The Ultrafast Vibrational Relaxation And Internal Conversion Dynamics Of The Polyatomic Molecules In Liquids Studied In Femtosecond Laser Pulse

With the development of the ultra-fast laser technology, molecular dynamics on the time scale of some femtoseconds is a subject of considerable interest. The advent of femtosecond optical laser technology has prompted a rapidly increasing amount of experimental and theoretical study considering the time-resolved spectroscopic investigation of ultra-fast molecular dynamics. Recently, the possible influence of environmental degrees of freedom has not been incorporated into the calculations of wave packet dynamics on investigating dynamical phenomena in molecular systems on a femtosecond time scale. Thus, the description of wave packet dynamics has been restricted to the gas phase. As almost all the biological chemical reaction take place in liquids, studying the ultra-fast dynamics of polyatomic molecules in liquids is the key to reveal the essence of the biological chemical reaction. The effects like dephasing and vibrational relaxation processes should be taken into account. In order to consider a coupling of the molecular system to a specific environment into the theoretical description one usually utilizes the density matrix formalism. The reduced density operator, the quantum master equation, etc. have been used to describe relaxational phenomena in molecular systems.The main works in this thesis are on the ultra-fast dynamics of polyatomic molecules in liquids using the perturbative density matrix method and the transient linear susceptibility theory. The fluorescence depletion spectrum of polyatomic molecules can be calculated within the density matrix models. According to the Vavilov theory, the fluorescence is generated by the transition from the level v=0 of the first excited electronic state S₁ to the ground electronic state S₀. The fluorescence intensity is proportional to the population of the excited electronic state. The fluorescence depletion reflects the vibrational relaxation dynamics in the electronic excited state of the molecule. Molecular dynamics can be monitored by the delay time between the pump and the probe pulses.Firstly, the fluorescence depletion spectra (FDS) are calculated to study the ultra-fast vibrational relaxation dynamics and the solvation effect of the excited state molecules. The effects of temperature and probe pulse parameters on the FDS are also investigated. A faster decay of the FDS with a few hundreds of femtoseconds reflects the vibrational relaxation time in the S₁ state and depends on the solute molecules. A slower decay process with a picosecond time scale reflects the solvation effect and depends on the property of solvent.Secondly, the FDS of chlorophyll-a (chl-a) in ethyl acetate are calculated and the internal conversion (IC) process is studied. The calculated FDS of chl-a agrees well with the experimental results. The IC time on the FDS is also discussed. With decreasing the IC time, the fluorescence depletion increases.Thirdly, the IC pathways and times between S₃ and S₁ states are investigated by simulating the FDS of chl-a in ethyl ether solvent using a theoretical model developed by us. The theoretical calculation shows that sequential IC process,S₃→S₂→S₁. agrees well with the experimental results.At last, the FDS and IC rates of the dye Rhodamine700 in methanol. ethanol and DMSO solvents are calculated using the density matrix theory. The effects of donor number, the hydrogen-bonding, the viscosity of the solvent on the FDS and IC times are also investigated.