Solvent dynamics in a glass-forming liquid from 300 K to 3 K: What photon echoes can teach us

The temperature dependence of the optical non-linear response of dye molecules dissolved in a glass-forming liquid over a temperature range that includes the glass transition is investigated. Cooling down to temperatures below the glass transition dramatically slows the diffusive motion of the solvent molecules, while the bath retains its strong amorphous character. The results of ultrafast experiments such as echo-peak shift and heterodyne-detected echo are presented. The temperature dependence of the optical response is discussed within the context of the multimode Brownian oscillator model that is commonly used to describe dynamics in liquids. Freezing out a large part of the bath fluctuations allows for testing of the physical interpretation associated with this model. A temperature-dependent spectral density is proposed that can describe the results at all temperatures over the explored temperature range. The temperature dependence follows readily from the physical processes that are associated with the various parts of the density of states, with the exception of the fastest modes. The temperature dependence of these modes is inspected closely and the spectral shape is reinterpreted. (C) 2007 Elsevier B.V. All rights reserved