The Dynamics Of Ultrafast Process In Liquids Studied By Femtosecond Laser

Femtosecond time-resolved studies using fluorescence depletion spectroscopy were performed on the vibrational relaxation; orientational relaxation and the intermolecular energy transfer of dye molecules in solutions under different conditions. The correlation of such ultrafast process with the system temperature and property of solvents is analyzed, which is useful for the further study about the ultrafast process of DNA, protein and other biological molecules, such as intra- and intermolecular electron, proton and energy transfer and photoisomerization.Firstly, a set of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD) is built, using the homemade Ti:sapphire femtosecond laser with regenerate amplifier. Some difficult technical problems, such as the measure of the zero-time point and the correlation function, are resolved. And the set have been improved to be the first polariton resolved FS TR SEP FD in the world, which can be used to study the orientation dynamics.Secondly, femtosecond time-resolved studies using fluorescence depletion spectroscopy were performed on Rhodamine 700 and Oxazine 750 in solutions to study the ultrafast dynamics, such as vibrational relaxation and the energy transfer from the solute to solvent. The orientational relaxation process of these dye molecules have been study in solution by the polariton resolved FS TR SEP FD.Lastly, the dynamic time constants of the vibrational relaxation and the energy transfer from the solute to solvent, which are hundreds fsand ranged from 1 to 50 ps respectively, were got by fitting the experimental data using a bi-exponential function. The intramolecular vibrational redistribution (IVR) of the dye molecules is insensitive to the character of solvents, and the time constants of them increase linearly as the temperature of environment decreases. The difference of the average vibrational energy of solute molecules in the ground state at different temperature is a possible reason that induces this IVR time constants temperature dependence. The cooling of the hot solute molecules by the collisional energy transfer to the surrounding strongly depends on the Lewis basicity of solvents. The more basic the solvent is, which mean the more interaction of the solute and the neighboring solvent shell, the more rapid of the intermolecular vibrational excess energy transfer from the solute to the surroundings is. The time constants of the energy transfer from vibrational hot solute molecules to the surrounding changed dramatically at lower temperature, nonlinearly increase with the decrease of temperature. Because of the C-H-0 hydrogen-bond between acetone molecules, it is more reasonable that acetone molecules start to be associated, which can influence the energy transfer between dye molecules and acetone molecules efficiently, even at the temperature far over the freezing point. Using the polariton resolved FS TR SEP FD, the orientational relaxation of the dye molecules in DMSO are studied. The time constants of them are less than 2 ps.