Characteristic Research Of Nanosecond Laser Driven Shock Wave Based On Anthracene Molecular Probe

The characteristic of shock wave is an important foundation of shock wave physics and plays an important role in the interaction between shock wave and materials.This paper aims to use the Raman and fluorescence spectrum of anthracene to establish some non-contact experimental methods which could obtain the pressure,temperature and energy of nano-laser driven shock wave,and reveal the physical mechanism behind those methods.To observe the propagation process of laser driven shock waves in the sample,a time-resolved Raman spectral detection system under shock loading was established in this paper.The time-resolved Raman spectra of shocked anthracene was analyzed by modified pressure distribution formula,the pressure distribution in the sample layer,the front spatial width,peak pressure,and velocity of shock wave were obtained in the processes of shock wave loading and unloading,thus giving a clearer picture of the detailed propagation process of shock wave in sample layer.Noted that the relative intensity of compressed Raman mode is proportional to the velocity of shock wave,the original calculation formula of shock velocity was revised based on the newly established shock wave loading physical model,and the modified calculation results were in good agreement with other methods.To study another important physical quantity of shock wave,temperature,the pressure and temperature effects on the fluorescence spectrum of anthracene were analyzed systematically in detail and the temperature in shocked material was discussed on this basis.The excitation wavelength dependence of fluorescence spectra of anthracene demonstrates that when the excitation wavelength gets close to the central wavelength of excimer spectrum,the anthracene molecule is more likely to form van der Waals dimers and generates structureless and broad excimer emission.When pressure is loaded,the excimer emission originated from pressure-induced van der Waals dimers is observed.The excitation wavelength dependence of the fluorescence spectra at high-pressure proves that excitation wavelength can promote the formation of van der Waals dimer,similar to the pressure effect.The research of the anthracene fluorescence at static pressure lays a solid foundation for its use as a temperature sensing probe in shocked materials.In order to find the fluorescence parameter that can be used for temperature sensing,the steady fluorescence spectra ranging from 300 K to 500 K were observed.It was found that the intensity of all the characteristic peaks in fluorescence spectrum increase with temperature and the logarithmic intensity ratio showed a linear correlation with the inverse of temperature,which was promising for temperature sensing.By the sensitivity and random uncertainty analysis,the intensity ratio of 2-0 transition to 1-1 transition,was proved to be the best comprehensive performance physical quantity for temperature sensing and can be used as temperature sensing probe in shocked materials.The temperature effects on the lifetime of different transition components suggest that the 2-0 and 1-1 transitions were originated from the second excited triplet state and the first excited singlet state,and the linear relationship between the corresponding intensity ratio and temperature can be ascribed to the thermally activated system crossing.Based on these results,the mean temperature can be detected when shock wave propagates completely through the sample layer,and therefore the injection energy dependence of the mean temperature in sample layer was obtained.The process of shock energy flowing into intra-molecular vibration modes is the core step for the rate of shock induced chemical reaction.After theoretical analysis,this paper proposed that there is another quicker shock energy transfer channel in shocked materials,through which the shock energy can directly flow into intra-molecular modes rather than go through the traditional multi-phonon up pumping process.This energy transfer channel is named as coherent transfer channel since the shock wave directly modulates the intra-molecular vibration modes and those modes will be coherently excited.Angle-resolved Raman scattering experiments found that the relative intensity and linewidth both reached their peak values around the phase matching angle,which demonstrates that the shock wave can directly generate coherent optical phonons in molecular crystal,in other words,there is a coherent energy transfer channel for shock energy injection.This research systematically studied the non-contact measure methods of pressure,temperature and energy in shocked materials based on the anthracene molecular probe under nano-laser driven shock wave.Not only the pressure distribution in sample layer during loading and unloading processes,but also the peak pressure and velocity of laser driven shock wave was obtained through the time-resolved Raman spectrum;a new fluorescence intensity ratio temperature sensing method for shocked materials was established based on the research of anthracene fluorescence spectra;the new coherent transfer channel for shock energy flowing into intra-molecular modes was proved by angle-resolved Raman scattering experiments.This work provide new knowledge about the characteristics of laser driven shock wave,and lay a solid theoretical and experimental foundation for exploring the interaction between shock wave and materials,especially in shock induced chemical reactions.