Tunneling negative-U centers and photo-induced reactions in solids

A model of tunneling centers which acquire the negative-U value as a result of a dependence shown by the electron-vibration interaction constant upon the center charge state is proposed. We address the question whether the model is of a sufficiently general character to be applied to the investigation of the tunneling probabilities for centers due to the reversible rearrangements brought about in lattice defects by optical pumping, photo-induced reactions occurring in semiconductors and photochromic materials, and by adsorbate photodesorption. The tunneling systems of the type being considered have been observed in the experimentally investigated gold centers in silicon.