Insights into Explicit Anharmonic Effects in Crystals via Intensity Analysis of THz-Range Raman Spectra

In optical spectroscopy, the harmonic approximation is commonly used to assign peaks and lineshapes to eigenstates, providing valuable information about the system's internal structure. However, understanding explicit anharmonic effects, such as eigenstate coupling and phase transformations, typically necessitates advanced techniques like 2D spectroscopy or calorimetric measurements, which may not provide detailed mechanistic insights.<br/>Nevertheless, the intensity of peaks in THz-range Raman spectroscopy measures the population of vibrational states and contains valuable information about anharmonic interactions. In this talk, I will illustrate how analyzing the temperature dependence of Raman peak intensities can provide valuable mechanistic insights into the coupling between eigenstates and phase transformations.<br/>The first example is in Bismuth Vanadate, where the soft mode loses intensity and depopulates towards its first-order phase transition, evident in the discontinuity of integrated intensity for all lattice modes. The second example is in alpha-glycine, where we discover a local phase transformation manifested by a change in the hydrogen bond free energy surface and accompanied by highly anharmonic lattice behavior. These findings represent an important initial step towards utilizing THz-range Raman spectroscopy as a direct probe for studying equilibrium and non-equilibrium dynamics.