An orientation-sensitive re-charging of tetragonal FeK-Oi centers under polarized optical irradiation was observed which resulted in the so-called alignment of the centers in a particular charge state. Also, the sign of the alignment changed during irradiation. A phenomenological model which involved only photo charge transfer processes, between the FeK-Oi centers and the host lattice, permitted a semi-quantitative explanation to be given for all of the observations. However, a considerable contribution from other mechanisms (such as local ionic reorientation) was possible and could not be ignored.

S.A.Basun, V.E.Bursian, H.Hesse, S.Kapphan, L.S.Sochava, V.S.Vikhnin: Materials Science Forum, 1997, 239-241, 345-8