FRAGMENTATION OF CH4 CAUSED BY FAST-PROTON IMPACT

The cross sections of the different breakup channels of CH4, 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 int o two charged fragments as well as for collisions where only a single charged molecular ion or fragment was produced. These relative abundan ces are compared to the ones measured for photodissociation and for el ectron and proton impact. Only the CH4+ ion survives long enough to be detected as a molecular ion, while the doubly charged CH42+ ion disso ciates rapidly. The most probable final product of the fragmentation o f doubly charged methane as formed by fast-proton impact is H++CH2++H. The abundance of H(m)++CH(n)+ (m+n less-than-or-equal-to 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 t he charged fragments so that two-body breakup holds approximately. The deviation from two-body breakup increases with increasing number of n eutral hydrogen atoms produced. The sensitivity of the experimental me thod enabled us to extend the study of the fragmentation pattern of CH 42+ to include small breakup channels such as CH42+-->H-3++CH+. Furthe rmore, some breakup channels of the triply charged CH43+ have been det ected as triple coincidences.