Negative-ion and electron emission from surfaces at low impact energies

Negative-ion and electron yields for collisions of positive alkali-metal ions with a gas-covered heterogeneous molybdenum surface have been measured for impact energies ranging from below threshold up to 300 eV. It is observed that the threshold energies are identical for the sputtering of negative ions and the emission of secondary electrons. The sputtered negative ions are found to be predominantly ${\mathrm{O}}_2^{\mathrm{-}}$ for collision energies in the near-threshold region. It is suggested that near threshold the mechanism for secondary-electron emission involves sputtering an unstable negative ion which subsequently decays, producing a free electron.