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  • 1
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A technique for selective characterization of the structure of free and buried thin-film interfaces by vibrationally resonant sum frequency generation spectroscopy is described. Manipulation of Fresnel coefficients by choice of film thickness on a reflecting substrate allows simultaneous optimization of the signal from the desired interface and minimization of the signal from other interfacial sources. This technique is demonstrated for the free polystyrene (PS)/air and the buried PS/spin-on glass interfaces. Our spectra show that the pendant phenyl group orientation is similar at the buried and free interfaces, with the phenyls pointing away from the bulk PS at each interface.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of a quantum-state-resolved study of the laser-induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption-laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the rest-atom localized, intrinsic surface state of the 7×7 reconstructed surface. It is suggested that the surface state hole drives the desorption by neutralization of a NO−δ adsorbate.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 477-490 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The coadsorption of CO and H2 on Rh(100) at 100 K has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), and temperature programmed EELS (TP-EELS). The preferred binding sites, long ranged order, and degree of segregation are dependent on the order of adsorption. When H2 is exposed to a CO preexposed surface, segregation of the surface species (atomic H and CO) is observed. The postdosed H2 causes isolated CO molecules to change from the top site to the bridge site, and compresses the c(2×2) CO islands that develop during the CO preexposure. When CO is exposed to a H2 saturated surface (one hydrogen per surface Rh atom) an intimately mixed c(2×2) CO and H structure is formed with all the CO molecules occupying the top site. Strong repulsive CO–H interactions in this mixed adlayer result in two new low temperature H2 TPD states. During the desorption of the lowest temperature H2 TPD peak, the c(2×2) LEED pattern streaks and the CO molecules shift from the top site to the bridge site. It is proposed that the preferred binding site for hydrogen in the c(2×2) bridge bonding CO structure is different from the fourfold hollow site preferred in the c(2×2) top bound CO structure. After the second H2 TPD peak, the remaining adsorbed H and CO segregate, and the CO regions are compressed. The compression is relaxed as the hydrogen desorbs. The development of the surface structures and their influence on the H2 TPD can be qualitatively understood in terms of precursor adsorption of both CO on H covered surfaces and H2 on CO covered surfaces, strong CO–H repulsions, and local poisoning of H2 dissociation by CO.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2569-2582 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The decomposition of methanol on a clean Ni(110) surface has been studied with temperature programmed reaction spectroscopy, low energy electron diffraction and temperature programmed electron energy loss spectroscopy: a new technique for the characterization of surface dynamic properties. Adsorption of methanol at 170 K results in the formation of a methoxy (CH3O) species. Near 270 K the methoxy decomposes into adsorbed hydrogen and carbon monoxide. For initial methoxy coverages greater than 0.20 monolayer (ML) the decomposition is rate limited by the desorption of product hydrogen. For initial methoxy coverages less than 0.20 ML the abstraction of the first methoxy hydrogen is rate limiting. This step is modeled by first order kinetics with an activation energy of 16 kcal mol−1 and a preexponential factor of 1012 s−1. A normal kinetic isotope effect (kH/kD〉1) is observed confirming that C–H bond scission is the rate limiting step at low coverages.
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  • 5
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The design, construction, and operation of a high-resolution electron energy-loss spectrometer incorporating a multidetector of 96 discrete anodes are described. In conjunction with temperature and pressure perturbations, this fast data-acquisition spectrometer allows studies of surface kinetics and reaction mechanisms by accumulating spectra in as fast as 2 ms. A comprehensive hardware and software system has been developed that permits experimental control and data acquisition with an LSI 11/23 microcomputer. A routine increase in data-acquisition speed of ≈10, when compared to a conventional electron energy-loss apparatus, has been achieved when the spectrometer is operated with 10–20-meV resolution.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 1469-1482 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The use of position-sensitive detectors (PSDs) has become common in many spectroscopies. The increased signal provided by the parallel detection of many spectral elements affords increases in sensitivity and/or time resolution. Recently, the use of a PSD to increase the temporal resolution of an electron energy loss (EEL) spectrometer has been demonstrated in our laboratory. This article summarizes the present development of EEL spectrometers and then presents a detailed review of four classes of PSDs in the context of their potential application to EEL spectroscopy (EELS). The four classes considered are (a) discrete channel detectors, (b) coincidence array detectors, (c) charge division detectors, and (d) optical detectors imaging phosphor screens. All four designs, as typically implemented, utilize microchannel plates (MCPs) as spatially imaging electron multipliers. The performance of MCPs is reviewed and the constraints that MCP behavior at the high count rates achievable in EELS places on acceptable PSD performance are outlined. The inability of present MCPs to maintain saturation gain at count rates above 1 MHz mm−2 implies that the PSD must tolerate significant gain reduction in the electron multiplier. The possibility of eliminating the MCP and using an electron-bombarded silicon gain mechanism is proposed. Such a device could, in principle, support count rates in excess of 100 MHz/channel, providing an order of magnitude increase in performance over the best existing detectors.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: State-resolved detection techniques have been used to characterize the ultraviolet photodecomposition dynamics of Mo(CO)6 on Si(111) 7×7 at 100 K. Details of the excitation/fragmentation mechanism including adsorbate energy transfer were examined by measuring the cross sections and the internal and translational energies of the photoejected CO from submonolayer through multilayer coverage regimes. The CO energy distributions are found to be independent of Mo(CO)6 coverage, and can be characterized by two components with markedly different mean energies. In contrast to the coverage independence of the measured energy disposal, the cross section was found to decrease by a factor of 3 from multilayer coverages to submonolayer coverages.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We demonstrate a method of acquiring near-field scanning optical microscopy data that allow for the construction of three different types of images from one data set: topographic, constantgap, and constant-height. This data set includes the topographic features of the surface and the optical response at various heights above the sample surface. Comparisons are made between the images recorded in this format and both conventional, constant-gap mode images, and pseudoconstant-height mode images constructed using a single retraction curve. Zmotion artifacts are identified by analyzing the optical intensity for a given image as a function of the sample topography. Using this procedure it is shown that significant z-motion artifacts exist in the constant-gap images of gold particles immobilized on a glass slide. These artifacts are avoided by constructing constant-height images.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2165-2169 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of H2CO on a clean Ni(110) surface have been studied by high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). At 95 K, H2CO adsorbs reactively, producing CO(a), CH3O(a), H(a), and small amounts of paraformaldehyde. Upon thermal treatment the CH3O decomposes to CO(a) and hydrogen while the paraformaldehyde reacts to form a surface intermediate which subsequently decomposes to CO2(g) and H2(g) at 350 K. Coadsorbed D2 is not incorporated into the CH3O(a) suggesting the methoxy is formed via a disproportionation reaction. Mixed paraformaldehyde and solid H2CO multilayers are formed at high exposures.
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  • 10
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A scanning near-field microscope that allows the fast acquisition of midinfrared absorption spectra is described. The microscope couples the nanoscale spatial resolution of a scanning probe microscope with the chemical specificity of vibrational spectroscopy. Key design elements of the microscope include a tunable broadband infrared light source; an infrared focal plane array-based spectrometer which allows parallel detection of the entire pulse bandwidth (200 cm−1); and a single mode, fluoride glass, near-field probe fabricated with a chemical etching protocol. Infrared transmission images of a micropatterned thin gold film are presented that demonstrate spatial resolution conservatively estimated to be λ/7.5 at 3.4 μm, in the absence of optical artifacts due to topography. Constant height mode images of a polymer nanocomposite demonstrate instrumental sensitivity to fractional transmission changes of 1×10−3. Near-field absorption spectra (λ=3.4 μm) of a 2 μm thick polystyrene film are presented which demonstrate the instrumental sensitivity required for high spatial resolution, near-field absorption imaging. These spectra are acquired in 2 s and indicate a film thickness detection limit for polystyrene of 200 nm. Measurements exploring the coupling between the infrared absorption magnitude and changes in tip–sample separation suggest that near-field absorption imaging is relatively insensitive to topographic artifacts.
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