The hyperfine structure of the lines of the arc spectrum of rubidium

The structure of the lines of the arc spectrum of rubidium was investigated with a reflexion echelon grating of high resolving power, the light source being a tube containing helium neon mixture at about ½ mm. pressure and a small quantity of rubidium; this tube was fitted with external electrodes and excited with an oscillator of very high frequency. The two resonance lines—7947 and 7800—were found to possess the same fine structure, and each possessed four components: at 0·00, —0·09, —0·19 and —0·23 cm.-1 ; the two outer components, 0·00 and —0·23 cm.-1 , are weak and the two inner components are strong. The lines 4215 and 4201 were found to possess three components; at 0·00, —0·09 and —0·20 cm.-1 . The line at the latter point was slightly broadened. This was presumed to correspond to the same structure as that observed in the resonance lines, the broadened line corresponding to —0·19 and —0·23 unresolved. This observed structure is explained as arising from the 5S1/2 level; the two outer, weak, components arising from the isotope 87 and corresponding to a separation into two levels, 0·23 cm.-1 apart, in the 5S1/2 level; while the two inner, stronger, components arise from the other isotope, 85, and correspond to a similar separation of 0·10 cm.-1 . The centre of gravity of the levels of Rb 85 is displaced about 0·02 cm.-1 from that of the levels of Rb 87. Measurements on the intensities of the components of 4201 show that the probable value for the nuclear moment of Rb 85 is 3/2, but the accuracy is not sufficiently high to enable the value for the scarcer isotope Rb 87 to be determined with certainty; the probable value is, however, 5/2. The magnetic moment of the nucleus of Rb 87 is shown to be approximately two and a half times as great as that of Rb 85.