Double Edge Optical Absorption By Silica Glass In The Vacuum Ultraviolet

Silica glass (v-Si02) shows an Urbach edge in the vacuum ultraviolet, some unconfirmed exciton peaks above the edge and an absorption tail below the edge sensitive to impurity. The glass Vycor 7913, 96% silica, the rest mainly boron, aluminium and sodium, shows similar excitonic peaks but two absorption edge and a distinctly different absorption tail. The curve, it is argued, represents Vycor 7913 as more disordered than v-Si02, a disorder tending to self trap excitons more strongly and some of the tail features imply that a substructure exists, of boron, aluminium and sodium.