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 sections), 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 nm) and are hence observable with a monochromator–photomultiplier-tube (PMT) system. A Fourier-transform spectrometer (FTS) 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.

• Received 4 August 1997

DOI:https://doi.org/10.1103/PhysRevA.57.267