Dissociative ionization of allyl bromide in 800 nm and 400 nm intense femtosecond laser fields
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摘要: 利用自行搭建的飞行时间质谱仪，在800 nm和400nm飞秒激光强场下对溴丙烯分子进行了电离解离过程的探究.通过分析离子产物的产率与激光功率的依赖关系并结合Keldysh因子计算，给出了实验中母体分子的电离机制；理论上，利用量子化学计算软件（Gaussian 09），对分子的化学键柔性力常数、反应通道出现势进行了计算，确认了溴丙烯分子的电离解离通道，发现了非共振多光子吸收导致的多个化学键的同时断裂，解释了母体分子离子电荷布局对反应路径的影响.Abstract: Using a homemade time-of-flight mass spectrometer, the ionization and dissociation of gaseous molecular allyl bromide in 800 nm and 400 nm femtosecond laser fields were studied. By analyzing the dependence of ion fragment yields and laser intensity, combined with calculation of the Keldysh parameter, it was shown that multi-photon ionization dominates the ionization process involved in our experiment. Based on a theoretical calculation with the Gaussian 09 software package, flexible force constants and appearance energies of different fragment ions were calculated and dissociation channels were identified. Meanwhile, the results demonstrate that non-resonant multi-photon absorption will lead to simultaneous cleavage of multi-chemical bonds; moreover, the charge distribution of parent ions will steer the reaction pathway.
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图 1 飞行时间质谱实验装置示意图
Fig. 1 Schematics of the time-of-flight mass spectrometer used in our experiment
图 2 离子的飞行时间与离子质荷比平方根的模拟图
Fig. 2 Simulated relationship between the ion flight time and the square root of ion mass ratio
图 3 Br离子的飞行时间与离子初动能的关系
Fig. 3 Simulated relationship between the time of flight of the Br ion and the initial kinetic energy of the same ion
图 4 溴丙烯分子在不同激光功率下的质谱图
Fig. 4 The TOF-MS of molecular allyl bromide at different laser power values
图 6 Gaussian 09优化得到的溴丙烯分子结构
Fig. 6 The optimized structure of allyl bromide molecule based on Gaussian 09
图 7 从头计算的一价母体离子C$_{3}$H$_{5}$Br$^{+}$的解离路径, 其中黑色虚线代表离子C$_{3}$H$_{5}^{+}$和C$_{3}$H$_{3}^{+}$反应路径, 红色虚线代表离子Br$^{+}$、CH$_{2}^{+}$和C$_{2}$H$_{3}^{+}$的反应路径
Fig. 7 Ab initio calculated dissociation pathways of singly-charged C$_{3}$H$_{5}$Br$^{+}$ ions. The black dashed lines represent the reaction channels for C$_{3}$H$_{5}^{+}$ and C$_{3}$H$_{3}^{+}$ ions, and the red dashed lines represent reaction channels for Br$^{+}$, CH$_{2}^{+}$, $^{ }$and C$_{2}$H$_{3}^{+}$ ions
表 1 800 nm激光中Keldysh因子$\gamma $的理论值
Tab. 1 The calculated Keldysh parameter $\gamma $ in a 800 nm laser field
功率/mW 光场强度
/($\times $10$^{14}$W$\cdot$cm$^{-2})$Keldysh因子$\gamma $ 1 500 8.68 0.309 4 1 400 8.00 0.322 2 1 300 7.32 0.336 8 1 200 6.65 0.353 5 1 100 5.97 0.372 9 1 000 3.64 0.477 7 900 1.96 0.650 5 800 1.49 0.746 3 700 1.04 0.890 4 600 0.93 0.944 9 500 0.81 1.010 8 450 0.75 1.049 3 400 0.70 1.092 7 350 0.64 1.141 9 300 0.58 1.198 4 250 0.52 1.264 2 200 0.46 1.342 2 150 0.40 1.436 8 100 0.34 1.554 5 注:激光光强由实验与文献[17]的实验结果对照得到 
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表 2 柔性力常数计算值
Tab. 2 Flexible force constant values
断键类别 断键位置 柔性力常数/(N$\cdot$Bohr$^{-1}$) C$_{3}$H$_{5}$Br C$_{3}$H$_{5}$Br$^{+}$ C-H 1-2 5.348 4.608 C-H 1-8 5.291 4.149 C-C 1-3 4.484 4.739 C-Br 1-9 2.020 2.469 C-H 3-4 5.464 5.525 C=C 3-5 9.615 6.944 C-H 5-6 5.525 5.587 C-H 5-7 5.587 5.650
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表 3 可能的反应通道及其反应出现势
Tab. 3 Estimated reaction channels and related appearance energies
反应通道 反应式 反应出现势/eV channel 1 C$_3$H$_5$Br$\to$C$_3$H$_5$Br$^{+}$+e$^{-}$ 9.43 channel 2 C$_3$H$_5$Br$\to$C$_3$H$^{+}_5$+Br+e$^{-}$ 10.42 channel 3 C$_3$H$_5$Br$\to$C$_3$H$^{+}_3$+Br+H$_{2+}$e$^{-}$ 11.68 channel 4 C$_3$H$_5$Br$\to$C$_3$H$_5$Br$^{+}$+e$^{-}$ 15.58 channel 5 C$_3$H$_5$Br$\to$C$_2$H$^{+}_3$+CH$_{2}$+Br+e$^{-}$ 16.56 channel 6 C$_3$H$_5$Br$\to$C$_2$H$_3$+CH$^{+}_{2}$+Br+e$^{-}$ 17.77 channel 7 C$_3$H$_5$Br$\to$C$_2$H$_3$+CH$^{+}_{2}$+Br$^{+}$+e$^{-}$ 21.19
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表 4 一价母体离子电荷布局分析
Tab. 4 Single parent ion charge population analysi
反应通道 电荷布局 电离能/eV channel 2 C$_3$H$_5$Br$\to$C$_3$H$^{+}_5$+Br+e$^{-}$ 8.06 (0.638-0.362) +1 0 channel 4 C$_3$H$_5$Br$\to$C$_3$H$_5$+Br$^{+}$+e$^{-}$ 13.21 (0.638-0.362) 0 +1
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