Structural deformation of nitro group of nitromethane molecule in liquid phase in an intense femtosecond laser field

The structural deformation of NO₂ group induced by an intense femtosecond laser field of liquid nitromethane (NM) molecule is detected by time- and frequency-resolved coherent anti-Stokes Raman spectroscopy (CARS) technique with the intense pump laser. Here, we present the mechanism of molecular alignment and deformation. The CARS spectra and its FFT spectra of liquid NM show that the NO₂ torsional mode couples with the CN symmetric stretching mode and that the NO₂ group undergoes ultrafast structural deformation with a relaxation time of 195 fs. The frequency of the NO₂ torsional mode in liquid NM (50.8±0.3 cm^-1) at room temperature is found. Our results prove the structural deformation of two groups in liquid NM molecule occur simultaneously in the intense laser field.