Metastable atom and electron density diagnostic in the initial stage of a pulsed discharge in Ar and other rare gases by emission spectroscopy

Abstract

Temporal measurements of the emission intensities of the Ar 419.8 and 420.1 nm spectral lines combined with Ar plasma modeling were used to examine the metastable atom and electron density behavior in the initial stage of a pulsed dc discharge. The emission intensity measurements of these spectral lines near the start of a pulsed dc discharge in Ar demonstrated a sharp growth of metastable atom and electron densities which was dependent on the applied reduced electric fields. For lower electric fields, the sharp growth of metastable atom density started earlier than the sharp electron density growth. The reverse situation was observed for larger electric fields. This presents the possibility for controlling plasma properties which may be useful for technological applications. Similar measurements with spectral lines of corresponding transitions in other rare gases are examined.

A Computationally Assisted Ar I Emission Line Ratio Technique to Infer Electron Energy Distribution and Determine Other Plasma Parameters in Pulsed Low-Temperature Plasma

“…The 420.1/419.8 nm emission line ratio is sensitive to stepwise excitation via the 420.1 nm emission and the presence of metastable atoms: initially R = 1, and the ratio will increase when a sufficient number of metastable atoms are present such that the 3p 9 state is populated by both direct and stepwise excitation. The ratio should be less than unity only in the presence of a significant population of high-energy (>30 eV) electrons, as argued in [2,3,6] and shown in [7]. Alternatively, the emission line ratio can be used in combination with the measured metastable atom density (e.g., by tunable diode laser absorption) to determine the EEDF by comparing the measured emission with the predicted emission for different EEDFs.…”

“…Another option is to use optical emission cross sections that include the branching ratio from selected higher-energy states down into the excited state of interest for the emission line ratio model. The excitation rates used in previous experiments [2,6] were calculated [3] using optical emission cross sections that accounted for the branching ratio as well as cascades into the excited state. The cascade contribution may be accounted for in the high-energy tail of the optical emission cross sections, as a function of pressure, in a procedure outlined in [7].…”

“…Passive optical diagnostics that are non-invasive and non-perturbative, like the 420.1/419.8 nm argon-emission line-height-ratio technique [2][3][4], are desirable because they can be implemented in real time and can be used to determine metastable-atom density [2,5] and effective electron temperature [4]. Emission spectra have been shown to correlate with high energy electrons [2,3,6,7]. The technique described here represents a computationally assisted argon emission line ratio technique to infer electron energy distribution and determine other plasma parameters in pulsed low-temperature plasma.…”

“…The 420.1/419.8 nm emission line ratio is suitably sensitive to (neutral) metastable-atom density because the upper state associated with 419.8 nm emission (3p 5 in Paschen's notation or 5p [1/2] 0 in Racah's notation) is populated almost entirely by direct excitation via electron-impact from the ground state while the upper state associated with 420.1 nm emission (3p 9 in Paschen's notation, 5p [5/2] 3 in Racah's notation) is populated both by direct excitation and stepwise excitation via electron-impact from the 1s 5 metastable state (4s[3/2] 2 in Racah's notation). The relative efficiency of direct, compared to stepwise, excitation is strongly influenced by the electron energy distribution f (E) because the cross section for direct excitation increases significantly as the effective electron temperature increases, while low energy electrons are mainly responsible for stepwise excitation.…”

“…where R is the observed 420.1/419.8 nm emission line intensity ratio, I 420.1 and I 419.8 are the observed intensity (W/m 2 ) of 420.1 nm and 419.8 nm emission, n M is the metastable atom density, n 0 is the neutral argon density, k 419.8 is the direct excitation rate from the ground state into the 3p 5 state (the upper state for 419.8 nm emission), which is equal in magnitude to the direct excitation rate from the ground state into the 3p 9 state (the upper state for 420.1 nm emission) [2,3], and k 420.1 is the stepwise excitation rate from the 1s 5 metastable state into the 3p 9 state. While the approximations made to obtain the final form expressed by Equation (1) are particular to the neutral argon transitions of interest, emission line pairs for neon, krypton, and xenon with similar sensitivity to the presence of metastable atoms are presented elsewhere [3] and may be used to expand the emission line ratio technique to other noble gases [8]. The utility of the 420.1/419.8 nm emission line ratio has been demonstrated in previous work [2,6].…”