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  • 2000-2004  (3)
  • 1985-1989
  • 2002  (3)
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  • 2000-2004  (3)
  • 1985-1989
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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 92 (2002), S. 1555-1563 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photoacoustic spectroscopy is an absorption spectroscopy technique that is currently used for low-level gas detection and catalyst characterization. It is a promising technique for chemical analysis in mesoscale analysis systems because the detection limit scales favorably with miniaturization. This work focuses on the scaling properties of photoacoustic spectroscopy, and on the miniaturization of gas-phase photoacoustic detection of propane in a nitrogen ambient. The detection system is modeled with a transmission line analogy, which is verified experimentally. The model includes the effects of acoustic leaks and absorption saturation. These two phenomena degrade the performance of the photoacoustic detector and must be controlled to realize the scaling advantages of photoacoustic systems. The miniature brass cells used to verify the model employ hearing aid microphones and optical excitation from a mechanically chopped, 3.39 μm He–Ne laser, transmitted into the cells with an optical fiber. These cells are able to detect 10 ppm of propane in nitrogen (a signal level of ∼1 Pa/W). We also describe the development of a miniaturized photoacoustic system formed by microfabrication. In this case, the pressure-driven deflection of the detection membrane is measured optically. These systems show that photoacoustic detection may be inappropriate for systems with large variations in gas concentration because of absorption saturation and changing gas acoustic properties. Nevertheless, photoacoustic spectroscopy is a promising technique for the analysis of dilute mixtures in miniature chemical systems. © 2002 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 117 (2002), S. 3711-3714 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The optical response of Li4+ and Li7+ exhibits three large maxima at photon energies between 1.9 and 3.6 eV, on which a fine structures could be resolved that has a peak to peak distance of about 30–40 meV. A high level ab initio calculation of Li4+ shows that this feature is due to vibrational structure of the excited electronic state. It is pointed out that the lifetime of the excited states exhibits a dramatic shortening with increasing particle size. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford UK : Blackwell Publishers
    The @journal of child psychology and psychiatry 43 (2002), S. 0 
    ISSN: 1469-7610
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: Background: An atypical EEG pattern of frontal brain activation has been found in infants and adults with emotional disorders. Eighty-two 8-year-old children and 56 11-year-old children were examined with regard to the following questions: 1. Do children who are diagnosed with anxiety disorder exhibit an atypical pattern of frontal brain activation? 2. Can this pattern be demonstrated in children of different ages? and 3. Are there gender differences in these patterns similar to those that have been demonstrated in adults? Baseline EEG activity was subjected to power spectral analysis. Results: In 8- and 11-year-old anxious girls, the well-known pattern of greater right than left frontal activation emerged that has been found previously in internalizing preschoolers. Healthy girls showed no frontal asymmetry at 8 years of age, and a greater left than right frontal brain activation at 11 years. In contrast, healthy boys demonstrated a significantly greater right than left frontal activation, whereas anxious boys displayed no frontal asymmetry at the age of 8, and a greater left than right frontal activation at the age of 11. Conclusions: Children suffering from anxiety disorders exhibited a significantly different pattern of frontal brain activation than healthy children without any lifetime diagnosis of mental disorder. Distinct gender differences in frontal activation were found in anxious as well as in healthy children. This could be demonstrated in two samples of different ages.
    Type of Medium: Electronic Resource
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