We describe the observation of a broad and strongly temperature-dependent peak in the terahertz absorption spectrum of several linear n-alkanes. On cooling from the liquid into the rotator phase, the absorption is enhanced within a very narrow temperature range at the temperature of the first-order phase transition. This peak is associated with an enhancement in the vibrational density of states, induced by the heterogeneous nucleation of the rotator phase from the liquid. This is similar to the well-known Boson peak in glassy solids, and provides a new experimental tool for the characterization of nucleation dynamics in non-polar molecular liquids. (C) 2010 Elsevier B. V. All rights reserved.