Sol-gel glasses (known since 1846) are a family of inorganic glasses or organic composites prepared at ambient temperature, usually from silicon alkoxides (sometimes mixed with other alkoxides). Hydrolysis and subsequent polycondensation of the precursor solution allows incorporation of specific cations or their pre-existing complexes. Detailed study of absorption and emission spectra, including lifetimes of their excited states, sometimes allows determination of the site symmetry and the coordination number, e.g., for cobalt (II). The ruthenium (II) tris(2,2'-bipyridine) cation in glass shows a much higher yield of fluorescence than in solution, partly because of less triplet quenching. Lanthanides can show very high luminescence yields because no collisions occur between the species in an excited state and in its ground state.