Fragmentation mechanisms for methane induced by 55 eV, 75 eV, and 100 eV electron impact.

The fragmentation of CH₄²⁺ dications following 55 eV, 75 eV, and 100 eV electron impact double ionization of methane was studied using a cold target recoil-ion momentum spectroscopy. From the measured momentum of each recoil ion, the momentum of the neutral particles has been deduced and the kinetic energy release distribution for the different fragmentation channels has been obtained. The doubly charged molecular ions break up into three or more fragments in one or twostep processes, resulting in different signatures in the data. We observed the fragmentation of CH₄²⁺ dications through different mechanisms according to the momentum of the neutral particles. For example, our result shows that there are three reaction channels to form CH₂⁺, H⁺, and H, one synchronous concerted reaction channel and two two-step reaction channels. For even more complicated fragmentation processes of CH₄²⁺ dications, the fragmentation mechanism can still be identified in the present measurements. The slopes of the peak in the ion-ion coincidence spectra were also estimated here, as they are also related to the fragmentation mechanism. [ABSTRACT FROM AUTHOR]