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Besides the known energy levels of Eu a+ at lattice sites of symmetry C z in YzO3, some transitions of this ion at lattice sites of symmetry Ca i in the same host crystal have been found and it was possible to construct the energy level scheme in part. Evidence for an energy transfer fl'om Eu 3+ (C 3 i) to Eu 3+ (C z) is given, the strength of which is rneasured as a function of the Eu203 concentration. The results lead to the assumption of a fixed distance R 0 for the interaction responsible for the energy transfer. A temperature-independent value of R0=8.7 ~ was found. The full explanation of the underlying quantum mechanical mechanism requires further experiments.

Polarized reflection spectra between 4.000 and 20 cm -1 and absorption spectra (at temperatures of liquid helium) between 2.000 and 650 cm -1 have been measured of the isomorphous crystals NaNO3, CaCOa, MgCO 3 (D~a) and of CaMg (CO3) 2 (C32i). The wavenumbers of the fundamental infrared transitions and of the twophonon-combinations have been determined. The combinationbands show a structure, which allows some insight into the wavevector dispersion of the phonon branches of these crystals. Selection rules have been worked out for several points in the interior and on the border of the Brillouin zone. By comparison with the spectra they give evidence for the presence of critical points of the phonon-branches for wavevectors parallel to the trigonal axis of the crystal.

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Kubische Kristalle sind gegeniiber elektrischer und magnetischer Dipolstrahlung sotrop, gegenfiber elektrischer und magnetischer Quadrupolstrahlung dagegen anisotrop. Man kann die Richtungsverteilung der Strahlungsintensit~t und der Polarisation bei Quadrupolstrahlung etwa als ,,optisch siebenachsig" bezeichnen, wobei die drei vierz/ihligen und die vier dreiz/~hligen Kristallachsen je emen Satz zusammengehSriger ,,optischer Achsen" bilden. Diese optische Symmetrie ist yon H. A. LORENTZ in einer h6heren N~herung seiner Elektronentheorie auch $iir den Brechungsindex hergeleitet worden. Hieraus wird geschlossen, dab der physikalische Sinn dieser h6heren NAherung die Mitberiicksichtigung yon Quadrupolstrahlung bei der,Dispersion ist. Die klassische Kristalloptik ist eine N/~herung allein ~tir Dipolstrahlung.