  2D-Raman-THz spectroscopy of liquid water, which has been presented recently
(Proc. Natl. Acad. Sci. USA 110, 20402 (2013)), directly probes the
intermolecular degrees of freedom of the hydrogen-bond network. However, being
a relatively new technique, its information content is not fully explored as to
date. While the spectroscopic signal can be simulated based on molecular
dynamics simulation in connection with a water force field, it is difficult to
relate spectroscopic signatures to the underlying microscopic features of the
force field. Here, a completely different approach is taken that starts from an
as simple as possible model, i.e., a single vibrational mode with electrical
and mechanical anharmonicity augmented with homogeneous and inhomogeneous
broadening. An intuitive Feynman diagram picture is developed for all possible
pulse sequences of hybrid 2D-Raman-THz spectroscopy. It is shown that the model
can explain the experimental data essentially quantitatively with a very small
set of parameters, and it is tentatively concluded that the experimental signal
originates from the hydrogen-bond stretching vibration around 170 cm$^{-1}$.
Furthermore, the echo observed in the experimental data can be quantified by
fitting the model. A dominant fraction of its linewidth is attributed to
quasi-inhomogeneous broadening in the slow-modulation limit with a correlation
time of 370 fs, reflecting the lifetime of the hydrogen-bond networks giving
rise the absorption band.
