Pandit Shubhrangshu, Preston Thomas J, King Simon J, Vallance Claire, Orr-Ewing Andrew J
School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom.
J Chem Phys. 2016 Jun 28;144(24):244312. doi: 10.1063/1.4954373.
Photodissociation of gaseous bromocyclopropane via its A-band continuum has been studied at excitation wavelengths ranging from 230 nm to 267 nm. Velocity-map images of ground-state bromine atoms (Br), spin-orbit excited bromine atoms (Br(∗)), and C3H5 hydrocarbon radicals reveal the kinetic energies of these various photofragments. Both Br and Br(∗) atoms are predominantly generated via repulsive excited electronic states in a prompt photodissociation process in which the hydrocarbon co-fragment is a cyclopropyl radical. However, the images obtained at the mass of the hydrocarbon radical fragment identify a channel with total kinetic energy greater than that deduced from the Br and Br(∗) images, and with a kinetic energy distribution that exceeds the energetic limit for Br + cyclopropyl radical products. The velocity-map images of these C3H5 fragments have lower angular anisotropies than measured for Br and Br(∗), indicating molecular restructuring during dissociation. The high kinetic energy C3H5 signals are assigned to allyl radicals generated by a minor photochemical pathway which involves concerted C-Br bond dissociation and cyclopropyl ring-opening following single ultraviolet (UV)-photon absorption. Slow photofragments also contribute to the velocity map images obtained at the C3H5 radical mass, but the corresponding slow Br atoms are not observed. These features in the images are attributed to C3H5 (+) from the photodissociation of the C3H5Br(+) molecular cation following two-photon ionization of the parent compound. This assignment is confirmed by 118-nm vacuum ultraviolet ionization studies that prepare the molecular cation in its ground electronic state prior to UV photodissociation.
通过A带连续光谱对气态溴环丙烷进行光解离的研究是在230纳米至267纳米的激发波长范围内进行的。基态溴原子（Br）、自旋轨道激发态溴原子（Br(∗)）和C3H5烃基自由基的速度映射图像揭示了这些不同光解离碎片的动能。Br和Br(∗)原子主要是在一个快速光解离过程中通过排斥性激发电子态产生的，在此过程中，烃类共碎片是一个环丙基自由基。然而，在烃基自由基碎片质量处获得的图像识别出一个通道，其总动能大于从Br和Br(∗)图像推断出的动能，并且动能分布超过了Br + 环丙基自由基产物的能量极限。这些C3H5碎片的速度映射图像的角各向异性比Br和Br(∗)的测量值低，表明解离过程中存在分子结构重组。高动能C3H5信号被归因于由一条次要光化学途径产生的烯丙基自由基，该途径涉及在单光子紫外（UV）吸收后C - Br键的协同解离和环丙基环的开环。慢速光解离碎片也对在C3H5自由基质量处获得的速度映射图像有贡献，但未观察到相应的慢速Br原子。图像中的这些特征归因于母体化合物双光子电离后C3H5Br(+)分子阳离子光解离产生的C3H5(+)。118纳米真空紫外电离研究证实了这一归属，该研究在紫外光解离之前将分子阳离子制备到其基态电子态。
