Visualizing the Effect of Phenyl Group on the Intra-or Intermolecular Vibrational Dynamics of Nitromethane, Nitrobenzene and Their Mixtures by Coherent Anti-Stokes Raman Scattering

We experimentally investigate the vibrational dynamics of nitromethane (NM), nitrobenzene (NB) and their mixtures using femtosecond time-resolved coherence anti-Stokes Raman scattering (CARS) spectroscopy. At first, we coherently excite Raman active modes of NM, NB and the beats arising from vibrational coupling can be clearly identified in CARS spectra. Results indicate that pairs of vibrational modes involved in the coupling correspond to different groups in one molecule. In a second stage, CARS spectroscopy is performed when vibrational modes of mixtures are collectively excited. NM/acetone and NM/methanol mixtures beats are not observed because the hydrogen bonds lead to a faster decoherence. On the other hand, evidences of beats and vibrational dephasing are found in NB/acetone and NB/methanol mixtures despite the fact that interactions in the mixture involve strong hydrogen bonds. The main reason behind this behavior is that phenyl group has a crucial influence on vibrational dynamics in mixtures. In particular, our results confirm that phenyl group bending mode at 425 cm−1 of NB is coupled with the other modes, and suggest may be instrumental in the energy transfer among molecules.