Electron impact ionization of ions trapped in a hollow electron beam

Abstract

Introduction of a molecular beam into a hollow cylindrical electron beam generated by a toroidal cathode produces positive ions which oscillate radially for periods of the order of a second: they are ionized by bombardment with an axial beam of electrons of variable energy; ions of different charge states are extracted axially through an orifice into a quadrupole mass-spectrometer and channeltron detector. In this way cross section functions for ionization of multiply charged ions are obtained for electron energies between threshold and 500 eV. The cross sections, not being absolute, are normalized against data obtained by crossed-beam techniques. The normalization factors which are governed by the ion trapping time, are independent of the energy of the axial beam but may depend on the residual gas pressure. They are used with +or-30% confidence for multiply charged ion cross section functions for which only the singly charged ion crossed beam data are available.