A new optical method for the precise determination of hyperfine structure and isotope shifts has been developed. A selective method of excitation, accomplished by means of a source of a single isotope operated in a magnetic field, allows the various hyperfine components to be recorded one or a few at a time. These features permit the use of long interferometers (ours were about 218 mm) of high resolving power where the whole order difference between two of the components may be very large. The isotope shifts in the 2537-Å line of mercury have been measured with a precision unequalled previously. The hyperfine structure of the odd isotopes has been measured with equal precision, though in this case the precision is less than that which has been achieved by radio frequency, double-resonance experiments. Our results are in good agreement with the double-resonance results, a fact which gives added confidence in our isotope shift results. The structure of the line relative to the isotope 198 is: 199A(−513.99±0.43); 204(−510.77±0.43); 201a(−488.96±0.33); 202(−336.96±0.15); 200(−160.29±0.15); 201b(−22.56±0.09); 198(0.00); 199B(224.40±0.23); 201c(229.23±0.51); where the units are millikaysers. The stated limits indicate the spread of the individual determinations about the mean value of the quantity as measured by their standard deviations. The magnetic dipole interaction constants for the 63P1° state are: A(199)=492.24±0.20 mK and A(201)=−181.88±0.13 mK. The nuclear magnetic moments calculated from these constants by means of the theory of Breit and Wills for intermediate coupling are: μI(199)=0.450 nm and μI(210)=−0.499 nm. The agreement between these values and the nuclear magnetic resonance values of Cagnac and Brossel (about 10%) is presumably an indication of the reliability of the theory for this case. The electric quadrupole interaction constant for the 63P1° state of Hg201 is B=−9.35±0.18 mK. This leads to the value for the quadrupole moment: Q=0.49 barn.
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