Negative-ion desorption from insulators by electron excitation of core levels

We have observed the desorption of ${\mathrm{O}}^{\mathrm{-}}$ and ${\mathrm{Si}}^{\mathrm{-}}$ ions during the electron bombardment of ${\mathrm{SiO}}_2$, and have determined ion yields and kinetic-energy distributions at electron energies >100 eV. The threshold energy for the ${\mathrm{O}}^{\mathrm{-}}$ yield corresponds to the excitation of Si-L-shell core levels. We propose that multielectron excitations cause the ejection of positive ions or neutral atoms from the surface, and that these species can capture electrons in the surface region to form negative ions. Alternatively, the creation of superexcited electronic states of an ${\mathrm{SiO}}_2$ surface complex may lead to the ejection of ${\mathrm{O}}^{\mathrm{-}}$.