Abstract
To study the effects of focusing distance on the characteristics of copper plasma, a picosecond laser is utilized to ablate a pure copper plate to generate a plasma spectrum. Following numerous experiments on the subject, three significant factors are determined: lens focal length, pulse energy, and the lens-to-sample distance. These factors are employed to analyze the spectral intensity, plasma temperature, and electron density in the local thermodynamic equilibrium (LTE) and optically thin condition. Due to the shielding effects of mixed plasma, the strongest spectral intensity is achieved in the prefocused case, no matter how much beam irradiance is employed. The more intensive the beam irradiance is, the more the optimal position is distant from the focal point. The variation of plasma temperature and electron density showed a peak in the prefocused case, which is consistent with the trend of spectral intensity. For the case of extremely high irradiance (on the focus), the shielding effects become seriously, and the resultant above three factors decreased sharply. When a longer-focal-length lens is employed, the spectral intensity exhibited an obvious bimodal trend. In the prefocused case, a longer-focal-length lens is helpful to eliminate the effects of the roughness of the target surface compared with a shorter one. Finally, the assumed LTE is validated by McWhirter relation, plasma relaxation time, and diffusion length, and the optically thin condition was also validated by spectral intensity ratio. We hope that this work could be an important reference for the future design of highly optimized experiments for Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS).
Highlights
Alongside the advancements in high-performance lasers and novel chemometrics methods in the last 30 years, laserinduced breakdown spectroscopy (LIBS) has become an attractive technique, with much potential, for the analysis of elements of materials [1]
Under the local thermodynamic equilibrium (LTE) and optically thin condition, we have observed the variation of plasma temperature and electron density with increased focusing distance
The pulse energy is set to 20 mJ, and a lens with f = 150 mm is chosen to illuminate the surface of copper plate
Summary
Alongside the advancements in high-performance lasers and novel chemometrics methods in the last 30 years, laserinduced breakdown spectroscopy (LIBS) has become an attractive technique, with much potential, for the analysis of elements of materials [1]. The above work mainly uses a nanosecond or a femtosecond laser to generate a plasma spectrum Compared with these two lasers, picosecond laser has appropriate pulse width with higher pulse energy, which can increase the ablation volume and spectral intensity (for femtosecond pulse), and may alleviate the effect of plasma shielding as well (for nanosecond pulse). The pulse energy of a picosecond laser is up to a few hundred mJ, which can increase the spectral intensity caused by plasma reheating or ablation mass [23]. For femtosecond and nanosecond laser-induced plasma, the variation of line intensity and plasma temperature shows a similar trend with the ranged lens-to-sample distance, while the plasma formation and shielding effects are not completely identical. Under the local thermodynamic equilibrium (LTE) and optically thin condition, we have observed the variation of plasma temperature and electron density with increased focusing distance
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