Time resolved study of excimer laser ablation of thin organic films from a metal surface

Abstract

The dynamics of pulsed excimer laser ablation of thin films of organic material from a metal surface was investigated using optically excited surface plasmons. The metal substrate is formed by a 500 Ǻ thick silver film adjacent to the base of a glass prism, which serves for optical excitation and detection of the surface plasmons propagating at the silver-vacuum interface. The organic films were prepared by cooling the sample to 77 K and depositing the materials (isopropanol, tetrafluoromethane, acetone) from the vapor phase; ablation of the films was accomplished by KrF excimer laser (λ = 248 nm). Detection of the surface plasmon resonance allows to monitor the ablation process on a nanosecond time scale and with a resolution far better than a monolayer. At the same time, the surface plasmons provide an insitu probe for time-resolved measurements of the substrate temperature. This allows to determinate the temperature at which the ablation sets in. For tetrafluoromethane, the ablation temperature was found to be independent of the laser fluence, suggesting a thermal desorption process. On the other hand, for isopropanol and acetone, a strong dependence of the ablation temperature of the influence was observed. From the large delay between the leading edge of the laser pulse and the onset of ablation, we conclude that ablative photodecomposition is not present in our experiment. There is, however, evidence for laser-induced shemical transformations in the organic films. Solid films of transformed material, which were stable at room-temperature and under atmospheric conditions, were formed during the isopropanol experiments. We suggest these transformation process to be connected to the observed fluence dependence of the ablation temperature.