Phonon attenuation

FT-RAIRS measurements of CO have also been used to identity facets of oxide supported Cu particles [78, 82]. The low sensitivity of RAIRS on single crystal ZnO(OOOl) prevented the observation of adsorbed CO or CO2, despite their observation in NEXAFS [78], although the local metallic dielectric allowed CO to be observed on the Cu particles. There appear to be no examples of HREELS being used to carry out vibrational spectroscopy of adsorbates on oxide supported metal particles. A HREELS study of Ag on MgO(lOO) films [95] was used only to characterise the Ag induced attenuation in the substrate Fuchs-Kliewer phonons, and the appearance of the metal/oxide interfacial plasmon at higher energies. HREELS has also been used to characterise the oxide/oxide interface between NiO and thin film MgO(lOO) [96]. Similar measurements of substrate phonon attenuation were made in HREELS studies on Pt films grown on ZnO(OOOl) [97]. 

In Fig. 4.12 we compare the experimental results of the internal fiiction with the numerical calculations for frequencies at 535 Hz and at 15 MHz using Efjke = 10 K, tq = 10 s, and a rather small distribution width relative value in comparison with the plateau [24]. It is worth pointing out, that for PMMA, since the changes produced by the free parameters Eq, tq and oq compete with each other, it is not possible to find another set of values which can explain the results as well as we can. It is important to note that the internal friction data for both polymers indicate a much smaller thermally activated contribution to the phonon attenuation for PMMA than for PS, in spite of the fact that in the quantum tunneling regime both samples show similar results. This difference may influence the low-temperature heat release because the number of excited states that are available after the cooling process will be different. 

In order to image the interaction of phonons with internal structure in a specimen, scattering by thermal phonons can be used to attenuate the acoustic beam in a cryogenic microscope (Foster 1984). Experiments to test this confirmed that the cryogenic microscope was indeed able to detect on one surface of a sapphire disc the arrival of thermal phonons generated by a... 

Brillouin scattering measures the velocity and attenuation of hypersonic thermal acoustic phonons. A theory of Brillouin scattering from polymer blends is presented and illustrated qualitatively by several examples. The study of blend compatibility is illustrated for the system PMMA-PVFS. The detection of inhomogeneous additives is shown for commercial PVC film and cellulose acetate, and simultaneous measurements on separated phases are presented for Mylar film. The main purpose of the paper is to stimulate further work in a potentially promising field. 

Surface biphonons could be investigated, for example, by the attenuated total reflection (ATR) method. In contrast to RSL by polaritons, this method is effective, as is well known, both for crystals with and without inversion center. In this sense, it is a more universal method. In conclusion we point out that in degenerate semiconductors Fermi resonance with plasmons (47) is also possible along with Fermi resonance with phonons and polaritons. The spectrum of plasmophonons has been measured in many semiconductors by the RSL method (see, e.g. Mooradian and McWhorter (48)). 

Above about 0.5 GHz, Brillouin scattering provides an elegant technique of measuring phonon mean free paths (which are inversely proportional to the sound attenuation coefficient) by evaluating the width of the Brillouin lines [53]. However, as a result of finite spectrometer resolution and contrast, this method is limited to aerogel densities above 180 kg/m. ... 

In BS, the energy transfer between photons and phonons is very small (5x10 Hz/5xl0 " Hz=10 ) and hence ki and ks have the same length. Provided, acoustic attenuation is small (Tosc f2) the phase-sound velocity can be calculated from simple geometric arguments. Equations (15, 16) yield Eq. (17), where n is the refractive index of the sample, for the sound velocity v for the longitudinal mode. Equivalent equations hold for other polarizations. 

Laser induced phonon spectroscopy has been used to study the ultrasonic properties of nematic sidechain LCEs [47, 48], It has been shown that both the ultrasonic velocity and the attenuation show a strong anisotropy and it was concluded that the coupling between density changes and reorientation-al motion of the sidechains takes place on a time scale of 10" -10" s [47]. 

Ultrasonic experiments using laser induced phonon spectroscopy have been performed in a nematic liquid single crystal elastomer [48]. The experiments reveal a dispersion step for the speed of sound and a strong anisotropy for the acoustic attenuation constant in the investigated frequency range (100 MHz -1 GHz). These results are consistent with a description of LCEs using macroscopic dynamics [54-56] and reflect a coupling between elastic effects and the nematic order parameter as analyzed in detail previously [48]. 

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