Theoretical kinetic analysis of biphotonic processes

Abstract

A simple kinetic scheme involving the successive absorption of two photons has been completely analysed and the solutions have been numerically computed within a wide range of parameter values. It is found that, aside from the classical monotonic behaviour described in many textbooks, a system can, under appropriate conditions, respond to photoexcitation in a number of unusual ways. In particular, for a given system and a single set of conditions more than one steady state can be achieved, not all of them being stable, and this results in the existence of critical light intensities around which the system behaves like a chemical amplifier or threshold discriminator. It is also shown by time-dependent analysis that the concentration in some species can be subject to overshoot or to damped oscillations before reaching its stationary value. Although a few experimental cases have been reported, such unusual phenomena have not been widely recognized in photochemistry, and it seems plausible that their possible occurrence in laboratories might have been overlooked or disregarded as being due to artefacts. The aim of this paper is to draw the attention of photochemists to the physical meaning of such unusual observations.