Tunnelling ionization of DX centers was observed in THz radiation fields. The tunnelling times were measured for autolocalized and on-site deep impurities. In one case the tunnelling time was shorter, and in the other case it was longer, than the reciprocal temperature multiplied by a universal constant. This difference was attributed to the effect of differing tunnelling trajectories. This permitted one to distinguish, in a direct manner, between the 2 types of configuration potential for impurities.

S.D.Ganichev, I.N.Yassievich, W.Prettl, J.Diener, B.K.Meyer, K.W.Benz: Materials Science Forum, 1995, 196-201, 1079-84