Two-body fragmentation of methane induced by extreme ultraviolet and high charge ions

Luo Yan Luo Yan,Zhu Xiao-Long Zhu Xiao-Long,Lei Jian-Ting Lei Jian-Ting,Yu Xuan Yu Xuan,Zhao Xiao-Hui Zhao Xiao-Hui,Yan Shun-Cheng Yan Shun-Cheng,3) (University Of Chinese Academy Of Sciences, Beijing 100049, China); ,Ma Xin-Wen Ma Xin-Wen,Zhang Shaofeng Zhang Shaofeng, ,Tao Chen-Yu Tao Chen-Yu

doi:10.7498/aps.73.20231377

Abstract

CH4 is abundant in planetary atmospheres, 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 CH42+→ CH3++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 the energy range of 25-44 eV and the collision experiment between 1 MeV Ne8+ and CH4 were carried out on the reaction microscope. The 3D momenta of CH3+ and H+ ions were measured in coincidence, the corresponding kinetic energy release (KER) was reconstructed, and fragmentation dynamics from the parent ion CH42+ to the CH3++H+ ion pair were investigated. In the photoionization experiment, we observed two peaks in the KER spectrum, one locates around 4.75 eV, and the other one lies at 6.09 eV. Benefiting from the conclusions of previous experiments and the theoretical calculations of Williams [19] et al, we discussed the corresponding mechanism of each KER peak. For the 6.09 eV peak, we attributed it to the CH42+ dissociation mediated by the Jahn-Teller effect, as this value is consistent with the energy difference between the CH42+ 1E initial state and the CH3+ /H+ final state involving the Jahn-Teller effect. For the 4.75 eV peak, we proposed that it may come from the direct dissociation of CH42+ without the Jahn-Teller effect. In more detail, Williams[19] 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. Based on the reflection approximation, we deduced the additional energy release from the internuclear distance of 8 a.u. to infinity. We found the sum KER is 4.7 eV, this is consistent with the experimental observation and suggests that the KER peak at 4.75 eV may arise from the direct dissociation of CH42+ without the Jahn-Teller effect. In addition, in the 1 MeV Ne8+ ion collision experiment, we observed three KER peaks with the mean kinetic energy release values of around 4.65, 5.75, and 7.94 eV. By comparing the branching ratio of each peak with the previous experimental results, it is suggested that the velocity effect is not significant in KER spectra.

Full Text