Off-resonant two-dimensional fifth-order Raman spectroscopy of liquid CS2: Detection of anharmonic dynamics

Off-resonant two-dimensional (2-D) fifth-order Raman signal of liquid CS2 is calculated by using a molecular dynamics (MD) simulation based on the fifth-order response function, which is characterized by a three-time correlation function of polarizability with the stability matrix of the phase space dynamics. The physical origin of the 2-D Raman signal pattern is investigated by analyzing anharmonic and nonlinear polarizability contributions. It is found that a rotation–rotation dynamical coupling arising from anharmonic dynamics is a dominant contribution to the fifth-order response function on the axis of the second time duration, t2. This coupling yields a novel feature of the signal, that is, a change in sign or a ridge in the vicinity along the t2 axis. The echo signal along t1=t2 obtained by the MD simulation accounting for the full dynamic effect, becomes blurred in comparison with that obtained by a harmonic description. The 2-D Raman signal at a very low temperature is also examined to find how the signal changes when structural transitions are suppressed. The 2-D Raman signal is also evaluated with a nonequilibrium MD simulation, which is found to be very similar to that with the equilibrium MD simulation based on the nonlinear response function formalism.