Excitation of the 1$s_5$,1$s_4$, 1$s_3$, and 1$s_2$ levels of argon by low-energy electrons

Excitation rate coefficients for the 1$s_5$, 1$s_4$, 1$s_3$, and 1$s_2$ levels of argon by collisions with low-energy electrons have been measured using a drift-tube technique. Time dependences of the absolute population densities of the excited levels were measured by an absorption method with a tunable diode laser as a light source. The absorption data were analyzed according to the rate equations for these levels and the excitation rate coefficient per unit length of electron drift and per argon-atom density was obtained for each level as a function of the electric field to gas density ratio E/N. The values for the 1$s_5$ level vary from 2.0×${10}^{\mathrm{-}19}$ to 2.5×${10}^{\mathrm{-}17}$ ${\mathrm{cm}}^2$ as E/N increases from 5×${10}^{\mathrm{-}17}$ to 5×${10}^{\mathrm{-}15}$ V ${\mathrm{cm}}^2$. In comparison with these values, those for the 1$s_3$ level are about one-fifth, those for the 1$s_4$ level are about the same, and those for the 1$s_2$ level are slightly larger in respective measured E/N ranges. In order to estimate the cascading effects from the higher-lying levels, excitation rate coefficients for the 2p and 3p levels have also been measured from absolute intensities of the line emissions. From a comparison of all the measured values of the excitation rate coefficients with those calculated from the Boltzmann analysis, a recommended set of cross sections for these levels has been deduced.