准一维单组分分子晶体中Frenkel激子和电荷转移激子的混合
Mixing of Frenkel and charge-transfer excitons in quasi-one-dimensional one-component molecular crystals
摘要：
在像MePTCDI和PTCDA这样的分子晶体中，分子规则地排列，产生准一维分子堆叠。堆叠中的分子间距离（约3.3 A）与激发分子中的电子-空穴距离相当。Frenkel激子（FE）和电荷转移激子（CTE）之间的混合对于两种晶体的激子和振动光谱都是非常重要的。本文对上述晶体的线性吸收光谱进行了模拟计算。基本的哈密顿量描述的FE-CTE混合在分子堆（点群$C_i$）引起的两个转移机制，特别是电子和空穴的邻居分子。的振动光谱由混合激子和一个振动模式的分子内振动线性耦合FE和CTE。使用振动的方法，我们计算的线性光学极化率的激子和一个声子振动区域的分子堆叠，以及包含两种类型的非等效堆栈的晶体。我们把PTCDA和MePTCDI晶体的激子和振动参数在以前的研究中拟合。分析了15000 - 23000 cm^{-1}范围内线性吸收光谱的一般结构和一些重要的线形特征。我们改变激子的线宽和线性激子-声子耦合的参数的值，并寻找再现类似于所研究的晶体的吸收光谱的吸收线型的值。我们的研究表明，在解释线性吸收光谱，特别是MePTCDI晶体的线性吸收光谱时，引入FE-CTE混合的必要性。
英文摘要：
In molecular crystals like MePTCDI and PTCDA the molecule are regularly arranged creating quasi-one-dimensional molecular stacks. The intermolecular distance in a stack (about 3.3 A) is comparable with the electron--hole distance in the excited molecule. The mixing between Frenkel excitons (FEs) and charge-transfer excitons (CTEs) is very essential for the excitonic and vibronic spectra of both crystals. In this paper, we make simulations of the linear absorption spectra of the abovementioned crystals. The basic Hamiltonian describes the FE--CTEs mixing in the molecular stack (point group $C_i$) caused by two transfer mechanisms, notably of the electron and the hole on the neighbor molecules. The vibronic spectra consist of mixed excitons and one vibrational mode of an intramolecular vibration linearly coupled with FE and CTEs. Using the vibronic approach, we calculate the linear optical susceptibility in the excitonic and one-phonon vibronic regions of the molecular stack, as well as of a crystal which contains two types of nonequivalent stacks. We put the excitonic and vibrational parameters for the crystals of PTCDA and MePTCDI fitted in the previous studies. We analyze the general structure and some important features of the lineshape of the linear absorption spectra in the spectral region of 15 000--23000 cm^{-1}. We vary the values of the excitonic linewidth and the parameters of the linear exciton--phonon coupling and look for the values which reproduce the absorption lineshape similar to the absorption spectra of the investigated crystals. Our study exhibits the necessity of introducing the FE--CTEs mixing in the interpretation of the linear absorption spectra, especially of the MePTCDI crystal.
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