bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic-energy release in the dissociation of NO2+
Authors: Masuoka Toshio
Published: July 2002

Kinetic-energy release distributions of the fragment ions (N+ and O+) produced in dissociative double photoionization of nitric oxide have been determined by analyzing the photoion–photoion coincidence (PIPICO) spectra measured in the region 42–100 eV by use of a time-of-flight mass spectrometer and synchrotron radiation. At low excitation energies (42–46 eV), low kinetic-energy components were observed, which cannot be explained by a simple framework whereby a doubly charged molecular ion is directly produced upon single photon absorption which then dissociates into two ionic fragments. The majority of the ion pairs are produced through indirect routes, in which highly excited NO*+ and NO** (double Rydberg) converging to the high-lying NO2+ electronic states autoionize before and after dissociation.
Photodesorption dynamics of Ba atoms from large Ar clusters
Authors: Schilling B. , Gaveau M. A. , Sublemontier O. , Mestdagh J. M. , Visticot J.-P. , Biquard X. , Berlande J.
Published: July 2002

Barium atoms at the surface of large argon clusters are excited into the 6s6p1P1 state leading either to ejection and subsequent fluorescence of the free barium atom or to fluorescence of solvated barium. Relative probabilities for these two processes as a function of the excitation energy and of the cluster size are presented. By comparison with the binding energy from a molecular dynamics (MD) calculation, we find that the barium atom is not excited above the limit for direct dissociation. The nonzero desorption probability is attributed to the gain of surface energy of the argon cluster which is partially transferred to the barium atom. Using linearly polarized laser light for the excitation, a positive polarization of the fluorescence of the ejected barium atoms is observed, whereas the polarization fluorescence of the solvated barium is negative. This indicates that the solvated barium atoms undergo a relaxation process which rotates the orientation of the excited orbital before the fluorescence. The degree of polarization of the fluorescence from ejected barium does not depend on the excitation energy or the cluster size.
Equilibrium polymerization and gelation. I. Integral-equation theory
Authors: Weist Annemarie Ott , Glandt Eduardo D.
Published: July 2002

This is the first of a series of papers on the application of Wertheim’s multidensity formalism to study the connectivity and related properties of equilibrium polymerizable particles. We first consider in detail the case in which each monomer has two sites available for covalent bonding. We modify Wertheim’s treatment to apply it to the case where the two sites on a monomer are indistinguishable, i.e., to homofunctional units. A study is presented of the connectedness behavior in these systems, including expressions for the weight-average molecular weight and cluster size. The application of these results to specific models is reported in subsequent parts of this series.
Hückel model for metal clusters: Ground states and low energy isomers
Authors: Yoshida A. , Do/ssing T. , Manninen M.
Published: July 2002

Hückel model with one s-electron per atom is used to study the geometries and electronic structures of clusters of 9 to 22 atoms. Two different optimization schemes for obtaining the ground states are used; (i) minimization of an approximate Hückel ground state energy starting from a random geometry and (ii) simulated annealing. Both methods give similar and new ground state geometries for clusters with 10 to 14 atoms. All clusters with more than 10 atoms will be distorted if the bond distance is allowed to vary ±5.5%. The ground states of clusters with atoms 10, 11, 12, and 14 are found to have the N=9 cluster as the basic building block, whereas the N=13 cluster is a distorted cuboctahedron. As a general trend, the deformation of clusters increases from atom number 8 to 14 and shrinks again from 15 to 20 atoms, in accordance with jellium model results.
Effect of molecular orientation on electron impact ionization and fragmentation
Authors: Aitken Craig G. , Blunt David A. , Harland Peter W.
Published: July 2002

Relative ionization cross sections have been determined for the production of the molecular ion CH3Cl+ and the fragmentation product CH+3 from the 200 eV electron impact ionization of spatially oriented CH3Cl molecules in a cross beam experiment. The ionization cross section for CH3Cl+ formation is higher at the positive end, or CH3 end, of the molecule while the cross section for formation of the CH+3 fragmentation product is independent of spatial orientation, within experimental uncertainty.
New experimental method for studying rotationally state-selected ion-molecule reactions
Authors: Mackenzie S. R. , Softley T. P.
Published: July 2002

A new apparatus is described in which a beam of molecular ions in a selected vibration–rotation state is prepared by field ionization of high Rydberg states, in an adaptation of the zero-kinetic-energy photoelectron technique. The state-selected ions undergo low energy reactive collisions within a molecular beam and the ionic products are detected in a quadrupole mass filter. The Rydberg states are populated by two-color stepwise multiphoton excitation, and by tuning to the pseudocontinuum of high-Rydberg states associated with different vibration–rotation states of the ion core, different states of the ion are selected and the effect on reactivity determined. Some preliminary results for the H2+H+2→H+3+H reaction are reported.
An analysis of the Ã 2Π–X̃ 2Σ+ band system of isotopically substituted calcium isocyanide CaNC
Authors: Scurlock C. T. , Fletcher D. A. , Steimle T. C.
Published: July 2002

The strong visible band systems of a supersonic molecular beam sample of isotopically substituted CaNC (13C and 15N individually) have been recorded at high resolution by laser-induced fluorescence spectroscopy. The spectra have the general appearance of the expected Ã 2Π–X̃ 2Σ+ band systems, but the energy levels of both the ‖Ω‖=1/2 and 3/2 excited substates have localized perturbations. Spectroscopic parameters for the X̃ 2Σ+ state of the isotopomers were extracted from a least-squares fit to the appropriate combinations differences of the observed transitions frequencies. Excited state effective parameters were extracted by directly fitting the unperturbed portions of the optical spectra. The rs-structure bond lengths for the ground electronic state were determined to be Ca–N=2.2065(58) Å and N–C=1.1186(58) Å. The permanent electric dipole moments for the Ca15NC isotopomer were determined to be 5.93(8) and 6.69(9) D for the Ã 2Π and X̃ 2Σ+ states, respectively.
Highly accurate relativistic universal Gaussian basis set: Dirac–Fock–Coulomb calculations for atomic systems up to nobelium
Authors: Malli G. L. , Da Silva A. B. F. , Ishikawa Yasuyuki
Published: July 2002

A universal Gaussian basis set is developed that leads to relativistic Dirac–Fock SCF energies of comparable accuracy as that obtained by the accurate numerical finite-difference method (GRASP2 package) [J. Phys. B 25, 1 (1992)]. The Gaussian-type functions of our universal basis set satisfy the relativistic boundary conditions associated with the finite nuclear model for a finite speed of light and conform to the so-called kinetic balance at the nonrelativistic limit. We attribute the exceptionally high accuracy obtained in our calculations to the fact that the representation of the relativistic dynamics of an electron in a spherical ball finite nucleus near the origin in terms of our universal Gaussian basis set is as accurate as that provided by the numerical finite-difference method. Results of the Dirac–Fock–Coulomb energies for a number of atoms up to No (Z=102) and some negative ions are presented and compared with the recent results obtained with the numerical finite-difference method and geometrical Gaussian basis sets by Parpia, Mohanty, and Clementi [J. Phys. B 25, 1 (1992)]. The accuracy of our calculations is estimated to be within a few parts in 109 for all the atomic systems studied.
Electron conduction in molecular wires. II. Application to scanning tunneling microscopy
Authors: Mujica V. , Kemp M. , Ratner M. A.
Published: July 2002

We use scattering methods to calculate the conductance of molecular wires. We show that three kinds of wire length dependences of the conductance arise: the decay can be exponential, polynomial, or very slow, depending on whether the reservoir Fermi level lies far from, in, or at the edge of the molecular energy band. We use the formalism to discuss simple models of tip-induced pressure and of imaging in scanning tunneling microscopy (STM), and point out a paradoxical situation in which the current can decrease with increased tip pressure. We also consider the connection of this formalism with the conventional theory of intramolecular, nonadiabatic electron transfer (ET).
Relativistic effects on sixth group hydrides
Authors: Pisani L. , Clementi E.
Published: July 2002

Dirac–Fock (DF) and Hartree–Fock (HF) calculations have been performed for the ground state configuration of the H2O, H2S, H2Se, H2Te, and H2Po molecules. Equilibrium geometries, atomization energies, and molecular orbitals energies are evaluated with both methods, compared and discussed with the help of population analysis and atomic orbital energies. Particular attention has been given to a qualitative understanding of the relativistic effects. Molecular spin–orbits corrections appear to be essential to a description of some in the sixth group hydrides set. A description of the relativistic computer program is presented elsewhere [L. Pisani and E. Clementi, J. Comput. Chem. (in press)].
Reactive scattering theory for molecular transitions in time-dependent fields
Authors: Peskin Uri , Miller William H.
Published: July 2002

A new approach is introduced for computing probabilities of molecular transitions in time-dependent fields. The method is based on the stationary (t,t′) representation of the Schrödinger equation and is shown to be equivalent to infinite order time-dependent perturbation theory. Bound-to-bound (i.e., photoexcitation) and bound-to-continuum (i.e., photoreaction) transitions are regarded as reactive collisions with the ‘‘time coordinate’’ as the reaction coordinate in an extended Hilbert space. A numerical method based on imposing absorbing boundary conditions for the time coordinate in a discrete variable representation framework is introduced. A single operation of the Green’s operator provides all the state-specific transition probabilities as well as partial state-resolved (inclusive) reaction probabilities. Illustrative numerical applications are given for model systems.
Nonresonant (VV) transfer between molecules dissolved in liquid N2, liquid Ar, and liquid N2/Ar mixtures
Authors: Wilson G. J. , Turnidge M. L. , Simpson C. J. S. M.
Published: July 2002

In order to investigate the effect of the solvent on nonresonant (VV) processes, rate constants have been measured for the deactivation of N2(v=1) and CO(v=1) in liquid N2/Ar mixtures at 85 K. The systems studied are relaxation of 14N2, 12C16O, and 13C16O by both O2 and CH4. Measurements were taken in liquid Ar, liquid N2, and in solvent mixtures. The rate constants in liquid Ar solution equal those in the gas phase at the same temperature. The rate constants fall nonlinearly with the addition of N2 to the solvent. The decrease on changing the solvent from liquid Ar to liquid N2 is 30% for the deactivation of N2(v=1) and 10% for CO(v=1). These results would not be predicted by current formulations of the isolated binary collision (IBC) hypothesis and suggest that one or more N2 molecules in the shell of solvent molecule surrounding vibrationally excited N2 or CO perturb the energy transfer process.
The van der Waals rovibronic spectrum of p-difluorobenzene–Ar up to 125 cm−1 intermolecular energy: Assignment and character of van der Waals modes
Authors: Sussmann R. , Neusser H. J.
Published: July 2002

The van der Waals (vdW) vibronic spectrum built on the electronic 000, S1(B2)←S0(A1) origin of p-difluorobenzene–Ar is investigated with rotational resolution (ΔνUV=60 MHz). For the first time vdW vibronic bands are detected up to a van der Waals energy of 125 cm−1 and assigned by a rotational analysis of the band structure. The band origin positions of the ten detected bands display a regular behavior with moderate anharmonicities and minor influences due to Fermi resonances. Using the concept of three-dimensional Kraitchman equations and of normalized effective planar moments characteristic data on the nuclear displacements in the two different bending coordinates are deduced from the measured rotational constants. The fundamental vdW vibronic states at low energies can be described in terms of one-dimensional normal modes while vdW states at higher energies display mixed mode character.
Rate coefficients for the N+O2 reaction computed on an ab initio potential energy surface
Authors: Suzzi Valli G. , Orrú R. , Clementi E. , Laganà A. , Crocchianti S.
Published: July 2002

Accurate calculations of the potential energy surface of the N+O2 reaction have been performed at complete active space self-consistent field (CASSCF) and multireference single–double configuration interaction (MR-SDCI) levels. Features of the calculated potential energy values are analyzed and compared with those of previous ab initio calculations and experimental data. The comparison has been extended to kinetic properties of the reaction.
Density functional calculations of lanthanide oxides
Authors: Wang S. G. , Pan D. K. , Schwarz W. H. E.
Published: July 2002

Density functional (DF) calculations have been performed on LaO, EuO, GdO, YbO and YbF. Gradient-exchange and correlation functionals work satisfactorily in the outer valence shells of these molecules, but less well for the localized lanthanide f-shells. Relativistic corrections to bond lengths, bond energies and vibrational frequencies are of quite different magnitudes and origins. The inner Ln 4f-shell has a fractional electron population in several molecular states. We corroborate the assignment of the 0+ ground state of YbO as configuration mixed Yb2+(f14/f13s)O2−. The effective charge distribution of the lanthanide oxides is at best approximated by Ln+O−.
Isotopic study of the intermolecular versus intramolecular energy transfer in the N2(W,A)+N2(X) reactions
Authors: Ottinger Ch. , Vilesov A. F. , Xu D. D.
Published: July 2002

The energy transfer from the long-lived states N2(W 3Δu, A 3Σ+u) to the radiating state N2(B 3Πg) in collisions with N2(X 1Σ+g) was studied under single collision conditions, employing a molecular beam/target gas cell arrangement. By means of using the isotopic species 14N2 in the metastable reactant beam and 15N2 in the target gas cell it was possible to differentiate between the intramolecular and the intermolecular energy transfer mechanisms, on the basis of well-resolved N2(B) product emission spectra. The overall contributions of the two reaction channels were found to be comparable, but they differ greatly in the vibrational product distributions. The intermolecular process populates preferentially the low vibrational levels of 15N2(B,v). The intramolecular process is most efficient for those 14N2(B,v) levels which are in close energy resonance with N2(A or W) vibrational levels.
Dynamic properties of molecular chains with variable stiffness
Authors: Harnau Ludger , Winkler Roland G. , Reineker Peter
Published: July 2002

The dynamics of a free-draining chain of variable stiffness in a dilute solution is investigated. The chain is considered as a differentiable space curve with stretching and bending elasticity. Second moments, like the mean square end-to-end distance, the radius of gyration, and the pair correlation function of the equilibrium distribution exactly agree with those of the well-known Kratky–Porod wormlike chain. The equation of motion of the chain is derived and solved by a normal mode analysis. In the limit of a flexible chain the model exhibits the well-known Rouse dynamics, whereas in the rod limit the eigenfunctions correspond to bending motion only. In addition, the rotational motion in the latter limit is naturally obtained within the model. The relaxation times obtained by the model are compared with experimental transient birefringence and dynamic light scattering data. In addition, electric dichroism measurements are interpreted in terms of the model. All of these experiments are in good agreement with the theoretical predictions.
Observation of the dipole-bound excited state of the I−⋅acetone ion-molecule complex
Authors: Dessent Caroline E. H. , Bailey Christopher G. , Johnson Mark A.
Published: July 2002

Photofragmentation action spectra of the I−⋅acetone ion-molecule complex reveal the existence of an excited state, located just below the electron detachment threshold, which is thought to correspond to a dipole-bound state derived primarily from the electric dipole moment of acetone. The excited state relaxes by fragmentation into the acetone anion, a dipole-bound ground state anion. This type of excited state should be a general property of ion-molecule complexes, X−⋅M, where M has a significant dipole moment and X does not, and is a microscopic precursor of the charge transfer to solvent bands observed for anions in dipolar solvents.
Diffusivity in a non-ionic surfactant/water mixture of critical composition
Authors: Lesemann M. , Zielesny A. , Belkoura L. , Woermann D.
Published: July 2002

Results of measurements of the temperature dependence of the mutual diffusion coefficient D of a tetraethyleneglycoloctylether (abbreviated C8E4) /water mixture of critical composition are reported (lower critical point, visually determined critical temperature Tc(vis)=40.587 °C; critical composition, mass fraction yc(C8E4)=0.071). The critical micelle concentration of C8E4 is smaller than the critical concentration, so that micelles are expected to be present in the mixture of critical composition. The reduced diffusion coefficients D*(=6πηξ/(kBT)⋅D) calculated from static and dynamic light scattering and viscosity data scale when plotted as function of the scaling variable x (=qξ) (η, shear viscosity; ξ, correlation length; D, diffusion coefficient; q, absolute value of scattering vector; kB, Boltzmann constant; T, temperature). They can be represented by the approximant of the dynamic scaling function proposed by Burstyn et al. [Phys. Rev. A 28, 1567 (1983)] assuming that the measured diffusion coefficient can be identified with the singular contribution (i.e., D=Dc). The influence of micelles on the critical properties of the system does not show up in the D* versus x plot.
Dynamic level-crossing model of antiphase electron spin polarization in spin-correlated radical pairs
Authors: Adrian Frank J.
Published: July 2002

The unusual antiphase electron spin polarizations, attributed to residual spin correlations in incompletely separated radical pairs, are treated by a dynamic model in which the polarized electron spin transitions occur as the radicals diffuse through the separations where the transitions are in resonance with the microwave field. This model shows that the inter-radical exchange interaction, and in some cases magnetic dipolar interactions as well, can produce the observed polarizations despite varying rapidly with diffusive motions of the radicals. However, the apparent fixed splitting of the emissive and absorptive components of the antiphase line is usually determined by the intrinsic width of the unpolarized electron spin resonance (ESR) line rather than by the inter-radical interactions. Surprisingly, a static model, in which the radicals are immobile during the short ESR observation period, yields the same polarization as the dynamic model under quite general conditions.
Pulsed gradient spin-echo nuclear magnetic resonance of confined Brownian particles
Authors: Terentjev E. M. , Callaghan P. T. , Warner M.
Published: July 2002

We present here a model calculation of the spin-echo gradient nuclear magnetic resonance signal of a Brownian particle diffusing in a confined environment, described by a harmonic potential well. The general expression reduces to characteristic scaling laws in limiting cases: long and short pulses, comparative to the spin-echo duration, and long, intermediate, and short durations of the spin phase reversal cycle, comparative to the internal system characteristic times scales. Spin-echo measurements in these limits enable one to detect independently the particle’s mass, viscous drag, and the confining potential strength.
H2O photodissociation dynamics based on potential energy surfaces from density functional calculations
Authors: Doublet M. L. , Kroes G. J. , Baerends E. J. , Rosa A.
Published: July 2002

We investigate the usefulness of density functional theory (DFT) for calculating excited state potential energy surfaces. In the DFT calculations, the generalized gradient approximation (GGA) is used. As a test case, the photodissociation of H2O through the first excited Ã 1B1 state was considered. Two-dimensional potential energy surfaces were obtained for both the X̃ 1A1 ground state and the first excited state. Wave packet calculations employing these surfaces were used to obtain both the absorption spectrum and partial photodissociation cross sections, which are resolved with respect to the final vibrational state of the OH fragment. Comparisons are made with a previously calculated high level ab initio potential energy surface, with dynamics calculations using that surface, and with experiment. The vertical excitation energy for the (X̃ 1A1→Ã 1B1) transition calculated using DFT is in good agreement with the previous ab initio calculations. The absorption spectrum and the partial cross sections obtained with the DFT treatment are in good agreement with experiment.
Femtosecond time-resolved studies of coherent vibrational Raman scattering in large gas-phase molecules
Authors: Hayden Carl C. , Chandler David W.
Published: July 2002

Results are presented from femtosecond time-resolved coherent Raman experiments in which we excite and monitor vibrational coherence in gas-phase samples of benzene and 1,3,5-hexatriene. Different physical mechanisms for coherence decay are seen in these two molecules. In benzene, where the Raman polarizability is largely isotropic, the Q branch of the vibrational Raman spectrum is the primary feature excited. Molecules in different rotational states have different Q-branch transition frequencies due to vibration–rotation interaction. Thus, the macroscopic polarization that is observed in these experiments decays because it has many frequency components from molecules in different rotational states, and these frequency components go out of phase with each other. In 1,3,5-hexatriene, the Raman excitation produces molecules in a coherent superposition of rotational states, through (O, P, R, and S branch) transitions that are strong due to the large anisotropy of the Raman polarizability. The coherent superposition of rotational states corresponds to initially spatially oriented, vibrationally excited, molecules that are freely rotating. The rotation of molecules away from the initial orientation is primarily responsible for the coherence decay in this case. These experiments produce large (∼10% efficiency) Raman shifted signals with modest excitation pulse energies (10 μJ) demonstrating the feasibility of this approach for a variety of gas phase studies.
Consistent contact pair correlation functions and equations of state for hard-sphere mixture
Authors: Hamad Esam Z.
Published: July 2002

General consistent equations for the contact values of the pair correlation functions of mixture of hard spheres are obtained using a recently proposed exact equation relating the mixture pair correlation integrals. Specific expressions are obtained when the general equations are combined with pure hard-sphere equations of state. The mixture equations reduce to the appropriate pure values for all the limiting values of the composition and the collision diameters. Consistent mixture equations are derived based on the Carnahan–Starling and the Kolafa pure equations of state. The resulting equations agree well with the exact mixture fourth virial coefficient and the compressibility factor simulation data.
Rotational analysis of the ν1 band of trichlorofluoromethane from high resolution Fourier transform and diode laser spectra of supersonic jets and isotopically enriched samples
Authors: Snels M. , Beil A. , Hollenstein H. , Quack M. , Schmitt U. , D’Amato F.
Published: July 2002

The spectrum of CCl3F (trichlorofluoromethane, CFC 11) has been measured in the region of the ν1 fundamental (1050–1120 cm−1) by high resolution interferometric Fourier-transform spectroscopy [0.004 cm−1 bandwidth full width at half maximum, apodized] and by diode laser spectroscopy (bandwidth 0.0008–0.0020 cm−1 FWHM) at room temperature, in cold cells and in supersonic jet expansions. Fourier-transform infrared and diode laser spectra of isotopically pure C35Cl3F have been recorded at room temperature in static cells. The C35Cl3F spectra allowed an analysis of the rotational structure to be started successfully for the first time. The rotational analysis of the isotopic species C35Cl237ClF and C35Cl37Cl2F, which are asymmetric rotors, was initiated from diode laser spectra of natural CCl3F with a rotational temperature of about 20 K (5% seeded in He) resulting from expansion in a supersonic pulsed slit jet. The rotational analysis yielded effective Hamiltonian constants including accurate band centers for the three most abundant isotopomers C35Cl3F (ν1=1081.2801 cm−1), C35Cl237ClF (ν1=1080.7330 cm−1), and C35Cl37Cl2F (ν1=1080.0663 cm−1), excited state rotational constants, and for C35Cl3F, quartic centrifugal distortion constants. The results are important for and discussed in relation to simulations of atmospheric absorption, line coincidences with CO2 lasers for sub-Doppler spectroscopy, as well as IR multiphoton excitation and infrared laser chemistry.
Mode-coupling theory of diffusion in block copolymer melts: Influence of concentration fluctuations
Authors: Tang Hai , Schweizer Kenneth S.
Published: July 2002

The polymer mode-coupling theory is extended to describe diffusion in block copolymer melts. Concentration fluctuations are shown to significantly slow down the diffusion near or below the order-disorder transition temperature. These effects are stronger for longer tracers, and for homopolymer tracers compared with block copolymer tracers. Quantitative agreement is obtained between our theory and recent experiments.
A classical path/forced rotor theory of state-to-state rotational energy transfer
Authors: Maricq M. Matti
Published: July 2002

The problem of rotational energy transfer (RET) is examined from a point of view intermediate between the current approaches based on empirical models or numerical solution of the coupled scattering equations. A semiclassical perspective is taken in which a classically described collision with an atom exerts a time dependent torque on the rotor and the resulting forced rotor dynamics is addressed quantum mechanically. By treating the anharmonicity in the rotational energy levels as a perturbation, a simple approximate expression is found for the inelastic transition probability. It reveals a marked difference between the distribution of final rotational states incurred from an individual collision trajectory as compared to trajectory averaged measures of RET, such as cross sections. The theory is applied to the scattering of Li2(A 1Σ+u) by Ne, Ar, Xe;Na2(A 1Σ+u) by He and Ne; N+2(X 2Σ+g) by He, and CN(X 2Σ+) by He. Its predictions compare well with those from a fully quantum mechanical description of rigid rotor scattering and with experiment. The insight into the energy transfer dynamics gained from the semiclassical approach is used to examine the assumptions underlying empirical models of rotational energy transfer.
Excitons in solid argon
Authors: Grünberg H. H. v. , Gabriel H.
Published: July 2002

A model for consistently describing the entire Ar exciton series is presented. Making use of a reliable approximation of the band structure in the whole first Brillouin zone, as provided by appropriately chosen Slater–Koster parameters, is an essential part of our approach. Its dominating effect on the final results is demonstrated by performing comparative studies based on several simpler band models. In fact, the energy defect of the n=1 exciton can to a large extent be understood as a consequence of the large-k variation of the bands. Our model, furthermore, makes allowance for the spin–orbit coupling of the valence bands, the exchange interaction and the dielectric function of the electron-hole potential. Without using any free parameter, the calculated Ar exciton series is in excellent agreement with the experimental binding energies.
Probing the (dual) repulsive wall in the interaction of O2, N2, and Ar with the Ag(111) surface
Authors: Raukema Age , Dirksen Ruud J. , Kleyn Aart W.
Published: July 2002

We have performed molecular beam scattering experiments of O2, N2, and Ar from the Ag(111) surface in the translational energy regime from 0.2 to 2.6 eV and at surface temperatures of 600 and 150 K. The experiments were carried out to probe the repulsive part in the particle–surface interaction potential. It is shown that the scattering dynamics of the systems N2–Ag(111) and Ar–Ag(111) is characteristic of physisorption systems. The scattering data for the system O2–Ag(111) shows that both the physisorption and chemisorption part of the interaction potential are probed in the experiment. For ‘‘normal incidence energies’’ exceeding a threshold energy, the barrier to the chemisorption part of the interaction potential can be crossed. An upper limit for this threshold is Ei cos2 θi=0.3 eV. Although the chemisorption region is probed, the (dissociative and nondissociative) chemisorption probability remains negligible.
Novel adiabatic invariant for the three-body Coulomb problem
Authors: Müller Jörg , Burgdörfer Joachim , Noid Donald W.
Published: July 2002

The two-(fixed) center Coulomb problem possesses in nonrelativistic mechanics, besides the ‘‘classical’’ constants of motion, an additional constant of motion associated with dynamical symmetry and separability of the problem. When the motion of the nuclei is taken into account the constant of motion is destroyed but reappears as an adiabatic invariant. We have identified this novel adiabatic invariant for the unrestricted three-body problem in the limit of two heavy particles and one light particle. Unlike all other known adiabatic invariants, it is globally conserved to order δ=m/M in the light- to heavy-mass ratio. The significance of this invariant for the semiclassical description of the chemical bond and for diatomic molecules beyond the Born–Oppenheimer approximation is discussed.
New integral equation for simple fluids
Authors: Kang Hong Seok , Ree Francis H.
Published: July 2002

We present a new integral equation for the radial distribution function of classical fluids. It employs the bridge function for a short-range repulsive reference system which was used earlier in our dense fluid perturbation theory. The bridge function is evaluated using Ballone et al.’s closure relation. Applications of the integral equation to the Lennard-Jones and inverse nth-power (n=12, 9, 6, and 4) repulsive systems show that it can predict thermodynamic and structural properties in close agreement with results from computer simulations and the reference-hypernetted-chain equation. We also discuss thermodynamic consistency tests on the new equation and comparisons with the integral equations of Rogers and Young and of Zerah and Hansen. The present equation has no parameter to adjust. This unique feature offers a significant advantage as it eliminates a time-consuming search to optimize such parameters appearing in other theories. It permits practical applications needing complex intermolecular potentials and for multicomponent systems.
Isomeric structures of the electronically excited acetylene⋅Ar complex: Spectroscopy and potential calculations
Authors: Ju S. S. , Cheng P. Y. , Hahn M. Y. , Dai H. L.
Published: July 2002

Acetylene⋅Ar complex in the S1 state has been characterized through laser fluorescence excitation spectra in the acetylene Ã←X̃, 3n0 (n=0–4) bands region. Two isomeric structures have been determined for the acetylene(Ã)⋅Ar complex from rotational band analysis, even though only one structure was known to exist for the ground state acetylene(X̃)⋅Ar. The in-plane isomer has the Ar atom situated in the molecular plane of the trans-bent acetylene, 3.77 Å from the acetylene center of mass and tilted from the H atoms. The out-of-plane isomeric structure, directly inverted from the rotational constants, has argon 3.76 Å away from the acetylene center of mass and 18° tilted from the C2 rotational axis. This structure is most likely due to large amplitude bending motions away from the equilibrium position which is along the C2 axis. Axis switching effect in the rotational band analysis for both isomers has been examined and found to be negligible. (Formulas for calculating the three-dimensional axis switching angles are detailed in the Appendix.) Three van der Waals vibrational mode frequencies have been determined from the vibrational progressions in the spectra; the stretching fundamental of the out-of-plane isomer is 28 cm−1; the in-plane bending fundamental, and the out-of-plane bending first overtone of the in-plane isomer are 11 and 17 cm−1, respectively.

The isomeric structures have been compared with the results from a pairwise-atom potential calculation with parameters transferred from the ones previously derived for C2H4⋅Ar potential calculations. It was found that when the set of parameters that most closely reflects the electronic density distribution in C2H2(Ã) orbitals was used, two potential minima mimicking the two isomeric structures were generated. This potential calculation can even qualitatively reproduce the complex spectral shift induced by the ν3 mode excitation in acetylene. Combining the observed spectral shifts and previous experimental and theoretical studies of acetylene(X̃)⋅Ar, we have estimated the binding energy of the out-of-plane C2H2(Ã)⋅Ar isomer to be 179 cm−1, and that of the in-plane isomer to be 170 cm−1.
Model derived reduced density matrix restrictions for correlated fermions
Authors: Sebold Joel H. , Percus J. K.
Published: July 2002

We illustrate the importance of restrictions in improving ground state energy lower bounds of a model of correlated electrons on a lattice. A reduced density matrix (RDM) formalism is employed. The restrictions are derived from closely related and exactly solved models. Such conditions raise the estimates without resorting to increasing the size of the physical space, thus improving computational efficiency. Our main motivation for this work is the problematic picture of Hohenberg–Kohn–Sham density functional theory for strongly correlated fermions. We find that using small cluster representations, errors can be reduced by more than 50% depending on the nature of the model and parameter regime studied. We obtain results for one- and two-dimensional lattices at half filling in the thermodynamic limit, although the method could be easily adapted to finite molecular structures as well.
Exact and semiclassical density matrix of a particle moving in a barrier potential with bound states
Authors: Weiper Franz Josef , Ankerhold Joachim , Grabert Hermann
Published: July 2002

We present a barrier potential with bound states that is exactly solvable and determine the eigenfunctions and eigenvalues of the Hamiltonian. The equilibrium density matrix of a particle moving at temperature T in this nonlinear barrier potential field is determined. The exact density matrix is compared with the result of the path integral approach in the semiclassical approximation. For opaque barriers the simple semiclassical approximation is found to be sufficient at high temperatures while at low temperatures the fluctuation paths may have a caustic depending on temperature and endpoints. Near the caustics the divergence of the simple semiclassical approximation of the density matrix is removed by a nonlinear fluctuation potential. For opaque barriers the improved semiclassical approximation is again in agreement with the exact result. In particular, bound states and the form of resonance states are described accurately by the semiclassical approach.
A mixed momentum-position space representation to study quantum vibrational energy transfer
Authors: Mitra Sami , Schwartz Steven D.
Published: July 2002

In this paper we describe a new technique that enables us to study vibrational energy transfer in linear hydrocarbon chains significantly more efficiently than by earlier approaches. The principal feature of our method is that the conjugate momentum operators that appear in the coupling terms in the Hamiltonian for the system are projected in the complete set of momentum states of the bonds. This allows us to express the expectation values of the time evolution operator in various energy eigenstates as one-dimensional momentum integrals which can be performed very rapidly and stored. All survival probabilities can then be expressed in terms of these stored integrals. We have evaluated the survival probability for HC2 and HC6 for up to eight time steps. Finally, we indicate how our approach may be extended to more general coupling terms.
Photon-stimulated desorption and photolysis of decaborane (B10H14) at semiconductor surfaces
Authors: Akazawa Housei , Utsumi Yuichi
Published: July 2002

Positive ions species resulting from photon-stimulated desorption (PSD) and photolysis from B10H14 at solid surfaces have been determined by means of time-of-flight mass spectroscopy using single-bunch operation of synchrotron radiation (SR) (hν≥100 eV). The ionic species desorbing from molecularly adsorbed B10H14 on Si(100) are H+2, BH+2, BH+3, and B2H+4. During SR excited chemical vapor deposition (SR–CVD) of boron films, the primary ion products created by photolysis of B10H14 are H+2, B10H+x, BH+x, and B9H+x. From the boron films deposited by SR–CVD, the additional PSD of B2H+3 and B2H+5 is observed and indicates that the boron film surface is terminated by several kinds of higher-order boron hydrides. The formation of BH+3 and B2H+4 is initiated by bulk-mediated secondary processes, whereas the formation of H+2 and B10H+x is a direct consequence of photoabsorption. The pressure dependence of the intensities of the ion species can be used to distinguish their gas and surface origins.
Observation of the weakly bound B(2s2p2 2D)–H2 complex by fluorescence depletion spectroscopy
Authors: Yang Xin , Hwang Eunsook , Dagdigian Paul J.
Published: July 2002

The fluorescence depletion method is used to observe electronic transitions in weakly bound complexes involving the B atom in the spectral range near the atomic 2s2p2 2D←2s22p 2P transition. The fluorescence depletion spectrum of the BNe C 2Δ, D 2Π←X 2Π transitions is compared with the previously reported fluorescence excitation spectrum [X. Yang, E. Hwang, and P. J. Dagdigian, J. Chem. Phys. 104, 599 (1996)]. This technique has also been employed to detect the corresponding electronic transitions in the B⋅⋅⋅H2 complex which cannot be detected by laser fluorescence excitation.
Structure and vibrational dynamics of sodium sulfate/sodium chromate solid solutions
Authors: Doyen Laurent , Frech Roger
Published: July 2002

Solid solutions of Na2SO4/Na2CrO4 are studied using laser Raman spectroscopy, infrared spectroscopy and differential scanning calorimetry. When the chromate ion concentration increases, the lattice expands continuously and a higher temperature sulfate phase, phase III, is stabilized to room temperature. The site group and correlation field splitting are also studied as a function of the relative sulfate\chromate ion concentration. The transition enthalpy from phase III to a higher temperature sulfate phase, phase I, has been measured. The calculated transition entropy is perfectly explained by a mixing entropy model.
Photochemistry of phosgene in the solid phase: Dissociation, ejection, and thermal desorption
Authors: Xin Q.-S. , Zhu X.-Y.
Published: July 2002

Understanding photochemistry and energy transfer mechanisms in molecular solid films is of interest to many scientific issues, ranging from matrix-assisted laser desorption ionization mass spectrometry to photochemical processes on polar stratospheric cloud particles. We present a study of a model system: the photochemistry (hν=1.2–6.4 eV) of a molecular Cl2CO solid film at low laser power density, 10 μJ–1 mJ/cm2 for ∼10 ns pulses. At hν≥3.5 eV, photon absorption by Cl2CO leads to a major photodissociation channel resulting in CO (g) and Cl (g) and a minor molecular Cl2CO ejection channel. Both photodissociation and molecular ejection are observed at the lowest laser power density and their yields depend linearly on pulse energy. This result establishes a single photon photoexcitation mechanism. The electronically excited Cl2CO in the surface region of the solid film can either dissociate or convert its electronic energy to translational motion in Cl2CO. The translational energy distribution of CO (g) from the photodissociation channel is bimodal: the flux-weighted mean translational energy of the fast channel is photon energy dependent (〈Etrans〉=210, 135, and ∼90 meV at hν=6.4, 5.0, and 3.5 eV, respectively), while the slow channel is independent of photon energy and corresponds to completely thermalized CO molecules (〈Etrans/2k〉=84±3 K). The mean translational energy of photoejected Cl2CO is 〈Etrans〉=220±20 meV. In addition to photoejection, there is also a distinctively different thermal desorption channel due to transient laser heating.
Relativistic potential energy surfaces of XH2 (X=C, Si, Ge, Sn, and Pb) molecules: Coupling of 1A1 and 3B1 states
Authors: Matsunaga Nikita , Koseki Shiro , Gordon Mark S.
Published: July 2002

Potential energy surfaces of the 1A1 and 3B1 states for XH2 molecules (X=C, Si, Ge, Sn, Pb) are investigated with ab initio full valence multiconfigurational self-consistent field wave functions, using effective core potentials. Spin–orbit coupling is also calculated to construct relativistic potential energy surfaces. The relativistic potential energy surfaces are compared with the adiabatic nonrelativistic potentials. Simple one dimensional Landau–Zener transition probabilities are calculated at the minimum energy crossing points of XH2 molecules to estimate the intersystem crossing probability.
Time-dependent Hartree configuration interaction study of the ICN photodissociation dynamics
Authors: Carmona-Novillo Estela , Caloto David , Campos-Martínez José
Published: July 2002

The application of the time-dependent Hartree configuration interaction (TDH-CI) method is examined for the ICN photodissociation process. Corrections to the approximate TDH dynamics are included up to a quantitative agreement with exact results. The validity of the TDH approximation, for computing some averaged magnitudes, is also tested against exact results.
Superthermal vibrational state distribution of CD3 thermally desorbed from GaAs(100)
Authors: Xin Q.-S. , Zhu X.-Y.
Published: July 2002

Vibrational population up to v″=3 in the umbrella mode of CD3 thermally desorbed from GaAs(100) has been determined by resonance enhanced multiphoton ionization (REMPI) spectroscopy. The population ratios are superthermal. The methyl radical must desorb promptly with a transition state geometry significantly different from the planar gas phase geometry.
Laser-reduced fluorescence study of the carbon monoxide nd triplet Rydberg series: Experimental results and multichannel quantum defect analysis
Authors: Mellinger A. , Vidal C. R. , Jungen Ch.
Published: July 2002

The ndσ and ndδ (n=4...9, v=0) triplet Rydberg series of carbon monoxide have been observed for the first time and rotationally resolved with an accuracy of 0.03 cm−1 using a three-step excitation scheme. A multichannel quantum defect theory (MQDT) analysis shows strong perturbations for n=5, 8, and 9 and predicts the positions of the sσ and dπ levels. There is significant mixing between the sσ and dσ states, with a mixing angle between 42 and 45 degrees, as well as some smaller interaction with the np series. The ionization limit for CO is found to be 113 027.5(3) cm−1.
Combined tight-binding and density functional molecular dynamics investigation of Si12 cluster structure
Authors: Ramakrishna Mushti V. , Bahel Atul
Published: July 2002

An extensive search for the lowest energy structure of Si12 has been carried out using a combination of simulated annealing studies based on tight-binding molecular dynamics and density functional based Car–Parrinello calculations. This investigation revealed three families of cluster structures that are low in energy. The potential energy surface in the vicinity of these structures has corrugated landscape, similar to that associated with the conformations of long chain polymers and proteins. The lowest energy structure is a hexacapped trigonal prism, which is a continuation of the growth pattern started at Si6, whereby the faces of a trigonal prism or anti prism seed are terminated by adatoms. This finding reveals emergence of a nucleation pattern in the growth of silicon clusters in the 6–13 atom size range.
Modeling the production and fragmentation of Ar+3 after threshold photon impact ionization of Ar3
Authors: Bastida Adolfo , Halberstadt Nadine , Beswick J. Alberto , Gadéa F. Xavier
Published: July 2002

Hybrid quantum/classical calculations of threshold photoionization of Ar3 are presented and the results are compared with measurements. Direct photoionization and autoionization mechanisms have been considered. The results indicate that, for the excitation wavelengths used in the experiments, autoionization is the dominant mechanism for the production of Ar+2+Ar.
Accurate calculation of quantum and diffusion propagators in arbitrary dimensions
Authors: Drozdov Alexander N.
Published: July 2002

A new approach to calculating the dynamics and equilibrium thermodynamics of an arbitrary (quantum or stochastic) system is presented. Its key points are representing the full propagator as a product of the harmonic-oscillator propagator with the configuration function, and expanding the configuration function (its exponent) in a power series in a given function of t. Recursion relations are obtained for the expansion coefficients which can be analytically evaluated for any number of degrees of freedom. This representation is particularly attractive for two reasons. Being structurally similar to the standard Taylorlike expansions for the propagator already known in the literature, it nevertheless shows a dramatic improvement over the latter in that it converges significantly better over a much broader range of t. Another attractive feature of the present expansion is that it is amenable to subsequent approximations. With this technique a minimal computational effort is required for constructing an improved global approximation for the propagator which is exact not only if t goes to zero, but also in the limit t→∞. Numerical applications to the coordinate space density matrix, quantum-mechanical time correlation function, and Fokker–Planck conditional probability show an accurate description of dynamical (statistical) properties to be already achieved for arbitrarily large times (small temperatures) with just the first term of the present expansion taken into account. Its use in a path integral means that a dramatic reduction of the number of integration variables which is required for convergence will be achieved even though simulations over very long times are desirable.