Tracking molecular structure deformation of nitrobenzene and its torsion–vibration coupling by intense pumping CARS

The structural deformation induced by intense laser field of liquid nitrobenzene (NB) molecule, a typical molecule with restricting internal rotation, is tracked by time- and frequency-resolved coherent anti-Stokes. Raman spectroscopy (CARS) technique with an intense pump laser. The CARS spectra of liquid NB show that the NO₂ torsional mode couples with the NO₂ symmetric stretching mode, and the NB molecule undergoes ultrafast structural deformation with a relaxation time of 265 fs. The frequency of NO₂ torsional mode in liquid NB (42 cm⁻¹) at room temperature is found from the sum and difference combination bands involving the NO₂ symmetric stretching mode and torsional mode in time- and frequency-resolved CARS spectra. (paper)