Park G Barratt, Baraban Joshua H, Field Robert W
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2014 Oct 7;141(13):134305. doi: 10.1063/1.4896533.
A full-dimensional Franck-Condon calculation has been applied to the à (1)Au-X̃ 1Σg+ transition in acetylene in the harmonic normal mode basis. Details of the calculation are discussed in Part I of this series. To our knowledge, this is the first full-dimensional Franck-Condon calculation on a tetra-atomic molecule undergoing a linear-to-bent geometry change. In the current work, the vibrational intensity factors for levels involving excitation in ungerade vibrational modes are evaluated. Because the Franck-Condon integral accumulates away from the linear geometry, we have been able to treat the out-of-plane component of trans bend (ν4('')) in the linear X̃ state in the rotational part of the problem, restoring the χ Euler angle and the a-axis Eckart conditions. A consequence of the Eckart conditions is that the out-of-plane component of ν4('') does not participate in the vibrational overlap integral. This affects the structure of the coordinate transformation and the symmetry of the vibrational wavefunctions used in the overlap integral, and results in propensity rules involving the bending modes of the X̃ state that were not previously understood. We explain the origin of some of the unexpected propensities observed in IR-UV laser-induced fluorescence spectra, and we calculate emission intensities from bending levels of the à state into bending levels of the X̃ state, using normal bending mode and local bending mode basis sets. Our calculations also reveal Franck-Condon propensities for the Cartesian components of the cis bend (ν5('')), and we predict that the best Ã-state vibrational levels for populating X̃-state levels with large amplitude bending motion localized in a single C-H bond (the acetylene↔vinylidene isomerization coordinate) involve a high degree of excitation in ν6(') (cis-bend). Mode ν4(') (torsion) populates levels with large amplitude counter-rotational motion of the two hydrogen atoms.
已在简正振动模式基组下对乙炔的Ã(1)Au - X̃1Σg +跃迁进行了全维弗兰克 - 康登计算。该计算细节在本系列的第一部分中讨论。据我们所知，这是对经历线性到弯曲几何结构变化的四原子分子进行的首次全维弗兰克 - 康登计算。在当前工作中，评估了涉及非对称振动模式激发的能级的振动强度因子。由于弗兰克 - 康登积分在远离线性几何结构处积累，我们能够在问题的转动部分中处理线性X̃态下反式弯曲（ν4('')）的面外分量，恢复χ欧拉角和a轴埃卡特条件。埃卡特条件的一个结果是ν4('')的面外分量不参与振动重叠积分。这影响了坐标变换的结构以及重叠积分中使用的振动波函数的对称性，并导致了涉及X̃态弯曲模式的选择定则，这些选择定则以前并不为人所知。我们解释了在红外 - 紫外激光诱导荧光光谱中观察到的一些意外选择定则的起源，并使用简正弯曲模式和局部弯曲模式基组计算了从Ã态的弯曲能级到X̃态的弯曲能级的发射强度。我们的计算还揭示了顺式弯曲（ν5('')）笛卡尔分量的弗兰克 - 康登选择定则，并且我们预测，用于填充X̃态能级且具有大振幅弯曲运动且局限于单个C - H键（乙炔↔亚乙烯基异构化坐标）的最佳Ã态振动能级涉及ν6(')（顺式弯曲）的高度激发。模式ν4(')（扭转）填充两个氢原子具有大振幅反向旋转运动的能级。
