Ultrafast dynamics of self-trapped excitons in silica revealed by ab initio simulations

Self-Trapped Excitons (STEs) govern the optical and electronic response of silica, yet their formation mechanisms remain unclear due to the interplay of electronic relaxation, lattice distortion, and defect generation. To investigate STE formation under ultrafast photoexcitation, we combine adiabatic and non-adiabatic time-dependent density functional theory calculations with surface-hopping dynamics. We show that when atomic movement is constrained, excited electrons relax primarily through internal conversion and intersystem crossing. In contrast, fully unconstrained structures undergo Si–O bond breaking that creates localized defect states within the bandgap. We identify a STE level at 2.74 eV with a relaxation rate consistent with experimentally observed photoluminescence. Non-adiabatic population dynamics reveals that relaxation proceeds predominantly along the S2 → S1 → GS pathway, with triplet channels contributing only under specific structural conditions. The relaxation time of 353 fs is consistent with the experimentally fitted electron trapping time. Moreover, we show that electron-phonon coupling plays a crucial role in STE formation.