Auger Electron Emission from Clean Mo Bombarded by Positive Ions. III. Effect of Electronically Excited Ions

Measurements of the effect of electronic excitation of the incident ions on Auger electron emission yield from clean molybdenum surfaces are reported. The influence of excited ions in the beam used in these measurements is studied by observing the variation in the measured Auger yield ${\gamma }_i$ with the energy of the electrons used to produce the ions in the electron bombardment ion source. The species ${\mathrm{O}}_2^{+}$, N${\mathrm{O}}^{+}$, ${\mathrm{N}}_2^{+}$, ${\mathrm{N}}^{+}$, and ${\mathrm{H}}_2^{+}$ were studied. Of these, only the ${\mathrm{O}}_2^{+}$ and N${\mathrm{O}}^{+}$ beams were found to contain electronically excited ions at a time ∼20 μsec after their formation. By operating the ion source at an electron energy below the threshold of the first excited state, a pure ground-state ion beam is produced. The electron yield for these ground-state ions bombarding an atomically clean molybdenum surface is reported over the range of ion kinetic energies from 30-400 eV. Methods of estimating the excited-state population in an ion beam are discussed and compared with one another. It is argued that in the present case, only a single excited state is present in the ${\mathrm{O}}_2^{+}$ and N${\mathrm{O}}^{+}$ beams, allowing an estimate of the excited-state electron yield to be made. The implications of these results with respect to previous measurements of electron yields in which excited states were not considered are discussed.