氟利昂F114B2分子在飞秒紫外辐射下的解离动力学

Multi-photon dissociation dynamics of Freon 114B2 under UV radiation by femtosecond laser pulse

含氟利昂在内的卤代烷烃在太阳光辐射下解离生成破坏臭氧的游离态卤素原子,是破坏大气臭氧层的主要元凶.利用飞秒激光技术结合飞行时间质谱以及离子速度成像探测技术研究了氟利昂F114B2(四氟二溴乙烷)分子在267 nm飞秒激光辐射作用下的多光子电离解离动力学.利用飞行时间质谱技术,得到了四氟二溴乙烷在267 nm飞秒激光脉冲作用下发生多光子解离产生的质谱,发现三个主要碎片离子C_2F_4Br^+,C_2F_4^+,和CF_2Br^+,分别对应了三种主要的解离机理:1)单个C—Br键断裂C_2F4_Br_2^+→C_2F_4Br^++Br;2)两个C—Br键断裂C_2F_4Br_2^+→C_2F_4^++2Br;3)C—C键断裂C_2F_4Br_2^+→CF_2Br^++CF_2Br.实验采用离子速度成像技术对最主要的碎片离子C_2F_4Br^+进行成像,发现该碎片离子的动能分布可由三个高斯分布曲线拟合,说明单个C—Br键断裂机理对应于三种解离通道.通过影像进一步分析还得到了三个通道的平动能和角分布各向异性参数等详尽的动力学信息.通过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.

The ozone layer which absorbs harmful solar UV radiation is a necessary umbrella for human beings and biosphere.A large amount of alkyl halide including freon exhausted by human into the atmosphere poses a great threat to the ozone layer. Freon dissociates into the product of halogen radical, like Br and Cl, induced by UV sunlight radiation,which is found to be the main culprit for the destruction of atmospheric ozone. In this paper, time-of-flight（TOF）mass spectrometry and velocity map imaging technique are employed for investigating the multiphoton dissociation dynamics of Freon F114B2（C_2F_4Br_2） induced by femtosecond UV radiation at 267 nm. Fragment mass spectra of C_2F_4Br_2 under UV radiation at 266 nm are obtained by TOF mass spectrometry. Three daughter ions C_2F_4Br~＋,C2F＋4 and CF_2Br＋ are discovered together with the parent ions C_2F_4Br_2~＋. And three corresponding photodissociation mechanisms are concluded as follows： 1） C_2F_4Br_2~＋→C_2F_4Br~＋＋Br with single C—Br bond breaking and direct production of Br radical; 2） C_2F_4Br_2~＋→C2F＋4＋2Br with double C—Br bonds breaking and production of two Br radical; 3）C_2F_4Br_2~＋→CF2Br＋＋CF2Br with C—C bond breaking. Velocity map images of the strongest daughter ion C_2F_4Br~＋with C—Br breaking are measured by imaging apparatus. The kinetic energy distribution of C_2F_4Br~＋ions is obtained from the measured velocity map images. And it can be well fitted by three Gaussian curves which describe normal distribution.It indicates that the production of the fragment C_2F_4Br~＋stems from three different dissociation channels. Additional photodissociation dynamics is obtained by analyzing the angular distribution of the measured image. The values of anisotropy parameter β are measured to be 0.1（for the low energy channel）, 0.8（for the middle energy channel） and1.4（for the high energy channel） for the fragment C_2F_4Br~＋, respectively. The ratios of parallel transition to perpendicular transition are determined for three different channels. In addition, density functional theory calculations are also performed for further analysis and discussion. The optimized geometries of ground state and ionic state of C_2F_4Br_2 are obtained and compared at the level of B3LYP/6-311G＋＋（d, p）. The calculated information about ionic states, including energy level and oscillator strength for the ionic excited states, are given.