Observation of a midinfrared fine-structure transition for the single trapped barium ion

We report observation of laser excitation of a narrow, magnetic-dipole-allowed, infrared transition in a single trapped ion. Quantum jumps of a Ba⁺ atom were observed using a diode laser tuned in resonance with the excited-state 5d ²{D}3/2$--5d $²D5/2 transition at 12.48 {μ}m. Single-ion resonance line shapes were obtained and a transition frequency of {ν}=24 012 033±{}11 MHz was determined. Calculated quantum jump rates of the D-D transition for the applied laser spectral brightness yield agreement with experimentally observed values.