Experiments on one- and two-photon laser excitation of
232Th
+ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the
(6d27s)J=3/2 ground state to the
(6d7s7p)J=5/2 state at 24874 cm${}^{\ensuremath{-}1}$ is driven by radiation from an extended-cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, which allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49 960 cm${}^{\ensuremath{-}1}$, among the highest-lying classified levels of Th
+. This is of interest as a test case for the search for higher-lying levels in the range above 55 000 cm${}^{\ensuremath{-}1}$ which can resonantly enhance the excitation of the
229Th
+ nuclear resonance through an inverse two-photon electronic bridge process.
The decay rate of the Hg 61P1 level was measured as a function of cold spot temperature (Hg density) and buffer gas pressure in cylindrical, sealed fused silica cells. The decay rates were obtained using a time-resolved laser-induced 185 nm fluorescence experiment with multi-step excitation. Cold spot temperatures from 25 to 100 °C were studied. The Hg densities for this temperature range and with no buffer gas yield the lowest possible decay rates due to radiation trapping with partial frequency redistribution. Decay rates with argon buffer gas pressures of 3 and 30 Torr were also studied. The results are in agreement with published data from a discharge afterglow experiment. Monte Carlo simulations of radiation transport in the cells, including the effects of hyperfine and isotope structure, the effects of foreign gas broadening, and partial frequency redistribution are compared to the experimental data. Reasonably good agreement is obtained, however there is evidence of quenching of Hg 61P1 atoms in collisions with ground state Hg and Ar atoms. An analytic formula for the fundamental mode trapped decay rate of the 61P1 level, which is applicable over a substantial region of parameter space, was devised from the Monte Carlo results.