Laser-induced plasma electron number density: Stark broadening method versus the Saha–Boltzmann equation

Abstract

We report spectroscopic studies on plasma electron number density of laser-induced plasma produced by ns-Nd:YAG laser light pulses on an aluminum sample in air at atmospheric pressure. The effect of different laser energy and the effect of different laser wavelengths were compared. The experimentally observed line profiles of neutral aluminum have been used to extract the excitation temperature using the Boltzmann plot method, whereas the electron number density has been determined from the Stark broadened as well as using the Saha-Boltzmann equation (SBE). Each approach was also carried out by using the Al emission line and Mg emission lines. It was observed that the SBE method generated a little higher electron number density value than the Stark broadening method, but within the experimental uncertainty range. Comparisons of N e determined by the two methods show the presence of a linear relation which is independent of laser energy or laser wavelength. These results show the applicability of the SBE method for N e determination, especially when the system does not have any pure emission lines whose electron impact factor is known. Also use of Mg lines gives superior results than Al lines.

Measurement of Dysprosium Stark Width and the Electron Impact Width Parameter

“…The electric field generated by the movement of charge carriers (electrons/ions) in the plasma disturbs the energy levels of the atoms, causing a broadening of the emission lines. 15 In a simplified form, 45 the line FWHM caused by the Stark effect ΔλStark is related to the electron density by the following equation. …”

“…The former provides the temperature and electron number density that, when substituted into the Stark linewidth equation, provide w for the studied target, while the latter can then be applied to measure electron number density in laser ablation experiments utilizing the same target. Sarkar and Singh, 15 using aluminum (which has well-known w values), showed that the results of both SBA and StB are compatible, to within an order of magnitude, for electron number density in laser ablation experiments employing nanosecond lasers. This result enables, in our understanding, the necessary confidence in Sternberg et al.’s method mentioned above.…”

“…Plasma temperature is usually measured using Boltzmann or the Saha–Boltzmann plot variant method, 13,14 while the determination of electron density can also be measured using Saha–Boltzmann (SBA) or, alternatively, by means of Stark broadening (StB). 15 These two last methods are known for intrinsic errors. 13…”

Continuum removal and self-absorption correction incalibration-free laser-induced breakdownspectroscopy for efficient elemental analysis

“…The emission line profile depends on the dominant broadening mechanism. In LIBS, Stark broadening is the major broadening mechanism compared to Doppler broadening and natural broadening, leading to the symmetric Lorenz profile to the emission lines [7]. While using a low-resolution spectrometer, the spectral profile is distorted and shows spectral interference between emission lines, which needs to be considered.…”

“…The Lorentzian fitted emission lines are used for finding plasma parameters and the elemental composition of bronze alloy. The electron number density n e is determined from the Stark broadening of the emission line at 521.820 nm (Cu I), and full width at half maximum (FWHM) is taken from the Lorentzian fitted emission line shown in figure 3b [7]. The value of n e in the bronze plasma is obtained to be 2.4 x 10 16 cm -3 .…”