Ultrafast Spectroscopy For Photoinduced Reaction Dynamics Of Molecular CH₃I

Recording and analyzing ultrafast molecular dynamics is a highlight in molecular dynamic researches. In this paper, we analyze the ultrafast dynamics of molecular CH3 I through methods of time-resolved spectra. As a non-contact measurement method, the optic spectrographic technique has more advantage than the mass spectrometric technique, where optic methods are more applicable to the ultrafast researches of liquid and solid sample. However, it is difficult to record a reaction of the condensed matter using a mass spectrometer. As well known, water environment is a one of the necessities of life. A lot of important biochemical reactions are reacted in liquid, such as photosynthesis and protein synthesis. In addition, most energetic materials are reacted in condensed matters, such as explosive, gunpowder and rocket propellant. The clear reaction dynamics will promote the development of such energetic materials, which has vast importance to the national defense industry. Development optic method to analyze ultrafast reaction mechanisms of condensed matter is a key technique which is pursued in the field of ultrafast molecular dynamics.The molecular CH3 I has become a typical sample for dissociation researches because of the less atomic number and various dissociation paths. A l ot of research of CH3 I dissociation in the past 20 years makes the CH3 I become a “standard sample” for dissociation dynamics. The research conclusions about CH3 I dynamics has good suitability and scalability, which could be extended to other condensed samples. In addition, recent studies have shown that the CH3 I has the potential to become an ideal active medium as the iodine-containing compound in the chemical oxygen iodine laser. The time-resolved spectrum is a feasible method to study the CH3 I ionization and dissociation dynamics. In comparison with time-of-flight mass spectrometry, the real-time emission spectroscopy is a more practical and convenient technique to monitor the CH3 I dissociation dynamic in chemical oxygen iodine laser researches.Molecular dynamic process in electronic ground state, electronic excited state and ionized state are the key reaction mechanisms, which have internal relations. We use three time-resolved spectrum methods in this work, including coherent anti-Stokes Raman Scattering, impulsive stimulated Raman scattering, and laser induced breakdown spectroscopy, by which the reaction dynamics of CH3 I molecule, such as the structural defamation dynamics, the liquid photo-dissociation dynamics, and the multiphoton ionization dynamics are analyzed.First, We use the coherent anti-Stokes Raman Scattering to research the CH3 I molecule structural defamation, in which the obtained molecular time-resolved rovibrational spectra are analyzed to track the molecular structural deformation and relaxation. By using the above-mentioned spectrum technology, We observe and obtain the CH3 I molecular structural deformation and relaxation in liquid for the first time, which provides new experimental evidence about the molecular deformation. Meanwhile, this research also provides the theoretical basis of classic dipole moment interaction and quantum dynamic Stark effect to explain the mechanisms of molecular structural deformation.Second, in the research of pure liquid CH3 I photo-dissociation dynamics, we design and build an impulsive stimulated Raman scattering spectroscopic system, in which the wave packet is created by the 266 nm pump excitation, and is tracked by the broadband white light continuum. The above spectrum technology overcomes the limitation of mass spectrometry used in liquid sample, and enables us to characterize the pure liquid CH3 I photo-dissociation dynamics for the first time, including the analysis of the binding wave packet due to the solvent cage and the assignments of dissociation products.Finally, in the research of ionization and dissociation of CH3 I, we find that the plasma production and evolution processes in the electric spark can be well simulated by applying the femtosecond laser pulse due to multiphoton ionization. We obtain and assign the characteristic emission peaks with high spectral resolution through femtosecond laser induced breakdown spectroscopy, by which the dissociation products are identified. Meanwhile, the obtained time-resolved spectra are used to analyse the plasma evolution and to evaluate the mechanism of I2 production.In this paper, the typical sample of CH3 I is used as the research object. The mentioned optic systems, the experimental methods, and the conclusions in this work have good suitability and scalability, which are suitable for other samples. The obtained results will be very useful in ultrafast dynamic researches on other condensed-phase materials.