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.
In version 1, we incorrectly used the phrases alpha and beta relaxation as understood in the glass community, leading to much confusion...! We now use orientational and translational relaxation instead, both of which follow the viscosity and are therefore (expressions of) alpha relaxation. What we referred to as the fast beta process, we now (less confusingly hopefully) refer to as the intermolecular mode. Also extra references have been added.
Boson peak - SI