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

Abstract

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 the 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 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.

Optical breakdown in aluminum vapor induced by ultraviolet laser radiation

“…The models ͑1͒-͑6͒ were used to study several action regimes close to certain experimental situations, where optical breakdown in aluminum vapor by laser radiation pulse was studied. [9][10][11] The pulses had different duration, temporal shapes and wavelengths. The effects associated with a finite duration of the action and the temporal shape of the laser pulse play a significant role and are to be taken into account.…”

“…In the experiments under consideration, [9][10][11] we note that the plasma formation near the target is always preceded by an intensive evaporation, and the optical breakdown is initiated just in the evaporated matter, but not in the environment gas. In the calculations, the stage of the target heating and evaporation is not considered explicitly but is modeled by giving the initial data corresponding to the parameters of intense evaporation of aluminum FIG.…”

“…According to the classical theory, the frequency dependence of the threshold intensity has the form G*ϳ l 2 . 7 At the same time, numerous experiments [8][9][10][11][12][13][14][15][16][17][18][19][20] point to the opposite. In the UV range, the threshold intensity has been found not to increase but to decrease.…”

“…Simultaneous consideration of the energy balance of two subsystems-the electronic and atomic-ionic onesmakes it possible to apply it for the description of optical breakdown in a wide frequency range ប l ͓0.1-10͔ eV that approximately corresponds to the radiation range of modern lasers ranging from the CO 2 laser to the ArF excimer laser. The developed kinetic model was used to model several interaction regimes close to experimental situations [9][10][11] in which optical breakdown in aluminum vapor by the excimer laser radiation was investigated.…”

Existence of Phase Explosion during Laser Ablation and Its Effects on Inductively Coupled Plasma-Mass Spectroscopy

“…Single-pulse ablation of the single crystal . Therefore, at longer wavelengths, substantially higher absorption of the incident radiation can occur by the plasma and less radiation energy will be transmitted to the sample surface [24][25][26] .…”