Noble-gas bubbles in metals: Molecular-dynamics simulations and positron states

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Abstract
A theoretical treatment of atomic structure and positron states in noble-gas bubbles in metals is presented. The Al-He and Cu-Kr systems are considered as specific examples. For large bubbles (radii above a few tens of angstroms) a calculational scheme is developed combining molecular-dynamics results for the metal–noble-gas interface with positron calculations. It is demonstrated that a positron is trapped at the surface of a noble-gas bubble, i.e., at the metal-gas interface. The annihilation rate with metal electrons is similar to that at a clean surface, while simultaneously there is a significant annihilation rate with gas-atom electrons. This enables relationships between the gas density and the positron lifetime to be obtained for the systems considered. Experimental evidence supports the theoretical relations. In the molecular-dynamics simulations a trend towards close-packed layer-by-layer ordering of the gas atoms close to the metal-gas interface is found even in the cases where the bulk gas is in a fluid phase. The positron-state calculations also cover the case of adsorbed noble-gas layers at surfaces. For vacancy–noble-gas clusters containing up to 13 vacancies complementary positron results obtained with the calculational method developed by Puska and Nieminen are presented.