Influence of beam intensity profile on the aerodynamic particle size distributions generated by femtosecond laser ablation

The dependence of nanoparticle size distributions on laser intensity profile was determined during infrared femtosecond laser ablation of silver targets in air. Laser parameters were adjusted to ablate at the same peak fluence with spatially homogeneous (Hat-top) and inhomogeneous (Gaussian) intensity distributions formed by diffractive optical elements. Aerodynamic particle size was measured online by an electric low-pressure cascade impactor. Narrower size distributions were detected for the flat-top intensity profile in the fluence range from 0.6 to 4.4 J/cm(2), while the Gaussian beam produced broad and bimodal distributions. The aerodynamic number frequency of the primary nanoparticulate fraction (40 nm) was equal to the number frequency of the submicron agglomerate fraction (200 urn) at laser fluence of 1 J/cm(2). The Feret diameter of primary particles was 80 nm. Geometrical interpretation of the irradiated spots at the corresponding laser fluence regimes explains the formation of lit nodal (submicron and nanoparticulate) size distribution M the case of Gaussian beams. The bimodality is attributed to different thermalization path ways during laser ablation.