bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated Raman probing of supercooling and phase transitions in large N2 clusters formed in free jet expansions
Authors: Beck Rainer D. , Hineman Max F. , Nibler Joseph W.
Published: July 2002

High resolution stimulated Raman spectroscopy (SRS) has been used to examine N2 and N2/He free jet expansions and also equilibrium samples of N2 from 15 to 110 K. The jet spectra show the formation of large liquid clusters which supercool and subsequently freeze to form crystalline β-N2 solid and, in He expansions, undergo a further transformation to a partially annealed α-N2 form. CW-SRS frequency and linewidth data obtained for equilibrium samples of the condensed phases of N2 yielded frequency–temperature relations used in deducing internal temperatures for the clusters produced in the expansion experiments. Analysis of the cooling curves indicates a mean cluster diameter of 35 nm and favors a prompt freezing process rather than a gradual conversion of liquid to solid in a single cluster on the microsecond time scale of the experiments. Supercooling limits of 34 to 44 K are deduced for the liquid, far below the triple point temperature of 63.2 K at which equilibrium samples freeze. Some evidence for surface versus bulk contributions to the spectra is seen in the asymmetric line shapes observed for liquid clusters in the condensation region. The results show that the high spectral and spatial resolution of nonlinear Raman methods such as SRS and CARS provide a unique probe of the condensation processes in free jet expansions.
CO2-laser induced photodissociation studies of size-selected small benzene clusters
Authors: de Meijere A. , Huisken F.
Published: July 2002

The infrared photodissociation of size-selected, small benzene clusters has been investigated in the region of the ν18 CH in-plane bend using a pulsed CO2 laser. By scattering the cluster beam with a secondary Ne beam and observing off-axis the effect of the laser irradiation with a rotatable mass spectrometer, cluster-specific spectroscopy is performed. The dependence of IR absorption and subsequent dissociation of (C6H6)n clusters has been investigated as a function of laser frequency and laser fluence for n=2, 3, and 4. The absorption profiles are structureless and show only little variation with cluster size. If, instead of He, Ne is used as carrier gas, the absorption profiles are distinctively narrower. This effect is attributed to a lower internal temperature achieved with Ne. In contrast to the benzene dimer and tetramer, the fluence dependence for the trimer dissociation is stronger than linear suggesting that more than one photon is needed to dissociate this cluster. In a computational approach, the structures of the benzene dimer, trimer, and tetramer have been calculated employing an energy minimization program. For the trimer a cyclic ring structure is determined. The computational results are in perfect agreement with the experimental findings.
A multichannel harpoon model for reactive quenching of Xe 5p5n
p by Cl2
Authors: Bruce M. R. , Layne W. B. , Keto J. W.
Published: July 2002

A harpoon reaction model employing multiple crossings based on the formalism of Bauer et al. [J. Chem. Phys. 51, 4173 (1969)] and Gislason and Sachs [J. Chem. Phys. 62, 2678 (1975)] is developed to explain the large cross sections (σ∼330–975 Å2) measured for the reaction Xe*(5p5np,np′, n=6,7)+Cl2. The model calculates the Landau–Zener transition probability for each intermediate ionic crossing with the covalent surface. The transition matrix elements are represented as a product of the electronic interaction (modeled by the empirical result of Olson et al. [Appl. Opt. 10, (1971)]) and a Franck–Condon factor for the Cl2→Cl−2 transition. The model predicts near unit probability for a transition to the ionic surface for impact parameters less than 20 Bohr. Once transfer occurs, the pair is captured by dissociation of Cl−2 to form XeCl*. The large temperature dependence observed qualitatively in the experiments is explained by the increased cross section for vibrationally excited Cl2. A simple model for orbiting collisions is used to estimate the effect of termolecular collisions.
Vibrational dynamics of the bifluoride ion. II. Adiabatic separation and proton dynamics
Authors: Epa V. C. , Thorson W. R.
Published: July 2002

Vibrational dynamics of the bifluoride ion FHF−, which exhibits strongly anharmonic and nonseparable vibrations, is studied using the extended ab initio model potential surface described in the first paper of this series. Adiabatic separation of the proton motion from the F–F (ν1) motion forms a zero-order basis for description, although strong coupling of adiabatic states by the ν1 motion is important in higher vibrational levels and must be considered to understand the spectrum. The adiabatic protonic eigenstates at F–F separations R from 3.75 to 6.40 a.u. have been determined using the self-consistent field approximation in prolate spheroidal coordinates to provide a basis set for configuration interaction expansion of the exact eigenstates. 78 SCF eigenstates (21 σg, 21 σu, 21 πu, and 15 πg) were computed by ‘‘exact’’ numerical solution of the SCF equations.

The adiabatic CI eigenstates are shown to be converged in energy to better than 1.0 cm−1 for the ground state of each symmetry type and usually better than 10 cm−1 for the lowest three to five states, and pass critical tests of accuracy such as the Hellmann–Feynman theorem. The resulting CI potential energy curves closely resemble corresponding SCF energy curves and justify the concept of mode separation even in this very anharmonic system. The adiabatic CI potential energy curves explain most aspects of the dynamics relevant to the IR and Raman spectra of FHF− (e.g., in KHF2), and calculations of ν1 dynamics within the adiabatic approximation suffice to assign most of the observed IR spectrum of KHF2(s) (to about 6000 cm−1). States corresponding qualitatively to modal overtone and combination levels such as 3ν2 and (ν2+2ν3) however exhibit avoided crossings in the neighborhood of the equilibrium configuration and ‘‘Fermi resonance’’ involving interactions of two or more such adiabatic states via the ν1 motion must be treated by close-coupling to predict both frequencies and intensities in the relevant portions of the IR spectrum. From the viewpoint of current interest in classical studies of vibrational dynamics, this system provides an interesting model problem markedly different from the more nearly harmonic models mainly studied in the past. The multiplicity of narrow avoided crossings between protonic levels and persistent success of the SCF approximation as a zero-order description of the proton dynamics except at crossings suggest that comparisons of classical trajectory studies of the system with the quantum mechanical results obtained here may be fruitful.
Ionic dynamics in computer simulated molten LiNO3. III. Effect of the potential well on the translational and reorientational motions
Authors: Katō Toshiko , Machida Katsunosuke , Oobatake Motohisa , Hayashi Soichi
Published: July 2002

In order to study the effect of the potential well on the single ion dynamics and the charge transport in molten LiNO3, a molecular dynamics simulation, which included all degrees of freedom of vibrating nitrate ions, was carried out. Simple Coulomb pair potential with Born-type repulsion was adopted as a standard, and a potential well of varying depth between Li+ and O of NO−3 was added to the standard potential. As the well depth increased, both the translational and orientational diffusion were found to slow down, leading to solid like dynamics. The potential dependence of the ionic dynamics was studied from the translational, orientational, and current correlation functions. Results of the simulation were compared with the experimental diffusion constants, static conductivity, and rotational behavior revealed by Raman spectroscopy. Using the relation between the infrared absorption spectrum and the autocorrelation function of the dipole moment (or the current) of the system, we carried out the first simulation of translational band shapes in an ionic melt, and demonstrated how the absorption spectrum was related to the velocity correlation functions of cations and anions. The infrared and Raman spectroscopy were found to give sensitive criteria for finding a good potential of an ionic liquid. The lifetime of local structures surrounding nitrate ions, which were characterized by the coordination number, was found to increase remarkably as the well depth increased. The coupling between the environmental structure and the translational and rotational behavior of a nitrate ion was examined through selectively sampled correlation functions.
Theoretical investigations of the structures and binding energies of Ben and Mgn(n=3–5) clusters
Authors: Lee Timothy J. , Rendell Alistair P. , Taylor Peter R.
Published: July 2002

We have determined the equilibrium geometries and binding energies of Be and Mg trimers, tetramers and pentamers using single and double excitation coupled cluster (CCSD) and complete active space self-consistent-field (CASSCF) multireference configuration interaction (MRCI) wave functions in conjunction with extended atomic basis sets. Our best estimates of the cluster binding energies are 24, 83, and 110 kcal/mol for Be3, Be4, and Be5; and 9, 31, and 41 kcal/mol for Mg3, Mg4, Mg5, respectively. A comparison of the MRCI and CCSD results shows that even the best single-reference approach (limited to single and double excitations) is not capable of quantitative accuracy in determining the binding energies of Be and Mg clusters.
Quantum localization and energy flow in many-dimensional Fermi resonant systems
Authors: Logan David E. , Wolynes Peter G.
Published: July 2002

The quantum mechanics of energy flow in many-dimensional Fermi resonant systems has several connections to the theory of Anderson localization in disordered solids. We argue that in high dimensional and highly quantum mechanical systems the energy flow can be modeled as coherent transport on a locally but weakly correlated random energy surface. This model exhibits a sharp but continuous transition from local to global energy flow characterized by critical exponents. Dephasing smears the transition and an interesting nonmonotonic dependence of energy flow rate on environmental coupling is predicted to occur near the transition.
H/D isotope exchange reaction of SiH+3 with SiD4 and SiD+3 with SiH4: Evidence for hydride stripping reaction
Authors: Reents W. D. , Mandich M. L.
Published: July 2002

We have measured the reaction rates and product distributions for SiHxD+3−x reactions with SiH4 and SiD4. The measured reaction rates for SiH+3 and SiD4 (26.1±1.0×10−10 cc/molecule s) and for SiD+3 and SiH4 (23.1±1.0×10−10 cc/molecule s) are greater than the calculated Langevin collision rate (12.3–12.4×10−10 cc/molecule s). Also, the product distribution observed for H/D exchange is nonstatistical. Dual, competing reaction mechanisms are invoked to account for these observations: reaction via formation of an ion-molecule complex and reaction via long-range hydride stripping. Using an expected product distribution calculated from reaction thermochemistries, the relative contributions of the two mechanisms is obtained for each reaction examined. The reaction rate for the ion-molecule complex mechanism is calculated to be at the Langevin collision rate within experimental error. The reaction rate for the stripping mechanism varies from 1–4×10−10 cc/molecule s (10–30% of the Langevin collision rate) for the mixed isotope ions SiH2D+ and SiHD+2 to 12–18×10−10 cc/molecule s (100%–150% of the Langevin collision rate) for the isotopically pure ions SiH+3 and SiD+3. The faster than Langevin reaction rates lower the expected low field mobility of SiH+3 in silane plasmas by 70% to ∼340 cm2 Torr/V s.
Coil to rod transitions in Monte Carlo simulations of a short polyelectrolyte. II. Natural screening and a new thermal effect
Authors: Brender Chava
Published: July 2002

A new thermal effect appears in a Monte Carlo simulation of a system of a single short polyelectrolyte chain together with counter ions in a dielectric continuum solvent. Interactions are taken to be Coulombic and the behavior at various temperatures is investigated. A ‘‘natural’’ screening effect is compared to the Debye–Hückel screening effect which was described in a previous article. Great differences between these effects are observed at 0<C<1 M, indicating the existence of counter ion condensation near the polyion, thus creating two kinds of concentrations: that in the bulk and that in the vicinity of the polyion.
Photon echo decays in optically dense media
Authors: Spano Francis C. , Warren Warren S.
Published: July 2002

In this paper we investigate the influence of optical density or superradiance on the photon echo decay using the coupled Maxwell–Bloch equations. Our theory applies to optically thick but dilute media, where the transition dipole–dipole interaction between nearby chromophores is negligible, but the far-field interaction is not. Corrections to the (normalized) echo decay in the optically thin limit, INe(tπ)=exp [−4tπ/T2] (tπ and T2 are the interpulse separation times and the coherence relaxation times, respectively), are found by expanding the polarization, excited state population and pulse electric fields in powers of ζ=αz, where α is the inverse Beer’s absorption length, and z is the coordinate along the pulse propagation axis. Numerical solutions for optically thick samples are also presented, and when ζ≫1 the normalized echo intensity is found to obey the simple analytic form: INe(tπ)=2 exp [−tπ/T1]−1, when tπ/T1<ln 2 and INe(tπ)=0 for longer times. (T1 is the population relaxation time.) In this limit the echo decay is independent of the coherence relaxation time T2, illustrating the profound effect intermolecular coupling may have on any nonlinear optical measurement. We show that at low temperatures, when T2=2T1, the echo decay rate increases as the optical density increases. However, for higher temperatures, when T2 is sufficiently shorter than 2T1, the opposite behavior results, that is, the echo decay rate decreases as the optical density increases.
Diode laser and Fourier transform infrared spectroscopy of jet-cooled nickel carbonyl
Authors: Davies Paul B. , Martin Nicholas A. , Nunes Michael D. , Pape David A. , Russell Douglas K.
Published: July 2002

Infrared absorption spectra of Ni(CO)4 in the region of its 510 band have been measured in supersonic jets using diode lasers and FTIR. The 510 band consisted of an intense Q branch and well-resolved P and R branches which clearly showed the effects of nuclear spin statistics. The analysis of 114 fundamental band transitions yielded the following effective parameters, which include small Coriolis effects (3σ uncertainties in parenthesis) ν̃0=2061.314 4(45) cm−1, B0=0.035 721 3(30) cm−1, B5=0.035 670 6(30) cm−1. The band center is blue-shifted by 3.5 cm−1 from the room temperature value due to the contribution of hot bands and higher-rotational distributions in the room temperature spectra. Weaker features near the 510 Q branch in the diode laser spectra are assigned to the Q branches of two hot bands. Their spectra, optimized using higher concentrations of Ni(CO)4 in He or Ar seed gas, were analyzed to give rotation, vibration–rotation and anharmonicity parameters. The effective hot band centers are: ν̃0(510411) =2060.996 4(50) cm−1, ν̃0(510811) =2061.114 8(48) cm−1.
Diffusion can explain the nonexponential rebinding of carbon monoxide to protoheme
Authors: Miers Jeffrey B. , Postlewaite Jay C. , Zyung Taehyoung , Chen Sheah , Roemig Gary R. , Wen Xiaoning , Dlott Dana D. , Szabo Attila
Published: July 2002

The recombination after flash photolysis of carbon monoxide (CO) to protoheme (PH) in glycerol: water is studied over ten decades in time (1 ps to 10 ms). The rebinding consists of an initial nonexponential geminate phase followed by a slower exponential bimolecular phase. The entire time course of this reaction between 260 and 300 K can be explained in a unified way using a simple, analytically tractable diffusion model involving just three parameters: the relative diffusion constant, the contact radius, and the intrinsic rate of reaction at contact.
Characteristics and some peculiarities of multiconfigurational self-consistent field stationary points of the Li− ground state
Authors: Rizzo Antonio , Yeager Danny L.
Published: July 2002

We examine in detail the multiconfigurational self-consistent field (MCSCF) energy hypersurface of the ground state of Li− using a previously reported Cartesian Gaussian basis set and several previously used different choices of complete active space (CAS). For many of the CASs we show there are many close-lying (in energy) stationary points that have the correct number of negative eigenvalues in the total Hessian with respect to real variations of the orbital and configurational parameters and correspond to the lowest energy eigenvector in the configuration interaction (CI) which uses the MCSCF orbitals and configurations. These stationary points may be obtained with any of the current, forefront MCSCF codes. More detailed examination of these stationary points shows that some of these are improper and that some do not correspond to the most ‘‘desired’’ stationary point. The presence of two nearby proper, desired MCSCF stationary points with the 2–3s, 2–3p CAS is troubling.
Isotropic coupling constant for the nitrogen atom from correlated calculations based on spin-unrestricted wave functions
Authors: Carmichael Ian
Published: July 2002

The isotropic hyperfine coupling constant for the ground electronic state of the nitrogen atom is computed from the quadratic configuration interaction correlation procedure based on a single unrestricted Hartree–Fock (UHF) determinant. The splitting is determined from the normalized spin density at the nucleus, derived by finite-field perturbation theory. Results obtained are compared with previous work based on spin-restricted reference spaces. Close accord is found between present estimates and full configuration interaction values for small basis sets. Contributions from shells of higher angular momentum functions in the basis set are similar in both approaches and for the largest unrestricted calculation the correlated results are in good accord with experiment. The UHF wave function is shown to provide a reasonable account of the K-shell contribution to the isotropic coupling and hence the UHF-based correlated calculations show much less sensitivity to the neglect of core–electron correlation than the dramatic effects observed in spin-restricted treatments.
Monte Carlo calculations of connectedness and mean cluster size for bidispersions of overlapping spheres
Authors: Kim In Chan , Torquato S.
Published: July 2002

The pair-connectedness function and mean cluster size are computed from Monte Carlo simulations for the continuum-percolation model of overlapping spheres with two different diameters. The percolation threshold of the spheres is found to be independent of the distribution (for the cases examined) and slightly higher than that for equisized spheres. Our simulation results are used to assess the accuracy of the Percus–Yevick approximation for the aforementioned cluster statistics.
Relativistic self-consistent-field calculations of spin–orbit splittings in diatomic hydrides
Authors: Baeck K. K. , Sup Lee Yoon
Published: July 2002

Spin–orbit splittings in a few diatomic hydrides are calculated as the differences between total energies of two Ω states of 2Π by the relativistic self-consistent-field method using Slater type functions as basis functions. The dependence of the spin–orbit splittings on the size and quality of basis sets is investigated and an optimal strategy for the selection of the basis set in the all-electron relativistic self-consistent-field calculation is suggested.Dipole moments are also calculated for the hydrides and the effects of the relativity and the spin–orbit coupling on dipole moments are discussed. The variation of the spin–orbit splittings with internuclear distances is investigated for the OH molecule. The present method is capable of yielding proper basis set limits and the calculations with a basis set of moderate size yield qualitatively good results although the Breit interaction term is omitted. The effects of the Breit term on the spin–orbit splittings are estimated to be considerably less than those of two-electron spin–orbit interaction terms.
Electron mobilities in low density helium and nitrogen gases: Momentum transfer cross sections at very low energies
Authors: Ramanan G. , Freeman Gordon R.
Published: July 2002

Electron mobilities have been measured in low density helium and nitrogen gases as reference standards in the ongoing study of effects of molecular shape and density on electron transport in fluids. Wider temperature ranges were used than heretofore. Electron mobilities in helium gas at temperatures T=11–466 K and densities n=4.9–7.3 (1025 molec/m3) are consistent with previously reported scattering cross sections. The value of σm at 10−23 J (10−4 eV) is 4.9×10−20 m2, corresponding to a scattering length of 6.2×10−11 m. Density normalized mobilities nμ in nitrogen gas at T=66–460 K and n=0.9–6.7 (1025 molec/m3) decrease with increasing density. Values of nμ extrapolated to low field strengths and zero density gave values which were used to estimate momentum transfer cross sections. Further study of electron scattering by nitrogen at energies <3×10−21 J (20 meV) is desired.
Surface vibrational energy relaxation by sum frequency generation: Five-wave mixing and coherent transients
Authors: Harris A. L. , Rothberg L.
Published: July 2002

Vibrational energy relaxation of molecular adsorbates at solid surfaces has been measured in several recent experiments by pumping vibrational transitions with resonant picosecond infrared laser pulses and probing the excited state vibrational populations with transient infrared–visible sum frequency generation (SFG). In this paper, we develop a description of the infrared-pump, SFG-probe experiment as a five-wave mixing process in the perturbation limit. Five-wave mixing describes the effect of finite vibrational coherent response times (T2) on the experimental time resolution and includes coherent interactions between the pump and probe infrared fields which contribute to transient coherent artifacts in the experimental signal. Neither of these effects is included in the previous steady state descriptions of the SFG vibrational probe. The five-wave mixing expression is developed for a three-vibrational-level model of the adsorbate molecule and is integrated numerically for Gaussian pulse shapes to illustrate the effects of coherent artifacts and to compare these artifacts with those observed in pump–probe absorption experiments, which are described by four-wave mixing. Numerical simulations of the three-level response for a recent measurement of the surface vibrational energy relaxation of methyl thiolate/Ag(111) illustrate the use of the five-wave mixing expression to distinguish unambiguously between rapid energy relaxation and coherent artifacts in the experiment.
Hyperfine structure transfer, fine structure transfer, and quenching of 113Cd atoms in the 5s5p 3P1 state by collisions with molecular gases
Authors: von der Haar Th. , Kramer C. , Baumann M.
Published: July 2002

Collisions of excited 113Cd(5 3P1) atoms with various molecular gases (N2, H2, D2, CO, CO2, CH4, C2H6, C2H4, O2) have been investigated using atomic fluorescence spectroscopy. By selective excitation of a hyperfine structure level with a pulsed dye laser and studying the time dependence of the fluorescence signals as a function of the buffer gas density the cross sections for quenching, fine structure transfer, and hyperfine structure transfer could be obtained. For N2 molecules the collisions can be described by a decoupling model as has been shown previously for the noble gases as collision partners.
Path integral studies of the 2D Hubbard model using a new projection operator
Authors: Hall Randall W.
Published: July 2002

Feynman’s path integral formulation of quantum mechanics, supplemented by an approximate projection operator (exact in the case of noninteracting particles), is used to study the 2D Hubbard model. The projection operator is designed to study Hamiltonians defined on a finite basis set, but extensions to continuous basis sets are suggested. The projection operator is shown to reduce the variance by a significant amount relative to straightforward Monte Carlo integration. Approximate calculations are usually within one standard deviation of exact results and virtually always within two to three standard deviations. In addition, the algorithm scales with the number of discretization points P as either P or P2 (depending on the method of implementation), rather than the P3 of the Hubbard–Stratonovich transformation. Accuracy to about 5%–10% in energies and spin–spin correlation functions are found using moderate amounts of computer time.
Small-angle x-ray scattering from the surfaces of reversed-phase silicas: Power-law scattering exponents of magnitudes greater than four
Authors: Schmidt Paul W. , Avnir David , Levy David , Höhr Axel , Steiner Mathias , Röll Armin
Published: July 2002

The small-angle x-ray scattering from fully and partially derivatized porous silicas has been studied. Power-law-scattering exponents of magnitude greater than 4 have been found in all cases. The magnitudes of the exponents increased with the alkyl chain length and with the degree of surface derivatization. In a preliminary model to explain these observations, a power-law-scattering exponent with magnitude greater than 4 is related to a ‘‘fuzzy’’ pore boundary, in which the density varies continuously at the pore boundary instead of changing discontinuously from a value of zero in the empty pore to the essentially constant density characteristic of the bulk silica, as is usually assumed in analyses of the small-angle scattering from porous silicas.
Behavior of collective time correlation functions in liquids composed of polyatomic molecules
Authors: Luo H. , Hoheisel C.
Published: July 2002

Recent molecular dynamics (MD) calculations for molecular liquids showed that the time correlation functions (TCFs) for thermal transport coefficients have a characteristic ‘‘chair form.’’ For dense liquid states, a weak positive branch of the TCFs of the shear and the bulk viscosity was found. This ‘‘long-time’’ behavior of the TCFs appeared to be of particular significance in model liquids of molecules of nonglobular shape, as n-butane, nitrogen or benzene. Molecular liquids of molecules of more spherical shape displayed this time tail of the TCF only to a much smaller extent. The origin of this microscopic long-time behavior of the TCFs governing the viscosity coefficients was unknown, particularly, due to the fact that it appeared also for states considerably far from the triple point state. In the present MD study we investigate in detail the behavior of the TCFs for the shear and the bulk viscosity in a liquid composed of diatomic molecules. Molecule number and potential cutoff effects are discussed for the two-center Lennard-Jones liquid. The long-time behavior of the TCF is explained by long-lived orientational correlations measured in terms of the reorientational CF of lowest even order. This explanation is corroborated by additional calculations for the dumbbell fluid at high density. Because of the occurrence of a long-ranged weak positive branch of the TCFs for the viscosity, accurate determination of the viscosity coefficients in liquids of considerably nonisotropic molecules requires a few hundred thousand MD integration steps. On the other hand, systems of a small number of molecules give TCFs not significantly different from those obtained with larger systems.
Collisions of NO(X 2Π) with a Ag(111) surface: New quantum scattering studies based on a semiempirical potential energy surface
Authors: Alexander Millard H.
Published: July 2002

We report the results of fully quantum close-coupled studies of collisions of NO(X 2Π) with a Ag(111) surface. The recent corrected effective medium potential energy surfaces (PES) of DePristo and Alexander [J. Chem. Phys. 94, 8454 (1991)] were used. The final state rotational distributions show evidence of at least four rotational rainbows, corresponding to scattering on (and interference between) the two PES which arise when the degeneracy of the NO molecule is lifted upon approach to the surface. A strong tendency is seen to populate the lower spin–orbit manifold at low to moderate final J, which disappears as J rises beyond 30.5 and the final states are better described in Hund’s case (b). Simultaneously, there exists a propensity to populate those Λ-doublet levels in which the electronic–rotational wave function is antisymmetric (ΠA″) with respect to reflection of the electronic coordinates in the plane of rotation of the scattered NO molecule. This feature is opposite to what has been seen experimentally. An approximate averaging over the lateral position of the NO molecule above the surface showed that although the rainbow oscillations are strongly sensitive to surface corrugation, the fine-structure propensities are not. This suggests that these latter are reflective of some fundamental characteristic of the NO–Ag interaction which is independent of the position of the NO molecule above the Ag(111) unit cell.
Structural and electronic properties of sodium microclusters (n=2–20) at low and high temperatures: New insights from a
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o molecular dynamics studies
Authors: Röthlisberger Ursula , Andreoni Wanda
Published: July 2002

We present the results of extensive computer simulations of several sodium microclusters, using the Car–Parrinello method (unified density-functional theory and molecular dynamics). Dynamical simulated annealing strategies are adopted in the search for low-energy minima of the potential energy surface. A detailed analysis of the results for both structural and electronic properties at temperatures in the 0–600 K range is carried out, which allows us for the first time to gain insight into the structural ‘‘growth’’ pattern, the extent of the validity of (spherical, spheroidal, and ellipsoidal) jellium models, and the effects of temperature. In particular, new and unforeseen structures are discovered for n=10, 13, 18, and 20 and we emphasize the constant presence of arrangements with local pentagonal symmetry for the low-energy isomers as well as the similarity of the structural pattern with that of Lennard-Jones systems. Shape transformations with increasing temperature are observed, ‘‘rigidity’’ and ‘‘nonrigidity’’ of the individual clusters examined, and the presence of distinct isomers is identified for the smaller ones. Closing of electronic shells is confirmed for Na8 and Na20 and—to a minor extent only—for Na18. Hybridization of cluster states of different angular momenta, which represents a deviation from the spherical shell model, is discovered in several cases and discussed in detail, also in correspondence with the presence of anisotropy of the electronic potential. In most cases, this hybridization is observed to increase with increasing temperature, in parallel with the increase of the eccentricity of the cluster shape. In spite of the relatively high atomic mobility, our results do not support a spherical liquid-droplet picture for the atomic distribution.
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o model potential study of local distortions around Cr+ and Cr3+ defects in fluorite
Authors: Seijo Luis , Barandiarán Zoila
Published: July 2002

In this paper we present the results of a theoretical study of the local geometry distortions produced by Cr+ and Cr3+ ions in fluorite, by means of the ab initio environment model potential method [J. Chem. Phys. 89, 5739 (1988)]. The valence energy and wave function of the (CrF8)q−(q=7,5) clusters embedded in the CaF2 lattice represented by 118 ab initio model potential ions plus 750 point-charge ions are calculated using a short CI, which includes all states related to the dn ionic configuration. A geometry optimization is performed for CaF2:(CrF8)7− within the cubic point symmetry and for CaF2:(CrF8)5− within the D3d point symmetry; the conclusions of analyses of the multimode Jahn–Teller coupling T1⊗(ε+τ2+τ′2) are used to address the geometry optimization, which is completed by a final unrestricted search. In an attempt to study all the structures proposed along previous experimental studies, the D3d structure corresponding to an octahedral cluster plus two axially displaced fluorines (octahedron+2) is investigated. Taking the ground state energy of the perfect cube as a reference, the results show that the most stable D3d distortion corresponds to an elongated cube in a 4A2g ground state, with an energy of about −1700 cm−1, while the compressed cube and the octahedron+2 structure energies are found to be −780 and +40 000 cm−1, respectively. A joint analysis of the available experimental data and the results of our calculations are presented.
Three-dimensional quantum theory of the H+H2 reaction in strong laser fields
Authors: Seideman Tamar , Shapiro Moshe
Published: July 2002

Three-dimensional quantum theory of triatomic exchange reactions in strong laser fields is presented. Our theory consists of an exact partitioning technique for treating the effects of optical fields on reactive scattering, based on approximate hindered-rotor adiabatic wave functions describing the pure nonradiative events. The method enables computations to be performed for an arbitrary number of field intensities with very little effort beyond that required for a single-intensity computation. Differential and integral cross sections for the H+H2 exchange reaction, involving the ground and first excited electronic states, in the presence of laser fields, are computed. The dependence of reactive nonlinear optical effects, and especially that of ‘‘laser catalysis,’’ on laser intensity; the way isolated and overlapping power-broadened resonances affect the optically induced reaction; the role of relative orientation of two incident molecular beams in crossed beams experiments are investigated. The three-dimensional computations confirm our previous expectations, based on a collinear model, that laser catalysis is achievable using only moderately high powered lasers. The above is expected to be true for all reactive systems (of which H+H2 is one) possessing optically allowed stable excited electronic states.
193 nm photodissociation of acetylene
Authors: Balko B. A. , Zhang J. , Lee Y. T.
Published: July 2002

The product translational energy distribution P(ET) for acetylene photodissociation at 193 nm was obtained from the time-of-flight spectrum of the H atom fragments. The P(ET) shows resolved structure from the vibrational and electronic excitation of the C2H fragment; comparison of the translational energy release for given excited states of C2H with the known energy levels of these states gives D0(HCC–H)=131.4±0.5 kcal/mol. This value is in agreement with that determined previously in this group from analogous studies of the C2H fragment and with the latest experimental and theoretical work. The high resolution of the experiment also reveals the nature of C2H internal excitation. A significant fraction of the H atoms detected at moderate laser power were from the secondary dissociation of C2H. The P(ET) derived for this channel indicates that most of the C2 is produced in excited electronic states.
Magic metastable decay of singly charged argon clusters
Authors: Foltin M. , Walder G. , Castleman A. W. , Märk T. D.
Published: July 2002

Singly charged argon cluster ions produed by electron impact ionization of a neutral argon cluster beam are found to decay in the metastable time regime by a new mechanism leading to the ejection of ‘‘magic’’ numbers of neutral argon atoms. The measured dependence of this new decay process on (i) the electron energy, (ii) cluster size and cluster properties, and (iii) the time since ion formation gives insight into the unique mechanism and the nature of this process. At a well-defined threshold energy of ca. 27 eV, the magic number loss mechanism occurs simultaneously with the well-known single monomer evaporation process which proceeds at all energies. Importantly, the new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.
Many dicationic states and two-hole population analysis as a bridge to Auger spectra: Strong localization phenomena in BF3
Authors: Tarantelli F. , Sgamellotti A. , Cederbaum L. S.
Published: July 2002

The outer valence double ionization spectrum of BF3 is investigated by computing 100 double ionization transitions with the Green’s function method. The results show that one-site and two-site hole localization on the fluorine atoms takes place to full extent and the energy split between clusters of states with similar hole distribution exceeds 10 eV. Double hole localization as a symmetry breaking phenomenon is discussed, pointing out the inherent inability of uncorrelated methods to describe it. A two-hole population analysis of the dicationic states for the quantitative measure of their localized character is presented. The strong effects of hole localization on Auger transition rates are discussed, showing that the two-hole population analysis can be used to develop a new statistical approach to the computation of Auger line shapes. Application of this method to the fluorine and boron KLL Auger spectra of BF3 permits, for the first time, an accurate reproduction of the spectral profiles and a complete interpretation of their features.
Gas–surface scattering cross section by the complex coordinate method
Authors: Engdahl Erik , Maniv Tsofar , Moiseyev Nimrod
Published: July 2002

The elastic scattering cross section of atomic beams from periodically rough solid surfaces is computed by means of a quantum-mechanical approach, based on the complex coordinate method. The effect of a threshold singularity in the specular scattering of helium beam from Cu(110) is found to yield a step structure in the scattering pattern, which should be observable experimentally. The calculated specular scattering pattern obtained, using a corrugated Morse potential with a simple one-dimensional corrugation function, is in a remarkable agreement with the experimental resonance line shapes of He beams from Cu(115) over the entire range of incident angles.
Neutral-stabilized electron–ion recombination in ambient helium gas
Authors: Cao Y. S. , Johnsen R.
Published: July 2002

We have measured rates of recombination of electrons with simple molecular ions in the presence of helium at densities from 2.5×1019 to 2.9×1020 cm−3 at reduced temperatures of 77, 125, and 150 K. The results agree surprisingly well with the neutral, collisional recombination mechanism proposed by Bates and Khare for atomic ions recombining in plasmas of high neutral density.
Adsorption on colloids and flocculation: The influence of salt
Authors: van Duijneveldt Jeroen S. , Beysens Daniel
Published: July 2002

The stability of colloidal silica particles in a mixture of water and 2,6-lutidine is investigated in the presence of salt. Close to the demixing temperature of the solvent, a reversible flocculation occurs, coupled with lutidine adsorption on the colloid surface. Addition of Mg(NO3)2 is shown to suppress the lutidine adsorption and to reverse it into water adsorption. The flocculation disappears at the same time. This behavior is interpreted as a modification of the properties of the colloid surface by the salt and is in accord with current wetting theory.
Photoionization spectroscopy of nickel clusters: The effect of ammonia adsorption on ionization potentials
Authors: Knickelbein Mark B. , Menezes Warren J. C.
Published: July 2002

The photoionization efficiency spectra of nickel clusters containing adsorbed ammonia have been recorded near threshold. Ionization potentials (IPs) have been assigned for 97 selected Nin(NH3)m species, with n up to 56 atoms. We observe that nickel cluster IPs decrease upon ammonia adsorption, with the magnitude of the IP shifts varying linearly with the number of adsorbed molecules, in good agreement with a simple electrostatic model. Deviations from this behavior are observed for Ni5(NH3)m and Ni6(NH3)m clusters, in which adsorption of the first ammonia molecule produces a significantly larger IP drop than do subsequently adsorbed molecules. Such behavior may indicate the presence of unique adsorption sites on Ni5 and Ni6 preferentially occupied by the first ammonia molecule adsorbed onto these clusters, or alternatively, may indicate cluster rearrangement occurring as the result of chemisorption. Our results are discussed in light of studies of ammonia adsorption onto single crystal nickel surfaces.
Driven Morse oscillator: The quantum problem simplified
Authors: Travlos S. Don , Boeyens Jan C. A.
Published: July 2002

The driven Morse oscillator in quantum formulation is a good model for the analysis of molecular dissociation by infrared laser light. A method is presented to transform the time-dependent Schrödinger equation of the system, by a Green’s function procedure, into a first-order Fredholm integral equation, more amenable to numerical solution.
Long-range poisoning of D2 dissociative chemisorption on Pt(111) by coadsorbed K
Authors: Brown J. K. , Luntz A. C. , Schultz Peter A.
Published: July 2002

We report detailed molecular beam studies of the effects of K adsorption on the dissociative chemisorption probabilities S0 for D2 on Pt(111). In contrast to conventional wisdom for many other molecular systems, we find that K is a very strong poison rather than promoter for H2 dissociation. S0 decreases roughly exponentially with K coverage ΘK. The effective cross section for poisoning per adsorbed K varies between 70 and 430 Å2, depending upon incident energy Ei. This suggests that an extremely long-range electronic perturbation is responsible for the poisoning. A theoretical model is developed to describe these sticking measurements. It is based on the fact that the adsorbed K lowers the work function of the surface. This enhances Pauli repulsion for the molecule-surface interaction which, in turn, increases the barrier to dissociation. When the model is generalized to include inhomogeneous effects through a local work function, excellent agreement is obtained between the model and experiments. This model also naturally accounts for the increased associative desorption temperatures for H2 when coadsorbed K is present.
Multiphoton mass spectrometry of clusters: Dissociation pathways and energetics of heterogeneous van der Waals cluster ions
Authors: Krause H. , Ernstberger B. , Bel Bruno J. J. , Neusser H. J.
Published: July 2002

In this work the metastable decay of heterogeneous benzene/toluene, benzene/cyclohexane, and benzene/para-difluorobenzene van der Waals cluster ions [(MiNk)+ with n=i+k<12] is investigated. Two-photon ionization leads to an internal energy of less than 1.5 eV. On the 100 μs time scale of the linear reflectron time-of-flight mass spectrometer used in this work, the main dissociation process of the cluster ions is the evaporation of a single molecule. A uniform decay behavior is observed for benzene/toluene and benzene/cyclohexane cluster ions; the only dissociation pathway being the evaporation of a single benzene or cyclohexane molecule, respectively. By contrast, the evaporation of either a benzene or a para-difluorobenzene molecule is observed for benzene/para-difluorobenzene cluster ions, depending on the size and the composition of the heterocluster. We apply an energetic model to explain the observed dissociation pathways. This model assumes the total miscibility of the different molecular species and a total dilution of charge. Despite these assumptions, the model predictions are in good agreement with our experimental data.
Tunneling rates in a two-dimensional symmetric double-well potential surface by the exterior scaling procedure
Authors: Rom Naomi , Moiseyev Nimrod , Lefebvre Roland
Published: July 2002

Time-independent methods which were recently developed [J. Chem. Phys. 91, 6246 (1989); 93, 3413 (1990)] to calculate tunneling rates in bound systems are applied to a two-dimensional symmetrical double-well surface proposed by Makri and Miller [J. Chem. Phys. 86, 1451 (1987)] to describe intramolecular H-atom transfer. The methods are based on exterior complex scaling either with basis-set expansions or with an iterative multichannel propagation and matching procedure. Exterior complex scaling is used to identify the initial well (characterized by an unscaled reaction coordinate) and the final well (with a complex-scaled reaction coordinate). The quantized energies of the system become complex, with imaginary parts providing the rates.
Spectroscopic constants and potential energy curves of Bi2 and Bi−2
Authors: Balasubramanian K. , Liao Dai-Wei
Published: July 2002

We compute the spectroscopic constants of 26 electronic states of Bi2 and six electronic states of Bi−2. In addition, the potential energy curves of electronic states of Bi2 dissociating into Bi(4S)+Bi(4S), Bi(4S)+Bi(2D), Bi(4S)+Bi(2P), Bi(2D)+Bi(2D), and Bi(4S)+Bi(4P) limits are computed. We use a complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by first-order configuration interaction (FOCI) and second-order configuration interaction (SOCI) methods. In addition, the spin–orbit effects are included through the relativistic configuration interaction (RCI) method. Our computed spectroscopic properties facilitate the assignment of recently observed negative ion photodetachment spectra as well as the electronic spectra accumulated up to now. The observed lifetime and transition moment dependence on internuclear distance are also explained based on computed potential energy curves.
The formation of large polyaromatic hydrocarbons via carbon cluster ion reactions
Authors: Zimmerman Jeffrey A. , Creasy William R.
Published: July 2002

Reactions of carbon cluster ions (C+n, 10<n<25) with polyaromatic hydrocarbons (PAHs) and substituted benzenes, studied using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, provide evidence for the formation of large, highly conjugated PAHs. Product ions consist of adducts formed by radiative association/collisional stabilization and adducts accompanied by the loss of a hydrogen. These two reaction pathways alternate between the even- and odd-numbered cluster ions dependent on reactant neutral employed. Adduct formation continues up to a maximum of five associations and to molecular weights in excess of 900 amu. Collisionally induced dissociation (CID) of product ions produced fragmentation indicative of species with multiply bonded carbon atoms and possibly fullerene-type structure. Complete dehydrogenation of the adduct ions was possible.