Doppler-Free, Linear, Single-Photon Spectroscopy of Metastable Atoms and Molecules

Linear, Doppler-free laser spectroscopy is applied to atoms or molecules having metastable states. A tightly collimated beam of metastables is crossed with a tunable-dye-laser beam. Absorption of light quenches the metastables beam. The method has been used to resolve and measure, for the first time, the fine structure of the $3s, 3d:{}_{}{}^3{}_{}{}^{}\Sigma , {}_{}{}^3{}_{}{}^{}\Pi , {}_{}{}^3{}_{}{}^{}\Delta$ complex of the hydrogen molecule. Isotope shifts have been measured in atomic neon. A discussion is given of the possibility of increasing the precision in the measurement of the Rydberg constant.