bluuebunny/crossref_metadata_2025 huggingface dataset:
On the nature of trapping and desorption at high surface temperatures. Theory and experiments for the Ar–Pt(111) system
Authors: Head-Gordon Martin , Tully John C. , Rettner Charles T. , Mullins C. Buddie , Auerbach Daniel J.
Published: July 2002

We report molecular dynamics calculations and molecular beam experiments on trapping and desorption as a function of surface temperature and initial gas conditions for the Ar–Pt(111) system. The trapping process involves very rapid equilibration of the normal component of incidence velocity but extremely slow accommodation of the parallel component. At high surface temperatures for which the residence time of Ar is sufficiently short (e.g., roughly 40 ps at 273 K), trapped atoms desorb before their incident parallel velocity is thermalized. Thus trapping in the usual sense of complete equilibration with the surface does not occur; instead these quasitrapped atoms are characterized by full accommodation of only the normal velocity component. In both simulations and experiments there is a range of temperatures for which the desorbing flux associated with quasitrapping is distinguishable from the flux due to direct inelastic scattering. These results are qualitatively reproduced by a simple model for the time-varying velocity distribution of quasitrapped atoms, which treats the different components of velocity independently.
Magnetic dipole transitions as standards for Judd–Ofelt parametrization in lanthanide spectra
Authors: Görller-Walrand C. , Fluyt L. , Ceulemans A. , Carnall W. T.
Published: July 2002

It is shown that the sum of the intensities for magnetic dipole transitions between crystal-field components of two free-ion levels in lanthanide spectra is almost independent of the symmetry of the environment. A mean theoretical sum value of 18×10−7𝒟2, 94×10−7𝒟2, and 9×10−7𝒟2 for, respectively, the 5D1←7F0, 5D0←7F1, and 5D2←7F1 dipole strengths has been found. Experimental values of the dipole strength for Eu3+ in different lattices support within reasonable limits the theoretically derived sum rule. We therefore propose to use these magnetic dipole transitions in the Eu3+ spectrum as standards for further Judd–Ofelt parametrization.
Direct calculation of collisional properties that require energy derivatives of the S matrix: Results for the reaction He+H+2⇌HeH++H
Authors: Darakjian Z. , Hayes E. F. , Parker G. A. , Butcher E. A. , Kress J. D.
Published: July 2002

The Darakjian–Hayes direct method for determining quantum lifetimes for three atoms scattering in three physical dimensions is used to determine accurate state-to-state time delays for the reaction of helium with H+2 for total angular momentum J=0. These results are compared with the time delays obtained by numerical differentiation of the S-matrix elements generated using the APH (adiabatically adjusting principal-axis hyperspherical) formulation of Pack and Parker. The direct method was found to be accurate and efficient for calculating the energy derivatives of the S matrix. The calculated eigenvalues of Smith’s collision lifetime matrix (eigen lifetimes) for this reaction predict numerous long-lived metastable states, many with lifetimes over 0.5 ps. The extent of the coupling of metastable states to specific scattering states provides an indication of the nature and magnitude of the time delays associated with particular state-to-state scattering processes. The direct method for calculating the energy derivatives of the S matrix is also found to be accurate and efficient for determining the energy derivative of the cumulative reaction probability.
Effect of the band degeneracy on the electronic structure: Application to the aluminum clusters
Authors: Tarento René-Jean , Joyes Pierre
Published: July 2002

Using a modified Gutzwiller technique, the effect of the band degeneracy on the electronic structure of dimer has been carried out. Aluminium clusters Aln (n≤6) have been investigated: The present calculation is in agreement with ab initio calculations; a singlet to triplet state transition is occurring for n=4.
On the coupling of rotovibrational motions in He–Li2 inelastic collisions
Authors: Gianturco F. A. , Serna S. , Delgado-Barrio G. , Villarreal P.
Published: July 2002

He–Li2 interaction potentials, which explicitly include both angular anisotropy and internal vibrational coordinates, have been proposed recently in the literature by extracting them from laser-selected, crossed beam experiments and from the use of various perturbation expansions. In view of the importance of assessing as accurately as possible the dynamical coupling of such internal degrees of freedom during inelastic, low-energy collisions, the present study undertakes a detailed computational comparison of the various possible decoupling schemes which can be employed to treat the above processes. It is found that because of the rather weak nature of the interaction in the title system, nearly all schemes work reasonably well and can be used in scattering calculations. It is, however, shown that the previously suggested potentials need to be modified extensively in order to yield cross sections which agree with the experimental findings.
State-selected reactive scattering. I. H+2+H2→H+3+H
Authors: Pollard J. E. , Johnson L. K. , Lichtin D. A. , Cohen R. B.
Published: July 2002

Cross sections for the reaction H+2+H2→H+3+H, differential in scattering angle and recoil energy, are measured in a molecular-beam experiment at c.m. collision energies of 1.5, 2.3, 3.5, and 5.3 eV. Resonantly enhanced four-photon ionization prepares H+2 in selected vibrational-state distributions, allowing a systematic exploration of the effects of reactant energy on the product angular and energy distributions. Angular data are interpreted on the basis of competition between H+3 formation and collision-induced dissociation. The nominal atom-transfer (AT) and proton-transfer (PT) processes are identified respectively with forward and backward scattered H+3. Effects of reactant energy on AT and PT cross sections in H+2+H2 are compared with previous observations on D+2+H2 and H+2+D2. The fraction of the available energy appearing as H+3+H recoil ranges from 26% to 39% depending on reactant conditions. Previous surface-hopping trajectory calculations successfully predict most of the observed trends. Evidence is given for the existence of long-lived H+3 with internal energy in excess of the dissociation limit.
The multireference coupled-cluster method in Hilbert space: An incomplete model space application to the LiH molecule
Authors: Balková A. , Kucharski S. A. , Meissner L. , Bartlett Rodney J.
Published: July 2002

The first results from a Hilbert space, multireference coupled-cluster (CC) method in an incomplete model (active) space are reported for the five lowest states of LiH. The active space is spanned by several configurations at the level of single and double excitations, where the configuration(s) causing intruder state problems are excluded from the complete Hilbert reference space. Full inclusion of single- and double-excitation operators is considered in the expansion for the cluster operator, with all quadratic terms in the renormalization part. The multireference CC results for the ground (X 1Σ+) and four low-lying excited states (a 3Σ+, A 1Σ+, a 3Π, A 1Π ) of LiH are compared with the corresponding full configuration-interaction (FCI) energies. The agreement between FCI and CC values within a few hundredths of mH for the Π states proves the feasibility of the present method to describe, quantitatively, the quasicomplete reference space problem. Deviations of the incomplete multireference results from the exact FCI energies for for excited Σ states vary with respect to the quasidegeneracies involved from a few tenths of mH for the nondegenerate (near-equilibrium) geometries to several mH in the dissociation region.
Electron transfer collisions between small water clusters and laser-excited Rydberg atoms
Authors: Desfrancois C. , Khelifa N. , Lisfi A. , Schermann J. P. , Eaton J. G. , Bowen K. H.
Published: July 2002

The relative formation rates of negative water clusters have been measured as a function of electron energy in the 8–170 meV range. The electron source is a beam of xenon atoms laser-excited in Rydberg states. Strong Rydberg electron energy dependences are observed for the attachment rates of the water dimer, hexamer, and heptamer.
Direct infrared and visible absorption spectroscopy of stoichiometric and nonstoichiometric clusters of indium phosphide
Authors: Rinnen K.-D. , Kolenbrander K. D. , DeSantolo A. M. , Mandich M. L.
Published: July 2002

Resonant two-color and one-color photodissociation spectroscopy is performed on neutral InxPy clusters with x+y=5 to 14 atoms. Absorption spectra of 29 different InxPy clusters are obtained for excitation energies between 0.84 and 1.84 eV. The absorption behavior is qualitatively similar for all of these clusters. An optical-gap-like absorption feature is observed at the blue end of the spectra, particularly for clusters which contain an even number of atoms. This feature shifts as a function of cluster size, but has an onset close to the band gap of bulk crystalline InP. Weaker absorptions are observed tailing to the red. A distinct absorption peak is also observed near the bulk InP band band gap for odd-numbered clusters with a stoichiometry of Inx+3Px. An empirical fitting of the low energy absorption tails reveals a correlation between the optical behaviors of these tiny clusters and amorphous semiconductors.
Naked organosulfur clusters: The infrared spectrum of the C2S molecule
Authors: Xie Yaoming , Schaefer Henry F.
Published: July 2002

Energetically low-lying states of C2S molecular systems are studied by ab initio self-consistent-field (SCF) theory and configuration interaction with all single and double excitations (CISD). Standard double-ζ plus polarization (DZP), triple-ζ plus double polarization (TZ2P), and TZ2P augmented by f-type function (TZ2P+f ) basis sets have been used for full geometry optimization and harmonic vibrational frequency analysis. For the linear 3Σ− ground state, the C–C bond length is predicted to be 1.304 Å, the C–S bond length is 1.550 Å and the harmonic vibrational frequencies are 1728, 917, and 337 cm−1 at the TZ2P+f CISD level of theory. Results for other linear and ring states are also reported.
Growth and structure of amorphous ice condensates: A computational study. II
Authors: Buch V.
Published: July 2002

Investigation is presented of the static and the dynamic properties of an amorphous cluster (H2O)450 at T≊10 K. The cluster was obtained in a simulation of slow condensation of gaseous water molecules, and is used here to obtain insights into molecular properties of low temperature amorphous ice deposits. The cluster is shown to contain a substantial fraction of molecules of hydrogen bond coordination less than 4. Low coordinated configurations are formed during condensation in a highly selective fashion, i.e., some configurations are strongly favored over others. Condensation dynamics is investigated, in an effort to clarify a link between molecular mechanisms of condensation and the resulting metastable structure. Physical reason is suggested for the experimentally measured very large surface area of the amorphous ice deposits.
On the mean spherical approximation for hard ions and dipoles
Authors: Blum L. , Vericat Fernando , Fawcett W. R.
Published: July 2002

The analytical solution of the mean spherical approximation for the case of equal size ions and different size solvent is reexamined using only two parameters: a polarization parameter λ and a screening parameter Γ. We show that the ion dipole cross energy parameter, which in previous work was obtained solving a cubic equation, can be obtained from a linear algebraic equation. Therefore, the inverse problem of calculating the reduced charge parameter d0 and the reduced dipole parameter d2 from λ and Γ is reduced to a system of two equations: a cubic for d0, and a linear for d2. Simpler expressions for the thermodynamic parameters are also obtained.
Simulation of photodissociation dynamics of HBr adsorbed on a LiF (001) surface
Authors: Huang Zhi-Hong , Guo Hua
Published: July 2002

Computer simulations are performed to study the dynamics of the photodissociation of a single HBr adsorbed on a LiF (001) surface. A cluster model, including 50 moving atoms in two layers surrounded by five static walls consisting of 274 fixed atoms, is used to represent the surface. Since only one HBr molecule is allowed to sit on the surface, interactions between coadsorbates and possible collisions between adsorbed species are not considered in this model. The equilibrium properties of the HBr molecule on the surface prior to the photon absorption are obtained through Monte Carlo simulations, while the subsequent dissociation dynamics is described by molecular dynamics. Two dissociation channels corresponding to the ground and excited spin–orbit states of bromine are explicitly considered and transitions from one excited-state surface to another are treated with the Landau–Zener model. The kinetic energy and angular distributions of the dissociated H atoms as well as the Br/Br* ratio are determined from 600 trajectories. At 100 K and 193 nm, the calculated kinetic-energy distribution of the H fragment agrees with experimental data very well, as does the Br/Br* ratio. Neither the H kinetic energy nor the Br/Br* ratio is significantly affected by the presence of the surface. The energy exchange between the H atom and the surface is minimal, but increases when H is replaced by deuterium. The calculated H fragment angular distribution deviates from experimental data by approximately 15°. The deviation can be attributed to the scattering of the dissociating H fragment by a coadsorbate. We have also investigated the influence of the bulk temperature and the incident photon wavelength on the dynamics.
The influence of diffusion on photoinduced electron transfer and geminate recombination
Authors: Dorfman R. C. , Fayer M. D.
Published: July 2002

The influence of diffusion on photoinduced electron transfer and geminate recombination in solutions of randomly distributed donors and acceptors is explored. The focus is on the effect diffusional motion has on geminate recombination. The reactive state (state following photoinduced electron transfer) probability is calculated as a function of diffusion constant and relative permittivity for three intermolecular potential cases: attractive, repulsive, and no Coulomb potentials. Also calculated are the reactive state yield and reactive state survival fraction. Both forward and back electron-transfer rates are distance dependent (not contact transfer). Any diffusion constant can be investigated, and donor–acceptor and acceptor–acceptor excluded volumes are taken into account. The model developed here is compared with slow and fast diffusion limits as well as with the theories of Smoluchowski, and Collins and Kimball.
Electronic structure of small GaAs clusters. II
Authors: Lou L. , Nordlander P. , Smalley R. E.
Published: July 2002

The electronic and geometrical structures of GaxAsy(x + y = 3–10) clusters in nonstoichiometric (x≠y) compositions have been calculated using the local-spin-density method. The results show that all even-numbered GaxAsy(x + y = even) clusters have closed shell electronic structures. The calculated ionization potential and electron affinity for the even-numbered clusters are distinctively different from the neighboring odd-numbered clusters, forming an even/odd alternating pattern with increasing cluster size. This calculation, combined with the results from an earlier calculation for stoichiometric compositions (x=y), shows that the electronic ground states of even-numbered gallium arsenide clusters in the size range 4–10 atoms are closed-shell singlet states with a substantial highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap.
Pseudospectral full configuration interaction
Authors: Martinez Todd J. , Mehta Aseem , Carter Emily A.
Published: July 2002

A pseudospectral formulation of the full configuration interaction method is presented in this paper. This represents the first application of the pseudospectral approximation to configuration interaction expansions. It is shown that a formal scaling advantage of n, the number of molecular orbital basis functions, is achieved. The spectral and pseudospectral total energies obtained for a series of first-row atoms and ions are compared. The relative operation counts of the spectral and pseudospectral methods are also discussed in this paper. Finally, two hybrid spectral/pseudospectral approximations that vastly improve the accuracy of the pseudospectral total energies are presented.
Theoretical study of the low-lying states of trans-1,3-butadiene
Authors: Serrano-Andrés Luis , Sánchez-Marín José , Nebot-Gil Ignacio
Published: July 2002

We present extensive ab initio calculations on the low-lying electronic states of trans-1,3-butadiene within the multireference configuration interaction (MRCI) framework by selecting the configurations with a perturbative criterion. The X 1Ag ground state and 1 3Bu, 1 3Ag, 2 1Ag, and 1 1Bu valence excited states have been calculated at a fixed geometry. The results obtained are in good agreement with previous experimental and calculated values, and could help to understand polyene spectroscopy, photochemistry, and photophysics. The advantages of a MRCI method where the most important contributions to the total MRCI wave function, perturbatively selected, are treated variationally, and the remaining terms are evaluated by means of a perturbational approach, are also discussed. Furthermore, a criterion in order to build a correlation-consistent configuration interaction space is stated and, therefore, a reliable approximation to achieve accurate energy differences is obtained. Several monoelectronic molecular-orbital basis functions are tried in order to select the most adequate to describe each state.
Treatment of rigid bodies by diffusion Monte Carlo: Application to the para-H2⋅⋅⋅H2O and ortho-H2⋅⋅⋅H2O clusters
Authors: Buch V.
Published: July 2002

Application of the diffusion Monte Carlo method is discussed for ground states of molecular systems in which molecules are treated as rigid bodies. The method is applied to the investigation of the para-H2...H2O and ortho-H2...H2O clusters. Significant differences are demonstrated in the bonding of para-H2 and ortho-H2 to the water molecule.
Quantum molecular dynamic simulation of an excess electron in dense helium
Authors: Boltjes B. , de Graaf C. , Rettschnick R. P. H. , de Leeuw S. W.
Published: July 2002

In this paper we present results of simulations of an electron in dense helium with ground state dominance, at 309 K and reduced densities ρ*=0.25, 0.50, and 0.90. Some results at ρ*=1.10 and T=309 K and at ρ*=0.25, T=77 K are also presented and discussed. From the mean square displacement of the electron, the diffusion coefficient and mobility was computed. Properties of six eigenstates with the lowest energy eigenvalues of instantaneous configurations are presented and discussed. The energy eigenvalues of the instantaneous potentials are compared with an eigenvalue spectrum computed with a spectral method. They are also compared with the energy eigenvalues obtained from the mean radial potential felt by the electron.
A new problem in the correlation of nuclear-spin relaxation and ionic conductivity in superionic glasses
Authors: Tatsumisago M. , Angell C. A. , Martin S. W.
Published: July 2002

Following the recent resolution of the longstanding problem of reconciling constant frequency nuclear-spin lattice relaxation (SLR) activation energies and d.c. conductivity activity energies in ion conducting glasses, we point out a new problem which seems not to have been discussed previously. We report conductivity data measured at a series of fixed frequencies and variable temperatures on a lithium chloroborate glass and compare them with SLR data on identically prepared samples, also using different fixed frequencies. While phenomenological similarities due to comparable departures from exponential relaxation are found in each case, pronounced differences in the most probable relaxation times themselves are observed. The conductivity relaxation at 500 K occurs on a time scale shorter by some 2 orders of magnitude than the 7Li SLR correlation, and has a significantly lower activation energy. We show from a literature review that this distinction is a common but unreported finding for highly decoupled (fast-ion conducting) systems, and that an inverse relationship is found in supercoupled salt/polymer ‘‘solid’’ electrolytes. In fast-ion conducting glasses, the slower SLR process would imply special features in the fast-ion motion which permit spin correlations to survive many more successive ion displacements than previously expected. It is conjectured that the SLR in superionic glasses depends on the existence of a class of low-lying traps infrequently visited by migrating ions.
Use of an improved ion–solvent potential-energy function to calculate the reaction rate and α-deuterium and microsolvation kinetic isotope effects for the gas-phase SN2 reaction of Cl−(H2O) with CH3Cl
Authors: Zhao Xin Gui , Lu Da-Hong , Liu Yi-Ping , Lynch Gillian C. , Truhlar Donald G.
Published: July 2002

We present calculations of the rate constants and secondary kinetic isotope effects for the gas-phase SN2 reaction Cl−(H2O)+CH3Cl based on a new chloride–water potential-energy function that has been specifically converged for heavy-water isotope effects. The results are compared to new calculations employing five chloride–water potential-energy functions that have been developed for simulations of aqueous solutions. In all calculations the ClCH3Cl− solute intramolecular potential is taken from a previous semiglobal fit to ab initio calculations including electron correlation. We also examine two different intramolecular water potentials, and we examine the effect of treating the CH3 internal rotation at the ClCH3Cl−(H2O) transition state as a hindered rotation. Both the CH3/CD3 (α-deuterium) and H2O/D2O (microsolvation) kinetic isotope effects are studied.
Intermolecular bonding and vibrations of phenol⋅H2O (D2O)
Authors: Schütz Martin , Bürgi Thomas , Leutwyler Samuel , Fischer Thomas
Published: July 2002

Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6-31G(d,p) and 6-311++G(d,p) basis sets. Fully energy-minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest-energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6-311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra- and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H-bond axis, and the β2 mode are strongly anharmonically coupled, and a two-dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h-phenol⋅H2O and d-phenol⋅D2O employing two-color resonance-two-photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes.
The geometry of large fullerene cages: C72 to C102
Authors: Zhang B. L. , Wang C. Z. , Ho K. M. , Xu C. H. , Chan C. T.
Published: July 2002

Combining an efficient simulated annealing scheme for generating closed, hollow, spheroidal cage structures with a tight-binding molecular-dynamics method for energy optimization, the ground-state structure of every even-numbered carbon fullerene from C72 to C102 is determined. As a general trend, most ground-state structures of the large fullerenes have relatively low symmetries. In many cases, several isomers of a fullerene are found to have competitively low energies, which suggests that a mixture of these isomers can be observed in experimental prepared samples.
The direct observation, assignment, and partial deperturbation of the ν4 and ν6 vibrational fundamentals in Ã 1A
u acetylene (C2H2)
Authors: Utz A. L. , Tobiason J. D. , Carrasquillo M. E. , Sanders L. J. , Crim F. F.
Published: July 2002

A pulsed-laser double resonance technique provides previously unavailable spectroscopic data on the rovibrational structure of Ã 1Au acetylene (C2H2). Our assignment and analysis of transitions to the Ã state ν4′ (torsion) and ν6′ (antisymmetric in-plane bend) vibrational fundamentals uncovers a strong Coriolis interaction between these two nearly degenerate modes and weaker Coriolis interactions between the ν4′/ν6′ pair and remote Ã state rovibrational levels. We deperturb the direct Coriolis interaction between ν4′ and ν6′ to obtain vibrational frequencies, Coriolis coupling constants and partially deperturbed rotational and centrifugal distortion constants for these previously unobserved fundamentals. Parity selection rules for the Ã←X̃ band permit an unambiguous assignment of the vibrations (ν4′=764.9±0.1 cm−1 and ν6′=768.3±0.2 cm−1). We use these new experimental values to reassign several Ã state vibrations and to assign previously unidentified Ã state levels. We also identify two vibrational resonances that seem to be important in determining the rovibrational structure of Ã 1Au C2H2.
A theoretical stochastic model for the A+1/2B2→0 reaction
Authors: Mai J. , Kuzovkov V. N. , von Niessen W.
Published: July 2002

A surface reaction model for the A+1/2B2→0 reaction is studied by a theoretical approach. To this end, we introduce a stochastical ansatz which describes the system behavior by master equations. These equations are solved numerically in the superposition approximation. The results of the theoretical description are in good agreement with the corresponding computer simulations of the system. We focus our interest on the study of oscillations. The correlation functions and the parameter ranges in which oscillations are possible are studied in detail. This model as well as the computer simulations describe some aspects of the heterogeneously catalyzed oxidation of CO on a Pt surface.
A combined theoretical and experimental study of the dissociation of benzene cation
Authors: Klippenstein Stephen J. , Faulk James D. , Dunbar Robert C.
Published: July 2002

Variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations of the energy and angular momentum dependence of the rate constant for the dissociation of C6H6+ into C6H5+ and an H atom are reported. In these variational calculations both the definition of the reaction coordinate and its value are independently optimized. A model potential-energy surface which interpolates between a Morse potential at short range and an ion-induced dipole potential at long range is employed in these variational calculations. The fully optimized variational results indicate that the transition state for this dissociation occurs at separation distances of about 3–4 Å and that the available phase space in the transition state is typically a factor of 5 lower than that predicted by phase space theory. Experimental measurements were made of the time-resolved product ion intensity resulting from the laser-induced dissociation of a thermal (≊375 K) distribution of benzene cations. An ion cyclotron resonance trap was used over a range of photolysis wavelengths from 266 to 285 nm. The observed time dependences in the product ion signals are a result of both dissociative and radiative relaxation processes with a deconvolution procedure yielding estimated dissociation rate constants. Satisfactory agreement between the theoretical and experimental results, including the previous experimental results of Neusser and co-workers [J. Phys. Chem. 93, 3897 (1989), and references cited therein] is obtained for an assumed dissociation energy of 3.88 eV to the lowest triplet state of C6H5+.
The reversible reaction A+B = C in solution. A system-size expansion approach on the base of reactive many-particle diffusion equations
Authors: Naumann Wolfgang
Published: July 2002

The diffusion-influenced reaction A+B■C is reconsidered by using an approach which starts directly from the reactive many-particle diffusion equations which govern the change in time of system states with a defined number of reactive particles. The classical problem is transformed into a more compact ‘‘quantum’’ one by using a second quantization procedure. In this way, by straightforward operator manipulations, exact state-specific evolution equations can be derived. To prove the conditions for an approximate deterministic description of macroscopic systems, a system-size expansion in the sense of van Kampen is applied to these equations. By approximating the triplet and quadruplet terms in the evolution equations, a rate equation, a Fokker–Planck equation for the particle number fluctuations, and an evolution equation for the AB-pair distribution function can be derived which are consistent with one another. The results of this approach are compared with those of other recent studies including the stochastic approach I used in [Chem. Phys. 150, 187 (1991)].
Empirical and semiempirical interaction potentials for rare gas–rare gas and rare gas–halide systems
Authors: Mansky E. J. , Flannery M. R.
Published: July 2002

The Tang–Toennies (TT) semiempirical model potentials for ion–atom systems is applied to the rare gas–halide negative ion exciplexes. The coefficients defining the repulsive Born–Mayer term in the TT semiempirical potentials are determined from the equilibrium bond length Re and dissociation energy De taken from ab initio calculations and from transport studies of these molecular ions. The damped dispersion and induction energy terms in the TT potentials are obtained from coupled Hartree–Fock calculations for the neutral rare gas atoms and F− and Cl− ions. The multipole polarizabilities for the heavier halogen atomic negative ions are estimated from a knowledge of polarizability ratios across isoelectronic sequences. The resultant semiempirical ionic potentials are compared to available ab initio calculations and the results of inversion of transport theory. To facilitate the comparison of the (sparse) ab initio data with the semiempirical potentials, a simple fitting procedure is presented for determining empirical potentials for diatomic molecules from a set of three constraint equations. The fitting procedure is applied to a total of 22 rare gas excimers and rare gas–halide exciplexes (both neutral and ionic) of interest to a variety of applications in gaseous discharges and excimer lasers. A three-term representation of the empirical potentials generated is accomplished with the use of a minimal data set which include the ‘‘geometric’’ parameters {R0,Re,De} and the additional parameters {αd, I.P., E.A.} needed for the dispersion and induction energy terms.

A novel feature of the empirical procedure is the formulation of the constraint equations at two nuclear displacements (one constraint at R0, wherein the potential passes through zero, and two constraints at Re the equilibrium separation) which yields an accurate fit to available ab initio data and greatly extends the range of internuclear separations R for which an accurate piecewise analytical empirical potential can be generated. To test the relative importance of the different terms in the fitted three-term empirical representations, the classical orbiting cross section Qorbit(E) is computed using the full empirical potential and compared against the standard Langevin orbiting cross section Qpol(E) for a pure polarization interaction.
CO on Pt(335): Vibrational overtones and site dependence of the vibrational Stark effect
Authors: Luo J. S. , Tobin R. G. , Lambert David K. , Fisher Galen B. , DiMaggio Craig L.
Published: July 2002

Electron energy loss spectroscopy is used to compare atop CO at two sites on Pt(335): on the flat (111) terrace and on the step edge. The cross section for the C–O stretch overtone of the terrace species is not significantly larger than that of the edge species. Previous experiments have shown that the vibrational frequency of terrace CO responds much less to applied electrostatic field than does edge CO, even though their ir cross sections are about the same. The present experiment shows that CO has similar molecular properties at the two sites: a chemical explanation for the different Stark tuning rates is ruled out. The difference could be due to spatial variation of the electric field. Local screening of almost all the static electric field at terrace sites is one possibility, but a simple jellium model is unable to explain the observed effect. Interaction of the molecular quadrupole moment with the spatially varying field near the surface is also important.
Pair interaction energy between deformable drops and bubbles
Authors: Danov K. D. , Petsev D. N. , Denkov N. D. , Borwankar R.
Published: July 2002

The potential energy of interaction between two deformed particles, having the shape of spherical segments and separated by a planar film, is considered. An exact explicit expression for the van der Waals interaction energy between such deformed particles of arbitrary size and deformation is derived by using the microscopic approach of Hamaker [Physica 4, 1058 (1937)]. By means of Derjaguin’s approximation [Kolloid Z. 69, 155 (1934)] explicit expressions for the electrostatic, steric, depletion, and other types of interaction are derived. The relative contributions of the interaction across the planar film and between the spherical surfaces surrounding the film, are analyzed for different cases. The surface deformation energy caused by an increase of the interfacial area during the particle deformation is also considered. The implication of the obtained formulas for treatment of the particle interactions in some particular systems like emulsions, microemulsions, and vesicles, as well as for the adhesion of such particles to a solid wall is briefly discussed.
Proton transfer dynamics in substituted 3-hydroxyflavones: Solvent polarization effects
Authors: Swinney T. C. , Kelley D. F.
Published: July 2002

The spectroscopy and excited state proton transfer (ESPT) dynamics of 4′-N,N-dimethylamino-3HF (I) and 4′-N,N-diethylamino-3HF (II) have been studied in acetonitrile/benzene solvent mixtures. Solvent composition-dependent spectral shifts are observed and can be understood in terms of an Onsager cavity model. Analysis of these spectral shifts accurately predicts solvent composition-dependent excited state equilibrium constants, which are also experimentally determined. The ESPT rates are analyzed within the framework of a transition state theory treatment of solvent polarization-mediated proton transfer. This treatment is analogous to electron transfer theory. In this treatment, the energetics of the transition state are largely determined by known solvent properties and the solvent-dependent spectroscopy. This analysis yields solvent-dependent ESPT activation energies. The corresponding calculated ESPT rates are in excellent agreement with the experimentally determined rates.
Shear viscosity of model mixtures by nonequilibrium molecular dynamics. I. Argon–krypton mixtures
Authors: Lee Song Hi , Cummings Peter T.
Published: July 2002

We present nonequilibrium molecular dynamics simulations of argon–krypton liquid mixtures at constant temperature and pressure (T=135 K and P=40 bar) as the base case for a consistent study of the role of intermolecular potentials on the transport properties of molecular liquids and their mixtures. Using Lennard-Jones models for the two species, very good agreement with experiment is obtained for the thermodynamic and transport properties of the two pure fluids. Simulations at constant temperature and density using the Barker–Fisher–Watts potentials for pure fluids yields predictions for thermodynamic properties and viscosity in excellent agreement with experiment.
Magnetic screening of nuclei by electrons as an effect of geometric vector potential
Authors: Yin Li , Alden Mead C.
Published: July 2002

It is known that the usual Born–Oppenheimer procedure, when applied to a molecular system in an external magnetic field, leads to an effective nuclear Hamiltonian in which the magnetic field acts on bare nuclei, so that, for instance, a neutral atom or molecule would be deflected in such a field as if the nuclei were the only charged particles. Elaborating on previous work, by others as well as ourselves, we discuss how this situation can be remedied by including the effect of the geometric phase and geometric vector potential. In particular, it is shown that inclusion of the geometric vector potential leads to results in accord with intuition and semiclassical calculations in simple cases, and that for all cases it provides a consistent Born–Oppenheimer-type formulation, free of obvious defects, for a molecular system in an external magnetic field.
Ab initiocomputation of semiempirical π-electron methods. I. Constrained, transferable valence spaces in ℋν calculations
Authors: Martin Charles H. , Freed Karl F.
Published: July 2002

Large basis set ℋν calculations are performed for ethylene, cyclobutadiene, and trans-butadiene in order to examine the accuracy of approximate ℋν calculations specifically designed to reproduce a Pariser–Parr–Pople-like (PPP) semiempirical model from first principles. This is the first ℋν study in which systematic, semiempirical-like approximations to ℋν are examined for their accuracy and basis set dependence in computations of vertical excitation energies and ionization and potentials for large, polyatomic molecules. We focus explicitly on two fundamental semiempirical-like approximations to ab initio ℋν calculations. First, it is shown that ab initio ℋν calculations can produce reasonably accurate vertical excitation energies and ionization potentials for the low-lying valencelike states when using valence orbitals ‘‘constrained’’ to be linear combinations of transferable, carbon atom pπ orbitals. The differences between full ab initio ℋν and constrained ℋν calculations for all states (with one exception each in trans- and cyclobutadiene) range between roughly 0.00 and 0.25 eV for all states and, furthermore, these constrained errors show no basis set dependence. Second, it is shown that the excitation energies and ionization potentials are not influenced significantly by the nonclassical ℋν three- and four-body operators. When neglecting these interactions, errors of only a few tenths of an electron volt are again encountered, and these errors also display no basis set dependence.

These conclusions are shown to be consistent with previous ab initio ℋν calculations. Because the three- and four-body ℋν terms are neglected as a leading approximation, the computed oneand two-body empirical parameters which appear in the PPP-like model. The present calculations explain the errors expected to occur in further, approximate ℋν calculations, the states which can be reasonably described with both approximate ℋν calculations and PPP-like models, and the ab initio basis set requirements for future ab initio ℋν studies of these models.
Structure of the propene⋅sulfur dioxide complex
Authors: Xu Li-Wei , Kuczkowski Robert L.
Published: July 2002

The rotational spectra of eight isotopomers of the propene⋅SO2 complex have been observed with a Fourier transform microwave spectrometer. The rotational constants of the normal species are A=4269.564 MHz, B=1577.2661 MHz, and C=1469.6335 MHz. The structure of the complex was derived from least-squares fitting of the 24 moments of inertia. It has a stacked, near-parallel planes configuration. The distance between the centers of mass of the two monomers is 3.26(5) Å. The sulfur atom is approximately above the propene double bond. The C2 axis of SO2 nearly eclipses the carbon–carbon single bond with the oxygen atoms towards the methyl group. The dipole moment of the complex was determined by Stark effect measurements to be μ=1.34(3) D. The binding energy is estimated to be 2.9 kcal/mol from the pseudodiatomic model. Both electrostatic and ab initio calculations have been carried out to rationalize the structure and properties of the complex. The effect of methyl group substitution on the structures and properties of the ethylene⋅SO2, propene⋅SO2, and toluene⋅SO2 complexes is discussed.
Instantaneous normal mode analysis of liquid water
Authors: Cho Minhaeng , Fleming Graham R. , Saito Shinji , Ohmine Iwao , Stratt Richard M.
Published: July 2002

We present an instantaneous-normal-mode analysis of liquid water at room temperature based on a computer simulated set of liquid configurations and we compare the results to analogous inherent-structure calculations. The separate translational and rotational contributions to each instantaneous normal mode are first obtained by computing the appropriate projectors from the eigenvectors. The extent of localization of the different kinds of modes is then quantified with the aid of the inverse participation ratio—roughly the reciprocal of the number of degrees of freedom involved in each mode. The instantaneous normal modes also carry along with them an implicit picture of how the topography of the potential surface changes as one moves from point to point in the very-high dimensional configuration space of a liquid. To help us understand this topography, we use the instantaneous normal modes to compute the predicted heights and locations of the nearest extrema of the potential. The net result is that in liquid water, at least, it is the low frequency modes that seem to reflect the largest-scale structural transitions. The detailed dynamics of such transitions are probably outside of the instantaneous-normal-mode formalism, but we do find that short-time dynamical quantities, such as the angular velocity autocorrelation functions, are described extraordinarily well by the instantaneous modes.
Thermal dissociation of NO on Pd surfaces: The influence of step sites
Authors: Ramsier R. D. , Gao Q. , Neergaard Waltenburg H. , Yates J. T.
Published: July 2002

The thermal behavior of NO on (flat) Pd(111) and (stepped) Pd(112) has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES) techniques. NO is shown to adsorb molecularly on both Pd(111) and Pd(112) in the temperature range 300–373 K. NO desorbs molecularly from Pd(111) near 500 K with evidence for slight NO dissociation. In contrast, on Pd(112), in addition to NO, relatively large amounts of N2(7x) and N2O(15x) are observed to desorb near 500 K, compared to Pd(111). This result indicates that the influence of the step sites on Pd(112) is to catalyze the decomposition of NO upon heating. This is a surprising result in light of the fact that NO molecules preferentially bind to terrace sites, instead of step sites, on Pd(112). HREELS measurements indicate the presence of small amounts of surface-bound O (resulting from NO thermal decomposition) coadsorbed with NO on both Pd(111) and Pd(112) after NO adsorption at 373 K, followed by 490 K annealing. In addition, HREELS provides evidence for the existence of subsurface O formed only on Pd(112) during this procedure. The presence of steps on Pd(112) presumably offers an efficient pathway for O incorporation within the outermost Pd layers, as no spectroscopic evidence for subsurface O exists for Pd(111). Annealing both surfaces to 550 K induces the diffusion of both surface O and subsurface O into the Pd bulk.
State-to-state rotational energy transfer measurements in methane (CHD3) by infrared double resonance with a tunable diode laser
Authors: Klaassen J. J. , Coy S. L. , Steinfeld J. I. , Roche Ch.
Published: July 2002

An infrared double-resonance laser spectroscopic technique is used to study state-resolved rotational energy transfer (RET), vibration–vibration (V–V) transfer, and symmetry-exchanging collisions in asymmetrically deuterated methane (CHD3). The molecules are prepared in selected rovibrational states of the {v3,v6}=1 dyad using coincidences between CO2 laser lines and dyad←ground state transitions. Measurements of both the total rate of depopulation by collisions and the rates of transfer into specific rovibrational (v,J,K) levels are carried out using time-resolved tunable diode laser absorption spectroscopy. Total excited-state depopulation and ground-state recovery rates range from 0.5 to 1.0 times the Lennard-Jones collision rate, consistent with relaxation due to short-range forces. V–V (ν6→ν3) processes contribute about 10% of the total relaxation rate, and symmetry-changing (A↔E) collisions occur at a rate another order of magnitude smaller, viz. (0.17±0.02) μs−1 Torr−1, corresponding to an effective cross section of 0.64 Å2, around 10−2 σLJ. The symmetry-exchanging collision efficiency for CHD3 as well as for other systems reported elsewhere (CD3Cl,CH3F) can be quantitatively estimated using a simple Förster resonant exchange mechanism. The state-to-state RET rates are modeled using a kinetic master equation. A strong propensity rule, ΔK=±3x (integer), similar to that found for highly dipolar symmetric tops such as ammonia, applies to CHD3 as well. We conclude that the flow of energy and angular momentum in molecular relaxation is dominated by the internal level structure of the molecule, rather than by specific details of the intermolecular potential.