Abstract
In traditional laser-induced breakdown spectroscopy (LIBS) applications, the line intensity and analysis capability are susceptible to plasma shielding. To investigate the shielding effects on the characteristics of Cu plasma in air, a ~120-picosecond laser with a wavelength of 1064 nm was employed to produce plasma. The plasma temperature and electron density were calculated under the condition of local thermal equilibrium (LTE) and optically thin, while the relationships between the line intensity, plasma temperature and electron density were analyzed. Moreover, the LTE condition was validated by the McWhirter relation, plasma relaxation time and diffusion length, and the optically thin condition was observed through the variation in line intensity. The results indicated that when the focal point was below the target surface, the plasma shielding was the weakest, and the highest line intensity could be obtained. In addition, there was a positive correlation between the increased plasma temperature and the degree of shielding effect. When the focal point was above the target surface, the high-irradiance pulse directly broke down the free air and produced a shock wave. Under the high pressure of the over-heated shock wave, the line intensity, plasma temperature and electron density increased again. This study provides an important insight into the experiments and applications of picosecond LIBS.
Highlights
Laser-induced breakdown spectroscopy (LIBS) originated in the second half of the20th century [1], and became a representative laser ablation technique in the discipline of chemical analysis [2]
The results indicated that the analytical performance of laser-induced breakdown spectroscopy (LIBS) depends on the degree of plasma shielding
Due to the effects of plasma shielding, the intensity of the Cu spectrum at d = 0 mm was lower than that of d = −2 mm and d = 2 mm. This phenomenon is similar to that in ns LIBS reported by Aguilera et al [15], while the physical mechanism of produced plasma shielding is not completely identical
Summary
Laser-induced breakdown spectroscopy (LIBS) originated in the second half of the20th century [1], and became a representative laser ablation technique in the discipline of chemical analysis [2]. LIBS is widely used in the fields of public safety [3], metallurgical engineering [6], space exploration [7], geology and archaeology [8] and water quality detection [9]. It is well-known that the spectral quality is susceptible to the wavelength, irradiance, pulse duration and gas atmosphere during the process of capturing spectra [10]. The laser irradiance is on the order of 107 –1011 W/cm
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