The results of implantation with O ions confirmed a previous theory; that the weak, approximately 3eV absorption band in this oxide was associated with positive charge that was trapped near to an Al vacancy. Photo-bleaching of the 5.4eV Ti4+ charge transfer band decorated an Al vacancy with a hole that was trapped on a neighboring O atom. The oscillator strength of the V-type band was estimated to be equal to 0.15.

B.D.Evans, L.S.Cain: Radiation Effects and Defects in Solids, 1995, 134, 329-32