  We demonstrate that halogenated methane 2D-Terahertz Terahertz Raman (2D-TTR)
spectra are determined by the complicated structure of the instrument response
function (IRF) along $f_1$ and by the molecular coherences along $f_2$.
Experimental improvements have helped increase the resolution and dynamic range
of the measurements, including accurate THz pulse shape characterization.
Sum-frequency excitations convolved with the IRF are found to quantitatively
reproduce the 2D-TTR signal. A new Reduced Density Matrix model which
incorporates sum-frequency pathways, with linear and harmonic operators fully
supports this (re)interpretation of the 2D-TTR spectra.
