ARTICLES Femtosecond Laser Ablation of Liquid Toluene: Molecular Mechanism Studied by Time-Resolved Absorption Spectroscopy Koji Hatanaka, †,‡,§ Tamitake Itoh, § Tsuyoshi Asahi, ‡ Nobuyuki Ichinose, § Shunichi Kawanishi, § Tsuneo Sasuga, § Hiroshi Fukumura, †,‡ and Hiroshi Masuhara* ,‡ Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565-0871, Japan, and AdVanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute, Neyagawa, Osaka 572-0019, Japan ReceiVed: July 23, 1999; In Final Form: October 15, 1999 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. Introduction Studies on the interactions between intense laser pulses and condensed matters are becoming the main stream of laser science beyond the conventional photophysics and photochemistry at present. Laser ablation is one of the phenomena which can be induced commonly in solids and liquids as a result of such interactions. From this viewpoint, we have studied laser ablation dynamics and clarified its molecular mechanism as a representa- tive of unconventional phenomena. Since the first reports on nanosecond laser ablation on polymers by Namba et al. 1 and Srinivasan and Mayne-Banton, 2 there have been a lot of reports on nanosecond and picosecond laser ablation. As stable femtosecond laser systems prevail to scientific fields, the pulse width of excitation laser light is shifted to the shorter. A lot of papers on femtosecond laser ablation have been published since the first reports by Srinivasan et al. 3 and Ku ¨per and Stuke. 4 The number of papers on time-resolved measurements is, however, limited to several such as a reflec- tivity measurement by Shank et al. 5 Especially, no report on a time-resolved absorption measurement has been published so far except our report on liquid benzyl chloride, 6 even though such a measurement is indispensable to clarify the molecular mechanism directly. We have adopted benzene derivatives such as benzyl chloride and toluene as samples to explore the laser ablation molecular mechanism since the first report by Tsuboi et al., 7 because such liquids are so much more simply structured compared with spaghetti-like polymers that it is easier to elucidate their molecular mechanism. Further, those sample liquids are known as reactive species to UV light irradiation leading to benzyl radical formation as follows: In the case of nanosecond 248 nm laser ablation, the relation of ablation thresholds decided quantitatively by time-resolved shadowgraphy 7 and photoacoustic measurement 7 are correlated well to the relation of photochemical reactivity reported by Porter and Wright 8 but not to the relation of boiling points. 7 We have also detected the benzyl radical absorption band clearly and estimated quantitatively the concentration of benzyl radical to induce laser ablation by time-resolved absorption measure- ment. 9 It should be noticed that the benzyl radical formation results in the volume expansion when the radical concentration is extremely high under laser ablation condition, as the suc- ceeding reactions produce gaseous molecules such as X 2 , XH, and so on. On the basis of those results, we have concluded that nanosecond laser ablation is induced by instantaneous photochemical volume expansion. In the case of solutions of the benzene derivatives, it is demonstrated experimentally that the molecular mechanism changes from photochemical to photothermal as the solute concentration decreases. 10 This is one example of the mechanism control by changing the solute concentration of sample solutions. Thus, we have constructed a high-power femtosecond Ti 3+ : Al 2 O 3 /KrF laser system and applied it for the study of * To whom all correspondence should be addressed. E-mail: masuhara@ ap.eng.osaka-u.ac.jp. † Present address: Department of Chemistry, Graduate School of Science at Tohoku University, Sendai, 980-8578, Japan. ‡ Osaka University. § Japan Atomic Energy Research Institute. φ-CH 2 X + hν(UV) f φ-CH 2 • + X • 11257 J. Phys. Chem. A 1999, 103, 11257-11263 10.1021/jp992565r CCC: $18.00 © 1999 American Chemical Society Published on Web 12/03/1999