Simulating two-dimensional correlation spectroscopies with third-order infrared and fifth-order infrared–Raman processes of liquid water

Abstract

To investigate the possibility of measuring the intermolecular and intramolecular anharmonic coupling of balk water, we calculate third-order two-dimensional (2D) infrared spectra and fifth-order 2D IR–IR–Raman–Raman spectra expressed in terms of four-body correlation functions of optical observables. For this purpose, a multimode Brownian oscillator model of four interacting anharmonic oscillators strongly coupled to their respective heat baths is employed. The nonlinearity of system–bath interactions is considered to describe thermal relaxation and vibrational dephasing. The linear and nonlinear spectra are then computed in a non-Markovian and nonperturbative regime in a rigorous manner using discretized hierarchical equations of motion in mixed Liouville–Wigner space. The calculated 2D spectra for stretching–bending, bending–librational, stretching–librational, and stretching–translational modes consist of various positive and negative peaks exhibiting essential details of intermolecular and intramolecular mode–mode interactions under thermal relaxation and dephasing at finite temperature.