Raman spectra of anatase have been observed in natural and synthetic crystals. Both crystals show the same spectral features. The Raman band occumng at 516 cm-' at room temperature is split into two peaks centred at 519 cm-' and 513 cm-' at low temperature (73 K). The six Raman active fundamentals predicted by group theory are all observed and assigned. The spectra are analyzed by a simple model considering only short-range forces and the calculated vibrational frequencies are in good agreement with the observed Raman frequencies.

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Coherent effects in synchrotron radiation (SR) have been observed for the first time from 180-MeV short electron bunches of 2.5 mm using the Tohoku 300-MeV Linac. The intensity of the coherent SR was about 10 5 times as strong as that of incoherent SR in the wavelengths of 0.4-2.2 mm. This enhancement factor corresponds to the number of electrons in a bunch. The SR intensity showed a quadratic dependence on the electron beam current. The radiation was mainly polarized in the orbital plane. The possibility of induced rf in a vacuum chamber was excluded experimentally.

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The far-infrared absorption for two types of silica glasses ͑containing р1 ppm and ϳ200 ppm of OH͒ has been quantitatively investigated in the region 10-100 cm Ϫ1 at room temperature. An absorption coefficient ␣͑͒ increased with increasing OH content and a broad peak on a plot of ␣()/ 2 vs , corresponding to a ''boson peak'' shifted from 41 to 36 cm Ϫ1 . The OH-related absorption increase ⌬␣͑͒, showed a monotonic increase with frequency in contrast to that previously published. The rate of the absorption increase ⌬␣͑͒/ ␣͑͒ showed a rapid decrease with frequency obeying a power-law ϰ Ϫ1.7 between ϳ17 and 51 cm Ϫ1 , whereas it decreased very slowly below ϳ17 cm Ϫ1 . It is suggested on the basis of a noncontinuous network model for the glass that OH ions are not uniformly distributed in silica glass. The light-vibration coupling coefficient determined experimentally is briefly discussed by some models proposed before. ͓S0163-1829͑98͒03009-4͔It is well known that glasses exhibit physical properties significantly different from crystalline solids, particularly in the low-energy region 1ϳ10 meV, for example, the excess of the low-frequency vibration density of states ͑VDOS͒ not described by the Debye approximation, low-temperature excess heat capacity, plateau in low-temperature thermal conductivity, low-frequency light scattering, far-infrared ͑FIR͒ absorption, etc. 1,2 These anomalous and universal properties are thought to be related to intermediate range order in glasses, but its origin is not yet clear. Silica glass is the most representative and probably most widely studied glass in amorphous solids. The low-energy properties in silica glass, and the effects of OH content on bulk properties such as dielectric constant, refractive index, density, elastic behavior, and thermal conductivity have also been studied. 3-5 Water in silica glass plays an important role and is associated with differences in physical and structural properties.Although the FIR absorption measurements on silica glass have been extensively made, 2,6-8 there are still discrepancies as to the frequency dependence of absorption coefficients ␣͑͒, in particular in the low-frequency region below ϳ30 cm Ϫ1 , and as to peak values in ␣()/ 2 vs plot, corresponding to ''boson peaks.'' Stolen 2 measured FIR absorption and low-frequency Raman scattering in SiO 2 , GeO 2 , and B 2 O 3 glasses, and indicated a similarity between the FIR absorption and Raman scattering in glasses. Hutt and co-workers 6 measured the FIR absorption on Spectrosil WF ͑a few ppm of OH͒ and Spectrosil B ͑ϳ1200 ppm of OH͒ at room temperature, 200 K, and 80 K using a FIR laser at intervals between 20 and 100 cm Ϫ1 ͑see Fig. 1, where open squares are for Spectrosil WF, plus signs for Spectrosil B͒. They found that the FIR absorption of Spectrosil WF was independent of temperature, and the existence of OH in silica glass increased the FIR absorption. They furthermore reported that this OH-related FIR absorption decreases with decreasing temperature. Ahmad 9 showed a similarity between...