Electron-impact excitation of the argon atom

Electron excitation functions of some thirty states of the $3{p}^{5}ns$, $3{p}^{5}np$,, and $3{p}^{5}nd$ configurations of the argon atom have been measured by the optical method. In order to evaluate the level cross sections from the optical excitation cross sections, we have determined the branching ratios experimentally by measuring in an argon discharge tube the relative intensities of the emission lines originating from the level of interest. This procedure gives more accurate level cross sections than does the conventional approach of using theoretical transition probabilities to form the branching ratios, since for an atom as complex as argon the wave functions of the excited states calculated by a semiempirical treatment of the fine structure are not always of sufficient accuracy to give reliable transition probabilites. Analysis of the shape of the excitation functions is facilitated by expressing the wave functions of the excited states of argon in terms of the $LS$ eigenfunctions, and the special features of the experimental data of excitation functions can be readily explained by generalizing the results of helium. Analogous to the case of neon, one can show theoretically that within a configuration $3{p}^{5}nl$,, the states with odd values of $J+l$ in general have larger excitation cross sections than the ones with even values at incident energies well above the threshold, and that in general $Q(J=0)>Q(J=2)$, $Q(J=1)>Q(J=3)$ for the $np$ states, and $Q(J=1)>Q(J=3)$, $Q(J=2)>Q(J=0,4)$ for $nd$. The experimental results of the excitation cross sections are found to be in good agreement with these rules.