ENERGY-LOSS OF HYDROGEN AND HELIUM-IONS IN HYDROGEN AND HELIUM GAS - LOOKING FOR EXCEPTIONS FROM VELOCITY PROPORTIONALITY

At projectile velocities upsilon(1) below the Bohr velocity the electronic stopping cross section epsilon of hydrogen (H2, D2) and helium gas for hydrogen isotopes (H, D) and of helium gas for helium projectiles has been measured by a time-of-flight technique. We find the hydrogen stopping in He to show a significant departure from the upsilon(1)-proportionality commonly assumed (epsilon is-proportional-to upsilon(1)3.3 at 4 keV per nucleon) [R. Golser and D. Semrad, Phys. Rev. Lett. 66 (1991) 1831]. Electronic stopping of H, D onto H-2, D2 (epsilon is-proportional-to upsilon(1)0.9 at 4 keV per nucleon) and He onto He (epsilon is-proportional-to upsilon(1)0.8 at 4 keV per nucleon, if nuclear stopping is not subtracted) may be considered velocity proportional. We attribute this behavior to the probability of charge changing processes, which is small if projectile and target ground state levels are strongly different. The helium to hydrogen stopping ratio of helium gas has the strange value 5.8 at 4 keV per nucleon; possible implications to the concept of "effective charge" are discussed.