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The photovoltaic current density in crystals without a center of symmetry depends bilinearly on the components of the light polarization vector via a third-rank tensor. For LiNb0,:Fe and LiNbO,: Cu the tensor properties are confirmed and three of the four non-vanishing components determined. The dependence on impurity concentration and on photon energy is also investigated.

I t is sho\\n from optical absorption nieasurements that under X-ray irradiation a t low temperatures Cu+ ions in Cu-doped KCI, KBr, and K I crystals trap electrons and holes. During this process Cu" and Cute centers are formed, which may be compared with Ag" and Ag++ centers in alkali halides. In the case of the Cut+ centersthesymmetryof the surrounding halogen ions is lowered by a Jahn-Teller distortion. This distortion, however, is different from that of the Ag++ centers studied by Delbecq, Hayes, O'Brien, and Yuster [9].hiit Hilfe von optischen Absorptionsmessungen wird gezeigt, daB die Cu+-Ionen in Cu-dotierten KCI-, KBrund KJ-Kristallen bei Riintgenbestrahlung bei tiefen Temperaturen Elektronen und Defektelektronen einfangen. Dabei entstehen Cu"und &++-Zentren, die wir mit den Ago-und Ag++-Zentren vergleichen. Auf Grund des Jahn-Teller-Effektes erniedrigt sich bei den Cut+-Zentren die Symmetric der umgebenden Halogen-Ionen. Diese Verzerrung fiillt jedoch anders aus als bei den Ag+t-Zentren, die Delbecq, Hayes, O'Brien und Yuster [9] untersucht haben.') Present address:

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