Spectrum of fast dynamics in glass forming liquids: Does the "knee'' exist?

A knee-shaped feature observed earlier in light scattering spectra of Ca0.4 K0.3(NO3)(1.4) (CKN) below T-c is used as a strong argument in favor of mod e- coupling theory of the glass transition (MCT). Our careful measurements reveal no "knee" in the spectra of two glass forming liquids, CKN and ortho - terphenyl. Instead of the knee the spectra show nontrivial broadening and an increase of the intensity with a temperature increase. Both variations are confirmed by neutron scattering measurements on CKN and are neither exp ected in the asymptotic MCT predictions nor in any other model. (C) 1999 Am erican Institute of Physics. [S0021-9606(99)51605-X].