Comparison of excitation mechanisms and the corresponding emission spectra in femto second and nano second laser-induced breakdown spectroscopy in reduced ambient air and their performances in surface analysis.

A study is conducted on the detailed static and dynamic characteristics of plasma and the resulting emission spectral qualities generated by a femtosecond (fs) laser irradiation compared with those produced by nanosecond (ns) laser irradiation at a reduced ambient air pressure of 0.65 kPa. It is shown that both plasmas feature a tiny primary and a much larger secondary plasmas, which share the same hemispherical shape of roughly the same size at the appropriately chosen pulse energies. The resulted emission spectra in both cases exhibit one of the shock wave characteristics marked by a much stronger Zn triplet than its singlet emission lines. Further measurement of log(r)–log(t) of Cu I 521.8 nm emission line yields a slope of around 0.4, which is in good agreement with Sedov's equation derived for shock wave plasma. While exhibiting similar pressure-dependent emission intensities of Cu and Zn emission lines, the fs induced emission intensities are consistently lower than those induced by ns laser plasma. The estimated average temperature of the ns laser induced plasma (10 200 K) is only slightly higher than that induced by the fs laser (9800 K). The lower integrated emission intensity of fs plasma is related to previously reported lower electron density in fs laser-induced breakdown spectroscopy (fs-LIBS) and the faster decay of the associated continuum background, implying rapid diminution of ionized atoms and hence lower integrated emission intensity. Therefore, apart from cases demanding minimal surface damages, the simpler and less expensive ns-LIBS should be considered as a more favorable alternative for spectrochemical analysis. However, the applications to surface analysis do show that the fs laser offers higher detection sensitivity. A judicial selection is, therefore, strongly recommended. [ABSTRACT FROM AUTHOR]