氟利昂F113分子在飞秒激光作用下的多光子电离解离动力学

大气臭氧层因吸收太阳紫外光, 是人类必不可少的保护伞. 氟利昂在太阳光辐射下解离生成破坏臭氧的游离态氯原子, 是破坏大气臭氧层的主要元凶之一. 本文利用飞行时间质谱技术和离子速度成像技术研究了氟利昂F113(三氟三氯乙烷)分子在800 nm 飞秒光作用下的多光子电离解离动力学. 利用飞行时间质谱探测技术, 得到了三氟三氯乙烷在该波长飞秒激光作用下发生多光子电离解离产生的碎片质谱. 通过荷质比对碎片质谱进行了详细的标定和分析. 在质谱上未发现母体离子, 所有观察到的离子都是由于激光脉冲作用下产生的碎片. 三个最主要的碎片离子是CFCl₂⁺, CF₂Cl⁺, C₂F₃Cl₂⁺. 通过飞行时间质谱标定, 发现并归属了多个解离通道. 三个主要的解离机理分别为: 1) C-Cl键断裂直接生产氯自由基的通道C₂F₃Cl₃⁺→C₂F₃Cl₂⁺+Cl; 2) C--C键断裂C₂F₃Cl₃⁺→CFCl₂⁺+CF₂Cl; 3) C--C键断裂C₂F₃Cl₃⁺→CF₂Cl⁺+CFCl₂. 利用离子速度成像技术对这三个主要通道产生的碎片离子进行成像, 得到了C₂F₃Cl₂⁺, CFCl₂⁺和CF₂Cl⁺离子的速度影像. 由C--Cl键断裂产生的碎片离子C₂F₃Cl₂^{+}的速度分布由两个高斯分布曲线拟合, 而由C--C键断裂产生的碎片离子CFCl₂⁺和CF₂Cl⁺可以用一个高斯曲线拟合. 通过影像分析得到了解离碎片的平动能分布和角向分布各向异性参数等详尽的动力学信息. 结合高精度密度泛函理论计算对解离动力学进行了进一步的分析和讨论.深入认识氟利昂的解离动力学可为进一步控制破坏臭氧层提供理论参考和实验依据.

Multiphoton ionization and dissociation dynamics of Freon-113 induced by femtosecond laser pulse

The ozone layer which absorbs harmful solar UV radiation is an essential umbrella for human. However, a large number of exhausts of Freon released by human activity into the atmosphere pose a great threat to the ozone layer. The UV sunlight radiation induced Freon dissociation produces chlorine radicals, which are found to be the main culprit for destroying the atmospheric ozone. In this paper, multiphoton ionization and dissociation dynamics of Freon-113 (CF₂ClCFCl₂) induced by femtosecond laser pulse are studied by time-of-flight mass spectrometry coupled with velocity map imaging technique. Fragment mass spectra of Freon-113 are measured by time-of-flight mass spectrometry. No parent ions are discovered in the time-of-flight mass spectra, and all the detected ions are from the fragmentation induced by the laser pulse. Daughter ions CFCl₂⁺, CF₂Cl⁺, C₂F₃Cl₂⁺ are found to be the three major fragmentation ions in the multi-photon ionization and dissociation. Several photodissociation channels are discussed and concluded by further analysis and calibration (via the ratio of mass to charge) of the measured time-of-flight mass spectra. Three main photodissociation mechanisms are found as follows: 1) C₂F₃Cl₃⁺→C₂F₃Cl₂⁺+Cl with breaking C--Cl bond and directly producing the Cl radical; 2) C₂F₃Cl₃⁺ →CFCl₂⁺+CF₂Cl with breaking the C--C; 3) C₂F₃Cl₃⁺ →CF₂Cl⁺+CFCl₂ with breaking the C--C bond. Ion images of the three main fragments C₂F₃Cl₂⁺, CFCl₂⁺ and CF₂Cl⁺ are measured by the velocity map imaging setup. The speed distributions of these three fragment ions are obtained from the velocity map imaging. The speed distribution of C₂F₃Cl₂⁺ with breaking C--Cl bond can be fitted by two Gaussian distributions while the speed distributions of both CFCl₂⁺ and CF₂Cl⁺ with breaking the C--C bond can be well fitted by one Gaussian distribution. The different fittings reflect different production channels. The detailed photodissociation dynamics is obtained by analyzing the kinetic energy distribution and angular distribution of the fragment ions. Additionally, density functional theory calculations on high-precision level are also performed on photodissociation dynamics for further analysis and discussion. An in-depth understanding of dissociation dynamics of freon can provide theoretical reference and experimental basis for further controlling the dissociation process that can do destruction to the ozone layer.