This image shows the structure of the self-trapped exciton in quartz (alpha-SiO2). Red balls represent Si atoms, blue balls O atoms, and black lines are bonds. The self-trapped exciton comprises an excited electron and hole that are localized and trapped via spontaneous symmetry breaking at a defect in the crystal lattice, here shown by the broken Si-O bond (for the pair at the center of the picture). The upper orange dumbbell shows an isosurface for the hole probability --- clearly inhabiting an oxygen 2p orbital. The lower green distribution is the isosurface of electron probability --- mainly localized on the silicon away from the oxygen, but also on the three remaining oxygen neighbors as well. The detailed positions and distributions tell us a great deal about the observable properties of this interesting defect. Reference: S. Ismail-Beigi and S. G. Louie, "Self-trapped Excitons in Silicon Dioxide: Mechanisms and Properties", Physical Review Letters 95, 156401 (2005).

Submitted by Sohrab Ismail-Beigi