Molecular Dynamics Of Typical Liquid Energetic Materials Studied By Time-and Frequency-resolved Coherent Spectroscopy

Energetic materials have been widely used in military,aerospace and civil applications,but they still cannot meet the increasing requirements in safety assessment and material performance prediction.This,to a great extent,relates to the lack of understanding of the microscopic mechanism of the reactions of energetic materials.Therefore,the study of the reaction mechanism of energetic materials from the molecular level is of great importance.So far,most of the research works on the decomposition mechanism of energetic materials are based on the direct detection of reaction products,and cannot to give the complete reaction path.Besides,the research objects in most of those research works are in gas phase with low pressure,but in practical applications the energetic materials are usually in condensed phase where the intermolecular interactions will influence the reaction.Moreover,in practical applications energetic materials will be mixed with catalyst and binder whose influence on the reaction of energetic materials should also be considered.With the maturity of the technology of ultrashort pulsed laser and the application of time-resolved spectroscopy,the ultrafast dynamics of chemical reactions can be observed on the time scale from femtoseconds to picoseconds and on the spatial scale of atoms or molecules.Therefore,for the status of the study on the reaction mechanism of energetic materials,time-resolved coherent spectroscopy techniques were employed to study the excited-state dynamics,intermolecular interaction and intramolecular and intermolecular vibrational coupling mechanisms.Liquid nitromethane and nitrobenzene were chosen as the research objects in this thesis.These two nitrocompounds containing methyl and phenyl functional group,respectively,are typical model molecules of energetic materials.They have been usually used as prototype systems to test the theoretical models,experimental methods and numerical simulations in previous research works.In the reaction mechanism study of condensed-phase materials,the traditional detection techniques do not work effectively because the initial reaction products cannot be separated.Thus,the dissociation mechanisms have not been effectively confirmed yet.To solve this problem,resonant pump and broadband probe transient grating(TG)technique was used to study the excited-state relaxation and dissociation dynamics.With this technique,the reactant in the excited state and the products can be monitored simultaneously.By combing the experimental results with quantum chemical calculation,the excited-state relaxation and dissociation path of liquid nitromethane and nitrobenzene was identified in this thesis.In liquid nitromethane,most of the excited molecules go back to the ground state through internal conversion and a small part of molecules dissociate in the excited state primarily to CH3 and NO2;while after excitation,nitrobenzene molecules first relax to the ground state and then dissociate in the ground state mainly in two channels: to C6H5O+NO and to C6H4(OH)+NO.The dissociation paths are basically the same for nitromethane in liquid phase and gas phase but different for nitrobenzene,which indicates that the intermolecular interaction ways are different in these two liquids.An experimental scheme was proposed to study the intermolecular interaction through monitoring the intermolecular dynamics by non-resonant transient grating technique.Experiments under different temperatures were performed on liquid nitromethane and nitrobenzene.The processes appeared in the TG signal of nitrobenzene were identified to arise from the orientation relaxation,dipole/induced dipole(DID)effect and molecular libration in liquid cage,and DID and molecular libration dynamics were identified in liquid nitromethane.Besides,the experimental result differences between TG and the traditional optical Kerr effect(OKE)experiment were analyzed,and TG was proved to be a powerful method to get more information about intermolecular coupling effect.The intramolecular vibrational coupling and intermolecular vibrational coupling are key factors to determine the chemical reaction path of condensed-phase molecular system.The femtosecond time-resolved multiplex coherent anti-Stokes Raman spectroscopy(CARS)technique was used in this thesis to study the intermolecular vibrational coupling in the sample nitromethane mixed with IR780 dye.Broadband white light continuum was used as the Stokes light,which makes it possible to excite multiple vibrational modes simultaneously,and then to observe the coherent vibrational coupling between different kinds of molecules.The vibrational coherence transfer(VCT)between special vibrational mode of nitromethane and IR780 molecules was observed.This thesis proposed that the selectivity of VCT arises from the coupling mode of molecular vibration with phonon.It provides new experimental evidence for explaining the mechanism of intermolecular vibrational energy transfer(VET).In this thesis,a set of coherent spectroscopy methods was built to study the ultrafast molecular dynamics to meet the research requirement of liquid-phase energetic material systems.With this set of methods,the electronic state,vibrational state,intermolecular motion behavior and the coupling between them in typical liquid energetic materials were obtained,which provides research support to further exploration.