Abstract
Three-photon resonance ionization of atomic manganese (Mn) in a hot-cavity ion source using
Ti:sapphire lasers has been demonstrated. Three-step ionization schemes employing different
intermediate levels and Rydberg or autoionizing (AI) states in the final ionization step are
established. Strong AI resonances were observed via the 3d54s5s f 6S5/2 level at 49 415.35 cm−1,
while Rydberg transitions were reached from the 3d54s4d e 6D9/2,7/2,5/2 levels at around
47 210 cm−1. Analyses of the strong Rydberg transitions associated with the 3d54s4d e 6D7/2
lower level indicate that they belong to the dipole-allowed 4d→nf 6F°9/2,7/2,5/2 series converging
to the 3d54s 7S3 ground state of Mn II. From this series, an ionization potential of
59 959.56 ± 0.01 cm−1 is obtained for Mn. At high ion source temperatures the semi-forbidden
4d→nf 8F°9/2,7/2,5/2 series was also observed. The overall ionization efficiency for Mn has been
measured to be about 0.9% when using the strong AI transition in the third excitation step and
0.3% when employing an intense Rydberg transition. Experimental data indicate that the
ionization efficiency was limited by the interaction of Mn atoms with ion source materials at high
temperatures.