Asymmetric line shapes for medium energy H and He ions undergoing a large angle collision

Asymmetric line shapes for medium energy H and He ions backscattered from topmost adatoms such as $\text{Si}(111)\text{\ensuremath{-}}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-Sb}$ and $\text{Ni}(111)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}2\text{-O}$ are measured by a toroidal electrostatic analyzer with an excellent energy resolution. The spectra exhibit a pronounced asymmetric nature and are well fitted by an exponentially modified Gaussian profile. It is found that the nonperturbative coupled-channel calculations reproduce well the observed asymmetric line shapes for ${\text{He}}^{+}$ impact on different materials, although slightly overestimate the asymmetry for ${\text{H}}^{+}$ impact on Au. On the other hand, the CASP 3.2 program (involving additional approximations) gives large underestimates for He ions and overestimates for H ions. This problem has been partially solved by modifying the order of the implementation of the shell corrections and higher-order effects in the CASP model.