Research On Molecular Coherent Vibrational Dynamics By Femtosecond CARS Spectroscopy

Coherent Anti-Stokes Raman Scattering spectroscopy(CARS spectroscopy) is a kind of third-order nonlinear effect which is related to the Raman vibrational energy level of molecules. Thanks to its advantages of high signal intensity and good directivity, it personally has a very important application prospect in analyzing the spectrum. As femtosecond laser pulses show the characteristics of narrow pulse, It can be well used and controlled to study molecular ultrafast dynamics of process. The Femtosecond Coherent Anti-Stokes Raman Spectroscopy combined with two available methods mentioned above could be used to explore the detailed information about coherent vibrational dynamics in different molecules as a result. With the development of femtosecond CARS spectroscopy, nowadays it has been widely applied to the combustion temperature, fields of biomedical, microscopic imaging and ultrafast dynamics. And it certainly has become a vital means of spectral measurements. The mainly research of this dissertation is the use of coherent vibrational dynamics.In this dissertation, the author firstly introduces the basic theory research and development of CARS at home and abroad,and then,detailed analyzes the condition that which factors mainly influence femtosecond CARS spectral intensity changed with the time including phase matching mode, the width of probe pulse, chirp effect of laser pulses, the polarization characteristics of the laser and quantum beats phenomenon.Based on the theoretical studies, these experiments use a pulse width of 40 fs femtosecond laser system as the laser source in novel hybrid fs/ps CARS spectroscopy and time-resolved CARS spectroscopy. in the experiment of hybrid fs/ps CARS spectroscopy, we can obtain the signal from covered-glass and ethanol. The experimental system is built and improved which can detect femtosecond time-resolved CARS signal from samples in form of liquid and vapor, such as methanol and ethanol, distilled water, BBO crystals. At last, the wavenumber of different Raman vibrational energy levels of the samples will be studied to deeply investigate molecular ultra-fast dynamics progress. Specifically, in the organic solution we have gained the information of CH bond stretching and anti-stretching vibration, measured kinetic information of water molecules with many vibrational modes and BO bonds stretching vibrations of the inner and outer ring of BBO crystal, etc. After that, an innovation has been showed by placing the Glan prism in the optical path to control the polarization properties of the laser pulse reducing the impact of non-resonant background noise of methanol and BBO crystal experiment. It broadens the scope of femtosecond CARS spectroscopy at the same time, which is super significant for the improvement of femtosecond CARS spectroscopy.