Investigating dissociation mechanism of ethane dication via three-body fragmentation
Authors: ZHANG Ziqi  , YAN Shuncheng  , TAO Chenyu  , YU Xuan  , ZHANG Shaofeng  , MA Xinwen  ,   1 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China;,   2 University of Chinese Academy of Sciences, Beijing 100049, China
Published: January 2025

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, its fragmentation may be involved in the generation of complex hydrocarbons, as well as the atmospheric escape processes on Titan.<br>In this paper, we carried out the experiment on ethane fragmentation by electron impact, focusing on the three-body fragmentation channel from C<sub>2</sub>H<sub>6</sub><sup>2+</sup> to CH<sub>3</sub><sup>+</sup>/CH<sub>2</sub><sup>+</sup>/H. We directly measured the three-dimensional momenta of CH<sub>3</sub><sup>+</sup> and CH<sub>2</sub><sup>+</sup> ions, and then reconstructed the momentum of the H atom using momentum conservation law. Based on these analyses, we investigated the kinetic energy release (KER) spectrum and the fragmentation mechanisms.<br>In the TOF coincidence map of the ions, we observed two channels: channel (1) represents the two-body dissociation generating CH<sub>3</sub><sup>+</sup>/CH<sub>3</sub><sup>+</sup>, and channel (2) represents the three-body dissociation generating CH<sub>3</sub><sup>+</sup>/CH<sub>2</sub><sup>+</sup>/H, which is mentioned above. It is found that the neutral H from channel (2) has a wide kinetic energy distribution, ranging from 0 eV up to more than 10 eV. This feature indicates 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 CH<sub>3</sub><sup>+</sup> and CH<sub>2</sub><sup>+</sup>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 CH<sub>2</sub><sup>+</sup> and CH<sub>3</sub><sup>+</sup>.<br>The corresponding fragmentation mechanism of channel (2) was also analyzed in this paper. We divided the overall KER spectrum into three parts, 0-6 eV, 6-9 eV, and 9-11 eV, and reconstructed the respective Dalitz plots and Newton diagrams under different KER conditions. In all Dalitz plots, there is a bright spot representing the concerted dissociation and a horizontal belt representing the sequential dissociation. The concerted dissociation is concluded as the main mechanism, while the sequential dissociation plays a minor role.<br>The bright spot in the Dalitz plot shifts from the center to the left as the KER increases. This feature arises from the following fact, the CH<sub>2</sub><sup>+</sup>lies between the H and the CH<sub>3</sub><sup>+</sup> in the concerted dissociation, and it feels the recoil both from H and CH<sub>3</sub><sup>+</sup>. Considering the Coulomb potential from CH<sub>3</sub><sup>+</sup> is constant, enhancing the C-H dissociation energy will decrease the CH<sub>2</sub><sup>+</sup> kinetic energy. The belt in the Dalitz suggests the sequential dissociation as a two-step process, the first step is the dissociation of C<sub>2</sub>H<sub>6</sub><sup>2+</sup> to generate H and metastable C<sub>2</sub>H<sub>5</sub><sup>2+</sup>, and the second step is the fragmentation from C<sub>2</sub>H<sub>5</sub><sup>2+</sup> to CH<sub>3</sub><sup>+</sup> and CH<sub>2</sub><sup>+</sup>.<br>We also reconstructed the Newton diagrams under different KER conditions to give further evidence of the sequential dissociation from the metastable C<sub>2</sub>H<sub>5</sub><sup>2+</sup>, rather than from the metastable CH<sub>3</sub><sup>+</sup>orCH<sub>4</sub><sup>+</sup>. Indeed, for the former case, the center positions of the two half circles in Newton diagrams appear correctly. Oppositely, for the latter two cases, the center positions notably deviate from the expected values. This means the sequential dissociation from C<sub>2</sub>H<sub>5</sub><sup>2+</sup> is dominant, which agrees excellently with the conclusion from the Dalitz plots.