SWITCHING FROM PHOTOCHEMICAL TO PHOTOTHERMAL MECHANISM IN LASER-ABLATION OF BENZENE SOLUTIONS

Nanosecond KrF excimer laser ablation of benzyl chloride, benzyl alcoh ol, toluene, ethylbenzene, and n-propylbenzene diluted in n-hexane, n- heptane, dichloromethane, and 1,2-dichloroethane was investigated by t ime-resolved photographic, photoacoustic, and absorbance measurements. Ablation threshold values, F-th, for high concentration solutions (al pha=250 cm(-1), 0.6-1 M) were confirmed to be correlated to photochemi cal reactivity (beta-bond cleavage) of the solute molecules, whereas n o distinct relation between F-th and boiling point of solvents was obs erved. Time-resolved absorbance at the laser wavelength was almost con stant during the excitation pulse, which means that the main light-abs orbing molecules were fixed to the ground-state solutes. It is conside red that this type of ablation is initiated by the photochemical fragm entation. On the contrary, F-th observed in relatively low concentrati on solutions (alpha = 25 cm(-1), 0.06-0.1 M) were about twice higher t han those for the high concentration solutions, and had no direct corr elation with the photochemical reactivity of the solute molecules. The time-resolved absorbance increased during the excitation pulse, and w as ascribed to the fact that benzyl radicals produced by the photodiss ociation of solute molecules absorbed the excitation photons and conve rted them into heat through ''a cyclic multiphotonic absorption proces s.'' Furthermore, morphological aspects observed in nanosecond photogr aphy exhibited appreciable differences by varying the solute concentra tions. These results clearly mean a concentration-dependent ablation m echanism; the ablation mechanism of the benzene derivative solutions s witches from photochemical to photothermal as the solute concentration decreases. (C) 1997 American Institute of Physics.