Signatures of Intra- and Intermolecular Vibrational Coupling in Halogenated Liquids Revealed by Two-Dimensional Raman-Terahertz Spectroscopy

Hybrid two-dimensional (2D) Raman-terahertz spectroscopy with the Raman-terahertz-terahertz (RTT) pulse sequence is used to explore the ultrafast intra-and intermolecular degrees of freedom of liquid bromoform (CHBr3) in the frequency range of 1-8 THz. Cross peaks observed in these 2D spectra are assigned to the coupling between the narrow intramolecular modes of the molecules and the much broader intermolecular degrees of freedom of the liquid. This assignment is based on the frequency position of the cross peaks; however, it is shown that these frequency positions can be deduced accurately only when properly taking into account the convolution of the molecular response with the instrument response function of the experimental setup, the latter of which distorts the 2D spectra considerably. The assignment is supported by additional experiments on diiodomethane (CH2I2), which has only one intramolecular mode in the frequency range of the experiment, and hence excludes the possibility of intramolecular couplings.