Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy
Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy
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通过时间分辨离子成像和光碎片激发光谱研究丙酮的光解离动力学
DOI：
10.1039/c7cp07320h
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发表时间：
2018
影响因子：
3.3
通讯作者：
Murray, Craig
中科院分区：
文献类型：
--
作者：
Toulson, Benjamin W.;Fishman, Dmitry A.;Murray, Craig
利用速度图离子成像和光碎片激发（PHOFEX）光谱在第一吸收波段（236-308 nm）的波长范围内研究了丙酮的光解动力学。采用单光子紫外电离技术，在118 nm处检测了Norrish I型解离的自由基产物、甲基和乙酰基以及分子产物烯酮。烯酮在所有波长下的产率都不可忽略，在280 nm处产率最大值为Φ≈0.3。适度的平动能释放与在S0表面的高势垒上的解离不一致，烯酮的形成暂定为一个漫游途径，涉及到自由基产物的解离受挫。在λ≤305 nm处检测到快速移动的自由基产物，总平动能分布扩展到能量极限，与系统间交叉后几乎只发生在T1表面的解离一致。在低于T1势位的能量处，观察到S0解离的统计成分，尽管与乙醛相比，在较短波长处不存在通过S1/S0锥形交叉的解离。在λ≤260 nm的PHOFEX光谱中，由于内激发的乙酰基自由基发生二次解离，甲基自由基的产率高于乙酰基。利用266 nm波长皮秒脉冲进行时间分辨离子成像实验，发现T1上形成的CH3自由基的出现时间常数为τ = 1490±140 ps。相关速率代表S1→T1系间交叉。在284nm处，CH3在T1上形成，具有两个不同的时间尺度：一个快的< 10ns的组分伴随着一个慢的τ = 42±7ns的组分。提出了一个两步机制，包括快速的内部转换，然后是较慢的系统间交叉（S1→S0→T1）来解释慢速成分。
The photodissociation dynamics of acetone has been investigated using velocity-map ion imaging and photofragment excitation (PHOFEX) spectroscopy across a range of wavelengths spanning the first absorption band (236–308 nm). The radical products of the Norrish Type I dissociation, methyl and acetyl, as well as the molecular product ketene have been detected by single-photon VUV ionization at 118 nm. Ketene appears to be formed with non-negligible yield at all wavelengths, with a maximum value of Φ ≈ 0.3 at 280 nm. The modest translational energy release is inconsistent with dissociation over high barriers on the S0 surface, and ketene formation is tentatively assigned to a roaming pathway involving frustrated dissociation to the radical products. Fast-moving radical products are detected at λ ≤ 305 nm with total translational energy distributions that extend to the energetic limit, consistent with dissociation occurring near-exclusively on the T1 surface following intersystem crossing. At energies below the T1 barrier a statistical component indicative of S0 dissociation is observed, although dissociation via the S1/S0 conical intersection is absent at shorter wavelengths, in contrast to acetaldehyde. The methyl radical yield is enhanced over that of acetyl in PHOFEX spectra at λ ≤ 260 nm due to the onset of secondary dissociation of internally excited acetyl radicals. Time-resolved ion imaging experiments using picosecond duration pulses at 266 nm find an appearance time constant of τ = 1490 ± 140 ps for CH3 radicals formed on T1. The associated rate is representative of S1 → T1 intersystem crossing. At 284 nm, CH3 is formed on T1 with two distinct timescales: a fast <10 ns component is accompanied by a slower component with τ = 42 ± 7 ns. A two-step mechanism involving fast internal conversion, followed by slower intersystem crossing (S1 → S0 → T1) is proposed to explain the slow component.