Hangzhou Institute of Advanced Studies, Zhejiang Normal University, 1108 Gengwen Road, Hangzhou 311231, Zhejiang Province, People's Republic of China.
Center for Advanced Chemical Physics and Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, Anhui Province, People's Republic of China.
J Chem Phys. 2019 Dec 7;151(21):214306. doi: 10.1063/1.5129764.
Understanding vacuum ultraviolet (VUV) photodissociation dynamics of CO is of considerable importance in the study of atmospheric chemistry and planetary chemistry. Yet, photodissociation dynamics of the spin-forbidden O(P) + CO(XΣ) channel has not been clearly understood so far. Here, we study the O(P) + CO(XΣ) dissociation processes in the VUV photodissociation of CO at the photolysis wavelengths between 129.02 and 134.67 nm by using the time-sliced velocity-mapped ion imaging technique. From the vibrational-resolved images of the O(P) photofragment, the total kinetic energy releases, the CO(XΣ) cofragment vibrational state distributions, and the product angular distributions have been derived, respectively. The experimental observations show that the total kinetic energy releases for the three P spin-orbit states (j = 2, 1, 0) exhibit a broad CO(XΣ) vibrational energy distribution with significant inverted characteristics, especially at short photoexcitation wavelengths, indicating that the VUV photodissociation could take place in a relatively linear geometry of the triplet state, with one C-O bond extended and the other compressed. Furthermore, a notable photolysis wavelength dependent feature has also been found in the product angular distributions of all three spin-orbit channels (j = 2, 1, 0): Only the vibrational-state specific anisotropy parameter β values at 130.18 nm behave more anisotropic, while all those at other photolysis wavelengths are near the value β = 0.5 for O(P) channels or β = 0.25 for the O(P) channel, with small fluctuations. This anomalous phenomenon suggests that the different nonadiabatic interactions, such as singlet-triplet coupling, may play a key role in the formation of O(P) + CO(XΣ) products, with strong photolysis wavelength dependence.
理解 CO 的真空紫外（VUV）光解动力学在大气化学和行星化学研究中具有重要意义。然而，迄今为止，自旋禁戒的 O(P) + CO(XΣ)通道的光解动力学尚未得到清晰理解。在这里，我们使用时间切片速度映射离子成像技术研究了 CO 在 129.02nm 至 134.67nm 光解波长范围内的 VUV 光解过程中 O(P) + CO(XΣ)的解离过程。通过 O(P)光碎片的振动分辨图像，分别得出了总动能释放、CO(XΣ)碎片振动态分布和产物角分布。实验观察表明，三个 P 自旋轨道态（j = 2、1、0）的总动能释放表现出具有显著反转特征的宽 CO(XΣ)振动能分布，特别是在短光激发波长下，表明 VUV 光解可能发生在三重态的相对线性几何形状中，一个 C-O 键伸展，另一个键压缩。此外，在所有三个自旋轨道通道（j = 2、1、0）的产物角分布中也发现了明显的光解波长依赖性特征：只有在 130.18nm 处的振动态特定各向异性参数β值表现出更大的各向异性，而在其他光解波长处，所有β值均接近 O(P)通道的β = 0.5 或 O(P)通道的β = 0.25，且波动较小。这种异常现象表明，不同的非绝热相互作用，如单重态-三重态耦合，可能在 O(P) + CO(XΣ)产物的形成中起关键作用，且对光解波长有很强的依赖性。
