Vibrational dynamics as an indicator of short-time interactions in glass-forming liquids and their possible relation to cooperativity

Abstract

We report on a vibrational dynamics study of two glass-forming liquids over a wide temperature range including the glassy, supercooled, and molten state. Our aim is to find possible sensitive indicators of short-time dynamics that experience characteristic changes when approaching the liquid–glass transition. The observed changes in vibrational dynamics are employed to track the cooperative behavior of the studied glass-forming liquids. It has been found that both strong and fragile liquids exhibit qualitative similarities in the vibrational relaxation and frequency modulation times as a function of temperature. The temperature dependence of the vibrational relaxation times τV experiences a break at the glass transition temperature Tg. On the contrary, the temperature dependence of the frequency modulation times τω exhibits an unexpected discontinuity at Tg, τω being shorter in the glassy phase than in the supercooled and liquid regime. Since microscopic vibrational dynamics depends upon the intermolecular interactions that ultimately are responsible for the cooperative (or sluggish) dynamics when approaching Tg, an attempt has been made to find a rationale between the unusual temperature dependence of τω and cooperative dynamics.