Low-frequency anharmonic couplings in bromoform revealed from 2D Raman-THz spectroscopy: From the liquid to the crystalline phase.

Two-dimensional (2D) Raman-THz spectroscopy in the frequency of up to 7 THz has been applied to study the crystalline β-phase of bromoform (CHBr ₃ ). As for liquid CHBr ₃ , cross peaks are observed, which, however, sharpen up in the crystalline sample and split into assignable sub-contributions. In the Raman dimension, the frequency positions of these cross peaks coincide with the intramolecular bending modes of the CHBr ₃ molecules and in the THz dimension with the IR-active lattice modes of the crystal. This work expands the applicability of this new 2D spectroscopic technique to solid samples at cryogenic temperatures. Furthermore, it provides new experimental evidence that the cross peaks, indeed, originate from the coupling between intra- and intermolecular vibrational modes.