Switching from photochemical to photothermal mechanism in laser ablation of benzene solutions

Nanosecond KrF excimer laser ablation of benzyl chloride, benzyl alcohol, toluene, ethylbenzene, and $n$-propylbenzene diluted in $n$-hexane, $n$-heptane, dichloromethane, and 1,2-dichloroethane was investigated by time-resolved photographic, photoacoustic, and absorbance measurements. Ablation threshold values, $Fth,$ for high concentration solutions (⁠$α=250 cm−1,$ 0.6–1 M) were confirmed to be correlated to photochemical reactivity (β-bond cleavage) of the solute molecules, whereas no distinct relation between $Fth$ and boiling point of solvents was observed. Time-resolved absorbance at the laser wavelength was almost constant during the excitation pulse, which means that the main light-absorbing molecules were fixed to the ground-state solutes. It is considered that this type of ablation is initiated by the photochemical fragmentation. On the contrary, $Fth$ observed in relatively low concentration solutions (⁠$α=25 cm−1,$ 0.06–0.1 M) were about twice higher than those for the high concentration solutions, and had no direct correlation with the photochemical reactivity of the solute molecules. The time-resolved absorbance increased during the excitation pulse, and was ascribed to the fact that benzyl radicals produced by the photodissociation of solute molecules absorbed the excitation photons and converted them into heat through “a cyclic multiphotonic absorption process.” Furthermore, morphological aspects observed in nanosecond photography exhibited appreciable differences by varying the solute concentrations. These results clearly mean a concentration-dependent ablation mechanism; the ablation mechanism of the benzene derivative solutions switches from photochemical to photothermal as the solute concentration decreases.