Secondary-ion emission from phenylalanine induced by atomic and molecular MeV ion beams

Relative yields of positive and negative secondary ions are measured for atomic and molecular MeV ion beam impinging on a phenylalanine target. The secondary ions were identified by a time-of-flight mass spectrometer. ${\mathrm{C}}^{+}$, ${\mathrm{O}}^{+}$, ${\mathrm{CO}}^{+}$, and ${\mathrm{CO}}_2^{+}$ were the beams used. The yields obtained for [M-H${]}^{\mathrm{-}}$, [M+H${]}^{+}$, and [M-COOH${]}^{+}$ secondary ions show that they are roughly proportional to the third power of the beam stopping power. Other experimental results and predictions of the molecular-expansion model for neutral ejected species are included in the discussion.