Inelastic neutron scattering experiments on the dynamics of a glass-forming material in mesoscopic confinement

In this article we present results on the microscopic dynamics of a glass-forming liquid (salol) confined in microporous silica glass (Gelsil) obtained by inelastic neutron scattering. By combining different methods we are able to cover a large dynamical range, from the low-frequency vibrations to the alpha-relaxation. The most prominent effect was observed on the 'Boson peak' in the vibrational spectrum. A strong reduction of the modes at lowest frequencies could be observed. This result can be explained qualitatively by a cut-off of long wavelength sound modes but yet defies a quantitative description. Especially, a series of samples with different pore sizes (25-200 Angstrom) did not show the expected scaling of the cut-off frequency with the diameter. The main effect on the alpha-relaxation is a broadening which we can explain by a tentative model of inhomogeneous relaxation time in the pores. According to this model the relaxation time in the core would be smaller. But due to the distribution the relaxation appears broadened and there is even a fraction of molecules which is 'frozen' on the time scale of quasielastic neutron scattering. (C) 2002 Elsevier Science B.V. All rights reserved.