Molecular mechanisms of nanosecond and femtosecond laser ablation and morphology-changing dynamics of neat liquid benzene derivatives, such as benzyl chloride and toluene, were investigated by photoacoustic measurement, nanosecond shadowgraphy, femtosecond surface light scattering imaging, and time-resolved ultraviolet-visible absorption spectroscopy. Ablation thresholds of the liquids were determined by photoacoustic measurement and shadowgraphy, whereas primary processes in ablation were elucidated by time-resolved absorption spectroscopy. Femtosecond surface light scattering imaging revealed how electronic excited/radical states evolved to nanometer morphological changes. In nanosecond laser ablation, ablation threshold value was related to photochemical reactivity producing benzyl radical; however, no correlation between the threshold and boiling point was confirmed. Indeed, a benzyl radical absorption band was clearly observed. Moreover, benzyl radical concentration at the threshold was estimated quantitatively as approximately 0.05 M in accordance with all sample liquids. Consequently, we concluded that nanosecond laser ablation of the liquids is induced photochemically by benzyl radical formation, and not by photothermal temperature elevation. In the case of femtosecond laser ablation, the relation between ablation threshold value and photochemical reactivity did not hold. Time-resolved absorption spectroscopy of liquid benzyl chloride clearly afforded benzyl radical absorption; in contrast, no radical absorption band was observed for toluene. The molecular ablation mechanism of toluene was thought to change into a photothermal mechanism upon application of femtosecond excitation. A double-pulse excitation experiment employing toluene demonstrated how the photothermal mechanism in femtosecond laser ablation was changed to the photochemical mechanism in nanosecond ablation.
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