Urbach absorption edge of silica: Reduction of glassy disorder by fluorine doping

The vacuum-ultraviolet fundamental absorption edge ('Urbach edge') of four types of synthetic silica glasses, 'wet', 'dry', and doped by 570 and 6010ppm wt. fluorine, was studied in the absorption coefficient range (1cm ^-1-500cm^-1) at room temperature. The absorption edge has exponential form in agreement with the Urbach's rule. The well-documented increase of vacuum-ultraviolet transparency upon fluorine doping is due to a steeper absorption edge (shorter 'Urbach tail') as compared to undoped silicas. The increase of the edge slope in F-doped silica occurs already the lower dopant concentration (570 ppm), the slope does not increase further in the 6010 ppm doped glass. These findings show that the improved vacuum UV transparency of F-doped silicas is due to the decreased contribution of the glassy disorder and strained Si-O bonds to the width of the absorption edge.