Field ionization of surface absorbates

When an auxiliary gas of low ionization potential, such as H2, is added to helium the energy spectrum of field-ionized helium ions displays a second spectral line of greater energy. This satellite line originates from the ionization of field-adsorbed He atoms following their excitation by the impact of electrons released during free-space ionization of the auxiliary gas. Using a magnetic-sector atom-probe field-ion microscope we have studied the field dependence of the difference in energies of the two spectral lines D(F), as a function of crystallographic plane, emitter material, and gas species. From our data and a simple expression for D(F) we have determined the position of the adsorbate with respect to the image plane and the ionic potential at the position of the adsorbate for the (111), (112), and (011) planes of tungsten. Results for Mo, Ir, and Re are also presented.