Spectral properties of laser-accelerated mid-$Z$ $MeV∕u$ ion beams^a)

Collimated jets of beryllium, carbon, oxygen, fluorine, and palladium ions with $>1MeV∕nucleon$ energies are observed from the rear surface of thin foils irradiated with laser intensities of up to $5×1019W∕cm2$⁠. The normally dominant proton acceleration is suppressed when the target is subjected to Joule heating to remove hydrogen-bearing contaminant. This inhibits screening effects and permits effective energy transfer to and acceleration of heavier ion species. The influence of remnant protons on the spectral shape of the next highest charge-to-mass ratio species is shown. Particle-in-cell simulations confirming the experimental findings are presented.