Structure evolution in metals bombarded with swift heavy ions

Abstract

Bombardment of pure metal targets with swift heavy ions (gigaelectronvolt energy) results in electronic excitation and latent track formation, beyond a threshold value of stopping power. Given the experimentally detected homogeneity of the core of the cylindrical damage track, the ion is considered to deposit homogeneously its energy over all atoms of the crystal involved in the interaction process. We suppose that energy deposition results in the ionization of the target, over a cylindrical region (ionization cylinder) coaxial with the damage cylinder, with the condition that each atom within the ionization cylinder is considered as isolated and undergoes n multiple ionization events. According to a criterion of minimum energy expense, it is progressively stripped of its electrons, beginning with the outer shell, and it changes its atomic configuration from Z_A to (Z_A - n)*. The ionized (Z_A - n)* atoms are ejected out of the ionization cylinder, being spread in the damage cylinder where they form a Z_A - (Z_A - n)* starting compound. In the frame of the segregation-charge-transfer model, at the interface between the crystalline Z_A matrix and a damage cylinder, containing the starting compound, segregation of either compound component occurs, leading to non-equilibrium profiles, both compositional and of electronic density. The local trend towards restoration of the charge-density profile is simulated by charge-transfer reactions. Each reaction product is a dimer, considered as a cluster of an effective compound. The energy cost to introduce in the matrix an effective compound dimer is calculated, and qualitative differences are found between metals undergoing amorphization or crystal structure formation under swift heavy-ion bombardment.