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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 1170-1177 
    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
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1020-1028 
    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.
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 85-87 
    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.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 1118-1120 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report a reduction of inhomogeneous broadening in CdSe-related quantum wells in ZnSe by employing a growth technique that uses a CdS-compound source instead of the standard Cd elemental source for molecular-beam epitaxy. Assisted by the low sticking coefficient of sulfur and possibly an exchange reaction between S and Se, only a small S contamination is observed. A comparison with standard layers reveals an increase in quality and homogeneity by a strong reduction of the photoluminescence (PL) linewidth. Samples obtained by our method show extremely little lateral confinement as indicated by a lack of sharp single dot emission lines in micro PL and the absence of the extensive redshift observed in temperature dependent PL of fluctuating well potentials. © 2001 American Institute of Physics.
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