Vibrational dynamics of liquid nitromethane at fundamental and overtone band studied by femtosecond time-resolved coherent anti-Stokes Raman spectroscopy

Time-resolved third-order and fifth-order CARS experiments were employed to systematically investigate the vibrational dynamics of liquid nitromethane (NM) molecules in both fundamental and overtone bands, and the overtone vibrational dephasing parameters of NM were reported for the first time. The experimental findings reveal that overtone vibrations exhibit a notably faster dephasing process than corresponding fundamental vibrations. This phenomenon arises from the higher energy levels of overtone vibrations, which render them more susceptible to strong intermolecular interactions within the condensed-phase environment. Among the main vibrational modes of NM, the dephasing lifetime of the C-N stretching vibration is remarkably longer than that of other modes, and even the overtone lifetime is longer than the fundamental lifetime of other modes. It is indicated that when NM is excited by external stimuli, energy has a high probability of depositing on the C-N bond, which in turn brings this bond to a high vibrational level. This inference helps account for the observation that C-N bond cleavage acts as the primary initial reaction channel in the pyrolysis and photolysis of NM. Molecular vibrations, especially the vibrations in high vibrational excited states, are closely associated with the chemical reactions of molecules. This study contributes to a better understanding of the vibrational energy localization mechanism and the subsequent molecular dissociation in condensed-phase energetic materials.