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 ${3p}^{5}5\overrightarrow{s}{3p}^{5}4p$ and ${3p}^{5}3\overrightarrow{d}{3p}^{5}4p$ 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 ${3p}^{5}4f$ 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.

• Received 13 May 1999

DOI:https://doi.org/10.1103/PhysRevA.60.3712