Two-body fragmentation of methane induced by extreme ultraviolet and high charge ions
Authors: Luo Yan  , Yu Xuan  , Lei Jian-Ting  , Tao Chen-Yu  , Zhang ShaoFeng  , Zhu Xiao-Long  , Ma Xin-Wen  , Yan Shun-Cheng  , Zhao Xiao-Hui  ,   1) (School of Physics Science and Technology, Hebei University, Baoding 071000, China);,   2) (Institute of Mordern Physics, Chinese Academy of Sciences, Lanzhou 730000, China);,   3) (University of Chinese Academy of Sciences, Beijing 100049, China);,   4) (School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China)
Published: November 2023

CH<sub>4</sub> is abundant in planetary atmospheres, and the study of CH<sub>4</sub> dissociation dynamics is of great importance and can help to understand the atmospheric evolution process in the universe. At present, the CH<sub>4</sub><sup>2+</sup>→ CH<sub>3</sub><sup>+</sup>+H<sup>+</sup> 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 CH<sub>4</sub> by extreme ultraviolet photon (XUV) in the energy range of 25-44 eV and the collision experiment between 1 MeV Ne<sup>8+</sup> and CH<sub>4</sub> were carried out on the reaction microscope. The 3D momenta of CH<sub>3</sub><sup>+</sup> and H<sup>+</sup> ions were measured in coincidence, the corresponding kinetic energy release (KER) was reconstructed, and fragmentation dynamics from the parent ion CH<sub>4</sub><sup>2+</sup> to the CH<sub>3</sub><sup>+</sup>+H<sup>+</sup> 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 CH<sub>4</sub><sup>2+</sup> dissociation mediated by the Jahn-Teller effect, as this value is consistent with the energy difference between the CH<sub>4</sub><sup>2+</sup> <sup>1</sup>E initial state and the CH<sub>3</sub><sup>+</sup> /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 CH<sub>4</sub><sup>2+</sup> without the Jahn-Teller effect. In more detail, Williams<sup>[19]</sup> 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 CH<sub>4</sub> 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 CH<sub>4</sub><sup>2+</sup> without the Jahn-Teller effect. In addition, in the 1 MeV Ne<sup>8+</sup> 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.