The Displacement of Principal Series Lines of Rubidium by the Addition of Rare Gases

The positions of the principal series lines of Rb displaced by spectroscopically pure helium, neon and argon are measured. The constant shift of the high terms reaches 37.24 cm^-1 towards violet under 12.75 atmospheres (at 570^∘{}K) for helium; 2.08 cm^-1 towards violet under 13.59 atmospheres (at 590^∘{}K) for neon; and 33.4 cm^-1 towards red under 7.12 atmospheres (at 563^∘{}K) for argon. The effective cross sections for all the atoms in 1 cm³ at 1 mm pressure calculated by Fermi's equation is 15.47 cm²/cm³ for helium, 0.24 cm²/cm³ for neon, and 23.7 cm²/cm³ for argon in good agreement with the results of F\"uchtbauer from his experiment with Na and K, and of Wahlin by direct electrical method. In the range of rare gas pressures above relative density 5 the mean shifts of the high members of the Rb principal series increases more rapidly than linearly with density.