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

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.