Toulson Benjamin W, Kapnas Kara M, Fishman Dmitry A, Murray Craig
Department of Chemistry, University of California, Irvine, Irvine, CA 92697, USA.
Phys Chem Chem Phys. 2017 Jun 7;19(22):14276-14288. doi: 10.1039/c7cp02573d.
Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265-328 nm. Ion images recorded probing CH radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH speed distributions become bimodal at λ < 292 nm. In addition to the translationally fast T products, a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CHCHO obtained probing CH and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S, accessed via a conical intersection between the S and S surfaces at extended C-C distances. Time-resolved ion images of CH radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T and Svia the conical intersection occurs with time constants of 240 ps and 560 ps, respectively.
已进行时间分辨离子成像测量，以探索乙醛在265 - 328 nm范围内光解波长下的光化学过程。用单光子真空紫外电离探测CH自由基记录的离子图像显示，在三个不同的波长区域有不同的解离动力学。在最长的光解波长，λ > 318 nm时，CH自由基在数十纳秒内形成，其速度分布与内转换后S表面上的统计单分子解离一致。在292 nm ≤ λ ≤ 318 nm范围内，解离几乎完全发生在系间窜越并越过势垒后的T表面，导致可用能量主要分配到光碎片反冲中。在λ < 292 nm时，CH速度分布变为双峰。除了平移速度快的T产物外，随着光解波长减小，一个新的平移速度慢但非统计性的组分出现并变得越来越重要。探测CH和HCO产物得到的CHCHO的光碎片激发（PHOFEX）光谱在整个吸收带内是相同的，表明三体碎裂不是非统计性慢组分的原因。相反，平移速度慢的产物归因于在S表面的解离，这是通过S和S表面在延长的C - C距离处的锥形交叉点实现的。使用在266 nm光解波长下工作的皮秒激光测量的CH自由基的时间分辨离子图像表明，通过锥形交叉点在T和S上形成产物的时间常数分别为240 ps和560 ps。
