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 lead s to an intense evaporation of target material. The ablated material c ompresses the surrounding gas and leads to the formation of a shock wa ve. The incident laser radiation interacts with the compressed, ionize d ambient gas behind:he shock front and the partially ionized material vapour. This interaction is responsible for extinction of the inciden t laser radiation and exerts effects on the processing result and the efficiency of the treatment. The transmission of the incident laser po wer through the laser-induced interaction zone was measured using a ta rget foil prepared with a small aperture within the area of irradiatio n. Extinction coefficients in the range 350-9500 m(-1) were measured f or PET, copper and aluminium. In order to explain the experimental res ults, a theoretical study of possible extinction mechanisms was perfor med on the basis of inverse Bremsstrahlung theory and scattering theor ies, The thermodynamic properties in the interaction zone were calcula ted by using a shock wave theory. Under the assumption that this theor y describes the thermodynamic properties in a physically correct manne r, it will be shown that inverse Bremsstrahlung cannot explain the mea sured small degrees of transmission. Interactions between the incident laser light and material clusters in the laser-induced material vapou r, like Rayleigh scattering, however, can lead to values comparable to the experimental findings. In order to classify the scattered fractio n from the incident laser power, information on the size of the scatte ring particles is necessary. Therefore, a simplified model of cluster condensation in the material vapour and the scattering of incident las er power by these clusters was developed. Theoretically obtained resul ts on this basis will be compared with the experimental data.