Interpretation of Channeled-Ion Energy-Loss Spectra

The energy-loss spectra produced in beams of energetic He and I ions transmitted through planar channels in thin Au monocrystals have been explained by a simple model in which the ions execute transverse anharmonic oscillations in the channel between a pair of adjacent planes of target atoms and in which the stopping power of the medium is a function of the distance of the ion from the crystal planes. A quantitative comparison of this model with recent experiments is made using a plane-averaged potential based on Moliere's approximation to the Thomas-Fermi screening function to describe the oscillatory motion. The stopping power is found to obey the same functional dependence on the coordinates, but with a doubled screening length. The parameters of the model may be deduced directly from the experimental data. For He in Au, these parameters agree well with those expected from Firsov's treatment of the interaction potential between neutral atoms. For I in Au, the high charge of the ion leads to parameters which differ significantly from Firsov's theory. The data also suggest that the energy dependence of the stopping power for channeled ions may be somewhat different from that for randomly directed ones.