Abstract
Absolute normalized differential electron-impact excitation cross sections (DCSs) are presented for the lowest three electronic states of argon: the lowest resonance 4s[3/2]1 (1s4 in Paschen's notation) state and two neighbouring metastable 4s[3/2]2 and 4s'[½]0 (1s5 and 1s3 in Paschen's notation) states. Direct, i.e. free of a cascade contribution, excitation cross sections were obtained experimentally for the 4s[3/2]1 state at 20, 40, 50 and 80 eV and for the 4s[3/2]2 and 4s'[½]0 states at 20 and 40 eV impact energies. The measurements were performed at scattering angles between 5° and 150° with an angular resolution of better than 2°. The ratios: r = DCS(4s[3/2]2)/DCS(4s'[½]0) and r' = DCS(4s[3/2]1)/DCS(4s'[½]1) were determined and compared with other available experimental and theoretical results. The absolute DCS scale was established with respect to DCSs of the 4s'[½]1 state (Filipovic et al 2000 J. Phys. B: At. Mol. Opt. Phys. 33 677) using the peak intensity ratios in the energy-loss spectra. The DCSs were extrapolated to 0° and 180° and numerically integrated to yield integral, momentum transfer and viscosity cross sections.