Double edge optical absorption by silica glass in the vacuum ultraviolet

Silica glass (v-SiO2) shows an (Urbach edge in the vacuum ultraviolet, some unconfirmed exciton peaks above the edge anti an absorption tail below the edge sensitive to impurity. The glass Vycor 7913, 96% silica, the rest mai nly boron, aluminium and sodium, shows similar excitonic peaks but two abso rption edge anti a distinctly different absorption tail. The curve, it is a rgued, represents Vycor 7913 as more disordered than v-SiO2, a disorder ten ding to self trap excitons more strongly and some of the tail features impl y that a substructure exists, of boron, aluminium and sodium.