Elemental fractionation of dielectric aerosols produced by near-infrared femtosecond laser ablation of silicate glasses

The composition and fractionation properties of dielectric aerosols generated by near-infrared femtosecond laser ablation of silicate glass (carrier gas: helium, 1 atm) have been examined. Aerosols were classified using low-pressure impaction of particles with diameters from 7 nm up to 7 mu m. The element-selective analyses of impacted material has been restricted to minor matrix constituents (nominal concentration similar to 4.5%) applying total reflection X-ray fluorescence. It has been found that for fluences larger than 5 J cm(-2) the total Zn-, Ca-, Sr-, Ba-, and Pb-specific composition of these aerosols corresponds to that of the bulk material even though the size-dependent particle composition strongly altered. Typical deviations were of the order of 5-10%. In contrast, fluences below 5 J cm(-2) usually resulted in stronger differences from the bulk composition indicating intensified fractionation during the ablation process. Our results furthermore demonstrate, that the major fraction of the aerosol mass is located within the mesoscopic size range, i.e. from 10 up to 100 nm, fairly independent on the fluence applied. However, the relative percentage of micrometer particles has been found to significantly decrease for higher fluences. Scanning electron microscopy of impacted brass particles moreover revealed a fractal-like structure of deposits. Implications for the classification of such structures using low-pressure impaction are discussed.