Stopping power and energy loss straggling data of 0.6–4.4 MeV H+ and He2+ swift ions in bismuth thin films: Influence of Heq+ ion charge-state effects

The stopping power and energy-loss straggling of H⁺ and He²⁺ swift ions in bismuth (Bi) thin film were investigated for a wide energy range from 0.6 to 4.4 MeV. The measurements were performed in a backscattering geometry for ∼ 120 nm Bi-films on silicon substrate with a very thin interface layer of copper deposited by electron-beam evaporation. The obtained stopping results were first compared to previous data available for the two ion-target systems (H⁺-Bi, He⁺-Bi), and again to numerical values generated by three different computer codes of SRIM-2013, PASS-2.0 and CasP-6.0. While those of energy-loss straggling were compared to some previous data available for the above two ion-target systems and to collisional straggling values derived by Bohr’s and Bethe-Livingston’s classical theories. The whole straggling data were then compared to numerical values generated by Yang et al.’s semi-empirical formula recently refined to account new experimental data. The influence of He^q+-ion charge-state and related charge-exchange effects on the measured stopping and straggling data was then discussed, and finally completed by comparison with the analytical charge-exchange formula of Efken et al. limited to just two ion charge-states q of + 1 and + 2.