ELECTRON-STIMULATED POSITIVE-ION EMISSION FROM NACL CRYSTAL-SURFACE

The energy and the mass distributions of positive ions emitted from th e surface of ultrathin epitaxial NaCl films under 1-5 keV electron bea m have been measured. Two peaks in the energy spectra have been found. The low energy ale, composed mostly of Na+, Cl+ and Cl-2(+) ions, is temperature dependent, and scales quadratically with the electron curr ent. This peak dearly corresponds to the gas-phase ionisation of neutr al atoms. The second, with a maximum around 1 eV, is composed exclusiv ely of Na+ ions, does not depend significantly on the crystal temperat ure, and scales linearly with che electron current. This last peak see ms to be due to ions ejected from the crystal surface as a result of t he Coulomb instability. Depending on the crystal temperature and the e lectron current density either the first or the second component may b ecome dominating, explaining the discrepancies in results obtained so far by different authors.