The structure of the resonance line ⋋553.5 nm of Ba ɪ has been studied by means of Doppler-free spectroscopy. Dye laser light was scattered resonantly from an atomic beam containing the natural mixture of isotopes; the spectra were recorded digitally and analysed by computer. The main object was to provide data to reduce uncertainties due to electronic calculations in the extraction of nuclear data from optical measurements. The isotope shifts (MHz), relative to
138
Ba, are
137
Ba, – 214.7 (5);
136
Ba, – 127.5 (13);
135
Ba, – 258.7 (7);
134
Ba, – 142.8 (12). The minus signs indicate that
138
Ba has the lower frequency in each case. These results are combined with muonic X-ray, electronic X-ray and other optical data to extract changes in mean square radius of the nuclear charge distribution. The hyperfine splitting factors
135
A
and
135
B
were found to be – 97.76 (14) MHz and 32.29 (47) MHz respectively. From an analysis of these and other results for the 6s6p configuration we obtain the quadrupole moment of
135
Ba (uncorrected for quadrupole shielding) to be 0.195 (40) barn (1 barn = 10
-28
m
2
).
Variations of Nuclear Charge Radii in Mercury Isotopes with A = 198, 199, 200, 201, 202, and 204 from X-Ray Isotope Shifts.