The spectral properties of liquids in the terahertz (THz) region play an important role in chemistry, as they reflect motions of large molecules and molecular complexes which, in turn, govern chemical reactivity and processes like solvation. These specific properties of liquids can be addressed either via resonant THz spectroscopy or Raman spectroscopy. However, due to the ultrafast dynamics and the large anharmonicity of intermolecular modes, low frequency one-dimensional spectra (both Raman and THz) are mostly blurred and indistinct. Multidimensional spectroscopy can enhance spectral resolution by spreading spectra in more than one frequency dimension, thus increasing significantly the amount of information that is possible to extract [1].

Here, we demonstrate the unique capabilities of 2D Raman-THz spectroscopy, a novel 2-dimensional spectroscopic technique developed in our group [2], of measuring coupling between low-frequency vibrational modes, information that are not present in a 1D linear spectrum. To that end, we recorded the 2D Raman-THz responses in the spectral range between 1 and ~ 8THz of liquid bromoform and dibromomethane. The spectra reveal cross peaks which we attribute to the coupling between the relatively sharp intramolecular modes and the much broader intermolecular degrees of freedom of these solvents.

[1] Hamm P., Zanni M.T. Concepts and Methods of 2D Infrared Spectroscopy, Cambridge University Press, 2011
[2] Savolainen J., Ahmed, S. & Hamm P., 110, 20402–20407 PNAS 2013