Structure of the Resonance Line of the Arc Spectrum of Barium

The structure of the line 5535 A, $6s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-6s6p{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$, of Ba i has been investigated by means of a multiple atomic beam of high collimation (28:1) and a single Fabry-Perot etalon with a spectral range of 62.5 mK; the combined instrumental and Doppler width was approximately 2 mK. The line was found to have four well resolved components at 0.0, +3.8, +9.9, and +18.4 mK with apparent intensities of approximately 20:4.5:2.5:1. This result is consistent with that of Kopfermann and Wessel, who, using a single atomic beam of collimation 10:1, found a structure consisting of a strong main component with wing extending to about +25 mK, but it cannot be reconciled with the results of Arroe obtained by comparing wavelengths of the line emitted by enriched isotopes in a hollow cathode tube.