bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of the interaction of AlH(X 1Σ+,A 1Π) with Ar: Potential energy surfaces and bend–stretch levels of the ArAlH(X,A) van der Waals complex
Authors: Yang Moonbong , Alexander Millard H. , Gregurick Susan , Dagdigian Paul J.
Published: July 2002

Multireference, configuration interaction potential energy surfaces (PES’s) are reported for the interaction of Ar with the AlH radical in its ground (X 1Σ+) and first singlet excited (A 1Π) electronic states. To assist in the assignment of the van der Waals bend–stretch vibrational quantum numbers in the accompanying experimental spectroscopic study of the ArAlH complex [E. Hwang and P. J. Dagdigian, J. Chem. Phys. 102, 2426 (1995)], slightly modified versions of these PES’s have been employed to calculate the vibrational energy levels of the ArAlH(X,A) van der Waals complex, both within the adiabatic bender model and in full variational calculations. Additionally, the band strengths for electronic transitions out of the ground and first vibrationally excited level of ArAlH(X) have been calculated for comparison with the relative band intensities measured by Hwang and Dagdigian.
Rate expressions for excitation transfer. III. An ab initio study of electronic factors in excitation transfer and exciton resonance interactions
Authors: Scholes Gregory D. , Harcourt Richard D. , Ghiggino Kenneth P.
Published: July 2002

A detailed theory for electronic aspects of electronic excitation (energy) transfer (EET) for sandwich dimers was derived in paper II of this series [J. Chem. Phys. 101, 10 521 (1994)]. In II, the electronic transfer matrix element for EET was evaluated, then simplified to various levels of approximation. The results of ab initio molecular orbital calculations on an ethene sandwich dimer are reported here in order to test and quantify the theory of II. The calculations were undertaken using a STO-6G basis set and localized molecular orbitals, with separations of 4, 5, and 6 Å between the molecules. It is demonstrated that the important electronic factors contributing to EET are the Coulombic interaction (for direct singlet–singlet transfer) and, for both singlet–singlet and triplet–triplet EET, orbital overlap-dependent interactions. The dominant orbital overlap-dependent terms arise from through-configuration interaction, which involves successive one-electron transfers mediated via bridging ionic configurations, first presented in II. The results confirm that the Dexter-type exchange interaction is small in comparison.
On the role of solvent electronic polarization in charge transfer reactions
Authors: Zhu Jianjun , Cukier R. I.
Published: July 2002

The effect of a solvent’s electronic polarization on the rate of a charge transfer reaction is studied in both continuum and discrete solvent models. An effective system Hamiltonian that contains the equilibrium solvation from the solvent electronic polarization is obtained, and leads to an effective matrix element Veff coupling the charge transfer states that is smaller than the gas phase value. Both the effective Hamiltonian and Veff are dependent on the solvent’s instantaneous nuclear configuration, and liquid state theory is used to carry out the configuration average. The solvent electronic polarization reduces the transition rate for both adiabatic and nonadiabatic reactions. A standard relation between the equilibrium solvation energy of the reactants and the solvent reorganization energy is established that permits evaluation of the effect of a molecular solvent (using a Drude model for the electronic degrees of freedom) on the rate by evaluating a partition function. This permits use of a path integral formulation for the mixed quantum (electronic polarization) classical (slow nuclear configuration) solvent that leads to the information required for the rate constant. In a strong coupling regime, where the coupling between the charge-transfer species would be so large as to preclude reactant and product species, we show that a new mechanism for charge localization arising from the solvating effect of the electronic polarization may occur, and formulate a rate constant expression for this regime. It has the form of a quantum Kramers rate and shows that the solvent provides a friction effect that will reduce the rate relative to the no-friction rate.
The Zimm plot and its analogs as indicators of vesicle and micelle size polydispersity
Authors: van Zanten John H.
Published: July 2002

The utility of using the Zimm plot and its analogs, the Debye and Berry plots, as a sensitive means of determining the degree of polydispersity present in solutions of surfactant aggregates such as vesicles or micelles is conclusively demonstrated. These methods of interpreting the excess scattered light intensity due to the surfactant aggregates lead to the determination of the weight-average molecular weight (Mw) and z-average mean square radius (〈r2g〉z), or radius of gyration, of the surfactant aggregates. These two parameters are very sensitive to any polydispersity which is present in the aggregate solution, since these two experimentally determined quantities represent different moments of the aggregate size distribution. For a given geometric model, apparent surfactant aggregate structural parameters, such as the radius and wall thickness in the case of surfactant vesicles, can be calculated under the assumption of monodisperse aggregates from 〈r2g〉z and Mw determined from a Zimm or Debye analysis of static light scattering spectra. For vesicles whose polydispersity is described by a Schulz distribution it is shown that the calculated apparent wall thickness and dispersion polydispersity are strongly correlated, thereby demonstrating the utility of this approach for determining vesicle dispersion polydispersity. Analogous approaches can be used to determine the degree of polydispersity present in surfactant solutions containing spherical, rod-like, or disk-like micelles.
Field dependence of the magic angle spinning nuclear magnetic resonance line shapes of paired spin-1/2 nuclei in solids
Authors: Kundla E. , Lippmaa E.
Published: July 2002

Proceeding from the average-Hamiltonian theory, the effects caused by nonsecular terms of the spin Hamiltonian in magic angle spinning nuclear magnetic resonance spectra of a coupled pair of rare spin-1/2 nuclei are described. Using doubly rotating coordinates with the Larmor frequencies (including isotropic screenings) measured in units of the rotation frequency, while the small frequency offsets between the actual Larmor frequencies and multiples of the rotation frequency were treated as small amplitudes within the Hamiltonian, a general formalism could be established that provides a universal description of the field-dependent line shifts, broadenings, and splittings. Analytical expressions are given for a continuous range of experimentally accessible sample rotation rates, including the rotational-resonance conditions where the Floquet theory fails. The second approximation includes the nonsecular terms and is sufficient to adequately describe the spectral features found. The anisotropic screening cross terms with the dipolar and J couplings generate in the high resolution spectrum of each single crystal at most four lines in the heteronuclear and up to eight lines in the homonuclear case. The positions and relative intensities of these lines depend both on the strength of the external magnetic field and on the mechanical sample rotation rate. In the powder pattern of a homonuclear pair, a broadened Pake-like doublet appears over a limited magnetic field range. This splitting persists even at the highest attainable rotation rates. The splittings and line shapes can provide useful data about the relative orientations of the two nuclear screening tensors.
Adiabatic pseudospectral calculation of vibrational states of four atom molecules: Application to hydrogen peroxide
Authors: Antikainen J. , Friesner R. , Leforestier C.
Published: July 2002

We use our adiabatic pseudospectral method (APS) to calculate energy levels of the H2O2 molecule up to 5000 cm−1. Reasonably high accuracy (a few wave numbers) is achieved for a fully six dimensional calculation in a few hours of CPU time on an IBM 580 workstation. This contrasts with previous calculations on the same system which required 50–100 times more computational effort for a similar level of accuracy. The method presented here is both general and robust, and will allow routine studies of six dimensional potential surfaces and the associated spectroscopy, while making calculations on still larger systems feasible.
Confirmation of long-range collision complex stabilization through the controlled relaxation of high internal excitation
Authors: Gole J. L. , Wang H. , Joiner J. S. , Dawson D. E.
Published: July 2002

A series of controlled multiple collision chemiluminescent and laser induced fluorescent studies confirm the long-range collisional stabilization of high temperature group IIA dihalide complexes of some considerable spatial extent. The relaxation process demonstrates that the pseudocontinuum emissions observed under near single collision conditions [J. Chem. Phys. 102, 7425 (1995)] correspond to the overlap of a closely spaced, highly excited, rovibronic distribution. Controlled relaxation reveals the first vibrationally resolved electronic emission for the dihalides. The vibronic structure of the observed emission spectra correlates well with expectations based on the molecular electronic structure of the ground and low-lying electronic states of the dihalides. The vibronically resolved emission from the Sr+IClHe→SrICl* and mixed halogen Sr+Cl2, Br2→HeSrCl2*+SrBr2* reactions provides strong support for the formation of a collisionally stabilized dihalide complex. These results, correlated with near single collision studies, form a basis for the discussion of (1) the kinetics of formation of the dihalide complexes and (2) the implications of long-range collisional stabilization. Current theories may not accurately model these observations. Dihalide complex formation as it influences the energy partitioning to metal monohalide excited states may account for the discrepancies between those monohalide bond strengths determined by mass spectrometry and chemiluminescent techniques.
Dynamics of molecular inversion: An instanton approach
Authors: Smedarchina Zorka , Siebrand Willem , Zgierski Marek Z. , Zerbetto Francesco
Published: July 2002

To describe tunneling of light atoms, a method is developed that takes account of the multidimensional nature of the process but remains tractable without becoming inaccurate. It combines the instanton formalism with ab initio potentials and force fields and makes effective use of a number of practical approximations suggested by the nature of the calculations. The tunneling potential is constructed from ab initio calculations that are fully optimized at stationary points. All other vibrations are represented by their harmonic force fields. Changes in the harmonic force fields between stationary points are expressed as couplings with the tunneling mode. The transfer rate is calculated for the instanton path, i.e., the path of least resistance, modulated by adjacent paths which define the damping required for nonoscillatory transfer. The multidimensional transfer integrals, involving all modes that change between the initial state and the transition state, are reduced to quasi-one-dimensional integrals by a number of approximation schemes. Modes with frequencies much higher than the tunneling mode are included in the adiabatic potential. Modes with frequencies much lower than the tunneling mode are treated classically. Modes that are linearly coupled to the tunneling mode are handled by separation of the variables followed by analytical integration. These approaches permit the calculation of most transfer rates without the explicit evaluation of the multidimensional instanton path.

They also specify the parts played by the various modes coupled to the tunneling mode. Totally symmetric modes generally promote tunneling by allowing a more favorable trajectory. Hence low-frequency symmetric modes tend to govern the temperature dependence of the transfer. Modes of the same symmetry as the transfer mode will generally contribute to the barrier and thus cause friction, represented by a Franck–Condon factor in the transfer integral. The method is applied to three molecular inversions whose rate constants have been deduced from magnetic resonance measurements. Inversion rate constants for aziridine and the oxiranyl radical together with their relevant deuterium analogs are calculated as a function of temperature and are found to be in a good agreement with the observed rates. For the dioxolanyl radical such agreement is obtained only after the introduction of an anharmonic correction. While inversions are generally low-frequency modes, the method is equally applicable to transfer governed by high-frequency XH stretch vibrations.
Monte Carlo simulations for the equation of state of athermal linear alkanes
Authors: Yethiraj Arun
Published: July 2002

Monte Carlo simulations are performed for the pressure of fused-hard-sphere freely rotating chains with fixed bond lengths and bond angles, in order to test various integral equation and mean-field generalized Flory-type equations of state. Due to a fortuitous canceling of errors, one of the generalized Flory theories is found to be the most accurate of all the theories tested.
Laser sputtering of highly oriented pyrolytic graphite at 248 nm
Authors: Krajnovich Douglas J.
Published: July 2002

The interaction of excimer laser pulses with a highly oriented pyrolytic graphite (HOPG) target has been studied. HOPG, a close approximation to single crystal graphite, was irradiated along a freshly cleaved basal plane in vacuum by pulses from a KrF excimer laser. The energy fluence was varied between 300–700 mJ/cm2, resulting in material removal rates of <0.01 Å/pulse to ∼100 Å/pulse. In this near-threshold regime, neutral carbon atoms, dimers, and trimers account for nearly all of the sputtered flux and collisional and plasma effects are minimized. Time-of-flight distributions of the neutral carbon atoms and small carbon clusters were measured and inverted to obtain translational energy flux distributions and relative sputtering yields as a function of fluence. The translational energy distributions are remarkably close to Maxwell–Boltzmann distributions over most of the fluence range studied. However, the mean translational energies are far too high to reconcile with a simple thermal vaporization model. For example, the mean translational energy of C3, the most abundant species, increases from 1.1 eV at 305 mJ/cm2 to 31.7 eV at 715 mJ/cm2. Explanations are considered for this curious mix of thermal and non-thermal behavior. At the high end of our fluence range, the mean translational energies of C1, C2, C3 converge to a 1:2:3 ratio, indicating that the velocity distributions are almost identical. This particular result can be interpreted as a gas dynamic effect. Prolonged sputtering of the same target spot results in a falloff in the sputtering yield and the mean translational energies, but little change in the cluster size distribution. These effects are related to impurity induced topography formation on the target surface.
Cavity ringdown laser absorption spectroscopy and time-of-flight mass spectroscopy of jet-cooled copper silicides
Authors: Scherer J. J. , Paul J. B. , Collier C. P. , Saykally R. J.
Published: July 2002

The cavity ringdown technique has been implemented for electronic spectroscopy of jet-cooled CuSi produced in a pulsed UV laser vaporization plasma reactor. A time-of-flight mass spectrometer is used to simultaneously monitor species produced in the supersonic expansion and allows correlation studies to be performed. Seven rotationally resolved vibronic bands have been measured near 400 nm, yielding spectroscopic constants for the 2Σ ground and excited states. Vibronic isotope shifts, together with rotational line positions, permit the unambiguous determination of the spectral carrier and vibronic assignment. Since no ab initio studies for the CuSi molecule exist, a comparison to ab initio studies of the related NiSi molecule is presented. Time-of-flight mass spectrometric measurements indicate the facile formation of CuxSiy clusters containing multiple copper atoms, in contrast to earlier mass spectrometric work on transition metal silicides.
Electronic orbital alignment effects in the reaction Mg*(3p 1P1)+CH4→MgH+CH3
Authors: Wong T. H. , Kleiber P. D.
Published: July 2002

We have measured the final state resolved far-wing action spectra for the MgCH4 reactive collision system. The results show a dramatic ‘‘Π-like’’ orbital alignment preference in the reaction channel. The reactive channel action spectra for different MgH rotational states in v=0 are identical, suggesting that the reaction follows from a single approach geometry, with the product rotational distribution determined by exit channel effects. Based on these observations and molecular orbital considerations, we propose that the reaction proceeds in η2 approach geometry through a triangular C–Mg–H transition state.
A πCI approach to the study of correlation effects on the nonlinear-optical properties in organic π-conjugated systems
Authors: Albert I. D. L. , Morley J. O. , Pugh D.
Published: July 2002

A π-electron method which allows for the systematic inclusion of configuration interaction of any order has been developed for the computation of electronic and optical properties of conjugated molecules. It has been used to study the effect of electron correlation on these properties in all trans finite polyenes of up to 16 carbon atoms. For smaller molecules it has been possible to carry out a complete set of CI calculations, from singly excited (SCI) to full configuration interaction (FCI). For the larger molecules the SCI and doubly excited CI (SDCI) calculations have been performed. The program permits the execution of a configuration interaction calculation of any order, n, in which all configurations involving the excitation of 1,2,...,n electrons from the occupied π-orbitals of the Hartree–Fock ground-state to the virtual π-orbitals are included. The set of π-orbitals is extracted from the ground state obtained from an all valence-electron, complete neglect of differential overlap (CNDO) calculation. The configurations are represented by binary integers so that their generation and storage is very rapid and efficient. The nonlinear optical properties have been computed mainly by the correction vector method but in some cases the sum-over-states (SOS) method has also been used to study the evolution of the THG coefficient as virtual states of increasing energy are added. The results obtained for the finite polyenes are found to be in very good agreement with both experimental and other theoretical values in literature. The results clearly show the effect of electron correlation, which is found to affect the electronic and optical properties of these systems both qualitatively and quantitatively.
Ab initio characterization of electronically excited states in highly unsaturated hydrocarbons
Authors: Sobolewski Andrzej L. , Adamowicz Ludwik
Published: July 2002

The results of extensive electronic structure calculations performed for a series of molecules: (CC)nH, H(CC)nH+ for n=2,3,4 and for (CC)3H− and H(CC)3H− are presented. The theoretical scheme is based on the complete-active-space self-consistent-field (CASSCF) method, with remaining dynamic electron correlation effects added in the subsequent step with the use of the second-order perturbation theory with the CASSCF wave function as the reference state (CASPT2). The results support the hypothesis of Fulara et al. [Nature 336, 439 (1993)] that the 2Π→2Π electronic transitions in the series of CnH radicals might be responsible for some of the ‘‘diffuse interstellar bands’’ observed in the visible and near-infrared spectra of stars located on the far side of diffuse interstellar clouds.
On the variation of the interfacial tension with cluster size in connection to homogeneous nucleation from the vapor phase
Authors: Kegel Willem K.
Published: July 2002

The size correction to the interfacial tension as used in phenomenological vapor phase nucleation theories is compared to expressions derived from Helfrich’s curvature free energy. This leads to a clear physical interpretation of the expansion parameters. It is shown that a set of parameters in the phenomenological free energy of a cluster are not independent. Consequences on existing theories of vapor phase nucleation are discussed.
A simplified released-node quantum Monte Carlo calculation of the ground state of LiH
Authors: Chen Bin , Anderson James B.
Published: July 2002

We report an exact ab initio calculation of the ground state of the LiH molecule using a simplified released-node Green’s function quantum Monte Carlo method. The energy determined for an internuclear separation of 3.015 bohr is −8.070 21±0.000 05 hartree, a value lower than that of the lowest-energy variational calculation, more accurate than that of prior quantum Monte Carlo calculations, and in excellent agreement with the nonrelativistic energy of −8.070 21 hartree determined from experimental measurements.
A resonance enhanced multiphoton ionization study of the CS2 molecule: The 4p Rydberg states
Authors: Baker Jacob , Konstantaki Maria , Couris Stelios
Published: July 2002

The resonance enhanced multiphoton ionization (REMPI) spectrum of jet-cooled CS2 has been recorded in the one-photon wavelength range 460–500 nm, corresponding to the three photon excitation energy range 60 000–65 000 cm−1. A previous assignment of one photon forbidden transitions in this region to 3d Rydberg states is shown to be incorrect and reassigned to the 4pπ 1,3Δu states. In fact all the observed states in this region can be assigned to 4p Rydberg states; the 4pσ 1,3Πu states at 62 768 and 62 083 cm−1, respectively, and the 4pπ 1,3Δu states at 64 214 and 63 698 cm−1, respectively. Another band at 64 374 cm−1 may be due to a three photon excitation to the 4pπ 1Σ+u Rydberg state. Our resolution is sufficient to resolve band shapes enabling symmetry assignments when coupled with their polarization behavior. The fact that the origin bands are not degraded and that Δν=0 sequence bands are strongly excited whereas Δν≠0 transitions are either absent or very weak implies that the upper states have a linear geometry similar to that of the ground state. A comparison of singlet–triplet splittings suggests the 4pπ Δu states have stronger Hund’s case (c) character than the 4pσ Πu state. While CS+2 was generally the predominant ion formed, resonance ionization through the 1Δu←X 1Σ+g origin band led to an unusual predominance of S+ and CS+ ions. This is explained by multiphoton fragmentation of CS+2 via an accidental one-photon resonance from the X 2Π1/2 (v=0) ionic ground state.
On the application of instantaneous normal mode analysis to long time dynamics of liquids
Authors: Vijayadamodar G. V. , Nitzan Abraham
Published: July 2002

While the applicability of instantaneous normal mode (INM) analysis of liquids to short time dynamics is in principle obvious, its relevance to long time dynamics is not clear. Recent attempts by Keyes and co-workers to apply information obtained from this analysis to self-diffusion in supercooled liquid argon is critically analyzed. By extending the range of frequencies studied we show that both imaginary and real branches of the density of modes are represented better, for large ω, by ln[ρ(ω)]∼ω2/T than by ln[ρ(ω)]∼ω4/T2 as advocated by Keyes [J. Chem. Phys. 101, 5081 (1994)]. However, since in the relevant frequency range the two fits almost overlap, the numerical results obtained by Keyes, showing good agreement with the simulation results for self-diffusion in supercooled liquid argon, remain valid even though implications for the frequency dependence of the barrier height distribution change. We also explore other possibilities for extracting information from the INM analysis: (1) The density of ‘‘zero force modes,’’ defined as the distribution of normal modes found at the bottom or top of their parabolic potential surfaces, can be computed with no appreciable additional numerical effort. This distribution provides a better representation than the total density of modes for the normal mode distribution at well bottoms and at saddles, however, we find that it makes little difference in quantitative analysis. (2) We suggest that the ratio ρu(ω)/ρs(ω) between the density of modes in the unstable and stable branches provide an estimate for the averaged barrier height distribution for large ω. Using this estimate in a transition state theory calculation of the average hopping time between locally stable liquid configurations and using the resulting time in a calculation of the self-diffusion coefficient yields a very good agreement with results of numerical simulation.
Accurate symplectic integrators via random sampling
Authors: Hoover William G. , Kum Oyeon , Owens Nancy E.
Published: July 2002

We develop a random-sampling method for finding accurate symplectic integrators which best match the exact trajectory of a one-dimensional harmonic oscillator. We recover several well-known algorithms. We demonstrate the usefulness of the random sampling method by finding and validating a new integrator, applying it to the classical many-body problem.
Molecular dynamics simulations of athermal polymer blends: Finite system size considerations
Authors: Stevenson Craig S. , McCoy John D. , Plimpton Steve J. , Curro John G.
Published: July 2002

Molecular dynamics simulations of binary, athermal blends of chains consisting of 50 tangent sites were carried out over a range of compositions at liquidlike packing fractions. The sites interact with repulsive Lennard-Jones potentials and have effective hard sphere diameters of approximately 1.0 and 1.2 for the two site types. The intrachain and interchain correlation functions were found and, except for the single component systems, the interchain correlations were seen to be strongly dependent on system size. Trivial, long range correlations due to the finite system size can be approximated from simple physical arguments, and the coarse, interchain, radial distribution functions can be ‘‘corrected’’ for these effects. The resulting correlation functions are seen to behave at large separation as would be expected of interchain radial distribution functions in an infinite sized system, permitting meaningful comparisons with the predictions of liquid state theory.
Study of correlation states of acetylene by synchrotron photoelectron spectroscopy
Authors: Moghaddam Maria Sabaye , Dejardins S. J. , Bawagan A. D. O. , Tan K. H. , Wang Y. , Davidson E. R.
Published: July 2002

The inner valence photoelectron spectra of acetylene (C2H2) and isotopically labeled acetylene (1,213C–C2H2) are obtained using high resolution synchrotron photoelectron spectroscopy. Four distinct correlation (satellite) peaks, consistent with previous x-ray photoelectron spectroscopy measurements, are resolved. The photon energy dependence of the intensity ratios of these satellites to the 2σ−1g main peak is observed over a wide photon energy range (32–72 eV). Three of these satellites (26.6, 28.0, and 29.8 eV binding energy) exhibited constant photon energy dependence while the fourth satellite (31.2 eV binding energy) showed enhancement of intensity towards the threshold. The photon energy dependence of correlation (satellite) peak 4 can be explained in either of two ways: (1) Peak 4 is a dynamic correlation peak associated with the 2σ−1g ionization process or (2) peak 4 is an intrinsic correlation peak associated with the 3σ−1g ionization process. A multireference singles and doubles configuration interaction (MRSDCI-ANO) calculation of the theoretical photoelectron inner valence spectrum using average natural orbitals indicates that the latter explanation (2) is more likely. Semiquantitative agreement (in terms of the peak positions and intensities) is also obtained between the experimental photoelectron spectrum and the MRSDCI(ANO) calculation.
Reduced electronic density matrices, effective Hamiltonians, and nonlinear susceptibilities of conjugated polyenes
Authors: Chen Guanhua , Mukamel Shaul
Published: July 2002

The ground state single-electron reduced density matrix is shown to contain the essential chemical bonding information relevant for calculating the nonlinear optical response of conjugated polymers. Applications to a series donor-acceptor substituted Hexatrienes demonstrate the interplay of electronic structure and dynamics, and the correlation between off-resonant polarizabilities and reduced density matrix. The construction of an effective Hamiltonian using the reduced density matrix is discussed.
Dynamics of a nematic liquid crystal constrained by a polymer network: A proton NMR study
Authors: Vilfan M. , Lahajnar G. , Zupančič I. , Žumer S. , Blinc R. , Crawford G. P. , Doane J. W.
Published: July 2002

We report on the proton spin–lattice relaxation measurements in the isotropic and nematic phases of the liquid crystal 4′-pentyl-4-cyanobiphenyl (5CB), constrained by a low-concentration polymer network. Above the nematic–isotropic transition, a relatively small impact of the network in the MHz regime and a huge increase in the relaxation rate in the kHz regime are explained consistently by processes in the weakly orientationally ordered surface layer. A dynamic process with the correlation time ∼4×10−6 s is detected. The orientational order parameter in the surface layer is found to be almost temperature independent, and is estimated as S0∼0.1. The large internal surface-to-volume ratio of the liquid crystal in the dispersion, required for the explanation of experimental data, supports the simplified model which envisions the network elements as tiny fibers with nanometer radius. Comparison between NMR and optical data reveals mutual cross linking of the fibers. Measurements of the molecular self-diffusion constant indicate that fibers are distributed inhomogeneously on the submicrometer scale.
A relativistic Kohn–Sham density functional procedure by means of direct perturbation theory
Authors: van Wüllen Christoph
Published: July 2002

The direct perturbation theory of relativistic effects is applied to the Kohn–Sham density functional procedure. The relativistic correction to the total energy is calculated through the leading order. A method for the determination of analytic energy derivatives of the resulting total energy expression is presented, which allows the calculation of the relativistic correction to molecular forces and other first-order properties like e.g. the dipole moment. The method is applied to the diatomic ‘‘benchmark’’ molecules AgH, AuH, and Au2, and to the transition metal carbonyls Mo(CO)6, Pd(CO)4, and W(CO)6.
Photodissociation of HCl+/MgO(001): The static and dynamic effects of the surface
Authors: Seideman Tamar , Guo Hua
Published: July 2002

The photodissociation dynamics of surface-adsorbed HCl+ is studied using both time-dependent and time-independent quantum mechanical methods. The photodissociation line shapes and photofragment angular distributions are calculated following excitation to the (2)2Σ and the (2)2Π electronic states, correlating with Cl++H and with Cl+H+, respectively. Complementary insight is gained by analyzing also the classical photodissociation dynamics. We find pronounced shifts and modifications of the photodissociation line shapes, which depend sensitively on the fragmentation channel and reflect surface-induced modification of the adsorbed state and the excited HCl+ potential energy surfaces. The quantum mechanical angular distributions exhibit a rich, energy-dependent structure, which differs substantially for the two fragmentation channels. Assisted by the classical trajectory results, we ascribe the structure to surface-induced rainbow scattering. The sensitivity of the photodissociation line shape and the angular distribution to the initial orientation of the adsorbate and to details of the substrate–adsorbate interaction in the excited manifold is illustrated.
The structure, spectroscopy, and excited state predissociation dynamics of GeH2
Authors: Karolczak J. , Harper Warren W. , Grev Roger S. , Clouthier Dennis J.
Published: July 2002

The spectroscopy and excited state dynamics of Ã 1B1 germylene (GeH2) have been investigated experimentally and theoretically. Jet-cooled laser-induced fluorescence spectra of GeH2 were obtained by subjecting germane (GeH4) to an electric discharge at the exit of a pulsed nozzle. The band origins of ten vibronic transitions were determined, giving values for the upper state fundamentals of ν1=783.0 cm−1 and ν2=1798.4 cm−1. Sufficient numbers of 000 band rovibronic transitions were observed to give the ground and excited state structures as r″=1.591(7) Å, θ″=91.2(8)° and r′=1.553(12) Å, θ′=123.4(19)°. Fluorescence lifetime measurements show that the 00,0 rotational levels decay radiatively; higher J rotational states in the 00 vibronic level decay much faster, due to a heterogeneous predissociation in the excited state. High quality ab initio studies are consistent with a model in which the lower vibronic levels of the Ã state predissociate through the ã 3B1 state to produce Ge(3P)+H2(1Σ+g). The transition state for this process has been located and the barrier to dissociation is 15.2 kcal/mol above the Ã 1B1 state, so that tunneling through the barrier must occur. Above 4000 cm−1 of vibrational energy in the Ã state, a breaking off of fluorescence is observed as a second predissociation channel involving GeH2(Ã 1B1)→Ge(1D)+H2(1Σ+g) becomes accessible. This process is also found to have a barrier, in contrast to previous theoretical studies of SiH2, where the analogous dissociation was predicted to be barrierless.
Perturbation theory for electronic excited states: The low lying Rydberg states of water
Authors: Warken Markus
Published: July 2002

The recently developed Λ-transformed perturbation theory offers a rigorous solution of the intruder state problem and can thus be employed for systematic construction of convergent perturbation series. The method is applied to the vertical excitation energies of the low lying Rydberg states of the water molecule and compared to results obtained by standard techniques for the calculation of excited electronic states. The values calculated by Λ-transformed perturbation theory are found to be of about the same quality as the best theoretical results available. It is shown that for practical applications the computational effort for a Λ-transformed perturbation series is only slightly greater than the cost for the original series.
Tight binding molecular dynamics study of Ni clusters
Authors: Lathiotakis N. N. , Andriotis A. N. , Menon M. , Connolly J.
Published: July 2002

A minimal parameter tight binding molecular dynamics scheme is used to study Nin clusters with n≤55. We present theoretical results for relaxed configurations of different symmetries, binding energies, and normal vibrational frequencies for these clusters. Our results are in good agreement with experiment and previous theoretical predictions. We also compare relative stabilities of fcc structures with icosahedral structures. In particular, we find that for clusters whose size allows them to form a close icosahedral geometry (normal or twinned), the closed icosahedral structures yield larger binding energies than fcc structures. The fcc structures, in turn, are found to be more stable than open icosahedral structures for n≤55. Additionally, results for normal vibrational frequencies and ionization energies for n≤10 are also presented. The present results, along with previous successful applications of the method on semiconductor systems, indicate that tight-binding molecular dynamics scheme can be relied on to provide a useful semiempirical scheme in modeling interactions in both covalent and metallic systems.
An evaluation of existing potential energy surfaces for CO2–Ar: Pressure broadening and high-resolution spectroscopy of van der Waals complexes
Authors: Roche Christine F. , Ernesti Andreas , Hutson Jeremy M. , Dickinson Alan S.
Published: July 2002

Nine different potential energy surfaces for CO2–Ar taken from the literature are tested for their ability to reproduce the spectra of the Ar–CO2 van der Waals complex and the pressure broadening of CO2 infrared lines by Ar. None of the potentials give a satisfactory account of all the experimental results. All the potentials are found to give significant discrepancies with at least some of the spectroscopic properties of the van der Waals complex. Coupled-states (CS) and infinite-order sudden (IOS) calculations of the pressure broadening cross sections are compared for a few of the potential energy surfaces. The IOS approximation is found to be seriously inaccurate for some potential surfaces, especially for high-j lines, so that CS calculations are essential when comparing with experimental line-shape data. CS calculations of line-broadening cross sections are therefore carried out on all nine different potential energy surfaces. For the pressure broadening coefficients, there are substantial uncertainties in the experimental results. Nevertheless, the only potential to give a satisfactory account of the pressure broadening is the electron gas potential of Preston and Pack, which is the least satisfactory of all for the spectra of the complex. It is concluded that a new potential that reconciles the different data sets is needed.
Current and condensate distributions in rotational excited states of quantum liquid clusters
Authors: Cheng E. , McMahon Michele A. , Whaley K. Birgitta
Published: July 2002

Recent quantum Monte Carlo calculations of small quantum clusters have shown that it is feasible to study their rotationally excited states directly. [M. A. McMahon, R. N. Barnett, and K. B. Whaley, J. Chem. Phys. 99, 8816 (1993).] We extend this work by sampling, from optimized variational wave functions, the current and Bose condensate distributions in (4He)7, (H2)7, and (4He)40 clusters. We demonstrate that these distributions are useful tools in identifying the location and nature of rotational excitations. With different constructions of rotational wave functions, the excitations can be either localized (as surface modes), or delocalized throughout the cluster analogous to the bulk superfluid vortex states. Condensate depletions in the excited states are observed, except for the delocalized excitation, where we find a higher condensate fraction than in the ground state.
Efficient recursive computation of molecular integrals for density functional methods
Authors: Köster Andreas M.
Published: July 2002

The expansion of the electronic density and the exchange-correlation potential in auxiliary functions is a successful technique to reduce the computational effort in linear-combination-of- Gaussian-type-orbitals density functional theory (LCGTO-DFT) methods. A new approach for the efficient calculation of the necessary three-center overlap and electron repulsion integrals for this technique is presented. A new set of recurrence relations is derived, which take advantage of the special structure of the auxiliary function sets (primitive functions with shared exponents). Pathway diagrams from uncontracted integrals over s functions to any given class of target integrals are presented. The efficiency of different paths is discussed on the basis of floating-point operations (FLOPs). The FLOP counting indicates that the new method represents a substantial improvement for the calculation of three-center overlap and electron repulsion integrals.
Guided ion beam studies of the reactions of Fe+n (n=2–15) with D2: Cluster–deuteride bond energies as a chemical probe of cluster structures
Authors: Conceição J. , Loh S. K. , Lian L. , Armentrout P. B.
Published: July 2002

The kinetic energy dependencies of the reactions of Fe+n (n=2–15) with D2 are studied in a guided ion beam mass spectrometer. The only products observed are FenD+ (n=2–15) and FenD+2 (n=9–15). All reactions are observed to exhibit thresholds, except for formation of Fe9D+2. Threshold analyses of the endothermic reactions lead to binding energies for the first deuterium atom to the cluster ions as a function of cluster size. The Fe+n–D bond energies are compared to previously determined metal–metal bond energies, D0(Fe+n–Fe). The bond energies of Fe+n–D vary nonmonotonically with n, and parallel those for Fe+n–Fe except for notable differences at n=5, 8, 12, and 14. These trends are rationalized in terms of electronic and geometric structures for the Fe+n clusters. Arguments are presented to suggest that the thresholds measured for FenD+2 production correspond to barriers for chemisorbtion.
Probing hydrogen bond potentials via combination band spectroscopy: A near infrared study of the geared bend/van der Waals stretch intermolecular modes in (HF)2
Authors: Anderson David T. , Davis Scott , Nesbitt David J.
Published: July 2002

High resolution near infrared spectra of the two lowest frequency intermolecular modes in HF-stretch excited states of (HF)2 have been characterized using a slit-jet infrared spectrometer. In the spectral region surveyed, ten vibration–rotation–tunneling (VRT) bands are observed and assigned to the low frequency ‘‘van der Waals stretch’’ (ν4) and ‘‘geared bend’’ (ν5) intermolecular modes, in combination with either the hydrogen bond acceptor (ν1) or donor (ν2) high-frequency intramolecular HF stretches. Analysis of the rotationally resolved spectra provide intermolecular frequencies, rotational constants, tunneling splittings, and predissociation rates for the ν4/ν5 intermolecular excited states. The intermolecular vibrational frequencies in the combination states display a systematic dependence on intramolecular redshift that allows far-IR intermolecular frequencies to be reliably extrapolated from the near-IR data. Approximately tenfold increases in the hydrogen bond interconversion tunneling splittings with either ν4 or ν5 excitation indicate that both intermolecular modes correlate strongly to the tunneling coordinate. The high resolution VRT line shapes reveal mode specific predissociation broadening sensitive predominantly to intramolecular excitation, with weaker but significant additional effects due to low frequency intermolecular excitation. Analysis of the high resolution spectroscopic data for these ν4 and ν5 combination bands suggests strong state mixing between what has previously been considered van der Waals stretch and geared bend degrees of freedom.
Simulating shear flow
Authors: Padilla Paz , Toxvaerd So/ren
Published: July 2002

In the present article, we have analyzed to which extent the steady states produced in simulations of fluids undergoing shear flow, can truly be representations of experimental steady states. For this purpose, we have performed nonequilibrium molecular dynamics (NEMD) simulations of two different fluid systems undergoing shear flow. One system is a Lennard-Jones (LJ) fluid where the viscous heat produced by shearing the system is eliminated only in certain regions of the simulation box. The other system is a polymer immersed in an atomic solvent. In this case, the viscous heat was removed by coupling a homogeneous thermostat to different degrees of freedom in the system. The results of these simulations show that at the shear rates commonly produced in simulation, the rate of production of viscous heat is very large. This heat is eliminated by the thermostat at rates that are higher than the rates of transport of heat across the fluid. Moreover, the heat has no time to redistribute into the different degrees of freedom of the system, and different steady states are reached depending on to which degrees of freedom the thermostat is coupled. As a conclusion of this investigation we believe that the efforts of simulating fluids undergoing shear flow should be directed to simulate lower shear rates.
Kinetic-energy release in the dissociation of CO22+
Authors: Masuoka Toshio , Nakamura Eiken , Hiraya Atsunari
Published: July 2002

The kinetic-energy release distributions (KERDs) of the fragment ion pairs (O++CO+ and C++O+) produced in dissociative double photoionization of carbon dioxide have been determined by analyzing the photoion–photoion coincidence spectra measured in the region hν=40–100 eV by use of a time-of-flight mass spectrometer and synchrotron radiation. The mechanism of the three-body dissociation (CO2+2→C++O++O) has been examined to take place sequentially via CO+ by a triple photoelectron–photoion–photoion coincidence experiment. The KERD observed for the O++CO+ and C++O+ channels at low excitation energies cannot be explained by a simple framework whereby a doubly charged molecular ion (AB2+) is directly produced on single photon absorption followed by the dissociation of AB2+ into two ionic fragments. Some of the ion pairs are produced through indirect processes in which highly excited CO2*+ and CO2** (double Rydberg) states converging to the high-lying CO2+2 electronic states autoionize before and after dissociation. From the observed KERDs for the O++CO+ and C++O+ channels of CO2+2, the range of the intercharge distances of two positive holes is estimated by assuming that the KER is given purely by Coulomb repulsion.
Two-band tight-binding model for push–pull polyenes
Authors: Suzuki Yasuo Y. , Beljonne D. , Brédas J. L.
Published: July 2002

We propose a two-band model for the description of the electronic structure of push–pull polyenes in order to analyze in simple ways their potentially useful electronic and optical features. The polyene part is described by two (valence and conduction) bands, which are coupled with two tight-binding frontier orbitals representing the donor and acceptor end groups. In this model, the ground state consists of the one-electron states of the (π) valence band hybridized with the donor orbitals, while the charge-transfer excited state is described as an excitation from the highest occupied molecular orbital to the lowest unoccupied molecular orbital, the latter being a hybridized one-electron state between the (π*) conduction band and the acceptor orbital. It is shown, by the Green’s function method, that the electron localizations (the partial density of states) at the end groups are determined by three factors; (1) the unperturbed energy levels of the frontier orbitals, (2) the density of states of the unperturbed polyene bands, and (3) the coupling constants between the π (π*) band and the donor (acceptor) orbital. Based on the results, a simple description is provided for the characteristic nonlinear optical responses and the intramolecular adiabatic charge-transfer mechanism of push–pull polyenes. In order to estimate the magnitude of the coupling constants, we compare the analytical results from the model with numerical calculations based on an established semiempirical method. This kind of modeling provides guidance for the design of functional push–pull polyenes.
Light-induced migration of Ag in Xe films and generation of sites
Authors: Kometer R. , Schwentner N.
Published: July 2002

Scattering losses and inhomogeneous broadening are reduced by an optimized cocondensation of laser evaporated Ag atoms with Xe gas allowing the determination of absolute quantum efficiencies in light-induced processes. A jump probability per absorbed photon increasing from 10% to about unity for higher temperatures is observed for the migration of Ag atoms originating from electronic excitation. A model based on a concerted host–guest motion is proposed. A well defined metastable site with a strongly red shifted absorption band and a generation probability of 1.5% which decreases with temperature is attributed to an intermediate position in the migration process.