  Terahertz molecular motions are often probed by high-frequency molecular
oscillators in different types of non-linear vibrational spectroscopy. Recently
developed two-dimensional terahertz-infrared-visible spectroscopy allows direct
measuring of this coupling and, thus, obtaining site-specific terahertz
vibrational spectrum. However, these data are affected by the intensity and
phase of the employed laser pulses. In this work, we develop a method of
extracting sample response - representing solely physical properties of a
material - from experimental spectra. Using dimethyl sulfoxide (DMSO) as a
model molecule to verify this method, we measure the coupling between C-H
stretch vibration of its methyl groups and terahertz intramolecular twist and
wagging modes.
