Two-photon spectroscopy of low-lying states of lithium: energy levels, hyperfine structure, and isotope shifts

Summary form only given. We have used non-resonant two-photon laser spectroscopy to observe the 2S-3S and 2S-4S transitions of Li. The structure of these levels is shown. As the laser scans across each of these transitions, four are observed. For each of these resonances we manually set the laser to the peak of a transition. A computer controlled vacuum Fabry-Perot wavemeter determines the laser frequency with respect to an I/sub 2/-stabilized He-Ne laser with an accuracy of a few parts in 10/sup 9/. From these measurements we obtain precise transition energies between the ground state and excited state hyperfine levels. Because the ground state hyperfine splitting is known to extremely high precision, we are able to calculate the excited state hyperfine constants for the 3S and 4S states with a precision of about 1 MHz for both isotopes. Our value for the 3S hyperfine constant resolves a discrepancy between an earlier high precision experimental determination and recent theoretical calculations.