Nanosecond and femtosecond laser photochemistry and ablation dynamics of neat liquid benzenes
Nanosecond and femtosecond laser photochemistry and ablation dynamics of neat liquid benzenes
复制标题
DOI：
10.1021/jp014332h
复制
发表时间：
2002-03-28
影响因子：
3.3
通讯作者：
Masuhara, H
中科院分区：
文献类型：
--
作者：
Hatanaka, K;Tsuboi, Y;Masuhara, H
采用光声测量、纳秒阴影照相、飞秒表面光散射成像和时间分辨紫外-可见吸收光谱等技术，研究了苯类衍生物（如氯化苄和甲苯）在纳秒和飞秒激光作用下的分子烧蚀机理和形貌变化动力学。烧蚀阈值的液体确定的光声测量和阴影，而在烧蚀的主要过程中阐明的时间分辨吸收光谱。飞秒表面光散射成像揭示了电子激发/自由基状态如何演变为纳米形态变化。在纳秒激光烧蚀中，烧蚀阈值与产生苄基自由基的光化学反应性有关，但与沸点无关。实际上，清楚地观察到苄基自由基吸收带。此外，根据所有样品液体，定量地估计阈值处的苄基自由基浓度为约0.05M。因此，我们得出结论，纳秒激光烧蚀的液体诱导光化学的苄基自由基的形成，而不是光热温度升高。在飞秒激光烧蚀的情况下，烧蚀阈值和光化学反应性之间的关系并不成立。液体氯化苄的时间分辨吸收光谱清楚地提供了苄基自由基吸收，相比之下，没有观察到甲苯的自由基吸收带。在飞秒激光的作用下，甲苯的分子烧蚀机制转变为光热烧蚀机制。采用甲苯的双脉冲激发实验演示了飞秒激光烧蚀中的光热机制如何转变为纳秒烧蚀中的光化学机制。
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.