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分子离子广泛存在于行星大气的电离层中, 其碎裂可以产生多个离子和中性碎片. 研究末态产物的动能分布和生成机理, 可以促进理解行星大气的逃逸等天文现象. 本文开展了电子碰撞乙烷的双电离碎裂实验, 重点研究了$ {{\text{C}}_2}{\text{H}}_6^{2 + } $离子C—C键断裂后形成$ {\text{CH}}_3^ + /{\text{CH}}_2^ + /{\text{H}} $ 的三体解离通道. 直接测量了$ {\text{CH}}_3^ + $和$ {\text{CH}}_2^ + $离子的三维动量, 然后利用动量守恒重构了H的动量. 通过动能释放谱、Dalitz图、牛顿图分析了三体碎裂的解离机制. 发现协同解离是产生该通道的主要机制, 另有部分次序解离的贡献, 其第一步是$ {{\text{C}}_2}{\text{H}}_6^{2 + } $解离生成H和亚稳态$ {{\text{C}}_2}{\text{H}}_5^{2 + } $, 第二步是$ {{\text{C}}_2}{\text{H}}_5^{2 + } $碎裂生成$ {\text{CH}}_3^ + $和$ {\text{CH}}_2^ + $. 实验发现H原子动能分布较广, 最高能量甚至达到10 eV, 远高于土卫六大气的逃逸能量, 因而该解离路径对H逃逸过程有贡献.
Molecular ions are widely distributed in the ionosphere of planetary atmospheres, and their fragmentations can generate different ions and neutral fragments. Studying the kinetic energy distribution and generation mechanism of the final products is helpful in understanding fundamental phenomena in astrophysics and plasma physics. In particular, ethane is an important molecule found in Titan and comet, and its fragmentation may be involved in the generation of complex hydrocarbons, as well as the atmospheric escape processes on Titan. In this paper, the experiment on ethane fragmentation by electron impact is carried out, focusing on the three-body fragmentation channel from $ {{\text{C}}_2}{\text{H}}_6^{2 + } $ to $ {\text{CH}}_3^ + /{\text{CH}}_2^ + /{\text{H}} $. The three-dimensional momenta of $ {\text{CH}}_3^ + $ and $ {\text{CH}}_2^ + $ ions are measured, and then the momentum of the H atom is reconstructed using momentum conservation law. Based on these analyses, the kinetic energy release (KER) spectrum and the fragmentation mechanisms are investigated. The time-of-flight (TOF) coincidence map of the ions shows two channels: channel (1) that represents the two-body dissociation generating $ {\text{CH}}_3^ + $/$ {\text{CH}}_3^ + $, and channel (2) that refers to the three-body dissociation generating $ {\text{CH}}_3^ + /{\text{CH}}_2^ + /{\text{H}} $. It is found that the neutral H from channel (2) has a wide kinetic energy distribution, ranging from 0 eV to more than 10 eV. This feature indicates that the dissociation of the C－H bond is from multiple electronic states. Since the escape threshold of H in Titan’s ionosphere is 0.02 eV, the vast majority of the H atoms produced in channel (2) can escape into outer space. In addition, the kinetic energy sum of $ {\text{CH}}_3^ + $ and $ {\text{CH}}_2^ + $ in channel (2) is found to be similar to the KER of channel (1), indicating that the C－H dissociation presents limited influence on the energy sum of the CH2+ and $ {\text{CH}}_3^ + $. The corresponding fragmentation mechanism of channel (2) is also analyzed in this work. the overall KER spectrum is divided into three parts: 0–6 eV, 6–9 eV, and 9–11 eV, and the respective Dalitz plots and Newton diagrams are reconstructed under different KER conditions. In all Dalitz plots, there are a bright spot representing the concerted dissociation and a horizontal belt representing the sequential dissociation. The concerted dissociation is considered as the main mechanism, while the sequential dissociation plays a secondary role. The bright spot in the Dalitz plot shifts from the center to the left as the KER increases. This feature arises from the fact that the $ {\text{CH}}_2^ + $ lies between the H and the $ {\text{CH}}_3^ + $ in the concerted dissociation, and it feels the recoil both from H and from $ {\text{CH}}_3^ + $. Considering that the Coulomb potential from $ {\text{CH}}_3^ + $ is constant, the increase of the C－H dissociation energy will reduce the $ {\text{CH}}_2^ + $ kinetic energy. The belt in the Dalitz indicates that the sequential dissociation is a two-step process, with the first step being the dissociation of $ {{\text{C}}_2}{\text{H}}_6^{2 + } $ to generate H and metastable $ {{\text{C}}_2}{\text{H}}_5^{2 + } $, and the second step being the fragmentation of $ {{\text{C}}_2}{\text{H}}_5^{2 + } $ into $ {\text{CH}}_3^ + $ and $ {\text{CH}}_2^ + $. The Newton diagrams under different KER conditions are also reconstructed to give further evidence of the sequential dissociation from the metastable $ {{\text{C}}_2}{\text{H}}_5^{2 + } $, rather than from the metastable $ {\text{CH}}_3^ + $ or $ {\text{CH}}_4^ + $. In fact, for the former case, the center positions of the two half circles in the Newton diagram are correct. Oppositely, for the latter two cases, the center positions notably deviate from the expected values. This means the sequential dissociation from $ {{\text{C}}_2}{\text{H}}_5^{2 + } $ is dominant, which agrees excellently with the conclusion from the Dalitz plots. -
Keywords:
- ethane /
- dissociative ionization /
- three-body fragmentation /
- reaction microscope
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