Many optical-based plasma diagnostic techniques require electron-impact excitation cross sections. In recent years, a considerable number of new results have become available for excitation of rare-gas atoms from both the ground state and metastable states. Using relatively simple techniques these cross sections can be combined with plasma emission measurements to extract many useful plasma parameters such as the electron temperature. Many of the limitations of simple plasma emission models such as the corona model can be overcome by using cross section measurements to select what particular emission lines to use in the analysis.

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To experimentally determine electron-impact excitation cross sections with the optical method, it is necessary to measure all transitions out of a level ͑the apparent cross sections͒, as well as the cascades into the level. In the case of the ten 3p 5 4p levels of argon, the emissions to lower levels lie in the visible and near infrared ͑660-1150 nm͒ and are hence observable with a monochromator-photomultiplier-tube ͑PMT͒ system. A Fourier-transform spectrometer ͑FTS͒ allows us to measure the previously uninvestigated cascades that lie in the infrared. For the incident electron energy range between onset and 300 eV, we have measured the apparent cross sections with a monochromator-PMT system, and the cascade cross sections with a weak emission FTS system. The magnitude of both the apparent and cascade cross sections increases with target gas pressure due to radiation trapping effects. By subtracting the cascade contributions from the apparent cross sections, we have determined the direct cross sections and verified that they do not vary with pressure in the 0.5-4-mTorr pressure range considered here.

Excitation and ionization of atoms out of the 4 energy levels of the excited np 5 (n + 1)s configuration of rare gases play an important role in many low temperature rare-gas plasmas. We compare two optical methods for measuring the number densities of atoms in these excited levels in an inductively coupled plasma under a variety of operating conditions (600 W, 1-25 mTorr). The first method is a standard white light absorption technique, whereas the second method exploits changes in the effective branching fractions of np 5 (n + 1)p → np 5 (n + 1)s emissions brought about by radiation trapping of atoms in np 5 (n + 1)s levels. The branching fraction method was found to produce results that agree well with the direct white light absorption method for both argon and neon plasmas using little more than a low-resolution spectrum of the plasma glow.

Optical modeling of emissions from low-temperature plasmas provides a non-invasive technique to measure the electron energy distribution function (EEDF) of the plasma. While many models assume the EEDF has a Maxwell-Boltzmann distribution, the EEDFs of numerous plasma systems deviate significantly from the Maxwellian form. In this paper, we present an optical emission model for the Ar(3p 5 4p → 3p 5 4s) emission array which is capable of capturing details of non-Maxwellian distributions. Our model combines previously measured electron-impact excitation cross sections with Ar(3p 5 4s) number density measurements and emission spectra. The model also includes corrections for radiation trapping of the Ar(3p 5 4p → 3p 5 4s) emission lines. Results obtained with this optical technique are compared with corresponding Langmuir probe measurements of the EEDF for Ar and Ar/N 2 inductively coupled plasma systems operating under a wide variety of source conditions (1-25 mTorr, 20-1000 W, %N 2 admixture). Both the optical emission method and probe measurements indicate the EEDF shapes are Maxwellian for low electron energies, but with depleted high energy tails.