Ba+Resonance-Line Excitation in Plasmas

Abstract

The rate of excitation by electron collisions of the $6{}_{}{}^2{}_{}{}^{}P$ levels of the ${\mathrm{Ba}}^{+}$ ion is experimentally determined as a function of electron temperature in the range 2500-6000° K. The measured rate is the sum of the excitation rates from the $6{}_{}{}^2{}_{}{}^{}S$ ground state and from the metastable $5{}_{}{}^2{}_{}{}^{}D$ states. The rate coefficients, as well as the corresponding electron temperatures, are obtained from population densities of the $6{}_{}{}^2{}_{}{}^{}P$, $6{}_{}{}^2{}_{}{}^{}S$, and $5{}_{}{}^2{}_{}{}^{}D$ levels. The $6{}_{}{}^2{}_{}{}^{}S$ and $5{}_{}{}^2{}_{}{}^{}D$ population densities are determined from resonance fluorescence measurements and the $6{}_{}{}^2{}_{}{}^{}P$ densities from the emitted light intensities. Approximate near-threshold excitation cross sections are deduced from the rate coefficients. Either the absolute rate coefficients or the ratio of excitation rates to the two $6{}_{}{}^2{}_{}{}^{}P$ levels is suitable for electron thermometry in low-density plasmas where photoexcitation and radiation trapping are negligible.