bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of ferromagnetic resonance in granular Cu–Co alloy
Authors: Lachowicz Henryk K. , Sienkiewicz Andrzej , Gierłowski Piotr , Ślawska-Waniewska Anna
Published: July 2002

Ferromagnetic resonance (FMR) spectra (derivatives of microwave power absorption) were measured in a granular, ribbon-shaped melt-spun Cu90Co10 alloy in a wide range of temperatures from 10 K up to room temperature. Prior to measurements the sample was annealed at 773 K for 1 h in order to create a well-developed granular structure. The results are interpreted using the Kittel FMR equations originally introduced for thin ferromagnetic films, as well as by using an Arrhenius-type exponential function for superparamagnetic relaxations. These results are also compared with the experimental data obtained from low-field susceptibility measurements in both, zero-field-cooling and field-cooling modes. It is shown that FMR spectroscopy is a useful tool for studying magnetic properties of granular magnets consisting of nanometer-scale single-domain particles, in particular, if the resonance spectra are measured in a wide range of temperatures, thus allowing for identification of the magnetic phases of the particles. The derived value of 10−13 s for the characteristic time of superparamagnetic relaxations is four orders of magnitude smaller than that commonly used while analyzing data yielded by just one measuring technique. The obtained results allowed us to estimate the first order constant of the magnetocrystalline anisotropy for fcc Co particles in the copper matrix K1=−4.2×106 erg/cm3.
Structural and photoluminescence properties of In0.9(Ga/Al)0.1As self-assembled quantum dots on InP substrate
Authors: Zhong-zhe Sun , Ju Wu , Feng-qi Liu , Huai-zhe Xu , Yong-hai Chen , Xiao-ling Ye , Wei-hong Jiang , Bo Xu , Zhan-guo Wang
Published: July 2002

Self-assembled In0.9Ga0.1As, In0.9Al0.1As, and InAs quantum dots (QD) were fabricated in an InAlAs matrix lattice-matched to an InP substrate by molecular beam epitaxy. Preliminary characterizations were performed using transmission electron microscopy, photoluminescence, and reflection high-energy electron diffraction. Experimental results reveal clear differences in QD formation, size distribution, and luminescence between the InAs and In0.9(Ga/Al)0.1As samples, which show the potential of introducing ternary compositions to adjust the structural and optical properties of QDs on an InP substrate.
An analytic solution of the dissociative and the kickout diffusion mechanisms
Authors: Kasuya Tadashi , Fuji Masao
Published: July 2002

An analytic solution of the dissociative and the kickout diffusion mechanisms has been presented for a one-dimensional region bounded by two parallel surfaces, assuming the rapid recovery of equilibrium conditions of vacancies and interstitial matrix atoms. This assumption makes the diffusion equation linear. The calculation results for gold diffusion into a silicon plate with high dislocation density using the literature data are demonstrated. The assumption of the rapid recovery can be verified using the present solution.
Unimolecular reaction rate theory for transition states of partial looseness. II. Implementation and analysis with applications to NO2 and C2H6 dissociations
Authors: Wardlaw David M. , Marcus R. A.
Published: July 2002

Implementation of RRKM theory for unimolecular dissociations having transition states of any degree of looseness is described for reactions involving dissociation into two fragments. The fragments may be atomic, diatomic, or polyatomic species. Action-angle and internal coordinates for the transitional modes of the reaction, transformations to Cartesian coordinates, and other calculational aspects are described. Results for the NO2→NO+O reaction are presented, including the dependence of the microcanonical rate constant on the bond fission and bending potentials for model potential energy surfaces. Illustrative calculations for the C2H6→2CH3 reaction are also given.
Rotationally inelastic collisions between a diatomic molecule in a 2S+1Σ electronic state and a 2S atom: The optimal choice for the total-𝒥 representation
Authors: Corey Gregory C. , Alexander Millard H.
Published: July 2002

The close-coupled formalism for rotationally inelastic collisions is extended to 2S+1 Σ state molecule-2S atom systems by using a Hund’s case (b) coupling scheme for the open-shell molecule. A novel total-𝒥 representation is introduced that is independent of 𝒥 and block diagonal in both the rotational-orbital and the electronic spin subspaces whenever the spin splitting of the molecular rotational levels is negligible. This total-𝒥 representation is based on an angular momentum vector recoupling algorithm that reduces the computing time required for the solution of the scattering problem by 94% for a doublet molecule and by 97% for a triplet molecule. The result of a lack of fine-structure resolution in a molecular beam scattering experiment is discussed, and it is demonstrated that rotationally inelastic cross sections for unresolved fine-structure transitions are equivalent to a degeneracy-weighted average of cross sections for scattering off the allowed adiabatic Born–Oppenheimer atom–molecule potential energy surfaces.
Axial motion of water–hydrogen halide and water–halogen complexes in argon matrices
Authors: Engdahl Anders , Nelander Bengt
Published: July 2002

The infrared spectra of the water (H2O,HDO,D2O) complexes with HCl, HBr, HI, Cl2, Br2 and I2 have been studied in argon matrices in the 11 to 25 K interval. In all cases, water acts as a lone-pair donor. The hydrogen halide complexes are effectively planar and rotate around the axis through the heavy atoms. For the water halogen complexes, the results are less clear, but a similar rotation, coupled to low wave number bending motions, seems possible.
The silicon analog of benzene–hexasilabenzene (Si6H6)
Authors: Clabo D. Allen , Schaefer Henry F.
Published: July 2002

A b initio molecular electronic structure theory has been applied to the D6h- symmetry isomer of Si6H6. Self-consistent-field (SCF) theory has been applied in conjunction with double zeta (DZ) and double zeta plus silicon 3d (DZ+d) basis sets. The DZ+d basis set may be designated Si(11s 7p 1d/6s 4p 1d), H (4s/2s). The structure of hexasilabenzene is predicted to be: re(Si–Si) =2.223 Å, re(Si–H) =1.476 Å (DZ SCF); re(Si–Si) =2.205 Å, re(Si–H) =1.463 Å (DZ+d SCF). Vibrational frequencies have been predicted at the DZ SCF level, at which the DZ SCF structure is a true minimum. However, the lowest vibrational frequency is predicted to be only 10 cm−1.
Quantum defect analysis of HD photoionization
Authors: Du Ning Yi , Greene Chris H.
Published: July 2002

A multichannel quantum defect calculation is shown to reproduce most observed features in several portions of the HD photoabsorption spectrum. The rovibrational frame transformation theory of Atabek, Dill, and Jungen is reformulated in terms of a quantum defect matrix. The calculation accounts for spectral regions far from dissociation thresholds despite its neglect of g–u mixing.
Deuterium isotope effects on photodecomposition of alkylbenzenes
Authors: Kajii Yoshizumi , Obi Kinichi , Tanaka Ikuzo , Ikeda Noriaki , Nakashima Nobuaki , Yoshihara Keitaro
Published: July 2002

Photodissociation of 11 alkylbenzenes (toluene, ethylbenzene, butylbenzene, and their deuterated compounds) have been studied by the ArF (193 nm) laser photolysis. Formation rate constants of benzyl radical were measured under collision free conditions. The major relaxation process of the photoexcited state is internal conversion to highly excited vibrational levels of the ground electronic state, i.e., hot alkylbenzene formation, followed by decomposition yielding benzyl radical. Deuterium isotope effects demonstrate that the bond fission occurs after intramolecular vibrational redistribution is complete.
Hexasilabenzene (Si6H6). Is the benzene-like D6h structure stable?
Authors: Nagase Shigeru , Teramae Hiroyuki , Kudo Takako
Published: July 2002

A b initio molecular orbital theory has been applied to the structure of hexasilabenzene (Si6H6), the silicon analog of benzene (C6H6). The minimum energy structure is calculated to be planar with D6h symmetry, as in the case of benzene, with flexible basis sets at the Hartree–Fock level of theory. Through the estimation of a significant effect of electron correlation, however, a chair-like puckered structure of D3d symmetry is proposed for hexasilabenzene. The preference of D3d over D6h is explained in terms of some ‘‘multiradical’’ character of the skeletal bonding (i.e., the antiferromagnetic-like localization of electrons onto each Si).
Dynamic solvent effects on outer-sphere electron transfer
Authors: Rips Ilya , Jortner Joshua
Published: July 2002

In this paper we present the results of a theoretical study of outer-sphere electron transfer (ET) in a polar solvent, with the modification of the nuclear states by the change in the charge distribution originating solely from the response of the exterior medium. The model Hamiltonian for the system corresponds to two parabolic diabatic potential surfaces with adiabatic coupling between them. The real-time path integral formalism is utilized to derive the general expressions for the influence functional of the medium in the Gaussian approximation and for the ET rate. The ET rate is explicitly evaluated for the particular case of a medium characterized by the Debye dielectric relaxation function. We explore the relation between the dynamics of the reaction coordinate and the character of the ET process, deriving an expression for the ET rate, which bridges between the nonadiabatic and the solvent-controlled adiabatic limits. We establish simple criteria for the validity range of various descriptions of ET dynamics, i.e., the transition state theory, the solvent-controlled ET as well as the adiabatic and nonadiabatic limits. The results are applied to intramolecular ET in alkanols, establishing the adiabatic nature of these processes, whose dynamics is dominated by the (slow) longitudinal dielectric relaxation rate of the solvent.
Metastable H+3 formation and decay in the reaction of highly excited H+2 with H2
Authors: Badenhoop Jay K. , Schatz George C. , Eaker Charles W.
Published: July 2002

This paper presents the detailed results of a quasiclassical trajectory surface hopping study of reaction of highly vibrationally excited H+2 with ground state H2 (and isotopic counterparts H+2 +D2, D+2 +H2, and D+2 +D2 ), with particular emphasis on the formation and decay of metastable H+3 products. A diatomics-in-molecules surface is used which has been successful in previous studies of H+2 (v) + H2 at low v. In the present study, we consider v=0–17, and find that metastable H+3 ’s are a major product for v≥13. Some of these metastables decay rapidly, showing exponential lifetime distributions with 2–7 ps lifetimes depending on v and on isotope. The remaining H+3 ’s have much longer lifetimes, and a number of methods are used to determine the origin of their stability. In no cases are any of these molecules found to be quasiperiodic, but a Fourier spectral analysis reveals some decoupling of H+ –H2 orbital motion from H2 rotational motion, and we find that many molecules have long lifetimes even though they have energies which are above any rigorous centrifugal barrier. The relation of these results to recent infrared absorption measurements on highly excited H+3 ’s is discussed.
Bound and rotational resonance states and the infrared spectrum of N2Ar
Authors: Brocks G.
Published: July 2002

Rotational and van der Waals vibrational states of N2Ar have been computed in the atom–diatom formalism by an expansion in L2 basis functions. The bound states are characterized by a strong coupling between the modes, which results in an irregular spacing of the bound levels. The lower frequency part of the spectrum (<25 cm−1) is therefore very complicated and sensitive to the detailed intermolecular potential. It is shown that the regular, free internal rotor pattern, which is observed in the higher frequency part of the gas phase infrared spectrum of N2Ar, is due to rotational resonances lying in the collision continuum. The resonances have a width in the range of 0.2–3.5 cm−1 and especially the narrow ones contribute to the free internal rotor structure in the spectrum.
Effective potentials and multiconfiguration wave functions in quantum Monte Carlo calculations
Authors: Christiansen P. A.
Published: July 2002

Using relativistic effective potentials to eliminate the electron density in the core region we have computed quantum Monte Carlo (QMC) atomic energies for Be which accurately reproduce the experimental values in the range from the ground state to the first ionization energy. These calculations required only a few hours on a MicroVAX computer to obtain statistical errors smaller than a millihartree. For the ground state we found that it was absolutely essential to employ a multiconfiguration trial wave function to define the local potential. In contrast with an all-electron QMC study, we find that the multiconfiguration trial wave function greatly reduces the statistical error for a given amount of sampling. Our work indicates that multiconfiguration wave functions should be used routinely in the context of effective potential QMC.
Approximate natural orbitals and the convergence of a second order multireference many-body perturbation theory (CIPSI) algorithm
Authors: Illas F. , Rubio J. , Ricart J. M.
Published: July 2002

In this paper we discuss a method to obtain approximate natural orbitals (ANOs) by means of the zero-order and first-order one-particle reduced density matrices. Convergence of the multireference second-order perturbation CI by means of CIPSI method, in its two-class and three-class versions, appears to be much improved when using these ANOs. The method requires only a single additional integral transformation. A comparison with canonical and valence virtual orbitals defined through the PHO procedure is also reported. Finally, the use of ANOs together with the multireference analog of the Davidson’s correction, as well as a suitable extrapolation procedure in the CIPSI method is discussed. Results are compared with available full CI calculations for H2O in a double-ξ and a double-ξ plus polarization basis set.
The benzene ground state potential surface. III. Analysis of b2u vibrational mode anharmonicity through two-photon intensity
Authors: Goodman Lionel , Berman Joel M. , Ozkabak Ali G.
Published: July 2002

The 1501/1401 vibronic two-photon cross section ratios are reported for a series of isotopically labeled benzenes in the Ã(1B2u)←X̃(1A1g) electronic transition. Predictions derived from the B2u force field are found to be in close agreement with the measured ratios. These ratios are shown to provide an excellent test of the B2u force field and mode forms as evidenced by the large variation over D6h labeled benzenes. In C6H6 the 1501/1401 cross section ratio is measured as 0.249±0.008 (equivalent to 0.180 for the theoretically testable ratio: 1501/1401[〈1‖Q14‖0〉/〈1‖ Q15‖0〉]2). The corresponding ratio in 13 C6H6 is 0.44±0.04 (equivalent to 0.36). The 13% disparity found between the measured and predicted C6H6 ratio (i.e., 0.206) is attributed to anharmonic coupling between the b2u modes: 2χ15,15=−9, χ14,15=4, and 2χ14,14=−4 cm−1. Two-photon intensities are proven to be useful in determining anharmonic interactions. The relatively small effects of the hydrogen motion provide an approach for solving the bifurcated B2u force constant problem in ground state benzene. The approach utilizes the contribution of harmonic C–C–H bending motions to the two-photon tensor controlling the 1501 and 1401 vibronic cross sections. This requires knowledge of the sign of the hydrogen motion term in the tensor. However, large anharmonic effects coupling the two b2u modes mask the small harmonic hydrogen contribution.
Systematic investigation of the phase behavior in binary fluid mixtures. I. Calculations based on the Redlich–Kwong equation of state
Authors: Deiters Ulrich K. , Pegg Ian L.
Published: July 2002

The original Redlich–Kwong equation, together with the usual quadratic mixing rules, has been used to calculate phase diagrams for binary fluid mixtures and to classify them according to the system of van Konynenburg and Scott. Global phase diagrams (maps) showing the extent of the various phase diagram classes in the space of the Redlich–Kwong parameters are presented. While for molecules of equal size the results are very similar to those known for the van der Waals equation, the maps become topologically different for molecules of unequal sizes; some complicated phase diagram classes, which otherwise cover small domains on the maps or cannot be found at all, become quite important.
Computer simulations studies of the catalytic oxidation of carbon monoxide on platinum metals
Authors: Kaukonen H.-P. , Nieminen R. M.
Published: July 2002

The steady-state catalytic oxidation process of carbon monoxide on platinum metal surfaces is studied using two irreversible kinetic computer simulation models: (a) An extended version of the model introduced by Ziff, Gulari, and Barshad (ZGB) with the effects of CO desorption and diffusion as well as finite reaction probability taken into account. The different physical processes, diffusion and desorption are studied independently and their effect on the equilibrium window, i.e., the regime where steady CO2 formation occurs is determined. (b) An interaction model where adatom–adatom nearest-neighbor (nn) interactions are taken explicitly into account through Boltzmann terms J1, J2, and J3 which are the energies of the CO–CO, O–O, and CO–O interactions, respectively. The phase diagrams in the temperature–CO-partial pressure (T,pCO−) plane are determined for different values of the nn interactions. The behavior of the system is dependent on the sign of J1(=J2 in the simulations) as well as the sign of the difference J1−J3. There is thus a clear analogy with a two-component equilibrium lattice gas with nn interactions.
Dissociation of cyclohexene and 1,4-cyclohexadiene in a molecular beam
Authors: Zhao Xinsheng , Continetti Robert E. , Yokoyama Atsushi , Hintsa Eric J. , Lee Yuan T.
Published: July 2002

Molecular-beam photofragmentation translational spectroscopy of cyclohexene and 1,4-cyclohexadiene was carried out using 193 nm and IR multiphoton excitation. At 193 nm, both the retro-Diels–Alder reaction of cyclohexene and H2 elimination from both molecules were observed in the ground electronic state, indicating the occurrence of internal conversion from the initially excited electronic states. The retro-Diels–Alder reaction is shown to be concerted up to an internal energy higher than 148 kcal/mol. Hydrogen-atom elimination was also observed for both molecules following 193 nm excitation. The H atom is eliminated from an excited state of cyclohexene and is assigned to be from a carbon adjacent to the double bond, with a corresponding C–H bond energy of 87±3 kcal/mol. It is shown that the peak of the translational energy distribution for concerted dissociation in the ground state is determined mainly by the dynamics of the potential-energy release along the reaction coordinate, and is not sensitive to either the amount of internal energy or the form of excitation.
State-resolved evidence for hot carrier driven surface reactions: Laser-induced desorption of NO from Pt(111)
Authors: Buntin S. A. , Richter L. J. , King D. S. , Cavanagh R. R.
Published: July 2002

State-specific diagnostics are used to characterize the laser-induced desorption of NO from Pt(111). Two desorption channels are observed; one is consistent with thermal activation, while the other is driven by adsorbate interactions with hot carriers. For this latter channel, the observed dependence of the desorption yield on the wavelength of the incident laser pulse (1907, 1064, 532, and 355 nm) and the wavelength dependence of the kinetic energy distributions establish the nonthermal nature of the excitation process. The inverted spin–orbit population, the non-Boltzmann rotational state distributions, and the vibrational state population are interpreted in terms of a desorption mechanism involving a temporary ion resonance.
Observation of highly vibrationally excited X̃ 1Σ+ HCP by stimulated emission pumping spectroscopy
Authors: Chen Yit-Tsong , Watt David M. , Field Robert W. , Lehmann Kevin K.
Published: July 2002

We have recorded stimulated emission pumping (SEP) spectra of highly vibrationally excited methinophosphide (HCP) at 16 000 cm−1–19 000 cm−1 above the zero-point energy of the linear X̃ 1Σ+ ground electronic state. These vibrational levels have most of their energy localized in the bending coordinate. The turning point in the bending vibration brings the H atom approximately halfway (∠HCP∼90°) from HCP to another chemical network, HPC. The SEP spectra of large-amplitude-bending states, (0,26,0), (0,28,0), and (0,30,0), and of bending–C≡P stretching combination states, (0,24,1) and (0,26,1), have enabled us to determine rotation–vibration spectroscopic constants for highly vibrationally excited X̃ 1Σ+ HCP. The rotational structure of HCP shows that B values change by<3% despite large amplitude motion corresponding to vibrations of over±90 degrees away from linearity. The g22 (eff ) constant, which reflects the energy difference between purely planar (l=0) versus slightly elliptical (l=2) bending, increases only 12% from (0,2,0) to (0,30,0), thus illustrating the surprisingly harmonic appearance of the vibrational motion even at this high energy.
Vibron–phonon excitations in the molecular crystals N2, O2, and CO by Fourier transform infrared and Raman studies
Authors: Löwen H. W. , Bier K. D. , Jodl H. J.
Published: July 2002

The influence of temperature on the vibron–phonon combination band in the Raman and infrared (ir) spectra of the N2 and O2 molecular crystals supports the possibility of an assignment of the main features in the sideband to strong contributions from translational and librational phonons in points of high symmetry in the reciprocal lattices. The temperature behavior in Raman and ir sideband spectra in α-N2 is attributed to distinct anharmonicities in the isotropic and anisotropic parts of the potential and to different coupling mechanisms, resulting in a librational and translational weighted one-phonon density of states (DOS). In contrast, such an interpretation is not feasible for the poorly structured CO sideband, although the crystal structures of CO and N2 are nearly identical. The difference is attributed to strong anharmonicities and the presence of a weak dipole moment in the former, which introduces strong lattice mode coupling. Crossing the α–β phase transition has marked effects on the sidebands (shape, intensity) in both N2 and O2 crystals, which reflects the orientational disorder in the β phase of the former and the importance of the change in magnetic interactions in the latter. For both molecular crystals, the vibron–phonon coupling to the electromagnetic field is stronger in the ir than in the Raman spectra and seems predominantly due to electrical anharmonicities.
The exact statistical thermodynamics of classical fluids
Authors: Edgal Uduzei F.
Published: July 2002

A scheme which may be considered ‘‘efficient’’ has been evolved for the ‘‘exact’’ determination of the free energy of classical fluids with arbitrary interparticle many-body interaction potentials. The judgment as to the extent of closeness of this scheme to an ‘‘ultimate’’ approach (which up until now has been sought by researchers for the investigation of equilibrium properties of fluids) is one presently expected to be subjective. The versatility of the ‘‘recipe,’’ however, is one that may be considered quite remarkable; and its ‘‘natural flavor’’ seems to suggest the approach is one that is forced upon us rather than the approach being a tool that has been artifically fashioned and forced by us into the problem. Application of the scheme to a few simple realistic fluid models has proved quite successful. However, more elaborate applications are expected in the future to provide an ultimate test of the suitability of the scheme.
Ultrafast vibrational dynamics of small organic molecules in solution
Authors: Bakker H. J. , Planken P. C. M. , Lagendijk A.
Published: July 2002

In this paper we present a time-resolved study of the vibrational relaxation after excitation of the asymmetric CH2 stretch vibration of dibromomethane and diiodomethane and the C–H stretch vibration of 1,1,2,2-tetrabromoethane. The experiments were performed in a polar and a nonpolar solvent in order to study the influence of the polarity of the solvent on the relaxation. We observe that in both types of solvent the vibrational energy transfer is successively intra- and intermolecular and that the intramolecular relaxation leads to a shift of the transition frequency of the excited molecular vibration. We discuss the experimentally determined time constants of the relaxation in terms of the energy differences between the molecular vibrations and the interactions with the solvent.
Relativistic effective potentials in transition metal quantum Monte Carlo simulations
Authors: Christiansen P. A.
Published: July 2002

Relativistic effective potentials are used in fixed-node quantum Monte Carlo simulations, including branching, to compute the lowest Sc and Y atomic excitation energies. Using simple SCF trial wave functions multiplied by pair correlation functions we obtain values of 1.5(3) and 1.4(2) eV, respectively, for Sc and Y as compared to experimental values of 1.43 and 1.36 eV. The simulations included eleven electrons in the valence space and gave valence electron correlation energies of 13.7(2) and 14.4(2) eV, respectively, for the ground states of Sc and Y. The simulations exhibited none of the instabilities predicted by other workers in the field. There appears to be no barrier to the use of conventional relativistic effective potentials in fixed-node quantum Monte Carlo simulations.
Molecular dynamics simulation of gas transport in amorphous polypropylene
Authors: Müller-Plathe Florian
Published: July 2002

Molecular dynamics simulations are performed on polypropylene-penetrant systems with the penetrant being molecular hydrogen, molecular oxygen, and methane. Well equilibrated initial configurations of the amorphous polypropylene are generated by the method of Theodorou and Suter. At the qualitative level, a hopping motion is found to be the underlying mechanism for penetrant diffusion. Coefficients for the diffusion of the penetrant molecules in polypropylene are obtained from their mean-square displacements. The agreement with experimental results, where available, is satisfactory. A relation between the penetrant’s diffusion coefficient and its size frequently observed in experiments is confirmed by the present results.
Long-time self-diffusion of spherical colloidal particles measured with fluorescence recovery after photobleaching
Authors: van Blaaderen A. , Peetermans J. , Maret G. , Dhont J. K. G.
Published: July 2002

Long-time self-diffusion coefficients in concentrated colloidal dispersions of silica spheres, with various interaction potentials, were measured with the fluorescence recovery after photobleaching technique. Charge stabilized spheres were measured in solutions of LiCl in dimethylformamide with varying ionic strength. Sterically stabilized hard-sphere-like stearyl silica dispersions were studied in cyclohexane. The fluorophore used, fluorescein-isothiocyanate, was covalently attached to the surface of the spheres (for the charged particles) or buried inside the silica core (for the hard spheres). The particles were characterized by electrophoresis, static and dynamic light scattering, and transmission electron microscopy. The experimental results are discussed and compared with existing theories on long-time self-diffusion.
Shadowing, rare events, and rubber bands. A variational Verlet algorithm for molecular dynamics
Authors: Gillilan Richard E. , Wilson Kent R.
Published: July 2002

We present a variational implementation of the Verlet algorithm for molecular dynamics which is both conceptually and computationally attractive. Given an approximate path, this variational Verlet algorithm computes an actual trajectory close to the path. Instead of initial conditions, we specify end-point conditions, which makes it possible to compute trajectories with fixed end points as well as trajectories that are exactly periodic (nonlinear modes of vibration). With a simple sign change, the method yields approximate reaction pathways. We present several applications which illustrate how this alternative approach to dynamics can yield unusual trajectories that are difficult to attain by conventional means. In our first model system, a simple double well, we examine the seemingly paradoxical shadowing lemma of classical mechanics which claims that, in spite of the cumulative numerical errors which cause rapid deviation from the correct path, a calculated trajectory can still be close to some true trajectory of the system over a relatively long time period. We explicitly calculate such so-called shadowing trajectories. We also study isomerization events in which trajectories cross from one potential well to another, obtaining guesses from simple line segments and from dynamics on a perturbed version of the double-well potential. A closed loop of briefly recurrent motion in a trajectory can serve as an initial guess in the computation of a large-amplitude nonlinear mode of vibration. Finally, we use a model AB+C reaction in rare-gas solution to demonstrate the feasibility of calculations on molecular systems with many degrees of freedom. A gas-phase reactive trajectory of the A, B, and C atoms is, with the variational Verlet algorithm, transformed into a similar solution-phase trajectory and vice versa. This calculation suggests that rare events, such as reactions in solution, may be obtained through refining initial paths of perturbed or decoupled systems where such events are more easily computable.
On the stability of microemulsions. II
Authors: Kahlweit M. , Jen J. , Busse G.
Published: July 2002

In literature, the stability of microemulsions has been attributed to the existence of a natural curvature as well as a bending energy of the saturated amphiphilic monolayers separating the polar and the nonpolar solvents. Alternatively, it has been suggested recently seeing the reason for the stability in the steep rise of the interfacial tension as soon as the interfacial layers become unsaturated. In this paper, we compare the predictions of the latter model with respect to the stability limits in ternary water–oil–nonionic amphiphile mixtures at temperatures below and above the three-phase interval. The agreement between theory and experiment is reasonable.
Molecular dynamics studies of polar/nonpolar fluid mixtures. II. Mixtures of Stockmayer and polarizable Lennard-Jones fluids
Authors: Mooij G. C. A. M. , De Leeuw S. W. , Smit B. , Williams C. P.
Published: July 2002

The results of canonical ensemble molecular dynamics simulations of mixtures of equisized Stockmayer and polarizable Lennard-Jones fluids are reported. The excess free energy of mixing shows a decrease with increasing polarizability reflecting an increased miscibility. For sufficiently large values of the polarizability, the miscibility gap, observed previously for mixtures of nonpolarizable Lennard-Jones and Stockmayer fluids, is shown to disappear. The results can be explained qualitatively with perturbation theory. The structure and orientational correlation functions of the mixture are discussed. The marked increase in local ordering at low concentrations of the polar component, so prominent in the nonpolarizable case, diminishes with increasing polarizability. As a result the asymmetry of excess properties with respect to composition becomes less pronounced.
A finite element implementation of exterior complex scaling for the accurate determination of resonance energies
Authors: Scrinzi Armin , Elander Nils
Published: July 2002

An implementation of exterior complex scaling using the finite elements method with high degree polynomials is presented. We apply the method to find the resonances of the potential 7.5r 2e−r and of a phenomenological coupled channel model of the CaH molecule. In both cases the method is quickly convergent and extremely stable numerically. Convergence could be pushed to the point where the real parts of most resonance energies were independent of the complex scaling angle and of the exterior scaling radius within machine precision (14 significant digits). All imaginary parts were stable to at least eight significant digits. Several resonances of CaH which had evaded searches with a finite difference method could be located.
An ab initio structural and spectroscopic study of acetone—An analysis of the far infrared torsional spectra of acetone-h6 and -d6
Authors: Smeyers Y. G. , Senent M. L. , Botella V. , Moule D. C.
Published: July 2002

The far infrared torsional spectra of acetone (CH3)2CO and (CD3)2CO have been determined from ab initio calculations, and the main features of the experimental data assigned. For this purpose, the potential energy surface for the double methyl rotation was determined with fully relaxed geometry into the RHF and RHF+MP2 approximations using a 6–31G(p,d) basis set. The energy values, as well as the kinetic parameters obtained from the optimized geometry, were fitted to double Fourier expansions as functions of the rotational angles in seven terms. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G36 nonrigid group, which factorize the Hamiltonian matrix into 16 boxes. The energy levels and torsional wave functions for each symmetry specie were then obtained diagonalizing each blocks separately. Intensities were obtained from the calculated electric dipole moment variations and the nuclear statistical weights, and were combined with the torsional frequencies to predict the spectra. The calculated band patterns show a multi- plet structure and reproduce the main features of the experimental data. The torsional bands of the infrared active ν17 mode were found to be clustered into quartets, (A1→A2, G→G, E1→E1, E3→E4), for the v=0→v=1 fundamental, and (A2→A1, G→G, E1→E1, E4→E3) for the v=1→v=2 first sequence transitions. The G→G transitions were found to be the more intense. The correlation between the calculated and observed spectra allows for an assignment of the major bands.
Critical phenomena in polymer solutions: Scaling of the free energy
Authors: Cherayil Binny J.
Published: July 2002

The thermodynamics of monodisperse solutions of polymers in the neighborhood of the phase separation temperature is studied by means of Wilson’s recursion relation approach, starting from an effective φ4 Hamiltonian derived from a continuum model of a many-chain system in poor solvents. Details of the chain statistics are contained in the coefficients of the field variables φ, so that the parameter space of the Hamiltonian includes the temperature, coupling constant, molecular weight, and excluded volume interaction. The recursion relations are solved under a series of simplifying assumptions, providing the scaling forms of the relevant parameters, which are then used to determine the scaling form of the free energy. The free energy, in turn, is used to calculate the other singular thermodynamic properties of the solution. These are characteristically power laws in the reduced temperature and molecular weight, with the temperature exponents being the same as those of the 3d Ising model. The molecular weight exponents are unique to polymer solutions, and the calculated values compare well with the available experimental data.
The interaction potential of H2+ calculated from the exact first-order wave function of the polarization perturbation theory
Authors: Guo G. L. , Tang K. T. , Toennies J. P. , Yiu C. L.
Published: July 2002

The interaction potential of the H2+ molecular ion is calculated with the exact first order wave function of the unsymmetrized Rayleigh–Schrödinger perturbation theory. An earlier asymptotic theory for the exchange energy based on the Holstein–Herring surface integral is extended to small distances by including the denominator of the expression. This exchange energy is modulated by the overlap integrals and combined with the polarization energy in the prescribed way of the generalized Heitler–London theory to give the ground state energy. Every term is evaluated analytically. The results are compared with the exact values of the potential and they agree to better than 5% over the entire range of distances from zero to infinity.
Vibrational and orientational relaxation of monomeric water molecules in liquids
Authors: Graener H. , Seifert G.
Published: July 2002

A polarization resolved infrared double resonance experiment with picosecond pulses has been used to measure the vibrational and orientational relaxation times of monomeric water and heavy water molecules in different organic solvents after excitation of the antisymmetric ν3 vibration. In all investigated systems a fast (but definitely time resolved) equilibration among the OH (OD) stretch vibrations was found, followed by a rather slow decay of this ensemble. Furthermore hints to a medium lived intermediate state, most likely the bending vibration, are reported. The principal relaxation scheme is very similar to that of water vapor. Comparing H2O and D2O in different solvents significant differences were found, which can be understood at least semiquantitatively. The orientational relaxation times of H2O and D2O differ by a factor of 2, which can be explained by the influence of weak hydrogen bonds of different strength for H2O and D2O on the orientational relaxation. Additionally interesting aspects of the transient spectra are discussed.
Approximate phase-space transport theory for vibrational predissociation
Authors: Buchachenko A. A. , Stepanov N. F.
Published: July 2002

Approximate version of the phase-space transport theory for vibrational predissociation of the van der Waals complexes with two degrees of freedom is proposed and tested. The theory uses the concept of phase-space bottlenecks which govern the phase-point stochastic diffusion. The global transport is treated as a sequence of the transitions across the bottlenecks within a kineticlike model. Simplified formulas for the transition probabilities across dissociation separatrix (intermolecular bottleneck) and Golden cantori (intramolecular bottlenecks) are obtained under the zeroth-order approximation. Applications to some model complexes indicate the ability of the model to reproduce correctly individual bottleneck parameters as well as the total predissociation rates. Some observations concerning the multibottleneck kinetic models are discussed.
Wetting transitions and other wetting properties of water–2,5 lutidine system
Authors: Amara M. , Privat M. , Bennes R. , Tronel-Peyroz E.
Published: July 2002

The wetting behavior of the water–2,5 lutidine system has been studied around the lower consolute point (Tc=13.1 °C). We have measured contact angles and surface tensions by varying the concentration and temperature. In the diphasic region, a wetting transition has been observed at 46–47 °C on a silica wall by direct observation of the solid–liquid–liquid contact angle. The perfect wetting occurs close to Tc, the wetting phase is water rich. At the liquid–vapor interface, the analysis of the values of the surface tensions shows that, close to Tc, they obey critical laws and that a lutidine rich phase perfectly wets the vapor and the water rich phase. These behaviors have been analyzed on the basis of the contact angles of the monophasic on a silica wall on both sides of the coexistence curve, and the general variation of the surface tensions. A second wetting transition has been shown on a differently washed glass surface. Reference is made to the theoretical and experimental works following the early work of Cahn.
Laser-induced micron size clustering in thiophenol vapor
Authors: Zafiropulos Vassilis , Kollia Zoe , Fotakis Costas , Stockdale John A. D.
Published: July 2002

Positively charged micron sized clusters are efficiently produced following irradiation of thiophenol vapor at 248 nm by a single KrF laser pulse of 30 ns duration. The production mechanism has been studied by examining the effect of N2, Ar, and He as diffusion media in mixtures with thiophenol, and by varying the laser pulse energy and excitation wavelength. A qualitative summary of possible mechanisms that may be responsible for the experimental results is presented providing a framework for understanding the observed cluster growth. The measurements indicate an optimum set of parameters for growth of large clusters and also show that the cluster charge can be controlled by selection of the buffer gas mixture.
Statistical mechanics and cooperative chemical kinetics
Authors: Poland Douglas
Published: July 2002

Cooperative interactions between reacting species (nonideal effects) enhance the nonlinear character of chemical reactions kinetics. In the limit that the equilibration between the solvent and the reactants and products is rapid compared to the rate of conversion of reactants to products (and vice versa), one can use grand partition function techniques to calculate the probabilities of the appropriate reactant and product complexes with solvent. This leads in turn to the rate equation in terms of the activities of the particles; relations from equilibrium statistical mechanics allow one to convert back and forth between concentrations and activities. For the example of chemistry occurring in a one-dimensional lattice gas, a grand partition function-like quantity is used to generate the rate equation (a sum over all solvent cages weighted with the appropriate rate constants). A general form is given for the linearized rate equation in nonideal systems arbitrarily far from equilibrium. As expected, it is found that nonideality as well as distance from equilibrium can lead to instabilities and oscillations. As an example, we treat a simple association reaction (2x↔y), where the only nonideal effect is excluded volume. If we make the system open by allowing x to enter the system with a rate proportional to the number of vacant lattice sites (and y to leave the system with first order kinetics), then far from equilibrium this reaction shows oscillations (stable spirals) with and without the effect of excluded volume. We discuss the use of the angle between the eigenvectors (in the appropriate coordinate system) as a measure of the distance to the onset of oscillation.
Ground-state splitting of S-state ion Cm3+ in LaCl3
Authors: Liu G. K. , Beitz J. V. , Huang Jin
Published: July 2002

We report optical-spectroscopic studies of the ground-state splitting of the nominal S-state ion Cm3+ in LaCl3. A total splitting of 1.97±0.05 cm−1 for the 8𝒮7/2 ground state of this system has been measured for the first time. Both electric and magnetic dipole transitions were observed. Energy levels of all four Kramers doublets of the 8𝒮7/2 manifold were resolved by means of the site-selective excitation and emission spectra of 8𝒮7/2↔6𝒟7/2 and 8𝒮7/2↔6𝒫5/2 transitions. Our results show that the lowest component of the ground manifold is a μ=5/2 state dominated by M=‖±7/2〉 in contrast to a previously reported μ=1/2 state. The crystal-field model, established through analyzing optical spectra of the excited states, predicts a ground-state splitting with the right ordering but greater magnitude. In comparison with the lanthanide S-state ion Gd3+, much stronger spin–orbit coupling and reduced electrostatic interaction in concert with greater crystal field interaction significantly influence the characteristics of the ground state of the actinide S-state ion. Contributions from within the 5f7 configuration account for most of the observed splitting.
Unraveling of transition dipole and dynamical contributions to resonance Raman intensities
Authors: Johnson Bruce R. , Kinsey James L.
Published: July 2002

The short-time behavior of resonance Raman correlation functions and the shape of the nuclear potential for the ground electronic state is shown to determine the shapes of both the excited state potential and the transition dipole moment. This is demonstrated for models involving large-amplitude vibrational motion.
Fourier transform microwave spectrum of the propane–water complex: A prototypical water-hydrophobe system.
Authors: Steyert D. W. , Elrod M. J. , Saykally R. J. , Lovas F. J. , Suenram R. D.
Published: July 2002

The Fourier transform microwave spectrum of the propane–water complex (C3H8–H2O) has been observed and analyzed. This spectrum includes transitions assigned to propane complexed with both the ortho and para nuclear spin confirmations of water. The rotational constants indicate that the vibrationally averaged structure has all four heavy atoms coplanar, with the water center of mass lying on or near the C2 axis of propane, inside the CCC angle, 3.76(±0.02) Å from the propane center-of-mass, and 4.35(±0.02) Å from the methylene carbon. The projection of the electric dipole onto the a inertial axis of the complex (0.732 D for the ortho state and 0.819 D for the para state) indicates that one of the protons of the water subunit lies on the C2 axis of the propane monomer, which is also the axis connecting the subunit centers of mass. The small projection of the dipole along the b axis (0.14 D for the ortho state and 0.38 D for the para state) is most consistent with an equilibrium structure in which all three atoms of the water lie in the CCC plane of propane, with torsional tunneling about the hydrogen bond occurring on the same time scale as the overall rotation. The small internal rotation tunneling splittings that occur in the rotational spectrum of the propane monomer are not observed in the spectrum of the complex.
Quadratic steepest descent on potential energy surfaces. II. Reaction path following without analytic Hessians
Authors: Sun Jun-Qiang , Ruedenberg Klaus
Published: July 2002

A second order method is developed for determining steepest descent lines of potential energy surfaces by following steepest curves of successive local quadratic surface approximations. The basic principle is similar to that of a previously developed method where, however, the availability of analytically calculated exact Hessians was assumed wherever needed. By contrast, only the analytically calculated exact values of the energy and its gradient are used here and this difference entails marked changes in strategy. Applications to the Gonzalez–Schlegel and the Müller–Brown surfaces show that the method compares favorably with existing methods.
A comparison between the long-time self-diffusion and low shear viscosity of concentrated dispersions of charged colloidal silica spheres
Authors: Imhof A. , van Blaaderen A. , Maret G. , Mellema J. , Dhont J. K. G.
Published: July 2002

Measurements are presented of the long-time self-diffusion coefficient and of the low shear limiting viscosity of dispersions of charge stabilized colloidal silica spheres. Long-time self-diffusion coefficients were measured using fluorescence recovery after photobleaching (FRAP), the theory of which is presented and generalized to Gaussian laser beams. The particles, suspended in solutions of LiCl in dimethylformamide, interacted via a screened Coulomb potential, the range of which was varied through the ionic strength. Measurements were made up to volume fractions beyond freezing where a coexistence occurred between a colloidal crystal and a colloidal fluid. It is often speculated that the long-time self-diffusion coefficient and the low shear viscosity of a dispersion are related through a simple Stokes–Einstein-like relation, but this expectation is not confirmed by the experiments. A slightly modified relation, however, does seem to provide a reasonable empirical description of the data.