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

a r t i c l e i n f o Keywords: Optical emission spectroscopy Laser-induced breakdown spectroscopy Laser-produced plasma Plasma dynamics Femtosecond laser ablation 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. The field of laser ablation and analysis of subsequent light emission has been growing since the development of the laser in the 1960s. One of the well-known applications of laser ablation is laser-induced breakdown spectroscopy (LIBS). LIBS is a technique commonly used to identify constituents of an unknown sample by ablating a small amount of the sample and studying the resultant optical emission [1–3]. LIBS has been applied to many fields as an analytical instrument, such as deep sea [4], extraterrestrial [5], artwork [6], explosive residue [7], and thin film [8] analysis. While LIBS experiments are primarily performed in an ambient atmosphere environment, the impact of low-pressure environments is also important for certain fields such as nuclear fusion reactor analysis [9] and space exploration [5,10]. As with all laser ablation, the properties of the LIBS plasma and its emission are strongly dependent on laser parameters such as energy , pulse length, and wavelength. Traditionally, LIBS systems have employed nanosecond (ns) laser systems, but with the recent developments in femtosecond (fs) laser systems, much attention has been turned to development of fs LIBS systems, as they offer greatly reduced thermal damage and heat affected zone (HAZ) due to negligible heat conduction and hydrodynamic motion during the laser pulse duration [11]. The differences …