Study On The Dynamics Spectroscopy Of Liquids Under Laser-driven Shockwave

When strong pulsed laser is applied to interact with materials,shockwave can be generated.It can be used to study the dynamics of materials under extreme conditions as shockwave generates high temperature and high pressure.The dynamics of liquids under extreme conditions is very complicated.Water solutions and dyes solutions,as a place for biochemical reaction and detection environmental changes,are particularly concerned.Although there have been a lot of related reasearches,how the ion fields and free electrons affect the properties of water molecules under extreme conditions is still unexplored.At the same time,the reaction dynamics of dyes solutions under extreme conditions is still lacking in understanding due to the limitations of the instrument.Therefore,this thesis mainly focuses on two points:i.Study on the electronic dynamics of the water solution under laser-induced breakdown shockwave and the effect of ion field on the shock dynamics of water molecules by Stimulated Raman Scattering(SRS);ii.Study on the dynamics of rhodamine alcohol solutions under laser-driven flyer shockwave using absorption and fluorescence spectroscopy.(1)SRS signals of H₂O₂ water solutions are obtained by using 532 nm Nd:YAG laser.It is found that the intensity of shockwave in H₂O₂ water solutions affects the transfer of excess electrons based on SRS.Our experiment has found that the Raman signal of water and H₂O₂ molecules can be alternatively changed with the pump power.The reason for this phenomenon is attributed to the transfer of the excess electrons in the mixed solution under shockwave.Electrons are transferred to water molecules when the pump power is lower than 90 m J and higher than 130 m J,thereby enhancing hydrogen-bond structure of water molecules.However,when the pump power between90 and 130 m J,the excess electrons transfer to H₂O₂ molecules and accordingly enhance their hydrogen-bond structure.The transfer of excess electrons between water and H₂O₂ molecules is dominated by the production rates of water and H₂O₂ molecules under high pressure and high temperature.Additionally,the 5^th-order stokes line is induced by the coupling of excess electrons and H₂O₂ molecules.This research has enriched the competition mechanism and the excess electron dynamics of aqueous solutions under shockwave.(2)The SRS of silicon quantum dots(Si QDs)water solution indicates that the direction of electron transfer under shockwave is affected by the emitted fluorescence.First,we measure the SRS spectrum of Si QDs water solution under 532 nm laser(non-fluorescence emission condition)and find that the Raman signal of water molecules with Si QDs is stronger than that of pure water molecules under the same laser pump energy.At the same time,compared to pure water,the laser energy for the phase transition from liquid water to ice-VII in Si QDs water solution is far smaller.These results imply that the non-fluorescent Si QDs under 532 nm laser pump can largely improve the stimulated Raman signal and reduce the threshold of laser induced breakdown for water.This is due to the transfer of excess electrons,which generated by the exciton effect of the mixed solution,from the Si QDs to the water molecules under shockwave.Then,355 nm pulsed laser is focused on Si QDs water solution(fluorescence emission)to obtain SRS.Since the fluorescence peak of Si QDs overlaps the Raman peaks of water molecules,the fluorescence of Si QDs should enhance the Raman signal of water molecules via the resonance effect,theoretically.However,SRS shows that the threshold of the mixed solution is higher than that of pure water,and the hydrogen bond strength is lower,which is an anomaly.Spectroscopic measurements and theoretical calculations indicate that the fluorescent Si QDs under 355 nm laser pump capture the excess electrons of water created by shockwave so that the hydrogen-bond structure of water is destroyed.These results can provide a reference for the influence of QDs fluorescence on SRS under shockwave condition,and the optical properties change of QDs.(3)Using SRS shows that the equivalent pressure,which generated under the action of the ion field induced by adding a certain concentration of NaOH into the water,can enhance shockwave of water solution.We measure the SRS spectra of water under different equivalent pressures controlled by NaOH concentrations,with the pump of532 nm Nd:YAG laser.We found the main Raman peak(?3400 cm^-1)of liquid water moves towards low frequency.Interestingly,a new peak is both observed in the forward and backward scattering directions,and its position is lower than that under ambient pressure.In particular,when the equivalent pressure is 400 MPa,a peak shows at the backward direction of 3140 cm^-1,implying the formation of like-ice structure.These results indicate that the equivalent pressure can improve the SRS signal of water molecules,which can be attributed to that the equivalent pressure enhances the shockwave compression and weakens the thermal disturbance.This study confirms that the SRS spectroscopy can be used to analyze the influence of environments on the shock dynamics of liquids.(4)Time-resolved absorption and fluorescence spectroscopy revealed the phonon transfer of rhodamine alcohol solution under shockwave.Absorption and fluorescence spectra are builded under a system where laser drives a flyer plate,and rhodamine dyes are used as molecular probes.We have found that the rhodamine-6G ethanol solution shows two peaks under the shock,which can be defined as shock peak(Redshift)and no-shocked peak(Intensity Loss).The result indicates that except to the polarization between the solute and solvent under the shock of high pressure causes the peak redshift(intensity loss),the thermal phonons of solvent transferred to the dye due to the high temperature during the shock also makes the peak redshift.In addition,rhodamine molecules of different structures have different redshift and intensity losses under shockwave.This indicates that the sensitivity of the molecular structure to shockwave can be used for labeling molecules.The experimental results indicate that the density of solvent affects the aggregation and thermal phonon transfer of dye molecules under shockwave.Our results reveal that the time-resolved absorption and fluorescence spectroscopy can clearly measure the shock dynamics of liquid molecules.Importantly,it has been proven that the absorption spectroscopy is much superior to the fluorescence one,because the fluorescence peak of dye is quenched under the strong shock.