a1 Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany
a2 Excellence Cluster REBIRTH, Hollerithallee 8, D-30419 Hannover, Germany
a3 General Physics Institute, 38 Vavilov street, 119991, Moscow, Russia
a4 Institute for Mathematical Modelling 4A Miusskaya Pl., 125047, Moscow, Russia
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 (flat-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/cm2, 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 nm) at laser fluence of 1 J/cm2. 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 bimodal (submicron and nanoparticulate) size distribution in the case of Gaussian beams. The bimodality is attributed to different thermalization pathways during laser ablation.
(Received August 03 2009)
(Accepted October 29 2009)