中文摘要：

本文使用不同激光能流（18 J/cm2—115 J/cm2）和脉冲宽度（50 fs—4 ps）的超短脉冲激光在真空中（4×10-4Pa）烧蚀高定向热解石墨.通过测量烧蚀喷射物的时间分辨发射光谱研究喷射物的超快时间演化.在喷射物发射光谱中,观察到了C2基团的天鹅带光谱系统,416 nm附近C15基团的由电子能级1Σ＋u和X1Σ＋g之间的振动跃迁产生的光谱峰以及连续谱.50 fs,115 J/cm2的脉冲激光烧蚀产生的喷射物的连续谱的强度衰减分为快速下降和慢速下降两个阶段（以20 ns时间延迟为分界）.这表明连续谱是由两种不同的组分贡献的.快速下降阶段,连续谱主要由碳等离子体通过韧致辐射产生;慢速下降阶段,连续谱主要由烧蚀后期产生的大颗粒碳簇的热辐射贡献.实验结果还揭示了激光能流的提高,会明显增加喷射物中碳等离子体和激发态C2的含量,但对质量稍大的C15的影响较小;此外,50 fs脉冲激光烧蚀产生的连续谱的存在时间会随着激光能流的减小而增大,这说明低能流更有利于在烧蚀后期产生碳簇.脉宽主要影响喷射物连续谱的时间演化.4 ps脉冲激光烧蚀产生的连续谱的整个时间演化过程明显慢于50 fs脉冲产生的连续谱.

英文摘要：

In this paper ultrashort laser pulses with different fluences（18 J/cm2–115 J/cm2） and pulse widths（50 fs–4 ps）are employed to ablate highly oriented pyrolytic graphite in vacuum（4 × 10-4Pa）. By recording the time-resolved emission spectra of the ablated plume, the ultrafast time evolution of the ablation process is investigated. The Swan bands of C2 radicals, the spectral band near 416 nm which may be assigned to the electronic transition from1Σ＋uto X1Σ＋gof C15 clusters, and the emission continuum ranging from 370–700 nm are observed. From the recorded timeresolved emission spectra of the ablated plume, it is seen that at larger time delays only the emission continuum is observed. The decay process of the emission continuum of the plume generated by 50 fs, 115 J/cm2 laser pulses can be divided into a fast decreasing stage（before 20 ns time delay） and a slow decreasing stage（after 20 ns time delay）,indicating that the emission continuum may come from two different compositions. During the fast decreasing process,the bremsstrahlung of the ablation-generated carbon plasma contributes to the major part of the continuum; while during the slow decreasing process, the thermal radiation of carbon clusters generated at a later stage of ablation mainly contributes to the continuum. In addition, the existence time of the continuum generated by 50 fs laser pulses increases with the decrease of laser fluence, indicating that laser pulses with lower fluences can generate more carbon clusters at later stages of ablation. It is also found that for the 50 fs pulses, when the laser fluence increases at the early stage of ablation, the quantities of carbon plasma and excited C2 radicals in the plume increase significantly, but the quantity of excited C15 radicals with larger mass only increases slightly. Therefore the laser fluence has a great impact on the concentrations of different compositions in the ejected plume, implying that different material removal mechanisms exist for ablation induced by