Excited-state evolution probed by convoy-electron emission in relativistic heavy-ion collisions

Abstract

We present a joint experimental and theoretical study of convoy-electron emission resulting from highly-charged-ion transport through carbon foils at moderately relativistic speeds. Energy spectra of electrons ejected at $0\ifmmode^\circ\else\textdegree\fi{}$ have been measured for 390 MeV/u hydrogen-like ${\mathrm{Ar}}^{17+}$ ions and 460 MeV/u $(\ensuremath{\beta}=v/c=0.74,\ensuremath{\gamma}=1.49)$ ${\mathrm{Fe}}^{25+}$ $(1s),$ ${\mathrm{Fe}}^{24+}$ ${(1s}^{2}),$ and ${\mathrm{Fe}}^{23+}$ ${(1s}^{2}2s)$ incident on carbon foils with thicknesses from 25 to $8700\ensuremath{\mu}{\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}.$ Due to this unprecedented wide range of thicknesses, the sequential excitation and ionization of initially deeply bound electrons to highly excited states and continuum states can be followed in considerable detail. The analysis of the spectra is aided by simulations based on the classical transport theory which has been extended to relativistic energies and to multielectron projectiles. The motion of the projectile electron inside the solid target is calculated taking into account the Coulomb potential of the projectile ion and the multiple stochastic collisions with target cores and target electrons. Different phases of the convoy-electron emissions can be disentangled: direct ejection to the continuum, the transient buildup of an excited-state wave packet followed by ionization, and postionization modification of the continuum spectrum. We find good agreement between experiment and simulation for the evolution of charge states and the emission spectrum.