Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
J Chem Phys. 2017 Jul 7;147(1):013907. doi: 10.1063/1.4977987.
Velocity map imaging has been used to characterize the angular and velocity distributions of O D photofragments arising from UV excitation of the simplest Criegee intermediate CHOO in the long wavelength tail region (364 to 417 nm) of the BA'-XA' spectrum. The O D images exhibit anisotropic angular distributions indicative of rapid dissociation to HCO XA + O D products, which occurs faster than the rotational period (ps) of CHOO. As a result, the broad oscillatory structure reported previously in the long wavelength region of the UV absorption spectrum is attributed to short-lived resonances associated with the excited BA' state of CHOO, which decay by nonadiabatic coupling to repulsive singlet states. The total kinetic energy distributions show that nearly half of the available energy, on average, is partitioned to product translation. The balance results in significant internal excitation of the HCO co-fragments. The product anisotropy and energy partitioning are unchanged across the UV spectrum and consistent with previously reported experimental and theoretical findings of the CHOO B-X transition moment and dissociation energy to HCO XA + O D products.
速度图成像是一种用于描述 O D 光碎片的角分布和速度分布的方法，这些 O D 光碎片是由最简单的 Criegee 中间体 CHOO 在 BA'-XA' 光谱的长波长尾部（364 到 417nm）区域的 UV 激发产生的。O D 图像显示出各向异性的角分布，表明 HCO XA + O D 产物的快速解离，这比 CHOO 的旋转周期（ps）快。因此，先前在 UV 吸收光谱的长波长区域报告的宽振荡结构归因于与 CHOO 的激发 BA'态相关的短寿命共振，这些共振通过非绝热耦合衰减到排斥的单线态。总动能分布表明，平均而言，近一半的可用能量分配给产物的平移。剩余的能量导致 HCO 碎片的显著内部激发。产物各向异性和能量分配在整个 UV 光谱中保持不变，与之前报道的 CHOO B-X 跃迁矩和到 HCO XA + O D 产物的解离能的实验和理论结果一致。
