Real-time Raman measurements were made of crystals under 5keV He+ bombardment. The phonon correlation length was deduced from the change in Raman spectral line-shape of the longitudinal optical phonon. The phonon correlation length decreased with increasing ion fluence. It was shown that the phonon correlation length corresponded to the mean distance between the defects in ion-bombarded material.

K.Ishioka, K.G.Nakamura, M.Kitajima: Physical Review B, 1995, 52[4], 2539-42