Gelfand Natalia, Komarova Ksenia, Remacle Francoise, Levine R D
The Fritz Haber Center for Molecular Dynamics, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Theoretical Physical Chemistry, UR MolSys B6c, University of Liège, B4000 Liège, Belgium.
Phys Chem Chem Phys. 2024 Jan 24;26(4):3274-3284. doi: 10.1039/d3cp04854c.
Vacuum ultraviolet (VUV) photodissociation of N molecules is a source of reactive N atoms in the interstellar medium. In the energy range of VUV optical excitation of N, the N-N triple bond cleavage leads to three types of atoms: ground-state N(S) and excited-state N(P) and N(D). The latter is the highest reactive and it is believed to be the primary participant in reactions with hydrocarbons in Titan's atmosphere. Experimental studies have observed a non-monotonic energy dependence and non-statistical character of the photodissociation of N. This implies different dissociation pathways and final atomic products for different wavelength regions in the sunlight spectrum. We here apply quantum chemical and nonadiabatic quantum dynamical techniques to follow the path of an electronic state from the excitation of a particular singlet Σ+u and Π vibronic level of N to its dissociation into different atomic products. We simulate dynamics for two isotopomers of the nitrogen molecule, N and NN for which experimental data on the branching are available. Our computations capture the non-monotonic energy dependence of the photodissociation branching ratios in the energy range 108 000-116 000 cm. Tracing the quantum dynamics in a bunch of electronic states enables us to identify the key components that determine the efficacy of singlet to triplet population transfer and therefore predissociation lifetimes and branching ratios for different energy regions.
N分子的真空紫外（VUV）光解离是星际介质中活性N原子的一个来源。在N的VUV光激发能量范围内，N-N三键断裂会产生三种类型的原子：基态N(S)以及激发态N(P)和N(D)。后者具有最高的反应活性，据信是泰坦大气中与碳氢化合物反应的主要参与者。实验研究观察到了N光解离的非单调能量依赖性和非统计特性。这意味着太阳光谱中不同波长区域存在不同的解离途径和最终原子产物。我们在此应用量子化学和非绝热量子动力学技术，追踪从N的特定单重态Σ+u和Π振转能级激发到其解离为不同原子产物的电子态路径。我们模拟了氮分子的两种同位素异构体N和N₂的动力学，它们有关于分支的实验数据。我们的计算捕捉到了在108000 - 116000 cm⁻¹能量范围内光解离分支比的非单调能量依赖性。追踪一系列电子态中的量子动力学使我们能够确定决定单重态到三重态布居转移效率的关键成分，从而确定不同能量区域的预解离寿命和分支比。
