3-PHOTON EXCITATION OF STRONTIUM RYDBERG LEVELS

We report here the first measurements of the three-photon excitation s pectrum of strontium using a Nd:YAG pumped dye laser in conjunction wi th an atomic beam apparatus as well as with a thermionic diode detecto r. The members of the 5snp P-1(1) and 5snf F-1(3) Rydberg series conve rging to the first ionization threshold are observed. A dominating two -photon resonant three-photon ionization through the 5s6s S-1(0) level of strontium lying adjacent to the three photon ionization threshold restricted the observation of the higher Rydberg members. (C) 1998 Els evier Science B.V. All rights reserved.