Comment on ‘Post UV irradiation annealing of E′ centers in silica controlled by H2 diffusion’ by M. Cannas et al. [J. Non-Cryst. Solids 337 (2004) 9–14]

In the paper by Cannas et al. [M. Cannas, S. Costa, R. Boscaino, F.M. Gelardi, J. Non-Cryst. Solids 337 (2004) 9] the simplest version of the Waite theory of diffusion limited reactions was applied to the post irradiation annealing of E′ centers by molecular hydrogen diffusing in a silica matrix. This version of the Waite theory does not take into account limitations arising from the rate of the chemical process between reactants but assumes that the annealing kinetics is limited by the diffusion only. An attempt to fit the annealing rate determined experimentally to that calculated from this version of the Waite theory led to an unrealistically small value of the capture radius. To clarify this issue, a more advanced version of the Waite theory, which takes into account the viability of the chemical reaction between H2 and E′ centers, should be used. In this comment such a version of the Waite theory is outlined and applied to an interpretation of the experimental data reported in [M. Cannas, S. Costa, R. Boscaino, F.M. Gelardi, J. Non-Cryst. Solids 337 (2004) 9]. A rigorous argument of the hypothesis introduced in paper [M. Cannas, S. Costa, R. Boscaino, F.M. Gelardi, J. Non-Cryst. Solids 337 (2004) 9] that the annealing kinetics of E′ centers is not limited by the diffusion of H2 molecules but is defined by the rate of the chemical reaction itself is presented.