NONLINEAR DYNAMICS, MULTIPLE STEADY-STATES, AND OSCILLATIONS IN PHOTOCHEMISTRY

Light is often considered as a type of chemical reagent. As a result, there is a natural tendency to infer that the rate equations describing the dynamics of photochemical systems should not differ significantly from their light-free counterparts. There are, however, two important differences between chemical and photochemical systems. The first one is that photochemical systems are nonisolated closed systems that are maintained away from their thermodynamic equilibrium by the photon flux. The second difference lies in the rather peculiar nature of the interaction between light and the physical medium in which the photoreaction takes place. This interaction is governed by the highly nonlinear Lambert-Beer's law and has no equivalent in light-free chemistry. In this paper, it is shown that these two differences are of more than academic importance and naturally lead to a variety of situations where instabilities such as multiple steady states and oscillations can be obtained in relatively simple photochemical schemes. Contrary to light-free chemistry, these instabilities can be observed in isothermal systems that are closed to mass flows and do not contain any autocatalytic steps. To support this thesis a number of models based on thermally reversible, consecutive, and cyclic photoreactions are analyzed. By use of these models, it is shown that features such as competitive absorption and excited state quenching can give rise to multiple steady states and sustained oscillations. The nature of the feedback mechanism responsible for the onset of these instabilities is clarified and shown to bear some similarity with substrate inhibition.