Comparison of optical emission from nanosecond and femtosecond laser produced plasma in atmosphere and vacuum conditions

• Emission from ns and fs LIBS plumes are compared under different pressure environments.
• Ablation mechanisms for each laser are used to explain different emission features.
• Ambient pressure plays a critical role in plume temperature & density evolution.
• Visible emission from fs LIBS plume is almost entirely from neutral species.
• Spectra collection time delay is shown to be very important in improving S/N & S/B.

In this study we examine the emission from brass plasma produced by ns and fs laser ablation under both vacuum and atmosphere environments using identical laser fluences in order to better understand the differences in emission features and plasma dynamics. Optical emission spectra show increased continuum and emission from lower-charged ions for ns laser-produced plasma (LPP), while fs plasma emission spectra show emission primarily from excited neutral species with negligible continuum. Plasma excitation temperature and electron density as a function of time show similar trends for both lasers, though fs LPP expansion appears to be approximately two times faster than ns LPP expansion for the conditions studied. Confinement by the ambient gas is shown to significantly enhance and maintain plasma temperature and density and hence, emission, at later times. ICCD images of plasma expansion showed a broader angular distribution for ns LPP, but narrower angular distribution for fs LPP. Images also confirm the significant effect that the ambient environment has in confining plume expansion.