The stopping power in {111} Au for channeled ions which have not changed charge has been measured for bare nuclei for $Z=1-5\mathrm{}$ at 3.5 MeV/amu, and for ions bearing 0, 1, and 2 bound electrons for $Z_1=1-9\mathrm{}$ at 2 MeV/amu. An effective screening of $\sim 0.9e$ per electron is obtained. For the bare nuclei, deviations from simple ${Z_1}^2$ scaling are reported. At 2 MeV/amu an increase of 3.5% in $\frac{S}{{Z_1}^2}$ is seen from H to He, 3% from He to Li; but no differences are seen from $Z_1=5-9\mathrm{}$. However, at 3.5-MeV/amu deviations of as much as 20% per $Z_1$ are observed.

• Received 3 March 1977

DOI:https://doi.org/10.1103/PhysRevLett.38.1145