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The dispersien-efthe volume plasmon of the valence electrons E (z) (< wave vector) is often used t o test the quality of a theoretically calculated dielectric function E(E, H). P F r o m the theoretical E (E, <) the energy-loss function -1m &-' (E, H) is calculated. The energy of the main maximum of this function is identified usually with E mission electron energy-loss (TEEL) measurements. The corresponding E (h) is obtained from scattering-angle dependent measurements. This energy can be determined experimentally directly from trans-P' P F o r some elemental and A ' ' $3' semiconductors E (c) curves have been P determined from T E E L measurements /1 to 5/. Generally for small lzl values (1z1< kc, kc cut-off wave number) a quadratic dependence was found ti2 -2 E (2) = E (0) +a; k P P with cx the dispersion coefficient. In the model of free electrons 3E F o ( =f 5E P, f 2 2 is valid. Here are E EF F e r m i energy. E ( k ) curves for G a P and I n P have not yet been published. F o s G a P only the dispersion coefficient o ( which has an extremely large value has been reported /4/. F o r I n P only E (0) = 1 4 . 5 eV is known from literature P = ( 4 n n e ti / m f l 2 free electron model plasmon energy, P, f , P /6/. 1 ) DDR-9200 Freiberg, GDR. 2 ) Permanent address: Zentralinstitut fur Elektronenphysik d e r Akademie d e r Wissenschaften d e r DDR, Hausvogteiplatz 5-7, DDR-1086 Berlin, GDR. 12 physica (b)

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The great importance of binary 111-V compounds as well f o r practical as for fundamental reasons led in recent y e a r s consequently t o the investigation of their ternary analogs, the AuBIvC; compounds. On one of them, CdGeP2, detailed modulation spectroscopic studies have been carried out in the energy range up to about 5 eV /1 t o 3/. Here we report on plasmon data (energy E and lifetime h/T of the volume plasmon), the dielectric function