bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic spectra of isolated cations in supersonic jets by mass-selected ion-dip spectroscopy. Cations of benzene, p-difluorobenzene, and 1,3,5-trifluorobenzene
Authors: Tsuchiya Yuko , Fujii Masaaki , Ito Mitsuo
Published: July 2002

The electronic transitions of the cations of benzene, p-difluorobenzene (p-DFB), and 1,3,5-trifluorobenzene (1,3,5-TFB) have been measured by mass-selected ion-dip spectroscopy which utilizes the dissociation of a parent cation in an excited state. This spectroscopy was successfully applied to the vibrational level selected cation in the ground electronic state which was prepared by 1+1 REMPI (resonant enhanced multiphoton ionization) of the neutral molecule in a supersonic jet. For all the cations, the spectra due to the transition from the ground state to the excited π,π state were observed. Ion-dip spectra having sharp vibrational structures were found for p-DFB and 1,3,5-TFB cations, while a broad spectrum was observed for a benzene cation. It was also found that the vibrational structure of the ion-dip spectrum of the 1,3,5-TFB cation is quite different from that of the fluorescence excitation spectrum. The assignments of the ion-dip spectra and dissociation mechanisms of the excited cations will be discussed.
Electrochemical electron transfer: Accounting for electron–hole excitations in the metal electrode
Authors: Sebastian K. L.
Published: July 2002

The transfer of an electron from a metal electrode to an ion in a polar liquid is probably the most important process in electrochemistry. As the electrons in the metal have a continuum of allowed energy levels, this transfer may be accompanied by the creation/annihilation of electron–hole excitations in the metal. Calculation of the rate of electron transfer, with these excitations and the solvent accounted for properly is a difficult problem to treat using the usual approaches of molecular reaction dynamics, as one now has a continuum of potential energy surfaces, on which the dynamics has to be considered. We suggest an approach in which the electron–hole excitations are treated as bosons. Using this, we have derived an expression for the rate, which accounts both for solvent dynamics and electron–hole excitations. Our analysis amounts to a solution of the problem of calculating the electronic transmission coefficient κ, for a continuum of crossing diabatic surfaces. Calculations of the rate using our expression are reported.
Chemisorption and thermal decomposition of ethylene on Pd(110): Electron energy loss spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy studies
Authors: Nishijima M. , Yoshinobu J. , Sekitani T. , Onchi M.
Published: July 2002

The adsorbed state of ethylene on Pd(110) at 90 K and its thermal decomposition in the temperature region between 90 and 600 K have been studied by the use of high resolution electron energy loss spectroscopy (EELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At 90 K, ethylene is π bonded to the Pd(110) surface and is adsorbed almost disorderedly. The c(2×2)-C2H4 patches are formed near the saturation coverage (which corresponds to 0.58 C2H4 molecule per surface Pd atom). By heating the C2H4-saturated Pd(110) surface to 260 K, some C2H4 admolecules are desorbed intact and the remaining admolecules rearrange their adsorbed sites to form the c(2×2)-C2H4 structure. At above 300 K, almost all the C2H4 admolecules are dehydrogenated, and the ethynyl (CCH) species, H adatoms and unstable dehydrogenated species [possibly, vinyl (CHCH2) species] are formed; the C2H4 desorption occurs by the recombination of H adatoms and dehydrogenated species. The remaining H adatoms are desorbed as β-H2 at ∼325 K. By heating to 450–520 K, ethynyl is decomposed, and only carbon adatoms remain on Pd(110); the decomposition is accompanied by the H2 desorption. Structural models of chemisorbed C2H4 and CCH on Pd(110) are proposed.
Wetting in mixtures of water, nonionic amphiphiles, and nonpolar solvents
Authors: Kahlweit M. , Busse G.
Published: July 2002

As is well known, medium- and long-chain alkanes do not spread across H2O–air interfaces, but shape a lens. In this paper it is shown that the same holds for the upper amphiphile-rich phase in binary H2O–amphiphile mixtures with medium- and long-chain nonionic amphiphiles that show a (closed) miscibility gap with water. This finding is somewhat unexpected because surfactants form monolayers at H2O–air interfaces which should facilitate the spreading of the amphiphile-rich phase. This wetting behavior corresponds to that in ternary H2O–oil–nonionic amphiphile mixtures with a three-phase body, in which the middle amphiphile-rich phase does not spread across the H2O–oil interface. The results may stimulate further studies on critical-point wetting [for a recent review see, e.g., S. Dietrich, in Phase Transitions and Critical Phenomena, edited by C. Domb and J. L. Lebowitz (Academic, London, 1988), Vol. 12, p. 1.], and may also help clarifying the properties of microemulsions [for a recent review see, e.g., M. Kahlweit, R. Strey, P. Firman, D. Haase, J. Jen, and R. Schomäcker, Langmuir 4, 499 (1988)].
Structure of swollen polydimethyl siloxane gels
Authors: Mallam Simon , Hecht Anne-Marie , Geissler Erik , Pruvost Pierre
Published: July 2002

Light and small angle neutron scattering results are presented for a series of poly(dimethyl siloxane) (PDMS) gels, cured in the dry state by endlinking with a trifunctional crosslinking agent, ethyl triacetoxy silane (ETAS). For gels swollen in octane, the scattered light and the smallest angle neutron response are dominated by scattering from aggregates of polymerized ETAS, the mean radius of which was found in this system to be ∼800 Å, and which have a smooth surface in the length scale between 500 and 100 Å. The internal surface area of this precipitate was roughly 200 cm2 per cm3 of the swollen gel. Only about one third of the free ETAS participates in this condensed phase. The rest is disseminated as oligomers or monomers throughout the gel. The gel itself appears as a solution-like structure with in addition permanent waves in the polymer concentration distribution. In this system, the permanent waves could be modeled by a Gaussian function of characteristic width ΔR=80 Å and relative amplitude 〈Δc2〉1/2/〈c〉≊0.1. The polymer–polymer correlation length ξ describing the shorter range fluctuations is substantially modified in comparison with an unperturbed polymer solution at the same overall concentration. For the uncrosslinked solution at volume fraction φ=0.16 it is found that ξ=10.3 Å, while for the crosslinked gel swollen to the same degree, ξ=18 Å.
Study of the electronic-to-vibrational energy transfer in the quenching process of Na*(3 2P) with N2(1Σ+g,v=0). A quantal close coupling calculation
Authors: Campos-Martinez J. , Roncero O. , Miret-Artés S. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

Electronic-to-vibrational energy transfer has been studied by solving numerically the close-coupling equations, in the T-shape configuration, on the two lowest electronic states of the Na–N2 system. The diabatic potential surfaces were taken from Archirel and Habitz while the interelectronic coupling was modeled by different Gaussian-type functions. Different sets of parameters for the coupling were used in order to study the final vibrational distributions of N2. Finally, partial quenching probabilities are presented and compared with previous theoretical and experimental works.
Thermodynamics of gas adsorption on fractal surfaces of heterogeneous microporous solids
Authors: Jaroniec Mieczyslaw , Lu Xiaochun , Madey Richard , Avnir David
Published: July 2002

Thermodynamic equations are derived for physical adsorption of gases and vapors on fractal surfaces of heterogeneous microporous solids. The influence of surface geometry on gas adsorption on a microporous solid is studied by analyzing the dependence of the differential molar enthalpy ΔH, the immersion enthalpy ΔHim, and the differential molar entropy ΔS on the fractal dimension D, which is used to characterize the surface irregularity. It is shown that −ΔH, −ΔHim, −ΔS, and the average adsorption potential Ā increase as the fractal dimension of the surface accessible for adsorption increases. The dependence of these thermodynamic quantities on the fractal dimension D is attributed to the fact that the fraction of small micropores in microporous solids increases as the fractal dimension D increases.
Wetting transition in a three-phase surfactant/oil/water microemulsion system
Authors: Smith Duane H. , Covatch G. L.
Published: July 2002

Experimental results have been found for a wetting transition involving a three-component (amphiphile/oil/water) microemulsion system that forms top, middle, and bottom liquid phases. For all temperatures between the wetting transition temperature (Tw) and the upper critical endpoint temperature (Tuc) the middle phase microemulsion of the C6H13(OC2H4)2OH/n-C16H34/H2O system appeared to wet the interface between the top and bottom phases, whereas below Tw it did not. Compositions of the phases below and above Tw are reported, also. Within the experimental uncertainties, Tw equaled the optimal temperature (at which the top–bottom tension passes through a minimum), as determined from the compositions of the middle phase. The wetting transition temperature, Tw=45.14±0.02 °C, was more than 15 °C below the upper critical endpoint temperature, Tuc=60.94±0.03 °C. This appears to be the first example of a temperature-induced wetting transition that involves a middle-phase microemulsion system.
Temperature and pressure dependence of ozone formation rates in the range 1–1000 bar and 90–370 K
Authors: Hippler H. , Rahn R. , Troe J.
Published: July 2002

The recombination O+O2+M→O3+M in the bath gases M=He, Ar, and N2 was studied over the temperature range 90–370 K and the pressure range 1–1000 bar. The temperature and pressure dependences of the reaction rates show an anomalous behavior which is attributed to superpositions of mechanisms involving energy transfer, complex formation and participation of weakly bound electronically excited O3 states. The results also show an analogy to oxygen isotope enhancements observed in ozone recombination and dissociation. Experiments in compressed liquid N2 were also made showing a transition to diffusion control.
Molecular dynamics algorithm for condensed systems with multiple time scales
Authors: Tuckerman Mark E. , Martyna Glenn J. , Berne Bruce J.
Published: July 2002

A frequently encountered problem in molecular dynamics simulations is the long runs required to study condensed systems consisting of both high frequency and low frequency degrees of freedom. Standard integrators require the choice of time step sufficiently small to guarantee stable solution of the highest frequency motion with the consequence that simulations require a very large number of central processing unit (CPU) cycles. In this note we present a new integrator that allows one to use a time step appropriate for the low frequency degrees of freedom without making any approximations related to the separation of time scales. This method is based on a choice of an analytically solvable reference system for the high frequency motion. We show how the analytical solution can be incorporated into a numerical integrator. The method is applied to two cases which are paradigms for this problem and it is shown that this approach and suitable generalizations should be very useful for future simulations of quantum and classical condensed matter systems.
Lattice dynamics and vibrational spectra of polythiophene. II. Effective coordinate theory, doping induced, and photoexcited spectra
Authors: Lopez Navarrete J. T. , Zerbi G.
Published: July 2002

It is shown that the infrared and Raman spectra of polythiophene, pristine, doped or photoexcited as well as the spectra of the oligomers pristine and doped are accounted for by the effective conjugation coordinate theory. An effective conjugation force constant associated with the delocalization of the π electrons is measured. It is proven that delocalization extends at least over five thiophene rings and that the samples of the polymer consist of a distribution of conjugation lengths with a confinement length of at least five thiophene rings. The infrared spectrum of the doped and photoexcited polymer is consistent with the existence of a bipolaronic conjugational defect confined within ∼5 thiophene rings.
Hartree–Fock second derivatives and electric field properties in a solvent reaction field: Theory and application
Authors: Wong Ming Wah , Wiberg Kenneth B. , Frisch Michael
Published: July 2002

A compact formalism for the second and third derivatives of the Hartree–Fock energy in the presence of an Onsager solvent reaction field is presented. All three standard algorithms (MO, AO, and direct) are extended to include the reaction field in a unified way. Predictions of the infrared spectrum of formaldehyde in a variety of solvents and of solvent-induced shifts in carbonyl stretching frequencies are presented along with the results of new measurements. As for the gas-phase case, analytical second derivatives are far more efficient than numerical ones. The reaction field provides very good predictions of solvent effects at negligible computational cost.
On the validity of the Gibbs–diMarzio theory of the glass transition of lattice polymers
Authors: Wittmann Hans-Peter
Published: July 2002

The derivation of the Gibbs–diMarzio theory is reconsidered in the framework of more general theories of complex fluids composed of polymers. Basic approximations are to model the polymers as nonreversal random walks and to reduce the equation of state to a van der Waals-like form. Taking the criticism of Milchev on Flory’s first order transition of semiflexible lattice polymers into account, it can be shown that the transition temperature of the Gibbs–diMarzio glass transition is shifted towards a lower temperature. For the limiting case of an infinitely high coordination number, the transition temperature is even vanishing.
Doppler spectroscopy of the OH fragment ejected by t
r
a
n
s HONO (Ã 1A″): Characterization of the Ã state resonances and determination of vibrational energy content of the NO fragment
Authors: Novicki S. W. , Vasudev R.
Published: July 2002

Resonances in the quasistable Ã 1A″ state of trans HONO are probed through photodissociation experiments. The OH fragment ejected by HONO excited to specific –N■O stretching vibrational levels of the Ã state is probed by high-resolution sub-Doppler spectroscopy of the R2(1) (1,0) A 2Σ+–X 2Π transition. The Doppler profiles provide information on the vibrational state dependence of HONO’s subpicosecond fragmentation time and OH fragment’s kinetic energy. The latter is used to deduce NO sister fragment’s translational and vibrational energy content. These provide quantitative probes for (a) V→T ‘‘energy transfer’’ from the photoselected N■O vibrational coordinate to the recoil O–N coordinate and (b) N■O ‘‘vibrational relaxation’’ during dissociation. The present results and our previous measurements on the HONO Ã–X̃ linewidths are discussed in terms of an excited-state potential energy surface in which (a) the –N■O coordinate is coupled to the reaction coordinate (O–N) and (b) there is a bottleneck in the exit channel.
An approximate classical unimolecular reaction rate theory
Authors: Zhao Meishan , Rice Stuart A.
Published: July 2002

We describe a classical theory of unimolecular reaction rate which is derived from the analysis of Davis and Gray by use of simplifying approximations. These approximations concern the calculation of the locations of, and the fluxes of phase points across, the bottlenecks to fragmentation and to intramolecular energy transfer. The bottleneck to fragment separation is represented as a vibration–rotation state dependent separatrix, which approximation is similar to but extends and improves the approximations for the separatrix introduced by Gray, Rice, and Davis and by Zhao and Rice. The novel feature in our analysis is the representation of the bottlenecks to intramolecular energy transfer as dividing surfaces in phase space; the locations of these dividing surfaces are determined by the same conditions as locate the remnants of robust tori with frequency ratios related to the golden mean (in a two degree of freedom system these are the cantori). The flux of phase points across each dividing surface is calculated with an analytic representation instead of a stroboscopic mapping. The rate of unimolecular reaction is identified with the net rate at which phase points escape from the region of quasiperiodic bounded motion to the region of free fragment motion by consecutively crossing the dividing surfaces for intramolecular energy exchange and the separatrix. This new theory generates predictions of the rates of predissociation of the van der Waals molecules HeI2, NeI2 and ArI2 which are in very good agreement with available experimental data.
Dissociation dynamics of core excited N2O
Authors: LeBrun T. , Lavollée M. , Simon M. , Morin P.
Published: July 2002

Fragmentation of N2O after selective core excitation of terminal and central nitrogen, has been studied by a multicoincidence technique (PEPIPICO) using synchrotron radiation. We show that dissociation dynamics is dependent upon the excited site, especially in the case of the ‘‘atomization’’ of the molecule. The central nitrogen is always found with very little kinetic energy, except after Nt→Π* transition. A bent intermediate geometry is proposed to explain this observation.
Lattice dynamics and vibrational spectra of pristine and doped polyconjugated polyfuran
Authors: Hernandez V. , Ramirez F. J. , Zotti G. , Lopez Navarrete J. T.
Published: July 2002

The interpretation of the infrared and Raman spectra of polyconjugated polyfuran prepared by electroreduction of 2,5-dibromofuran is presented as the result of lattice dynamical calculations. Geometries and vibrational force fields are derived from semiempirical calculations carried out on oligomers and the parameters are used for the polymer. Finally, it is shown that the infrared and Raman spectra of polyfuran pristine and doped are accounted for by effective conjugation coordinate theory, thus showing that this material also constitutes a polyconjugated material.
On the solid–fluid interface of adhesive spheres
Authors: Marr D. W. , Gast A. P.
Published: July 2002

The adhesive-sphere interaction potential provides a good model system to study the influence of the attractive well depth on phase behavior and interfacial phenomena. We investigate the solid–fluid phase behavior of adhesive spheres with the modified weighted density approximation (MWDA) of Denton and Ashcroft. We then apply a planar-averaged density functional approach (PWDA) to determine interfacial properties. We find both a narrowing of the interface between fluid and coexisting fcc solid and an increase in the interfacial energy with increasing attractive interaction strength in accord with the empirical relation γ≊ 0.47ΔHρ2/3s. In addition, we investigate metastable solid nucleation through calculation of metastable bcc solid–fluid interfacial tensions and find results suggesting the possibility of such a route to stable solid formation.
A general discrete variable method to calculate vibrational energy levels of three- and four-atom molecules
Authors: Bramley Matthew J. , Carrington Tucker
Published: July 2002

We present a general variational method to calculate vibrational energy levels of polyatomic molecules without dynamical approximation. The method is based on a Lanczos algorithm, which does not require storage of the Hamiltonian matrix. The rate-determining step of each Lanczos iteration is the evaluation of the product of the matrix and a trial vector. We use simple product basis functions and write the Hamiltonian as a sum of factorizable terms. With n one-dimensional functions in each of f dimensions, the matrix-vector product requires no more than cnf+1 multiplications for a single term involving c coordinates. Choosing a (potential optimized) discrete variable representation (DVR) in each dimension, the potential energy matrix is diagonal. The rate-determining step is now the multiplication of a vector by the kinetic energy matrix and c is effectively (with rare exceptions) at most two. The nf+1 scaling holds for both diagonal and mixed second derivative operators. The method is directly applicable to any three-atom and any nonlinear four-atom molecule. We use a variety of coordinate systems (Jacobi, Radau, a hybrid of the two, and bond), for which the total number of factorizable terms in the exact kinetic energy operator is never large, to calculate very well-converged band origins of H2O up to 22 000 cm−1, of H+3 up to 18 000 cm−1, and of CH2O up to 5700 cm−1; and low-lying levels of H2O2. The results for CH2O are new, and those for H+3 clarify the causes of discrepancies in published work. The product basis results in very large matrices (up to 500 000×500 000 for four atoms), but the cost is within an order of magnitude of that of contracted-basis approaches using explicit diagonalization. While contracted basis approaches are molecule and Hamiltonian specific, it was possible to apply the DVR-Lanczos method to all the examples presented here with a single computer program. The principal advantage of our method is thus its generality, and in this context it is efficient, with the cost scaling slowly with basis size. It is also easily parallelized.
The H–NCO bond energy and ΔH
f (NCO) from photoionization mass spectrometric studies of HNCO and NCO
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

A photoionization mass spectrometric study of HNCO yields the ionization potential (I.P.) (HNCO)=11.595±0.005 eV and the appearance potential (A.P.) (NCO+/HNCO)≤16.532±0.011 eV at 0 K. A similar study of NCO (generated by F+HNCO) gives I.P. (NCO)=11.759±0.006 eV. These observations lead to D0 (H–NCO)≤110.1±0.3 kcal/mol. Additional analysis enables one to infer 28.4±0.5 kcal/mol ≤ ΔHf00 (NCO)≤32.8±0.7 kcal/mol. The implication of these results for kinetic modeling of the processes for reduction of NOx is discussed.
Luminescence properties of Mn5+ in a variety of host lattices: Effects of chemical and structural variation
Authors: Oetliker Ueli , Herren Markus , Güdel Hans U. , Kesper Ute , Albrecht Christa , Reinen Dirk
Published: July 2002

The optical spectroscopic properties of Mn5+ doped into ten host lattices have been studied. The influence of both chemical variation [phosphates(V), arsenates(V), and vanadates(V)] and structural variation (spodiosites, apatites, and Li3PO4-type salts) on the luminescence properties is studied in detail. By varying the host lattice, the position of the sharp luminescence band can be varied from 8300 to 8960 cm−1. Excited state splittings, luminescence lifetimes, and sideband intensities are correlated with the geometrical distortion of the MnO3−4 tetrahedron. In all the host lattices, the luminescence originates from the 1E excited state at all temperatures. It is essentially unquenched at room temperature in apatite and Li3PO4-type lattices, whereas in spodiosites up to 90% of the luminescence may be quenched at room temperature.
Exploration of a quadruple critical point in a quaternary liquid mixture
Authors: Narayanan T. , Kumar A. , Venkatachalam S. , Jacob J. , Prafulla B. V.
Published: July 2002

We report the preliminary findings concerning the nature of a quadruple critical point that evolves when two independent double critical points merge in a thermodynamic field space. This special critical point is realized in a quasibinary mixture of 3-methylpyridine, water, heavy water, and KI. The critical behavior of osmotic susceptibility was deduced from the scattered intensity measurements at 90°. Our results suggest that the nature of a quadruple critical point is similar to that of a normal double critical point. In addition, we find that the significance of the leading correction-to-scaling term is markedly increased as a result of the presence of the salt (KI) in the system.
Theory of nonadiabatic transition for general two-state curve crossing problems. II. Landau–Zener case
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

New accurate and compact formulas are established for general two-state curve crossing problems in the Landau–Zener case, in which the two diabatic potentials cross with the same sign of slopes. These formulas can cover practically the whole range of energy and coupling strength, and can be directly applied to various problems involving the curve crossing. All the basic potential parameters can be estimated directly from the adiabatic potentials and nonunique diabatization procedure is not required. Complex contour integrals are not necessary to evaluate the nonadiabatic transition parameter; thus the whole theory is very convenient for various applications. The compact formula for the Landau–Zener transition probability, which is far better than the famous Landau–Zener formula, is proposed. Now, together with the previous paper [Zhu and Nakamura, J. Chem. Phys. 101, 10 630 (1994)], the present semiclassical theory can present a complete set of solutions of the the two-state curve crossing problems.
The heat flux vector for highly inhomogeneous nonequilibrium fluids in very narrow pores
Authors: Todd B. D. , Evans Denis J.
Published: July 2002

In this paper we present the results of nonequilibrium molecular dynamics simulations of an atomic fluid undergoing planar Poiseuille flow through a narrow slit pore. In particular we examine the heat flux vector for such a fluid and find that it displays weak oscillations with wavelengths that are characteristic of the van der Waals particle diameter. Furthermore, we examine the possibility of a coupling of the heat flux vector with the gradient of the square of the strain rate tensor and find that if such a coupling exists it would be undetectable by an examination of the heat flux vector alone. Its existence only manifests itself in the temperature profile, where a quadratic term would appear in addition to the usual quartic profile predicted by classical Navier–Stokes hydrodynamics.
Soret and mass diffusion coefficients of toluene/n-hexane mixtures
Authors: Köhler W. , Müller B.
Published: July 2002

Soret coefficients and mass diffusion coefficients of mixtures of toluene and n-hexane have been measured by thermal diffusion forced Rayleigh scattering (TDFRS) and are compared with literature data. For the mass diffusion coefficients a good agreement between different experimental techniques is found. For the Soret coefficients contradictory results are obtained from thermogravitational column measurements, small angle Rayleigh scattering, and TDFRS. Arguments supporting the TDFRS data are presented.
Resonance Raman spectroscopy of the S1 and S2 states of pyrazine: Experiment and first principles calculation of spectra
Authors: Stock Gerhard , Woywod Clemens , Domcke Wolfgang , Swinney Tim , Hudson Bruce S.
Published: July 2002

New experimental and theoretical data on the resonance Raman (RR) spectroscopy of the S1 and S2 states of pyrazine are presented. Based on recent ab initio CASSCF (complete- active-space-self-consistent-field) and MRCI (multireference configuration interaction) calculations of Woywod et al. [J. Chem. Phys. 100, 1400 (1994)], we construct a vibronic coupling model of the conically intersecting S1 and S2 states of pyrazine, which includes the seven most relevant vibrational degrees of freedom of the molecule. Employing a time-dependent approach that treats the intramolecular couplings in a nonperturbative manner, we calculate RR cross sections for this model, taking explicitly into account the nonseparability of all vibrational modes. The combination of high-level ab initio calculations and multimode propagation techniques makes it possible, for the first time, to make first-principles predictions of RR spectra for vibronically coupled electronic states of an aromatic molecule. The theoretical data are compared to experimental gas-phase RR spectra which have been obtained for five different excitation wavelengths. The comparison reveals that the ab initio predictions match the experimental results in almost every detail.
Semiclassical coupled wave packet study of the nonadiabatic collisions Ar+(J)+H2: Zero angular momentum case
Authors: Aguillon F. , Sizun M. , Sidis V. , Billing G. D. , Marković N.
Published: July 2002

The title reaction is investigated for total angular momentum ℐ=0 using a semiclassical coupled wave packet method where the Smith–Whitten-type hyperspherical angles θ and φ are treated quantally, and the hyperspherical radius ρ is treated classically. The wave function is expanded over an electronic basis set which includes 28 states. The diabatic potential energy surfaces are determined by DIMZO calculations. Probabilities for reaction, charge transfer, collision induced dissociation, dissociative charge transfer, and fine structure transitions are obtained in the energy range 0.3 eV≤E≤30 eV. A comprehensive analysis of the reaction mechanisms is presented.
Quantum adiabatic umbrella sampling: The excited state free energy surface of an electron-atom pair in solution
Authors: Borgis Daniel , Staib Arnulf
Published: July 2002

A quantum umbrella sampling method to generate free energy surfaces for excited-state as well as ground state reactions in solution is presented. The approach is applied to the photodetachment of an electron from aqueous chloride in the lowest excited state channel (corresponding to the lowest charge-transfer-to-solvent state). An activation free energy barrier due to solvent reorganization is identified and discussed.
Construction of the Foldy–Wouthuysen transformation and solution of the Dirac equation using large components only
Authors: van Lenthe E. , Baerends E. J. , Snijders J. G.
Published: July 2002

It is shown that it is possible to construct, within the framework of a basis set expansion method, the full Foldy–Wouthuysen transformation (i.e., to all orders in the inverse velocity of light) for an arbitrary potential once the Dirac equation has been solved. On this basis an iterative procedure to solve the Dirac equation is suggested that involves only the large component, obviating the time-consuming (at least in molecular calculations) introduction of large basis sets for a proper description of just the small components. The methods are used to compare the expectation value of the radial distance operator in the Dirac picture and in the Schrödinger picture for the orbitals of the Uranium atom.
Strongly mixed resonances in the photofragmentation of HeBr2 near Br2(B) dissociation: Stabilization and close-coupling studies
Authors: González-Lezana Tomás , Hernández Marta I. , Delgado-Barrio Gerardo , Villarreal Pablo
Published: July 2002

The photofragmentation of the He79Br2 van der Waals complex is studied for the transition from the ground to the excited electronic state B and where the Br2 subunit is also excited to the neighborhood of the v0=45 vibrational state. At this vibrational level the bromine molecule is close to its dissociation limit and the Δv=−1 channel is closed for dissociation of the complex. In a previous work it was suggested that energy in the HeBr2(B, v0=45) quasibound state may be internally redistributed (exciting van der Waals modes at the expense of the bromine excitation) prior to dissociation. Such mechanisms are more deeply studied in this work by means of the stabilization method, which works with square-integrable wave functions and is an appropriate approach to perform quasibound state analysis. Stabilization total cross sections compare fairly well with close-coupling ones, where the proper asymptotic behaviour of the continuum wave functions is taken into account. By inspection of the quasibound state wave functions, it is seen that energy is redistributed to several excited states belonging to the v0−1 manifold. In addition, it is shown that such excited states also carry oscillator strength in the transition from the ground electronic state and thus interference effects in the excitation process are significant. HeBr2 near the halogen dissociation limit is found to be a rather strongly coupled system where the quasibound states involved can only approximately be assigned to quantum numbers corresponding to interhalogen and van der Waals vibrational excitations.
Nonradiative lifetimes for LiH in the
A
state using adiabatic and diabatic schemes
Authors: Gadéa F. X. , Berriche H. , Roncero O. , Villarreal P. , Delgado Barrio G.
Published: July 2002

Accurate positions and nonradiative lifetimes of states belonging to the adiabatic A state of LiH are estimated. The results coming from a Golden Rule treatment in the adiabatic scheme present excellent agreement with those obtained through a diabatic close coupling calculation. That confirms the accuracy reached in both approaches and also in the treatment of the diabatic–adiabatic transformation. It involves, in particular, an effective phase control that is needed to properly estimate nonadiabatic couplings. Also, a powerful numerical procedure to obtain energy profiles in the diabatic close coupling frame is described and applied in this work.
The dissociation of diatomic molecules on vibrating surfaces: A semiclassical generalized Langevin approach
Authors: Dohle Michael , Saalfrank Peter , Uzer T.
Published: July 2002

The dissociative sticking of a diatomic molecule on a vibrating surface is a complex many-body process. Here we investigate the dissociation of N2 on a model Fe substrate with the help of a semiclassical generalized Langevin equation (GLE) scheme, in which the molecule is described by a quantum wave packet, and the substrate idealized by a set of classical Langevin oscillators. In a first part of the paper, cold surfaces at surface temperature Ts=0 K are considered. Here, we investigate (i) the validity of the classical approximation(s) and (ii) errors contained in the widely used single-oscillator approximation. Furthermore, a systematic analysis (iii) of the substrate atom motion and the energy transfer from the molecular (quantum) degrees of freedom to the substrate (classical) modes, and (iv) of effects of increasing “hardness” of the substrate and/or of increasing surface atom masses, is presented. We find that the complex many-body dynamics is more accurately captured by treating many surface oscillators in an approximate fashion, rather than treating a single one as exactly as possible. In a second part of the paper, the formalism is generalized to the Ts>0 K case, and effects of finite surface temperatures are studied. It is found that the semiclassical GLE approach correctly accounts for the thermal broadening of the sticking probability vs kinetic energy curve, well-known from experiments on the activated dissociation of diatomic molecules on surfaces.
Photoionization spectroscopy of dichromium and dimolybdenum: Ionization potentials and bond energies
Authors: Simard Benoit , Lebeault-Dorget Marie-Ange , Marijnissen Adrian , ter Meulen J. J.
Published: July 2002

Photoionization spectroscopy has been used to probe molecular beams of laser-vaporized chromium (Cr2) and molybdenum (Mo2) dimers. Two-color photoionization efficiency spectroscopy has been used to determine the adiabatic ionization potential (IP) of Cr2 and Mo2 to be 56 449±8 cm−1 and 56 042±8 cm−1, respectively. The IP of Cr2 is combined with the IP of Cr [54 575.6±0.3 cm−1, Huber et al., Proc. R. Soc. London, Ser. A 342, 431 (1975)] and the bond energy of Cr2+ [10 200±500 cm−1, Su et al., Chem. Phys. Lett. 201, 199 (1993)] to yield a bond energy of 12 400±500 cm−1 for Cr2. One-color resonant two-photon ionization (R2PI) spectroscopy has been employed to probe the molybdenum dimer molecule in the energy region where its dissociation should occur. The dissociation limit has been ascribed to the threshold observed at 36 100±80 cm−1. This value is combined with the IP of Mo [57 204.3±0.3 cm−1, Rayner et al., J. Opt. Soc. Am. B 4, 900 (1987)] and Mo2 to yield a bond energy of 37 260±80 cm−1 for Mo2+.
Thermochemistry of HNC, HNC+, and CF3+
Authors: Hansel A. , Scheiring Ch. , Glantschnig M. , Lindinger W. , Ferguson E. E.
Published: July 2002

In the course of mechanistic studies on HCN+ isomerization in reaction with CO and CO2, we have fortuitously determined the heats of formation of HNC and HNC+ with good precision. The uncertainty is the uncertainty in the heat of formation of HCN, ±1 kcal mol−1. This appears to be the first precise experimental determination of these energies. Several prior theoretical calculations have produced reasonable energy values. We also observed the reaction of HCN+ with CF4 to be slightly endothermic, which allows the determination of an upper limit on the threshold energy for CF4→CF3++F of 14.28 eV, in excellent agreement with several prior determinations but in sharp disagreement with the most recent determination.
Controlling the alignment of neutral molecules by a strong laser field
Authors: Sakai Hirofumi , Safvan C. P. , Larsen Jakob Juul , Hilligso/e Karen Marie , Hald Kasper , Stapelfeldt Henrik
Published: July 2002

A strong nonresonant nanosecond laser pulse is used to align neutral iodine molecules. The technique, applicable to both polar and nonpolar molecules, relies on the interaction between the strong laser field and the induced dipole moment of the molecules. The degree of alignment is enhanced by lowering the initial rotational energy of the molecules or by increasing the laser intensity. The alignment is measured by photodissociating the molecules with a femtosecond laser pulse and detecting the direction of the photofragments by imaging techniques. The strongest degree of alignment observed is 〈cos2 θ〉=0.81.
Minimum energy configurations of repelling particles in two dimensions
Authors: Jagla E. A.
Published: July 2002

Geometrical arrangements of minimum energy of a system of identical repelling particles in two dimensions are studied for different forms of the interaction potential. Stability conditions for the triangular structure are derived, and some potentials that do not satisfy them are discussed. It is shown that in addition to the triangular lattice, other structures may appear (some of them with nontrivial unit cells, and nonequivalent positions of the particles) even for simple choices of the interaction. The same qualitative behavior is expected in three dimensions.
Temperature dependence of amplitudes of libration motion of guest spin-probe molecules in organic glasses
Authors: Paschenko S. V. , Toropov Yu. V. , Dzuba S. A. , Tsvetkov Yu. D. , Vorobiev A. Kh.
Published: July 2002

Electron paramagnetic resonance (EPR) lineshapes of nitroxide spin probes in a variety of organic supercooled liquids and glasses were analyzed within the framework of the fast librational motion model. Computer simulation confirmed that the EPR lineshape may be fitted well by the librational model. For all systems studied, a mean-squared amplitude of this motion, 〈α2〉, was found to depend linearly on temperature in the low-temperature region, which is typical of harmonic solids. The slope of the linear dependence varies from glass to glass and does not change for different nitroxides in the same glass. This slope seems to correlate with the strength of the intermolecular bonds and with a degree of the fragility of the glass. Steep rises in 〈α2〉 were observed at high temperature.
Comparative configurational study for He, Ne, and Ar trimers
Authors: González-Lezana T. , Rubayo-Soneira J. , Miret-Artés S. , Gianturco F. A. , Delgado-Barrio G. , Villarreal P.
Published: July 2002

Helium trimer bound states are calculated by means of a variational method described in terms of atom pair coordinates and distributed Gaussian basis functions for zero total angular momentum. To show the feasibility of this method, we also apply it to the calculation of the first vibrational levels of the Ar3 and Ne3 clusters. Special emphasis is made on the study of the possible Efimov behavior of the first excited state found in the He34 trimer. Geometrical configurations of the ground and first excited states of these rare gas trimers have been exhaustively studied owing to the proper symmetry of the coordinates chosen.
A formulation and numerical approach to molecular systems by the Green function method without the Born–Oppenheimer approximation
Authors: Shigeta Yasuteru , Nagao Hidemi , Nishikawa Kiyoshi , Yamaguchi Kizashi
Published: July 2002

We have proposed a new numerical scheme for the non-Born–Oppenheimer density functional calculation based upon the Green function techniques within the GW approximation for evaluating molecular properties in the full quantum mechanical treatment. We numerically calculate the physical properties of the individual motion in a hydrogen molecule and a muon molecule by means of this method and discuss the isotope effect on the properties in relation to correlation effects. It is concluded that the GW approximation is work well not only for calculation of the electronic state but also for that of nuclear state.
Energy landscape of a model protein
Authors: Miller Mark A. , Wales David J.
Published: July 2002

The potential energy surface of an off-lattice model protein is characterized in detail by constructing a disconnectivity graph and by examining the organization of pathways on the surface. The results clearly reveal the frustration exhibited by this system and explain why it does not fold efficiently to the global potential energy minimum. In contrast, when the frustration is removed by constructing a “Gō-type”. model, the resulting graph exhibits the characteristics expected of a folding funnel.
Symplectic algorithm for constant-pressure molecular dynamics using a Nosé–Poincaré thermostat
Authors: Sturgeon Jess B. , Laird Brian B.
Published: July 2002

We present a new algorithm for isothermal–isobaric molecular-dynamics simulation. The method uses an extended Hamiltonian with an Andersen piston combined with the Nosé–Poincaré thermostat, recently developed by Bond, Leimkuhler, and Laird [J. Comp. Phys. 151, 114 (1999)]. This Nosé–Poincaré–Andersen (NPA) formulation has advantages over the Nosé-Hoover-Andersen approach in that the NPA is Hamiltonian and can take advantage of symplectic integration schemes, which lead to enhanced stability for long-time simulations. The equations of motion are integrated using a generalized leapfrog algorithm (GLA) and the method is easy to implement, symplectic, explicit, and time reversible. To demonstrate the superior stability of the method we show results for test simulations using a model for aluminum and compare it to a recently developed time-reversible algorithm for Nosé–Hoover–Anderson. In addition, an extension of the NPA to multiple time steps is outlined and a symplectic and time-reversible integration algorithm, based on the GLA, is given.
Modeling absorption and segregation of magnesium and cadmium ions to calcite surfaces: Introducing MgCO3 and CdCO3 potential models
Authors: de Leeuw N. H. , Parker S. C.
Published: July 2002

We introduce a potential model for MgCO3 and CdCO3 which reproduces experimental structural parameters and solution energies. We employed atomistic simulation techniques to model the absorption and segregation of magnesium and cadmium ions to the low-index surfaces of calcite. Both magnesium and cadmium ions absorb at the surfaces from solution. The absorption energies are surface dependent, due to distinct relaxations of the different surfaces, and ion dependent with the smaller sized magnesium ion showing larger relaxations. The absorption energies are larger for magnesium than for cadmium, mainly due to the closer coordination of the water molecules to the magnesium substituted surfaces. The cadmium ions segregate to the bulk crystal more easily than the magnesium ions which prefer to remain at the surface. However, segregation energies for both cations in second and further layers of some surfaces are positive, indicating that once a calcium carbonate layer has overgrown the substituted surface layer, segregation to the bulk is energetically possible.
Driving wave packet recurrences with optimally modulated laser pulses
Authors: Goodson Boyd M. , Goswami Debabrata , Rabitz Herschel , Warren Warren S.
Published: July 2002

In the weak-field limit, laser pulses optimized to induce vibrational wave packet recurrences in excited state potentials were calculated for Morse oscillators and for a real system [the X 1Σ+ and A 3Π(1) states of IBr]. The performance of the optimized pulses was studied via simulated wave packet propagation. Such optimal light fields may be computationally generated given only the form of the electronic potential surfaces, knowledge of the particular ground state supplying population, and simple molecular constants. Thus it should be possible to use the modulation of light fields experimentally optimized to achieve recurrences in order to obtain substantial information regarding previously uncharacterized potential surfaces in both diatomic and polyatomic molecules. Moreover, it should be possible to generalize this approach to the strong-field limit.
Symmetries of variable coefficient Korteweg–de Vries equations
Authors: Gazeau J. P. , Winternitz P.
Published: July 2002

The Lie point symmetries of the equation ut+f(x, t)uux+g(x,t)uxxx=0 are studied. The symmetry group is shown to be, at most, four dimensional, and this occurs if and only if the equation is equivalent, under local point transformations, to the KdV equation with f=g=1. For nine different classes of functions f and g, the symmetry group turns out to be three dimensional. Two-dimensional and one-dimensional symmetry groups occur for 11 and 15 classes of equations, respectively.
Hamiltonian formalism for nonlocal Lagrangians
Authors: Llosa J. , Vives J.
Published: July 2002

A Hamiltonian formalism is set up for nonlocal Lagrangian systems. The method is based on obtaining an equivalent singular first order Lagrangian, which is processed according to the standard Legendre transformation and then, the resulting Hamiltonian formalism is pulled back onto the phase space defined by the corresponding constraints. Finally, the standard results for local Lagrangians of any order are obtained as a particular case.
On the solution of the inverse scattering problem, at fixed energy, for the class of Yukawian potentials
Authors: Fioravanti R. , Viano G. A.
Published: July 2002

In this paper we prove that if in the Fourier–Legendre expansion of the scattering amplitude the partial-waves fl satisfy a Hausdorff-type condition and are exponentially bounded in l, then this expansion converges uniformly to a function f(θ) (θ=u+iv is the complexified scattering angle) analytic in a strip contained in the θ-plane; furthermore we prove that this function has a holomorphic extension to a ‘‘cut-domain.’’ From the discontinuity of f(θ) across the cuts we can reconstruct the spectral density σ(μ), μ=k{2(cosh v−1)}1/2, associated with the class of Yukawian potentials. We give also an inversion algorithm which is able to reconstruct the density σ(μ), via an iterative procedure, starting from the partial-waves {fl}∞0. Since the data fl are necessarily affected by noise, and we know only a finite number of them, then the data set is given by {f(ε)l}N0, where ε characterizes the noise. It follows that there is a certain degree of ‘‘irreversibility’’ in the inverse problem, due to its ill-posedness. All these questions, as well as the various problems connected with the convergence of the algorithm, are analyzed in details. Finally, the numerical efficiency of the algorithm is tested on some numerical examples.
Classification of spherically symmetric static space–times by their curvature collineations
Authors: Bokhari Ashfaque H. , Qadir Asghar , Shahan Ahmed M. , Asghar Mohammad
Published: July 2002

A complete classification of all spherically symmetric static space–times according to their curvature collineations is presented and compared with Ricci collineations of corresponding space–times.