Park G Barratt, Steeves Adam H, Baraban Joshua H, Field Robert W
Department of Chemistry, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
J Phys Chem A. 2015 Feb 5;119(5):857-65. doi: 10.1021/jp5113608. Epub 2015 Jan 27.
The acetylene emission spectrum from the trans-bent electronically excited à state to the linear ground electronic X̃ state has attracted considerable attention because it grants Franck–Condon access to local bending vibrational levels of the X̃ state with large-amplitude motion along the acetylene ⇌ vinylidene isomerization coordinate. For emission from the ground vibrational level of the à state, there is a simplifying set of Franck–Condon propensity rules that gives rise to only one zero-order bright state per conserved vibrational polyad of the X̃ state. Unfortunately, when the upper level involves excitation in the highly admixed ungerade bending modes, ν4′ and ν6′, the simplifying Franck–Condon propensity rule breaks down--as long as the usual polar basis (with v and l quantum numbers) is used to describe the degenerate bending vibrations of the X̃ state--and the intrapolyad intensities result from complicated interference patterns between many zero-order bright states. In this article, we show that, when the degenerate bending levels are instead treated in the Cartesian two-dimensional harmonic oscillator basis (with vx and vy quantum numbers), the propensity for only one zero-order bright state (in the Cartesian basis) is restored, and the intrapolyad intensities are simple to model, as long as corrections are made for anharmonic interactions. As a result of trans ⇌ cis isomerization in the à state, intrapolyad emission patterns from overtones of ν4′ and ν6′ evolve as quanta of trans bend (ν3′) are added, so the emission intensities are not only relevant to the ground-state acetylene ⇌ vinylidene isomerization, they are also a direct reporter of isomerization in the electronically excited state.
从反式弯曲的电子激发态到线性基态电子态的乙炔发射光谱引起了相当大的关注，因为它通过弗兰克-康登原理能够访问基态的局部弯曲振动能级，这些能级沿着乙炔⇌亚乙烯基异构化坐标有大幅度运动。对于从激发态的基振动能级发射的情况，有一组简化的弗兰克-康登倾向规则，对于基态的每个守恒振动多重组，只产生一个零阶明亮态。不幸的是，当上层能级涉及在高度混合的非对称弯曲模式ν4′和ν6′中的激发时，只要使用通常的极坐标基（具有v和l量子数）来描述基态的简并弯曲振动，简化的弗兰克-康登倾向规则就会失效，并且多重组内的强度来自许多零阶明亮态之间复杂的干涉图案。在本文中，我们表明，当简并弯曲能级改用笛卡尔二维谐振子基（具有vx和vy量子数）处理时，（在笛卡尔基中）只产生一个零阶明亮态的倾向得以恢复，并且只要对非谐相互作用进行修正，多重组内的强度就易于建模。由于激发态中的反式⇌顺式异构化，随着ν4′和ν6′泛音的多重组发射图案随着反式弯曲（ν3′）量子的增加而演变，因此发射强度不仅与基态乙炔⇌亚乙烯基异构化有关，它们也是电子激发态异构化的直接报告者。
