The irradiation of tungsten with metallic diatomic molecular ions: atomic resolution observations of depleted zones

Direct evidence, on an atomic scale, is presented for the enhancement of damage production per projectile ion in diatomic metallic molecular ion (dimer) irradiations of tungsten as compared to monatomic metallic ion (monomer) irradations. Irradiations were performed in situ at ≤10 K, in a field-ion microscope, employing 20 keV ag⁺ or W⁺ monomer ions and the results are compared with 40 keV W₂⁺ or Ag₂⁺ dimer ion bombardments; the average energy per ion was 20 keV. Firstly, in the near-surface region the depleted zones produced by the W₂⁺ dimer ions give rise to void-like contrast effects. The W⁺ monomer ions do not produce this void-like damage. The existence of voids was explained employing a classical nucleation and diffusion-limited growth model which suggests that the growth can occur on a time scale of <10^-10 s, if the effective diffusivity of an atom in the fully-developed collision cascade is > 30 × 10^-4 cm² s^-. Secondly, by counting the number of vacancies in individual depleted zones, produced by the different ions, it was demonstrated that the number of vacancies produced per incoming ion of the dimer is 1.55 time the number of vacancies produced per monomer ion. This constitutes direct evidence for non linear (or spike) effects during the creation of collision cascades by the metallic dimer ions. Further information is presented concerning the dimensions of the depleted zones and the values of the average concentration of vacancies in depleted zones.