Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Boulevard, Flushing, New York 11367, USA.
J Chem Phys. 2013 Apr 7;138(13):135101. doi: 10.1063/1.4795824.
The emission lineshape of the B850 band in the light harvesting complex 2 of purple bacteria is calculated by extending the approach of 2nd order time-nonlocal quantum master equation [S. Jang and R. J. Silbey, J. Chem. Phys. 118, 9312 (2003)]. The initial condition for the emission process corresponds to the stationary excited state density where exciton states are entangled with the bath modes in equilibrium. This exciton-bath coupling, which is not diagonal in either site excitation or exciton basis, results in a new inhomogeneous term that is absent in the expression for the absorption lineshape. Careful treatment of all the 2nd order terms are made, and explicit expressions are derived for both full 2nd order lineshape expression and the one based on secular approximation that neglects off-diagonal components in the exciton basis. Numerical results are presented for a few representative cases of disorder and temperature. Comparison of emission line shape with the absorption line shape is also made. It is shown that the inhomogeneous term coming from the entanglement of the system and bath degrees of freedom makes significant contributions to the lineshape. It is also found that the perturbative nature of the theory can result in negative portion of lineshape in some situations, which can be removed significantly by inclusion of the inhomogeneous term and completely by using the secular approximation. Comparison of the emission and absorption lineshapes at different temperatures demonstrates the role of thermal population of different exciton states and exciton-phonon couplings.
通过扩展二阶非局域量子主方程方法[Jang and Silbey, J. Chem. Phys. 118, 9312 (2003)]，计算了细菌光合作用 2 号复合物中 B850 带的发射谱线形状。发射过程的初始条件对应于平衡时激子态与浴模纠缠的稳态激发态密度。这种在局域激发或激子基中都不是对角的激子-浴耦合，导致了一个新的非均匀项，而在吸收谱线形状的表达式中不存在该非均匀项。对所有二阶项进行了仔细处理，并推导出了完整的二阶谱线形状表达式和基于忽略激子基中非对角分量的定态近似的表达式。针对几种代表性的无序和温度情况给出了数值结果。还比较了发射谱线形状与吸收谱线形状。结果表明，系统和浴自由度的纠缠产生的非均匀项对谱线形状有重要贡献。还发现，在某些情况下，理论的微扰性质可能导致谱线形状的负部分，通过包含非均匀项和完全使用定态近似可以显著消除。比较不同温度下的发射和吸收谱线形状，展示了不同激子态的热占据和激子-声子耦合的作用。
