Evidence of a Submegahertz Acoustic Dispersion in Liquid and Glassy O-Terphenyl

An optoacoustic technique is used to measure at about 1 MHz the sound velocity and absorption of the molecular glass former o-terphenyl in a temperature range covering the normal and supercooled liquid phases. These data show the clear signature of the well-known structural relaxation. More interestingly, the comparison of the present data with thermodynamic data clearly shows the existence, in both the supercooled liquid and the glass, of an additional acoustic dispersion in the submegahertz frequency range. This finding implies that also molecular glass formers such as o-terphenyl, which are usually considered as simple in nature, are actually characterized by a complex pattern of relaxations.