Direct detection of atomic ions from molecular photofragmentation during nonresonant multiphoton ionization of sputtered species

The photoionization of sputtered Cu, Al, and Ru atoms and dimers was investigated by measuring velocity distributions using both resonant and nonresonant photoionization. Nonresonant ionization produced an atomic distribution that peaked at the same velocity as the respective dimer distribution, indicating that virtually all the nonresonant atomic ion signal is from photofragmented dimers. Various mechanisms of dimer photofragmentation are discussed. 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 nonresonant ionization experiments at easily achievable laser power densities ({le} 10{sup 9} W/cm{sup 2}).