MOLECULAR PHOTOFRAGMENTATION DURING NONRESONANT 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, photofragmentation of clusters was found to be significant. Atomic velocity distributions measured using nonresonant ionization peaked at the same velocity as the respective dimer distribution, indicating that the majority of the non-resonant atomic ion signal is from photofragmented dimers. The contribution to the dimer ion signal from photofragmentation 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 (less than or equal to 10(8) W cm(-2)). To minimize photofragmentation effects, atoms should be ionized resonantly and clusters should be one-photon ionized.