Discrimination effects for ions with high initial kinetic energy in a Nier-type ion source and partial and total electron ionization cross-sections of CF 4

Abstract

Fragment ions produced by dissociative electron impact ionization of molecules often have large amounts of kinetic energy (up to several electronvolts). Presently performed computer simulations of the ion trajectories in a Nier-type ion source show strong discrimination effects in the extraction characteristics of such ions, leading to huge errors in measured fragmentation patterns and absolute partial ionization cross-section functions. Using the simulation results a correction function (extraction efficiency vs. fragment kinetic energy) is claculated here for a given potential distribution of a Nier-type ion source extraction system. This correction procedure is applied to previously measured partial ionization cross-sections of CF 4. The corrected partial and total CF 4 cross-sections are in excellent agreement with other recent measurements using advanced methods.