VELOCITY DEPENDENCE OF IONIZATION AND FRAGMENTATION OF METHANE CAUSED BY FAST-PROTON IMPACT

The velocity dependence of methane ionization and fragmentation have been studied using a coincidence time-of-flight technique. The relative yields of single-ion, ion-pair, and ion-triple channels have been determined for 1- to 12-MeV proton impact. The multiple-ionization cross sections have been determined and are in accord with our model semiclassical Coulomb approximation calculations. The single-ion channels resulting from the breakup of CH4+* were found to be independent of the collision velocity. Good agreement with previous proton-impact measurements and with electron-impact measurements is observed. On the other hand, the ion-pair breakup channels of CH42+* show a surprising dependence on the collision velocity. The:ion-pair data are in reasonable agreement with fast-electron-impact measurements at 10 keV while differing significantly with a lower, 1-keV, electron-impact measurement.