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

Abstract

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 <inline-formula><tex-math id="M6">\begin{document}$ {\text{CH}}_4^{2 + } \to {\text{CH}}_3^ + + {{\text{H}}^ + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M6.png"/></alternatives></inline-formula> channel has been extensively studied, but the explanation of the dissociation mechanism for this channel is controversial. In this work, the double-photoionization experiment of CH₄ by extreme ultraviolet photon (XUV) in an energy range of 25－44 eV and the collision experiment between 1 MeV Ne⁸⁺ and CH₄ are carried out by using the reaction microscope. The three-dimensional (3D) momenta of <inline-formula><tex-math id="M7">\begin{document}$ {\text{CH}}_3^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M7.png"/></alternatives></inline-formula> and H⁺ ions are measured in coincidence, and the corresponding kinetic energy release (KER) is reconstructed, and fragmentation dynamics from the parent ion <inline-formula><tex-math id="M8">\begin{document}$ {\text{CH}}_4^{2 + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M8.png"/></alternatives></inline-formula> to the <inline-formula><tex-math id="M9">\begin{document}$ {\text{CH}}_3^ + + {{\text{H}}^ + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M9.png"/></alternatives></inline-formula> ion pair are investigated. In the photoionization experiment, two peaks in the KER spectrum are observed: one is located around 4.75 eV, and the other lies at 6.09 eV. Following the conclusions of previous experiments and the theoretical calculations of Williams et al. (Williams J B, Trevisan C S, Schöffler M S, Jahnke T, Bocharova I, Kim H, Ulrich B, Wallauer R, Sturm F, Rescigno T N, Belkacem A, Dörner R, Weber T, McCurdy C W, Landers A L 2012 <i>J. Phys. B At. Mol. Opt. Phys.</i> <b>45</b> 194003), we discuss the corresponding mechanism of each KER peak. For the 6.09 eV peak, we attribute it to the <inline-formula><tex-math id="M10">\begin{document}$ {\text{CH}}_4^{2 + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M10.png"/></alternatives></inline-formula> dissociation caused by the Jahn-Teller effect, because this value is consistent with the energy difference in energy between the <inline-formula><tex-math id="M11">\begin{document}$ {\text{CH}}_4^{2 + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M11.png"/></alternatives></inline-formula> ¹E initial state and the <inline-formula><tex-math id="M12">\begin{document}$ {\text{CH}}_3^ + /{{\text{H}}^ + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M12.png"/></alternatives></inline-formula> final state involving the Jahn-Teller effect. For the 4.75 eV peak, we believe that it may come from the direct dissociation of <inline-formula><tex-math id="M13">\begin{document}$ {\text{CH}}_4^{2 + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M13.png"/></alternatives></inline-formula> without contribution from the Jahn-Teller effect. More specifically, Williams et al. presented the potential energy curve for one C-H bond stretching to 8 a.u., while other C—H bonds are fixed at the initial geometry of the CH₄ molecule. In the reflection approximation, we infer that the extra energy is released from the internuclear distance of 8 a.u. to infinity. It is found that the KER is 4.7 eV, which is consistent with the experimental observation, suggesting that the KER peak at 4.75 eV may arise from the direct dissociation of <inline-formula><tex-math id="M14">\begin{document}$ {\text{CH}}_4^{2 + } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231377_M14.png"/></alternatives></inline-formula> 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, 5.75, 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.