Influence of the chemical state on the stopping of protons and He-ions in some oxides

We present stopping cross section data of Al2O3 and SiO2 for hydrogen- and helium-ions in the energy range 2-1000 keV, measured in transmission and in backscattering geometry. To interpret the data, we discuss the high velocity and the low velocity limit of so-called chemical effects commonly defined as the difference in stopping of the compound and of a mixture of its constituents. as calculated by applying Bragg's rule. At high velocities, the projectiles are point charges and only changes in the target electron stales contribute to the chemical effect. In addition, at low velocities the charge states of the projectiles and the screening by the target valence electrons may differ in the compound and in the mixture, due to different electron densities. (C) 1998 Elsevier Science B.V.