Note: Inverted time-ordering in two-dimensional-Raman-terahertz spectroscopy of water

The inverted time-ordered pulse sequence, in which the first coherence is generated by a direct THz excitation, while the switching of coherences is achieved by a Raman interaction, was studied. The dependence of the polarizibility on the nuclear coordinates is expected to be more nonlinear compared to that of the dipole moment. The hybrid equilibrium-non-equilibrium approach introduced by Tanimura and co-workers is used to compute this response function from an all-atom MD simulation. An echo arises when a coherence dephases very quickly, and then rephases upon a second perturbation. Traces of a photon echo appear in simulations of 2D-Raman spectroscopy, but only when approximating the signal with instantaneous or quenched normal modes. It is concluded that indeed the inverted pulse sequence more sensitively measures the coupling between the various degrees of freedom of water.