DOPPLER-FREE TWO-PHOTON LASER SPECTROSCOPY OF RYDBERG STATES IN ALKALINE-EARTH ELEMENTS WITH THERMIONIC DETECTION R. Beigang and A. Timmermann J. Phys. Colloques, 44 C7 (1983) C7-137-C7-148 DOI: 10.1051/jphyscol:1983712