Photoconductivity and optical quenching of the photoconductivity were measured. The photoneutralization of the Fe^{3+} centres has been found to occur for photon energies down to 0.8 eV (i.e. the threshold can not be at 1.1 eV, as often quoted). The conclusion from our previous paper that the lattice relaxation energy is small, has been confirmed. It has to be smaller than 0.1 eV.
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