  Applying coherent X-rays by the method of atomic-scale X-ray Photon
Correlation Spectroscopy results in beam-induced dynamics in a number of oxide
glasses. Here these studies are extended to rubidium and caesium borates with
varying alkali contents. While no cumulative beam damage is observed, the
observed rate of structural rearrangements shows a linear relation to the dose
rate. In agreement with the increasing glass transition temperature, the rate
of dynamics at given dose rate decreases with increasing alkali content, while
the shape of the decay of correlations becomes progressively stretched. This
behavior is explained in terms of faster dynamics of the alkali positions
compared to the borate network. Finally, the q-dependent behavior of the
correlation decay rate implies the observed dynamics to proceed via small-scale
atomic displacements subject to de Gennes narrowing.
