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  • 11
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9733-9740 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Small-angle x-ray and neutron scattering are widely used techniques to study the structure of colloidal particles in the size range up to 100 nm. The indirect Fourier transformation technique is well established to obtain model free real space information, but the interpretation of the results is limited to cases where particle interaction can be neglected. The extended generalized indirect Fourier transform (GIFT) allows one to separate inter- and intraparticle effects, but needs models for the particle interaction. We present the application of three different models for the calculation of interaction effects of charged particles, represented by the structure factor. With this extension, useful real space information can be obtained by the GIFT method for solutions with volume fractions up to about 0.3 without any assumption for the shape of the particles. Only the interaction effects need a model assumption, and the parameters determined from this model can give some additional information. Simulations show that it is impossible to determine charge and ionic strength simultaneously. There exists another ambiguity between the parameter sets for charge, radius, and volume fraction, but we show how this problem can be overcome in most cases. The practical applicability of the method is demonstrated by means of the micellar system CTAB in different concentrations from 1% up to 20% and with varying amounts of added salt to screen the charges and change the particle shape. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
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  • 12
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 1477-1486 
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: The detailed description, the experimental demonstration, and the design of single-mode microwave cavity applicators are presented. These microwave applicators utilize an internally tuned, single-mode (or selective multimode) cylindrical cavity to focus and match microwave energy into cylindrical rod, disk-, or slab-shaped material loads. The combination of mode focus control and variable, internal cavity matching allows the efficient coupling of microwave energy into a wide range of solid material loads. The experimental operation of these applicators is demonstrated for low loss to lossy materials, which include nylon, water, and semiconducting silicon wafers. Overall microwave coupling efficiencies of 70%–95% were measured and experiments demonstrated the ability to precisely control material heating. The single-mode fields can be used to heat and diagnose during processing, and on-line process diagnosis was experimentally demonstrated using cavity perturbation theory. When using these cavity applicators, many system variables such as tuning control, cavity electric field strength, input, and reflected power are available as potential process system feedback signals for automated intelligent processing. These applicators can be used to carefully study the fundamentals of electromagnetic interactions of many materials. Applications include the electromagnetic heating of foods, semiconducting and biological materials, and the heating and curing of epoxy and composite materials.
    Materialart: Digitale Medien
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  • 13
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 3380-3392 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Experiments have characterized the operation of a LaB6 triode gun in a standard three-lens column of the type used for Gaussian electron-beam lithography and scanning electron microscopy. A series of images representing cross sections of the three-dimensional spatial distribution of current emitted from the gun is obtained by configuring the electron optics as a scanning confocal microscope. The gun acts as an immersion objective whose image is scanned by deflection coils and focused by the condenser lenses onto a pinhole transmission detector. Characteristics of the emission distribution include an emission image of the cathode surface situated between two distinct beam crossovers whose origin is either the apex (001) and {310} planes or the large {110} planes on the machined 90° cone angle of the cathode surface. Virtual objects are imaged when the back focal plane of the condensers falls inside the high-field region of the gun. The target axial brightness is dependent on gun excitation and angular acceptance angle. The temperature-dependent brightness of the cathode is used to determine its effective emission area, work function, and surface electric field. Beam positional stability of three-carbon-mounted LaB6 directly heated cathodes is measured. However, for measurement times ≤100 h the drift rate is found to be limited by thermal expansion of the movable anode assembly and not the particular cathode mounting technique.
    Materialart: Digitale Medien
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  • 14
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3530-3536 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Heat capacity of a pure rubidium cyanide crystal has been measured between 14 and 330 K with an adiabatic calorimeter. A first order phase transition was found at (132.30±0.05) K in agreement with the previous dielectric study. The transition enthalpy and entropy are 1280 J mol−1 and 10.2 J K−1 mol−1, respectively, of which (742±2) J mol−1 and (5.65±0.02) J K−1 mol−1 are the discontinuous parts. Under the assumption that the orientational disorder of the cyanide ion in the cubic phase is eightfold, a residual entropy of 5.43 J K−1 mol−1 was derived from the transition entropy and related to the head-to-tail disorder of the anions. The disorder causes a glass transition at 30 K. The enthalpy relaxation time determined by spectroscopic use of the adiabatic calorimeter agrees very well with the dielectric relaxation time extrapolated to the lower temperature, giving evidence for the frozen disordered state to be a dipolar glass.
    Materialart: Digitale Medien
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  • 15
    Digitale Medien
    Digitale Medien
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 2227-2229 
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Simultaneous measurement of both the conduction- and valence-band dispersion curves in single strained-layer structures is presented. These measurements rely on the application of recent observations regarding breaking of the usual selection rules for interband magnetoluminescence transitions in modulation-doped structures. Low-temperature magneto-luminescence data for three representative InGaAs/GaAs n-type single-strained quantum well structures are presented. For energies approaching 50 meV above the band gap, we find that the conduction band is parabolic with an effective mass of 0.071m0. Over the same energy range, the valence bands are highly nonparabolic.
    Materialart: Digitale Medien
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  • 16
    Digitale Medien
    Digitale Medien
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 1877-1878 
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Conduction-band and valence-band energies are presented for ternary III-V compounds in a novel way. These data are used to evaluate new material combinations for heterostructure devices.
    Materialart: Digitale Medien
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  • 17
    Digitale Medien
    Digitale Medien
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 1324-1326 
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report low-field electroreflectance (ER) spectra of an all-semiconductor multilayer optical mirror structure. The structure, consisting of alternating blocks of AlAs/Al0.5Ga0.5 As and Al0.5Ga0.5As/GaAs multiple quantum well layers, was grown by molecular beam epitaxy without wafer rotation. Thickness variations across the wafer produce a position-dependent reflectance spectrum. The observed line shape of the band-edge exciton depends on its wavelength position relative to the mirror spectrum and cannot be explained by ordinary ER theory, due to the rapidly varying background mirror reflectance. Computer simulations, using the matrix method to calculate the reflectance for different layer thicknesses and exciton energies, agree qualitatively with the data. A strong enhancement in ER response is predicted near the minima in the mirror spectrum. This enhancement is important in electo-optic reflectance modulators.
    Materialart: Digitale Medien
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  • 18
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 9642-9647 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Vibrational overtone excitation followed by laser-induced fluorescence (LIF) interrogation determines state-to-state rotational and vibrational relaxation rates of highly energetic acetylene molecules by argon. We initially excite a single rotational state of the vibration 3ν3 (three quanta of C–H stretch, Evib=9640 cm−1) and detect the state (3ν3, Jf=20) in rotational relaxation measurements or the state (ν1+ν2+ν3+2ν4, l=0, Jf=4) in vibrational relaxation measurements. The data, which we acquire under single-collision conditions, show that argon rotationally relaxes acetylene almost half as efficiently as acetylene itself but is relatively inefficient in vibrational relaxation. © 1994 American Institute of Physics.
    Materialart: Digitale Medien
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  • 19
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 6079-6085 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Electron stimulated desorption (ESD) of fragment ions in the energy range between 0 and 18 eV from SF6 adsorbed on rare-gas films (Kr, Xe) is reported. The ESD results are compared with previous experiments on dissociative electron attachment (DA) to gas-phase SF6. At energies characteristic for the respective rare-gas substrate strong resonant enhancements in the ESD yield of F− are observed. This enhancement is explained by the appearance of an "electron–exciton complex" in the rare-gas film (the analogue to the anionic Feshbach resonances in single atoms) which couples to the first dipole allowed excitation of the SF6 molecule. After electron and energy transfer, the highly excited SF6*− ion dissociates at the surface resulting in the desorption of F− fragments. At low electron energies (in the range from 0 to 0.6 eV) charging of the rare-gas film covered with SF6 is observed. From these experiments a charging cross section of 2.1(±1.8)×10−15 cm2 is derived. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
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  • 20
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 3472-3478 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We present the first photoacoustic absorption spectra of higher C–H stretching overtones in formaldehyde up to the visible spectral region. The analysis of the coarse rotational structure on the basis of asymmetric rotor simulations provides band centers up to the fifth overtone (vCH=6). A two dimensional normal coordinate model for the C–H stretching degrees of freedom explains the observed polyad structure of the spectrum using ab initio (MP2/6-311G**) potential energy and electric dipole functions. The observed intensity distribution reflects the increasing local mode character of higher overtone wave functions within the normal coordinate subspace. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
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