Οπτική εξέταση μορφολογικών αλλαγών κατά την UV ακτινοβόληση van der waals υμενίων
Explosive boiling has been proposed, by extensive theoretical and experimental work, to be one of the dominant mechanisms in the UV nanosecond laser ablation. For a better understanding of the processes involved in material ejection, simple systems have to be examined. Van der Waals molecular solids provide a unique medium for studying these issues. This thesis concerns optical examination of the temporal evolution of morphological changes induced to van der Waals films, upon irradiation below and above the ablation threshold, in order to quantitatively describe the explosive boiling process. Neat toluene film is studied extensively, because it is highly transparent and, thus, optical examination is possible, via optical probing techniques. This thesis continues the previous work to examine in quantitative way the explosive boiling. Parameters examined: a. the dependence on laser pulses. It is shown that this is a critical parameter for the reproducibility of the results and that there is a high sensitivity of bubble formation on the structure/morphology of the film. b. neat toluene is compared with mixtures composed of toluene with a dopant ((CH3)2O or c- C6H12) to show how the phenomenon depends on chemical composition. The most important result is that bubble formation is much lower for the more volatile dopant, which is opposite to what a simple theory of explosive boiling predicts. This directly demonstrates the competition between explosive boiling and surface desorption. c. a preliminary examination of solid vs. liquid toluene. Bubble formation and growth is indicated to be consistent with material ejection. However, several discrepancies exist between theoretical estimations and experiment. The most important one is that ablation is predicted to occur at temperatures close to the spinodal limit, whereas, experimentally, ablation is observed at temperatures well below that limit. Overall, we claim that the quantitative description of explosive boiling is not so well described by the theoretical models. Therefore, theoretical estimations will have to be improved, by taking into account all these factors that affect bubble formation and growth.