A time-resolved absorption spectroscopic measurement of liquid toluene under femtosecond UV (300-500 fs, 248 nm) laser ablation conditions was carried out, and its molecular mechanism was studied. The lowest excited singlet state of toluene monomer and toluene excimer were clearly observed through the delay time by 19 ns, while benzyl radical was not detected unexpectedly under any condition no matter how high the laser fluence was. This indicates that the femtosecond laser ablation is based on a photothermal mechanism. A femtosecond double-pulse excitation, on the other hand, induced benzyl radical formation, which is consistent with the photochemical mechanism in the case of nanosecond laser ablation. The result that molecular mechanism is dependent on the excitation laser pulse width suggests a possible mechanism control of laser ablation.
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