氟利昂F1110分子在飞秒激光脉冲作用下的多光子解离动力学

含氟利昂在内的含氯化合物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. 本文利用飞行时间质谱技术及离子速度成像技术研究了氟利昂F1110(四氯乙烯)分子在800 nm飞秒脉冲光作用下的多光子解离动力学. 利用飞行时间质谱探测技术, 得到了四氯乙烯在800 nm飞秒激光脉冲作用下发生多光子解离产生的碎片质谱, 发现了两个主要碎片离子C₂Cl₃⁺和 C₂Cl₂⁺. 对应的解离机理分别为单个C-Cl键断裂直接生产氯自由基C₂Cl₄⁺→C₂Cl₃⁺ +Cl 和两个C—Cl 键断裂C₂Cl₄⁺→C₂Cl₂⁺+2Cl: 利用离子速度成像技术对这两种机理产生的碎片离子进行成像, 得到了C₂Cl₃⁺ 和C₂Cl₂⁺ 离子的速度影像. 分析发现这两个碎片离子的动能分布均可由两个高斯分布曲线拟合, 说明这两种解离机理分别还对应了两种解离通道. 通过影像分析得到了解离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. 通过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.

Multi-photon dissociation dynamics of Freon 1110 induced by femtosecond laser pulse

The ozone layer which absorbs harmful solar UV radiation is an essential umbrella for human beings. However, a large number of exhausts of chlorine compounds including freon released by people in the atmosphere pose a great threat to the ozone layer. Freon dissociates into the product of chlorine radicals induced by UV sunlight radiation, which are found to be the main culprit for the destruction of atmospheric ozone. In this paper, time-of-flight mass spectrometry and velocity map imaging technique are coupled for investigating the multiphoton dissociation dynamics of Freon 1110 (C₂Cl₄, Tetrachloroethylene) induced by ultrafast short laser pulse on a femtosecond time scale at 800 nm. Fragments mass spectra of C₂Cl₄ are measured by time-of-flight mass spectrometry. Together with the parent ion C₂Cl₄⁺, two dominant fragment ions C₂Cl₃⁺ and C₂Cl₂⁺ are discovered in the multi-photon ionization and dissociation process in the experiment. By analyzing the above mass spectra, two corresponding photodissociation mechanisms are discussed and listed as follows: 1) C₂Cl₄⁺→C₂Cl₃⁺ +Cl with single C—Cl bond breaking and direct production of Cl radical; 2) C₂Cl₄⁺→C₂Cl₂⁺ +2Cl with double C—Cl bonds breaking and production of two Cl radicals. Also, ion images of these two observed fragment ions C₂Cl₃⁺ and C₂Cl₂⁺ are measured by velocity map imaging apparatus. The kinetic energy distributions of these two fragment ions are determined from the measured velocity map images. The kinetic energy distributions of both C₂Cl₃⁺ and C₂Cl₂⁺ can be well fitted by two Gaussion distributions. It indicates that both fragments C₂Cl₃⁺ and C₂Cl₂⁺ are from two production channels. The peak energies for each channel are fitted. More detailed photodissociation dynamics is obtained by analyzing the angular distribution of the generated fragment ions. The anisotropy parameter β values are measured to be 0.46 (low energy channel) and 0.52 (high energy channel) for the fragment C₂Cl₃⁺, and 0.41 (low energy channel) and 0.66 (high energy channel) for the fragment C₂Cl₂⁺, respectively. The ratios between parallel transition and perpendicular transition are determined for all the observed channels for producing fragments C₂Cl₃⁺ and C₂Cl₂⁺. In addition, density functional theory calculations at a high-precision level are also performed on photodissociation dynamics for further analysis and discussion. The optimized geometries of ground state and ionic state of C₂Cl₄ are obtained and compared with density functional theory calculation at the level of B3LYP/6-311G++(d,p). The different structures of the ground and ionic states are given and discussed. The calculated information about ionic states of C₂Cl₄, including energy level and oscillator strength for the ionic excited states, is also given for analyzing the photodissociation dynamics of the C₂Cl₄ ions.