COLLISIONAL POPULATION TRAPPING AND OPTICAL DEEXCITATION OF YTTERBIUM IONS IN A RADIOFREQUENCY TRAP

We describe and discuss experiments related to the phenomenon of population trapping of ytterbium ions in the extremely long-lived 4f13 6s2 2F7/2 level. Ion clouds are confined in a radiofrequency trap in the presence of helium buffer gas. The experimental set-up permits electronic ion detection, the observation of resonance fluorescence, and optical excitation at alternating wavelengths to excite either the 2S1/2-2P1/2 resonance transition, or transitions of similar wavelength which provide optical deexcitation of ions from the 2F7/2 level to the ground state. The observed dependence of the population trapping rates on helium pressure is attributed to the effect of fine structure mixing collisions. The transitions which deexcite ions from the 2F7/2 state show Lorentzian lineshapes with widths of 6-10 GHz and resonance wavelengths which are in significant disagreement with tabulated level positions.