Molecular photofragmentation during non‐resonant multiphoton ionization of sputtered species

To assess the quantitative analytical capabilities of laser post-ionization of sputtered neutrals, the photoionization of sputtered Cu, Al and Ru atoms and clusters was investigated. By measuring velocity distributions using both resonant and non-resonant photoionization of the atoms and one-photon and two-photon ionization of the clusters, photogragmentation of clusters was found to be significant. Atomic velocity distributions measured using non-resonant ionization peaked at the same velocity as the respective dimer distribution, indicating that the majority of the non-resonant atomic ion signal is from photogragmented dimers. The contribution to the dimer ion signal from photogragmentation of the trimer varied with the two-photon laser power density. Domination of the atomic photoion channel by molecule fragmentation appears to be a general phenomenon that must be accounted for in all gas-phase multiphoton non-resonant ionization experiments at easily achievable laser power densities (≤ 108 W cm−2). To minize photogragmentation effects, atoms should be ionized resonantly and clusters should be onephoton ionized.