VELOCITY DEPENDENCE OF IONIZATION AND FRAGMENTATION OF METHANE CAUSEDBY FAST-PROTON IMPACT

The velocity dependence of methane ionization and fragmentation have b een studied using a coincidence time-of-flight technique. The relative yields of single-ion, ion-pair, and ion-triple channels have been det ermined for 1- to 12-MeV proton impact. The multiple-ionization cross sections have been determined and are in accord with our model semicla ssical Coulomb approximation calculations. The single-ion channels res ulting from the breakup of CH4+ were found to be independent of the c ollision velocity. Good agreement with previous proton-impact measurem ents and with electron-impact measurements is observed. On the other h and, the ion-pair breakup channels of CH42+ show a surprising depende nce on the collision velocity. The:ion-pair data are in reasonable agr eement with fast-electron-impact measurements at 10 keV while differin g significantly with a lower, 1-keV, electron-impact measurement.