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  • Wiley-Blackwell  (261)
  • American Institute of Physics (AIP)  (96)
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  • 1
    Electronic Resource
    Electronic Resource
    Excitation of chemical waves in a surface reaction by laser-induced thermal desorption: CO oxidation on Pt(100) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5002-5010 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: If a Pt(100) sample is kept at steady-state conditions of O2 and CO partial pressures and temperature which are similar to those for the occurrence of autonomous temporal oscillations in the rate of CO2 formation, then the surface will be largely covered by adsorbed CO which inhibits oxygen adsorption and keeps the catalytic rate low. Irradiation of a small spot with a high power laser pulse causes momentarily local thermal desorption of CO and creation of a reaction front which propagates as a chemical wave across the surface area, as was monitored by the excursion of the integral reaction rate and locally by means of a work function (=oxygen coverage) probe placed at several mm distance from the spot of irradiation. The velocity for wave propagation rises from about 2 mm/min at 480 K to 4 mm/min at 507 K and is not noticeably dependent on the partial pressures. The mechanism is closely related to that for self-sustained kinetic oscillations of this system and exhibits the typical features of trigger waves: Coupling between autocatalytic reaction and diffusion, as well as the occurrence of a refractory period during which the system is "dead,'' and of a threshold for the intensity of the excitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456740
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  • 2
    Electronic Resource
    Electronic Resource
    Polyparaphenylene under pressure: Optical absorption and vibrational modes (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1930-1934 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Optical absorption, Raman, and infrared (IR) spectra of undoped polyparaphenylene (PPP) have been studied under pressure up to 20 GPa at 300 K. PPP shows stable behavior, in marked contrast to the reactivity observed in trans-polyacetylene under pressure. The absorption edge of PPP shifts to lower energy with a total shift of 0.55 eV at 20 GPa. Raman spectra reveal a new band at about 820 cm−1 which becomes active above ∼5 GPa. At high pressure we find a considerable broadening of the infrared-active out-of-plane mode at 805 cm−1. The results are in part interpreted in terms of the larger interchain coupling induced by pressure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456035
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  • 3
    Electronic Resource
    Electronic Resource
    An electron diffraction study of alkali iodide vapors (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6053-6057 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures of NaI, KI, RbI, and CsI in the vapor phase were determined via standard electron diffraction counting techniques with nozzle temperatures ranging from 770–866 K. The data analysis yielded the structure parameters and the ratio of the monomeric to dimeric compounds. The mean amplitudes of vibration for the monomeric molecules were held at values based on microwave frequencies and the harmonic potential approximations. The corrected monomer distances re agree well with microwave-spectroscopy results. Rhombic geometries with bonded dimer distances ranging from 3.00 to 3.57 A(ring) and I–M–I angles that ranged from 95° to 102° were observed. The dimer mole fraction in the vapor varied from 3% to 18%. There was no significant amount of any trimeric species observed in the vapor. The bonded distance of KI was found to be equal to the interionic separation of the crystal. The observed dimer mole fractions were compared with various theoretical predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455419
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  • 4
    Electronic Resource
    Electronic Resource
    An electron diffraction study of alkali fluoride vapors (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6058-6063 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures of NaF, KF, RbF, and CsF in the vapor phase were determined via standard electron diffraction counting techniques with nozzle temperatures ranging from 798 to 1123 K. The reactivity of the fluorides combined with the higher temperature requirements necessitated a new nozzle design as well as modification to the oven. The data analysis yielded the structure parameters and the ratio of monomeric to dimeric compounds. The mean amplitudes of vibration of the monomeric molecules were held at values based on microwave frequencies and the harmonic potential approximation. The corrected monomer distances re agree well with microwave-spectroscopy results. Rhombic geometries with bonded dimer distances ranging from 2.08 to 2.70 A(ring) were observed. The F–M–F bond angles were found to be nearly constant, varying only from 82.2° to 85.5°. The dimer mole fractions in the vapor were observed to vary from 5% to 18%. There were no significant amounts of any trimeric species observed in the vapor. The observed dimer mole fractions were compared with existing measurements from velocity selection experiments and found to be in excellent agreement. The dimer mole fractions were also compared with various theoretical predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455420
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  • 5
    Electronic Resource
    Electronic Resource
    An electron diffraction study of alkali bromide vapors (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5477-5482 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures of NaBr, KBr, RbBr, and CsBr in the vapor phase were determined via standard electron diffraction counting techniques with nozzle temperatures ranging from 820–920 K. The data analysis yielded the structure parameters and the ratio of the monomeric to dimeric compounds. The mean amplitudes of vibration for the monomeric molecules were held at values based on microwave frequencies and the harmonic potential approximations. The corrected monomer distances re agree well with microwave–spectroscopy results. Rhombic geometries with bonded dimer distances ranging from 2.74 to 3.37 A(ring) and Br–M–Br angles that ranged from 85° to 101.6° were observed. The percentage of dimer in the vapor was found to vary from 7% to 18%. The most surprising result was the large value obtained for the KBr dimer bonded distance. It is substantially larger than predicted by current theories and is nearly as large as the interionic distance in the crystal. Thermodynamic calculations were performed to allow comparisons of the dimer mole fractions observed with values derived from existing structure models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453666
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  • 6
    Electronic Resource
    Electronic Resource
    A combined quantum chemistry and classical molecular interaction energy method for the determination of crystal geometries and energies (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 747-754 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical molecular solid crystal structure optimization presents many computational challenges. Calculation of both intramolecular interactions and intermolecular interactions are major obstacles. In this study, the intramolecular interactions are treated quantum mechanically and the intermolecular interactions are approximated by a force field, which is in part determined by a partial charge analysis of the quantum treatment. This combined approach, called the quantum coupled unit cell description (QCUCD) method, treats short-range and long-range intermolecular interactions with convergence-accelerated lattice sum techniques. QCUCD finds the internal molecular geometry; then crystal parameters are optimized until a fully consistent solution between the unit cell description and the quantum chemistry of the molecular electronic structure is achieved. The single molecule experiences the periodically repeating potential at one molecular site of the crystal. The solution of the electronic structure of the molecule under the QCUCD Hamiltonian determines the atomic partial charges that are positioned in a periodic array to establish the electrostatic contribution to the potential. The parameters for the Lennard-Jones interactions are taken from the force field of the widely used molecular simulation program AMBER as a convenient source for a large number of atom-based interactions. All geometric parameters for definition of the molecular crystal are optimized: the lattice constants, the orientational and positional degrees of freedom of the molecules at their crystal lattice sites, and the internal molecular geometry. QCUCD correctly determines the crystalline structures of the simple molecular solids N2, CO, and CO2 and also obtains the correct crystal packing orders of two different phases of N2 crystal in terms of crystal packing energies. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1424316
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  • 7
    Electronic Resource
    Electronic Resource
    Photodissociation of the dibromomethane cation at 355 nm by means of ion velocity imaging (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 6012-6017 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of the dibromomethane cation, CH2Br2+, have been studied by means of ion velocity imaging and time-of-flight mass spectroscopy methods at 355 nm. The dibromomethane cation is produced through the direct ionization of the neutral molecule with a pulsed 118 nm laser. The translational energy distribution shows that the CH2Br+ fragment is formed in highly vibrationally excited states with two distinguished dissociation channels following a parallel excitation from 2b2 to 3b2 of the parent ion. The broad fast speed distribution is fit with two Gaussian functions, from which a branching ratio of Br*(2P1/2) to Br(2P3/2) is determined as 2.2:1. The sharp peak with very slow speed was modeled with a Boltzmann distribution with a temperature of 300 K. This channel contributes ∼4.5% to the reaction and is proposed to proceed on the ground state surface following internal conversion. Ab initio calculations for both the parent and the fragment ions have been performed that strongly support the proposed dissociation mechanisms. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1402993
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  • 8
    Electronic Resource
    Electronic Resource
    The resonant Auger electron spectrum of C 1s−1π* excited ethene: A combined theoretical and experimental investigation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6666-6677 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The resonant Auger electron spectrum for ethene has been calculated with an ab initio approach using configuration-interaction energies and wave functions for the intermediate core-excited and final states. The transition rates were determined by the "one-center approximation." The role of vibrational relaxation on the line shapes was described by a moment method which considers the case of symmetric core holes and their localization due to the vibrational relaxation of the core-excited state. The core hole localization is investigated in some detail and is found to be extremely efficient in the C 1s−1π* excited state of ethene. Another property of the core-excited state is found to be the polarization of the valence electron density toward the core hole. We demonstrate this by using three different symmetric configuration interaction representations and one nonsymmetric Hartree–Fock representation for this state. A modified improved virtual orbitals method is described and employed to obtain virtual orbitals which give a compact description of this effect. The theoretical spectra obtained in this way are compared with a measured spectrum and assignment of the structures in the spectrum to electronic configurations is made. We find strong configuration mixing in the higher excited final states which is evidence for the breakdown of the one-particle picture. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481241
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  • 9
    Electronic Resource
    Electronic Resource
    The vibrationally averaged, temperature-dependent structure of polyatomic molecules. III. N2O (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 939-944 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The thermally averaged molecular structure of gas phase N2O has been measured at temperatures between 293 and 932 K. Excellent agreement with the predictions of force field calculations based on the variational wave function method and the moment method is found. The stretch–bend coupling constant in the force field is of particular importance in describing the temperature dependence of the vibrationally averaged structure parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449420
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  • 10
    Electronic Resource
    Electronic Resource
    Ethylene adsorption on Ge(100)-(2×1): A combined angle-resolved photoemission and thermal desorption spectroscopy study (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2768-2775 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ethylene adsorption on vicinal, single-domain Ge(100)-(2×1) has been investigated by thermal desorption spectroscopy (TPD) and angle-resolved photoemission (ARUPS) using linearly polarized synchrotron radiation. Thermal desorption experiments show that chemisorbed C2H4 desorbs from Ge(100) nondissociatively around 393 K with a high temperature shoulder which is tentatively assigned to step site desorption. The ethylene saturation coverage is strongly temperature dependent. Adsorption at 90 K saturates at 0.38 monolayer (ML), whereas adsorption at 170 K leads to a saturation coverage of approximately 1 ML. This behavior is explained by an adsorption barrier for coverages exceeding 0.38 ML. ARUP spectra for a dilute and the saturated ethylene monolayer reveal clear differences. Using photoemission selection rules a highly (C2v) symmetric adsorption geometry with a C–C bond axis parallel to the Ge–Ge dimer axis is found for the dilute layer; whereas a reduced C2 adsorption symmetry is found for the saturated ethylene layer. The comparison of photoemission spectra for C2H4 on Ge(100) and Si(100) shows that C2H4 is di-σ bound to the dangling bonds of a single Ge–Ge dimer. For two molecular orbitals, 1b3u and 1b2g, one-dimensional band structures with dispersion widths of 0.5 and 0.39 eV, respectively, along the Ge–Ge dimer rows are found which present a straightforward explanation for the observed symmetry reduction and adsorption behavior. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1384552
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