Ejectedelectron-ion coincidence measurements of multiple ionization ofargon by 10-24 keV electron impact

Abstract

The charge state spectra of argon ions produced in singlecollisions by electrons having energies from 10 to 24 keVwith argon atoms have been measured by using the ejectedelectron-ion coincidence technique. Charge analysis of theions is accomplished by a time-of-flight mass spectrometer equipped with a channeltron while the ejected electrons aredetected by another channeltron, both being used in thepulse-counting mode. The ions and the electrons are detected at90° to the incident electron beam direction. Therelative abundance of argon ions in charge state n up to n = 4 + was deduced from the experiment. The charge state fractionsFn of ions with charge states n are found to be constantin the investigated impact energy range. However, their valuesare seen to decrease rapidly with n. A good agreement isobtained between our experimental results and the calculationsbased on inner shell ionization cross sections folded with charge state abundances, which result from the decay of thecorresponding initial inner shell vacancies. This agreementsupports the idea that the multiple ionization of argon atomsby energetic electrons is caused mainly due to inner shellvacancy production followed by relaxation processes resultingin an increase of the charge state of the generated ion. Further,our results show that the Ar+ ions are produced mainlythrough dipole transitions while ions having n>1 + aredominantly produced by the non-dipole transitions in the entireenergy range of investigation. The partial ionizationcross sections for Arn + ions (n = 1-4) are found tobe in close agreement with the results of other workers in the overlapping region of impact energies.