In-Flight Decay of a Foil-Excited Ion Beam

700 keV ⁷Li⁺ beam (velocity 4.4 mm/ns) incident on a thin (3 mg/cm²) carbon foil. The blue light near the foil is from the H-like 4f-5g transition manifold in Li²⁺ (wavelength 4500 Å, lifetime 3 ns, decay length 1.3 cm). The green light downstream is from the He-like 1s2s-1s2p triplet transition in Li⁺ (wavelength 5485 Å, lifetime 44 ns, decay length 19 cm).