Gardiner Sara H, Karsili Tolga N V, Lipciuc M Laura, Wilman Edward, Ashfold Michael N R, Vallance Claire
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Rd, Oxford OX1 3TA, UK.
Phys Chem Chem Phys. 2014 Feb 7;16(5):2167-78. doi: 10.1039/c3cp53970a.
The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br(+) and C2H5I(+)) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br(+) cations undergo prompt C-Br bond fission to form predominantly C2H5(+) + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C2H3(+) + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5(+) products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I(+), along with an additional energetically accessible C-I bond fission channel to form C2H5 + I(+) products. HX (X = Br, I) elimination from the highly internally excited C2H5X(+) cation is deemed the most probable route to forming the C2H4(+) fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH2X(+) + CH3 products.
对溴乙烷和碘乙烷阳离子（C2H5Br(+)和C2H5I(+)）的光解离动力学进行了研究。卤代乙烷阳离子通过在118.2 nm处对各自的中性母体分子进行真空紫外（VUV）光电离形成，并在多个紫外（UV）光解波长下进行光解，包括355 nm以及236至266 nm范围内的波长。已获取所有碎片离子以及基态（Br）和自旋轨道激发态（Br*）溴原子产物的飞行时间质谱和速度映射图像，从而能够研究多种碎片化途径。通过对各自阳离子的基态和前几个激发态沿势能面（沿着RC-Br/I伸缩坐标）进行自旋轨道分辨的从头算计算，对实验研究进行了补充。对速度映射图像的分析表明，光激发的C2H5Br(+)阳离子迅速发生C-Br键断裂，主要形成C2H5(+) + Br*产物，具有接近极限的“平行”反冲速度分布。观察到的C2H3(+) + H2 + Br产物通道被认为是由光子吸收产生的初始激发态通过无辐射转移形成的高度内激发C2H5(+)产物的单分子衰变产生的。在C2H5I(+)的情况下观察到大致相似的行为，同时还有一个额外的能量上可及的C-I键断裂通道，形成C2H5 + I(+)产物。从高度内激发的C2H5X(+)阳离子中消除HX（X = Br，I）被认为是形成从两种阳离子中观察到的C2H4(+)碎片离子的最可能途径。最后，两种卤代乙烷阳离子还显示出存在少量C-C键断裂过程以形成CH2X(+) + CH3产物的证据。
