From microscopic fluctuations to susceptibility spectra: Single-molecule relaxation in glassy media

Single-molecule (SM) rotational dynamics of fluorescent probes in polystyrene near the glass transition temperature (Tg) are investigated over long times to reconstruct susceptibility spectra. The loss spectrum, commonly recorded using external field-driven (frequency-domain) spectroscopy, such as dielectric spectroscopy, is reconstructed from purely thermal SM rotational fluctuations. The results reproduce time-temperature superposition in the temperature range explored and show that the ensemble spectrum is comprised of individual molecular responses to distinct environments.