From the general polariton dispersion theory it can be shown that in an uniaxial crystal the frequencies of optical phonons * which are identical to those of the short wavelength polaritons depend on the angle 0 between the optic axis and the phonon wave vector. For <9=0 and &=jx/2 the phonons are exactly transversal or longitudinal so that they can be assigned to be of totally symmetric or twofold degenerate type. Careful measurements of the directional dispersion of all phonon modes of LiNb03 form a firm basis for a new complete assignment. 6 of the total number of 13 dispersion brandies previously given in the literature had to be reassigned.

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In a paper on the quantitative determination of sources of the electro-optic effect in LiNb03 and LiTaO, by Kaminow and Johnston, a complete assignment of all optical phonon modes in LiNb03 has been given. Assignments have earlier also been given by Schaufele and %cher, and Barker and Loudon. ' Because of important technical applications of the material in nonlinear optical experiments, as for instance second-harmonic generation and parametric oscillators, LiNb03 has frequently been used as a model crystal for calculations of different optical properties. In most papers of this kind the assignments given by Ref. 1 have been used as a fundamental set of parameters.Recent measurements concerning the directional dispersion of the extraordinary polaritons, however, showed that this set predicted dispersion branches which could not be experimentally verified. In extending this work, the directional dispersion brariches of all optical phonons have been carefully measured by Raman scattering in steps of 5 from 8=0 to 8= & m, where 8 denotes the angle between the phonon wave vector and the optic axis of the crystal. Spectral series have been recorded in backward as well as right-angle scattering for different polarizations of the incident and scattered radiation, and at room temperature as well as at liquid-nitrogen temperature.Furthermore, Raman spectra of the polariton region have been recorded in order to check the data.These experiments formed a firm basis for a new complete assignment. Our new assignments which fit the rightangle scattering and angular dispersion are reproduced in Table I, where the data can be compared with those given in Refs. 1-3. A detailed discussion of the experiments will be given. " %e want to thank the Deutsche Forschungsgemeiuschaft for financial support.lief. 1 E(T) 155

A dispersion relation for long‐wave phonons and phonon‐polaritons in monoclinic polyatomic crystals is derived. It can be brought in the form of a generalized Fresnel equation for the wave normal referring to the principal dielectric axes at a fixed frequency in the visible. Angular dispersion of coupled phonon‐ and polariton‐modes in the crystallographic plane with directional dispersion of the dielectric axes, and the possibility of experiments are discussed.

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.