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.

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Cross sections for direct electron-impact excitation from the ground level into the 3p 5 5s and 3p 5 3d levels are measured for incident electron energies from threshold to 200 eV. The optical cross sections for the 3p 5 5s→3p 5 4p and 3p 5 3d→3p 5 4p emissions yield the apparent excitation cross sections. The cascade cross sections are obtained by measuring the emission intensities for transitions into 3p 5 5s and 3p 5 3d from the higher levels, and are subtracted from the apparent excitation cross sections to give the direct excitation cross sections. Most of the emission lines are in the infrared region, requiring the use of Fourier-transform spectroscopic techniques. The general trends of the cross-section results are discussed. Excitation cross sections for the 3p 5 4 f levels are also reported. Because our experiments do not extend to the vacuum ultraviolet, the direct excitation cross sections for the levels optically connected to the ground state were not obtained.

We report cross section results for electron excitation out of the 2 3 S metastable level of He into the 5 3 P, 6