Recoil ion mass spectrometry. Part 2. Formation of slow, multiply charged recoil ions in collisions of fast negative ions with Ar and Kr atoms

Abstract

Mass spectrometric studies have been conducted on slow, multiply charged recoil ions produced in collisions of O^-, Si^-, C^- and C^-₂ ions with neutral beams of Ar and Kr atoms at impact energies in the range 40-70 keV. A crossed-beams apparatus incorporating a quadrupole mass spectrometer has been coupled to the mass-selective negative ion injection system of a tandem heavy-ion accelerator for these studies. In contrast to the situation pertaining to multiple ionisation in collisions with fast positively charged projectiles, where indirect ionisation mechanisms, such as excitation- autoionisation and Auger rearrangement processes may dominate multielectron ejection, the dependence of the relative multiple ionisation cross sections on the nature of the projectile species in the present experiments appears to indicate that in ionisation by negative ions, the overall electron density distribution in the projectile-target system may play a very significant role.