Excitation of the 1s5,1s4, 1s3, and 1s2levels of argon by low-energy electrons

Abstract

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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ to 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$ as E/N increases from 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ 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.