Measurement of electron-impact excitation cross sections out of metastable levels of argon and comparison with ground-state excitation

This paper reports the results of measurements of cross sections for electron excitation out of the ${1s}_3$ and ${1s}_5$ metastable levels of argon (the $J=0\mathrm{}$ and 2 levels, respectively, of the ${3p}^{5}4s$ configuration) into eight of the ten levels of the ${3p}^{5}4p$ manifold. The metastable atoms were generated by two methods: (a) an atomic beam emerging from a hollow-cathode discharge, and (b) charge-exchange collisions between a fast argon-ion beam and cesium atoms. The metastable argon atoms are excited by a crossed electron beam into the ${3p}^{5}4p$ levels and the emissions from these levels are utilized to determine the cross sections. Removal of the ${1s}_5$ atoms in the hollow-cathode discharge experiment by means of laser pumping allows us to determine the separate contributions from each metastable level to the observed fluorescence signal. The magnitudes of the cross sections for excitation out of the metastable levels into the different levels of the ${3p}^{5}4p$ manifold vary vastly. The patterns of the observed variations are interpreted by means of a multipole analysis. This multipole model is also used to discuss the comparison of excitation cross sections out of the metastable levels with those out of the ground level.