Comparative OHD-RIKES and THz-TDS Probes of Ultrafast Structural Dynamics in Molecular Liquids

We compare terahertz time domain (THz-TDS) and optical heterodyne-detected Raman-induced Kerr effect (OHD-RIKES) spectroscopic techniques as probes of molecular and vibrational dynamics in the frequency range of 0.1 to 6 THz (3 to 200 cm -1 ). The spectra of molecular liquids benzene, toluene, and bromobenzene, with dipole moments in the range of 0 to 1.7 D, are analyzed within a multimode spectral decomposition to examine the relative sensitivities of the techniques to distinguishable dynamical coordinates. We emphasize the use of minimum modal bases primarily for the purpose of comparing the spectra. Significant differences are found between the THz and OHD-RIKES spectra of benzene and bromobenzene that cannot be traced to rescaled oscillator strengths within a set of fixed-frequency modes.