This article reviews a variety of methods to obtain the electron temperature and density by the emission line ratios for low-temperature plasmas containing argon or nitrogen gas. Based on the collisional–radiative model of excited particles, the underlying principle of each of these methods is described, along with the criterion on how to select an appropriate line-ratio method according to the discharge conditions. Limitations on the application of each line-ratio technique are also discussed.

A simple collisional–radiative model for the Paschen 1s and 2p levels is proposed for low-temperature argon discharges. This model can predict the population distribution of 1s and 2p levels over a wide discharge pressure range 1–105 Pa and ionization ratio range 10−6–10−3. The modelling results are found to be in good agreement with observed optical emissions from several different types of argon discharges at 1, 100 and 105 Pa. By using the model, the dominant kinetic processes of 1s and 2p levels are investigated for an electron beam plasma, an inductively coupled plasma, a capacitively coupled plasma and a microwave microplasma. A kinetic diagram is given, which can be used to identify the kinetic state of 1s and 2p levels in many low-temperature argon discharges reported in the literature. This model is also useful for obtaining discharge parameters from optical emissions in low-temperature argon discharges.

Effects of gas flow rate on the length of atmospheric pressure plasma jets have been investigated using a capillary dielectric barrier discharge configuration. For the discharge in only downstream region, three distinguishable modes of plasma jet length versus argon gas flow rate, namely, laminar, transition, and turbulent jet mode, have been identified. For the case of discharge in both downstream and upstream regions, the curve of length versus flow rate has significant “dent” in the laminar jet mode for pure helium, neon, and argon flow gas spraying into air ambient.

A new collisional-radiative model for atmospheric-pressure low-temperature argon discharges is proposed, which illustrates the significant effect of electron density on the excited atom population distribution. This makes it possible to determine the electron density from the intensity ratio of emission lines of excited atoms. Results of this new method in several types of atmospheric-pressure discharges are found to be in agreement with those of the Stark broadening method and the electric model over a wide electron density range 10 11 -10 16 cm −3 .

Gas temperature, electron density and electron temperature of a microwave excited microplasma are measured by optical emission spectroscopy. This microplasma is generated in the small gap of a microstrip split-ring resonator in argon at near atmospheric pressure. When less than 100 ppm of water is present in the plasma, the gas temperature can be obtained from the rotational temperature of the hydroxyl molecule (A 2Σ+, v = 0) and the electron density can be measured by the Stark broadening of the hydrogen Balmer β line. According to a collisional–radiative model, the electron temperature can be estimated from the measured excitation temperature of argon 4p and 5p levels. It is found that the values of these parameters (gas temperature, electron density and temperature) increase when the gap width of the resonator is reduced. However, when the microwave power increases, these parameters, especially the electron density, do not vary significantly. Discussions on this phenomenon, being very different from that in the low-pressure bounded discharges, are provided.