bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscopic expression for dielectric friction on a moving ion
Authors: Bagchi Biman
Published: July 2002

A theoretical study of the dielectric friction on an ion moving through a dipolar liquid is presented. A microscopic expression for the dielectric friction is derived from linear response theory for a Brownian ion. This expression supports the picture of a ‘‘dynamic solventberg’’ model in the sense that much of the contribution to dielectric friction (ζDF) comes from the nearest-neighbor molecules. The translational modes of the solvent are found to have a very strong influence on the dielectric friction, in agreement with the observation of Colonomos and Wolynes [J. Chem. Phys. 71, 2644 (1979)]. In fact, except in the limit of small ion size, the microscopic ζDF is significantly larger than the continuum ζDF in the absence of the translational motion of the solvent molecules, but the reverse is obtained in the presence of a substantial translational contribution. It is found that the recovery of the continuum limit results from the molecular expression requires serious assumptions, some of which are hard to justify. It is also found that the point dipole approximation for the dipolar solvent molecule leads to improper results for ζDF because this approximation gives a wrong wave vector (k) dependence of the wave vector dependent dielectric function (ε(k)) of the liquid at large k (kσ≫2π, where σ is the solvent molecular diameter). We show that within a linear equilibrium theory for dipolar liquids, the cross correlations between the short ranged hard force and the long ranged dipolar force is zero so the calculations of Colonomos and Wolynes are internally consistent. However, this cross correlation can be quite important if the soft force also contains a spherically symmetric part. The similarity between the solvent role in ζDF and in the time-dependent fluorescence Stokes shift is discussed. The limitations of the present theory are also pointed out.
Quantitative determination of H2, HD, and D2 internal-state distributions by (2+1) resonance-enhanced multiphoton ionization
Authors: Rinnen Klaus-Dieter , Buntine Mark A. , Kliner Dahv A. V. , Zare Richard N. , Huo Winifred M.
Published: July 2002

The relationship between quantum-state populations and ion signals in (2+1) resonance-enhanced multiphoton ionization (REMPI) detection of H2, HD, and D2 via the E, F 1Σ+g (v′E=0, J′=J″)–X 1Σ+g (v″,J″) transition is determined by calibration against a thermal effusive source. Correction factors are obtained for 102 rovibrational levels for v″=0, 1, and 2 and J″ ranging from 0 to 17. Within a given v″, rotational correction factors are nearly unity except for the highest J″ levels. The vibrational correction factors vary with v″; (2+1) REMPI detection is 2–3 times more sensitive to v″=1 and 2 than to v″=0. Experimental correction factors are compared with those derived from a theoretical calculation of the two-photon transition moments by Huo et al. [J. Chem. Phys. 95, xxxx (1991)]. In general, the agreement is excellent, which suggests that theoretical correction factors may be used when experimental ones are unavailable.
Rotational branching ratios and photoelectron angular distributions in resonance enhanced multiphoton ionization of diatomic molecules
Authors: Wang Kwanghsi , McKoy V.
Published: July 2002

In this paper we extend a previous formulation of molecular resonance enhanced multiphoton ionization (REMPI) photoelectron spectra to explicitly include multiplet-specific final state wave functions and intermediate coupling schemes. The results of this formulation should be well suited and helpful in quantitative theoretical studies of rotationally resolved REMPI spectra in many diatomic molecules of interest. As an example, we use this formulation to study the rotational branching ratios and photoelectron angular distributions for (3+1) REMPI of NH via the 3 3Π Rydberg resonant state. The predicted anomalous rotational distributions are interpreted as arising from a Cooper minimum in the l=2 component of the kπ photoionization channel. A number of other results are obtained and discussed.
Vibrational relaxation on metal surfaces: Molecular-orbital theory and application to CO/Cu(100)
Authors: Head-Gordon Martin , Tully John C.
Published: July 2002

A nonempirical theory of vibrational relaxation at metal surfaces via nonadiabatic coupling to conduction electrons is presented. Using a single determinant Hartree–Fock (HF) description of the electronic states of the system, an expression for the lifetime of an excited vibration is obtained. Under certain additional assumptions, all the quantities necessary to calculate the lifetime can be obtained from the results of ab initio HF calculations on cluster models of the adsorbate-metal system. As a practical test of this procedure, the lifetime of the excited v=1 vibrational state of CO on Cu(100) is calculated using clusters of 6, 10, and 14 copper atoms. Results ranging between 1.1 and 3.5 ps are obtained, with our preferred procedure yielding 1.7 ps for the largest cluster, in good agreement with experiment. Extensions of this approach may also be valuable for treating other nonadiabatic phenomena at metal surfaces.
Collisional effects in the stimulated Raman Q branch of O2 and O2–N2
Authors: Millot G. , Saint-Loup R. , Santos J. , Chaux R. , Berger H. , Bonamy J.
Published: July 2002

The fundamental isotropic Raman Q branch of oxygen at pressures up to 2 atm and for temperatures between 295 and 1350 K has been recorded using stimulated Raman gain spectroscopy (SRGS) for collisions with oxygen and nitrogen. The line broadening and line shifting coefficients have been determined for several rotational quantum numbers (up to N=55 at 1350 K). The temperature dependence of these coefficients has also been studied for most of the rotational lines. The line parameters (widths and shifts) have been then calculated a priori through a semiclassical model. A good agreement between experimental and theoretical data has been observed. Another theoretical approach based on fitting and scaling law has been used to calculate the line broadening coefficients. It is shown that a modified exponential energy gap model (MEG) and an energy corrected sudden law (ECS) for the state-to-state rotationally inelastic rates, account for the rotational and temperature dependences of the observed linewidths. With regard to the energy corrected sudden law, the best results are obtained when the basis rate constants are modeled with a hybrid exponential-power fitting law (EP). The line broadening and shifting coefficients of the oxygen–nitrogen mixture are very close to those found for pure oxygen.
Photodepletion probes of Na5, Na6, and Na7. Molecular dimensionality transition (2D→3D)?
Authors: Wang C. R. Chris , Pollack Stuart , Dahlseid Tina A. , Koretsky Geoffrey M. , Kappes Manfred M.
Published: July 2002

Photodepletion spectroscopy was used to obtain electronic absorption data for Na5, Na6, and Na7 in a wavelength range from 420–770 nm. Measurements were rationalized using several models, ranging from classical electrostatic to ab initio large-scale configuration-interaction calculations. As in previous studies of Na4, Na8, and Na20, classical Mie–Drude theory applied to appropriately dimensioned metal droplets describes the gross but not a number of fine-structure features of the spectra observed. In contrast, ab initio large-scale configuration-interaction predictions of vertical electronic excitations from specific ground-state geometries can more fully account for the measurements. Theory–experiment comparison suggests that Na6 is predominantly formed under our molecular-beam conditions in a planar or near-planar structure. In contrast, the spectrum obtained for Na7 is consistent with a three-dimensional topology.
Collision-induced dissociation of Ni+n (n=2–18) with Xe: Bond energies, geometrical structures, and dissociation pathways
Authors: Lian Li , Su C.-X. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the collision-induced dissociation (CID) of Ni+n (n=2–18) with xenon is studied by using a guided ion beam mass spectrometer. Bond energies of nickel cluster ions, D0(Ni+n−1–Ni), are determined from measurements of the CID thresholds. Bond energies for neutral nickel clusters, D0(Nin−1–Ni), are derived by combining these ionic bond energies with literature values of ionization energies for Nin. Both D0(Ni+n−1–Ni) and D0(Nin−1–Ni) are found to increase nonmonotonically as a function of cluster size, with local maxima at n=3, 7, and 13 for ionic clusters and at n=6 and 13 for neutral clusters. Examination of the cluster size dependence of nickel cluster bond energies leads to speculations on the likely cluster geometric structures. Examination of the general dissociation behavior over a broad collision energy range shows that nickel cluster ions dissociate primarily by sequential atom loss, although exceptions are noted.
Collision energy dependence of product branching in the ionization of HCl molecules by collision with metastable neon atoms
Authors: Navarro A. Aguilar , Brunetti B. , Falcinelli S. , Gonzalez M. , Vecchiocattivi F.
Published: July 2002

The ionization of HCl by collision with metastable neon atoms has been studied in a crossed beam-mass spectrometric experiment in the thermal energy range. It was found that the ionization leads to HCl+, NeH+, and NeHCl+ ion products. The HCl+ ion is the main product. Its cross section shows a decreasing trend in the energy range investigated. The NeH+ cross section is about one order of magnitude lower and decreases with collision energy more rapidly than that of HCl+. The NeHCl+ ion is the minor product with a cross section decreasing dramatically with collision energy. The production of these ions can be explained as the result of the postionization dynamics of the two possible ionic complexes formed when the electron is ejected: the ground state [Ne⋅⋅⋅HCl+(X)] ionic complex dissociates into Ne+HCl+(X) or remains as a stable NeHCl+ ion; the excited [Ne⋅⋅⋅HCl+(A)] complex dissociates into Ne+HCl+(A) or reacts to NeH++Cl. The theoretical model used to analyze the experimental results gives a satisfactory qualitative account of the cross sections and of their collision energy dependence. The model combines the semiclassical treatment for Penning and associative ionization with the Langevin criterion for the ion–molecule reactions. It assumes a local complex spherical potential for the Ne*–HCl interaction, the probability for the formation of each of the four possible product ions, HCl+(X), HCl+(A), NeH+, and NeHCl+ being obtained from the distribution of electronic states, kinetic energy, angular momentum, and intermolecular distance of the two nascent [Ne⋅⋅⋅HCl+(X,A)] complexes formed when the electron is emitted by the system.
Modeling the potential of a charge distribution
Authors: Gill Peter M. W. , Johnson Benny G. , Pople John A. , Taylor Stephen W.
Published: July 2002

We describe a numerically straightforward method for obtaining a model distribution whose potential is as close as possible, in a least-squares sense, to that of a known charge distribution.
High resolution photoelectron spectra of the NO dimer
Authors: Fischer Ingo , Strobel Andreas , Staecker Jan , Niedner-Schatteburg Gereon , Müller-Dethlefs Klaus , Bondybey Vladimir E.
Published: July 2002

High resolution zero kinetic energy (ZEKE) photoelectron spectra of the NO dimer are measured. They provide information about the ionization energy of the neutral, as well as about the binding energy, vibrations and structure of the ionized dimer indicating considerable structural reorganization of the dimer upon ionization.
Structures of small metal clusters. I. Low temperature behavior
Authors: Vlachos D. G. , Schmidt L. D. , Aris R.
Published: July 2002

The binding energy, surface energy, surface atom coordination numbers, and bond lengths and angles of small clusters at low temperatures are calculated with parameters fitted to Ni using both the Lennard-Jones (LJ) potential and the embedded atom (EA) potential to assess the sensitivity of interatomic potential on cluster structure. Simulations are performed by implementation of the simulated annealing method in a canonical ensemble Monte Carlo technique. We examine clusters with n≤34 atoms and we find that they are noncrystalline (with the exception of n=6). The most stable structure of clusters consisting of n≤15 atoms and n=19 atoms is the same for both potentials (with the exception of n=8). However, the most stable structure of clusters with n≥16 atoms is different for the two potentials (with the exception of n=19). Smeared angular distribution and pair distribution functions are found for many EA clusters whereas sharp, well defined peaks exist for LJ clusters. A discontinuous transition from polyicosahedral to quasicrystalline structure is found from n=30 to n=31 atoms for the LJ potential. This transition occurs at smaller n for the EA potential. Surface atom coordination numbers are found to be nonmonotonic functions of cluster size. The existence of multiple structures of small clusters and the effect of quenching rate during crystallization on the final shape of clusters are also examined.
Electron impact dissociation of the NH3 molecule: Absolute and relative cross sections for NH*(c 1Π,A 3Π), NH+*(C 2Σ+, B 2Δ,A 2Σ−), and NH*2(Ã 2A1) fragment emissions
Authors: Müller U. , Schulz G.
Published: July 2002

The emission spectrum between 2700 and 5000 Å following single electron impact on ammonia has been investigated. Emission bands of the electronically excited NH(c 1Π,A 3Π), NH+(C 2Σ+,B 2Δ,A 2Σ−), and NH2(Ã 2A1) fragments have been identified and absolute emission cross sections have been determined at 100 eV incident energy. Relative cross sections as a function of the impact energy reveal information about the excitation mechanisms of the parent molecule. Conclusions can be drawn on the dissociation process by a comparison of the observed appearance potentials with thermochemical thresholds and with the results of photodissociation experiments.
Liquid water with an ab initio potential: X-ray and neutron scattering from 238 to 368 K
Authors: Corongiu Giorgina , Clementi Enrico
Published: July 2002

Molecular dynamics simulations with the ‘‘Nieser–Corongiu–Clementi flexible water molecules’’ (NCC-vib) ab initio potential are reported at a number of temperatures ranging from 242 to 361 K for H2O and from 238 to 368 K for D2O. Each simulation has been carried out, after equilibration, for 12 ps; the computed trajectories have been used to study structural properties as a function of the temperature. Water geometry in the liquid, pair correlation functions, coordination numbers, x-ray and neutron scattering structure functions are compared with available experiments. The overall agreement is quite satisfactory, validating the reliability of the NCC-vib interaction potential. Concerning the pair correlation [gαβ(r,T)] and the neutron scattering structure functions, our simulation validates more Soper et al. [Chem. Phys. 107 (1986)] than Thiessen et al. [J. Chem. Phys. 77 (1982)] data. Plots of the Δgαβ(r,ΔT) and of the temperature variations in the coordination numbers are favorably compared with the experimental findings by Bosio et al. [Phys. Rev. A 27 (1983)]. Temperature dependent shifts in peak position and intensity of x-ray and neutron structural functions are in excellent agreement with the experimental finding of Bosio et al. [Phys. Rev. A 27 (1983)] and Chen et al. [Adv. Chem. Phys. 64 (1988)]. These simulations point out that for liquid water, in the temperature range ∼238 to 368 K, at the density of 0.997 g/cm3, the NOO oxygen coordination number grows, with a well defined transition, from 4 to 5 while the NOH decreases from 5 to 4.
The susceptibility critical exponent for a nonaqueous ionic binary mixture near a consolute point
Authors: Zhang Kai C. , Briggs Matthew E. , Gammon Robert W. , Levelt Sengers J. M. H.
Published: July 2002

We report turbidity measurements of a nonaqueous ionic solution of triethyl n-hexylammonium triethyl n-hexylboride in diphenyl ether. A classical susceptibility critical exponent γ=1.01±0.01 is obtained over the reduced temperature range 10−4≤ t≤10−1. The best fits of the sample transmission had a standard deviation of 0.39% over this range. Ising and spherical model critical exponents are firmly excluded. The correlation length amplitude ξ0 from fitting is 1.0±0.2 nm which is much larger than values found in neutral fluids and some aqueous binary mixtures.
Transient grating effects in absorbing colloidal suspensions
Authors: McEwan K. J. , Madden Paul A.
Published: July 2002

The time dependence of the refractive index change induced in an absorbing colloidal suspension by a nanosecond laser pulse is studied by the four-wave mixing method. A very large transient is observed even in very dilute suspensions whose amplitude oscillates on a time scale much shorter than the laser pulse duration; the effect is found in a variety of systems. The behavior of the transient suggests that it is associated with an acoustic wave in the suspending fluid excited by an explosive event at the surface of the particles. By comparing the predictions of various hydrodynamic models with the observations and by studying the effect of changing the experimental conditions on the form of the transient, it is proposed that the behavior arises from the very rapid growth of vapor bubbles around those particles which are heated sufficiently strongly; the four-wave mixing signal monitors the coherent superposition of the resulting pressure pulses in the fluid.
Applicability of valence-universal multireference coupled-cluster theories to quasidegenerate electronic states. I. Models involving at most two-body amplitudes
Authors: Jankowski K. , Paldus J. , Grabowski I. , Kowalski K.
Published: July 2002

The recently developed algebraic formulation of valence-universal coupled-cluster (VU-CC) theories [Jeziorski and Paldus, J. Chem. Phys. 90, 2714 (1989)] for open-shell systems has been employed in a systematic derivation of explicit equations defining cluster amplitudes assuming Lindgren’s normal ordered exponential ansatz for the wave operator. The latter is approximated by its one- and two-electron components. Various aspects of the applicability of this version of the VU-CC theory to quasidegenerate electronic states are studied for a model system consisting of two slightly stretched, interacting hydrogen molecules. A single parameter that determines the geometry of this system makes it possible to vary the extent of quasidegeneracy of the two lowest-energy states over a wide range. Along with the complete theory, the linear version (VU-LCC) is also examined. The results are compared with the full configuration interaction results as well as with those obtained using other approaches. It was found that, at least in the strongly quasidegenerate region, the VU-CC energies are less accurate than those obtained with other multireference CC theories. It is shown that the VU-CC equations for cluster amplitudes possess multiple solutions representing various pairs of states. The individual solutions that arise for a system possessing n-valence electrons may be characterized in terms of a genealogical scheme that involves states of systems with 0,1,...,n valence electrons.
Structure and dynamics of polymer brushes near the Θ point: A Monte Carlo simulation
Authors: Lai Pik-Yin , Binder Kurt
Published: July 2002

Grafted polymer layers under variable solvent conditions are studied by Monte Carlo simulations using the bond fluctuation model. Structural information such as monomer density profiles, brush thickness, mean-square displacement of monomers, and positions of the monomers along the chain are obtained for temperatures above, at, and below the Θ point. In particular, the scaling of the brush thickness is formulated and verified by the simulation data. At the Θ point, more extensive simulations are performed to investigate the structural and dynamical properties. While the brush thickness at the Θ point agrees very well with the scaling and self-consistent field predictions, the latter deviate from our results for the monomer density profile and the distribution function of the free chain ends. It is not clear whether these deviations result from our chains being too short or from a basic inadequacy of the theory. For the dynamics at the Θ point, both the relaxation of the chain configurations and the mean-square time displacement of the chains are studied. The relaxation time τ for a polymer brush of chain length N and surface coverage σ are found to behave as τ∼N3σb with an apparent exponent b≂1.6. For poor solvent conditions below the Θ point, we find that the layer (with randomly fixed grafting sites) develops considerable lateral inhomogenity in its density, which has not been predicted by previous theories.
Study of C6− and C6 with threshold photodetachment spectroscopy and autodetachment spectroscopy
Authors: Arnold Caroline C. , Zhao Yuexing , Kitsopoulos Theofanis N. , Neumark Daniel M.
Published: July 2002

The C6− anion and C6 neutral have been studied using both threshold photodetachment (zero electron kinetic energy) spectroscopy and autodetachment spectroscopy of C6−. The threshold photodetachment spectrum yields the electron affinity of linear C6 to high accuracy, along with the three symmetric stretch frequencies for linear C6 and the spin–orbit splitting in the ground 2Πu state of the anion. Two of the symmetric stretch frequencies are significantly lower than previous ab initio predictions. A simple model force field is used to calculate stretching force constants and estimate bond length changes between the anion and neutral. In addition, using autodetachment spectroscopy, we have located an excited electronic state of C6− that lies 43 cm−1 below the detachment threshold. This state is very similar in geometry to neutral C6. Excited vibrational levels of this state autodetach with rates that depend strongly on the available autodetachment channels. The excited state is tentatively assigned to a valence state, rather than an electrostatically bound state.
Hierarchical analysis of molecular spectra
Authors: Davis Michael J.
Published: July 2002

A method of characterizing molecular spectra is proposed here. Hierarchical trees are generated from molecular spectra by following the branching of peaks as resolution is changed continuously. The trees are analyzed using several techniques developed in the classification and multivariate analysis literature. The information obtained from these techniques includes (1) dimensionality of geometrical representations of the trees; (2) clustering statistics; and (3) number of distinct time scales inherent in the spectra. Several model spectra are studied to establish the utility of the hierarchical analysis and a portion of a vibronic spectrum of NO2 is also investigated.
Applications of surface resistivity to atomic scale friction, to the migration of ‘‘hot’’ adatoms, and to electrochemistry
Authors: Persson B. N. J.
Published: July 2002

This work focuses on the concept of surface resistivity. The change in the resistivity of thin metallic films upon adsorption of atoms and molecules is discussed and the resistivity change is related to the nature of the adsorption bond. With this background, I discuss applications to atomic scale friction, surface migration of ‘‘hot’’ adatoms, and electrochemistry.
Structural properties of sodium microclusters (n=4–34) using a Monte Carlo growth method
Authors: Poteau Romuald , Spiegelmann Fernand
Published: July 2002

The structural and electronic properties of small sodium clusters are investigated using a distance-dependent extension of the tight-binding (Hückel) model and a Monte Carlo growth algorithm for the search of the lowest energy isomers. The efficiency and advantages of the Monte Carlo growth algorithm are discussed and the building scheme of sodium microclusters around constituting seeds is explained in details. The pentagonal-based seeds (pentagonal bipyramids and icosahedral structures) are shown to play an increasing role beyond n=12. Optimized geometries of Nan clusters are obtained in the range n=4–21 and for n=34. In particular, Na20 is found to have C3 symmetry, hardly prolate with all axial ratios almost equivalent, whereas Na34 has D5h symmetry and consists of a doubly icosahedral seed of 19 atoms surrounded by a ring of 15 atoms. Stabilities, fragmentation channels, and one-electron orbital levels are derived for the lowest isomers and shown to be characterized by a regular odd–even alternation. The present results are in generally good correspondence with previous nuclei-based calculations when available. The global shapes of clusters, as well as the shape-induced fine structure splitting of the spherical electronic jellium shell are found, with a few exceptions, to be also consistent with the ellipsoidal or spheroidal versions of the jellium model.
Nuclear magnetic relaxation spectroscopy of polymers and anomalous segment diffusion. Reorientations mediated by translational displacements
Authors: Kimmich Rainer , Weber Hans Werner
Published: July 2002

Chain dynamics of polymer melts was investigated by field cycling and rotating-frame nuclear magnetic resonance (NMR) relaxation spectroscopy in a frequency range from 103 to 3×108 Hz. Far above the critical molecular weight, the frequency dependencies of the spin–lattice relaxation times T1 and T1ρ are characterized by a sequence of power laws ∝ν0.5, ∝ν0.25, and ∝ν0.5 occurring in ranges analogous to the Doi/Edwards limits of the anomalous time dependencies of the mean-square displacement of segments ∝t1/2, ∝t1/4, and ∝t1/2. The T1 dispersion data clearly contradict the dominance of Rouse dynamics within the Doi/Edwards tube. The ν3/4 frequency dependence predicted by de Gennes for the regime of Rouse relaxation along the tube was not observed. The spin–lattice relaxation behavior can, however, be derived from the Doi/Edwards mean-square displacement limits assuming a correlation between segment orientation and displacement direction. A corresponding formalism is presented. On the other hand, the spin–lattice relaxation in dilute polymer solutions and in melts of polymers with molecular weights below the critical value can be described perfectly by the Khazanovich NMR relaxation theory for the Rouse model.
High resolution electronic spectroscopy of ZnCH3 and CdCH3
Authors: Cerny Timothy M. , Tan Xue Qing , Williamson James M. , Robles Eric S. J. , Ellis Andrew M. , Miller Terry A.
Published: July 2002

ZnCH3 and CdCH3 radicals have been prepared in a cold supersonic free jet expansion and their laser-induced-fluorescence spectrum recorded for the Ã 2E←X̃ 2A1 electronic transition. These spectra show well resolved rotational and spin structure, which has been completely analyzed. This analysis yields the rotational constants and the components of the spin–rotation tensors in the Ã and X̃ states of both radicals. The observed constants are discussed in terms of the electronic structure of the radicals. It is demonstrated that the upper F2 spin–orbit component of the Ã 2E state of CdCH3 is strongly perturbed by another, dissociative electronic state. This leads to some predissociation of the Ã 2E3/2 component and a broadening of its lines. The rotational and fine structure in this state is also quite perturbed leading to an unusual, but still interpretable spectrum.
Comparison of theoretical vibrational and rotational energies of the HCP molecule with experimental values
Authors: Chen Yit-Tsong , Chong Delano P.
Published: July 2002

Using the deMon density functional program, we calculated 150 points on the potential-energy surface for the ground electronic state (X̃ 1Σ+) of the HCP molecule. An analytical function was fitted through these points and used in two different computer programs (called triatom and dvr) to calculate the vibration-rotational energies of the HCP and DCP molecules. The computed vibrational fundamentals of ν1, ν2, and ν3 for HCP (DCP) are 3134 (2365), 654 (509), and 1269 (1227) cm−1, respectively, in good agreement with the observed values of 3217 (2419), 675 (525), and 1278 (1231) cm−1. The calculated vibrational energies of HCP show a pronounced regularity, in accordance with the observed harmonic-vibrational characteristic of the molecule concerned. The average energy deviation of the computed vibration-rotational energies of HCP up to 8200 cm−1 from the 58 corresponding observations is 90±32 cm−1, thus demonstrating the deMon potential-energy function to be of good quality.
Bimolecular surface photochemistry: Mechanisms of CO oxidation on Pt(111) at 85 K
Authors: Mieher W. D. , Ho W.
Published: July 2002

Results from a photoinduced bimolecular surface reaction are presented. The reaction, occurring from CO coadsorbed with O2 on Pt(111) at 85 K, is O2+CO+hν→O+CO2. Surface analysis techniques employed include electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS), photon-induced desorption spectroscopy (PID), and low energy electron diffraction (LEED). The incident power, photon energy, and polarization dependences of the photochemical processes, O2 photodesorption and CO2 photoproduction, were characterized, with the cross section for both processes being 3×10−19 cm2 at 240 nm. Electronic EELS studies were performed to acquire information on the electronic structure of O2 on Pt(111). The experimental results are compared to predictions of models describing direct dipole excitation of the O2–Pt system and substrate mediated hot carrier mechanisms. Reaction mechanisms involving photogenerated hot O atoms or excited O2 molecules on the surface are considered. The implications of this work on surface reaction dynamics are discussed.
Test of a simple model of the intermolecular potential for the condensed phases of C70
Authors: Pickholz M. , Gamba Z.
Published: July 2002

We test a simple atom–atom model of the intermolecular potential for the condensed phases of C70. The model, previously developed for the crystalline phases of C60, consists of 12 Lennard-Jones interactions sites. Here we transfer it to C70 crystals without further refinement of its potential parameters, and good agreement with experimental data on the low temperature crystal structure is obtained.
On the quantum cross sections in dilute gases
Authors: Meeks F. R. , Cleland T. J. , Hutchinson K. E. , Taylor W. L.
Published: July 2002

The quantal cross section Q(N)qm for collision in dilute gases is given for any order N, where previously in the literature only values up to N=4 were to be found. The quantum-statistical character of Q(N)qm is discussed and calculations are presented for Q(1) through Q(6) for Boltzmann, Fermi–Dirac, and Bose–Einstein cases for 3He–4He, 3He–3He, and 4He–4He using extensive tables of phase shifts.
Isothermal glass transitions in supercooled and overcompressed liquids
Authors: Alba-Simionesco C.
Published: July 2002

We report the first calorimetric characterization of isothermal glass transitions obtained by changing the pressure at constant temperature. This isothermal glass transition is described in the case of m-fluoroaniline, a so-called fragile liquid. The method employed is based on Maxwell’s equations and involves measurement of the heat of compression of a sample enclosed in a hydrostatic pressure cell under isothermal and reversible conditions. A discontinuity of the thermodynamic quantity αV, where α is the expansivity and V is the molar volume, defines the glass transition pressure Pg at which ergodicity is broken on the time scale of the experiments (300–1000 s) which depends on the pressure step employed. The step in αV is found initially to decrease rapidly with increasing pressure. The slope (∂Pg/∂T)τ seems smaller than that determined by the usual isobaric methods, implying some distinction in the degrees of freedom involved and associated differences in defining the values of Tg. Because of the high sensitivity and stability of the calorimeter, we can also observe annealing effects in the overcompressed glass at a pressure above Pg; the slow dynamic processes associated are called entropic relaxation because it is the entropy which is measured directly. An approximate relaxation function and a model for the compression data are discussed. Keeping in mind the overall pattern of ‘‘strong and fragile’’ liquid behavior, isothermal pressure variations appear to offer a useful tool to connect these extremes and may constrain the glass transition of a molecular liquid in a particular range of the P–T diagram. The concept of Kauzmann pressure PK is introduced, but it is noted that PK must become less well defined the further from ambient pressure it lies due to the decrease in ΔαV which is observed with increasing pressure.
A general analysis of approximate nonlinear lumping in chemical kinetics. II. Constrained lumping
Authors: Tomlin Alison S. , Li Genyuan , Rabitz Herschel , Tóth János
Published: July 2002

A general analysis of approximate constrained nonlinear lumping is presented for a chemical kinetic system described by an n-dimensional set of first order ordinary differential equations dy/dt=f(y). There is a one-to-one relation between the differential equation system and the linear partial differential operator A=∑ni=1fi(y)(∂/∂yi). The algebraic method in nonlinear perturbation theory for lumping is extended to include constrained nonlinear lumping, in which the operator A is only transformed to a partially canonical form and some variables are left unlumped. A singular perturbation method is used to provide approximate analytical expressions for the solutions of the lumped variables. The resulting expressions can then be substituted into the equations describing the unlumped species, leading to a low dimensional system. The method is illustrated by application to a simple model describing the nonisothermal oxidation of hydrogen in a closed vessel. The results show that the method of constrained lumping leads to an accurate representation of the ignition features and maximum temperature rise given by the full model. The singular perturbation technique is proved to be only a special case of a general constrained lumping approach based on the algebraic method in nonlinear perturbation theory when the equations are linear in the deleted variables. Consequently the quasisteady-state approximation (QSSA) is the zeroth order approximation within the slow invariant manifold of the constrained approach. In cases where QSSA is not a good approximation, the first order correction generally provides significant improvement of the results.
Model calculations for the analysis of scattering data from layered structures
Authors: Loose Werner , Ackerson Bruce J.
Published: July 2002

Model calculations for the interpretation of scattering patterns from layered structures are presented. They assume a rigid configuration of the layers which are then stacked in a deterministic or stochastic manner. Results for stacking faults of close-packed crystals and for sheared sliding layers are presented. The method can easily be applied to other stacking sequences.
Hierarchical characterization of energy landscapes using Gaussian packet states
Authors: Orešič Matej , Shalloway David
Published: July 2002

We present a new method for exploring multidimensional energy landscapes by using Gaussian packets to characterize metastable states. Focusing on spatial scaling properties of the energy function, we derive nonlinear self-consistent packet equations which determine the parameters of these Gaussian packets. This provides a unique definition of the thermodynamic properties of metastable states and a computational prescription for their calculation. The packet equations allow bifurcation and enable us to follow the ‘‘trajectories’’ of states as functions of temperature and to determine the hierarchical relationships between states. The method is demonstrated on simple one-dimensional models and on Lennard-Jones six-, seven-, and eight-atom microclusters. We find that the microcluster state trajectories that are connected to lowest zero-temperature energy conformations are also connected to the lowest free-energy trajectories up to their destabilization temperatures. That is, the global minima have a ‘‘strong scaling’’ property that can accelerate searches for global minima by scale-dependent annealing methods.
Nucleation of bubbles in binary fluids
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

We have applied density functional methods to predict the nucleation rates of bubbles in superheated, stretched, or supersaturated binary fluid mixtures. Our model uses Lennard-Jones mixtures, with mixing rules chosen to allow either ideal or nonideal solution behavior. Deviations from the predictions of classical nucleation theory are in general quite large, with the locus of observable bubble nucleation (the kinetic stability limit) following the spinodal (the thermodynamic stability limit) reasonably closely. Comparisons are made with a variety of experiments, and puzzling earlier results are explained, such as the increase in solubility of some gases with temperature at the kinetic stability limit. Further experiments are needed to explore the variety of behavior predicted by the present calculations.
Ab initio energy-adjusted pseudopotentials for the noble gases Ne through Xe: Calculation of atomic dipole and quadrupole polarizabilities
Authors: Nicklass Andreas , Dolg Michael , Stoll Hermann , Preuss Heinzwerner
Published: July 2002

Nonrelativistic and one-component relativistic energy-adjusted ab initio pseudopotentials for the noble gases neon through xenon are presented together with corresponding optimized valence basis sets. To account for nonscalar relativistic effects the relativistic pseudopotentials are supplemented with effective spin–orbit potentials. The reliability of the presented pseudopotentials is demonstrated in atomic test calculations on ionization potentials and spin–orbit splittings in comparison with nonrelativistic and relativistic all-electron calculations as well as experimental data. Together with extended valence basis sets the pseudopotentials are applied in calculations on the static dipole and quadrupole polarizabilities of the noble gas atoms. The best values, computed at the coupled-cluster level of theory [CCSD(T)], for the dipole and quadrupole polarizabilities of the noble gases are 2.69a30 and 7.52a50 for Ne, 11.07a30 and 52.25a50 for Ar, 17.06a30 and 97.39a50 for Kr, and 27.66a30 and 209.85a50 for Xe.
Simulations of three-dimensional θ polymers
Authors: Grassberger Peter , Hegger Rainer
Published: July 2002

Using a new recursive sampling algorithm, we present simulation results for single three-dimensional chain polymers at and below the θ point. These simulations extend far into hitherto inaccessible regions as concerns chain length (up to N=5000 for free chains, up to N=300 000 for chains in a finite volume with periodic boundary conditions) and temperature. They show that the collapse is very sudden for large N, leading, e.g., to a nonmonotonic dependence of the end-to-end distance on N for fixed temperatures slightly below Tθ. As expected, the critical exponents describing the size of the chain and the partition sum at T=Tθ are of mean-field type. But the logarithmic corrections to this mean-field behavior are very different from those predicted by field theoretic methods. Also, the critical exponent which describes how the monomer density deep inside a collapsed chain increases with Tθ−T is not as predicted by mean-field theory.
On the scattering of low-energy electrons by sulphur hexafluoride
Authors: Gianturco F. A. , Lucchese R. R. , Sanna N.
Published: July 2002

The collision between a beam of electrons and gaseous molecules of sulphur hexafluoride (SF6) are studied from a theoretical and computational point of view, by solving the multichannel scattering problem within the close-coupling (CC) approach that uses a symmetry-adapted, single-center-expanded (SCE) description of the continuum functions. The static and exchange interactions are treated exactly within the basis set expansion and correlation forces are introduced in local form using a density functional model (FEG) without empirical parameters. Elastic observables (rotationally summed) are obtained over a broad range of energies, both for the integral and differential cross sections, and compared with the existing experiments. The symmetry and nature of the observed resonances are analyzed and discussed. One finds, in general, very satisfactory accord between the present calculations and the measured cross sections.
An ab initio determination of the bending–torsion–torsion spectrum of dimethyl ether, CH3OCH3 and CD3OCD3
Authors: Senent M. L. , Moule D. C. , Smeyers Y. G.
Published: July 2002

We have calculated the potential energy hypersurface of dimethyl ether with respect to the COC bending coordinate α and the torsional angles of the two methyl groups, θ1 and θ2. Two sets of ab initio calculations were carried out. The first was made at the level MP2/6-31G(d,p) in which the structural coordinates were fully relaxed except for the grid points on the hypersurface. More extensive calculation were carried out with MP4 corrections for electron correlation with the same molecular structure. The torsional bending Hamiltonian matrix was symmetrized by the operations of the G36 nonrigid group and was solved variationally. The effect of explicitly considering the bending mode in the three-dimensional treatment was determined by a comparison to the two-dimensional model in which the flexibility of the frame was absorbed into the calculation by the fully relaxed method. It was found that the three-dimensional calculation gave a much better account of the sin(3θ1)sin(θ2) intermode coupling than the two-dimensional treatment.
Nematic-isotropic phase transition: Hard spheroids with London interaction
Authors: Linder Bruno , Kromhout Robert A.
Published: July 2002

Cluster expansions for the thermodynamic functions of ordered systems are specialized to nematic systems and used in an augmented generalized van der Waals calculation based on hard spheroids of axial ratio 5 with full London attractive interaction. The convergence of the cluster expansion is accelerated by use of the y expansion [B. Barboy and W. M. Gelbart, J. Chem. Phys. 71, 3053 (1979)]. The correlation functions are hard spheroid Percus–Yevick values from the literature. The orientational distribution functions are determined by minimizing the Helmholtz free energy. An isotropic-nematic transition is found at one atmosphere pressure with characteristics in approximate agreement with the experimental values for paraazoxyanisole (PAA). The importance of the attractive interaction for reproducing experimental pressures and temperatures is discussed, and some speculations are made as to how agreement with experiment might be further im- proved.
Characterization of order fluctuations in liquid crystals by the dipolar-correlation effect of the stimulated echo
Authors: Grinberg Farida , Kimmich Rainer
Published: July 2002

A NMR method is presented which permits one to probe order fluctuations in ordered materials such as nematics. Information on the order parameter is also accessible. The principle is based on the standard stimulated-echo three-pulse sequence. Both the stimulated and the primary echo are recorded. The quotient of the echo amplitudes is measured as a function of the spacing of the first two rf pulses. The attenuation curves of the quotient corrected for spin-lattice relaxation show strong modulations directly reflecting the order parameter of the material. This dipolar-correlation effect moreover provides information on the distribution of the director fluctuation modes. The time scale exceeds the millisecond range by far. A formalism has been developed which permits us the quantitative description of experimental data measured in nematic 5CB.
Variational discrete variable representation
Authors: Corey Gregory C. , Tromp John W.
Published: July 2002

In developing a pseudospectral transform between a nondirect product basis of spherical harmonics and a direct product grid, Corey and Lemoine [J. Chem. Phys. 97, 4115 (1992)] generalized the Fourier method of Kosloff and the discrete variable representation (DVR) of Light by introducing more grid points than spectral basis functions. Assuming that the potential energy matrix is diagonal on the grid destroys the variational principle in the Fourier and DVR methods. In the present article we (1) demonstrate that the extra grid points in the generalized discrete variable representation (GDVR) act as dealiasing functions that restore the variational principle and make a pseudospectral calculation equivalent to a purely spectral one, (2) describe the general principles for extending the GDVR to other nondirect product spectral bases of orthogonal polynomials, and (3) establish the relation between the GDVR and the least squares method exploited in the pseudospectral electronic structure and adiabatic pseudospectral bound state calculations of Friesner and collaborators.
Hyperfine polarization quantum beats in cyanogen
Authors: Cool Terrill A. , Hemmi Naoki
Published: July 2002

Hyperfine polarization quantum beats caused by the reversible interchange of molecular polarization and nuclear spin polarization are observed in the fluorescence of C2N2, following laser absorption on the 410Ã(1Σ−u)←X̃(1Σ+g) band at 219 nm. Cross sections for collisional quenching and alignment depolarization of the fluorescence, determined by quantum beat spectroscopy, are 88 and 285 Å2, respectively.
Group theoretical study of the radical cation of methane: The effect of tunneling motions on the hyperfine interaction
Authors: Matsushita Michio , Momose Takamasa , Shida Tadamasa , Knight Lon B.
Published: July 2002

The electron spin resonance (ESR) spectrum of CH+4 indicates that some large amplitude tunneling motions among Jahn–Teller distorted structures make the four protons equivalent. A group theoretical study using the permutation–inversion (PI) group is performed to analyze the hyperfine interaction of the nonrigid CH+4. It is shown that three patterns of the interaction are possible depending upon the type of tunneling motions. Only one of the three patterns is consistent with the experimental spectrum, which is presented in the accompanying paper [J. Chem. Phys. 103, 3377 (1995)].
Solution of the polymer Percus–Yevick approximation for the multicomponent totally flexible sticky two-point model of polymerizing fluid
Authors: Kalyuzhnyi Yu. V. , Cummings P. T.
Published: July 2002

The analytic solution of the polymer Percus–Yevick approximation for the multicomponent version of the totally flexible sticky two-point model of Wertheim is obtained in closed form. The model consists of an n-component mixture of hard spheres with two sticky points of the type A and B randomly placed on the surface of each hard sphere. The solution of the problem has been reduced to solving a set of 5n algebraic equations. An iterative scheme of the solution of this set of equations is proposed.