Ab Initio Modeling of Optical Excitations Associated with Intrinsic Defects in a-SiO2

A systematic study of the structure and optical properties of diamagnetic neutral defects in silicon dioxide is conducted using ab initio quantum-chemical methods. It is found that the =Si---Si= defect, formed by two weakly bonded twofold-coordinated silicon atoms, and the =Si(O2) defect containing a dioxasilirane group are capable of absorbing intense visible-range laser radiation. The potential contribution of these defects to the initiation of laser-induced damage is discussed. It is also shown that bond elongation in certain defects leads to a reduction in excitation energies, shifting them from the ultraviolet to the visible range.