Photoinduced anomalous heat generation in AgCl and AgBr single crystals at liquid-helium temperature

Anomalous heat generation induced by band-gip light illumination was observed in AgCl and AgBr single crystals by a sensitive photocalorimetric method at liquid-helium temperature. The generated heat per second was estimated to be one to two orders of magnitude larger than the absorbed energy per second. This heat generation lasted for more than 10 h after temination of the band-gap light illumination, indicating that exothermic chain reactions were triggered by the absorbed photons. In a crystal which was not annealed, total emitted heat energy was estimated to be as high as 0.17 eV per unit cell. Consecutive nonradiative recombination of interstitial silver ions and vacancies mediated by the photoelectrons are tentatively proposed to be the responsible chain reactions. Although most of the features of the heat generation can be explained by the proposed mechanism, this mechanism requires an unrealistically high concentration of interstitial-vacancy pairs to explain the total emitted energy, leaving the observed anomalous phenomenon unexplained.