MOLECULAR PHOTOFRAGMENTATION DURING NONRESONANT MULTIPHOTON IONIZATION OF SPUTTERED SPECIES

To assess the quantitative analytical capabilities of laser post-ioniz ation of sputtered neutrals, the photoionization of sputtered Cu, Al a nd Ru atoms and clusters was investigated. By measuring velocity distr ibutions using both resonant and non-resonant photoionization of the a toms and one-photon and two-photon ionization of the clusters, photofr agmentation of clusters was found to be significant. Atomic velocity d istributions measured using nonresonant ionization peaked at the same velocity as the respective dimer distribution, indicating that the maj ority of the non-resonant atomic ion signal is from photofragmented di mers. The contribution to the dimer ion signal from photofragmentation of the trimer varied with the two-photon laser power density. Dominat ion of the atomic photoion channel by molecule fragmentation appears t o be a general phenomenon that must be accounted for in all gas-phase multiphoton non-resonant ionization experiments at easily achievable l aser power densities (less than or equal to 10(8) W cm(-2)). To minimi ze photofragmentation effects, atoms should be ionized resonantly and clusters should be one-photon ionized.