Stopping Power and Valence States

Based on the dynamic Thomas-Fermi model of the atom, the Bloch theory of the mean excitation potential, $I$, predicts that $I=KZ$, where $K$ is Bloch's adjustable constant and $Z$ the atomic number. Stopping experiments with 340-Mev protons reveal an increase of $K$ for decreasing $Z$, with superimposed fluctuations. These fluctuations are shown quantitatively to be caused by valence state and polarization effects. For isolated atoms, $\frac{I}{Z}$ increases smoothly with decreasing $Z$, confirming the $Z$-dependence of $K$ predicted by Jensen for the dynamic Thomas-Fermi-Dirac model.