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Superlattices consisting of alternating layers of organic and inorganic materials have been fabricated from 3,4,9,10-perylenetetracarboxylic diimide (PTCDI), copper phthalocyanine (&PC), and MgF, by molecular-beam deposition. From the small-angle x-ray diffraction patterns of PTCDVMgF, and CuPc/MgFz superlattices, a periodically layered structure is confirmed through the entire stack. The PTCDI layers in the PTCDVMgF, superlattices have a high degree of structural ordering, in which molecular planes are nearly parallel to the substrate surface. On the other hand, the CuPc/MgF2 superlattices have a poorly ordered structure in the CuPc layers. It is found that the optical absorption spectra of these superlattices are different from those of organic single films and significantly change with varying organic layer thickness. Such spectral changes can be interpreted by the effect of the aggregate size and the coexistence of two different crystal forms. 0 1995 American Institute of Phy+cs.