Theoretical approach to optical response properties of photoprecipitation in irradiated colloid systems

In this work we describe some theoretical results of the photoexcitation process in colloidal systems in spatio-temporal terms of their optical absorption constant α(x, t). Experimentally, in some colloidal systems a photoprecipitation effect is observed which is enhanced autocatalytically leading to an amplification effect. The amplification is 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 to the window highly photoexcited but also obstructs the light penetration into the bulk of the photoreactor. From basic principles, we show in a unidimensional spatial model that the amplification effect contains a positive as well as a negative feedback term, which can be traced to the physical phenomena occurring due to photoexcitation.