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首页> 外文期刊>The Journal of Chemical Physics >First principles simulation of the UV absorption spectrum of ethylene using the vertical Franck-Condon approach
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First principles simulation of the UV absorption spectrum of ethylene using the vertical Franck-Condon approach

机译：使用垂直Franck-Condon方法模拟乙烯的UV吸收光谱的第一原理

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A new method which we refer to as vertical Franck-Condon is proposed to calculate electronic absorption spectra of polyatomic molecules.In accord with the short-time picture of spectroscopy,the excited-state potential energy surface is expanded at the ground-state equilibrium geometry and the focus of the approach is more on the overall shape of the spectrum and the positions of the band maxima,rather than the precise position of the 0-0 lines.The Born-Oppenheimer approximation and the separability of the excited-state potential energy surface along the excited-state normal mode coordinates are assumed.However,the potential surface is not necessarily approximated as harmonic oscillator potentials along the individual normal modes.Instead,depending upon the nature of the potential surface along a particular normal mode,it is treated either in the harmonic approximation or the full one-dimensional potential is considered along this mode.The vertical Franck-Condon approach is applicable therefore even in cases where the excited state potential energy surface is highly anharmonic and the conventional harmonic Franck-Condon approach is inadequate.As an application of the method,the ultraviolet spectrum of ethylene between 6.2 eV (50 000 cm~(-1)) and 8.7 eV (70 000 cm~(-1)) is simulated,using the Similarity Transformed Equation of Motion Coupled-Cluster method to describe the required features of the potential energy surfaces.The spectrum is shown to be a result of sharp doublet structures stemming from the pi->3s (Rydberg) state superimposed on top of a broad band resulting from the pi-pi~* (valence) state.For the Rydberg state,the symmetric C-C stretch and the torsion mode contribute to the spectrum,while the broad valence band results from excitation into the C=C stretch,CH_2 scissors,and the torsion mode.For both states,the potential along the torsion mode is highly anharmonic and the full treatment of the potential along this mode in the vertical Franck-Condon method is required.
机译：提出了一种称为垂直弗兰克-康登的垂直计算多原子分子电子吸收光谱的新方法。根据光谱学的短时图，激发态势能面在基态平衡几何下扩展该方法的重点更多地放在频谱的整体形状和谱带最大值的位置上，而不是0-0线的精确位置上。Born-Oppenheimer逼近和激发态势能的可分离性假定沿着激发态法向模坐标的表面。但是，不一定将势能面近似为沿着单个法则模的谐波振荡器势。相反，取决于沿着特定法则模的势面的性质，将其处理沿该模式考虑谐波近似或完整的一维电势。垂直Franck-Condon方法适用于因此，即使在激发态势能面高度非谐且常规谐波Franck-Condon方法也不充分的情况下。作为该方法的应用，乙烯的紫外光谱在6.2 eV（50 000 cm〜（-1））之间用运动耦合簇方法的相似变换方程描述了势能面的所需特征，并模拟了8.7 eV（70 000 cm〜（-1））。该光谱表明是尖锐的双峰结构的结果源自pi-> 3s（Rydberg）状态叠加在pi-pi〜*（价）状态所产生的宽带之上。对于Rydberg状态，对称CC拉伸和扭转模式有助于频谱，而宽价带是由激发进入C = C拉伸，CH_2剪刀和扭转模式产生的。对于这两种状态，沿扭转模式的电势高度不谐和，并且在垂直Franck-中充分处理了沿该模式的电势康登法 是必须的。

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