High-resolution three-photon laser spectroscopy has been performed for the first time to gain new spectroscopic information. Starting from the $6s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ ground state of Yb, $6s15p{}_{}{}^{1,3}{}_{}{}^{}P_1^{}{}_{}{}^{}$ Rydberg states were populated by two-photon-resonant, three-photon absorption via the $6s7s{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ intermediate level. As a result of two-photon line narrowing Doppler-free spectra, showing well resolved isotope shifts and hyperfine structures, were obtained. From the hyperfine coupling constants ${}_{}{}^{171,173}{}_{}{}^{}A\left({}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}\right)$ the singlet-triplet mixing was derived.

• Received 15 July 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.1340