Influence of laser pulse width on the ablation of Zinc in nitrogen ambient

Full text: We report time of flight (TOF) spectral measurements of laser produced plasmas(LPP) generated by irradiating a solid Zinc target with lasers of nanosecond (7 ns, 1064 nm) and femtosecond (100 fs, 796 nm) pulse durations respectively, carried out in a broad ambient pressure range of 0.05 to 100 Torr of Nitrogen. At the relatively high input fluence of ~ 16 J/cm² used, fast and slow atomic species are found to appear at different times in the TOF spectra, the dynamics of which are primarily determined by the pulse durations of the irradiating laser. In fs LPP, the average speed of fast species is unaffected by an increase in ambient pressure, while in ns LPP average speed is found to reduce with pressure. Slow species shows a sharp peak in TOF spectra with a narrow velocity distribution for fs LPP, indicating a large number density (𝘕_𝑒) and low electron temperature (𝘛_𝑒), which is confirmed through optical emission spectroscopic studies. Whereas for ns LPP, the TOF of slow species is found to have more broadened profile due to strong plume-laser interaction. The dynamics of slow species is heavily influenced by the presence of shock waves, which leads to the occurrence of a much slower species at larger pressures. The optimum ambient pressure for maximum emission intensity and emission life time is determined to be 10 Torr. (author)