Precision two-photon spectroscopy of alkali elements

In this paper, we have briefly reviewed the work on two-photon spectroscopy of alkali elements and its applications. The technique of Doppler-free two-photon spectroscopy is briefly summarized. A review of various techniques adopted for measuring absolute frequencies of the atomic transitions and precision measurements of isotope shifts and hyperfine structures (HFS) is presented. Some of the recent works on precision measurements of HFS constants of 6 s 2 S 1/2 level of 39 K and 41 K, 9 s 2 S 1/2 level and 7 d 2 D 3/2 level of 133 Cs are also discussed.