Tracking asymmetric intramolecular vibrational redistribution of nitromethane

Abstract

Femtosecond time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy is proposed to selectively excite parent vibrational modes and detect energy flow to daughter modes in liquid nitromethane (NM, CH3NO2). The NO symmetric bending mode (δsNO2~657cm−1, symmetry: A1), CN symmetric stretching mode (νs(CN)~918cm−1, symmetry: A1), CH3 rocking mode (ρ(CH3)~1104cm−1, symmetry: B1), nitro group stretching modes (νs(NO2)~1379cm−1, symmetry: A1 and νa(NO2)~1560cm−1, symmetry: B1) of nitromethane (NM) are selectively excited in an orderly manner, vibrational energy flow to the neighboring higher ones located outside the excitation range is observed. Counterintuitive up-hill energy flow from lower frequency modes to higher ones is related to the vibrational symmetry. What's more, it is also found that up-hill intramolecular vibrational energy redistribution efficiencies decrease monotonically with the increase of the frequency differences.