bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic solvent effects on activated chemical reactions. I. Classical effects of reaction path curvature
Authors: Schenter Gregory K. , McRae Robin P. , Garrett Bruce C.
Published: July 2002

In gas phase reactions, dynamical recrossings across a phase space dividing surface induced by nonlinear reaction path curvature coupling leads to the breakdown of the fundamental dynamical approximation of classical transition state theory (TST). In the following study, we examine the nature of this breakdown for chemical reaction dynamics occurring in solution. As a model system, we consider the collinear A+BC reaction where reaction path curvature increases as the mass of B becomes small compared to the mass of A and C. We use a London–Eyring–Polanyi–Sato (LEPS) potential to describe the solute interaction and model the influence of the solvent by using a generalized Langevin equation that is further represented by a system of coupled harmonic oscillators. Exact classical rate constants are compared to those obtained from conventional TST and canonical variational transition state theory (CVT) as a function of solvent friction coupling. A harmonic TST analysis at the saddle point of the full system (solute plus solvent) with an optimum dividing surface containing both solute and solvent degrees of freedom returns the Grote–Hynes expression for the rate. For the case of no solvent coupling, both TST and CVT are identical and fail to account for the dynamical recrossings induced by reaction path curvature. At intermediate couplings, CVT provides an improvement to the TST estimate and agrees with dynamical simulation results. All estimates of the rate constant approach each other asymptotically at large couplings. The results are interpreted in terms of recrossings in the extended system (solute and solvent coordinates).
Rotational structure in the Hopfield series of N2
Authors: Huber K. P. , Stark G. , Ito K.
Published: July 2002

The Hopfield Rydberg series of 14N2 have been reinvestigated both at room temperature under equilibrium conditions and at a rotational temperature of ∼20 K in supersonically expanding nitrogen. High-resolution absorption spectra of the preionizing resonances in the region from 143 000 to 151 100 cm−1 have been recorded photographically as well as photoelectrically using, respectively, the 10.6 m vuv spectrograph of the National Research Council in Ottawa, Canada, and the 6.65 m scanning spectrometer at the Photon Factory synchrotron facility in Tsukuba, Japan. The photoelectric measurements have been reduced to absolute photoabsorption cross sections. The rotationally cold spectra show strikingly regular patterns of absorption peaks and window resonances that are accounted for, semiquantitatively, by the calculated total ionization cross sections for the nonrotating molecule [M. Raoult et al., J. Phys. B 16, 4601 (1983), and unpublished work by H. Le Rouzo and M. Raoult, quoted by C. H. Greene and Ch. Jungen, in Adv. At. Mol. Phys. 21, 51 (1985)]. The complexity of the room temperature spectra is rotationally induced; it does not call for an interpretation in terms of strong interactions with an unidentified perturber state as suggested by Baig and Connerade [J. Phys. B 19, L605 (1986)].
Substrate dependence of the surface structure and chain packing of docosyl mercaptan self-assembled on the (111), (110), and (100) faces of single crystal gold
Authors: Camillone Nicholas , Chidsey Christopher E. D. , Liu Gang-yu , Scoles Giacinto
Published: July 2002

Low-energy helium diffraction has been used to study the surface structure, chain packing, and thermal vibrations of docosyl mercaptan [CH3(CH2)21SH] self-assembled on single crystal Au(111), (110), and (100). The docosyl mercaptan molecules form monolayers with different periodicity on the different surfaces of gold. On Au(111) at low temperatures (≤100 K), the terminal methyl groups of the docosyl mercaptan molecules form domains of a hexagonal lattice with a unit mesh constant of 5.01±0.02 Å. The sulfur head groups are arranged in a commensurate (√3×√3)R30° structure and are believed to adsorb on the triple hollow sites of the Au(111) lattice. The unit mesh parameters for CH3(CH2)21S/Au(110) are a=b=4.99±0.08 Å and α=109.5°, suggesting that the chemisorbed sulfur atoms remove the ‘‘missing row’’ reconstruction of the Au(110) surface and form a commensurate c(2×2) lattice. The adsorption of docosyl mercaptan molecules on a Au(100) surface results in a complicated diffraction pattern. Analysis of the data reveals an oblique unit mesh with a=b=5.97±0.09 Å and α=95°±5° with four kinds of equivalent domains present because of the fourfold symmetry of Au(100). The above results confirm that the sulfur–substrate interaction plays an important role in determining the periodicity and the packing density of the molecules within the monolayers. The estimated average domain size of the terminal methyl groups is 22.8, 38.6, and 23.4 Å for CH3(CH2)21SH self-assembled on Au(111), (110), and (100) faces, respectively. The chain packing and orientation within the unit cell are also discussed in this paper in conjunction with the latest results obtained via other techniques such as reflection IR spectroscopy and low-energy electron diffraction.
Apparatus for coupled 3D wave-packet solution of reactive scattering problems in hyperspherical coordinates
Authors: Billing Gert D. , Marković Nikola
Published: July 2002

The apparatus for a coupled 3D wave-packet solution of the reactive three particle scattering problem is given. The method uses hyperspherical coordinates with the hyperradius and the two hyperangles propagated as a 3D wave packet. The coupling of these wave packets arises through the rotation of the three particle plane by the Euler angles. The necessary formulas for the transformation to Cartesian coordinates and the projection on asymptotic states are also given.
Molecular dynamics simulation of adatom diffusion on metal surfaces
Authors: Shiang Keh-Dong
Published: July 2002

We propose, in this paper, a theoretical model to investigate surface self-diffusion of single adatoms on the face-centered-cubic metals. Calculations are performed on both close packed (111) and loosely packed (001) planes of rhodium and nickel. Two realistic model potentials are applied to describe the interatomic interaction of the adatom/substrate systems. The first model is a Morse-type potential, which involves several empirical fitting of bulk properties of solid. The second newly popular potential was introduced by Sutton and Chen, which incorporates with many-body effects. With these potentials, conventional molecular dynamics (MD) is employed to obtain trajectories of the atoms. The averaged square displacements are computed for a range of initial kinetic energies, and the surface diffusion constants can be obtained by means of the Einstein relation. The estimated random walk exponential prefactors and activation energies exhibit Arrhenius behavior, which are confirmed with the previous results. Surface migration by an exchange mechanism for self-diffusion on the Ni(001) plane is also discussed.
Theoretical study of the silicon–oxygen hydrides SiOHn (n=0–4) and SiOH+n (n=0–5): Dissociation energies, ionization energies, enthalpies of formation, and proton affinities
Authors: Lucas David J. , Curtiss Larry A. , Pople John A.
Published: July 2002

The energies of silicon–oxygen hydrides SiOHn (n=0–4) and their cations SiOHn+ (n=0–5) have been studied using Gaussian-2 (G2) theory. Atomization energies, enthalpies of formation, bond dissociation energies, and ionization energies have been evaluated. There is little experimental thermochemical data available for these species, although many of them have been detected. In silanol, the O–H bond dissociation energy is 119.3 kcal/mol and the first Si–H bond dissociation energy is 92.6 kcal/mol. The O–H bond is significantly stronger than the same bond in methanol (105.0 kcal/mol) and the S–H bond in methylmercapton (86.0 kcal/mol). The silicon–oxygen systems tend to show a preference for isomers containing O–H bonds as opposed to H–Si bonds because of the strong O–H bond. The G2 proton affinity of SiO (191.1 kcal/mol at 298 K) is in good agreement with a recently measured value (189.3±2.6 kcal/mol at 295 K). The G2 ionization energy of SiO is 11.58 eV, in agreement with the observed value (11.61 eV) from the vacuum ultraviolet photoelectron spectrum.
Transition state structures and reaction profiles from constrained optimization procedure. Implementation in the framework of density functional theory
Authors: Abashkin Y. , Russo N.
Published: July 2002

A method for finding the transition structures (TS) based on constrained optimization techniques is proposed. The algorithm can be considered as a step-by-step walking uphill process along the minimum energy path, followed by a refining procedure of TS parameters in the saddle point vicinity. By accounting for the constraint conditions in a straightforward manner, it is possible to use efficient quasi-Newton algorithms at every geometry reoptimization step and to manage the moving direction in a reaction valley. This approach may be suitable in the framework of density functional theory. Tests including a potential energy surface model, HNC→HCN and N2H2 trans–cis isomerizations and LiBH4 rearrangements are given. The possible future development of the approach is discussed.
Pulsed neutron diffraction study on lithium (I) hydration in supercooled aqueous chloride solutions
Authors: Yamagami M. , Yamaguchi T. , Wakita H. , Misawa M.
Published: July 2002

Pulsed neutron diffraction measurements have been performed on aqueous LiCl solutions (mole ratio D2O:LiCl=5) in the supercooled state (258, 213, and 173 K) and at room temperature (295 K). An isotopic substitution method with respect to Li has been used to derive the Li+-related radial distribution function. The neutron diffraction data at all the temperatures have shown the Li–O and Li–D distances to be 2.02±0.05 and 2.61±0.05 Å, respectively, and the most likely orientation of the coordinated water molecules has been found to be such that the four atoms in a Li+–D2O unit is pyramidal. It has also been found that at temperatures from 295 K down to 213 K the Li+ hydration is represented mostly by the primary hydration shell of about four coordinated water molecules, whereas at 173 K, about 40 K above the glass transition temperature, the definite second hydration shell of Li+ is formed. The obtained structural characteristics of the supercooled solutions are discussed in connection with nucleation of ice, an isotropic reorientational motion of water, crystallization, and the partial recovery of hydrogen bonds in the supercooled solution.
Mechanical and molecular properties of ice VIII from crystal-orbital ab initio calculations
Authors: Ojamäe Lars , Hermansson Kersti , Dovesi Roberto , Roetti Carla , Saunders V. R.
Published: July 2002

The proton-ordered ice VIII structure has been investigated by ab initio periodic Hartree–Fock calculations in the pressure interval from 0 to 30 GPa using a 6-31G** basis set. The structure was optimized by energy-minimization at different volumes, and from the resulting energy vs volume relationship, the equation of state of ice VIII was derived. The variation of the structure, intramolecular geometry, Mulliken charges, electron density, Raman spectrum, and infrared stretching vibrations with varying pressure were investigated. The agreement with existing experimental data is generally good. Nearest-neighbor hydrogen-bonded O...O distances decrease from 2.88 to 2.57 Å as the pressure is increased from 0 to 30 GPa. For the same pressure range, the intramolecular OH bond increases from 0.951 to 0.955 Å (giving a drOH/dP value of 0.000 14 Å/GPA), the Mulliken charge on H increases from +0.386 to +0.452, the calculated bulk modulus increases from ∼25 to ∼160 GPa (corresponding experimental values are ∼25 at 2.4 GPa and ∼135 at 30 GPa), and the electron density redistribution is considerably enhanced. The frequency downshift of the OH stretching vibration varies from −200 cm−1 at 2.4 GPa to −500 cm−1 at 20 GPa; the corresponding experimental values are −300 and −650 cm−1. Electronic density-of-states diagrams are presented.
Electronic structure of BaLi. I. Theoretical study
Authors: Allouche A. R. , Aubert-Frécon M.
Published: July 2002

Results of the first theoretical study of the electronic structure of all the molecular states of BaLi dissociating into the six lowest limits Ba+Li are reported. The method used is similar to that used previously with success to describe the molecule BaH [J. Chem. Phys. 96, 7646 (1992)]. For the bound states 2,4Λ(±) (spin–orbit effects neglected) and Ω(±) (spin–orbit effects included), all previously unknown values of a set of spectroscopic constants are displayed. They have been very helpful in the analysis of the (2)2Π→X 2Σ+ system of BaLi recently observed in our laboratory and presented in the following paper. An agreement of ≊1.5% for the energy Te of the (2)2Π state, as well as for rotational constants Bv=0 of both states (2)2Π and X 2Σ+ and of ≊9% for the spin–orbit parameter Av=0 of the (2)2Π state, is obtained between theoretical predictions and experimental observations.
Wentzel–Kramers–Brillouin theory of multidimensional tunneling: General theory for energy splitting
Authors: Takada Shoji , Nakamura Hiroki
Published: July 2002

A general Wentzel–Kramers–Brillouin (WKB) theory of multidimensional tunneling is formulated and an illuminating physical picture of the effects of multidimensionality is provided. Two basic problems are solved: (i) Maslov’s semiclassical wave function in the classically accessible region is connected to the wave function in the classically inaccessible region and (ii) the latter is propagated into the deep tunneling region. It is found that there exist two distinct types of tunneling: pure tunneling and mixed tunneling. The former is the usual one in which the tunneling path can be defined by a certain classical trajectory on the inverted potential and its associated action is pure imaginary. In the latter case, no tunneling path can be defined and the Huygens-type wave propagation should be carried out. In this case, tunneling is always accompanied by classical motion in the transversal direction and the associated action is complex. A general procedure is presented for the evaluation of energy splitting ΔE in the double well. Moreover, under the locally separable linear approximation, a simple and convenient formula for ΔE is derived and is confirmed to work well by comparison with the exact numerical calculations.
The chemical bonds in CuH, Cu2, NiH, and Ni2 studied with multiconfigurational second order perturbation theory
Authors: Pou-Amérigo Rosendo , Merchán Manuela , Nebot-Gil Ignacio , Malmqvist Per-Åke , Roos Björn O.
Published: July 2002

The performance of multiconfigurational second order perturbation theory has been analyzed for the description of the bonding in CuH, Cu2, NiH, and Ni2. Large basis sets based on atomic natural orbitals (ANOS) were employed. The effects of enlarging the active space and including the core-valence correlation contributions have also been analyzed. Spectroscopic constants have been computed for the corresponding ground state. The Ni2 molecule has been found to have a 0+g ground state with a computed dissociation energy of 2.10 eV, exp. 2.09 eV, and a bond distance of 2.23 Å. The dipole moments of NiH and CuH are computed to be 2.34 (exp. 2.4±0.1) and 2.66 D, respectively.
Solvent interaction with the (2p3s) Rydberg state of hexamethylenetetramine: Energetics and relaxation dynamics
Authors: Shang Q. Y. , Dion C. , Bernstein E. R.
Published: July 2002

The (1+1) mass resolved excitation spectra are reported for the (2p3s)←(2p)2 Rydberg transition of the tricyclic, high symmetry molecule hexamethylenetetramine [HMT (C6H12N4)] and its van der Waals clusters. The solvent molecules employed include both nonpolar (Ar, CH4) and polar (NH3, CH3OCH3) species. HMT and its clusters are generated and cooled in a supersonic expansion. The observed electronic transition is assigned as T2←A1 within the Td print group. A transition blue shift of 52 cm−1 for each Ar atom and 65 cm−1 for each methane molecule in the HMT van der Waals cluster is characterized. These shifts are caused by an excited state repulsive interaction between the excited Rydberg electron and the closed shell solvent which reduces the attractive dispersion interaction between the HMT and nonpolar solvent species in the van der Waals cluster. A transition red shift of more than 600 cm−1 for NH3 and CH3OCH3 solvent/HMT clusters is observed; this large increase in interaction energy for the excited Rydberg state of HMT with respect to the ground state of HMT is associated with the delocalization of the excited electron into available (virtual) Rydberg orbitals of the solvent molecules. The interaction is characterized as an electron transfer interaction. These results and assignments are consistent with previously reported ones for DABCO/solvent clusters. Relaxation dynamics of excited HMT and its clusters are investigated via a pump/probe (ionization) technique. Relaxation of the clusters is dominated by an intersystem crossing mechanism resulting in an excited state singlet lifetime of ∼5 ns compared to a bare molecule HMT excited state lifetime of ∼1.0 μs. A triplet state of HMT lies 255 cm−1 below the first excited singlet Rydberg state as determined by two-color threshold ionization studies.
Ab initio calculations of anharmonic vibrational circular dichroism intensities of trans-2,3-dideuteriooxirane
Authors: Bak Keld L. , Bludský Ota , Jo/rgensen Poul
Published: July 2002

A priori theory is derived for anharmonic calculations of vibrational circular dichroism (VCD). The anharmonic VCD expression is gauge origin independent and reduce to the magnetic field perturbation theory expression in the double-harmonic approximation. The theory has been implemented using second-order contact transformations for the vibration problem. Zeroth, first, and diagonal second derivatives of the atomic axial tensor are needed at the molecular equilibrium geometry. Ab initio calculations have been carried out for trans-2,3-dideuteriooxirane using self consistent field theory for the atomic axial tensors and using second-order Mo/ller–Plesset theory for the atomic polar tensors and the force fields. The changes of the vibrational rotatory strengths from anharmonicities are small, and do not explain the previously observed large discrepancies between the double-harmonic results and the experimental values for three out of the 15 vibrational modes.
On the low lying excited states of methyl amine
Authors: Taylor D. P. , Bernstein E. R.
Published: July 2002

Mass resolved excitation spectroscopy (MRES) and high level ab initio calculations are employed to explore the low lying excited states of methyl amine, CH3NH2. Both (1+1) and (2+2) MRES of CH3NH2 produce well resolved vibronic features in the energy region around 39 770 to 46 000 cm−1. A complete data set in this region for (2+2) MRES is presented for the isotopic series CH3NH2, CD3NH2, CH3ND2, and CD3ND2. Two apparent Franck–Condon progressions can be qualitatively characterized in these spectra. In order to identify the excited state vibrations active in these spectra and to identify the nature of the excited electronic state(s) accessed, a rather extensive set of ab initio calculations are undertaken. An open shell Hartree–Fock force constant calculation proves central to assigning the observed vibrations. Agreement between the predicted and observed vibrational frequencies provide the strongest evidence to date for a planar excited state C–NH2 geometry. Using combinations and overtones of only two vibrations, the amine wag and scissors modes, all the major features of the low energy region of the spectra can be assigned for all the isotopically substituted methyl amines. Ab initio calculations indicate that the lowest A′ excited state is an A′ 3s Rydberg and the lowest A″ excited state is a valence electronic state. An additional A′ 3s Rydberg state is also found in this region, which because of its geometry, can be implicated in the methyl hydrogen elimination photodissociation reaction of methyl amine. Complete active space self-consistent field (CASSCF) calculations alone, and augmented by many body perturbation theory (MBPT), are also performed. The spectra are consistent with two excited electronic states in the 40 000 cm−1 region. This new characterization of the low energy absorption spectra, and the interpretation of the high energy region in terms of an addition electronic state, challenge the long held view of the nature of the methyl amine excited states.
Second-order many-body perturbation-theory calculations in extended systems
Authors: Sun Jun-Qiang , Bartlett Rodney J.
Published: July 2002

Explicit expressions for electron correlation at the second-order many-body perturbation-theory [MBPT(2)] level are presented and implemented for the total energy per unit cell and for the band structure of extended systems. In the latter case, a formula is presented for a direct evaluation of the band gap rather than obtaining it as a difference of two large numbers. Application is made to alternating trans-polyacetylene. We assess the convergence of MBPT(2) with the number of unit cells (N) included in the lattice summations, the number of k-points (K) taken for the integrals over k in the first Brillouin zone, and the cutoff threshold (10−C) for the two-electron integrals. The MBPT(2) correlation correction to the band structure converges very slowly with N and demands a large K while the MBPT(2) correction to the total energy per unit cell converges much faster with N and needs a much smaller K. Neither MBPT(2) correction is sensitive to the cutoff of the two-electron atomic orbital integrals, 10−C, when C≥5. For polyacetylene, the MBPT(2) band gap is much improved from the SCF result, but does not agree with previous numerical data. Analysis shows that the previous MBPT(2) results were obtained either with too few unit cells such that the convergence with N had not been reached, or that the zeroth-order Hartree–Fock results were inadequately converged. MBPT(2) with a DZP basis improves the Hartree–Fock band gap from 5.57 to 3.22 eV at the experimentally estimated geometry, compared to the measured ∼2 eV peak in the absorption spectrum of the system. We verify our results with three independent programs. We also study the band gap as a function of geometry.
Quantum confinement effects in semiconductor clusters. II
Authors: Tomasulo Antonietta , Ramakrishna Mushti V.
Published: July 2002

The band gaps and spectral shifts of CdS, CdSe, CdTe, AlP, GaP, GaAs, and InP semiconductor clusters are calculated from band structure calculations using accurate local and nonlocal empirical pseudopotentials. The effect of spin-orbit coupling on the band structures is included in the calculations when they are important. The complete set of pseudopotential parameters and full computational details are reported for all these semiconductors. The calculated spectral shifts of zinc-blende and wurtzite CdS, wurtzite CdSe, zinc-blende CdTe, and zinc-blende InP clusters are in good agreement with experiments over a range of cluster sizes. The effect of crystal structure on the band gaps is small in large clusters but becomes important in small clusters. Spin-orbit coupling splits the valence band into A, B, and C sub-bands and we identify transitions arising from these sub-bands in the spectra of both CdSe and CdTe clusters. These results demonstrate that the empirical pseudopotential method yields unique insights into the quantum confinement effects and is a powerful quantitative tool for calculating the spectral shifts of semiconductor clusters.
The dissociative chemisorption of cyclopropane on Ir(110)
Authors: Kelly D. , Weinberg W. H.
Published: July 2002

We have employed molecular beam techniques to investigate the dissociative chemisorption of cyclopropane on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 48 kcal/mol, and surface temperature, Ts, from 85 to 1200 K. For Ts=85 K, c-C3H6 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, of 0.97 at Ei=1.5 kcal/mol and ξ=0.90 at Ei=5 kcal/mol. For Ei≤5 kcal/mol, c-C3H6 is dissociatively adsorbed through a mechanism of trapping-mediated chemisorption, with initial probabilities of chemisorption, Pa, decreasing with increasing surface temperature from the intrinsic trapping probability at Ts=150 K, to Pa<0.05 above Ts=1000 K. The activation energy for trapping-mediated chemisorption of c-C3H6, referenced to the bottom of the physically adsorbed well and attributed to C–C bond cleavage, is 3.6±0.2 kcal/mol. For Ei≥10 kcal/mol, direct dissociative chemisorption increasingly contributes to the overall measured initial probability of chemisorption of cyclopropane. The initial probability of direct dissociative chemisorption of c-C3H6 increases approximately linearly from Pa=0.1 at Ei=10 kcal/mol, to Pa=0.5 at Ei=45 kcal/mol. No isotope effect is observed for the direct dissociative chemisorption of c-C3D6 for beam translational energies of 17 to 48 kcal/mol, indicating that C–C bond cleavage is the initial reaction coordinate for direct chemisorption of cyclopropane on Ir(110).
Structure and vibrational dynamics of aniline and aniline–Ar from high resolution electronic spectroscopy in the gas phase
Authors: Sinclair Wayne E. , Pratt David W.
Published: July 2002

Rotationally resolved S1←S0 electronic spectra of aniline and its single atom van der Waals complex with argon (An–Ar) have been observed. Analysis of these spectra leads to a determination of the vibrationally averaged structures of the bare molecule and the complex in the two electronic states. Aniline itself is pyramidally distorted at the NH2 group in the S0 state. Attachment of the Ar atom on the side of the ring opposite the two N–H bonds converts the symmetric double well along the inversion coordinate into an asymmetric one, in the ground state. The excited state is quasiplanar along this coordinate. Analyses of the spectra of An–Ar at higher energies in the S1 state provide a probe of the vibrational predissociation (VP) behavior of the complex. We observe in these spectra line broadenings and spectral perturbations from which the important role of intra–intermolecular mode mixing (i.e., IVR) in promoting the VP process is elucidated.
Kinetics and structure of irreversibly adsorbed polymer layers
Authors: Jia L.-C. , Lai Pik-Yin
Published: July 2002

The kinetics of formation and the structure of an adsorbed layer formed by homopolymer chains irreversibly adsorbed onto a flat wall are studied by Monto Carlo simulation using the bond fluctuation model. The rapid initial adsorption is followed by slow kinetics at later times. The surface coverage can be described by an empirical law of stretched exponential approach to the final value. The formation time constant τ can be interpreted by scaling theories of diffusion inside a dense pseudo-brush. The detailed structure of the layer in a good solvent saturated by the adsorbed chains is also measured. The extension of the layer and the distributions of the loops and tails are also analyzed in light of recent scaling theories.
Calorimetric investigation of NO and CO adsorption on Pd{100} and the influence of preadsorbed carbon
Authors: Yeo Y. Y. , Vattuone L. , King D. A.
Published: July 2002

The coverage dependencies of the differential heats of adsorption for CO and NO on Pd{100}, at room temperature, are reported using single crystal adsorption calorimetry (SCAC). The initial heat for CO is 165 kJ/mol, falling markedly with increasing coverage due to anomalously strong repulsive interactions between molecules. The formation of a c(2×4) overlayer at half monolayer coverage places severe constraints on the magnitudes of lateral interactions between adsorbates which can fit the data. A realistic lateral interaction model is proposed. The influence of predosed carbon on the heat of adsorption of CO was also investigated. As little as 0.05 ML of C causes a decrease in the initial heat of about 20 kJ/mol; 0.2 ML reduces the heat to 120 kJ/mol. The results suggest long range interactions between C atoms and CO molecules. The initial heat for NO is 155 kJ/mol, decreasing smoothly to about 105 kJ/mol at saturation. The coverage dependencies of the sticking probabilities for both NO and CO exhibits precursor behavior which is considerably diminished in the presence of a high C precoverage.
Molecular beam studies of sticking of oxygen on the Rh(111) surface
Authors: Brault Pascal , Range Heiko , Toennies J. Peter
Published: July 2002

The sticking coefficients of oxygen on the Rh(111) surface have been measured using O2 molecular beams seeded with different rare gases to achieve beam energies in the range Ei=25–600 meV. With increasing O2 incident beam energy, the adsorption evolves continuously from a molecular precursor-mediated process at low energies to a direct adsorption process described by Langmuir kinetics at high energies. The analysis of the dependence of the initial sticking coefficient on the incident kinetic energy and the angle of incidence as well as on the surface temperature provide information on important parameters of the potential energy surface. Further insight into the nature of the adsorbed species could be obtained from the UPS measurements of the adsorption induced work function changes and from helium atom diffraction measurements of structural modifications as a function of the kinetic energy angle of incidence of the oxygen molecules, and the surface temperature. The “simple washboard” model accounts for the trapping well of the molecular precursor and is able to describe many of the observed features in the precursor region.
Preferential formation of C10− upon tandem irradiation of graphite with IR and UV laser pulses
Authors: Wakabayashi Tomonari , Momose Takamasa , Shida Tadamasa , Shiromaru Haruo , Ohara Michiaki , Achiba Yohji
Published: July 2002

Infrared (IR) pulse laser ablation of graphite followed by tightly focused ultraviolet (UV) pulse laser irradiation yields C10− predominantly when the delay time between the two laser pulses is set at about 50–200 μs. The mechanism of the formation of C10− is deduced to be the attachment of slow photoelectrons to neutral C10 produced by fragmentation of hot neutral clusters of larger sizes. There is an indication that C10 and its anion have cyclic structures in contrast to the other chainlike clusters.
IR–microwave double resonance studies of dipole moments in the ν1 and ν3 states of ammonia
Authors: Marshall Mark D. , Izgi Kamil Can , Muenter J. S.
Published: July 2002

Infrared laser–microwave double resonance spectroscopy is used to observe the Stark effect of tunneling transitions within the ν1 and ν3 hydrogen stretching states of NH3. Dipole moments for 11 J,K states of ν1 and 3 J,K states of ν3 are measured to high accuracy. These data, combined with previous measurements in the ground and excited bending states, give a dipole moment function (in Debye) of: μv1v2v3v4=1.5610+7.2×10−3(v1+1/2)−2.271×10−1(v2+1/2)+3.75×10−2(v3+1)−1.65×10−2(v4+1). μe=1.561 D  is the first experimental measurement of the NH3 equilibrium moment. These results are also used to analyze and recalculate the dipole moment measured by Shimizu and co-workers in a vNH=5 excited state of NH3.
The high density equation of state and solid-fluid equilibrium in systems of freely jointed chains of tangent hard spheres
Authors: Malanoski A. P. , Monson P. A.
Published: July 2002

We present a study of the solid-fluid phase equilibrium for systems of freely jointed chains of tangent hard spheres using Monte Carlo computer simulation. Extensive calculations of the fluid and solid phase equations of state have been made. Our results for the fluid properties are compared with various theoretical equations of state which have been proposed for these systems. A method for calculating the solid phase free energy of chain molecules has been developed based on finding a reversible path to an Einstein crystal. The stable solid phase structure of these systems approximates cubic close packing of the atoms without long range orientational ordering of the bond vectors. The contribution to the solid phase free energy from the configurational degeneracy has been approximated by the expression developed by Huggins. The dependence of the solid phase thermodynamics upon the chain conformation has been studied and solid-fluid phase diagrams for chains with lengths ranging from three through eight atoms have been determined.
Transient relaxation of a charged polymer chain subject to an external field in a random tube
Authors: Burlatsky S. F. , Deutch John M.
Published: July 2002

Transient relaxation of a charged polymer chain in a random media is studied theoretically. We consider a chain in a random tube and present scaling results for relaxation times, for displacement of the chain segments inside the tube, and for the dynamics of leaking from the tube ends.
Elimination of Coulombic infinities through transformation of the Hamiltonian
Authors: Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

It is demonstrated that Coulombic infinities of both nuclear–electron and electron–electron type can be eliminated through a transformation of the Hamiltonian. The transformed Hamiltonian is no longer self-adjoint and will contain three-particle interactions. The new pair interaction, depending only on the interparticle distance, can be chosen almost at will, however. If the new pair-potential terms are chosen to not contain an infinity, the corresponding right-hand wave function no longer contains a cusp, and we expect this feature to help improve the convergence of ab initio quantum chemical calculations with respect to the one-particle basis set. We limit ourselves to an exposition of the idea, illustrated with some examples for the Hydrogen atom.
Hydrogenated and deuterated iron clusters: Infrared spectra and density functional calculations
Authors: Knickelbein Mark B. , Koretsky Geoffrey M. , Jackson Koblar A. , Pederson Mark R. , Hajnal Zoltan
Published: July 2002

Iron clusters react sequentially with hydrogen molecules to form multiply hydrogenated products. The increases in cluster ionization potential upon reaction verify that hydrogen chemisorbs dissociatively to form iron cluster–hydride complexes, FenHm. At low source temperatures, the cluster–hydride complexes take up additional hydrogen molecules which are shown to be physisorbed onto the underlying FenHm complexes to form FenHm(H2)p species. The infrared spectra of FenHm and FenDm (n=9–20) were obtained by the photodissociation action spectroscopic method in which depletion of the FenHm(H2)p and FenDm(D2)p species was the signature of absorption. The spectra, recorded in the 885–1090 cm−1 region, consist of several overlapping bands, each approximately 20 cm−1 in width. The dissimilarity of each FenHm(H2)p spectrum with the corresponding FenDm(D2)p spectrum indicates that the carrier involves hydrogen and is not merely due to absorption by the underlying iron cluster. Density functional calculations were performed on model complexes, Fe13H14 and Fe13D14, the iron portion of which was assumed to have Th symmetry. The infrared-active vibrational frequencies involving hydrogen bending and deuterium stretching are predicted to lie within the experimental frequency range of the experiment, well removed from the skeletal modes of the underlying iron cluster. The complexity of the observed spectra as compared to simulations based on the assumed (high-symmetry) model imply that the experimentally produced complexes possess low symmetry.
Local volume fraction fluctuations in periodic heterogeneous media
Authors: Quintanilla J. , Torquato S.
Published: July 2002

Although the volume fraction is a constant for statistically homogeneous media, on a spatially local level it fluctuates and depends on the observation window size. In this article, we develop exact analytical expressions for the full local-volume fraction distribution function of periodic arrangements of rods, rectangles, and cubes in a matrix. These formulas depend on the inclusion density and window size.
Some aspects of the adsorption of a Lennard-Jones gas on a rough surface
Authors: Bryk Paweł , Sokołowski Stefan , Henderson Douglas
Published: July 2002

We study the adsorption, including wetting, of a Lennard-Jones gas on a rough surface consisting of rough layer on a smooth substrate with which the gas interacts via a 9-3 potential. The rough layer is two molecular diameters thick and consists of a disordered quenched matrix of hard spheres. As well as interacting with the other matrix spheres by the hard-sphere potential, the matrix molecules interact with the gas molecules by the hard-sphere potential. Hence, on average, the degree to which the gas molecules can approach the substrate depends only on the density of the matrix layer. The density of this rough layer has a significant effect. As the density of the matrix layer increases, the adsorption isotherms pass from wetting to a prewetting transition, and, if the layer is dense enough, to partial wetting. It is interesting that the prewetting transition remains first order.
Phase diagram of random lattice gases in the annealed limit
Authors: Vieira A. P. , Gonçalves L. L.
Published: July 2002

An analysis of the random lattice gas in the annealed limit is presented. The statistical mechanics of disordered lattice systems is briefly reviewed. For the case of the lattice gas with an arbitrary uniform interaction potential and random short-range interactions the annealed limit is discussed in detail. By identifying and extracting an entropy of mixing term, a correct physical expression for the pressure is explicitly given. The one-dimensional lattice gas with uniform long-range interactions and random short-range interactions satisfying a bimodal annealed probability distribution is discussed. The model is exactly solved and is shown to present interesting behavior in the presence of competition between interactions, such as the presence of three phase transitions with different critical temperatures and the occurrence of triple and quadruple points.
Photodissociation of N2O: J-dependent anisotropy revealed in N2 photofragment images
Authors: Neyer David W. , Heck Albert J. R. , Chandler David W.
Published: July 2002

Photofragment imaging is used to measure the state-specific angular distributions of N2 produced in the ultraviolet photodissociation of N2O. 2+1 REMPI (resonance enhanced multiphoton ionization) through the a″(1Σg+) state is used to ionize individual rotational states of N2(ν=0) between J=40 and J=90 produced when N2O is dissociated by light of approximately 203 nm. The anisotropy parameter, β, used to characterize the angular distributions of the N2 ion images, is greater than one for the lower rotational levels probed, and decreases to approximately zero for the highest rotational levels of N2. The positive beta parameter indicates that the products arise primarily from a parallel transition to the 2 1A′(1Δ) state in the parent molecule, and the decrease in β with increasing J of N2 is due to increased bending in the parent N2O during dissociation. The ion images also allow us to assign transitions in a spectroscopically perturbed region of the N2 photofragment REMPI spectrum and to reveal the production of N2 products in the first vibrationally excited state.
Noniterative algorithms for finding quantum optimal controls
Authors: Zhu Wusheng , Rabitz Herschel
Published: July 2002

Iterative methods are generally necessary for solving the design equations to identify optimal quantum controls. Since iteration can be computationally intensive, it is significant to develop good approximate noniterative methods. In this paper we present noniterative techniques for achieving quantum optimal control over the expectation value of positive semidefinite operators. The noniterative methods are characterized by an order index. Zeroth-order methods involving no feedback from the objective are generally found to be inadequate. A proposed first-order noniterative algorithm is expected to often be a good approximation. Numerical tests verify the noniterative capabilities of the algorithm.
Rotationally resolved differential scattering cross sections for the reaction F+para-H2 (v=0, j=0)→HF(v′=2, 3, j′)+H
Authors: Baer M. , Faubel M. , Martı́nez-Haya B. , Rusin L. , Tappe U. , Toennies J. P.
Published: July 2002

Time-of-flight spectra of HF products in the v′=2 vibrational state from reactive scattering of F atoms from para-H2 exhibit at least four smaller peaks which are assigned to the rotational states j′=7, 8, 9, and 10. The center-of-mass rotational distributions are in good agreement with accurate quantum mechanical and approximate coupled states calculations.
Birefringence and second-order nonlinear optics as probes of polymer cooperative segmental mobility: Demonstration of Debye-type relaxation
Authors: Hooker Jacob C. , Burghardt Wesley R. , Torkelson John M.
Published: July 2002

Two complementary optical techniques, second-harmonic generation (SHG), and electric birefringence are used to observe chromophore dynamics in an amorphous polymer above Tg. In particular, a random copolymer of isobutyl methacrylate and a disperse red 1-labeled methacrylate (mole ratio 94:6) is quantitatively studied over a range of time scales and temperatures. In doing so, two models of reorientation are tested: one considers the chromophores motions as small correlated diffusive steps (Debye rotation) and the other describes the motions in terms of uncorrelated instantaneous jumps. Given that SHG is sensitive to the first moment of the orientational autocorrelation function (OACF), or 〈cos θ〉 (where θ is the polar angle between the poling field and the permanent dipole moment directions), and birefringence to the second, or 〈cos2 θ〉, the ability of the models to describe chromophore motion is tested by comparing their predictions of the relationship between the OACFs and the measured properties. By investigating the same polymer system under equivalent conditions, it is shown that chromophore reorientation is explained by a modified Debye model that takes into account a distribution of relaxation times.
Quantum calculations of highly excited vibrational spectrum of sulfur dioxide. II. Normal to local mode transition and quantum stochasticity
Authors: Ma Guobin , Guo Hua
Published: July 2002

We report a quantum mechanical calculation of highly excited vibrational spectrum of SO2 up to 25 000 cm−1, using a filter-diagonalization method based on the Chebyshev propagation. Our results indicate a graduate transition from a normal mode regime at low energies to a local mode regime near 25 000 cm−1, which is marked by a decreasing energy gap between the (n1,0,0) and (n1−1,0,1) states and bifurcation of the corresponding wave functions. Approximately 4700 vibrational levels are found below 25 000 cm−1 and statistical analysis reveals that the SO2 vibration in this energy range is largely regular although the existence of chaos cannot be excluded.
One-particle resonances in low-energy electron scattering from C60
Authors: Gianturco F. A. , Lucchese Robert R.
Published: July 2002

The quantum dynamics of low-energy electron scattering from C60 molecules is carried out using the full anisotropic interaction between the molecule at its equilibrium geometry and the impinging electron. The interaction is constructed using a nonempirical model potential for the short-range dynamical correlation between the scattered electron and the bound electrons, while both the static interaction and the nonlocal exchange interaction are treated correctly. The fairly large size of the carbon cage, and the hollow structure which is created inside it, are unique features of the C60 molecule that are shown here to play a very important role in forming a broad variety of metastable electron-attachment states without electronic excitation being involved. In spite of various aspects of the resonant process which are not considered in the present treatment, the calculations show very clearly the occurrence of several trapping processes for the scattered electrons which could be amenable to experimental observation.
Reproducing kernel technique for extracting accurate potentials from spectral data: Potential curves of the two lowest states X 1Σg+ and a 3Σu+ of the sodium dimer
Authors: Ho T.-S. , Rabitz H. , Scoles G.
Published: July 2002

This work describes an extension of the Reproducing Kernel Hilbert Space (RKHS) method, in conjunction with the Tikhonov regularization, for constructing potential energy surfaces, with correct asymptotic forms, from high quality experimental measurements. The method is applied to the construction of new, global potential energy curves of the two lowest states X 1Σg+ and a 3Σu+ of the sodium dimer using rovibrational spectral measurements. The exchange interaction of Na2 at intermediate and long ranges is accordingly derived and adopted for determining the ionization energy of the corresponding valence electron. It is found that the resulting ground-state X 1Σg+ dissociation energy 6022.025 (±0.049) cm−1 of Na2 agrees within the experimental errors with the most recent experimental value [6022.0286 (±0.0053) cm−1, Jones et al., Phys. Rev. A 54, R1006 (1996)]. The well depth of the a 3Σu+ state is determined to be 174.96 (±1.18) cm−1, compared to the Rydberg–Klein–Rees (RKR) value of 174.45 (±0.36) cm−1 [Li et al., J. Chem. Phys. 82, 1178 (1985)]. Moreover, the equilibrium positions of both RKHS potential curves, 3.0796 (±0.0010) Å for the X 1Σg+ state and 5.089 (±0.062) Å for the a 3Σu+ state, are in excellent agreement with previously determined RKR results of 3.079 53 Å [Babaky and Hussein, Can. J. Phys. 67, 912 (1989)] and 5.0911 Å (Li et al.), respectively. The experimentally determined values of the equilibrium position and well depth for the a 3Σu+ state differ from recent theoretical values of 5.192 Å and 177.7 cm−1 obtained by highly accurate ab initio calculations [Gutowski, J. Chem. Phys. 110, 4695 (1999)]. Finally, both RKHS potential curves at large distances reproduce very recent theoretical dispersion coefficients within 1.0×10−5 percentage errors.
The KRb ground electronic state potential up to 10 Å
Authors: Amiot C. , Vergès J.
Published: July 2002

High resolution spectra of the A 1Σ+→X 1Σ+ system of the KRb molecule, obtained after excitation using a titanium-doped sapphire laser, were recorded on a Connes-type Fourier transform interferometer. Molecular constants of the first 88 vibrational levels of the X 1Σ+ state are determined, and the potential energy curve is derived up to an internuclear distance of 10.419 Å (99.3% of the potential well depth). The energy of the dissociation limit is found as De=4217.3 (15) cm−1 (referred to the bottom of the potential curve well), in good agreement with the theoretical predictions of Jenč and Brandt [J. Mol. Spectrosc. 154, 226 (1992)] who gave De=4220 (20) cm−1 and the value De=4217.4 (8) cm−1 reported by Kasahara et al. [J. Chem. Phys. 111, 8857 (1999)].