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  • 1
    Electronic Resource
    Electronic Resource
    Observation of growth modes during metal-organic chemical vapor deposition of GaN (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 3326-3328 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present real-time surface x-ray scattering measurements during homoepitaxial growth of GaN by metal-organic chemical vapor deposition. We observed intensity oscillations corresponding to the completion of each monolayer during layer-by-layer growth. The growth rate was found to be temperature independent and Ga-transport limited. Transitions between step-flow, layer-by-layer, and three-dimensional growth modes were determined as a function of temperature and growth rate. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.123333
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  • 2
    Electronic Resource
    Electronic Resource
    Statistical and nonstatistical effects in bond fission reactions of SiH2 and Si2H6 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4219-4229 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient implementation of microcanonical, classical variational transition-state theory based on the use of the efficient microcanonical sampling (EMS) procedure is applied to simple bond fissions in SiH2 and Si2H6 using recently constructed global potential-energy surfaces. Comparison is made with results of trajectory calculations performed on the same potential-energy surfaces. The predictions of the statistical theory agree well with and provide an upper bound to the trajectory derived rate constants for SiH2→SiH+H. In the case of Si2H6, agreement between the statistical theory and trajectory results for Si–Si and Si–H bond fission is poor with differences as large as a factor of 72. Moreover, at the lower energies studied, the statistical calculations predict considerably slower rates of bond fission than those calculated from trajectories. These results indicate that the statistical assumptions inherent in the transition-state theory method are not valid for disilane in spite of the fact that many of the mode-to-mode rate constants for intramolecular energy transfer in this molecule are large relative to the Si–Si and Si–H bond fission rates. There are indications that such behavior may be widespread among large, polyatomic molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460630
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  • 3
    Electronic Resource
    Electronic Resource
    Adsorbate effects on photoemission from Ag (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 3974-3977 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used pyridine and a number of related molecules to lower the workfunction of thin Ag films so as to compare the effects of various molecular properties on the photoemission process and on surface electronic modes. We find that a nitrogen lone pair electronic orbital is essential for a large threshold shift. Other molecular properties, such as dipole moment and polarizability have a very small effect. The films are rough, therefore we see strong photoyields at the energy of the surface plasma mode, 3.6 eV. There is also a peak in the photoyield at 3.85 eV, the energy of the bulk plasmon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460674
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  • 4
    Electronic Resource
    Electronic Resource
    Classical trajectory studies of the unimolecular decomposition of the 2-chloroethyl radical (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4077-4093 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectories have been employed in a study of the intramolecular dynamics and unimolecular decomposition of the 2-chloroethyl radical. A potential-energy surface was constructed by using the available experimental data and theoretical results. The following reaction channels were included in the study: ⋅CH2CH2Cl→CH2=CH2+⋅Cl, ⋅CH2CH2Cl→CH2=CHCl+⋅H. Mode-specific behavior was investigated by computing ensembles of trajectories for initial conditions (1) in which the normal-mode vibrations of the radical were assigned zero-point energies and a single C–H local stretch on the radical end of the system was excited, and (2) in which the normal modes were all excited so as to distribute the total energy uniformly throughout the radical. First-order rate coefficients were calculated both for the disappearance of the reactant and for the two chemically distinct reaction channels. The results do not indicate significant, if any, mode-specific effects. Energy transfer from and into local C–H stretching modes was studied. Relaxation of an initially excited C–H bond is observed to be irreversible and complete within about 0.6 ps.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458740
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  • 5
    Electronic Resource
    Electronic Resource
    X-ray reflectivity on perfluoropolyether polymer molecules on amorphous carbon (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3781-3793 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mass density profile from thin (∼25 A(ring)) layers of polymer molecules adsorbed on amorphous carbon thin films is determined from measurements of the specular reflectivity of x rays. Important information on the conformation of the polymer molecules is obtained from the density profile (average density as a function of position above the surface). The polymer is a perfluoropolyether (PFPE) with piperonyl end groups and has a molecular weight of 2000–3000 Daltons. Three samples are measured: a nonbonded sample with an approximately 25 A(ring) layer applied by dipping, a bonded sample with the layer applied by dipping but followed by a process to attach the molecule to the carbon, and a cleaned sample which is the nonbonded sample after the polymer had been removed. The density profile of the nonbonded polymer shows that the density within 10 A(ring) of the carbon surface is ∼1.6 gm/cc, which is smaller than the density in the region between 10 and 25 A(ring) (∼1.8 gm/cc). This indicates that the less dense piperonyl end groups are preferentially adsorbed near the carbon surface, which provides a possible explanation of the observation that the addition of the piperonyl end group to PFPE improves adhesion of the polymer to surfaces. The bulk-like density in the region between 10 and 25 A(ring) and the compactness of the layer suggest that the preponderance of molecules have both end groups preferentially adsorbed; consequently, the PFPE chains remain close to the surface. The data from the bonded and nonbonded polymer layers are essentially the same, which indicates the molecular conformation is not strongly affected by the bonding treatment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457836
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  • 6
    Electronic Resource
    Electronic Resource
    Computational studies of heterogeneous reactions of SiH2 on reconstructed Si(111)–(7×7) and Si(111)–(1×1) surfaces (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5021-5029 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of chemisorption and decomposition of SiH2 on Si(111)–(1×1) and recontructed Si(111)–(7×7) surfaces have been investigated using classical trajectories on a previously described [Surf. Sci. 195, 283 (1988)] potential-energy surface modified to yield the experimental bending frequencies for chemisorbed hydrogen atoms and to incorporate the results of ab initio calculations of the repulsive interaction between SiH2 and closed-shell lattice atoms. The Binnig et al. model is employed for the (7×7) reconstruction. Sticking probabilities are found to be unity on the (1×1) surface and near unity on Si(111)–(7×7). The major mode of surface decomposition on the (7×7) surface is by direct molecular elimination of H2 into the gas phase. Hydrogen atom dissociation to adjacent lattice sites is a much slower process and the chemisorbed hydrogen atoms thus formed exhibit very short lifetimes on the order of (1.13–10.6)×10−13 s. The calculated rate coefficients for these two decomposition modes are 3.4×1010 and 0.79×1010 s−1 , respectively. The rate coefficients for the corresponding reactions on the (1×1) surface are 6.6×1010 and 5.3×1010 s−1 , respectively. The rates on the (1×1) surface are faster due to the increased exothermicity released by the formation of two tetrahedral Si–Si bonds upon chemisorption compared to a single Si–Si bond on the (7×7) surface. Molecular beam deposition/decomposition experiments of SiH4 on Si(111)– (7×7) surfaces reported by Farnaam and Olander [Surf. Sci. 145, 390 (1984)] indicate that chemisorbed hydrogen atoms are not formed in the SiH4 decomposition process whereas the present calculations suggest that such a reaction, although slow, does occur subsequent to SiH2 chemisorption. It is suggested that energetic differences between SiH4 and SiH2 chemisorption are responsible for these differences.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457618
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  • 7
    Electronic Resource
    Electronic Resource
    Cis–trans isomerization of methyl nitrite (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2299-2307 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular vibrational energy redistribution and intramolecular cis→trans and trans→cis conversions have been investigated as a function of initial excitations of C–H local stretch modes and some bending normal modes in methyl nitrite by using quasiclassical trajectories. Harmonic and anharmonic valence force-field potential-energy surfaces with parameter values based on the available experimental and ab initio results were used in the calculations. The anharmonic potential gives rates of energy transfer and isomerization that are significantly faster than those for the harmonic potential. The rate of cis→trans isomerization is significantly larger than the rate of trans→cis. The energy flow out of excited C–H local stretch modes is irreversible and essentially complete in less than 0.5 ps.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457037
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  • 8
    Electronic Resource
    Electronic Resource
    Magnetic field effects on triplet exciton fission and fusion in a polydiacetylene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6642-6646 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied the origin and decay dynamics of triplet excitons in the conjugated polymer poly(4BCMU) in its sol(yellow) and gel(red) phases. Wavelength and intensity dependencies of the triplet yield show that the triplet exciton cannot be produced by excitation into the singlet exciton edge but only from higher lying states. The observed lifetime of the triplet state, coupled with the magnetic field dependence of the triplet state production and decay, indicate that the triplet state is created by a fission process from the excited singlet. The time and magnetic field dependence of the triplet exciton decay indicate that the triplet exciton decay in the red phase occurs by diffusive bimolecular fusion, but in the yellow phase the triplet exciton decay is nondiffusive. We postulate that the unimolecular decay of the triplet exciton in the yellow phase results from exciton pinning by conformational disorder.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456281
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  • 9
    Electronic Resource
    Electronic Resource
    Trajectory studies of unimolecular reactions of Si2H4 and SiH2 on a global potential surface fitted to ab initio and experimental data (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 741-750 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unimolecular decomposition dynamics of Si2H4 have been investigated using classical trajectory methods on a global potential-energy surface fitted to the results of ab initio calculations and the available experimental data. The required phase-space averages are computed using Metropolis sampling techniques. It is found that unless the parameters of the Markov walk are adjusted for each different type of atom present, extremely long Markov walks are required to adequately cover the phase space of the system. Microcanonical rate coefficients for the decomposition of Si2H4 into all open channels are reported at energies in the range 5.0〈E〈9.0 eV. The most important dissociation channel over this energy range is three-center elimination of molecular hydrogen leading to H2 Si=Si. At energies below 7.0 eV, the other channels are, in order of importance, Si–Si bond rupture, four-center H2 elimination, and simple Si–H bond rupture. At or above 8.0 eV, four-center H2 elimination replaces Si–Si bond rupture as the second most important decomposition channel. The energy dependence of the rate coefficients is well described by an RRK expression. Three-center H2 elimination involves a simultaneous rupture of both Si–H bonds whereas the four-center elimination is found to proceed by a hydrogen atom transfer process followed by H2 elimination. Except for a small propensity to form H2 with excess rotational energy, the energy partitioning among the products is nearly statistical. A comparison study of the decomposition of Si2H4 complexes formed by the recombination of two SiH2 molecules shows that the rates for both three- and four-center H2 elimination are in agreement with those computed using a statistical distribution of the same internal energy. The rate for Si–Si bond rupture, however, is significantly larger for Si2H4 complexes formed by SiH2 recombination than for Si2H4 molecules with the same internal energy randomly distributed. The decomposition dynamics of SiH2 on the global surface are also reported.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455197
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  • 10
    Electronic Resource
    Electronic Resource
    Classical dynamics study of intramolecular energy flow in benzene (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 1848-1853 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The flow of energy from highly excited CH overtones in benzene is investigated by quasiclassical trajectory methods for atomic motions restricted to the molecular plane. Various initial conditions and potential-energy surfaces (harmonic and anharmonic) are examined. The results are in accord with the rapid energy transfer rates observed in experiments. Comparisons are made with other theoretical studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451186
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