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Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations
The Journal of Physical Chemistry A ( IF 2.9 ) Pub Date : 2017-11-07 00:00:00 , DOI: 10.1021/acs.jpca.7b09185
Kristin Munkerup 1, 2 , Dmitri Romanov , Timothy Bohinski , Anne B. Stephansen 3 , Robert J Levis , Theis I. Sølling 2
Affiliation  

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1 surface with little excess energy at the equilibrium neutral geometry. These D1 ions are preferably formed because in this case D1 and D0 switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D1 surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D1/D0 conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0 becomes populated with molecules that are energized mainly along the cis–trans isomerization coordinate, with the kinetic energy above the cis–trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0 surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole–lone pair interaction that drives the molecule along the cistrans isomerization coordinate to facilitate the transition between the involved electronic states.

中文翻译:

内部转换过程中保持相干性和存储能量:顺式反式偶氮苯自由基阳离子的光诱导动力学

数十年来,通过在偶氮苯中储能来收集光一直是一个关键主题,光激发后分子自由度上的能量分布过程仍有待了解。当光激发系统仅由几个自由度驱动时,其动力学就会表现出高度的非遍历性。通常,由于分子内能量在统计上重新分布在所有分子自由度上,内部转换导致动力学这种局部化的损失。在这里,我们提出了一种独特的情况,即使内部转换之后,激发能量仍然保持局部化。强场电离用于制备D 1上的顺式反式偶氮苯自由基阳离子平衡中性几何体上几乎没有多余能量的表面。优选地形成这些D 1离子,因为在这种情况下,D 1和D 0在强激光场的存在下切换位置。后电离动力学是由势能态决定的。D 1表面沿着/异构化坐标并在D 1 / D 0圆锥形相交区域中朝向两个异构体共有的最小最小值陡峭下坡。相干/反式沿着该扭转运动坐标在离子瞬变由余弦调制表现。在这种情况下,D0充满了主要沿顺反异构化坐标激发的分子,并且动能高于顺反互变壁垒。这些活化的偶氮苯分子很容易沿D 0表面来回循环,并在失去相干性之前产生了多个调制信号周期。内部转化过程中内部能量的这种持久定位是由陡峭的下坡势能面,较小的初始内部能量含量以及强烈的空穴-孤对相互作用引起的,该相互作用使分子沿着-异构化坐标移动,从而促进了分子之间的过渡。涉及的电子状态。
更新日期:2017-11-08
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