In the presence of heavy foreign gases, such as Kr and Xe, diffused absorption bands were observed at both the short wavelength and the long wavelength side of the first four absorption doublets of the Rb principal series. The red bands were observable when the Rb vapor pressure in the absorption tube was relatively low (10-3 to 10-2. mm Hg). The violet bands were not intense enough until the Rb vapor pressure was increased to 0.1–1 mm Hg. With other lighter gases (H2, He, Ne, A, N2) only violet bands were observed. The positions and the absorption contours of the bands in relation to the member of the absorption series and to the nature of the foreign gases are discussed.
SHANG-YI CH’EN, ROBERT B. BENNETT, and OLEG JEFIMENKO, "Absorption Bands of Rubidium due to the Presence of Foreign Gases," J. Opt. Soc. Am. 46, 182-185 (1956)