Atomic resolution observations of nonlinear depleted zones in tungsten irradiated with metallic diatomic molecular ions

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Abstract
Direct evidence, on an atomic scale, is presented for the enhancement of damage production per projectile ion in metallic diatomic molecular ion (dimer) irradiations of tungsten as compared to monatomic ion (monomer) irradiations. Irradiations were performed in situ at ≲10 K, in a field-ion microscope, employing 20 keV Ag+ and W+ ions and the results are compared with 40 keV W+2 and Ag+2 ion bombardments; the average energy per ion is 20 keV. The energy and mass of the dimers are such that the collision cascades are nonlinear. By directly 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 times greater than the number of vacancies produced per monomer. The depleted zones produced in the near-surface region, by the W+2 dimers, give rise to void-like contrast effects. Whereas the W+ monomers do not produce the void-like contrast of the W+2 ions. This result can only be understood if significant rearrangement of vacancies had occurred after the dynamic development of the collision cascade. The presence of voids is explained employing a nucleation and growth model. The average diameter (〈λ〉) of the depleted zones created by 20 keV Ag+ and W+ monomers is 17.5 Å, while 〈λ〉 is 28.5 Å for the Ag+2 and W+2 dimers. Thus the depleted zones created by the dimers are larger than those produced by the monomers, but not twice as large. This indicates that the collision cascades, in the case of the dimer irradiations, overlapped to a certain degree. In addition, the radial distribution and the first nearest-neighbor clusters functions for the vacancies in the depleted zones are presented and compared for the monomer and dimer ion irradiations.