, ,BM~TY~.llhHbIX" IIOB~pXIIOCTIIbIX IIOJIRpHTOIIOl3 CorJIaCyeTCR C TeOpIleli.

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zum 85. Geburtstag gewidmet.I n a continuation of the study of surface polaritons with "stop points" on u-quartz, experimental ATR spectra for various thicknesses of the air gap between prism and a-quartz sample were@compared to theoretical ones. The relations between surface polaritons, stop points, anisotropic reststrahlen bands, longitudinal phonons, and bulk polaritons were studied. It is shown that the dispersion of bulk polaritons may be measured by ATR as well as the dispersion of surface polaritons. R,esults are presented for the dispersion of two bulk polaritons, one of which is Raman-inactive.In Fortfuhrung der Untersuchung von Oberflachenpolaritonen mit ,,Stoppunkten" an a-Quarz wurden experimentelle und theoretische ATR-Spektren fur verschiedene Dicken des Luftspaltes zwischen Prisma und u-Quarz-Kristall verglichen. Die Beziehungen zwischen Oberflachenpolaritonen, Stoppunkten, snisotropen Reststrahlenbanden, longitudinalen Phononen und Volumenpolaritonen wurden untersucht. Es wird gezeigt, dafl die Dispersion von Volumenpolaritonen neben der Dispersion von Oberflachenpolaritonen durch ATR gemessen werden kann. Zur Dispersion von zwei Volumenpolaritonen, von denen eines Raman-inaktiv ist, werden Ergebnisse mitgeteilt.

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The assignments of fundamental mode frequencies in LiNbO3 determined by IR-and Raman scattering-experiments show some characteristic differences. Comparative frustrated total reflection (FTR) -and Raman scattering-studies were carried out in order to find the origin of these discrepancies. The most serious deviation was due to the fact that in one case the ordinary phonon is observed by Raman scattering whereas the extraordinary mode is recorded only by FTR. It has been shown, on the other hand, that the oblique phonon dispersion technique did not provide arguments for the correct assignments. The main limit of the method is due to the fact that the influence of exciton states on polaritons hitherto is taken into account only by a constant high frequency dielectric 'function' (ε∞). The uncertainty in the choice of a suitable ε∞-value allows fittings of quite diverging sets of experimentally determined fundamental frequencies.