Evidence for an ultrafast breakdown of the BeO band structure due to swift argon and xenon ions.

Auger-electron spectra associated with Be atoms in the pure metal lattice and in the stoichiometric oxide have been investigated for different incident charged particles. For fast incident electrons, for ${\mathrm{Ar}}^{7+}$ and ${\mathrm{Ar}}^{15+}$ ions as well as ${\mathrm{Xe}}^{15+}$ and ${\mathrm{Xe}}^{31+}$ ions at velocities of 6% to 10% the speed of light, there are strong differences in the corresponding spectral distributions of Be-K Auger lines. These differences are related to changes in the local electronic band structure of BeO on a femtosecond time scale after the passage of highly charged heavy ions.