Criegee反应中瞬态中间体的慢电子速度成像研究

Haze is a prominent environmental problem to human society today. The generation of Criegee intermediates (CIs) is one of the key processes for the formation of haze. CIs mainly stem from the reactions of ozone (O3) with alkenes in the atmosphere. They significantly contribute to the formation of OH radical and secondary organic aerosols, which can further lead to the formation of acid rain and haze. The particulate pollution is threatening the environment, climate, and human health. However, these short-lived and highly active CIs have baffled directly spectral detection in the gas phase. In the present proposal, CIs are produced by UV laser photolysis of haloalkane molecules in abundant oxygen in a cold supersonic molecular beam in the gas phase. The research focuses on performing the slow electron velocity-map imaging detection of CIs with high sensitivity to obtain the high-resolution photoelectron spectra and photoelectron angular distributions via resonance-enhanced multiphoton ionization (REMPI)-slow electron velocity-map imaging method. Combined with theoretical calculations, the aim of the research is to study the geometrical structures, electronic structures, and chemical bond nature of CIs. And the studies can further help us investigate the isomerization course in the ground state and excited states with a view to understand the unimolecular isomerization mechanism at the molecular level. The research will provide essential scientific basis for the atmospheric chemical process relevant to the CIs.

雾霾是当前人类社会面临的一个突出的环境问题，其中克里奇(Criegee)中间体是雾霾形成的关键环节之一。Criegee中间体主要来源于大气中烯烃与臭氧反应，是OH自由基和二次有机气溶胶的重要来源，导致酸雨和雾霾的形成，对环境、气候以及人类健康产生至关重要的影响。但由于其寿命短、活性高等固有属性，目前直接气相谱学研究尚存在诸多难题。本项目拟以紫外光解卤代烷烃分子并与氧气反应产生分子束冷却的Criegee中间体为研究对象，重点是利用共振增强多光子电离-慢电子速度成像方法对Criegee中间体进行高灵敏的电子影像研究，得到高分辨电子谱和角分布。项目目标是结合理论计算，研究该类中间体的基态和激发态的构型、电子结构和化学键性质，并深入考察Criegee中间体的基态和激发态的异构化反应过程，期望在分子层面上深入理解单分子异构化机理，为进一步研究与Criegee相关的大气化学反应过程提供重要的科学依据。

本项目主要结合电喷雾臭氧化离子反应源、低能电子贴附反应负离子源和电子速度成像技术，探测一系列大气环境和燃烧过程中反应前驱物和反应中间体等物种的谱学信息。a）通过电喷雾臭氧化离子反应源装置和电子速度成像谱测量了大气环境中重要的三元反应系统：I–/S2O32–+O3中ISO3–, IS2O3–和IS2O4–瞬态中间体的电子态，并结合理论计算确认了该类瞬态中间体的稳定结构形式为碘-硫键结合的形式。b）通过低能电子贴附负离子源和慢电子速度成像技术精确测量大气和燃烧过程中一类反应中间体CS, S2, HCS2, CH2S2和CH3S2的电子亲合势，振动频率和各项异性参数，S2, HCS2和CH3S2的低能激发态的精细电子-振动谱。结合高精度理论计算对基态和激发态进行了详尽的归属。c）结合共振增强多光子电离质谱-慢电子速度成像技术对一系列大气臭氧化反应的前驱物（与大气不饱和烃臭氧化反应相关的如苯乙烯、4-甲基苯乙烯等烯烃类和相关的重要的卤代芳香烃类）进行了系统地共振增强光电离谱测量和慢电子速度成像分析，结合高精度理论计算对其中性激发态和离子态的电子结构进行了细致的表征和归属。