Roaming反应通道的激发态动力学研究

The transition state theory has been central to chemistry because the products, rates, and dynamics of a reaction are often determined by the "transition state" (TS) configuration. In 2004, however, a non-TS mechanism, named as "roaming mechanism", in which the reaction products and kinetics that cannot be predicted by current TS theories, was reported and challenged our current understanding of reaction rate theories. Up to now, it is generally believed that roaming occurs on the ground state potential energy surface (PES). Interestingly, a novel roaming mechanism, which evolves along the excited-state PES, has been found for the first time in the photodissociation of NO3 (Science, Vol. 335, page 1075, 2012). This discovery marks a significant advance in the study of roaming dynamics and, at the same time, poses many open questions on this excited-state roaming mechanism. One of them,for example, is how fast the roaming is on the excited state PES? Since it can not be predicted by current TS theories, it is necessary to provide trustable experimental data. In this proposal, roaming pathways in dissociation of NO3 and C6H5NO2 will be studied using time-resolved mass spectra in combination with time-resolved slice image technique. With our method, one can distinguish the reaction pathways by the size of the image ring and provide a femtosecond scale time resolution by the pump-probe method. For the NO3, we will measure the roaming reaction rate on the excited state (and the ground state); while for C6H5NO2, the reaction rate of the roaming pathway and the unknown reaction pathway will be measured, and the assignment of the unknown reaction pathway will be discussed. Our results will offer valuable parameters which are essential for new theory, and moreover, shed new light on excited-state dynamics.

过渡态理论是化学反应动力学最基本的理论之一,它能够预言反应的速率、机理等。然而2004年，过渡态理论的例外- - "徘徊"(roaming)反应机理的发现，激发了人们对该理论不足之处的思考。最近(2012年)，人们首次发现激发态上也存在roaming反应通道，这一重大发现深化了我们对roaming反应机理的认识。然而，激发态上roaming 的反应速率是多少这一直观问题并没有得到解答。鉴于此，我们提出采用时间分辨的质谱结合时间分辨的切影像技术，通过测量不同反应通道生成产物的时间来测量反应的速率。我们将研究NO3和硝基苯解离时roaming生成NO这两个代表性反应,并直接回答NO3激发态(和基态)上roaming的反应速率问题；而对于硝基苯，我们将得到基态和未知通道上生成NO的速率并探讨未知通道来自激发态的可能性。本研究不仅为新的化学反应理论提供可靠的参数，还将启发我们对激发态动学展开新的思考。

分子激发态势能面上roaming（漫游）机理的研究对激发态动力学和roaming反应机理都有着重要意义。在本项目的支持下，我们围绕分子激发态上roaming机理（如硝基苯、NO3等分子）展开了研究。实验首先使用氯丙烯和碘甲烷对本实验室的时间分辨影像装置进行了校准。在校准仪器过程中，我们首次测量了氯丙烯分子几个光解通道的寿命，更加深入的揭示了其势能面特征和光解机理，该成果发表在JPCA杂志上(J. Phys. Chem. A, 118(2014)4444 )。随后，我们展开了硝基苯等分子激发态上漫游机理的探索。实验发现，在飞秒激光作用下，硝基苯分子离子极容易布局到不稳定的离子态形成碎解，导致母体离子信号极弱。同时，NO+来源于母体离子的碎解而非NO的共振电离，难以准确给出roaming通道的信息。NO3在形成分子束时容易堵住脉冲阀导致实验难以顺利进行。跟踪roaming机理研究的最新文献发现，时间分辨的瞬态吸收方法是新发现的一种有效的研究漫游机理的方法(Nature Chemistry, 7(2015)562)。该方法主要关注解离产物的吸收，可以避免离子信号对中性碎片的干扰。鉴于此，我们对研究方法进行了调整，积极建立瞬态吸收光谱方法。在完成装置和方法的建立后，我们首先选择了相对熟悉的6-氮尿嘧啶（6AU）体系。对6AU的研究澄清了其S2态上的激发态动力学，还系统揭示了动力学过程对溶剂的依赖关系，此成果已经整理成文章发表在JPCA杂志上(J. Phys. Chem. A, 119(2015)12985)。该研究的展开为后续展开漫游机理的瞬态吸收研究打下了坚实的基础。在此基金的支持下，本项目共发表高质量文章2篇。