Kosma K, Trushin S A, Fuss W, Schmid W E
Max-Planck-Institut für Quantenoptik, D-85741, Garching, Germany.
Phys Chem Chem Phys. 2009 Jan 7;11(1):172-81. doi: 10.1039/b814201g. Epub 2008 Nov 6.
The third harmonic (270 nm, 11 fs), produced in a short argon cell from Ti-sapphire laser pulses (810 nm, 12 fs), was used to excite 1,3-cyclohexadiene to its lowest pipi* state (1B). Probing was done by transient ionization by the 810 nm pulses, measuring the yields of the parent and a fragment ion. As previously found with 10 times longer pulses, the molecule leaves in two steps (time constants tau(1), tau(2)) from the spectroscopic (1B) to a dark (2A) state and from there (within tau(3)) to the ground-state surface. In addition to slightly improved values for tau(1)-tau(3), we found in all three locations (L(1)-L(3)) on the potentials coherent oscillations, which can be assigned to vibrations. They are stimulated by slopes (driving forces) of the potentials, and the vibrational coordinates indicate the slope directions. From them we can infer the path following the initial excitation: the molecule is first not only accelerated towards CC stretching in the pi system but also along a symmetric C[double bond, length as m-dash]C twist. The latter motion-after some excursion-also erects and stretches the CH(2)-CH(2) bond, so that Woodward-Hoffmann interactions are activated after this delay (in L(2)). On leaving L(2) (the 1B minimum) around the lower cone of the 1B/2A conical intersection, the wave packet is rapidly accelerated along an antisymmetric coordinate, which breaks the C(2) symmetry of the molecule and eventually leads in a ballistic path to (and through) the last (2A/1A) conical intersection. The ring opening begins already on the 1B surface; near the 2A minimum it is already far advanced, but is only completed on the ground-state surface.
由钛宝石激光脉冲（810纳米，12飞秒）在一个短氩气池中产生的三次谐波（270纳米，11飞秒），用于将1,3 - 环己二烯激发到其最低的ππ*态（1B）。通过810纳米脉冲的瞬态电离进行探测，测量母体离子和一个碎片离子的产率。正如之前用长10倍的脉冲所发现的那样，分子分两步（时间常数τ(1)，τ(2)）从光谱态（1B）跃迁到暗态（2A），并从那里（在τ(3)内）跃迁到基态表面。除了τ(1) - τ(3)的值略有改善外，我们在势能的所有三个位置（L(1) - L(3)）都发现了相干振荡，其可归因于振动。它们受到势能斜率（驱动力）的激发，并且振动坐标指示斜率方向。由此我们可以推断初始激发后的路径：分子首先不仅在π体系中朝着碳 - 碳（C - C）键拉伸加速，而且还沿着对称的碳 - 碳（C═C）扭转方向加速。后者的运动在经过一些偏移后，也会使亚甲基（CH₂ - CH₂）键竖起并拉伸，从而在这种延迟之后（在L(2)处）激活伍德沃德 - 霍夫曼相互作用。在离开1B/2A锥形交叉点下锥附近的L(2)（1B最小值）时，波包沿着反对称坐标迅速加速，这打破了分子的C₂对称性，并最终以弹道路径通向（并穿过）最后一个（2A/1A）锥形交叉点。开环在1B表面就已开始；在2A最小值附近它已经进展得很远，但仅在基态表面完成。
