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A comprehensive analysis of the stopping power of antiprotons and negative muons in He and gas targets for projectile velocities (equivalent antiproton energies) ranging from about 0.1 to 10 au (0.25 keV to 2.5 MeV) is performed. Recent experimental data are contrasted with theoretical results obtained from different approaches. The electronic stopping power is evaluated within the coupled-state atomic-orbital method and the distorted-wave Born approximation as well as, for low projectile velocities, within a generalized adiabatic-ionization model that takes into account collisional-broadening effects. The departure of the antiproton stopping power from the proton stopping power (`Barkas effect'), observed for intermediate projectile velocities, is discussed. The contribution to the stopping power arising from energy transfer to the translational degrees of freedom of the target system (`nuclear stopping') is evaluated. Our analysis results in a good understanding of the stopping mechanisms of negative heavy particles in gases, in particular in He. Discrepancies between theory and experiment in the case are attributed to effects of the molecular structure of the target.

In this work we describe a model for the electronic energy loss of bare ions at high velocities. Starting from first-order perturbation theory we propose a simple formula to calculate the impact-parameter dependence of the electronic energy loss for all impact parameters. The physical inputs are the electron density and oscillators strengths of the atoms. A very good agreement is obtained with full first-order calculations.

A single-center coupled-channel code based on an expansion in terms of atomic wave functions that includes dynamic curved projectile trajectories is applied to the calculation of stopping powers. Stopping powers and differential ionization cross sections are evaluated for p, H+, He +, and Li'+ projectiles penetrating atomic H and He targets at energies of 10-500 keV/u. The results are compared to experimental data, to predictions of the first-order plane-wave Born approximation, and to results of calculation for excitation of a harmonic oscillator including Barkas corrections. The improvement of the present model to first-order or second-order perturbative treatments as well as the effect of polarization on the projectile trajectories is discussed.

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