Tunnel ionization of semiconductor deep impurity centres has been investigated in a field of far-infrared radiation where photon energies are several factors of ten smaller than the binding energy of the impurities. Depending on the
radiation electric field strength, ionization is caused by phonon-assisted tunnel ionization or direct electron tunnelling. Applying high-power pulsed lasers, two types of impurities have been studied: substitutional on-site acceptors in Ge and autolocalized DX− centres in AlxGa1−x Sb. The experimental results are analysed in terms of the theory of multiphonon and cold carrier emission of deep impurities in the adiabatic approximation. Tunnelling times have been measured for both types of impurities. Due to different tunnelling trajectories of on-site and autolocalized centres, the tunnelling time is in the first case larger and in the other case smaller than the reciprocal temperature multiplied by universal constants. This allows us to distinguish in a direct way between the two types of configuration potentials of impurities. The results demonstrate that high-frequency far-infrared laser pulses may be used to study the elementary process of tunnelling in extremely large electric field strengths, avoiding contact phenomena and avalanche breakdown.