Parsons Bradley F, Hulce Martin R, Ackerman John R, Reardon Kylie A, Pappas Emerson S, Kettler Lauren E
Department of Chemistry and Biochemistry 2500 California Plaza, Creighton University, Omaha, Nebraska 68178, United States.
J Phys Chem A. 2024 Apr 18;128(15):2971-2981. doi: 10.1021/acs.jpca.4c00143. Epub 2024 Apr 5.
In our experiment, a trace amount of an organic molecule (M = 1-phenalen-1-one, 9-fluorenone, pyridine, or acridine) was seeded into a gas mix consisting of 3% O with a rare gas buffer (He or Ar) and then supersonically expanded. We excited the resulting molecular beam with ultraviolet light at either 355 nm (1-phenalen-1-one, 9-fluorenone, or acridine) or 266 nm (pyridine) and used resonance enhanced multiphoton ionization (REMPI) spectroscopy to probe for the formation of O in the a-Δ state, O. For all systems, the REMPI spectra demonstrate that ultraviolet excitation results in the formation of O and the oxygen product is confirmed to be in the ground vibrational state and with an effective rotational temperature below 80 K. We then recorded the velocity map ion image of the O product. From the ion images, we determined the center-of-mass translational energy distribution, (), assuming photodissociation of a bimolecular M-O complex. We also report results from electronic structure calculations that allow for a determination of the M-O ground state binding energy. We use the complex binding energy, the energy to form O, and the adiabatic triplet energy for each organic molecule to determine the available energy following photodissociation. For dissociation of a bimolecular complex, this available energy may be partitioned into either center-of-mass recoil or internal degrees of freedom of the organic moiety. We use the available energy to generate a Prior distribution, which predicts statistical energy partitioning during dissociation. For low available energies, less than 0.2 eV, we find that the statistical prediction is in reasonable agreement with the experimental observations. However, at higher available energies, the experimental distribution is biased to lower center-of-mass kinetic energies compared with the statistical prediction, which suggests the complex undergoes vibrational predissociation.
在我们的实验中，将痕量的有机分子（M = 1-菲-1-酮、9-芴酮、吡啶或吖啶）引入由3% O与稀有气体缓冲剂（He或Ar）组成的气体混合物中，然后进行超声膨胀。我们用355 nm（1-菲-1-酮、9-芴酮或吖啶）或266 nm（吡啶）的紫外光激发所得分子束，并使用共振增强多光子电离（REMPI）光谱来探测a-Δ态的O（O）的形成。对于所有系统，REMPI光谱表明紫外激发导致O的形成，并且确认氧产物处于基态振动状态，有效转动温度低于80 K。然后我们记录了O产物的速度成像离子图像。从离子图像中，我们确定了质心平动能量分布（），假设双分子M - O络合物发生光解离。我们还报告了电子结构计算的结果，这些结果有助于确定M - O基态结合能。我们使用每个有机分子的络合物结合能、形成O的能量和绝热三重态能量来确定光解离后的可用能量。对于双分子络合物的解离，该可用能量可以分配为质心反冲或有机部分的内自由度。我们使用可用能量生成一个先验分布，该分布预测解离过程中的统计能量分配。对于低于0.2 eV的低可用能量，我们发现统计预测与实验观察结果合理一致。然而，在较高的可用能量下，与统计预测相比，实验分布偏向较低的质心动能，这表明络合物发生了振动预解离。
