CARs Spectroscopy For Intramolecular Vibrational Energy Redistribution Of Small Polyatomic Molecules

Intramolecular vibrational energy redistribution?IVR?is the first step in chemical reaction process and decides the speed of chemical reaction and the change of chemical structure.A clear understanding of IVR is contributed to exploration of biomacromolecule,dissociation of organic molecular materials,development and preservation of energetic or industrial materials.Intramolecular enegry flow from one mode to another occurs on an ultrafast femtosecond or picosecond time scale.Therefore,it is difficult to detect and record the ultrafast time-resolved dynamic process.Development up optic method to detect IVR and analyze ultrafast reaction mechanisms is a key technique which is pursued in the scientific research.It is difficult to selective excitation of one vibrational mode and detection of ultrafast vibrational energy flow from one mode to another one.Aiming at such problems,we use femtosecond time-resolved coherent anti-Stokes Raman Scattering?CARS?spectroscopic technique to seletively excite different frequency modes of highly symmetric small molecules in the condensed phase,then detect intramolecular energy flow from the mode in excitation range to another mode which locate outside of excitation range.Target molecules have the simple structrue and can be easily acquired general conclusions about the relationship between the key structure and performance.Hence,we regared them as the“standard samples”.Moreover,the higher frequency CH stretching vibrational modes and lower frequency substituent-dependent modes in these model molecules are really easy to identify.Therefore,using the results of IVR in these polyatomic small molecules as a reference and guidance for exploring enegry redistribution in biomacromolecule or organic polymer before chemical reactions is meaningful.Firstly,the higher frequency CH stretching vibrational modes around 3000 cm^-1 of benzene and nitrobenzene are selectively excited.It can be conformed that the femtosecond time-resolved CARS technique can selective excitaion of vibrational modes and detection of ultrafast IVR processes.Intramolecular energy can flow from high frequency CH stretching vibrational modes to lower frequency ones which locates outside of excitation range.Intramolecular energy flow between the modes with same vibrational symmetry,it means that IVR has selectivity.Dynamic process of selectively excite vibrational modes and detect IVR is tracked at the first time.Secondly,the low frequency vibrational modes and higher ones are selectively excited,respectively,and the“up-hill”IVR processes are detected in liquid acetonitrile and nitromethane.Cohernt coupling information is tracked in femtosecond time-resolved CARS spectra.From the experimental results of acetonitrile and nitromethane,vibrational energy can flow from low frequency modes to higher ones and the“up-hill”energy redistribution is firstly confirmed in the ultrafast spectroscopy experiments.The efficiencies of intramolecular energy redistribution are evaluated,it is found that efficiencies of intramolecular“up-hill”energy redistribution decrease monotonically with the increase of the frequency differences.Finally,the substituent-dependent modes,CC stretching vibrational modes and CH stretching vibrational modes of phenyl are selectively excited in an orderly manner,and“up-hill”vibrational energy redistribution is controlled by the mass of the halide.From the results of intramolecular energy redistribution efficiencies,we find that the substituent-dependent modes are selectively excited,efficiencies of vibrational energy redistribution are increased with increasing relative molecular mass ratio;On the other hand,the phenyl modes of the aryl halides are selectively excited,efficiencies of vibrational energy redistribution are decreased with the increasing relative molecular mass ratio.The effects of halides mass on the intramolecular energy redistribution are described at the first time.In this paper,selective excitation of different frequency vibrational modes,then detection of IVR of polyatomic small molecules in condensed phase are performed by using femtosecond time-resolved CARS spectroscopy.The mentioned optic systems,the experimental methods,and the conclusions of IVR in this work have good suitability and scalability,which are suitable for complex biopolymer molecules or energetic materials.