Tracking Intramolecular Vibrational Redistribution in Polyatomic Small-Molecule Liquids by Ultrafast Time-Frequency-Resolved CARS

Selective excitation of C-H stretching vibrational modes, detection of intramolecular vibrational energy redistribution (IVR), and vibrational modes coupling in the electronic ground state of benzene are performed by using femtosecond time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy. Both of the parent modes in the Raman-active bands are coherently excited by an ultrafast stimulated Raman pump, giving initial excitations of 3056 cm^-1 (A_1g) and 3074 cm^-1 (E_2g) and subsequent IVR from the parent modes to daughter modes of 1181 and 992 cm^-1, and the coherent vibrational coupling of the relevant modes is tracked. The directionality and selectivity of IVR and coherent coupling among all of the relevant vibrational modes are discussed in the view of molecular symmetry.