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circulation. As their contributions are much stronger than the contribution from the circulation in the weak central field of the vacancy, a negative spin-orbit splitting results. Thus the observed negative sign and magnitude of 6 indicate that the Z, -center electron is vacancy centered and overlaps the surrounding lattice similar to the F-center electron. Moreover the insensitivity of 6 against Ca++-Sr++ substitution suggests a non-nearest-neighbor site of the impurity ion at the vacancy. Similar conclusions have been drawn from the corresponding insensitivity of optical-absorption and ENDQH, 'o measurements.Thus the Faraday-rotation results lend strongest support to a vacancy-centered model of type C or D for the Z, center. The absorption band at 2.4 eV accompanying the Z, band [Fig. 2(a)] resembles in relative location and spectral shape very much the K band of the F center. The fundara. ental absorption edge and the exciton structure of zinc oxide (Zno) was first studied systematically by Thomas, ' and the observation of exitons as polaritons has also been reported by Hopfield and Thomas. ' The latter work was probably stimulated by results on the fundamental spectra of ZnO reported by Park et al. ' Different interpretations for much of the experimentally observed structure have been given by these workers; a particular consequence of this is a discrepancy in the values quoted for both the spin-orbit interaction energy and the exciton binding energies. The difficulty derives from the interpretation of the reflection spectra which are highly complex near the absorption edge. Transmission experiments using polarized light have been made on single crystals of ZnO at liquid-helium, liquid-nitrogen, and roomtemperatures. By using crystals as thin as 0.1 p, m, transmission rneasurernents have been made to photon energies greater than the band gap, and the method is similar to that used for wurtzite zinc selenide. Thin platelets of ZnO were grown by the vapor transport method, using argon as the carrier gas. These crystals have the wurtzite structure with the c axis lying along the growth plane. Transmission electron-microscopy and -diffraction studies showed that most of these platelets were good single crystals free from def ects such as stacking faults, though isolated crystals 59

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