  The distribution of Ge-O-Ge and Si-O-Si bond angles alpha in amorphous
germania and silica is re-determined on the basis of diffraction experiments.
The bond angle alpha joining adjacent tetrahedra is the central parameter of
any continuous random network description (CRN) of these glasses. New high
energy photon diffraction experiments on amorphous germania (at photon energies
of 97 and 149 keV) are presented, covering the momentum transfer 0.6-33.5
AA^{-1}. In photon diffraction experiments on GeO2 the contribution of the OO
pairs is very small. To obtain a similar information for amorphous SiO2, high
energy photon diffraction experiments have been combined with neutron
diffraction data on amorphous silica in order to eliminate the OO- partial
structure factor. With this technique it is shown that the Si-O-Si angle
distribution is fairly narrow (sigma=7.5 degree) and in fact comparable in
width to the Ge-O-Ge angle distribution (sigma=8.3 degree), a result which
differs from current opinion. The narrower distribution found in this study are
in much better agreement to the determinations based on 29Si-MAS-NMR. Among the
various models relating the chemical shift to the bond angle, best agreement is
found with those models based on the secant model. Sharp components in the bond
angle distribution can be excluded within the reached real space resolution of
0.09 AA.
