LAMB SHIFTS AND HYPERFINE-STRUCTURE IN LI-6(-7(+) - THEORY AND EXPERIMENT() AND LI)

High-precision laser-resonance measurements accurate to +/-0.5 MHz, or better are reported for transitions among the 1s2s S-3(1)-1s2p P-3(J) hyperfine manifolds for each of J = 0, 1, and 2 in both Li-6(+) and L i-7(+). A detailed analysis of hyperfine structure is performed for bo th the S and P states, using newly calculated values for the magnetic dipole and electric quadrupole coupling constants, and the hyperfine s hifts subtracted from the measurements. The resulting transition frequ encies are then analysed on three different levels. First, the isotope shifts in the fine-structure splittings are calculated from the relat ivistic reduced mass and recoil terms in the Breit interaction, and co mpared with experiment at the +/-0.5-MHz level of accuracy. This compa rison is particularly significant because J-independent theoretical un certainties reduce through cancellation to the +/-0.01-MHz level. Seco nd, the isotope shifts in the full transition frequencies are used to deduce the difference in rms nuclear radii. The result is R(rms)(Li-6) - R(rms)(Li-7) = 0.15 +/- 0.01 fm, in agreement With nuclear scatteri ng data, but with substantially improved accuracy. Third, high-precisi on calculations of the low-order non-QED contributions to the transiti on frequencies are subtracted from the measurements to obtain the resi dual QED shifts. The isotope-averaged and spin-averaged effective shif t for Li-7(+) is 37 429.40 +/- 0.39 MHz, with an additional uncertaint y of +/-1.5 MHs due to finite nuclear size corrections. The accuracy o f 11 parts per million is the best two-electron Lamb shift measurement in the literature, and is comparable to the accuracies achieved in hy drogen. Theoretical contributions to the two-electron Lamb shift are d iscussed, including terms of order (alpha Z)(4) recently obtained by C hen, Cheng, and Johnson [Phys. Rev, A 47, 3692 (1993)], and the result s used to extract a QED shift for the 2 S-3(1) state. The result of 30 254 +/- 12 MHz is shown to be in good accord with theory (30 250 +/- 30 MHz) when two-electron corrections to the Bethe logarithm are taken into account by a 1/Z expansion method.