THEORETICAL APPROACH TO OPTICAL-RESPONSE PROPERTIES OF PHOTOPRECIPITATION IN IRRADIATED COLLOID SYSTEMS

In this work we describe some theoretical results of the photoexcitati on process in colloidal systems in spatio-temporal terms of their opti cal absorption constant alpha(x, t), Experimentally, in some colloidal systems a photoprecipitation effect is observed which is enhanced aut ocatalytically leading to an amplification effect. The amplification i s understood here in the sense that new chromophores are created by a positive feedback loop of photoprecipitation - photon absorption. This causes an 'avalanche process' which makes the irradiated zone close t o the window highly photoexcited but also obstructs the right penetrat ion into the bulk of the photoreactor. From basic principles, we show in a unidimensional spatial model that the amplification effect contai ns a positive as well as a negative feedback term, which can be traced to the physical phenomena occurring due to photoexcitation.