiled derivative. Excitation profiles of the three derivatives also show an increase in nonradiative decay at high energies when the polymer assumes a more coiled comformation. Thus, the quantum yields are dependent on pump energy.
FeCn− and FeCnH− (n=3,4): A photoelectron spectroscopic and density functional study
Authors: Fan Jiawen , Lou Liang , Wang Lai-Sheng
Published: July 2002

Photoelectron spectra of the title molecules are reported at 3.49 eV photon energy. Vibrational structures are resolved in the spectra of FeC−3 and FeC3H−. The FeC−4 spectrum is unusually broad, indicating a large equilibrium geometry change from the anion to the neutral states. The FeC4H− spectrum exhibits a single strong feature. Theoretical studies using the density functional theory are carried out to determine the structures and bonding of these clusters. All the molecules in the anion ground states are found to be linear with the Fe atom bonded at one end. The Fe and C bonding involves strong Fe 4s and C sp interactions as well as considerable Fe 3d and C π interactions. The n=3 species can be best characterized by cumulenic types of bonding with FeC3H also having an acetylenic isomer. The n=4 species in the linear structures can be approximately described by diacetylenic types of bonding. Mulliken charge analyses indicate that the extra charge in all the anions enters mainly into the Fe 4s antibonding orbital, in agreement with the assignment that the threshold detachment takes place from the σ* orbital mainly between the Fe and C atoms. The vibrational structure resolved in the FeC−3 spectrum yields a Fe–C stretching frequency of 700 (150) cm−1 for the first excited state of FeC3, in agreement with the Fe–C multiple bonding character.
Chemical bonding in water clusters
Authors: Lee Chengteh , Chen Han , Fitzgerald George
Published: July 2002

Different sizes of water clusters from a dimer to twenty water molecules are studied using density functional theory. The binding energies of water clusters are calculated, and a relationship in terms of a simple function has been found between binding energy and the size of the water clusters. The interpolation of this correlation function reproduces the binding energies for the other water clusters to an accuracy within 1 kcal/mol. The extrapolation of the function gives the binding energy, −11.38 kcal/mol, which agrees very well with the experimental binding energy of ice, −11.35 kcal/mol. We also find small water clusters composed of mainly planar four membered rings to be more stable, implying the existence of magic numbers for water clusters with sizes of 4, 8, and 12.
Quantum adsorption dynamics of a diatomic molecule on surface: Four-dimensional fixed-site model for H2 on Cu(111)
Authors: Dai Jiqiong , Zhang John Z. H.
Published: July 2002

We presented a detailed quantum dynamics study for dissociative adsorption of H2 at different sites of Cu(111) surface to investigate the effect of surface corrugation and site specificity. The theoretical study employed a four-dimensional (4-D) ‘‘fixed-site’’ model, in which the lateral coordinates (X,Y) of the center of mass of the diatom are fixed at the impact site, but the remaining four degrees of freedom are explicitly treated in quantum calculations. The inclusion of the azimuthal angle φ in the present 4-D model is a significant step forward in theoretical studies beyond the 3-D ‘‘flat surface’’ model. This 4-D ‘‘fixed-site’’ model allows us to investigate explicitly the local corrugation effect that was not possible using the 3-D flat-surface model. We incorporated the latest ab initio data of Hammer et al. in constructing the LEPS potential energy surface, which gives the lowest dissociation barrier over the bridge site. 4-D dynamics calculations are performed in the present study to mimic a normal incidence of H2 at three symmetric sites on Cu(111): bridge, atop, and center sites with the corresponding rotation symmetries. Our results show that a hydrogen impact at a high symmetry site (six-fold atop site) shows little corrugation effect while impact at low symmetry site (two-fold bridge site) shows a large corrugation effect. In particular, our calculation shows that the inclusion of surface corrugation preserves the strong rotational orientation effect observed in flat-surface model calculations. The effect of homonuclear symmetry persists at high symmetry atop site, and to a lesser degree at a low symmetry bridge site. The contour plot of the wavefunction in the current 4-D model shows explicitly that hydrogen atoms following the dissociation of H2 over the bridge site do not settle at the neighboring center site, but migrate to the next available center site. Our study demonstrated that the 4-D fixed-site model is very useful in investigating surface corrugation and molecule site specificity in model-surface reactions.
The structure and dynamics of CO2 on NaCl(001) studied by helium atom scattering
Authors: Lange G. , Schmicker D. , Toennies J. P. , Vollmer R. , Weiss H.
Published: July 2002

The structure and dynamics of physisorbed carbon dioxide on in situ cleaved single crystal sodium chloride surfaces was studied by means of elastic as well as inelastic helium atom scattering. At Tsurface=80–83.5 K the diffraction patterns indicate a commensurate (2×1) monolayer superstructure on the (001) plane of the substrate, the unit cell containing a glide plane. This is in agreement with results obtained from low energy electron diffraction and infrared spectroscopy. In time-of-flight experiments single phonon low-energy loss and gain features were observed which can be attributed to acoustic and optical modes. Two higher-energy features are probably due to the first combination modes observed by helium atom scattering so far. The growth of solid CO2 adsorbed on NaCl(001) was also studied.
Perturbative treatment of the similarity transformed Hamiltonian in equation-of-motion coupled-cluster approximations
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

A series of size-consistent approximations to the equation-of-motion coupled cluster method in the singles and doubles approximation (EOM-CCSD) are developed by subjecting the similarity transformed Hamiltonian H̄=exp(−T)H exp(T) to a perturbation expansion. Attention is directed to N and N−1 electron final state realizations of the method defined by truncation of H̄ at second order. Explicit spin–orbital equations for the energy and its first derivative are documented for both approaches [EOMEE-CCSD(2) and EOMIP-CCSD(2), respectively], and have been implemented in a large-scale quantum chemistry program. Vertical ionization potentials calculated by EOMIP-CCSD(2) are shown to be equivalent to those of an approach presented recently by Nooijen and Snijders [J. Chem. Phys. 102, 1681 (1995)]. Applications of both EOMIP-CCSD(2) and EOMEE-CCSD(2) provide results for final state properties that compare favorably with those obtained in full EOM-CCSD calculations. Analysis of the computational aspects of the approximate and full EOM-CCSD methods shows that the cost of EOMIP-CCSD(2) energy and gradient calculations scales in proportion to the fifth power of the basis set size, a significant savings over the sixth power dependence of EOMIP-CCSD. This feature is of great practical importance, as it shows that this N−1 electron final state approach has a large domain of applicability and is therefore likely to become a valuable tool for application calculations. On the other hand, the same cannot be said for EOMEE-CCSD(2) since its asymptotic computational dependence and storage requirements are the same as the full EOMEE-CCSD method.
Hydrodynamic calculation of the frequency dependent friction on the bond of a diatomic molecule
Authors: Mishra B. , Berne B. J.
Published: July 2002

In this paper molecular hydrodynamics is used to calculate the dynamic friction on the intramolecular vibrational coordinate of a homonuclear diatomic molecule dissolved in a simple liquid. The predicted dynamic friction is then compared to recent computer experiments. Agreement with the experimental dynamic function is obtained when the linearized hydrodynamics is modified to include Gaussian viscoelasticity and compressibility. The hydrodynamic friction on the bond appears to agree qualitatively very well, although quantitative agreement is not found at high frequencies. Various limits of the hydrodynamic friction are discussed.
Photofragment μ-v-j correlation measured by 1+n′ resonance-enhanced multiphoton ionization: Selective probing of bipolar moments and detection of chiral dynamics
Authors: Uberna Radoslaw , Hinchliffe Robert D. , Cline Joseph I.
Published: July 2002

A practical procedure is described to measure photofragment μ-v-j correlations using polarized 1+n′ resonance-enhanced multiphoton ionization with a time-of-flight mass spectrometer detector. Following the theory of Dixon [R. N. Dixon, J. Chem. Phys. 85, 1866 (1986)], the correlations are expressed as the moments of a bipolar harmonic expansion of the correlated angular distribution of photofragment velocity and angular momentum (v and j) about the parent molecule transition dipole, μ. At a fixed detection geometry and on a single rotational transition, polarization control of the dissociating or probing light permits selective determination of targeted moments of the bipolar harmonic expansion. The velocity-dependent spherical tensor moments of the angular momentum distribution depend upon these bipolar moments and are given for a general experimental geometry and for general elliptical polarization of the probing light. Several practical experimental geometries are described that isolate and measure targeted bipolar moments. The bipolar moments that can be measured using elliptical probe polarizations are described. A method for analyzing the symmetries of dissociation dynamics is proposed and used to identify the moments unique to the dynamics of chiral systems.
Approximately extensive modifications of the multireference configuration interaction method: A theoretical and practical analysis
Authors: Szalay Péter G. , Bartlett Rodney J.
Published: July 2002

The extensivity error of configuration interaction (CI) is well understood and unlinked diagram corrections must be applied to get reliable results. Besides the well known a posteriori Davidson-type corrections, several methods attempt to modify the CI equations a priori to obtain nearly extensive results, while retaining the convenience of working in a configuration space. Such unlinked diagram corrections are particularly important for multireference cases for which coupled-cluster (CC) calculations, which require a many-body, integral-based calculation, are more difficult. Several such multireference methods have been presented recently, ranging from the multireference linearized coupled cluster method (MR-LCCM), averaged coupled pair functional (MR-ACPF), through various quasidegenerate variational perturbation theory (QD-VPT), MR-coupled electron pair method (MR-CEPA) to size-consistent, self-consistent, selected CI [(SC)2SCI]. We analyze all of these methods theoretically and numerically, paying particular attention to the new multireference averaged quadratic CC method (MR-AQCC), and demonstrate its comparative quality of performance even when using small references spaces. We consider several demanding molecular examples that benefit from a multireference description, like bond stretching in H2O; N2 and C2; the insertion of Be into H2; and the singlet–triplet splitting in CH2. We also investigate the extensivity error.
Size-consistent approach and density analysis of hyperpolarizability: Second hyperpolarizabilities of polymeric systems with and without defects
Authors: Nakano Masayoshi , Shigemoto Isamu , Yamada Satoru , Yamaguchi Kizashi
Published: July 2002

Various size-consistent approaches to the calculation of molecular hyperpolarizabilities are analyzed based on the double perturbation theory. General equations for the nth-order response property with respect to an external time-independent field are derived on the basis of the Rayleigh–Schrödinger perturbation theory (RSPT) and the coupled-cluster (CC) theory. The corresponding equations for the time-dependent case are also derived by the CC formalism, which is referred to as the time-dependent CC (TDCC) method. In order to clarify the spatial characteristics of polarizability and hyperpolarizability, we present an analysis method using a new concept ‘‘the polarizability and hyperpolarizability densities.’’ As an application of the size-consistent methods, the static second hyperpolarizabilities (γ) of π-conjugated polymeric systems are calculated by the use of the uncoupled (UCHF), and coupled-Hartree–Fock (CHF) methods combined with the semiempirical INDO approximation. Characteristics of γ values calculated for regular polyenes, solitonlike polyenes and donor(D)–acceptor(A) disubstituted polyenes are investigated, particularly in relation to the chain-length effect. Further, we employ γ density analysis method, in which the third derivatives of the Mulliken charge densities against applied electric fields are plotted for exploring the local contributions of the constituent atoms to γ values. Results for the finite polyenes are also extrapolated to an infinity of the chain length to predict the intrinsic γ values per unit carbon–carbon (CC) bond of polymeric chains. Furthermore, the CHF+Mo/ller–Plesset second-order perturbation (MP2) method in the PPP approximation is applied to polymeric systems with larger chain length in order to elucidate the electron correlation effects on the chain-length dependencies of γ values.