Target dependence of effective projectile charge in stopping powers

The theory of the effective ion charge, Z₁*e, in stopping powers is tested for ion velocities v₁ near v₀=e² through measurements of relative stopping powers for He⁺ and D⁺ in foils of Au(rₛ=1.49), C (1.66), Al(2.12), and Cs(5.88), where rₛ is related to the conduction-electron density, nₑ, as rₛ=(3/4πnₑa₀³)1/3. One finds a strong material dependence of Z₁*. It can be accounted for by referring to the relative ion-electron velocities, vᵣ, which include the Fermi velocity vF=1.919rₛ^-1 (a.u.) of the target electrons. The results may pertain to the stopping of charged particles in materials at very high pressures and temperatures.