Non-linear spectroscopy, on the other hand, provides much more detailed information that is not available in linear spectra. More detailed dynamical information can be extracted (compared to the linear spectra) by perturbing the system multiple times with consecutive laser pulses. Each additional laser pulse adds a higherorder non-linear perturbation to the overall density matrix, which can be interpreted from a specific pathway in the Liouville space. For a general two-dimensional spectroscopy experiment, three laser pulses are used with different time delays and thus, in the impulsive limit, the 3 rd order response can be calculated from a four-point correlation function