Description Of Molecular Ultrafast Dynamics Process Using The Density Matrix Theory

There is great interest to study molecular ultrafast dynamics using femtosecond laser pulse. To elucidate the experiments of time-resolved spectroscopy, investigating the dynamics of molecules with appropriate theoretical approach becomes a crucial topic. In gas phase, an external field factor should be involved in the schr(o|¨)dinger equation to account for the field molecule interaction. For solid or liquid phases, some environmental effects, such as dephasing and vibrational relaxation, should be taken into account for studying molecular dynamics. The density matrix is used to interpret the molecule environment coupling dynamics rather than the schr(o|¨)dinger equation. Since most chemical reactions are carried out in the liquid phase, the density matrix theory has been widely used in the study of molecular reaction dynamics.In this thesis, based on the density matrix theory, the ultrafast dynamics process of the polyatomic molecules in liquids have been investigated using femtosecond laser pulse. The selective excitation of the high ground vibrational states of HF in an environment by a pulse train is also studied. The main works are as follows.(1) To accurately and efficiently simulate molecular dynamics, perturbative density matrix method and the transient linear susceptibility theory can be used to investigate the fluorescence depletion spectrum. The fluorescence depletion spectrum (FDS) of Oxazine 750 in acetone are calculated within the reduced density matrix. The theoretical calculation agrees well with the experimental results. The fluorescence depletion reflects the molecular dynamics of the excited states. Here, a faster decay of the FDS reflects the vibrational relaxation time in the S₁ state and a slower decay process reflects the solvation effects. The effects of the probe pulse frequency and system-reservoir coupling on the FDS are also discussed. When the probing frequency increases, the faster decay process of the fluorescence depletion signal becomes quicker. With the increase of the system-reservoir coupling, the population transfers more quickly from high vibrational levels to the ground vibrational level of the excited electronic state, and the faster decay process of the FDS becomes faster.(2) The femtosecond time-resolved FDS involving IC process of Chlorophyll a (chl a) in solvents is studied. The calculated FDS of chl a agrees well with the experimental results. The effects of diabatic coupling between two electronic states on the FDS and the IC time are investigated. With the increase of the intersite coupling parameter, the internal conversion time decreases and the fluorescence depletion intensity in a long delay time region decreases. When the characteristic inverse length increases, the internal conversion time increases and the fluorescence depletion intensity in a large decay time region increases. With the increase of the molecule-reservoir coupling, the internal conversion time increases.(3) The selective excitation of the high ground vibrational state of rotationless HF in an unobserved quasi-resonant thermal environment under the control of a single pulse and pulse train is studied using the reduced density matrix theory. The results have shown that the probability of transferring population to the target state can be improved for the pulse train or the use of overlapping pulses. We have also studied the effects of the environment frequency distribution and the molecule-environment coupling intensity on the relaxation rate. With broadening of the environment frequency distribution or increasing molecule-environment coupling intensity, the relaxation rate increases. When the molecule-environment coupling is taken into account, the target state occupation will obviously decrease.(4) The femtosecond stimulated Raman spectroscopy of rhodamine 6G (R6G)of the 13 modes is applied by the perturbation theory of stimulated Raman scattering with harmonic potentials. And the absorption spectrum of R6G is also calculated. The calculation compared very well with the R6G experimental results for off-resonance and resonance femtosecond stimulated Raman scattering spectra spanning both Stokes and anti-Stokes bands, and for negative and positive pump-probe delay times on resonance.