Z2 structure and gas-solid effect in the stopping of slow ions

A recent claim by Paul of a systematic gas-solid difference in stopping cross sections for ions such as nitrogen and oxygen in the velocity range v ≅ v0 is studied on the basis of existing experimental data. We find that all existing data support the commonly known Z2 structure which, by and large, follows the valence structure of the target material. Existing experimental evidence is not found to support a specific gas-solid difference in the velocity range under consideration. The possibility of such an effect due to a gas-solid difference in charge state is rejected on theoretical grounds. Data for compound gases and solids are found to be well described by the Bragg additivity rule. We have also studied nitrogen/helium and oxygen/helium stopping ratios which determine the so-called effective-charge ratio. Taking into account the scatter of experimental data, we do not find clear evidence against Northcliffe's assumption of a stopping ratio independent of Z2 and common for gases and solids in the considered velocity range, although the absolute value appears too high