【作者】 牛凯；

【导师】 丛书林；

【作者基本信息】 大连理工大学 ， 原子与分子物理， 2009， 博士

【摘要】 利用飞秒脉冲激光研究分子的超快动力学过程引起人们的广泛关注。为解释时间分辨及频率分辨光谱实验现象,用适当的理论方法探讨飞秒激光场中分子超快动力学成为一项重要的课题。在气相条件下,理论上需要在含时Schr(o|¨)dinger方程中加入外场与分子相互作用势来解释外场对分子动力学的影响。对于固相或液相情况,需要加入一些环境效应诸如退相、振动弛豫因子来描述分子动力学过程。环境效应的理论描述不便于用含时Schr(o|¨)dinger方程,人们使用密度矩阵来研究含有分子与环境耦合的量子系统。由于多数化学反应在液相中进行,故密度矩阵理论被广泛地应用于研究液相分子反应动力学。本论文基于密度矩阵理论来模拟、研究液相多原子分子的超快动力学过程,探讨利用超快激光脉冲优化控制双原子分子的选态过程。主要工作包括以下几个方面:(1)基于微扰密度矩阵方法和瞬态线性极化率理论,通过自编计算机程序,我们计算丙酮溶液中Oxazine 750分子的飞秒时间分辨荧光亏蚀光谱,理论计算结果与实验光谱吻合很好。荧光亏蚀光谱反映了分子在液相中的激发动力学过程。荧光亏蚀谱的快衰减过程描述了激发态的振动弛豫过程,慢衰减过程反映了溶剂化效应。研究了探测脉冲参数及分子与环境的耦合对荧光亏蚀谱的影响。当探测脉冲频率增加时,亏蚀谱的快衰减信号衰减得更快。随着分子与环境耦合强度的增加,分子布居从高振动能级跃迁到低振动能级所需时间缩短,亏蚀谱快衰减过程加快。(2)通过计算飞秒时间分辨荧光亏蚀谱,研究了液相中叶绿素a分子的内转换过程,理论计算结果与实验符合较好。探讨了两电子态间非绝热耦合对内转换时间和亏蚀谱的影响。随着两态之间耦合强度的增加,内转换时间减少,荧光亏蚀强度减弱。当特征长度的倒数增加时,内转换时间增加,荧光亏蚀强度增强。随着分子与环境耦合强度的增加,内转换时间也增加。(3)我们利用约化密度矩阵理论计算了热浴中氟化氢分子的基电子态的高振动态的选择激发过程,计算结果表明使用脉冲链可以增加目标振动态的选态跃迁几率,且采用合适的重叠脉冲也可增加在考虑弛豫效应时的选态跃迁几率。还研究了环境的频率分布函数及分子与环境的耦合强度对弛豫过程的影响。随着环境频率分布函数带宽的增加,弛豫速率增加。当分子与环境耦合强度增加时,弛豫速率也增加。当考虑环境的弛豫效应时,目标态的跃迁几率会减少。(4)通过应用可分离谐振子近似下的受激Raman散射含时微扰理论计算了rhodamine 6G分子的十三个模式的飞秒受激Raman光谱,并计算了吸收光谱。理论计算在不同延迟时间下在共振、非共振、Stokes、anti-Stokes带等几个方面与实验光谱符合较好。更多还原

【Abstract】 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.更多还原