Fragmentation of CH4 caused by fast-proton impact.

Abstract

The cross sections of the different breakup channels of ${\mathrm{CH}}_{4}$, produced by 4-MeV proton impact, have been measured using the coincidence time-of-flight technique. The relative abundances of the different breakup channels were evaluated for collisions in which the molecule broke into two charged fragments as well as for collisions where only a single charged molecular ion or fragment was produced. These relative abundances are compared to the ones measured for photodissociation, and for electron and proton impact. Only the ${\mathrm{CH}}_{4}^{+}$ ion survives long enough to be detected as a molecular ion, while the doubly charged ${\mathrm{CH}}_{4}^{2+}$ ion dissociates rapidly. The most probable final product of the fragmentation of doubly charged methane as formed by fast-proton impact is ${\mathrm{H}}^{+}$+${\mathrm{CH}}_{2}^{+}$+H. The abundance of ${\mathrm{H}}_{\mathit{m}}^{+}$+${\mathrm{CH}}_{\mathit{n}}^{+}$ (m+n\ensuremath{\le}4) ion pairs decreases rapidly with increasing m, as suggested by Siegbahn [Chem. Phys. 66, 443 (1982)]. The momentum of neutral fragments, in channels where they are produced, is small in comparison with the momentum of the charged fragments so that two-body breakup holds approximately. The deviation from two-body breakup increases with increasing number of neutral hydrogen atoms produced. The sensitivity of the experimental method enabled us to extend the study of the fragmentation pattern of ${\mathrm{CH}}_{4}^{2+}$ to include small breakup channels such as ${\mathrm{CH}}_{4}^{2+}$\ensuremath{\rightarrow}${\mathrm{H}}_{3}^{+}$+${\mathrm{CH}}^{+}$. Furthermore, some breakup channels of the triply charged ${\mathrm{CH}}_{4}^{3+}$ have been detected as triple coincidences.