Multielectron effects in capture of antiprotons and muons by helium and neon

Antiproton $\left(\overline{p}\right)$ and negative muon $\left({\mu }^{-}\right)$ captures by helium and neon atoms are treated using the fermion- molecular-dynamics method, yielding capture cross sections, initial quantum numbers, and ejected-electron energies. The calculated angular-momentum distributions tend to pile up at $l=n-1$ and are not well fitted by the form ${\mathrm{}(2l+1)e}^{\alpha l},$ which is often assumed in experimental analyses. The residual electrons are generally left in a “shake-up” state. When capture is accompanied by multiple ionization, the second and later electrons escape with increasing kinetic energies, a process that is not well described as quasiadiabatic. In a 50:50 mixture of helium and neon, the calculated Ne:He capture ratios are 3.36 for $\overline{p}$ and 3.69 for ${\mu }^{-}.$ The ${\mu }^{-}$ per-atom capture probability is almost independent of the neon fraction and is in overall agreement with several ${\mu }^{-}$ and ${\pi }^{-}$ experiments; for $\overline{p}$ there is a rather strong dependence on neon fraction, but there are as yet no experiments. The possible experimental indication of a strong isotope effect on capture of ${\pi }^{-}$ in mixtures of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ is not supported by the present calculation.