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CH4广泛存在于行星大气之中, 研究CH4的解离动力学对了解宇宙中气体演化的过程具有重要的价值. 目前,
$ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $ 碎裂通道已被大量研究, 但针对该通道的解离机制的解释尚存在一定争议. 本实验利用高分辨反应显微成像谱仪, 开展了25—44 eV的极紫外 (extreme ultraviolet, XUV) 光电离实验及1 MeV Ne8+与CH4的碰撞实验. 通过符合测量得到了$ {\text{CH}}_3^ + $ 和 H+两种离子的动能, 重构了两体解离的动能释放 (kinetic energy release, KER), 并研究了$ {\text{CH}}_4^{2 + } $ 解离产生$ {\text{CH}}_3^ + + {{\text{H}}^ + } $ 解离路径下的碎裂动力学过程. 在光电离实验中, 观测到KER谱上存在4.75 eV和6.09 eV两个峰, 结合前人的工作及XUV的能量范围, 对每个峰的机制归属进行讨论. 特别是4.75 eV峰, 分析认为可能来自于$ {\text{CH}}_4^{2 + } $ 直接解离机制的贡献. 另外, 在1 MeV Ne8+离子碰撞实验中, 可观测到3个KER峰, 将每个峰的分支比与以往的实验结果进行对比, 未发现速度效应对KER谱的显著影响.CH4 is abundant in planetary atmosphere, and the study of CH4 dissociation dynamics is of great importance and can help to understand the atmospheric evolution process in the universe. At present, the$ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $ channel has been extensively studied, but the explanation of the dissociation mechanism for this channel is controversial. In this work, the double-photoionization experiment of CH4 by extreme ultraviolet photon (XUV) in an energy range of 25－44 eV and the collision experiment between 1 MeV Ne8+ and CH4 are carried out by using the reaction microscope. The three-dimensional (3D) momenta of$ {\text{CH}}_3^ + $ and H+ ions are measured in coincidence, the corresponding kinetic energy release (KER) is reconstructed, and fragmentation dynamics from the parent ion$ {\text{CH}}_4^{2 + } $ to the$ {\text{CH}}_3^ + + {{\text{H}}^ + } $ ion pair are investigated. In the photoionization experiment, two peaks in the KER spectrum are observed: one is located around 4.75 eV, and the other lies at 6.09 eV. Following the conclusions of previous experiments and the theoretical calculations of Williams et al. (Williams J B, Trevisan C S, Schöffler M S, et al. 2012 J. Phys. B At. Mol. Opt. Phys. 45 194003), we discuss the corresponding mechanism of each KER peak. For the 6.09 eV peak, we attribute it to the$ {\text{CH}}_4^{2 + } $ dissociation caused by the Jahn-Teller effect, because this value is consistent with the energy difference in energy between the$ {\text{CH}}_4^{2 + } $ 1E initial state and the$ {\text{CH}}_3^ + /{{\text{H}}^ + } $ final state involving the Jahn-Teller effect. For the 4.75 eV peak, we believe that it may come from the direct dissociation of$ {\text{CH}}_4^{2 + } $ without contribution from the Jahn-Teller effect. More specifically, Williams et al. presented the potential energy curve for one C—H bond stretching to 8 a.u., while other C—H bonds are fixed at the initial geometry of the CH4 molecule. In the reflection approximation, we infer that the extra energy is released from the internuclear distance of 8 a.u. to infinity. It is found that the KER is 4.7 eV, which is consistent with the experimental observation, suggesting that the KER peak at 4.75 eV may arise from the direct dissociation of$ {\text{CH}}_4^{2 + } $ without contribution from the Jahn-Teller effect. In addition, in the 1 MeV Ne8+ ion collision experiment, it is observed that the released energy values corresponding to the three KER peaks are about 4.65, 5.76, and 7.94 eV. By comparing the branching ratio of each peak with the previous experimental result, it is suggested that the velocity effect is not significant in KER spectra.-
Keywords:
- extreme ultraviolet /
- kinetic energy release /
- isomerization /
- Coulomb explosion
[1] Casner A, Caillaud T, Darbon S, Duval A, Thfouin I, Jadaud J P, LeBreton J P, Reverdin C, Rosse B, Rosch R, Blanchot N, Villette B, Wrobel R, Miquel J L 2015 10th Int. Conf. High Energy Density Lab. Astrophys. 17 2
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图 1 $ {\text{CH}}_4^{2 + } $离子部分初态和末态的能级, 由文献[19]的势能曲线导出. 其中$ ({\text{CH}}_4^{2 + })^*_{R=8~\rm a.u.} $是指将一个C—H键拉伸至8 a.u. 而其他C—H键长不变 (处于中性CH4分子平衡结构) 的$ {\text{CH}}_4^{2 + } $离子; ($ {\text{CH}}_3^ + $)fin+H+表示末态解离产物. 1E(opt)态表示在Jahn-Teller效应影响下解离形成末态$ {\text{CH}}_3^ + $的能级位置; 1E(dir) 表示未受Jahn-Teller效应影响而通过直接解离形成末态$ {\text{CH}}_3^ + $的能级位置, 该能级根据反冲近似得到
Fig. 1. Partial initial energy levels of $ {\text{CH}}_4^{2 + } $ and final energy levels of $ {\text{CH}}_3^ + $, derived from the potential energy curve according to Ref. [19]. $ ({\text{CH}}_4^{2 + })^*_{R=8~\rm a.u.} $ represents the $ {\text{CH}}_4^{2 + } $ ion with one C—H bond stretching to 8 a.u., while other C—H bonds frozen in the initial geomertry of CH4 molecule. ($ {\text{CH}}_3^ + $)fin + H+ represents the final dissociation products. 1E(opt) represents the energy level position of the $ {\text{CH}}_3^ + $ final state influenced by Jahn-Teller effect; 1Edir represents the energy level position of the $ {\text{CH}}_4^{2 + } $ final state of directly dissociating without being influenced by Jahn-Teller effect, the energy level is obtained based on the reflection approximation.
图 2 XUV作用下的$ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $通道的二维飞行时间谱
Fig. 2. Two-dimensional time-of-flight (TOF) spectra for $ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $ channel by XUV.
图 3 1 MeV Ne8+ (a) 和 (b) 25—44 eV (b) XUV作用下$ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $的KER分布
Fig. 3. KER distribution for $ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $ by 1 MeV Ne8+ (a) and 25—44 eV XUV (b).
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[1] Casner A, Caillaud T, Darbon S, Duval A, Thfouin I, Jadaud J P, LeBreton J P, Reverdin C, Rosse B, Rosch R, Blanchot N, Villette B, Wrobel R, Miquel J L 2015 10th Int. Conf. High Energy Density Lab. Astrophys. 17 2
Google Scholar
[2] Xie H B, Li C, He N, Wang C, Zhang S, Chen J 2014 Environ. Sci. Technol. 48 1700
Google Scholar
[3] Oghbaie S, Gisselbrecht M, Månsson E P, Laksman J, Stråhlman C, Sankari A, Sorensen S L 2017 Phys. Chem. Chem. Phys. 19 19631
Google Scholar
[4] Zhang M, Najjari B, Hai B, Zhao D M, Lei J T, Dong D P, Zhang S F, Ma X W 2020 Chin. Phys. B 29 063302
Google Scholar
[5] Zhang Y, Wang B, Wei L, Jiang T, Yu W, Hutton R, Zou Y, Chen L, Wei B R 2019 J. Chem. Phys. 150 204303
Google Scholar
[6] Oghbaie S, Gisselbrecht M, Laksman J, Månsson E P, Sankari A, Sorensen S L 2015 J. Chem. Phys. 143 114309
Google Scholar
[7] Cornaggia C 2016 J. Phys. B At. Mol. Opt. Phys. 49 19LT01
Google Scholar
[8] 李桃桃, 苑航, 王兴, 张震, 郭大龙, 朱小龙, 闫顺成, 赵冬梅, 张少锋, 许慎跃, 马新文 2022 物理学报 71 093401
Google Scholar
Li T T, Yuan H, Wang X, Zhang Z, Guo D L, Zhu X L, Yan S C, Zhao D M, Zhang S F, Xu S Y, Ma X W 2022 Acta Phys. Sin. 71 093401
Google Scholar
[9] Yousefi M, Donne S 2022 Int. J. Hydrog. Energy 47 699
Google Scholar
[10] Ren Z J 2017 Nat. Energy 2 17093
Google Scholar
[11] Lefèvre F, Forget F 2009 Nature 460 720
Google Scholar
[12] Reay D S, Smith P, Christensen T R, James R H, Clark H 2018 Annu. Rev. Environ. Resour. 43 165
Google Scholar
[13] Ortenburger I B, Bagus P S 1975 Phys. Rev. A 11 1501
Google Scholar
[14] Wong M W, Radom L 1989 J. Am. Chem. Soc. 111 1155
Google Scholar
[15] Flammini R, Satta M, Fainelli E, Alberti G, Maracci F, Avaldi L 2009 New J. Phys. 11 083006
Google Scholar
[16] Gonçalves C E M, Levine R D, Remacle F 2021 Phys. Chem. Chem. Phys. 23 12051
Google Scholar
[17] Dujardin G, Winkoun D, Leach S 1985 Phys. Rev. A 31 3027
Google Scholar
[18] Werner U, Siegmann B, Lebius H, Huber B, Lutz H O 2003 11th Int. Conf. Phys. Highly Charg. Ions 205 639
[19] Williams J B, Trevisan C S, Schöffler M S, Jahnke T, Bocharova I, Kim H, Ulrich B, Wallauer R, Sturm F, Rescigno T N, Belkacem A, Dörner R, Weber T, McCurdy C W, Landers A L 2012 J. Phys. B At. Mol. Opt. Phys. 45 194003
Google Scholar
[20] Rajput J, Garg D, Cassimi A, Méry A, Fléchard X, Rangama J, Guillous S, Iskandar W, Agnihotri A N, Matsumoto J, Ahuja R, Safvan C P 2022 J. Chem. Phys. 156 054301
Google Scholar
[21] Li M, Zhang M, Vendrell O, Guo Z N, Zhu Q R, Gao X, Cao L S, Guo K Y, Su Q Q, Cao W, Luo S Q, Yan J Q, Zhou Y M, Liu Y Q, Li Z, Lu P X 2021 Nat. Commun. 12 4233
Google Scholar
[22] Mathur D, Rajgara F A 2006 J. Chem. Phys. 124 194308
Google Scholar
[23] Folkerts H O, Hoekstra R, Morgenstern R 1996 Phys. Rev. Lett. 77 3339
Google Scholar
[24] Lei J T, Shi G Q, Tao C Y, Sun S H, Yan S C, Ma X W, Ding J J , Zhang S F 2023 Chin. Phys. B 32 53205
Google Scholar
[25] Xu S, Zhu X L, Feng W T, Guo D L, Zhao Q, Yan S, Zhang P, Zhao D M, Gao Y, Zhang S F, Yang J, Ma X 2018 Phys. Rev. A 97 062701
Google Scholar
[26] Singh R, Bhatt P, Yadav N, Shanker R 2013 Phys. Rev. A 87 062706
Google Scholar
[27] Carroll T X, Berrah N, Bozek J, Hahne J, Kukk E, Sæthre L J, Thomas T D 1999 Phys. Rev. A 59 3386
Google Scholar
[28] Gaumnitz T, Jain A, Pertot Y, Huppert M, Jordan I, Ardana-Lamas F, Wörner H J 2017 Opt. Express 25 27506
Google Scholar
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