Measurements of cross sections for electron-impact excitation into the metastable levels of argon and number densities of metastable argon atoms

The technique of laser-induced fluorescence (LIF) has been applied to measure the cross sections for electron-impact excitation into the metastable levels of argon atoms as well as the number densities of the argon metastable atoms produced by electron excitation. A monoenergetic electron beam excites the ground-state atoms into the 3p⁵4s ³P₀,³P₂ metastable levels (1s₃ and 1s₅ in Paschen’s notation) and a pulsed laser pumps the atoms of a metastable level to a level in the 3p⁵4p configuration (2p in Paschen’s notation). The transient LIF from the 2p level is shown to be proportional to the apparent cross section of the metastable level and to the metastable number densities so that measurements of LIF enable us to determine both metastable cross sections and number densities. Methods for absolute calibration are described. We obtain both the apparent and direct excitation cross sections for each of the two metastable levels for electron energies from threshold to 200 eV and the results are compared with previous experimental and theoretical works. The pulsed LIF technique is also used to determine the disappearance rates of the metastable atoms after the electron beam is turned off. Two distinct decay modes are found and the observed behaviors are consistent with calculations based on diffusion theory.