Doubly versus singly positively charged oxygen ions back-scattered from a silicon surface under dynamic O2+ bombardment

Mass-resolved (and emission-charge-state-resolved) low-energy ion back-scattering during dynamic O2+ bombardment of a silicon surface was applied in a Cameca IMS-3f secondary ion mass spectrometry (SIMS) instrument to determine the bombarding energy dependence of the ratio of back-scattered O2+ versus O+. While the ratio of O2+ versus O+ drops significantly at reduced bombarding energies, O2+ back-scattered from silicon was still detectable at an impact energy (in the lab frame) as low as about 1.6 keV per oxygen atom. Assuming neutralization prior to impact, O2+ ion formation in an asymmetric 16O→28Si collision is expected to take place via ‘collisional double ionization’ (i.e. by promotion of two outer O 2p electrons) rather than by the production of an inner-shell (O 2s or O 1s) core hole followed by Auger-type de-excitation during or after ejection. A molecular orbital (MO) correlation diagram calculated for a binary ‘head-on’ O–Si collision supports this interpretation.