Resonant phonon scattering by radiation induced crystal defects in CaF2

Abstract

After irradiation of CaF2 single crystals with electrons in the range of 500 keV to 1 MeV we find by phonon spectroscopy with superconducting tunneling junctions phonon scattering lines at 398 GHz and 628 GHz. The line strength at 398 GHz decreases with isochronal annealing correlated with the decrease of the optical absorption at 550 nm in these samples due to Ca clusters of an average radius of 6 nm. We ascribe the 398 GHz line to acoustic resonances of these clusters and the line at 628 GHz to resonant phonon scattering at localized excitons of F- and H-center pairs.