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  • American Institute of Physics (AIP)  (12)
  • 2000-2004  (4)
  • 1995-1999  (8)
  • 1960-1964
  • 1
    Electronic Resource
    Electronic Resource
    Absolute total and partial cross sections for the electron impact ionization of tetrafluorosilane (SiF4) (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 1170-1177 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on SiF4 from threshold to 900 eV using a time-of-flight mass spectrometer. Dissociative ionization was found to be the dominant process, although we found evidence of the presence of the SiF4+ parent ion in our experiment. The SiF3+ fragment ion has the largest partial ionization cross section with a maximum value of 4.3×10−16 cm2 at 90 eV. All other singly charged fragment ion cross sections are about one order of magnitude smaller at this impact energy. The cross-section values of the doubly charged ions with the exception of SiF2++ are about two orders of magnitude smaller. A comparison is made with available previously measured data. Additional measurements using a sector-field mass spectrometer revealed that all fragment ions are formed with excess kinetic energy. The experimentally determined total single ionization cross section of SiF4 is compared with results of semiempirical and semiclassical calculations and reasonable agreement is found. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1333018
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  • 2
    Electronic Resource
    Electronic Resource
    Quasi-elastic Raman scattering and free volume in isotropic pressure-densified atactic poly(methyl methacrylate) glasses (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1020-1028 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have made a series of isotropic pressure-densified atactic poly(methyl methacrylate) (PMMA) glasses with densities ranging from 1.1823 g/cm3 to 1.1963 g/cm3, in order to investigate how the glassy dynamics, measured by Raman scattering in the frequency range of 0.1–10 THz, depends on the thermomechanical history of the glasses. In particular, we investigate whether there is a correlation between the strength of the fast relaxation dynamics and the free-volume characteristics, such as the average free-volume size and the free-volume fraction. The fast relaxation strength is deduced from the quasielastic light scattering (QES) intensity; the free-volume size is measured with positron annihilation lifetime spectroscopy (PALS); and the free-volume fraction is determined from pressure–volume–temperature (PVT) data and the Simha–Somcynsky equation-of-state (EOS) theory. Our temperature-dependent Raman and PALS measurements show that both the QES intensity and the ortho-positronium (o-Ps) lifetime increase smoothly with the temperature. On the other hand, the relaxation strength of the densified glasses at room temperature is, within experimental error, independent of the density, whereas both the o-Ps lifetime from PALS and the free volume fraction from the PVT data decrease significantly with pressure densification. We therefore conclude that the fast relaxation intensity cannot be explained on the basis of free-volume quantities, contrary to a recently suggested correlation between the QES intensity and the free volume [V. N. Novikov et al., J. Chem. Phys. 107, 1057 (1997)]. On the other hand, the observed behavior is consistent with another recently suggested model [V. N. Novikov, Phys. Rev. B 58, 8367 (1998)] which attributes the fast relaxations to vibration anharmonicity. Moreover, we show that the measured increase in the refractive index of the pressure-densified glasses fits the Lorentz–Lorenz equation and compares well to predictions of the relative change in refractive index with formation pressure from our own as well as published PVT data. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480625
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  • 3
    Electronic Resource
    Electronic Resource
    Electron impact ionization of the SO2 molecule (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 211-218 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report measurements of the absolute cross sections for the electron-impact ionization of SO2 from threshold to 200 eV. Absolute cross sections for the formation of the SO+2 parent ions and of the SO+, S+, O+, and O+2 fragment ions were obtained independently in two different laboratories using two different experimental techniques with uncertainties ranging from ±18% to ±25%. The level of agreement between the absolute cross sections (at 70 eV) obtained by the two techniques ranges from about 10% for SO+2 and SO+ to 20% for (S++O+2) and O+, which in all cases is well within the combined error margins of the two measurements. The high resolution capability of the mass spectrometer employed in one experiment enabled the separation of the S+ and O+2 fragment ions, which are separated by only 0.017 76 atomic mass units (amu), for the first time. The single positive ion formation is the dominant process for all observed product ions. The total single SO2 ionization cross section obtained by the two techniques agreed to within 8%. A comparison of the experimentally determined total SO2 single ionization cross sections with calculated cross sections based on a modified additivity rule revealed agreement to within 20%. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469634
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  • 4
    Electronic Resource
    Electronic Resource
    New assignments in the UV spectroscopy of the small benzene–argonn clusters: The effects of a structure-selective vibrational predissociation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1676-1686 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, new experimental results using UV spectroscopy of the small benzene–Arn clusters are presented. We have found evidence for the vibrational predissociation of the S1 state of some of these species on a nanosecond time scale and we propose a new assignment for the UV spectral features of this system. This assignment is consistent with other experimental data and it accounts additionally for the previously reported spectral anomalies, in particular the ionization potential measurements of these species. The two-color R2PI spectra performed can thus be considered as nanosecond pump–probe experiments that allow us to estimate the relaxation rate of the S1state of these clusters. The vibrational relaxation is found to be strongly size and structure dependent: The two isomers [(1/1) and (2/0)] of the n=2 species exhibit different lifetimes differing by at least one order of magnitude. The size dependence of the relaxation process within the one-sided isomer series appears also paradoxical since the relaxation rates of the (3/0) and (4/0) species are found to be smaller than that of the (2/0). These properties are discussed in relationship with the symmetry of these species. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473321
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  • 5
    Electronic Resource
    Electronic Resource
    Electrochemical tuning of the lifetime of the CO stretching vibration for CO/Pt(111) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2438-2445 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used the large electric fields at the interface of a Pt(111) electrode and an acetonitrile electrolyte solution to tune the interaction between adsorbed CO and the Pt(111) surface. The electrode potential is varied over a 2.5 Volt range. As the electrode potential is made more positive, the CO vibrational frequency increases and the vibrational lifetime decreases. Over the potential range investigated, the tuning is about 35–40 cm−1 and the lifetime varies from ≈2.1 to ≈1.5 ps. Ab initio calculations performed for CO/Cu(100) predicted the opposite trend for the lifetimes for that system [M. Head-Gordon and J. C. Jully, Chem. Phys. 175, 37 (1993)]. Within an empirical model of nonadiabatic charge transfer [B. N. J. Persson and M. Persson, Solid State Commun. 36, 175 (1980)], our observations can be explained by a decreasing 2π-derived density of states of the Pt/CO complex at the Fermi level. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470938
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  • 6
    Electronic Resource
    Electronic Resource
    On the interaction between two oppositely signed, shielded, monopolar vortices (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 10 (1998), S. 3099-3110 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The formation two-dimensional dipolar vortices by the interaction between two shielded monopolar vortices with opposite vorticity, as shown in a numerical study by Couder and Basdevant,〈citeref RID="R1" STYLE="SUPERIOR"〉1 is investigated in detail, both experimentally, in a nonrotating stratified fluid and numerically by direct solutions of the two-dimensional Navier–Stokes equations. A comparative study between the laboratory experiments and numerical simulations is performed. The vorticity distribution measured in the early stage of the evolution in the laboratory is used as initial data for the simulations, and an additional damping term in the Navier–Stokes equations, that accounts for the vertical diffusion in the laboratory experiments, is used. The results show that, depending on the initial separation between the vortices, the shields of the monopoles are peeled off and indeed a compact dipole with a linear (ω,ψ)-relationship is formed, or when the monopoles are further apart the shields of the monopoles are perturbed and two tripoles are formed. The characteristics of the emerged dipole are analyzed and a dye visualization of the dipole formation is performed. A second, more general numerical study yields a relationship between the formation time of the dipole and the initial separation distance between the monopoles and it shows that the deshielding process can be explained by the domination of strain over vorticity. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869838
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  • 7
    Electronic Resource
    Electronic Resource
    Dynamics of a nonlinear dipole vortex (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 2220-2229 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A localized stationary dipole solution to the Euler equations with a relationship between the vorticity and streamfunction given as ω=−ψ+ψ3 is presented. By numerical integration of the Euler equations this dipole is shown to be unstable. However, the initially unstable dipole reorganizes itself into a new nonlinear dipole, which is found to be stable. This new structure has a functional relationship given as ω=αψ+βψ3−γψ5. Such dipoles are stable to head-on collisions and they are capable of creating tripolar structures when colliding off axis. The effects of increasing Newtonian viscosity on the nonlinear dipole is studied revealing that even though the nonlinearity is weakening, the dipole does not relax towards a Lamb dipole. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868470
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  • 8
    Electronic Resource
    Electronic Resource
    New and versatile ultrahigh vacuum scanning tunneling microscope for film growth experiments (1998)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 2696-2703 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: With the increasing popularity of the scanning tunneling microscope (STM) in surface science, many ideas for additional and new technical features have been proposed. The work herein contributes to this evolution with a special STM design. The STM described is part of an experimental apparatus for thin film growth investigations in ultrahigh vacuum. Besides the STM, the apparatus includes facilities for thermal desorption spectroscopy and Auger electron spectroscopy and a Kelvin probe for measuring dynamic work function changes. The Kelvin probe is optimized for gas adsorption experiments as well as for in situ film growth investigations during metal deposition. These added features combined with the STM and easy sample transfer yield a new powerful tool for in situ controlled preparation and extensive characterization of thin films. In the present work we describe the novel features of this STM and we demonstrate the efficiency of the whole system by giving a few representative results. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149001
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  • 9
    Electronic Resource
    Electronic Resource
    A versatile Kelvin probe for dynamic work function change measurements during gas adsorption and in situ film growth experiments (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 3866-3871 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In the present work, we describe a new Kelvin probe for dynamical work function change (ΔΦ) measurements in ultrahigh vacuum. The construction of the Kelvin probe is especially optimized to meet the experimental conditions for gas-adsorption experiments as well as for in situfilm growth investigations during metal deposition. This is realized by a new setup which enables a change of the geometrical orientation of the vibrating reference electrode with respect to the sample surface. The Kelvin probe combined with thermal desorption spectroscopy, Auger electron spectroscopy, and scanning tunneling microscopy facilities, forms a powerful tool for film growth analysis. The performance of the instrumentation is demonstrated with some representative test experiments for copper deposition on Pt(111). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148039
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  • 10
    Electronic Resource
    Electronic Resource
    Polarized luminescence in CdS/ZnSe quantum-well structures (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 85-87 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The photoluminescence from type II CdS/ZnSe quantum-well structures is found to be polarized with respect to the 〈110〉 directions with polarization degrees up to 20%. The absolute polarization direction is related to the interface bond directions in samples with differently prepared interfaces. The observations are explained by the detailed analysis of the epitaxial growth process and polarization sensitive luminescence experiments. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.126885
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