Study of the time-resolved emission spectra of the ejected plume generated by ultrashort laser ablation of graphite

Abstract

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 (4×10-4 Pa). 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 from 1Σu+ to X1Σg+ of C15 clusters, and the emission continuum ranging from 370-700 nm are observed. From the recorded time-resolved 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 laser pulses with different laser fluences. Finally, pulse width plays an important role in the time evolution manner of the emission continuum. As the laser pulse width increases, the two-stage decay process of the emission continuum gradually changes into one-stage process, indicating that the existence time intervals of carbon plasma and carbon clusters overlap each other for longer laser pulse width. And the whole evolution process of the emission continuum induced by 4 ps laser pulses is much slower than that induced by 50 fs laser pulses. Longer laser pulse width also causes the decrease of the spectral intensity of C2 radicals, and thus higher laser intensity favors the generation of excited C2 radicals.