Absorption and saturation mechanisms in aluminium laser ablated plasmas

We have measured time-of-flight (TOF) distributions of positive ions in plasmas produced by laser ablation of Al targets at the second (lambda = 532 nm) and third (lambda = 355 nm) harmonic of Nd:YAG laser radiation. The investigation has been carried out in a regime of relatively high laser fluences (1-60 Jcm(-2)). The interpretation of the ion TOF distributions in terms of theoretical shifted Maxwell-Boltzmann distributions produces a good agreement with the experimental data. This has allowed us to infer the ion flow velocity and temperature associated with the measured TOF distributions, as well as the ion kinetic energies as a function of the laser fluence. We have also studied the total ion yield at different laser fluences. Our results show that all the plume parameters investigated are increasing functions of the laser fluence until a saturation plateau is reached at high fluences (> 20 Jcm(-2)). We ascribe this saturation behaviour to strong absorption and partial, or total, reflection of the laser light by the hot plasma produced by the leading edge of the intense laser pulse. This interpretation is supported by a semi-quantitative analysis of the laser photon absorption and ionization mechanisms in Al plasma, at both laser wavelengths.