Is there a Ioffe-Regel limit for sound propagation in glasses?

New neutron spectroscopic results on glassy selenium are presented. For the first time both Brillouin scattering under kinematic conditions and the bo son peak were observed. Both thermal neutron three-axis and cold neutron ti me-of-flight spectroscopies were necessary. Their absolute calibration reve als the strength of the various spectral contributions in this frequency ra nge. The most plausible interpretation of the overall data is that longitud inal acoustic waves enter a regime of strong scattering between 3 and 4 meV . Umklapp scattering of the neutrons via the static structure factor is als o seen. It provides unique information on transverse acoustic modes. Its fa st disappearance beyond 5 meV is another strong indication that acoustic ex citations cease to propagate. (C) 1999 Elsevier Science B.V. All rights res erved.