bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvation of Hg(3P1) in the HgArn clusters through resonant enhanced multiphoton ionization: Evidence for nonadditive effects in the excited state potentials
Authors: Martrenchard-Barra S. , Jouvet C. , Lardeux-Dedonder C. , Solgadi D.
Published: July 2002

Mass resolved resonance enhanced excitation spectra of HgArn have been measured in the vicinity of the Hg(6s6p) 3P1→(6s2) 1S0 transition. The spectra are compared with calculations based on additive potentials for both the ground and the excited state. These potentials give reasonable agreement with the experiments. Deviation from this simple model occurs in the excited electronic state mainly for structures where the argon atoms are in the closest approach from the Hg.
A comparison of unrestricted Hartree–Fock- and restricted open-shell Hartree–Fock-based methods for determining the magnetic hyperfine parameters of NO (X 2Π)
Authors: Feller David , Glendening Eric D. , McCullough E. A. , Miller R. J.
Published: July 2002

The magnetic hyperfine structure parameters of NO X 2Π have been determined through a variety of ab initio methods based on restricted and unrestricted Hartree–Fock zeroth order wave functions. Examples of the former include singles configuration interaction (CI), multireference CI, and averaged coupled pair functional theory. Examples of the latter include Mo/ller–Plesset perturbation theory (through fifth order, with estimates to infinite order), coupled cluster methods, and quadratic CI (with approximate inclusion of triple and quadruple excitations). The performance of the various methods in reproducing the difficult-to-describe 14N and 17O isotropic hyperfine interactions is judged in light of both experimental data, where available, and estimated full CI values. The full CI limit was approached through a systematic sequence of ever-more-extensive, selected multireference CI wave functions that would, in principle, include the full CI as its final element. While the isotropic coupling constants were found to converge very slowly along this sequence, at least in comparison to other one-electron properties, the selected CI approach was efficient enough in its recovery of correlation effects to be used with large basis sets. The biggest calculation in the sequence of CI wave functions included over two million configurations. Energies and properties exhibited sufficient regularity to allow fitting with simple functional forms. The error arising from the lack of basis set completeness is estimated by comparison to fully numerical, partial-wave self-consistent field (SCF) and singles CI results. Effects due to vibrational motion are accounted for by numerical integration of the one-dimensional Schrödinger equation.
Derivative Hartree–Fock approach to molecular Sternheimer shielding. Calculation of intermolecular influence on nuclear quadrupole coupling
Authors: Augspurger Joseph D. , Dykstra Clifford E.
Published: July 2002

Molecular Sternheimer shielding constants, γ, the proportionality constants relating the electric field gradient at a quadrupolar nucleus to an external electric field gradient are usually introduced phenomenologically. In this report, we take a comprehensive view of the sensitivity of the electric field gradient at a nucleus to arbitrary external electrical potentials and we show how the response can be obtained from analytically determined properties via derivative Hartree–Fock theory. From application of this ab initio technique, values have been obtained for the first and second order changes in nuclear quadrupole coupling with respect to external fields and field gradients, as well as nearby ideal multipole moments, for HCN and HCl. These values have been used to evaluate the change in the nuclear quadrupole coupling for several weakly bound complexes and to provide a nonempirical approach to relative effects on Sternheimer shielding. In weak molecular complexes, the effect of uniform fields can be as sizable as the effect of external field gradients in the overall change in nuclear quadrupole coupling, and so the underlying issue of convergence of multipolar expansions is considered over a range of geometries. This is important for structural interpretations of both nuclear magnetic resonance (NMR) and microwave data, and a simple formula, representing a practical point of truncation, is presented for quadrupole coupling analysis.
Topological properties of supercooled polymeric liquid
Authors: Montero M. J. Ruiz , Mohanty Udayan , Brey J. J.
Published: July 2002

We construct an isothermal–isobaric partition function of glass-forming polymeric liquids by separating the local minima of the potential energy hypersurface from the anharmonic vibrations about these packings. The partition function is evaluated by a maximum term method. A suitable approximation to the partition function below a temperature T2 is required since the maximum term approach is not valid in this range of temperatures. We obtained the following results: (a) The density of states including thermodynamic quantities such as entropy and heat capacity reveal an Ehrenfest second-order transition at T2; (b) T2<T2,c where T2,c is the temperature of the transition when considering only configurational contributions; (c) if the density of states at the minimum energy is finite, then T2,c does not vanish; (d) the stability conditions [Eqs. (17)–(20)] necessary to derive (b); (e) the discontinuity of various thermodynamic quantities such as heat capacity, thermal expansion coefficient and compressibility at T2.
The effect of internal rotation on the methyl CH-stretching overtone spectra of toluene and the xylenes
Authors: Anastasakos Louis , Wildman Timothy A.
Published: July 2002

The structure of methyl CH-stretching overtone bands in the vibrational spectra of methylbenzenes was investigated theoretically. The anharmonic CH-stretching vibration, described by a Morse potential, was represented in terms of a harmonic basis while hindered internal rotation of the methyl group was represented by a rigid rotor attached to an infinitely massive frame. Relatively weak coupling between the anharmonic CH vibration and the hindered internal rotation is sufficient to shift the positions of rovibrational lines from a PQR-like rotational contour to patterns similar to those observed experimentally. For high rotational barriers, as in o-xylene, the rovibrational transitions form two bands associated with conformationally nonequivalent CH-bonds, consistent with the conformational preference established by microwave spectroscopy and molecular orbital calculations. For nearly free internal rotation, as in toluene, m-xylene and p-xylene, a prominent middle band is also present. This ‘‘free rotor’’ band corresponds to rotational transitions between states high above the barrier and disappears as the barrier height increases. The outer bands correspond to transitions for which either the initial or the final state is below or near the barrier height in energy. Contrary to earlier suggestions, the band structure is not indicative of the conformational preference of the methyl group in toluene. In fact, the calculated spectra of nearly free internal rotors are insensitive to this preference.
Dynamics of CS2 in the large spectral bandwidth stimulated Rayleigh-wing scattering
Authors: Rivoire Geneviève , Wang Dadi
Published: July 2002

A theoretical model for the large bandwidth stimulated Rayleigh-wing scattering [D. Wang and G. Rivoire, J. Chem. Phys. 98, 9279 (1993)] has been built, based on the existence of a translational coherent collective movement driven by an intense optical field. Its rise and decay times deduced from the comparison with the experimental spectral observation are as short as 115 fs.
Relaxation of vibrationally excited HCl molecules in the H2O–HCl collision complex
Authors: Ree J. , Kim Y. H. , Shin H. K.
Published: July 2002

The temperature dependence of the relaxation of HCl(v=1) by H2O in a complex-mode collision is studied in a semiclassical approach. The de-excitation probability takes a maximum value near room temperature, and it decreases logarithmically with increasing temperature. The dependence is nearly linear. Below room temperature, the relaxation becomes less efficient. This unusual temperature dependence is a result of the vibrational relaxation occurring in complex-mode collisions, which are dominated by large impact parameter interactions. The principal pathway for the removal of vibrational energy is the H–Cl oscillatory and librational motions along the O–H–Cl configuration. When these hindered motions gain the energy, they undergo transitions to free rotational states. The decreased energy transfer efficiency at low temperature is due to the slowing of rotational motions. Energy transfer to the O–Cl large-amplitude motion is of minor importance.
Application of the time-dependent Hartree grid–configuration interaction method to the desorption of diatomic molecules from solid surfaces
Authors: Campos-Martínez José , Coalson Rob D.
Published: July 2002

We study UV laser induced photodesorption of diatomic molecules from solid surfaces by means of the time-dependent Hartree grid–configuration interaction (TDHG–CI) method [J. Chem. Phys. 93, 4740 (1990)]. Converged partial and total absorption cross sections are computed in several cases to demonstrate the flexibility and accuracy of the method. Comparison to TDHG results without CI corrections is also made. The failure of simple TDHG dynamics to reproduce state resolved rotational cross sections in various strong coupling limits is noted as a justification for the development of TDHG–CI algorithms.
Effect of pressure on intermolecular and intramolecular phonons in solid C60
Authors: Yu Jin , Kalia Rajiv K. , Vashishta Priya
Published: July 2002

The effect of pressure on inter- and intramolecular phonons in solid C60 is studied using a unified model which consists of a tight-binding potential for the intramolecular interaction and a Lennard-Jones and bond charge model for the intermolecular interaction. At various pressures (up to 56 kbar), the phonon dispersion and density of states of solid C60 are calculated in the energy range from 0 to 210 meV. At zero pressure, the intermolecular phonon density of states shows peaks around 2.3 and 3.7 meV, and extends to 7.6 meV. Not only the intermolecular phonon modes but also the intramolecular modes show significant dispersions, especially those modes with energy below 70 meV. Under pressure, the intermolecular phonon spectrum shows strong broadening. The libron modes shift to higher frequencies at a rate of 0.40 cm−1/kbar. The effect of pressure on intramolecular modes manifests in two ways: (i) the lower-frequency modes broaden and shift toward higher energies, and (ii) the higher-frequency modes split and shift upward in energy. Intramolecular modes shift to higher energy at a rate up to 0.88 cm−1/kbar. Most Raman and infrared active modes show strong pressure dependence. Inter- and intramolecular phonon spectra at various pressures are presented and results are compared with experiments.
Intracluster rearrangement of protonated nitric acid: Infrared spectroscopic studies of H+(HNO3)(H2O)n
Authors: Cao Yibin , Choi Jong-Ho , Haas Bernd-Michael , Johnson Matthew S. , Okumura Mitchio
Published: July 2002

Infrared spectra of clusters of protonated nitric acid and water exhibit a marked change with cluster size, indicating that an intracluster reaction occurs with sufficient solvation. In small clusters, H2O binds to a nitronium ion core, but at a critical cluster size the NO+2 reacts. A lower bound of 174 kcal/mol is found for the proton affinity of HNO3.
Electronic friction and electron transfer rates at metallic electrodes
Authors: Smith Barton B. , Hynes James T.
Published: July 2002

A theory is presented for the rate constant k for electron transfer between a metal electrode and a redox couple solute in solution, in or near the electronically adiabatic regime. The departure of k from its electronically adiabatic transition state theory limit kTST is described via Grote–Hynes theory, and includes two sources of friction. The electronic friction arises from excitation of electron hole pairs in the metal, i.e., electronic nonadiabaticity effects. The solvent friction arises from solvent dynamical effects. Both features can result in significant reduction of k below kTST, and their interplay can lead to interesting nonmonotonic variations with reaction overpotential.
Integral equations of the correlation functions for polymeric liquids
Authors: Eu Byung Chan , Gan Hin Hark
Published: July 2002

The integral equations for intramolecular and intermolecular correlation functions are derived for nonrigid polymeric (polyatomic) liquids by the device of the Kirkwood charging parameters. These integral equations are cast into mean-field-type equations by using the potential elimination method, reported previously for dense simple fluids. Based on the mean-field integral equations, we examine the superposition approximations for various levels of correlation. The present theory provides a means to make systematic corrections for superposition approximations for correlation functions of various orders. Upon using the superposition approximations for the triplet correlation functions in the Kirkwood hierarchy and an assumption or another concerning the charging parameter dependence of the cavity functions, we derive a set of generalized Percus–Yevick and hypernetted chain integral equations for the intramolecular and intermolecular pair correlation functions for beads (sites) of polymeric (polyatomic) liquids. This set of integral equations allows the intramolecular and intermolecular correlation functions to be determined self-consistently. The connection of this set of integral equations to the bead–bead (molecular) Ornstein–Zernike relation is pointed out. The integral equations for the intramolecular correlation functions will be numerically solved for some properties of a single polymer chain in the infinite dilution limit in the sequel to this paper.
A combined density functional and intrinsic reaction coordinate study on the ground state energy surface of H2CO
Authors: Deng Liqun , Ziegler Tom , Fan Liangyou
Published: July 2002

Approximate density functional theory (DFT) has been used to study four elementary processes on the singlet ground state energy surface of H2CO. The elementary steps include the elimination of H2 and CO from formaldehyde, the transformation of formaldehyde to trans-hydroxymethylene, the isomerization of trans-hydroxymethylene to cis-hydroxymethylene, and the 1,2 elimination of H2 from cis-hydroxymethylene. The DFT studies were based on the local density approximation (LDA) as well as a nonlocal self-consistent field (NL-SCF) extension in which Perdew’s correlation correction and Becke’s exchange correction were added to LDA. Fully optimized structures as well as harmonic vibrational frequencies have been evaluated for all of the stationary points corresponding to the reactants, products, and transition states for the four reactions within the LDA and NL-SCF approximations. The four reactions have in addition been studied by the intrinsic reaction coordinate (IRC) method in which stationary points on the potential energy surface are connected by a steepest energy path. It is concluded that the NL-SCF method affords as good an overall fit to experiment as the fourth-order Mo/ller–Plesset single, double, triple, and quadruple excitations [MP4(SDTQ)]/6-31G**//MP2/6-31G* scheme for the S0 surface of H2CO. The DFT methods tend to afford energies for the stationary points which are too low compared to the separate species H2+CO, whereas the corresponding MP4 energies are too high. This is in particular the case for the local LDA method. We attribute this trend to a bias in favor of bond formation among the DFT methods. The reaction paths for LDA and NL-SCF were found to be very similar, and it is suggested that one might combine geometries evaluated by the less demanding LDA method with energies obtained by the more accurate and involved NL-SCF scheme.
Relativistic effects in gold chemistry. V. Group 11 dipole polarizabilities and weak bonding in monocarbonyl compounds
Authors: Schwerdtfeger Peter , Bowmaker Graham A.
Published: July 2002

Hartree–Fock and Mo/ller–Plesset second order (MP2) calculations have been carried out in order to study the stability and structure of open-shell group 11 monocarbonyl compounds MCO (M=Cu,Ag,Au). AgCO is calculated to be a very weakly bound molecule with a dissociation energy of less than 1 kJ/mol, and this casts some doubt on the previously reported identification of this species in matrix isolation studies. AuCO is stable only if relativistic effects are included, which explains the recently observed anomaly in the strength of the metal–CO interaction within the group 11 series. The metal–carbonyl interactions in CuCO and AuCO are relatively weak, with dissociation energies of about 30 kJ/mol and may be best described as a combination of dispersion, donor–acceptor (charge-transfer) and repulsive interactions. The MP2 Cu–CO dissociation energy of 32 kJ/mol is in good agreement with the estimated experimental value of 23±6 kJ/mol. At the highest level of theory, AuCO possesses a bent geometry with a bond angle of 151° at the MP2 level, but this arrangement lies only 2 kJ/mol below the linear structure. The basis set superposition error and the role of metal f functions are discussed. Dipole polarizabilities are calculated for the group 11 elements at various levels of electron correlation in order to discuss dispersion interactions. These are found to be very sensitive to relativistic and electron correlation effects. For the neutral gold atom, relativistic effects in the dipole polarizability become comparable to electron correlation contributions. The multielectron adjusted group 11 pseudopotentials give reliable results for the atomic and molecular properties discussed.
The compositional dependence of thermodynamic interactions in blends of model polyolefins
Authors: Krishnamoorti R. , Graessley W. W. , Balsara N. P. , Lohse D. J.
Published: July 2002

We have investigated the thermodynamic interactions as functions of component volume fraction φ and temperature T in six binary polymer blend systems. The components in all cases were model polyolefins, made by saturating the double bonds of nearly monodisperse polydienes. Small-angle neutron scattering measurements for single-phase melts, analyzed with the incompressible random phase approximation, were expressed in terms of the Flory–Huggins interaction parameter χ for 27 °C≤T≤167 °C and 0.1≤φ≤0.9. In most systems we found a characteristic upturn in χ at the composition extremes that diminished with increasing temperature. We also found that the component chain dimensions were unaffected by the strength of the interactions. Several theoretical attempts to explain the dependence of χ on component volume fraction were examined. Most were qualitatively inconsistent with the results, and none were fully satisfactory.
Bound excited electronic states of anions
Authors: Brinkman Elizabeth A. , Günther Erhard , Schafer Olivier , Brauman John I.
Published: July 2002

Bound excited electronic states are observed in the anions of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,5-dimethyl-N,N′-dicyanoquinodiimine (Me2–DCNQI), chloranil, and hexacyano-butadiene using electron photodetachment spectroscopy. Intensity dependence studies and two color experiments are consistent with a two-photon detachment mechanism. A mechanism is proposed to explain the observed photodetachment. The nature of the electronic state responsible for these excited states is discussed and other anions are suggested which should possess bound excited electronic states.
Pathways for H2 elimination from ethylene: A theoretical study
Authors: Jensen Jan H. , Morokuma Keiji , Gordon Mark S.
Published: July 2002

Ab initio quantum chemical methods are applied to the study of ethylene decomposition to acetylene and molecular hydrogen in the ground electronic state. Results are reported on three different pathways for ethylene decomposition—two stepwise processes involving a hydrogen transfer followed by 1,1 elimination of H2, or vice versa, and a 1,2 elimination. The latter proceeds through an energy maximum with two imaginary frequencies, rather than one as for conventional transition states. Ethylidene and vinylidene are predicted to be stationary points on the C2H4 and C2H2 potential energy surfaces, respectively. Recent photochemical studies have observed rotationally hot H2. It is shown that due to the excess energy available in the photochemical experiments, all three mechanisms can give rise to rotationally hot H2 when proper account is taken of the transverse vibrational modes along the reaction paths.
Theoretical ab initio investigation of the dipole polarizabilities of oligothiophenes and polythiophene
Authors: Champagne B. , Mosley D. H. , André J.-M.
Published: July 2002

Ab initio coupled and uncoupled Hartree–Fock polarizabilities are computed for increasingly large oligothiophenes using the 3–21G, 6–31G**, 6–311G**, and the Sadlej medium-size polarized atomic basis sets. The comparison with experimental values highlights the large sensitivity of the measured polarizabilities with respect to working conditions. By extrapolating to the infinite chain limit the polarizability values of increasingly large oligomers, we compute the asymptotic polarizabilities per thiophene ring. The transversal, perpendicular, and longitudinal asymptotic coupled Hartree–Fock polarizabilities per thiophene ring are, using the 3–21G (6–31G**) atomic basis set, 49.7 (52.5), 15.2 (20.6), and 184.1 (190.0) a.u., respectively. Polythiophene is more polarizable than polydiacetylene, polyyne, and polysilane but less polarizable than polyacetylene which, in contrast to polythiophene, is much more difficult to process. Using a procedure which relates the polarizability and the topology of the one-particle states, the major contribution to the polarizability is attributed to π-symmetry crystalline orbitals located at the top of the valence bands, which look like an alternation of segments of the highest occupied crystalline orbitals of the all-trans and trans-cisoïd polyacetylene. However, a sizeable σ contribution comes from σ-conjugated segments formed by the sulfur atoms and their two adjacent carbon atoms.
Chemical potentials of hard sphere solutes in hard sphere solvents. Monte Carlo simulations and analytical approximations
Authors: de Souza Luís E. S. , Stamatopoulou Argyroula , Ben-Amotz Dor
Published: July 2002

We report Monte Carlo simulation results for the excess chemical potentials of infinitely dilute hard spheres, and the distribution of cavity sizes, in a hard sphere fluid. The results are compared with previous simulations and analytical expressions derived from the Boublik–Mansoori–Carnahan–Starling–Leland equation of state and scaled particle theory.
Efficient polynomial expansion of the scattering Green’s function: Application to the D+H2(v=1) rate constant
Authors: Auerbach Scott M. , Miller William H.
Published: July 2002

We apply the absorbing boundary condition (ABC) discrete variable representation (DVR) theory of quantum reactive scattering to the initial state selected D+H2(v=1, j)→DH+H reaction. The ABC-DVR Green’s function is efficiently computed by a Newton polynomial expansion. We compute accurate reaction probabilities for the total energies and angular momenta required to obtain the thermal rate constants kv=1, j(T). At T=310 K, a thermal average over j=(0,1,2,3) is performed to yield the final result kv=1(310 K)=1.87×10−13 cm3 molecule−1 s−1, in quantitative agreement with the most recent experimental value (1.9±0.2)×10−13 cm3 molecule−1 s−1. The J-shifting approximation using accurate J=0 reaction probabilities is tested against the exact results. It reliably predicts kv=1(T) for temperatures up to 700 K, but individual (v=1, j) selected rate constants are in error by as much as 41%.
Dissipative structures in the CO oxidation on Pt(100)
Authors: Andrade R. F. S. , Lima D. , Dewel G. , Borckmans P.
Published: July 2002

A model for the homogeneous kinetic oscillations in the CO oxidation on Pt(100) is extended to describe space dependent situations by the introduction of two diffusive-like processes. As is well known these increase the size of the instability domain. Depending on the values of the diffusion coefficients, they lead to the existence, when the surface is homogeneous, of stationary periodic space patterns besides the uniform oscillating solutions. This may be shown both theoretically through linear stability analysis or numerical computation.
Electron spin resonance investigation of small magnesium cluster cation radicals, Mg+N, in neon and argon matrices at 4 K: Comparison with ab initio calculations
Authors: Knight Lon B. , Cleveland Cheryl B. , Frey Regina F. , Davidson Ernest R.
Published: July 2002

A series of cationic magnesium radicals (Mg+N, N=1–6) was studied by electron spin resonance (ESR) spectroscopy. The ESR data indicate that all Mg nuclei are equivalent on the ESR time scale in each of these clusters. The nuclear hyperfine interactions obtained for 25Mg+N (N=1–3) in neon and argon matrices from ESR measurements were compared with ab initio calculations. The minimum energy structures for Mg+2 and Mg+3 and the transition-state structure for Mg+3 were optimized using complete active space self-consistent field (CASSCF) wave functions. The minimum energy structure for Mg+3 was determined to be linear and the barrier for the pseudorotation between equivalent minima was calculated. Good agreement between theory and experiment is obtained if averaging between the center and end position of Mg+3 is assumed to occur in the ESR experiment in spite of the 6 kcal/mol calculated energy barrier.
Kinetics of optically detected magnetic resonance of single molecules
Authors: Brown R. , Wrachtrup J. , Orrit M. , Bernard J. , von Borczyskowski C.
Published: July 2002

Various time-resolved experiments on optically detected magnetic resonance (ODMR) of single pentacene molecules in p-terphenyl at 1.8 K are described and discussed with the help of a model based on the optical-microwave Bloch equations. Intersystem crossing rates for this system are deduced from the ODMR spectra, from the fluorescence correlation function, which varies nonmonotonically with the microwave power, and from the analysis of the fluorescence recovery transients. We also discuss the enhancement of the ODMR effect on using fluorescence photons from single molecules as a time base to trigger the microwaves in fluorescence recovery experiments.
Solving the single-reference coupled-cluster equations involving highly excited clusters in quasidegenerate situations
Authors: Piecuch Piotr , Adamowicz Ludwik
Published: July 2002

A new procedure for improving the convergence of the reduced linear equation method for solving the single-reference (SR) coupled-cluster (CC) equations in highly degenerate cases, where the convergence of the iterative process is often destabilized by ‘‘dangerous’’ denominators, is discussed and implemented. Emphasis is placed on the SR CC methods involving highly excited clusters, such as the CC method with singles, doubles, triples, and quadruples (CCSDTQ). The new algorithm is based on rearranging and quasilinearizing the nonlinear terms and correcting the energy denominators by the quasilinearized terms. The method is tested in the SR CCSDTQ calculations for a dissociating single bond.
Critical temperature shifts in thin polymer blend films
Authors: Tang Hai , Szleifer Igal , Kumar Sanat K.
Published: July 2002

We present the first comprehensive theory for the phase behavior of thin polymer blend films. Based on the Landau–Ginzburg free energy functional, our mean field analysis incorporates the influence of finite size effects and surface interactions, and explicitly considers surface segregation. The procedure for calculating the full phase diagram is provided. In symmetric blends with neutral surfaces, the reduced critical temperature shifts t are obtained in exact analytical forms. Our predictions are in good agreement with our simulations. For polymers with N≳100 (N being the polymerization index) in films much thinner than fully extended chain dimensions Nl, a unique scaling behavior t∝L−1 (i.e., the inverse film critical temperature depends linearly on 1/L) is found. When L≫Nl, an Ising-type behavior t∝N0.59L−1.59 is expected.
Edge effects on the random sequential adsorption of hot dimers
Authors: Monetti R. A. , Albano E. V. , Pereyra V. D.
Published: July 2002

Both jamming coverage and percolating probability profiles are studied for the random sequential adsorption of hot dimers in two dimensions. Jamming profiles show that the effect of the sample edge dies off after a few lattice spaces. On the contrary, in percolating profiles the effect propagates into the bulk
Multiconfiguration time-dependent Hartree studies of the CH3I/MgO photodissociation dynamics
Authors: Fang Jian-Yun , Guo Hua
Published: July 2002

A multiconfiguration time-dependent Hartree method is applied to study the photodissociation dynamics of methyl iodide on a MgO surface. The surface is assumed to be rigid and the dissociation is restricted in orientations parallel to the surface normal. Two active mathematical dimensions and two electronic states are included in our model. The dynamics of the fragments (the methyl radical and iodine atom) are complicated by the so-called ‘‘chattering effect’’ and by nonadiabatic transitions between the two dissociative states of CH3I. It is shown from the comparison with exact calculations that the single configuration time-dependent Hartree approximation fails to give an accurate description of the dynamics. The failure of the Hartree approximation, which is due to the neglect of direct spatial correlations between modes, can be effectively corrected by including a few additional configurations. Our results show that four configurations are sufficient to provide an accurate description of the complex dissociation dynamics of the CH3I/MgO system.
Evaluation of S2 for correlated wave functions and spin projection of unrestricted Mo/ller–Plesset perturbation theory
Authors: Chen Wei , Schlegel H. Bernhard
Published: July 2002

The value of S2 can be an important diagnostic tool for judging the quality of correlated wave functions. A production code has been developed to evaluate S2 for unrestricted Mo/ller–Plesset perturbation theory (UMPn), coupled clusters (UCCSD), quadratic configuration interaction (UQCISD), and Brueckner doubles (UBD) methods, and to evaluate UMP3 and UMP4 energies with spin projection. The code has been used to examine the bond dissociation potentials for HF→H+F and CH4→CH3+H. For both systems, the onset of the RBD-UBD instabitity occurs near S2≊0.35 for UCCSD or UQCISD calculations. Maximum errors in the UMP4, UCCSD(T), UQCISD(T), and UBD(T) single bond dissociation curves are near S2≊0.5. The behavior of S2 for UCCSD and UQCISD is closer to BD than MP4. Projected MP4 energies are in good agreement with full CI calculations, but between the onset of the RHF-UHF instability and the RBD-UBD instability, CCSD(T), QCISD(T), and BD(T) are significantly better. If ANorm≳1.2 for restricted calculations on single bond dissociations, it is better to use a spin-unrestricted method.
Stable multiply negative ionic chains: How many excess electrons can a finite quasilinear molecule hold?
Authors: Scheller M. K. , Cederbaum L. S.
Published: July 2002

A recently introduced construction principle is investigated with the objective of designing stable multiply negative quasilinear ions. The discussion is exemplified for chainlike potassium fluorides for which explicit calculations are carried out. The stability of the newly designed chains to fragmentation and to electron autodetachment is studied and found to depend on the terminal groups of the chains. The shortest chain which can hold three excess electrons contains five fluorine atoms; the one which can stabilize seven excess electrons has 19 fluorine atoms.
Optical absorption spectra of Au7, Au9, Au11, and Au13, and their cations: Gold clusters with 6, 7, 8, 9, 10, 11, 12, and 13 s-electrons
Authors: Collings B. A. , Athanassenas K. , Lacombe D. , Rayner D. M. , Hackett P. A.
Published: July 2002

The optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size-specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd–even alternation in the spectra of gold clusters with differing numbers of valence s electrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au13 is in reasonable agreement with Dirac scattered-wave molecular orbital considerations for icosahedral Au13 [A. F. Ramos, R. Arratia-Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is contrasted with other models based upon jellium potentials and delocalized excitations that have been used to describe small clusters of alkali metals and silver. The bonding in gold clusters is influenced by relativistic effects that increase the degree of sd hybridization in the molecular orbitals. Even though gold clusters can be described in this way, some evidence for electron shells is also presented. Thus, it is concluded that structural motifs other than jellium potentials can lead to shell structure in cluster properties.
The vibrational spectrum of isolated AlH4−: An infrared matrix isolation and ab initio study
Authors: Pullumbi Pluton , Bouteiller Yves , Manceron Laurent
Published: July 2002

An aluminium tetrahydride species has been formed after electronic excitation at about 5 eV of AlH2(H2) van der Waals aggregates isolated in argon matrices. Isotopic studies characterized the five isotopic species (corresponding to −H4, −H3D, −H2D2, −HD3 and −D4 containing molecules) and demonstrated that this species has four equivalent Al–H bonds with a tetrahedral or very nearly tetrahedral structure. Ab initio studies using the coupled-cluster singles and doubles [CCSD (T,E4T)] and density functional theory (DFT) methods confirmed the highly unstable character of an hypothetical neutral AlH4 species as previously established by other workers, which would spontaneously dissociate back into AlH2(H2). On the other hand, the calculated vibrational spectrum of the tetrahydroaluminate anion, AlH−4, would perfectly fit that of the observed species. Identification of the isolated AlH−4 is thus proposed.
A new ion detection scheme for ion spectroscopy equivalent to zero kinetic energy photoelectron spectroscopy
Authors: Dietrich Hans-Jörg , Lindner Reiner , Müller-Dethlefs Klaus
Published: July 2002

A new scheme for the detection of ions produced by delayed pulsed field ionization of long-lived high-n Rydberg states (ZEKE Rydberg states) is presented. The equivalence of this new ion detection scheme with high resolution zero kinetic energy photoelectron (ZEKE) spectroscopy is proven by the simultaneous measurement of electrons and ions originating from the same ZEKE Rydberg states, taking into account the effects of pulsed and static low electric fields on the ZEKE Rydberg states. Using this ion detection, a rotationally resolved mass analyzed threshold ionization (MATI) spectrum of benzene with the same resolution (the rotational structure of the benzene cation) as the simultaneously recorded ZEKE photoelectron spectrum has been obtained.
The structures, energies, vibrational, and rotational frequencies, and dissociation energy of GeH+5
Authors: Schreiner Peter R. , Schaefer Henry F. , von Ragué Schleyer Paul
Published: July 2002

Eight protonated germane structures (GeH+5) were examined in detail using the self-consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included triple excitations [CCSD(T)] methods in conjunction with a double-ζ plus polarization (DZP) and a triple-ζ plus polarization (including f functions on germanium TZP+f) basis set. The C4v and the D3h isomers are high in energy [84 and 48 kcal mol−1, respectively, relative to the Cs(I) structure at DZP SCF]. Although structure Cs(I) is the global minimum, both Cs structures, where GeH+5 is comprised of a nearly planar germyl cation and a hydrogen molecule, are essentially equal in energy and allow virtually free hydrogen moiety rotation. Complete hydrogen scrambling does not occur, as the energy of the C2v(I) structure [33 kcal mol−1 (TZP+f CCSD)] is higher than the dissociation energy of GeH+5 into GeH+3 and H2 (D0=10 kcal mol−1). D0 of GeH+5 is almost the same as for SiH+5 due to a favorable highest occupied molecular orbital (HOMO) (σH2)−lowest unoccupied molecular orbital (LUMO) (pGe) interaction in GeH+5. The global minimum was characterized by vibrational frequency analyses (up to TZP+f CCSD), and we also report on the rotational constants as well as the proton affinity of germane (156 kcal mol−1).
A simple prediction of approximate transition states on potential energy surfaces
Authors: Ruedenberg Klaus , Sun Jun-Qiang
Published: July 2002

Given the locations, the energies, and the force constants of a reactant minimum and a product minimum and assuming that no other information is available, an analytical algorithm is formulated for determining the optimal conjecture for a surmized transition state between them. It is based on a model surface obtained by combining the two quadratic basin approximations. The method is illustrated by applications to transition states on an analytical surface and on an ab initio surface.
Calculated rotational spectrum of Ar...CO from an ab initio potential energy surface: A very floppy van der Waals molecule
Authors: Castells Victoria , Halberstadt Nadine , Shin Seung Koo , Beaudet Robert A. , Wittig Curt
Published: July 2002

Using the ab initio potential of Shin et al. (to be published), we have calculated the bound states and infrared absorption spectrum of the van der Waals complex Ar...CO. The results show that Ar...CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the spectrum, and its frequency varies with Ω (the projection quantum number of the total angular momentum onto the intermolecular axis going from the center of mass of CO to the Ar atom). It is also shown that, although the spectrum cannot be analyzed by the use of a rigid rotor model, rotational ‘‘constants’’ can still be defined for each value of Ω. This is consistent with the available experimental data and the predicted bending excitation can account for unassigned transitions in the infrared spectrum of this complex. Finally, a sensitivity analysis of the calculated spectrum with respect to the potential anisotropy has been performed.
Vibrational energy transfer from resummed evolution operators
Authors: Schwartz Steven D.
Published: July 2002

This paper describes the application of our recently derived infinite order evolution operator expansion and resummation technique to the problem of vibrational energy redistribution in molecules. For a standard mass tensor coupled model of a linear hydrocarbon we show how the resummation technique allows the derivation of an approximate evolution operator that in a single time step accurately reproduces vibrational dynamics for over 25 fs in hydrocarbons. This single time evolution operator can be calculated efficiently enough so that long time dynamics with multiple time steps seem to now be within reach. In addition, the structure of the theory is such that longer chain hydrocarbons can be efficiently ‘‘built up’’ from shorter chain molecules. The theory starts with an adiabatic approximation which describes coupled vibrational degrees of freedom by uncoupled but guage shifted evolution operators. A modified version of this adiabatic approximation shows promise for application to molecules of a size too large to be handled exactly.
State specific electronic quenching rates for the N2 a 1Πg(v=0) level from collisions with He, Ar, and N2
Authors: Katayama Daniel H. , Dentamaro Anthony V. , Welsh Judith A.
Published: July 2002

The technique of double, resonance enhanced multiphoton ionization (REMPI) spectroscopy has been used to observe collision induced electronic transitions (CIET) and rotational energy transfer (RET) from a selectively excited rotational level of the N2 a 1Πg (v=0) rotational manifold. A direct comparison of these two processes under the same experimental and single collision conditions is demonstrated for the first time and the results show clearly that CIET to the a′ 1∑−u(v=0) rotational levels occur prior to rotational equilibration in the a 1Πg manifold. The collision partners are He, Ar, and N2. There is a general propensity for ΔJ≊0 but detailed changes in the collisional selection rules for CIET are difficult to observe for the different colliding partners except for N2 where there is a fraction of E–E transfer that allows antisymmetric to symmetric transitions to be observable. The double REMPI experiments allow the determination of collisional deactivation paths and energy gaps. CIET from the a 1Πg(v=0) to the a′ 1∑−u(v=1) level which is endothermic by 295 cm−1 is observed for the first time at room temperature. From the determination of these deactivation paths a collisional quenching model is derived and state specific quenching rates are determined from pressure dependent radiative decay curves as a function of temperature for each of the collision partners.
Photofragment vibrational, rotational, and translational distributions for N2–HF (v=1)
Authors: Bemish R. J. , Bohac E. J. , Wu M. , Miller R. E.
Published: July 2002

Two new methods have been developed for studying the vibrational predissociation dynamics of weakly bound complexes at the state-to-state level. The first is based upon the use of two F-center lasers, one to photodissociate the complex by single quantum state excitation and the second to probe the final states of the resulting fragments. These pump–probe experiments are carried out with angular resolution of the photofragments, giving information on the associated vibrational, rotational and translational distributions. The second method makes use of a dc electric field to orient the parent molecules prior to photodissociation. In this way the two fragments recoil to different sides of the apparatus, allowing us to detect them independently. These experiments also provide us with valuable information on the nature of the photofragment angular distribution in the molecule fixed frame. The present application is to the study of the vibrational predissociation dynamics of N2–HF (v=1). A combination of the two methods has enabled us to determine that the complex dissociates via two distinctly different channels with similar probability, namely one that produces vN2= 1, jHF=7 and the other vN2= 0, jHF=12. The rotational distribution of the N2 fragment is also determined by fitting the angular distributions. Conservation of energy is used to determine the dissociation energy of the complex (D0) to be 398±2 cm−1.
Near-infrared vibronic spectrum of the CH2 b̃ 1B1←ã 1A1 transition
Authors: Chang Bor-Chen , Wu Ming , Hall Gregory E. , Sears Trevor J.
Published: July 2002

A Doppler-limited high resolution vibronic spectrum of the methylene (CH2) b̃ 1B1←ã 1A1 transition in the near-infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low-J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 states given by statistical phase space theory. The vibrationally excited coincident CO appears, however, to be produced about 2–3 times more efficiently than is predicted by various statistical theories. A frequency modulation (FM) technique has been adopted to improve the sensitivity of the transient absorption experiment. There is a significant improvement in the observed signal to noise ratio of the CH2 spectrum over the dual beam method previously used.
S
1 excitation and zero kinetic energy spectra of partly deuterated 1:1 phenol–water complexes
Authors: Dopfer Otto , Müller-Dethlefs Klaus
Published: July 2002

Two-photon, two-color resonant-enhanced multiphoton ionization (REMPI) spectra of the S1 state of isotopic 1:1 hydrogen-bonded phenol–water clusters have been recorded. Up to three deuterium atoms are introduced in the phenolic OH group and/or the water molecule. The intermolecular vibrational structure found is in reasonable agreement with previously reported one-color REMPI spectra, however, a partly different interpretation of the spectra is presented here. Zero kinetic energy photoelectron (ZEKE) spectra have been obtained via different intermediate S1 levels of the various isotopic complexes. The analysis of both the REMPI and the ZEKE spectra supports the new assignment of several vibrational bands observed in the REMPI spectra of the deuterated complexes where one or two hydrogen atoms are substituted by deuterium. For these deuterated complexes, the reassignment given here is based on the assumption that two different nonequivalent isomeric configurations are responsible for the structure observed in the REMPI spectra. This result is in clear contrast to the previously given interpretation where the spectra were analyzed in terms of only one isomer and the occurrence of Fermi resonances. Furthermore, accurate ionization energies are determined for all possible isomers of the various isotopic complexes and propensity rules for these values as a function of site-specific deuteration have been found. In addition, the analysis of the intermolecular vibrational structure of the complex cations confirmed the assignment of the intermolecular stretch vibration.