Fast local motion around tg in a molecular glass as observed by incoherent neutron scattering

Incoherent quasi-elastic neutron scattering experiments on a molecular glass (1, 3, 5-tri-α-naphtylbenzene, C36H24) are reported. We find clear evidence for an anomalously strong decrease of the elastic and a corresponding increase of the inelastic scattering around the glass transition temperature. The line shape of this extra inelastic intensity gives evidence for a quasi-elastic scattering implying the existence of a localized and fast (τc.10^-12s) molecular motion. From the elastic incoherent structure factor (EISF) a mean jump length is estimated. The significance of this motion as precursor of the glass instability is discussed within the framework of dynamic glass transition theories.