Determination of transition frequencies in a single ¹³⁸Ba⁺ ion

Transition frequencies between low-lying energy levels in a single trapped Ba-138(+) ion have been measured with laser spectroscopy referenced to an optical frequency comb. By extracting the frequencies of one-photon and two-photon components of the line shape using an eight-level optical Bloch model, we achieved 0.1 MHz accuracy for the 5d D-2(3/2)-6p P-2(1/2) and 6s S-2(1/2)-5d D-2(3/2) transition frequencies, and 0.2MHz for the 6s S-2(1/2)-6p P-2(1/2) transition frequency.