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  • 11
    Electronic Resource
    Electronic Resource
    The effect of interactive noise on the driven Brusselator model (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 416-427 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the effects of interactive noise, i.e., noise which is processed by the system, on the Brusselator, a nonlinear oscillator. The Brusselator is investigated for three types of motion: periodic, quasiperiodic, and chaotic. Fluctuations are imposed on the system variables (Type V noise). The average fluctuation amplitudes are chosen between 10 and 10 000 ppm (1%) and they are Gaussian distributed. The simulated time series are analyzed by autocorrelation functions, Fourier spectra, Poincaré sections, one-dimensional maps, maximum Lyapunov exponents, and correlation dimensions. As a result, noisy periodic and quasiperiodic motion can be distinguished from deterministic chaos if the fluctuation amplitude is sufficiently small. The generic structure of the attractor can be recognized when Lyapunov exponents or correlation dimensions are extrapolated to zero fluctuation amplitude. Quasiperiodic attractors in the Brusselator are obscured even by small amounts of noise. Chaos in the Brusselator, on the other hand, is found to be robust against noise. For periodic motion we show that points close to a bifurcation exhibit a stronger sensitivity towards noise than points far away. In the log–log plots for the correlation dimension we observed break points for noisy periodic and quasiperiodic motion. They separate the noise from the purely deterministic part of the motion. For increasing noise levels the break points move to higher length scales of the attractor. Break points were not found for chaos in the Brusselator nor in the Lorenz and Rössler models. In the Brusselator very large noise levels beyond 1% obscure the deterministic structure even of a chaotic attractor so that any clear distinction between chaos and noise induced (statistical) aperiodicity is no longer possible. Implications on experimental systems are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459541
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  • 12
    Electronic Resource
    Electronic Resource
    Atoms in molecules in external fields (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6989-7001 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of atoms in molecules is extended to the case where the molecule is in the presence of an electromagnetic field. This theory is based upon a generalization of quantum mechanics to an open system, as obtained through a corresponding extension of Schwinger's principle of stationary action. The extension of this principle is possible only if the open system satisfies a particular boundary condition, one which is expressed as a constraint on the variation of the action integral. This is the condition that it be bounded by a surface of zero flux in the gradient vector field of the charge density, the definition of an atom in a molecule. It is shown that this boundary constraint again suffices to define an atom as a quantum subsystem when the molecule is in the presence of an electromagnetic field. The mechanics of an open system and its properties are determined by the fluxes in corresponding vector current densities through its surface. As in the fieldfree case, the obtainment of these currents from the variation of the action integral is a direct result of the variation of the atomic surface and of the imposition of the variational constraint on its boundary. The currents in this case consist of a paramagnetic and a diamagnetic contribution, currents whose presence are a necessary requirement for the description of the properties of a system in the presence of external fields. The variational statement of the Heisenberg equation of motion obtained from the principle of stationary action is used to derive the Ehrenfest force and virial theorems for an atom in a molecule in the presence of external electric and magnetic fields. In this case, there are forces acting on the interior of the atom which arise from the magnetic pressures acting on its surface. It is shown that the molecular electric polarizability and magnetic susceptibility, like other properties, are rigorously expressible as a sum of atomic contributions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457315
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  • 13
    Electronic Resource
    Electronic Resource
    Effect of electron correlation on the topological properties of molecular charge distributions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3792-3804 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper reports on the effect of electron correlation on the topological properties of the charge density and its associated gradient vector and Laplacian fields. The properties of these fields define the atoms, their reactivity, and the structure of a molecular system within the theory of atoms in molecules. The singlet and triplet states of CH2, CF2, and SiH2 are investigated using a configuration interaction method which includes all single and double excitations with respect to suitable zero-order reference wave functions, together with a number of hydrocarbon molecules including unsaturated and geometrically strained systems for which the correlation is introduced via the generalized valence bond approach. It is found that the correlated charge distributions possess the same number and kind of critical points in both the ρ(r) and ∇2ρ(r) fields as are found for SCF charge distributions. Thus the topology of a charge distribution and the structure it defines are unaffected by the addition of Coulomb correlation. The quantitative changes in the properties of the charge density at the critical points in both ρ(r) and ∇2ρ(r) induced by correlation are found to be small in magnitude and to be more pronounced for shared or covalent atomic interactions than for systems with pronounced charge transfer between the atoms. The properties of the atoms in these molecules also exhibit correspondingly small changes in value.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453879
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  • 14
    Electronic Resource
    Electronic Resource
    Erratum: Oscillatory femtosecond relaxation of photoexcited organic molecules [J. Chem. Phys. 86, 2827 (1987)] (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 4241-4241 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453749
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  • 15
    Electronic Resource
    Electronic Resource
    Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) of Pt–Re and Pd–Re clusters (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6354-6363 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Extended x-ray absorption fine structure (EXAFS) studies were conducted on catalysts containing platinum and rhenium, or palladium and rhenium, on alumina. The atomic ratio of rhenium to either platinum or palladium was close to one in the catalysts investigated. The metallic entities in the catalysts were characterized by analyses of the EXAFS associated with the LI absorption edge of platinum, the LIII edge of rhenium, and the K absorption edge of palladium. It was concluded that Pt–Re and Pd–Re bimetallic clusters are present in the catalysts. However, the clusters have regions rich in rhenium and other regions which are rich in either platinum or palladium. Exposure of Pt–Re clusters to sulfur has little influence on their structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453465
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  • 16
    Electronic Resource
    Electronic Resource
    Reply to Comments of Li and Parr on Bader's definition of an atom (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3133-3134 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450977
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  • 17
    Electronic Resource
    Electronic Resource
    Analysis of complex molecular dynamics in an organic liquid by polarization selective subpicosecond transient grating experiments (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2269-2279 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a subpicosecond time resolved polarization selective transient grating (TG) investigation of pentylcyanobiphenyl (5CB) in its isotropic liquid phase. This system shows complex molecular dynamics with various contributions to the TG signal. With an excitation wavelength λexc=665 nm one induces an anisotropy in the sample from the electronic and nuclear Kerr effects. Exciting with λexc=575 nm generates an excited state grating because of two photon absorption. Solvent relaxation around the excited molecule on a few ps time scale is observed because it spectrally shifts the excited state–excited state transition which is monitored by the TG signal. In addition, radiationless relaxation leads to local heating of the sample in a spatial pattern which mimics the optical interference pattern which established the TG. The heating generates an acoustic standing wave. It is demonstrated that with a single TG setup, polarization selection allows separation and identification of all of the above-mentioned physical processes as they are characterized by different symmetries of the nonlinear susceptibility tensor χ(3).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457034
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  • 18
    Electronic Resource
    Electronic Resource
    Properties of atoms in molecules: Dipole moments and transferability of properties (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 1142-1152 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper uses the theory of atoms in molecules to investigate the origin of molecular dipole moments. The dipole moment is given by a sum over the net charge and first moment of every atom in a molecule. The first term leads to a charge transfer contribution μc, the second to an atomic polarization contribution μa. It is shown that both terms are, in general, of equal importance in determining both the static molecular dipole moment and the moment induced by a nuclear displacement. Models which imploy only point charges and corresponding bond moments which follow rigidly the nuclear framework, i.e., models which approximate μc and ignore μa, are shown to lead to results that are incompatible with the changes that are found to occur in a molecular charge distribution during a nuclear vibration. The dipole moment is shown to be another group property that is transferable between molecules in the normal hydrocarbons, This property, along with the net charge, the energy, the correlation energy (expressed as a functional of the charge density), and the atomic volumes are transferable for methyl and methylene groups because of a corresponding transferability of the distribution of charge over the basins of the atoms in these groups up to their surfaces of zero flux in the gradient vector of the charge density. Transferability of group properties is not a result of the transferability of individual orbitals or geminals, which necessarily have tails lying outside of the group in question. Atoms have no tails and their energy, as is known from experiment, can in certain instances be transferred between systems with changes of less than 1 kcal/mol. The properties of atoms in molecules can be determined experimentally and quantum mechanics predicts these properties, just as it predicts the properties of a total system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453294
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  • 19
    Electronic Resource
    Electronic Resource
    Oscillatory femtosecond relaxation of photoexcited organic molecules (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 2827-2832 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast relaxation of large organic molecules in solution has been investigated with the transmission correlation technique. The existence of damped sinusoidal decays after photoexcitation has been verified for several organic dyes. This behavior (the first such decay observed on a femtosecond time scale for any material) is explained as quantum beats. For all molecules examined, we have also observed fast initial exponential decays (〈100 fs) in addition to previously known picosecond processes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452032
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  • 20
    Electronic Resource
    Electronic Resource
    Visible absorption and magnetic-rotation spectroscopy of 1CH2: Analysis of the 1A1 state and the 1A1–3B1 coupling (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 1189-1205 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In methylene, the rotational levels of the a˜ 1A1 (0,0,0) and (0,1,0) vibronic states are so heavily perturbed by nearby rovibrational levels of the ground triplet state (X˜ 3B1) that standard rotation–vibration Hamiltonians do not provide a satisfactory fit or any indication as to which levels are strongly perturbed and which are less perturbed. Recent spectroscopic and theoretical work gives triplet state term values and the singlet–triplet energy difference with an accuracy of a few tens of wave numbers. Using these term values and ab initio spin-orbit matrix elements it is shown that all Ka=1, 3, and 4 levels of 1A1 (0,0,0) and all Ka=1 levels of 1A1 (0,1,0) are strongly perturbed by 3B1 (0,v2,0) levels with 2≤v2≤4. Individual levels in the other Ka stacks are also perturbed but most can be fit satisfactorily with a Watson Hamiltonian. The shifts between the observed term values and those calculated from the Watson Hamiltonian are reproduced in each Ka stack by a spin-orbit matrix element value only 5%–30% larger than the ab initio value. Over 60% of the measured term values show shifts greater than 0.2 cm−1. Shifts of the 322 and 000 levels can only be explained by interaction with levels in either (1,0,0) or (0,0,1) states; possible values for the vibrational frequencies of ν1 and ν3 of 3B1 are given. Molecular constants for 1A1 (0,0,0) and (0,1,0) are derived and quantitative estimates of spin-orbit mixing for individual levels of 1A1 (0,0,0) and (0,1,0) are given. From a chemical point of view singlet methylene is never in a pure spin state and always has some triplet character in its wave function. These data provide a basis for proper modeling of the kinetics of chemical reactions of "singlet'' and "triplet'' methylene species and their interconversion by "intersystem crossing.''
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452264
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