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  • 1
    Electronic Resource
    Electronic Resource
    Measuring the structure of etched silicon surfaces with Raman spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8055-8072 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the unenhanced, nonresonant surface Raman spectra of one monolayer of hydrogen bound to flat and stepped Si(111) surfaces prepared using a novel, aqueous fluorine etch. The orientation and normal mode composition of adsorbate vibrations are obtained from polarized, angle-resolved Raman spectra using a 3-layer dielectric model. This approach requires the experimental determination of both the anisotropy in the dynamic polarizability of the adsorbate bond and the effective dielectric constant in the vicinity of the adsorbate. The measured Si–H bond anisotropy is 0.263±0.028 in good agreement with gas phase measurements. The adsorbate dielectric constant is measured to be 3.78±0.20; this response is clearly nonlocal and predominantly due to polarization of the underlying silicon lattice. Using this technique, we find that the step dihydride on a Si[6(111)-(1¯1¯2)] surface is rotated 37°±4° from the surface normal in good agreement with the 31° predicted by ab initio cluster techniques, but significantly larger than the 12.5° predicted by pseudopotential slab calculations. In contrast to both theoretical predictions, the normal modes of this step dihydride display little concerted motion indicating that subsurface relaxation near the step edge reduces steric interactions much further than predicted. The observed anisotropic etch rates, evidenced by the production of atomically straight steps, are explained in terms of the measured distortion at the step edge.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468232
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  • 2
    Electronic Resource
    Electronic Resource
    Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 788-806 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Picosecond transient absorption measurements from 1000–295 nm are used to monitor the recombination dynamics of iodine after photodissociation in a variety of inert solvents. The high time resolution and signal-to-noise ratio of these measurements permits the development of a detailed model of this reaction, which should resolve disagreements over the time scales of geminate recombination and vibrational relaxation and over the role of excited electronic state trapping. Most of the atoms which undergo geminate recombination do so in ≤15 ps, in agreement with the predictions of existing molecular dynamics simulations. The subsequent vibrational and electronic energy relaxation of the recombined molecule is relatively slow and accounts for most of the transient absorption dynamics. The relaxing X-state vibrational population distribution is extracted with an approximate method using calculated spectra of the excited vibrational levels and is compared to recent models. Vibrational relaxation times vary from ∼15 ps near the middle of the ground state well to ∼150 ps for complete relaxation to v=0. The vibrational relaxation rates do not provide support for the predicted role of resonant vibration-to-vibration energy transfer to chlorinated methane solvents, but some evidence for this mechanism is found in alkane solvents. B-state predissociation times of 10–15 ps and A'-state lifetimes of 65–2700 ps are found depending on the solvent. Current theory is not able to satisfactorily explain the large variation of the A'-state lifetime in various solvents.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450578
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  • 3
    Electronic Resource
    Electronic Resource
    Erratum: Ground state densities from electron propagators: Optimized Thomas–Fermi approximation for short wavelength modes [J. Chem. Phys. 92, 6687 (1990)] (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4493-4493 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459725
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  • 4
    Electronic Resource
    Electronic Resource
    Generalized Brownian dynamics. I. Numerical integration of the generalized Langevin equation through autoregressive modeling of the memory function (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1304-1311 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method is presented for numerical integration of the generalized Langevin equation (GLE) based on modeling of the "random force'' as a discrete autoregressive process. This modeling procedure, drawn from digital signal processing and spectral estimation methods which have been used extensively in electrical engineering applications, provides for efficient evaluation of the friction integral in the GLE as well as for generation of a random force process with the desired spectrum. The method is shown, through comparison with molecular dynamics results, to be effective in simulating the force autocorrelation function of an iodine atom dissolved in Lennard-Jones (LJ) xenon. In a companion paper this method is applied in a simulation of the vibrational relaxation of I2 in LJ xenon at two very different densities and found to perform well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458140
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  • 5
    Electronic Resource
    Electronic Resource
    Recoil effects in surface dissociation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 7610-7617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using real-time wave packet propagation we consider the effects of lattice recoil, inelasticity and surface temperature in strongly activated dissociation reactions of diatomic molecules at surfaces. The energy diagram governing the dissociation, modeled as suggested by electronic structure calculations for H2 dissociation at Cu surfaces, consisted of an entrance channel barrier separated from the chemisorption region by a ridge, where dissociation takes place. Lattice recoil is simulated by coupling this "stiff-barrier'' PES to a harmonic oscillator. Calculations were carried out for masses and potential parameters appropriate to H2/D2 dissociation on Cu and N2 dissociation on Fe. Barrier recoil was found to suppress the dissociation probability as compared with its stiff-barrier value. The effect, marginal for H2 and D2 but pronounced in the case of N2, can be understood in terms of dynamical increases in the barrier width and height. Simulations where the N2–Fe barrier was excited in the initial state showed that the influence of surface temperature on the dissociation can be quite dramatic and led to a strong enhancement in the tunneling region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458198
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  • 6
    Electronic Resource
    Electronic Resource
    Ground state densities from electron propagators: Optimized Thomas–Fermi approximation for short wavelength modes (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6687-6696 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron densities obtained from a ground state path integral approach to density functional theory using a primitive Monte Carlo method display large statistical uncertainties when short wavelength fluctuations of the paths are considered directly. An optimized Thomas–Fermi approximation is developed to eliminate these degrees of freedom in a systematic and physically motivated fashion. Beyond improving the precision of the numerical results, this theoretical development permits a simple qualitative discussion of how the calculation pushes electron density into tunneling regions by iteratively renormalizing the Fermi wavelength appropriate to the remaining integrations which involve only long wavelength coordinates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458304
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  • 7
    Electronic Resource
    Electronic Resource
    A molecular dynamics study of the structure of a model Langmuir monolayer of amphiphile molecules (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5898-5908 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the thermodynamics and structure of a model monomolecular film of pentadecanoic acid on water. In this model the water is treated as a uniform polarizable continuum and the pentadecanoic acid molecules as chains of 15 pseudoatom with internal bond constraints, angle bending and torsional intramolecular interactions, and Lennard-Jones atom–atom intermolecular interactions. The only low pressure phases exhibited by this model at temperatures between 300 and 400 K are the very low density vapor phase and a well ordered condensed phase with a reciprocal density ≤23 A(ring)2 per molecule. This is in sharp contrast with experimental studies which show the existence of a stable liquid-expanded phase with a density of about 35 A(ring)2 per molecule at pressures 〈15 dyn cm−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455541
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  • 8
    Electronic Resource
    Electronic Resource
    Laser spectroscopy of NiO: The 3Σ− ground state (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2748-2753 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of the first rotational analysis of NiO are reported. Several bands in the green spectra region were resolved and analyzed using laser induced fluorescence (LIF). It was found that the symmetry of the well behaved ground electronic state is 3Σ− as was predicted by ab initio calculations. Moreover, the analysis showed that the angular momentum coupling scheme can be described as intermediate between Hund's coupling cases (b) and (c). A least-squares fit of the ground state combination differences gave re=1.627 A(ring).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455711
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  • 9
    Electronic Resource
    Electronic Resource
    Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4001-4011 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A(ring) diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454833
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  • 10
    Electronic Resource
    Electronic Resource
    On the structure of the liquid metal–polar adsorbate interface: Monte Carlo simulations (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2427-2434 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show, by use of self-consistent Monte Carlo simulations, that contact between a liquid metal and a polar adsorbate alters the longitudinal density distribution in the liquid metal side of the interface. In particular, the amplitude of modulation of the stratification of the metal in the interface is increased by the polar adsorbate, which will lead to changes in the work function, transverse electrical conductivity, etc. The results of calculations of the x-ray reflectivity as a function of angle of incidence on the liquid metal–polar adsorbate interface suggest that the change induced in the interface longitudinal density distribution of the metal is measurable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455037
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