Ultrafast Dynamics Of Molecular Deformation And Functional Group Rotation For Liquid Energetic Materials

Energetic materials play an important role in many areas of the national defense and economics.The general knowledge of the reaction dynamics process and the chemical reaction mechanism is the prerequisite for designing and producing energetic materials with specific properties and as well as evaluating their safety and reliability.Although a large number of studies have been performed in this field,the microscopic reaction mechanisms for energetic materials are far from reaching an agreement.Lots of theoretical simulations of condensed phase energetic materials were performed on the atom and molecule levels.However,most of experiments focus on the dissociation products of the gas phase molecules or the macroscopic detonation process,and the experimental research on microscopic reaction mechanism of energetic materials in the condensed phase is insufficient.Thus the experimental and theoretical results can hardly match with each other.The external energy can induce the molecular deformation,the rotation of functional group and the coupling of molecular vibrational energy,which plays crucial roles in the reaction of energetic materials.However,these processes have seldom been researched in the experimental aspect until now.The nonresonant intense pumping timeand frequency resolved broadband coherent anti-Stokes Raman scattering spectroscopy?CARS?technique was used in this work to study the dynamics induced by the laser field in two kinds of energetic materials nitrobenzene and nitromethane.The purpose is to obtain the physical parameters of the dynamic process in the ground electronic state,such as the relaxation time of molecular deformation,the rotation of functional group,and the coupling of vibrational mode.These researches can provide the experimental support in building microscopic molecular reaction model and predicting reaction tunnels.The physical mechanism of molecular orientation and deformation driven by nonresonance laser is presented.The traditional CARS spectroscopy technique is optimized and improved,and the intense pumping and time-and frequency resolved broadband CARS spectroscopy is established.The testing experiment on liquid nitromethane is carried on.The observed spectral narrowing is ascribed to the stimulated Raman scattering,and the threshold of the stimulated Raman scattering is determined.Based on the theory and experiments above,the dynamics induced by intense laser field of liquid nitrobenzene molecule is tracked by intense pumping and time-and frequency resolved broadband CARS technique.The quantum beating coming from the coherently coupling of the synchronously excited vibrational modes is observed in the frequency range of 800-1200 cm-1.The CARS spectra in the 1200-1500 cm-1 frequency range show that the NO₂ torsional mode couples with the NO₂ symmetric stretching mode.The ultrafast structural deformation and the relaxation process of nitrobenzene molecule are tracked.The angle of O-N-O of nitro-group increases and then recovers with the relaxation time of 265 fs.The frequency of NO₂ torsional mode in liquid nitrobenzene at room temperature is 40±2 cm-1.According to these observed molecular deformation and vibrational energy transferring dynamics,the initial reaction of liquid nitrobenzene may be the breakage of the C-N bond.The structural deformation of liquid nitromethane molecule induced by intense laser field is tracked by intense pumping and time-and frequency broadband resolved CARS technique.The femtosecond pulse and picosecond pulse are respectively used as the probe pulse in CARS experiment.In the experiment that the femtosecond pulse is used as probe,the results are similar with that in the nitrobenzene.The structural deformation of nitro-group induced by intense femtosecond laser field of liquid nitromethane molecule is detected.The NO₂ torsional mode couples with CN symmetric stretching vibration mode,the nitro-group undergoes the ultrafast deformation with the relaxation time of 195 fs,and the frequency of NO₂ torsional mode in liquid nitromethane at room temperature is 50.8±0.3 cm-1.In the experiment that the picosecond pulse is used as probe pulse,the CARS spectra reveal that the internal rotation of the molecule can couple with the CN symmetric stretching mode.The molecules undergo ultrafast structural deformation in the methyl group from ?opened umbrella? to ?closed umbrella? shape with the relaxation time of 720 fs.The rotational constant of CH3 group in liquid nitromethane at room temperature is 5.5±0.4 cm-1,which is nearly free rotation.This result suggests that the intermolecular interaction in liquid nitromethane is quite weak and liquid nitromethane prefers proton transferring reaction under shock compression.The intense pumping and time-and frequency resolved broadband CARS technique is proposed and set up in this thesis.The molecular deformation,response of functional group,the coupling of molecular vibrational energy,and the internal rotation energy induced by the nonresonant intense femtosecond laser field of typical liquid energetic materials nitrobenzene and nitromethane are tracked successfully and the related dynamic parameters are obtained.The results provide a deeper insight into the molecular dynamics process under the intense femtosecond laser field,and provide new experimental evidence for the microcosmic reaction mechanism of energetic materials.