Keywords: 
• 氟利昂 /
• 光解离 /
• 飞行时间质谱 /
• 离子速度成像

Abstract: 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 C2Cl+4 , two dominant fragment ions C2Cl+3 and C2Cl+2 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) C2Cl+4 →C2Cl+3+Cl with single C—Cl bond breaking and direct production of Cl radical;2) C2Cl+4 →C2Cl+2+2Cl with double C—Cl bonds breaking and production of two Cl radicals. Also, ion images of these two observed fragment ions C2Cl+3 and C2Cl+2 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 C2Cl+3 and C2Cl+2 can be well fitted by two Gaussion distributions. It indicates that both fragments C2Cl+3 and C2Cl+2 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 parameterβ values are measured to be 0.46 (low energy channel) and 0.52 (high energy channel) for the fragment C2Cl+3 , and 0.41 (low energy channel) and 0.66 (high energy channel) for the fragment C2Cl+2 , respectively. The ratios between parallel transition and perpendicular transition are determined for all the observed channels for producing fragments C2Cl+3 and C2Cl+2 . 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.