Recently, it has been established that dangling bonds in ordered vacancy compounds play a crucial role in optical activity. With this view a spectroscopic investigation of ZnGaS_4, where stands for ordered vacancy, has been carried out at 10 K. The optical absorption spectra were recorded under polarized radiation. Absorption maxima (in perpendicular direction of polarization absorption minima) were obtained at 2.51 and 1.9 e V and they were associated with the transitions from A _1 state originated from the vacancy (in D2 symmetry) to the mid-band gap states of the ordered vacancy compounds. These transitions are optically active because the bond bending near the vacancy creates a situation where the component of the magnetic dipole moment is parallel to the electric dipole moment and hence their scalar product is non-zero.
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