Characterisation of the Boson Peak from the Glass into the Liquid

Phenomena ranging from vitrification to crystal nucleation are governed by locally ordered structures, in otherwise disordered phases, that can either inhibit or favour the growth of macroscopic order. However, such structures are ephemeral, do not typically have distinct spectral features, and are therefore critically important but largely unobservable by current methods. Illuminating these structures therefore presents the single greatest challenge in physical chemistry. The boson peak is characteristic of glasses and represents the locally ordered structures inhibiting crystallisation but is typically obscured by other spectral contributions. Here we show that depolarised Raman scattering—obtained using femtosecond optical Kerr-effect spectroscopy—in liquids consisting of highly symmetric molecules can be used to isolate the boson peak thereby allowing detailed characterisation of the intermolecular potential-energy landscape for the first time.