Tatchen Jörg, Pollak Eli
Chemical Physics Department, Weizmann Institute of Science, 76100 Rehovot, Israel.
J Chem Phys. 2008 Apr 28;128(16):164303. doi: 10.1063/1.2895041.
We present a theoretical study of the S(0)-->S(1) and S(0)<--S(1) vibronic spectra for trans-stilbene. Franck-Condon spectra in the harmonic approximation are generated for the complete system with 72 degrees of freedom by means of an analytic time-dependent approach accounting for Dushinsky rotations and thermal effects. The force fields are computed by means of density functional theory (DFT) and time-dependent DFT, on the one hand, and ab initio complete active space self-consistent field theory, on the other hand. The B3LYP functional shows that almost planar potential energy surface minima are found for the S(0) and S(1) state. Imposing C(2h) symmetry constraints, we obtain low-temperature high-resolution Franck-Condon spectra for both absorption and emission which are in reasonably good agreement with the experimental spectra measured by Syage et al. [J. Chem. Phys. 81, 4685 (1984)] in supersonic jets. Due to thermal population of low-energy modes, the room temperature absorption spectrum is very broad. An almost structureless band which extends over several thousand cm(-1) is obtained from the present theory and agrees with the experimental absorption band shape. Finally, within the harmonic model, we study the effect of photoexcitation on the energy distribution in the excited S(1) state. We find noticeable cooling of approximately 20 K within a frequency interval spanning from -400 to 200 cm(-1) around the 0-0 transition. This indicates that photoinduced cooling must be taken into account when considering the dynamics of the photoinduced isomerization of trans-stilbene. Moreover, this is not the final word, as anharmonicity of the low frequency modes must be taken into account to obtain a full picture which would explain both the energy dependence of the isomerization rates as well as the dependence on the pressure of an external buffer gas.
我们对反式芪的S(0)-->S(1)和S(0)<--S(1)振子光谱进行了理论研究。借助考虑杜什insky旋转和热效应的解析含时方法，为具有72个自由度的完整系统生成了谐波近似下的弗兰克 - 康登光谱。一方面，通过密度泛函理论（DFT）和含时DFT计算力场；另一方面，通过从头算完全活性空间自洽场理论计算力场。B3LYP泛函表明，在S(0)和S(1)态发现了几乎平面的势能面极小值。施加C(2h)对称约束后，我们获得了吸收和发射的低温高分辨率弗兰克 - 康登光谱，这些光谱与Syage等人[《化学物理杂志》81, 4685 (1984)]在超声速射流中测量的实验光谱相当吻合。由于低能模式的热占据，室温吸收光谱非常宽。从当前理论得到了一个延伸超过数千cm(-1)的几乎无结构的带，并且与实验吸收带形状相符。最后，在谐波模型内，我们研究了光激发对激发态S(1)中能量分布的影响。我们发现在0 - 0跃迁周围从 - 400到200 cm(-1)的频率区间内有大约20 K的明显冷却。这表明在考虑反式芪光致异构化的动力学时必须考虑光致冷却。此外，这并非最终结论，因为必须考虑低频模式的非谐性才能获得一个完整的图像，从而既能解释异构化速率的能量依赖性，又能解释对外部缓冲气体压力的依赖性。
