Exciton mechanisms and modeling of the ionoluminescence in silica

A theoretical model is presented in order to discuss detailed kinetic data describing the evolution of the two main ionoluminescence bands at 650 nm (1.9 eV) and 460 nm (2.7 eV) in silica as a function of the irradiation fluence at room temperature. The model is based on the generation of self-trapped excitons (STEs), their hopping migration through the silica network and their recombination at non-bridging oxygen hole and Type II oxygen-deficient centers to produce the red and blue emission bands, respectively. For heavy ions, which have a high electronic stopping power, the two emission yields experience a fast initial growth with fluence up to a maximum value and then decrease at a fairly comparable rate. The fluence for such a maximum strongly increases on decreasing ion mass and stopping power and, finally, for H and He it is not observed for any of the investigated energy and fluence range. This kinetic behavior is explained in terms of the strong structural distortions (compaction) induced by the heavy-ion irradiations. In particular, it is proposed that these strong structural distortions cause a significant decrease in the STE migration length and, consequently, in the recombination rates at the two active recombination centers. The model offers a good quantitative accordance with detailed infrared spectroscopy reporting on the changes in the frequency ω ₄ of a first-order vibrational mode in the SiO₂ network as a function of irradiation fluence. (paper)