Investigations of extinction coefficients during excimer laser ablation and their interpretation in terms of Rayleigh scattering

KrF excimer laser ablation in air at an ambient pressure of 1 bar leads to an intense evaporation of 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 compressed, ionized ambient gas behind:he shock front and the partially ionized material vapour. This interaction is responsible for extinction of the incident laser radiation and exerts effects on the processing result and the efficiency of the treatment. The transmission of the incident laser power through the laser-induced interaction zone was measured using a target foil prepared with a small aperture within the area of irradiation. Extinction coefficients in the range 350-9500 m(-1) were measured for PET, copper and aluminium. In order to explain the experimental results, a theoretical study of possible extinction mechanisms was performed on the basis of inverse Bremsstrahlung theory and scattering theories, The thermodynamic properties in the interaction zone were calculated by using a shock wave theory. Under the assumption that this theory describes the thermodynamic properties in a physically correct manner, it will be shown that inverse Bremsstrahlung cannot explain the measured small degrees of transmission. Interactions between the incident laser light and material clusters in the laser-induced material vapour, like Rayleigh scattering, however, can lead to values comparable to the experimental findings. In order to classify the scattered fraction from the incident laser power, information on the size of the scattering particles is necessary. Therefore, a simplified model of cluster condensation in the material vapour and the scattering of incident laser power by these clusters was developed. Theoretically obtained results on this basis will be compared with the experimental data.