Isotope shifts in the arc spectrum of xenon

Isotope shifts have been measured in the lines of the spectrum xenon I for all of the nine stable isotopes. For the five more abundant (heavier) isotopes of even mass number the measurements include 37 lines in the visible and the photographic infrared; but for the lightest isotopes $^{126}$Xe and $^{124}$Xe measurements could be made only in respectively 3 and 12 lines, on account of the very small amounts of enriched isotopes which were available. For the isotopes of odd mass-number precise measurements could be made only when the hyperfine structures were resolved. This was possible for 22 lines of $^{129}$Xe, which has a nuclear spin of $\frac{1}{2}$ with correspondingly simple hyperfine structures but for only 10 lines of $^{131}$Xe, which has a nuclear spin of $\frac{3}{2}$ with more complex hyperfine structures, with much narrower separations. The lines for which isotope shifts were measured were (with two exceptions) due to transitions either between the electron configurations 5p$^{5}$, 6 or 7p and 5p$^{5}$6s or the configurations 5p$^{5}$, 6, 7, 8 or 9d and 5p$^{5}$6p. The shifts in 24 lines due to the first type of transitions are invariably in the opposite sense to the Bohr shift and from two to six times greater while those in 11 lines due to the second type of transitions are in the same sense as the Bohr shift, and two to three times greater. The isotope shifts show departures from a linear relation between the neutron number and the isotope shift which resemble those found in lines of barium and caesium. The large number of lines in which isotope shifts have been measured permits an extensive test of the hypothesis that the ratio of the field-shifts of the isotopes should be constant in all lines; the validity of this hypothesis is confirmed.