Ultrafast Process Of Coherent Vibrations Of Molecules And Lattice Studied By Femtosecond CARS Spectroscopy

With the development of ultrashort pulse laser and time-resolved spectroscopytechnique, people can gain information about the molecular dynamics in the disorderedmolecular materials and coherent lattice vibrational, i.e. coherent phonon dynamics inordered crystalline materials in the ultrafast time-regime. The investigations onmolecular and lattice coherent vibration dynamic is of great value in many fields, suchas studying the interaction between element excitations, monitoring chemical reactionprocess, studying the changes of molecular and phonon surrounding environment, etc.Femtosecond time-resolved coherent anti-Stokes Raman scattering (TR-CARS)technique has proved to be a valuable tool to study the dynamics of molecular systemsboth in electronic ground state and excited state. It has been widely used to studycoherent phonon dynamics in nano-materials. So far, there are many achievements onthe theoretical and experimental investigations of femtosecond TR-CARS. However, thetheoretical investigations are not enough in-depth, and there are a lot of deficiency inexperimental investigations. Therefore, in the present thesis, the theory of femtosecondTR-CARS are analysed in depth. Based on the temporal chirp characteristics of thewhite-light continuum, we have designed the experimental system of femtosecondTR-CARS. The ultrabroadband white-light continuum (WLC) is used for the Stokespulse, due to the temporal chirp characteristics of the WLC, We have achieved theselective excitation of vibrational modes, no complicated laser system is required. Wehave studied the molecular dynamics in electronic ground state in small moleculetoluene and polymer PVK, and coherent phonon dynamics in bulk sapphire crystals.At present, the theoretical investigations of femtosecond TR-CARS technique arenot enough in-depth, there are still certain lacks of theoretical models, and there are a lotof difficulties in analyzing and explain experimental results. We analyse femtosecondTR-CARS process in depth by semiclassical theory. Considering the characteristics ofthe femtosecond laser pulses, we have built the theoretical model of femtosecondTR-CARS, explained the quantum beat signals in emtosecond TR-CARS procss. Thiswork provides important theoretical basis for analyzing and explaining the experimentalresults, and obtaining dynamic information of vibrational modes.In the field of molecular vibrational dynamics, it is insufficient on the study ofcoherent coupling between vibrational modes. This affects the integrated developmentof molecular dynamics. By femtosecond TR-CARS technique, we have achieved theselective excitation of CH stretching vibrational modes near3000cm-1in liquid tolueneand PVK film samples, and and probe the dynamics process of the vibrational modes.The theoretical formulas are in good agreement with the experimental results. Thedephasing times of the excited modes are obtained. Our investigations demonstrate that there exists coherent coupling between vibrational modes of the same kinds of chemicalbonds at different positions in molecules. This conclusion is especially helpful for us tounderstand quantum coherence and develop quantum coherence techniques.It is a subject of special significance for the study of phonon in crystals. At present,however, there is no report on coherent phonon dynamics in bulk crystals studied byfemtosecond TR-CARS technique. In order to fill the gap of research in this field, wehave studied the coherent phonon dynamics in sapphire crystals by femtosecondTR-CARS. The multiple phonon modes in300~600cm-1region are simultaneouslyexcitated and probed. The quantum beat signal is observed. We have also achieved theselective excitation of the379cm-1and418cm-1phonon mode in the low-frequencyregion, and detected the coherent phonon dynamics process. The experimental resultsare fitted using the theoretical model, the dephasing times of the two phonon modes areobtained. This work is improtant for understanding coherent phonon and developingapplied research.Defects will influence many properties of crystals.Therefore, it is important todevelop non-destructive detection technique of lattice defects. Phonon dynamicsDefects in crystals will influence phonon dynamics. We have achieved the selectiveexcitation of the645cm-1and750cm-1phonon mode in the high-frequency region, andstudied the ultrafast dynamics of coherent phonons in sapphire crystals irradiated with60Co γ-rays for three different doses. The dephasing times of the two modes bothdecrease with increasing γ-ray irradiation dose, which is due to that the dephasingprocesses of coherent phonons are accelerated because of the scattering of phonons bythe defects introduced by γ-ray irradiation. This investigation indicates that fs-CARStechnique is a powerful tool to study the effect of defects on the dynamics of coherentphonons, and it provides the possibility of developing a new non-destructive detectiontechnique to predict the damage degree of crystals.This work is of great valable for enriching the theory contents of femtosecondTR-CARS process, broadening the research scope of femtosecond TR-CARStechnology, and developing new technologies.