Mathematical model for the measurement of pseudo-first-order rate constants in laser flash photolysis experiments

A two-dimensional macroscopic model for a laser flash photolysis experiment in a collinear arrangement is proposed and solved. Concentration profiles for the reagents, intermediates and products are calculated and errors in the evaluation of pseudo-first-order constants from absorbance measurements of the intermediates are estimated. These errors result from diffusion of species outside the monitoring beam and an inhomogeneous concentration owing to either Beer's law or the non-uniformity of the excitation beam. It is found that the latter may cause considerable error and that the error owing to diffusion is evident at slow rates of reaction.