Measurements of wavelength-dependent transmission in excimer laser-induced plasma plumes and their interpretation

Short-pulse laser ablation in air at 0.1 MPa leads to intense evaporation of the target material. The ablated material compresses the surrounding gas and leads to the formation of a shock wave. The incident laser radiation interacts with the partially ionized material vapour and the condensed material clusters embedded therein and affects the efficiency and quality of the ablation. Methods to increase the efficiency and quality of the ablation process require knowledge of these mechanisms. Therefore, the transmissivity of a laser-induced plasma plume was investigated in the wavelength range between 440 nm and 690 nm with a spatial resolution of about . The results show a weak dependence of the extinction coefficients over the investigated wavelength range. The spatial resolution allowed us to identify the regions behind the shock wave with the highest extinction for the visible wavelength range probed. These regions correspond to areas with high free-electron densities. To understand the mechanisms that are responsible for the heating and ionization of the vapour at the start of the excimer laser pulse, a simplified stationary model was applied. The experimental results were interpreted using Mie scattering theory on condensed material clusters, inverse bremsstrahlung absorption and absorption due to photoionization of excited material vapour atoms. The modelling shows that extinction of the laser light in a plasma with the assumed thermodynamic parameters is dominated by Mie absorption on condensed material clusters for wavelengths less than about 430 nm and is dominated by photoionization absorption and inverse bremsstrahlung above 430 nm.