Experimental energy loss of slow H{sup +} and H{sub 2}{sup +} in channeling conditions

The interactions of hydrogen molecular ions (H-2(+)) and protons in the low energy range (E<7.5 keV/u) with very thin foils of monocrystalline gold are experimentally studied. Measurements of energy loss distributions of molecular fragments, recombined molecules, and protons channeled in the <100> direction of a monocrystalline gold thin film have been performed. From the energy loss data we determine the stopping power ratio, which shows a significant negative "vicinage effect," giving a reduced energy loss of molecular fragments and recombined molecules as compared to the energy loss of protons. This effect is more pronounced in the case of detected molecules. Computational simulations, where nonlinear models are included, qualitatively agree with our experimental results and suggest that some pairs of ions travel and emerge from the crystal with appropriate internuclear distances and relative velocities and are able to recombine at the exit.