Isotropic Brillouin spectra of the two chemically similar van der Waals glass forming liquids, 1,1-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC) and 1,1-bis(p-methoxy-phenyl)cyclohexane (BMPC) and ortho-terphenyl (OTP), were studied in a broad temperature and pressure range in order to characterize the effect of internal relaxations on the damping of longitudinal phonons. Such relaxations are present in BMPC, while in BMMPC and OTP they are strongly hindered. The authors show that in BMPC (with strong internal relaxations) the damping (broadening) of longitudinal phonons (Brillouin peaks) is much stronger than in BMMPC and OTP (with weaker internal relaxations). The contributions of the internal and structural relaxations to the phonon damping can be separated using high pressure, due to their very different pressure dependences. They show that internal relaxations strongly contribute to the damping of longitudinal phonons at all temperatures and should be taken into account in theoretical models describing the Brillouin spectra of supercooled liquids.

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