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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 3220-3236 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thin epitaxial single-crystalline B-type CoSi2 films (twin-oriented) have been grown in ultrahigh vacuum by stoichiometric codeposition of Co and Si on slightly misoriented (0.1°–0.3°) Si(111) substrates. The microstructure as well as the nature of interfacial defects has been investigated in detail by transmission electron microscopy. The defect structure is found to depend closely on the initial deposition parameters, annealing temperature, and the topography of the Si substrate. It will be shown that even during the early stages of layer growth, loss of coherence is obtained and lattice strain already starts to occur with the introduction of misfit dislocations with Burgers vector b=a/2〈110〉 inclined to the interface or with Burgers vector b=a/6〈112〉 parallel to it. It is demonstrated that ultrathin CoSi2 films with thickness of about 1 nm grown on slightly misoriented substrates with parallel surface steps, exhibit quite different defect structures at annealing temperatures between 300 °C and 550 °C. Control of the dislocation density has been obtained by applying a two-step growth procedure. CoSi2 layers grown to a thickness 〈hc (4–5 nm) exhibit line defects with Burgers vector b=a/6〈112〉 associated with interfacial misorientation-related steps. Above this thickness additional dislocations in the three equivalent directions are formed, indicating biaxial strain relaxation. In addition, calculations of the critical thickness hc of biaxial strain relaxation based on thermodynamic equilibrium theory are presented. It is shown that the observed critical thickness hc is in qualitative agreement with theoretical predictions.
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A family of resistivity curves with carrier concentrations approaching the metal insulator transition is obtained in a single sample of the dilute magnetic persistent photoconductor Cd0.91Mn0.09Te:In. In zero applied magnetic field, these curves exhibit a cross-over from an exp[(T0/T)1/2] for variable range hopping with interactions to an exp(EH/T) form upon reducing temperature. The energy EH is associated with a hard gap in the density of states which is magnetic in origin. All of the data for different carrier concentrations are shown to scale on to a single curve.1 The localization length is found to have the same critical dependence as the electronic part of the dielectric constant which has been previously measured.2 In a magnetic field, the hard gap is suppressed, and only variable range hopping is observed. The orientation of the Mn spins by the carriers contributes both to the localization (bound magnetic polaron) and is the relaxation responsible for the hard gap. A large peak in the magnetoresistance is also understood within this model. The localization length ξ and hence T0=2.8e2/κξk and the resistivity are field dependent. The magnetic field first aids the BMP formation and then destroys it. The result is first a peak and then in some cases negative magnetoresistance.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 3082-3091 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study a model of the CO oxidation on percolation clusters which represent the surface of our system in the Monte Carlo simulation. We observe two phase transitions where the surface is completely covered (poisoned) by one species (in our system CO or O). These phase transitions are described by their order and the values of the mole fraction yCO of CO in the gas phase at y1 (O poisoning) and at y2 (CO poisoning). The interval (y1,y2) represents the reactive regime. The influence of the occupation probability p for generating a spanning cluster on the underlying square lattice, of the diffusion of CO and of the lattice size on the value and the character of the kinetic phase transitions is studied. Increasing p leads to a shift of y2 to larger values of yCO for all ratios of adsorption to diffusion events but the value of y1 is increased to a larger value of yCO only if diffusion is not allowed. In the case of diffusion the value of y1 is maximally independent of p. A change in the character of the phase transition at y2 from first order on regular lattices to second order is observed on percolation clusters (p≥pC=0.592 75). Here pC is the percolation threshold on the square lattice. The character becomes again first order if diffusion is allowed or if p exceeds 0.97. The observed effects may be understood as a result of the different ramification of the percolation clusters which strongly depends on p. The cluster structures are characterized by pair correlation functions. The lattice size shows for p near unity no significant effect on the values of y1 and y2 but its influence increases with decreasing p. We will show that the fractal dimension of the surface is not a useful quantity for predicting the positions and the character of the phase transitions for the reaction system studied here.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a theoretical and experimental investigation of the emission spectrum of dissociating water after excitation in the second absorption band (X˜ 1A1→B˜ 1A1). The calculations are performed in the time-dependent wave packet formalism employing an ab initio potential energy surface. All three degrees of freedom (the two OH stretching modes and the HOH bending mode) are taken into account. The B˜ 1A1 potential energy surface depends strongly on the HOH bending angle which leads to very fast opening of this angle after the water molecule is promoted to the excited electronic state. As a consequence, we observe, both experimentally and theoretically, the excitation of high bending states in the X˜ ground state. According to the wave packet study the emission spectrum is determined in the first ten femtoseconds of the motion in the excited state. The agreement with the measured spectrum for an excitation wavelength of 141.2 nm is good.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 3716-3720 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The closely related Cs(1), Cs(2), and C2v(3) structures of CH5+ have been reinvestigated with high level ab initio theory through the coupled cluster with single and double substitutions (CCSD), and CCSD with perturbatively included connected triple excitations [CCSD(T)] levels, employing a triple-ζ plus double polarization functions basis set, with f-functions on carbon as well as d-functions on the hydrogens [TZ2P(f,d)]. Vibrational frequencies have been computed up to TZ2P+f CCSD; the inclusion of f-functions on carbon is critical for the configuration interaction with single and double excitations (CISD) and coupled cluster methods using the triple-ζ basis sets. The changes in geometries between the CISD and CCSD levels are very small, e.g., the C–H bond lengths vary by at most 0.005 A(ring). Thus, the optimizations are essentially converged within theoretical limits. The differences in energies of 1, 2, and 3 decrease and essentially vanish at the most sophisticated levels when the zero point vibrational energy corrections are applied. Hence, there is essentially no barrier to complete hydrogen scrambling.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8793-8804 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio calculations are used to provide bond lengths, harmonic frequencies, and dissociation energies of low-lying electronic states for LiX, LiX+, and LiX− (with X=Li through F and Na through Cl). Most of these species represent hitherto experimentally unknown molecules or ions, which provides the focus of the work presented here. All of these species are stable to dissociation and the anions are stable to loss of an electron. Differences among the electronic structures of the valence isoelectronic LiX; and HX, LiX+, and HX+; and LiX− and HX− species are analyzed. Optimized geometries, dissociation energies, ionization potentials, and electron affinities were calculated for the following ground states of the respective species: 1Σ+ for Li2(1Σ+g) LiNa, LiBe+, LiBe−, LiMg+, LiMg−, LiF, LiAl, LiS−, and LiCl; 2Σ+ for Li+2(2Σ+g), Li−2(2Σ+u) LiBe, LiB+, LiF−, LiNa+, LiNa−, LiMg, LiAl+, and LiCl−; 2Πr for LiB−, LiAl−; 2Πi for LiO, LiF+, LiS, and LiCl+; 3Πr for LiB, LiC+, and LiSi+; 3Σ− for LiN, LiO+, LiSi−, LiP, and LiS+; 4Σ− for LiC, LiN+, LiN−, LiSi, LiP+, and LiP−; and 5Σ− for LiC−.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 2916-2926 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time-domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210〈T〈320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130〈T〈160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet- and probably singlet-initiated ET routes can concurrently be detected, and the free energy of the charge-separated state is estimated from the spectral dynamics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 2032-2037 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A cellular-automaton model for the diffusion on a lattice is introduced. The spatial and temporal correlations that appear in the model are compared with the results of the correlated random walk, which has been obtained by Monte Carlo simulations. Very good agreement was found between these completely different simulations for the spatial correlations. The temporal correlations are quite different because of the nonequivalent temporal evolution of the two models. This new cellular automaton model is used to describe a reaction-diffusion system that represents the catalytic oxidation of CO on a metal surface.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 3945-3951 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the results of all-electron relativistic and nonrelativistic ab initio calculations on ionization potentials, electron affinities, and excitation energies for the gold atom and the gold hydride molecule. Values obtained from selecting multireference configuration interaction (CI) methods, full-class singles–doubles multireference CI, averaged coupled pair functional (ACPF), and various Green's function methods {outer valence Green's function (OVGF), algebraic diagrammatic construction in third order [ADC(3)]} are compared and generally found to be in good agreement. Relativistic effects are taken into account by means of the spin-free no-pair Hamiltonian obtained from a second-order Douglas–Kroll transformation. The influence of relativistic effects and electron correlation on the investigated properties is discussed. As far as experiments are available, our calculations are in good agreement with the measurements. Our best estimate for the ionization potential of the gold atom is 9.08 eV (experiment 9.22 eV) and 2.28 eV for the electron affinity (experiment 2.31 eV).
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 1271-1278 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vertical and adiabatic electron affinities and vertical ionization energies are presented for the clusters Sin, n=3,10 obtained from ab initio calculations with large basis sets. Electron correlation effects are taken into account by configuration interaction (CI) and Green function techniques. The clusters exhibit a complex behavior upon electron attachment. For each cluster there are several affinity states which result from capture of an electron into different orbitals. In some cases shake-up affinities are calculated which are positive. Si3 C2v is found to have at least three, Si4 D2h four, Si5 D3h two, Si5 C4v one, Si6 C2v three, Si7 D5h two, Si8 C2h five, Si9 Cs four, and Si10 C3v two affinity states (vertical processes, Sin+e−→Sin−+hν). The effects of electron correlation on the electron affinities are very large. In several cases the differences between the adiabatic and vertical electron affinities are exceedingly large amounting up to 1.5 eV.
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