Ion-electron recombination on silica gel surfaces: experiment and modelling

Kinetics on silica gel and other solid, porous surfaces are often complex. In this paper we have studied the decay kinetics of radical cations produced following multiphoton ionisation on silica gel, and have characterised these using an empirical model. Trends in kinetics have been observed both as a function of concentration and of temperature. Concentration dependent studies suggest heterogeneity of surface adsorption, both in terms of the nature of adsorption sites and aggregation effects. Temperature dependent studies show that the activation energies for surface diffusion correlate with the size of the radical cation, suggesting that its movement rather than that of the electron dominates the observed kinetics. Monte Carlo simulations have been shown to give useful qualitative insights into the interpretation of the extracted parameters, in particular into how apparent distributions of rate constants can arise as a result of low surface dimensionality.