We study the formation and destabilization of dark states in a single trapped
88Sr+ ion caused by the cooling and repumping laser fields required for Doppler cooling and fluorescence detection of the ion. By numerically solving the time-dependent density matrix equations for the eight-level system consisting of the sublevels of the
5s2S1/2,
5p2P1/2, and
4d2D3/2 states, we analyze the different types of dark states and how to prevent them in order to maximize the scattering rate, which is crucial for both the cooling and the detection of the ion. The influence of the laser linewidths and ion motion on the scattering rate and the dark resonances is studied. The calculations are then compared with experimental results obtained with an endcap ion trap system located at the National Research Council of Canada and found to be in good agreement. The results are applicable also to other alkaline-earth-metal ions and isotopes without hyperfine structure.
Doppler laser cooling of a three-level ladder system using two near-resonant laser fields is analyzed in the case of the intermediate level being metastable while the upper level is short lived. Analytical as well as numerical results, e.g., obtainable scattering rates and achievable temperatures are presented. When appropriate, comparisons with two-level single-photon Doppler laser cooling is made. These results are relevant to recent experimental Doppler laser cooling investigations addressing intercombination lines in alkaline-earth-metal atoms and quadrupole transitions in alkaline-earth-metal ions.
The study of the creation of doubly charged ions upon the laser-induced multiphoton ionization of alkaline-earth metals is reviewed. The results show that the features of the creation of the doubly charged ions depend on the spectral range of the ionizing laser radiation. In the ionization of the alkaline-earth atoms by the visible laser radiation, the cascade creation of the doubly charged ions is predominantly implemented when the ions result from the multiphoton ionization of singly charged ions created in the presence of the same laser pulse. In the multiphoton ionization of the alkaline-earth atoms by the IR laser radiation, the two-electron process occurs when the doubly charged ions are directly created from neutral atoms owing to the ionization via induced resonances with strongly perturbed states of these atoms.