UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization
UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization
复制标题
单光子真空紫外电离制备乙醛阳离子的紫外光裂解动力学
DOI：
10.1039/c8cp06640j
复制
发表时间：
2019
影响因子：
3.3
通讯作者：
Murray, Craig
中科院分区：
文献类型：
--
作者：
Kapnas, Kara M.;McCaslin, Laura M.;Murray, Craig
乙醛阳离子（CH 3CHO+）的制备使用单光子真空紫外电离的CH 3CHO在分子束和碎片动力学研究的光解波长范围390-210 nm，使用速度图离子成像和光碎片产率（PHOFY）光谱。四个裂解通道：CH 3CHO + → C2 H3 O + + H（I），CH 3CHO + → HCO+ + CH 3（II），CH 3CHO + → CH 3 + + HCO（III），CH 3CHO + → CH 4 + + CO（IV）。在整个光解波长范围内观察到通道（I）、（II）和（IV），而通道（III）仅在λ < 317 nm处观察到。在λ 250 nm处获得最大碎片离子产率。记录了316-228 nm范围内的离子图像，该范围对应于CH 3CHO+的2A '和2A'态的初始激发。的速度和角分布是明显不同的每个检测到的离子，并显示统计和动力学碎裂途径的证据。在较长的波长下，通过通道（I）的碎裂导致适度的平移能（ET），与在小势垒上的解离和高度内部激发的CH 3CO+的产生一致。EINT大于CH 3CO+二级解离阈值的其他组分出现在较短的波长处，并被分配给乙烯醇阳离子或环氧乙烷阳离子通过能量化CH 3CHO+的异构化形成的碎片化产物。通道（III）的产品观察到的ET分布峰值为零，但显着冷比相空间理论预测的，特别是在较长的光解波长。冷比统计ET分布归因于从二次碎裂的能量CH 3CO+通过通道（I），这是衰减的CH 3CHO+异构化在较短的波长形成的贡献。通过通道（II）和（IV）的碎片在定性上类似的结果，在低ET和各向异性的组件，由于在较高的ET激发态动力学的各向同性的统计成分的证据。
Acetaldehyde cations (CH3CHO+) were prepared using single-photon vacuum ultraviolet ionization of CH3CHO in a molecular beam and the fragmentation dynamics explored over the photolysis wavelength range 390–210 nm using velocity-map ion imaging and photofragment yield (PHOFY) spectroscopy. Four fragmentation channels are characterized: CH3CHO+ → C2H3O+ + H (I), CH3CHO+ → HCO+ + CH3 (II), CH3CHO+ → CH3+ + HCO (III), CH3CHO+ → CH4+ + CO (IV). Channels (I), (II), and (IV) are observed across the full photolysis wavelength range while channel (III) is observed only at λ < 317 nm. Maximum fragment ion yields are obtained at ∼250 nm. Ion images were recorded over the range 316–228 nm, which corresponds to initial excitation to the 2A′ and 2A′ states of CH3CHO+. The speed and angular distributions are distinctly different for each detected ion and show evidence of both statistical and dynamical fragmentation pathways. At longer wavelengths, fragmentation via channel (I) leads to modest translational energies (ET), consistent with dissociation over a small barrier and production of highly internally excited CH3CO+. Additional components with EINT greater than the CH3CO+ secondary dissociation threshold appear at shorter wavelengths and are assigned to fragmentation products of vinyl alcohol cation or oxirane cation formed by isomerization of energized CH3CHO+. The ET distribution observed for channel (III) products peaks at zero but is notably colder than that predicted by phase space theory, particularly at longer photolysis wavelengths. The colder-than-statistical ET distributions are attributed to contributions from secondary fragmentation of energized CH3CO+ formed via channel (I), which are attenuated by CH3CHO+ isomerization at shorter wavelengths. Fragmentation via channels (II) and (IV) results in qualitatively similar outcomes, with evidence of isotropic statistical components at low-ET and anisotropic components due to excited state dynamics at higher ET.