Molecular SIMS Ionization Probability Studied with Laser Postionization: Influence of the Projectile Cluster

The formation probability of (quasi-)molecular secondary ions released from an organic film under bombardment with a 20 keV cluster ion beam is investigated using combined time-of-flight secondary ion and neutral mass spectrometry (ToF-SIMS/SNMS) experiments. The emitted neutral molecules are postionized after their ejection using strong-field photoionization in an intense short infrared laser pulse. Comparing the (quasi)-molecular secondary ion signal with that of the corresponding neutral molecules, the ionization probability of sputtered intact coronene and guanine molecules is determined. The results are compared between two different projectile cluster ions, namely (i) C-60(+) and (ii) Ar-n(+) with n similar to 1000. It is shown that both projectiles deliver different SNMS spectra, indicating pronounced differences in the collision-induced fragmentation of the emitted molecules. For guanine, the ionization probability obtained with both projectiles is of the same order of magnitude (similar to 10(-3)), with the fullerene cluster producing a slightly larger ionization efficiency than the rare gas cluster. For coronene, on the other hand, a substantially lower ionization efficiency is found for the gas cluster projectile.